The OCaml system

release 5.3
Documentation and user’s manual

Xavier Leroy,
Damien Doligez, Alain Frisch, Jacques Garrigue,
Didier Rémy, KC Sivaramakrishnan and Jérôme Vouillon

January 8, 2025

Copyright © 2025 Institut National de Recherche en Informatique et en Automatique

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Contents

I An introduction to OCaml 13

1 The core language 15

1.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.2 Data types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3 Functions as values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.4 Records and variants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.5 Imperative features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.6 Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1.7 Lazy expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
1.8 Symbolic processing of expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1.9 Pretty-printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
1.10 Printf formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
1.11 Standalone OCaml programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2 The module system 35

2.1 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.2 Signatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.3 Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4 Functors and type abstraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.5 Modules and separate compilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3 Objects in OCaml 45
3.1 Classes and objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2 Immediate objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3 Reference to self . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.4 Initializers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.5 Virtual methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6 Private methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.7 Class interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.8 Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.9 Multiple inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.10 Parameterized classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.11 Polymorphic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.12 Using coercions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.13 Functional objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

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3.14 Cloning objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3.15 Recursive classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.16 Binary methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.17 Friends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4 Labeled arguments 77
4.1 Optional arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.2 Labels and type inference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.3 Suggestions for labeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5 Polymorphic variants 83
5.1 Basic use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.2 Advanced use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.3 Weaknesses of polymorphic variants . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

6 Polymorphism and its limitations 87

6.1 Weak polymorphism and mutation . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.2 Polymorphic recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.3 Higher-rank polymorphic functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

7 Generalized algebraic datatypes 97

7.1 Recursive functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
7.2 Type inference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
7.3 Refutation cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.4 Advanced examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
7.5 Existential type names in error messages . . . . . . . . . . . . . . . . . . . . . . . . 101
7.6 Explicit naming of existentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
7.7 Equations on non-local abstract types . . . . . . . . . . . . . . . . . . . . . . . . . . 102

8 Advanced examples with classes and modules 103

8.1 Extended example: bank accounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
8.2 Simple modules as classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
8.3 The subject/observer pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

9 Parallel programming 119

9.1 Domains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
9.2 Domainslib: A library for nested-parallel programming . . . . . . . . . . . . . . . . 121
9.3 Parallel garbage collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
9.4 Memory model: The easy bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
9.5 Blocking synchronisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
9.6 Interaction with C bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
9.7 Atomics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
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10 Memory model: The hard bits 133

10.1 Why weakly consistent memory? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
10.2 Data race freedom implies sequential consistency . . . . . . . . . . . . . . . . . . . 136
10.3 Reasoning with DRF-SC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
10.4 Local data race freedom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
10.5 An operational view of the memory model . . . . . . . . . . . . . . . . . . . . . . . 141
10.6 Non-compliant operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

II The OCaml language 147

11 The OCaml language 149

11.1 Lexical conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
11.2 Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
11.3 Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
11.4 Type expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.5 Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
11.6 Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
11.7 Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
11.8 Type and exception definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
11.9 Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
11.10 Module types (module specifications) . . . . . . . . . . . . . . . . . . . . . . . . . . 203
11.11 Module expressions (module implementations) . . . . . . . . . . . . . . . . . . . . . 208
11.12 Compilation units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

12 Language extensions 213

12.1 Recursive definitions of values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
12.2 Recursive modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
12.3 Private types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
12.4 Locally abstract types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
12.5 First-class modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
12.6 Recovering the type of a module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
12.7 Substituting inside a signature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
12.8 Type-level module aliases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
12.9 Overriding in open statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
12.10 Generalized algebraic datatypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
12.11 Syntax for Bigarray access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
12.12 Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
12.13 Extension nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
12.14 Extensible variant types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
12.15 Generative functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
12.16 Extension-only syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
12.17 Inline records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
12.18 Documentation comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
12.19 Extended indexing operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
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12.20 Empty variant types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

12.21 Alerts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
12.22 Generalized open statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
12.23 Binding operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
12.24 Effect handlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

III The OCaml tools 269

13 Batch compilation (ocamlc) 271

13.1 Overview of the compiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
13.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
13.3 Modules and the file system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287
13.4 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
13.5 Warning reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290

14 The toplevel system or REPL (ocaml) 297

14.1 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
14.2 Toplevel directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
14.3 The toplevel and the module system . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
14.4 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
14.5 Building custom toplevel systems: ocamlmktop . . . . . . . . . . . . . . . . . . . . . 312
14.6 The native toplevel: ocamlnat (experimental) . . . . . . . . . . . . . . . . . . . . . 313

15 The runtime system (ocamlrun) 315

15.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
15.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
15.3 Dynamic loading of shared libraries . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
15.4 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

16 Native-code compilation (ocamlopt) 321

16.1 Overview of the compiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
16.2 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322
16.3 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338
16.4 Running executables produced by ocamlopt . . . . . . . . . . . . . . . . . . . . . . 338
16.5 Compatibility with the bytecode compiler . . . . . . . . . . . . . . . . . . . . . . . 338

17 Lexer and parser generators (ocamllex, ocamlyacc) 341

17.1 Overview of ocamllex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
17.2 Syntax of lexer definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
17.3 Overview of ocamlyacc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.4 Syntax of grammar definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
17.5 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
17.6 A complete example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
17.7 Common errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
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18 Dependency generator (ocamldep) 355

18.1 Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
18.2 A typical Makefile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

19 The documentation generator (ocamldoc) 361

19.1 Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
19.2 Syntax of documentation comments . . . . . . . . . . . . . . . . . . . . . . . . . . . 368
19.3 Custom generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
19.4 Adding command line options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

20 The debugger (ocamldebug) 383

20.1 Compiling for debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
20.2 Invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383
20.3 Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
20.4 Executing a program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
20.5 Breakpoints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388
20.6 The call stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
20.7 Examining variable values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389
20.8 Controlling the debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390
20.9 Miscellaneous commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394
20.10 Running the debugger under Emacs . . . . . . . . . . . . . . . . . . . . . . . . . . . 394

21 Profiling (ocamlprof) 397

21.1 Compiling for profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397
21.2 Profiling an execution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398
21.3 Printing profiling information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398
21.4 Time profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399

22 Interfacing C with OCaml 401

22.1 Overview and compilation information . . . . . . . . . . . . . . . . . . . . . . . . . 401
22.2 The value type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407
22.3 Representation of OCaml data types . . . . . . . . . . . . . . . . . . . . . . . . . . 409
22.4 Operations on values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
22.5 Living in harmony with the garbage collector . . . . . . . . . . . . . . . . . . . . . 416
22.6 A complete example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
22.7 Advanced topic: callbacks from C to OCaml . . . . . . . . . . . . . . . . . . . . . . 425
22.8 Advanced example with callbacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433
22.9 Advanced topic: custom blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
22.10 Advanced topic: Bigarrays and the OCaml-C interface . . . . . . . . . . . . . . . . 440
22.11 Advanced topic: cheaper C call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
22.12 Advanced topic: multithreading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444
22.13 Advanced topic: interfacing with Windows Unicode APIs . . . . . . . . . . . . . . . 447
22.14 Building mixed C/OCaml libraries: ocamlmklib . . . . . . . . . . . . . . . . . . . . 449
22.15 Cautionary words: the internal runtime API . . . . . . . . . . . . . . . . . . . . . . 451
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23 Optimisation with Flambda 453

23.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453
23.2 Command-line flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453
23.3 Inlining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
23.4 Specialisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461
23.5 Default settings of parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464
23.6 Manual control of inlining and specialisation . . . . . . . . . . . . . . . . . . . . . . 465
23.7 Simplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466
23.8 Other code motion transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . 467
23.9 Unboxing transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468
23.10 Removal of unused code and values . . . . . . . . . . . . . . . . . . . . . . . . . . . 472
23.11 Other code transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472
23.12 Treatment of effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
23.13 Compilation of statically-allocated modules . . . . . . . . . . . . . . . . . . . . . . . 474
23.14 Inhibition of optimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474
23.15 Use of unsafe operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474
23.16 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475

24 Fuzzing with afl-fuzz 477

24.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477
24.2 Generating instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477
24.3 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

25 Runtime tracing with runtime events 479

25.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479
25.2 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480
25.3 Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481
25.4 Custom events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

26 The “Tail Modulo Constructor” program transformation 489

26.1 Disambiguation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492
26.2 Danger: getting out of tail-mod-cons . . . . . . . . . . . . . . . . . . . . . . . . . . 493
26.3 Details on the transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496
26.4 Current limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498

27 Runtime detection of data races with ThreadSanitizer 501

27.1 Overview and usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501
27.2 Performance implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503
27.3 False negatives and false positives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503
27.4 Runtime options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
27.5 Guidelines for linking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
27.6 Changes in the delivery of signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505
 7

IV The OCaml library 507

28 The core library 509

28.1 Built-in types and predefined exceptions . . . . . . . . . . . . . . . . . . . . . . . . 509
28.2 Module Stdlib : The OCaml Standard library. . . . . . . . . . . . . . . . . . . . . 512

29 The standard library 541

29.1 Module Arg : Parsing of command line arguments. . . . . . . . . . . . . . . . . . . 543
29.2 Module Array : Array operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 548
29.3 Module ArrayLabels : Array operations. . . . . . . . . . . . . . . . . . . . . . . . . 556
29.4 Module Atomic : Atomic references. . . . . . . . . . . . . . . . . . . . . . . . . . . . 564
29.5 Module Bigarray : Large, multi-dimensional, numerical arrays. . . . . . . . . . . . 567
29.6 Module Bool : Boolean values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589
29.7 Module Buffer : Extensible buffers. . . . . . . . . . . . . . . . . . . . . . . . . . . . 590
29.8 Module Bytes : Byte sequence operations. . . . . . . . . . . . . . . . . . . . . . . . 595
29.9 Module BytesLabels : Byte sequence operations. . . . . . . . . . . . . . . . . . . . 609
29.10 Module Callback : Registering OCaml values with the C runtime. . . . . . . . . . 623
29.11 Module Char : Character operations. . . . . . . . . . . . . . . . . . . . . . . . . . . 623
29.12 Module Complex : Complex numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . 624
29.13 Module Condition : Condition variables. . . . . . . . . . . . . . . . . . . . . . . . . 626
29.14 Module Domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 628
29.15 Module Digest : Message digest. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631
29.16 Module Dynarray : Dynamic arrays. . . . . . . . . . . . . . . . . . . . . . . . . . . 635
29.17 Module Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 646
29.18 Module Either : Either type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649
29.19 Module Ephemeron : Ephemerons and weak hash tables. . . . . . . . . . . . . . . . 651
29.20 Module Filename : Operations on file names. . . . . . . . . . . . . . . . . . . . . . 657
29.21 Module Float : Floating-point arithmetic. . . . . . . . . . . . . . . . . . . . . . . . 661
29.22 Module Format : Pretty-printing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683
29.23 Module Fun : Function manipulation. . . . . . . . . . . . . . . . . . . . . . . . . . . 711
29.24 Module Gc : Memory management control and statistics; finalised values. . . . . . . 712
29.25 Module Hashtbl : Hash tables and hash functions. . . . . . . . . . . . . . . . . . . 722
29.26 Module In_channel : Input channels. . . . . . . . . . . . . . . . . . . . . . . . . . . 734
29.27 Module Int : Integer values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739
29.28 Module Int32 : 32-bit integers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742
29.29 Module Int64 : 64-bit integers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 746
29.30 Module Lazy : Deferred computations. . . . . . . . . . . . . . . . . . . . . . . . . . 751
29.31 Module Lexing : The run-time library for lexers generated by ocamllex. . . . . . . 753
29.32 Module List : List operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 756
29.33 Module ListLabels : List operations. . . . . . . . . . . . . . . . . . . . . . . . . . 765
29.34 Module Map : Association tables over ordered types. . . . . . . . . . . . . . . . . . . 774
29.35 Module Marshal : Marshaling of data structures. . . . . . . . . . . . . . . . . . . . 782
29.36 Module MoreLabels : Extra labeled libraries. . . . . . . . . . . . . . . . . . . . . . 785
29.37 Module Mutex : Locks for mutual exclusion. . . . . . . . . . . . . . . . . . . . . . . 812
29.38 Module Nativeint : Processor-native integers. . . . . . . . . . . . . . . . . . . . . . 813
8

29.39 Module Oo : Operations on objects . . . . . . . . . . . . . . . . . . . . . . . . . . . 817

29.40 Module Option : Option values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 818
29.41 Module Out_channel : Output channels. . . . . . . . . . . . . . . . . . . . . . . . . 819
29.42 Module Parsing : The run-time library for parsers generated by ocamlyacc. . . . . 824
29.43 Module Printexc : Facilities for printing exceptions and inspecting current call stack.825
29.44 Module Printf : Formatted output functions. . . . . . . . . . . . . . . . . . . . . . 832
29.45 Module Queue : First-in first-out queues. . . . . . . . . . . . . . . . . . . . . . . . . 835
29.46 Module Random : Pseudo-random number generators (PRNG). . . . . . . . . . . . . 839
29.47 Module Result : Result values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844
29.48 Module Scanf : Formatted input functions. . . . . . . . . . . . . . . . . . . . . . . 846
29.49 Module Seq : Sequences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 856
29.50 Module Set : Sets over ordered types. . . . . . . . . . . . . . . . . . . . . . . . . . . 869
29.51 Module Semaphore : Semaphores . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875
29.52 Module Stack : Last-in first-out stacks. . . . . . . . . . . . . . . . . . . . . . . . . . 877
29.53 Module StdLabels : Standard labeled libraries. . . . . . . . . . . . . . . . . . . . . 879
29.54 Module String : Strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 880
29.55 Module StringLabels : Strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 889
29.56 Module Sys : System interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 898
29.57 Module Type : Type introspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . 906
29.58 Module Uchar : Unicode characters. . . . . . . . . . . . . . . . . . . . . . . . . . . . 909
29.59 Module Unit : Unit values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 911
29.60 Module Weak : Arrays of weak pointers and hash sets of weak pointers. . . . . . . . 912
29.61 Ocaml_operators : Precedence level and associativity of operators . . . . . . . . . . 915

30 The compiler front-end 917

30.1 Module Ast_mapper : The interface of a -ppx rewriter . . . . . . . . . . . . . . . . 917
30.2 Module Asttypes : Auxiliary AST types used by parsetree and typedtree. . . . . . 921
30.3 Module Location : Source code locations (ranges of positions), used in parsetree. . 923
30.4 Module Longident : Long identifiers, used in parsetree. . . . . . . . . . . . . . . . . 930
30.5 Module Parse : Entry points in the parser . . . . . . . . . . . . . . . . . . . . . . . 930
30.6 Module Parsetree : Abstract syntax tree produced by parsing . . . . . . . . . . . 932
30.7 Module Pprintast : Pretty-printers for Parsetree[30.6] . . . . . . . . . . . . . . . 957

31 The unix library: Unix system calls 959

31.1 Module Unix : Interface to the Unix system. . . . . . . . . . . . . . . . . . . . . . . 959
31.2 Module UnixLabels: labelized version of the interface . . . . . . . . . . . . . . . . . 1003

32 The str library: regular expressions and string processing 1007

32.1 Module Str : Regular expressions and high-level string processing . . . . . . . . . . 1007

33 The runtime_events library 1015

33.1 Module Runtime_events : Runtime events - ring buffer-based runtime tracing . . . 1015

34 The threads library 1029

34.1 Module Thread : Lightweight threads for Posix 1003.1c and Win32. . . . . . . . . 1029
34.2 Module Event : First-class synchronous communication. . . . . . . . . . . . . . . . 1032
 9

35 The dynlink library: dynamic loading and linking of object files 1035
35.1 Module Dynlink : Dynamic loading of .cmo, .cma and .cmxs files. . . . . . . . . . . 1035

36 Recently removed or moved libraries (Graphics, Bigarray, Num, LablTk) 1039

36.1 The Graphics Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039
36.2 The Bigarray Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1039
36.3 The Num Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1040
36.4 The Labltk Library and OCamlBrowser . . . . . . . . . . . . . . . . . . . . . . . . . 1040

V Indexes 1041

Index to the library 1043

Index of keywords 1064

10
Foreword

This manual documents the release 5.3 of the OCaml system. It is organized as follows.

• Part I, “An introduction to OCaml”, gives an overview of the language.

• Part II, “The OCaml language”, is the reference description of the language.

• Part III, “The OCaml tools”, documents the compilers, toplevel system, and programming
utilities.

• Part IV, “The OCaml library”, describes the modules provided in the standard library.

• Part V, “Indexes”, contains an index of all identifiers defined in the standard library, and an
index of keywords.

Conventions
OCaml runs on several operating systems. The parts of this manual that are specific to one operating
system are presented as shown below:

Unix:
This is material specific to the Unix family of operating systems, including Linux and macOS.

Windows:
This is material specific to Microsoft Windows (Vista, 7, 8, 10, 11).

License
The OCaml system is copyright © 1996–2025 Institut National de Recherche en Informatique et en
Automatique (INRIA). INRIA holds all ownership rights to the OCaml system.
The OCaml system is open source and can be freely redistributed. See the file LICENSE in the
distribution for licensing information.
The OCaml documentation and user’s manual is copyright © 2025 Institut National de Recherche
en Informatique et en Automatique (INRIA).
The OCaml documentation and user’s manual is licensed under a Creative Commons Attribution-
ShareAlike 4.0 International License (CC BY-SA 4.0), https://creativecommons.org/licenses/
by-sa/4.0/.

11
12 Foreword

The sample code in the user’s manual and in the reference documentation of the standard library
is licensed under a Creative Commons CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
License, https://creativecommons.org/publicdomain/zero/1.0/.

Availability
The complete OCaml distribution can be accessed via the website https://ocaml.org/. This site
contains a lot of additional information on OCaml.
 Part I

An introduction to OCaml

13
Chapter 1

The core language

This part of the manual is a tutorial introduction to the OCaml language. A good familiarity with
programming in a conventional languages (say, C or Java) is assumed, but no prior exposure to
functional languages is required. The present chapter introduces the core language. Chapter 2
deals with the module system, chapter 3 with the object-oriented features, chapter 4 with labeled
arguments, chapter 5 with polymorphic variants, chapter 6 with the limitations of polymorphism,
and chapter 8 gives some advanced examples.

1.1 Basics
For this overview of OCaml, we use the interactive system, which is started by running ocaml from
the Unix shell or Windows command prompt. This tutorial is presented as the transcript of a
session with the interactive system: lines starting with # represent user input; the system responses
are printed below, without a leading #.
Under the interactive system, the user types OCaml phrases terminated by ;; in response to
the # prompt, and the system compiles them on the fly, executes them, and prints the outcome of
evaluation. Phrases are either simple expressions, or let definitions of identifiers (either values or
functions).
# 1 + 2 * 3;;
- : int = 7

# let pi = 4.0 *. atan 1.0;;

val pi : float = 3.14159265358979312

# let square x = x *. x;;

val square : float -> float = <fun>

# square (sin pi) +. square (cos pi);;

- : float = 1.
The OCaml system computes both the value and the type for each phrase. Even function parameters
need no explicit type declaration: the system infers their types from their usage in the function.
Notice also that integers and floating-point numbers are distinct types, with distinct operators: +
and * operate on integers, but +. and *. operate on floats.

15
16

# 1.0 * 2;;
Error : The constant 1.0 has type float but an expression was expected of type
int
Recursive functions are defined with the let rec binding:
# let rec fib n =
if n < 2 then n else fib (n - 1) + fib (n - 2);;
val fib : int -> int = <fun>

# fib 10;;
- : int = 55

1.2 Data types

In addition to integers and floating-point numbers, OCaml offers the usual basic data types:
• booleans
# (1 < 2) = false;;
- : bool = false

# let one = if true then 1 else 2;;

val one : int = 1

• characters
# 'a';;
- : char = 'a'

# int_of_char '\n';;
- : int = 10

• immutable character strings

# "Hello" ^ " " ^ "world";;
- : string = "Hello world"

# {|This is a quoted string, here, neither \ nor " are special characters|};;
- : string =
"This is a quoted string, here, neither \\ nor \" are special characters"

# {|"\\"|}="\"\\\\\"";;
- : bool = true

# {delimiter|the end of this|}quoted string is here|delimiter}

= "the end of this|}quoted string is here";;
- : bool = true
Chapter 1. The core language 17

Predefined data structures include tuples, arrays, and lists. There are also general mechanisms
for defining your own data structures, such as records and variants, which will be covered in more
detail later; for now, we concentrate on lists. Lists are either given in extension as a bracketed list of
semicolon-separated elements, or built from the empty list [] (pronounce “nil”) by adding elements
in front using the :: (“cons”) operator.
# let l = ["is"; "a"; "tale"; "told"; "etc."];;
val l : string list = ["is"; "a"; "tale"; "told"; "etc."]

# "Life" :: l;;
- : string list = ["Life"; "is"; "a"; "tale"; "told"; "etc."]
As with all other OCaml data structures, lists do not need to be explicitly allocated and deallocated
from memory: all memory management is entirely automatic in OCaml. Similarly, there is no
explicit handling of pointers: the OCaml compiler silently introduces pointers where necessary.
As with most OCaml data structures, inspecting and destructuring lists is performed by pattern-
matching. List patterns have exactly the same form as list expressions, with identifiers representing
unspecified parts of the list. As an example, here is insertion sort on a list:
# let rec sort lst =
match lst with
[] -> []
| head :: tail -> insert head (sort tail)
and insert elt lst =
match lst with
[] -> [elt]
| head :: tail -> if elt <= head then elt :: lst else head :: insert elt tail
;;
val sort : 'a list -> 'a list = <fun>
val insert : 'a -> 'a list -> 'a list = <fun>

# sort l;;
- : string list = ["a"; "etc."; "is"; "tale"; "told"]
The type inferred for sort, 'a list -> 'a list, means that sort can actually apply to lists
of any type, and returns a list of the same type. The type 'a is a type variable, and stands for any
given type. The reason why sort can apply to lists of any type is that the comparisons (=, <=, etc.)
are polymorphic in OCaml: they operate between any two values of the same type. This makes
sort itself polymorphic over all list types.
# sort [6; 2; 5; 3];;
- : int list = [2; 3; 5; 6]

# sort [3.14; 2.718];;

- : float list = [2.718; 3.14]
The sort function above does not modify its input list: it builds and returns a new list containing
the same elements as the input list, in ascending order. There is actually no way in OCaml to
modify a list in-place once it is built: we say that lists are immutable data structures. Most OCaml
18

data structures are immutable, but a few (most notably arrays) are mutable, meaning that they can
be modified in-place at any time.
The OCaml notation for the type of a function with multiple arguments is
arg1_type -> arg2_type -> ... -> return_type. For example, the type inferred for insert,
'a -> 'a list -> 'a list, means that insert takes two arguments, an element of any type 'a
and a list with elements of the same type 'a and returns a list of the same type.

1.3 Functions as values

OCaml is a functional language: functions in the full mathematical sense are supported and can be
passed around freely just as any other piece of data. For instance, here is a deriv function that
takes any float function as argument and returns an approximation of its derivative function:
# let deriv f dx = fun x -> (f (x +. dx) -. f x) /. dx;;
val deriv : (float -> float) -> float -> float -> float = <fun>

# let sin' = deriv sin 1e-6;;

val sin' : float -> float = <fun>

# sin' pi;;
- : float = -1.00000000013961143
Even function composition is definable:
# let compose f g = fun x -> f (g x);;
val compose : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b = <fun>

# let cos2 = compose square cos;;

val cos2 : float -> float = <fun>
Functions that take other functions as arguments are called “functionals”, or “higher-order
functions”. Functionals are especially useful to provide iterators or similar generic operations over
a data structure. For instance, the standard OCaml library provides a List.map functional that
applies a given function to each element of a list, and returns the list of the results:
# List.map (fun n -> n * 2 + 1) [0;1;2;3;4];;
- : int list = [1; 3; 5; 7; 9]
This functional, along with a number of other list and array functionals, is predefined because it is
often useful, but there is nothing magic with it: it can easily be defined as follows.
# let rec map f l =
match l with
[] -> []
| hd :: tl -> f hd :: map f tl;;
val map : ('a -> 'b) -> 'a list -> 'b list = <fun>
Chapter 1. The core language 19

1.4 Records and variants

User-defined data structures include records and variants. Both are defined with the type declaration.
Here, we declare a record type to represent rational numbers.
# type ratio = {num: int; denom: int};;
type ratio = { num : int; denom : int; }

# let add_ratio r1 r2 =
{num = r1.num * r2.denom + r2.num * r1.denom;
denom = r1.denom * r2.denom};;
val add_ratio : ratio -> ratio -> ratio = <fun>

# add_ratio {num=1; denom=3} {num=2; denom=5};;

- : ratio = {num = 11; denom = 15}
Record fields can also be accessed through pattern-matching:
# let integer_part r =
match r with
{num=num; denom=denom} -> num / denom;;
val integer_part : ratio -> int = <fun>
Since there is only one case in this pattern matching, it is safe to expand directly the argument r in
a record pattern:
# let integer_part {num=num; denom=denom} = num / denom;;
val integer_part : ratio -> int = <fun>
Unneeded fields can be omitted:
# let get_denom {denom=denom} = denom;;
val get_denom : ratio -> int = <fun>
Optionally, missing fields can be made explicit by ending the list of fields with a trailing wildcard
_::
# let get_num {num=num; _ } = num;;
val get_num : ratio -> int = <fun>
When both sides of the = sign are the same, it is possible to avoid repeating the field name by
eliding the =field part:
# let integer_part {num; denom} = num / denom;;
val integer_part : ratio -> int = <fun>
This short notation for fields also works when constructing records:
# let ratio num denom = {num; denom};;
val ratio : int -> int -> ratio = <fun>
At last, it is possible to update few fields of a record at once:
# let integer_product integer ratio = { ratio with num = integer * ratio.num };;
20

val integer_product : int -> ratio -> ratio = <fun>

With this functional update notation, the record on the left-hand side of with is copied except for
the fields on the right-hand side which are updated.
The declaration of a variant type lists all possible forms for values of that type. Each case is
identified by a name, called a constructor, which serves both for constructing values of the variant
type and inspecting them by pattern-matching. Constructor names are capitalized to distinguish
them from variable names (which must start with a lowercase letter). For instance, here is a variant
type for doing mixed arithmetic (integers and floats):
# type number = Int of int | Float of float | Error;;
type number = Int of int | Float of float | Error
This declaration expresses that a value of type number is either an integer, a floating-point number,
or the constant Error representing the result of an invalid operation (e.g. a division by zero).
Enumerated types are a special case of variant types, where all alternatives are constants:
# type sign = Positive | Negative;;
type sign = Positive | Negative

# let sign_int n = if n >= 0 then Positive else Negative;;

val sign_int : int -> sign = <fun>
To define arithmetic operations for the number type, we use pattern-matching on the two numbers
involved:
# let add_num n1 n2 =
match (n1, n2) with
(Int i1, Int i2) ->
(∗ Check for overflow of integer addition ∗)
if sign_int i1 = sign_int i2 && sign_int (i1 + i2) <> sign_int i1
then Float(float i1 +. float i2)
else Int(i1 + i2)
| (Int i1, Float f2) -> Float(float i1 +. f2)
| (Float f1, Int i2) -> Float(f1 +. float i2)
| (Float f1, Float f2) -> Float(f1 +. f2)
| (Error, _) -> Error
| (_, Error) -> Error;;
val add_num : number -> number -> number = <fun>

# add_num (Int 123) (Float 3.14159);;

- : number = Float 126.14159
Another interesting example of variant type is the built-in 'a option type which represents
either a value of type 'a or an absence of value:
# type 'a option = Some of 'a | None;;
type 'a option = Some of 'a | None
This type is particularly useful when defining function that can fail in common situations, for
instance
Chapter 1. The core language 21

# let safe_square_root x = if x >= 0. then Some(sqrt x) else None;;

val safe_square_root : float -> float option = <fun>
The most common usage of variant types is to describe recursive data structures. Consider for
example the type of binary trees:
# type 'a btree = Empty | Node of 'a * 'a btree * 'a btree;;
type 'a btree = Empty | Node of 'a * 'a btree * 'a btree
This definition reads as follows: a binary tree containing values of type 'a (an arbitrary type) is
either empty, or is a node containing one value of type 'a and two subtrees also containing values
of type 'a, that is, two 'a btree.
Operations on binary trees are naturally expressed as recursive functions following the same
structure as the type definition itself. For instance, here are functions performing lookup and
insertion in ordered binary trees (elements increase from left to right):
# let rec member x btree =
match btree with
Empty -> false
| Node(y, left, right) ->
if x = y then true else
if x < y then member x left else member x right;;
val member : 'a -> 'a btree -> bool = <fun>

# let rec insert x btree =

match btree with
Empty -> Node(x, Empty, Empty)
| Node(y, left, right) ->
if x <= y then Node(y, insert x left, right)
else Node(y, left, insert x right);;
val insert : 'a -> 'a btree -> 'a btree = <fun>

1.4.1 Record and variant disambiguation

( This subsection can be skipped on the first reading )
Astute readers may have wondered what happens when two or more record fields or constructors
share the same name
# type first_record = { x:int; y:int; z:int }
type middle_record = { x:int; z:int }
type last_record = { x:int };;
# type first_variant = A | B | C
type last_variant = A;;
The answer is that when confronted with multiple options, OCaml tries to use locally available
information to disambiguate between the various fields and constructors. First, if the type of the
record or variant is known, OCaml can pick unambiguously the corresponding field or constructor.
For instance:
22

# let look_at_x_then_z (r:first_record) =

let x = r.x in
x + r.z;;
val look_at_x_then_z : first_record -> int = <fun>

# let permute (x:first_variant) = match x with

| A -> (B:first_variant)
| B -> A
| C -> C;;
val permute : first_variant -> first_variant = <fun>

# type wrapped = First of first_record

let f (First r) = r, r.x;;
type wrapped = First of first_record
val f : wrapped -> first_record * int = <fun>
In the first example, (r:first_record) is an explicit annotation telling OCaml that the type
of r is first_record. With this annotation, Ocaml knows that r.x refers to the x field of the first
record type. Similarly, the type annotation in the second example makes it clear to OCaml that the
constructors A, B and C come from the first variant type. Contrarily, in the last example, OCaml has
inferred by itself that the type of r can only be first_record and there are no needs for explicit
type annotations.
Those explicit type annotations can in fact be used anywhere. Most of the time they are
unnecessary, but they are useful to guide disambiguation, to debug unexpected type errors, or
combined with some of the more advanced features of OCaml described in later chapters.
Secondly, for records, OCaml can also deduce the right record type by looking at the whole set
of fields used in a expression or pattern:
# let project_and_rotate {x; y; _} = { x= - y; y = x; z = 0} ;;
val project_and_rotate : first_record -> first_record = <fun>
Since the fields x and y can only appear simultaneously in the first record type, OCaml infers that
the type of project_and_rotate is first_record -> first_record.
In last resort, if there is not enough information to disambiguate between different fields or
constructors, Ocaml picks the last defined type amongst all locally valid choices:
# let look_at_xz {x; z} = x;;
val look_at_xz : middle_record -> int = <fun>
Here, OCaml has inferred that the possible choices for the type of {x;z} are first_record
and middle_record, since the type last_record has no field z. Ocaml then picks the type
middle_record as the last defined type between the two possibilities.
Beware that this last resort disambiguation is local: once Ocaml has chosen a disambiguation, it
sticks to this choice, even if it leads to an ulterior type error:
# let look_at_x_then_y r =
let x = r.x in (∗ Ocaml deduces [r: last_record] ∗)
x + r.y;;
Chapter 1. The core language 23

Error : This expression has type last_record

There is no field y within type last_record

# let is_a_or_b x = match x with

| A -> true (∗ OCaml infers [x: last_variant] ∗)
| B -> true;;
Error : This variant pattern is expected to have type last_variant
There is no constructor B within type last_variant
Moreover, being the last defined type is a quite unstable position that may change surreptitiously
after adding or moving around a type definition, or after opening a module (see chapter 2).
Consequently, adding explicit type annotations to guide disambiguation is more robust than relying
on the last defined type disambiguation.

1.5 Imperative features

Though all examples so far were written in purely applicative style, OCaml is also equipped with
full imperative features. This includes the usual while and for loops, as well as mutable data
structures such as arrays. Arrays are either created by listing semicolon-separated element values
between [| and |] brackets, or allocated and initialized with the Array.make function, then filled
up later by assignments. For instance, the function below sums two vectors (represented as float
arrays) componentwise.
# let add_vect v1 v2 =
let len = min (Array.length v1) (Array.length v2) in
let res = Array.make len 0.0 in
for i = 0 to len - 1 do
res.(i) <- v1.(i) +. v2.(i)
done;
res;;
val add_vect : float array -> float array -> float array = <fun>

# add_vect [| 1.0; 2.0 |] [| 3.0; 4.0 |];;

- : float array = [|4.; 6.|]
Record fields can also be modified by assignment, provided they are declared mutable in the
definition of the record type:
# type mutable_point = { mutable x: float; mutable y: float };;
type mutable_point = { mutable x : float; mutable y : float; }

# let translate p dx dy =
p.x <- p.x +. dx; p.y <- p.y +. dy;;
val translate : mutable_point -> float -> float -> unit = <fun>

# let mypoint = { x = 0.0; y = 0.0 };;

val mypoint : mutable_point = {x = 0.; y = 0.}
24

# translate mypoint 1.0 2.0;;

- : unit = ()

# mypoint;;
- : mutable_point = {x = 1.; y = 2.}
OCaml has no built-in notion of variable – identifiers whose current value can be changed by
assignment. (The let binding is not an assignment, it introduces a new identifier with a new
scope.) However, the standard library provides references, which are mutable indirection cells, with
operators ! to fetch the current contents of the reference and := to assign the contents. Variables
can then be emulated by let-binding a reference. For instance, here is an in-place insertion sort
over arrays:
# let insertion_sort a =
for i = 1 to Array.length a - 1 do
let val_i = a.(i) in
let j = ref i in
while !j > 0 && val_i < a.(!j - 1) do
a.(!j) <- a.(!j - 1);
j := !j - 1
done;
a.(!j) <- val_i
done;;
val insertion_sort : 'a array -> unit = <fun>
References are also useful to write functions that maintain a current state between two calls to
the function. For instance, the following pseudo-random number generator keeps the last returned
number in a reference:
# let current_rand = ref 0;;
val current_rand : int ref = {contents = 0}

# let random () =
current_rand := !current_rand * 25713 + 1345;
!current_rand;;
val random : unit -> int = <fun>
Again, there is nothing magical with references: they are implemented as a single-field mutable
record, as follows.
# type 'a ref = { mutable contents: 'a };;
type 'a ref = { mutable contents : 'a; }

# let ( ! ) r = r.contents;;
val ( ! ) : 'a ref -> 'a = <fun>

# let ( := ) r newval = r.contents <- newval;;

val ( := ) : 'a ref -> 'a -> unit = <fun>
Chapter 1. The core language 25

In some special cases, you may need to store a polymorphic function in a data structure, keeping
its polymorphism. Doing this requires user-provided type annotations, since polymorphism is only
introduced automatically for global definitions. However, you can explicitly give polymorphic types
to record fields.
# type idref = { mutable id: 'a. 'a -> 'a };;
type idref = { mutable id : 'a. 'a -> 'a; }

# let r = {id = fun x -> x};;

val r : idref = {id = <fun>}

# let g s = (s.id 1, s.id true);;

val g : idref -> int * bool = <fun>

# r.id <- (fun x -> print_string "called id\n"; x);;

- : unit = ()

# g r;;
called id
called id
- : int * bool = (1, true)

1.6 Exceptions
OCaml provides exceptions for signalling and handling exceptional conditions. Exceptions can also
be used as a general-purpose non-local control structure, although this should not be overused since
it can make the code harder to understand. Exceptions are declared with the exception construct,
and signalled with the raise operator. For instance, the function below for taking the head of a list
uses an exception to signal the case where an empty list is given.
# exception Empty_list;;
exception Empty_list

# let head l =
match l with
[] -> raise Empty_list
| hd :: tl -> hd;;
val head : 'a list -> 'a = <fun>

# head [1; 2];;

- : int = 1

# head [];;
Exception: Empty_list.
Exceptions are used throughout the standard library to signal cases where the library functions
cannot complete normally. For instance, the List.assoc function, which returns the data associated
with a given key in a list of (key, data) pairs, raises the predefined exception Not_found when the
key does not appear in the list:
26

# List.assoc 1 [(0, "zero"); (1, "one")];;

- : string = "one"

# List.assoc 2 [(0, "zero"); (1, "one")];;

Exception: Not_found.
Exceptions can be trapped with the try…with construct:
# let name_of_binary_digit digit =
try
List.assoc digit [0, "zero"; 1, "one"]
with Not_found ->
"not a binary digit";;
val name_of_binary_digit : int -> string = <fun>

# name_of_binary_digit 0;;
- : string = "zero"

# name_of_binary_digit (-1);;
- : string = "not a binary digit"
The with part does pattern matching on the exception value with the same syntax and behavior
as match. Thus, several exceptions can be caught by one try…with construct:
# let rec first_named_value values names =
try
List.assoc (head values) names
with
| Empty_list -> "no named value"
| Not_found -> first_named_value (List.tl values) names;;
val first_named_value : 'a list -> ('a * string) list -> string = <fun>

# first_named_value [0; 10] [1, "one"; 10, "ten"];;

- : string = "ten"
Also, finalization can be performed by trapping all exceptions, performing the finalization, then
re-raising the exception:
# let temporarily_set_reference ref newval funct =
let oldval = !ref in
try
ref := newval;
let res = funct () in
ref := oldval;
res
with x ->
ref := oldval;
raise x;;
val temporarily_set_reference : 'a ref -> 'a -> (unit -> 'b) -> 'b = <fun>
Chapter 1. The core language 27

An alternative to try…with is to catch the exception while pattern matching:

# let assoc_may_map f x l =
match List.assoc x l with
| exception Not_found -> None
| y -> f y;;
val assoc_may_map : ('a -> 'b option) -> 'c -> ('c * 'a) list -> 'b option =
<fun>
Note that this construction is only useful if the exception is raised between match…with. Exception
patterns can be combined with ordinary patterns at the toplevel,
# let flat_assoc_opt x l =
match List.assoc x l with
| None | exception Not_found -> None
| Some _ as v -> v;;
val flat_assoc_opt : 'a -> ('a * 'b option) list -> 'b option = <fun>
but they cannot be nested inside other patterns. For instance, the pattern Some (exception A) is
invalid.
When exceptions are used as a control structure, it can be useful to make them as local as
possible by using a locally defined exception. For instance, with
# let fixpoint f x =
let exception Done in
let x = ref x in
try while true do
let y = f !x in
if !x = y then raise Done else x := y
done
with Done -> !x;;
val fixpoint : ('a -> 'a) -> 'a -> 'a = <fun>
the function f cannot raise a Done exception, which removes an entire class of misbehaving functions.

1.7 Lazy expressions

OCaml allows us to defer some computation until later when we need the result of that computation.
We use lazy (expr) to delay the evaluation of some expression expr. For example, we can
defer the computation of 1+1 until we need the result of that expression, 2. Let us see how we
initialize a lazy expression.
# let lazy_two = lazy (print_endline "lazy_two evaluation"; 1 + 1);;
val lazy_two : int lazy_t = <lazy>
We added print_endline "lazy_two evaluation" to see when the lazy expression is being
evaluated.
The value of lazy_two is displayed as <lazy>, which means the expression has not been evaluated
yet, and its final value is unknown.
28

Note that lazy_two has type int lazy_t. However, the type 'a lazy_t is an internal type
name, so the type 'a Lazy.t should be preferred when possible.
When we finally need the result of a lazy expression, we can call Lazy.force on that expression
to force its evaluation. The function force comes from standard-library module Lazy[29.30].
# Lazy.force lazy_two;;
lazy_two evaluation
- : int = 2
Notice that our function call above prints “lazy_two evaluation” and then returns the plain
value of the computation.
Now if we look at the value of lazy_two, we see that it is not displayed as <lazy> anymore but
as lazy 2.
# lazy_two;;
- : int lazy_t = lazy 2
This is because Lazy.force memoizes the result of the forced expression. In other words, every
subsequent call of Lazy.force on that expression returns the result of the first computation without
recomputing the lazy expression. Let us force lazy_two once again.
# Lazy.force lazy_two;;
- : int = 2
The expression is not evaluated this time; notice that “lazy_two evaluation” is not printed. The
result of the initial computation is simply returned.
Lazy patterns provide another way to force a lazy expression.
# let lazy_l = lazy ([1; 2] @ [3; 4]);;
val lazy_l : int list lazy_t = <lazy>

# let lazy l = lazy_l;;

val l : int list = [1; 2; 3; 4]
We can also use lazy patterns in pattern matching.
# let maybe_eval lazy_guard lazy_expr =
match lazy_guard, lazy_expr with
| lazy false, _ -> "matches if (Lazy.force lazy_guard = false); lazy_expr not forced"
| lazy true, lazy _ -> "matches if (Lazy.force lazy_guard = true); lazy_expr forced";;
val maybe_eval : bool lazy_t -> 'a lazy_t -> string = <fun>
The lazy expression lazy_expr is forced only if the lazy_guard value yields true once computed.
Indeed, a simple wildcard pattern (not lazy) never forces the lazy expression’s evaluation. However,
a pattern with keyword lazy, even if it is wildcard, always forces the evaluation of the deferred
computation.

1.8 Symbolic processing of expressions

We finish this introduction with a more complete example representative of the use of OCaml for
symbolic processing: formal manipulations of arithmetic expressions containing variables. The
following variant type describes the expressions we shall manipulate:
Chapter 1. The core language 29

# type expression =
Const of float
| Var of string
| Sum of expression * expression (∗ e1 + e2 ∗)
| Diff of expression * expression (∗ e1 − e2 ∗)
| Prod of expression * expression (∗ e1 ∗ e2 ∗)
| Quot of expression * expression (∗ e1 / e2 ∗)
;;
type expression =
Const of float
| Var of string
| Sum of expression * expression
| Diff of expression * expression
| Prod of expression * expression
| Quot of expression * expression
We first define a function to evaluate an expression given an environment that maps variable
names to their values. For simplicity, the environment is represented as an association list.
# exception Unbound_variable of string;;
exception Unbound_variable of string

# let rec eval env exp =

match exp with
Const c -> c
| Var v ->
(try List.assoc v env with Not_found -> raise (Unbound_variable v))
| Sum(f, g) -> eval env f +. eval env g
| Diff(f, g) -> eval env f -. eval env g
| Prod(f, g) -> eval env f *. eval env g
| Quot(f, g) -> eval env f /. eval env g;;
val eval : (string * float) list -> expression -> float = <fun>

# eval [("x", 1.0); ("y", 3.14)] (Prod(Sum(Var "x", Const 2.0), Var "y"));;
- : float = 9.42
Now for a real symbolic processing, we define the derivative of an expression with respect to a
variable dv:
# let rec deriv exp dv =
match exp with
Const c -> Const 0.0
| Var v -> if v = dv then Const 1.0 else Const 0.0
| Sum(f, g) -> Sum(deriv f dv, deriv g dv)
| Diff(f, g) -> Diff(deriv f dv, deriv g dv)
| Prod(f, g) -> Sum(Prod(f, deriv g dv), Prod(deriv f dv, g))
| Quot(f, g) -> Quot(Diff(Prod(deriv f dv, g), Prod(f, deriv g dv)),
Prod(g, g))
;;
30

val deriv : expression -> string -> expression = <fun>

# deriv (Quot(Const 1.0, Var "x")) "x";;

- : expression =
Quot (Diff (Prod (Const 0., Var "x"), Prod (Const 1., Const 1.)),
Prod (Var "x", Var "x"))

1.9 Pretty-printing
As shown in the examples above, the internal representation (also called abstract syntax) of
expressions quickly becomes hard to read and write as the expressions get larger. We need a printer
and a parser to go back and forth between the abstract syntax and the concrete syntax, which in
the case of expressions is the familiar algebraic notation (e.g. 2*x+1).
For the printing function, we take into account the usual precedence rules (i.e. * binds tighter
than +) to avoid printing unnecessary parentheses. To this end, we maintain the current operator
precedence and print parentheses around an operator only if its precedence is less than the current
precedence.
# let print_expr exp =
(∗ Local function definitions ∗)
let open_paren prec op_prec =
if prec > op_prec then print_string "(" in
let close_paren prec op_prec =
if prec > op_prec then print_string ")" in
let rec print prec exp = (∗ prec is the current precedence ∗)
match exp with
Const c -> print_float c
| Var v -> print_string v
| Sum(f, g) ->
open_paren prec 0;
print 0 f; print_string " + "; print 0 g;
close_paren prec 0
| Diff(f, g) ->
open_paren prec 0;
print 0 f; print_string " - "; print 1 g;
close_paren prec 0
| Prod(f, g) ->
open_paren prec 2;
print 2 f; print_string " * "; print 2 g;
close_paren prec 2
| Quot(f, g) ->
open_paren prec 2;
print 2 f; print_string " / "; print 3 g;
close_paren prec 2
in print 0 exp;;
Chapter 1. The core language 31

val print_expr : expression -> unit = <fun>

# let e = Sum(Prod(Const 2.0, Var "x"), Const 1.0);;

val e : expression = Sum (Prod (Const 2., Var "x"), Const 1.)

# print_expr e; print_newline ();;

2. * x + 1.
- : unit = ()

# print_expr (deriv e "x"); print_newline ();;

2. * 1. + 0. * x + 0.
- : unit = ()

1.10 Printf formats

There is a printf function in the Printf[29.44] module (see chapter 2) that allows you to make
formatted output more concisely. It follows the behavior of the printf function from the C standard
library. The printf function takes a format string that describes the desired output as a text
interspersed with specifiers (for instance %d, %f). Next, the specifiers are substituted by the following
arguments in their order of apparition in the format string:
# Printf.printf "%i + %i is an integer value, %F * %F is a float, %S\n"
3 2 4.5 1. "this is a string";;
3 + 2 is an integer value, 4.5 * 1. is a float, "this is a string"
- : unit = ()
The OCaml type system checks that the type of the arguments and the specifiers are compatible. If
you pass it an argument of a type that does not correspond to the format specifier, the compiler
will display an error message:
# Printf.printf "Float value: %F" 42;;
Error : The constant 42 has type int but an expression was expected of type
float
Hint: Did you mean 42.?
The fprintf function is like printf except that it takes an output channel as the first argument.
The %a specifier can be useful to define custom printers (for custom types). For instance, we can
create a printing template that converts an integer argument to signed decimal:
# let pp_int ppf n = Printf.fprintf ppf "%d" n;;
val pp_int : out_channel -> int -> unit = <fun>

# Printf.printf "Outputting an integer using a custom printer: %a " pp_int 42;;

Outputting an integer using a custom printer: 42 - : unit = ()
The advantage of those printers based on the %a specifier is that they can be composed together to
create more complex printers step by step. We can define a combinator that can turn a printer for
'a type into a printer for 'a optional:
32

# let pp_option printer ppf = function

| None -> Printf.fprintf ppf "None"
| Some v -> Printf.fprintf ppf "Some(%a)" printer v;;
val pp_option :
(out_channel -> 'a -> unit) -> out_channel -> 'a option -> unit = <fun>

# Printf.fprintf stdout
"The current setting is %a. \nThere is only %a\n"
(pp_option pp_int) (Some 3)
(pp_option pp_int) None
;;
The current setting is Some(3).
There is only None
- : unit = ()
If the value of its argument its None, the printer returned by pp_option printer prints None otherwise
it uses the provided printer to print Some .
Here is how to rewrite the pretty-printer using fprintf:
# let pp_expr ppf expr =
let open_paren prec op_prec output =
if prec > op_prec then Printf.fprintf output "%s" "(" in
let close_paren prec op_prec output =
if prec > op_prec then Printf.fprintf output "%s" ")" in
let rec print prec ppf expr =
match expr with
| Const c -> Printf.fprintf ppf "%F" c
| Var v -> Printf.fprintf ppf "%s" v
| Sum(f, g) ->
open_paren prec 0 ppf;
Printf.fprintf ppf "%a + %a" (print 0) f (print 0) g;
close_paren prec 0 ppf
| Diff(f, g) ->
open_paren prec 0 ppf;
Printf.fprintf ppf "%a - %a" (print 0) f (print 1) g;
close_paren prec 0 ppf
| Prod(f, g) ->
open_paren prec 2 ppf;
Printf.fprintf ppf "%a * %a" (print 2) f (print 2) g;
close_paren prec 2 ppf
| Quot(f, g) ->
open_paren prec 2 ppf;
Printf.fprintf ppf "%a / %a" (print 2) f (print 3) g;
close_paren prec 2 ppf
in print 0 ppf expr;;
val pp_expr : out_channel -> expression -> unit = <fun>
Chapter 1. The core language 33

# pp_expr stdout e; print_newline ();;

2. * x + 1.
- : unit = ()

# pp_expr stdout (deriv e "x"); print_newline ();;

2. * 1. + 0. * x + 0.
- : unit = ()
Due to the way that format strings are built, storing a format string requires an explicit type
annotation:
# let str : _ format =
"%i is an integer value, %F is a float, %S\n";;
# Printf.printf str 3 4.5 "string value";;
3 is an integer value, 4.5 is a float, "string value"
- : unit = ()

1.11 Standalone OCaml programs

All examples given so far were executed under the interactive system. OCaml code can also be
compiled separately and executed non-interactively using the batch compilers ocamlc and ocamlopt.
The source code must be put in a file with extension .ml. It consists of a sequence of phrases, which
will be evaluated at runtime in their order of appearance in the source file. Unlike in interactive
mode, types and values are not printed automatically; the program must call printing functions
explicitly to produce some output. The ;; used in the interactive examples is not required in
source files created for use with OCaml compilers, but can be helpful to mark the end of a top-level
expression unambiguously even when there are syntax errors. Here is a sample standalone program
to print the greatest common divisor (gcd) of two numbers:

(* File gcd.ml *)
let rec gcd a b =
if b = 0 then a
else gcd b (a mod b);;

let main () =
let a = int_of_string Sys.argv.(1) in
let b = int_of_string Sys.argv.(2) in
Printf.printf "%d\n" (gcd a b);
exit 0;;
main ();;

Sys.argv is an array of strings containing the command-line parameters. Sys.argv.(1) is thus

the first command-line parameter. The program above is compiled and executed with the following
shell commands:

$ ocamlc -o gcd gcd.ml

$ ./gcd 6 9
34

3
$ ./gcd 7 11
1

More complex standalone OCaml programs are typically composed of multiple source files, and
can link with precompiled libraries. Chapters 13 and 16 explain how to use the batch compilers
ocamlc and ocamlopt. Recompilation of multi-file OCaml projects can be automated using third-
party build systems, such as dune.
Chapter 2

The module system

This chapter introduces the module system of OCaml.

2.1 Structures
A primary motivation for modules is to package together related definitions (such as the definitions
of a data type and associated operations over that type) and enforce a consistent naming scheme
for these definitions. This avoids running out of names or accidentally confusing names. Such a
package is called a structure and is introduced by the struct…end construct, which contains an
arbitrary sequence of definitions. The structure is usually given a name with the module binding.
For instance, here is a structure packaging together a type of FIFO queues and their operations:
# module Fifo =
struct
type 'a queue = { front: 'a list; rear: 'a list }
let make front rear =
match front with
| [] -> { front = List.rev rear; rear = [] }
| _ -> { front; rear }
let empty = { front = []; rear = [] }
let is_empty = function { front = []; _ } -> true | _ -> false
let add x q = make q.front (x :: q.rear)
exception Empty
let top = function
| { front = []; _ } -> raise Empty
| { front = x :: _; _ } -> x
let pop = function
| { front = []; _ } -> raise Empty
| { front = _ :: f; rear = r } -> make f r
end;;
module Fifo :
sig
type 'a queue = { front : 'a list; rear : 'a list; }

35
36

val make : 'a list -> 'a list -> 'a queue
val empty : 'a queue
val is_empty : 'a queue -> bool
val add : 'a -> 'a queue -> 'a queue
exception Empty
val top : 'a queue -> 'a
val pop : 'a queue -> 'a queue
end
Outside the structure, its components can be referred to using the “dot notation”, that is, identifiers
qualified by a structure name. For instance, Fifo.add is the function add defined inside the structure
Fifo and Fifo.queue is the type queue defined in Fifo.
# Fifo.add "hello" Fifo.empty;;
- : string Fifo.queue = {Fifo.front = ["hello"]; rear = []}
Another possibility is to open the module, which brings all identifiers defined inside the module
into the scope of the current structure.
# open Fifo;;
# add "hello" empty;;
- : string Fifo.queue = {front = ["hello"]; rear = []}
Opening a module enables lighter access to its components, at the cost of making it harder to
identify in which module an identifier has been defined. In particular, opened modules can shadow
identifiers present in the current scope, potentially leading to confusing errors:
# let empty = []
open Fifo;;
val empty : 'a list = []

# let x = 1 :: empty ;;
Error : The value empty has type 'a Fifo.queue
but an expression was expected of type int list
A partial solution to this conundrum is to open modules locally, making the components of the
module available only in the concerned expression. This can also make the code both easier to
read (since the open statement is closer to where it is used) and easier to refactor (since the code
fragment is more self-contained). Two constructions are available for this purpose:
# let open Fifo in
add "hello" empty;;
- : string Fifo.queue = {front = ["hello"]; rear = []}
and
# Fifo.(add "hello" empty);;
- : string Fifo.queue = {front = ["hello"]; rear = []}
In the second form, when the body of a local open is itself delimited by parentheses, braces or
bracket, the parentheses of the local open can be omitted. For instance,
Chapter 2. The module system 37

# Fifo.[empty] = Fifo.([empty]);;
- : bool = true

# Fifo.[|empty|] = Fifo.([|empty|]);;
- : bool = true

# Fifo.{ contents = empty } = Fifo.({ contents = empty });;

- : bool = true
This second form also works for patterns:
# let at_most_one_element x = match x with
| Fifo.{ front = ([] | [_]); rear = [] } -> true
| _ -> false ;;
val at_most_one_element : 'a Fifo.queue -> bool = <fun>
It is also possible to copy the components of a module inside another module by using an
include statement. This can be particularly useful to extend existing modules. As an illustration,
we could add functions that return an optional value rather than an exception when the queue is
empty.
# module FifoOpt =
struct
include Fifo
let top_opt q = if is_empty q then None else Some(top q)
let pop_opt q = if is_empty q then None else Some(pop q)
end;;
module FifoOpt :
sig
type 'a queue = 'a Fifo.queue = { front : 'a list; rear : 'a list; }
val make : 'a list -> 'a list -> 'a queue
val empty : 'a queue
val is_empty : 'a queue -> bool
val add : 'a -> 'a queue -> 'a queue
exception Empty
val top : 'a queue -> 'a
val pop : 'a queue -> 'a queue
val top_opt : 'a queue -> 'a option
val pop_opt : 'a queue -> 'a queue option
end

2.2 Signatures
Signatures are interfaces for structures. A signature specifies which components of a structure are
accessible from the outside, and with which type. It can be used to hide some components of a
structure (e.g. local function definitions) or export some components with a restricted type. For
instance, the signature below specifies the queue operations empty, add, top and pop, but not the
auxiliary function make. Similarly, it makes the queue type abstract (by not providing its actual
38

representation as a concrete type). This ensures that users of the Fifo module cannot violate data
structure invariants that operations rely on, such as “if the front list is empty, the rear list must
also be empty”.
# module type FIFO =
sig
type 'a queue (∗ now an abstract type ∗)
val empty : 'a queue
val add : 'a -> 'a queue -> 'a queue
val top : 'a queue -> 'a
val pop : 'a queue -> 'a queue
exception Empty
end;;
module type FIFO =
sig
type 'a queue
val empty : 'a queue
val add : 'a -> 'a queue -> 'a queue
val top : 'a queue -> 'a
val pop : 'a queue -> 'a queue
exception Empty
end
Restricting the Fifo structure to this signature results in another view of the Fifo structure where
the make function is not accessible and the actual representation of queues is hidden:
# module AbstractQueue = (Fifo : FIFO);;
module AbstractQueue : FIFO

# AbstractQueue.make [1] [2;3] ;;

Error : Unbound value AbstractQueue .make

# AbstractQueue.add "hello" AbstractQueue.empty;;

- : string AbstractQueue.queue = <abstr>
The restriction can also be performed during the definition of the structure, as in
module Fifo = (struct ... end : FIFO);;
An alternate syntax is provided for the above:
module Fifo : FIFO = struct ... end;;
Like for modules, it is possible to include a signature to copy its components inside the current
signature. For instance, we can extend the FIFO signature with the top_opt and pop_opt functions:
# module type FIFO_WITH_OPT =
sig
include FIFO
val top_opt: 'a queue -> 'a option
val pop_opt: 'a queue -> 'a queue option
end;;
Chapter 2. The module system 39

module type FIFO_WITH_OPT =

sig
type 'a queue
val empty : 'a queue
val add : 'a -> 'a queue -> 'a queue
val top : 'a queue -> 'a
val pop : 'a queue -> 'a queue
exception Empty
val top_opt : 'a queue -> 'a option
val pop_opt : 'a queue -> 'a queue option
end

2.3 Functors
Functors are “functions” from modules to modules. Functors let you create parameterized modules
and then provide other modules as parameter(s) to get a specific implementation. For instance, a
Set module implementing sets as sorted lists could be parameterized to work with any module that
provides an element type and a comparison function compare (such as OrderedString):
# type comparison = Less | Equal | Greater;;
type comparison = Less | Equal | Greater

# module type ORDERED_TYPE =

sig
type t
val compare: t -> t -> comparison
end;;
module type ORDERED_TYPE = sig type t val compare : t -> t -> comparison end

# module Set =
functor (Elt: ORDERED_TYPE) ->
struct
type element = Elt.t
type set = element list
let empty = []
let rec add x s =
match s with
[] -> [x]
| hd::tl ->
match Elt.compare x hd with
Equal -> s (∗ x is already in s ∗)
| Less -> x :: s (∗ x is smaller than all elements of s ∗)
| Greater -> hd :: add x tl
let rec member x s =
match s with
[] -> false
40

| hd::tl ->
match Elt.compare x hd with
Equal -> true (∗ x belongs to s ∗)
| Less -> false (∗ x is smaller than all elements of s ∗)
| Greater -> member x tl
end;;
module Set :
(Elt : ORDERED_TYPE) ->
sig
type element = Elt.t
type set = element list
val empty : 'a list
val add : Elt.t -> Elt.t list -> Elt.t list
val member : Elt.t -> Elt.t list -> bool
end
By applying the Set functor to a structure implementing an ordered type, we obtain set operations
for this type:
# module OrderedString =
struct
type t = string
let compare x y = if x = y then Equal else if x < y then Less else Greater
end;;
module OrderedString :
sig type t = string val compare : 'a -> 'a -> comparison end

# module StringSet = Set(OrderedString);;

module StringSet :
sig
type element = OrderedString.t
type set = element list
val empty : 'a list
val add : OrderedString.t -> OrderedString.t list -> OrderedString.t list
val member : OrderedString.t -> OrderedString.t list -> bool
end

# StringSet.member "bar" (StringSet.add "foo" StringSet.empty);;

- : bool = false

2.4 Functors and type abstraction

As in the Fifo example, it would be good style to hide the actual implementation of the type set,
so that users of the structure will not rely on sets being lists, and we can switch later to another,
more efficient representation of sets without breaking their code. This can be achieved by restricting
Set by a suitable functor signature:
# module type SETFUNCTOR =
Chapter 2. The module system 41

(Elt: ORDERED_TYPE) ->

sig
type element = Elt.t (∗ concrete ∗)
type set (∗ abstract ∗)
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end;;
module type SETFUNCTOR =
(Elt : ORDERED_TYPE) ->
sig
type element = Elt.t
type set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end

# module AbstractSet = (Set : SETFUNCTOR);;

module AbstractSet : SETFUNCTOR

# module AbstractStringSet = AbstractSet(OrderedString);;

module AbstractStringSet :
sig
type element = OrderedString.t
type set = AbstractSet(OrderedString).set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end

# AbstractStringSet.add "gee" AbstractStringSet.empty;;

- : AbstractStringSet.set = <abstr>
In an attempt to write the type constraint above more elegantly, one may wish to name the
signature of the structure returned by the functor, then use that signature in the constraint:
# module type SET =
sig
type element
type set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end;;
module type SET =
sig
type element
type set
42

val empty : set

val add : element -> set -> set
val member : element -> set -> bool
end

# module WrongSet = (Set : (Elt: ORDERED_TYPE) -> SET);;

module WrongSet : (Elt : ORDERED_TYPE) -> SET

# module WrongStringSet = WrongSet(OrderedString);;

module WrongStringSet :
sig
type element = WrongSet(OrderedString).element
type set = WrongSet(OrderedString).set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end

# WrongStringSet.add "gee" WrongStringSet.empty ;;

Error : This constant has type string but an expression was expected of type
WrongStringSet . element = WrongSet ( OrderedString ). element
The problem here is that SET specifies the type element abstractly, so that the type equality
between element in the result of the functor and t in its argument is forgotten. Consequently,
WrongStringSet.element is not the same type as string, and the operations of WrongStringSet
cannot be applied to strings. As demonstrated above, it is important that the type element in the
signature SET be declared equal to Elt.t; unfortunately, this is impossible above since SET is defined
in a context where Elt does not exist. To overcome this difficulty, OCaml provides a with type
construct over signatures that allows enriching a signature with extra type equalities:
# module AbstractSet2 =
(Set : (Elt: ORDERED_TYPE) -> (SET with type element = Elt.t));;
module AbstractSet2 :
(Elt : ORDERED_TYPE) ->
sig
type element = Elt.t
type set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end
As in the case of simple structures, an alternate syntax is provided for defining functors and
restricting their result:

module AbstractSet2(Elt: ORDERED_TYPE) : (SET with type element = Elt.t) =

struct ... end;;

Abstracting a type component in a functor result is a powerful technique that provides a high
degree of type safety, as we now illustrate. Consider an ordering over character strings that is
Chapter 2. The module system 43

different from the standard ordering implemented in the OrderedString structure. For instance,
we compare strings without distinguishing upper and lower case.
# module NoCaseString =
struct
type t = string
let compare s1 s2 =
OrderedString.compare (String.lowercase_ascii s1) (String.lowercase_ascii s2)
end;;
module NoCaseString :
sig type t = string val compare : string -> string -> comparison end

# module NoCaseStringSet = AbstractSet(NoCaseString);;

module NoCaseStringSet :
sig
type element = NoCaseString.t
type set = AbstractSet(NoCaseString).set
val empty : set
val add : element -> set -> set
val member : element -> set -> bool
end

# NoCaseStringSet.add "FOO" AbstractStringSet.empty ;;

Error : The value AbstractStringSet .empty has type
AbstractStringSet .set = AbstractSet ( OrderedString ). set
but an expression was expected of type
NoCaseStringSet .set = AbstractSet ( NoCaseString ). set
Note that the two types AbstractStringSet.set and NoCaseStringSet.set are not compatible,
and values of these two types do not match. This is the correct behavior: even though both set types
contain elements of the same type (strings), they are built upon different orderings of that type, and
different invariants need to be maintained by the operations (being strictly increasing for the standard
ordering and for the case-insensitive ordering). Applying operations from AbstractStringSet to
values of type NoCaseStringSet.set could give incorrect results, or build lists that violate the
invariants of NoCaseStringSet.

2.5 Modules and separate compilation

All examples of modules so far have been given in the context of the interactive system. However,
modules are most useful for large, batch-compiled programs. For these programs, it is a practical
necessity to split the source into several files, called compilation units, that can be compiled
separately, thus minimizing recompilation after changes.
In OCaml, compilation units are special cases of structures and signatures, and the relationship
between the units can be explained easily in terms of the module system. A compilation unit A
comprises two files:
• the implementation file A.ml, which contains a sequence of definitions, analogous to the inside
of a struct…end construct;
44

• the interface file A.mli, which contains a sequence of specifications, analogous to the inside of
a sig…end construct.

These two files together define a structure named A as if the following definition was entered at
top-level:

module A: sig (* contents of file A.mli *) end

= struct (* contents of file A.ml *) end;;

The files that define the compilation units can be compiled separately using the ocamlc -c
command (the -c option means “compile only, do not try to link”); this produces compiled interface
files (with extension .cmi) and compiled object code files (with extension .cmo). When all units
have been compiled, their .cmo files are linked together using the ocamlc command. For instance,
the following commands compile and link a program composed of two compilation units Aux and
Main:

$ ocamlc -c Aux.mli # produces aux.cmi

$ ocamlc -c Aux.ml # produces aux.cmo
$ ocamlc -c Main.mli # produces main.cmi
$ ocamlc -c Main.ml # produces main.cmo
$ ocamlc -o theprogram Aux.cmo Main.cmo

The program behaves exactly as if the following phrases were entered at top-level:

module Aux: sig (* contents of Aux.mli *) end

= struct (* contents of Aux.ml *) end;;
module Main: sig (* contents of Main.mli *) end
= struct (* contents of Main.ml *) end;;

In particular, Main can refer to Aux: the definitions and declarations contained in Main.ml and
Main.mli can refer to definition in Aux.ml, using the Aux.ident notation, provided these definitions
are exported in Aux.mli.
The order in which the .cmo files are given to ocamlc during the linking phase determines the
order in which the module definitions occur. Hence, in the example above, Aux appears first and
Main can refer to it, but Aux cannot refer to Main.
Note that only top-level structures can be mapped to separately-compiled files, but neither
functors nor module types. However, all module-class objects can appear as components of a
structure, so the solution is to put the functor or module type inside a structure, which can then be
mapped to a file.
Chapter 3

Objects in OCaml

(Chapter written by Jérôme Vouillon, Didier Rémy and Jacques Garrigue)

This chapter gives an overview of the object-oriented features of OCaml.

Note that the relationship between object, class and type in OCaml is different than in mainstream
object-oriented languages such as Java and C++, so you shouldn’t assume that similar keywords
mean the same thing. Object-oriented features are used much less frequently in OCaml than in
those languages. OCaml has alternatives that are often more appropriate, such as modules and
functors. Indeed, many OCaml programs do not use objects at all.

3.1 Classes and objects

The class point below defines one instance variable x and two methods get_x and move. The initial
value of the instance variable is 0. The variable x is declared mutable, so the method move can
change its value.
# class point =
object
val mutable x = 0
method get_x = x
method move d = x <- x + d
end;;
class point :
object val mutable x : int method get_x : int method move : int -> unit end
We now create a new point p, instance of the point class.
# let p = new point;;
val p : point = <obj>
Note that the type of p is point. This is an abbreviation automatically defined by the class definition
above. It stands for the object type <get_x : int; move : int -> unit>, listing the methods of
class point along with their types.
We now invoke some methods of p:
# p#get_x;;

45
46

- : int = 0

# p#move 3;;
- : unit = ()

# p#get_x;;
- : int = 3
The evaluation of the body of a class only takes place at object creation time. Therefore, in
the following example, the instance variable x is initialized to different values for two different
objects.
# let x0 = ref 0;;
val x0 : int ref = {contents = 0}

# class point =
object
val mutable x = incr x0; !x0
method get_x = x
method move d = x <- x + d
end;;
class point :
object val mutable x : int method get_x : int method move : int -> unit end

# new point#get_x;;
- : int = 1

# new point#get_x;;
- : int = 2
The class point can also be abstracted over the initial values of the x coordinate.
# class point = fun x_init ->
object
val mutable x = x_init
method get_x = x
method move d = x <- x + d
end;;
class point :
int ->
object val mutable x : int method get_x : int method move : int -> unit end
Like in function definitions, the definition above can be abbreviated as:
# class point x_init =
object
val mutable x = x_init
method get_x = x
method move d = x <- x + d
end;;
Chapter 3. Objects in OCaml 47

class point :
int ->
object val mutable x : int method get_x : int method move : int -> unit end
An instance of the class point is now a function that expects an initial parameter to create a point
object:
# new point;;
- : int -> point = <fun>

# let p = new point 7;;

val p : point = <obj>
The parameter x_init is, of course, visible in the whole body of the definition, including methods.
For instance, the method get_offset in the class below returns the position of the object relative
to its initial position.
# class point x_init =
object
val mutable x = x_init
method get_x = x
method get_offset = x - x_init
method move d = x <- x + d
end;;
class point :
int ->
object
val mutable x : int
method get_offset : int
method get_x : int
method move : int -> unit
end
Expressions can be evaluated and bound before defining the object body of the class. This is useful
to enforce invariants. For instance, points can be automatically adjusted to the nearest point on a
grid, as follows:
# class adjusted_point x_init =
let origin = (x_init / 10) * 10 in
object
val mutable x = origin
method get_x = x
method get_offset = x - origin
method move d = x <- x + d
end;;
class adjusted_point :
int ->
object
val mutable x : int
method get_offset : int
48

method get_x : int

method move : int -> unit
end
(One could also raise an exception if the x_init coordinate is not on the grid.) In fact, the same effect
could be obtained here by calling the definition of class point with the value of the origin.
# class adjusted_point x_init = point ((x_init / 10) * 10);;
class adjusted_point : int -> point
An alternate solution would have been to define the adjustment in a special allocation function:
# let new_adjusted_point x_init = new point ((x_init / 10) * 10);;
val new_adjusted_point : int -> point = <fun>
However, the former pattern is generally more appropriate, since the code for adjustment is part of
the definition of the class and will be inherited.
This ability provides class constructors as can be found in other languages. Several constructors
can be defined this way to build objects of the same class but with different initialization patterns;
an alternative is to use initializers, as described below in section 3.4.

3.2 Immediate objects

There is another, more direct way to create an object: create it without going through a class.
The syntax is exactly the same as for class expressions, but the result is a single object rather
than a class. All the constructs described in the rest of this section also apply to immediate
objects.
# let p =
object
val mutable x = 0
method get_x = x
method move d = x <- x + d
end;;
val p : < get_x : int; move : int -> unit > = <obj>

# p#get_x;;
- : int = 0

# p#move 3;;
- : unit = ()

# p#get_x;;
- : int = 3
Unlike classes, which cannot be defined inside an expression, immediate objects can appear
anywhere, using variables from their environment.
# let minmax x y =
if x < y then object method min = x method max = y end
else object method min = y method max = x end;;
Chapter 3. Objects in OCaml 49

val minmax : 'a -> 'a -> < max : 'a; min : 'a > = <fun>
Immediate objects have two weaknesses compared to classes: their types are not abbreviated,
and you cannot inherit from them. But these two weaknesses can be advantages in some situations,
as we will see in sections 3.3 and 3.10.

3.3 Reference to self

A method or an initializer can invoke methods on self (that is, the current object). For that, self
must be explicitly bound, here to the variable s (s could be any identifier, even though we will often
choose the name self.)
# class printable_point x_init =
object (s)
val mutable x = x_init
method get_x = x
method move d = x <- x + d
method print = print_int s#get_x
end;;
class printable_point :
int ->
object
val mutable x : int
method get_x : int
method move : int -> unit
method print : unit
end

# let p = new printable_point 7;;

val p : printable_point = <obj>

# p#print;;
7- : unit = ()
Dynamically, the variable s is bound at the invocation of a method. In particular, when the class
printable_point is inherited, the variable s will be correctly bound to the object of the subclass.
A common problem with self is that, as its type may be extended in subclasses, you cannot fix
it in advance. Here is a simple example.
# let ints = ref [];;
val ints : '_weak1 list ref = {contents = []}

# class my_int =
object (self)
method n = 1
method register = ints := self :: !ints
end ;;
50

Error : The value self has type < n : int; register : 'a; .. >
but an expression was expected of type 'weak1
Self type cannot escape its class
You can ignore the first two lines of the error message. What matters is the last one: putting self
into an external reference would make it impossible to extend it through inheritance. We will see in
section 3.12 a workaround to this problem. Note however that, since immediate objects are not
extensible, the problem does not occur with them.
# let my_int =
object (self)
method n = 1
method register = ints := self :: !ints
end;;
val my_int : < n : int; register : unit > = <obj>

3.4 Initializers
Let-bindings within class definitions are evaluated before the object is constructed. It is also possible
to evaluate an expression immediately after the object has been built. Such code is written as
an anonymous hidden method called an initializer. Therefore, it can access self and the instance
variables.
# class printable_point x_init =
let origin = (x_init / 10) * 10 in
object (self)
val mutable x = origin
method get_x = x
method move d = x <- x + d
method print = print_int self#get_x
initializer print_string "new point at "; self#print; print_newline ()
end;;
class printable_point :
int ->
object
val mutable x : int
method get_x : int
method move : int -> unit
method print : unit
end

# let p = new printable_point 17;;

new point at 10
val p : printable_point = <obj>
Initializers cannot be overridden. On the contrary, all initializers are evaluated sequentially.
Initializers are particularly useful to enforce invariants. Another example can be seen in section 8.1.
Chapter 3. Objects in OCaml 51

3.5 Virtual methods

It is possible to declare a method without actually defining it, using the keyword virtual. This
method will be provided later in subclasses. A class containing virtual methods must be flagged
virtual, and cannot be instantiated (that is, no object of this class can be created). It still defines
type abbreviations (treating virtual methods as other methods.)
# class virtual abstract_point x_init =
object (self)
method virtual get_x : int
method get_offset = self#get_x - x_init
method virtual move : int -> unit
end;;
class virtual abstract_point :
int ->
object
method get_offset : int
method virtual get_x : int
method virtual move : int -> unit
end

# class point x_init =

object
inherit abstract_point x_init
val mutable x = x_init
method get_x = x
method move d = x <- x + d
end;;
class point :
int ->
object
val mutable x : int
method get_offset : int
method get_x : int
method move : int -> unit
end
Instance variables can also be declared as virtual, with the same effect as with methods.
# class virtual abstract_point2 =
object
val mutable virtual x : int
method move d = x <- x + d
end;;
class virtual abstract_point2 :
object val mutable virtual x : int method move : int -> unit end

# class point2 x_init =

object
52

inherit abstract_point2
val mutable x = x_init
method get_offset = x - x_init
end;;
class point2 :
int ->
object
val mutable x : int
method get_offset : int
method move : int -> unit
end

3.6 Private methods

Private methods are methods that do not appear in object interfaces. They can only be invoked
from other methods of the same object.
# class restricted_point x_init =
object (self)
val mutable x = x_init
method get_x = x
method private move d = x <- x + d
method bump = self#move 1
end;;
class restricted_point :
int ->
object
val mutable x : int
method bump : unit
method get_x : int
method private move : int -> unit
end

# let p = new restricted_point 0;;

val p : restricted_point = <obj>

# p#move 10 ;;
Error : This expression has type restricted_point
It has no method move

# p#bump;;
- : unit = ()
Note that this is not the same thing as private and protected methods in Java or C++, which can
be called from other objects of the same class. This is a direct consequence of the independence
between types and classes in OCaml: two unrelated classes may produce objects of the same type,
and there is no way at the type level to ensure that an object comes from a specific class. However
a possible encoding of friend methods is given in section 3.17.
Chapter 3. Objects in OCaml 53

Private methods are inherited (they are by default visible in subclasses), unless they are hidden
by signature matching, as described below.
Private methods can be made public in a subclass.
# class point_again x =
object (self)
inherit restricted_point x
method virtual move : _
end;;
class point_again :
int ->
object
val mutable x : int
method bump : unit
method get_x : int
method move : int -> unit
end
The annotation virtual here is only used to mention a method without providing its definition.
Since we didn’t add the private annotation, this makes the method public, keeping the original
definition.
An alternative definition is
# class point_again x =
object (self : < move : _; ..> )
inherit restricted_point x
end;;
class point_again :
int ->
object
val mutable x : int
method bump : unit
method get_x : int
method move : int -> unit
end
The constraint on self’s type is requiring a public move method, and this is sufficient to override
private.
One could think that a private method should remain private in a subclass. However, since the
method is visible in a subclass, it is always possible to pick its code and define a method of the
same name that runs that code, so yet another (heavier) solution would be:
# class point_again x =
object
inherit restricted_point x as super
method move = super#move
end;;
class point_again :
int ->
object
54

val mutable x : int

method bump : unit
method get_x : int
method move : int -> unit
end
Of course, private methods can also be virtual. Then, the keywords must appear in this order:
method private virtual.

3.7 Class interfaces

Class interfaces are inferred from class definitions. They may also be defined directly and used to
restrict the type of a class. Like class declarations, they also define a new type abbreviation.
# class type restricted_point_type =
object
method get_x : int
method bump : unit
end;;
class type restricted_point_type =
object method bump : unit method get_x : int end

# fun (x : restricted_point_type) -> x;;

- : restricted_point_type -> restricted_point_type = <fun>
In addition to program documentation, class interfaces can be used to constrain the type of a
class. Both concrete instance variables and concrete private methods can be hidden by a class type
constraint. Public methods and virtual members, however, cannot.
# class restricted_point' x = (restricted_point x : restricted_point_type);;
class restricted_point' : int -> restricted_point_type
Or, equivalently:
# class restricted_point' = (restricted_point : int -> restricted_point_type);;
class restricted_point' : int -> restricted_point_type
The interface of a class can also be specified in a module signature, and used to restrict the inferred
signature of a module.
# module type POINT = sig
class restricted_point' : int ->
object
method get_x : int
method bump : unit
end
end;;
module type POINT =
sig
class restricted_point' :
Chapter 3. Objects in OCaml 55

int -> object method bump : unit method get_x : int end
end

# module Point : POINT = struct

class restricted_point' = restricted_point
end;;
module Point : POINT

3.8 Inheritance
We illustrate inheritance by defining a class of colored points that inherits from the class of points.
This class has all instance variables and all methods of class point, plus a new instance variable c
and a new method color.
# class colored_point x (c : string) =
object
inherit point x
val c = c
method color = c
end;;
class colored_point :
int ->
string ->
object
val c : string
val mutable x : int
method color : string
method get_offset : int
method get_x : int
method move : int -> unit
end

# let p' = new colored_point 5 "red";;

val p' : colored_point = <obj>

# p'#get_x, p'#color;;
- : int * string = (5, "red")
A point and a colored point have incompatible types, since a point has no method color. However,
the function get_succ_x below is a generic function applying method get_x to any object p that
has this method (and possibly some others, which are represented by an ellipsis in the type). Thus,
it applies to both points and colored points.
# let get_succ_x p = p#get_x + 1;;
val get_succ_x : < get_x : int; .. > -> int = <fun>

# get_succ_x p + get_succ_x p';;

- : int = 8
56

Methods need not be declared previously, as shown by the example:

# let set_x p = p#set_x;;
val set_x : < set_x : 'a; .. > -> 'a = <fun>

# let incr p = set_x p (get_succ_x p);;

val incr : < get_x : int; set_x : int -> 'a; .. > -> 'a = <fun>

3.9 Multiple inheritance

Multiple inheritance is allowed. Only the last definition of a method is kept: the redefinition in
a subclass of a method that was visible in the parent class overrides the definition in the parent
class. Previous definitions of a method can be reused by binding the related ancestor. Below, super
is bound to the ancestor printable_point. The name super is a pseudo value identifier that can
only be used to invoke a super-class method, as in super#print.
# class printable_colored_point y c =
object (self)
val c = c
method color = c
inherit printable_point y as super
method! print =
print_string "(";
super#print;
print_string ", ";
print_string (self#color);
print_string ")"
end;;
class printable_colored_point :
int ->
string ->
object
val c : string
val mutable x : int
method color : string
method get_x : int
method move : int -> unit
method print : unit
end

# let p' = new printable_colored_point 17 "red";;

new point at (10, red)
val p' : printable_colored_point = <obj>

# p'#print;;
(10, red)- : unit = ()
Chapter 3. Objects in OCaml 57

A private method that has been hidden in the parent class is no longer visible, and is thus not
overridden. Since initializers are treated as private methods, all initializers along the class hierarchy
are evaluated, in the order they are introduced.
Note that for clarity’s sake, the method print is explicitly marked as overriding another definition
by annotating the method keyword with an exclamation mark !. If the method print were not
overriding the print method of printable_point, the compiler would raise an error:
# object
method! m = ()
end;;
Error : The method m has no previous definition
This explicit overriding annotation also works for val and inherit:
# class another_printable_colored_point y c c' =
object (self)
inherit printable_point y
inherit! printable_colored_point y c
val! c = c'
end;;
class another_printable_colored_point :
int ->
string ->
string ->
object
val c : string
val mutable x : int
method color : string
method get_x : int
method move : int -> unit
method print : unit
end

3.10 Parameterized classes

Reference cells can be implemented as objects. The naive definition fails to typecheck:
# class oref x_init =
object
val mutable x = x_init
method get = x
method set y = x <- y
end;;
Error : Some type variables are unbound in this type:
class oref :
'a ->
object
val mutable x : 'a
58

method get : 'a

method set : 'a -> unit
end
The method get has type 'a where 'a is unbound
The reason is that at least one of the methods has a polymorphic type (here, the type of the value
stored in the reference cell), thus either the class should be parametric, or the method type should
be constrained to a monomorphic type. A monomorphic instance of the class could be defined
by:
# class oref (x_init:int) =
object
val mutable x = x_init
method get = x
method set y = x <- y
end;;
class oref :
int ->
object val mutable x : int method get : int method set : int -> unit end
Note that since immediate objects do not define a class type, they have no such restriction.
# let new_oref x_init =
object
val mutable x = x_init
method get = x
method set y = x <- y
end;;
val new_oref : 'a -> < get : 'a; set : 'a -> unit > = <fun>
On the other hand, a class for polymorphic references must explicitly list the type parameters in its
declaration. Class type parameters are listed between [ and ]. The type parameters must also be
bound somewhere in the class body by a type constraint.
# class ['a] oref x_init =
object
val mutable x = (x_init : 'a)
method get = x
method set y = x <- y
end;;
class ['a] oref :
'a -> object val mutable x : 'a method get : 'a method set : 'a -> unit end

# let r = new oref 1 in r#set 2; (r#get);;

- : int = 2
The type parameter in the declaration may actually be constrained in the body of the class definition.
In the class type, the actual value of the type parameter is displayed in the constraint clause.
# class ['a] oref_succ (x_init:'a) =
object
Chapter 3. Objects in OCaml 59

val mutable x = x_init + 1

method get = x
method set y = x <- y
end;;
class ['a] oref_succ :
'a ->
object
constraint 'a = int
val mutable x : int
method get : int
method set : int -> unit
end
Let us consider a more complex example: define a circle, whose center may be any kind of point. We
put an additional type constraint in method move, since no free variables must remain unaccounted
for by the class type parameters.
# class ['a] circle (c : 'a) =
object
val mutable center = c
method center = center
method set_center c = center <- c
method move = (center#move : int -> unit)
end;;
class ['a] circle :
'a ->
object
constraint 'a = < move : int -> unit; .. >
val mutable center : 'a
method center : 'a
method move : int -> unit
method set_center : 'a -> unit
end
An alternate definition of circle, using a constraint clause in the class definition, is shown below.
The type #point used below in the constraint clause is an abbreviation produced by the definition
of class point. This abbreviation unifies with the type of any object belonging to a subclass of
class point. It actually expands to < get_x : int; move : int -> unit; .. >. This leads to
the following alternate definition of circle, which has slightly stronger constraints on its argument,
as we now expect center to have a method get_x.
# class ['a] circle (c : 'a) =
object
constraint 'a = #point
val mutable center = c
method center = center
method set_center c = center <- c
method move = center#move
end;;
60

class ['a] circle :

'a ->
object
constraint 'a = #point
val mutable center : 'a
method center : 'a
method move : int -> unit
method set_center : 'a -> unit
end
The class colored_circle is a specialized version of class circle that requires the type of the
center to unify with #colored_point, and adds a method color. Note that when specializing a
parameterized class, the instance of type parameter must always be explicitly given. It is again
written between [ and ].
# class ['a] colored_circle c =
object
constraint 'a = #colored_point
inherit ['a] circle c
method color = center#color
end;;
class ['a] colored_circle :
'a ->
object
constraint 'a = #colored_point
val mutable center : 'a
method center : 'a
method color : string
method move : int -> unit
method set_center : 'a -> unit
end

3.11 Polymorphic methods

While parameterized classes may be polymorphic in their contents, they are not enough to allow
polymorphism of method use.
A classical example is defining an iterator.
# List.fold_left;;
- : ('acc -> 'a -> 'acc) -> 'acc -> 'a list -> 'acc = <fun>

# class ['a] intlist (l : int list) =

object
method empty = (l = [])
method fold f (accu : 'a) = List.fold_left f accu l
end;;
class ['a] intlist :
int list ->
object method empty : bool method fold : ('a -> int -> 'a) -> 'a -> 'a end
Chapter 3. Objects in OCaml 61

At first look, we seem to have a polymorphic iterator, however this does not work in practice.
# let l = new intlist [1; 2; 3];;
val l : '_weak2 intlist = <obj>

# l#fold (fun x y -> x+y) 0;;

- : int = 6

# l;;
- : int intlist = <obj>

# l#fold (fun s x -> s ^ Int.to_string x ^ " ") "" ;;

Error : The value s has type int but an expression was expected of type string
Our iterator works, as shows its first use for summation. However, since objects themselves are not
polymorphic (only their constructors are), using the fold method fixes its type for this individual
object. Our next attempt to use it as a string iterator fails.
The problem here is that quantification was wrongly located: it is not the class we want to be
polymorphic, but the fold method. This can be achieved by giving an explicitly polymorphic type
in the method definition.
# class intlist (l : int list) =
object
method empty = (l = [])
method fold : 'a. ('a -> int -> 'a) -> 'a -> 'a =
fun f accu -> List.fold_left f accu l
end;;
class intlist :
int list ->
object method empty : bool method fold : ('a -> int -> 'a) -> 'a -> 'a end

# let l = new intlist [1; 2; 3];;

val l : intlist = <obj>

# l#fold (fun x y -> x+y) 0;;

- : int = 6

# l#fold (fun s x -> s ^ Int.to_string x ^ " ") "";;

- : string = "1 2 3 "
As you can see in the class type shown by the compiler, while polymorphic method types must be
fully explicit in class definitions (appearing immediately after the method name), quantified type
variables can be left implicit in class descriptions. Why require types to be explicit? The problem is
that (int -> int -> int) -> int -> int would also be a valid type for fold, and it happens
to be incompatible with the polymorphic type we gave (automatic instantiation only works for
toplevel types variables, not for inner quantifiers, where it becomes an undecidable problem.) So
the compiler cannot choose between those two types, and must be helped.
However, the type can be completely omitted in the class definition if it is already known,
through inheritance or type constraints on self. Here is an example of method overriding.
62

# class intlist_rev l =
object
inherit intlist l
method! fold f accu = List.fold_left f accu (List.rev l)
end;;
The following idiom separates description and definition.
# class type ['a] iterator =
object method fold : ('b -> 'a -> 'b) -> 'b -> 'b end;;
# class intlist' l =
object (self : int #iterator)
method empty = (l = [])
method fold f accu = List.fold_left f accu l
end;;
Note here the (self : int #iterator) idiom, which ensures that this object implements the
interface iterator.
Polymorphic methods are called in exactly the same way as normal methods, but you should be
aware of some limitations of type inference. Namely, a polymorphic method can only be called if its
type is known at the call site. Otherwise, the method will be assumed to be monomorphic, and
given an incompatible type.
# let sum lst = lst#fold (fun x y -> x+y) 0;;
val sum : < fold : (int -> int -> int) -> int -> 'a; .. > -> 'a = <fun>

# sum l ;;
Error : The value l has type
intlist = < empty : bool; fold : 'a. ('a -> int -> 'a) -> 'a -> 'a >
but an expression was expected of type
< fold : (int -> int -> int) -> int -> 'b; .. >
The method fold has type 'a. ('a -> int -> 'a) -> 'a -> 'a,
but the expected method type was (int -> int -> int) -> int -> 'b
The workaround is easy: you should put a type constraint on the parameter.
# let sum (lst : _ #iterator) = lst#fold (fun x y -> x+y) 0;;
val sum : int #iterator -> int = <fun>
Of course the constraint may also be an explicit method type. Only occurrences of quantified
variables are required.
# let sum lst =
(lst : < fold : 'a. ('a -> _ -> 'a) -> 'a -> 'a; .. >)#fold (+) 0;;
val sum : < fold : 'a. ('a -> int -> 'a) -> 'a -> 'a; .. > -> int = <fun>
Another use of polymorphic methods is to allow some form of implicit subtyping in method
arguments. We have already seen in section 3.8 how some functions may be polymorphic in the
class of their argument. This can be extended to methods.
# class type point0 = object method get_x : int end;;
class type point0 = object method get_x : int end
Chapter 3. Objects in OCaml 63

# class distance_point x =
object
inherit point x
method distance : 'a. (#point0 as 'a) -> int =
fun other -> abs (other#get_x - x)
end;;
class distance_point :
int ->
object
val mutable x : int
method distance : #point0 -> int
method get_offset : int
method get_x : int
method move : int -> unit
end

# let p = new distance_point 3 in

(p#distance (new point 8), p#distance (new colored_point 1 "blue"));;
- : int * int = (5, 2)
Note here the special syntax (#point0 as 'a) we have to use to quantify the extensible part
of #point0. As for the variable binder, it can be omitted in class specifications. If you want
polymorphism inside object field it must be quantified independently.
# class multi_poly =
object
method m1 : 'a. (< n1 : 'b. 'b -> 'b; .. > as 'a) -> _ =
fun o -> o#n1 true, o#n1 "hello"
method m2 : 'a 'b. (< n2 : 'b -> bool; .. > as 'a) -> 'b -> _ =
fun o x -> o#n2 x
end;;
class multi_poly :
object
method m1 : < n1 : 'b. 'b -> 'b; .. > -> bool * string
method m2 : < n2 : 'b -> bool; .. > -> 'b -> bool
end
In method m1, o must be an object with at least a method n1, itself polymorphic. In method m2,
the argument of n2 and x must have the same type, which is quantified at the same level as 'a.

3.12 Using coercions

Subtyping is never implicit. There are, however, two ways to perform subtyping. The most general
construction is fully explicit: both the domain and the codomain of the type coercion must be given.
We have seen that points and colored points have incompatible types. For instance, they cannot
be mixed in the same list. However, a colored point can be coerced to a point, hiding its color
method:
# let colored_point_to_point cp = (cp : colored_point :> point);;
64

val colored_point_to_point : colored_point -> point = <fun>

# let p = new point 3 and q = new colored_point 4 "blue";;

val p : point = <obj>
val q : colored_point = <obj>

# let l = [p; (colored_point_to_point q)];;

val l : point list = [<obj>; <obj>]
An object of type t can be seen as an object of type t' only if t is a subtype of t'. For instance, a
point cannot be seen as a colored point.
# (p : point :> colored_point);;
Error : Type point = < get_offset : int; get_x : int; move : int -> unit >
is not a subtype of
colored_point =
< color : string ; get_offset : int; get_x : int;
move : int -> unit >
The first object type has no method color
Indeed, narrowing coercions without runtime checks would be unsafe. Runtime type checks might
raise exceptions, and they would require the presence of type information at runtime, which is
not the case in the OCaml system. For these reasons, there is no such operation available in the
language.
Be aware that subtyping and inheritance are not related. Inheritance is a syntactic relation
between classes while subtyping is a semantic relation between types. For instance, the class of
colored points could have been defined directly, without inheriting from the class of points; the type
of colored points would remain unchanged and thus still be a subtype of points.
The domain of a coercion can often be omitted. For instance, one can define:
# let to_point cp = (cp :> point);;
val to_point : #point -> point = <fun>
In this case, the function colored_point_to_point is an instance of the function to_point. This
is not always true, however. The fully explicit coercion is more precise and is sometimes unavoidable.
Consider, for example, the following class:
# class c0 = object method m = {< >} method n = 0 end;;
class c0 : object ('a) method m : 'a method n : int end
The object type c0 is an abbreviation for <m : 'a; n : int> as 'a. Consider now the type
declaration:
# class type c1 = object method m : c1 end;;
class type c1 = object method m : c1 end
The object type c1 is an abbreviation for the type <m : 'a> as 'a. The coercion from an object
of type c0 to an object of type c1 is correct:
# fun (x:c0) -> (x : c0 :> c1);;
- : c0 -> c1 = <fun>
Chapter 3. Objects in OCaml 65

However, the domain of the coercion cannot always be omitted. In that case, the solution is to use
the explicit form. Sometimes, a change in the class-type definition can also solve the problem
# class type c2 = object ('a) method m : 'a end;;
class type c2 = object ('a) method m : 'a end

# fun (x:c0) -> (x :> c2);;

- : c0 -> c2 = <fun>
While class types c1 and c2 are different, both object types c1 and c2 expand to the same object
type (same method names and types). Yet, when the domain of a coercion is left implicit and its
co-domain is an abbreviation of a known class type, then the class type, rather than the object type,
is used to derive the coercion function. This allows leaving the domain implicit in most cases when
coercing from a subclass to its superclass. The type of a coercion can always be seen as below:
# let to_c1 x = (x :> c1);;
val to_c1 : < m : #c1; .. > -> c1 = <fun>

# let to_c2 x = (x :> c2);;

val to_c2 : #c2 -> c2 = <fun>
Note the difference between these two coercions: in the case of to_c2, the type
#c2 = < m : 'a; .. > as 'a is polymorphically recursive (according to the explicit re-
cursion in the class type of c2); hence the success of applying this coercion to an object of
class c0. On the other hand, in the first case, c1 was only expanded and unrolled twice to
obtain < m : < m : c1; .. >; .. > (remember #c1 = < m : c1; .. >), without introducing
recursion. You may also note that the type of to_c2 is #c2 -> c2 while the type of to_c1 is
more general than #c1 -> c1. This is not always true, since there are class types for which some
instances of #c are not subtypes of c, as explained in section 3.16. Yet, for parameterless classes the
coercion (_ :> c) is always more general than (_ : #c :> c).
A common problem may occur when one tries to define a coercion to a class c while defining
class c. The problem is due to the type abbreviation not being completely defined yet, and so its
subtypes are not clearly known. Then, a coercion (_ :> c) or (_ : #c :> c) is taken to be the
identity function, as in
# fun x -> (x :> 'a);;
- : 'a -> 'a = <fun>
As a consequence, if the coercion is applied to self, as in the following example, the type of self is
unified with the closed type c (a closed object type is an object type without ellipsis). This would
constrain the type of self be closed and is thus rejected. Indeed, the type of self cannot be closed:
this would prevent any further extension of the class. Therefore, a type error is generated when the
unification of this type with another type would result in a closed object type.
# class c = object method m = 1 end
and d = object (self)
inherit c
method n = 2
method as_c = (self :> c)
end;;
66

Error : This expression cannot be coerced to type c = < m : int >; it has type
< as_c : c; m : int; n : int; .. >
but is here used with type c
Self type cannot escape its class
However, the most common instance of this problem, coercing self to its current class, is detected as
a special case by the type checker, and properly typed.
# class c = object (self) method m = (self :> c) end;;
class c : object method m : c end
This allows the following idiom, keeping a list of all objects belonging to a class or its subclasses:
# let all_c = ref [];;
val all_c : '_weak3 list ref = {contents = []}

# class c (m : int) =
object (self)
method m = m
initializer all_c := (self :> c) :: !all_c
end;;
class c : int -> object method m : int end
This idiom can in turn be used to retrieve an object whose type has been weakened:
# let rec lookup_obj obj = function [] -> raise Not_found
| obj' :: l ->
if (obj :> < >) = (obj' :> < >) then obj' else lookup_obj obj l ;;
val lookup_obj : < .. > -> (< .. > as 'a) list -> 'a = <fun>

# let lookup_c obj = lookup_obj obj !all_c;;

val lookup_c : < .. > -> < m : int > = <fun>
The type < m : int > we see here is just the expansion of c, due to the use of a reference; we have
succeeded in getting back an object of type c.
The previous coercion problem can often be avoided by first defining the abbreviation, using a
class type:
# class type c' = object method m : int end;;
class type c' = object method m : int end

# class c : c' = object method m = 1 end

and d = object (self)
inherit c
method n = 2
method as_c = (self :> c')
end;;
class c : c'
and d : object method as_c : c' method m : int method n : int end
It is also possible to use a virtual class. Inheriting from this class simultaneously forces all methods
of c to have the same type as the methods of c'.
Chapter 3. Objects in OCaml 67

# class virtual c' = object method virtual m : int end;;

class virtual c' : object method virtual m : int end

# class c = object (self) inherit c' method m = 1 end;;

class c : object method m : int end
One could think of defining the type abbreviation directly:
# type c' = <m : int>;;
However, the abbreviation #c' cannot be defined directly in a similar way. It can only be defined
by a class or a class-type definition. This is because a #-abbreviation carries an implicit anonymous
variable .. that cannot be explicitly named. The closer you get to it is:
# type 'a c'_class = 'a constraint 'a = < m : int; .. >;;
with an extra type variable capturing the open object type.

3.13 Functional objects

It is possible to write a version of class point without assignments on the instance variables. The
override construct {< ... >} returns a copy of “self” (that is, the current object), possibly changing
the value of some instance variables.
# class functional_point y =
object
val x = y
method get_x = x
method move d = {< x = x + d >}
method move_to x = {< x >}
end;;
class functional_point :
int ->
object ('a)
val x : int
method get_x : int
method move : int -> 'a
method move_to : int -> 'a
end

# let p = new functional_point 7;;

val p : functional_point = <obj>

# p#get_x;;
- : int = 7

# (p#move 3)#get_x;;
- : int = 10

# (p#move_to 15)#get_x;;
- : int = 15
68

# p#get_x;;
- : int = 7
As with records, the form {< x >} is an elided version of {< x = x >} which avoids the repetition
of the instance variable name. Note that the type abbreviation functional_point is recursive,
which can be seen in the class type of functional_point: the type of self is 'a and 'a appears
inside the type of the method move.
The above definition of functional_point is not equivalent to the following:
# class bad_functional_point y =
object
val x = y
method get_x = x
method move d = new bad_functional_point (x+d)
method move_to x = new bad_functional_point x
end;;
class bad_functional_point :
int ->
object
val x : int
method get_x : int
method move : int -> bad_functional_point
method move_to : int -> bad_functional_point
end
While objects of either class will behave the same, objects of their subclasses will be different. In a
subclass of bad_functional_point, the method move will keep returning an object of the parent
class. On the contrary, in a subclass of functional_point, the method move will return an object
of the subclass.
Functional update is often used in conjunction with binary methods as illustrated in section 8.2.1.

3.14 Cloning objects

Objects can also be cloned, whether they are functional or imperative. The library function Oo.copy
makes a shallow copy of an object. That is, it returns a new object that has the same methods
and instance variables as its argument. The instance variables are copied but their contents are
shared. Assigning a new value to an instance variable of the copy (using a method call) will not
affect instance variables of the original, and conversely. A deeper assignment (for example if the
instance variable is a reference cell) will of course affect both the original and the copy.
The type of Oo.copy is the following:
# Oo.copy;;
- : (< .. > as 'a) -> 'a = <fun>
The keyword as in that type binds the type variable 'a to the object type < .. >. Therefore,
Oo.copy takes an object with any methods (represented by the ellipsis), and returns an object
of the same type. The type of Oo.copy is different from type < .. > -> < .. > as each ellipsis
represents a different set of methods. Ellipsis actually behaves as a type variable.
Chapter 3. Objects in OCaml 69

# let p = new point 5;;

val p : point = <obj>

# let q = Oo.copy p;;

val q : point = <obj>

# q#move 7; (p#get_x, q#get_x);;

- : int * int = (5, 12)
In fact, Oo.copy p will behave as p#copy assuming that a public method copy with body {< >}
has been defined in the class of p.
Objects can be compared using the generic comparison functions = and <>. Two objects are equal
if and only if they are physically equal. In particular, an object and its copy are not equal.
# let q = Oo.copy p;;
val q : point = <obj>

# p = q, p = p;;
- : bool * bool = (false, true)
Other generic comparisons such as (<, <=, ...) can also be used on objects. The relation < defines an
unspecified but strict ordering on objects. The ordering relationship between two objects is fixed
permanently once the two objects have been created, and it is not affected by mutation of fields.
Cloning and override have a non empty intersection. They are interchangeable when used within
an object and without overriding any field:
# class copy =
object
method copy = {< >}
end;;
class copy : object ('a) method copy : 'a end

# class copy =
object (self)
method copy = Oo.copy self
end;;
class copy : object ('a) method copy : 'a end
Only the override can be used to actually override fields, and only the Oo.copy primitive can be
used externally.
Cloning can also be used to provide facilities for saving and restoring the state of objects.
# class backup =
object (self : 'mytype)
val mutable copy = None
method save = copy <- Some {< copy = None >}
method restore = match copy with Some x -> x | None -> self
end;;
70

class backup :
object ('a)
val mutable copy : 'a option
method restore : 'a
method save : unit
end
The above definition will only backup one level. The backup facility can be added to any class by
using multiple inheritance.
# class ['a] backup_ref x = object inherit ['a] oref x inherit backup end;;
class ['a] backup_ref :
'a ->
object ('b)
val mutable copy : 'b option
val mutable x : 'a
method get : 'a
method restore : 'b
method save : unit
method set : 'a -> unit
end

# let rec get p n = if n = 0 then p # get else get (p # restore) (n-1);;

val get : (< get : 'b; restore : 'a; .. > as 'a) -> int -> 'b = <fun>

# let p = new backup_ref 0 in

p # save; p # set 1; p # save; p # set 2;
[get p 0; get p 1; get p 2; get p 3; get p 4];;
- : int list = [2; 1; 1; 1; 1]
We can define a variant of backup that retains all copies. (We also add a method clear to manually
erase all copies.)
# class backup =
object (self : 'mytype)
val mutable copy = None
method save = copy <- Some {< >}
method restore = match copy with Some x -> x | None -> self
method clear = copy <- None
end;;
class backup :
object ('a)
val mutable copy : 'a option
method clear : unit
method restore : 'a
method save : unit
end

# class ['a] backup_ref x = object inherit ['a] oref x inherit backup end;;
Chapter 3. Objects in OCaml 71

class ['a] backup_ref :

'a ->
object ('b)
val mutable copy : 'b option
val mutable x : 'a
method clear : unit
method get : 'a
method restore : 'b
method save : unit
method set : 'a -> unit
end

# let p = new backup_ref 0 in

p # save; p # set 1; p # save; p # set 2;
[get p 0; get p 1; get p 2; get p 3; get p 4];;
- : int list = [2; 1; 0; 0; 0]

3.15 Recursive classes

Recursive classes can be used to define objects whose types are mutually recursive.
# class window =
object
val mutable top_widget = (None : widget option)
method top_widget = top_widget
end
and widget (w : window) =
object
val window = w
method window = window
end;;
class window :
object
val mutable top_widget : widget option
method top_widget : widget option
end
and widget : window -> object val window : window method window : window end
Although their types are mutually recursive, the classes widget and window are themselves inde-
pendent.

3.16 Binary methods

A binary method is a method which takes an argument of the same type as self. The class
comparable below is a template for classes with a binary method leq of type 'a -> bool
where the type variable 'a is bound to the type of self. Therefore, #comparable expands to
< leq : 'a -> bool; .. > as 'a. We see here that the binder as also allows writing recursive
types.
72

# class virtual comparable =

object (_ : 'a)
method virtual leq : 'a -> bool
end;;
class virtual comparable : object ('a) method virtual leq : 'a -> bool end
We then define a subclass money of comparable. The class money simply wraps floats as comparable
objects.1 We will extend money below with more operations. We have to use a type constraint on
the class parameter x because the primitive <= is a polymorphic function in OCaml. The inherit
clause ensures that the type of objects of this class is an instance of #comparable.
# class money (x : float) =
object
inherit comparable
val repr = x
method value = repr
method leq p = repr <= p#value
end;;
class money :
float ->
object ('a)
val repr : float
method leq : 'a -> bool
method value : float
end
Note that the type money is not a subtype of type comparable, as the self type appears in
contravariant position in the type of method leq. Indeed, an object m of class money has a method
leq that expects an argument of type money since it accesses its value method. Considering m of
type comparable would allow a call to method leq on m with an argument that does not have a
method value, which would be an error.
Similarly, the type money2 below is not a subtype of type money.
# class money2 x =
object
inherit money x
method times k = {< repr = k *. repr >}
end;;
class money2 :
float ->
object ('a)
val repr : float
method leq : 'a -> bool
method times : float -> 'a
method value : float
end
1
floats are an approximation of decimal numbers, they are unsuitable for use in most monetary calculations as they
may introduce errors.
Chapter 3. Objects in OCaml 73

It is however possible to define functions that manipulate objects of type either money or money2: the
function min will return the minimum of any two objects whose type unifies with #comparable. The
type of min is not the same as #comparable -> #comparable -> #comparable, as the abbreviation
#comparable hides a type variable (an ellipsis). Each occurrence of this abbreviation generates a
new variable.
# let min (x : #comparable) y =
if x#leq y then x else y;;
val min : (#comparable as 'a) -> 'a -> 'a = <fun>
This function can be applied to objects of type money or money2.
# (min (new money 1.3) (new money 3.1))#value;;
- : float = 1.3

# (min (new money2 5.0) (new money2 3.14))#value;;

- : float = 3.14
More examples of binary methods can be found in sections 8.2.1 and 8.2.4.
Note the use of override for method times. Writing new money2 (k *. repr) instead of
{< repr = k *. repr >} would not behave well with inheritance: in a subclass money3 of money2
the times method would return an object of class money2 but not of class money3 as would be
expected.
The class money could naturally carry another binary method. Here is a direct definition:
# class money x =
object (self : 'a)
val repr = x
method value = repr
method print = print_float repr
method times k = {< repr = k *. x >}
method leq (p : 'a) = repr <= p#value
method plus (p : 'a) = {< repr = x +. p#value >}
end;;
class money :
float ->
object ('a)
val repr : float
method leq : 'a -> bool
method plus : 'a -> 'a
method print : unit
method times : float -> 'a
method value : float
end

3.17 Friends
The above class money reveals a problem that often occurs with binary methods. In order to interact
with other objects of the same class, the representation of money objects must be revealed, using a
74

method such as value. If we remove all binary methods (here plus and leq), the representation
can easily be hidden inside objects by removing the method value as well. However, this is not
possible as soon as some binary method requires access to the representation of objects of the same
class (other than self).
# class safe_money x =
object (self : 'a)
val repr = x
method print = print_float repr
method times k = {< repr = k *. x >}
end;;
class safe_money :
float ->
object ('a)
val repr : float
method print : unit
method times : float -> 'a
end
Here, the representation of the object is known only to a particular object. To make it available to
other objects of the same class, we are forced to make it available to the whole world. However we
can easily restrict the visibility of the representation using the module system.
# module type MONEY =
sig
type t
class c : float ->
object ('a)
val repr : t
method value : t
method print : unit
method times : float -> 'a
method leq : 'a -> bool
method plus : 'a -> 'a
end
end;;
# module Euro : MONEY =
struct
type t = float
class c x =
object (self : 'a)
val repr = x
method value = repr
method print = print_float repr
method times k = {< repr = k *. x >}
method leq (p : 'a) = repr <= p#value
method plus (p : 'a) = {< repr = x +. p#value >}
Chapter 3. Objects in OCaml 75

end
end;;
Another example of friend functions may be found in section 8.2.4. These examples occur when
a group of objects (here objects of the same class) and functions should see each others internal
representation, while their representation should be hidden from the outside. The solution is always
to define all friends in the same module, give access to the representation and use a signature
constraint to make the representation abstract outside the module.
76
Chapter 4

Labeled arguments

(Chapter written by Jacques Garrigue)

If you have a look at modules ending in Labels in the standard library, you will see that function
types have annotations you did not have in the functions you defined yourself.
# ListLabels.map;;
- : f:('a -> 'b) -> 'a list -> 'b list = <fun>

# StringLabels.sub;;
- : string -> pos:int -> len:int -> string = <fun>
Such annotations of the form name: are called labels. They are meant to document the code,
allow more checking, and give more flexibility to function application. You can give such names to
arguments in your programs, by prefixing them with a tilde ~.
# let f ~x ~y = x - y;;
val f : x:int -> y:int -> int = <fun>

# let x = 3 and y = 2 in f ~x ~y;;

- : int = 1
When you want to use distinct names for the variable and the label appearing in the type, you
can use a naming label of the form ~name:. This also applies when the argument is not a variable.
# let f ~x:x1 ~y:y1 = x1 - y1;;
val f : x:int -> y:int -> int = <fun>

# f ~x:3 ~y:2;;
- : int = 1
Labels obey the same rules as other identifiers in OCaml, that is you cannot use a reserved
keyword (like in or to) as a label.
Formal parameters and arguments are matched according to their respective labels, the absence
of label being interpreted as the empty label. This allows commuting arguments in applications.
One can also partially apply a function on any argument, creating a new function of the remaining
parameters.
# let f ~x ~y = x - y;;

77
78

val f : x:int -> y:int -> int = <fun>

# f ~y:2 ~x:3;;
- : int = 1

# ListLabels.fold_left;;
- : f:('acc -> 'a -> 'acc) -> init:'acc -> 'a list -> 'acc = <fun>

# ListLabels.fold_left [1;2;3] ~init:0 ~f:( + );;

- : int = 6

# ListLabels.fold_left ~init:0;;
- : f:(int -> 'a -> int) -> 'a list -> int = <fun>
If several arguments of a function bear the same label (or no label), they will not commute
among themselves, and order matters. But they can still commute with other arguments.
# let hline ~x:x1 ~x:x2 ~y = (x1, x2, y);;
val hline : x:'a -> x:'b -> y:'c -> 'a * 'b * 'c = <fun>

# hline ~x:3 ~y:2 ~x:5;;

- : int * int * int = (3, 5, 2)

4.1 Optional arguments

An interesting feature of labeled arguments is that they can be made optional. For optional
parameters, the question mark ? replaces the tilde ~ of non-optional ones, and the label is also
prefixed by ? in the function type. Default values may be given for such optional parameters.
# let bump ?(step = 1) x = x + step;;
val bump : ?step:int -> int -> int = <fun>

# bump 2;;
- : int = 3

# bump ~step:3 2;;

- : int = 5
A function taking some optional arguments must also take at least one non-optional argument.
The criterion for deciding whether an optional argument has been omitted is the non-labeled
application of an argument appearing after this optional argument in the function type. Note that if
that argument is labeled, you will only be able to eliminate optional arguments by totally applying
the function, omitting all optional arguments and omitting all labels for all remaining arguments.
# let test ?(x = 0) ?(y = 0) () ?(z = 0) () = (x, y, z);;
val test : ?x:int -> ?y:int -> unit -> ?z:int -> unit -> int * int * int =
<fun>

# test ();;
Chapter 4. Labeled arguments 79

- : ?z:int -> unit -> int * int * int = <fun>

# test ~x:2 () ~z:3 ();;

- : int * int * int = (2, 0, 3)
Optional parameters may also commute with non-optional or unlabeled ones, as long as they are
applied simultaneously. By nature, optional arguments do not commute with unlabeled arguments
applied independently.
# test ~y:2 ~x:3 () ();;
- : int * int * int = (3, 2, 0)

# test () () ~z:1 ~y:2 ~x:3;;

- : int * int * int = (3, 2, 1)

# (test () ()) ~z:1 ;;

Error : This expression has type int * int * int
This is not a function ; it cannot be applied .
Here (test () ()) is already (0,0,0) and cannot be further applied.
Optional arguments are actually implemented as option types. If you do not give a default value,
you have access to their internal representation, type 'a option = None | Some of 'a. You can
then provide different behaviors when an argument is present or not.
# let bump ?step x =
match step with
| None -> x * 2
| Some y -> x + y
;;
val bump : ?step:int -> int -> int = <fun>
It may also be useful to relay an optional argument from a function call to another. This can
be done by prefixing the applied argument with ?. This question mark disables the wrapping of
optional argument in an option type.
# let test2 ?x ?y () = test ?x ?y () ();;
val test2 : ?x:int -> ?y:int -> unit -> int * int * int = <fun>

# test2 ?x:None;;
- : ?y:int -> unit -> int * int * int = <fun>

4.2 Labels and type inference

While they provide an increased comfort for writing function applications, labels and optional
arguments have the pitfall that they cannot be inferred as completely as the rest of the language.
You can see it in the following two examples.
# let h' g = g ~y:2 ~x:3;;
val h' : (y:int -> x:int -> 'a) -> 'a = <fun>
80

# h' f ;;
Error : The value f has type x:int -> y:int -> int
but an expression was expected of type y:int -> x:int -> 'a
Labels x and y do not match

# let bump_it bump x =

bump ~step:2 x;;
val bump_it : (step:int -> 'a -> 'b) -> 'a -> 'b = <fun>

# bump_it bump 1 ;;
Error : The value bump has type ?step:int -> int -> int
but an expression was expected of type step:int -> 'a -> 'b
The label ?step was expected to not be optional
The first case is simple: g is passed ~y and then ~x, but f expects ~x and then ~y. This is correctly
handled if we know the type of g to be x:int -> y:int -> int in advance, but otherwise this
causes the above type clash. The simplest workaround is to apply formal parameters in a standard
order.
The second example is more subtle: while we intended the argument bump to be of type
?step:int -> int -> int, it is inferred as step:int -> int -> 'a. These two types being
incompatible (internally normal and optional arguments are different), a type error occurs when
applying bump_it to the real bump.
We will not try here to explain in detail how type inference works. One must just understand
that there is not enough information in the above program to deduce the correct type of g or bump.
That is, there is no way to know whether an argument is optional or not, or which is the correct
order, by looking only at how a function is applied. The strategy used by the compiler is to assume
that there are no optional arguments, and that applications are done in the right order.
The right way to solve this problem for optional parameters is to add a type annotation to the
argument bump.
# let bump_it (bump : ?step:int -> int -> int) x =
bump ~step:2 x;;
val bump_it : (?step:int -> int -> int) -> int -> int = <fun>

# bump_it bump 1;;

- : int = 3
In practice, such problems appear mostly when using objects whose methods have optional arguments,
so writing the type of object arguments is often a good idea.
Normally the compiler generates a type error if you attempt to pass to a function a parameter
whose type is different from the expected one. However, in the specific case where the expected type
is a non-labeled function type, and the argument is a function expecting optional parameters, the
compiler will attempt to transform the argument to have it match the expected type, by passing
None for all optional parameters.
# let twice f (x : int) = f(f x);;
val twice : (int -> int) -> int -> int = <fun>
Chapter 4. Labeled arguments 81

# twice bump 2;;

- : int = 8
This transformation is coherent with the intended semantics, including side-effects. That is, if
the application of optional parameters shall produce side-effects, these are delayed until the received
function is really applied to an argument.

4.3 Suggestions for labeling

Like for names, choosing labels for functions is not an easy task. A good labeling is one which

• makes programs more readable,

• is easy to remember,

• when possible, allows useful partial applications.

We explain here the rules we applied when labeling OCaml libraries.

To speak in an “object-oriented” way, one can consider that each function has a main argument,
its object, and other arguments related with its action, the parameters. To permit the combination
of functions through functionals in commuting label mode, the object will not be labeled. Its role is
clear from the function itself. The parameters are labeled with names reminding of their nature
or their role. The best labels combine nature and role. When this is not possible the role is to be
preferred, since the nature will often be given by the type itself. Obscure abbreviations should be
avoided.

ListLabels.map : f:('a -> 'b) -> 'a list -> 'b list
UnixLabels.write : file_descr -> buf:bytes -> pos:int -> len:int -> unit

When there are several objects of same nature and role, they are all left unlabeled.

ListLabels.iter2 : f:('a -> 'b -> unit) -> 'a list -> 'b list -> unit

When there is no preferable object, all arguments are labeled.

BytesLabels.blit :
src:bytes -> src_pos:int -> dst:bytes -> dst_pos:int -> len:int -> unit

However, when there is only one argument, it is often left unlabeled.

BytesLabels.create : int -> bytes

This principle also applies to functions of several arguments whose return type is a type variable,
as long as the role of each argument is not ambiguous. Labeling such functions may lead to
awkward error messages when one attempts to omit labels in an application, as we have seen with
ListLabels.fold_left.
Here are some of the label names you will find throughout the libraries.
82

Label Meaning
f: a function to be applied
pos: a position in a string, array or byte sequence
len: a length
buf: a byte sequence or string used as buffer
src: the source of an operation
dst: the destination of an operation
init: the initial value for an iterator
cmp: a comparison function, e.g. Stdlib.compare
mode: an operation mode or a flag list

All these are only suggestions, but keep in mind that the choice of labels is essential for readability.
Bizarre choices will make the program harder to maintain.
In the ideal, the right function name with right labels should be enough to understand the
function’s meaning. Since one can get this information with OCamlBrowser or the ocaml toplevel,
the documentation is only used when a more detailed specification is needed.
Chapter 5

Polymorphic variants

(Chapter written by Jacques Garrigue)

Variants as presented in section 1.4 are a powerful tool to build data structures and algorithms.
However they sometimes lack flexibility when used in modular programming. This is due to the
fact that every constructor is assigned to a unique type when defined and used. Even if the same
name appears in the definition of multiple types, the constructor itself belongs to only one type.
Therefore, one cannot decide that a given constructor belongs to multiple types, or consider a value
of some type to belong to some other type with more constructors.
With polymorphic variants, this original assumption is removed. That is, a variant tag does
not belong to any type in particular, the type system will just check that it is an admissible value
according to its use. You need not define a type before using a variant tag. A variant type will be
inferred independently for each of its uses.

5.1 Basic use

In programs, polymorphic variants work like usual ones. You just have to prefix their names with a
backquote character `.
# [`On; `Off];;
- : [> `Off | `On ] list = [`On; `Off]

# `Number 1;;
- : [> `Number of int ] = `Number 1

# let f = function `On -> 1 | `Off -> 0 | `Number n -> n;;

val f : [< `Number of int | `Off | `On ] -> int = <fun>

# List.map f [`On; `Off];;

- : int list = [1; 0]
[>`Off|`On] list means that to match this list, you should at least be able to match `Off and
`On, without argument. [<`On|`Off|`Number of int] means that f may be applied to `Off, `On
(both without argument), or `Number n where n is an integer. The > and < inside the variant types
show that they may still be refined, either by defining more tags or by allowing less. As such, they

83
84

contain an implicit type variable. Because each of the variant types appears only once in the whole
type, their implicit type variables are not shown.
The above variant types were polymorphic, allowing further refinement. When writing type
annotations, one will most often describe fixed variant types, that is types that cannot be refined.
This is also the case for type abbreviations. Such types do not contain < or >, but just an enumeration
of the tags and their associated types, just like in a normal datatype definition.
# type 'a vlist = [`Nil | `Cons of 'a * 'a vlist];;
type 'a vlist = [ `Cons of 'a * 'a vlist | `Nil ]

# let rec map f : 'a vlist -> 'b vlist = function

| `Nil -> `Nil
| `Cons(a, l) -> `Cons(f a, map f l)
;;
val map : ('a -> 'b) -> 'a vlist -> 'b vlist = <fun>

5.2 Advanced use

Type-checking polymorphic variants is a subtle thing, and some expressions may result in more
complex type information.
# let f = function `A -> `C | `B -> `D | x -> x;;
val f : ([> `A | `B | `C | `D ] as 'a) -> 'a = <fun>

# f `E;;
- : [> `A | `B | `C | `D | `E ] = `E
Here we are seeing two phenomena. First, since this matching is open (the last case catches any
tag), we obtain the type [> `A | `B] rather than [< `A | `B] in a closed matching. Then, since
x is returned as is, input and return types are identical. The notation as 'a denotes such type
sharing. If we apply f to yet another tag `E, it gets added to the list.
# let f1 = function `A x -> x = 1 | `B -> true | `C -> false
let f2 = function `A x -> x = "a" | `B -> true ;;
val f1 : [< `A of int | `B | `C ] -> bool = <fun>
val f2 : [< `A of string | `B ] -> bool = <fun>

# let f x = f1 x && f2 x;;

val f : [< `A of string & int | `B ] -> bool = <fun>
Here f1 and f2 both accept the variant tags `A and `B, but the argument of `A is int for f1 and
string for f2. In f’s type `C, only accepted by f1, disappears, but both argument types appear
for `A as int & string. This means that if we pass the variant tag `A to f, its argument should
be both int and string. Since there is no such value, f cannot be applied to `A, and `B is the only
accepted input.
Even if a value has a fixed variant type, one can still give it a larger type through coercions.
Coercions are normally written with both the source type and the destination type, but in simple
cases the source type may be omitted.
Chapter 5. Polymorphic variants 85

# type 'a wlist = [`Nil | `Cons of 'a * 'a wlist | `Snoc of 'a wlist * 'a];;
type 'a wlist = [ `Cons of 'a * 'a wlist | `Nil | `Snoc of 'a wlist * 'a ]

# let wlist_of_vlist l = (l : 'a vlist :> 'a wlist);;

val wlist_of_vlist : 'a vlist -> 'a wlist = <fun>

# let open_vlist l = (l : 'a vlist :> [> 'a vlist]);;

val open_vlist : 'a vlist -> [> 'a vlist ] = <fun>

# fun x -> (x :> [`A|`B|`C]);;

- : [< `A | `B | `C ] -> [ `A | `B | `C ] = <fun>
You may also selectively coerce values through pattern matching.
# let split_cases = function
| `Nil | `Cons _ as x -> `A x
| `Snoc _ as x -> `B x
;;
val split_cases :
[< `Cons of 'a | `Nil | `Snoc of 'b ] ->
[> `A of [> `Cons of 'a | `Nil ] | `B of [> `Snoc of 'b ] ] = <fun>
When an or-pattern composed of variant tags is wrapped inside an alias-pattern, the alias is given a
type containing only the tags enumerated in the or-pattern. This allows for many useful idioms,
like incremental definition of functions.
# let num x = `Num x
let eval1 eval (`Num x) = x
let rec eval x = eval1 eval x ;;
val num : 'a -> [> `Num of 'a ] = <fun>
val eval1 : 'a -> [< `Num of 'b ] -> 'b = <fun>
val eval : [< `Num of 'a ] -> 'a = <fun>

# let plus x y = `Plus(x,y)

let eval2 eval = function
| `Plus(x,y) -> eval x + eval y
| `Num _ as x -> eval1 eval x
let rec eval x = eval2 eval x ;;
val plus : 'a -> 'b -> [> `Plus of 'a * 'b ] = <fun>
val eval2 : ('a -> int) -> [< `Num of int | `Plus of 'a * 'a ] -> int = <fun>
val eval : ([< `Num of int | `Plus of 'a * 'a ] as 'a) -> int = <fun>
To make this even more comfortable, you may use type definitions as abbreviations for or-
patterns. That is, if you have defined type myvariant = [`Tag1 of int | `Tag2 of bool],
then the pattern #myvariant is equivalent to writing (`Tag1(_ : int) | `Tag2(_ : bool)).
Such abbreviations may be used alone,
# let f = function
| #myvariant -> "myvariant"
| `Tag3 -> "Tag3";;
86

val f : [< `Tag1 of int | `Tag2 of bool | `Tag3 ] -> string = <fun>
or combined with with aliases.
# let g1 = function `Tag1 _ -> "Tag1" | `Tag2 _ -> "Tag2";;
val g1 : [< `Tag1 of 'a | `Tag2 of 'b ] -> string = <fun>

# let g = function
| #myvariant as x -> g1 x
| `Tag3 -> "Tag3";;
val g : [< `Tag1 of int | `Tag2 of bool | `Tag3 ] -> string = <fun>

5.3 Weaknesses of polymorphic variants

After seeing the power of polymorphic variants, one may wonder why they were added to core
language variants, rather than replacing them.
The answer is twofold. The first aspect is that while being pretty efficient, the lack of static type
information allows for less optimizations, and makes polymorphic variants slightly heavier than core
language ones. However noticeable differences would only appear on huge data structures.
More important is the fact that polymorphic variants, while being type-safe, result in a weaker
type discipline. That is, core language variants do actually much more than ensuring type-safety, they
also check that you use only declared constructors, that all constructors present in a data-structure
are compatible, and they enforce typing constraints to their parameters.
For this reason, you must be more careful about making types explicit when you use polymorphic
variants. When you write a library, this is easy since you can describe exact types in interfaces, but
for simple programs you are probably better off with core language variants.
Beware also that some idioms make trivial errors very hard to find. For instance, the following
code is probably wrong but the compiler has no way to see it.
# type abc = [`A | `B | `C] ;;
type abc = [ `A | `B | `C ]

# let f = function
| `As -> "A"
| #abc -> "other" ;;
val f : [< `A | `As | `B | `C ] -> string = <fun>

# let f : abc -> string = f ;;

val f : abc -> string = <fun>
You can avoid such risks by annotating the definition itself.
# let f : abc -> string = function
| `As -> "A"
| #abc -> "other" ;;
Error : This pattern matches values of type [? `As ]
but a pattern was expected which matches values of type abc
The second variant type does not allow tag(s) `As
Chapter 6

Polymorphism and its limitations

This chapter covers more advanced questions related to the limitations of polymorphic functions and
types. There are some situations in OCaml where the type inferred by the type checker may be less
generic than expected. Such non-genericity can stem either from interactions between side-effects
and typing or the difficulties of implicit polymorphic recursion and higher-rank polymorphism.
This chapter details each of these situations and, if it is possible, how to recover genericity.

6.1 Weak polymorphism and mutation

6.1.1 Weakly polymorphic types
Maybe the most frequent examples of non-genericity derive from the interactions between polymor-
phic types and mutation. A simple example appears when typing the following expression
# let store = ref None ;;
val store : '_weak1 option ref = {contents = None}
Since the type of None is 'a option and the function ref has type 'b -> 'b ref, a natu-
ral deduction for the type of store would be 'a option ref. However, the inferred type,
'_weak1 option ref, is different. Type variables whose names start with a _weak prefix like
'_weak1 are weakly polymorphic type variables, sometimes shortened to “weak type variables”. A
weak type variable is a placeholder for a single type that is currently unknown. Once the specific
type t behind the placeholder type '_weak1 is known, all occurrences of '_weak1 will be replaced
by t. For instance, we can define another option reference and store an int inside:
# let another_store = ref None ;;
val another_store : '_weak2 option ref = {contents = None}

# another_store := Some 0;
another_store ;;
- : int option ref = {contents = Some 0}
After storing an int inside another_store, the type of another_store has been updated from
'_weak2 option ref to int option ref. This distinction between weakly and generic polymorphic
type variable protects OCaml programs from unsoundness and runtime errors. To understand from

87
88

where unsoundness might come, consider this simple function which swaps a value x with the value
stored inside a store reference, if there is such value:
# let swap store x = match !store with
| None -> store := Some x; x
| Some y -> store := Some x; y;;
val swap : 'a option ref -> 'a -> 'a = <fun>
We can apply this function to our store
# let one = swap store 1
let one_again = swap store 2
let two = swap store 3;;
val one : int = 1
val one_again : int = 1
val two : int = 2
After these three swaps the stored value is 3. Everything is fine up to now. We can then try to
swap 3 with a more interesting value, for instance a function:
# let error = swap store (fun x -> x);;
Error : This expression should not be a function , the expected type is int
At this point, the type checker rightfully complains that it is not possible to swap an integer and a
function, and that an int should always be traded for another int. Furthermore, the type checker
prevents us from manually changing the type of the value stored by store:
# store := Some (fun x -> x);;
Error : This expression should not be a function , the expected type is int
Indeed, looking at the type of store, we see that the weak type '_weak1 has been replaced by the
type int
# store;;
- : int option ref = {contents = Some 3}
Therefore, after placing an int in store, we cannot use it to store any value other than an int.
More generally, weak types protect the program from undue mutation of values with a polymorphic
type.
Moreover, weak types cannot appear in the signature of toplevel modules: types must be known
at compilation time. Otherwise, different compilation units could replace the weak type with
different and incompatible types. For this reason, compiling the following small piece of code

let option_ref = ref None

yields a compilation error

Error: The type of this expression, '_weak1 option ref,

contains type variables that cannot be generalized

To solve this error, it is enough to add an explicit type annotation to specify the type at
declaration time:
Chapter 6. Polymorphism and its limitations 89

let option_ref: int option ref = ref None

This is in any case a good practice for such global mutable variables. Otherwise, they will pick
out the type of first use. If there is a mistake at this point, it can result in confusing type errors
when later, correct uses are flagged as errors.

6.1.2 The value restriction

Identifying the exact context in which polymorphic types should be replaced by weak types in
a modular way is a difficult question. Indeed the type system must handle the possibility that
functions may hide persistent mutable states. For instance, the following function uses an internal
reference to implement a delayed identity function
# let make_fake_id () =
let store = ref None in
fun x -> swap store x ;;
val make_fake_id : unit -> 'a -> 'a = <fun>

# let fake_id = make_fake_id();;

val fake_id : '_weak3 -> '_weak3 = <fun>
It would be unsound to apply this fake_id function to values with different types. The function
fake_id is therefore rightfully assigned the type '_weak3 -> '_weak3 rather than 'a -> 'a. At
the same time, it ought to be possible to use a local mutable state without impacting the type of a
function.
To circumvent these dual difficulties, the type checker considers that any value returned by a
function might rely on persistent mutable states behind the scene and should be given a weak type.
This restriction on the type of mutable values and the results of function application is called the
value restriction. Note that this value restriction is conservative: there are situations where the
value restriction is too cautious and gives a weak type to a value that could be safely generalized to
a polymorphic type:
# let not_id = (fun x -> x) (fun x -> x);;
val not_id : '_weak4 -> '_weak4 = <fun>
Quite often, this happens when defining functions using higher order functions. To avoid this
problem, a solution is to add an explicit argument to the function:
# let id_again = fun x -> (fun x -> x) (fun x -> x) x;;
val id_again : 'a -> 'a = <fun>
With this argument, id_again is seen as a function definition by the type checker and can therefore
be generalized. This kind of manipulation is called eta-expansion in lambda calculus and is sometimes
referred under this name.

6.1.3 The relaxed value restriction

There is another partial solution to the problem of unnecessary weak types, which is implemented
directly within the type checker. Briefly, it is possible to prove that weak types that only appear as
type parameters in covariant positions –also called positive positions– can be safely generalized to
polymorphic types. For instance, the type 'a list is covariant in 'a:
90

# let f () = [];;
val f : unit -> 'a list = <fun>

# let empty = f ();;

val empty : 'a list = []
Note that the type inferred for empty is 'a list and not the '_weak5 list that should have
occurred with the value restriction.
The value restriction combined with this generalization for covariant type parameters is called
the relaxed value restriction.

6.1.4 Variance and value restriction

Variance describes how type constructors behave with respect to subtyping. Consider for instance a
pair of type x and xy with x a subtype of xy, denoted x :> xy:
# type x = [ `X ];;
type x = [ `X ]

# type xy = [ `X | `Y ];;
type xy = [ `X | `Y ]
As x is a subtype of xy, we can convert a value of type x to a value of type xy:
# let x:x = `X;;
val x : x = `X

# let x' = ( x :> xy);;

val x' : xy = `X
Similarly, if we have a value of type x list, we can convert it to a value of type xy list, since we
could convert each element one by one:
# let l:x list = [`X; `X];;
val l : x list = [`X; `X]

# let l' = ( l :> xy list);;

val l' : xy list = [`X; `X]
In other words, x :> xy implies that x list :> xy list, therefore the type constructor 'a list
is covariant (it preserves subtyping) in its parameter 'a.
Contrarily, if we have a function that can handle values of type xy
# let f: xy -> unit = function
| `X -> ()
| `Y -> ();;
val f : xy -> unit = <fun>
it can also handle values of type x:
# let f' = (f :> x -> unit);;
val f' : x -> unit = <fun>
Chapter 6. Polymorphism and its limitations 91

Note that we can rewrite the type of f and f' as

# type 'a proc = 'a -> unit
let f' = (f: xy proc :> x proc);;
type 'a proc = 'a -> unit
val f' : x proc = <fun>
In this case, we have x :> xy implies xy proc :> x proc. Notice that the second subtyping relation
reverse the order of x and xy: the type constructor 'a proc is contravariant in its parameter 'a.
More generally, the function type constructor 'a -> 'b is covariant in its return type 'b and
contravariant in its argument type 'a.
A type constructor can also be invariant in some of its type parameters, neither covariant nor
contravariant. A typical example is a reference:
# let x: x ref = ref `X;;
val x : x ref = {contents = `X}
If we were able to coerce x to the type xy ref as a variable xy, we could use xy to store the value
`Y inside the reference and then use the x value to read this content as a value of type x, which
would break the type system.
More generally, as soon as a type variable appears in a position describing mutable state it becomes
invariant. As a corollary, covariant variables will never denote mutable locations and can be safely
generalized. For a better description, interested readers can consult the original article by Jacques
Garrigue on http://www.math.nagoya-u.ac.jp/~garrigue/papers/morepoly-long.pdf
Together, the relaxed value restriction and type parameter covariance help to avoid eta-expansion
in many situations.

6.1.5 Abstract data types

Moreover, when the type definitions are exposed, the type checker is able to infer variance information
on its own and one can benefit from the relaxed value restriction even unknowingly. However, this is
not the case anymore when defining new abstract types. As an illustration, we can define a module
type collection as:
# module type COLLECTION = sig
type 'a t
val empty: unit -> 'a t
end

module Implementation = struct

type 'a t = 'a list
let empty ()= []
end;;
module type COLLECTION = sig type 'a t val empty : unit -> 'a t end
module Implementation :
sig type 'a t = 'a list val empty : unit -> 'a list end

# module List2: COLLECTION = Implementation;;

module List2 : COLLECTION
92

In this situation, when coercing the module List2 to the module type COLLECTION, the type
checker forgets that 'a List2.t was covariant in 'a. Consequently, the relaxed value restriction
does not apply anymore:
# List2.empty ();;
- : '_weak5 List2.t = <abstr>
To keep the relaxed value restriction, we need to declare the abstract type 'a COLLECTION.t as
covariant in 'a:
# module type COLLECTION = sig
type +'a t
val empty: unit -> 'a t
end

module List2: COLLECTION = Implementation;;

module type COLLECTION = sig type +'a t val empty : unit -> 'a t end
module List2 : COLLECTION
We then recover polymorphism:
# List2.empty ();;
- : 'a List2.t = <abstr>

6.2 Polymorphic recursion

The second major class of non-genericity is directly related to the problem of type inference for
polymorphic functions. In some circumstances, the type inferred by OCaml might be not general
enough to allow the definition of some recursive functions, in particular for recursive functions acting
on non-regular algebraic data types.
With a regular polymorphic algebraic data type, the type parameters of the type constructor
are constant within the definition of the type. For instance, we can look at arbitrarily nested list
defined as:
# type 'a regular_nested = List of 'a list | Nested of 'a regular_nested list;;
type 'a regular_nested = List of 'a list | Nested of 'a regular_nested list

# let l = Nested[ List [1]; Nested [List[2;3]]; Nested[Nested[]] ];;

val l : int regular_nested =
Nested [List [1]; Nested [List [2; 3]]; Nested [Nested []]]
Note that the type constructor regular_nested always appears as 'a regular_nested in the
definition above, with the same parameter 'a. Equipped with this type, one can compute a depth
with a classic recursive function
# let rec regular_depth = function
| List _ -> 1
| Nested n -> 1 + List.fold_left max 1 (List.map regular_depth n);;
val regular_depth : 'a regular_nested -> int = <fun>
Chapter 6. Polymorphism and its limitations 93

Non-regular recursive algebraic data types correspond to polymorphic algebraic data types whose
parameter types vary between the left and right side of the type definition. For instance, it might
be interesting to define a datatype that ensures that all lists are nested at the same depth:
# type 'a nested = List of 'a list | Nested of 'a list nested;;
type 'a nested = List of 'a list | Nested of 'a list nested
Intuitively, a value of type 'a nested is a list of list …of list of elements a with k nested list. We
can then adapt the regular_depth function defined on regular_nested into a depth function that
computes this k. As a first try, we may define
# let rec depth = function
| List _ -> 1
| Nested n -> 1 + depth n;;
Error : The value n has type 'a list nested
but an expression was expected of type 'a nested
The type variable 'a occurs inside 'a list
The type error here comes from the fact that during the definition of depth, the type checker
first assigns to depth the type 'a -> 'b . When typing the pattern matching, 'a -> 'b becomes
'a nested -> 'b, then 'a nested -> int once the List branch is typed. However, when typing
the application depth n in the Nested branch, the type checker encounters a problem: depth n is
applied to 'a list nested, it must therefore have the type 'a list nested -> 'b. Unifying this
constraint with the previous one leads to the impossible constraint 'a list nested = 'a nested.
In other words, within its definition, the recursive function depth is applied to values of type 'a t
with different types 'a due to the non-regularity of the type constructor nested. This creates a
problem because the type checker had introduced a new type variable 'a only at the definition of
the function depth whereas, here, we need a different type variable for every application of the
function depth.

6.2.1 Explicitly polymorphic annotations

The solution of this conundrum is to use an explicitly polymorphic type annotation for the type
'a:
# let rec depth: 'a. 'a nested -> int = function
| List _ -> 1
| Nested n -> 1 + depth n;;
val depth : 'a nested -> int = <fun>

# depth ( Nested(List [ [7]; [8] ]) );;

- : int = 2
In the type of depth, 'a.'a nested -> int, the type variable 'a is universally quantified. In other
words, 'a.'a nested -> int reads as “for all type 'a, depth maps 'a nested values to integers”.
Whereas the standard type 'a nested -> int can be interpreted as “let be a type variable 'a,
then depth maps 'a nested values to integers”. There are two major differences with these two
type expressions. First, the explicit polymorphic annotation indicates to the type checker that it
94

needs to introduce a new type variable every time the function depth is applied. This solves our
problem with the definition of the function depth.
Second, it also notifies the type checker that the type of the function should be polymorphic.
Indeed, without explicit polymorphic type annotation, the following type annotation is perfectly
valid
# let sum: 'a -> 'b -> 'c = fun x y -> x + y;;
val sum : int -> int -> int = <fun>
since 'a,'b and 'c denote type variables that may or may not be polymorphic. Whereas, it is an
error to unify an explicitly polymorphic type with a non-polymorphic type:
# let sum: 'a 'b 'c. 'a -> 'b -> 'c = fun x y -> x + y;;
Error : This definition has type int -> int -> int which is less general than
'a 'b 'c. 'a -> 'b -> 'c
An important remark here is that it is not needed to explicit fully the type of depth: it is
sufficient to add annotations only for the universally quantified type variables:
# let rec depth: 'a. 'a nested -> _ = function
| List _ -> 1
| Nested n -> 1 + depth n;;
val depth : 'a nested -> int = <fun>

# depth ( Nested(List [ [7]; [8] ]) );;

- : int = 2

6.2.2 More examples

With explicit polymorphic annotations, it becomes possible to implement any recursive function
that depends only on the structure of the nested lists and not on the type of the elements. For
instance, a more complex example would be to compute the total number of elements of the nested
lists:
# let len nested =
let map_and_sum f = List.fold_left (fun acc x -> acc + f x) 0 in
let rec len: 'a. ('a list -> int ) -> 'a nested -> int =
fun nested_len n ->
match n with
| List l -> nested_len l
| Nested n -> len (map_and_sum nested_len) n
in
len List.length nested;;
val len : 'a nested -> int = <fun>

# len (Nested(Nested(List [ [ [1;2]; [3] ]; [ []; [4]; [5;6;7]]; [[]] ])));;

- : int = 7
Similarly, it may be necessary to use more than one explicitly polymorphic type variables, like
for computing the nested list of list lengths of the nested list:
Chapter 6. Polymorphism and its limitations 95

# let shape n =
let rec shape: 'a 'b. ('a nested -> int nested) ->
('b list list -> 'a list) -> 'b nested -> int nested
= fun nest nested_shape ->
function
| List l -> raise
(Invalid_argument "shape requires nested_list of depth greater than 1")
| Nested (List l) -> nest @@ List (nested_shape l)
| Nested n ->
let nested_shape = List.map nested_shape in
let nest x = nest (Nested x) in
shape nest nested_shape n in
shape (fun n -> n ) (fun l -> List.map List.length l ) n;;
val shape : 'a nested -> int nested = <fun>

# shape (Nested(Nested(List [ [ [1;2]; [3] ]; [ []; [4]; [5;6;7]]; [[]] ])));;

- : int nested = Nested (List [[2; 1]; [0; 1; 3]; [0]])

6.3 Higher-rank polymorphic functions

Explicit polymorphic annotations are however not sufficient to cover all the cases where the inferred
type of a function is less general than expected. A similar problem arises when using polymorphic
functions as arguments of higher-order functions. For instance, we may want to compute the average
depth or length of two nested lists:
# let average_depth x y = (depth x + depth y) / 2;;
val average_depth : 'a nested -> 'b nested -> int = <fun>

# let average_len x y = (len x + len y) / 2;;

val average_len : 'a nested -> 'b nested -> int = <fun>

# let one = average_len (List [2]) (List [[]]);;

val one : int = 1
It would be natural to factorize these two definitions as:
# let average f x y = (f x + f y) / 2;;
val average : ('a -> int) -> 'a -> 'a -> int = <fun>
However, the type of average len is less generic than the type of average_len, since it requires
the type of the first and second argument to be the same:
# average_len (List [2]) (List [[]]);;
- : int = 1

# average len (List [2]) (List [[]]);;

Error : The constructor [] has type 'a list
but an expression was expected of type int
96

As previously with polymorphic recursion, the problem stems from the fact that type variables
are introduced only at the start of the let definitions. When we compute both f x and f y, the
type of x and y are unified together. To avoid this unification, we need to indicate to the type
checker that f is polymorphic in its first argument. In some sense, we would want average to have
type

val average: ('a. 'a nested -> int) -> 'a nested -> 'b nested -> int

Note that this syntax is not valid within OCaml: average has an universally quantified type 'a
inside the type of one of its argument whereas for polymorphic recursion the universally quantified
type was introduced before the rest of the type. This position of the universally quantified type
means that average is a second-rank polymorphic function. This kind of higher-rank functions is
not directly supported by OCaml: type inference for second-rank polymorphic function and beyond
is undecidable; therefore using this kind of higher-rank functions requires to handle manually these
universally quantified types.
In OCaml, there are two ways to introduce this kind of explicit universally quantified types:
universally quantified record fields,
# type 'a nested_reduction = { f:'elt. 'elt nested -> 'a };;
type 'a nested_reduction = { f : 'elt. 'elt nested -> 'a; }

# let boxed_len = { f = len };;

val boxed_len : int nested_reduction = {f = <fun>}
and universally quantified object methods:
# let obj_len = object method f:'a. 'a nested -> 'b = len end;;
val obj_len : < f : 'a. 'a nested -> int > = <obj>
To solve our problem, we can therefore use either the record solution:
# let average nsm x y = (nsm.f x + nsm.f y) / 2 ;;
val average : int nested_reduction -> 'a nested -> 'b nested -> int = <fun>
or the object one:
# let average (obj:<f:'a. 'a nested -> _ > ) x y = (obj#f x + obj#f y) / 2 ;;
val average : < f : 'a. 'a nested -> int > -> 'b nested -> 'c nested -> int =
<fun>
Chapter 7

Generalized algebraic datatypes

Generalized algebraic datatypes, or GADTs, extend usual sum types in two ways: constraints on
type parameters may change depending on the value constructor, and some type variables may be
existentially quantified. Adding constraints is done by giving an explicit return type, where type
parameters are instantiated:
type _ term =
| Int : int -> int term
| Add : (int -> int -> int) term
| App : ('b -> 'a) term * 'b term -> 'a term
This return type must use the same type constructor as the type being defined, and have the
same number of parameters. Variables are made existential when they appear inside a constructor’s
argument, but not in its return type. Since the use of a return type often eliminates the need
to name type parameters in the left-hand side of a type definition, one can replace them with
anonymous types _ in that case.
The constraints associated to each constructor can be recovered through pattern-matching.
Namely, if the type of the scrutinee of a pattern-matching contains a locally abstract type, this type
can be refined according to the constructor used. These extra constraints are only valid inside the
corresponding branch of the pattern-matching. If a constructor has some existential variables, fresh
locally abstract types are generated, and they must not escape the scope of this branch.

7.1 Recursive functions

We write an eval function:
let rec eval : type a. a term -> a = function
| Int n -> n (∗ a = int ∗)
| Add -> (fun x y -> x+y) (∗ a = int −> int −> int ∗)
| App(f,x) -> (eval f) (eval x)
(∗ eval called at types (b−>a) and b for fresh b ∗)
And use it:
let two = eval (App (App (Add, Int 1), Int 1))
val two : int = 2

97
98

It is important to remark that the function eval is using the polymorphic syntax for locally abstract
types. When defining a recursive function that manipulates a GADT, explicit polymorphic recursion
should generally be used. For instance, the following definition fails with a type error:
let rec eval (type a) : a term -> a = function
| Int n -> n
| Add -> (fun x y -> x+y)
| App(f,x) -> (eval f) (eval x)
Error : The value f has type ($b -> a) term
but an expression was expected of type 'a
The type constructor $b would escape its scope
Hint: $b is an existential type bound by the constructor App.
In absence of an explicit polymorphic annotation, a monomorphic type is inferred for the recursive
function. If a recursive call occurs inside the function definition at a type that involves an existential
GADT type variable, this variable flows to the type of the recursive function, and thus escapes its
scope. In the above example, this happens in the branch App(f,x) when eval is called with f as an
argument. In this branch, the type of f is ($App_'b -> a) term. The prefix $ in $App_'b denotes
an existential type named by the compiler (see 7.5). Since the type of eval is 'a term -> 'a, the
call eval f makes the existential type $App_'b flow to the type variable 'a and escape its scope.
This triggers the above error.

7.2 Type inference

Type inference for GADTs is notoriously hard. This is due to the fact some types may become
ambiguous when escaping from a branch. For instance, in the Int case above, n could have either
type int or a, and they are not equivalent outside of that branch. As a first approximation, type
inference will always work if a pattern-matching is annotated with types containing no free type
variables (both on the scrutinee and the return type). This is the case in the above example, thanks
to the type annotation containing only locally abstract types.
In practice, type inference is a bit more clever than that: type annotations do not need to be
immediately on the pattern-matching, and the types do not have to be always closed. As a result,
it is usually enough to only annotate functions, as in the example above. Type annotations are
propagated in two ways: for the scrutinee, they follow the flow of type inference, in a way similar to
polymorphic methods; for the return type, they follow the structure of the program, they are split
on functions, propagated to all branches of a pattern matching, and go through tuples, records, and
sum types. Moreover, the notion of ambiguity used is stronger: a type is only seen as ambiguous if
it was mixed with incompatible types (equated by constraints), without type annotations between
them. For instance, the following program types correctly.
let rec sum : type a. a term -> _ = fun x ->
let y =
match x with
| Int n -> n
| Add -> 0
| App(f,x) -> sum f + sum x
in y + 1
Chapter 7. Generalized algebraic datatypes 99

val sum : 'a term -> int = <fun>

Here the return type int is never mixed with a, so it is seen as non-ambiguous, and can be inferred.
When using such partial type annotations we strongly suggest specifying the -principal mode, to
check that inference is principal.
The exhaustiveness check is aware of GADT constraints, and can automatically infer that some
cases cannot happen. For instance, the following pattern matching is correctly seen as exhaustive
(the Add case cannot happen).
let get_int : int term -> int = function
| Int n -> n
| App(_,_) -> 0

7.3 Refutation cases

Usually, the exhaustiveness check only tries to check whether the cases omitted from the pattern
matching are typable or not. However, you can force it to try harder by adding refutation cases,
written as a full stop. In the presence of a refutation case, the exhaustiveness check will first compute
the intersection of the pattern with the complement of the cases preceding it. It then checks whether
the resulting patterns can really match any concrete values by trying to type-check them. Wild
cards in the generated patterns are handled in a special way: if their type is a variant type with
only GADT constructors, then the pattern is split into the different constructors, in order to check
whether any of them is possible (this splitting is not done for arguments of these constructors, to
avoid non-termination). We also split tuples and variant types with only one case, since they may
contain GADTs inside. For instance, the following code is deemed exhaustive:
type _ t =
| Int : int t
| Bool : bool t

let deep : (char t * int) option -> char = function

| None -> 'c'
| _ -> .
Namely, the inferred remaining case is Some _, which is split into Some (Int, _) and
Some (Bool, _), which are both untypable because deep expects a non-existing char t as the
first element of the tuple. Note that the refutation case could be omitted here, because it is
automatically added when there is only one case in the pattern matching.
Another addition is that the redundancy check is now aware of GADTs: a case will be detected
as redundant if it could be replaced by a refutation case using the same pattern.

7.4 Advanced examples

The term type we have defined above is an indexed type, where a type parameter reflects a property
of the value contents. Another use of GADTs is singleton types, where a GADT value represents
exactly one type. This value can be used as runtime representation for this type, and a function
receiving it can have a polytypic behavior.
100

Here is an example of a polymorphic function that takes the runtime representation of some
type t and a value of the same type, then pretty-prints the value as a string:
type _ typ =
| Int : int typ
| String : string typ
| Pair : 'a typ * 'b typ -> ('a * 'b) typ

let rec to_string: type t. t typ -> t -> string =

fun t x ->
match t with
| Int -> Int.to_string x
| String -> Printf.sprintf "%S" x
| Pair(t1,t2) ->
let (x1, x2) = x in
Printf.sprintf "(%s,%s)" (to_string t1 x1) (to_string t2 x2)
Another frequent application of GADTs is equality witnesses.
type (_,_) eq = Eq : ('a,'a) eq

let cast : type a b. (a,b) eq -> a -> b = fun Eq x -> x

Here type eq has only one constructor, and by matching on it one adds a local constraint allowing
the conversion between a and b. By building such equality witnesses, one can make equal types
which are syntactically different.
Here is an example using both singleton types and equality witnesses to implement dynamic
types.
let rec eq_type : type a b. a typ -> b typ -> (a,b) eq option =
fun a b ->
match a, b with
| Int, Int -> Some Eq
| String, String -> Some Eq
| Pair(a1,a2), Pair(b1,b2) ->
begin match eq_type a1 b1, eq_type a2 b2 with
| Some Eq, Some Eq -> Some Eq
| _ -> None
end
| _ -> None

type dyn = Dyn : 'a typ * 'a -> dyn

let get_dyn : type a. a typ -> dyn -> a option =

fun a (Dyn(b,x)) ->
match eq_type a b with
| None -> None
| Some Eq -> Some x
Chapter 7. Generalized algebraic datatypes 101

7.5 Existential type names in error messages

The typing of pattern matching in the presence of GADTs can generate many existential types.
When necessary, error messages refer to these existential types using compiler-generated names.
Currently, the compiler generates these names according to the following nomenclature:
• First, types whose name starts with a $ are existentials.
• $a denotes an existential type introduced for the type variable 'a of a GADT constructor:
type any = Any : 'name -> any
let escape (Any x) = x
Error : The value x has type $name but an expression was expected of type 'a
The type constructor $name would escape its scope
Hint: $name is an existential type bound by the constructor Any.

• $'a if the existential variable was unified with the type variable 'a during typing:
type ('arg,'result,'aux) fn =
| Fun: ('a ->'b) -> ('a,'b,unit) fn
| Mem1: ('a ->'b) * 'a * 'b -> ('a, 'b, 'a * 'b) fn
let apply: ('arg,'result, _ ) fn -> 'arg -> 'result = fun f x ->
match f with
| Fun f -> f x
| Mem1 (f,y,fy) -> if x = y then fy else f x
Error : This pattern matches values of type
($'arg , 'result , $'arg * 'result ) fn
but a pattern was expected which matches values of type
($'arg , 'result , unit) fn
The type constructor $'arg would escape its scope

• $n (n a number) is an internally generated existential which could not be named using one of
the previous schemes.
As shown by the last item, the current behavior is imperfect and may be improved in future
versions.

7.6 Explicit naming of existentials

As explained above, pattern-matching on a GADT constructor may introduce existential types.
Syntax has been introduced which allows them to be named explicitly. For instance, the following
code names the type of the argument of f and uses this name.
type _ closure = Closure : ('a -> 'b) * 'a -> 'b closure
let eval = fun (Closure (type a) (f, x : (a -> _) * _)) -> f (x : a)
All existential type variables of the constructor must be introduced by the (type ...) construct and
bound by a type annotation on the outside of the constructor argument.
One can additionally bind existentials that were freshly introduced by the refinement of another
existential type, if they appear in the type of the arguments.
102

type _ ty =
| Int : int ty
| Pair : 'b ty * 'c ty -> ('b * 'c) ty
let rec default : type a. a ty -> a = function
| Int -> 0
| Pair (type b c) (b, c : b ty * c ty) ->
(default b : b), (default c : c)

7.7 Equations on non-local abstract types

GADT pattern-matching may also add type equations to non-local abstract types. The behaviour
is the same as with local abstract types. Reusing the above eq type, one can write:
module M : sig type t val x : t val e : (t,int) eq end = struct
type t = int
let x = 33
let e = Eq
end

let x : int = let Eq = M.e in M.x

Of course, not all abstract types can be refined, as this would contradict the exhaustiveness
check. Namely, builtin types (those defined by the compiler itself, such as int or array), and
abstract types defined by the local module, are non-instantiable, and as such cause a type error
rather than introduce an equation.
Chapter 8

Advanced examples with classes and

modules

(Chapter written by Didier Rémy)

In this chapter, we show some larger examples using objects, classes and modules. We review
many of the object features simultaneously on the example of a bank account. We show how modules
taken from the standard library can be expressed as classes. Lastly, we describe a programming
pattern known as virtual types through the example of window managers.

8.1 Extended example: bank accounts

In this section, we illustrate most aspects of Object and inheritance by refining, debugging, and
specializing the following initial naive definition of a simple bank account. (We reuse the module
Euro defined at the end of chapter 3.)
# let euro = new Euro.c;;
val euro : float -> Euro.c = <fun>

# let zero = euro 0.;;

val zero : Euro.c = <obj>

# let neg x = x#times (-1.);;

val neg : < times : float -> 'a; .. > -> 'a = <fun>

# class account =
object
val mutable balance = zero
method balance = balance
method deposit x = balance <- balance # plus x
method withdraw x =
if x#leq balance then (balance <- balance # plus (neg x); x) else zero
end;;

103
104

class account :
object
val mutable balance : Euro.c
method balance : Euro.c
method deposit : Euro.c -> unit
method withdraw : Euro.c -> Euro.c
end

# let c = new account in c # deposit (euro 100.); c # withdraw (euro 50.);;

- : Euro.c = <obj>
We now refine this definition with a method to compute interest.
# class account_with_interests =
object (self)
inherit account
method private interest = self # deposit (self # balance # times 0.03)
end;;
class account_with_interests :
object
val mutable balance : Euro.c
method balance : Euro.c
method deposit : Euro.c -> unit
method private interest : unit
method withdraw : Euro.c -> Euro.c
end
We make the method interest private, since clearly it should not be called freely from the outside.
Here, it is only made accessible to subclasses that will manage monthly or yearly updates of the
account.
We should soon fix a bug in the current definition: the deposit method can be used for
withdrawing money by depositing negative amounts. We can fix this directly:
# class safe_account =
object
inherit account
method deposit x = if zero#leq x then balance <- balance#plus x
end;;
class safe_account :
object
val mutable balance : Euro.c
method balance : Euro.c
method deposit : Euro.c -> unit
method withdraw : Euro.c -> Euro.c
end
However, the bug might be fixed more safely by the following definition:
# class safe_account =
object
inherit account as unsafe
Chapter 8. Advanced examples with classes and modules 105

method deposit x =
if zero#leq x then unsafe # deposit x
else raise (Invalid_argument "deposit")
end;;
class safe_account :
object
val mutable balance : Euro.c
method balance : Euro.c
method deposit : Euro.c -> unit
method withdraw : Euro.c -> Euro.c
end
In particular, this does not require the knowledge of the implementation of the method deposit.
To keep track of operations, we extend the class with a mutable field history and a private
method trace to add an operation in the log. Then each method to be traced is redefined.
# type 'a operation = Deposit of 'a | Retrieval of 'a;;
type 'a operation = Deposit of 'a | Retrieval of 'a

# class account_with_history =
object (self)
inherit safe_account as super
val mutable history = []
method private trace x = history <- x :: history
method deposit x = self#trace (Deposit x); super#deposit x
method withdraw x = self#trace (Retrieval x); super#withdraw x
method history = List.rev history
end;;
class account_with_history :
object
val mutable balance : Euro.c
val mutable history : Euro.c operation list
method balance : Euro.c
method deposit : Euro.c -> unit
method history : Euro.c operation list
method private trace : Euro.c operation -> unit
method withdraw : Euro.c -> Euro.c
end
One may wish to open an account and simultaneously deposit some initial amount. Although the
initial implementation did not address this requirement, it can be achieved by using an initializer.
# class account_with_deposit x =
object
inherit account_with_history
initializer balance <- x
end;;
class account_with_deposit :
Euro.c ->
object
106

val mutable balance : Euro.c

val mutable history : Euro.c operation list
method balance : Euro.c
method deposit : Euro.c -> unit
method history : Euro.c operation list
method private trace : Euro.c operation -> unit
method withdraw : Euro.c -> Euro.c
end
A better alternative is:
# class account_with_deposit x =
object (self)
inherit account_with_history
initializer self#deposit x
end;;
class account_with_deposit :
Euro.c ->
object
val mutable balance : Euro.c
val mutable history : Euro.c operation list
method balance : Euro.c
method deposit : Euro.c -> unit
method history : Euro.c operation list
method private trace : Euro.c operation -> unit
method withdraw : Euro.c -> Euro.c
end
Indeed, the latter is safer since the call to deposit will automatically benefit from safety checks
and from the trace. Let’s test it:
# let ccp = new account_with_deposit (euro 100.) in
let _balance = ccp#withdraw (euro 50.) in
ccp#history;;
- : Euro.c operation list = [Deposit <obj>; Retrieval <obj>]
Closing an account can be done with the following polymorphic function:
# let close c = c#withdraw c#balance;;
val close : < balance : 'a; withdraw : 'a -> 'b; .. > -> 'b = <fun>
Of course, this applies to all sorts of accounts.
Finally, we gather several versions of the account into a module Account abstracted over some
currency.
# let today () = (01,01,2000) (∗ an approximation ∗)
module Account (M:MONEY) =
struct
type m = M.c
let m = new M.c
let zero = m 0.
Chapter 8. Advanced examples with classes and modules 107

class bank =
object (self)
val mutable balance = zero
method balance = balance
val mutable history = []
method private trace x = history <- x::history
method deposit x =
self#trace (Deposit x);
if zero#leq x then balance <- balance # plus x
else raise (Invalid_argument "deposit")
method withdraw x =
if x#leq balance then
(balance <- balance # plus (neg x); self#trace (Retrieval x); x)
else zero
method history = List.rev history
end

class type client_view =

object
method deposit : m -> unit
method history : m operation list
method withdraw : m -> m
method balance : m
end

class virtual check_client x =

let y = if (m 100.)#leq x then x
else raise (Failure "Insufficient initial deposit") in
object (self)
initializer self#deposit y
method virtual deposit: m -> unit
end

module Client (B : sig class bank : client_view end) =

struct
class account x : client_view =
object
inherit B.bank
inherit check_client x
end

let discount x =
let c = new account x in
if today() < (1998,10,30) then c # deposit (m 100.); c
end
108

end;;
This shows the use of modules to group several class definitions that can in fact be thought of as a
single unit. This unit would be provided by a bank for both internal and external uses. This is
implemented as a functor that abstracts over the currency so that the same code can be used to
provide accounts in different currencies.
The class bank is the real implementation of the bank account (it could have been inlined). This
is the one that will be used for further extensions, refinements, etc. Conversely, the client will only
be given the client view.
# module Euro_account = Account(Euro);;
# module Client = Euro_account.Client (Euro_account);;
# new Client.account (new Euro.c 100.);;
Hence, the clients do not have direct access to the balance, nor the history of their own accounts.
Their only way to change their balance is to deposit or withdraw money. It is important to give
the clients a class and not just the ability to create accounts (such as the promotional discount
account), so that they can personalize their account. For instance, a client may refine the deposit
and withdraw methods so as to do his own financial bookkeeping, automatically. On the other
hand, the function discount is given as such, with no possibility for further personalization.
It is important to provide the client’s view as a functor Client so that client accounts can still
be built after a possible specialization of the bank. The functor Client may remain unchanged and
be passed the new definition to initialize a client’s view of the extended account.
# module Investment_account (M : MONEY) =
struct
type m = M.c
module A = Account(M)

class bank =
object
inherit A.bank as super
method deposit x =
if (new M.c 1000.)#leq x then
print_string "Would you like to invest?";
super#deposit x
end

module Client = A.Client

end;;
The functor Client may also be redefined when some new features of the account can be given to
the client.
# module Internet_account (M : MONEY) =
struct
type m = M.c
module A = Account(M)
Chapter 8. Advanced examples with classes and modules 109

class bank =
object
inherit A.bank
method mail s = print_string s
end

class type client_view =

object
method deposit : m -> unit
method history : m operation list
method withdraw : m -> m
method balance : m
method mail : string -> unit
end

module Client (B : sig class bank : client_view end) =

struct
class account x : client_view =
object
inherit B.bank
inherit A.check_client x
end
end
end;;

8.2 Simple modules as classes

One may wonder whether it is possible to treat primitive types such as integers and strings as
objects. Although this is usually uninteresting for integers or strings, there may be some situations
where this is desirable. The class money above is such an example. We show here how to do it for
strings.

8.2.1 Strings
A naive definition of strings as objects could be:
# class ostring s =
object
method get n = String.get s n
method print = print_string s
method escaped = new ostring (String.escaped s)
end;;
class ostring :
string ->
object
method escaped : ostring
110

method get : int -> char

method print : unit
end
However, the method escaped returns an object of the class ostring, and not an object of the
current class. Hence, if the class is further extended, the method escaped will only return an object
of the parent class.
# class sub_string s =
object
inherit ostring s
method sub start len = new sub_string (String.sub s start len)
end;;
class sub_string :
string ->
object
method escaped : ostring
method get : int -> char
method print : unit
method sub : int -> int -> sub_string
end
As seen in section 3.16, the solution is to use functional update instead. We need to create an
instance variable containing the representation s of the string.
# class better_string s =
object
val repr = s
method get n = String.get repr n
method print = print_string repr
method escaped = {< repr = String.escaped repr >}
method sub start len = {< repr = String.sub s start len >}
end;;
class better_string :
string ->
object ('a)
val repr : string
method escaped : 'a
method get : int -> char
method print : unit
method sub : int -> int -> 'a
end
As shown in the inferred type, the methods escaped and sub now return objects of the same type
as the one of the class.
Another difficulty is the implementation of the method concat. In order to concatenate a string
with another string of the same class, one must be able to access the instance variable externally.
Thus, a method repr returning s must be defined. Here is the correct definition of strings:
# class ostring s =
object (self : 'mytype)
Chapter 8. Advanced examples with classes and modules 111

val repr = s
method repr = repr
method get n = String.get repr n
method print = print_string repr
method escaped = {< repr = String.escaped repr >}
method sub start len = {< repr = String.sub s start len >}
method concat (t : 'mytype) = {< repr = repr ^ t#repr >}
end;;
class ostring :
string ->
object ('a)
val repr : string
method concat : 'a -> 'a
method escaped : 'a
method get : int -> char
method print : unit
method repr : string
method sub : int -> int -> 'a
end
Another constructor of the class string can be defined to return a new string of a given length:
# class cstring n = ostring (String.make n ' ');;
class cstring : int -> ostring
Here, exposing the representation of strings is probably harmless. We do could also hide the
representation of strings as we hid the currency in the class money of section 3.17.

8.2.2 Stacks
There is sometimes an alternative between using modules or classes for parametric data types.
Indeed, there are situations when the two approaches are quite similar. For instance, a stack can be
straightforwardly implemented as a class:
# exception Empty;;
exception Empty

# class ['a] stack =

object
val mutable l = ([] : 'a list)
method push x = l <- x::l
method pop = match l with [] -> raise Empty | a::l' -> l <- l'; a
method clear = l <- []
method length = List.length l
end;;
class ['a] stack :
object
val mutable l : 'a list
method clear : unit
method length : int
112

method pop : 'a

method push : 'a -> unit
end
However, writing a method for iterating over a stack is more problematic. A method fold would
have type ('b -> 'a -> 'b) -> 'b -> 'b. Here 'a is the parameter of the stack. The parameter
'b is not related to the class 'a stack but to the argument that will be passed to the method fold.
A naive approach is to make 'b an extra parameter of class stack:
# class ['a, 'b] stack2 =
object
inherit ['a] stack
method fold f (x : 'b) = List.fold_left f x l
end;;
class ['a, 'b] stack2 :
object
val mutable l : 'a list
method clear : unit
method fold : ('b -> 'a -> 'b) -> 'b -> 'b
method length : int
method pop : 'a
method push : 'a -> unit
end
However, the method fold of a given object can only be applied to functions that all have the same
type:
# let s = new stack2;;
val s : ('_weak1, '_weak2) stack2 = <obj>

# s#fold ( + ) 0;;
- : int = 0

# s;;
- : (int, int) stack2 = <obj>
A better solution is to use polymorphic methods, which were introduced in OCaml version 3.05.
Polymorphic methods makes it possible to treat the type variable 'b in the type of fold as universally
quantified, giving fold the polymorphic type Forall 'b. ('b -> 'a -> 'b) -> 'b -> 'b. An
explicit type declaration on the method fold is required, since the type checker cannot infer the
polymorphic type by itself.
# class ['a] stack3 =
object
inherit ['a] stack
method fold : 'b. ('b -> 'a -> 'b) -> 'b -> 'b
= fun f x -> List.fold_left f x l
end;;
class ['a] stack3 :
object
Chapter 8. Advanced examples with classes and modules 113

val mutable l : 'a list

method clear : unit
method fold : ('b -> 'a -> 'b) -> 'b -> 'b
method length : int
method pop : 'a
method push : 'a -> unit
end

8.2.3 Hashtbl
A simplified version of object-oriented hash tables should have the following class type.
# class type ['a, 'b] hash_table =
object
method find : 'a -> 'b
method add : 'a -> 'b -> unit
end;;
class type ['a, 'b] hash_table =
object method add : 'a -> 'b -> unit method find : 'a -> 'b end
A simple implementation, which is quite reasonable for small hash tables is to use an association
list:
# class ['a, 'b] small_hashtbl : ['a, 'b] hash_table =
object
val mutable table = []
method find key = List.assoc key table
method add key value = table <- (key, value) :: table
end;;
class ['a, 'b] small_hashtbl : ['a, 'b] hash_table
A better implementation, and one that scales up better, is to use a true hash table… whose elements
are small hash tables!
# class ['a, 'b] hashtbl size : ['a, 'b] hash_table =
object (self)
val table = Array.init size (fun i -> new small_hashtbl)
method private hash key =
(Hashtbl.hash key) mod (Array.length table)
method find key = table.(self#hash key) # find key
method add key = table.(self#hash key) # add key
end;;
class ['a, 'b] hashtbl : int -> ['a, 'b] hash_table

8.2.4 Sets
Implementing sets leads to another difficulty. Indeed, the method union needs to be able to access
the internal representation of another object of the same class.
114

This is another instance of friend functions as seen in section 3.17. Indeed, this is the same
mechanism used in the module Set in the absence of objects.
In the object-oriented version of sets, we only need to add an additional method tag to return
the representation of a set. Since sets are parametric in the type of elements, the method tag has a
parametric type 'a tag, concrete within the module definition but abstract in its signature. From
outside, it will then be guaranteed that two objects with a method tag of the same type will share
the same representation.
# module type SET =
sig
type 'a tag
class ['a] c :
object ('b)
method is_empty : bool
method mem : 'a -> bool
method add : 'a -> 'b
method union : 'b -> 'b
method iter : ('a -> unit) -> unit
method tag : 'a tag
end
end;;
# module Set : SET =
struct
let rec merge l1 l2 =
match l1 with
[] -> l2
| h1 :: t1 ->
match l2 with
[] -> l1
| h2 :: t2 ->
if h1 < h2 then h1 :: merge t1 l2
else if h1 > h2 then h2 :: merge l1 t2
else merge t1 l2
type 'a tag = 'a list
class ['a] c =
object (_ : 'b)
val repr = ([] : 'a list)
method is_empty = (repr = [])
method mem x = List.exists (( = ) x) repr
method add x = {< repr = merge [x] repr >}
method union (s : 'b) = {< repr = merge repr s#tag >}
method iter (f : 'a -> unit) = List.iter f repr
method tag = repr
end
end;;
Chapter 8. Advanced examples with classes and modules 115

8.3 The subject/observer pattern

The following example, known as the subject/observer pattern, is often presented in the literature
as a difficult inheritance problem with inter-connected classes. The general pattern amounts to the
definition a pair of two classes that recursively interact with one another.
The class observer has a distinguished method notify that requires two arguments, a subject
and an event to execute an action.
# class virtual ['subject, 'event] observer =
object
method virtual notify : 'subject -> 'event -> unit
end;;
class virtual ['subject, 'event] observer :
object method virtual notify : 'subject -> 'event -> unit end
The class subject remembers a list of observers in an instance variable, and has a distinguished
method notify_observers to broadcast the message notify to all observers with a particular
event e.
# class ['observer, 'event] subject =
object (self)
val mutable observers = ([]:'observer list)
method add_observer obs = observers <- (obs :: observers)
method notify_observers (e : 'event) =
List.iter (fun x -> x#notify self e) observers
end;;
class ['a, 'event] subject :
object ('b)
constraint 'a = < notify : 'b -> 'event -> unit; .. >
val mutable observers : 'a list
method add_observer : 'a -> unit
method notify_observers : 'event -> unit
end
The difficulty usually lies in defining instances of the pattern above by inheritance. This can be
done in a natural and obvious manner in OCaml, as shown on the following example manipulating
windows.
# type event = Raise | Resize | Move;;
type event = Raise | Resize | Move

# let string_of_event = function

Raise -> "Raise" | Resize -> "Resize" | Move -> "Move";;
val string_of_event : event -> string = <fun>

# let count = ref 0;;

val count : int ref = {contents = 0}

# class ['observer] window_subject =

let id = count := succ !count; !count in
116

object (self)
inherit ['observer, event] subject
val mutable position = 0
method identity = id
method move x = position <- position + x; self#notify_observers Move
method draw = Printf.printf "{Position = %d}\n" position;
end;;
class ['a] window_subject :
object ('b)
constraint 'a = < notify : 'b -> event -> unit; .. >
val mutable observers : 'a list
val mutable position : int
method add_observer : 'a -> unit
method draw : unit
method identity : int
method move : int -> unit
method notify_observers : event -> unit
end

# class ['subject] window_observer =

object
inherit ['subject, event] observer
method notify s e = s#draw
end;;
class ['a] window_observer :
object
constraint 'a = < draw : unit; .. >
method notify : 'a -> event -> unit
end
As can be expected, the type of window is recursive.
# let window = new window_subject;;
val window :
(< notify : 'a -> event -> unit; .. > as '_weak3) window_subject as 'a =
<obj>
However, the two classes of window_subject and window_observer are not mutually recursive.
# let window_observer = new window_observer;;
val window_observer : (< draw : unit; .. > as '_weak4) window_observer =
<obj>

# window#add_observer window_observer;;
- : unit = ()

# window#move 1;;
{Position = 1}
- : unit = ()
Chapter 8. Advanced examples with classes and modules 117

Classes window_observer and window_subject can still be extended by inheritance. For

instance, one may enrich the subject with new behaviors and refine the behavior of the observer.
# class ['observer] richer_window_subject =
object (self)
inherit ['observer] window_subject
val mutable size = 1
method resize x = size <- size + x; self#notify_observers Resize
val mutable top = false
method raise = top <- true; self#notify_observers Raise
method draw = Printf.printf "{Position = %d; Size = %d}\n" position size;
end;;
class ['a] richer_window_subject :
object ('b)
constraint 'a = < notify : 'b -> event -> unit; .. >
val mutable observers : 'a list
val mutable position : int
val mutable size : int
val mutable top : bool
method add_observer : 'a -> unit
method draw : unit
method identity : int
method move : int -> unit
method notify_observers : event -> unit
method raise : unit
method resize : int -> unit
end

# class ['subject] richer_window_observer =

object
inherit ['subject] window_observer as super
method notify s e = if e <> Raise then s#raise; super#notify s e
end;;
class ['a] richer_window_observer :
object
constraint 'a = < draw : unit; raise : unit; .. >
method notify : 'a -> event -> unit
end
We can also create a different kind of observer:
# class ['subject] trace_observer =
object
inherit ['subject, event] observer
method notify s e =
Printf.printf
"<Window %d <== %s>\n" s#identity (string_of_event e)
end;;
class ['a] trace_observer :
118

object
constraint 'a = < identity : int; .. >
method notify : 'a -> event -> unit
end
and attach several observers to the same object:
# let window = new richer_window_subject;;
val window :
(< notify : 'a -> event -> unit; .. > as '_weak5) richer_window_subject
as 'a = <obj>

# window#add_observer (new richer_window_observer);;

- : unit = ()

# window#add_observer (new trace_observer);;

- : unit = ()

# window#move 1; window#resize 2;;

<Window 1 <== Move>
<Window 1 <== Raise>
{Position = 1; Size = 1}
{Position = 1; Size = 1}
<Window 1 <== Resize>
<Window 1 <== Raise>
{Position = 1; Size = 3}
{Position = 1; Size = 3}
- : unit = ()
Chapter 9

Parallel programming

In this chapter we look at the parallel programming facilities in OCaml. The OCaml standard
library exposes low-level primitives for parallel programming. We recommend that users make
use of higher-level parallel programming libraries such as domainslib. This tutorial will first cover
high-level parallel programming using domainslib followed by low-level primitives exposed by the
compiler.
OCaml distinguishes between concurrency and parallelism and provides distinct mechanisms for
expressing them. Concurrency is interleaved execution of tasks (section 12.24.2) whereas parallelism
is simultaneous execution of tasks. In particular, parallel tasks overlap in time but concurrent tasks
may or may not overlap in time. Tasks may execute concurrently by yielding control to each other.
While concurrency is a program structuring mechanism, parallelism is a mechanism to make your
programs run faster. If you are interested in the concurrent programming mechanisms in OCaml,
please refer to section 12.24 on effect handlers and chapter 34 on the threads library.

9.1 Domains
Domains are the units of parallelism in OCaml. The module Domain[29.14] provides the primitives
to create and manage domains. New domains can be spawned using the spawn function.
Domain.spawn (fun _ -> print_endline "I ran in parallel")
I ran in parallel
- : unit Domain.t = <abstr>
The spawn function executes the given computation in parallel with the calling domain.
Domains are heavy-weight entities. Each domain maps 1:1 to an operating system thread. Each
domain also has its own runtime state, which includes domain-local structures for allocating memory.
Hence, they are relatively expensive to create and tear down.
It is recommended that programs do not spawn more domains than the number of
available cores.
In this tutorial we will be implementing, running and measuring the performance of parallel
programs. The results observed are dependent on the number of cores available on the target
machine. This tutorial was written on a 2.3 GHz Quad-Core Intel Core i7 MacBook Pro with 4
cores and 8 hardware threads. It is reasonable to expect roughly 4x performance on 4 domains
for parallel programs with little coordination between the domains, and when the machine is not

119
120

under load. Beyond 4 domains, the speedup is likely to be less than linear. We will also use the
command-line benchmarking tool hyperfine to benchmark our programs.

9.1.1 Joining domains

We will write a program to compute the nth Fibonacci number using recursion as a running example.
The sequential program for computing the nth Fibonacci number is given below.
(∗ fib.ml ∗)
let n = try int_of_string Sys.argv.(1) with _ -> 1

let rec fib n = if n < 2 then 1 else fib (n - 1) + fib (n - 2)

let main () =
let r = fib n in
Printf.printf "fib(%d) = %d\n%!" n r

let _ = main ()
The program can be compiled and benchmarked as follows:

$ ocamlopt -o fib.exe fib.ml

$ ./fib.exe 42
fib(42) = 433494437
$ hyperfine './fib.exe 42' # Benchmarking
Benchmark 1: ./fib.exe 42
Time (mean ± sd): 1.193 s ± 0.006 s [User: 1.186 s, System: 0.003 s]
Range (min … max): 1.181 s … 1.202 s 10 runs

We see that it takes around 1.2 seconds to compute the 42nd Fibonacci number.
Spawned domains can be joined using the join function to get their results. The join function
waits for target domain to terminate. The following program computes the nth Fibonacci number
twice in parallel.
(∗ fib_twice.ml ∗)
let n = int_of_string Sys.argv.(1)

let rec fib n = if n < 2 then 1 else fib (n - 1) + fib (n - 2)

let main () =
let d1 = Domain.spawn (fun _ -> fib n) in
let d2 = Domain.spawn (fun _ -> fib n) in
let r1 = Domain.join d1 in
Printf.printf "fib(%d) = %d\n%!" n r1;
let r2 = Domain.join d2 in
Printf.printf "fib(%d) = %d\n%!" n r2

let _ = main ()
Chapter 9. Parallel programming 121

The program spawns two domains which compute the nth Fibonacci number. The spawn
function returns a Domain.t value which can be joined to get the result of the parallel computation.
The join function blocks until the computation runs to completion.
$ ocamlopt -o fib_twice.exe fib_twice.ml
$ ./fib_twice.exe 42
fib(42) = 433494437
fib(42) = 433494437
$ hyperfine './fib_twice.exe 42'
Benchmark 1: ./fib_twice.exe 42
Time (mean ± sd): 1.249 s ± 0.025 s [User: 2.451 s, System: 0.012 s]
Range (min … max): 1.221 s … 1.290 s 10 runs
As one can see, computing the nth Fibonacci number twice takes almost the same time as
computing it once, thanks to parallelism.

9.2 Domainslib: A library for nested-parallel programming

Let us attempt to parallelise the Fibonacci function. The two recursive calls may be executed in
parallel. However, naively parallelising the recursive calls by spawning domains for each one will
not work as it spawns too many domains.
(∗ fib_par1.ml ∗)
let n = try int_of_string Sys.argv.(1) with _ -> 1

let rec fib n =

if n < 2 then 1 else begin
let d1 = Domain.spawn (fun _ -> fib (n - 1)) in
let d2 = Domain.spawn (fun _ -> fib (n - 2)) in
Domain.join d1 + Domain.join d2
end

let main () =
let r = fib n in
Printf.printf "fib(%d) = %d\n%!" n r

let _ = main ()
fib(1) = 1
val n : int = 1
val fib : int -> int = <fun>
val main : unit -> unit = <fun>

$ ocamlopt -o fib_par1.exe fib_par1.ml

$ ./fib_par1.exe 42
Fatal error: exception Failure("failed to allocate domain")
OCaml has a limit of 128 domains that can be active at the same time. An attempt to spawn
more domains will raise an exception. How then can we parallelise the Fibonacci function?
122

9.2.1 Parallelising Fibonacci using domainslib

The OCaml standard library provides only low-level primitives for concurrent and parallel pro-
gramming, leaving high-level programming libraries to be developed and distributed outside the
core compiler distribution. Domainslib is such a library for nested-parallel programming, which
is epitomised by the parallelism available in the recursive Fibonacci computation. Let us use
domainslib to parallelise the recursive Fibonacci program. It is recommended that you install
domainslib using the opam package manager. This tutorial uses domainslib version 0.5.0.
Domainslib provides an async/await mechanism for spawning parallel tasks and awaiting their
results. On top of this mechanism, domainslib provides parallel iterators. At its core, domainslib
has an efficient implementation of work-stealing queues in order to efficiently share tasks with other
domains. A parallel implementation of the Fibonacci program follows:

(* fib_par2.ml *)
let num_domains = int_of_string Sys.argv.(1)
let n = int_of_string Sys.argv.(2)

let rec fib n = if n < 2 then 1 else fib (n - 1) + fib (n - 2)

module T = Domainslib.Task

let rec fib_par pool n =

if n > 20 then begin
let a = T.async pool (fun _ -> fib_par pool (n-1)) in
let b = T.async pool (fun _ -> fib_par pool (n-2)) in
T.await pool a + T.await pool b
end else fib n

let main () =
let pool = T.setup_pool ~num_domains:(num_domains - 1) () in
let res = T.run pool (fun _ -> fib_par pool n) in
T.teardown_pool pool;
Printf.printf "fib(%d) = %d\n" n res

let _ = main ()

The program takes the number of domains and the input to the Fibonacci function as the first
and the second command-line arguments respectively.
Let us start with the main function. First, we set up a pool of domains on which the nested
parallel tasks will run. The domain invoking the run function will also participate in executing the
tasks submitted to the pool. We invoke the parallel Fibonacci function fib_par in the run function.
Finally, we tear down the pool and print the result.
For sufficiently large inputs (n > 20), the fib_par function spawns the left and the right
recursive calls asynchronously in the pool using the async function. The async function returns
a promise for the result. The result of an asynchronous computation is obtained by awaiting the
promise using the await function. The await function call blocks until the promise is resolved.
Chapter 9. Parallel programming 123

For small inputs, the fib_par function simply calls the sequential Fibonacci function fib. It is
important to switch to sequential mode for small problem sizes. If not, the cost of parallelisation
will outweigh the work available.
For simplicity, we use ocamlfind to compile this program. In general, it is recommended that
users use dune to build programs that use libraries installed through opam.

$ ocamlfind ocamlopt -package domainslib -linkpkg -o fib_par2.exe fib_par2.ml

$ ./fib_par2.exe 1 42
fib(42) = 433494437
$ hyperfine './fib.exe 42' './fib_par2.exe 2 42' \
'./fib_par2.exe 4 42' './fib_par2.exe 8 42'
Benchmark 1: ./fib.exe 42
Time (mean ± sd): 1.217 s ± 0.018 s [User: 1.203 s, System: 0.004 s]
Range (min … max): 1.202 s … 1.261 s 10 runs

Benchmark 2: ./fib_par2.exe 2 42
Time (mean ± sd): 628.2 ms ± 2.9 ms [User: 1243.1 ms, System: 4.9 ms]
Range (min … max): 625.7 ms … 634.5 ms 10 runs

Benchmark 3: ./fib_par2.exe 4 42
Time (mean ± sd): 337.6 ms ± 23.4 ms [User: 1321.8 ms, System: 8.4 ms]
Range (min … max): 318.5 ms … 377.6 ms 10 runs

Benchmark 4: ./fib_par2.exe 8 42
Time (mean ± sd): 250.0 ms ± 9.4 ms [User: 1877.1 ms, System: 12.6 ms]
Range (min … max): 242.5 ms … 277.3 ms 11 runs

Summary
'./fib_par2.exe 8 42' ran
1.35 ± 0.11 times faster than './fib_par2.exe 4 42'
2.51 ± 0.10 times faster than './fib_par2.exe 2 42'
4.87 ± 0.20 times faster than './fib.exe 42'

The results show that, with 8 domains, the parallel Fibonacci program runs 4.87 times faster
than the sequential version.

9.2.2 Parallel iteration constructs

Many numerical algorithms use for-loops. The parallel-for primitive provides a straight-forward
way to parallelise such code. Let us take the spectral-norm benchmark from the computer language
benchmarks game and parallelise it. The sequential version of the program is given below.
(∗ spectralnorm.ml ∗)
let n = try int_of_string Sys.argv.(1) with _ -> 32

let eval_A i j = 1. /. float((i+j)*(i+j+1)/2+i+1)

124

let eval_A_times_u u v =
let n = Array.length v - 1 in
for i = 0 to n do
let vi = ref 0. in
for j = 0 to n do vi := !vi +. eval_A i j *. u.(j) done;
v.(i) <- !vi
done

let eval_At_times_u u v =
let n = Array.length v - 1 in
for i = 0 to n do
let vi = ref 0. in
for j = 0 to n do vi := !vi +. eval_A j i *. u.(j) done;
v.(i) <- !vi
done

let eval_AtA_times_u u v =
let w = Array.make (Array.length u) 0.0 in
eval_A_times_u u w; eval_At_times_u w v

let () =
let u = Array.make n 1.0 and v = Array.make n 0.0 in
for _i = 0 to 9 do
eval_AtA_times_u u v; eval_AtA_times_u v u
done;

let vv = ref 0.0 and vBv = ref 0.0 in

for i=0 to n-1 do
vv := !vv +. v.(i) *. v.(i);
vBv := !vBv +. u.(i) *. v.(i)
done;
Printf.printf "%0.9f\n" (sqrt(!vBv /. !vv))
Observe that the program has nested loops in eval_A_times_u and eval_At_times_u. Each
iteration of the outer loop body reads from u but writes to disjoint memory locations in v. Hence,
the iterations of the outer loop are not dependent on each other and can be executed in parallel.
The parallel version of spectral norm is shown below.

(* spectralnorm_par.ml *)
let num_domains = try int_of_string Sys.argv.(1) with _ -> 1
let n = try int_of_string Sys.argv.(2) with _ -> 32

let eval_A i j = 1. /. float((i+j)*(i+j+1)/2+i+1)

module T = Domainslib.Task
Chapter 9. Parallel programming 125

let eval_A_times_u pool u v =

let n = Array.length v - 1 in
T.parallel_for pool ~start:0 ~finish:n ~body:(fun i ->
let vi = ref 0. in
for j = 0 to n do vi := !vi +. eval_A i j *. u.(j) done;
v.(i) <- !vi
)

let eval_At_times_u pool u v =

let n = Array.length v - 1 in
T.parallel_for pool ~start:0 ~finish:n ~body:(fun i ->
let vi = ref 0. in
for j = 0 to n do vi := !vi +. eval_A j i *. u.(j) done;
v.(i) <- !vi
)

let eval_AtA_times_u pool u v =

let w = Array.make (Array.length u) 0.0 in
eval_A_times_u pool u w; eval_At_times_u pool w v

let () =
let pool = T.setup_pool ~num_domains:(num_domains - 1) () in
let u = Array.make n 1.0 and v = Array.make n 0.0 in
T.run pool (fun _ ->
for _i = 0 to 9 do
eval_AtA_times_u pool u v; eval_AtA_times_u pool v u
done);

let vv = ref 0.0 and vBv = ref 0.0 in

for i=0 to n-1 do
vv := !vv +. v.(i) *. v.(i);
vBv := !vBv +. u.(i) *. v.(i)
done;
T.teardown_pool pool;
Printf.printf "%0.9f\n" (sqrt(!vBv /. !vv))

Observe that the parallel_for function is isomorphic to the for-loop in the sequential version.
No other change is required except for the boilerplate code to set up and tear down the pools.

$ ocamlopt -o spectralnorm.exe spectralnorm.ml

$ ocamlfind ocamlopt -package domainslib -linkpkg -o spectralnorm_par.exe \
spectralnorm_par.ml
$ hyperfine './spectralnorm.exe 4096' './spectralnorm_par.exe 2 4096' \
'./spectralnorm_par.exe 4 4096' './spectralnorm_par.exe 8 4096'
Benchmark 1: ./spectralnorm.exe 4096
126

Time (mean ± sd): 1.989 s ± 0.013 s [User: 1.972 s, System: 0.007 s]

Range (min … max): 1.975 s … 2.018 s 10 runs

Benchmark 2: ./spectralnorm_par.exe 2 4096

Time (mean ± sd): 1.083 s ± 0.015 s [User: 2.140 s, System: 0.009 s]
Range (min … max): 1.064 s … 1.102 s 10 runs

Benchmark 3: ./spectralnorm_par.exe 4 4096

Time (mean ± sd): 698.7 ms ± 10.3 ms [User: 2730.8 ms, System: 18.3 ms]
Range (min … max): 680.9 ms … 721.7 ms 10 runs

Benchmark 4: ./spectralnorm_par.exe 8 4096

Time (mean ± sd): 921.8 ms ± 52.1 ms [User: 6711.6 ms, System: 51.0 ms]
Range (min … max): 838.6 ms … 989.2 ms 10 runs

Summary
'./spectralnorm_par.exe 4 4096' ran
1.32 ± 0.08 times faster than './spectralnorm_par.exe 8 4096'
1.55 ± 0.03 times faster than './spectralnorm_par.exe 2 4096'
2.85 ± 0.05 times faster than './spectralnorm.exe 4096'

On the author’s machine, the program scales reasonably well up to 4 domains but performs
worse with 8 domains. Recall that the machine only has 4 physical cores. Debugging and fixing this
performance issue is beyond the scope of this tutorial.

9.3 Parallel garbage collection

An important aspect of the scalability of parallel OCaml programs is the scalability of the garbage
collector (GC). The OCaml GC is designed to have both low latency and good parallel scalability.
OCaml has a generational garbage collector with a small minor heap and a large major heap. New
objects (upto a certain size) are allocated in the minor heap. Each domain has its own domain-local
minor heap arena into which new objects are allocated without synchronising with the other domains.
When a domain exhausts its minor heap arena, it calls for a stop-the-world collection of the minor
heaps. In the stop-the-world section, all the domains collect their minor heap arenas in parallel
evacuating the survivors to the major heap.
For the major heap, each domain maintains domain-local, size-segmented pools of memory into
which large objects and survivors from the minor collection are allocated. Having domain-local pools
avoids synchronisation for most major heap allocations. The major heap is collected by a concurrent
mark-and-sweep algorithm that involves a few short stop-the-world pauses for each major cycle.
Overall, users should expect the garbage collector to scale well with as the number of domains in-
creases, with latency remaining low. For more information on the design and evaluation of the garbage
collector, please have a look at the ICFP 2020 paper on Retrofitting Parallelism onto OCaml.
Chapter 9. Parallel programming 127

9.4 Memory model: The easy bits

Modern processors and compilers aggressively optimise programs. These optimisations accelerate
without otherwise affecting sequential programs, but cause surprising behaviours to be visible in
parallel programs. To benefit from these optimisations, OCaml adopts a relaxed memory model that
precisely specifies which of these relaxed behaviours programs may observe. While these models are
difficult to program against directly, the OCaml memory model provides recipes that retain the
simplicity of sequential reasoning.
Firstly, immutable values may be freely shared between multiple domains and may be accessed
in parallel. For mutable data structures such as reference cells, arrays and mutable record fields,
programmers should avoid data races. Reference cells, arrays and mutable record fields are said to
be non-atomic data structures. A data race is said to occur when two domains concurrently access
a non-atomic memory location without synchronisation and at least one of the accesses is a write.
OCaml provides a number of ways to introduce synchronisation including atomic variables (section
9.7) and mutexes (section 9.5).
Importantly, for data race free (DRF) programs, OCaml provides sequentially con-
sistent (SC) semantics – the observed behaviour of such programs can be explained by the
interleaving of operations from different domains. This property is known as the DRF-SC guarantee.
Moreover, in OCaml, the DRF-SC guarantee is modular – if a part of a program is data race free,
then the OCaml memory model ensures that those parts have sequential consistency despite other
parts of the program having data races. Even for programs with data races, OCaml provides strong
guarantees. While the user may observe non-sequentially consistent behaviours, there are no crashes.
For more details on the relaxed behaviours in the presence of data races, please have a look at
the chapter on the hard bits of the memory model (chapter 10).

9.5 Blocking synchronisation

Domains may perform blocking synchronisation with the help of Mutex[29.37], Condition[29.13]
and Semaphore[29.51] modules. These modules are the same as those used to synchronise threads
created by the threads library (chapter 34). For clarity, in the rest of this chapter, we refer to
the threads created by the threads library as systhreads. The following program implements a
concurrent stack using mutex and condition variables.
module Blocking_stack : sig
type 'a t
val make : unit -> 'a t
val push : 'a t -> 'a -> unit
val pop : 'a t -> 'a
end = struct
type 'a t = {
mutable contents: 'a list;
mutex : Mutex.t;
nonempty : Condition.t
}
128

let make () = {
contents = [];
mutex = Mutex.create ();
nonempty = Condition.create ()
}

let push r v =
Mutex.lock r.mutex;
r.contents <- v::r.contents;
Condition.signal r.nonempty;
Mutex.unlock r.mutex

let pop r =
Mutex.lock r.mutex;
let rec loop () =
match r.contents with
| [] ->
Condition.wait r.nonempty r.mutex;
loop ()
| x::xs -> r.contents <- xs; x
in
let res = loop () in
Mutex.unlock r.mutex;
res
end
The concurrent stack is implemented using a record with three fields: a mutable field contents
which stores the elements in the stack, a mutex to control access to the contents field, and a
condition variable nonempty, which is used to signal blocked domains waiting for the stack to
become non-empty.
The push operation locks the mutex, updates the contents field with a new list whose head is
the element being pushed and the tail is the old list. The condition variable nonempty is signalled
while the lock is held in order to wake up any domains waiting on this condition. If there are waiting
domains, one of the domains is woken up. If there are none, then the signal operation has no
effect.
The pop operation locks the mutex and checks whether the stack is empty. If so, the calling
domain waits on the condition variable nonempty using the wait primitive. The wait call atomically
suspends the execution of the current domain and unlocks the mutex. When this domain is woken
up again (when the wait call returns), it holds the lock on mutex. The domain tries to read the
contents of the stack again. If the pop operation sees that the stack is non-empty, it updates the
contents to the tail of the old list, and returns the head.
The use of mutex to control access to the shared resource contents introduces sufficient
synchronisation between multiple domains using the stack. Hence, there are no data races when
multiple domains use the stack in parallel.
Chapter 9. Parallel programming 129

9.5.1 Interaction with systhreads

How do systhreads interact with domains? The systhreads created on a particular domain remain
pinned to that domain. Only one systhread at a time is allowed to run OCaml code on a particular
domain. However, systhreads belonging to a particular domain may run C library or system code in
parallel. Systhreads belonging to different domains may execute in parallel.
When using systhreads, the thread created for executing the computation given to Domain.spawn
is also treated as a systhread. For example, the following program creates in total two domains
(including the initial domain) with two systhreads each (including the initial systhread for each of
the domains).

(* dom_thr.ml *)
let m = Mutex.create ()
let r = ref None (* protected by m *)

let task () =
let my_thr_id = Thread.(id (self ())) in
let my_dom_id :> int = Domain.self () in
Mutex.lock m;
begin match !r with
| None ->
Printf.printf "Thread %d running on domain %d saw initial write\n%!"
my_thr_id my_dom_id
| Some their_thr_id ->
Printf.printf "Thread %d running on domain %d saw the write by thread %d\n%!"
my_thr_id my_dom_id their_thr_id;
end;
r := Some my_thr_id;
Mutex.unlock m

let task' () =
let t = Thread.create task () in
task ();
Thread.join t

let main () =
let d = Domain.spawn task' in
task' ();
Domain.join d

let _ = main ()

$ ocamlopt -I +threads -I +unix unix.cmxa threads.cmxa -o dom_thr.exe dom_thr.ml

$ ./dom_thr.exe
Thread 1 running on domain 1 saw initial write
Thread 0 running on domain 0 saw the write by thread 1
130

Thread 2 running on domain 1 saw the write by thread 0

Thread 3 running on domain 0 saw the write by thread 2

This program uses a shared reference cell protected by a mutex to communicate between the
different systhreads running on two different domains. The systhread identifiers uniquely identify
systhreads in the program. The initial domain gets the domain id 0 and thread id 0. The newly
spawned domain gets the domain id 1.

9.6 Interaction with C bindings

During parallel execution with multiple domains, C code running on a domain may run in parallel
with any C code running in other domains even if neither of them has released the “domain lock”.
Prior to OCaml 5.0, C bindings may have assumed that if the OCaml runtime lock is not released,
then it would be safe to manipulate global C state (e.g. initialise a function-local static value). This
is no longer true in the presence of parallel execution with multiple domains.

9.7 Atomics
Mutexes, condition variables and semaphores are used to implement blocking synchronisation
between domains. For non-blocking synchronisation, OCaml provides Atomic[29.4] variables. As
the name suggests, non-blocking synchronisation does not provide mechanisms for suspending and
waking up domains. On the other hand, primitives used in non-blocking synchronisation are often
compiled to atomic read-modify-write primitives that the hardware provides. As an example, the
following program increments a non-atomic counter and an atomic counter in parallel.
(∗ incr.ml ∗)
let twice_in_parallel f =
let d1 = Domain.spawn f in
let d2 = Domain.spawn f in
Domain.join d1;
Domain.join d2

let plain_ref n =
let r = ref 0 in
let f () = for _i=1 to n do incr r done in
twice_in_parallel f;
Printf.printf "Non-atomic ref count: %d\n" !r

let atomic_ref n =
let r = Atomic.make 0 in
let f () = for _i=1 to n do Atomic.incr r done in
twice_in_parallel f;
Printf.printf "Atomic ref count: %d\n" (Atomic.get r)

let main () =
Chapter 9. Parallel programming 131

let n = try int_of_string Sys.argv.(1) with _ -> 1 in

plain_ref n;
atomic_ref n

let _ = main ()

$ ocamlopt -o incr.exe incr.ml

$ ./incr.exe 1_000_000
Non-atomic ref count: 1187193
Atomic ref count: 2000000

Observe that the result from using the non-atomic counter is lower than what one would naively
expect. This is because the non-atomic incr function is equivalent to:
let incr r =
let curr = !r in
r := curr + 1
Observe that the load and the store are two separate operations, and the increment operation as
a whole is not performed atomically. When two domains execute this code in parallel, both of them
may read the same value of the counter curr and update it to curr + 1. Hence, instead of two
increments, the effect will be that of a single increment. On the other hand, the atomic counter
performs the load and the store atomically with the help of hardware support for atomicity. The
atomic counter returns the expected result.
Atomic variables can be used for low-level synchronisation between domains. The following
example uses an atomic variable to exchange a message between two domains.
let r = Atomic.make None

let sender () = Atomic.set r (Some "Hello")

let rec receiver () =

match Atomic.get r with
| None -> Domain.cpu_relax (); receiver ()
| Some m -> print_endline m

let main () =
let s = Domain.spawn sender in
let d = Domain.spawn receiver in
Domain.join s;
Domain.join d

let _ = main ()
Hello
val r : string option Atomic.t = <abstr>
val sender : unit -> unit = <fun>
val receiver : unit -> unit = <fun>
val main : unit -> unit = <fun>
132

Although the sender and the receiver compete to access r, this is not a data race since r is an
atomic reference.

9.7.1 Lock-free stack

The Atomic module is used to implement non-blocking, lock-free data structures. The following
program implements a lock-free stack.
module Lockfree_stack : sig
type 'a t
val make : unit -> 'a t
val push : 'a t -> 'a -> unit
val pop : 'a t -> 'a option
end = struct
type 'a t = 'a list Atomic.t

let make () = Atomic.make []

let rec push r v =

let s = Atomic.get r in
if Atomic.compare_and_set r s (v::s) then ()
else (Domain.cpu_relax (); push r v)

let rec pop r =

let s = Atomic.get r in
match s with
| [] -> None
| x::xs ->
if Atomic.compare_and_set r s xs then Some x
else (Domain.cpu_relax (); pop r)
end
The atomic stack is represented by an atomic reference that holds a list. The push and pop
operations use the compare_and_set primitive to attempt to atomically update the atomic reference.
The expression compare_and_set r seen v sets the value of r to v if and only if its current value
is physically equal to seen. Importantly, the comparison and the update occur atomically. The
expression evaluates to true if the comparison succeeded (and the update happened) and false
otherwise.
If the compare_and_set fails, then some other domain is also attempting to update the atomic
reference at the same time. In this case, the push and pop operations call Domain.cpu_relax to
back off for a short duration allowing competing domains to make progress before retrying the failed
operation. This lock-free stack implementation is called a Treiber stack.
Chapter 10

Memory model: The hard bits

This chapter describes the details of OCaml relaxed memory model. The relaxed memory model
describes what values an OCaml program is allowed to witness when reading a memory location. If
you are interested in high-level parallel programming in OCaml, please have a look at the parallel
programming chapter 9.
This chapter is aimed at experts who would like to understand the details of the OCaml memory
model from a practitioner’s perspective. For a formal definition of the OCaml memory model, its
guarantees and the compilation to hardware memory models, please have a look at the PLDI 2018
paper on Bounding Data Races in Space and Time. The memory model presented in this chapter is
an extension of the one presented in the PLDI 2018 paper. This chapter also covers some pragmatic
aspects of the memory model that are not covered in the paper.

10.1 Why weakly consistent memory?

The simplest memory model that we could give to our programs is sequential consistency. Under
sequential consistency, the values observed by the program can be explained through some interleaving
of the operations from different domains in the program. For example, consider the following program
with two domains d1 and d2 executing in parallel:
let d1 a b =
let r1 = !a * 2 in
let r2 = !b in
let r3 = !a * 2 in
(r1, r2, r3)

let d2 b = b := 0

let main () =
let a = ref 1 in
let b = ref 1 in
let h = Domain.spawn (fun _ ->
let r1, r2, r3 = d1 a b in
Printf.printf "r1 = %d, r2 = %d, r3 = %d\n" r1 r2 r3)

133
134

in
d2 b;
Domain.join h
The reference cells a and b are initially 1. The user may observe r1 = 2, r2 = 0, r3 = 2
if the write to b in d2 occurred before the read of b in d1. Here, the observed behaviour can be
explained in terms of interleaving of the operations from different domains.
Let us now assume that a and b are aliases of each other.
let d1 a b =
let r1 = !a * 2 in
let r2 = !b in
let r3 = !a * 2 in
(r1, r2, r3)

let d2 b = b := 0

let main () =
let ab = ref 1 in
let h = Domain.spawn (fun _ ->
let r1, r2, r3 = d1 ab ab in
assert (not (r1 = 2 && r2 = 0 && r3 = 2)))
in
d2 ab;
Domain.join h
In the above program, the variables ab, a and b refer to the same reference cell. One would
expect that the assertion in the main function will never fail. The reasoning is that if r2 is 0, then
the write in d2 occurred before the read of b in d1. Given that a and b are aliases, the second read
of a in d1 should also return 0.

10.1.1 Compiler optimisations

Surprisingly, this assertion may fail in OCaml due to compiler optimisations. The OCaml compiler
observes the common sub-expression !a * 2 in d1 and optimises the program to:
let d1 a b =
let r1 = !a * 2 in
let r2 = !b in
let r3 = r1 in (∗ CSE: !a ∗ 2 ==> r1 ∗)
(r1, r2, r3)

let d2 b = b := 0

let main () =
let ab = ref 1 in
let h = Domain.spawn (fun _ ->
let r1, r2, r3 = d1 ab ab in
assert (not (r1 = 2 && r2 = 0 && r3 = 2)))
Chapter 10. Memory model: The hard bits 135

in
d2 ab;
Domain.join h
This optimisation is known as the common sub-expression elimination (CSE). Such optimisations
are valid and necessary for good performance, and do not change the sequential meaning of the
program. However, CSE breaks sequential reasoning.
In the optimized program above, even if the write to b in d2 occurs between the first and the
second reads in d1, the program will observe the value 2 for r3, causing the assertion to fail. The
observed behaviour cannot be explained by interleaving of operations from different domains in the
source program. Thus, CSE optimization is said to be invalid under sequential consistency.
One way to explain the observed behaviour is as if the operations performed on a domain were
reordered. For example, if the second and the third reads from d1 were reordered,
let d1 a b =
let r1 = !a * 2 in
let r3 = !a * 2 in
let r2 = !b in
(r1, r2, r3)
then we can explain the observed behaviour (2,0,2) returned by d1.

10.1.2 Hardware optimisations

The other source of reordering is by the hardware. Modern hardware architectures have complex
cache hierarchies with multiple levels of cache. While cache coherence ensures that reads and writes
to a single memory location respect sequential consistency, the guarantees on programs that operate
on different memory locations are much weaker. Consider the following program:
let a = ref 0
and b = ref 0

let d1 () =
a := 1;
!b

let d2 () =
b := 1;
!a

let main () =
let h = Domain.spawn d2 in
let r1 = d1 () in
let r2 = Domain.join h in
assert (not (r1 = 0 && r2 = 0))
Under sequential consistency, we would never expect the assertion to fail. However, even on
x86, which offers much stronger guarantees than ARM, the writes performed at a CPU core are not
immediately published to all of the other cores. Since a and b are different memory locations, the
reads of a and b may both witness the initial values, leading to the assertion failure.
136

This behaviour can be explained if a load is allowed to be reordered before a preceding store to
a different memory location. This reordering can happen due to the presence of in-core store-buffers
on modern processors. Each core effectively has a FIFO buffer of pending writes to avoid the need
to block while a write completes. The writes to a and b may be in the store-buffers of cores c1 and
c2 running the domains d1 and d2, respectively. The reads of b and a running on the cores c1 and
c2, respectively, will not see the writes if the writes have not propagated from the buffers to the
main memory.

10.2 Data race freedom implies sequential consistency

The aim of the OCaml relaxed memory model is to precisely describe which orders are preserved by
the OCaml program. The compiler and the hardware are free to optimize the program as long as
they respect the ordering guarantees of the memory model. While programming directly under the
relaxed memory model is difficult, the memory model also describes the conditions under which a
program will only exhibit sequentially consistent behaviours. This guarantee is known as data race
freedom implies sequential consistency (DRF-SC). In this section, we shall describe this guarantee.
In order to do this, we first need a number of definitions.

10.2.1 Memory locations

OCaml classifies memory locations into atomic and non-atomic locations. Reference cells, array
fields and mutable record fields are non-atomic memory locations. Immutable objects are non-atomic
locations with an initialising write but no further updates. Atomic memory locations are those that
are created using the Atomic[29.4] module.

10.2.2 Happens-before relation

Let us imagine that the OCaml programs are executed by an abstract machine that executes one
action at a time, arbitrarily picking one of the available domains at each step. We classify actions
into two: inter-domain and intra-domain. An inter-domain action is one which can be observed and
be influenced by actions on other domains. There are several inter-domain actions:

• Reads and writes of atomic and non-atomic locations.

• Spawn and join of domains.

• Operations on mutexes.

On the other hand, intra-domain actions can neither be observed nor influence the execution
of other domains. Examples include evaluating an arithmetic expression, calling a function, etc.
The memory model specification ignores such intra-domain actions. In the sequel, we use the term
action to indicate inter-domain actions.
A totally ordered list of actions executed by the abstract machine is called an execution trace.
There might be several possible execution traces for a given program due to non-determinism.
For a given execution trace, we define an irreflexive, transitive happens-before relation that
captures the causality between actions in the OCaml program. The happens-before relation is
defined as the smallest transitive relation satisfying the following properties:
Chapter 10. Memory model: The hard bits 137

• We define the order in which a domain executes its actions as the program order. If an action
x precedes another action y in program order, then x precedes y in happens-before order.
• If x is a write to an atomic location and y is a subsequent read or write to that memory
location in the execution trace, then x precedes y in happens-before order. For atomic locations,
compare_and_set, fetch_and_add, exchange, incr and decr are considered to perform both
a read and a write.
• If x is Domain.spawn f and y is the first action in the newly spawned domain executing f,
then x precedes y in happens-before order.
• If x is the last action in a domain d and y is Domain.join d, then x precedes y in
happens-before order.
• If x is an unlock operation on a mutex, and y is any subsequent operation on the mutex in
the execution trace, then x precedes y in happens-before order.

10.2.3 Data race

In a given trace, two actions are said to be conflicting if they access the same non-atomic location,
at least one is a write and neither is an initialising write to that location.
We say that a program has a data race if there exists some execution trace of the program
with two conflicting actions and there does not exist a happens-before relationship between the
conflicting accesses. A program without data races is said to be correctly synchronised.

10.2.4 DRF-SC
DRF-SC guarantee: A program without data races will only exhibit sequentially consistent
behaviours.
DRF-SC is a strong guarantee for the programmers. Programmers can use sequential reasoning
i.e., reasoning by executing one inter-domain action after the other, to identify whether their
program has a data race. In particular, they do not need to reason about reorderings described in
section 10.1 in order to determine whether their program has a data race. Once the determination
that a particular program is data race free is made, they do not need to worry about reorderings in
their code.

10.3 Reasoning with DRF-SC

In this section, we will look at examples of using DRF-SC for program reasoning. In this section, we
will use the functions with names dN to represent domains executing in parallel with other domains.
That is, we assume that there is a main function that runs the dN functions in parallel as follows:
let main () =
let h1 = Domain.spawn d1 in
let h2 = Domain.spawn d2 in
...
ignore @@ Domain.join h1;
ignore @@ Domain.join h2
138

Here is a simple example with a data race:

(∗ Has data race ∗)
let r = ref 0
let d1 () = r := 1
let d2 () = !r
r is a non-atomic reference. The two domains race to access the reference, and d1 is a write.
Since there is no happens-before relationship between the conflicting accesses, there is a data race.
Both of the programs that we had seen in the section 10.1 have data races. It is no surprise that
they exhibit non sequentially consistent behaviours.
Accessing disjoint array indices and fields of a record in parallel is not a data race. For example,
(∗ No data race ∗)
let a = [| 0; 1 |]
let d1 () = a.(0) <- 42
let d2 () = a.(1) <- 42
(∗ No data race ∗)
type t = {
mutable a : int;
mutable b : int
}
let r = {a = 0; b = 1}
let d1 () = r.a <- 42
let d2 () = r.b <- 42
do not have data races.
Races on atomic locations do not lead to a data race.
(∗ No data race ∗)
let r = Atomic.make 0
let d1 () = Atomic.set r 1
let d2 () = Atomic.get r

10.3.1 Message-passing
Atomic variables may be used for implementing non-blocking communication between domains.
(∗ No data race ∗)
let msg = ref 0
let flag = Atomic.make false
let d1 () =
msg := 42; (∗ a ∗)
Atomic.set flag true (∗ b ∗)
let d2 () =
if Atomic.get flag (∗ c ∗) then
!msg (∗ d ∗)
else 0
Chapter 10. Memory model: The hard bits 139

Observe that the actions a and d write and read from the same non-atomic location msg,
respectively, and hence are conflicting. We need to establish that a and d have a happens-before
relationship in order to show that this program does not have a data race.
The action a precedes b in program order, and hence, a happens-before b. Similarly, c happens-
before d. If d2 observes the atomic variable flag to be true, then b precedes c in happens-before
order. Since happens-before is transitive, the conflicting actions a and d are in happens-before order.
If d2 observes the flag to be false, then the read of msg is not done. Hence, there is no conflicting
access in this execution trace. Hence, the program does not have a data race.
The following modified version of the message passing program does have a data race.
(∗ Has data race ∗)
let msg = ref 0
let flag = Atomic.make false
let d1 () =
msg := 42; (∗ a ∗)
Atomic.set flag true (∗ b ∗)
let d2 () =
ignore (Atomic.get flag); (∗ c ∗)
!msg (∗ d ∗)
The domain d2 now unconditionally reads the non-atomic reference msg. Consider the execution
trace:

Atomic.get flag; (* c *)
!msg; (* d *)
msg := 42; (* a *)
Atomic.set flag true (* b *)

In this trace, d and a are conflicting operations. But there is no happens-before relationship
between them. Hence, this program has a data race.

10.4 Local data race freedom

The OCaml memory model offers strong guarantees even for programs with data races. It offers what
is known as local data race freedom sequential consistency (LDRF-SC) guarantee. A formal definition
of this property is beyond the scope of this manual chapter. Interested readers are encouraged to
read the PLDI 2018 paper on Bounding Data Races in Space and Time.
Informally, LDRF-SC says that the data race free parts of the program remain sequentially
consistent. That is, even if the program has data races, those parts of the program that are disjoint
from the parts with data races are amenable to sequential reasoning.
Consider the following snippet:
let snippet () =
let c = ref 0 in
c := 42;
let a = !c in
(a, c)
140

Observe that c is a newly allocated reference. Can the read of c return a value which is not 42?
That is, can a ever be not 42? Surprisingly, in the C++ and Java memory models, the answer is yes.
With the C++ memory model, if the program has a data race, even in unrelated parts, then the
semantics is undefined. If this snippet were linked with a library that had a data race, then, under
the C++ memory model, the read may return any value. Since data races on unrelated locations
can affect program behaviour, we say that C++ memory model is not bounded in space.
Unlike C++, Java memory model is bounded in space. But Java memory model is not bounded in
time; data races in the future will affect the past behaviour. For example, consider the translation of
this example to Java. We assume a prior definition of Class c {int x;} and a shared non-volatile
variable C g. Now the snippet may be part of a larger program with parallel threads:

(* Thread 1 *)
C c = new C();
c.x = 42;
a = c.x;
g = c;

(* Thread 2 *)
g.x = 7;

The read of c.x and the write of g in the first thread are done on separate memory locations.
Hence, the Java memory model allows them to be reordered. As a result, the write in the second
thread may occur before the read of c.x, and hence, c.x returns 7.
The OCaml equivalent of the Java code above is:
let g = ref None

let snippet () =
let c = ref 0 in
c := 42;
let a = !c in
(a, c)

let d1 () =
let (a,c) = snippet () in
g := Some c;
a

let d2 () =
match !g with
| None -> ()
| Some c -> c := 7
Observe that there is a data race on both g and c. Consider only the first three instructions in
snippet:

let c = ref 0 in
Chapter 10. Memory model: The hard bits 141

c := 42;
let a = !c in
...

The OCaml memory model is bounded both in space and time. The only memory location here
is c. Reasoning only about this snippet, there is neither the data race in space (the race on g) nor
in time (the future race on c). Hence, the snippet will have sequentially consistent behaviour, and
the value returned by !c will be 42.
The OCaml memory model guarantees that even for programs with data races, memory safety
is preserved. While programs with data races may observe non-sequentially consistent behaviours,
they will not crash.

10.5 An operational view of the memory model

In this section, we describe the semantics of the OCaml memory model. A formal definition of
the operational view of the memory model is presented in section 3 of the PLDI 2018 paper on
Bounding Data Races in Space and Time. This section presents an informal description of the
memory model with the help of an example.
Given an OCaml program, which may possibly contain data races, the operational semantics
tells you the values that may be observed by the read of a memory location. For simplicity, we
restrict the intra-thread actions to just the accesses to atomic and non-atomic locations, ignoring
domain spawn and join operations, and the operations on mutexes.
We describe the semantics of the OCaml memory model in a straightforward small-step op-
erational manner. That is, the semantics is described by an abstract machine that executes one
action at a time, arbitrarily picking one of the available domains at each step. This is similar to the
abstract machine that we had used to describe the happens-before relationship in section 10.2.2.

10.5.1 Non-atomic locations

In the semantics, we model non-atomic locations as finite maps from timestamps t to values v. We
take timestamps to be rational numbers. The timestamps are totally ordered but dense; there is a
timestamp between any two others.
For example,

a: [t1 -> 1; t2 -> 2]

b: [t3 -> 3; t4 -> 4; t5 -> 5]
c: [t6 -> 5; t7 -> 6; t8 -> 7]

represents three non-atomic locations a, b and c and their histories. The location a has two writes at
timestamps t1 and t2 with values 1 and 2, respectively. When we write a: [t1 -> 1; t2 -> 2],
we assume that t1 < t2. We assume that the locations are initialised with a history that has a
single entry at timestamp 0 that maps to the initial value.
142

10.5.2 Domains
Each domain is equipped with a frontier, which is a map from non-atomic locations to timestamps.
Intuitively, each domain’s frontier records, for each non-atomic location, the latest write known to
the thread. More recent writes may have occurred, but are not guaranteed to be visible.
For example,

d1: [a -> t1; b -> t3; c -> t7]

d2: [a -> t1; b -> t4; c -> t7]

represents two domains d1 and d2 and their frontiers.

10.5.3 Non-atomic accesses

Let us now define the semantics of non-atomic reads and writes. Suppose domain d1 performs the
read of b. For non-atomic reads, the domains may read an arbitrary element of the history for
that location, as long as it is not older than the timestamp in the domains’s frontier. In this case,
since d1 frontier at b is at t3, the read may return the value 3, 4 or 5. A non-atomic read does not
change the frontier of the current domain.
Suppose domain d2 writes the value 10 to c (c := 10). We pick a new timestamp t9 for this
write such that it is later than d2’s frontier at c. Note a subtlety here: this new timestamp might
not be later than everything else in the history, but merely later than any other write known to
the writing domain. Hence, t9 may be inserted in c’s history either (a) between t7 and t8 or (b)
after t8. Let us pick the former option for our discussion. Since the new write appears after all the
writes known by the domain d2 to the location c, d2’s frontier at c is also updated. The new state
of the abstract machine is:

(* Non-atomic locations *)
a: [t1 -> 1; t2 -> 2]
b: [t3 -> 3; t4 -> 4; t5 -> 5]
c: [t6 -> 5; t7 -> 6; t9 -> 10; t8 -> 7] (* new write at t9 *)

(* Domains *)
d1: [a -> t1; b -> t3; c -> t7]
d2: [a -> t1; b -> t4; c -> t9] (* frontier updated at c *)

10.5.4 Atomic accesses

Atomic locations carry not only values but also synchronization information. We model atomic
locations as a pair of the value held by that location and a frontier. The frontier models the
synchronization information, which is merged with the frontiers of threads that operate on the
location. In this way, non-atomic writes made by one thread can become known to another by
communicating via an atomic location.
For example,

(* Atomic locations *)
A: 10, [a -> t1; b -> t5; c -> t7]
B: 5, [a -> t2; b -> t4; c -> t6]
Chapter 10. Memory model: The hard bits 143

shows two atomic variables A and B with values 10 and 5, respectively, and frontiers of their own.
We use upper-case variable names to indicate atomic locations.
During atomic reads, the frontier of the location is merged into that of the domain performing
the read. For example, suppose d1 reads B. The read returns 5, and d1’s frontier updated by
merging it with B’s frontier, choosing the later timestamp for each location. The abstract machine
state before the atomic read is:

(* Non-atomic locations *)
a: [t1 -> 1; t2 -> 2]
b: [t3 -> 3; t4 -> 4; t5 -> 5]
c: [t6 -> 5; t7 -> 6; t9 -> 10; t8 -> 7]

(* Domains *)
d1: [a -> t1; b -> t3; c -> t7]
d2: [a -> t1; b -> t4; c -> t9]

(* Atomic locations *)
A: 10, [a -> t1; b -> t5; c -> t7]
B: 5, [a -> t2; b -> t4; c -> t6]

As a result of the atomic read, the abstract machine state is updated to:

(* Non-atomic locations *)
a: [t1 -> 1; t2 -> 2]
b: [t3 -> 3; t4 -> 4; t5 -> 5]
c: [t6 -> 5; t7 -> 6; t9 -> 10; t8 -> 7]

(* Domains *)
d1: [a -> t2; b -> t4; c -> t7] (* frontier updated at a and b *)
d2: [a -> t1; b -> t4; c -> t9]

(* Atomic locations *)
A: 10, [a -> t1; b -> t5; c -> t7]
B: 5, [a -> t2; b -> t4; c -> t6]

During atomic writes, the value held by the atomic location is updated. The frontiers of both
the writing domain and that of the location being written to are updated to the merge of the two
frontiers. For example, if d2 writes 20 to A in the current machine state, the machine state is
updated to:

(* Non-atomic locations *)
a: [t1 -> 1; t2 -> 2]
b: [t3 -> 3; t4 -> 4; t5 -> 5]
c: [t6 -> 5; t7 -> 6; t9 -> 10; t8 -> 7]

(* Domains *)
144

d1: [a -> t2; b -> t4; c -> t7]

d2: [a -> t1; b -> t5; c -> t9] (* frontier updated at b *)

(* Atomic locations *)
A: 20, [a -> t1; b -> t5; c -> t9] (* value updated. frontier updated at c. *)
B: 5, [a -> t2; b -> t4; c -> t6]

10.5.5 Reasoning with the semantics

Let us revisit an example from earlier (section 10.1).
let a = ref 0
and b = ref 0

let d1 () =
a := 1;
!b

let d2 () =
b := 1;
!a

let main () =
let h = Domain.spawn d2 in
let r1 = d1 () in
let r2 = Domain.join h in
assert (not (r1 = 0 && r2 = 0))
This program has a data race on a and b, and hence, the program may exhibit non sequen-
tially consistent behaviour. Let us use the semantics to show that the program may exhibit
r1 = 0 && r2 = 0.
The initial state of the abstract machine is:

(* Non-atomic locations *)
a: [t0 -> 0]
b: [t1 -> 0]

(* Domains *)
d1: [a -> t0; b -> t1]
d2: [a -> t0; b -> t1]

There are several possible schedules for executing this program. Let us consider the following
schedule:

1: a := 1 @ d1
2: b := 1 @ d2
3: !b @ d1
4: !a @ d2
Chapter 10. Memory model: The hard bits 145

After the first action a:=1 by d1, the machine state is:

(* Non-atomic locations *)
a: [t0 -> 0; t2 -> 1] (* new write at t2 *)
b: [t1 -> 0]

(* Domains *)
d1: [a -> t2; b -> t1] (* frontier updated at a *)
d2: [a -> t0; b -> t1]

After the second action b:=1 by d2, the machine state is:

(* Non-atomic locations *)
a: [t0 -> 0; t2 -> 1]
b: [t1 -> 0; t3 -> 1] (* new write at t3 *)

(* Domains *)
d1: [a -> t2; b -> t1]
d2: [a -> t0; b -> t3] (* frontier updated at b *)

Now, for the third action !b by d1, observe that d1’s frontier at b is at t1. Hence, the read may
return either 0 or 1. Let us assume that it returns 0. The machine state is not updated by the
non-atomic read.
Similarly, for the fourth action !a by d2, d2’s frontier at a is at t0. Hence, this read may also
return either 0 or 1. Let us assume that it returns 0. Hence, the assertion in the original program,
assert (not (r1 = 0 && r2 = 0)), will fail for this particular execution.

10.6 Non-compliant operations

There are certain operations which are not memory model compliant.

• Array.blit function on float arrays may cause tearing. When an unsynchronized blit operation
runs concurrently with some overlapping write to the fields of the same float array, the field
may end up with bits from either of the writes.

• With flat-float arrays or records with only float fields on 32-bit architectures, getting or setting
a field involves two separate memory accesses. In the presence of data races, the user may
observe tearing.

• The Bytes module Bytes[29.8] permits mixed-mode accesses where reads and writes may be
of different sizes. Unsynchronized mixed-mode accesses lead to tearing.
146
 Part II

The OCaml language

147
Chapter 11

The OCaml language

Foreword
This document is intended as a reference manual for the OCaml language. It lists the language
constructs, and gives their precise syntax and informal semantics. It is by no means a tutorial
introduction to the language. A good working knowledge of OCaml is assumed.
No attempt has been made at mathematical rigor: words are employed with their intuitive
meaning, without further definition. As a consequence, the typing rules have been left out, by lack
of the mathematical framework required to express them, while they are definitely part of a full
formal definition of the language.

Notations
The syntax of the language is given in BNF-like notation. Terminal symbols are set in typewriter font
(like this). Non-terminal symbols are set in italic font (like that). Square brackets [ . . .] denote
optional components. Curly brackets { . . .} denotes zero, one or several repetitions of the enclosed
components. Curly brackets with a trailing plus sign { . . .}+ denote one or several repetitions of the
enclosed components. Parentheses ( . . .) denote grouping.

11.1 Lexical conventions

Source file encoding
OCaml source files are expected to be valid UTF-8 encoded Unicode text. The interpretation of
source files which are not UTF-8 encoded is unspecified. Such source files may be rejected in the
future.

Blanks
The following characters are considered as blanks: space, horizontal tabulation, carriage return, line
feed and form feed. Blanks are ignored, but they separate adjacent identifiers, literals and keywords
that would otherwise be confused as one single identifier, literal or keyword.

149
150

Comments
Comments are introduced by the two characters (*, with no intervening blanks, and terminated by
the characters *), with no intervening blanks. Comments are treated as blank characters. Comments
do not occur inside string or character literals. Nested comments are handled correctly.
(∗ single line comment ∗)

(∗ multiple line comment, commenting out part of a program, and containing a

nested comment:
let f = function
| 'A'..'Z' −> ”Uppercase”
(∗ Add other cases later... ∗)
∗)

Identifiers
ident ::= (letter | _) {letter | 0 . . . 9 | _ | '}
capitalized-ident ::= uppercase-letter {letter | 0 . . . 9 | _ | '}
lowercase-ident ::= (lowercase-letter | _) {letter | 0 . . . 9 | _ | '}
letter ::= uppercase-letter | lowercase-letter
lowercase-letter ::= a . . . z | U+00DF . . . U+00F6 | U+00F8 . . . U+00FF | U+0153 | U+0161 | U+017E
uppercase-letter ::= A . . . Z | U+00C0 . . . U+00D6 | U+00D8 . . . U+00DE
| U+0152 | U+0160 | U+017D | U+0178 | U+1E9E

Identifiers are sequences of letters, digits, _ (the underscore character), and ' (the single quote),
starting with a letter or an underscore. Letters contain the 52 lowercase and uppercase letters from
the ASCII set, letters ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ
from the Latin-1 Supplement block, letters ŠšŽžŒœŸ from the Latin Extended-A block and upper
case SS (U+189E). Any byte sequence which is equivalent to one of these Unicode characters under
NFC1 is supported too.
All characters in an identifier are meaningful. The current implementation accepts identifiers up
to 16000000 characters in length.
In many places, OCaml makes a distinction between capitalized identifiers and identifiers that
begin with a lowercase letter. The underscore character is considered a lowercase letter for this
purpose.
1
Normalization Form C
Chapter 11. The OCaml language 151

Integer literals
integer-literal ::= [-] (0 . . . 9) {0 . . . 9 | _}
| [-] (0x | 0X) (0 . . . 9 | A . . . F | a . . . f) {0 . . . 9 | A . . . F | a . . . f | _}
| [-] (0o | 0O) (0 . . . 7) {0 . . . 7 | _}
| [-] (0b | 0B) (0 . . . 1) {0 . . . 1 | _}
int32-literal ::= integer-literal l
int64-literal ::= integer-literal L
nativeint-literal ::= integer-literal n
An integer literal is a sequence of one or more digits, optionally preceded by a minus sign. By
default, integer literals are in decimal (radix 10). The following prefixes select a different radix:

Prefix Radix
0x, 0X hexadecimal (radix 16)
0o, 0O octal (radix 8)
0b, 0B binary (radix 2)

(The initial 0 is the digit zero; the O for octal is the letter O.) An integer literal can be followed
by one of the letters l, L or n to indicate that this integer has type int32, int64 or nativeint
respectively, instead of the default type int for integer literals. The interpretation of integer literals
that fall outside the range of representable integer values is undefined.
For convenience and readability, underscore characters (_) are accepted (and ignored) within
integer literals.
# let house_number = 37
let million = 1_000_000
let copyright = 0x00A9
let counter64bit = ref 0L;;
val house_number : int = 37
val million : int = 1000000
val copyright : int = 169
val counter64bit : int64 ref = {contents = 0L}

Floating-point literals
float-literal ::= [-] (0 . . . 9) {0 . . . 9 | _} [. {0 . . . 9 | _}] [(e | E) [+ | -] (0 . . . 9) {0 . . . 9 | _}]
| [-] (0x | 0X) (0 . . . 9 | A . . . F | a . . . f) {0 . . . 9 | A . . . F | a . . . f | _}
[. {0 . . . 9 | A . . . F | a . . . f | _}] [(p | P) [+ | -] (0 . . . 9) {0 . . . 9 | _}]
Floating-point decimal literals consist in an integer part, a fractional part and an exponent part.
The integer part is a sequence of one or more digits, optionally preceded by a minus sign. The
fractional part is a decimal point followed by zero, one or more digits. The exponent part is the
character e or E followed by an optional + or - sign, followed by one or more digits. It is interpreted
as a power of 10. The fractional part or the exponent part can be omitted but not both, to avoid
ambiguity with integer literals. The interpretation of floating-point literals that fall outside the
range of representable floating-point values is undefined.
152

Floating-point hexadecimal literals are denoted with the 0x or 0X prefix. The syntax is similar
to that of floating-point decimal literals, with the following differences. The integer part and the
fractional part use hexadecimal digits. The exponent part starts with the character p or P. It is
written in decimal and interpreted as a power of 2.
For convenience and readability, underscore characters (_) are accepted (and ignored) within
floating-point literals.
# let pi = 3.141_592_653_589_793_12
let small_negative = -1e-5
let machine_epsilon = 0x1p-52;;
val pi : float = 3.14159265358979312
val small_negative : float = -1e-05
val machine_epsilon : float = 2.22044604925031308e-16

Character literals
char-literal ::= ' regular-char '
| ' escape-sequence '
escape-sequence ::= \ (\ | " | ' | n | t | b | r | space)
| \ (0 . . . 9) (0 . . . 9) (0 . . . 9)
| \x (0 . . . 9 | A . . . F | a . . . f) (0 . . . 9 | A . . . F | a . . . f)
| \o (0 . . . 3) (0 . . . 7) (0 . . . 7)
Character literals are delimited by ' (single quote) characters. The two single quotes enclose
either one character different from ' and \, or one of the escape sequences below:

Sequence Character denoted

\\ backslash (\)
\" double quote (")
\' single quote (')
\n linefeed (LF)
\r carriage return (CR)
\t horizontal tabulation (TAB)
\b backspace (BS)
\space space (SPC)
\ddd the character with ASCII code ddd in decimal
\xhh the character with ASCII code hh in hexadecimal
\oooo the character with ASCII code ooo in octal

# let a = 'a'
let single_quote = '\''
let copyright = '\xA9';;
val a : char = 'a'
val single_quote : char = '\''
val copyright : char = '\169'
Chapter 11. The OCaml language 153

String literals
string-literal ::= " {string-character} "
| { quoted-string-id | {newline
| any-char} | quoted-string-id }
quoted-string-id ::= {lowercase-letter | _}
string-character ::= regular-string-char
| escape-sequence
| \u{ {0 . . . 9 | A . . . F | a . . . f}+ }
| newline
| \ newline {space | tab}
String literals are delimited by " (double quote) characters. The two double quotes enclose a
sequence of either characters different from " and \, or escape sequences from the table given above
for character literals, or a Unicode character escape sequence.
A Unicode character escape sequence is substituted by the UTF-8 encoding of the specified
Unicode scalar value. The Unicode scalar value, an integer in the ranges 0x0000...0xD7FF or
0xE000...0x10FFFF, is defined using 1 to 6 hexadecimal digits; leading zeros are allowed.
# let greeting = "Hello, World!\n"
let superscript_plus = "\u{207A}";;
val greeting : string = "Hello, World!\n"
val superscript_plus : string = "�"
A newline sequence is a line feed optionally preceded by a carriage return. Since OCaml 5.2, a
newline sequence occurring in a string literal is normalized into a single line feed character.
To allow splitting long string literals across lines, the sequence \newline spaces-or-tabs (a
backslash at the end of a line followed by any number of spaces and horizontal tabulations at the
beginning of the next line) is ignored inside string literals.
# let longstr =
"Call me Ishmael. Some years ago — never mind how long \
precisely — having little or no money in my purse, and \
nothing particular to interest me on shore, I thought I\
\ would sail about a little and see the watery part of t\
he world.";;
val longstr : string =
"Call me Ishmael. Some years ago — never mind how long precisely — having little or no money in my pur
Escaped newlines provide more convenient behavior than non-escaped newlines, as the indentation
is not considered part of the string literal.
# let contains_unexpected_spaces =
"This multiline literal
contains three consecutive spaces."

let no_unexpected_spaces =
"This multiline literal \n\
uses a single space between all words.";;
154

val contains_unexpected_spaces : string =

"This multiline literal\n contains three consecutive spaces."
val no_unexpected_spaces : string =
"This multiline literal \nuses a single space between all words."
Quoted string literals provide an alternative lexical syntax for string literals. They are useful to
represent strings of arbitrary content without escaping. Quoted strings are delimited by a matching
pair of { quoted-string-id | and | quoted-string-id } with the same quoted-string-id on both sides.
Quoted strings do not interpret any character in a special way2 but requires that the sequence
| quoted-string-id } does not occur in the string itself. The identifier quoted-string-id is a (possibly
empty) sequence of lowercase letters and underscores that can be freely chosen to avoid such issue.
# let quoted_greeting = {|"Hello, World!"|}
let nested = {ext|hello {|world|}|ext};;
val quoted_greeting : string = "\"Hello, World!\""
val nested : string = "hello {|world|}"
The current implementation places practically no restrictions on the length of string literals.

Naming labels
To avoid ambiguities, naming labels in expressions cannot just be defined syntactically as the
sequence of the three tokens ~, ident and :, and have to be defined at the lexical level.

label-name ::= lowercase-ident

label ::= ~ label-name :
optlabel ::= ? label-name :
Naming labels come in two flavours: label for normal arguments and optlabel for optional ones.
They are simply distinguished by their first character, either ~ or ?.
Despite label and optlabel being lexical entities in expressions, their expansions ~ label-name :
and ? label-name : will be used in grammars, for the sake of readability. Note also that inside type
expressions, this expansion can be taken literally, i.e. there are really 3 tokens, with optional blanks
between them.

Prefix and infix symbols

infix-symbol ::= (core-operator-char | % | <) {operator-char}
| # {operator-char}+
prefix-symbol ::= ! {operator-char}
| (? | ~) {operator-char}+
operator-char ::= ~ | ! | ? | core-operator-char | % | < | : | .
core-operator-char ::= $ | & | * | + | - | / | = | > | @ | ^ | |
See also the following language extensions: extension operators, extended indexing operators, and
binding operators.
2
Except for the normalization of newline sequences into a single line feed mentioned earlier.
Chapter 11. The OCaml language 155

Sequences of “operator characters”, such as <=> or !!, are read as a single token from the
infix-symbol or prefix-symbol class. These symbols are parsed as prefix and infix operators inside
expressions, but otherwise behave like normal identifiers.

Keywords
The identifiers below are reserved as keywords, and cannot be employed otherwise:

and as assert asr begin class

constraint do done downto else end
exception external false for fun function
functor if in include inherit initializer
land lazy let lor lsl lsr
lxor match method mod module mutable
new nonrec object of open or
private rec sig struct then to
true try type val virtual when
while with

The following character sequences are also keywords:

!= # & && ' ( ) * + , -

-. -> . .. .~ : :: := :> ; ;;
< <- = > >] >} ? [ [< [> [|
] _ ` { {< | |] || } ~

Note that the following identifiers are keywords of the now unmaintained Camlp4 system and
should be avoided for backwards compatibility reasons.

parser value $ $$ $: <: << >> ??

Ambiguities
Lexical ambiguities are resolved according to the “longest match” rule: when a character sequence
can be decomposed into two tokens in several different ways, the decomposition retained is the one
with the longest first token.

Line number directives

linenum-directive ::= # {0 . . . 9}+ " {string-character} "
Preprocessors that generate OCaml source code can insert line number directives in their output
so that error messages produced by the compiler contain line numbers and file names referring to
the source file before preprocessing, instead of after preprocessing. A line number directive starts at
the beginning of a line, is composed of a # (sharp sign), followed by a positive integer (the source
line number), followed by a character string (the source file name). Line number directives are
treated as blanks during lexical analysis.
156

11.2 Values
This section describes the kinds of values that are manipulated by OCaml programs.

11.2.1 Base values

Integer numbers
Integer values are integer numbers from −230 to 230 − 1, that is −1073741824 to 1073741823. The
implementation may support a wider range of integer values: on 64-bit platforms, the current
implementation supports integers ranging from −262 to 262 − 1.

Floating-point numbers
Floating-point values are numbers in floating-point representation. The current implementation
uses double-precision floating-point numbers conforming to the IEEE 754 standard, with 53 bits of
mantissa and an exponent ranging from −1022 to 1023.

Characters
Character values are represented as 8-bit integers between 0 and 255. Character codes between
0 and 127 are interpreted following the ASCII standard. The current implementation interprets
character codes between 128 and 255 following the ISO 8859-1 standard.

Character strings
String values are finite sequences of characters. The current implementation supports strings
containing up to 224 − 5 characters (16777211 characters); on 64-bit platforms, the limit is 257 − 9.

11.2.2 Tuples
Tuples of values are written (v 1 , . . . ,v n ), standing for the n-tuple of values v 1 to v n . The current
implementation supports tuple of up to 222 − 1 elements (4194303 elements).

11.2.3 Records
Record values are labeled tuples of values. The record value written { field1 =v 1 ; . . . ; fieldn =v n }
associates the value v i to the record field fieldi , for i = 1 . . . n. The current implementation supports
records with up to 222 − 1 fields (4194303 fields).

11.2.4 Arrays
Arrays are finite, variable-sized sequences of values of the same type. The current implementation
supports arrays containing up to 222 − 1 elements (4194303 elements) unless the elements are
floating-point numbers (2097151 elements in this case); on 64-bit platforms, the limit is 254 − 1 for
all arrays.
Chapter 11. The OCaml language 157

11.2.5 Variant values

Variant values are either a constant constructor, or a non-constant constructor applied to a number
of values. The former case is written constr; the latter case is written constr (v 1 ,... ,v n ), where
the v i are said to be the arguments of the non-constant constructor constr. The parentheses may
be omitted if there is only one argument.
The following constants are treated like built-in constant constructors:

Constant Constructor
false the boolean false
true the boolean true
() the “unit” value
[] the empty list

The current implementation limits each variant type to have at most 246 non-constant construc-
tors and 230 − 1 constant constructors.

11.2.6 Polymorphic variants

Polymorphic variants are an alternate form of variant values, not belonging explicitly to a predefined
variant type, and following specific typing rules. They can be either constant, written ` tag-name,
or non-constant, written ` tag-name (v ).

11.2.7 Functions
Functional values are mappings from values to values.

11.2.8 Objects
Objects are composed of a hidden internal state which is a record of instance variables, and a set of
methods for accessing and modifying these variables. The structure of an object is described by the
toplevel class that created it.

11.3 Names
Identifiers are used to give names to several classes of language objects and refer to these objects by
name later:

• value names (syntactic class value-name),

• value constructors and exception constructors (class constr-name),

• labels (label-name, defined in section 11.1),

• polymorphic variant tags (tag-name),

• type constructors (typeconstr-name),

• record fields (field-name),

158

• class names (class-name),

• method names (method-name),

• instance variable names (inst-var-name),

• module names (module-name),

• module type names (modtype-name).

These eleven name spaces are distinguished both by the context and by the capitalization of the
identifier: whether the first letter of the identifier is in lowercase (written lowercase-ident below)
or in uppercase (written capitalized-ident). Underscore is considered a lowercase letter for this
purpose.

Naming objects
value-name ::= lowercase-ident
| ( operator-name )
operator-name ::= prefix-symbol | infix-op
infix-op ::= infix-symbol
| * | + | - | -. | = | != | < | > | or | || | & | && | :=
| mod | land | lor | lxor | lsl | lsr | asr
constr-name ::= capitalized-ident
tag-name ::= capitalized-ident
typeconstr-name ::= lowercase-ident
field-name ::= lowercase-ident
module-name ::= capitalized-ident
modtype-name ::= ident
class-name ::= lowercase-ident
inst-var-name ::= lowercase-ident
method-name ::= lowercase-ident
See also the following language extension: extended indexing operators.
As shown above, prefix and infix symbols as well as some keywords can be used as value names,
provided they are written between parentheses. The capitalization rules are summarized in the
table below.
Chapter 11. The OCaml language 159

Name space Case of first letter

Values lowercase
Constructors uppercase
Labels lowercase
Polymorphic variant tags uppercase
Exceptions uppercase
Type constructors lowercase
Record fields lowercase
Classes lowercase
Instance variables lowercase
Methods lowercase
Modules uppercase
Module types any

Note on polymorphic variant tags: the current implementation accepts lowercase variant tags in
addition to capitalized variant tags, but we suggest you avoid lowercase variant tags for portability
and compatibility with future OCaml versions.

Referring to named objects

value-path ::= [module-path .] value-name
constr ::= [module-path .] constr-name
typeconstr ::= [extended-module-path .] typeconstr-name
field ::= [module-path .] field-name
modtype-path ::= [extended-module-path .] modtype-name
class-path ::= [module-path .] class-name
classtype-path ::= [extended-module-path .] class-name
module-path ::= module-name {. module-name}
extended-module-path ::= extended-module-name {. extended-module-name}
extended-module-name ::= module-name {( extended-module-path )}
A named object can be referred to either by its name (following the usual static scoping rules
for names) or by an access path prefix . name, where prefix designates a module and name is
the name of an object defined in that module. The first component of the path, prefix, is either
a simple module name or an access path name 1 . name 2 . . ., in case the defining module is itself
nested inside other modules. For referring to type constructors, module types, or class types, the
prefix can also contain simple functor applications (as in the syntactic class extended-module-path
above) in case the defining module is the result of a functor application.
Label names, tag names, method names and instance variable names need not be qualified: the
former three are global labels, while the latter are local to a class.
160

11.4 Type expressions

typexpr ::= ' ident
| _
| ( typexpr )
| [[?] label-name :] typexpr -> typexpr
| typexpr {* typexpr}+
| typeconstr
| typexpr typeconstr
| ( typexpr {, typexpr} ) typeconstr
| typexpr as ' ident
| polymorphic-variant-type
| < [..] >
| < method-type {; method-type} [; | ; ..] >
| # classtype-path
| typexpr # class-path
| ( typexpr {, typexpr} ) # class-path
poly-typexpr ::= typexpr
| {' ident}+ . typexpr
method-type ::= method-name : poly-typexpr
See also the following language extensions: first-class modules, attributes and extension nodes.
The table below shows the relative precedences and associativity of operators and non-closed
type constructions. The constructions with higher precedences come first.

Operator Associativity
Type constructor application –
# –
* –
-> right
as –

Type expressions denote types in definitions of data types as well as in type constraints over
patterns and expressions.

Type variables
The type expression ' ident stands for the type variable named ident. The type expression _ stands
for either an anonymous type variable or anonymous type parameters. In data type definitions, type
variables are names for the data type parameters. In type constraints, they represent unspecified
types that can be instantiated by any type to satisfy the type constraint. In general the scope of a
named type variable is the whole top-level phrase where it appears, and it can only be generalized
when leaving this scope. Anonymous variables have no such restriction. In the following cases,
the scope of named type variables is restricted to the type expression where they appear: 1) for
universal (explicitly polymorphic) type variables; 2) for type variables that only appear in public
method specifications (as those variables will be made universal, as described in section 11.9.1); 3)
Chapter 11. The OCaml language 161

for variables used as aliases, when the type they are aliased to would be invalid in the scope of the
enclosing definition (i.e. when it contains free universal type variables, or locally defined types.)

Parenthesized types
The type expression ( typexpr ) denotes the same type as typexpr.

Function types
The type expression typexpr 1 -> typexpr 2 denotes the type of functions mapping arguments of type
typexpr 1 to results of type typexpr 2 .
label-name : typexpr 1 -> typexpr 2 denotes the same function type, but the argument is labeled
label.
? label-name : typexpr 1 -> typexpr 2 denotes the type of functions mapping an optional labeled
argument of type typexpr 1 to results of type typexpr 2 . That is, the physical type of the function
will be typexpr 1 option -> typexpr 2 .

Tuple types
The type expression typexpr 1 * . . . * typexpr n denotes the type of tuples whose elements belong to
types typexpr 1 , . . . typexpr n respectively.

Constructed types
Type constructors with no parameter, as in typeconstr, are type expressions.
The type expression typexpr typeconstr, where typeconstr is a type constructor with one
parameter, denotes the application of the unary type constructor typeconstr to the type typexpr.
The type expression (typexpr 1 , . . . , typexpr n ) typeconstr, where typeconstr is a type constructor
with n parameters, denotes the application of the n-ary type constructor typeconstr to the types
typexpr 1 through typexpr n .
In the type expression _ typeconstr, the anonymous type expression _ stands in for anonymous
type parameters and is equivalent to (_, . . . , _) with as many repetitions of _ as the arity of
typeconstr.

Aliased and recursive types

The type expression typexpr as ' ident denotes the same type as typexpr, and also binds the type
variable ident to type typexpr both in typexpr and in other types. In general the scope of an alias
is the same as for a named type variable, and covers the whole enclosing definition. If the type
variable ident actually occurs in typexpr, a recursive type is created. Recursive types for which
there exists a recursive path that does not contain an object or polymorphic variant type constructor
are rejected, except when the -rectypes mode is selected.
If ' ident denotes an explicit polymorphic variable, and typexpr denotes either an object or
polymorphic variant type, the row variable of typexpr is captured by ' ident, and quantified upon.
162

Polymorphic variant types

polymorphic-variant-type ::= [ tag-spec-first {| tag-spec} ]
| [> [tag-spec] {| tag-spec} ]
| [< [|] tag-spec-full {| tag-spec-full} [> {` tag-name}+ ] ]
tag-spec-first ::= ` tag-name [of typexpr]
| [typexpr] | tag-spec
tag-spec ::= ` tag-name [of typexpr]
| typexpr
tag-spec-full ::= ` tag-name [of [&] typexpr {& typexpr}]
| typexpr
Polymorphic variant types describe the values a polymorphic variant may take.
The first case is an exact variant type: all possible tags are known, with their associated types,
and they can all be present. Its structure is fully known.
The second case is an open variant type, describing a polymorphic variant value: it gives the list
of all tags the value could take, with their associated types. This type is still compatible with a
variant type containing more tags. A special case is the unknown type, which does not define any
tag, and is compatible with any variant type.
The third case is a closed variant type. It gives information about all the possible tags and
their associated types, and which tags are known to potentially appear in values. The exact variant
type (first case) is just an abbreviation for a closed variant type where all possible tags are also
potentially present.
In all three cases, tags may be either specified directly in the ` tag-name [of typexpr] form,
or indirectly through a type expression, which must expand to an exact variant type, whose tag
specifications are inserted in its place.
Full specifications of variant tags are only used for non-exact closed types. They can be
understood as a conjunctive type for the argument: it is intended to have all the types enumerated
in the specification.
Such conjunctive constraints may be unsatisfiable. In such a case the corresponding tag may
not be used in a value of this type. This does not mean that the whole type is not valid: one can
still use other available tags. Conjunctive constraints are mainly intended as output from the type
checker. When they are used in source programs, unsolvable constraints may cause early failures.

Object types
An object type < [method-type {; method-type}] > is a record of method types.
Each method may have an explicit polymorphic type: {' ident}+ . typexpr. Explicit poly-
morphic variables have a local scope, and an explicit polymorphic type can only be unified to an
equivalent one, where only the order and names of polymorphic variables may change.
The type < {method-type ;} .. > is the type of an object whose method names and types are
described by method-type 1 , . . . , method-type n , and possibly some other methods represented by the
ellipsis. This ellipsis actually is a special kind of type variable (called row variable in the literature)
that stands for any number of extra method types.
Chapter 11. The OCaml language 163

#-types
The type # classtype-path is a special kind of abbreviation. This abbreviation unifies with the type
of any object belonging to a subclass of the class type classtype-path. It is handled in a special
way as it usually hides a type variable (an ellipsis, representing the methods that may be added
in a subclass). In particular, it vanishes when the ellipsis gets instantiated. Each type expression
# classtype-path defines a new type variable, so type # classtype-path -> # classtype-path is usually
not the same as type (# classtype-path as ' ident) -> ' ident.

Variant and record types

There are no type expressions describing (defined) variant types nor record types, since those are
always named, i.e. defined before use and referred to by name. Type definitions are described in
section 11.8.1.

11.5 Constants
constant ::= integer-literal
| int32-literal
| int64-literal
| nativeint-literal
| float-literal
| char-literal
| string-literal
| constr
| false
| true
| ()
| begin end
| []
| [| |]
| ` tag-name
See also the following language extension: extension literals.
The syntactic class of constants comprises literals from the four base types (integers, floating-
point numbers, characters, character strings), the integer variants, and constant constructors from
both normal and polymorphic variants, as well as the special constants false, true, ( ), [ ], and
[| |], which behave like constant constructors, and begin end, which is equivalent to ( ).
164

11.6 Patterns
pattern ::= value-name
| _
| constant
| pattern as value-name
| ( pattern )
| ( pattern : typexpr )
| pattern | pattern
| constr pattern
| ` tag-name pattern
| # typeconstr
| pattern {, pattern}+
| { field [: typexpr] [= pattern] {; field [: typexpr] [= pattern]} [; _] [;] }
| [ pattern {; pattern} [;] ]
| pattern :: pattern
| [| pattern {; pattern} [;] |]
| char-literal .. char-literal
| lazy pattern
| exception pattern
| module-path .( pattern )
| module-path .[ pattern ]
| module-path .[| pattern |]
| module-path .{ pattern }
See also the following language extensions: first-class modules, attributes, extension nodes and effect
handlers.
The table below shows the relative precedences and associativity of operators and non-closed
pattern constructions. The constructions with higher precedences come first.
Operator Associativity
.. –
lazy (see section 11.6.1) –
Constructor application, Tag application right
:: right
, –
| left
as –
Patterns are templates that allow selecting data structures of a given shape, and binding
identifiers to components of the data structure. This selection operation is called pattern matching;
its outcome is either “this value does not match this pattern”, or “this value matches this pattern,
resulting in the following bindings of names to values”.

Variable patterns
A pattern that consists in a value name matches any value, binding the name to the value. The
pattern _ also matches any value, but does not bind any name.
Chapter 11. The OCaml language 165

# let is_empty = function

| [] -> true
| _ :: _ -> false;;
val is_empty : 'a list -> bool = <fun>
Patterns are linear: a variable cannot be bound several times by a given pattern. In particular,
there is no way to test for equality between two parts of a data structure using only a pattern:
# let pair_equal = function
| x, x -> true
| x, y -> false;;
Error : Variable x is bound several times in this matching
However, we can use a when guard for this purpose:
# let pair_equal = function
| x, y when x = y -> true
| _ -> false;;
val pair_equal : 'a * 'a -> bool = <fun>

Constant patterns
A pattern consisting in a constant matches the values that are equal to this constant.
# let bool_of_string = function
| "true" -> true
| "false" -> false
| _ -> raise (Invalid_argument "bool_of_string");;
val bool_of_string : string -> bool = <fun>

Alias patterns
The pattern pattern1 as value-name matches the same values as pattern1 . If the matching against
pattern1 is successful, the name value-name is bound to the matched value, in addition to the
bindings performed by the matching against pattern1 .
# let sort_pair ((x, y) as p) =
if x <= y then p else (y, x);;
val sort_pair : 'a * 'a -> 'a * 'a = <fun>

Parenthesized patterns
The pattern ( pattern1 ) matches the same values as pattern1 . A type constraint can appear in a
parenthesized pattern, as in ( pattern1 : typexpr ). This constraint forces the type of pattern1 to
be compatible with typexpr.
# let int_triple_is_ordered ((a, b, c) : int * int * int) =
a <= b && b <= c;;
val int_triple_is_ordered : int * int * int -> bool = <fun>
166

“Or” patterns
The pattern pattern1 | pattern2 represents the logical “or” of the two patterns pattern1 and
pattern2 . A value matches pattern1 | pattern2 if it matches pattern1 or pattern2 . The two
sub-patterns pattern1 and pattern2 must bind exactly the same identifiers to values having the
same types. Matching is performed from left to right. More precisely, in case some value v
matches pattern1 | pattern2 , the bindings performed are those of pattern1 when v matches pattern1 .
Otherwise, value v matches pattern2 whose bindings are performed.
# type shape = Square of float | Rect of (float * float) | Circle of float

let is_rectangular = function

| Square _ | Rect _ -> true
| Circle _ -> false;;
type shape = Square of float | Rect of (float * float) | Circle of float
val is_rectangular : shape -> bool = <fun>

Variant patterns
The pattern constr ( pattern1 , . . . , patternn ) matches all variants whose constructor is equal to
constr, and whose arguments match pattern1 . . . patternn . It is a type error if n is not the number
of arguments expected by the constructor.
The pattern constr _ matches all variants whose constructor is constr.
# type 'a tree = Lf | Br of 'a tree * 'a * 'a tree

let rec total = function

| Br (l, x, r) -> total l + x + total r
| Lf -> 0;;
type 'a tree = Lf | Br of 'a tree * 'a * 'a tree
val total : int tree -> int = <fun>
The pattern pattern1 :: pattern2 matches non-empty lists whose heads match pattern1 , and
whose tails match pattern2 .
The pattern [ pattern1 ; . . . ; patternn ] matches lists of length n whose elements match
pattern1 …patternn , respectively. This pattern behaves like pattern1 :: . . . :: patternn :: [].
# let rec destutter = function
| [] -> []
| [a] -> [a]
| a :: b :: t -> if a = b then destutter (b :: t) else a :: destutter (b :: t);;
val destutter : 'a list -> 'a list = <fun>

Polymorphic variant patterns

The pattern ` tag-name pattern1 matches all polymorphic variants whose tag is equal to tag-name,
and whose argument matches pattern1 .
Chapter 11. The OCaml language 167

# let rec split = function

| [] -> ([], [])
| h :: t ->
let ss, gs = split t in
match h with
| `Sheep _ as s -> (s :: ss, gs)
| `Goat _ as g -> (ss, g :: gs);;
val split :
[< `Goat of 'a | `Sheep of 'b ] list ->
[> `Sheep of 'b ] list * [> `Goat of 'a ] list = <fun>

Polymorphic variant abbreviation patterns

If the type [('a,'b, . . . )] typeconstr = [ ` tag-name 1 typexpr 1 | . . . | ` tag-name n typexpr n ]
is defined, then the pattern # typeconstr is a shorthand for the following or-pattern:
( ` tag-name 1 (_ : typexpr 1 ) | . . . | ` tag-name n (_ : typexpr n )). It matches all values of type
[< typeconstr ].
# type 'a rectangle = [`Square of 'a | `Rectangle of 'a * 'a]
type 'a shape = [`Circle of 'a | 'a rectangle]

let try_rectangle = function

| #rectangle as r -> Some r
| `Circle _ -> None;;
type 'a rectangle = [ `Rectangle of 'a * 'a | `Square of 'a ]
type 'a shape = [ `Circle of 'a | `Rectangle of 'a * 'a | `Square of 'a ]
val try_rectangle :
[< `Circle of 'a | `Rectangle of 'b * 'b | `Square of 'b ] ->
[> `Rectangle of 'b * 'b | `Square of 'b ] option = <fun>

Tuple patterns
The pattern pattern1 , . . . , patternn matches n-tuples whose components match the patterns
pattern1 through patternn . That is, the pattern matches the tuple values (v1 , . . . , vn ) such that
patterni matches vi for i = 1, . . . , n.
# let vector (x0, y0) (x1, y1) =
(x1 -. x0, y1 -. y0);;
val vector : float * float -> float * float -> float * float = <fun>

Record patterns
The pattern { field1 [= pattern1 ] ; . . . ; fieldn [= patternn ] } matches records that define at least the
fields field1 through fieldn , and such that the value associated to fieldi matches the pattern patterni ,
for i = 1, . . . , n. A single identifier fieldk stands for fieldk = fieldk , and a single qualified identifier
module-path . fieldk stands for module-path . fieldk = fieldk . The record value can define more
fields than field1 …fieldn ; the values associated to these extra fields are not taken into account for
168

matching. Optionally, a record pattern can be terminated by ; _ to convey the fact that not all fields
of the record type are listed in the record pattern and that it is intentional. Optional type constraints
can be added field by field with { field1 : typexpr 1 = pattern1 ; . . . ; fieldn : typexpr n = patternn }
to force the type of fieldk to be compatible with typexpr k .
# let bytes_allocated
{Gc.minor_words = minor;
Gc.major_words = major;
Gc.promoted_words = prom;
_}
=
(Sys.word_size / 4) * int_of_float (minor +. major -. prom);;
val bytes_allocated : Gc.stat -> int = <fun>

Array patterns
The pattern [| pattern1 ; . . . ; patternn |] matches arrays of length n such that the i-th array
element matches the pattern patterni , for i = 1, . . . , n.
# let matrix3_is_symmetric = function
| [|[|_; b; c|];
[|d; _; f|];
[|g; h; _|]|] -> b = d && c = g && f = h
| _ -> failwith "matrix3_is_symmetric: not a 3x3 matrix";;
val matrix3_is_symmetric : 'a array array -> bool = <fun>

Range patterns
The pattern 'c ' .. 'd ' is a shorthand for the pattern

'c ' | 'c 1 ' | 'c 2 ' | . . . | 'c n ' | 'd '

where c1 , c2 , …, cn are the characters that occur between c and d in the ASCII character set. For
instance, the pattern '0'..'9' matches all characters that are digits.
# type char_class = Uppercase | Lowercase | Digit | Other

let classify_char = function

| 'A'..'Z' -> Uppercase
| 'a'..'z' -> Lowercase
| '0'..'9' -> Digit
| _ -> Other;;
type char_class = Uppercase | Lowercase | Digit | Other
val classify_char : char -> char_class = <fun>
Chapter 11. The OCaml language 169

11.6.1 Lazy patterns

(Introduced in Objective Caml 3.11)
The pattern lazy pattern matches a value v of type Lazy.t, provided pattern matches the
result of forcing v with Lazy.force. A successful match of a pattern containing lazy sub-patterns
forces the corresponding parts of the value being matched, even those that imply no test such
as lazy value-name or lazy _. Matching a value with a pattern-matching where some patterns
contain lazy sub-patterns may imply forcing parts of the value, even when the pattern selected in
the end has no lazy sub-pattern.
# let force_opt = function
| Some (lazy n) -> n
| None -> 0;;
val force_opt : int lazy_t option -> int = <fun>
For more information, see the description of module Lazy in the standard library (module
Lazy[29.30]).

Exception patterns
(Introduced in OCaml 4.02)
A new form of exception pattern, exception pattern, is allowed only as a toplevel pattern or
inside a toplevel or-pattern under a match...with pattern-matching (other occurrences are rejected
by the type-checker).
Cases with such a toplevel pattern are called “exception cases”, as opposed to regular “value
cases”. Exception cases are applied when the evaluation of the matched expression raises an exception.
The exception value is then matched against all the exception cases and re-raised if none of them
accept the exception (as with a try...with block). Since the bodies of all exception and value cases
are outside the scope of the exception handler, they are all considered to be in tail-position: if the
match...with block itself is in tail position in the current function, any function call in tail position
in one of the case bodies results in an actual tail call.
A pattern match must contain at least one value case. It is an error if all cases are exceptions,
because there would be no code to handle the return of a value.
# let find_opt p l =
match List.find p l with
| exception Not_found -> None
| x -> Some x;;
val find_opt : ('a -> bool) -> 'a list -> 'a option = <fun>

Local opens for patterns

(Introduced in OCaml 4.04)
For patterns, local opens are limited to the module-path .( pattern ) construction. This
construction locally opens the module referred to by the module path module-path in the scope of
the pattern pattern.
170

When the body of a local open pattern is delimited by [ ], [| |], or { }, the parentheses can
be omitted. For example, module-path .[ pattern ] is equivalent to module-path .([ pattern ]),
and module-path .[| pattern |] is equivalent to module-path .([| pattern |]).
# let bytes_allocated Gc.{minor_words; major_words; promoted_words; _} =
(Sys.word_size / 4)
* int_of_float (minor_words +. major_words -. promoted_words);;
val bytes_allocated : Gc.stat -> int = <fun>
Chapter 11. The OCaml language 171

11.7 Expressions
expr ::= value-path
| constant
| ( expr )
| begin expr end
| ( expr : typexpr )
| expr {, expr}+
| constr expr
| ` tag-name expr
| expr :: expr
| [ expr {; expr} [;] ]
| [| expr {; expr} [;] |]
| { field [: typexpr] [= expr] {; field [: typexpr] [= expr]} [;] }
| { expr with field [: typexpr] [= expr] {; field [: typexpr] [= expr]} [;] }
| expr {argument}+
| prefix-symbol expr
| - expr
| -. expr
| expr infix-op expr
| expr . field
| expr . field <- expr
| expr .( expr )
| expr .( expr ) <- expr
| expr .[ expr ]
| expr .[ expr ] <- expr
| if expr then expr [else expr]
| while expr do expr done
| for value-name = expr (to | downto) expr do expr done
| expr ; expr
| match expr with pattern-matching
| function pattern-matching
| fun {parameter}+ [: typexpr] -> expr
| try expr with pattern-matching
| let [rec] let-binding {and let-binding} in expr
| let exception constr-decl in expr
| let module module-name {( module-name : module-type )} [: module-type]
= module-expr in expr
| ( expr :> typexpr )
| ( expr : typexpr :> typexpr )
| assert expr
| lazy expr
| local-open
| object-expr
172

argument ::= expr

| ~ label-name
| ~ label-name : expr
| ? label-name
| ? label-name : expr
pattern-matching ::= [|] pattern [when expr] -> expr {| pattern [when expr] -> expr}
let-binding ::= pattern = expr
| value-name {parameter} [: typexpr] [:> typexpr] = expr
| value-name : poly-typexpr = expr
parameter ::= pattern
| ~ label-name
| ~ ( label-name [: typexpr] )
| ~ label-name : pattern
| ? label-name
| ? ( label-name [: typexpr] [= expr] )
| ? label-name : pattern
| ? label-name : ( pattern [: typexpr] [= expr] )
local-open ::=
| let open module-path in expr
| module-path .( expr )
| module-path .[ expr ]
| module-path .[| expr |]
| module-path .{ expr }
| module-path .{< expr >}
object-expr ::=
| new class-path
| object class-body end
| expr # method-name
| inst-var-name
| inst-var-name <- expr
| {< [inst-var-name [= expr] {; inst-var-name [= expr]} [;]] >}
See also the following language extensions: first-class modules, overriding in open statements, syntax
for Bigarray access, attributes, extension nodes and extended indexing operators.

11.7.1 Precedence and associativity

The table below shows the relative precedences and associativity of operators and non-closed
constructions. The constructions with higher precedence come first. For infix and prefix symbols,
we write “*…” to mean “any symbol starting with *”.
Chapter 11. The OCaml language 173

Construction or operator Associativity

prefix-symbol –
. .( .[ .{ (see section 12.11) –
#… left
function application, constructor application, tag application, assert, lazy left
- -. (prefix) –
**… lsl lsr asr right
*… /… %… mod land lor lxor left
+… -… left
:: right
@… ^… right
=… <… >… |… &… $… != left
& && right
or || right
, –
<- := right
if –
; right
let match fun function try –

It is simple to test or refresh one’s understanding:

# 3 + 3 mod 2, 3 + (3 mod 2), (3 + 3) mod 2;;
- : int * int * int = (4, 4, 0)

11.7.2 Basic expressions

Constants
An expression consisting in a constant evaluates to this constant. For example, 3.14 or [||].

Value paths
An expression consisting in an access path evaluates to the value bound to this path in the current
evaluation environment. The path can be either a value name or an access path to a value component
of a module.
# Float.ArrayLabels.to_list;;
- : Float.ArrayLabels.t -> float list = <fun>

Parenthesized expressions
The expressions ( expr ) and begin expr end have the same value as expr. The two constructs are
semantically equivalent, but it is good style to use begin . . . end inside control structures:

if … then begin … ; … end else begin … ; … end

and ( . . . ) for the other grouping situations.

174

# let x = 1 + 2 * 3
let y = (1 + 2) * 3;;
val x : int = 7
val y : int = 9

# let f a b =
if a = b then
print_endline "Equal"
else begin
print_string "Not Equal: ";
print_int a;
print_string " and ";
print_int b;
print_newline ()
end;;
val f : int -> int -> unit = <fun>
Parenthesized expressions can contain a type constraint, as in ( expr : typexpr ). This constraint
forces the type of expr to be compatible with typexpr.
Parenthesized expressions can also contain coercions ( expr [: typexpr] :> typexpr ) (see
subsection 11.7.9 below).

Function application
Function application is denoted by juxtaposition of (possibly labeled) expressions. The expression
expr argument1 . . . argumentn evaluates the expression expr and those appearing in argument1 to
argumentn . The expression expr must evaluate to a functional value f , which is then applied to the
values of argument1 , . . . , argumentn .
The order in which the expressions expr, argument1 , . . . , argumentn are evaluated is not specified.
# List.fold_left ( + ) 0 [1; 2; 3; 4; 5];;
- : int = 15
Arguments and parameters are matched according to their respective labels. Argument order is
irrelevant, except among arguments with the same label, or no label.
# ListLabels.fold_left ~f:( @ ) ~init:[] [[1; 2; 3]; [4; 5; 6]; [7; 8; 9]];;
- : int list = [1; 2; 3; 4; 5; 6; 7; 8; 9]
If a parameter is specified as optional (label prefixed by ?) in the type of expr, the corresponding
argument will be automatically wrapped with the constructor Some, except if the argument itself is
also prefixed by ?, in which case it is passed as is.
# let fullname ?title first second =
match title with
| Some t -> t ^ " " ^ first ^ " " ^ second
| None -> first ^ " " ^ second

let name = fullname ~title:"Mrs" "Jane" "Fisher"

Chapter 11. The OCaml language 175

let address ?title first second town =

fullname ?title first second ^ "\n" ^ town;;
val fullname : ?title:string -> string -> string -> string = <fun>
val name : string = "Mrs Jane Fisher"
val address : ?title:string -> string -> string -> string -> string = <fun>
If a non-labeled argument is passed, and its corresponding parameter is preceded by one or
several optional parameters, then these parameters are defaulted, i.e. the value None will be passed
for them. All other missing parameters (without corresponding argument), both optional and
non-optional, will be kept, and the result of the function will still be a function of these missing
parameters to the body of f .
# let fullname ?title first second =
match title with
| Some t -> t ^ " " ^ first ^ " " ^ second
| None -> first ^ " " ^ second

let name = fullname "Jane" "Fisher";;

val fullname : ?title:string -> string -> string -> string = <fun>
val name : string = "Jane Fisher"
In all cases but exact match of order and labels, without optional parameters, the function
type should be known at the application point. This can be ensured by adding a type constraint.
Principality of the derivation can be checked in the -principal mode.
As a special case, OCaml supports labels-omitted full applications: if the function has a
known arity, all the arguments are unlabeled, and their number matches the number of non-optional
parameters, then labels are ignored and non-optional parameters are matched in their definition
order. Optional arguments are defaulted. This omission of labels is discouraged and results in a
warning, see 13.5.1.

Function definition
Two syntactic forms are provided to define functions. The first form is introduced by the keyword
function:

function pattern1 -> expr1

| ...
| patternn -> exprn
This expression evaluates to a functional value with one argument. When this function is applied to
a value v, this value is matched against each pattern pattern1 to patternn . If one of these matchings
succeeds, that is, if the value v matches the pattern patterni for some i, then the expression expr i
associated to the selected pattern is evaluated, and its value becomes the value of the function
application. The evaluation of expr i takes place in an environment enriched by the bindings
performed during the matching.
If several patterns match the argument v, the one that occurs first in the function definition is
selected. If none of the patterns matches the argument, the exception Match_failure is raised.
176

# (function (0, 0) -> "both zero"

| (0, _) -> "first only zero"
| (_, 0) -> "second only zero"
| (_, _) -> "neither zero")
(7, 0);;
- : string = "second only zero"
The other form of function definition is introduced by the keyword fun:
fun parameter 1 . . . parameter n -> expr
This expression is equivalent to:
fun parameter 1 -> . . . fun parameter n -> expr

# let f = (fun a -> fun b -> fun c -> a + b + c)

let g = (fun a b c -> a + b + c);;
val f : int -> int -> int -> int = <fun>
val g : int -> int -> int -> int = <fun>
An optional type constraint typexpr can be added before -> to enforce the type of the result to
be compatible with the constraint typexpr:
fun parameter 1 . . . parameter n : typexpr -> expr
is equivalent to
fun parameter 1 -> . . . fun parameter n -> (expr : typexpr)
Beware of the small syntactic difference between a type constraint on the last parameter
fun parameter 1 . . . (parameter n : typexpr) -> expr
and one on the result
fun parameter 1 . . . parameter n : typexpr -> expr

# let eq = fun (a : int) (b : int) -> a = b

let eq2 = fun a b : bool -> a = b
let eq3 = fun (a : int) (b : int) : bool -> a = b;;
val eq : int -> int -> bool = <fun>
val eq2 : 'a -> 'a -> bool = <fun>
val eq3 : int -> int -> bool = <fun>
The parameter patterns ~ lab and ~( lab [: typ] ) are shorthands for respectively ~ lab : lab
and ~ lab :( lab [: typ] ), and similarly for their optional counterparts.
# let bool_map ~cmp:(cmp : int -> int -> bool) l =
List.map cmp l

let bool_map' ~(cmp : int -> int -> bool) l =

List.map cmp l;;
Chapter 11. The OCaml language 177

val bool_map : cmp:(int -> int -> bool) -> int list -> (int -> bool) list =
<fun>
val bool_map' : cmp:(int -> int -> bool) -> int list -> (int -> bool) list =
<fun>
A function of the form fun ? lab :( pattern = expr 0 ) -> expr is equivalent to

fun ? lab : ident -> let pattern = match ident with Some ident -> ident | None -> expr 0 in expr

where ident is a fresh variable, except that it is unspecified when expr 0 is evaluated.
# let open_file_for_input ?binary filename =
match binary with
| Some true -> open_in_bin filename
| Some false | None -> open_in filename

let open_file_for_input' ?(binary=false) filename =

if binary then open_in_bin filename else open_in filename;;
val open_file_for_input : ?binary:bool -> string -> in_channel = <fun>
val open_file_for_input' : ?binary:bool -> string -> in_channel = <fun>
After these two transformations, expressions are of the form

fun [label 1 ] pattern1 -> . . . fun [label n ] patternn -> expr

If we ignore labels, which will only be meaningful at function application, this is equivalent to

function pattern1 -> . . . function patternn -> expr

That is, the fun expression above evaluates to a curried function with n arguments: after applying
this function n times to the values v 1 . . . v n , the values will be matched in parallel against the
patterns pattern1 . . . patternn . If the matching succeeds, the function returns the value of expr in
an environment enriched by the bindings performed during the matchings. If the matching fails, the
exception Match_failure is raised.

Guards in pattern-matchings
The cases of a pattern matching (in the function, match and try constructs) can include guard
expressions, which are arbitrary boolean expressions that must evaluate to true for the match case
to be selected. Guards occur just before the -> token and are introduced by the when keyword:

function pattern1 [when cond1 ] -> expr 1

| ...
| patternn [when condn ] -> expr n
Matching proceeds as described before, except that if the value matches some pattern patterni
which has a guard condi , then the expression condi is evaluated (in an environment enriched by
the bindings performed during matching). If condi evaluates to true, then expr i is evaluated and
its value returned as the result of the matching, as usual. But if condi evaluates to false, the
matching is resumed against the patterns following patterni .
178

# let rec repeat f = function

| 0 -> ()
| n when n > 0 -> f (); repeat f (n - 1)
| _ -> raise (Invalid_argument "repeat");;
val repeat : (unit -> 'a) -> int -> unit = <fun>

Local definitions
The let and let rec constructs bind value names locally. The construct

let pattern1 = expr 1 and . . . and patternn = expr n in expr

evaluates expr 1 . . . expr n in some unspecified order and matches their values against the patterns
pattern1 . . . patternn . If the matchings succeed, expr is evaluated in the environment enriched by
the bindings performed during matching, and the value of expr is returned as the value of the whole
let expression. If one of the matchings fails, the exception Match_failure is raised.
# let v =
let x = 1 in [x; x; x]

let v' =
let a, b = (1, 2) in a + b

let v'' =
let a = 1 and b = 2 in a + b;;
val v : int list = [1; 1; 1]
val v' : int = 3
val v'' : int = 3
An alternate syntax is provided to bind variables to functional values: instead of writing

let ident = fun parameter 1 . . . parameter m -> expr

in a let expression, one may instead write

let ident parameter 1 . . . parameter m = expr

# let f = fun x -> fun y -> fun z -> x + y + z

let f' = fun x y z -> x + y + z

let f'' x y z = x + y + z;;

val f : int -> int -> int -> int = <fun>
val f' : int -> int -> int -> int = <fun>
val f'' : int -> int -> int -> int = <fun>
Recursive definitions of names are introduced by let rec:

let rec pattern1 = expr 1 and . . . and patternn = expr n in expr

Chapter 11. The OCaml language 179

The only difference with the let construct described above is that the bindings of names to
values performed by the pattern-matching are considered already performed when the expressions
expr 1 to expr n are evaluated. That is, the expressions expr 1 to expr n can reference identifiers that
are bound by one of the patterns pattern1 , . . . , patternn , and expect them to have the same value
as in expr, the body of the let rec construct.
# let rec even =
function 0 -> true | n -> odd (n - 1)
and odd =
function 0 -> false | n -> even (n - 1)
in
even 1000;;
- : bool = true
The recursive definition is guaranteed to behave as described above if the expressions expr 1 to
expr n are function definitions (fun . . . or function . . .), and the patterns pattern1 . . . patternn are
just value names, as in:
let rec name 1 = fun . . . and . . . and name n = fun . . . in expr
This defines name 1 . . . name n as mutually recursive functions local to expr.
The behavior of other forms of let rec definitions is implementation-dependent. The current
implementation also supports a certain class of recursive definitions of non-functional values, as
explained in section 12.1.

11.7.3 Local exceptions

(Introduced in OCaml 4.04)
It is possible to define local exceptions in expressions: let exception constr-decl in expr .
# let map_empty_on_negative f l =
let exception Negative in
let aux x = if x < 0 then raise Negative else f x in
try List.map aux l with Negative -> [];;
val map_empty_on_negative : (int -> 'a) -> int list -> 'a list = <fun>
The syntactic scope of the exception constructor is the inner expression, but nothing prevents
exception values created with this constructor from escaping this scope. Two executions of the
definition above result in two incompatible exception constructors (as for any exception definition).
For instance:
# let gen () = let exception A in A

let () = assert(gen () = gen ());;

Exception: Assert_failure ("expr.etex", 3, 9).

11.7.4 Explicit polymorphic type annotations

(Introduced in OCaml 3.12)
Polymorphic type annotations in let-definitions behave in a way similar to polymorphic methods:
180

let pattern1 : typ 1 . . . typ n . typexpr = expr

These annotations explicitly require the defined value to be polymorphic, and allow one to use
this polymorphism in recursive occurrences (when using let rec). Note however that this is a
normal polymorphic type, unifiable with any instance of itself.

11.7.5 Control structures

Sequence
The expression expr 1 ; expr 2 evaluates expr 1 first, then expr 2 , and returns the value of expr 2 .
# let print_pair (a, b) =
print_string "(";
print_string (string_of_int a);
print_string ",";
print_string (string_of_int b);
print_endline ")";;
val print_pair : int * int -> unit = <fun>

Conditional
The expression if expr 1 then expr 2 else expr 3 evaluates to the value of expr 2 if expr 1 evaluates
to the boolean true, and to the value of expr 3 if expr 1 evaluates to the boolean false.
# let rec factorial x =
if x <= 1 then 1 else x * factorial (x - 1);;
val factorial : int -> int = <fun>
The else expr 3 part can be omitted, in which case it defaults to else ().
# let debug = ref false

let log msg =

if !debug then prerr_endline msg;;
val debug : bool ref = {contents = false}
val log : string -> unit = <fun>

Case expression
The expression
match expr
with pattern1 -> expr1
| ...
| patternn -> exprn
matches the value of expr against the patterns pattern1 to patternn . If the matching against
patterni succeeds, the associated expression expr i is evaluated, and its value becomes the value of
the whole match expression. The evaluation of expr i takes place in an environment enriched by
Chapter 11. The OCaml language 181

the bindings performed during matching. If several patterns match the value of expr, the one that
occurs first in the match expression is selected.
# let rec sum l =
match l with
| [] -> 0
| h :: t -> h + sum t;;
val sum : int list -> int = <fun>
If none of the patterns match the value of expr, the exception Match_failure is raised.
# let unoption o =
match o with
| Some x -> x ;;
Warning 8 [partial -match ]: this pattern - matching is not exhaustive .
Here is an example of a case that is not matched :
None

val unoption : 'a option -> 'a = <fun >

# let l = List.map unoption [Some 1; Some 10; None; Some 2];;

Exception: Match_failure ("expr.etex", 2, 2).

Boolean operators
The expression expr 1 && expr 2 evaluates to true if both expr 1 and expr 2 evaluate to true; otherwise,
it evaluates to false. The first component, expr 1 , is evaluated first. The second component, expr 2 ,
is not evaluated if the first component evaluates to false. Hence, the expression expr 1 && expr 2
behaves exactly as

if expr 1 then expr 2 else false.

The expression expr 1 || expr 2 evaluates to true if one of the expressions expr 1 and expr 2
evaluates to true; otherwise, it evaluates to false. The first component, expr 1 , is evaluated first.
The second component, expr 2 , is not evaluated if the first component evaluates to true. Hence, the
expression expr 1 || expr 2 behaves exactly as

if expr 1 then true else expr 2 .

The boolean operators & and or are deprecated synonyms for (respectively) && and ||.
# let xor a b =
(a || b) && not (a && b);;
val xor : bool -> bool -> bool = <fun>
182

Loops
The expression while expr 1 do expr 2 done repeatedly evaluates expr 2 while expr 1 evaluates to
true. The loop condition expr 1 is evaluated and tested at the beginning of each iteration. The
whole while . . . done expression evaluates to the unit value ().
# let chars_of_string s =
let i = ref 0 in
let chars = ref [] in
while !i < String.length s do
chars := s.[!i] :: !chars;
i := !i + 1
done;
List.rev !chars;;
val chars_of_string : string -> char list = <fun>
As a special case, while true do expr done is given a polymorphic type, allowing it to be used
in place of any expression (for example as a branch of any pattern-matching).
The expression for name = expr 1 to expr 2 do expr 3 done first evaluates the expressions expr 1
and expr 2 (the boundaries) into integer values n and p. Then, the loop body expr 3 is repeatedly
evaluated in an environment where name is successively bound to the values n, n + 1, …, p − 1, p.
The loop body is never evaluated if n > p.
# let chars_of_string s =
let l = ref [] in
for p = 0 to String.length s - 1 do
l := s.[p] :: !l
done;
List.rev !l;;
val chars_of_string : string -> char list = <fun>
The expression for name = expr 1 downto expr 2 do expr 3 done evaluates similarly, except that
name is successively bound to the values n, n − 1, …, p + 1, p. The loop body is never evaluated if
n < p.
# let chars_of_string s =
let l = ref [] in
for p = String.length s - 1 downto 0 do
l := s.[p] :: !l
done;
!l;;
val chars_of_string : string -> char list = <fun>
In both cases, the whole for expression evaluates to the unit value ().
Chapter 11. The OCaml language 183

Exception handling
The expression
try expr
with pattern1 -> expr1
| ...
| patternn -> exprn
evaluates the expression expr and returns its value if the evaluation of expr does not raise any
exception. If the evaluation of expr raises an exception, the exception value is matched against the
patterns pattern1 to patternn . If the matching against patterni succeeds, the associated expression
expr i is evaluated, and its value becomes the value of the whole try expression. The evaluation of
expr i takes place in an environment enriched by the bindings performed during matching. If several
patterns match the value of expr, the one that occurs first in the try expression is selected. If none
of the patterns matches the value of expr, the exception value is raised again, thereby transparently
“passing through” the try construct.
# let find_opt p l =
try Some (List.find p l) with Not_found -> None;;
val find_opt : ('a -> bool) -> 'a list -> 'a option = <fun>

11.7.6 Operations on data structures

Products
The expression expr 1 , . . . , expr n evaluates to the n-tuple of the values of expressions expr 1 to
expr n . The evaluation order of the subexpressions is not specified.
# (1 + 2 * 3, (1 + 2) * 3, 1 + (2 * 3));;
- : int * int * int = (7, 9, 7)

Variants
The expression constr expr evaluates to the unary variant value whose constructor is constr, and
whose argument is the value of expr. Similarly, the expression constr ( expr 1 , . . . , expr n )
evaluates to the n-ary variant value whose constructor is constr and whose arguments are the values
of expr 1 , . . . , expr n .
The expression constr ( expr 1 , . . . , expr n ) evaluates to the variant value whose constructor is
constr, and whose arguments are the values of expr 1 . . . expr n .
# type t = Var of string | Not of t | And of t * t | Or of t * t

let test = And (Var "x", Not (Or (Var "y", Var "z")));;
type t = Var of string | Not of t | And of t * t | Or of t * t
val test : t = And (Var "x", Not (Or (Var "y", Var "z")))
For lists, some syntactic sugar is provided. The expression expr 1 :: expr 2 stands for the
constructor ( :: ) applied to the arguments ( expr 1 , expr 2 ), and therefore evaluates to
the list whose head is the value of expr 1 and whose tail is the value of expr 2 . The expression
184

[ expr 1 ; . . . ; expr n ] is equivalent to expr 1 :: . . . :: expr n :: [], and therefore evaluates to the
list whose elements are the values of expr 1 to expr n .
# 0 :: [1; 2; 3] = 0 :: 1 :: 2 :: 3 :: [];;
- : bool = true

Polymorphic variants
The expression ` tag-name expr evaluates to the polymorphic variant value whose tag is tag-name,
and whose argument is the value of expr.
# let with_counter x = `V (x, ref 0);;
val with_counter : 'a -> [> `V of 'a * int ref ] = <fun>

Records
The expression { field1 [= expr 1 ] ; . . . ; fieldn [= expr n ]} evaluates to the record value {f ield1 =
v1 ; . . . ; f ieldn = vn } where vi is the value of expr i for i = 1, . . . , n. A single identifier fieldk stands for
fieldk = fieldk , and a qualified identifier module-path . fieldk stands for module-path . fieldk = fieldk .
The fields field1 to fieldn must all belong to the same record type; each field of this record type
must appear exactly once in the record expression, though they can appear in any order. The order
in which expr 1 to expr n are evaluated is not specified. Optional type constraints can be added after
each field { field1 : typexpr 1 = expr 1 ; . . . ; fieldn : typexpr n = expr n } to force the type of fieldk
to be compatible with typexpr k .
# type t = {house_no : int; street : string; town : string; postcode : string}

let address x =
Printf.sprintf "The occupier\n%i %s\n%s\n%s"
x.house_no x.street x.town x.postcode;;
type t = {
house_no : int;
street : string;
town : string;
postcode : string;
}
val address : t -> string = <fun>
The expression { expr with field1 [= expr 1 ] ; . . . ; fieldn [= expr n ] } builds a fresh record
with fields field1 . . . fieldn equal to expr 1 . . . expr n , and all other fields having the same value
as in the record expr. In other terms, it returns a shallow copy of the record expr, except
for the fields field1 . . . fieldn , which are initialized to expr 1 . . . expr n . As previously, single
identifier fieldk stands for fieldk = fieldk , a qualified identifier module-path . fieldk stands for
module-path . fieldk = fieldk and it is possible to add an optional type constraint on each field
being updated with { expr with field1 : typexpr 1 = expr 1 ; . . . ; fieldn : typexpr n = expr n }.
# type t = {house_no : int; street : string; town : string; postcode : string}
Chapter 11. The OCaml language 185

let uppercase_town address =

{address with town = String.uppercase_ascii address.town};;
type t = {
house_no : int;
street : string;
town : string;
postcode : string;
}
val uppercase_town : t -> t = <fun>
The expression expr 1 . field evaluates expr 1 to a record value, and returns the value associated
to field in this record value.
The expression expr 1 . field <- expr 2 evaluates expr 1 to a record value, which is then modified
in-place by replacing the value associated to field in this record by the value of expr 2 . This operation
is permitted only if field has been declared mutable in the definition of the record type. The whole
expression expr 1 . field <- expr 2 evaluates to the unit value ().
# type t = {mutable upper : int; mutable lower : int; mutable other : int}

let stats = {upper = 0; lower = 0; other = 0}

let collect =
String.iter
(function
| 'A'..'Z' -> stats.upper <- stats.upper + 1
| 'a'..'z' -> stats.lower <- stats.lower + 1
| _ -> stats.other <- stats.other + 1);;
type t = { mutable upper : int; mutable lower : int; mutable other : int; }
val stats : t = {upper = 0; lower = 0; other = 0}
val collect : string -> unit = <fun>

Arrays
The expression [| expr 1 ; . . . ; expr n |] evaluates to a n-element array, whose elements are
initialized with the values of expr 1 to expr n respectively. The order in which these expressions are
evaluated is unspecified.
The expression expr 1 .( expr 2 ) returns the value of element number expr 2 in the array denoted
by expr 1 . The first element has number 0; the last element has number n − 1, where n is the size of
the array. The exception Invalid_argument is raised if the access is out of bounds.
The expression expr 1 .( expr 2 ) <- expr 3 modifies in-place the array denoted by expr 1 , replacing
element number expr 2 by the value of expr 3 . The exception Invalid_argument is raised if the
access is out of bounds. The value of the whole expression is ().
# let scale arr n =
for x = 0 to Array.length arr - 1 do
arr.(x) <- arr.(x) * n
done
186

let x = [|1; 10; 100|]

let _ = scale x 2;;
val scale : int array -> int -> unit = <fun>
val x : int array = [|2; 20; 200|]

Strings
The expression expr 1 .[ expr 2 ] returns the value of character number expr 2 in the string denoted
by expr 1 . The first character has number 0; the last character has number n − 1, where n is the
length of the string. The exception Invalid_argument is raised if the access is out of bounds.
# let iter f s =
for x = 0 to String.length s - 1 do f s.[x] done;;
val iter : (char -> 'a) -> string -> unit = <fun>
The expression expr 1 .[ expr 2 ] <- expr 3 modifies in-place the string denoted by expr 1 ,
replacing character number expr 2 by the value of expr 3 . The exception Invalid_argument is raised
if the access is out of bounds. The value of the whole expression is (). Note: this possibility is
offered only for backward compatibility with older versions of OCaml and will be removed in a
future version. New code should use byte sequences and the Bytes.set function.

11.7.7 Operators
Symbols from the class infix-symbol, as well as the keywords *, +, -, -., =, !=, <, >, or, ||, &, &&,
:=, mod, land, lor, lxor, lsl, lsr, and asr can appear in infix position (between two expressions).
Symbols from the class prefix-symbol, as well as the keywords - and -. can appear in prefix position
(in front of an expression).
# (( * ), ( := ), ( || ));;
- : (int -> int -> int) * ('a ref -> 'a -> unit) * (bool -> bool -> bool) =
(<fun>, <fun>, <fun>)
Infix and prefix symbols do not have a fixed meaning: they are simply interpreted as
applications of functions bound to the names corresponding to the symbols. The expression
prefix-symbol expr is interpreted as the application ( prefix-symbol ) expr. Similarly, the
expression expr 1 infix-symbol expr 2 is interpreted as the application ( infix-symbol ) expr 1 expr 2 .
The table below lists the symbols defined in the initial environment and their initial meaning.
(See the description of the core library module Stdlib in chapter 28 for more details). Their meaning
may be changed at any time using let ( infix-op ) name 1 name 2 = . . .
# let ( + ), ( - ), ( * ), ( / ) = Int64.(add, sub, mul, div);;
val ( + ) : int64 -> int64 -> int64 = <fun>
val ( - ) : int64 -> int64 -> int64 = <fun>
val ( * ) : int64 -> int64 -> int64 = <fun>
val ( / ) : int64 -> int64 -> int64 = <fun>
Note: the operators &&, ||, and ~- are handled specially and it is not advisable to change their
meaning.
The keywords - and -. can appear both as infix and prefix operators. When they appear as
prefix operators, they are interpreted respectively as the functions (~-) and (~-.).
Chapter 11. The OCaml language 187

Operator Initial meaning

+ Integer addition.
- (infix) Integer subtraction.
~- - (prefix) Integer negation.
* Integer multiplication.
/ Integer division. Raise Division_by_zero if second argument is zero.
mod Integer modulus. Raise Division_by_zero if second argument is zero.
land Bitwise logical “and” on integers.
lor Bitwise logical “or” on integers.
lxor Bitwise logical “exclusive or” on integers.
lsl Bitwise logical shift left on integers.
lsr Bitwise logical shift right on integers.
asr Bitwise arithmetic shift right on integers.
+. Floating-point addition.
-. (infix) Floating-point subtraction.
~-. -. (prefix) Floating-point negation.
*. Floating-point multiplication.
/. Floating-point division.
** Floating-point exponentiation.
@ List concatenation.
^ String concatenation.
! Dereferencing (return the current contents of a reference).
:= Reference assignment (update the reference given as first argument with
the value of the second argument).
= Structural equality test.
<> Structural inequality test.
== Physical equality test.
!= Physical inequality test.
< Test “less than”.
<= Test “less than or equal”.
> Test “greater than”.
>= Test “greater than or equal”.
&& & Boolean conjunction.
|| or Boolean disjunction.

11.7.8 Objects
Object creation
When class-path evaluates to a class body, new class-path evaluates to a new object containing the
instance variables and methods of this class.
# class of_list (lst : int list) = object
val mutable l = lst
method next =
match l with
188

| [] -> raise (Failure "empty list");

| h::t -> l <- t; h
end

let a = new of_list [1; 1; 2; 3; 5; 8; 13]

let b = new of_list;;

class of_list :
int list -> object val mutable l : int list method next : int end
val a : of_list = <obj>
val b : int list -> of_list = <fun>
When class-path evaluates to a class function, new class-path evaluates to a function expecting
the same number of arguments and returning a new object of this class.

Immediate object creation

Creating directly an object through the object class-body end construct is operationally equivalent
to defining locally a class class-name = object class-body end —see sections 11.9.2 and following
for the syntax of class-body— and immediately creating a single object from it by new class-name.
# let o =
object
val secret = 99
val password = "unlock"
method get guess = if guess <> password then None else Some secret
end;;
val o : < get : string -> int option > = <obj>
The typing of immediate objects is slightly different from explicitly defining a class in two
respects. First, the inferred object type may contain free type variables. Second, since the class
body of an immediate object will never be extended, its self type can be unified with a closed object
type.

Method invocation
The expression expr # method-name invokes the method method-name of the object denoted by
expr.
# class of_list (lst : int list) = object
val mutable l = lst
method next =
match l with
| [] -> raise (Failure "empty list");
| h::t -> l <- t; h
end

let a = new of_list [1; 1; 2; 3; 5; 8; 13]

Chapter 11. The OCaml language 189

let third = ignore a#next; ignore a#next; a#next;;

class of_list :
int list -> object val mutable l : int list method next : int end
val a : of_list = <obj>
val third : int = 2
If method-name is a polymorphic method, its type should be known at the invocation site. This
is true for instance if expr is the name of a fresh object (let ident = new class-path . . .) or if there
is a type constraint. Principality of the derivation can be checked in the -principal mode.

Accessing and modifying instance variables

The instance variables of a class are visible only in the body of the methods defined in the same class
or a class that inherits from the class defining the instance variables. The expression inst-var-name
evaluates to the value of the given instance variable. The expression inst-var-name <- expr assigns
the value of expr to the instance variable inst-var-name, which must be mutable. The whole
expression inst-var-name <- expr evaluates to ().
# class of_list (lst : int list) = object
val mutable l = lst
method next =
match l with (∗ access instance variable ∗)
| [] -> raise (Failure "empty list");
| h::t -> l <- t; h (∗ modify instance variable ∗)
end;;
class of_list :
int list -> object val mutable l : int list method next : int end

Object duplication
An object can be duplicated using the library function Oo.copy (see module Oo[29.39]). Inside a
method, the expression {< [inst-var-name [= expr] {; inst-var-name [= expr]}] >} returns a copy of
self with the given instance variables replaced by the values of the associated expressions. A single
instance variable name id stands for id = id. Other instance variables have the same value in the
returned object as in self.
# let o =
object
val secret = 99
val password = "unlock"
method get guess = if guess <> password then None else Some secret
method with_new_secret s = {< secret = s >}
end;;
val o : < get : string -> int option; with_new_secret : int -> 'a > as 'a =
<obj>
190

11.7.9 Coercions
Expressions whose type contains object or polymorphic variant types can be explicitly coerced
(weakened) to a supertype. The expression ( expr :> typexpr ) coerces the expression expr to type
typexpr. The expression ( expr : typexpr 1 :> typexpr 2 ) coerces the expression expr from type
typexpr 1 to type typexpr 2 .
The former operator will sometimes fail to coerce an expression expr from a type typ 1 to a type
typ 2 even if type typ 1 is a subtype of type typ 2 : in the current implementation it only expands two
levels of type abbreviations containing objects and/or polymorphic variants, keeping only recursion
when it is explicit in the class type (for objects). As an exception to the above algorithm, if both the
inferred type of expr and typ are ground (i.e. do not contain type variables), the former operator
behaves as the latter one, taking the inferred type of expr as typ 1 . In case of failure with the former
operator, the latter one should be used.
It is only possible to coerce an expression expr from type typ 1 to type typ 2 , if the type of expr
is an instance of typ 1 (like for a type annotation), and typ 1 is a subtype of typ 2 . The type of the
coerced expression is an instance of typ 2 . If the types contain variables, they may be instantiated
by the subtyping algorithm, but this is only done after determining whether typ 1 is a potential
subtype of typ 2 . This means that typing may fail during this latter unification step, even if some
instance of typ 1 is a subtype of some instance of typ 2 . In the following paragraphs we describe the
subtyping relation used.

Object types
A fixed object type admits as subtype any object type that includes all its methods. The types of
the methods shall be subtypes of those in the supertype. Namely,

< met1 : typ 1 ; . . . ; metn : typ n >

is a supertype of

< met1 : typ 01 ; . . . ; metn : typ 0n ; metn+1 : typ 0n+1 ; . . . ; metn+m : typ 0n+m [; ..] >

which may contain an ellipsis .. if every typ i is a supertype of the corresponding typ 0i .
A monomorphic method type can be a supertype of a polymorphic method type. Namely, if typ
is an instance of typ 0 , then 'a1 . . . 'an . typ 0 is a subtype of typ.
Inside a class definition, newly defined types are not available for subtyping, as the type
abbreviations are not yet completely defined. There is an exception for coercing self to the (exact)
type of its class: this is allowed if the type of self does not appear in a contravariant position in the
class type, i.e. if there are no binary methods.

Polymorphic variant types

A polymorphic variant type typ is a subtype of another polymorphic variant type typ 0 if the upper
bound of typ (i.e. the maximum set of constructors that may appear in an instance of typ) is
included in the lower bound of typ 0 , and the types of arguments for the constructors of typ are
subtypes of those in typ 0 . Namely,

[ [<] ` C 1 of typ 1 | . . . | ` C n of typ n ]

Chapter 11. The OCaml language 191

which may be a shrinkable type, is a subtype of

[ [>] ` C 1 of typ 01 | . . . | ` C n of typ 0n | ` C n+1 of typ 0n+1 | . . . | ` C n+m of typ 0n+m ]

which may be an extensible type, if every typ i is a subtype of typ 0i .

Variance
Other types do not introduce new subtyping, but they may propagate the subtyping of their
arguments. For instance, typ 1 * typ 2 is a subtype of typ 01 * typ 02 when typ 1 and typ 2 are respectively
subtypes of typ 01 and typ 02 . For function types, the relation is more subtle: typ 1 -> typ 2 is a subtype
of typ 01 -> typ 02 if typ 1 is a supertype of typ 01 and typ 2 is a subtype of typ 02 . For this reason, function
types are covariant in their second argument (like tuples), but contravariant in their first argument.
Mutable types, like array or ref are neither covariant nor contravariant, they are nonvariant, that
is they do not propagate subtyping.
For user-defined types, the variance is automatically inferred: a parameter is covariant if it has
only covariant occurrences, contravariant if it has only contravariant occurrences, variance-free if it
has no occurrences, and nonvariant otherwise. A variance-free parameter may change freely through
subtyping, it does not have to be a subtype or a supertype. For abstract and private types, the
variance must be given explicitly (see section 11.8.1), otherwise the default is nonvariant. This is
also the case for constrained arguments in type definitions.

11.7.10 Other
Assertion checking
OCaml supports the assert construct to check debugging assertions. The expression assert expr
evaluates the expression expr and returns () if expr evaluates to true. If it evaluates to false the
exception Assert_failure is raised with the source file name and the location of expr as arguments.
Assertion checking can be turned off with the -noassert compiler option. In this case, expr is not
evaluated at all.
# let f a b c =
assert (a <= b && b <= c);
(b -. a) /. (c -. b);;
val f : float -> float -> float -> float = <fun>
As a special case, assert false is reduced to raise (Assert_failure ...), which gives it
a polymorphic type. This means that it can be used in place of any expression (for example as
a branch of any pattern-matching). It also means that the assert false “assertions” cannot be
turned off by the -noassert option.
# let min_known_nonempty = function
| [] -> assert false
| l -> List.hd (List.sort compare l);;
val min_known_nonempty : 'a list -> 'a = <fun>
192

Lazy expressions
The expression lazy expr returns a value v of type Lazy.t that encapsulates the computation of
expr. The argument expr is not evaluated at this point in the program. Instead, its evaluation will
be performed the first time the function Lazy.force is applied to the value v, returning the actual
value of expr. Subsequent applications of Lazy.force to v do not evaluate expr again. Applications
of Lazy.force may be implicit through pattern matching (see 11.6.1).
# let lazy_greeter = lazy (print_string "Hello, World!\n");;
val lazy_greeter : unit lazy_t = <lazy>

# Lazy.force lazy_greeter;;
Hello, World!
- : unit = ()

Local modules
The expression let module module-name = module-expr in expr locally binds the module expres-
sion module-expr to the identifier module-name during the evaluation of the expression expr. It
then returns the value of expr. For example:
# let remove_duplicates comparison_fun string_list =
let module StringSet =
Set.Make(struct type t = string
let compare = comparison_fun end)
in
StringSet.elements
(List.fold_right StringSet.add string_list StringSet.empty);;
val remove_duplicates :
(string -> string -> int) -> string list -> string list = <fun>

Local opens
The expressions let open module-path in expr and module-path .( expr ) are strictly equivalent.
These constructions locally open the module referred to by the module path module-path in the
respective scope of the expression expr.
# let map_3d_matrix f m =
let open Array in
map (map (map f)) m

let map_3d_matrix' f =
Array.(map (map (map f)));;
val map_3d_matrix :
('a -> 'b) -> 'a array array array -> 'b array array array = <fun>
val map_3d_matrix' :
('a -> 'b) -> 'a array array array -> 'b array array array = <fun>
Chapter 11. The OCaml language 193

When the body of a local open expression is delimited by [ ], [| |], or { }, the parenthe-
ses can be omitted. For expression, parentheses can also be omitted for {< >}. For example,
module-path .[ expr ] is equivalent to module-path .([ expr ]), and module-path .[| expr |] is
equivalent to module-path .([| expr |]).
# let vector = Random.[|int 255; int 255; int 255; int 255|];;
val vector : int array = [|220; 90; 247; 144|]

11.8 Type and exception definitions

11.8.1 Type definitions
Type definitions bind type constructors to data types: either variant types, record types, type
abbreviations, or abstract data types. They also bind the value constructors and record fields
associated with the definition.

type-definition ::= type [nonrec] typedef {and typedef }

typedef ::= [type-params] typeconstr-name type-information
type-information ::= [type-equation] [type-representation] {type-constraint}
type-equation ::= = typexpr
type-representation ::= = [|] constr-decl {| constr-decl}
| = record-decl
| =|
type-params ::= type-param
| ( type-param {, type-param} )
type-param ::= [ext-variance] ' ident
ext-variance ::= variance [injectivity]
| injectivity [variance]
variance ::= +
| -
injectivity ::= !
record-decl ::= { field-decl {; field-decl} [;] }
constr-decl ::= (constr-name | [] | (::)) [of constr-args]
constr-args ::= typexpr {* typexpr}
field-decl ::= [mutable] field-name : poly-typexpr
type-constraint ::= constraint typexpr = typexpr

See also the following language extensions: private types, generalized algebraic datatypes, attributes,
extension nodes, extensible variant types and inline records.
194

Type definitions are introduced by the type keyword, and consist in one or several simple
definitions, possibly mutually recursive, separated by the and keyword. Each simple definition
defines one type constructor.
A simple definition consists in a lowercase identifier, possibly preceded by one or several type
parameters, and followed by an optional type equation, then an optional type representation, and
then a constraint clause. The identifier is the name of the type constructor being defined.

type colour =
| Red | Green | Blue | Yellow | Black | White
| RGB of {r : int; g : int; b : int}

type 'a tree = Lf | Br of 'a * 'a tree * 'a;;

type t = {decoration : string; substance : t'}

and t' = Int of int | List of t list

In the right-hand side of type definitions, references to one of the type constructor name being
defined are considered as recursive, unless type is followed by nonrec. The nonrec keyword was
introduced in OCaml 4.02.2.
The optional type parameters are either one type variable ' ident, for type constructors with one
parameter, or a list of type variables (' ident1 , . . . , ' identn ), for type constructors with several
parameters. Each type parameter may be prefixed by a variance constraint + (resp. -) indicating
that the parameter is covariant (resp. contravariant), and an injectivity annotation ! indicating that
the parameter can be deduced from the whole type. These type parameters can appear in the type
expressions of the right-hand side of the definition, optionally restricted by a variance constraint ;
i.e. a covariant parameter may only appear on the right side of a functional arrow (more precisely,
follow the left branch of an even number of arrows), and a contravariant parameter only the left
side (left branch of an odd number of arrows). If the type has a representation or an equation,
and the parameter is free (i.e. not bound via a type constraint to a constructed type), its variance
constraint is checked but subtyping etc. will use the inferred variance of the parameter, which may
be less restrictive; otherwise (i.e. for abstract types or non-free parameters), the variance must be
given explicitly, and the parameter is invariant if no variance is given.
The optional type equation = typexpr makes the defined type equivalent to the type expression
typexpr: one can be substituted for the other during typing. If no type equation is given, a new
type is generated: the defined type is incompatible with any other type.
The optional type representation describes the data structure representing the defined type, by
giving the list of associated constructors (if it is a variant type) or associated fields (if it is a record
type). If no type representation is given, nothing is assumed on the structure of the type besides
what is stated in the optional type equation.
The type representation = [|] constr-decl {| constr-decl} describes a variant type. The construc-
tor declarations constr-decl 1 , . . . , constr-decl n describe the constructors associated to this variant
type. The constructor declaration constr-name of typexpr 1 * . . . * typexpr n declares the name
constr-name as a non-constant constructor, whose arguments have types typexpr 1 …typexpr n . The
constructor declaration constr-name declares the name constr-name as a constant constructor.
Constructor names must be capitalized.
Chapter 11. The OCaml language 195

The type representation = { field-decl {; field-decl} [;] } describes a record type. The field
declarations field-decl 1 , . . . , field-decl n describe the fields associated to this record type. The field
declaration field-name : poly-typexpr declares field-name as a field whose argument has type
poly-typexpr. The field declaration mutable field-name : poly-typexpr behaves similarly; in
addition, it allows physical modification of this field. Immutable fields are covariant, mutable fields
are non-variant. Both mutable and immutable fields may have explicitly polymorphic types. The
polymorphism of the contents is statically checked whenever a record value is created or modified.
Extracted values may have their types instantiated.
The two components of a type definition, the optional equation and the optional representation,
can be combined independently, giving rise to four typical situations:

Abstract type: no equation, no representation.

When appearing in a module signature, this definition specifies nothing on the type constructor,
besides its number of parameters: its representation is hidden and it is assumed incompatible
with any other type.

Type abbreviation: an equation, no representation.

This defines the type constructor as an abbreviation for the type expression on the right of
the = sign.

New variant type or record type: no equation, a representation.

This generates a new type constructor and defines associated constructors or fields, through
which values of that type can be directly built or inspected.

Re-exported variant type or record type: an equation, a representation.

In this case, the type constructor is defined as an abbreviation for the type expression given
in the equation, but in addition the constructors or fields given in the representation remain
attached to the defined type constructor. The type expression in the equation part must agree
with the representation: it must be of the same kind (record or variant) and have exactly the
same constructors or fields, in the same order, with the same arguments. Moreover, the new
type constructor must have the same arity and the same type constraints as the original type
constructor.

The type variables appearing as type parameters can optionally be prefixed by + or - to indicate
that the type constructor is covariant or contravariant with respect to this parameter. This variance
information is used to decide subtyping relations when checking the validity of :> coercions (see
section 11.7.9).
For instance, type +'a t declares t as an abstract type that is covariant in its parameter;
this means that if the type τ is a subtype of the type σ, then τ t is a subtype of σ t. Similarly,
type -'a t declares that the abstract type t is contravariant in its parameter: if τ is a subtype of
σ, then σ t is a subtype of τ t. If no + or - variance annotation is given, the type constructor is
assumed non-variant in the corresponding parameter. For instance, the abstract type declaration
type 'a t means that τ t is neither a subtype nor a supertype of σ t if τ is subtype of σ.
The variance indicated by the + and - annotations on parameters is enforced only for abstract and
private types, or when there are type constraints. Otherwise, for abbreviations, variant and record
types without type constraints, the variance properties of the type constructor are inferred from its
definition, and the variance annotations are only checked for conformance with the definition.
196

Injectivity annotations are only necessary for abstract types and private row types, since they
can otherwise be deduced from the type declaration: all parameters are injective for record and
variant type declarations (including extensible types); for type abbreviations a parameter is injective
if it has an injective occurrence in its defining equation (be it private or not). For constrained type
parameters in type abbreviations, they are injective if either they appear at an injective position in
the body, or if all their type variables are injective; in particular, if a constrained type parameter
contains a variable that doesn’t appear in the body, it cannot be injective.
The construct constraint ' ident = typexpr allows the specification of type parameters. Any
actual type argument corresponding to the type parameter ident has to be an instance of typexpr
(more precisely, ident and typexpr are unified). Type variables of typexpr can appear in the type
equation and the type declaration.

11.8.2 Exception definitions

exception-definition ::= exception constr-decl
| exception constr-name = constr
Exception definitions add new constructors to the built-in variant type exn of exception values.
The constructors are declared as for a definition of a variant type.
# exception E of int * string;;
exception E of int * string
The form exception constr-decl generates a new exception, distinct from all other exceptions
in the system. The form exception constr-name = constr gives an alternate name to an existing
exception.
# exception E of int * string

exception F = E

let eq =
E (1, "one") = F (1, "one");;
exception E of int * string
exception F of int * string
val eq : bool = true

11.9 Classes
Classes are defined using a small language, similar to the module language.

11.9.1 Class types

Class types are the class-level equivalent of type expressions: they specify the general shape and
type properties of classes.
Chapter 11. The OCaml language 197

class-type ::= [[?] label-name :] typexpr -> class-type

| class-body-type
class-body-type ::= object [( typexpr )] {class-field-spec} end
| [[ typexpr {, typexpr} ]] classtype-path
| let open module-path in class-body-type
class-field-spec ::= inherit class-body-type
| val [mutable] [virtual] inst-var-name : typexpr
| val virtual mutable inst-var-name : typexpr
| method [private] [virtual] method-name : poly-typexpr
| method virtual private method-name : poly-typexpr
| constraint typexpr = typexpr

See also the following language extensions: attributes and extension nodes.

Simple class expressions

The expression classtype-path is equivalent to the class type bound to the name classtype-path.
Similarly, the expression [ typexpr 1 , . . . typexpr n ] classtype-path is equivalent to the parametric
class type bound to the name classtype-path, in which type parameters have been instantiated to
respectively typexpr 1 , …typexpr n .

Class function type

The class type expression typexpr -> class-type is the type of class functions (functions from values
to classes) that take as argument a value of type typexpr and return as result a class of type
class-type.

Class body type

The class type expression object [( typexpr )] {class-field-spec} end is the type of a class body. It
specifies its instance variables and methods. In this type, typexpr is matched against the self type,
therefore providing a name for the self type.
A class body will match a class body type if it provides definitions for all the components
specified in the class body type, and these definitions meet the type requirements given in the
class body type. Furthermore, all methods either virtual or public present in the class body must
also be present in the class body type (on the other hand, some instance variables and concrete
private methods may be omitted). A virtual method will match a concrete method, which makes
it possible to forget its implementation. An immutable instance variable will match a mutable
instance variable.

Local opens
Local opens are supported in class types since OCaml 4.06.
198

Inheritance
The inheritance construct inherit class-body-type provides for inclusion of methods and instance
variables from other class types. The instance variable and method types from class-body-type are
added into the current class type.

Instance variable specification

A specification of an instance variable is written val [mutable] [virtual] inst-var-name : typexpr,
where inst-var-name is the name of the instance variable and typexpr its expected type. The flag
mutable indicates whether this instance variable can be physically modified. The flag virtual
indicates that this instance variable is not initialized. It can be initialized later through inheritance.
An instance variable specification will hide any previous specification of an instance variable of
the same name.

Method specification
The specification of a method is written method [private] method-name : poly-typexpr, where
method-name is the name of the method and poly-typexpr its expected type, possibly polymorphic.
The flag private indicates that the method cannot be accessed from outside the object.
The polymorphism may be left implicit in public method specifications: any type variable which
is not bound to a class parameter and does not appear elsewhere inside the class specification will be
assumed to be universal, and made polymorphic in the resulting method type. Writing an explicit
polymorphic type will disable this behaviour.
If several specifications are present for the same method, they must have compatible types. Any
non-private specification of a method forces it to be public.

Virtual method specification

A virtual method specification is written method [private] virtual method-name : poly-typexpr,
where method-name is the name of the method and poly-typexpr its expected type.

Constraints on type parameters

The construct constraint typexpr 1 = typexpr 2 forces the two type expressions to be equal. This is
typically used to specify type parameters: in this way, they can be bound to specific type expressions.

11.9.2 Class expressions

Class expressions are the class-level equivalent of value expressions: they evaluate to classes, thus
providing implementations for the specifications expressed in class types.
Chapter 11. The OCaml language 199

class-expr ::= class-path

| [ typexpr {, typexpr} ] class-path
| ( class-expr )
| ( class-expr : class-type )
| class-expr {argument}+
| fun {parameter}+ -> class-expr
| let [rec] let-binding {and let-binding} in class-expr
| object class-body end
| let open module-path in class-expr

class-field ::= inherit class-expr [as lowercase-ident]

| inherit! class-expr [as lowercase-ident]
| val [mutable] inst-var-name [: typexpr] = expr
| val! [mutable] inst-var-name [: typexpr] = expr
| val [mutable] virtual inst-var-name : typexpr
| val virtual mutable inst-var-name : typexpr
| method [private] method-name {parameter} [: typexpr] = expr
| method! [private] method-name {parameter} [: typexpr] = expr
| method [private] method-name : poly-typexpr = expr
| method! [private] method-name : poly-typexpr = expr
| method [private] virtual method-name : poly-typexpr
| method virtual private method-name : poly-typexpr
| constraint typexpr = typexpr
| initializer expr
See also the following language extensions: locally abstract types, attributes and extension nodes.

Simple class expressions

The expression class-path evaluates to the class bound to the name class-path. Similarly, the
expression [ typexpr 1 , . . . typexpr n ] class-path evaluates to the parametric class bound to
the name class-path, in which type parameters have been instantiated respectively to typexpr 1 ,
…typexpr n .
The expression ( class-expr ) evaluates to the same module as class-expr.
The expression ( class-expr : class-type ) checks that class-type matches the type of class-expr
(that is, that the implementation class-expr meets the type specification class-type). The whole
expression evaluates to the same class as class-expr, except that all components not specified in
class-type are hidden and can no longer be accessed.

Class application
Class application is denoted by juxtaposition of (possibly labeled) expressions. It denotes the
class whose constructor is the first expression applied to the given arguments. The arguments are
evaluated as for expression application, but the constructor itself will only be evaluated when objects
200

are created. In particular, side-effects caused by the application of the constructor will only occur
at object creation time.

Class function
The expression fun [[?] label-name :] pattern -> class-expr evaluates to a function from values to
classes. When this function is applied to a value v, this value is matched against the pattern pattern
and the result is the result of the evaluation of class-expr in the extended environment.
Conversion from functions with default values to functions with patterns only works identically
for class functions as for normal functions.
The expression

fun parameter 1 . . . parameter n -> class-expr

is a short form for

fun parameter 1 -> . . . fun parameter n -> expr

Local definitions
The let and let rec constructs bind value names locally, as for the core language expressions.
If a local definition occurs at the very beginning of a class definition, it will be evaluated when
the class is created (just as if the definition was outside of the class). Otherwise, it will be evaluated
when the object constructor is called.

Local opens
Local opens are supported in class expressions since OCaml 4.06.

Class body
class-body ::= [( pattern [: typexpr] )] {class-field}
The expression object class-body end denotes a class body. This is the prototype for an object : it
lists the instance variables and methods of an object of this class.
A class body is a class value: it is not evaluated at once. Rather, its components are evaluated
each time an object is created.
In a class body, the pattern ( pattern [: typexpr] ) is matched against self, therefore providing
a binding for self and self type. Self can only be used in method and initializers.
Self type cannot be a closed object type, so that the class remains extensible.
Since OCaml 4.01, it is an error if the same method or instance variable name is defined several
times in the same class body.
Chapter 11. The OCaml language 201

Inheritance
The inheritance construct inherit class-expr allows reusing methods and instance variables from
other classes. The class expression class-expr must evaluate to a class body. The instance variables,
methods and initializers from this class body are added into the current class. The addition of a
method will override any previously defined method of the same name.
An ancestor can be bound by appending as lowercase-ident to the inheritance construct.
lowercase-ident is not a true variable and can only be used to select a method, i.e. in an ex-
pression lowercase-ident # method-name. This gives access to the method method-name as it was
defined in the parent class even if it is redefined in the current class. The scope of this ancestor
binding is limited to the current class. The ancestor method may be called from a subclass but only
indirectly.

Instance variable definition

The definition val [mutable] inst-var-name = expr adds an instance variable inst-var-name whose
initial value is the value of expression expr. The flag mutable allows physical modification of this
variable by methods.
An instance variable can only be used in the methods and initializers that follow its definition.
Since version 3.10, redefinitions of a visible instance variable with the same name do not create
a new variable, but are merged, using the last value for initialization. They must have identical
types and mutability. However, if an instance variable is hidden by omitting it from an interface, it
will be kept distinct from other instance variables with the same name.

Virtual instance variable definition

A variable specification is written val [mutable] virtual inst-var-name : typexpr. It specifies
whether the variable is modifiable, and gives its type.
Virtual instance variables were added in version 3.10.

Method definition
A method definition is written method method-name = expr. The definition of a method overrides
any previous definition of this method. The method will be public (that is, not private) if any of
the definition states so.
A private method, method private method-name = expr, is a method that can only be invoked
on self (from other methods of the same object, defined in this class or one of its subclasses). This
invocation is performed using the expression value-name # method-name, where value-name is
directly bound to self at the beginning of the class definition. Private methods do not appear in
object types. A method may have both public and private definitions, but as soon as there is a
public one, all subsequent definitions will be made public.
Methods may have an explicitly polymorphic type, allowing them to be used polymorphically in
programs (even for the same object). The explicit declaration may be done in one of three ways: (1)
by giving an explicit polymorphic type in the method definition, immediately after the method name,
i.e. method [private] method-name : {' ident}+ . typexpr = expr; (2) by a forward declaration
202

of the explicit polymorphic type through a virtual method definition; (3) by importing such a
declaration through inheritance and/or constraining the type of self.
Some special expressions are available in method bodies for manipulating instance variables and
duplicating self:
expr ::= . . .
| inst-var-name <- expr
| {< [inst-var-name = expr {; inst-var-name = expr} [;]] >}
The expression inst-var-name <- expr modifies in-place the current object by replacing the value
associated to inst-var-name by the value of expr. Of course, this instance variable must have been
declared mutable.
The expression {< inst-var-name 1 = expr 1 ; . . . ; inst-var-name n = expr n >} evaluates to a copy
of the current object in which the values of instance variables inst-var-name 1 , . . . , inst-var-name n
have been replaced by the values of the corresponding expressions expr 1 , . . . , expr n .

Virtual method definition

A method specification is written method [private] virtual method-name : poly-typexpr. It
specifies whether the method is public or private, and gives its type. If the method is intended to
be polymorphic, the type must be explicitly polymorphic.

Explicit overriding
Since Ocaml 3.12, the keywords inherit!, val! and method! have the same semantics as inherit,
val and method, but they additionally require the definition they introduce to be overriding. Namely,
method! requires method-name to be already defined in this class, val! requires inst-var-name to
be already defined in this class, and inherit! requires class-expr to override some definitions. If
no such overriding occurs, an error is signaled.
As a side-effect, these 3 keywords avoid the warnings 7 (method override) and 13 (instance
variable override). Note that warning 7 is disabled by default.

Constraints on type parameters

The construct constraint typexpr 1 = typexpr 2 forces the two type expressions to be equals. This is
typically used to specify type parameters: in that way they can be bound to specific type expressions.

Initializers
A class initializer initializer expr specifies an expression that will be evaluated whenever an
object is created from the class, once all its instance variables have been initialized.

11.9.3 Class definitions

class-definition ::= class class-binding {and class-binding}
class-binding ::= [virtual] [[ type-parameters ]] class-name {parameter} [: class-type]
= class-expr
type-parameters ::= ' ident {, ' ident}
Chapter 11. The OCaml language 203

A class definition class class-binding {and class-binding} is recursive. Each class-binding

defines a class-name that can be used in the whole expression except for inheritance. It can also be
used for inheritance, but only in the definitions that follow its own.
A class binding binds the class name class-name to the value of expression class-expr. It also
binds the class type class-name to the type of the class, and defines two type abbreviations :
class-name and # class-name. The first one is the type of objects of this class, while the second is
more general as it unifies with the type of any object belonging to a subclass (see section 11.4).

Virtual class
A class must be flagged virtual if one of its methods is virtual (that is, appears in the class type,
but is not actually defined). Objects cannot be created from a virtual class.

Type parameters
The class type parameters correspond to the ones of the class type and of the two type abbreviations
defined by the class binding. They must be bound to actual types in the class definition using type
constraints. So that the abbreviations are well-formed, type variables of the inferred type of the
class must either be type parameters or be bound in the constraint clause.

11.9.4 Class specifications

class-specification ::= class class-spec {and class-spec}
class-spec ::= [virtual] [[ type-parameters ]] class-name : class-type
This is the counterpart in signatures of class definitions. A class specification matches a class
definition if they have the same type parameters and their types match.

11.9.5 Class type definitions

classtype-definition ::= class type classtype-def {and classtype-def }
classtype-def ::= [virtual] [[ type-parameters ]] class-name = class-body-type
A class type definition class class-name = class-body-type defines an abbreviation class-name
for the class body type class-body-type. As for class definitions, two type abbreviations class-name
and # class-name are also defined. The definition can be parameterized by some type parameters.
If any method in the class type body is virtual, the definition must be flagged virtual.
Two class type definitions match if they have the same type parameters and they expand to
matching types.

11.10 Module types (module specifications)

Module types are the module-level equivalent of type expressions: they specify the general shape
and type properties of modules.
204

module-type ::= modtype-path

| sig {specification [;;]} end
| [functor] ( module-name : module-type ) -> module-type
| module-type -> module-type
| module-type with mod-constraint {and mod-constraint}
| ( module-type )
mod-constraint ::= type [type-params] typeconstr type-equation {type-constraint}
| module module-path = extended-module-path

specification ::= val value-name : typexpr

| external value-name : typexpr = external-declaration
| type-definition
| exception constr-decl
| class-specification
| classtype-definition
| module module-name : module-type
| module module-name {( module-name : module-type )} : module-type
| module type modtype-name
| module type modtype-name = module-type
| open module-path
| include module-type
See also the following language extensions: recovering the type of a module, substitution inside a
signature, type-level module aliases, attributes, extension nodes, generative functors, and module
type substitutions.

11.10.1 Simple module types

The expression modtype-path is equivalent to the module type bound to the name modtype-path.
The expression ( module-type ) denotes the same type as module-type.

11.10.2 Signatures
Signatures are type specifications for structures. Signatures sig . . . end are collections of type
specifications for value names, type names, exceptions, module names and module type names.
A structure will match a signature if the structure provides definitions (implementations) for all
the names specified in the signature (and possibly more), and these definitions meet the type
requirements given in the signature.
An optional ;; is allowed after each specification in a signature. It serves as a syntactic separator
with no semantic meaning.
Chapter 11. The OCaml language 205

Value specifications
A specification of a value component in a signature is written val value-name : typexpr, where
value-name is the name of the value and typexpr its expected type.
The form external value-name : typexpr = external-declaration is similar, except that
it requires in addition the name to be implemented as the external function specified in
external-declaration (see chapter 22).

Type specifications
A specification of one or several type components in a signature is written type typedef {and typedef }
and consists of a sequence of mutually recursive definitions of type names.
Each type definition in the signature specifies an optional type equation = typexpr and an
optional type representation = constr-decl . . . or = { field-decl . . . }. The implementation of the type
name in a matching structure must be compatible with the type expression specified in the equation
(if given), and have the specified representation (if given). Conversely, users of that signature will
be able to rely on the type equation or type representation, if given. More precisely, we have the
following four situations:

Abstract type: no equation, no representation.

Names that are defined as abstract types in a signature can be implemented in a matching
structure by any kind of type definition (provided it has the same number of type parameters).
The exact implementation of the type will be hidden to the users of the structure. In particular,
if the type is implemented as a variant type or record type, the associated constructors and
fields will not be accessible to the users; if the type is implemented as an abbreviation, the type
equality between the type name and the right-hand side of the abbreviation will be hidden
from the users of the structure. Users of the structure consider that type as incompatible with
any other type: a fresh type has been generated.
Type abbreviation: an equation = typexpr, no representation.
The type name must be implemented by a type compatible with typexpr. All users of the
structure know that the type name is compatible with typexpr.
New variant type or record type: no equation, a representation.
The type name must be implemented by a variant type or record type with exactly the
constructors or fields specified. All users of the structure have access to the constructors
or fields, and can use them to create or inspect values of that type. However, users of the
structure consider that type as incompatible with any other type: a fresh type has been
generated.
Re-exported variant type or record type: an equation, a representation.
This case combines the previous two: the representation of the type is made visible to all
users, and no fresh type is generated.

Exception specification
The specification exception constr-decl in a signature requires the matching structure to provide
an exception with the name and arguments specified in the definition, and makes the exception
206

available to all users of the structure.

Class specifications
A specification of one or several classes in a signature is written class class-spec {and class-spec}
and consists of a sequence of mutually recursive definitions of class names.
Class specifications are described more precisely in section 11.9.4.

Class type specifications

A specification of one or several class types in a signature is written class type classtype-def
{and classtype-def } and consists of a sequence of mutually recursive definitions of class type names.
Class type specifications are described more precisely in section 11.9.5.

Module specifications
A specification of a module component in a signature is written module module-name : module-type,
where module-name is the name of the module component and module-type its expected type.
Modules can be nested arbitrarily; in particular, functors can appear as components of structures
and functor types as components of signatures.
For specifying a module component that is a functor, one may write

module module-name ( name 1 : module-type 1 ) . . . ( name n : module-type n ) : module-type

instead of

module module-name : ( name 1 : module-type 1 ) . . . ( name n : module-type n ) -> module-type

Module type specifications

A module type component of a signature can be specified either as a manifest module type or as an
abstract module type.
An abstract module type specification module type modtype-name allows the name
modtype-name to be implemented by any module type in a matching signature, but hides the
implementation of the module type to all users of the signature.
A manifest module type specification module type modtype-name = module-type requires the
name modtype-name to be implemented by the module type module-type in a matching signature,
but makes the equality between modtype-name and module-type apparent to all users of the
signature.

11.10.3 Opening a module path

The expression open module-path in a signature does not specify any components. It simply
affects the parsing of the following items of the signature, allowing components of the module
denoted by module-path to be referred to by their simple names name instead of path accesses
module-path . name. The scope of the open stops at the end of the signature expression.
Chapter 11. The OCaml language 207

11.10.4 Including a signature

The expression include module-type in a signature performs textual inclusion of the components
of the signature denoted by module-type. It behaves as if the components of the included signature
were copied at the location of the include. The module-type argument must refer to a module
type that is a signature, not a functor type.

11.10.5 Functor types

The module type expression [functor] ( module-name : module-type 1 ) -> module-type 2 is the
type of functors (functions from modules to modules) that take as argument a module of type
module-type 1 and return as result a module of type module-type 2 . The module type module-type 2
can use the name module-name to refer to type components of the actual argument of the functor.
If the type module-type 2 does not depend on type components of module-name, the module type
expression can be simplified with the alternative short syntax module-type 1 -> module-type 2 . No
restrictions are placed on the type of the functor argument; in particular, a functor may take another
functor as argument (“higher-order” functor).
When the result module type is itself a functor,
( name 1 : module-type 1 ) -> . . . -> ( name n : module-type n ) -> module-type
one may use the abbreviated form
( name 1 : module-type 1 ) . . . ( name n : module-type n ) -> module-type

11.10.6 The with operator

Assuming module-type denotes a signature, the expression module-type with mod-constraint
{and mod-constraint} denotes the same signature where type equations have been added to some
of the type specifications, as described by the constraints following the with keyword. The
constraint type [type-parameters] typeconstr = typexpr adds the type equation = typexpr to the
specification of the type component named typeconstr of the constrained signature. The constraint
module module-path = extended-module-path adds type equations to all type components of the sub-
structure denoted by module-path, making them equivalent to the corresponding type components
of the structure denoted by extended-module-path.
For instance, if the module type name S is bound to the signature
sig type t module M: (sig type u end) end
then S with type t=int denotes the signature
sig type t=int module M: (sig type u end) end
and S with module M = N denotes the signature
sig type t module M: (sig type u=N.u end) end
A functor taking two arguments of type S that share their t component is written
functor (A: S) (B: S with type t = A.t) ...
Constraints are added left to right. After each constraint has been applied, the resulting signature
must be a subtype of the signature before the constraint was applied. Thus, the with operator can
only add information on the type components of a signature, but never remove information.
208

11.11 Module expressions (module implementations)

Module expressions are the module-level equivalent of value expressions: they evaluate to modules,
thus providing implementations for the specifications expressed in module types.

module-expr ::= module-path

| struct [module-items] end
| functor ( module-name : module-type ) -> module-expr
| module-expr ( module-expr )
| ( module-expr )
| ( module-expr : module-type )
module-items ::= {;;} (definition | expr) {{;;} (definition | ;; expr)} {;;}
definition ::= let [rec] let-binding {and let-binding}
| external value-name : typexpr = external-declaration
| type-definition
| exception-definition
| class-definition
| classtype-definition
| module module-name {( module-name : module-type )} [: module-type]
= module-expr
| module type modtype-name = module-type
| open module-path
| include module-expr

See also the following language extensions: recursive modules, first-class modules, overriding in open
statements, attributes, extension nodes and generative functors.

11.11.1 Simple module expressions

The expression module-path evaluates to the module bound to the name module-path.
The expression ( module-expr ) evaluates to the same module as module-expr.
The expression ( module-expr : module-type ) checks that the type of module-expr is a
subtype of module-type, that is, that all components specified in module-type are implemented
in module-expr, and their implementation meets the requirements given in module-type. In other
terms, it checks that the implementation module-expr meets the type specification module-type.
The whole expression evaluates to the same module as module-expr, except that all components
not specified in module-type are hidden and can no longer be accessed.

11.11.2 Structures
Structures struct . . . end are collections of definitions for value names, type names, exceptions,
module names and module type names. The definitions are evaluated in the order in which they
appear in the structure. The scopes of the bindings performed by the definitions extend to the end
of the structure. As a consequence, a definition may refer to names bound by earlier definitions in
the same structure.
Chapter 11. The OCaml language 209

For compatibility with toplevel phrases (chapter 14), optional ;; are allowed after and before
each definition in a structure. These ;; have no semantic meanings. Similarly, an expr preceded by
;; is allowed as a component of a structure. It is equivalent to let _ = expr, i.e. expr is evaluated
for its side-effects but is not bound to any identifier. If expr is the first component of a structure,
the preceding ;; can be omitted.

Value definitions
A value definition let [rec] let-binding {and let-binding} bind value names in the same way as a
let . . . in . . . expression (see section 11.7.2). The value names appearing in the left-hand sides of
the bindings are bound to the corresponding values in the right-hand sides.
A value definition external value-name : typexpr = external-declaration implements
value-name as the external function specified in external-declaration (see chapter 22).

Type definitions
A definition of one or several type components is written type typedef {and typedef } and consists
of a sequence of mutually recursive definitions of type names.

Exception definitions
Exceptions are defined with the syntax exception constr-decl or exception constr-name = constr.

Class definitions
A definition of one or several classes is written class class-binding {and class-binding} and consists
of a sequence of mutually recursive definitions of class names. Class definitions are described more
precisely in section 11.9.3.

Class type definitions

A definition of one or several classes is written class type classtype-def {and classtype-def } and
consists of a sequence of mutually recursive definitions of class type names. Class type definitions
are described more precisely in section 11.9.5.

Module definitions
The basic form for defining a module component is module module-name = module-expr, which
evaluates module-expr and binds the result to the name module-name.
One can write

module module-name : module-type = module-expr

instead of

module module-name = ( module-expr : module-type ).

Another derived form is

210

module module-name ( name 1 : module-type 1 ) . . . ( name n : module-type n ) = module-expr

which is equivalent to

module module-name = functor ( name 1 : module-type 1 ) -> . . . -> module-expr

Module type definitions

A definition for a module type is written module type modtype-name = module-type. It binds the
name modtype-name to the module type denoted by the expression module-type.

Opening a module path

The expression open module-path in a structure does not define any components nor perform any
bindings. It simply affects the parsing of the following items of the structure, allowing components
of the module denoted by module-path to be referred to by their simple names name instead of path
accesses module-path . name. The scope of the open stops at the end of the structure expression.

Including the components of another structure

The expression include module-expr in a structure re-exports in the current structure all definitions
of the structure denoted by module-expr. For instance, if you define a module S as below
module S = struct type t = int let x = 2 end
defining the module B as
module B = struct include S let y = (x + 1 : t) end
is equivalent to defining it as
module B = struct type t = S.t let x = S.x let y = (x + 1 : t) end
The difference between open and include is that open simply provides short names for the
components of the opened structure, without defining any components of the current structure,
while include also adds definitions for the components of the included structure.

11.11.3 Functors
Functor definition
The expression functor ( module-name : module-type ) -> module-expr evaluates to a functor
that takes as argument modules of the type module-type 1 , binds module-name to these modules,
evaluates module-expr in the extended environment, and returns the resulting modules as results.
No restrictions are placed on the type of the functor argument; in particular, a functor may take
another functor as argument (“higher-order” functor).
When the result module expression is itself a functor,

functor ( name 1 : module-type 1 ) -> . . . -> functor ( name n : module-type n ) -> module-expr

one may use the abbreviated form

functor ( name 1 : module-type 1 ) . . . ( name n : module-type n ) -> module-expr

Chapter 11. The OCaml language 211

Functor application
The expression module-expr 1 ( module-expr 2 ) evaluates module-expr 1 to a functor and
module-expr 2 to a module, and applies the former to the latter. The type of module-expr 2 must
match the type expected for the arguments of the functor module-expr 1 .

11.12 Compilation units

unit-interface ::= {specification [;;]}
unit-implementation ::= [module-items]
Compilation units bridge the module system and the separate compilation system. A compilation
unit is composed of two parts: an interface and an implementation. The interface contains a sequence
of specifications, just as the inside of a sig . . . end signature expression. The implementation contains
a sequence of definitions and expressions, just as the inside of a struct . . . end module expression.
A compilation unit also has a name unit-name, derived from the names of the files containing the
interface and the implementation (see chapter 13 for more details). A compilation unit behaves
roughly as the module definition

module unit-name : sig unit-interface end = struct unit-implementation end

A compilation unit can refer to other compilation units by their names, as if they were regular
modules. For instance, if U is a compilation unit that defines a type t, other compilation units can
refer to that type under the name U.t; they can also refer to U as a whole structure. Except for
names of other compilation units, a unit interface or unit implementation must not have any other
free variables. In other terms, the type-checking and compilation of an interface or implementation
proceeds in the initial environment

name 1 : sig specification1 end . . . name n : sig specificationn end

where name 1 . . . name n are the names of the other compilation units available in the search path
(see chapter 13 for more details) and specification1 . . . specificationn are their respective interfaces.
212
Chapter 12

Language extensions

This chapter describes language extensions and convenience features that are implemented in OCaml,
but not described in chapter 11.

12.1 Recursive definitions of values

(Introduced in Objective Caml 1.00)
As mentioned in section 11.7.2, the let rec binding construct, in addition to the definition of
recursive functions, also supports a certain class of recursive definitions of non-functional values,
such as

let rec name 1 = 1 :: name 2 and name 2 = 2 :: name 1 in expr

which binds name 1 to the cyclic list 1::2::1::2::…, and name 2 to the cyclic list 2::1::2::1::…In-
formally, the class of accepted definitions consists of those definitions where the defined names occur
only inside function bodies or as argument to a data constructor.
More precisely, consider the expression:

let rec name 1 = expr 1 and . . . and name n = expr n in expr

It will be accepted if each one of expr 1 . . . expr n is statically constructive with respect to
name 1 . . . name n , is not immediately linked to any of name 1 . . . name n , and is not an array
constructor whose arguments have abstract type.
An expression e is said to be statically constructive with respect to the variables name 1 . . . name n
if at least one of the following conditions is true:

• e has no free occurrence of any of name 1 . . . name n

• e is a variable

• e has the form fun . . . -> . . .

• e has the form function . . . -> . . .

• e has the form lazy ( . . . )

213
214

• e has one of the following forms, where each one of expr 1 . . . expr m is statically construc-
tive with respect to name 1 . . . name n , and expr 0 is statically constructive with respect to
name 1 . . . name n , xname 1 . . . xname m :

– let [rec] xname 1 = expr 1 and . . . and xname m = expr m in expr 0

– let module . . . in expr 1
– constr ( expr 1 , . . . , expr m )
– ` tag-name ( expr 1 , . . . , expr m )
– [| expr 1 ; . . . ; expr m |]
– { field1 = expr 1 ; . . . ; fieldm = expr m }
– { expr 1 with field2 = expr 2 ; . . . ; fieldm = expr m } where expr 1 is not immediately
linked to name 1 . . . name n
– ( expr 1 , . . . , expr m )
– expr 1 ; . . . ; expr m

An expression e is said to be immediately linked to the variable name in the following cases:

• e is name

• e has the form expr 1 ; . . . ; expr m where expr m is immediately linked to name

• e has the form let [rec] xname 1 = expr 1 and . . . and xname m = expr m in expr 0 where expr 0
is immediately linked to name or to one of the xname i such that expr i is immediately linked
to name.

12.2 Recursive modules

(Introduced in Objective Caml 3.07)

definition ::= ...

| module rec module-name : module-type = module-expr
{and module-name : module-type = module-expr}
specification ::= ...
| module rec module-name : module-type {and module-name : module-type}

Recursive module definitions, introduced by the module rec …and … construction, general-
ize regular module definitions module module-name = module-expr and module specifications
module module-name : module-type by allowing the defining module-expr and the module-type to
refer recursively to the module identifiers being defined. A typical example of a recursive module
definition is:
module rec A : sig
type t = Leaf of string | Node of ASet.t
val compare: t -> t -> int
Chapter 12. Language extensions 215

end = struct
type t = Leaf of string | Node of ASet.t
let compare t1 t2 =
match (t1, t2) with
| (Leaf s1, Leaf s2) -> Stdlib.compare s1 s2
| (Leaf _, Node _) -> 1
| (Node _, Leaf _) -> -1
| (Node n1, Node n2) -> ASet.compare n1 n2
end
and ASet
: Set.S with type elt = A.t
= Set.Make(A)
It can be given the following specification:
module rec A : sig
type t = Leaf of string | Node of ASet.t
val compare: t -> t -> int
end
and ASet : Set.S with type elt = A.t
This is an experimental extension of OCaml: the class of recursive definitions accepted, as well
as its dynamic semantics are not final and subject to change in future releases.
Currently, the compiler requires that all dependency cycles between the recursively-defined
module identifiers go through at least one “safe” module. A module is “safe” if all value definitions
that it contains have function types typexpr 1 -> typexpr 2 . Evaluation of a recursive module
definition proceeds by building initial values for the safe modules involved, binding all (functional)
values to fun _ -> raiseUndefined_recursive_module. The defining module expressions are then
evaluated, and the initial values for the safe modules are replaced by the values thus computed. If a
function component of a safe module is applied during this computation (which corresponds to an
ill-founded recursive definition), the Undefined_recursive_module exception is raised at runtime:
module rec M: sig val f: unit -> int end = struct let f () = N.x end
and N:sig val x: int end = struct let x = M.f () end
Exception:
Undefined_recursive_module ("extensions/recursivemodules.etex", 1, 43).
If there are no safe modules along a dependency cycle, an error is raised
module rec M: sig val x: int end = struct let x = N.y end
and N:sig val x: int val y:int end = struct let x = M.x let y = 0 end
Error : Cannot safely evaluate the definition of the following cycle
of recursively - defined modules : M -> N -> M.
There are no safe modules in this cycle (see manual section 12.2).
Module M defines an unsafe value , x .
Module N defines an unsafe value , x .
Note that, in the specification case, the module-types must be parenthesized if they use the
with mod-constraint construct.
216

12.3 Private types

Private type declarations in module signatures, of the form type t = private ..., enable libraries
to reveal some, but not all aspects of the implementation of a type to clients of the library. In this
respect, they strike a middle ground between abstract type declarations, where no information is
revealed on the type implementation, and data type definitions and type abbreviations, where all
aspects of the type implementation are publicized. Private type declarations come in three flavors:
for variant and record types (section 12.3.1), for type abbreviations (section 12.3.2), and for row
types (section 12.3.3).

12.3.1 Private variant and record types

(Introduced in Objective Caml 3.07)

type-representation ::= ...

| = private [|] constr-decl {| constr-decl}
| = private record-decl
Values of a variant or record type declared private can be de-structured normally in pattern-
matching or via the expr . field notation for record accesses. However, values of these types cannot
be constructed directly by constructor application or record construction. Moreover, assignment on
a mutable field of a private record type is not allowed.
The typical use of private types is in the export signature of a module, to ensure that construction
of values of the private type always go through the functions provided by the module, while still
allowing pattern-matching outside the defining module. For example:
module M : sig
type t = private A | B of int
val a : t
val b : int -> t
end = struct
type t = A | B of int
let a = A
let b n = assert (n > 0); B n
end
Here, the private declaration ensures that in any value of type M.t, the argument to the B
constructor is always a positive integer.
With respect to the variance of their parameters, private types are handled like abstract types.
That is, if a private type has parameters, their variance is the one explicitly given by prefixing the
parameter by a ‘+’ or a ‘-’, it is invariant otherwise.

12.3.2 Private type abbreviations

(Introduced in Objective Caml 3.11)

type-equation ::= ...

| = private typexpr
Chapter 12. Language extensions 217

Unlike a regular type abbreviation, a private type abbreviation declares a type that is distinct
from its implementation type typexpr. However, coercions from the type to typexpr are permitted.
Moreover, the compiler “knows” the implementation type and can take advantage of this knowledge
to perform type-directed optimizations.
The following example uses a private type abbreviation to define a module of nonnegative
integers:
module N : sig
type t = private int
val of_int: int -> t
val to_int: t -> int
end = struct
type t = int
let of_int n = assert (n >= 0); n
let to_int n = n
end
The type N.t is incompatible with int, ensuring that nonnegative integers and regular integers
are not confused. However, if x has type N.t, the coercion (x :> int) is legal and returns the
underlying integer, just like N.to_int x. Deep coercions are also supported: if l has type N.t list,
the coercion (l :> int list) returns the list of underlying integers, like List.map N.to_int l
but without copying the list l.
Note that the coercion ( expr :> typexpr ) is actually an abbreviated form, and will only work in
presence of private abbreviations if neither the type of expr nor typexpr contain any type variables. If
they do, you must use the full form ( expr : typexpr 1 :> typexpr 2 ) where typexpr 1 is the expected
type of expr. Concretely, this would be (x : N.t :> int) and (l : N.t list :> int list) for
the above examples.

12.3.3 Private row types

(Introduced in Objective Caml 3.09)

type-equation ::= ...

| = private typexpr
Private row types are type abbreviations where part of the structure of the type is left abstract.
Concretely typexpr in the above should denote either an object type or a polymorphic variant
type, with some possibility of refinement left. If the private declaration is used in an interface, the
corresponding implementation may either provide a ground instance, or a refined private type.
module M : sig type c = private < x : int; .. > val o : c end =
struct
class c = object method x = 3 method y = 2 end
let o = new c
end
This declaration does more than hiding the y method, it also makes the type c incompatible with
any other closed object type, meaning that only o will be of type c. In that respect it behaves
similarly to private record types. But private row types are more flexible with respect to incremental
refinement. This feature can be used in combination with functors.
218

module F(X : sig type c = private < x : int; .. > end) =

struct
let get_x (o : X.c) = o#x
end
module G(X : sig type c = private < x : int; y : int; .. > end) =
struct
include F(X)
let get_y (o : X.c) = o#y
end
A polymorphic variant type [t], for example
type t = [ `A of int | `B of bool ]
can be refined in two ways. A definition [u] may add new field to [t], and the declaration
type u = private [> t]
will keep those new fields abstract. Construction of values of type [u] is possible using the known
variants of [t], but any pattern-matching will require a default case to handle the potential extra
fields. Dually, a declaration [u] may restrict the fields of [t] through abstraction: the declaration
type v = private [< t > `A]
corresponds to private variant types. One cannot create a value of the private type [v], except
using the constructors that are explicitly listed as present, (`A n) in this example; yet, when
pattern-matching on a [v], one should assume that any of the constructors of [t] could be present.
Similarly to abstract types, the variance of type parameters is not inferred, and must be given
explicitly.

12.4 Locally abstract types

(Introduced in OCaml 3.12, short syntax added in 4.03)

parameter ::= ...

| ( type {typeconstr-name}+ )
The expression fun ( type typeconstr-name ) -> expr introduces a type constructor named
typeconstr-name which is considered abstract in the scope of the sub-expression, but then replaced
by a fresh type variable. Note that contrary to what the syntax could suggest, the expression
fun ( type typeconstr-name ) -> expr itself does not suspend the evaluation of expr as a regular
abstraction would. The syntax has been chosen to fit nicely in the context of function declarations,
where it is generally used. It is possible to freely mix regular function parameters with pseudo type
parameters, as in:
let f = fun (type t) (foo : t list) -> …
and even use the alternative syntax for declaring functions:
let f (type t) (foo : t list) = …
If several locally abstract types need to be introduced, it is possible to use the syn-
tax fun ( type typeconstr-name 1 . . . typeconstr-name n ) -> expr as syntactic sugar for
fun ( type typeconstr-name 1 ) -> . . . -> fun ( type typeconstr-name n ) -> expr. For instance,
Chapter 12. Language extensions 219

let f = fun (type t u v) -> fun (foo : (t * u * v) list) -> …

let f' (type t u v) (foo : (t * u * v) list) = …
This construction is useful because the type constructors it introduces can be used in places
where a type variable is not allowed. For instance, one can use it to define an exception in a local
module within a polymorphic function.
let f (type t) () =
let module M = struct exception E of t end in
(fun x -> M.E x), (function M.E x -> Some x | _ -> None)
Here is another example:
let sort_uniq (type s) (cmp : s -> s -> int) =
let module S = Set.Make(struct type t = s let compare = cmp end) in
fun l ->
S.elements (List.fold_right S.add l S.empty)
It is also extremely useful for first-class modules (see section 12.5) and generalized algebraic
datatypes (GADTs: see section 12.10).

Polymorphic syntax (Introduced in OCaml 4.00)

let-binding ::= ...

| value-name : type {typeconstr-name}+ . typexpr = expr
class-field ::= ...
| method [private] method-name : type {typeconstr-name}+ . typexpr = expr
| method! [private] method-name : type {typeconstr-name}+ . typexpr = expr

The (type typeconstr-name ) syntax construction by itself does not make polymorphic the type
variable it introduces, but it can be combined with explicit polymorphic annotations where needed.
The above rule is provided as syntactic sugar to make this easier:
let rec f : type t1 t2. t1 * t2 list -> t1 = …
is automatically expanded into
let rec f : 't1 't2. 't1 * 't2 list -> 't1 =
fun (type t1) (type t2) -> ( … : t1 * t2 list -> t1)
This syntax can be very useful when defining recursive functions involving GADTs, see the sec-
tion 12.10 for a more detailed explanation.
The same feature is provided for method definitions.

12.5 First-class modules

(Introduced in OCaml 3.12; pattern syntax and package type inference introduced in 4.00; structural
comparison of package types introduced in 4.02.; fewer parens required starting from 4.05)
220

typexpr ::= ...

| ( module package-type )
module-expr ::= ...
| ( val expr [: package-type] )
expr ::= ...
| ( module module-expr [: package-type] )
pattern ::= ...
| ( module module-name [: package-type] )
package-type ::= modtype-path
| modtype-path with package-constraint {and package-constraint}
package-constraint ::= type typeconstr = typexpr

Modules are typically thought of as static components. This extension makes it possible to pack
a module as a first-class value, which can later be dynamically unpacked into a module.
The expression ( module module-expr : package-type ) converts the module (structure or
functor) denoted by module expression module-expr to a value of the core language that encapsulates
this module. The type of this core language value is ( module package-type ). The package-type
annotation can be omitted if it can be inferred from the context.
Conversely, the module expression ( val expr : package-type ) evaluates the core language
expression expr to a value, which must have type module package-type, and extracts the module
that was encapsulated in this value. Again package-type can be omitted if the type of expr is known.
If the module expression is already parenthesized, like the arguments of functors are, no additional
parens are needed: Map.Make(val key).
The pattern ( module module-name : package-type ) matches a package with type package-type
and binds it to module-name. It is not allowed in toplevel let bindings. Again package-type can be
omitted if it can be inferred from the enclosing pattern.
The package-type syntactic class appearing in the ( module package-type ) type expression and
in the annotated forms represents a subset of module types. This subset consists of named module
types with optional constraints of a limited form: only non-parametrized types can be specified.
For type-checking purposes (and starting from OCaml 4.02), package types are compared using
the structural comparison of module types.
In general, the module expression ( val expr : package-type ) cannot be used in the body of
a functor, because this could cause unsoundness in conjunction with applicative functors. Since
OCaml 4.02, this is relaxed in two ways: if package-type does not contain nominal type decla-
rations (i.e. types that are created with a proper identity), then this expression can be used
anywhere, and even if it contains such types it can be used inside the body of a generative functor,
described in section 12.15. It can also be used anywhere in the context of a local module binding
let module module-name = ( val expr 1 : package-type ) in expr 2 .

Basic example A typical use of first-class modules is to select at run-time among several
implementations of a signature. Each implementation is a structure that we can encapsulate as a
Chapter 12. Language extensions 221

first-class module, then store in a data structure such as a hash table:

type picture = …
module type DEVICE = sig
val draw : picture -> unit
…
end
let devices : (string, (module DEVICE)) Hashtbl.t = Hashtbl.create 17

module SVG = struct … end

let _ = Hashtbl.add devices "SVG" (module SVG : DEVICE)

module PDF = struct … end

let _ = Hashtbl.add devices "PDF" (module PDF : DEVICE)
We can then select one implementation based on command-line arguments, for instance:
let parse_cmdline () = …
module Device =
(val (let device_name = parse_cmdline () in
try Hashtbl.find devices device_name
with Not_found ->
Printf.eprintf "Unknown device %s\n" device_name;
exit 2)
: DEVICE)
Alternatively, the selection can be performed within a function:
let draw_using_device device_name picture =
let module Device =
(val (Hashtbl.find devices device_name) : DEVICE)
in
Device.draw picture

Advanced examples With first-class modules, it is possible to parametrize some code over the
implementation of a module without using a functor.
let sort (type s) (module Set : Set.S with type elt = s) l =
Set.elements (List.fold_right Set.add l Set.empty)
val sort : (module Set.S with type elt = 's) -> 's list -> 's list = <fun>
To use this function, one can wrap the Set.Make functor:
let make_set (type s) cmp =
let module S = Set.Make(struct
type t = s
let compare = cmp
end) in
(module S : Set.S with type elt = s)
val make_set : ('s -> 's -> int) -> (module Set.S with type elt = 's) = <fun>
222

12.6 Recovering the type of a module

(Introduced in OCaml 3.12)

module-type ::= ...

| module type of module-expr
The construction module type of module-expr expands to the module type (signature or functor
type) inferred for the module expression module-expr. To make this module type reusable in many
situations, it is intentionally not strengthened: abstract types and datatypes are not explicitly
related with the types of the original module. For the same reason, module aliases in the inferred
type are expanded.
A typical use, in conjunction with the signature-level include construct, is to extend the
signature of an existing structure. In that case, one wants to keep the types equal to types in the
original module. This can done using the following idiom.
module type MYHASH = sig
include module type of struct include Hashtbl end
val replace: ('a, 'b) t -> 'a -> 'b -> unit
end
The signature MYHASH then contains all the fields of the signature of the module Hashtbl (with
strengthened type definitions), plus the new field replace. An implementation of this signature can
be obtained easily by using the include construct again, but this time at the structure level:
module MyHash : MYHASH = struct
include Hashtbl
let replace t k v = remove t k; add t k v
end
Another application where the absence of strengthening comes handy, is to provide an alternative
implementation for an existing module.
module MySet : module type of Set = struct
…
end
This idiom guarantees that Myset is compatible with Set, but allows it to represent sets internally
in a different way.

12.7 Substituting inside a signature

12.7.1 Destructive substitutions
(Introduced in OCaml 3.12, generalized in 4.06)

mod-constraint ::= ...

| type [type-params] typeconstr-name := typexpr
| module module-path := extended-module-path
A “destructive” substitution (with... :=...) behaves essentially like normal signature constraints
(with... =...), but it additionally removes the redefined type or module from the signature.
Chapter 12. Language extensions 223

Prior to OCaml 4.06, there were a number of restrictions: one could only remove types and
modules at the outermost level (not inside submodules), and in the case of with type the definition
had to be another type constructor with the same type parameters.
A natural application of destructive substitution is merging two signatures sharing a type
name.
module type Printable = sig
type t
val print : Format.formatter -> t -> unit
end
module type Comparable = sig
type t
val compare : t -> t -> int
end
module type PrintableComparable = sig
include Printable
include Comparable with type t := t
end
One can also use this to completely remove a field:
module type S = Comparable with type t := int
module type S = sig val compare : int -> int -> int end
or to rename one:
module type S = sig
type u
include Comparable with type t := u
end
module type S = sig type u val compare : u -> u -> int end
Note that you can also remove manifest types, by substituting with the same type.
module type ComparableInt = Comparable with type t = int ;;
module type ComparableInt = sig type t = int val compare : t -> t -> int end

module type CompareInt = ComparableInt with type t := int

module type CompareInt = sig val compare : int -> int -> int end

12.7.2 Local substitution declarations

(Introduced in OCaml 4.08, module type substitution introduced in 4.13)

specification ::= ...

| type type-subst {and type-subst}
| module module-name := extended-module-path
| module type module-name := module-type
type-subst ::= [type-params] typeconstr-name := typexpr {type-constraint}
224

Local substitutions behave like destructive substitutions (with... :=...) but instead of being
applied to a whole signature after the fact, they are introduced during the specification of the
signature, and will apply to all the items that follow.
This provides a convenient way to introduce local names for types and modules when defining a
signature:
module type S = sig
type t
module Sub : sig
type outer := t
type t
val to_outer : t -> outer
end
end
module type S =
sig type t module Sub : sig type t val to_outer : t -> t/2 end end
Note that, unlike type declarations, type substitution declarations are not recursive, so substitu-
tions like the following are rejected:
# module type S = sig
type 'a poly_list := [ `Cons of 'a * 'a poly_list | `Nil ]
end ;;
Error : Unbound type constructor poly_list
Local substitutions can also be used to give a local name to a type or a module type introduced
by a functor application:
# module type F = sig
type set := Set.Make(Int).t

module type Type = sig type t end

module Nest : Type -> sig module type T = Type end

module type T := Nest(Int).T

val set: set

val m : (module T)
end;;
module type F =
sig
module type Type = sig type t end
module Nest : Type -> sig module type T = Type end
val set : Set.Make(Int).t
val m : (module Nest(Int).T)
end
Local module type substitutions are subject to the same limitations as module type substitutions,
see section 12.7.3.
Chapter 12. Language extensions 225

12.7.3 Module type substitutions

(Introduced in OCaml 4.13)

mod-constraint ::= ...

| module type modtype-path = module-type
| module type modtype-path := module-type
Module type substitution essentially behaves like type substitutions. They are useful to refine
an abstract module type in a signature into a concrete module type,
# module type ENDO = sig
module type T
module F: T -> T
end
module Endo(X: sig module type T end): ENDO with module type T = X.T =
struct
module type T = X.T
module F(X:T) = X
end;;
module type ENDO = sig module type T module F : T -> T end
module Endo :
(X : sig module type T end) ->
sig module type T = X.T module F : T -> T end
It is also possible to substitute a concrete module type with an equivalent module types.
module type A = sig
type x
module type R = sig
type a = A of x
type b
end
end
module type S = sig
type a = A of int
type b
end
module type B = A with type x = int and module type R = S
However, such substitutions are never necessary.
Destructive module type substitution removes the module type substitution from the signature
# module type ENDO' = ENDO with module type T := ENDO;;
module type ENDO' = sig module F : ENDO -> ENDO end

Limitations
If the right hand side of a module type substitution or a local module type substitution is not a
modtype-path, then the destructive substitution is only valid if the left-hand side of the substitution
is never used as the type of a first-class module in the original module type.
226

module type T = sig module type S val x: (module S) end

module type Error = T with module type S := sig end
Error : This with constraint S := sig end makes a packed module ill - formed .
(see manual section 12.7.3)

module type T = sig module type S := sig end val x: (module S) end
Error : The module type S is not a valid type for a packed module :
it is defined as a local substitution ( temporary name)
for an anonymous module type. (see manual section 12.7.3)

12.8 Type-level module aliases

(Introduced in OCaml 4.02)

specification ::= ...

| module module-name = module-path
The above specification, inside a signature, only matches a module definition equal to
module-path. Conversely, a type-level module alias can be matched by itself, or by any supertype
of the type of the module it references.
There are several restrictions on module-path:

1. it should be of the form M0 .M1 ...Mn (i.e. without functor applications);

2. inside the body of a functor, M0 should not be one of the functor parameters;

3. inside a recursive module definition, M0 should not be one of the recursively defined modules.

Such specifications are also inferred. Namely, when P is a path satisfying the above constraints,
module N = P
has type
module N = P
Type-level module aliases are used when checking module path equalities. That is, in a context
where module name N is known to be an alias for P, not only these two module paths check as
equal, but F (N ) and F (P) are also recognized as equal. In the default compilation mode, this is
the only difference with the previous approach of module aliases having just the same module type
as the module they reference.
When the compiler flag -no-alias-deps is enabled, type-level module aliases are also exploited
to avoid introducing dependencies between compilation units. Namely, a module alias referring
to a module inside another compilation unit does not introduce a link-time dependency on that
compilation unit, as long as it is not dereferenced; it still introduces a compile-time dependency
if the interface needs to be read, i.e. if the module is a submodule of the compilation unit, or if
some type components are referred to. Additionally, accessing a module alias introduces a link-time
dependency on the compilation unit containing the module referenced by the alias, rather than
the compilation unit containing the alias. Note that these differences in link-time behavior may be
Chapter 12. Language extensions 227

incompatible with the previous behavior, as some compilation units might not be extracted from
libraries, and their side-effects ignored.
These weakened dependencies make possible to use module aliases in place of the -pack
mechanism. Suppose that you have a library Mylib composed of modules A and B. Using -pack,
one would issue the command line

ocamlc -pack a.cmo b.cmo -o mylib.cmo

and as a result obtain a Mylib compilation unit, containing physically A and B as submodules,
and with no dependencies on their respective compilation units. Here is a concrete example of a
possible alternative approach:

1. Rename the files containing A and B to Mylib__A and Mylib__B.

2. Create a packing interface Mylib.ml, containing the following lines.

module A = Mylib__A
module B = Mylib__B

3. Compile Mylib.ml using -no-alias-deps, and the other files using -no-alias-deps and
-open Mylib (the last one is equivalent to adding the line open! Mylib at the top of each file).

ocamlc -c -no-alias-deps Mylib.ml

ocamlc -c -no-alias-deps -open Mylib Mylib__*.mli Mylib__*.ml

4. Finally, create a library containing all the compilation units, and export all the compiled
interfaces.

ocamlc -a Mylib*.cmo -o Mylib.cma

This approach lets you access A and B directly inside the library, and as Mylib.A and Mylib.B from
outside. It also has the advantage that Mylib is no longer monolithic: if you use Mylib.A, only
Mylib__A will be linked in, not Mylib__B.
Note the use of double underscores in Mylib__A and Mylib__B. These were chosen on purpose; the
compiler uses the following heuristic when printing paths: given a path Lib__fooBar, if Lib.FooBar
exists and is an alias for Lib__fooBar, then the compiler will always display Lib.FooBar instead of
Lib__fooBar. This way the long Mylib__ names stay hidden and all the user sees is the nicer dot
names. This is how the OCaml standard library is compiled.

12.9 Overriding in open statements

(Introduced in OCaml 4.01)
228

definition ::= ...

| open! module-path
specification ::= ...
| open! module-path
expr ::= ...
| let open! module-path in expr
class-body-type ::= ...
| let open! module-path in class-body-type
class-expr ::= ...
| let open! module-path in class-expr

Since OCaml 4.01, open statements shadowing an existing identifier (which is later used) trigger
the warning 44. Adding a ! character after the open keyword indicates that such a shadowing is
intentional and should not trigger the warning.
This is also available (since OCaml 4.06) for local opens in class expressions and class type
expressions.

12.10 Generalized algebraic datatypes

Generalized algebraic datatypes, or GADTs, extend usual sum types in two ways: constraints on
type parameters may change depending on the value constructor, and some type variables may be
existentially quantified. They are described in chapter 7.
(Introduced in OCaml 4.00)

constr-decl ::= ...

| constr-name : [constr-args ->] typexpr
type-param ::= ...
| [variance] _
Refutation cases. (Introduced in OCaml 4.03)

matching-case ::= pattern [when expr] -> expr

| pattern -> .
Explicit naming of existentials. (Introduced in OCaml 4.13.0)

pattern ::= ...

| constr ( type {typeconstr-name}+ ) ( pattern )
Chapter 12. Language extensions 229

12.11 Syntax for Bigarray access

(Introduced in Objective Caml 3.00)

expr ::= ...

| expr .{ expr {, expr} }
| expr .{ expr {, expr} } <- expr
This extension provides syntactic sugar for getting and setting elements in the arrays provided
by the Bigarray[29.5] module.
The short expressions are translated into calls to functions of the Bigarray module as described
in the following table.

expression translation
expr 0 .{ expr 1 } Bigarray.Array1.get expr 0 expr 1
expr 0 .{ expr 1 } <- expr Bigarray.Array1.set expr 0 expr 1 expr
expr 0 .{ expr 1 , expr 2 } Bigarray.Array2.get expr 0 expr 1 expr 2
expr 0 .{ expr 1 , expr 2 } <- expr Bigarray.Array2.set expr 0 expr 1 expr 2 expr
expr 0 .{ expr 1 , expr 2 , expr 3 } Bigarray.Array3.get expr 0 expr 1 expr 2 expr 3
expr 0 .{ expr 1 , expr 2 , expr 3 } <- expr Bigarray.Array3.set expr 0 expr 1 expr 2 expr 3 expr
expr 0 .{ expr 1 , . . . , expr n } Bigarray.Genarray.get expr 0 [| expr 1 , . . . , expr n |]
expr 0 .{ expr 1 , . . . , expr n } <- expr Bigarray.Genarray.set expr 0 [| expr 1 , . . . , expr n |] expr

The last two entries are valid for any n > 3.

12.12 Attributes
(Introduced in OCaml 4.02, infix notations for constructs other than expressions added in 4.03)
Attributes are “decorations” of the syntax tree which are mostly ignored by the type-checker but
can be used by external tools. An attribute is made of an identifier and a payload, which can be a
structure, a type expression (prefixed with :), a signature (prefixed with :) or a pattern (prefixed
with ?) optionally followed by a when clause:

attr-id ::= lowercase-ident

| capitalized-ident
| attr-id . attr-id
attr-payload ::= [module-items]
| : typexpr
| : [specification]
| ? pattern [when expr]

The first form of attributes is attached with a postfix notation on “algebraic” categories:
230

attribute ::= [@ attr-id attr-payload ]

expr ::= ...
| expr attribute
typexpr ::= ...
| typexpr attribute
pattern ::= ...
| pattern attribute
module-expr ::= ...
| module-expr attribute
module-type ::= ...
| module-type attribute
class-expr ::= ...
| class-expr attribute
class-type ::= ...
| class-type attribute

This form of attributes can also be inserted after the ` tag-name in polymorphic variant type
expressions (tag-spec-first, tag-spec, tag-spec-full) or after the method-name in method-type.
The same syntactic form is also used to attach attributes to labels and constructors in type
declarations:

field-decl ::= [mutable] field-name : poly-typexpr {attribute}

constr-decl ::= (constr-name | ()) [of constr-args] {attribute}

Note: when a label declaration is followed by a semi-colon, attributes can also be put after the
semi-colon (in which case they are merged to those specified before).
The second form of attributes are attached to “blocks” such as type declarations, class fields, etc:
Chapter 12. Language extensions 231

item-attribute ::= [@@ attr-id attr-payload ]

typedef ::= ...
| typedef item-attribute
exception-definition ::= exception constr-decl
| exception constr-name = constr
module-items ::= [;;] (definition | expr {item-attribute}) {[;;] definition | ;; expr {item-attribute}} [;;
class-binding ::= ...
| class-binding item-attribute
class-spec ::= ...
| class-spec item-attribute
classtype-def ::= ...
| classtype-def item-attribute
definition ::= let [rec] let-binding {and let-binding}
| external value-name : typexpr = external-declaration {item-attribute}
| type-definition
| exception-definition {item-attribute}
| class-definition
| classtype-definition
| module module-name {( module-name : module-type )} [: module-type]
= module-expr {item-attribute}
| module type modtype-name = module-type {item-attribute}
| open module-path {item-attribute}
| include module-expr {item-attribute}
| module rec module-name : module-type =
module-expr {item-attribute}
{and module-name : module-type = module-expr
{item-attribute}}
specification ::= val value-name : typexpr {item-attribute}
| external value-name : typexpr = external-declaration {item-attribute}
| type-definition
| exception constr-decl {item-attribute}
| class-specification
| classtype-definition
| module module-name : module-type {item-attribute}
| module module-name {( module-name : module-type )} : module-type {item-attribu
| module type modtype-name {item-attribute}
| module type modtype-name = module-type {item-attribute}
| open module-path {item-attribute}
| include module-type {item-attribute}
class-field-spec ::= ...
| class-field-spec item-attribute
class-field ::= ...
| class-field item-attribute
232

A third form of attributes appears as stand-alone structure or signature items in the module or
class sub-languages. They are not attached to any specific node in the syntax tree:

floating-attribute ::= [@@@ attr-id attr-payload ]

definition ::= ...
| floating-attribute
specification ::= ...
| floating-attribute
class-field-spec ::= ...
| floating-attribute
class-field ::= ...
| floating-attribute

(Note: contrary to what the grammar above describes, item-attributes cannot be attached to
these floating attributes in class-field-spec and class-field.)
It is also possible to specify attributes using an infix syntax. For instance:

let[@foo] x = 2 in x + 1 === (let x = 2 [@@foo] in x + 1)

begin[@foo][@bar x] ... end === (begin ... end)[@foo][@bar x]
module[@foo] M = ... === module M = ... [@@foo]
type[@foo] t = T === type t = T [@@foo]
method[@foo] m = ... === method m = ... [@@foo]

For let, the attributes are applied to each bindings:

let[@foo] x = 2 and y = 3 in x + y === (let x = 2 [@@foo] and y = 3 in x + y)

let[@foo] x = 2
and[@bar] y = 3 in x + y === (let x = 2 [@@foo] and y = 3 [@@bar] in x + y)

12.12.1 Built-in attributes

Some attributes are understood by the compiler:
• “ocaml.warning” or “warning”, with a string literal payload. This can be used as floating
attributes in a signature/structure/object/object type. The string is parsed and has the same
effect as the -w command-line option, in the scope between the attribute and the end of the
current signature/structure/object/object type. The attribute can also be attached to any
kind of syntactic item which support attributes (such as an expression, or a type expression)
in which case its scope is limited to that item. Note that it is not well-defined which scope
is used for a specific warning. This is implementation dependent and can change between
versions. Some warnings are even completely outside the control of “ocaml.warning” (for
instance, warnings 1, 2, 14, 29 and 50).

• “ocaml.warnerror” or “warnerror”, with a string literal payload. Same as “ocaml.warning”, for

the -warn-error command-line option.
Chapter 12. Language extensions 233

• “ocaml.alert” or “alert”: see section 12.21.

• “ocaml.deprecated” or “deprecated”: alias for the “deprecated” alert, see section 12.21.

• “ocaml.deprecated_mutable” or “deprecated_mutable”. Can be applied to a mutable record

label. If the label is later used to modify the field (with “expr.l <- expr”), the “deprecated”
alert will be triggered. If the payload of the attribute is a string literal, the alert message
includes this text.

• “ocaml.ppwarning” or “ppwarning”, in any context, with a string literal payload. The text
is reported as warning (22) by the compiler (currently, the warning location is the location
of the string payload). This is mostly useful for preprocessors which need to communicate
warnings to the user. This could also be used to mark explicitly some code location for further
inspection.

• “ocaml.warn_on_literal_pattern” or “warn_on_literal_pattern” annotate constructors in

type definition. A warning (52) is then emitted when this constructor is pattern matched with
a constant literal as argument. This attribute denotes constructors whose argument is purely
informative and may change in the future. Therefore, pattern matching on this argument with
a constant literal is unreliable. For instance, all built-in exception constructors are marked as
“warn_on_literal_pattern”. Note that, due to an implementation limitation, this warning
(52) is only triggered for single argument constructor.

• “ocaml.tailcall” or “tailcall” can be applied to function application in order to check that the
call is tailcall optimized. If it it not the case, a warning (51) is emitted.

• “ocaml.inline” or “inline” take either “never”, “always” or nothing as payload on a function

or functor definition. If no payload is provided, the default value is “always”. This payload
controls when applications of the annotated functions should be inlined.

• “ocaml.inlined” or “inlined” can be applied to any function or functor application to check

that the call is inlined by the compiler. If the call is not inlined, a warning (55) is emitted.

• “ocaml.noalloc”, “ocaml.unboxed”and “ocaml.untagged” or “noalloc”, “unboxed” and “un-

tagged” can be used on external definitions to obtain finer control over the C-to-OCaml
interface. See 22.11 for more details.

• “ocaml.immediate” or “immediate” applied on an abstract type mark the type as having

a non-pointer implementation (e.g. “int”, “bool”, “char” or enumerated types). Mutation
of these immediate types does not activate the garbage collector’s write barrier, which can
significantly boost performance in programs relying heavily on mutable state.

• “ocaml.immediate64” or “immediate64” applied on an abstract type mark the type as hav-

ing a non-pointer implementation on 64 bit platforms. No assumption is made on other
platforms. In order to produce a type with the “immediate64“ attribute, one must use
“Sys.Immediate64.Make“ functor.

• ocaml.unboxed or unboxed can be used on a type definition if the type is a single-field record
or a concrete type with a single constructor that has a single argument. It tells the compiler
234

to optimize the representation of the type by removing the block that represents the record or
the constructor (i.e. a value of this type is physically equal to its argument). In the case of
GADTs, an additional restriction applies: the argument must not be an existential variable,
represented by an existential type variable, or an abstract type constructor applied to an
existential type variable.

• ocaml.boxed or boxed can be used on type definitions to mean the opposite of ocaml.unboxed:
keep the unoptimized representation of the type. When there is no annotation, the default is
currently boxed but it may change in the future.

• ocaml.local or local take either never, always, maybe or nothing as payload on a function
definition. If no payload is provided, the default is always. The attribute controls an
optimization which consists in compiling a function into a static continuation. Contrary to
inlining, this optimization does not duplicate the function’s body. This is possible when
all references to the function are full applications, all sharing the same continuation (for
instance, the returned value of several branches of a pattern matching). never disables the
optimization, always asserts that the optimization applies (otherwise a warning 55 is emitted)
and maybe lets the optimization apply when possible (this is the default behavior when the
attribute is not specified). The optimization is implicitly disabled when using the bytecode
compiler in debug mode (-g), and for functions marked with an ocaml.inline always or
ocaml.unrolled attribute which supersede ocaml.local.

• ocaml.poll or poll with an error payload on a function definition emits an error whenever
the compiler inserts a runtime polling point in the body of the annotated function.

module X = struct
[@@@warning "+9"] (∗ locally enable warning 9 in this structure ∗)
…
end
[@@deprecated "Please use module 'Y' instead."]

let x = begin[@warning "+9"] […] end

type t = A | B
[@@deprecated "Please use type 's' instead."]
let fires_warning_22 x =
assert (x >= 0) [@ppwarning "TODO: remove this later"]
Warning 22 [ preprocessor ]: TODO: remove this later

let rec is_a_tail_call = function

| [] -> ()
| _ :: q -> (is_a_tail_call[@tailcall]) q

let rec not_a_tail_call = function

| [] -> []
| x :: q -> x :: (not_a_tail_call[@tailcall]) q
Chapter 12. Language extensions 235

Warning 51 [wrong -tailcall - expectation ]: expected tailcall

let f x = x [@@inline]

let () = (f[@inlined]) ()
type fragile =
| Int of int [@warn_on_literal_pattern]
| String of string [@warn_on_literal_pattern]
let fragile_match_1 = function
| Int 0 -> ()
| _ -> ()
Warning 52 [fragile -literal - pattern ]: Code should not depend on the actual values of
this constructor 's arguments . They are only for information
and may change in future versions . (see manual section 13.5.3)

val fragile_match_1 : fragile -> unit = <fun >

let fragile_match_2 = function

| String "constant" -> ()
| _ -> ()
Warning 52 [fragile -literal - pattern ]: Code should not depend on the actual values of
this constructor 's arguments . They are only for information
and may change in future versions . (see manual section 13.5.3)

val fragile_match_2 : fragile -> unit = <fun >

module Immediate: sig

type t [@@immediate]
val x: t ref
end = struct
type t = A | B
let x = ref A
end
module Int_or_int64 : sig
type t [@@immediate64]
val zero : t
val one : t
val add : t -> t -> t
end = struct

include Sys.Immediate64.Make(Int)(Int64)

module type S = sig

val zero : t
236

val one : t
val add : t -> t -> t
end

let impl : (module S) =

match repr with
| Immediate ->
(module Int : S)
| Non_immediate ->
(module Int64 : S)

include (val impl : S)

end

12.13 Extension nodes

(Introduced in OCaml 4.02, infix notations for constructs other than expressions added in 4.03, infix
notation (e1 ;%ext e2) added in 4.04. )
Extension nodes are generic placeholders in the syntax tree. They are rejected by the type-checker
and are intended to be “expanded” by external tools such as -ppx rewriters.
Extension nodes share the same notion of identifier and payload as attributes 12.12.
The first form of extension node is used for “algebraic” categories:

extension ::= [% attr-id attr-payload ]

expr ::= ...
| extension
typexpr ::= ...
| extension
pattern ::= ...
| extension
module-expr ::= ...
| extension
module-type ::= ...
| extension
class-expr ::= ...
| extension
class-type ::= ...
| extension

A second form of extension node can be used in structures and signatures, both in the module
and object languages:
Chapter 12. Language extensions 237

item-extension ::= [%% attr-id attr-payload ]

definition ::= ...
| item-extension
specification ::= ...
| item-extension
class-field-spec ::= ...
| item-extension
class-field ::= ...
| item-extension

An infix form is available for extension nodes when the payload is of the same kind (expression
with expression, pattern with pattern ...).
Examples:

let%foo x = 2 in x + 1 === [%foo let x = 2 in x + 1]

begin%foo ... end === [%foo begin ... end]
x ;%foo 2 === [%foo x; 2]
module%foo M = .. === [%%foo module M = ... ]
val%foo x : t === [%%foo: val x : t]

When this form is used together with the infix syntax for attributes, the attributes are considered
to apply to the payload:

fun%foo[@bar] x -> x + 1 === [%foo (fun x -> x + 1)[@bar ] ];

An additional shorthand let%foo x in ... is available for convenience when extension nodes
are used to implement binding operators (See 12.23.4).
Furthermore, quoted strings {|...|} can be combined with extension nodes to embed foreign
syntax fragments. Those fragments can be interpreted by a preprocessor and turned into OCaml
code without requiring escaping quotes. A syntax shortcut is available for them:

{%%foo|...|} === [%%foo{|...|}]

let x = {%foo|...|} === let x = [%foo{|...|}]
let y = {%foo bar|...|bar} === let y = [%foo{bar|...|bar}]

For instance, you can use {%sql|...|} to represent arbitrary SQL statements – assuming you
have a ppx-rewriter that recognizes the %sql extension.
Note that the word-delimited form, for example {sql|...|sql}, should not be used for signaling
that an extension is in use. Indeed, the user cannot see from the code whether this string literal has
different semantics than they expect. Moreover, giving semantics to a specific delimiter limits the
freedom to change the delimiter to avoid escaping issues.
238

12.13.1 Built-in extension nodes

(Introduced in OCaml 4.03)
Some extension nodes are understood by the compiler itself:

• “ocaml.extension_constructor” or “extension_constructor” take as payload a constructor from

an extensible variant type (see 12.14) and return its extension constructor slot.

type t = ..
type t += X of int | Y of string
let x = [%extension_constructor X]
let y = [%extension_constructor Y]
# x <> y;;
- : bool = true

12.14 Extensible variant types

(Introduced in OCaml 4.02)

type-representation ::= ...

| = ..
specification ::= ...
| type [type-params] typeconstr type-extension-spec
definition ::= ...
| type [type-params] typeconstr type-extension-def
type-extension-spec ::= += [private] [|] constr-decl {| constr-decl}
type-extension-def ::= += [private] [|] constr-def {| constr-def }
constr-def ::= constr-decl
| constr-name = constr

Extensible variant types are variant types which can be extended with new variant constructors.
Extensible variant types are defined using ... New variant constructors are added using +=.
module Expr = struct
type attr = ..

type attr += Str of string

type attr +=
| Int of int
| Float of float
end
Chapter 12. Language extensions 239

Pattern matching on an extensible variant type requires a default case to handle unknown variant
constructors:
let to_string = function
| Expr.Str s -> s
| Expr.Int i -> Int.to_string i
| Expr.Float f -> string_of_float f
| _ -> "?"
A preexisting example of an extensible variant type is the built-in exn type used for exceptions.
Indeed, exception constructors can be declared using the type extension syntax:
type exn += Exc of int
Extensible variant constructors can be rebound to a different name. This allows exporting
variants from another module.
# let not_in_scope = Str "Foo";;
Error : Unbound constructor Str

type Expr.attr += Str = Expr.Str

# let now_works = Str "foo";;
val now_works : Expr.attr = Expr.Str "foo"
Extensible variant constructors can be declared private. As with regular variants, this prevents
them from being constructed directly by constructor application while still allowing them to be
de-structured in pattern-matching.
module B : sig
type Expr.attr += private Bool of int
val bool : bool -> Expr.attr
end = struct
type Expr.attr += Bool of int
let bool p = if p then Bool 1 else Bool 0
end
# let inspection_works = function
| B.Bool p -> (p = 1)
| _ -> true;;
val inspection_works : Expr.attr -> bool = <fun>

# let construction_is_forbidden = B.Bool 1;;

Error : Cannot use private constructor Bool to create values of type Expr.attr

12.14.1 Private extensible variant types

(Introduced in OCaml 4.06)

type-representation ::= ...

| = private ..
240

Extensible variant types can be declared private. This prevents new constructors from being
declared directly, but allows extension constructors to be referred to in interfaces.
module Msg : sig
type t = private ..
module MkConstr (X : sig type t end) : sig
type t += C of X.t
end
end = struct
type t = ..
module MkConstr (X : sig type t end) = struct
type t += C of X.t
end
end

12.15 Generative functors

(Introduced in OCaml 4.02)

module-expr ::= ...

| functor () -> module-expr
| module-expr ()
definition ::= ...
| module module-name {( module-name : module-type ) | ()} [: module-type]
= module-expr
module-type ::= ...
| [functor] () -> module-type
specification ::= ...
| module module-name {( module-name : module-type ) | ()} : module-type

A generative functor takes a unit () argument. In order to use it, one must necessarily apply it
to this unit argument, ensuring that all type components in the result of the functor behave in a
generative way, i.e. they are different from types obtained by other applications of the same functor.
This is equivalent to taking an argument of signature sig end, and always applying to struct end,
but not to some defined module (in the latter case, applying twice to the same module would return
identical types).
As a side-effect of this generativity, one is allowed to unpack first-class modules in the body of
generative functors.

12.16 Extension-only syntax

(Introduced in OCaml 4.02.2, extended in 4.03)
Chapter 12. Language extensions 241

Some syntactic constructions are accepted during parsing and rejected during type checking.
These syntactic constructions can therefore not be used directly in vanilla OCaml. However, -ppx
rewriters and other external tools can exploit this parser leniency to extend the language with these
new syntactic constructions by rewriting them to vanilla constructions.

12.16.1 Extension operators

(Introduced in OCaml 4.02.2, extended to unary operators in OCaml 4.12.0)

infix-symbol ::= ...

| # {operator-char} # {operator-char | #}
prefix-symbol ::= ...
| (? | ~ | !) {operator-char} # {operator-char | #}

There are two classes of operators available for extensions: infix operators with a name starting
with a # character and containing more than one # character, and unary operators with a name
(starting with a ?, ~, or ! character) containing at least one # character.
For instance:
# let infix x y = x##y;;
Error : ## is not a valid value identifier .

# let prefix x = !#x;;

Error : !# is not a valid value identifier .
Note that both ## and !# must be eliminated by a ppx rewriter to make this example valid.

12.16.2 Extension literals

(Introduced in OCaml 4.03)

float-literal ::= ...

| [-] (0 . . . 9) {0 . . . 9 | _} [. {0 . . . 9 | _}] [(e | E) [+ | -] (0 . . . 9) {0 . . . 9 | _}] [g . . . z | G . . . Z]
| [-] (0x | 0X) (0 . . . 9 | A . . . F | a . . . f) {0 . . . 9 | A . . . F | a . . . f | _}
[. {0 . . . 9 | A . . . F | a . . . f | _}] [(p | P) [+ | -] (0 . . . 9) {0 . . . 9 | _}] [g . . . z | G . . . Z]
int-literal ::= ...
| [-] (0 . . . 9) {0 . . . 9 | _} [g . . . z | G . . . Z]
| [-] (0x | 0X) (0 . . . 9 | A . . . F | a . . . f) {0 . . . 9 | A . . . F | a . . . f | _} [g . . . z | G . . . Z]
| [-] (0o | 0O) (0 . . . 7) {0 . . . 7 | _} [g . . . z | G . . . Z]
| [-] (0b | 0B) (0 . . . 1) {0 . . . 1 | _} [g . . . z | G . . . Z]

Int and float literals followed by an one-letter identifier in the range [g.. z | G.. Z] are extension-only
literals.
242

12.17 Inline records

(Introduced in OCaml 4.03)
constr-args ::= ...
| record-decl

The arguments of sum-type constructors can now be defined using the same syntax as records.
Mutable and polymorphic fields are allowed. GADT syntax is supported. Attributes can be specified
on individual fields.
Syntactically, building or matching constructors with such an inline record argument is similar
to working with a unary constructor whose unique argument is a declared record type. A pattern
can bind the inline record as a pseudo-value, but the record cannot escape the scope of the binding
and can only be used with the dot-notation to extract or modify fields or to build new constructor
values.
type t =
| Point of {width: int; mutable x: float; mutable y: float}
| Other

let v = Point {width = 10; x = 0.; y = 0.}

let scale l = function

| Point p -> Point {p with x = l *. p.x; y = l *. p.y}
| Other -> Other

let print = function

| Point {x; y; _} -> Printf.printf "%f/%f" x y
| Other -> ()

let reset = function

| Point p -> p.x <- 0.; p.y <- 0.
| Other -> ()
let invalid = function
| Point p -> p
Error : This form is not allowed as the type of the inlined record could escape .

12.18 Documentation comments

(Introduced in OCaml 4.03)
Comments which start with ** are treated specially by the compiler. They are automatically
converted during parsing into attributes (see 12.12) to allow tools to process them as documentation.
Such comments can take three forms: floating comments, item comments and label comments.
Any comment starting with ** which does not match one of these forms will cause the compiler to
emit warning 50.
Chapter 12. Language extensions 243

Comments which start with ** are also used by the ocamldoc documentation generator (see
19). The three comment forms recognised by the compiler are a subset of the forms accepted by
ocamldoc (see 19.2).

12.18.1 Floating comments

Comments surrounded by blank lines that appear within structures, signatures, classes or class
types are converted into floating-attributes. For example:
type t = T

(∗∗ Now some definitions for [t] ∗)

let mkT = T
will be converted to:
type t = T

[@@@ocaml.text " Now some definitions for [t] "]

let mkT = T

12.18.2 Item comments

Comments which appear immediately before or immediately after a structure item, signature item,
class item or class type item are converted into item-attributes. Immediately before or immediately
after means that there must be no blank lines, ;;, or other documentation comments between them.
For example:
type t = T
(∗∗ A description of [t] ∗)
or
(∗∗ A description of [t] ∗)
type t = T
will be converted to:
type t = T
[@@ocaml.doc " A description of [t] "]
Note that, if a comment appears immediately next to multiple items, as in:
type t = T
(∗∗ An ambiguous comment ∗)
type s = S
then it will be attached to both items:
type t = T
[@@ocaml.doc " An ambiguous comment "]
type s = S
[@@ocaml.doc " An ambiguous comment "]
and the compiler will emit warning 50.
244

12.18.3 Label comments

Comments which appear immediately after a labelled argument, record field, variant constructor,
object method or polymorphic variant constructor are are converted into attributes. Immediately
after means that there must be no blank lines or other documentation comments between them.
For example:
type t1 = lbl:int (∗∗ Labelled argument ∗) -> unit

type t2 = {
fld: int; (∗∗ Record field ∗)
fld2: float;
}

type t3 =
| Cstr of string (∗∗ Variant constructor ∗)
| Cstr2 of string

type t4 = < meth: int * int; (∗∗ Object method ∗) >

type t5 = [
`PCstr (∗∗ Polymorphic variant constructor ∗)
]
will be converted to:
type t1 = lbl:(int [@ocaml.doc " Labelled argument "]) -> unit

type t2 = {
fld: int [@ocaml.doc " Record field "];
fld2: float;
}

type t3 =
| Cstr of string [@ocaml.doc " Variant constructor "]
| Cstr2 of string

type t4 = < meth : int * int [@ocaml.doc " Object method "] >

type t5 = [
`PCstr [@ocaml.doc " Polymorphic variant constructor "]
]
Note that label comments take precedence over item comments, so:
type t = T of string
(∗∗ Attaches to T not t ∗)
will be converted to:
type t = T of string [@ocaml.doc " Attaches to T not t "]
Chapter 12. Language extensions 245

whilst:
type t = T of string
(∗∗ Attaches to T not t ∗)
(∗∗ Attaches to t ∗)
will be converted to:
type t = T of string [@ocaml.doc " Attaches to T not t "]
[@@ocaml.doc " Attaches to t "]
In the absence of meaningful comment on the last constructor of a type, an empty comment (**)
can be used instead:
type t = T of string
(∗∗)
(∗∗ Attaches to t ∗)
will be converted directly to
type t = T of string
[@@ocaml.doc " Attaches to t "]

12.19 Extended indexing operators

(Introduced in 4.06)

dot-ext ::=
| dot-operator-char {operator-char}
dot-operator-char ::= ! | ? | core-operator-char | % | :
expr ::= ...
| expr . [module-path .] dot-ext (( expr ) | [ expr ] | { expr }) [<- expr]
operator-name ::= ...
| . dot-ext (() | [] | {}) [<-]

This extension provides syntactic sugar for getting and setting elements for user-defined indexed
types. For instance, we can define python-like dictionaries with
module Dict = struct
include Hashtbl
let ( .%{} ) tabl index = find tabl index
let ( .%{}<- ) tabl index value = add tabl index value
end
let dict =
let dict = Dict.create 10 in
let () =
dict.Dict.%{"one"} <- 1;
let open Dict in
246

dict.%{"two"} <- 2 in
dict
# dict.Dict.%{"one"};;
- : int = 1

# let open Dict in dict.%{"two"};;

- : int = 2

12.19.1 Multi-index notation

expr ::= ...
| expr . [module-path .] dot-ext ( expr {; expr}+ ) [<- expr]
| expr . [module-path .] dot-ext [ expr {; expr}+ ] [<- expr]
| expr . [module-path .] dot-ext { expr {; expr}+ } [<- expr]
operator-name ::= ...
| . dot-ext ((;..) | [;..] | {;..}) [<-]

Multi-index are also supported through a second variant of indexing operators

let (.%[;..]) = Bigarray.Genarray.get
let (.%{;..}) = Bigarray.Genarray.get
let (.%(;..)) = Bigarray.Genarray.get
which is called when an index literals contain a semicolon separated list of expressions with two
and more elements:
let sum x y = x.%[1;2;3] + y.%[1;2]
(∗ is equivalent to ∗)
let sum x y = (.%[;..]) x [|1;2;3|] + (.%[;..]) y [|1;2|]
In particular this multi-index notation makes it possible to uniformly handle indexing Genarray
and other implementations of multidimensional arrays.
module A = Bigarray.Genarray
let (.%{;..}) = A.get
let (.%{;..}<- ) = A.set
let (.%{ }) a k = A.get a [|k|]
let (.%{ }<-) a k x = A.set a [|k|] x
let syntax_compare vec mat t3 t4 =
vec.%{0} = A.get vec [|0|]
&& mat.%{0;0} = A.get mat [|0;0|]
&& t3.%{0;0;0} = A.get t3 [|0;0;0|]
&& t4.%{0;0;0;0} = t4.{0,0,0,0}
Beware that the differentiation between the multi-index and single index operators is purely
syntactic: multi-index operators are restricted to index expressions that contain one or more
semicolons ;. For instance,
let pair vec mat = vec.%{0}, mat.%{0;0}
Chapter 12. Language extensions 247

is equivalent to
let pair vec mat = (.%{ }) vec 0, (.%{;..}) mat [|0;0|]
Notice that in the vec case, we are calling the single index operator, (.%{}), and not the multi-index
variant, (.{;..}). For this reason, it is expected that most users of multi-index operators will need
to define conjointly a single index variant
let (.%{;..}) = A.get
let (.%{ }) a k = A.get a [|k|]
to handle both cases uniformly.

12.20 Empty variant types

(Introduced in 4.07.0)

type-representation ::= ...

| =|
This extension allows user to define empty variants. Empty variant type can be eliminated by
refutation case of pattern matching.
type t = |
let f (x: t) = match x with _ -> .

12.21 Alerts
(Introduced in 4.08)
Since OCaml 4.08, it is possible to mark components (such as value or type declarations) in
signatures with “alerts” that will be reported when those components are referenced. This generalizes
the notion of “deprecated” components which were previously reported as warning 3. Those alerts
can be used for instance to report usage of unsafe features, or of features which are only available
on some platforms, etc.
Alert categories are identified by a symbolic identifier (a lowercase identifier, following the usual
lexical rules) and an optional message. The identifier is used to control which alerts are enabled,
and which ones are turned into fatal errors. The message is reported to the user when the alert is
triggered (i.e. when the marked component is referenced).
The ocaml.alert or alert attribute serves two purposes: (i) to mark component with an alert
to be triggered when the component is referenced, and (ii) to control which alert names are enabled.
In the first form, the attribute takes an identifier possibly followed by a message. Here is an example
of a value declaration marked with an alert:

module U: sig
val fork: unit -> bool
[@@alert unix "This function is only available under Unix."]
end
248

Here unix is the identifier for the alert. If this alert category is enabled, any reference to U.fork
will produce a message at compile time, which can be turned or not into a fatal error.
And here is another example as a floating attribute on top of an “.mli” file (i.e. before any other
non-attribute item) or on top of an “.ml” file without a corresponding interface file, so that any
reference to that unit will trigger the alert:

[@@@alert unsafe "This module is unsafe!"]

Controlling which alerts are enabled and whether they are turned into fatal errors is done either
through the compiler’s command-line option -alert <spec> or locally in the code through the
alert or ocaml.alert attribute taking a single string payload <spec>. In both cases, the syntax
for <spec> is a concatenation of items of the form:

• +id enables alert id.

• -id disables alert id.
• ++id turns alert id into a fatal error.
• --id turns alert id into non-fatal mode.
• @id equivalent to ++id+id (enables id and turns it into a fatal-error)

As a special case, if id is all, it stands for all alerts.

Here are some examples:

(* Disable all alerts, reenables just unix (as a soft alert) and window
(as a fatal-error), for the rest of the current structure *)

[@@@alert "-all--all+unix@window"]
...

let x =
(* Locally disable the window alert *)
begin[@alert "-window"]
...
end

Before OCaml 4.08, there was support for a single kind of deprecation alert. It is now known
as the deprecated alert, but legacy attributes to trigger it and the legacy ways to control it as
warning 3 are still supported. For instance, passing -w +3 on the command-line is equivalent to
-alert +deprecated, and:

val x: int
[@@ocaml.deprecated "Please do something else"]

is equivalent to:

val x: int
[@@ocaml.alert deprecated "Please do something else"]
Chapter 12. Language extensions 249

12.22 Generalized open statements

(Introduced in 4.08)

definition ::= ...

| open module-expr
| open! module-expr
specification ::= ...
| open extended-module-path
| open! extended-module-path
expr ::= ...
| let open module-expr in expr
| let open! module-expr in expr

This extension makes it possible to open any module expression in module structures and
expressions. A similar mechanism is also available inside module types, but only for extended
module paths (e.g. F(X).G(Y)).
For instance, a module can be constrained when opened with
module M = struct let x = 0 let hidden = 1 end
open (M:sig val x: int end)
let y = hidden
Error : Unbound value hidden
Another possibility is to immediately open the result of a functor application
let sort (type x) (x:x list) =
let open Set.Make(struct type t = x let compare=compare end) in
elements (of_list x)
val sort : 'x list -> 'x list = <fun>
Going further, this construction can introduce local components inside a structure,
module M = struct
let x = 0
open! struct
let x = 0
let y = 1
end
let w = x + y
end
module M : sig val x : int val w : int end
One important restriction is that types introduced by open struct... end cannot appear in the
signature of the enclosing structure, unless they are defined equal to some non-local type. So:
module M = struct
open struct type 'a t = 'a option = None | Some of 'a end
250

let x : int t = Some 1

end
module M : sig val x : int option end
is OK, but:
module M = struct
open struct type t = A end
let x = A
end
Error : The type t introduced by this open appears in the signature .
The value x has no valid type if t is hidden .
is not because x cannot be given any type other than t, which only exists locally. Although the
above would be OK if x too was local:
module M: sig end = struct
open struct
type t = A
end
…
open struct let x = A end
…
end
module M : sig end
Inside signatures, extended opens are limited to extended module paths,
module type S = sig
module F: sig end -> sig type t end
module X: sig end
open F(X)
val f: t
end
module type S =
sig
module F : sig end -> sig type t end
module X : sig end
val f : F(X).t
end
and not

open struct type t = int end

In those situations, local substitutions(see 12.7.2) can be used instead.

Beware that this extension is not available inside class definitions:

class c =
let open Set.Make(Int) in
...
Chapter 12. Language extensions 251

12.23 Binding operators

(Introduced in 4.08.0)

let-operator ::=
| let (core-operator-char | <) {dot-operator-char}
and-operator ::=
| and (core-operator-char | <) {dot-operator-char}
operator-name ::= ...
| let-operator
| and-operator
letop-binding ::= pattern = expr
| value-name
expr ::= ...
| let-operator letop-binding {and-operator letop-binding} in expr

Binding operators offer syntactic sugar to expose library functions under (a variant of) the
familiar syntax of standard keywords. Currently supported “binding operators” are let<op> and
and<op>, where <op> is an operator symbol, for example and+$.
Binding operators were introduced to offer convenient syntax for working with monads and
applicative functors; for those, we propose conventions using operators * and + respectively. They
may be used for other purposes, but one should keep in mind that each new unfamiliar notation
introduced makes programs harder to understand for non-experts. We expect that new conventions
will be developed over time on other families of operator.

12.23.1 Examples
Users can define let operators:
let ( let* ) o f =
match o with
| None -> None
| Some x -> f x

let return x = Some x

val ( let* ) : 'a option -> ('a -> 'b option) -> 'b option = <fun>
val return : 'a -> 'a option = <fun>
and then apply them using this convenient syntax:
let find_and_sum tbl k1 k2 =
let* x1 = Hashtbl.find_opt tbl k1 in
let* x2 = Hashtbl.find_opt tbl k2 in
return (x1 + x2)
252

val find_and_sum : ('a, int) Hashtbl.t -> 'a -> 'a -> int option = <fun>
which is equivalent to this expanded form:
let find_and_sum tbl k1 k2 =
( let* ) (Hashtbl.find_opt tbl k1)
(fun x1 ->
( let* ) (Hashtbl.find_opt tbl k2)
(fun x2 -> return (x1 + x2)))
val find_and_sum : ('a, int) Hashtbl.t -> 'a -> 'a -> int option = <fun>
Users can also define and operators:
module ZipSeq = struct

type 'a t = 'a Seq.t

open Seq

let rec return x =

fun () -> Cons(x, return x)

let rec prod a b =

fun () ->
match a (), b () with
| Nil, _ | _, Nil -> Nil
| Cons(x, a), Cons(y, b) -> Cons((x, y), prod a b)

let ( let+ ) f s = map s f

let ( and+ ) a b = prod a b

end
module ZipSeq :
sig
type 'a t = 'a Seq.t
val return : 'a -> 'a Seq.t
val prod : 'a Seq.t -> 'b Seq.t -> ('a * 'b) Seq.t
val ( let+ ) : 'a Seq.t -> ('a -> 'b) -> 'b Seq.t
val ( and+ ) : 'a Seq.t -> 'b Seq.t -> ('a * 'b) Seq.t
end
to support the syntax:
open ZipSeq
let sum3 z1 z2 z3 =
let+ x1 = z1
and+ x2 = z2
and+ x3 = z3 in
x1 + x2 + x3
Chapter 12. Language extensions 253

val sum3 : int Seq.t -> int Seq.t -> int Seq.t -> int Seq.t = <fun>
which is equivalent to this expanded form:
open ZipSeq
let sum3 z1 z2 z3 =
( let+ ) (( and+ ) (( and+ ) z1 z2) z3)
(fun ((x1, x2), x3) -> x1 + x2 + x3)
val sum3 : int Seq.t -> int Seq.t -> int Seq.t -> int Seq.t = <fun>

12.23.2 Conventions
An applicative functor should provide a module implementing the following interface:
module type Applicative_syntax = sig
type 'a t
val ( let+ ) : 'a t -> ('a -> 'b) -> 'b t
val ( and+ ): 'a t -> 'b t -> ('a * 'b) t
end
where (let+) is bound to the map operation and (and+) is bound to the monoidal product
operation.
A monad should provide a module implementing the following interface:
module type Monad_syntax = sig
include Applicative_syntax
val ( let* ) : 'a t -> ('a -> 'b t) -> 'b t
val ( and* ): 'a t -> 'b t -> ('a * 'b) t
end
where (let*) is bound to the bind operation, and (and*) is also bound to the monoidal product
operation.

12.23.3 General desugaring rules

The form

let<op0>
x1 = e1
and<op1>
x2 = e2
and<op2>
x3 = e3
in e

desugars into

( let<op0> )
(( and<op2> )
(( and<op1> )
254

e1
e2)
e3)
(fun ((x1, x2), x3) -> e)

This of course works for any number of nested and-operators. One can express the general rule
by repeating the following simplification steps:

• The first and-operator in

let<op0> x1 = e1 and<op1> x2 = e2 and... in e

can be desugared into a function application

let<op0> (x1, x2) = ( and<op1> ) e1 e2 and... in e.

• Once all and-operators have been simplified away, the let-operator in

let<op> x1 = e1 in e

can be desugared into an application

( let<op> ) e1 (fun x1 -> e).

Note that the grammar allows mixing different operator symbols in the same binding (<op0>,
<op1>, <op2> may be distinct), but we strongly recommend APIs where let-operators and and-
operators working together use the same symbol.

12.23.4 Short notation for variable bindings (let-punning)

(Introduced in 4.13.0)
When the expression being bound is a variable, it can be convenient to use the shorthand
notation let+ x in ..., which expands to let+ x = x in .... This notation, also known as
let-punning, allows the sum3 function above can be written more concisely as:
open ZipSeq
let sum3 z1 z2 z3 =
let+ z1 and+ z2 and+ z3 in
z1 + z2 + z3
val sum3 : int Seq.t -> int Seq.t -> int Seq.t -> int Seq.t = <fun>
This notation is also supported for extension nodes, expanding let%foo x in ... to
let%foo x = x in .... However, to avoid confusion, this notation is not supported for plain let
bindings.
Chapter 12. Language extensions 255

12.24 Effect handlers

(Introduced in 5.0. The syntax support for deep handlers was introduced in 5.3.)

pattern ::= ...

| effect pattern, value-name

Effect handlers are a mechanism for modular programming with user-defined effects. Effect
handlers allow the programmers to describe computations that perform effectful operations, whose
meaning is described by handlers that enclose the computations. Effect handlers are a generalization
of exception handlers and enable non-local control-flow mechanisms such as resumable exceptions,
lightweight threads, coroutines, generators and asynchronous I/O to be composably expressed. In
this tutorial, we shall see how some of these mechanisms can be built using effect handlers.

12.24.1 Basics
To understand the basics, let us define an effect (that is, an operation) that takes an integer argument
and returns an integer result. We name this effect Xchg.
open Effect
open Effect.Deep

type _ Effect.t += Xchg: int -> int t

let comp1 () = perform (Xchg 0) + perform (Xchg 1)
We declare the exchange effect Xchg by extending the pre-defined extensible variant type
Effect.t with a new constructor Xchg: int -> int t. The declaration may be intuitively read
as “the Xchg effect takes an integer parameter, and when this effect is performed, it returns an
integer”. The computation comp1 performs the effect twice using the perform primitive and returns
their sum.
We can handle the Xchg effect by implementing a handler that always returns the successor of
the offered value:
try comp1 () with
| effect (Xchg n), k -> continue k (n+1)
- : int = 3
We run the computation comp1 () under an effect handler that handles the Xchg effect with a
continuation bound to k. Here effect is a keyword which signifies that the Xchg n pattern matches
effects and not exceptions. As mentioned earlier, effect handlers are a generalization of exception
handlers. Similar to exception handlers, when the computation performs the Xchg effect, the control
jumps to the corresponding handler, and unhandled effects are forwarded to the outer handler.
However, unlike exception handlers, the handler is also provided with the delimited continuation k,
which represents the suspended computation between the point of perform and this handler.
The handler uses the continue primitive to resume the suspended computation with the successor
of the offered value. In this example, the computation comp1 performs Xchg 0 and Xchg 1 and
receives the values 1 and 2 from the handler respectively. Hence, the whole expression evaluates to
3.
256

In this example, we use the deep version of the effect handlers here as opposed to the shallow
version. A deep handler monitors a computation until the computation terminates (either normally
or via an exception), and handles all of the effects performed (in sequence) by the computation. In
contrast, a shallow handler monitors a computation until either the computation terminates or the
computation performs one effect, and it handles this single effect only. In situations where they
are applicable, deep handlers are usually preferred. An example that utilises shallow handlers is
discussed later in 12.24.13.

12.24.2 Concurrency
The expressive power of effect handlers comes from the delimited continuation. While the previous
example immediately resumed the computation, the computation may be resumed later, running
some other computation in the interim. Let us extend the previous example and implement message-
passing concurrency between two concurrent computations using the Xchg effect. We call these
concurrent computations tasks.
A task either is in a suspended state or is completed. We represent the task status as follows:
type 'a status =
Complete of 'a
| Suspended of {msg: int; cont: (int, 'a status) continuation}
A task either is complete, with a result of type 'a, or is suspended with the message msg to send
and the continuation cont. The type (int,'a status) continuation says that the suspended
delimited computation expects an int value to resume and returns a value of type 'a status when
resumed.
Next, we define a step function that executes one step of computation until it completes or
suspends:
let step (f : unit -> 'a) () : 'a status =
match f () with
| v -> Complete v
| effect (Xchg msg), cont -> Suspended {msg; cont}
The argument to the step function, f, is a computation that can perform an Xchg effect and
returns a result of type 'a. The step function itself returns a value of type 'a status. Similar to
exception patterns in a match ... with expression (11.6.1), OCaml also supports effect patterns.
Here, we pattern match the result of running the computation f. If the computation returns with a
value v, we return Complete v. Instead, if the computation performs the effect Xchg msg with the
continuation cont, then we return Suspended {msg;cont}. In this case, the continuation cont is
not immediately invoked by the handler; instead, it is stored in a data structure for later use.
Since the step function handles the Xchg effect, step f is a computation that does not perform
the Xchg effect. It may however perform other effects. Moreover, since we are using deep handlers,
the continuation cont stored in the status does not perform the Xchg effect.
We can now write a simple scheduler that runs a pair of tasks to completion:
let rec run_both a b =
match a (), b () with
| Complete va, Complete vb -> (va, vb)
| Suspended {msg = m1; cont = k1},
Chapter 12. Language extensions 257

Suspended {msg = m2; cont = k2} ->

run_both (fun () -> continue k1 m2)
(fun () -> continue k2 m1)
| _ -> failwith "Improper synchronization"
Both of the tasks may run to completion, or both may offer to exchange a message. In the
latter case, each computation receives the value offered by the other computation. The situation
where one computation offers an exchange while the other computation terminates is regarded as a
programmer error, and causes the handler to raise an exception
We can now define a second computation that also exchanges two messages:
let comp2 () = perform (Xchg 21) * perform (Xchg 21)
Finally, we can run the two computations together:
run_both (step comp1) (step comp2)
- : int * int = (42, 0)
The computation comp1 offers the values 0 and 1 and in exchange receives the values 21 and 21,
which it adds, producing 42. The computation comp2 offers the values 21 and 21 and in exchange
receives the values 0 and 1, which it multiplies, producing 0. The communication between the two
computations is programmed entirely inside run_both. Indeed, the definitions of comp1 and comp2,
alone, do not assign any meaning to the Xchg effect.

12.24.3 User-level threads

Let us extend the previous example for an arbitrary number of tasks. Many languages such as GHC
Haskell and Go provide user-level threads as a primitive feature implemented in the runtime system.
With effect handlers, user-level threads and their schedulers can be implemented in OCaml itself.
Typically, user-level threading systems provide a fork primitive to spawn off a new concurrent task
and a yield primitive to yield control to some other task. Correspondingly, we shall declare two
effects as follows:
type _ Effect.t += Fork : (unit -> unit) -> unit t
| Yield : unit t
The Fork effect takes a thunk (a suspended computation, represented as a function of type
unit -> unit) and returns a unit to the performer. The Yield effect is unparameterized and
returns a unit when performed. Let us consider that a task performing an Xchg effect may match
with any other task also offering to exchange a value.
We shall also define helper functions that simply perform these effects:
let fork f = perform (Fork f)
let yield () = perform Yield
let xchg v = perform (Xchg v)
A top-level run function defines the scheduler:
(∗ A concurrent round−robin scheduler ∗)
let run (main : unit -> unit) : unit =
let exchanger : (int * (int, unit) continuation) option ref =
ref None (∗ waiting exchanger ∗)
in
258

let run_q = Queue.create () in (∗ scheduler queue ∗)

let enqueue k v =
let task () = continue k v in
Queue.push task run_q
in
let dequeue () =
if Queue.is_empty run_q then () (∗ done ∗)
else begin
let task = Queue.pop run_q in
task ()
end
in
let rec spawn (f : unit -> unit) : unit =
match f () with
| () -> dequeue ()
| exception e ->
print_endline (Printexc.to_string e);
dequeue ()
| effect Yield, k -> enqueue k (); dequeue ()
| effect (Fork f), k -> enqueue k (); spawn f
| effect (Xchg n), k ->
begin match !exchanger with
| Some (n', k') -> exchanger := None; enqueue k' n; continue k n'
| None -> exchanger := Some (n, k); dequeue ()
end
in
spawn main
We use a mutable queue run_q to hold the scheduler queue. The FIFO queue enables round-robin
scheduling of tasks in the scheduler. enqueue inserts tasks into the queue, and dequeue extracts
tasks from the queue and runs them. The reference cell exchanger holds a (suspended) task offering
to exchange a value. At any time, there is either zero or one suspended task that is offering an
exchange.
The heavy lifting is done by the spawn function. The spawn function runs the given computation
f in an effect handler. If f returns with unit value, we dequeue and run the next task from the
scheduler queue. If the computation f raises any exception, we print the exception to the standard
output and run the next task from the scheduler.
The computation f may also perform effects. If f performs the Yield effect, the current task is
suspended (inserted into the queue of ready tasks), and the next task from the scheduler queue
is run. If the effect is Fork f, then the current task is suspended, and the new task f is executed
immediately via a tail call to spawn f. Note that this choice to run the new task first is arbitrary.
We could very well have chosen instead to insert the task for f into the ready queue and resumed k
immediately.
If the effect is Xchg, then we first check whether there is a task waiting to exchange. If so, we
enqueue the waiting task with the current value being offered and immediately resume the current
task with the value being offered. If not, we make the current task the waiting exchanger, and run
Chapter 12. Language extensions 259

the next task from the scheduler queue.

Now we can write a concurrent program that utilises the newly defined operations:
open Printf

let _ = run (fun _ ->

fork (fun _ ->
printf "[t1] Sending 0\n";
let v = xchg 0 in
printf "[t1] received %d\n" v);
fork (fun _ ->
printf "[t2] Sending 1\n";
let v = xchg 1 in
printf "[t2] received %d\n" v))
[t1] Sending 0
[t2] Sending 1
[t2] received 0
[t1] received 1
Observe that the messages from the two tasks are interleaved. Notice also that the snippet
above makes no reference to the effect handlers and is in direct style (no monadic operations). This
example illustrates that, with effect handlers, the user code in a concurrent program can remain in
simple direct style, and the use of effect handlers can be fully contained within the concurrency
library implementation.

12.24.4 Resuming with an exception

In addition to resuming a continuation with a value, effect handlers also permit resuming by raising
an effect at the point of perform. This is done with the help of the discontinue primitive. The
discontinue primitive helps ensure that resources are always eventually deallocated, even in the
presence of effects.
For example, consider the dequeue operation in the previous example reproduced below:
…
let dequeue () =
if Queue.is_empty run_q then () (∗ done ∗)
else (Queue.pop run_q) ()
If the scheduler queue is empty, dequeue considers that the scheduler is done and returns to
the caller. However, there may still be a task waiting to exchange a value (stored in the reference
cell exchanger), which remains blocked forever! If the blocked task holds onto resources, these
resources are leaked. For example, consider the following task:
let leaky_task () =
fork (fun _ ->
let oc = open_out "secret.txt" in
Fun.protect ~finally:(fun _ -> close_out oc) (fun _ ->
output_value oc (xchg 0)))
260

The task writes the received message to the file secret.txt. It uses Fun.protect to ensure
that the output channel oc is closed on both normal and exceptional return cases. Unfortunately,
this is not sufficient. If the exchange effect xchg 0 cannot be matched with an exchange effect
performed by some other thread, then this task remains blocked forever. Thus, the output channel
oc is never closed.
To avoid this problem, one must adhere to a simple discipline: every continuation must be
eventually either continued or discontinued. Here, we use discontinue to ensure that the
blocked task does not remain blocked forever. By discontinuing this task, we force it to terminate
(with an exception):
exception Improper_synchronization

let dequeue () =
if Queue.is_empty run_q then begin
match !exchanger with
| None -> () (∗ done ∗)
| Some (n, k) ->
exchanger := None;
discontinue k Improper_synchronization
end else (Queue.pop run_q) ()
When the scheduler queue is empty and there is a blocked exchanger thread, the dequeue function
discontinues the blocked thread with an Improper_synchronization exception. This exception is
raised at the blocked xchg function call, which causes the finally block to be run and closes the
output channel oc. From the point of view of the user, it seems as though the function call xchg 0
raises the exception Improper_synchronization.

12.24.5 Control inversion

When it comes to performing traversals on a data structure, there are two fundamental ways
depending on whether the producer or the consumer has the control over the traversal. For example,
in List.iter f l, the producer List.iter has the control and pushes the element to the consumer
f who processes them. On the other hand, the Seq[29.49] module provides a mechanism similar to
delayed lists where the consumer controls the traversal. For example, Seq.forever Random.bool
returns an infinite sequence of random bits where every bit is produced (on demand) when queried
by the consumer.
Naturally, producers such as List.iter are easier to write in the former style. The latter style
is ergonomically better for the consumer since it is preferable and more natural to be in control. To
have the best of both worlds, we would like to write a producer in the former style and automatically
convert it to the latter style. The conversion can be written once and for all as a library function,
thanks to effect handlers. Let us name this function invert. We will first look at how to use the
invert function before looking at its implementation details. The type of this function is given
below:
val invert : iter:(('a -> unit) -> unit) -> 'a Seq.t
The invert function takes an iter function (a producer that pushes elements to the consumer)
and returns a sequence (where the consumer has the control). For example,
Chapter 12. Language extensions 261

let lst_iter = Fun.flip List.iter [1;2;3]

val lst_iter : (int -> unit) -> unit = <fun>
is an iter function with type (int -> unit) -> unit. The expression lst_iter f pushes the
elements 1, 2 and 3 to the consumer f. For example,
lst_iter (fun i -> Printf.printf "%d\n" i)
1
2
3
- : unit = ()
The expression invert lst_iter returns a sequence that allows the consumer to traverse the
list on demand. For example,
let s = invert ~iter:lst_iter
let next = Seq.to_dispenser s;;
val s : int Seq.t = <fun>
val next : unit -> int option = <fun>

next();;
- : int option = Some 1

next();;
- : int option = Some 2

next();;
- : int option = Some 3

next();;
- : int option = None
We can use the same invert function on any iter function. For example,
let s = invert ~iter:(Fun.flip String.iter "OCaml")
let next = Seq.to_dispenser s;;
val s : char Seq.t = <fun>
val next : unit -> char option = <fun>

next();;
- : char option = Some 'O'

next();;
- : char option = Some 'C'

next();;
- : char option = Some 'a'

next();;
- : char option = Some 'm'
262

next();;
- : char option = Some 'l'

next();;
- : char option = None

12.24.6 Implementing control inversion

The implementation of the invert function is given below:
let invert (type a) ~(iter : (a -> unit) -> unit) : a Seq.t =
let module M = struct
type _ Effect.t += Yield : a -> unit t
end in
let yield v = perform (M.Yield v) in
fun () -> match iter yield with
| () -> Seq.Nil
| effect M.Yield v, k -> Seq.Cons (v, continue k)
The invert function declares an effect Yield that takes the element to be yielded as a parameter.
The yield function performs the Yield effect. The lambda abstraction fun () -> ... delays all
action until the first element of the sequence is demanded. Once this happens, the computation
iter yield is executed under an effect handler. Every time the iter function pushes an element to
the yield function, the computation is interrupted by the Yield effect. The Yield effect is handled
by returning the value Seq.Cons(v,continue k) to the consumer. The consumer gets the element
v as well as the suspended computation, which in the consumer’s eyes is just the tail of sequence.
When the consumer demands the next element from the sequence (by applying it to ()), the
continuation k is resumed. This allows the computation iter yield to make progress, until it either
yields another element or terminates normally. In the latter case, the value Seq.Nil is returned,
indicating to the consumer that the iteration is over.
It is important to note that the sequence returned by the invert function is ephemeral (as
defined by the Seq[29.49] module) i.e., the sequence must be used at most once. Additionally,
the sequence must be fully consumed (i.e., used at least once) so as to ensure that the captured
continuation is used linearly.

12.24.7 Semantics
In this section, we shall see the semantics of effect handlers with the help of examples.

12.24.8 Nesting handlers

Like exception handlers, effect handlers can be nested.
type _ Effect.t += E : int t
| F : string t

let foo () = perform F

Chapter 12. Language extensions 263

let bar () =
try foo () with
| effect E, k -> failwith "impossible"

let baz () =
try bar () with
| effect F, k -> continue k "Hello, world!"
In this example, the computation foo performs F, the inner handler handles only E and the
outer handler handles F. The call to baz returns Hello, world!.
baz ()
- : string = "Hello, world!"

12.24.9 Fibers
It is useful to know a little bit about the implementation of effect handlers to appreciate the design
choices and their performance characteristics. Effect handlers are implemented with the help of
runtime-managed, dynamically growing segments of stack called fibers. The program stack in OCaml
is a linked list of such fibers.
A new fiber is allocated for evaluating the computation enclosed by an effect handler. The fiber
is freed when the computation returns to the caller either normally by returning a value or by
raising an exception.
At the point of perform in foo in the previous example, the program stack looks like this:
+-----+ +-----+ +-----+
| | | | | |
| baz |<--| bar |<--| foo |
| | | | | |
| | | | | |
+-----+ +-----+ +-----+ <- stack_pointer
The two links correspond to the two effect handlers in the program. When the effect F is handled
in baz, the program state looks as follows:
+-----+ +-----+ +-----+
| | | | | | +-+
| baz | | bar |<--| foo |<--|k|
| | | | | | +-+
+-----+ <- stack_pointer +-----+ +-----+
The delimited continuation k is an object on the heap that refers to the segment of the stack that
corresponds to the suspended computation. Capturing a continuation does not involve copying stack
frames. When the continuation is resumed, the stack is restored to the previous state by linking
together the segment pointed to by k to the current stack. Since neither continuation capture nor
resumption requires copying stack frames, suspending the execution using perform and resuming it
using either continue or discontinue are fast.
264

12.24.10 Unhandled effects

Unlike languages such as Eff and Koka, effect handlers in OCaml do not provide effect safety; the
compiler does not statically ensure that all the effects performed by the program are handled. If
effects do not have a matching handler, then an Effect.Unhandled exception is raised at the point
of the corresponding perform. For example, in the previous example, bar does not handle the effect
F. Hence, we will get an Effect.Unhandled F exception when we run bar.
try bar () with Effect.Unhandled F -> "Saw Effect.Unhandled exception"
- : string = "Saw Effect.Unhandled exception"

12.24.11 Linear continuations

As discussed earlier 12.24.4, the delimited continuations in OCaml must be used linearly – every
captured continuation must be resumed either with a continue or discontinue exactly
once. Attempting to use a continuation more than once raises a Continuation_already_resumed
exception. For example:
try perform (Xchg 0) with
| effect Xchg n, k -> continue k 21 + continue k 21
Exception: Stdlib.Effect.Continuation_already_resumed.
The primary motivation for adding effect handlers to OCaml is to enable concurrent programming.
One-shot continuations are sufficient for almost all concurrent programming needs. They are also
much cheaper to implement compared to multi-shot continuations since they do not require stack
frames to be copied. Moreover, OCaml programs may also manipulate linear resources such as
sockets and file descriptors. The linearity discipline is easily broken if the continuations are allowed
to resume more than once. It would be quite hard to debug such linearity violations on resources
due to the lack of static checks for linearity and the non-local nature of control flow. Hence, OCaml
does not support multi-shot continuations.
While the “at most once resumption” property of continuations is ensured with a dynamic check,
there is no check to ensure that the continuations are resumed “at least once”. It is left to the user to
ensure that the captured continuations are resumed at least once. Not resuming continuations will
leak the memory allocated for the fibers as well as any resources that the suspended computation
may hold.
One may install a finaliser on the captured continuation to ensure that the resources are freed:
exception Unwind
Gc.finalise (fun k ->
try ignore (discontinue k Unwind) with _ -> ()) k
In this case, if k becomes unreachable, then the finaliser ensures that the continuation stack is
unwound by discontinuing with an Unwind exception, allowing the computation to free up resources.
However, the runtime cost of finalisers is much more than the cost of capturing a continuation.
Hence, it is recommended that the user take care of resuming the continuation exactly once rather
than relying on the finaliser.
Chapter 12. Language extensions 265

12.24.12 Limitations
OCaml’s effects are synchronous: It is not possible to perform an effect asynchronously from a
signal handler, a finaliser, a memprof callback, or a GC alarm, and catch it from the main part of
the code. Instead, this would result in an Effect.Unhandled exception (12.24.10).
Similarly, effects are incompatible with the use of callbacks from C to OCaml (section 22.7).
It is not possible for an effect to cross a call to caml_callback, this would instead result in an
Effect.Unhandled exception. In particular, care must be taken when mixing libraries that use
callbacks from C to OCaml and libraries that use effects.

12.24.13 Shallow handlers

The examples that we have seen so far have used deep handlers. A deep handler handles all the
effects performed (in sequence) by the computation. Whenever a continuation is captured in a
deep handler, the captured continuation also includes the handler. This means that, when the
continuation is resumed, the effect handler is automatically re-installed, and will handle the effect(s)
that the computation may perform in the future.
OCaml also provides shallow handlers. Compared to deep handlers, a shallow handler handles
only the first effect performed by the computation. The continuation captured in a shallow handler
does not include the handler. This means that, when the continuation is resumed, the handler is no
longer present. For this reason, when the continuation is resumed, the user is expected to provide a
new effect handler (possibly a different one) to handle the next effect that the computation may
perform.
Shallow handlers make it easier to express certain kinds of programs. Let us implement a
shallow handler that enforces a particular sequence of effects (a protocol) on a computation. For
this example, let us consider that the computation may perform the following effects:
type _ Effect.t += Send : int -> unit Effect.t
| Recv : int Effect.t
Let us assume that we want to enforce a protocol that only permits an alternating sequence
of Send and Recv effects that conform to the regular expression (Send;Recv)*;Send?. Hence, the
sequence of effects [] (the empty sequence), [Send], [Send;Recv], [Send;Recv;Send], etc., are
allowed, but not [Recv], [Send;Send], [Send;Recv;Recv], etc. The key observation here is that
the set of effects handled evolves over time. We can enforce this protocol quite naturally using
shallow handlers as shown below:
open Effect.Shallow

let run (comp: unit -> unit) : unit =

let rec loop_send : type a. (a,unit) continuation -> a -> unit = fun k v ->
continue_with k v
{ retc = Fun.id;
exnc = raise;
effc = fun (type b) (eff : b Effect.t) ->
match eff with
| Send n -> Some (fun (k: (b,_) continuation) ->
loop_recv n k ())
266

| Recv -> failwith "protocol violation"

| _ -> None }
and loop_recv : type a. int -> (a,unit) continuation -> a -> unit = fun n k v ->
continue_with k v
{ retc = Fun.id;
exnc = raise;
effc = fun (type b) (eff : b Effect.t) ->
match eff with
| Recv -> Some (fun (k: (b,_) continuation) ->
loop_send k n)
| Send v -> failwith "protocol violation"
| _ -> None }
in
loop_send (fiber comp) ()
The run function executes the computation comp ensuring that it can only perform an alternating
sequence of Send and Recv effects. The shallow handler uses a different set of primitives compared
to the deep handler. The primitive fiber (on the last line) takes an 'a -> 'b function and returns
a ('a,'b) Effect.Shallow.continuation.
Unlike deep handlers, OCaml does not provide syntax support for shallow handlers. The
expression continue_with k v h resumes the continuation k with value v under the handler h.
The handler here is a record with three fields for the value case (retc), the exceptional case (exnc)
and the effect case (effc).
The mutually recursive functions loop_send and loop_recv resume the given continuation k
with value v under different handlers. The loop_send function handles the Send effect and tail calls
the loop_recv function. If the computation performs the Recv effect, then loop_send aborts the
computation by raising an exception. Similarly, the loop_recv function handles the Recv effect and
tail calls the loop_send function. If the computation performs the Send effect, then loop_recv
aborts the computation. Given that the continuation captured in the shallow handler do not include
the handler, there is only ever one handler installed in the dynamic scope of the computation comp.
Note that unlike deep handlers with syntax support, explicit type annotations are necessary
for the shallow handler. We must use a locally abstract type (type b) in the effect handler (effc)
and explicitly type annotate the effect argument eff and the continuation k in each of the effect
cases. Another point to note is that the catch-all effect case “| _ -> None” is necessary. This case
may be intuitively read as “forward the unhandled effects to the outer handler”. The standard
library module Effect[29.17] also provides a non-syntactic version of deep handlers, where similar
annotations are necessary.
The computation is initially executed by the loop_send function (see last line in the code above)
which ensures that the first effect that the computation is allowed to perform is the Send effect.
Note that the computation is free to perform effects other than Send and Recv, which may be
handled by an outer handler.
We can see that the run function will permit a computation that follows the protocol:
run (fun () ->
printf "Send 42\n";
perform (Send 42);
printf "Recv: %d\n" (perform Recv);
Chapter 12. Language extensions 267

printf "Send 43\n";

perform (Send 43);
printf "Recv: %d\n" (perform Recv))
Send 42
Recv: 42
Send 43
Recv: 43
- : unit = ()
and aborts those that do not:
run (fun () ->
Printf.printf "Send 0\n";
perform (Send 0);
Printf.printf "Send 1\n";
perform (Send 1) (∗ protocol violation ∗))
Send 0
Send 1
Exception: Failure "protocol violation".
We may implement the same example using deep handlers using reference cells (easy, but
unsatisfying) or without them (harder). We leave this as an exercise to the reader.
268
 Part III

The OCaml tools

269
Chapter 13

Batch compilation (ocamlc)

This chapter describes the OCaml batch compiler ocamlc, which compiles OCaml source files to
bytecode object files and links these object files to produce standalone bytecode executable files.
These executable files are then run by the bytecode interpreter ocamlrun.

13.1 Overview of the compiler

The ocamlc command has a command-line interface similar to the one of most C compilers. It
accepts several types of arguments and processes them sequentially, after all options have been
processed:

• Arguments ending in .mli are taken to be source files for compilation unit interfaces. Interfaces
specify the names exported by compilation units: they declare value names with their types,
define public data types, declare abstract data types, and so on. From the file x.mli, the
ocamlc compiler produces a compiled interface in the file x.cmi.

• Arguments ending in .ml are taken to be source files for compilation unit implementations.
Implementations provide definitions for the names exported by the unit, and also contain
expressions to be evaluated for their side-effects. From the file x.ml, the ocamlc compiler
produces compiled object bytecode in the file x.cmo.
If the interface file x.mli exists, the implementation x.ml is checked against the corresponding
compiled interface x.cmi, which is assumed to exist. If no interface x.mli is provided, the
compilation of x.ml produces a compiled interface file x.cmi in addition to the compiled object
code file x.cmo. The file x.cmi produced corresponds to an interface that exports everything
that is defined in the implementation x.ml.

• Arguments ending in .cmo are taken to be compiled object bytecode. These files are linked
together, along with the object files obtained by compiling .ml arguments (if any), and the
OCaml standard library, to produce a standalone executable program. The order in which
.cmo and .ml arguments are presented on the command line is relevant: compilation units
are initialized in that order at run-time, and it is a link-time error to use a component of a
unit before having initialized it. Hence, a given x.cmo file must come before all .cmo files that
refer to the unit x.

271
272

• Arguments ending in .cma are taken to be libraries of object bytecode. A library of object
bytecode packs in a single file a set of object bytecode files (.cmo files). Libraries are built
with ocamlc -a (see the description of the -a option below). The object files contained in the
library are linked as regular .cmo files (see above), in the order specified when the .cma file
was built. The only difference is that if an object file contained in a library is not referenced
anywhere in the program, then it is not linked in.

• Arguments ending in .c are passed to the C compiler, which generates a .o object file (.obj
under Windows). This object file is linked with the program if the -custom flag is set (see the
description of -custom below).

• Arguments ending in .o or .a (.obj or .lib under Windows) are assumed to be C object

files and libraries. They are passed to the C linker when linking in -custom mode (see the
description of -custom below).

• Arguments ending in .so (.dll under Windows) are assumed to be C shared libraries (DLLs).
During linking, they are searched for external C functions referenced from the OCaml code,
and their names are written in the generated bytecode executable. The run-time system
ocamlrun then loads them dynamically at program start-up time.

The output of the linking phase is a file containing compiled bytecode that can be executed by
the OCaml bytecode interpreter: the command named ocamlrun. If a.out is the name of the file
produced by the linking phase, the command

ocamlrun a.out arg 1 arg 2 … arg n

executes the compiled code contained in a.out, passing it as arguments the character strings
arg 1 to arg n . (See chapter 15 for more details.)
On most systems, the file produced by the linking phase can be run directly, as in:

./a.out arg 1 arg 2 … arg n

The produced file has the executable bit set, and it manages to launch the bytecode interpreter
by itself.
The compiler is able to emit some information on its internal stages. It can output .cmt files for
the implementation of the compilation unit and .cmti for signatures if the option -bin-annot is
passed to it (see the description of -bin-annot below). Each such file contains a typed abstract
syntax tree (AST), that is produced during the type checking procedure. This tree contains all
available information about the location and the specific type of each term in the source file. The
AST is partial if type checking was unsuccessful.
These .cmt and .cmti files are typically useful for code inspection tools.

13.2 Options
The following command-line options are recognized by ocamlc. The options -pack, -a, -c,
-output-obj and -output-complete-obj are mutually exclusive.
Chapter 13. Batch compilation (ocamlc) 273

-a Build a library(.cma file) with the object files ( .cmo files) given on the command line, instead
of linking them into an executable file. The name of the library must be set with the -o
option.
If -custom, -cclib or -ccopt options are passed on the command line, these options are
stored in the resulting .cmalibrary. Then, linking with this library automatically adds back
the -custom, -cclib and -ccopt options as if they had been provided on the command line,
unless the -noautolink option is given.

-absname
Force error messages to show absolute paths for file names.

-no-absname
Do not try to show absolute filenames in error messages.

-annot
Deprecated since OCaml 4.11. Please use -bin-annot instead.

-args filename
Read additional newline-terminated command line arguments from filename.

-args0 filename
Read additional null character terminated command line arguments from filename.

-bin-annot
Dump detailed information about the compilation (types, bindings, tail-calls, etc) in binary
format. The information for file src.ml (resp. src.mli) is put into file src.cmt (resp. src.cmti).
In case of a type error, dump all the information inferred by the type-checker before the error.
The *.cmt and *.cmti files produced by -bin-annot contain more information and are much
more compact than the files produced by -annot.

-c Compile only. Suppress the linking phase of the compilation. Source code files are turned into
compiled files, but no executable file is produced. This option is useful to compile modules
separately.

-cc ccomp
Use ccomp as the C linker when linking in “custom runtime” mode (see the -custom option)
and as the C compiler for compiling .c source files. When linking object files produced by a
C++ compiler (such as g++ or clang++), it is recommended to use -cc c++.

-cclib -llibname
Pass the -llibname option to the C linker when linking in “custom runtime” mode (see the
-custom option). This causes the given C library to be linked with the program.

-ccopt option
Pass the given option to the C compiler and linker. When linking in “custom runtime” mode,
for instance -ccopt -Ldir causes the C linker to search for C libraries in directory dir. (See
the -custom option.)
274

-cmi-file filename
Use the given interface file to type-check the ML source file to compile. When this option is not
specified, the compiler looks for a .mli file with the same base name than the implementation
it is compiling and in the same directory. If such a file is found, the compiler looks for a
corresponding .cmi file in the included directories and reports an error if it fails to find one.

-color mode
Enable or disable colors in compiler messages (especially warnings and errors). The following
modes are supported:

auto
use heuristics to enable colors only if the output supports them (an ANSI-compatible tty
terminal);
always
enable colors unconditionally;
never
disable color output.

The environment variable OCAML_COLOR is considered if -color is not provided. Its values are
auto/always/never as above.
If -color is not provided, OCAML_COLOR is not set and the environment variable NO_COLOR is
set, then color output is disabled. Otherwise, the default setting is ’auto’, and the current
heuristic checks that the TERM environment variable exists and is not empty or dumb, and that
’isatty(stderr)’ holds.

-error-style mode
Control the way error messages and warnings are printed. The following modes are supported:

short
only print the error and its location;
contextual
like short, but also display the source code snippet corresponding to the location of the
error.

The default setting is contextual.

The environment variable OCAML_ERROR_STYLE is considered if -error-style is not provided.
Its values are short/contextual as above.

-compat-32
Check that the generated bytecode executable can run on 32-bit platforms and signal an error
if it cannot. This is useful when compiling bytecode on a 64-bit machine.

-config
Print the version number of ocamlc and a detailed summary of its configuration, then exit.

-config-var var
Print the value of a specific configuration variable from the -config output, then exit. If the
Chapter 13. Batch compilation (ocamlc) 275

variable does not exist, the exit code is non-zero. This option is only available since OCaml
4.08, so script authors should have a fallback for older versions.

-custom
Link in “custom runtime” mode. In the default linking mode, the linker produces bytecode
that is intended to be executed with the shared runtime system, ocamlrun. In the custom
runtime mode, the linker produces an output file that contains both the runtime system and
the bytecode for the program. The resulting file is larger, but it can be executed directly, even
if the ocamlrun command is not installed. Moreover, the “custom runtime” mode enables
static linking of OCaml code with user-defined C functions, as described in chapter 22.

Unix:

Never use the strip command on executables produced by ocamlc -custom,

this would remove the bytecode part of the executable.

Unix:

Security warning: never set the “setuid” or “setgid” bits on executables pro-
duced by ocamlc -custom, this would make them vulnerable to attacks.

-depend ocamldep-args
Compute dependencies, as the ocamldep command would do. The remaining arguments are
interpreted as if they were given to the ocamldep command.

-dllib -llibname
Arrange for the C shared library dlllibname.so (dlllibname.dll under Windows) to be
loaded dynamically by the run-time system ocamlrun at program start-up time.

-dllpath dir
Adds the directory dir to the run-time search path for shared C libraries. At link-time, shared
libraries are searched in the standard search path (the one corresponding to the -I option).
The -dllpath option simply stores dir in the produced executable file, where ocamlrun can
find it and use it as described in section 15.3.

-for-pack module-path
Generate an object file (.cmo) that can later be included as a sub-module (with the given
access path) of a compilation unit constructed with -pack. For instance, ocamlc -for-pack P
-c A.ml will generate a..cmo that can later be used with ocamlc -pack -o P.cmo a.cmo. Note:
you can still pack a module that was compiled without -for-pack but in this case exceptions
will be printed with the wrong names.

-g Add debugging information while compiling and linking. This option is required in order
to be able to debug the program with ocamldebug (see chapter 20), and to produce stack
backtraces when the program terminates on an uncaught exception (see section 15.2).

-no-g
Do not record debugging information (default).
276

-i Cause the compiler to print all defined names (with their inferred types or their definitions)
when compiling an implementation (.ml file). No compiled files (.cmo and .cmi files) are
produced. This can be useful to check the types inferred by the compiler. Also, since the
output follows the syntax of interfaces, it can help in writing an explicit interface (.mli file)
for a file: just redirect the standard output of the compiler to a .mli file, and edit that file to
remove all declarations of unexported names.

-I directory
Add the given directory to the list of directories searched for compiled interface files (.cmi),
compiled object code files .cmo, libraries (.cma) and C libraries specified with -cclib -lxxx.
By default, the current directory is searched first, then the standard library directory. Di-
rectories added with -I are searched after the current directory, in the order in which they
were given on the command line, but before the standard library directory. See also option
-nostdlib.
If the given directory starts with +, it is taken relative to the standard library directory. For
instance, -I +unix adds the subdirectory unix of the standard library to the search path.

-H directory
Behaves identically to -I, except that (a) programs may not directly refer to modules added
to the search path this way, and (b) these directories are searched after any -I directories.
This makes it possible to provide the compiler with compiled interface and object code files for
the current program’s transitive dependencies (the dependencies of its dependencies) without
allowing them to silently become direct dependencies.

-impl filename
Compile the file filename as an implementation file, even if its extension is not .ml.

-intf filename
Compile the file filename as an interface file, even if its extension is not .mli.

-intf-suffix string
Recognize file names ending with string as interface files (instead of the default .mli).

-keywords version+list
Set keywords according to the version+list specification.
This specification starts with an optional version number, defining the base set of keywords,
followed by a +-separated list of additional keywords to add to this base set.
Without an explicit version number, the base set of keywords is the set of keywords in the
current version of OCaml. Additional keywords that do not match any known keyword in the
current version of the language trigger an error whenever they are present in the source code.

-labels
Labels are not ignored in types, labels may be used in applications, and labelled parameters
can be given in any order. This is the default.

-linkall
Force all modules contained in libraries to be linked in. If this flag is not given, unreferenced
Chapter 13. Batch compilation (ocamlc) 277

modules are not linked in. When building a library (option -a), setting the -linkall option
forces all subsequent links of programs involving that library to link all the modules contained
in the library. When compiling a module (option -c), setting the -linkall option ensures
that this module will always be linked if it is put in a library and this library is linked.

-make-runtime
Build a custom runtime system (in the file specified by option -o) incorporating the C object
files and libraries given on the command line. This custom runtime system can be used later to
execute bytecode executables produced with the ocamlc -use-runtime runtime-name option.
See section 22.1.6 for more information.

-match-context-rows
Set the number of rows of context used for optimization during pattern matching compilation.
The default value is 32. Lower values cause faster compilation, but less optimized code. This
advanced option is meant for use in the event that a pattern-match-heavy program leads to
significant increases in compilation time.

-no-alias-deps
Do not record dependencies for module aliases. See section 12.8 for more information.

-no-app-funct
Deactivates the applicative behaviour of functors. With this option, each functor application
generates new types in its result and applying the same functor twice to the same argument
yields two incompatible structures.

-noassert
Do not compile assertion checks. Note that the special form assert false is always compiled
because it is typed specially. This flag has no effect when linking already-compiled files.

-noautolink
When linking .cmalibraries, ignore -custom, -cclib and -ccopt options potentially contained
in the libraries (if these options were given when building the libraries). This can be useful
if a library contains incorrect specifications of C libraries or C options; in this case, during
linking, set -noautolink and pass the correct C libraries and options on the command line.

-nolabels
Ignore non-optional labels in types. Labels cannot be used in applications, and parameter
order becomes strict.

-nostdlib
Do not include the standard library directory in the list of directories searched for compiled
interface files (.cmi), compiled object code files (.cmo), libraries (.cma), and C libraries
specified with -cclib -lxxx. See also option -I.

-o output-file
Specify the name of the output file to produce. For executable files, the default output name
is a.out under Unix and camlprog.exe under Windows. If the -a option is given, specify the
name of the library produced. If the -pack option is given, specify the name of the packed
278

object file produced. If the -output-obj or -output-complete-obj options are given, specify
the name of the produced object file. If the -c option is given, specify the name of the object
file produced for the next source file that appears on the command line.

-opaque
When the native compiler compiles an implementation, by default it produces a .cmx file
containing information for cross-module optimization. It also expects .cmx files to be present
for the dependencies of the currently compiled source, and uses them for optimization. Since
OCaml 4.03, the compiler will emit a warning if it is unable to locate the .cmx file of one of
those dependencies.
The -opaque option, available since 4.04, disables cross-module optimization information
for the currently compiled unit. When compiling .mli interface, using -opaque marks the
compiled .cmi interface so that subsequent compilations of modules that depend on it will
not rely on the corresponding .cmx file, nor warn if it is absent. When the native compiler
compiles a .ml implementation, using -opaque generates a .cmx that does not contain any
cross-module optimization information.
Using this option may degrade the quality of generated code, but it reduces compilation
time, both on clean and incremental builds. Indeed, with the native compiler, when the
implementation of a compilation unit changes, all the units that depend on it may need to
be recompiled – because the cross-module information may have changed. If the compilation
unit whose implementation changed was compiled with -opaque, no such recompilation needs
to occur. This option can thus be used, for example, to get faster edit-compile-test feedback
loops.

-open Module
Opens the given module before processing the interface or implementation files. If several
-open options are given, they are processed in order, just as if the statements open! Module1;;
... open! ModuleN;; were added at the top of each file.

-output-obj
Cause the linker to produce a C object file instead of a bytecode executable file. This is
useful to wrap OCaml code as a C library, callable from any C program. See chapter 22,
section 22.7.6. The name of the output object file must be set with the -o option. This option
can also be used to produce a C source file (.c extension) or a compiled shared/dynamic
library (.so extension, .dll under Windows).

-output-complete-exe
Build a self-contained executable by linking a C object file containing the bytecode program,
the OCaml runtime system and any other static C code given to ocamlc. The resulting effect
is similar to -custom, except that the bytecode is embedded in the C code so it is no longer
accessible to tools such as ocamldebug. On the other hand, the resulting binary is resistant
to strip.

-output-complete-obj
Same as -output-obj options except the object file produced includes the runtime and autolink
libraries.
Chapter 13. Batch compilation (ocamlc) 279

-pack
Build a bytecode object file (.cmo file) and its associated compiled interface (.cmi) that
combines the object files given on the command line, making them appear as sub-modules of
the output .cmo file. The name of the output .cmo file must be given with the -o option. For
instance,

ocamlc -pack -o p.cmo a.cmo b.cmo c.cmo

generates compiled files p.cmo and p.cmi describing a compilation unit having three sub-
modules A, B and C, corresponding to the contents of the object files a.cmo, b.cmo and c.cmo.
These contents can be referenced as P.A, P.B and P.C in the remainder of the program.

-pp command
Cause the compiler to call the given command as a preprocessor for each source file. The
output of command is redirected to an intermediate file, which is compiled. If there are no
compilation errors, the intermediate file is deleted afterwards.

-ppx command
After parsing, pipe the abstract syntax tree through the preprocessor command. The module
Ast_mapper, described in section 30.1, implements the external interface of a preprocessor.

-principal
Check information path during type-checking, to make sure that all types are derived in
a principal way. When using labelled arguments and/or polymorphic methods, this flag is
required to ensure future versions of the compiler will be able to infer types correctly, even if
internal algorithms change. All programs accepted in -principal mode are also accepted in
the default mode with equivalent types, but different binary signatures, and this may slow
down type checking; yet it is a good idea to use it once before publishing source code.

-rectypes
Allow arbitrary recursive types during type-checking. By default, only recursive types where
the recursion goes through an object type are supported. Note that once you have created an
interface using this flag, you must use it again for all dependencies.

-runtime-variant suffix
Add the suffix string to the name of the runtime library used by the program. Currently, only
one such suffix is supported: d, and only if the OCaml compiler was configured with option
-with-debug-runtime. This suffix gives the debug version of the runtime, which is useful for
debugging pointer problems in low-level code such as C stubs.

-safe-string
Enforce the separation between types string and bytes, thereby making strings read-only.
This is the default, and enforced since OCaml 5.0.

-safer-matching
Do not use type information to optimize pattern-matching. This allows to detect match
failures even if a pattern-matching was wrongly assumed to be exhaustive. This only impacts
GADT and polymorphic variant compilation.
280

-short-paths
When a type is visible under several module-paths, use the shortest one when printing the
type’s name in inferred interfaces and error and warning messages. Identifier names starting
with an underscore _ or containing double underscores __ incur a penalty of +10 when
computing their length.
-stop-after pass
Stop compilation after the given compilation pass. The currently supported passes are:
parsing, typing.
-strict-sequence
Force the left-hand part of each sequence to have type unit.
-strict-formats
Reject invalid formats that were accepted in legacy format implementations. You should use
this flag to detect and fix such invalid formats, as they will be rejected by future OCaml
versions.
-unboxed-types
When a type is unboxable (i.e. a record with a single argument or a concrete datatype with a
single constructor of one argument) it will be unboxed unless annotated with [@@ocaml.boxed].
-no-unboxed-types
When a type is unboxable it will be boxed unless annotated with [@@ocaml.unboxed]. This
is the default.
-unsafe
Turn bound checking off for array and string accesses (the v.(i) and s.[i] constructs).
Programs compiled with -unsafe are therefore slightly faster, but unsafe: anything can
happen if the program accesses an array or string outside of its bounds. Additionally, turn
off the check for zero divisor in integer division and modulus operations. With -unsafe, an
integer division (or modulus) by zero can halt the program or continue with an unspecified
result instead of raising a Division_by_zero exception.
-unsafe-string
Identify the types string and bytes, thereby making strings writable. This is intended for
compatibility with old source code and should not be used with new software. This option
raises an error unconditionally since OCaml 5.0.
-use-runtime runtime-name
Generate a bytecode executable file that can be executed on the custom runtime system
runtime-name, built earlier with ocamlc -make-runtime runtime-name. See section 22.1.6
for more information.
-v Print the version number of the compiler and the location of the standard library directory,
then exit.
-verbose
Print all external commands before they are executed, in particular invocations of the C
compiler and linker in -custom mode. Useful to debug C library problems.
Chapter 13. Batch compilation (ocamlc) 281

-version or -vnum
Print the version number of the compiler in short form (e.g. 3.11.0), then exit.

-w warning-list
Enable, disable, or mark as fatal the warnings specified by the argument warning-list. Each
warning can be enabled or disabled, and each warning can be fatal or non-fatal. If a warning is
disabled, it isn’t displayed and doesn’t affect compilation in any way (even if it is fatal). If a
warning is enabled, it is displayed normally by the compiler whenever the source code triggers
it. If it is enabled and fatal, the compiler will also stop with an error after displaying it.
The warning-list argument is a sequence of warning specifiers, with no separators between
them. A warning specifier is one of the following:

+num
Enable warning number num.
-num
Disable warning number num.
@num
Enable and mark as fatal warning number num.
+num1..num2
Enable warnings in the given range.
-num1..num2
Disable warnings in the given range.
@num1..num2
Enable and mark as fatal warnings in the given range.
+letter
Enable the set of warnings corresponding to letter. The letter may be uppercase or
lowercase.
-letter
Disable the set of warnings corresponding to letter. The letter may be uppercase or
lowercase.
@letter
Enable and mark as fatal the set of warnings corresponding to letter. The letter may be
uppercase or lowercase.
uppercase-letter
Enable the set of warnings corresponding to uppercase-letter.
lowercase-letter
Disable the set of warnings corresponding to lowercase-letter.

Alternatively, warning-list can specify a single warning using its mnemonic name (see below),
as follows:

+name
Enable warning name.
282

-name
Disable warning name.
@name
Enable and mark as fatal warning name.

Warning numbers, letters and names which are not currently defined are ignored. The warnings
are as follows (the name following each number specifies the mnemonic for that warning).

1 comment-start
Suspicious-looking start-of-comment mark.
2 comment-not-end
Suspicious-looking end-of-comment mark.
3 Deprecated synonym for the ’deprecated’ alert.
4 fragile-match
Fragile pattern matching: matching that will remain complete even if additional con-
structors are added to one of the variant types matched.
5 ignored-partial-application
Partially applied function: expression whose result has function type and is ignored.
6 labels-omitted
Label omitted in function application.
7 method-override
Method overridden.
8 partial-match
Partial match: missing cases in pattern-matching.
9 missing-record-field-pattern
Missing fields in a record pattern.
10 non-unit-statement
Expression on the left-hand side of a sequence that doesn’t have type unit (and that is
not a function, see warning number 5).
11 redundant-case
Redundant case in a pattern matching (unused match case).
12 redundant-subpat
Redundant sub-pattern in a pattern-matching.
13 instance-variable-override
Instance variable overridden.
14 illegal-backslash
Illegal backslash escape in a string constant.
15 implicit-public-methods
Private method made public implicitly.
16 unerasable-optional-argument
Unerasable optional argument.
Chapter 13. Batch compilation (ocamlc) 283

17 undeclared-virtual-method
Undeclared virtual method.
18 not-principal
Non-principal type.
19 non-principal-labels
Type without principality.
20 ignored-extra-argument
Unused function argument.
21 nonreturning-statement
Non-returning statement.
22 preprocessor
Preprocessor warning.
23 useless-record-with
Useless record with clause.
24 bad-module-name
Bad module name: the source file name is not a valid OCaml module name.
25 Ignored: now part of warning 8.
26 unused-var
Suspicious unused variable: unused variable that is bound with let or as, and doesn’t
start with an underscore (_) character.
27 unused-var-strict
Innocuous unused variable: unused variable that is not bound with let nor as, and
doesn’t start with an underscore (_) character.
28 wildcard-arg-to-constant-constr
Wildcard pattern given as argument to a constant constructor.
29 eol-in-string
Unescaped end-of-line in a string constant (non-portable code).
30 duplicate-definitions
Two labels or constructors of the same name are defined in two mutually recursive types.
31 module-linked-twice
A module is linked twice in the same executable.
I gnored: now a hard error (since 5.1).
32 unused-value-declaration
Unused value declaration. (since 4.00)
33 unused-open
Unused open statement. (since 4.00)
34 unused-type-declaration
Unused type declaration. (since 4.00)
35 unused-for-index
Unused for-loop index. (since 4.00)
284

36 unused-ancestor
Unused ancestor variable. (since 4.00)
37 unused-constructor
Unused constructor. (since 4.00)
38 unused-extension
Unused extension constructor. (since 4.00)
39 unused-rec-flag
Unused rec flag. (since 4.00)
40 name-out-of-scope
Constructor or label name used out of scope. (since 4.01)
41 ambiguous-name
Ambiguous constructor or label name. (since 4.01)
42 disambiguated-name
Disambiguated constructor or label name (compatibility warning). (since 4.01)
43 nonoptional-label
Nonoptional label applied as optional. (since 4.01)
44 open-shadow-identifier
Open statement shadows an already defined identifier. (since 4.01)
45 open-shadow-label-constructor
Open statement shadows an already defined label or constructor. (since 4.01)
46 bad-env-variable
Error in environment variable. (since 4.01)
47 attribute-payload
Illegal attribute payload. (since 4.02)
48 eliminated-optional-arguments
Implicit elimination of optional arguments. (since 4.02)
49 no-cmi-file
Absent cmi file when looking up module alias. (since 4.02)
50 unexpected-docstring
Unexpected documentation comment. (since 4.03)
51 wrong-tailcall-expectation
Function call annotated with an incorrect @tailcall attribute. (since 4.03)
52 fragile-literal-pattern (see 13.5.3)
Fragile constant pattern. (since 4.03)
53 misplaced-attribute
Attribute cannot appear in this context. (since 4.03)
54 duplicated-attribute
Attribute used more than once on an expression. (since 4.03)
55 inlining-impossible
Inlining impossible. (since 4.03)
Chapter 13. Batch compilation (ocamlc) 285

56 unreachable-case
Unreachable case in a pattern-matching (based on type information). (since 4.03)
57 ambiguous-var-in-pattern-guard (see 13.5.4)
Ambiguous or-pattern variables under guard. (since 4.03)
58 no-cmx-file
Missing cmx file. (since 4.03)
59 flambda-assignment-to-non-mutable-value
Assignment to non-mutable value. (since 4.03)
60 unused-module
Unused module declaration. (since 4.04)
61 unboxable-type-in-prim-decl
Unboxable type in primitive declaration. (since 4.04)
62 constraint-on-gadt
Type constraint on GADT type declaration. (since 4.06)
63 erroneous-printed-signature
Erroneous printed signature. (since 4.08)
64 unsafe-array-syntax-without-parsing
-unsafe used with a preprocessor returning a syntax tree. (since 4.08)
65 redefining-unit
Type declaration defining a new ’()’ constructor. (since 4.08)
66 unused-open-bang
Unused open! statement. (since 4.08)
67 unused-functor-parameter
Unused functor parameter. (since 4.10)
68 match-on-mutable-state-prevent-uncurry
Pattern-matching depending on mutable state prevents the remaining arguments from
being uncurried. (since 4.12)
69 unused-field
Unused record field. (since 4.13)
70 missing-mli
Missing interface file. (since 4.13)
71 unused-tmc-attribute
Unused @tail_mod_cons attribute. (since 4.14)
72 tmc-breaks-tailcall
A tail call is turned into a non-tail call by the @tail_mod_cons transformation. (since
4.14)
73 generative-application-expects-unit
A generative functor is applied to an empty structure (struct end) rather than to ().
(since 5.1)
74 degraded-to-partial-match
A pattern-matching is compiled as partial even if it appears to be total. (since 5.3)
286

A all warnings
C warnings 1, 2.
D Alias for warning 3.
E Alias for warning 4.
F Alias for warning 5.
K warnings 32, 33, 34, 35, 36, 37, 38, 39.
L Alias for warning 6.
M Alias for warning 7.
P Alias for warning 8.
R Alias for warning 9.
S Alias for warning 10.
U warnings 11, 12.
V Alias for warning 13.
X warnings 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 30.
Y Alias for warning 26.
Z Alias for warning 27.

The default setting is -w +a-4-6-7-9-27-29-32..42-44-45-48-50-60. It is displayed by

ocamlc -help. Note that warnings 5 and 10 are not always triggered, depending on the internals
of the type checker.

-warn-error warning-list
Mark as fatal the warnings specified in the argument warning-list. The compiler will stop
with an error when one of these warnings is emitted. The warning-list has the same meaning
as for the -w option: a + sign (or an uppercase letter) marks the corresponding warnings as
fatal, a - sign (or a lowercase letter) turns them back into non-fatal warnings, and a @ sign
both enables and marks as fatal the corresponding warnings.
Note: it is not recommended to use warning sets (i.e. letters) as arguments to -warn-error
in production code, because this can break your build when future versions of OCaml add
some new warnings.
The default setting is -warn-error -a (no warning is fatal).

-warn-help
Show the description of all available warning numbers.

-where
Print the location of the standard library, then exit.

-with-runtime
The pair of options -with-runtime and -without-runtime give precise control over the way
the runtime is linked. -with-runtime is the default. In summary, it instructs to include the
runtime system, or a reference to the default path of the runtime system, in the generated
Chapter 13. Batch compilation (ocamlc) 287

program/executable/object file. The detailed behaviour depends on the compiler and options
used:
For ocamlc, in its default linking mode (no use of -custom or -output-*), -with-runtime
creates a file which can be executed, whereas -without-runtime creates a pure bytecode
image which must be explicitly passed to a runtime (i.e. ./foo vs ocamlrun ./foo).
For all other uses of ocamlc and ocamlopt, -with-runtime and -without-runtime control
whether the compiler passes flags for linking with the installed runtime (-with-runtime) or
whether the user is required to pass them (-without-runtime).
For more information about the options -custom and -output-*, see their documentation
and section 22.7.6 of the manual.

-without-runtime
The compiler does not include the runtime system (nor a reference to it) in the generated
program, executable or object file; it must be supplied separately. See option -with-runtime
for details.

- file
Process file as a file name, even if it starts with a dash (-) character.

-help or --help
Display a short usage summary and exit.

contextual-cli-control Contextual control of command-line options

The compiler command line can be modified “from the outside” with the following mechanisms.
These are experimental and subject to change. They should be used only for experimental and
development work, not in released packages.

OCAMLPARAM (environment variable)

A set of arguments that will be inserted before or after the arguments from the command
line. Arguments are specified in a comma-separated list of name=value pairs. A _ is used to
specify the position of the command line arguments, i.e. a=x,_,b=y means that a=x should
be executed before parsing the arguments, and b=y after. Finally, an alternative separator
can be specified as the first character of the string, within the set :|; ,.

ocaml_compiler_internal_params (file in the stdlib directory)

A mapping of file names to lists of arguments that will be added to the command line (and
OCAMLPARAM) arguments.

13.3 Modules and the file system

This short section is intended to clarify the relationship between the names of the modules corre-
sponding to compilation units and the names of the files that contain their compiled interface and
compiled implementation.
The compiler always derives the module name by taking the capitalized base name of the source
file (.ml or .mli file). That is, it strips the leading directory name, if any, as well as the .ml or
288

.mli suffix; then, it set the first letter to uppercase, in order to comply with the requirement that
module names must be capitalized. For instance, compiling the file mylib/misc.ml provides an
implementation for the module named Misc. Other compilation units may refer to components
defined in mylib/misc.ml under the names Misc.name; they can also do open Misc, then use
unqualified names name.
The .cmi and .cmo files produced by the compiler have the same base name as the source file.
Hence, the compiled files always have their base name equal (modulo capitalization of the first
letter) to the name of the module they describe (for .cmi files) or implement (for .cmo files).
When the compiler encounters a reference to a free module identifier Mod, it looks in the search
path for a file named Mod.cmi or mod.cmi and loads the compiled interface contained in that file. As
a consequence, renaming .cmi files is not advised: the name of a .cmi file must always correspond to
the name of the compilation unit it implements. It is admissible to move them to another directory,
if their base name is preserved, and the correct -I options are given to the compiler. The compiler
will flag an error if it loads a .cmi file that has been renamed.
Compiled bytecode files (.cmo files), on the other hand, can be freely renamed once created.
That’s because the linker never attempts to find by itself the .cmo file that implements a module
with a given name: it relies instead on the user providing the list of .cmo files by hand.

13.4 Common errors

This section describes and explains the most frequently encountered error messages.

Cannot find file filename

The named file could not be found in the current directory, nor in the directories of the search
path. The filename is either a compiled interface file (.cmi file), or a compiled bytecode file
(.cmo file). If filename has the format mod.cmi, this means you are trying to compile a file
that references identifiers from module mod, but you have not yet compiled an interface for
module mod. Fix: compile mod.mli or mod.ml first, to create the compiled interface mod.cmi.
If filename has the format mod.cmo, this means you are trying to link a bytecode object file
that does not exist yet. Fix: compile mod.ml first.
If your program spans several directories, this error can also appear because you haven’t
specified the directories to look into. Fix: add the correct -I options to the command line.

Corrupted compiled interface filename

The compiler produces this error when it tries to read a compiled interface file (.cmi file) that
has the wrong structure. This means something went wrong when this .cmi file was written:
the disk was full, the compiler was interrupted in the middle of the file creation, and so on.
This error can also appear if a .cmi file is modified after its creation by the compiler. Fix:
remove the corrupted .cmi file, and rebuild it.

This expression has type t1 , but is used with type t2

This is by far the most common type error in programs. Type t1 is the type inferred for the
expression (the part of the program that is displayed in the error message), by looking at the
expression itself. Type t2 is the type expected by the context of the expression; it is deduced
Chapter 13. Batch compilation (ocamlc) 289

by looking at how the value of this expression is used in the rest of the program. If the two
types t1 and t2 are not compatible, then the error above is produced.
In some cases, it is hard to understand why the two types t1 and t2 are incompatible. For
instance, the compiler can report that “expression of type foo cannot be used with type foo”,
and it really seems that the two types foo are compatible. This is not always true. Two type
constructors can have the same name, but actually represent different types. This can happen
if a type constructor is redefined. Example:

type foo = A | B
let f = function A -> 0 | B -> 1
type foo = C | D
f C

This result in the error message “expression C of type foo cannot be used with type foo”.

The type of this expression, t, contains type variables that cannot be generalized
Type variables ('a, 'b, …) in a type t can be in either of two states: generalized (which means
that the type t is valid for all possible instantiations of the variables) and not generalized
(which means that the type t is valid only for one instantiation of the variables). In a let
binding let name = expr, the type-checker normally generalizes as many type variables as
possible in the type of expr. However, this leads to unsoundness (a well-typed program can
crash) in conjunction with polymorphic mutable data structures. To avoid this, generalization
is performed at let bindings only if the bound expression expr belongs to the class of “syntactic
values”, which includes constants, identifiers, functions, tuples of syntactic values, etc. In
all other cases (for instance, expr is a function application), a polymorphic mutable could
have been created and generalization is therefore turned off for all variables occurring in
contravariant or non-variant branches of the type. For instance, if the type of a non-value
is 'a list the variable is generalizable (list is a covariant type constructor), but not in
'a list -> 'a list (the left branch of -> is contravariant) or 'a ref (ref is non-variant).
Non-generalized type variables in a type cause no difficulties inside a given structure or
compilation unit (the contents of a .ml file, or an interactive session), but they cannot be
allowed inside signatures nor in compiled interfaces (.cmi file), because they could be used
inconsistently later. Therefore, the compiler flags an error when a structure or compilation
unit defines a value name whose type contains non-generalized type variables. There are two
ways to fix this error:

• Add a type constraint or a .mli file to give a monomorphic type (without type variables)
to name. For instance, instead of writing
let sort_int_list = List.sort Stdlib.compare
(* inferred type 'a list -> 'a list, with 'a not generalized *)
write
let sort_int_list = (List.sort Stdlib.compare : int list -> int list);;
• If you really need name to have a polymorphic type, turn its defining expression into a
function by adding an extra parameter. For instance, instead of writing
290

let map_length = List.map Array.length

(* inferred type 'a array list -> int list, with 'a not generalized *)
write
let map_length lv = List.map Array.length lv
Reference to undefined global mod
This error appears when trying to link an incomplete or incorrectly ordered set of files. Either
you have forgotten to provide an implementation for the compilation unit named mod on the
command line (typically, the file named mod.cmo, or a library containing that file). Fix: add
the missing .ml or .cmo file to the command line. Or, you have provided an implementation
for the module named mod, but it comes too late on the command line: the implementation
of mod must come before all bytecode object files that reference mod. Fix: change the order of
.ml and .cmo files on the command line.
Of course, you will always encounter this error if you have mutually recursive functions across
modules. That is, function Mod1.f calls function Mod2.g, and function Mod2.g calls function
Mod1.f. In this case, no matter what permutations you perform on the command line, the
program will be rejected at link-time. Fixes:
• Put f and g in the same module.
• Parameterize one function by the other. That is, instead of having
mod1.ml: let f x = ... Mod2.g ...
mod2.ml: let g y = ... Mod1.f ...
define
mod1.ml: let f g x = ... g ...
mod2.ml: let rec g y = ... Mod1.f g ...
and link mod1.cmo before mod2.cmo.
• Use a reference to hold one of the two functions, as in :
mod1.ml: let forward_g =
ref((fun x -> failwith "forward_g") : <type>)
let f x = ... !forward_g ...
mod2.ml: let g y = ... Mod1.f ...
let _ = Mod1.forward_g := g
The external function f is not available
This error appears when trying to link code that calls external functions written in C. As
explained in chapter 22, such code must be linked with C libraries that implement the required
f C function. If the C libraries in question are not shared libraries (DLLs), the code must be
linked in “custom runtime” mode. Fix: add the required C libraries to the command line, and
possibly the -custom option.

13.5 Warning reference

This section describes and explains in detail some warnings:
Chapter 13. Batch compilation (ocamlc) 291

13.5.1 Warning 6: Label omitted in function application

OCaml supports labels-omitted full applications: if the function has a known arity, all the
arguments are unlabeled, and their number matches the number of non-optional parameters, then
labels are ignored and non-optional parameters are matched in their definition order. Optional
arguments are defaulted.

let f ~x ~y = x + y
let test = f 2 3

> let test = f 2 3

> ^
> Warning 6 [labels-omitted]: labels x, y were omitted in the application of this function.

This support for labels-omitted application was introduced when labels were added to OCaml,
to ease the progressive introduction of labels in a codebase. However, it has the downside of
weakening the labeling discipline: if you use labels to prevent callers from mistakenly reordering
two parameters of the same type, labels-omitted make this mistake possible again.
Warning 6 warns when labels-omitted applications are used, to discourage their use. When
labels were introduced, this warning was not enabled by default, so users would use labels-omitted
applications, often without noticing.
Over time, it has become idiomatic to enable this warning to avoid argument-order mistakes. The
warning is now on by default, since OCaml 4.13. Labels-omitted applications are not recommended
anymore, but users wishing to preserve this transitory style can disable the warning explicitly.

13.5.2 Warning 9: missing fields in a record pattern

When pattern matching on records, it can be useful to match only few fields of a record. Eliding
fields can be done either implicitly or explicitly by ending the record pattern with ; _. However,
implicit field elision is at odd with pattern matching exhaustiveness checks. Enabling warning
9 prioritizes exhaustiveness checks over the convenience of implicit field elision and will warn on
implicit field elision in record patterns. In particular, this warning can help to spot exhaustive
record pattern that may need to be updated after the addition of new fields to a record type.

type 'a point = {x : 'a; y : 'a}

let dx { x } = x (* implicit field elision: trigger warning 9 *)
let dy { y; _ } = y (* explicit field elision: do not trigger warning 9 *)

13.5.3 Warning 52: fragile constant pattern

Some constructors, such as the exception constructors Failure and Invalid_argument, take as
parameter a string value holding a text message intended for the user.
These text messages are usually not stable over time: call sites building these constructors may
refine the message in a future version to make it more explicit, etc. Therefore, it is dangerous to
match over the precise value of the message. For example, until OCaml 4.02, Array.iter2 would
raise the exception
292

Invalid_argument "arrays must have the same length"

Since 4.03 it raises the more helpful message

Invalid_argument "Array.iter2: arrays must have the same length"

but this means that any code of the form

try ...
with Invalid_argument "arrays must have the same length" -> ...

is now broken and may suffer from uncaught exceptions.

Warning 52 is there to prevent users from writing such fragile code in the first place. It does not
occur on every matching on a literal string, but only in the case in which library authors expressed
their intent to possibly change the constructor parameter value in the future, by using the attribute
ocaml.warn_on_literal_pattern (see the manual section on builtin attributes in 12.12.1):
type t =
| Foo of string [@ocaml.warn_on_literal_pattern]
| Bar of string

let no_warning = function

| Bar "specific value" -> 0
| _ -> 1

let warning = function

| Foo "specific value" -> 0
| _ -> 1
Warning 52 [fragile -literal - pattern ]: Code should not depend on the actual values of
this constructor 's arguments . They are only for information
and may change in future versions . (see manual section 13.5.3)
In particular, all built-in exceptions with a string argument have this attribute set:
Invalid_argument, Failure, Sys_error will all raise this warning if you match for a specific
string argument.
Additionally, built-in exceptions with a structured argument that includes a string also have the
attribute set: Assert_failure and Match_failure will raise the warning for a pattern that uses a
literal string to match the first element of their tuple argument.
If your code raises this warning, you should not change the way you test for the specific string
to avoid the warning (for example using a string equality inside the right-hand-side instead of a
literal pattern), as your code would remain fragile. You should instead enlarge the scope of the
pattern by matching on all possible values.

let warning = function

| Foo _ -> 0
| _ -> 1
Chapter 13. Batch compilation (ocamlc) 293

This may require some care: if the scrutinee may return several different cases of the same
pattern, or raise distinct instances of the same exception, you may need to modify your code to
separate those several cases.
For example,

try (int_of_string count_str, bool_of_string choice_str) with

| Failure "int_of_string" -> (0, true)
| Failure "bool_of_string" -> (-1, false)

should be rewritten into more atomic tests. For example, using the exception patterns
documented in Section 11.6.1, one can write:

match int_of_string count_str with

| exception (Failure _) -> (0, true)
| count ->
begin match bool_of_string choice_str with
| exception (Failure _) -> (-1, false)
| choice -> (count, choice)
end

The only case where that transformation is not possible is if a given function call may raise
distinct exceptions with the same constructor but different string values. In this case, you will have
to check for specific string values. This is dangerous API design and it should be discouraged: it’s
better to define more precise exception constructors than store useful information in strings.

13.5.4 Warning 57: Ambiguous or-pattern variables under guard

The semantics of or-patterns in OCaml is specified with a left-to-right bias: a value v matches the
pattern p | q if it matches p or q, but if it matches both, the environment captured by the match is
the environment captured by p, never the one captured by q.
While this property is generally intuitive, there is at least one specific case where a different
semantics might be expected. Consider a pattern followed by a when-guard: | p when g -> e, for
example:

| ((Const x, _) | (_, Const x)) when is_neutral x -> branch

The semantics is clear: match the scrutinee against the pattern, if it matches, test the guard, and
if the guard passes, take the branch. In particular, consider the input (Const a, Const b), where a
fails the test is_neutral a, while b passes the test is_neutral b. With the left-to-right semantics,
the clause above is not taken by its input: matching (Const a, Const b) against the or-pattern
succeeds in the left branch, it returns the environment x -> a, and then the guard is_neutral a is
tested and fails, the branch is not taken.
However, another semantics may be considered more natural here: any pair that has one side
passing the test will take the branch. With this semantics the previous code fragment would be
equivalent to

| (Const x, _) when is_neutral x -> branch

| (_, Const x) when is_neutral x -> branch
294

This is not the semantics adopted by OCaml.

Warning 57 is dedicated to these confusing cases where the specified left-to-right semantics is
not equivalent to a non-deterministic semantics (any branch can be taken) relatively to a specific
guard. More precisely, it warns when guard uses “ambiguous” variables, that are bound to different
parts of the scrutinees by different sides of a or-pattern.

13.5.5 Warning 74: Pattern matching degraded to partial.

The OCaml type-checker performs a totality analysis to distinguish matches that are “total”, all
possible input values are handled by a clause, from matches that are “partial”, some values may
match none of the clause and raise a Match_failure exception. Total matches generate slightly
better code.
There are a few rare situations where this totality analysis can be wrong: the type-checker
believes the pattern-matching to be total, but certain values may try clauses in order and fail to
match any of them. When the pattern-matching compiler detects that it may be in this situation, it
conservatively compiles the pattern-matching as partial, and emits this warning. In some cases, this
conservative assumption is necessary (the pattern-matching was partial), and in some other cases it
is in fact overly conservative (a more fine-grained analysis would see that it is in fact total).
This situation happens rarely: the only known case requires matching on mutable fields. Moreover,
the vast majority of programs will see no performance difference at all if it were to happen. For
these reasons, the warning is disabled by default: we believe that the right approach when this
situation occurs is to do nothing at all.
However, some expert users might still be interested in finding out whether some pattern-
matchings in their codebase may be affected by this issue, for two reasons:
1. Users may want to reason with certainty about which parts of the code may raise a
Match_failure exception.
2. Performance-conscious expert users might want to inspect the -dlambda generated code if the
affected pattern-matching is in a performance-critical section.
In the rest of this section we explain the only known situation where this pessimization occurs.

Repeated matches in mutable positions

When a pattern-matching contains patterns on mutable fields, it is possible in theory that the
value of these fields may be mutated concurrently with the matching logic, and this mutation the
remaining clauses failing to match, even though the pattern-matching appears to be total – it does
not raise warning 8.
Consider for example:
let f : bool * bool ref -> unit = function
| (true, {contents = true}) -> ()
| (_, r) when (r.contents <- true; false) -> assert false
| (false, _) -> ()
| (_, {contents = false}) -> ()

let () = f (true, ref false)

(* Exception: Match_failure(...) *)
Chapter 13. Batch compilation (ocamlc) 295

The OCaml type-checker performs its totality analysis without considering the possibility of
concurrent mutations of the scrutinee, and it believes that the function f is total. The function f is
partial due to side-effects on mutable fields.
Note that the following version may also be affected:

let g : bool * bool ref -> unit = function

| (true, {contents = true}) -> ()
| (false, _) -> ()
| (_, {contents = false}) -> ()

In this example, there is no when guard mutating the scrutinee, but it is possible that a data race
coming from another domain would mutate the second field of the input at exactly the wrong time,
resulting in the same Match_failure behavior. In other words, the function g might be partial in
presence of side-effects on mutable fields.
Most patterns with mutable fields are not affected by this issue, because their mutable field is
read only once by the control-flow code generated by the pattern-matching compiler. For example,
the following simplification of our example does not suffer from this issue:

let h : bool ref -> unit = function

| ({contents = true}) -> ()
| r when (r.contents <- true; false) -> assert false
| ({contents = false}) -> ()

Whether a mutable field will be read once or several times depends on the implementation of
the pattern-matching compiler, it is not a property of the source-level pattern matching.
When the compiler detects that this situation is possible – that the pattern is partial or that it
might be partial as in the examples above – it will raise warning 74 if enabled.
Note: this warning was introduced in OCaml 5.3; earlier versions of the OCaml compiler would
not warn, and they would not ensure that a Match_failure case is generated in such cases: they
would generate (slightly more efficient but) incorrect code that could violate type- and memory-safety
in the case of concurrent mutation during matching.
296
Chapter 14

The toplevel system or REPL (ocaml)

This chapter describes ocaml, the toplevel system for OCaml, that permits interactive use of the
OCaml system through a read-eval-print loop (REPL). In this mode, the system repeatedly reads
OCaml phrases from the input, then typechecks, compile and evaluate them, then prints the inferred
type and result value, if any. End-of-file on standard input terminates ocaml.
Input to the toplevel can span several lines. It begins after the # (sharp) prompt printed by
the system and is terminated by ;; (a double-semicolon) followed by optional white space and
comments and an end of line. The toplevel input consists in one or several toplevel phrases, with
the following syntax:

toplevel-input ::= {definition} ;;

| expr ;;
| # ident [directive-argument] ;;
directive-argument ::= string-literal
| integer-literal
| value-path
| true | false
A phrase can consist of a sequence of definitions, like those found in implementations of
compilation units or in struct . . . end module expressions. The definitions can bind value names,
type names, exceptions, module names, or module type names. The toplevel system performs the
bindings, then prints the types and values (if any) for the names thus defined.
A phrase may also consist in a value expression (section 11.7). It is simply evaluated without
performing any bindings, and its value is printed.
Finally, a phrase can also consist in a toplevel directive, starting with # (the sharp sign). These
directives control the behavior of the toplevel; they are listed below in section 14.2.

Unix:
The toplevel system is started by the command ocaml, as follows:

ocaml options objects # interactive mode

ocaml options objects scriptfile # script mode

297
298

options are described below. objects are filenames ending in .cmo or .cma; they are loaded
into the interpreter immediately after options are set. scriptfile is any file name not ending in
.cmo or .cma.
If no scriptfile is given on the command line, the toplevel system enters interactive mode:
phrases are read on standard input, results are printed on standard output, errors on stan-
dard error. End-of-file on standard input terminates ocaml (see also the #quit directive in
section 14.2).
On start-up (before the first phrase is read), the contents of initialization file are read as a
sequence of OCaml phrases and executed as per the #use directive described in section 14.2.
The evaluation outcode for each phrase are not displayed.
The initialization file is the first found of:
1. .ocamlinit in the current directory;
2. XDG_CONFIG_HOME/ocaml/init.ml, if XDG_CONFIG_HOME is an absolute path;
3. otherwise, on Unix, HOME/ocaml/init.ml or, on Windows, ocaml\init.ml under
LocalAppData (e.g. C:\Users\Bactrian\AppData\Local\ocaml\init.ml);
4. ocaml/init.ml under any of the absolute paths in XDG_CONFIG_DIRS. Paths in
XDG_CONFIG_DIRS are colon-delimited on Unix, and semicolon-delimited on Windows;
5. if XDG_CONFIG_DIRS contained no absolute paths, /usr/xdg/ocaml/init.ml on Unix or,
ocaml\init.ml under any of LocalAppData (e.g. C:\Users\Bactrian\AppData\Local),
RoamingAppData (e.g. C:\Users\Bactrian\AppData\Roaming), or ProgramData (e.g.
C:\ProgramData) on Windows;
6. HOME/.ocamlinit, if HOME is non-empty;
The toplevel system does not perform line editing, but it can easily be used in conjunction
with an external line editor such as ledit, or rlwrap. An improved toplevel, utop, is also
available. Another option is to use ocaml under Gnu Emacs, which gives the full editing power
of Emacs (command run-caml from library inf-caml).
At any point, the parsing, compilation or evaluation of the current phrase can be interrupted
by pressing ctrl-C (or, more precisely, by sending the INTR signal to the ocaml process). The
toplevel then immediately returns to the # prompt.
If scriptfile is given on the command-line to ocaml, the toplevel system enters script mode:
the contents of the file are read as a sequence of OCaml phrases and executed, as per the #use
directive (section 14.2). The outcome of the evaluation is not printed. On reaching the end
of file, the ocaml command exits immediately. No commands are read from standard input.
Sys.argv is transformed, ignoring all OCaml parameters, and starting with the script file
name in Sys.argv.(0).
In script mode, the first line of the script is ignored if it starts with #!. Thus, it should be
possible to make the script itself executable and put as first line #!/usr/local/bin/ocaml,
thus calling the toplevel system automatically when the script is run. However, ocaml itself is
a #! script on most installations of OCaml, and Unix kernels usually do not handle nested #!
scripts. A better solution is to put the following as the first line of the script:

#!/usr/local/bin/ocamlrun /usr/local/bin/ocaml
Chapter 14. The toplevel system or REPL (ocaml) 299

14.1 Options
The following command-line options are recognized by the ocaml command.

-absname
Force error messages to show absolute paths for file names.

-no-absname
Do not try to show absolute filenames in error messages.

-args filename
Read additional newline-terminated command line arguments from filename. It is not possible
to pass a scriptfile via file to the toplevel.

-args0 filename
Read additional null character terminated command line arguments from filename. It is not
possible to pass a scriptfile via file to the toplevel.

-I directory
Add the given directory to the list of directories searched for source and compiled files. By
default, the current directory is searched first, then the standard library directory. Directories
added with -I are searched after the current directory, in the order in which they were given
on the command line, but before the standard library directory. See also option -nostdlib.
If the given directory starts with +, it is taken relative to the standard library directory. For
instance, -I +unix adds the subdirectory unix of the standard library to the search path.
Directories can also be added to the list once the toplevel is running with the #directory
directive (section 14.2).

-init file
Load the given file instead of the default initialization file. The default initialization file is the
first found of:

1. .ocamlinit in the current directory;

2. XDG_CONFIG_HOME/ocaml/init.ml, if XDG_CONFIG_HOME is an absolute path;
3. otherwise, on Unix, HOME/ocaml/init.ml or, on Windows, ocaml\init.ml under
LocalAppData (e.g. C:\Users\Bactrian\AppData\Local\ocaml\init.ml);
4. ocaml/init.ml under any of the absolute paths in XDG_CONFIG_DIRS. Paths in
XDG_CONFIG_DIRS are colon-delimited on Unix, and semicolon-delimited on Windows;
5. if XDG_CONFIG_DIRS contained no absolute paths, /usr/xdg/ocaml/init.ml on Unix or,
ocaml\init.ml under any of LocalAppData (e.g. C:\Users\Bactrian\AppData\Local),
RoamingAppData (e.g. C:\Users\Bactrian\AppData\Roaming), or ProgramData (e.g.
C:\ProgramData) on Windows;
6. HOME/.ocamlinit, if HOME is non-empty;
300

-keywords version+list
Set keywords according to the version+list specification.
This specification starts with an optional version number, defining the base set of keywords,
followed by a +-separated list of additional keywords to add to this base set.
Without an explicit version number, the base set of keywords is the set of keywords in the
current version of OCaml. Additional keywords that do not match any known keyword in the
current version of the language trigger an error whenever they are present in the source code.

-labels
Labels are not ignored in types, labels may be used in applications, and labelled parameters
can be given in any order. This is the default.

-no-app-funct
Deactivates the applicative behaviour of functors. With this option, each functor application
generates new types in its result and applying the same functor twice to the same argument
yields two incompatible structures.

-noassert
Do not compile assertion checks. Note that the special form assert false is always compiled
because it is typed specially.

-nolabels
Ignore non-optional labels in types. Labels cannot be used in applications, and parameter
order becomes strict.

-noprompt
Do not display any prompt when waiting for input.

-nopromptcont
Do not display the secondary prompt when waiting for continuation lines in multi-line inputs.
This should be used e.g. when running ocaml in an emacs window.

-nostdlib
Do not include the standard library directory in the list of directories searched for source and
compiled files.

-ppx command
After parsing, pipe the abstract syntax tree through the preprocessor command. The module
Ast_mapper, described in section 30.1, implements the external interface of a preprocessor.

-principal
Check information path during type-checking, to make sure that all types are derived in
a principal way. When using labelled arguments and/or polymorphic methods, this flag is
required to ensure future versions of the compiler will be able to infer types correctly, even if
internal algorithms change. All programs accepted in -principal mode are also accepted in
the default mode with equivalent types, but different binary signatures, and this may slow
down type checking; yet it is a good idea to use it once before publishing source code.
Chapter 14. The toplevel system or REPL (ocaml) 301

-rectypes
Allow arbitrary recursive types during type-checking. By default, only recursive types where
the recursion goes through an object type are supported.

-safe-string
Enforce the separation between types string and bytes, thereby making strings read-only.
This is the default, and enforced since OCaml 5.0.

-safer-matching
Do not use type information to optimize pattern-matching. This allows to detect match
failures even if a pattern-matching was wrongly assumed to be exhaustive. This only impacts
GADT and polymorphic variant compilation.

-short-paths
When a type is visible under several module-paths, use the shortest one when printing the
type’s name in inferred interfaces and error and warning messages. Identifier names starting
with an underscore _ or containing double underscores __ incur a penalty of +10 when
computing their length.

-stdin
Read the standard input as a script file rather than starting an interactive session.

-strict-sequence
Force the left-hand part of each sequence to have type unit.

-strict-formats
Reject invalid formats that were accepted in legacy format implementations. You should use
this flag to detect and fix such invalid formats, as they will be rejected by future OCaml
versions.

-unsafe
Turn bound checking off for array and string accesses (the v.(i) and s.[i] constructs).
Programs compiled with -unsafe are therefore faster, but unsafe: anything can happen if
the program accesses an array or string outside of its bounds.

-unsafe-string
Identify the types string and bytes, thereby making strings writable. This is intended for
compatibility with old source code and should not be used with new software. This option
raises an error unconditionally since OCaml 5.0.

-v Print the version number of the compiler and the location of the standard library directory,
then exit.

-verbose
Print all external commands before they are executed, Useful to debug C library problems.

-version
Print version string and exit.
302

-vnum
Print short version number and exit.

-no-version
Do not print the version banner at startup.

-w warning-list
Enable, disable, or mark as fatal the warnings specified by the argument warning-list. Each
warning can be enabled or disabled, and each warning can be fatal or non-fatal. If a warning is
disabled, it isn’t displayed and doesn’t affect compilation in any way (even if it is fatal). If a
warning is enabled, it is displayed normally by the compiler whenever the source code triggers
it. If it is enabled and fatal, the compiler will also stop with an error after displaying it.
The warning-list argument is a sequence of warning specifiers, with no separators between
them. A warning specifier is one of the following:

+num
Enable warning number num.
-num
Disable warning number num.
@num
Enable and mark as fatal warning number num.
+num1..num2
Enable warnings in the given range.
-num1..num2
Disable warnings in the given range.
@num1..num2
Enable and mark as fatal warnings in the given range.
+letter
Enable the set of warnings corresponding to letter. The letter may be uppercase or
lowercase.
-letter
Disable the set of warnings corresponding to letter. The letter may be uppercase or
lowercase.
@letter
Enable and mark as fatal the set of warnings corresponding to letter. The letter may be
uppercase or lowercase.
uppercase-letter
Enable the set of warnings corresponding to uppercase-letter.
lowercase-letter
Disable the set of warnings corresponding to lowercase-letter.

Alternatively, warning-list can specify a single warning using its mnemonic name (see below),
as follows:
Chapter 14. The toplevel system or REPL (ocaml) 303

+name
Enable warning name.
-name
Disable warning name.
@name
Enable and mark as fatal warning name.

Warning numbers, letters and names which are not currently defined are ignored. The warnings
are as follows (the name following each number specifies the mnemonic for that warning).

1 comment-start
Suspicious-looking start-of-comment mark.
2 comment-not-end
Suspicious-looking end-of-comment mark.
3 Deprecated synonym for the ’deprecated’ alert.
4 fragile-match
Fragile pattern matching: matching that will remain complete even if additional con-
structors are added to one of the variant types matched.
5 ignored-partial-application
Partially applied function: expression whose result has function type and is ignored.
6 labels-omitted
Label omitted in function application.
7 method-override
Method overridden.
8 partial-match
Partial match: missing cases in pattern-matching.
9 missing-record-field-pattern
Missing fields in a record pattern.
10 non-unit-statement
Expression on the left-hand side of a sequence that doesn’t have type unit (and that is
not a function, see warning number 5).
11 redundant-case
Redundant case in a pattern matching (unused match case).
12 redundant-subpat
Redundant sub-pattern in a pattern-matching.
13 instance-variable-override
Instance variable overridden.
14 illegal-backslash
Illegal backslash escape in a string constant.
15 implicit-public-methods
Private method made public implicitly.
304

16 unerasable-optional-argument
Unerasable optional argument.
17 undeclared-virtual-method
Undeclared virtual method.
18 not-principal
Non-principal type.
19 non-principal-labels
Type without principality.
20 ignored-extra-argument
Unused function argument.
21 nonreturning-statement
Non-returning statement.
22 preprocessor
Preprocessor warning.
23 useless-record-with
Useless record with clause.
24 bad-module-name
Bad module name: the source file name is not a valid OCaml module name.
25 Ignored: now part of warning 8.
26 unused-var
Suspicious unused variable: unused variable that is bound with let or as, and doesn’t
start with an underscore (_) character.
27 unused-var-strict
Innocuous unused variable: unused variable that is not bound with let nor as, and
doesn’t start with an underscore (_) character.
28 wildcard-arg-to-constant-constr
Wildcard pattern given as argument to a constant constructor.
29 eol-in-string
Unescaped end-of-line in a string constant (non-portable code).
30 duplicate-definitions
Two labels or constructors of the same name are defined in two mutually recursive types.
31 module-linked-twice
A module is linked twice in the same executable.
I gnored: now a hard error (since 5.1).
32 unused-value-declaration
Unused value declaration. (since 4.00)
33 unused-open
Unused open statement. (since 4.00)
34 unused-type-declaration
Unused type declaration. (since 4.00)
Chapter 14. The toplevel system or REPL (ocaml) 305

35 unused-for-index
Unused for-loop index. (since 4.00)
36 unused-ancestor
Unused ancestor variable. (since 4.00)
37 unused-constructor
Unused constructor. (since 4.00)
38 unused-extension
Unused extension constructor. (since 4.00)
39 unused-rec-flag
Unused rec flag. (since 4.00)
40 name-out-of-scope
Constructor or label name used out of scope. (since 4.01)
41 ambiguous-name
Ambiguous constructor or label name. (since 4.01)
42 disambiguated-name
Disambiguated constructor or label name (compatibility warning). (since 4.01)
43 nonoptional-label
Nonoptional label applied as optional. (since 4.01)
44 open-shadow-identifier
Open statement shadows an already defined identifier. (since 4.01)
45 open-shadow-label-constructor
Open statement shadows an already defined label or constructor. (since 4.01)
46 bad-env-variable
Error in environment variable. (since 4.01)
47 attribute-payload
Illegal attribute payload. (since 4.02)
48 eliminated-optional-arguments
Implicit elimination of optional arguments. (since 4.02)
49 no-cmi-file
Absent cmi file when looking up module alias. (since 4.02)
50 unexpected-docstring
Unexpected documentation comment. (since 4.03)
51 wrong-tailcall-expectation
Function call annotated with an incorrect @tailcall attribute. (since 4.03)
52 fragile-literal-pattern (see 13.5.3)
Fragile constant pattern. (since 4.03)
53 misplaced-attribute
Attribute cannot appear in this context. (since 4.03)
54 duplicated-attribute
Attribute used more than once on an expression. (since 4.03)
306

55 inlining-impossible
Inlining impossible. (since 4.03)
56 unreachable-case
Unreachable case in a pattern-matching (based on type information). (since 4.03)
57 ambiguous-var-in-pattern-guard (see 13.5.4)
Ambiguous or-pattern variables under guard. (since 4.03)
58 no-cmx-file
Missing cmx file. (since 4.03)
59 flambda-assignment-to-non-mutable-value
Assignment to non-mutable value. (since 4.03)
60 unused-module
Unused module declaration. (since 4.04)
61 unboxable-type-in-prim-decl
Unboxable type in primitive declaration. (since 4.04)
62 constraint-on-gadt
Type constraint on GADT type declaration. (since 4.06)
63 erroneous-printed-signature
Erroneous printed signature. (since 4.08)
64 unsafe-array-syntax-without-parsing
-unsafe used with a preprocessor returning a syntax tree. (since 4.08)
65 redefining-unit
Type declaration defining a new ’()’ constructor. (since 4.08)
66 unused-open-bang
Unused open! statement. (since 4.08)
67 unused-functor-parameter
Unused functor parameter. (since 4.10)
68 match-on-mutable-state-prevent-uncurry
Pattern-matching depending on mutable state prevents the remaining arguments from
being uncurried. (since 4.12)
69 unused-field
Unused record field. (since 4.13)
70 missing-mli
Missing interface file. (since 4.13)
71 unused-tmc-attribute
Unused @tail_mod_cons attribute. (since 4.14)
72 tmc-breaks-tailcall
A tail call is turned into a non-tail call by the @tail_mod_cons transformation. (since
4.14)
73 generative-application-expects-unit
A generative functor is applied to an empty structure (struct end) rather than to ().
(since 5.1)
Chapter 14. The toplevel system or REPL (ocaml) 307

74 degraded-to-partial-match
A pattern-matching is compiled as partial even if it appears to be total. (since 5.3)
A all warnings
C warnings 1, 2.
D Alias for warning 3.
E Alias for warning 4.
F Alias for warning 5.
K warnings 32, 33, 34, 35, 36, 37, 38, 39.
L Alias for warning 6.
M Alias for warning 7.
P Alias for warning 8.
R Alias for warning 9.
S Alias for warning 10.
U warnings 11, 12.
V Alias for warning 13.
X warnings 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 30.
Y Alias for warning 26.
Z Alias for warning 27.

The default setting is -w +a-4-6-7-9-27-29-32..42-44-45-48-50-60. It is displayed by

-help. Note that warnings 5 and 10 are not always triggered, depending on the internals of
the type checker.

-warn-error warning-list
Mark as fatal the warnings specified in the argument warning-list. The compiler will stop
with an error when one of these warnings is emitted. The warning-list has the same meaning
as for the -w option: a + sign (or an uppercase letter) marks the corresponding warnings as
fatal, a - sign (or a lowercase letter) turns them back into non-fatal warnings, and a @ sign
both enables and marks as fatal the corresponding warnings.
Note: it is not recommended to use warning sets (i.e. letters) as arguments to -warn-error
in production code, because this can break your build when future versions of OCaml add
some new warnings.
The default setting is -warn-error -a (no warning is fatal).

-warn-help
Show the description of all available warning numbers.

- file
Use file as a script file name, even when it starts with a hyphen (-).

-help or --help
Display a short usage summary and exit.
308

Unix:
The following environment variables are also consulted:

OCAMLTOP_INCLUDE_PATH
Additional directories to search for compiled object code files (.cmi, .cmo and .cma).
The specified directories are considered from left to right, after the include directories
specified on the command line via -I have been searched. Available since OCaml 4.08.
OCAMLTOP_UTF_8
When printing string values, non-ascii bytes ( > \0x7E) are printed as decimal escape
sequence if OCAMLTOP_UTF_8 is set to false. Otherwise, they are printed unescaped.
TERM
When printing error messages, the toplevel system attempts to underline visually the
location of the error. It consults the TERM variable to determines the type of output
terminal and look up its capabilities in the terminal database.
XDG_CONFIG_HOME, HOME, XDG_CONFIG_DIRS
Initialization file lookup procedure (see above).

14.2 Toplevel directives

The following directives control the toplevel behavior, load files in memory, and trace program
execution.
Note: all directives start with a # (sharp) symbol. This # must be typed before the directive,
and must not be confused with the # prompt displayed by the interactive loop. For instance, typing
#quit;; will exit the toplevel loop, but typing quit;; will result in an “unbound value quit” error.

General

#help;;
Prints a list of all available directives, with corresponding argument type if appropriate.
#quit;;
Exit the toplevel loop and terminate the ocaml command.

Loading codes

#cd "dir-name";;
Change the current working directory.
#directory "dir-name";;
Add the given directory to the list of directories searched for source and compiled files.
#remove_directory "dir-name";;
Remove the given directory from the list of directories searched for source and compiled
files. Do nothing if the list does not contain the given directory.
Chapter 14. The toplevel system or REPL (ocaml) 309

#load "file-name";;
Load in memory a bytecode object file (.cmo file) or library file (.cma file) produced by
the batch compiler ocamlc.
#load_rec "file-name";;
Load in memory a bytecode object file (.cmo file) or library file (.cma file) produced by
the batch compiler ocamlc. When loading an object file that depends on other modules
which have not been loaded yet, the .cmo files for these modules are searched and loaded
as well, recursively. The loading order is not specified.
#use "file-name";;
Read, compile and execute source phrases from the given file. This is textual inclusion:
phrases are processed just as if they were typed on standard input. The reading of the
file stops at the first error encountered.
#use_output "command";;
Execute a command and evaluate its output as if it had been captured to a file and
passed to #use.
#mod_use "file-name";;
Similar to #use but also wrap the code into a top-level module of the same name as
capitalized file name without extensions, following semantics of the compiler.

For directives that take file names as arguments, if the given file name specifies no directory,
the file is searched in the following directories:

1. In script mode, the directory containing the script currently executing; in interactive
mode, the current working directory.
2. Directories added with the #directory directive.
3. Directories given on the command line with -I options.
4. The standard library directory.

Environment queries

#show_class class-path;;
#show_class_type class-path;;
#show_exception ident;;
#show_module module-path;;
#show_module_type modtype-path;;
#show_type typeconstr;;
#show_val value-path;;
Print the signature of the corresponding component.
#show ident;;
Print the signatures of components with name ident in all the above categories.

Pretty-printing
310

#install_printer printer-name;;
This directive registers the function named printer-name (a value path) as a printer for
values whose types match the argument type of the function. That is, the toplevel loop
will call printer-name when it has such a value to print.
The printing function printer-name should have type Format.formatter ->t -> unit,
where t is the type for the values to be printed, and should output its textual representation
for the value of type t on the given formatter, using the functions provided by the Format
library. For backward compatibility, printer-name can also have type t-> unit and
should then output on the standard formatter, but this usage is deprecated.
#print_depth n;;
Limit the printing of values to a maximal depth of n. The parts of values whose depth
exceeds n are printed as ... (ellipsis).
#print_length n;;
Limit the number of value nodes printed to at most n. Remaining parts of values are
printed as ... (ellipsis).
#remove_printer printer-name;;
Remove the named function from the table of toplevel printers.

Tracing

#trace function-name;;
After executing this directive, all calls to the function named function-name will be
“traced”. That is, the argument and the result are displayed for each call, as well as the
exceptions escaping out of the function, raised either by the function itself or by another
function it calls. If the function is curried, each argument is printed as it is passed to the
function.
#untrace function-name;;
Stop tracing the given function.
#untrace_all;;
Stop tracing all functions traced so far.

Compiler options

#debug bool;;
Turn on/off the insertion of debugging events. Default is true.
#labels bool;;
Ignore labels in function types if argument is false, or switch back to default behaviour
(commuting style) if argument is true.
#ppx "file-name";;
After parsing, pipe the abstract syntax tree through the preprocessor command.
#principal bool;;
If the argument is true, check information paths during type-checking, to make sure
Chapter 14. The toplevel system or REPL (ocaml) 311

that all types are derived in a principal way. If the argument is false, do not check
information paths.
#rectypes;;
Allow arbitrary recursive types during type-checking. Note: once enabled, this option
cannot be disabled because that would lead to unsoundness of the type system.
#warn_error "warning-list";;
Treat as errors the warnings enabled by the argument and as normal warnings the
warnings disabled by the argument.
#warnings "warning-list";;
Enable or disable warnings according to the argument.

14.3 The toplevel and the module system

Toplevel phrases can refer to identifiers defined in compilation units with the same mechanisms
as for separately compiled units: either by using qualified names (Modulename.localname), or by
using the open construct and unqualified names (see section 11.3).
However, before referencing another compilation unit, an implementation of that unit must be
present in memory. At start-up, the toplevel system contains implementations for all the modules in
the the standard library. Implementations for user modules can be entered with the #load directive
described above. Referencing a unit for which no implementation has been provided results in the
error Reference to undefined global `...'.
Note that entering open Mod merely accesses the compiled interface (.cmi file) for Mod, but
does not load the implementation of Mod, and does not cause any error if no implementation of
Mod has been loaded. The error “reference to undefined global Mod” will occur only when executing
a value or module definition that refers to Mod.

14.4 Common errors

This section describes and explains the most frequently encountered error messages.

Cannot find file filename

The named file could not be found in the current directory, nor in the directories of the search
path.
If filename has the format mod.cmi, this means you have referenced the compilation unit
mod, but its compiled interface could not be found. Fix: compile mod.mli or mod.ml first, to
create the compiled interface mod.cmi.
If filename has the format mod.cmo, this means you are trying to load with #load a bytecode
object file that does not exist yet. Fix: compile mod.ml first.
If your program spans several directories, this error can also appear because you haven’t
specified the directories to look into. Fix: use the #directory directive to add the correct
directories to the search path.
This expression has type t1 , but is used with type t2
See section 13.4.
312

Reference to undefined global mod

You have neglected to load in memory an implementation for a module with #load. See
section 14.3 above.

14.5 Building custom toplevel systems: ocamlmktop

The ocamlmktop command builds OCaml toplevels that contain user code preloaded at start-up.
The ocamlmktop command takes as argument a set of .cmo and .cma files, and links them with
the object files that implement the OCaml toplevel. The typical use is:
ocamlmktop -o mytoplevel foo.cmo bar.cmo gee.cmo
This creates the bytecode file mytoplevel, containing the OCaml toplevel system, plus the code
from the three .cmo files. This toplevel is directly executable and is started by:
./mytoplevel
This enters a regular toplevel loop, except that the code from foo.cmo, bar.cmo and gee.cmo
is already loaded in memory, just as if you had typed:
#load "foo.cmo";;
#load "bar.cmo";;
#load "gee.cmo";;
on entrance to the toplevel. The modules Foo, Bar and Gee are not opened, though; you still
have to do
open Foo;;
yourself, if this is what you wish.

14.5.1 Options
The following command-line options are recognized by ocamlmktop.

-cclib libname
Pass the -llibname option to the C linker when linking in “custom runtime” mode. See the
corresponding option for ocamlc, in chapter 13.
-ccopt option
Pass the given option to the C compiler and linker, when linking in “custom runtime” mode.
See the corresponding option for ocamlc, in chapter 13.
-custom
Link in “custom runtime” mode. See the corresponding option for ocamlc, in chapter 13.
-I directory
Add the given directory to the list of directories searched for compiled object code files (.cmo
and .cma).
-o exec-file
Specify the name of the toplevel file produced by the linker. The default is a.out.
Chapter 14. The toplevel system or REPL (ocaml) 313

14.6 The native toplevel: ocamlnat (experimental)

This section describes a tool that is not yet officially supported but may be found
useful.
OCaml code executing in the traditional toplevel system uses the bytecode interpreter. When
increased performance is required, or for testing programs that will only execute correctly when
compiled to native code, the native toplevel may be used instead.
For the majority of installations the native toplevel will not have been installed along with
the rest of the OCaml toolchain. In such circumstances it will be necessary to build the OCaml
distribution from source. From the built source tree of the distribution you may use make natruntop
to build and execute a native toplevel. (Alternatively make ocamlnat can be used, which just
performs the build step.)
If the make install command is run after having built the native toplevel then the ocamlnat
program (either from the source or the installation directory) may be invoked directly rather than
using make natruntop.
314
Chapter 15

The runtime system (ocamlrun)

The ocamlrun command executes bytecode files produced by the linking phase of the ocamlc
command.

15.1 Overview
The ocamlrun command comprises three main parts: the bytecode interpreter, that actually executes
bytecode files; the memory allocator and garbage collector; and a set of C functions that implement
primitive operations such as input/output.
The usage for ocamlrun is:

ocamlrun options bytecode-executable arg 1 ... arg n

The first non-option argument is taken to be the name of the file containing the executable
bytecode. (That file is searched in the executable path as well as in the current directory.) The
remaining arguments are passed to the OCaml program, in the string array Sys.argv. Element 0 of
this array is the name of the bytecode executable file; elements 1 to n are the remaining arguments
arg 1 to arg n .
As mentioned in chapter 13, the bytecode executable files produced by the ocamlc command are
self-executable, and manage to launch the ocamlrun command on themselves automatically. That
is, assuming a.out is a bytecode executable file,

a.out arg 1 ... arg n

works exactly as

ocamlrun a.out arg 1 ... arg n

Notice that it is not possible to pass options to ocamlrun when invoking a.out directly.

Windows:
Under several versions of Windows, bytecode executable files are self-executable only if their
name ends in .exe. It is recommended to always give .exe names to bytecode executables,
e.g. compile with ocamlc -o myprog.exe ... rather than ocamlc -o myprog ....

315
316

15.2 Options
The following command-line options are recognized by ocamlrun.

-b When the program aborts due to an uncaught exception, print a detailed “back trace” of the
execution, showing where the exception was raised and which function calls were outstanding
at this point. The back trace is printed only if the bytecode executable contains debugging
information, i.e. was compiled and linked with the -g option to ocamlc set. This is equivalent
to setting the b flag in the OCAMLRUNPARAM environment variable (see below).

-config
Print the version number of ocamlrun and a detailed summary of its configuration, then exit.

-I dir
Search the directory dir for dynamically-loaded libraries, in addition to the standard search
path (see section 15.3).

-m Print the magic number of the bytecode executable given as argument and exit.

-M Print the magic number expected for bytecode executables by this version of the runtime and
exit.

-p Print the names of the primitives known to this version of ocamlrun and exit.

-t Increments the trace level for the debug runtime (ignored otherwise).

-v Direct the memory manager to print some progress messages on standard error. This is
equivalent to setting v=61 in the OCAMLRUNPARAM environment variable (see below).

-version
Print version string and exit.

-vnum
Print short version number and exit.

The following environment variables are also consulted:

CAML_LD_LIBRARY_PATH
Additional directories to search for dynamically-loaded libraries (see section 15.3).

OCAMLLIB
The directory containing the OCaml standard library. (If OCAMLLIB is not set, CAMLLIB will
be used instead.) Used to locate the ld.conf configuration file for dynamic loading (see
section 15.3). If not set, default to the library directory specified when compiling OCaml.

OCAMLRUNPARAM
Set the runtime system options and garbage collection parameters. (If OCAMLRUNPARAM is
not set, CAMLRUNPARAM will be used instead.) This variable must be a sequence of parameter
specifications separated by commas. For convenience, commas at the beginning of the variable
are ignored, and multiple runs of commas are interpreted as a single one. A parameter
Chapter 15. The runtime system (ocamlrun) 317

specification is an option letter followed by an = sign, a decimal number (or an hexadecimal

number prefixed by 0x), and an optional multiplier. The options are documented below;
the options l, m, M, n, o, s, v correspond to fields of the control record documented in
section 29.24.

b (backtrace) Trigger the printing of a stack backtrace when an uncaught exception aborts
the program. An optional argument can be provided: b=0 turns backtrace printing off;
b=1 is equivalent to b and turns backtrace printing on; b=2 turns backtrace printing
on and forces the runtime system to load debugging information at program startup
time instead of at backtrace printing time. b=2 can be used if the runtime is unable to
load debugging information at backtrace printing time, for example if there are no file
descriptors available.
c (cleanup_on_exit) Shut the runtime down gracefully on exit (see caml_shutdown in
section 22.7.6). The option also enables pooling (as in caml_startup_pooled). This
mode can be used to detect leaks with a third-party memory debugger.
d (max_domains) Maximum number of domains that can be active concurrently. Defaults
to 128 on 64-bit platforms and 16 on 32-bit platforms.
e (runtime_events_log_wsize) Size of the per-domain runtime events ring buffers in log
powers of two words. Defaults to 16, giving 64k word or 512kb buffers on 64-bit systems.
l (stack_limit) The limit (in words) of the stack size. This is relevant to both the
byte-code runtime and the native code runtime: OCaml always uses its own stack and
not the operating system’s stack.
m (custom_minor_ratio) Bound on floating garbage for out-of-heap memory held by
custom values in the minor heap. A minor GC is triggered when this much memory is held
by custom values located in the minor heap. Expressed as a percentage of minor heap size.
Default: 100. Note: this only applies to values allocated with caml_alloc_custom_mem.
M (custom_major_ratio) Target ratio of floating garbage to major heap size for out-of-heap
memory held by custom values (e.g. bigarrays) located in the major heap. The GC speed
is adjusted to try to use this much memory for dead values that are not yet collected.
Expressed as a percentage of major heap size. Default: 44. Note: this only applies to
values allocated with caml_alloc_custom_mem.
n (custom_minor_max_size) Maximum amount of out-of-heap memory for each custom
value allocated in the minor heap. When a custom value is allocated on the minor heap and
holds more than this many bytes, only this value is counted against custom_minor_ratio
and the rest is directly counted against custom_major_ratio. Default: 8192 bytes. Note:
this only applies to values allocated with caml_alloc_custom_mem.

The multiplier is k, M, or G, for multiplication by 210 , 220 , and 230 respectively.

o (space_overhead) The major GC speed setting. See the Gc module documentation for details.

p (parser trace) Turn on debugging support for ocamlyacc-generated parsers. When this option
is on, the pushdown automaton that executes the parsers prints a trace of its actions. This
option takes no argument.
318

R (randomize) Turn on randomization of all hash tables by default (see section 29.25). This
option takes no argument.
s (minor_heap_size) Size of the minor heap. (in words)
t Set the trace level for the debug runtime (ignored by the standard runtime).
v (verbose) What GC messages to print to stderr. This is a sum of values selected from the
following:
1 (= 0x001)
Start and end of major GC cycle.
2 (= 0x002)
Minor collection and major GC slice.
4 (= 0x004)
Growing and shrinking of the heap.
8 (= 0x008)
Resizing of stacks and memory manager tables.
16 (= 0x010)
Heap compaction.
32 (= 0x020)
Change of GC parameters.
64 (= 0x040)
Computation of major GC slice size.
128 (= 0x080)
Calling of finalization functions
256 (= 0x100)
Startup messages (loading the bytecode executable file, resolving shared libraries).
512 (= 0x200)
Computation of compaction-triggering condition.
1024 (= 0x400)
Output GC statistics at program exit.
2048 (= 0x800)
GC debugging messages.
4096 (= 0x1000)
Address space reservation changes.
V (verify_heap) runs an integrity check on the heap just after the completion of a major GC
cycle
W Print runtime warnings to stderr (such as Channel opened on file dies without being closed,
unflushed data, etc.)
If the option letter is not recognized, the whole parameter is ignored; if the equal sign or the
number is missing, the value is taken as 1; if the multiplier is not recognized, it is ignored.
For example, on a 32-bit machine, under bash the command
Chapter 15. The runtime system (ocamlrun) 319

export OCAMLRUNPARAM='b,s=256k,v=0x015'

tells a subsequent ocamlrun to print backtraces for uncaught exceptions, set its initial minor
heap size to 1 megabyte and print a message at the start of each major GC cycle, when the
heap size changes, and when compaction is triggered.

CAMLRUNPARAM
If OCAMLRUNPARAM is not found in the environment, then CAMLRUNPARAM will be used instead.
If CAMLRUNPARAM is also not found, then the default values will be used.

PATH
List of directories searched to find the bytecode executable file.

15.3 Dynamic loading of shared libraries

On platforms that support dynamic loading, ocamlrun can link dynamically with C shared libraries
(DLLs) providing additional C primitives beyond those provided by the standard runtime system.
The names for these libraries are provided at link time as described in section 22.1.4), and recorded
in the bytecode executable file; ocamlrun, then, locates these libraries and resolves references to
their primitives when the bytecode executable program starts.
The ocamlrun command searches shared libraries in the following directories, in the order
indicated:

1. Directories specified on the ocamlrun command line with the -I option.

2. Directories specified in the CAML_LD_LIBRARY_PATH environment variable.

3. Directories specified at link-time via the -dllpath option to ocamlc. (These directories are
recorded in the bytecode executable file.)

4. Directories specified in the file ld.conf. This file resides in the OCaml standard library
directory, and lists directory names (one per line) to be searched. Typically, it contains only
one line naming the stublibs subdirectory of the OCaml standard library directory. Users can
add there the names of other directories containing frequently-used shared libraries; however,
for consistency of installation, we recommend that shared libraries are installed directly in the
system stublibs directory, rather than adding lines to the ld.conf file.

5. Default directories searched by the system dynamic loader. Under Unix, these generally
include /lib and /usr/lib, plus the directories listed in the file /etc/ld.so.conf and the
environment variable LD_LIBRARY_PATH. Under Windows, these include the Windows system
directories, plus the directories listed in the PATH environment variable.

15.4 Common errors

This section describes and explains the most frequently encountered error messages.
320

filename: no such file or directory

If filename is the name of a self-executable bytecode file, this means that either that file does
not exist, or that it failed to run the ocamlrun bytecode interpreter on itself. The second
possibility indicates that OCaml has not been properly installed on your system.

Cannot exec ocamlrun

(When launching a self-executable bytecode file.) The ocamlrun could not be found in the
executable path. Check that OCaml has been properly installed on your system.

Cannot find the bytecode file

The file that ocamlrun is trying to execute (e.g. the file given as first non-option argument to
ocamlrun) either does not exist, or is not a valid executable bytecode file.

Truncated bytecode file

The file that ocamlrun is trying to execute is not a valid executable bytecode file. Probably it
has been truncated or mangled since created. Erase and rebuild it.

Uncaught exception
The program being executed contains a “stray” exception. That is, it raises an exception at
some point, and this exception is never caught. This causes immediate termination of the
program. The name of the exception is printed, along with its string, byte sequence, and
integer arguments (arguments of more complex types are not correctly printed). To locate the
context of the uncaught exception, compile the program with the -g option and either run it
again under the ocamldebug debugger (see chapter 20), or run it with ocamlrun -b or with
the OCAMLRUNPARAM environment variable set to b=1.

Out of memory
The program being executed requires more memory than available. Either the program builds
excessively large data structures; or the program contains too many nested function calls, and
the stack overflows. In some cases, your program is perfectly correct, it just requires more
memory than your machine provides. In other cases, the “out of memory” message reveals an
error in your program: non-terminating recursive function, allocation of an excessively large
array, string or byte sequence, attempts to build an infinite list or other data structure, …
To help you diagnose this error, run your program with the -v option to ocamlrun, or with
the OCAMLRUNPARAM environment variable set to v=63. If it displays lots of “Growing stack…”
messages, this is probably a looping recursive function. If it displays lots of “Growing heap…”
messages, with the heap size growing slowly, this is probably an attempt to construct a
data structure with too many (infinitely many?) cells. If it displays few “Growing heap…”
messages, but with a huge increment in the heap size, this is probably an attempt to build an
excessively large array, string or byte sequence.
Chapter 16

Native-code compilation (ocamlopt)

This chapter describes the OCaml high-performance native-code compiler ocamlopt, which compiles
OCaml source files to native code object files and links these object files to produce standalone
executables.
The native-code compiler is only available on certain platforms. It produces code that runs faster
than the bytecode produced by ocamlc, at the cost of increased compilation time and executable
code size. Compatibility with the bytecode compiler is extremely high: the same source code should
run identically when compiled with ocamlc and ocamlopt.
It is not possible to mix native-code object files produced by ocamlopt with bytecode object files
produced by ocamlc: a program must be compiled entirely with ocamlopt or entirely with ocamlc.
Native-code object files produced by ocamlopt cannot be loaded in the toplevel system ocaml.

16.1 Overview of the compiler

The ocamlopt command has a command-line interface very close to that of ocamlc. It accepts the
same types of arguments, and processes them sequentially, after all options have been processed:

• Arguments ending in .mli are taken to be source files for compilation unit interfaces. Interfaces
specify the names exported by compilation units: they declare value names with their types,
define public data types, declare abstract data types, and so on. From the file x.mli, the
ocamlopt compiler produces a compiled interface in the file x.cmi. The interface produced is
identical to that produced by the bytecode compiler ocamlc.

• Arguments ending in .ml are taken to be source files for compilation unit implementations.
Implementations provide definitions for the names exported by the unit, and also contain
expressions to be evaluated for their side-effects. From the file x.ml, the ocamlopt compiler
produces two files: x.o, containing native object code, and x.cmx, containing extra information
for linking and optimization of the clients of the unit. The compiled implementation should
always be referred to under the name x.cmx (when given a .o or .obj file, ocamlopt assumes
that it contains code compiled from C, not from OCaml).
The implementation is checked against the interface file x.mli (if it exists) as described in the
manual for ocamlc (chapter 13).

321
322

• Arguments ending in .cmx are taken to be compiled object code. These files are linked together,
along with the object files obtained by compiling .ml arguments (if any), and the OCaml
standard library, to produce a native-code executable program. The order in which .cmx and
.ml arguments are presented on the command line is relevant: compilation units are initialized
in that order at run-time, and it is a link-time error to use a component of a unit before having
initialized it. Hence, a given x.cmx file must come before all .cmx files that refer to the unit x.

• Arguments ending in .cmxa are taken to be libraries of object code. Such a library packs in
two files (lib.cmxa and lib.a/.lib) a set of object files (.cmx and .o/.obj files). Libraries
are build with ocamlopt -a (see the description of the -a option below). The object files
contained in the library are linked as regular .cmx files (see above), in the order specified
when the library was built. The only difference is that if an object file contained in a library
is not referenced anywhere in the program, then it is not linked in.

• Arguments ending in .c are passed to the C compiler, which generates a .o/.obj object file.
This object file is linked with the program.

• Arguments ending in .o, .a or .so (.obj, .lib and .dll under Windows) are assumed to be
C object files and libraries. They are linked with the program.

The output of the linking phase is a regular Unix or Windows executable file. It does not need
ocamlrun to run.
The compiler is able to emit some information on its internal stages:

• .cmt files for the implementation of the compilation unit and .cmti for signatures if the option
-bin-annot is passed to it (see the description of -bin-annot below). Each such file contains
a typed abstract syntax tree (AST), that is produced during the type checking procedure.
This tree contains all available information about the location and the specific type of each
term in the source file. The AST is partial if type checking was unsuccessful.
These .cmt and .cmti files are typically useful for code inspection tools.

• .cmir-linear files for the implementation of the compilation unit if the option
-save-ir-after scheduling is passed to it. Each such file contains a low-level intermediate
representation, produced by the instruction scheduling pass.
An external tool can perform low-level optimisations, such as code layout, by transforming a
.cmir-linear file. To continue compilation, the compiler can be invoked with (a possibly
modified) .cmir-linear file as an argument, instead of the corresponding source file.

16.2 Options
The following command-line options are recognized by ocamlopt. The options -pack, -a, -shared,
-c, -output-obj and -output-complete-obj are mutually exclusive.

-a Build a library(.cmxa and .a/.lib files) with the object files (.cmx and .o/.obj files) given
on the command line, instead of linking them into an executable file. The name of the library
must be set with the -o option.
Chapter 16. Native-code compilation (ocamlopt) 323

If -cclib or -ccopt options are passed on the command line, these options are stored in the
resulting .cmxalibrary. Then, linking with this library automatically adds back the -cclib
and -ccopt options as if they had been provided on the command line, unless the -noautolink
option is given.

-absname
Force error messages to show absolute paths for file names.

-no-absname
Do not try to show absolute filenames in error messages.

-annot
Deprecated since OCaml 4.11. Please use -bin-annot instead.

-args filename
Read additional newline-terminated command line arguments from filename.

-args0 filename
Read additional null character terminated command line arguments from filename.

-bin-annot
Dump detailed information about the compilation (types, bindings, tail-calls, etc) in binary
format. The information for file src.ml (resp. src.mli) is put into file src.cmt (resp. src.cmti).
In case of a type error, dump all the information inferred by the type-checker before the error.
The *.cmt and *.cmti files produced by -bin-annot contain more information and are much
more compact than the files produced by -annot.

-c Compile only. Suppress the linking phase of the compilation. Source code files are turned into
compiled files, but no executable file is produced. This option is useful to compile modules
separately.

-cc ccomp
Use ccomp as the C linker called to build the final executable and as the C compiler for
compiling .c source files. When linking object files produced by a C++ compiler (such as g++
or clang++), it is recommended to use -cc c++.

-cclib -llibname
Pass the -llibname option to the linker . This causes the given C library to be linked with
the program.

-ccopt option
Pass the given option to the C compiler and linker. For instance, -ccopt -Ldir causes the C
linker to search for C libraries in directory dir.

-cmi-file filename
Use the given interface file to type-check the ML source file to compile. When this option is not
specified, the compiler looks for a .mli file with the same base name than the implementation
it is compiling and in the same directory. If such a file is found, the compiler looks for a
corresponding .cmi file in the included directories and reports an error if it fails to find one.
324

-color mode
Enable or disable colors in compiler messages (especially warnings and errors). The following
modes are supported:

auto
use heuristics to enable colors only if the output supports them (an ANSI-compatible tty
terminal);
always
enable colors unconditionally;
never
disable color output.

The environment variable OCAML_COLOR is considered if -color is not provided. Its values are
auto/always/never as above.
If -color is not provided, OCAML_COLOR is not set and the environment variable NO_COLOR is
set, then color output is disabled. Otherwise, the default setting is ’auto’, and the current
heuristic checks that the TERM environment variable exists and is not empty or dumb, and that
’isatty(stderr)’ holds.

-error-style mode
Control the way error messages and warnings are printed. The following modes are supported:

short
only print the error and its location;
contextual
like short, but also display the source code snippet corresponding to the location of the
error.

The default setting is contextual.

The environment variable OCAML_ERROR_STYLE is considered if -error-style is not provided.
Its values are short/contextual as above.

-compact
Optimize the produced code for space rather than for time. This results in slightly smaller
but slightly slower programs. The default is to optimize for speed.

-config
Print the version number of ocamlopt and a detailed summary of its configuration, then exit.

-config-var var
Print the value of a specific configuration variable from the -config output, then exit. If the
variable does not exist, the exit code is non-zero. This option is only available since OCaml
4.08, so script authors should have a fallback for older versions.

-depend ocamldep-args
Compute dependencies, as the ocamldep command would do. The remaining arguments are
interpreted as if they were given to the ocamldep command.
Chapter 16. Native-code compilation (ocamlopt) 325

-for-pack module-path
Generate an object file (.cmx and .o/.obj files) that can later be included as a sub-module
(with the given access path) of a compilation unit constructed with -pack. For instance,
ocamlopt -for-pack P -c A.ml will generate a..cmx and a.o files that can later be used with
ocamlopt -pack -o P.cmx a.cmx. Note: you can still pack a module that was compiled without
-for-pack but in this case exceptions will be printed with the wrong names.

-g Add debugging information while compiling and linking. This option is required in order
to produce stack backtraces when the program terminates on an uncaught exception (see
section 15.2).

-no-g
Do not record debugging information (default).

-i Cause the compiler to print all defined names (with their inferred types or their definitions)
when compiling an implementation (.ml file). No compiled files (.cmo and .cmi files) are
produced. This can be useful to check the types inferred by the compiler. Also, since the
output follows the syntax of interfaces, it can help in writing an explicit interface (.mli file)
for a file: just redirect the standard output of the compiler to a .mli file, and edit that file to
remove all declarations of unexported names.

-I directory
Add the given directory to the list of directories searched for compiled interface files (.cmi),
compiled object code files (.cmx), and libraries (.cmxa). By default, the current directory is
searched first, then the standard library directory. Directories added with -I are searched
after the current directory, in the order in which they were given on the command line, but
before the standard library directory. See also option -nostdlib.
If the given directory starts with +, it is taken relative to the standard library directory. For
instance, -I +unix adds the subdirectory unix of the standard library to the search path.

-H directory
Behaves identically to -I, except that (a) programs may not directly refer to modules added
to the search path this way, and (b) these directories are searched after any -I directories.
This makes it possible to provide the compiler with compiled interface and object code files for
the current program’s transitive dependencies (the dependencies of its dependencies) without
allowing them to silently become direct dependencies.

-impl filename
Compile the file filename as an implementation file, even if its extension is not .ml.

-inline n
Set aggressiveness of inlining to n, where n is a positive integer. Specifying -inline 0 prevents
all functions from being inlined, except those whose body is smaller than the call site. Thus,
inlining causes no expansion in code size. The default aggressiveness, -inline 1, allows
slightly larger functions to be inlined, resulting in a slight expansion in code size. Higher values
for the -inline option cause larger and larger functions to become candidate for inlining, but
can result in a serious increase in code size.
326

-intf filename
Compile the file filename as an interface file, even if its extension is not .mli.

-intf-suffix string
Recognize file names ending with string as interface files (instead of the default .mli).

-keywords version+list
Set keywords according to the version+list specification.
This specification starts with an optional version number, defining the base set of keywords,
followed by a +-separated list of additional keywords to add to this base set.
Without an explicit version number, the base set of keywords is the set of keywords in the
current version of OCaml. Additional keywords that do not match any known keyword in the
current version of the language trigger an error whenever they are present in the source code.

-labels
Labels are not ignored in types, labels may be used in applications, and labelled parameters
can be given in any order. This is the default.

-linkall
Force all modules contained in libraries to be linked in. If this flag is not given, unreferenced
modules are not linked in. When building a library (option -a), setting the -linkall option
forces all subsequent links of programs involving that library to link all the modules contained
in the library. When compiling a module (option -c), setting the -linkall option ensures
that this module will always be linked if it is put in a library and this library is linked.

-linscan
Use linear scan register allocation. Compiling with this allocator is faster than with the usual
graph coloring allocator, sometimes quite drastically so for long functions and modules. On
the other hand, the generated code can be a bit slower.

-match-context-rows
Set the number of rows of context used for optimization during pattern matching compilation.
The default value is 32. Lower values cause faster compilation, but less optimized code. This
advanced option is meant for use in the event that a pattern-match-heavy program leads to
significant increases in compilation time.

-no-alias-deps
Do not record dependencies for module aliases. See section 12.8 for more information.

-no-app-funct
Deactivates the applicative behaviour of functors. With this option, each functor application
generates new types in its result and applying the same functor twice to the same argument
yields two incompatible structures.

-no-float-const-prop
Deactivates the constant propagation for floating-point operations. This option should be
given if the program changes the float rounding mode during its execution.
Chapter 16. Native-code compilation (ocamlopt) 327

-noassert
Do not compile assertion checks. Note that the special form assert false is always compiled
because it is typed specially. This flag has no effect when linking already-compiled files.

-noautolink
When linking .cmxalibraries, ignore -cclib and -ccopt options potentially contained in the
libraries (if these options were given when building the libraries). This can be useful if a
library contains incorrect specifications of C libraries or C options; in this case, during linking,
set -noautolink and pass the correct C libraries and options on the command line.

-nodynlink
Allow the compiler to use some optimizations that are valid only for code that is statically linked
to produce a non-relocatable executable. The generated code cannot be linked to produce a
shared library nor a position-independent executable (PIE). Many operating systems produce
PIEs by default, causing errors when linking code compiled with -nodynlink. Either do not
use -nodynlink or pass the option -ccopt -no-pie at link-time.

-nolabels
Ignore non-optional labels in types. Labels cannot be used in applications, and parameter
order becomes strict.

-nostdlib
Do not automatically add the standard library directory to the list of directories searched for
compiled interface files (.cmi), compiled object code files (.cmx), and libraries (.cmxa). See
also option -I.

-o output-file
Specify the name of the output file to produce. For executable files, the default output name
is a.out under Unix and camlprog.exe under Windows. If the -a option is given, specify the
name of the library produced. If the -pack option is given, specify the name of the packed
object file produced. If the -output-obj or -output-complete-obj options are given, specify
the name of the produced object file. If the -shared option is given, specify the name of
plugin file produced.

-opaque
When the native compiler compiles an implementation, by default it produces a .cmx file
containing information for cross-module optimization. It also expects .cmx files to be present
for the dependencies of the currently compiled source, and uses them for optimization. Since
OCaml 4.03, the compiler will emit a warning if it is unable to locate the .cmx file of one of
those dependencies.
The -opaque option, available since 4.04, disables cross-module optimization information
for the currently compiled unit. When compiling .mli interface, using -opaque marks the
compiled .cmi interface so that subsequent compilations of modules that depend on it will
not rely on the corresponding .cmx file, nor warn if it is absent. When the native compiler
compiles a .ml implementation, using -opaque generates a .cmx that does not contain any
cross-module optimization information.
328

Using this option may degrade the quality of generated code, but it reduces compilation
time, both on clean and incremental builds. Indeed, with the native compiler, when the
implementation of a compilation unit changes, all the units that depend on it may need to
be recompiled – because the cross-module information may have changed. If the compilation
unit whose implementation changed was compiled with -opaque, no such recompilation needs
to occur. This option can thus be used, for example, to get faster edit-compile-test feedback
loops.

-open Module
Opens the given module before processing the interface or implementation files. If several
-open options are given, they are processed in order, just as if the statements open! Module1;;
... open! ModuleN;; were added at the top of each file.

-output-obj
Cause the linker to produce a C object file instead of an executable file. This is useful to wrap
OCaml code as a C library, callable from any C program. See chapter 22, section 22.7.6. The
name of the output object file must be set with the -o option. This option can also be used
to produce a compiled shared/dynamic library (.so extension, .dll under Windows).

-output-complete-obj
Same as -output-obj options except the object file produced includes the runtime and autolink
libraries.

-pack
Build an object file (.cmx and .o/.obj files) and its associated compiled interface (.cmi) that
combines the .cmx object files given on the command line, making them appear as sub-modules
of the output .cmx file. The name of the output .cmx file must be given with the -o option.
For instance,

ocamlopt -pack -o P.cmx A.cmx B.cmx C.cmx

generates compiled files P.cmx, P.o and P.cmi describing a compilation unit having three
sub-modules A, B and C, corresponding to the contents of the object files A.cmx, B.cmx and
C.cmx. These contents can be referenced as P.A, P.B and P.C in the remainder of the program.
The .cmx object files being combined must have been compiled with the appropriate -for-pack
option. In the example above, A.cmx, B.cmx and C.cmx must have been compiled with
ocamlopt -for-pack P.
Multiple levels of packing can be achieved by combining -pack with -for-pack. Consider the
following example:

ocamlopt -for-pack P.Q -c A.ml ocamlopt -pack -o Q.cmx -for-pack P A.cmx

ocamlopt -for-pack P -c B.ml ocamlopt -pack -o P.cmx Q.cmx B.cmx

The resulting P.cmx object file has sub-modules P.Q, P.Q.A and P.B.
Chapter 16. Native-code compilation (ocamlopt) 329

-pp command
Cause the compiler to call the given command as a preprocessor for each source file. The
output of command is redirected to an intermediate file, which is compiled. If there are no
compilation errors, the intermediate file is deleted afterwards.

-ppx command
After parsing, pipe the abstract syntax tree through the preprocessor command. The module
Ast_mapper, described in section 30.1, implements the external interface of a preprocessor.

-principal
Check information path during type-checking, to make sure that all types are derived in
a principal way. When using labelled arguments and/or polymorphic methods, this flag is
required to ensure future versions of the compiler will be able to infer types correctly, even if
internal algorithms change. All programs accepted in -principal mode are also accepted in
the default mode with equivalent types, but different binary signatures, and this may slow
down type checking; yet it is a good idea to use it once before publishing source code.

-rectypes
Allow arbitrary recursive types during type-checking. By default, only recursive types where
the recursion goes through an object type are supported. Note that once you have created an
interface using this flag, you must use it again for all dependencies.

-runtime-variant suffix
Add the suffix string to the name of the runtime library used by the program. Currently, only
one such suffix is supported: d, and only if the OCaml compiler was configured with option
-with-debug-runtime. This suffix gives the debug version of the runtime, which is useful for
debugging pointer problems in low-level code such as C stubs.

-S Keep the assembly code produced during the compilation. The assembly code for the source
file x.ml is saved in the file x.s.

-safe-string
Enforce the separation between types string and bytes, thereby making strings read-only.
This is the default, and enforced since OCaml 5.0.

-safer-matching
Do not use type information to optimize pattern-matching. This allows to detect match
failures even if a pattern-matching was wrongly assumed to be exhaustive. This only impacts
GADT and polymorphic variant compilation.

-save-ir-after pass
Save intermediate representation after the given compilation pass to a file. The currently
supported passes and the corresponding file extensions are: scheduling (.cmir-linear).
This experimental feature enables external tools to inspect and manipulate compiler’s inter-
mediate representation of the program using compiler-libs library (see section 30).

-shared
Build a plugin (usually .cmxs) that can be dynamically loaded with the Dynlink module. The
330

name of the plugin must be set with the -o option. A plugin can include a number of OCaml
modules and libraries, and extra native objects (.o, .obj, .a, .lib files). Building native
plugins is only supported for some operating system. Under some systems (currently, only
Linux AMD 64), all the OCaml code linked in a plugin must have been compiled without the
-nodynlink flag. Some constraints might also apply to the way the extra native objects have
been compiled (under Linux AMD 64, they must contain only position-independent code).

-short-paths
When a type is visible under several module-paths, use the shortest one when printing the
type’s name in inferred interfaces and error and warning messages. Identifier names starting
with an underscore _ or containing double underscores __ incur a penalty of +10 when
computing their length.

-stop-after pass
Stop compilation after the given compilation pass. The currently supported passes are:
parsing, typing, scheduling, emit.

-strict-sequence
Force the left-hand part of each sequence to have type unit.

-strict-formats
Reject invalid formats that were accepted in legacy format implementations. You should use
this flag to detect and fix such invalid formats, as they will be rejected by future OCaml
versions.

-unboxed-types
When a type is unboxable (i.e. a record with a single argument or a concrete datatype with a
single constructor of one argument) it will be unboxed unless annotated with [@@ocaml.boxed].

-no-unboxed-types
When a type is unboxable it will be boxed unless annotated with [@@ocaml.unboxed]. This
is the default.

-unsafe
Turn bound checking off for array and string accesses (the v.(i) and s.[i] constructs).
Programs compiled with -unsafe are therefore faster, but unsafe: anything can happen if
the program accesses an array or string outside of its bounds. Additionally, turn off the check
for zero divisor in integer division and modulus operations. With -unsafe, an integer division
(or modulus) by zero can halt the program or continue with an unspecified result instead of
raising a Division_by_zero exception.

-unsafe-string
Identify the types string and bytes, thereby making strings writable. This is intended for
compatibility with old source code and should not be used with new software. This option
raises an error unconditionally since OCaml 5.0.

-v Print the version number of the compiler and the location of the standard library directory,
then exit.
Chapter 16. Native-code compilation (ocamlopt) 331

-verbose
Print all external commands before they are executed, in particular invocations of the assembler,
C compiler, and linker. Useful to debug C library problems.
-version or -vnum
Print the version number of the compiler in short form (e.g. 3.11.0), then exit.
-w warning-list
Enable, disable, or mark as fatal the warnings specified by the argument warning-list. Each
warning can be enabled or disabled, and each warning can be fatal or non-fatal. If a warning is
disabled, it isn’t displayed and doesn’t affect compilation in any way (even if it is fatal). If a
warning is enabled, it is displayed normally by the compiler whenever the source code triggers
it. If it is enabled and fatal, the compiler will also stop with an error after displaying it.
The warning-list argument is a sequence of warning specifiers, with no separators between
them. A warning specifier is one of the following:

+num
Enable warning number num.
-num
Disable warning number num.
@num
Enable and mark as fatal warning number num.
+num1..num2
Enable warnings in the given range.
-num1..num2
Disable warnings in the given range.
@num1..num2
Enable and mark as fatal warnings in the given range.
+letter
Enable the set of warnings corresponding to letter. The letter may be uppercase or
lowercase.
-letter
Disable the set of warnings corresponding to letter. The letter may be uppercase or
lowercase.
@letter
Enable and mark as fatal the set of warnings corresponding to letter. The letter may be
uppercase or lowercase.
uppercase-letter
Enable the set of warnings corresponding to uppercase-letter.
lowercase-letter
Disable the set of warnings corresponding to lowercase-letter.

Alternatively, warning-list can specify a single warning using its mnemonic name (see below),
as follows:
332

+name
Enable warning name.
-name
Disable warning name.
@name
Enable and mark as fatal warning name.

Warning numbers, letters and names which are not currently defined are ignored. The warnings
are as follows (the name following each number specifies the mnemonic for that warning).

1 comment-start
Suspicious-looking start-of-comment mark.
2 comment-not-end
Suspicious-looking end-of-comment mark.
3 Deprecated synonym for the ’deprecated’ alert.
4 fragile-match
Fragile pattern matching: matching that will remain complete even if additional con-
structors are added to one of the variant types matched.
5 ignored-partial-application
Partially applied function: expression whose result has function type and is ignored.
6 labels-omitted
Label omitted in function application.
7 method-override
Method overridden.
8 partial-match
Partial match: missing cases in pattern-matching.
9 missing-record-field-pattern
Missing fields in a record pattern.
10 non-unit-statement
Expression on the left-hand side of a sequence that doesn’t have type unit (and that is
not a function, see warning number 5).
11 redundant-case
Redundant case in a pattern matching (unused match case).
12 redundant-subpat
Redundant sub-pattern in a pattern-matching.
13 instance-variable-override
Instance variable overridden.
14 illegal-backslash
Illegal backslash escape in a string constant.
15 implicit-public-methods
Private method made public implicitly.
Chapter 16. Native-code compilation (ocamlopt) 333

16 unerasable-optional-argument
Unerasable optional argument.
17 undeclared-virtual-method
Undeclared virtual method.
18 not-principal
Non-principal type.
19 non-principal-labels
Type without principality.
20 ignored-extra-argument
Unused function argument.
21 nonreturning-statement
Non-returning statement.
22 preprocessor
Preprocessor warning.
23 useless-record-with
Useless record with clause.
24 bad-module-name
Bad module name: the source file name is not a valid OCaml module name.
25 Ignored: now part of warning 8.
26 unused-var
Suspicious unused variable: unused variable that is bound with let or as, and doesn’t
start with an underscore (_) character.
27 unused-var-strict
Innocuous unused variable: unused variable that is not bound with let nor as, and
doesn’t start with an underscore (_) character.
28 wildcard-arg-to-constant-constr
Wildcard pattern given as argument to a constant constructor.
29 eol-in-string
Unescaped end-of-line in a string constant (non-portable code).
30 duplicate-definitions
Two labels or constructors of the same name are defined in two mutually recursive types.
31 module-linked-twice
A module is linked twice in the same executable.
I gnored: now a hard error (since 5.1).
32 unused-value-declaration
Unused value declaration. (since 4.00)
33 unused-open
Unused open statement. (since 4.00)
34 unused-type-declaration
Unused type declaration. (since 4.00)
334

35 unused-for-index
Unused for-loop index. (since 4.00)
36 unused-ancestor
Unused ancestor variable. (since 4.00)
37 unused-constructor
Unused constructor. (since 4.00)
38 unused-extension
Unused extension constructor. (since 4.00)
39 unused-rec-flag
Unused rec flag. (since 4.00)
40 name-out-of-scope
Constructor or label name used out of scope. (since 4.01)
41 ambiguous-name
Ambiguous constructor or label name. (since 4.01)
42 disambiguated-name
Disambiguated constructor or label name (compatibility warning). (since 4.01)
43 nonoptional-label
Nonoptional label applied as optional. (since 4.01)
44 open-shadow-identifier
Open statement shadows an already defined identifier. (since 4.01)
45 open-shadow-label-constructor
Open statement shadows an already defined label or constructor. (since 4.01)
46 bad-env-variable
Error in environment variable. (since 4.01)
47 attribute-payload
Illegal attribute payload. (since 4.02)
48 eliminated-optional-arguments
Implicit elimination of optional arguments. (since 4.02)
49 no-cmi-file
Absent cmi file when looking up module alias. (since 4.02)
50 unexpected-docstring
Unexpected documentation comment. (since 4.03)
51 wrong-tailcall-expectation
Function call annotated with an incorrect @tailcall attribute. (since 4.03)
52 fragile-literal-pattern (see 13.5.3)
Fragile constant pattern. (since 4.03)
53 misplaced-attribute
Attribute cannot appear in this context. (since 4.03)
54 duplicated-attribute
Attribute used more than once on an expression. (since 4.03)
Chapter 16. Native-code compilation (ocamlopt) 335

55 inlining-impossible
Inlining impossible. (since 4.03)
56 unreachable-case
Unreachable case in a pattern-matching (based on type information). (since 4.03)
57 ambiguous-var-in-pattern-guard (see 13.5.4)
Ambiguous or-pattern variables under guard. (since 4.03)
58 no-cmx-file
Missing cmx file. (since 4.03)
59 flambda-assignment-to-non-mutable-value
Assignment to non-mutable value. (since 4.03)
60 unused-module
Unused module declaration. (since 4.04)
61 unboxable-type-in-prim-decl
Unboxable type in primitive declaration. (since 4.04)
62 constraint-on-gadt
Type constraint on GADT type declaration. (since 4.06)
63 erroneous-printed-signature
Erroneous printed signature. (since 4.08)
64 unsafe-array-syntax-without-parsing
-unsafe used with a preprocessor returning a syntax tree. (since 4.08)
65 redefining-unit
Type declaration defining a new ’()’ constructor. (since 4.08)
66 unused-open-bang
Unused open! statement. (since 4.08)
67 unused-functor-parameter
Unused functor parameter. (since 4.10)
68 match-on-mutable-state-prevent-uncurry
Pattern-matching depending on mutable state prevents the remaining arguments from
being uncurried. (since 4.12)
69 unused-field
Unused record field. (since 4.13)
70 missing-mli
Missing interface file. (since 4.13)
71 unused-tmc-attribute
Unused @tail_mod_cons attribute. (since 4.14)
72 tmc-breaks-tailcall
A tail call is turned into a non-tail call by the @tail_mod_cons transformation. (since
4.14)
73 generative-application-expects-unit
A generative functor is applied to an empty structure (struct end) rather than to ().
(since 5.1)
336

74 degraded-to-partial-match
A pattern-matching is compiled as partial even if it appears to be total. (since 5.3)
A all warnings
C warnings 1, 2.
D Alias for warning 3.
E Alias for warning 4.
F Alias for warning 5.
K warnings 32, 33, 34, 35, 36, 37, 38, 39.
L Alias for warning 6.
M Alias for warning 7.
P Alias for warning 8.
R Alias for warning 9.
S Alias for warning 10.
U warnings 11, 12.
V Alias for warning 13.
X warnings 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 30.
Y Alias for warning 26.
Z Alias for warning 27.

The default setting is -w +a-4-6-7-9-27-29-32..42-44-45-48-50-60. It is displayed by

ocamlopt -help. Note that warnings 5 and 10 are not always triggered, depending on the
internals of the type checker.

-warn-error warning-list
Mark as fatal the warnings specified in the argument warning-list. The compiler will stop
with an error when one of these warnings is emitted. The warning-list has the same meaning
as for the -w option: a + sign (or an uppercase letter) marks the corresponding warnings as
fatal, a - sign (or a lowercase letter) turns them back into non-fatal warnings, and a @ sign
both enables and marks as fatal the corresponding warnings.
Note: it is not recommended to use warning sets (i.e. letters) as arguments to -warn-error
in production code, because this can break your build when future versions of OCaml add
some new warnings.
The default setting is -warn-error -a (no warning is fatal).

-warn-help
Show the description of all available warning numbers.

-where
Print the location of the standard library, then exit.
Chapter 16. Native-code compilation (ocamlopt) 337

-with-runtime
The pair of options -with-runtime and -without-runtime give precise control over the way
the runtime is linked. -with-runtime is the default. In summary, it instructs to include the
runtime system, or a reference to the default path of the runtime system, in the generated
program/executable/object file. The detailed behaviour depends on the compiler and options
used:
For ocamlc, in its default linking mode (no use of -custom or -output-*), -with-runtime
creates a file which can be executed, whereas -without-runtime creates a pure bytecode
image which must be explicitly passed to a runtime (i.e. ./foo vs ocamlrun ./foo).
For all other uses of ocamlc and ocamlopt, -with-runtime and -without-runtime control
whether the compiler passes flags for linking with the installed runtime (-with-runtime) or
whether the user is required to pass them (-without-runtime).
For more information about the options -custom and -output-*, see their documentation
and section 22.7.6 of the manual.

-without-runtime
The compiler does not include the runtime system (nor a reference to it) in the generated
program, executable or object file; it must be supplied separately. See option -with-runtime
for details.

- file
Process file as a file name, even if it starts with a dash (-) character.

-help or --help
Display a short usage summary and exit.

Options for the 64-bit x86 architecture The 64-bit code generator for Intel/AMD x86
processors (amd64 architecture) supports the following additional options:

-fPIC
Generate position-independent machine code. This is the default.

-fno-PIC
Generate position-dependent machine code.

Contextual control of command-line options

The compiler command line can be modified “from the outside” with the following mechanisms.
These are experimental and subject to change. They should be used only for experimental and
development work, not in released packages.

OCAMLPARAM (environment variable)

A set of arguments that will be inserted before or after the arguments from the command
line. Arguments are specified in a comma-separated list of name=value pairs. A _ is used to
specify the position of the command line arguments, i.e. a=x,_,b=y means that a=x should
be executed before parsing the arguments, and b=y after. Finally, an alternative separator
can be specified as the first character of the string, within the set :|; ,.
338

ocaml_compiler_internal_params (file in the stdlib directory)

A mapping of file names to lists of arguments that will be added to the command line (and
OCAMLPARAM) arguments.

16.3 Common errors

The error messages are almost identical to those of ocamlc. See section 13.4.

16.4 Running executables produced by ocamlopt

Executables generated by ocamlopt are native, stand-alone executable files that can be invoked
directly. They do not depend on the ocamlrun bytecode runtime system nor on dynamically-loaded
C/OCaml stub libraries.
During execution of an ocamlopt-generated executable, the following environment variables are
also consulted:

OCAMLRUNPARAM
Same usage as in ocamlrun (see section 15.2), except that option l is ignored (the operating
system’s stack size limit is used instead).

CAMLRUNPARAM
If OCAMLRUNPARAM is not found in the environment, then CAMLRUNPARAM will be used instead.
If CAMLRUNPARAM is not found, then the default values will be used.

16.5 Compatibility with the bytecode compiler

This section lists the known incompatibilities between the bytecode compiler and the native-code
compiler. Except on those points, the two compilers should generate code that behave identically.

• Signals are detected only when the program performs an allocation in the heap. That is, if a
signal is delivered while in a piece of code that does not allocate, its handler will not be called
until the next heap allocation.

• On ARM and PowerPC processors (32 and 64 bits), fused multiply-add (FMA) instructions
can be generated for a floating-point multiplication followed by a floating-point addition or
subtraction, as in x *. y +. z. The FMA instruction avoids rounding the intermediate result
x *. y, which is generally beneficial, but produces floating-point results that differ slightly
from those produced by the bytecode interpreter.

• The native-code compiler performs a number of optimizations that the bytecode compiler does
not perform, especially when the Flambda optimizer is active. In particular, the native-code
compiler identifies and eliminates “dead code”, i.e. computations that do not contribute to
the results of the program. For example,

let _ = ignore M.f

Chapter 16. Native-code compilation (ocamlopt) 339

contains a reference to compilation unit M when compiled to bytecode. This reference forces M
to be linked and its initialization code to be executed. The native-code compiler eliminates the
reference to M, hence the compilation unit M may not be linked and executed. A workaround
is to compile M with the -linkall flag so that it will always be linked and executed, even if
not referenced. See also the Sys.opaque_identity function from the Sys standard library
module.

• Before 4.10, stack overflows, typically caused by excessively deep recursion, are not always
turned into a Stack_overflow exception like with the bytecode compiler. The runtime system
makes a best effort to trap stack overflows and raise the Stack_overflow exception, but
sometimes it fails and a “segmentation fault” or another system fault occurs instead.
340
Chapter 17

Lexer and parser generators (ocamllex,

ocamlyacc)

This chapter describes two program generators: ocamllex, that produces a lexical analyzer from a
set of regular expressions with associated semantic actions, and ocamlyacc, that produces a parser
from a grammar with associated semantic actions.
These program generators are very close to the well-known lex and yacc commands that can be
found in most C programming environments. This chapter assumes a working knowledge of lex and
yacc: while it describes the input syntax for ocamllex and ocamlyacc and the main differences
with lex and yacc, it does not explain the basics of writing a lexer or parser description in lex and
yacc. Readers unfamiliar with lex and yacc are referred to “Compilers: principles, techniques, and
tools” by Aho, Lam, Sethi and Ullman (Pearson, 2006), or “Lex & Yacc”, by Levine, Mason and
Brown (O’Reilly, 1992).

17.1 Overview of ocamllex

The ocamllex command produces a lexical analyzer from a set of regular expressions with attached
semantic actions, in the style of lex. Assuming the input file is lexer.mll, executing

ocamllex lexer.mll

produces OCaml code for a lexical analyzer in file lexer.ml. This file defines one lexing
function per entry point in the lexer definition. These functions have the same names as the entry
points. Lexing functions take as argument a lexer buffer, and return the semantic attribute of the
corresponding entry point.
Lexer buffers are an abstract data type implemented in the standard library module Lexing. The
functions Lexing.from_channel, Lexing.from_string and Lexing.from_function create lexer
buffers that read from an input channel, a character string, or any reading function, respectively.
(See the description of module Lexing in chapter 29.)
When used in conjunction with a parser generated by ocamlyacc, the semantic actions compute
a value belonging to the type token defined by the generated parsing module. (See the description
of ocamlyacc below.)

341
342

17.1.1 Options
The following command-line options are recognized by ocamllex.

-ml Output code that does not use OCaml’s built-in automata interpreter. Instead, the automaton
is encoded by OCaml functions. This option improves performance when using the native
compiler, but decreases it when using the bytecode compiler.

-o output-file
Specify the name of the output file produced by ocamllex. The default is the input file name
with its extension replaced by .ml.

-q Quiet mode. ocamllex normally outputs informational messages to standard output. They
are suppressed if option -q is used.

-v or -version
Print version string and exit.

-vnum
Print short version number and exit.

-help or --help
Display a short usage summary and exit.

17.2 Syntax of lexer definitions

The format of lexer definitions is as follows:

{ header }
let ident = regexp …
[refill { refill-handler }]
rule entrypoint [arg 1 … arg n ] =
parse regexp { action }
| …
| regexp { action }
and entrypoint [arg 1 … arg n ] =
parse …
and …
{ trailer }

Comments are delimited by (* and *), as in OCaml. The parse keyword, can be replaced by
the shortest keyword, with the semantic consequences explained below.
Refill handlers are a recent (optional) feature introduced in 4.02, documented below in subsec-
tion 17.2.7.
Chapter 17. Lexer and parser generators (ocamllex, ocamlyacc) 343

17.2.1 Header and trailer

The header and trailer sections are arbitrary OCaml text enclosed in curly braces. Either or both
can be omitted. If present, the header text is copied as is at the beginning of the output file and
the trailer text at the end. Typically, the header section contains the open directives required by
the actions, and possibly some auxiliary functions used in the actions.

17.2.2 Naming regular expressions

Between the header and the entry points, one can give names to frequently-occurring regular
expressions. This is written let ident = regexp. In regular expressions that follow this declaration,
the identifier ident can be used as shorthand for regexp.

17.2.3 Entry points

The names of the entry points must be valid identifiers for OCaml values (starting with a lowercase
letter). Similarly, the arguments arg1 … argn must be valid identifiers for OCaml. Each entry point
becomes an OCaml function that takes n + 1 arguments, the extra implicit last argument being of
type Lexing.lexbuf. Characters are read from the Lexing.lexbuf argument and matched against
the regular expressions provided in the rule, until a prefix of the input matches one of the rule. The
corresponding action is then evaluated and returned as the result of the function.
If several regular expressions match a prefix of the input, the “longest match” rule applies: the
regular expression that matches the longest prefix of the input is selected. In case of tie, the regular
expression that occurs earlier in the rule is selected.
However, if lexer rules are introduced with the shortest keyword in place of the parse keyword,
then the “shortest match” rule applies: the shortest prefix of the input is selected. In case of tie, the
regular expression that occurs earlier in the rule is still selected. This feature is not intended for use
in ordinary lexical analyzers, it may facilitate the use of ocamllex as a simple text processing tool.

17.2.4 Regular expressions

The regular expressions are in the style of lex, with a more OCaml-like syntax.
regexp ::= ...
' regular-char | escape-sequence '
A character constant, with the same syntax as OCaml character constants. Match the denoted
character.
_ (underscore) Match any character.
eof Match the end of the lexer input.
Note: On some systems, with interactive input, an end-of-file may be followed by more
characters. However, ocamllex will not correctly handle regular expressions that contain eof
followed by something else.
" {string-character} "
A string constant, with the same syntax as OCaml string constants. Match the corresponding
sequence of characters.
344

[ character-set ]
Match any single character belonging to the given character set. Valid character sets are:
single character constants 'c '; ranges of characters 'c 1 ' - 'c 2 ' (all characters between c1
and c2 , inclusive); and the union of two or more character sets, denoted by concatenation.

[ ^ character-set ]
Match any single character not belonging to the given character set.

regexp 1 # regexp 2
(difference of character sets) Regular expressions regexp 1 and regexp 2 must be character sets
defined with [ . . . ] (or a single character expression or underscore _). Match the difference of
the two specified character sets.

regexp *
(repetition) Match the concatenation of zero or more strings that match regexp.

regexp +
(strict repetition) Match the concatenation of one or more strings that match regexp.

regexp ?
(option) Match the empty string, or a string matching regexp.

regexp 1 | regexp 2
(alternative) Match any string that matches regexp 1 or regexp 2 . If both regexp 1 and regexp 2
are character sets, this constructions produces another character set, obtained by taking the
union of regexp 1 and regexp 2 .

regexp 1 regexp 2
(concatenation) Match the concatenation of two strings, the first matching regexp 1 , the second
matching regexp 2 .

( regexp )
Match the same strings as regexp.

ident
Reference the regular expression bound to ident by an earlier let ident = regexp definition.

regexp as ident
Bind the substring matched by regexp to identifier ident.

Concerning the precedences of operators, # has the highest precedence, followed by *, + and ?,
then concatenation, then | (alternation), then as.

17.2.5 Actions
The actions are arbitrary OCaml expressions. They are evaluated in a context where the identifiers
defined by using the as construct are bound to subparts of the matched string. Additionally,
lexbuf is bound to the current lexer buffer. Some typical uses for lexbuf, in conjunction with the
operations on lexer buffers provided by the Lexing standard library module, are listed below.
Chapter 17. Lexer and parser generators (ocamllex, ocamlyacc) 345

Lexing.lexeme lexbuf
Return the matched string.

Lexing.lexeme_char lexbuf n
Return the n character in the matched string. The first character corresponds to n = 0.

Lexing.lexeme_start lexbuf
Return the absolute position in the input text of the beginning of the matched string (i.e. the
offset of the first character of the matched string). The first character read from the input
text has offset 0.

Lexing.lexeme_end lexbuf
Return the absolute position in the input text of the end of the matched string (i.e. the offset
of the first character after the matched string). The first character read from the input text
has offset 0.

entrypoint [exp1 … expn ] lexbuf

(Where entrypoint is the name of another entry point in the same lexer definition.) Recursively
call the lexer on the given entry point. Notice that lexbuf is the last argument. Useful for
lexing nested comments, for example.

17.2.6 Variables in regular expressions

The as construct is similar to “groups” as provided by numerous regular expression packages. The
type of these variables can be string, char, string option or char option.
We first consider the case of linear patterns, that is the case when all as bound variables are
distinct. In regexp as ident, the type of ident normally is string (or string option) except when
regexp is a character constant, an underscore, a string constant of length one, a character set
specification, or an alternation of those. Then, the type of ident is char (or char option). Option
types are introduced when overall rule matching does not imply matching of the bound sub-pattern.
This is in particular the case of ( regexp as ident ) ? and of regexp 1 | ( regexp 2 as ident ).
There is no linearity restriction over as bound variables. When a variable is bound more than
once, the previous rules are to be extended as follows:

• A variable is a char variable when all its occurrences bind char occurrences in the previous
sense.

• A variable is an option variable when the overall expression can be matched without binding
this variable.

For instance, in ('a' as x) | ( 'a' (_ as x) ) the variable x is of type char, whereas in

("ab" as x) | ( 'a' (_ as x) ? ) the variable x is of type string option.
In some cases, a successful match may not yield a unique set of bindings. For instance the
matching of aba by the regular expression (('a'|"ab") as x) (("ba"|'a') as y) may result
in binding either x to "ab" and y to "a", or x to "a" and y to "ba". The automata produced
ocamllex on such ambiguous regular expressions will select one of the possible resulting sets of
bindings. The selected set of bindings is purposely left unspecified.
346

17.2.7 Refill handlers

By default, when ocamllex reaches the end of its lexing buffer, it will silently call the refill_buff
function of lexbuf structure and continue lexing. It is sometimes useful to be able to take control
of refilling action; typically, if you use a library for asynchronous computation, you may want to
wrap the refilling action in a delaying function to avoid blocking synchronous operations.
Since OCaml 4.02, it is possible to specify a refill-handler, a function that will be called when
refill happens. It is passed the continuation of the lexing, on which it has total control. The OCaml
expression used as refill action should have a type that is an instance of

(Lexing.lexbuf -> 'a) -> Lexing.lexbuf -> 'a

where the first argument is the continuation which captures the processing ocamllex would
usually perform (refilling the buffer, then calling the lexing function again), and the result type that
instantiates [’a] should unify with the result type of all lexing rules.
As an example, consider the following lexer that is parametrized over an arbitrary monad:

{
type token = EOL | INT of int | PLUS

module Make (M : sig

type 'a t
val return: 'a -> 'a t
val bind: 'a t -> ('a -> 'b t) -> 'b t
val fail : string -> 'a t

(* Set up lexbuf *)
val on_refill : Lexing.lexbuf -> unit t
end)
= struct

let refill_handler k lexbuf =

M.bind (M.on_refill lexbuf) (fun () -> k lexbuf)

refill {refill_handler}

rule token = parse

| [' ' '\t']
{ token lexbuf }
| '\n'
{ M.return EOL }
| ['0'-'9']+ as i
{ M.return (INT (int_of_string i)) }
| '+'
{ M.return PLUS }
Chapter 17. Lexer and parser generators (ocamllex, ocamlyacc) 347

| _
{ M.fail "unexpected character" }
{
end
}

17.2.8 Reserved identifiers

All identifiers starting with __ocaml_lex are reserved for use by ocamllex; do not use any such
identifier in your programs.

17.3 Overview of ocamlyacc

The ocamlyacc command produces a parser from a context-free grammar specification with attached
semantic actions, in the style of yacc. Assuming the input file is grammar.mly, executing

ocamlyacc options grammar.mly

produces OCaml code for a parser in the file grammar.ml, and its interface in file grammar.mli.
The generated module defines one parsing function per entry point in the grammar. These
functions have the same names as the entry points. Parsing functions take as arguments a lexical
analyzer (a function from lexer buffers to tokens) and a lexer buffer, and return the semantic
attribute of the corresponding entry point. Lexical analyzer functions are usually generated from a
lexer specification by the ocamllex program. Lexer buffers are an abstract data type implemented
in the standard library module Lexing. Tokens are values from the concrete type token, defined in
the interface file grammar.mli produced by ocamlyacc.

17.4 Syntax of grammar definitions

Grammar definitions have the following format:

%{
header
%}
declarations
%%
rules
%%
trailer

Comments are delimited by (* and *), as in OCaml. Additionally, comments can be delimited
by /* and */, as in C, in the “declarations” and “rules” sections. C-style comments do not nest,
but OCaml-style comments do.
348

17.4.1 Header and trailer

The header and the trailer sections are OCaml code that is copied as is into file grammar.ml. Both
sections are optional. The header goes at the beginning of the output file; it usually contains open
directives and auxiliary functions required by the semantic actions of the rules. The trailer goes at
the end of the output file.

17.4.2 Declarations
Declarations are given one per line. They all start with a % sign.

%token constr . . . constr

Declare the given symbols constr . . . constr as tokens (terminal symbols). These symbols are
added as constant constructors for the token concrete type.

%token < typexpr > constr . . . constr

Declare the given symbols constr . . . constr as tokens with an attached attribute of the given
type. These symbols are added as constructors with arguments of the given type for the
token concrete type. The typexpr part is an arbitrary OCaml type expression, except that all
type constructor names must be fully qualified (e.g. Modname.typename) for all types except
standard built-in types, even if the proper open directives (e.g. open Modname) were given in
the header section. That’s because the header is copied only to the .ml output file, but not to
the .mli output file, while the typexpr part of a %token declaration is copied to both.

%start symbol . . . symbol

Declare the given symbols as entry points for the grammar. For each entry point, a parsing
function with the same name is defined in the output module. Non-terminals that are not
declared as entry points have no such parsing function. Start symbols must be given a type
with the %type directive below.

%type < typexpr > symbol . . . symbol

Specify the type of the semantic attributes for the given symbols. This is mandatory for start
symbols only. Other nonterminal symbols need not be given types by hand: these types will
be inferred when running the output files through the OCaml compiler (unless the -s option
is in effect). The typexpr part is an arbitrary OCaml type expression, except that all type
constructor names must be fully qualified, as explained above for %token.

%left symbol . . . symbol

%right symbol . . . symbol

%nonassoc symbol . . . symbol

Associate precedences and associativities to the given symbols. All symbols on the same line
are given the same precedence. They have higher precedence than symbols declared before
in a %left, %right or %nonassoc line. They have lower precedence than symbols declared
Chapter 17. Lexer and parser generators (ocamllex, ocamlyacc) 349

after in a %left, %right or %nonassoc line. The symbols are declared to associate to the
left (%left), to the right (%right), or to be non-associative (%nonassoc). The symbols are
usually tokens. They can also be dummy nonterminals, for use with the %prec directive inside
the rules.
The precedence declarations are used in the following way to resolve reduce/reduce and
shift/reduce conflicts:

• Tokens and rules have precedences. By default, the precedence of a rule is the precedence
of its rightmost terminal. You can override this default by using the %prec directive in
the rule.
• A reduce/reduce conflict is resolved in favor of the first rule (in the order given by the
source file), and ocamlyacc outputs a warning.
• A shift/reduce conflict is resolved by comparing the precedence of the rule to be reduced
with the precedence of the token to be shifted. If the precedence of the rule is higher,
then the rule will be reduced; if the precedence of the token is higher, then the token will
be shifted.
• A shift/reduce conflict between a rule and a token with the same precedence will be
resolved using the associativity: if the token is left-associative, then the parser will reduce;
if the token is right-associative, then the parser will shift. If the token is non-associative,
then the parser will declare a syntax error.
• When a shift/reduce conflict cannot be resolved using the above method, then ocamlyacc
will output a warning and the parser will always shift.

17.4.3 Rules
The syntax for rules is as usual:

nonterminal :
symbol … symbol { semantic-action }
| …
| symbol … symbol { semantic-action }
;

Rules can also contain the %prec symbol directive in the right-hand side part, to override the
default precedence and associativity of the rule with the precedence and associativity of the given
symbol.
Semantic actions are arbitrary OCaml expressions, that are evaluated to produce the semantic
attribute attached to the defined nonterminal. The semantic actions can access the semantic
attributes of the symbols in the right-hand side of the rule with the $ notation: $1 is the attribute
for the first (leftmost) symbol, $2 is the attribute for the second symbol, etc.
The rules may contain the special symbol error to indicate resynchronization points, as in yacc.
Actions occurring in the middle of rules are not supported.
Nonterminal symbols are like regular OCaml symbols, except that they cannot end with ' (single
quote).
350

17.4.4 Error handling

Error recovery is supported as follows: when the parser reaches an error state (no grammar rules can
apply), it calls a function named parse_error with the string "syntax error" as argument. The
default parse_error function does nothing and returns, thus initiating error recovery (see below).
The user can define a customized parse_error function in the header section of the grammar file.
The parser also enters error recovery mode if one of the grammar actions raises the
Parsing.Parse_error exception.
In error recovery mode, the parser discards states from the stack until it reaches a place where
the error token can be shifted. It then discards tokens from the input until it finds three successive
tokens that can be accepted, and starts processing with the first of these. If no state can be uncovered
where the error token can be shifted, then the parser aborts by raising the Parsing.Parse_error
exception.
Refer to documentation on yacc for more details and guidance in how to use error recovery.

17.5 Options
The ocamlyacc command recognizes the following options:

-bprefix
Name the output files prefix.ml, prefix.mli, prefix.output, instead of the default naming
convention.

-q This option has no effect.

-v Generate a description of the parsing tables and a report on conflicts resulting from ambiguities
in the grammar. The description is put in file grammar.output.

-version
Print version string and exit.

-vnum
Print short version number and exit.

- Read the grammar specification from standard input. The default output file names are
stdin.ml and stdin.mli.

-- file
Process file as the grammar specification, even if its name starts with a dash (-) character.
This option must be the last on the command line.

At run-time, the ocamlyacc-generated parser can be debugged by setting the p option in the
OCAMLRUNPARAM environment variable (see section 15.2). This causes the pushdown automaton
executing the parser to print a trace of its action (tokens shifted, rules reduced, etc). The
trace mentions rule numbers and state numbers that can be interpreted by looking at the file
grammar.output generated by ocamlyacc -v.
Chapter 17. Lexer and parser generators (ocamllex, ocamlyacc) 351

17.6 A complete example

The all-time favorite: a desk calculator. This program reads arithmetic expressions on standard
input, one per line, and prints their values. Here is the grammar definition:
/* File parser.mly */
%token <int> INT
%token PLUS MINUS TIMES DIV
%token LPAREN RPAREN
%token EOL
%left PLUS MINUS /* lowest precedence */
%left TIMES DIV /* medium precedence */
%nonassoc UMINUS /* highest precedence */
%start main /* the entry point */
%type <int> main
%%
main:
expr EOL { $1 }
;
expr:
INT { $1 }
| LPAREN expr RPAREN { $2 }
| expr PLUS expr { $1 + $3 }
| expr MINUS expr { $1 - $3 }
| expr TIMES expr { $1 * $3 }
| expr DIV expr { $1 / $3 }
| MINUS expr %prec UMINUS { - $2 }
;
Here is the definition for the corresponding lexer:
(* File lexer.mll *)
{
open Parser (* The type token is defined in parser.mli *)
exception Eof
}
rule token = parse
[' ' '\t'] { token lexbuf } (* skip blanks *)
| ['\n' ] { EOL }
| ['0'-'9']+ as lxm { INT(int_of_string lxm) }
| '+' { PLUS }
| '-' { MINUS }
| '*' { TIMES }
| '/' { DIV }
| '(' { LPAREN }
| ')' { RPAREN }
| eof { raise Eof }
352

Here is the main program, that combines the parser with the lexer:

(* File calc.ml *)
let _ =
try
let lexbuf = Lexing.from_channel stdin in
while true do
let result = Parser.main Lexer.token lexbuf in
print_int result; print_newline(); flush stdout
done
with Lexer.Eof ->
exit 0

To compile everything, execute:

ocamllex lexer.mll # generates lexer.ml

ocamlyacc parser.mly # generates parser.ml and parser.mli
ocamlc -c parser.mli
ocamlc -c lexer.ml
ocamlc -c parser.ml
ocamlc -c calc.ml
ocamlc -o calc lexer.cmo parser.cmo calc.cmo

17.7 Common errors

ocamllex: transition table overflow, automaton is too big

The deterministic automata generated by ocamllex are limited to at most 32767 transitions.
The message above indicates that your lexer definition is too complex and overflows this limit.
This is commonly caused by lexer definitions that have separate rules for each of the alphabetic
keywords of the language, as in the following example.

rule token = parse

"keyword1" { KWD1 }
| "keyword2" { KWD2 }
| ...
| "keyword100" { KWD100 }
| ['A'-'Z' 'a'-'z'] ['A'-'Z' 'a'-'z' '0'-'9' '_'] * as id
{ IDENT id}

To keep the generated automata small, rewrite those definitions with only one general “identi-
fier” rule, followed by a hashtable lookup to separate keywords from identifiers:

{ let keyword_table = Hashtbl.create 53

let _ =
List.iter (fun (kwd, tok) -> Hashtbl.add keyword_table kwd tok)
Chapter 17. Lexer and parser generators (ocamllex, ocamlyacc) 353

[ "keyword1", KWD1;
"keyword2", KWD2; ...
"keyword100", KWD100 ]
}
rule token = parse
['A'-'Z' 'a'-'z'] ['A'-'Z' 'a'-'z' '0'-'9' '_'] * as id
{ try
Hashtbl.find keyword_table id
with Not_found ->
IDENT id }

ocamllex: Position memory overflow, too many bindings

The deterministic automata generated by ocamllex maintain a table of positions inside the
scanned lexer buffer. The size of this table is limited to at most 255 cells. This error should
not show up in normal situations.

ocamlyacc: concurrency safety

Parsers generated by ocamlyacc are not thread-safe. Those parsers rely on an internal work
state which is shared by all ocamlyacc generated parsers. The menhir parser generator is a
better option if you want thread-safe parsers.
354
Chapter 18

Dependency generator (ocamldep)

The ocamldep command scans a set of OCaml source files (.ml and .mli files) for references to
external compilation units, and outputs dependency lines in a format suitable for the make utility.
This ensures that make will compile the source files in the correct order, and recompile those files
that need to when a source file is modified.
The typical usage is:

ocamldep options *.mli *.ml > .depend

where *.mli *.ml expands to all source files in the current directory and .depend is the file
that should contain the dependencies. (See below for a typical Makefile.)
Dependencies are generated both for compiling with the bytecode compiler ocamlc and with the
native-code compiler ocamlopt.

18.1 Options
The following command-line options are recognized by ocamldep.

-absname
Show absolute filenames in error messages.

-all
Generate dependencies on all required files, rather than assuming implicit dependencies.

-allow-approx
Allow falling back on a lexer-based approximation when parsing fails.

-args filename
Read additional newline-terminated command line arguments from filename.

-args0 filename
Read additional null character terminated command line arguments from filename.

-as-map
For the following files, do not include delayed dependencies for module aliases. This option

355
356

assumes that they are compiled using options -no-alias-deps -w -49, and that those files
or their interface are passed with the -map option when computing dependencies for other
files. Note also that for dependencies to be correct in the implementation of a map file, its
interface should not coerce any of the aliases it contains.

-debug-map
Dump the delayed dependency map for each map file.

-I directory
Add the given directory to the list of directories searched for source files. If a source file
foo.ml mentions an external compilation unit Bar, a dependency on that unit’s interface
bar.cmi is generated only if the source for bar is found in the current directory or in one of
the directories specified with -I. Otherwise, Bar is assumed to be a module from the standard
library, and no dependencies are generated. For programs that span multiple directories, it is
recommended to pass ocamldep the same -I options that are passed to the compiler.

-H directory
Behaves identically to -I, except that the -H directories are searched last. This flag is included
to make it easier to invoke ocamldep with the same options as the compiler, where -H is used
for transitive dependencies that the program should not directly mention.

-nocwd
Do not add current working directory to the list of include directories.

-impl file
Process file as a .ml file.

-intf file
Process file as a .mli file.

-map file
Read and propagate the delayed dependencies for module aliases in file, so that the following
files will depend on the exported aliased modules if they use them. See the example below.

-ml-synonym .ext
Consider the given extension (with leading dot) to be a synonym for .ml.

-mli-synonym .ext
Consider the given extension (with leading dot) to be a synonym for .mli.

-modules
Output raw dependencies of the form

filename: Module1 Module2 ... ModuleN

where Module1, …, ModuleN are the names of the compilation units referenced within the file
filename, but these names are not resolved to source file names. Such raw dependencies
cannot be used by make, but can be post-processed by other tools such as Omake.
Chapter 18. Dependency generator (ocamldep) 357

-native
Generate dependencies for a pure native-code program (no bytecode version). When an
implementation file (.ml file) has no explicit interface file (.mli file), ocamldep generates
dependencies on the bytecode compiled file (.cmo file) to reflect interface changes. This can
cause unnecessary bytecode recompilations for programs that are compiled to native-code only.
The flag -native causes dependencies on native compiled files (.cmx) to be generated instead
of on .cmo files. (This flag makes no difference if all source files have explicit .mli interface
files.)

-one-line
Output one line per file, regardless of the length.

-open module
Assume that module module is opened before parsing each of the following files.

-pp command
Cause ocamldep to call the given command as a preprocessor for each source file.

-ppx command
Pipe abstract syntax trees through preprocessor command.

-shared
Generate dependencies for native plugin files (.cmxs) in addition to native compiled files
(.cmx).

-slash
Under Windows, use a forward slash (/) as the path separator instead of the usual backward
slash (\). Under Unix, this option does nothing.

-sort
Sort files according to their dependencies.

-version
Print version string and exit.

-vnum
Print short version number and exit.

-help or --help
Display a short usage summary and exit.

18.2 A typical Makefile

Here is a template Makefile for a OCaml program.

OCAMLC=ocamlc
OCAMLOPT=ocamlopt
OCAMLDEP=ocamldep
358

INCLUDES= # all relevant -I options here

OCAMLFLAGS=$(INCLUDES) # add other options for ocamlc here
OCAMLOPTFLAGS=$(INCLUDES) # add other options for ocamlopt here

# prog1 should be compiled to bytecode, and is composed of three

# units: mod1, mod2 and mod3.

# The list of object files for prog1

PROG1_OBJS=mod1.cmo mod2.cmo mod3.cmo

prog1: $(PROG1_OBJS)
$(OCAMLC) -o prog1 $(OCAMLFLAGS) $(PROG1_OBJS)

# prog2 should be compiled to native-code, and is composed of two

# units: mod4 and mod5.

# The list of object files for prog2

PROG2_OBJS=mod4.cmx mod5.cmx

prog2: $(PROG2_OBJS)
$(OCAMLOPT) -o prog2 $(OCAMLFLAGS) $(PROG2_OBJS)

# Common rules

%.cmo: %.ml
$(OCAMLC) $(OCAMLFLAGS) -c $<

%.cmi: %.mli
$(OCAMLC) $(OCAMLFLAGS) -c $<

%.cmx: %.ml
$(OCAMLOPT) $(OCAMLOPTFLAGS) -c $<

# Clean up
clean:
rm -f prog1 prog2
rm -f *.cm[iox]

# Dependencies
depend:
$(OCAMLDEP) $(INCLUDES) *.mli *.ml > .depend

include .depend

If you use module aliases to give shorter names to modules, you need to change the above
Chapter 18. Dependency generator (ocamldep) 359

definitions. Assuming that your map file is called mylib.mli, here are minimal modifications.

OCAMLFLAGS=$(INCLUDES) -open Mylib

mylib.cmi: mylib.mli
$(OCAMLC) $(INCLUDES) -no-alias-deps -w -49 -c $<

depend:
$(OCAMLDEP) $(INCLUDES) -map mylib.mli $(PROG1_OBJS:.cmo=.ml) > .depend

Note that in this case you should not compute dependencies for mylib.mli together with the
other files, hence the need to pass explicitly the list of files to process. If mylib.mli itself has
dependencies, you should compute them using -as-map.
360
Chapter 19

The documentation generator

(ocamldoc)

This chapter describes OCamldoc, a tool that generates documentation from special comments
embedded in source files. The comments used by OCamldoc are of the form (**…*) and follow the
format described in section 19.2.
OCamldoc can produce documentation in various formats: HTML, LATEX, TeXinfo, Unix man
pages, and dot dependency graphs. Moreover, users can add their own custom generators, as
explained in section 19.3.
In this chapter, we use the word element to refer to any of the following parts of an OCaml
source file: a type declaration, a value, a module, an exception, a module type, a type constructor,
a record field, a class, a class type, a class method, a class value or a class inheritance clause.

19.1 Usage
19.1.1 Invocation
OCamldoc is invoked via the command ocamldoc, as follows:

ocamldoc options sourcefiles

Options for choosing the output format

The following options determine the format for the generated documentation.

-html
Generate documentation in HTML default format. The generated HTML pages are stored in
the current directory, or in the directory specified with the -d option. You can customize the
style of the generated pages by editing the generated style.css file, or by providing your
own style sheet using option -css-style. The file style.css is not generated if it already
exists or if -css-style is used.

-latex
Generate documentation in LATEX default format. The generated LATEX document is saved in

361
362

file ocamldoc.out, or in the file specified with the -o option. The document uses the style
file ocamldoc.sty. This file is generated when using the -latex option, if it does not already
exist. You can change this file to customize the style of your LATEX documentation.

-texi
Generate documentation in TeXinfo default format. The generated LATEX document is saved
in file ocamldoc.out, or in the file specified with the -o option.

-man
Generate documentation as a set of Unix man pages. The generated pages are stored in the
current directory, or in the directory specified with the -d option.

-dot
Generate a dependency graph for the toplevel modules, in a format suitable for displaying
and processing by dot. The dot tool is available from https://graphviz.org/. The textual
representation of the graph is written to the file ocamldoc.out, or to the file specified with
the -o option. Use dot ocamldoc.out to display it.

-g file.cm[o,a,xs]
Dynamically load the given file, which defines a custom documentation generator. See section
19.4.1. This option is supported by the ocamldoc command (to load .cmo and .cma files) and
by its native-code version ocamldoc.opt (to load .cmxs files). If the given file is a simple
one and does not exist in the current directory, then ocamldoc looks for it in the custom
generators default directory, and in the directories specified with optional -i options.

-customdir
Display the custom generators default directory.

-i directory
Add the given directory to the path where to look for custom generators.

General options
-d dir
Generate files in directory dir, rather than the current directory.

-dump file
Dump collected information into file. This information can be read with the -load option in
a subsequent invocation of ocamldoc.

-hide modules
Hide the given complete module names in the generated documentation. modules is a list of
complete module names separated by ’,’, without blanks. For instance: Stdlib,M2.M3.

-inv-merge-ml-mli
Reverse the precedence of implementations and interfaces when merging. All elements in
implementation files are kept, and the -m option indicates which parts of the comments in
interface files are merged with the comments in implementation files.
Chapter 19. The documentation generator (ocamldoc) 363

-keep-code
Always keep the source code for values, methods and instance variables, when available.

-load file
Load information from file, which has been produced by ocamldoc -dump. Several -load
options can be given.

-m flags
Specify merge options between interfaces and implementations. (see section 19.1.2 for details).
flags can be one or several of the following characters:

d merge description
a merge @author
v merge @version
l merge @see
s merge @since
b merge @before
o merge @deprecated
p merge @param
e merge @raise
r merge @return
A merge everything

-no-custom-tags
Do not allow custom @-tags (see section 19.2.12).

-no-stop
Keep elements placed after/between the (**/**) special comment(s) (see section 19.2).

-o file
Output the generated documentation to file instead of ocamldoc.out. This option is mean-
ingful only in conjunction with the -latex, -texi, or -dot options.

-pp command
Pipe sources through preprocessor command.

-impl filename
Process the file filename as an implementation file, even if its extension is not .ml.

-intf filename
Process the file filename as an interface file, even if its extension is not .mli.

-text filename
Process the file filename as a text file, even if its extension is not .txt.

-sort
Sort the list of top-level modules before generating the documentation.
364

-stars
Remove blank characters until the first asterisk (’*’) in each line of comments.

-t title
Use title as the title for the generated documentation.

-intro file
Use content of file as ocamldoc text to use as introduction (HTML, LATEX and TeXinfo only).
For HTML, the file is used to create the whole index.html file.

-v Verbose mode. Display progress information.

-version
Print version string and exit.

-vnum
Print short version number and exit.

-warn-error
Treat Ocamldoc warnings as errors.

-hide-warnings
Do not print OCamldoc warnings.

-help or --help
Display a short usage summary and exit.

Type-checking options
OCamldoc calls the OCaml type-checker to obtain type information. The following options impact
the type-checking phase. They have the same meaning as for the ocamlc and ocamlopt commands.

-I directory
Add directory to the list of directories search for compiled interface files (.cmi files).

-H directory
Like -I, but the -H directories are searched last and the program may not directly refer to
the modules added to the search path this way.

-nolabels
Ignore non-optional labels in types.

-rectypes
Allow arbitrary recursive types. (See the -rectypes option to ocamlc.)
Chapter 19. The documentation generator (ocamldoc) 365

Options for generating HTML pages

The following options apply in conjunction with the -html option:

-all-params
Display the complete list of parameters for functions and methods.

-charset charset
Add information about character encoding being charset (default is iso-8859-1).

-colorize-code
Colorize the OCaml code enclosed in [ ] and {[ ]}, using colors to emphasize keywords, etc.
If the code fragments are not syntactically correct, no color is added.

-css-style filename
Use filename as the Cascading Style Sheet file.

-index-only
Generate only index files.

-short-functors
Use a short form to display functors:

module M : functor (A:Module) -> functor (B:Module2) -> sig .. end

is displayed as:

module M (A:Module) (B:Module2) : sig .. end

Options for generating LATEX files

The following options apply in conjunction with the -latex option:

-latex-value-prefix prefix
Give a prefix to use for the labels of the values in the generated LATEX document. The
default prefix is the empty string. You can also use the options -latex-type-prefix,
-latex-exception-prefix, -latex-module-prefix, -latex-module-type-prefix,
-latex-class-prefix, -latex-class-type-prefix, -latex-attribute-prefix and
-latex-method-prefix.
These options are useful when you have, for example, a type and a value with the same name.
If you do not specify prefixes, LATEX will complain about multiply defined labels.

-latextitle n,style
Associate style number n to the given LATEX sectioning command style, e.g. section or
subsection. (LATEX only.) This is useful when including the generated document in another
LATEX document, at a given sectioning level. The default association is 1 for section, 2 for
subsection, 3 for subsubsection, 4 for paragraph and 5 for subparagraph.
366

-noheader
Suppress header in generated documentation.

-notoc
Do not generate a table of contents.

-notrailer
Suppress trailer in generated documentation.

-sepfiles
Generate one .tex file per toplevel module, instead of the global ocamldoc.out file.

Options for generating TeXinfo files

The following options apply in conjunction with the -texi option:

-esc8
Escape accented characters in Info files.

-info-entry
Specify Info directory entry.

-info-section
Specify section of Info directory.

-noheader
Suppress header in generated documentation.

-noindex
Do not build index for Info files.

-notrailer
Suppress trailer in generated documentation.

Options for generating dot graphs

The following options apply in conjunction with the -dot option:

-dot-colors colors
Specify the colors to use in the generated dot code. When generating module dependencies,
ocamldoc uses different colors for modules, depending on the directories in which they reside.
When generating types dependencies, ocamldoc uses different colors for types, depending on
the modules in which they are defined. colors is a list of color names separated by ’,’, as in
Red,Blue,Green. The available colors are the ones supported by the dot tool.

-dot-include-all
Include all modules in the dot output, not only modules given on the command line or loaded
with the -load option.
Chapter 19. The documentation generator (ocamldoc) 367

-dot-reduce
Perform a transitive reduction of the dependency graph before outputting the dot code. This
can be useful if there are a lot of transitive dependencies that clutter the graph.

-dot-types
Output dot code describing the type dependency graph instead of the module dependency
graph.

Options for generating man files

The following options apply in conjunction with the -man option:

-man-mini
Generate man pages only for modules, module types, classes and class types, instead of pages
for all elements.

-man-suffix suffix
Set the suffix used for generated man filenames. Default is ’3o’, as in List.3o.

-man-section section
Set the section number used for generated man filenames. Default is ’3’.

19.1.2 Merging of module information

Information on a module can be extracted either from the .mli or .ml file, or both, depending on
the files given on the command line. When both .mli and .ml files are given for the same module,
information extracted from these files is merged according to the following rules:

• Only elements (values, types, classes, ...) declared in the .mli file are kept. In other terms,
definitions from the .ml file that are not exported in the .mli file are not documented.

• Descriptions of elements and descriptions in @-tags are handled as follows. If a description for
the same element or in the same @-tag of the same element is present in both files, then the
description of the .ml file is concatenated to the one in the .mli file, if the corresponding -m
flag is given on the command line. If a description is present in the .ml file and not in the
.mli file, the .ml description is kept. In either case, all the information given in the .mli file
is kept.

19.1.3 Coding rules

The following rules must be respected in order to avoid name clashes resulting in cross-reference
errors:

• In a module, there must not be two modules, two module types or a module and a module
type with the same name. In the default HTML generator, modules ab and AB will be printed
to the same file on case insensitive file systems.

• In a module, there must not be two classes, two class types or a class and a class type with
the same name.
368

• In a module, there must not be two values, two types, or two exceptions with the same name.

• Values defined in tuple, as in let (x,y,z) = (1,2,3) are not kept by OCamldoc.

• Avoid the following construction:

open Foo (∗ which has a module Bar with a value x ∗)
module Foo =
struct
module Bar =
struct
let x = 1
end
end
let dummy = Bar.x
In this case, OCamldoc will associate Bar.x to the x of module Foo defined just above, instead
of to the Bar.x defined in the opened module Foo.

19.2 Syntax of documentation comments

Comments containing documentation material are called special comments and are written between
(** and *). Special comments must start exactly with (**. Comments beginning with ( and more
than two * are ignored.

19.2.1 Placement of documentation comments

OCamldoc can associate comments to some elements of the language encountered in the source
files. The association is made according to the locations of comments with respect to the language
elements. The locations of comments in .mli and .ml files are different.

19.2.2 Comments in .mli files

A special comment is associated to an element if it is placed before or after the element.
A special comment before an element is associated to this element if :

• There is no blank line or another special comment between the special comment and the
element. However, a regular comment can occur between the special comment and the element.

• The special comment is not already associated to the previous element.

• The special comment is not the first one of a toplevel module.

A special comment after an element is associated to this element if there is no blank line or
comment between the special comment and the element.
There are two exceptions: for constructors and record fields in type definitions, the associated
comment can only be placed after the constructor or field definition, without blank lines or other
comments between them. The special comment for a constructor with another constructor following
must be placed before the ’|’ character separating the two constructors.
Chapter 19. The documentation generator (ocamldoc) 369

The following sample interface file foo.mli illustrates the placement rules for comments in .mli
files.
(∗∗ The first special comment of the file is the comment associated
with the whole module.∗)

(∗∗ Special comments can be placed between elements and are kept
by the OCamldoc tool, but are not associated to any element.
@−tags in these comments are ignored.∗)

(∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗)
(∗∗ Comments like the one above, with more than two asterisks,
are ignored. ∗)

(∗∗ The comment for function f. ∗)

val f : int -> int -> int
(∗∗ The continuation of the comment for function f. ∗)

(∗∗ Comment for exception My_exception, even with a simple comment

between the special comment and the exception.∗)
(∗ Hello, I'm a simple comment :−) ∗)
exception My_exception of (int -> int) * int

(∗∗ Comment for type weather ∗)

type weather =
| Rain of int (∗∗ The comment for constructor Rain ∗)
| Sun (∗∗ The comment for constructor Sun ∗)

(∗∗ Comment for type weather2 ∗)

type weather2 =
| Rain of int (∗∗ The comment for constructor Rain ∗)
| Sun (∗∗ The comment for constructor Sun ∗)
(∗∗ I can continue the comment for type weather2 here
because there is already a comment associated to the last constructor.∗)

(∗∗ The comment for type my_record ∗)

type my_record = {
foo : int ; (∗∗ Comment for field foo ∗)
bar : string ; (∗∗ Comment for field bar ∗)
}
(∗∗ Continuation of comment for type my_record ∗)

(∗∗ Comment for foo ∗)

val foo : string
(∗∗ This comment is associated to foo and not to bar. ∗)
370

val bar : string

(∗∗ This comment is associated to bar. ∗)

(∗∗ The comment for class my_class ∗)

class my_class :
object
(∗∗ A comment to describe inheritance from cl ∗)
inherit cl

(∗∗ The comment for attribute tutu ∗)

val mutable tutu : string

(∗∗ The comment for attribute toto. ∗)

val toto : int

(∗∗ This comment is not attached to titi since

there is a blank line before titi, but is kept
as a comment in the class. ∗)

val titi : string

(∗∗ Comment for method toto ∗)

method toto : string

(∗∗ Comment for method m ∗)

method m : float -> int
end

(∗∗ The comment for the class type my_class_type ∗)

class type my_class_type =
object
(∗∗ The comment for variable x. ∗)
val mutable x : int

(∗∗ The comment for method m. ∗)

method m : int -> int
end

(∗∗ The comment for module Foo ∗)

module Foo :
sig
(∗∗ The comment for x ∗)
val x : int

(∗∗ A special comment that is kept but not associated to any element ∗)
Chapter 19. The documentation generator (ocamldoc) 371

end

(∗∗ The comment for module type my_module_type. ∗)

module type my_module_type =
sig
(∗∗ The comment for value x. ∗)
val x : int

(∗∗ The comment for module M. ∗)

module M :
sig
(∗∗ The comment for value y. ∗)
val y : int

(∗ ... ∗)
end

end

19.2.3 Comments in .ml files

A special comment is associated to an element if it is placed before the element and there is no blank
line between the comment and the element. Meanwhile, there can be a simple comment between the
special comment and the element. There are two exceptions, for constructors and record fields in
type definitions, whose associated comment must be placed after the constructor or field definition,
without blank line between them. The special comment for a constructor with another constructor
following must be placed before the ’|’ character separating the two constructors.
The following example of file toto.ml shows where to place comments in a .ml file.
(∗∗ The first special comment of the file is the comment associated
to the whole module. ∗)

(∗∗ The comment for function f ∗)

let f x y = x + y

(∗∗ This comment is not attached to any element since there is another
special comment just before the next element. ∗)

(∗∗ Comment for exception My_exception, even with a simple comment

between the special comment and the exception.∗)
(∗ A simple comment. ∗)
exception My_exception of (int -> int) * int

(∗∗ Comment for type weather ∗)

type weather =
| Rain of int (∗∗ The comment for constructor Rain ∗)
372

| Sun (∗∗ The comment for constructor Sun ∗)

(∗∗ The comment for type my_record ∗)

type my_record = {
foo : int ; (∗∗ Comment for field foo ∗)
bar : string ; (∗∗ Comment for field bar ∗)
}

(∗∗ The comment for class my_class ∗)

class my_class =
object
(∗∗ A comment to describe inheritance from cl ∗)
inherit cl

(∗∗ The comment for the instance variable tutu ∗)

val mutable tutu = "tutu"
(∗∗ The comment for toto ∗)
val toto = 1
val titi = "titi"
(∗∗ Comment for method toto ∗)
method toto = tutu ^ "!"
(∗∗ Comment for method m ∗)
method m (f : float) = 1
end

(∗∗ The comment for class type my_class_type ∗)

class type my_class_type =
object
(∗∗ The comment for the instance variable x. ∗)
val mutable x : int
(∗∗ The comment for method m. ∗)
method m : int -> int
end

(∗∗ The comment for module Foo ∗)

module Foo =
struct
(∗∗ The comment for x ∗)
let x = 0
(∗∗ A special comment in the class, but not associated to any element. ∗)
end

(∗∗ The comment for module type my_module_type. ∗)

module type my_module_type =
sig
Chapter 19. The documentation generator (ocamldoc) 373

(∗ Comment for value x. ∗)

val x : int
(∗ ... ∗)
end

19.2.4 The Stop special comment

The special comment (**/**) tells OCamldoc to discard elements placed after this comment, up to
the end of the current class, class type, module or module type, or up to the next stop comment.
For instance:
class type foo =
object
(∗∗ comment for method m ∗)
method m : string

(∗∗/∗∗)

(∗∗ This method won't appear in the documentation ∗)

method bar : int
end

(∗∗ This value appears in the documentation, since the Stop special comment
in the class does not affect the parent module of the class.∗)
val foo : string

(∗∗/∗∗)
(∗∗ The value bar does not appear in the documentation.∗)
val bar : string
(∗∗/∗∗)

(∗∗ The type t appears since in the documentation since the previous stop comment
toggled off the ”no documentation mode”. ∗)
type t = string
The -no-stop option to ocamldoc causes the Stop special comments to be ignored.

19.2.5 Syntax of documentation comments

The inside of documentation comments (**…*) consists of free-form text with optional formatting
annotations, followed by optional tags giving more specific information about parameters, version,
authors, … The tags are distinguished by a leading @ character. Thus, a documentation comment
has the following shape:
(** The comment begins with a description, which is text formatted
according to the rules described in the next section.
The description continues until the first non-escaped '@' character.
@author Mr Smith
374

@param x description for parameter x

*)

Some elements support only a subset of all @-tags. Tags that are not relevant to the documented
element are simply ignored. For instance, all tags are ignored when documenting type constructors,
record fields, and class inheritance clauses. Similarly, a @param tag on a class instance variable is
ignored.
At last, (**) is the empty documentation comment.

19.2.6 Text formatting

Here is the BNF grammar for the simple markup language used to format text descriptions.
Chapter 19. The documentation generator (ocamldoc) 375

text ::= {text-element}+

inline-text ::= {inline-text-element}+

text-element ::=
| inline-text-element
| blank-line force a new line.
inline-text-element ::=
| { {0 . . . 9}+ inline-text } format text as a section header; the integer following { indi-
cates the sectioning level.
| { {0 . . . 9} : label inline-text } same, but also associate the name label to the current point.
+

This point can be referenced by its fully-qualified label in a

{! command, just like any other element.
| {b inline-text } set text in bold.
| {i inline-text } set text in italic.
| {e inline-text } emphasize text.
| {C inline-text } center text.
| {L inline-text } left align text.
| {R inline-text } right align text.
| {ul list } build a list.
| {ol list } build an enumerated list.
| {{: string } inline-text } put a link to the given address (given as string) on the given
text.
| [ string ] set the given string in source code style.
| {[ string ]} set the given string in preformatted source code style.
| {v string v} set the given string in verbatim style.
| {% string %} target-specific content (LATEX code by default, see details in
19.2.10)
| {! string } insert a cross-reference to an element (see section 19.2.8 for
the syntax of cross-references).
| {{! string } inline-text } insert a cross-reference with the given text.
| {!modules: string string... } insert an index table for the given module names. Used in
HTML only.
| {!indexlist} insert a table of links to the various indexes (types, values,
modules, ...). Used in HTML only.
| {^ inline-text } set text in superscript.
| {_ inline-text } set text in subscript.
| escaped-string typeset the given string as is; special characters (’{’, ’}’, ’[’,
’]’ and ’@’) must be escaped by a ’\’
376

19.2.7 List formatting

list ::=
| {{- inline-text }}+
| {{li inline-text }}+
A shortcut syntax exists for lists and enumerated lists:

(** Here is a {b list}

- item 1
- item 2
- item 3

The list is ended by the blank line.*)

is equivalent to:

(** Here is a {b list}

{ul {- item 1}
{- item 2}
{- item 3}}
The list is ended by the blank line.*)

The same shortcut is available for enumerated lists, using ’+’ instead of ’-’. Note that only one
list can be defined by this shortcut in nested lists.

19.2.8 Cross-reference formatting

Cross-references are fully qualified element names, as in the example {!Foo.Bar.t}. This is an
ambiguous reference as it may designate a type name, a value name, a class name, etc. It is possible
to make explicit the intended syntactic class, using {!type:Foo.Bar.t} to designate a type, and
{!val:Foo.Bar.t} a value of the same name.
The list of possible syntactic class is as follows:
tag syntactic class
module: module
modtype: module type
class: class
classtype: class type
val: value
type: type
exception: exception
attribute: attribute
method: class method
section: ocamldoc section
const: variant constructor
recfield: record field
Chapter 19. The documentation generator (ocamldoc) 377

In the case of variant constructors or record fields, the constructor or field name should be
preceded by the name of the corresponding type to avoid the ambiguity of several types having the
same constructor names. For example, the constructor Node of the type tree will be referenced
as {!tree.Node} or {!const:tree.Node}, or possibly {!Mod1.Mod2.tree.Node} from outside the
module.

19.2.9 First sentence

In the description of a value, type, exception, module, module type, class or class type, the first
sentence is sometimes used in indexes, or when just a part of the description is needed. The first
sentence is composed of the first characters of the description, until

• the first dot followed by a blank, or

• the first blank line

outside of the following text formatting : {ul list }, {ol list }, [ string ], {[ string ]}, {v string v},
{% string %}, {! string }, {^ text }, {_ text }.

19.2.10 Target-specific formatting

The content inside {%foo: ... %} is target-specific and will be interpreted only by the backend
foo, and ignored by other backends. The backends of the distribution are latex, html, texi and
man. If no target is specified (syntax {% ... %}), latex is chosen by default. Custom generators
may support their own target prefix.

19.2.11 Recognized HTML tags

The HTML tags <b>..</b>, <code>..</code>, <i>..</i>, <ul>..</ul>, <ol>..</ol>,
<li>..</li>, <center>..</center> and <h[0-9]>..</h[0-9]> can be used instead of,
respectively, {b ..}, [..], {i ..}, {ul ..}, {ol ..}, {li ..}, {C ..} and {[0-9] ..}.

19.2.12 Documentation tags (@-tags)

19.2.13 Predefined tags
The following table gives the list of predefined @-tags, with their syntax and meaning.
378

@author string The author of the element. One author per @author tag.
There may be several @author tags for the same element.
@deprecated text The text should describe when the element was deprecated,
what to use as a replacement, and possibly the reason for
deprecation.
@param id text Associate the given description (text) to the given parameter
name id. This tag is used for functions, methods, classes and
functors.
@raise Exc text Explain that the element may raise the exception Exc.
@return text Describe the return value and its possible values. This tag is
used for functions and methods.
@see < URL > text Add a reference to the URL with the given text as comment.
@see 'filename' text Add a reference to the given file name (written between single
quotes), with the given text as comment.
@see "document-name" text Add a reference to the given document name (written between
double quotes), with the given text as comment.
@since string Indicate when the element was introduced.
@before version text Associate the given description (text) to the given version in
order to document compatibility issues.
@version string The version number for the element.

19.2.14 Custom tags

You can use custom tags in the documentation comments, but they will have no effect if the generator
used does not handle them. To use a custom tag, for example foo, just put @foo with some text in
your comment, as in:
(** My comment to show you a custom tag.
@foo this is the text argument to the [foo] custom tag.
*)
To handle custom tags, you need to define a custom generator, as explained in section 19.3.2.

19.3 Custom generators

OCamldoc operates in two steps:
1. analysis of the source files;
2. generation of documentation, through a documentation generator, which is an object of class
Odoc_args.class_generator.
Users can provide their own documentation generator to be used during step 2 instead of the default
generators. All the information retrieved during the analysis step is available through the Odoc_info
module, which gives access to all the types and functions representing the elements found in the
given modules, with their associated description.
The files you can use to define custom generators are installed in the ocamldoc sub-directory of
the OCaml standard library.
Chapter 19. The documentation generator (ocamldoc) 379

19.3.1 The generator modules

The type of a generator module depends on the kind of generated documentation. Here is the list of
generator module types, with the name of the generator class in the module :
• for HTML : Odoc_html.Html_generator (class html),

• for LATEX : Odoc_latex.Latex_generator (class latex),

• for TeXinfo : Odoc_texi.Texi_generator (class texi),

• for man pages : Odoc_man.Man_generator (class man),

• for graphviz (dot) : Odoc_dot.Dot_generator (class dot),

• for other kinds : Odoc_gen.Base (class generator).

That is, to define a new generator, one must implement a module with the expected signature, and
with the given generator class, providing the generate method as entry point to make the generator
generates documentation for a given list of modules :

method generate : Odoc_info.Module.t_module list -> unit

This method will be called with the list of analysed and possibly merged Odoc_info.t_module
structures.
It is recommended to inherit from the current generator of the same kind as the one you want to
define. Doing so, it is possible to load various custom generators to combine improvements brought
by each one.
This is done using first class modules (see chapter 12.5).
The easiest way to define a custom generator is the following this example, here extending the
current HTML generator. We don’t have to know if this is the original HTML generator defined in
ocamldoc or if it has been extended already by a previously loaded custom generator :

module Generator (G : Odoc_html.Html_generator) =

struct
class html =
object(self)
inherit G.html as html
(* ... *)

method generate module_list =

(* ... *)
()

(* ... *)
end
end;;

let _ = Odoc_args.extend_html_generator (module Generator : Odoc_gen.Html_functor);;

380

To know which methods to override and/or which methods are available, have a look at the
different base implementations, depending on the kind of generator you are extending :

• for HTML : odoc_html.ml,

• for LATEX : odoc_latex.ml,

• for TeXinfo : odoc_texi.ml,

• for man pages : odoc_man.ml,

• for graphviz (dot) : odoc_dot.ml.

19.3.2 Handling custom tags

Making a custom generator handle custom tags (see 19.2.14) is very simple.

For HTML
Here is how to develop a HTML generator handling your custom tags.
The class Odoc_html.Generator.html inherits from the class Odoc_html.info, containing a
field tag_functions which is a list pairs composed of a custom tag (e.g. "foo") and a function
taking a text and returning HTML code (of type string). To handle a new tag bar, extend the
current HTML generator and complete the tag_functions field:

module Generator (G : Odoc_html.Html_generator) =

struct
class html =
object(self)
inherit G.html

(** Return HTML code for the given text of a bar tag. *)
method html_of_bar t = (* your code here *)

initializer
tag_functions <- ("bar", self#html_of_bar) :: tag_functions
end
end
let _ = Odoc_args.extend_html_generator (module Generator : Odoc_gen.Html_functor);;

Another method of the class Odoc_html.info will look for the function associated to a custom
tag and apply it to the text given to the tag. If no function is associated to a custom tag, then the
method prints a warning message on stderr.

19.3.3 For other generators

You can act the same way for other kinds of generators.
Chapter 19. The documentation generator (ocamldoc) 381

19.4 Adding command line options

The command line analysis is performed after loading the module containing the documentation
generator, thus allowing command line options to be added to the list of existing ones. Adding an
option can be done with the function

Odoc_args.add_option : string * Arg.spec * string -> unit

Note: Existing command line options can be redefined using this function.

19.4.1 Compilation and usage

19.4.2 Defining a custom generator class in one file
Let custom.ml be the file defining a new generator class. Compilation of custom.ml can be
performed by the following command :

ocamlc -I +ocamldoc -c custom.ml

The file custom.cmo is created and can be used this way :

ocamldoc -g custom.cmo other-options source-files

Options selecting a built-in generator to ocamldoc, such as -html, have no effect if a custom
generator of the same kind is provided using -g. If the kinds do not match, the selected built-in
generator is used and the custom one is ignored.

19.4.3 Defining a custom generator class in several files

It is possible to define a generator class in several modules, which are defined in several files
file1 .ml[i], file2 .ml[i], ..., filen .ml[i]. A .cma library file must be created, including all these files.
The following commands create the custom.cma file from files file1 .ml[i], ..., filen .ml[i] :

ocamlc -I +ocamldoc -c file1 .ml[i]

ocamlc -I +ocamldoc -c file2 .ml[i]
...
ocamlc -I +ocamldoc -c filen .ml[i]
ocamlc -o custom.cma -a file1 .cmo file2 .cmo ... filen .cmo

Then, the following command uses custom.cma as custom generator:

ocamldoc -g custom.cma other-options source-files

382
Chapter 20

The debugger (ocamldebug)

This chapter describes the OCaml source-level replay debugger ocamldebug.

The debugger is available on systems that provide BSD sockets, including Windows. Under the
native Windows ports, the replay functions are not enabled.

20.1 Compiling for debugging

Before the debugger can be used, the program must be compiled and linked with the -g option: all
.cmo and .cma files that are part of the program should have been created with ocamlc -g, and
they must be linked together with ocamlc -g.
Compiling with -g entails no penalty on the running time of programs: object files and bytecode
executable files are bigger and take longer to produce, but the executable files run at exactly the
same speed as if they had been compiled without -g.

20.2 Invocation
20.2.1 Starting the debugger
The OCaml debugger is invoked by running the program ocamldebug with the name of the bytecode
executable file as first argument:

ocamldebug [options] program [arguments]

The arguments following program are optional, and are passed as command-line arguments to
the program being debugged. (See also the set arguments command.)
The following command-line options are recognized:

-c count
Set the maximum number of simultaneously live checkpoints to count.

-cd dir
Run the debugger program from the working directory dir, instead of the current directory.
(See also the cd command.)

383
384

-emacs
Tell the debugger it is executed under Emacs. (See section 20.10 for information on how to
run the debugger under Emacs.)
-I directory
Add directory to the list of directories searched for source files and compiled files. (See also
the directory command.)
-s socket
Use socket for communicating with the debugged program. See the description of the command
set socket (section 20.8.8) for the format of socket.
-version
Print version string and exit.
-vnum
Print short version number and exit.
-help or --help
Display a short usage summary and exit.

20.2.2 Initialization file

On start-up, the debugger will read commands from an initialization file before giving control to the
user. The default file is .ocamldebug in the current directory if it exists, otherwise .ocamldebug in
the user’s home directory.

20.2.3 Exiting the debugger

The command quit exits the debugger. You can also exit the debugger by typing an end-of-file
character (usually ctrl-D).
Typing an interrupt character (usually ctrl-C) will not exit the debugger, but will terminate
the action of any debugger command that is in progress and return to the debugger command level.

20.3 Commands
A debugger command is a single line of input. It starts with a command name, which is followed by
arguments depending on this name. Examples:
run
goto 1000
set arguments arg1 arg2
A command name can be truncated as long as there is no ambiguity. For instance, go 1000
is understood as goto 1000, since there are no other commands whose name starts with go. For
the most frequently used commands, ambiguous abbreviations are allowed. For instance, r stands
for run even though there are others commands starting with r. You can test the validity of an
abbreviation using the help command.
If the previous command has been successful, a blank line (typing just RET) will repeat it.
Chapter 20. The debugger (ocamldebug) 385

20.3.1 Getting help

The OCaml debugger has a simple on-line help system, which gives a brief description of each
command and variable.

help
Print the list of commands.

help command
Give help about the command command.

help set variable, help show variable

Give help about the variable variable. The list of all debugger variables can be obtained with
help set.

help info topic

Give help about topic. Use help info to get a list of known topics.

20.3.2 Accessing the debugger state

set variable value
Set the debugger variable variable to the value value.

show variable
Print the value of the debugger variable variable.

info subject
Give information about the given subject. For instance, info breakpoints will print the list
of all breakpoints.

20.4 Executing a program

20.4.1 Events
Events are “interesting” locations in the source code, corresponding to the beginning or end of
evaluation of “interesting” sub-expressions. Events are the unit of single-stepping (stepping goes to
the next or previous event encountered in the program execution). Also, breakpoints can only be
set at events. Thus, events play the role of line numbers in debuggers for conventional languages.
During program execution, a counter is incremented at each event encountered. The value of
this counter is referred as the current time. Thanks to reverse execution, it is possible to jump back
and forth to any time of the execution.
Here is where the debugger events (written ./) are located in the source code:

• Following a function application:

(f arg)./

• On entrance to a function:
386

fun x y z -> ./ ...

• On each case of a pattern-matching definition (function, match…with construct, try…with

construct):

function pat1 -> ./ expr1

| ...
| patN -> ./ exprN

• Between subexpressions of a sequence:

expr1; ./ expr2; ./ ...; ./ exprN

• In the two branches of a conditional expression:

if cond then ./ expr1 else ./ expr2

• At the beginning of each iteration of a loop:

while cond do ./ body done

for i = a to b do ./ body done

Exceptions: A function application followed by a function return is replaced by the compiler by a

jump (tail-call optimization). In this case, no event is put after the function application.

20.4.2 Starting the debugged program

The debugger starts executing the debugged program only when needed. This allows setting
breakpoints or assigning debugger variables before execution starts. There are several ways to start
execution:

run Run the program until a breakpoint is hit, or the program terminates.

goto 0
Load the program and stop on the first event.

goto time
Load the program and execute it until the given time. Useful when you already know
approximately at what time the problem appears. Also useful to set breakpoints on function
values that have not been computed at time 0 (see section 20.5).

The execution of a program is affected by certain information it receives when the debugger starts
it, such as the command-line arguments to the program and its working directory. The debugger
provides commands to specify this information (set arguments and cd). These commands must be
used before program execution starts. If you try to change the arguments or the working directory
after starting your program, the debugger will kill the program (after asking for confirmation).
Chapter 20. The debugger (ocamldebug) 387

20.4.3 Running the program

The following commands execute the program forward or backward, starting at the current time.
The execution will stop either when specified by the command or when a breakpoint is encountered.

run Execute the program forward from current time. Stops at next breakpoint or when the
program terminates.

reverse
Execute the program backward from current time. Mostly useful to go to the last breakpoint
encountered before the current time.

step [count]
Run the program and stop at the next event. With an argument, do it count times. If count
is 0, run until the program terminates or a breakpoint is hit.

backstep [count]
Run the program backward and stop at the previous event. With an argument, do it count
times.

next [count]
Run the program and stop at the next event, skipping over function calls. With an argument,
do it count times.

previous [count]
Run the program backward and stop at the previous event, skipping over function calls. With
an argument, do it count times.

finish
Run the program until the current function returns.

start
Run the program backward and stop at the first event before the current function invocation.

20.4.4 Time travel

You can jump directly to a given time, without stopping on breakpoints, using the goto command.
As you move through the program, the debugger maintains an history of the successive times
you stop at. The last command can be used to revisit these times: each last command moves one
step back through the history. That is useful mainly to undo commands such as step and next.

goto time
Jump to the given time.

last [count]
Go back to the latest time recorded in the execution history. With an argument, do it count
times.

set history size

Set the size of the execution history.
388

20.4.5 Killing the program

kill
Kill the program being executed. This command is mainly useful if you wish to recompile the
program without leaving the debugger.

20.5 Breakpoints
A breakpoint causes the program to stop whenever a certain point in the program is reached. It can
be set in several ways using the break command. Breakpoints are assigned numbers when set, for
further reference. The most comfortable way to set breakpoints is through the Emacs interface (see
section 20.10).

break
Set a breakpoint at the current position in the program execution. The current position must
be on an event (i.e., neither at the beginning, nor at the end of the program).

break function
Set a breakpoint at the beginning of function. This works only when the functional value of
the identifier function has been computed and assigned to the identifier. Hence this command
cannot be used at the very beginning of the program execution, when all identifiers are still
undefined; use goto time to advance execution until the functional value is available.

break @ [module] line

Set a breakpoint in module module (or in the current module if module is not given), at the
first event of line line.

break @ [module] line column

Set a breakpoint in module module (or in the current module if module is not given), at the
event closest to line line, column column.

break @ [module] # character

Set a breakpoint in module module at the event closest to character number character.

break frag:pc, break pc

Set a breakpoint at code address frag:pc. The integer frag is the identifier of a code fragment,
a set of modules that have been loaded at once, either initially or with the Dynlink module.
The integer pc is the instruction counter within this code fragment. If frag is omitted, it
defaults to 0, which is the code fragment of the program loaded initially.

delete [breakpoint-numbers]
Delete the specified breakpoints. Without argument, all breakpoints are deleted (after asking
for confirmation).

info breakpoints
Print the list of all breakpoints.
Chapter 20. The debugger (ocamldebug) 389

20.6 The call stack

Each time the program performs a function application, it saves the location of the application (the
return address) in a block of data called a stack frame. The frame also contains the local variables
of the caller function. All the frames are allocated in a region of memory called the call stack. The
command backtrace (or bt) displays parts of the call stack.
At any time, one of the stack frames is “selected” by the debugger; several debugger commands
refer implicitly to the selected frame. In particular, whenever you ask the debugger for the value of
a local variable, the value is found in the selected frame. The commands frame, up and down select
whichever frame you are interested in.
When the program stops, the debugger automatically selects the currently executing frame and
describes it briefly as the frame command does.

frame
Describe the currently selected stack frame.

frame frame-number
Select a stack frame by number and describe it. The frame currently executing when the
program stopped has number 0; its caller has number 1; and so on up the call stack.

backtrace [count], bt [count]

Print the call stack. This is useful to see which sequence of function calls led to the currently
executing frame. With a positive argument, print only the innermost count frames. With a
negative argument, print only the outermost -count frames.

up [count]
Select and display the stack frame just “above” the selected frame, that is, the frame that
called the selected frame. An argument says how many frames to go up.

down [count]
Select and display the stack frame just “below” the selected frame, that is, the frame that was
called by the selected frame. An argument says how many frames to go down.

20.7 Examining variable values

The debugger can print the current value of simple expressions. The expressions can involve program
variables: all the identifiers that are in scope at the selected program point can be accessed.
Expressions that can be printed are a subset of OCaml expressions, as described by the following
390

grammar:
simple-expr ::= lowercase-ident
| {capitalized-ident .} lowercase-ident
| *
| $ integer
| simple-expr . lowercase-ident
| simple-expr .( integer )
| simple-expr .[ integer ]
| ! simple-expr
| ( simple-expr )
The first two cases refer to a value identifier, either unqualified or qualified by the path to the
structure that define it. * refers to the result just computed (typically, the value of a function
application), and is valid only if the selected event is an “after” event (typically, a function
application). $ integer refer to a previously printed value. The remaining four forms select part
of an expression: respectively, a record field, an array element, a string element, and the current
contents of a reference.

print variables
Print the values of the given variables. print can be abbreviated as p.

display variables
Same as print, but limit the depth of printing to 1. Useful to browse large data structures
without printing them in full. display can be abbreviated as d.

When printing a complex expression, a name of the form $integer is automatically assigned to
its value. Such names are also assigned to parts of the value that cannot be printed because the
maximal printing depth is exceeded. Named values can be printed later on with the commands
p $integer or d $integer. Named values are valid only as long as the program is stopped. They are
forgotten as soon as the program resumes execution.

set print_depth d
Limit the printing of values to a maximal depth of d.

set print_length l
Limit the printing of values to at most l nodes printed.

20.8 Controlling the debugger

20.8.1 Setting the program name and arguments
set program file
Set the program name to file.

set arguments arguments

Give arguments as command-line arguments for the program.
Chapter 20. The debugger (ocamldebug) 391

A shell is used to pass the arguments to the debugged program. You can therefore use wildcards,
shell variables, and file redirections inside the arguments. To debug programs that read from standard
input, it is recommended to redirect their input from a file (using set arguments < input-file),
otherwise input to the program and input to the debugger are not properly separated, and inputs
are not properly replayed when running the program backwards.

20.8.2 How programs are loaded

The loadingmode variable controls how the program is executed.

set loadingmode direct

The program is run directly by the debugger. This is the default mode.
set loadingmode runtime
The debugger execute the OCaml runtime ocamlrun on the program. Rarely useful; moreover
it prevents the debugging of programs compiled in “custom runtime” mode.
set loadingmode manual
The user starts manually the program, when asked by the debugger. Allows remote debugging
(see section 20.8.8).

20.8.3 Search path for files

The debugger searches for source files and compiled interface files in a list of directories, the search
path. The search path initially contains the current directory . and the standard library directory.
The directory command adds directories to the path.
Whenever the search path is modified, the debugger will clear any information it may have
cached about the files.

directory directorynames
Add the given directories to the search path. These directories are added at the front, and
will therefore be searched first.
directory directorynames for modulename
Same as directory directorynames, but the given directories will be searched only when
looking for the source file of a module that has been packed into modulename.
directory
Reset the search path. This requires confirmation.

20.8.4 Working directory

Each time a program is started in the debugger, it inherits its working directory from the current
working directory of the debugger. This working directory is initially whatever it inherited from its
parent process (typically the shell), but you can specify a new working directory in the debugger
with the cd command or the -cd command-line option.

cd directory
Set the working directory for ocamldebug to directory.
392

pwd Print the working directory for ocamldebug.

20.8.5 Turning reverse execution on and off

In some cases, you may want to turn reverse execution off. This speeds up the program execution,
and is also sometimes useful for interactive programs.
Normally, the debugger takes checkpoints of the program state from time to time. That is, it
makes a copy of the current state of the program (using the Unix system call fork). If the variable
checkpoints is set to off, the debugger will not take any checkpoints.

set checkpoints on/off

Select whether the debugger makes checkpoints or not.

20.8.6 Behavior of the debugger with respect to fork

When the program issues a call to fork, the debugger can either follow the child or the parent.
By default, the debugger follows the parent process. The variable follow_fork_mode controls this
behavior:

set follow_fork_mode child/parent

Select whether to follow the child or the parent in case of a call to fork.

20.8.7 Stopping execution when new code is loaded

The debugger is compatible with the Dynlink module. However, when an external module is not
yet loaded, it is impossible to set a breakpoint in its code. In order to facilitate setting breakpoints
in dynamically loaded code, the debugger stops the program each time new modules are loaded.
This behavior can be disabled using the break_on_load variable:

set break_on_load on/off

Select whether to stop after loading new code.

20.8.8 Communication between the debugger and the program

The debugger communicate with the program being debugged through a Unix socket. You may
need to change the socket name, for example if you need to run the debugger on a machine and
your program on another.

set socket socket

Use socket for communication with the program. socket can be either a file name, or an
Internet port specification host:port, where host is a host name or an Internet address in dot
notation, and port is a port number on the host.

On the debugged program side, the socket name is passed through the CAML_DEBUG_SOCKET
environment variable.
Chapter 20. The debugger (ocamldebug) 393

20.8.9 Fine-tuning the debugger

Several variables enables to fine-tune the debugger. Reasonable defaults are provided, and you
should normally not have to change them.

set processcount count

Set the maximum number of checkpoints to count. More checkpoints facilitate going far back
in time, but use more memory and create more Unix processes.

As checkpointing is quite expensive, it must not be done too often. On the other hand, backward
execution is faster when checkpoints are taken more often. In particular, backward single-stepping is
more responsive when many checkpoints have been taken just before the current time. To fine-tune
the checkpointing strategy, the debugger does not take checkpoints at the same frequency for long
displacements (e.g. run) and small ones (e.g. step). The two variables bigstep and smallstep
contain the number of events between two checkpoints in each case.

set bigstep count

Set the number of events between two checkpoints for long displacements.

set smallstep count

Set the number of events between two checkpoints for small displacements.

The following commands display information on checkpoints and events:

info checkpoints
Print a list of checkpoints.

info events [module]

Print the list of events in the given module (the current module, by default).

20.8.10 User-defined printers

Just as in the toplevel system (section 14.2), the user can register functions for printing values of
certain types. For technical reasons, the debugger cannot call printing functions that reside in the
program being debugged. The code for the printing functions must therefore be loaded explicitly in
the debugger.

load_printer "file-name"
Load in the debugger the indicated .cmo or .cma object file. The file is loaded in an environment
consisting only of the OCaml standard library plus the definitions provided by object files
previously loaded using load_printer. If this file depends on other object files not yet loaded,
the debugger automatically loads them if it is able to find them in the search path. The loaded
file does not have direct access to the modules of the program being debugged.

install_printer printer-name
Register the function named printer-name (a value path) as a printer for objects whose types
match the argument type of the function. That is, the debugger will call printer-name when it
has such an object to print. The printing function printer-name must use the Format library
394

module to produce its output, otherwise its output will not be correctly located in the values
printed by the toplevel loop.
The value path printer-name must refer to one of the functions defined by the object files
loaded using load_printer. It cannot reference the functions of the program being debugged.

remove_printer printer-name
Remove the named function from the table of value printers.

20.9 Miscellaneous commands

list [module] [beginning] [end]
List the source of module module, from line number beginning to line number end. By default,
20 lines of the current module are displayed, starting 10 lines before the current position.

source filename
Read debugger commands from the script filename.

20.10 Running the debugger under Emacs

The most user-friendly way to use the debugger is to run it under Emacs with the OCaml mode
available through MELPA and also at https://github.com/ocaml/caml-mode.
The OCaml debugger is started under Emacs by the command M-x camldebug, with argument
the name of the executable file progname to debug. Communication with the debugger takes place
in an Emacs buffer named *camldebug-progname*. The editing and history facilities of Shell mode
are available for interacting with the debugger.
In addition, Emacs displays the source files containing the current event (the current position
in the program execution) and highlights the location of the event. This display is updated
synchronously with the debugger action.
The following bindings for the most common debugger commands are available in the
*camldebug-progname* buffer:

C-c C-s
(command step): execute the program one step forward.

C-c C-k
(command backstep): execute the program one step backward.

C-c C-n
(command next): execute the program one step forward, skipping over function calls.

Middle mouse button

(command display): display named value. $n under mouse cursor (support incremental
browsing of large data structures).

C-c C-p
(command print): print value of identifier at point.
Chapter 20. The debugger (ocamldebug) 395

C-c C-d
(command display): display value of identifier at point.

C-c C-r
(command run): execute the program forward to next breakpoint.

C-c C-v
(command reverse): execute the program backward to latest breakpoint.

C-c C-l
(command last): go back one step in the command history.

C-c C-t
(command backtrace): display backtrace of function calls.

C-c C-f
(command finish): run forward till the current function returns.

C-c <
(command up): select the stack frame below the current frame.

C-c >
(command down): select the stack frame above the current frame.

In all buffers in OCaml editing mode, the following debugger commands are also available:

C-x C-a C-b

(command break): set a breakpoint at event closest to point

C-x C-a C-p

(command print): print value of identifier at point

C-x C-a C-d

(command display): display value of identifier at point
396
Chapter 21

Profiling (ocamlprof)

This chapter describes how the execution of OCaml programs can be profiled, by recording how
many times functions are called, branches of conditionals are taken, …

21.1 Compiling for profiling

Before profiling an execution, the program must be compiled in profiling mode, using the ocamlcp
front-end to the ocamlc compiler (see chapter 13) or the ocamloptp front-end to the ocamlopt
compiler (see chapter 16). When compiling modules separately, ocamlcp or ocamloptp must be
used when compiling the modules (production of .cmo or .cmx files), and can also be used (though
this is not strictly necessary) when linking them together.

Note If a module (.ml file) doesn’t have a corresponding interface (.mli file), then compiling
it with ocamlcp will produce object files (.cmi and .cmo) that are not compatible with the ones
produced by ocamlc, which may lead to problems (if the .cmi or .cmo is still around) when switching
between profiling and non-profiling compilations. To avoid this problem, you should always have a
.mli file for each .ml file. The same problem exists with ocamloptp.

Note To make sure your programs can be compiled in profiling mode, avoid using any identifier
that begins with __ocaml_prof.
The amount of profiling information can be controlled through the -P option to ocamlcp or
ocamloptp, followed by one or several letters indicating which parts of the program should be
profiled:

a all options

f function calls : a count point is set at the beginning of each function body

i if …then …else … : count points are set in both then branch and else branch

l while, for loops: a count point is set at the beginning of the loop body

m match branches: a count point is set at the beginning of the body of each branch

397
398

t try …with … branches: a count point is set at the beginning of the body of each branch

For instance, compiling with ocamlcp -P film profiles function calls, if…then…else…, loops and
pattern matching.
Calling ocamlcp or ocamloptp without the -P option defaults to -P fm, meaning that only
function calls and pattern matching are profiled.

Note For compatibility with previous releases, ocamlcp also accepts the -p option, with the same
arguments and behaviour as -P.
The ocamlcp and ocamloptp commands also accept all the options of the corresponding ocamlc
or ocamlopt compiler, except the -pp (preprocessing) option.

21.2 Profiling an execution

Running an executable that has been compiled with ocamlcp or ocamloptp records the execution
counts for the specified parts of the program and saves them in a file called ocamlprof.dump in the
current directory.
If the environment variable OCAMLPROF_DUMP is set when the program exits, its value is used as
the file name instead of ocamlprof.dump.
The dump file is written only if the program terminates normally (by calling exit or by falling
through). It is not written if the program terminates with an uncaught exception.
If a compatible dump file already exists in the current directory, then the profiling information
is accumulated in this dump file. This allows, for instance, the profiling of several executions of a
program on different inputs. Note that dump files produced by byte-code executables (compiled
with ocamlcp) are compatible with the dump files produced by native executables (compiled with
ocamloptp).

21.3 Printing profiling information

The ocamlprof command produces a source listing of the program modules where execution counts
have been inserted as comments. For instance,
ocamlprof foo.ml
prints the source code for the foo module, with comments indicating how many times the
functions in this module have been called. Naturally, this information is accurate only if the source
file has not been modified after it was compiled.
The following options are recognized by ocamlprof:

-args filename
Read additional newline-terminated command line arguments from filename.
-args0 filename
Read additional null character terminated command line arguments from filename.
-f dumpfile
Specifies an alternate dump file of profiling information to be read.
Chapter 21. Profiling (ocamlprof) 399

-F string
Specifies an additional string to be output with profiling information. By default, ocamlprof
will annotate programs with comments of the form (* n *) where n is the counter value for
a profiling point. With option -F s, the annotation will be (* sn *).

-impl filename
Process the file filename as an implementation file, even if its extension is not .ml.

-intf filename
Process the file filename as an interface file, even if its extension is not .mli.

-version
Print version string and exit.

-vnum
Print short version number and exit.

-help or --help
Display a short usage summary and exit.

21.4 Time profiling

Profiling with ocamlprof only records execution counts, not the actual time spent within each
function. There is currently no way to perform time profiling on bytecode programs generated by
ocamlc. For time profiling of native code, users are recommended to use standard tools such as perf
(on Linux), Instruments (on macOS) and DTrace. Profiling with gprof is no longer supported.
400
Chapter 22

Interfacing C with OCaml

This chapter describes how user-defined primitives, written in C, can be linked with OCaml code
and called from OCaml functions, and how these C functions can call back to OCaml code.

22.1 Overview and compilation information

22.1.1 Declaring primitives
definition ::= ...
| external value-name : typexpr = external-declaration
external-declaration ::= string-literal [string-literal [string-literal]]
User primitives are declared in an implementation file or struct . . . end module expression using
the external keyword:

external name : type = C-function-name

This defines the value name name as a function with type type that executes by calling the given
C function. For instance, here is how the seek_in primitive is declared in the standard library
module Stdlib:

external seek_in : in_channel -> int -> unit = "caml_ml_seek_in"

Primitives with several arguments are always curried. The C function does not necessarily have
the same name as the ML function.
External functions thus defined can be specified in interface files or sig . . . end signatures either
as regular values

val name : type

thus hiding their implementation as C functions, or explicitly as “manifest” external functions

external name : type = C-function-name

401
402

The latter is slightly more efficient, as it allows clients of the module to call directly the C
function instead of going through the corresponding OCaml function. On the other hand, it should
not be used in library modules if they have side-effects at toplevel, as this direct call interferes with
the linker’s algorithm for removing unused modules from libraries at link-time.
The arity (number of arguments) of a primitive is automatically determined from its OCaml type
in the external declaration, by counting the number of function arrows in the type. For instance,
seek_in above has arity 2, and the caml_ml_seek_in C function is called with two arguments.
Similarly,

external seek_in_pair: in_channel * int -> unit = "caml_ml_seek_in_pair"

has arity 1, and the caml_ml_seek_in_pair C function receives one argument (which is a pair
of OCaml values).
Type abbreviations are not expanded when determining the arity of a primitive. For instance,

type int_endo = int -> int

external f : int_endo -> int_endo = "f"
external g : (int -> int) -> (int -> int) = "f"

f has arity 1, but g has arity 2. This allows a primitive to return a functional value (as in the f
example above): just remember to name the functional return type in a type abbreviation.
The language accepts external declarations with one or two flag strings in addition to the C
function’s name. These flags are reserved for the implementation of the standard library.

22.1.2 Implementing primitives

User primitives with arity n ≤ 5 are implemented by C functions that take n arguments of type
value, and return a result of type value. The type value is the type of the representations for
OCaml values. It encodes objects of several base types (integers, floating-point numbers, strings, …)
as well as OCaml data structures. The type value and the associated conversion functions and
macros are described in detail below. For instance, here is the declaration for the C function
implementing the In_channel.input primitive, which takes 4 arguments:

CAMLprim value input(value channel, value buffer, value offset, value length)
{
...
}

When the primitive function is applied in an OCaml program, the C function is called with the
values of the expressions to which the primitive is applied as arguments. The value returned by the
function is passed back to the OCaml program as the result of the function application.
User primitives with arity greater than 5 should be implemented by two C functions. The first
function, to be used in conjunction with the bytecode compiler ocamlc, receives two arguments: a
pointer to an array of OCaml values (the values for the arguments), and an integer which is the
number of arguments provided. The other function, to be used in conjunction with the native-code
compiler ocamlopt, takes its arguments directly. For instance, here are the two C functions for the
7-argument primitive Nat.add_nat:
Chapter 22. Interfacing C with OCaml 403

CAMLprim value add_nat_native(value nat1, value ofs1, value len1,

value nat2, value ofs2, value len2,
value carry_in)
{
...
}
CAMLprim value add_nat_bytecode(value * argv, int argn)
{
return add_nat_native(argv[0], argv[1], argv[2], argv[3],
argv[4], argv[5], argv[6]);
}

The names of the two C functions must be given in the primitive declaration, as follows:

external name : type =

bytecode-C-function-name native-code-C-function-name

For instance, in the case of add_nat, the declaration is:

external add_nat: nat -> int -> int -> nat -> int -> int -> int -> int
= "add_nat_bytecode" "add_nat_native"

Implementing a user primitive is actually two separate tasks: on the one hand, decoding the
arguments to extract C values from the given OCaml values, and encoding the return value as an
OCaml value; on the other hand, actually computing the result from the arguments. Except for
very simple primitives, it is often preferable to have two distinct C functions to implement these two
tasks. The first function actually implements the primitive, taking native C values as arguments and
returning a native C value. The second function, often called the “stub code”, is a simple wrapper
around the first function that converts its arguments from OCaml values to C values, calls the first
function, and converts the returned C value to an OCaml value. For instance, here is the stub code
for the Int64.float_of_bits primitive:

CAMLprim value caml_int64_float_of_bits(value vi)

{
return caml_copy_double(caml_int64_float_of_bits_unboxed(Int64_val(vi)));
}

(Here, caml_copy_double and Int64_val are conversion functions and macros for the type
value, that will be described later. The CAMLprim macro expands to the required compiler directives
to ensure that the function is exported and accessible from OCaml.) The hard work is performed
by the function caml_int64_float_of_bits_unboxed, which is declared as:

double caml_int64_float_of_bits_unboxed(int64_t i)
{
...
}

To write C code that operates on OCaml values, the following include files are provided:
404

Include file Provides

caml/mlvalues.h definition of the value type, and conversion macros
caml/alloc.h allocation functions (to create structured OCaml objects)
caml/memory.h miscellaneous memory-related functions and macros (for GC interface,
in-place modification of structures, etc).
caml/fail.h functions for raising exceptions (see section 22.4.7)
caml/callback.h callback from C to OCaml (see section 22.7).
caml/custom.h operations on custom blocks (see section 22.9).
caml/intext.h operations for writing user-defined serialization and deserialization func-
tions for custom blocks (see section 22.9).
caml/threads.h operations for interfacing in the presence of multiple threads (see sec-
tion 22.12).

These files reside in the caml/ subdirectory of the OCaml standard library directory, which is
returned by the command ocamlc -where (usually /usr/local/lib/ocaml or /usr/lib/ocaml).

22.1.3 Statically linking C code with OCaml code

The OCaml runtime system comprises three main parts: the bytecode interpreter, the memory
manager, and a set of C functions that implement the primitive operations. Some bytecode
instructions are provided to call these C functions, designated by their offset in a table of functions
(the table of primitives).
In the default mode, the OCaml linker produces bytecode for the standard runtime system,
with a standard set of primitives. References to primitives that are not in this standard set result
in the “unavailable C primitive” error. (Unless dynamic loading of C libraries is supported – see
section 22.1.4 below.)
In the “custom runtime” mode, the OCaml linker scans the object files and determines the set
of required primitives. Then, it builds a suitable runtime system, by calling the native code linker
with:

• the table of the required primitives;

• a library that provides the bytecode interpreter, the memory manager, and the standard
primitives;

• libraries and object code files (.o files) mentioned on the command line for the OCaml linker,
that provide implementations for the user’s primitives.

This builds a runtime system with the required primitives. The OCaml linker generates bytecode for
this custom runtime system. The bytecode is appended to the end of the custom runtime system,
so that it will be automatically executed when the output file (custom runtime + bytecode) is
launched.
To link in “custom runtime” mode, execute the ocamlc command with:

• the -custom option;

• the names of the desired OCaml object files (.cmo and .cma files) ;
Chapter 22. Interfacing C with OCaml 405

• the names of the C object files and libraries (.o and .a files) that implement the required
primitives. Under Unix and Windows, a library named libname.a (respectively, .lib)
residing in one of the standard library directories can also be specified as -cclib -lname.
If you are using the native-code compiler ocamlopt, the -custom flag is not needed, as the final
linking phase of ocamlopt always builds a standalone executable. To build a mixed OCaml/C
executable, execute the ocamlopt command with:
• the names of the desired OCaml native object files (.cmx and .cmxa files);
• the names of the C object files and libraries (.o, .a, .so or .dll files) that implement the
required primitives.
Starting with Objective Caml 3.00, it is possible to record the -custom option as well as the
names of C libraries in an OCaml library file .cma or .cmxa. For instance, consider an OCaml
library mylib.cma, built from the OCaml object files a.cmo and b.cmo, which reference C code in
libmylib.a. If the library is built as follows:
ocamlc -a -o mylib.cma -custom a.cmo b.cmo -cclib -lmylib
users of the library can simply link with mylib.cma:
ocamlc -o myprog mylib.cma ...
and the system will automatically add the -custom and -cclib -lmylib options, achieving the
same effect as
ocamlc -o myprog -custom a.cmo b.cmo ... -cclib -lmylib
The alternative is of course to build the library without extra options:
ocamlc -a -o mylib.cma a.cmo b.cmo
and then ask users to provide the -custom and -cclib -lmylib options themselves at link-time:
ocamlc -o myprog -custom mylib.cma ... -cclib -lmylib
The former alternative is more convenient for the final users of the library, however.

22.1.4 Dynamically linking C code with OCaml code

Starting with Objective Caml 3.03, an alternative to static linking of C code using the -custom code
is provided. In this mode, the OCaml linker generates a pure bytecode executable (no embedded
custom runtime system) that simply records the names of dynamically-loaded libraries containing
the C code. The standard OCaml runtime system ocamlrun then loads dynamically these libraries,
and resolves references to the required primitives, before executing the bytecode.
This facility is currently available on all platforms supported by OCaml except Cygwin 64 bits.
To dynamically link C code with OCaml code, the C code must first be compiled into a
shared library (under Unix) or DLL (under Windows). This involves 1- compiling the C files
with appropriate C compiler flags for producing position-independent code (when required by the
operating system), and 2- building a shared library from the resulting object files. The resulting
shared library or DLL file must be installed in a place where ocamlrun can find it later at program
start-up time (see section 15.3). Finally (step 3), execute the ocamlc command with
406

• the names of the desired OCaml object files (.cmo and .cma files) ;

• the names of the C shared libraries (.so or .dll files) that implement the required primitives.
Under Unix and Windows, a library named dllname.so (respectively, .dll) residing in one
of the standard library directories can also be specified as -dllib -lname.

Do not set the -custom flag, otherwise you’re back to static linking as described in section 22.1.3.
The ocamlmklib tool (see section 22.14) automates steps 2 and 3.
As in the case of static linking, it is possible (and recommended) to record the names of C
libraries in an OCaml .cma library archive. Consider again an OCaml library mylib.cma, built from
the OCaml object files a.cmo and b.cmo, which reference C code in dllmylib.so. If the library is
built as follows:

ocamlc -a -o mylib.cma a.cmo b.cmo -dllib -lmylib

users of the library can simply link with mylib.cma:

ocamlc -o myprog mylib.cma ...

and the system will automatically add the -dllib -lmylib option, achieving the same effect as

ocamlc -o myprog a.cmo b.cmo ... -dllib -lmylib

Using this mechanism, users of the library mylib.cma do not need to know that it references C
code, nor whether this C code must be statically linked (using -custom) or dynamically linked.

22.1.5 Choosing between static linking and dynamic linking

After having described two different ways of linking C code with OCaml code, we now review the
pros and cons of each, to help developers of mixed OCaml/C libraries decide.
The main advantage of dynamic linking is that it preserves the platform-independence of bytecode
executables. That is, the bytecode executable contains no machine code, and can therefore be
compiled on platform A and executed on other platforms B, C, …, as long as the required shared
libraries are available on all these platforms. In contrast, executables generated by ocamlc -custom
run only on the platform on which they were created, because they embark a custom-tailored
runtime system specific to that platform. In addition, dynamic linking results in smaller executables.
Another advantage of dynamic linking is that the final users of the library do not need to have a
C compiler, C linker, and C runtime libraries installed on their machines. This is no big deal under
Unix and Cygwin, but many Windows users are reluctant to install Microsoft Visual C just to be
able to do ocamlc -custom.
There are two drawbacks to dynamic linking. The first is that the resulting executable is
not stand-alone: it requires the shared libraries, as well as ocamlrun, to be installed on the
machine executing the code. If you wish to distribute a stand-alone executable, it is better to
link it statically, using ocamlc -custom -ccopt -static or ocamlopt -ccopt -static. Dynamic
linking also raises the “DLL hell” problem: some care must be taken to ensure that the right versions
of the shared libraries are found at start-up time.
The second drawback of dynamic linking is that it complicates the construction of the library.
The C compiler and linker flags to compile to position-independent code and build a shared library
Chapter 22. Interfacing C with OCaml 407

vary wildly between different Unix systems. Also, dynamic linking is not supported on all Unix
systems, requiring a fall-back case to static linking in the Makefile for the library. The ocamlmklib
command (see section 22.14) tries to hide some of these system dependencies.
In conclusion: dynamic linking is highly recommended under the native Windows port, because
there are no portability problems and it is much more convenient for the end users. Under Unix,
dynamic linking should be considered for mature, frequently used libraries because it enhances
platform-independence of bytecode executables. For new or rarely-used libraries, static linking is
much simpler to set up in a portable way.

22.1.6 Building standalone custom runtime systems

It is sometimes inconvenient to build a custom runtime system each time OCaml code is linked
with C libraries, like ocamlc -custom does. For one thing, the building of the runtime system is
slow on some systems (that have bad linkers or slow remote file systems); for another thing, the
platform-independence of bytecode files is lost, forcing to perform one ocamlc -custom link per
platform of interest.
An alternative to ocamlc -custom is to build separately a custom runtime system integrating
the desired C libraries, then generate “pure” bytecode executables (not containing their own
runtime system) that can run on this custom runtime. This is achieved by the -make-runtime and
-use-runtime flags to ocamlc. For example, to build a custom runtime system integrating the C
parts of the “Unix” and “Threads” libraries, do:

ocamlc -make-runtime -o /home/me/ocamlunixrun unix.cma threads.cma

To generate a bytecode executable that runs on this runtime system, do:

ocamlc -use-runtime /home/me/ocamlunixrun -o myprog \

unix.cma threads.cma your .cmo and .cma files

The bytecode executable myprog can then be launched as usual: myprog args or
/home/me/ocamlunixrun myprog args.
Notice that the bytecode libraries unix.cma and threads.cma must be given twice: when
building the runtime system (so that ocamlc knows which C primitives are required) and also when
building the bytecode executable (so that the bytecode from unix.cma and threads.cma is actually
linked in).

22.2 The value type

All OCaml objects are represented by the C type value, defined in the include file caml/mlvalues.h,
along with macros to manipulate values of that type. An object of type value is either:

• an unboxed integer;

• or a pointer to a block inside the heap, allocated through one of the caml_alloc_* functions
described in section 22.4.4.
408

22.2.1 Integer values

Integer values encode 63-bit signed integers (31-bit on 32-bit architectures). They are unboxed
(unallocated).

22.2.2 Blocks
Blocks in the heap are garbage-collected, and therefore have strict structure constraints. Each block
includes a header containing the size of the block (in words), and the tag of the block. The tag
governs how the contents of the blocks are structured. A tag lower than No_scan_tag indicates
a structured block, containing well-formed values, which is recursively traversed by the garbage
collector. A tag greater than or equal to No_scan_tag indicates a raw block, whose contents are not
scanned by the garbage collector. For the benefit of ad-hoc polymorphic primitives such as equality
and structured input-output, structured and raw blocks are further classified according to their tags
as follows:
Tag Contents of the block
0 to No_scan_tag − 1 A structured block (an array of OCaml objects). Each field
is a value.
Closure_tag A closure representing a functional value. The first word is
a pointer to a piece of code, the remaining words are value
containing the environment.
String_tag A character string or a byte sequence.
Double_tag A double-precision floating-point number.
Double_array_tag An array or record of double-precision floating-point numbers.
Abstract_tag A block representing an abstract datatype.
Custom_tag A block representing an abstract datatype with user-defined
finalization, comparison, hashing, serialization and deserial-
ization functions attached.

22.2.3 Pointers outside the heap

In earlier versions of OCaml, it was possible to use word-aligned pointers to addresses outside the
heap as OCaml values, just by casting the pointer to type value. This usage is no longer supported
since OCaml 5.0.
A correct way to manipulate pointers to out-of-heap blocks from OCaml is to store those pointers
in OCaml blocks with tag Abstract_tag or Custom_tag, then use the blocks as the OCaml values.
Here is an example of encapsulation of out-of-heap pointers of C type ty * inside Abstract_tag
blocks. Section 22.6 gives a more complete example using Custom_tag blocks.

/* Create an OCaml value encapsulating the pointer p */

static value val_of_typtr(ty * p)
{
value v = caml_alloc(1, Abstract_tag);
*((ty **) Data_abstract_val(v)) = p;
return v;
}
Chapter 22. Interfacing C with OCaml 409

/* Extract the pointer encapsulated in the given OCaml value */

static ty * typtr_of_val(value v)
{
return *((ty **) Data_abstract_val(v));
}

Alternatively, out-of-heap pointers can be treated as “native” integers, that is, boxed 32-bit
integers on a 32-bit platform and boxed 64-bit integers on a 64-bit platform.

/* Create an OCaml value encapsulating the pointer p */

static value val_of_typtr(ty * p)
{
return caml_copy_nativeint((intnat) p);
}

/* Extract the pointer encapsulated in the given OCaml value */

static ty * typtr_of_val(value v)
{
return (ty *) Nativeint_val(v);
}

For pointers that are at least 2-aligned (the low bit is guaranteed to be zero), we have yet
another valid representation as an OCaml tagged integer.

/* Create an OCaml value encapsulating the pointer p */

static value val_of_typtr(ty * p)
{
assert (((uintptr_t) p & 1) == 0); /* check correct alignment */
return (value) p | 1;
}

/* Extract the pointer encapsulated in the given OCaml value */

static ty * typtr_of_val(value v)
{
return (ty *) (v & ~1);
}

22.3 Representation of OCaml data types

This section describes how OCaml data types are encoded in the value type.
410

22.3.1 Atomic types

OCaml type Encoding
int Unboxed integer values.
char Unboxed integer values (ASCII code).
float Blocks with tag Double_tag.
bytes Blocks with tag String_tag.
string Blocks with tag String_tag.
int32 Blocks with tag Custom_tag.
int64 Blocks with tag Custom_tag.
nativeint Blocks with tag Custom_tag.

22.3.2 Tuples and records

Tuples are represented by pointers to blocks, with tag 0.
Records are also represented by zero-tagged blocks. The ordering of labels in the record type
declaration determines the layout of the record fields: the value associated to the label declared first
is stored in field 0 of the block, the value associated to the second label goes in field 1, and so on.
As an optimization, records whose fields all have static type float are represented as arrays of
floating-point numbers, with tag Double_array_tag. (See the section below on arrays.)
As another optimization, unboxable record types are represented specially; unboxable record
types are the immutable record types that have only one field. An unboxable type will be represented
in one of two ways: boxed or unboxed. Boxed record types are represented as described above (by a
block with tag 0 or Double_array_tag). An unboxed record type is represented directly by the
value of its field (i.e. there is no block to represent the record itself).
The representation is chosen according to the following, in decreasing order of priority:

• An attribute ([@@boxed] or [@@unboxed]) on the type declaration.

• A compiler option (-unboxed-types or -no-unboxed-types).

• The default representation. In the present version of OCaml, the default is the boxed
representation.

22.3.3 Arrays
Arrays of integers and pointers are represented like tuples and records, that is, as pointers to blocks
tagged 0. They are accessed with the Field macro for reading and the caml_modify function for
writing.
Values of type floatarray (as manipulated by the Float.Array module), as well as records
whose declaration contains only float fields, use an efficient unboxed representation: blocks with tag
Double_array_tag whose content consist of raw double values, which are not themselves valid OCaml
values. They should be accessed using the Double_flat_field and Store_double_flat_field
macros.
Finally, arrays of type float array may use either the boxed or the unboxed representation de-
pending on the how the compiler is configured. They currently use the unboxed representation by de-
fault, but can be made to use the boxed representation by passing the --disable-flat-float-array
Chapter 22. Interfacing C with OCaml 411

flag to the ‘configure‘ script. They should be accessed using the Double_array_field and
Store_double_array_field macros, which will work correctly under both modes.

22.3.4 Concrete data types

Constructed terms are represented either by unboxed integers (for constant constructors) or by
blocks whose tag encode the constructor (for non-constant constructors). The constant constructors
and the non-constant constructors for a given concrete type are numbered separately, starting from
0, in the order in which they appear in the concrete type declaration. A constant constructor is
represented by the unboxed integer equal to its constructor number. A non-constant constructor
declared with n arguments is represented by a block of size n, tagged with the constructor number;
the n fields contain its arguments. Example:

Constructed term Representation

() Val_int(0)
false Val_int(0)
true Val_int(1)
[] Val_int(0)
h::t Block with size = 2 and tag = 0; first field con-
tains h, second field t.

As a convenience, caml/mlvalues.h defines the macros Val_unit, Val_false, Val_true and

Val_emptylist to refer to (), false, true and [].
The following example illustrates the assignment of integers and block tags to constructors:

type t =
| A (* First constant constructor -> integer "Val_int(0)" *)
| B of string (* First non-constant constructor -> block with tag 0 *)
| C (* Second constant constructor -> integer "Val_int(1)" *)
| D of bool (* Second non-constant constructor -> block with tag 1 *)
| E of t * t (* Third non-constant constructor -> block with tag 2 *)

As an optimization, unboxable concrete data types are represented specially; a concrete data
type is unboxable if it has exactly one constructor and this constructor has exactly one argument.
Unboxable concrete data types are represented in the same ways as unboxable record types: see the
description in section 22.3.2.

22.3.5 Objects
Objects are represented as blocks with tag Object_tag. The first field of the block refers to the
object’s class and associated method suite, in a format that cannot easily be exploited from C. The
second field contains a unique object ID, used for comparisons. The remaining fields of the object
contain the values of the instance variables of the object. It is unsafe to access directly instance
variables, as the type system provides no guarantee about the instance variables contained by an
object.
One may extract a public method from an object using the C function caml_get_public_method
(declared in <caml/mlvalues.h>.) Since public method tags are hashed in the same way as variant
412

tags, and methods are functions taking self as first argument, if you want to do the method call
foo#bar from the C side, you should call:
callback(caml_get_public_method(foo, hash_variant("bar")), foo);

22.3.6 Polymorphic variants

Like constructed terms, polymorphic variant values are represented either as integers (for polymorphic
variants without argument), or as blocks (for polymorphic variants with an argument). Unlike con-
structed terms, variant constructors are not numbered starting from 0, but identified by a hash value
(an OCaml integer), as computed by the C function hash_variant (declared in <caml/mlvalues.h>):
the hash value for a variant constructor named, say, VConstr is hash_variant("VConstr").
The variant value `VConstr is represented by hash_variant("VConstr"). The variant value
`VConstr(v) is represented by a block of size 2 and tag 0, with field number 0 containing
hash_variant("VConstr") and field number 1 containing v.
Unlike constructed values, polymorphic variant values taking several arguments are not flattened.
That is, `VConstr(v, w) is represented by a block of size 2, whose field number 1 contains the
representation of the pair (v, w), rather than a block of size 3 containing v and w in fields 1 and 2.

22.4 Operations on values

22.4.1 Kind tests
• Is_long(v) is true if value v is an immediate integer, false otherwise
• Is_block(v) is true if value v is a pointer to a block, and false if it is an immediate integer.
• Is_none(v) is true if value v is None.
• Is_some(v) is true if value v (assumed to be of option type) corresponds to the Some con-
structor.

22.4.2 Operations on integers

• Val_long(l) returns the value encoding the long int l.
• Long_val(v) returns the long int encoded in value v.
• Val_int(i) returns the value encoding the int i.
• Int_val(v) returns the int encoded in value v.
• Val_bool(x) returns the OCaml boolean representing the truth value of the C integer x.
• Bool_val(v) returns 0 if v is the OCaml boolean false, 1 if v is true.
• Val_true, Val_false represent the OCaml booleans true and false.
• Val_emptylist, Val_emptylist represents the empty list.
• Val_none represents the OCaml value None.
Chapter 22. Interfacing C with OCaml 413

22.4.3 Accessing blocks

• Wosize_val(v) returns the size of the block v, in words, excluding the header.

• Tag_val(v) returns the tag of the block v.

• Field(v, n) returns the value contained in the n field of the structured block v. Fields are
numbered from 0 to Wosize_val(v) − 1.

• Store_field(b, n, v) stores the value v in the field number n of value b, which must be a
structured block.

• Code_val(v) returns the code part of the closure v.

• caml_string_length(v) returns the length (number of bytes) of the string or byte sequence
v.

• Byte(v, n) returns the n byte of the string or byte sequence v, with type char. Bytes are
numbered from 0 to string_length(v) − 1.

• Byte_u(v, n) returns the n byte of the string or byte sequence v, with type unsigned char.
Bytes are numbered from 0 to string_length(v) − 1.

• String_val(v) returns a pointer to the first byte of the string v, with type const char *.
This pointer is a valid C string: there is a null byte after the last byte in the string. However,
OCaml strings can contain embedded null bytes, which will confuse the usual C functions over
strings. The function caml_string_is_c_safe(v) returns true if the OCaml string v does
not contain any embedded null bytes.

• Bytes_val(v) returns a pointer to the first byte of the byte sequence v, with type
unsigned char *.

• Double_val(v) returns the floating-point number contained in value v, with type double.

• Double_array_field(v, n) returns the n element of a float array v.

• Store_double_array_field(v, n, d) stores the double precision floating-point number d

in the n element of a float array v.

• Double_flat_field(v, n) returns the n element of a floatarray or a record of floats v (an

unboxed block tagged Double_array_tag).

• Store_double_flat_field(v, n, d) stores the double precision floating-point number d in

the n element of a floatarray or a record of floats v.

• Data_custom_val(v) returns a pointer to the data part of the custom block v. This pointer
has type void * and must be cast to the type of the data contained in the custom block.

• Int32_val(v) returns the 32-bit integer contained in the int32 v.

• Int64_val(v) returns the 64-bit integer contained in the int64 v.

414

• Nativeint_val(v) returns the long integer contained in the nativeint v.

• caml_field_unboxed(v) returns the value of the field of a value v of any unboxed type (record
or concrete data type).

• caml_field_boxed(v) returns the value of the field of a value v of any boxed type (record or
concrete data type).

• caml_field_unboxable(v) calls either caml_field_unboxed or caml_field_boxed accord-

ing to the default representation of unboxable types in the current version of OCaml.

• Some_val(v) returns the argument \var{x} of a value v of the form Some(x).

The expressions Field(v, n), Byte(v, n) and Byte_u(v, n) are valid l-values. Hence, they can
be assigned to, resulting in an in-place modification of value v. Assigning directly to Field(v, n)
must be done with care to avoid confusing the garbage collector (see below).

22.4.4 Allocating blocks

22.4.5 Simple interface
• Atom(t) returns an “atom” (zero-sized block) with tag t. Zero-sized blocks are preallocated
outside of the heap. It is incorrect to try and allocate a zero-sized block using the functions
below. For instance, Atom(0) represents the empty array.

• caml_alloc(n, t) returns a fresh block of size n with tag t. If t is less than No_scan_tag, then
the fields of the block are initialized with a valid value in order to satisfy the GC constraints.

• caml_alloc_tuple(n) returns a fresh block of size n words, with tag 0.

• caml_alloc_string(n) returns a byte sequence (or string) value of length n bytes. The
sequence initially contains uninitialized bytes.

• caml_alloc_initialized_string(n, p) returns a byte sequence (or string) value of length

n bytes. The value is initialized from the n bytes starting at address p.

• caml_copy_string(s) returns a string or byte sequence value containing a copy of the null-
terminated C string s (a char *).

• caml_copy_double(d) returns a floating-point value initialized with the double d.

• caml_copy_int32(i), caml_copy_int64(i) and caml_copy_nativeint(i) return a value of

OCaml type int32, int64 and nativeint, respectively, initialized with the integer i.

• caml_alloc_array(f, a) allocates an array of values, calling function f over each element of

the input array a to transform it into a value. The array a is an array of pointers terminated
by the null pointer. The function f receives each pointer as argument, and returns a value.
The zero-tagged block returned by alloc_array(f, a) is filled with the values returned by
the successive calls to f. (This function must not be used to build an array of floating-point
numbers.)
Chapter 22. Interfacing C with OCaml 415

• caml_copy_string_array(p) allocates an array of strings or byte sequences, copied from the

pointer to a string array p (a char **). p must be NULL-terminated.

• caml_alloc_float_array(n) allocates an array of floating point numbers of size n. The

array initially contains uninitialized values.

• caml_alloc_unboxed(v) returns the value (of any unboxed type) whose field is the value v.

• caml_alloc_boxed(v) allocates and returns a value (of any boxed type) whose field is the
value v.

• caml_alloc_unboxable(v) calls either caml_alloc_unboxed or caml_alloc_boxed accord-

ing to the default representation of unboxable types in the current version of OCaml.

• caml_alloc_some(v) allocates a block representing Some(v).

22.4.6 Low-level interface

The following functions are slightly more efficient than caml_alloc, but also much more difficult to
use.
From the standpoint of the allocation functions, blocks are divided according to their size as
zero-sized blocks, small blocks (with size less than or equal to Max_young_wosize), and large blocks
(with size greater than Max_young_wosize). The constant Max_young_wosize is declared in the
include file mlvalues.h. It is guaranteed to be at least 64 (words), so that any block with constant
size less than or equal to 64 can be assumed to be small. For blocks whose size is computed at
run-time, the size must be compared against Max_young_wosize to determine the correct allocation
procedure.

• caml_alloc_small(n, t) returns a fresh small block of size n ≤ Max_young_wosize words,

with tag t. If this block is a structured block (i.e. if t < No_scan_tag), then the fields of
the block (initially containing garbage) must be initialized with legal values (using direct
assignment to the fields of the block) before the next allocation.

• caml_alloc_shr(n, t) returns a fresh block of size n, with tag t. The size of the block can
be greater than Max_young_wosize. (It can also be smaller, but in this case it is more efficient
to call caml_alloc_small instead of caml_alloc_shr.) If this block is a structured block
(i.e. if t < No_scan_tag), then the fields of the block (initially containing garbage) must be
initialized with legal values (using the caml_initialize function described below) before the
next allocation.

22.4.7 Raising exceptions

Two functions are provided to raise two standard exceptions:

• caml_failwith(s), where s is a null-terminated C string (with type char *), raises exception
Failure with argument s.

• caml_invalid_argument(s), where s is a null-terminated C string (with type char *), raises

exception Invalid_argument with argument s.
416

Raising arbitrary exceptions from C is more delicate: the exception constructor is dynamically
allocated by the OCaml program, and therefore must be communicated to the C function using the
registration facility described below in section 22.7.4. Once the exception constructor is recovered
in C, the following functions actually raise the exception:
• caml_raise_constant(id) raises the exception id with no argument;
• caml_raise_with_arg(id, v) raises the exception id with the OCaml value v as argument;
• caml_raise_with_args(id, n, v) raises the exception id with the OCaml values v[0], …,
v[n-1] as arguments;
• caml_raise_with_string(id, s), where s is a null-terminated C string, raises the exception
id with a copy of the C string s as argument.
Sometimes, it is necessary to clean-up state and release resources before actually raising the
exception back into OCaml. To this end, alternative functions that return the exception instead
of raising it directly (caml_exception_failure, caml_exception_invalid_argument, etc.) are
provided. The type caml_result represents either an OCaml value or an exception; see section 22.7.2.

22.5 Living in harmony with the garbage collector

Unused blocks in the heap are automatically reclaimed by the garbage collector. This requires some
cooperation from C code that manipulates heap-allocated blocks.

22.5.1 Simple interface

All the macros described in this section are declared in the memory.h header file.
Rule 1 A function that has parameters or local variables of type value must begin with a call to
one of the CAMLparam macros and return with CAMLreturn, CAMLreturn0, or CAMLreturnT.
There are six CAMLparam macros: CAMLparam0 to CAMLparam5, which take zero to five arguments
respectively. If your function has no more than 5 parameters of type value, use the corresponding
macros with these parameters as arguments. If your function has more than 5 parameters of type
value, use CAMLparam5 with five of these parameters, and use one or more calls to the CAMLxparam
macros for the remaining parameters (CAMLxparam1 to CAMLxparam5). If the function has an
argument x which is an array of values of length n, use CAMLparamN (x, n) to declare it (or
CAMLxparamN (x, n) if you already have a call to CAMLparam for some other arguments).
The macros CAMLreturn, CAMLreturn0, and CAMLreturnT are used to replace the C keyword
return; any function using a CAMLparam macro on entry should use CAMLreturn on all exit points.
C functions exported as OCaml externals must return a value, and they should use CAMLreturn (x)
instead of return x. Some helper functions may manipulate OCaml values yet return void or
another datatype. void-returning procedures should explicitly use CAMLreturn0, and not have any
implicit return. Helper functions returning C data of some type t should use CAMLreturnT (t, x)
instead of return x.

Note: Some C compilers give bogus warnings about unused variables caml__dummy_xxx at each
use of CAMLparam and CAMLlocal. You should ignore them.
Chapter 22. Interfacing C with OCaml 417

Examples:

CAMLprim value my_external (value v1, value v2, value v3)

{
CAMLparam3 (v1, v2, v3);
...
CAMLreturn (Val_unit);
}

static void helper_procedure (value v1, value v2)

{
CAMLparam2 (v1, v2);
...
CAMLreturn0;
}

static int helper_function (value v1, value v2)

{
CAMLparam2 (v1, v2);
...
CAMLreturnT (int, 0);
}

Note: If your function is a primitive with more than 5 arguments for use with the byte-code
runtime, its arguments are not values and must not be declared (they have types value * and
int).

Warning: CAMLreturn0 should only be used for internal procedures that return void.
CAMLreturn(Val_unit) should be used for functions that return an OCaml unit value. Primitives
(C functions that can be called from OCaml) should never return void.

Rule 2 Local variables of type value must be declared with one of the CAMLlocal macros. Arrays of
values are declared with CAMLlocalN. These macros must be used at the beginning of the function,
not in a nested block. Temporaries are equivalent to local variables, but they cannot be registered
with the garbage collector, so a temporary of type value must not be live when a garbage collection
may occur.

The macros CAMLlocal1 to CAMLlocal5 declare and initialize one to five local variables of type
value. The variable names are given as arguments to the macros. CAMLlocalN(x, n) declares and
initializes a local variable of type value [n]. You can use several calls to these macros if you have
more than 5 local variables.
Example:

CAMLprim value bar (value v1, value v2, value v3)

{
418

CAMLparam3 (v1, v2, v3);

CAMLlocal1 (result);
result = caml_alloc (3, 0);
...
CAMLreturn (result);
}

Warning: CAMLlocal (and CAMLxparam) can only be called after CAMLparam. If a function
declares local values but takes no value argument, it should start with CAMLparam0 ().

static value foo (int n)

{
CAMLparam0 ();;
CAMLlocal (result);
...
CAMLreturn (result);
}

Rule 3 Assignments to the fields of structured blocks must be done with the Store_field
macro (for normal blocks), Store_double_array_field macro (for float array values)
or Store_double_flat_field (for floatarray values and records of floating-point num-
bers). Other assignments must not use Store_field, Store_double_array_field nor
Store_double_flat_field.

Store_field (b, n, v) stores the value v in the field number n of value b, which must be a
block (i.e. Is_block(b) must be true).
Example:

CAMLprim value bar (value v1, value v2, value v3)

{
CAMLparam3 (v1, v2, v3);
CAMLlocal1 (result);
result = caml_alloc (3, 0);
Store_field (result, 0, v1);
Store_field (result, 1, v2);
Store_field (result, 2, v3);
CAMLreturn (result);
}

Warning: The first argument of Store_field and Store_double_field must be a variable

declared by CAMLparam* or a parameter declared by CAMLlocal* to ensure that a garbage collection
triggered by the evaluation of the other arguments will not invalidate the first argument after it is
computed.
Chapter 22. Interfacing C with OCaml 419

Use with CAMLlocalN: Arrays of values declared using CAMLlocalN must not be written to
using Store_field. Use the normal C array syntax instead.

Rule 4 Global variables containing values must be registered with the garbage collector using the
caml_register_global_root function, save that global variables and locations that will only ever
contain OCaml integers (and never pointers) do not have to be registered.
The same is true for any memory location outside the OCaml heap that contains a value and is
not guaranteed to be reachable—for as long as it contains such value—from either another registered
global variable or location, local variable declared with CAMLlocal or function parameter declared
with CAMLparam.

Registration of a global variable v is achieved by calling caml_register_global_root(&v) just

before or just after a valid value is stored in v for the first time; likewise, registration of an arbitrary
location p is achieved by calling caml_register_global_root(p).
You must not call any of the OCaml runtime functions or macros between registering and storing
the value. Neither must you store anything in the variable v (likewise, the location p) that is not a
valid value.
The registration causes the contents of the variable or memory location to be updated by the
garbage collector whenever the value in such variable or location is moved within the OCaml heap.
In the presence of threads care must be taken to ensure appropriate synchronisation with the OCaml
runtime to avoid a race condition against the garbage collector when reading or writing the value.
(See section 22.12.2.)
A registered global variable v can be un-registered by calling caml_remove_global_root(&v).
If the contents of the global variable v are seldom modified after registration, better performance
can be achieved by calling caml_register_generational_global_root(&v) to register v (after
its initialization with a valid value, but before any allocation or call to the GC functions), and
caml_remove_generational_global_root(&v) to un-register it. In this case, you must not modify
the value of v directly, but you must use caml_modify_generational_global_root(&v,x) to set
it to x. The garbage collector takes advantage of the guarantee that v is not modified between calls
to caml_modify_generational_global_root to scan it less often. This improves performance if
the modifications of v happen less often than minor collections.

Note: The CAML macros use identifiers (local variables, type identifiers, structure tags) that start
with caml__. Do not use any identifier starting with caml__ in your programs.

22.5.2 Low-level interface

We now give the GC rules corresponding to the low-level allocation functions caml_alloc_small
and caml_alloc_shr. You can ignore those rules if you stick to the simplified allocation function
caml_alloc.

Rule 5 After a structured block (a block with tag less than No_scan_tag) is allocated with the
low-level functions, all fields of this block must be filled with well-formed values before the next
allocation operation. If the block has been allocated with caml_alloc_small, filling is performed by
direct assignment to the fields of the block:
420

Field(v, n) = vn ;

If the block has been allocated with caml_alloc_shr, filling is performed through the
caml_initialize function:

caml_initialize(&Field(v, n), vn );

The next allocation can trigger a garbage collection. The garbage collector assumes that all
structured blocks contain well-formed values. Newly created blocks contain random data, which
generally do not represent well-formed values.
If you really need to allocate before the fields can receive their final value, first initialize with a
constant value (e.g. Val_unit), then allocate, then modify the fields with the correct value (see
rule 6).

Rule 6 Direct assignment to a field of a block, as in

Field(v, n) = w;

is safe only if v is a block newly allocated by caml_alloc_small; that is, if no allocation took
place between the allocation of v and the assignment to the field. In all other cases, never assign
directly. If the block has just been allocated by caml_alloc_shr, use caml_initialize to assign a
value to a field for the first time:

caml_initialize(&Field(v, n), w);

Otherwise, you are updating a field that previously contained a well-formed value; then, call the
caml_modify function:

caml_modify(&Field(v, n), w);

To illustrate the rules above, here is a C function that builds and returns a list containing the
two integers given as parameters. First, we write it using the simplified allocation functions:

value alloc_list_int(int i1, int i2)

{
CAMLparam0 ();
CAMLlocal2 (result, r);

r = caml_alloc(2, 0); /* Allocate a cons cell */

Store_field(r, 0, Val_int(i2)); /* car = the integer i2 */
Store_field(r, 1, Val_emptylist); /* cdr = the empty list [] */
result = caml_alloc(2, 0); /* Allocate the other cons cell */
Store_field(result, 0, Val_int(i1)); /* car = the integer i1 */
Store_field(result, 1, r); /* cdr = the first cons cell */
CAMLreturn (result);
}
Chapter 22. Interfacing C with OCaml 421

Here, the registering of result is not strictly needed, because no allocation takes place after it
gets its value, but it’s easier and safer to simply register all the local variables that have type value.
Here is the same function written using the low-level allocation functions. We notice that
the cons cells are small blocks and can be allocated with caml_alloc_small, and filled by direct
assignments on their fields.

value alloc_list_int(int i1, int i2)

{
CAMLparam0 ();
CAMLlocal2 (result, r);

r = caml_alloc_small(2, 0); /* Allocate a cons cell */

Field(r, 0) = Val_int(i2); /* car = the integer i2 */
Field(r, 1) = Val_emptylist; /* cdr = the empty list [] */
result = caml_alloc_small(2, 0); /* Allocate the other cons cell */
Field(result, 0) = Val_int(i1); /* car = the integer i1 */
Field(result, 1) = r; /* cdr = the first cons cell */
CAMLreturn (result);
}

In the two examples above, the list is built bottom-up. Here is an alternate way, that proceeds
top-down. It is less efficient, but illustrates the use of caml_modify.

value alloc_list_int(int i1, int i2)

{
CAMLparam0 ();
CAMLlocal2 (tail, r);

r = caml_alloc_small(2, 0); /* Allocate a cons cell */

Field(r, 0) = Val_int(i1); /* car = the integer i1 */
Field(r, 1) = Val_int(0); /* A dummy value
tail = caml_alloc_small(2, 0); /* Allocate the other cons cell */
Field(tail, 0) = Val_int(i2); /* car = the integer i2 */
Field(tail, 1) = Val_emptylist; /* cdr = the empty list [] */
caml_modify(&Field(r, 1), tail); /* cdr of the result = tail */
CAMLreturn (r);
}

It would be incorrect to perform Field(r, 1) = tail directly, because the allocation of tail
has taken place since r was allocated.

22.5.3 Pending actions and asynchronous exceptions

Since 4.10, allocation functions are guaranteed not to run any OCaml code, including finalisers,
signal handlers and other threads running on the same domain. Instead, their execution is delayed
to a later safe point.
422

The function caml_process_pending_actions from <caml/signals.h> executes any pending

signal handlers and finalisers, Memprof callbacks, preemptive systhread switching, and requested
minor and major garbage collections. In particular, it can raise asynchronous exceptions and cause
mutations on the OCaml heap from the same domain. It is recommended to call it regularly at safe
points inside long-running non-blocking C code.
The function caml_process_pending_actions_res returns the exception instead of raising it
directly into OCaml code. This is represented by a different C type, caml_result rather than
value. The result can be tested using caml_result_is_exception, followed by some cleanup logic,
and finally caml_get_value_or_raise which returns the value (here just a unit value, ignored) or
raises the exception.

CAMLlocalresult(res);
...
res = caml_process_pending_actions_res();
if(caml_result_is_exception(res)) {
...cleanup...
(void)caml_get_value_or_raise(res);
}

For more details on caml_result, see section 22.7.2.

22.6 A complete example

This section outlines how the functions from the Unix curses library can be made available to
OCaml programs. First of all, here is the interface curses.ml that declares the curses primitives
and data types:
(* File curses.ml -- declaration of primitives and data types *)
type window (* The type "window" remains abstract *)
external initscr: unit -> window = "caml_curses_initscr"
external endwin: unit -> unit = "caml_curses_endwin"
external refresh: unit -> unit = "caml_curses_refresh"
external wrefresh : window -> unit = "caml_curses_wrefresh"
external newwin: int -> int -> int -> int -> window = "caml_curses_newwin"
external addch: char -> unit = "caml_curses_addch"
external mvwaddch: window -> int -> int -> char -> unit = "caml_curses_mvwaddch"
external addstr: string -> unit = "caml_curses_addstr"
external mvwaddstr: window -> int -> int -> string -> unit
= "caml_curses_mvwaddstr"
(* lots more omitted *)
To compile this interface:
ocamlc -c curses.ml
To implement these functions, we just have to provide the stub code; the core functions are
already implemented in the curses library. The stub code file, curses_stubs.c, looks like this:
Chapter 22. Interfacing C with OCaml 423

/* File curses_stubs.c -- stub code for curses */

#include <curses.h>
#include <caml/mlvalues.h>
#include <caml/memory.h>
#include <caml/alloc.h>
#include <caml/custom.h>

/* Encapsulation of opaque window handles (of type WINDOW *)

as OCaml custom blocks. */

static struct custom_operations curses_window_ops = {

"fr.inria.caml.curses_windows",
custom_finalize_default,
custom_compare_default,
custom_hash_default,
custom_serialize_default,
custom_deserialize_default,
custom_compare_ext_default,
custom_fixed_length_default
};

/* Accessing the WINDOW * part of an OCaml custom block */

#define Window_val(v) (*((WINDOW **) Data_custom_val(v)))

/* Allocating an OCaml custom block to hold the given WINDOW * */

static value alloc_window(WINDOW * w)
{
value v = caml_alloc_custom(&curses_window_ops, sizeof(WINDOW *), 0, 1);
Window_val(v) = w;
return v;
}

CAMLprim value caml_curses_initscr(value unit)

{
CAMLparam1 (unit);
CAMLreturn (alloc_window(initscr()));
}

CAMLprim value caml_curses_endwin(value unit)

{
CAMLparam1 (unit);
endwin();
CAMLreturn (Val_unit);
}
424

CAMLprim value caml_curses_refresh(value unit)

{
CAMLparam1 (unit);
refresh();
CAMLreturn (Val_unit);
}

CAMLprim value caml_curses_wrefresh(value win)

{
CAMLparam1 (win);
wrefresh(Window_val(win));
CAMLreturn (Val_unit);
}

CAMLprim value caml_curses_newwin(value nlines, value ncols, value x0, value y0)
{
CAMLparam4 (nlines, ncols, x0, y0);
CAMLreturn (alloc_window(newwin(Int_val(nlines), Int_val(ncols),
Int_val(x0), Int_val(y0))));
}

CAMLprim value caml_curses_addch(value c)

{
CAMLparam1 (c);
addch(Int_val(c)); /* Characters are encoded like integers */
CAMLreturn (Val_unit);
}

CAMLprim value caml_curses_mvwaddch(value win, value x, value y, value c)

{
CAMLparam4 (win, x, y, c);
mvwaddch(Window_val(win), Int_val(x), Int_val(y), Int_val(c));
CAMLreturn (Val_unit);
}

CAMLprim value caml_curses_addstr(value s)

{
CAMLparam1 (s);
addstr(String_val(s));
CAMLreturn (Val_unit);
}

CAMLprim value caml_curses_mvwaddstr(value win, value x, value y, value s)

{
CAMLparam4 (win, x, y, s);
Chapter 22. Interfacing C with OCaml 425

mvwaddstr(Window_val(win), Int_val(x), Int_val(y), String_val(s));

CAMLreturn (Val_unit);
}

/* This goes on for pages. */

The file curses_stubs.c can be compiled with:

cc -c -I`ocamlc -where` curses_stubs.c

or, even simpler,

ocamlc -c curses_stubs.c

(When passed a .c file, the ocamlc command simply calls the C compiler on that file, with the
right -I option.)
Now, here is a sample OCaml program prog.ml that uses the curses module:

(* File prog.ml -- main program using curses *)

open Curses;;
let main_window = initscr () in
let small_window = newwin 10 5 20 10 in
mvwaddstr main_window 10 2 "Hello";
mvwaddstr small_window 4 3 "world";
refresh();
Unix.sleep 5;
endwin()

To compile and link this program, run:

ocamlc -custom -o prog unix.cma curses.cmo prog.ml curses_stubs.o -cclib -

lcurses

(On some machines, you may need to put -cclib -lcurses -cclib -ltermcap or
-cclib -ltermcap instead of -cclib -lcurses.)

22.7 Advanced topic: callbacks from C to OCaml

So far, we have described how to call C functions from OCaml. In this section, we show how C
functions can call OCaml functions, either as callbacks (OCaml calls C which calls OCaml), or with
the main program written in C.

22.7.1 Applying OCaml closures from C

C functions can apply OCaml function values (closures) to OCaml values. The following functions
are provided to perform the applications:

• caml_callback(f, a) applies the functional value f to the value a and returns the value
returned by f.
426

• caml_callback2(f, a, b) applies the functional value f (which is assumed to be a curried

OCaml function with two arguments) to a and b.

• caml_callback3(f, a, b, c) applies the functional value f (a curried OCaml function with

three arguments) to a, b and c.

• caml_callbackN(f, n, args) applies the functional value f to the n arguments contained in

the C array of values args.

If the function f does not return, but raises an exception that escapes the scope of the application,
then this exception is propagated to the next enclosing OCaml code, skipping over the C code. That
is, if an OCaml function f calls a C function g that calls back an OCaml function h that raises a
stray exception, then the execution of g is interrupted and the exception is propagated back into f.

22.7.2 caml_result: resource cleanup on OCaml exceptions

If the OCaml function called with caml_callback (or caml_process_pending_actions as men-
tioned in Section 22.5.3) raises an exception, the C caller will be interrupted immediately to return
to the closest OCaml exception handler. This is often the wrong behavior if the C caller needs to
run some resource cleanup logic to terminate safely.
The OCaml FFI provides an alternative API to call OCaml code from C that returns a value
of type caml_result instead of value. A C value of type caml_result is either an OCaml value
returned correctly by the OCaml function, or an OCaml exception raised by the OCaml function.
The C caller can run any cleanup logic before re-raising the exception (if any) or continuing the
computation.
caml_result values can be manipulated using the following functions and macros:

• value caml_get_value_or_raise(caml_result \var{res}) (in fail.h) returns

the value contained in res or reraises the exception it contains. In particular,
(void)caml_get_value_or_raise(res) can be used to ignore an OCaml result of type
unit, yet propagate exceptions to the caller.

• Result_value(value \var{v}) (in mlvalues.h) is the result that represents returning the
value v.

• Result_exception(value \var{exn}) (in mlvalues.h) is the result that represents raising

the exception exn.

• int caml_result_is_exception(caml_result \var{res}) (in mlvalues.h) is true if res

represents an exception.

• The macro CAMLlocalresult(foo) (in memory.h) is the caml_result counterpart of

CAMLlocal1: it declares a local variable of type caml_result, whose content is tracked by
the OCaml GC. Just like CAMLlocal1, it can only be used between a CAMLparam macro and
CAMLreturn or CAMLreturnT macros. There is no equivalent of CAMLlocal2, CAMLlocal3,
etc., but the macro can be used several times.
Chapter 22. Interfacing C with OCaml 427

For convenience, Result_unit is defined as Result_value(Val_unit).

By convention, result-returning C functions have their name suffixed with _res, for example
caml_callback2_res and caml_process_pending_actions_res.
Some examples:
#include <caml/mlvalues.h> // for the caml_result type, caml_result_is_exception
#include <caml/memory.h> // CAMLlocalresult
#include <caml/callback.h> // caml_callback_res
#include <caml/fail.h> // caml_get_value_or_raise
#include <caml/alloc.h> // caml_alloc_2

/* This function calls an OCaml callback,

and returns the value or reraises the exception. */
value logging_callback(value f, value arg) {
CAMLparam2(f, arg);
CAMLlocalresult(res);
printf("Start callback.\n");
res = caml_callback_res(f, arg);
printf("End callback.\n");
CAMLreturn (caml_get_value_or_raise(res));
}

/* This function assumes that the callback returns

a 'unit' value and ignores it -- or reraises
its exception. It must be called by C, as it
does not return a value, but the caller cannot
handle exceptions as they are raised directly. */
void logging_unit_callback(value f, value arg) {
CAMLparam2(f, arg);
CAMLlocalresult(res);
printf("Start unit callback.\n");
res = caml_callback_res(f, arg);
printf("End callback.\n");
(void)caml_get_value_or_raise(res);
CAMLreturn0;
}

/* This function calls a callback on two arguments in turn,

and returns the pair of values;
it does not reraise exceptions, but returns a caml_result instead, letting
its own C caller perform its own cleanup. */
caml_result two_callbacks_res(value f, value arg1, value arg2) {
CAMLparam3(f, arg1, arg2);
CAMLlocalresult(res);
CAMLlocal3(v1, v2, pair);
428

res = caml_callback_res(f, arg1);

// early exit on exception
if (caml_result_is_exception(res))
CAMLreturnT(caml_result, res);
v1 = caml_get_value_or_raise(res);

res = caml_callback_res(f, arg2);

if (caml_result_is_exception(res))
CAMLreturnT(caml_result, res);
v2 = caml_get_value_or_raise(res);

// build the pair of values, and

// wrap it in a caml_result
pair = caml_alloc_2(0, v1, v2);
res = Result_value(pair);
CAMLreturnT(caml_result, res);
}

Compatibility: The caml_result type is available since OCaml 5.3. Older versions of OCaml
use an unsafe concept of encoded exceptions (suffix _exn, operations Is_exception_result and
Extract_exception) which are of type value but are not valid OCaml values and can crash the
GC if they are not extracted immediately by the caller. We strongly recommend using caml_result
instead, to have a clear separation between valid values and reified exceptions at distinct C types,
but the older approach remains available for backwards-compatibility.

22.7.3 Obtaining or registering OCaml closures for use in C functions

There are two ways to obtain OCaml function values (closures) to be passed to the callback
functions described above. One way is to pass the OCaml function as an argument to a primitive
function. For example, if the OCaml code contains the declaration
external apply : ('a -> 'b) -> 'a -> 'b = "caml_apply"
the corresponding C stub can be written as follows:
CAMLprim value caml_apply(value vf, value vx)
{
CAMLparam2(vf, vx);
CAMLlocal1(vy);
vy = caml_callback(vf, vx);
CAMLreturn(vy);
}
Another possibility is to use the registration mechanism provided by OCaml. This registration
mechanism enables OCaml code to register OCaml functions under some global name, and C code
to retrieve the corresponding closure by this global name.
On the OCaml side, registration is performed by evaluating Callback.register n v. Here, n is
the global name (an arbitrary string) and v the OCaml value. For instance:
Chapter 22. Interfacing C with OCaml 429

let f x = print_string "f is applied to "; print_int x; print_newline()

let _ = Callback.register "test function" f

On the C side, a pointer to the value registered under name n is obtained by calling
caml_named_value(n). The returned pointer must then be dereferenced to recover the actual
OCaml value. If no value is registered under the name n, the null pointer is returned. For example,
here is a C wrapper that calls the OCaml function f above:

void call_caml_f(int arg)

{
caml_callback(*caml_named_value("test function"), Val_int(arg));
}

The pointer returned by caml_named_value is constant and can safely be cached in a C variable
to avoid repeated name lookups. The value pointed to cannot be changed from C. However, it might
change during garbage collection, so must always be recomputed at the point of use. Here is a more
efficient variant of call_caml_f above that calls caml_named_value only once:

void call_caml_f(int arg)

{
static const value * closure_f = NULL;
if (closure_f == NULL) {
/* First time around, look up by name */
closure_f = caml_named_value("test function");
}
caml_callback(*closure_f, Val_int(arg));
}

22.7.4 Registering OCaml exceptions for use in C functions

The registration mechanism described above can also be used to communicate exception con-
structors from OCaml to C. The OCaml code registers the exception constructor by evaluating
Callback.register_exception n exn, where n is an arbitrary name and exn is an exception value
with the exception constructor to register. For example:

exception Error of string

let _ = Callback.register_exception "test exception" (Error "any string")

The C code can then recover the exception constructor using caml_named_value and pass it
as first argument to the functions raise_constant, raise_with_arg, and raise_with_string
(described in section 22.4.7) to actually raise the exception. For example, here is a C function that
raises the Error exception with the given argument:

void raise_error(char * msg)

{
caml_raise_with_string(*caml_named_value("test exception"), msg);
}
430

22.7.5 Main program in C

In normal operation, a mixed OCaml/C program starts by executing the OCaml initialization code,
which then may proceed to call C functions. We say that the main program is the OCaml code. In
some applications, it is desirable that the C code plays the role of the main program, calling OCaml
functions when needed. This can be achieved as follows:
• The C part of the program must provide a main function, which will override the default main
function provided by the OCaml runtime system. Execution will start in the user-defined
main function just like for a regular C program.
• At some point, the C code must call caml_main(argv) to initialize the OCaml code. The
argv argument is a C array of strings (type char **), terminated with a NULL pointer, which
represents the command-line arguments, as passed as second argument to main. The OCaml
array Sys.argv will be initialized from this parameter. For the bytecode compiler, argv[0]
and argv[1] are also consulted to find the file containing the bytecode.
• The call to caml_main initializes the OCaml runtime system, loads the bytecode (in the
case of the bytecode compiler), and executes the initialization code of the OCaml program.
Typically, this initialization code registers callback functions using Callback.register. Once
the OCaml initialization code is complete, control returns to the C code that called caml_main.
• The C code can then invoke OCaml functions using the callback mechanism (see section 22.7.1).

22.7.6 Embedding the OCaml code in the C code

The bytecode compiler in custom runtime mode (ocamlc -custom) normally appends the bytecode
to the executable file containing the custom runtime. This has two consequences. First, the final
linking step must be performed by ocamlc. Second, the OCaml runtime library must be able to find
the name of the executable file from the command-line arguments. When using caml_main(argv)
as in section 22.7.5, this means that argv[0] or argv[1] must contain the executable file name.
An alternative is to embed the bytecode in the C code. The -output-obj and
-output-complete-obj options to ocamlc are provided for this purpose. They cause the ocamlc
compiler to output a C object file (.o file, .obj under Windows) containing the bytecode for the
OCaml part of the program, as well as a caml_startup function. The C object file produced by
ocamlc -output-complete-obj also contains the runtime and autolink libraries. The C object file
produced by ocamlc -output-obj or ocamlc -output-complete-obj can then be linked with C
code using the standard C compiler, or stored in a C library.
The caml_startup function must be called from the main C program in order to initialize the
OCaml runtime and execute the OCaml initialization code. Just like caml_main, it takes one argv
parameter containing the command-line parameters. Unlike caml_main, this argv parameter is
used only to initialize Sys.argv, but not for finding the name of the executable file.
The caml_startup function calls the uncaught exception handler (or enters the debugger, if
running under ocamldebug) if an exception escapes from a top-level module initialiser. Such
exceptions may be caught in the C code by instead using the caml_startup_exn function and
testing the result using Is_exception_result (followed by Extract_exception if appropriate).
The -output-obj and -output-complete-obj options can also be used to obtain the C
source file. More interestingly, these options can also produce directly a shared library (.so
Chapter 22. Interfacing C with OCaml 431

file, .dll under Windows) that contains the OCaml code, the OCaml runtime system and any other
static C code given to ocamlc (.o, .a, respectively, .obj, .lib). This use of -output-obj and
-output-complete-obj is very similar to a normal linking step, but instead of producing a main
program that automatically runs the OCaml code, it produces a shared library that can run the
OCaml code on demand. The three possible behaviors of -output-obj and -output-complete-obj
(to produce a C source code .c, a C object file .o, a shared library .so), are selected according to
the extension of the resulting file (given with -o).
The native-code compiler ocamlopt also supports the -output-obj and -output-complete-obj
options, causing it to output a C object file or a shared library containing the native code for all
OCaml modules on the command-line, as well as the OCaml startup code. Initialization is performed
by calling caml_startup (or caml_startup_exn) as in the case of the bytecode compiler. The file
produced by ocamlopt -output-complete-obj also contains the runtime and autolink libraries.
For the final linking phase, in addition to the object file produced by -output-obj, you will have
to provide the OCaml runtime library (libcamlrun.a for bytecode, libasmrun.a for native-code),
as well as all C libraries that are required by the OCaml libraries used. For instance, assume the
OCaml part of your program uses the Unix library. With ocamlc, you should do:

ocamlc -output-obj -o camlcode.o unix.cma other .cmo and .cma files

cc -o myprog C objects and libraries \
camlcode.o -L`ocamlc -where` -lunix -lcamlrun

With ocamlopt, you should do:

ocamlopt -output-obj -o camlcode.o unix.cmxa other .cmx and .cmxa files

cc -o myprog C objects and libraries \
camlcode.o -L`ocamlc -where` -lunix -lasmrun

For the final linking phase, in addition to the object file produced by -output-complete-obj,
you will have only to provide the C libraries required by the OCaml runtime.
For instance, assume the OCaml part of your program uses the Unix library. With ocamlc, you
should do:

ocamlc -output-complete-obj -o camlcode.o unix.cma other .cmo and .cma files

cc -o myprog C objects and libraries \
camlcode.o C libraries required by the runtime, eg -lm -ldl -lcurses -lpthread

With ocamlopt, you should do:

ocamlopt -output-complete-obj -o camlcode.o unix.cmxa other .cmx and .cmxa files

cc -o myprog C objects and libraries \
camlcode.o C libraries required by the runtime, eg -lm -ldl

Warning: On some ports, special options are required on the final linking phase that links
together the object file produced by the -output-obj and -output-complete-obj options and the
remainder of the program. Those options are shown in the configuration file Makefile.config
generated during compilation of OCaml, as the variable OC_LDFLAGS.
432

• Windows with the MSVC compiler: the object file produced by OCaml have been compiled
with the /MD flag, and therefore all other object files linked with it should also be compiled
with /MD.

• other systems: you may have to add one or both of -lm and -ldl, depending on your OS and
C compiler.

Stack backtraces. When OCaml bytecode produced by ocamlc -g is embedded in a C program,

no debugging information is included, and therefore it is impossible to print stack backtraces
on uncaught exceptions. This is not the case when native code produced by ocamlopt -g is
embedded in a C program: stack backtrace information is available, but the backtrace mechanism
needs to be turned on programmatically. This can be achieved from the OCaml side by calling
Printexc.record_backtrace true in the initialization of one of the OCaml modules. This can
also be achieved from the C side by calling caml_record_backtraces(1); in the OCaml-C glue
code. (caml_record_backtraces is declared in backtrace.h)

Unloading the runtime.

In case the shared library produced with -output-obj is to be loaded and unloaded repeatedly
by a single process, care must be taken to unload the OCaml runtime explicitly, in order to avoid
various system resource leaks.
Since 4.05, caml_shutdown function can be used to shut the runtime down gracefully, which
equals the following:

• Running the functions that were registered with Stdlib.at_exit.

• Triggering finalization of allocated custom blocks (see section 22.9). For example,
Stdlib.in_channel and Stdlib.out_channel are represented by custom blocks that enclose
file descriptors, which are to be released.

• Unloading the dependent shared libraries that were loaded by the runtime, including dynlink
plugins.

• Freeing the memory blocks that were allocated by the runtime with malloc. Inside C
primitives, it is advised to use caml_stat_* functions from memory.h for managing static
(that is, non-moving) blocks of heap memory, as all the blocks allocated with these functions
are automatically freed by caml_shutdown. For ensuring compatibility with legacy C stubs
that have used caml_stat_* incorrectly, this behaviour is only enabled if the runtime is
started with a specialized caml_startup_pooled function.

As a shared library may have several clients simultaneously, it is made for convenience that
caml_startup (and caml_startup_pooled) may be called multiple times, given that each such call
is paired with a corresponding call to caml_shutdown (in a nested fashion). The runtime will be
unloaded once there are no outstanding calls to caml_startup.
Once a runtime is unloaded, it cannot be started up again without reloading the shared library
and reinitializing its static data. Therefore, at the moment, the facility is only useful for building
reloadable shared libraries.
Chapter 22. Interfacing C with OCaml 433

Unix signal handling. Depending on the target platform and operating system, the native-
code runtime system may install signal handlers for one or several of the SIGSEGV, SIGTRAP and
SIGFPE signals when caml_startup is called, and reset these signals to their default behaviors when
caml_shutdown is called. The main program written in C should not try to handle these signals
itself.

22.8 Advanced example with callbacks

This section illustrates the callback facilities described in section 22.7. We are going to package
some OCaml functions in such a way that they can be linked with C code and called from C just
like any C functions. The OCaml functions are defined in the following mod.ml OCaml source:

(* File mod.ml -- some "useful" OCaml functions *)

let rec fib n = if n < 2 then 1 else fib(n-1) + fib(n-2)

let format_result n = Printf.sprintf "Result is: %d\n" n

(* Export those two functions to C *)

let _ = Callback.register "fib" fib

let _ = Callback.register "format_result" format_result

Here is the C stub code for calling these functions from C:

/* File modwrap.c -- wrappers around the OCaml functions */

#include <stdio.h>
#include <string.h>
#include <caml/mlvalues.h>
#include <caml/callback.h>

int fib(int n)
{
static const value * fib_closure = NULL;
if (fib_closure == NULL) fib_closure = caml_named_value("fib");
return Int_val(caml_callback(*fib_closure, Val_int(n)));
}

char * format_result(int n)
{
static const value * format_result_closure = NULL;
if (format_result_closure == NULL)
format_result_closure = caml_named_value("format_result");
return strdup(String_val(caml_callback(*format_result_closure, Val_int(n))));
434

/* We copy the C string returned by String_val to the C heap

so that it remains valid after garbage collection. */
}

We now compile the OCaml code to a C object file and put it in a C library along with the stub
code in modwrap.c and the OCaml runtime system:

ocamlc -custom -output-obj -o modcaml.o mod.ml

ocamlc -c modwrap.c
cp `ocamlc -where`/libcamlrun.a mod.a && chmod +w mod.a
ar r mod.a modcaml.o modwrap.o

(One can also use ocamlopt -output-obj instead of ocamlc -custom -output-obj. In this
case, replace libcamlrun.a (the bytecode runtime library) by libasmrun.a (the native-code runtime
library).)
Now, we can use the two functions fib and format_result in any C program, just like regular
C functions. Just remember to call caml_startup (or caml_startup_exn) once before.

/* File main.c -- a sample client for the OCaml functions */

#include <stdio.h>
#include <caml/callback.h>

extern int fib(int n);

extern char * format_result(int n);

int main(int argc, char ** argv)

{
int result;

/* Initialize OCaml code */

caml_startup(argv);
/* Do some computation */
result = fib(10);
printf("fib(10) = %s\n", format_result(result));
return 0;
}

To build the whole program, just invoke the C compiler as follows:

cc -o prog -I `ocamlc -where` main.c mod.a -lcurses

(On some machines, you may need to put -ltermcap or -lcurses -ltermcap instead of
-lcurses.)
Chapter 22. Interfacing C with OCaml 435

22.9 Advanced topic: custom blocks

Blocks with tag Custom_tag start with a pointer to a C struct with type struct custom_operations
that associates user-provided finalization, comparison, hashing, serialization and deserialization
functions for this block.
After this pointer to custom operations, there can be arbitrary data of zero, one or more words
whose semantics is defined by the user (for example this can be just a pointer to a C struct or
a C struct itself). Use Data_custom_val on the block value to get a pointer on the start of this
arbitrary data.

22.9.1 The struct custom_operations

The struct custom_operations is defined in <caml/custom.h> and contains the following fields:
• char *identifier
A zero-terminated character string serving as an identifier for serialization and deserialization
operations.
• void (*finalize)(value v)
The finalize field contains a pointer to a C function that is called when the block becomes
unreachable and is about to be reclaimed. The block is passed as first argument to the
function. The finalize field can also be custom_finalize_default to indicate that no
finalization function is associated with the block. Note the caution below: there are many
restrictions on the behaviour of these custom block finalizers; they must not allocate on
the Caml heap, or call OCaml code or allow it to be called (for example, they should not
call caml_release_runtime_system()). For more powerful and flexible finalization, use
Gc.finalise (see Gc[29.24]).
• int (*compare)(value v1, value v2)
The compare field contains a pointer to a C function that is called whenever two custom blocks
are compared using OCaml’s generic comparison operators (=, <>, <=, >=, <, > and compare).
The C function should return 0 if the data contained in the two blocks are structurally equal,
a negative integer if the data from the first block is less than the data from the second block,
and a positive integer if the data from the first block is greater than the data from the second
block.
The compare field can be set to custom_compare_default; this default comparison function
simply raises Failure.
• int (*compare_ext)(value v1, value v2)
(Since 3.12.1) The compare_ext field contains a pointer to a C function that is called whenever
one custom block and one unboxed integer are compared using OCaml’s generic comparison
operators (=, <>, <=, >=, <, > and compare). As in the case of the compare field, the C function
should return 0 if the two arguments are structurally equal, a negative integer if the first
argument compares less than the second argument, and a positive integer if the first argument
compares greater than the second argument.
The compare_ext field can be set to custom_compare_ext_default; this default comparison
function simply raises Failure.
436

• intnat (*hash)(value v)
The hash field contains a pointer to a C function that is called whenever OCaml’s generic
hash operator (see module Hashtbl[29.25]) is applied to a custom block. The C function can
return an arbitrary integer representing the hash value of the data contained in the given
custom block. The hash value must be compatible with the compare function, in the sense
that two structurally equal data (that is, two custom blocks for which compare returns 0)
must have the same hash value.
The hash field can be set to custom_hash_default, in which case the custom block is ignored
during hash computation.

• void (*serialize)(value v, uintnat * bsize_32, uintnat * bsize_64)

The serialize field contains a pointer to a C function that is called whenever the cus-
tom block needs to be serialized (marshaled) using the OCaml functions output_value or
Marshal.to_.... For a custom block, those functions first write the identifier of the block (as
given by the identifier field) to the output stream, then call the user-provided serialize
function. That function is responsible for writing the data contained in the custom block, using
the serialize_... functions defined in <caml/intext.h> and listed in section 22.9.4. The
user-provided serialize function must then store in its bsize_32 and bsize_64 parameters
the sizes in bytes of the data part of the custom block on a 32-bit architecture and on a 64-bit
architecture, respectively.
The serialize field can be set to custom_serialize_default, in which case the Failure
exception is raised when attempting to serialize the custom block.

• uintnat (*deserialize)(void * dst)

The deserialize field contains a pointer to a C function that is called whenever a custom
block with identifier identifier needs to be deserialized (un-marshaled) using the OCaml
functions input_value or Marshal.from_.... This user-provided function is responsible
for reading back the data written by the serialize operation, using the deserialize_...
functions defined in <caml/intext.h> and listed in section 22.9.4. It must then rebuild the
data part of the custom block and store it at the pointer given as the dst argument. Finally,
it returns the size in bytes of the data part of the custom block. This size must be identical to
the bsize_32 result of the serialize operation if the architecture is 32 bits, or bsize_64 if
the architecture is 64 bits.
The deserialize field can be set to custom_deserialize_default to indicate that deserial-
ization is not supported. In this case, do not register the struct custom_operations with
the deserializer using register_custom_operations (see below).

• const struct custom_fixed_length* fixed_length

(Since 4.08.0) Normally, space in the serialized output is reserved to write the bsize_32 and
bsize_64 fields returned by serialize. However, for very short custom blocks, this space can
be larger than the data itself! As a space optimisation, if serialize always returns the same
values for bsize_32 and bsize_64, then these values may be specified in the fixed_length
structure, and do not consume space in the serialized output.
Caution: the finalize, compare, hash, serialize, and deserialize functions attached to
custom blocks descriptors are only allowed limited interactions with the OCaml runtime. Within
Chapter 22. Interfacing C with OCaml 437

these functions, do not call any of the OCaml allocation functions, and do not perform any callback
into OCaml code. Do not use CAMLparam to register the parameters to these functions, and do not use
CAMLreturn to return the result. Do not raise exceptions (to signal an error during deserialization,
use caml_deserialize_error). Do not remove global roots. When in doubt, err on the side of
caution. Within serialize and deserialize functions, use of the corresponding functions from
section 22.9.4 is allowed (and even recommended).

22.9.2 Allocating custom blocks

Custom blocks must be allocated via caml_alloc_custom or caml_alloc_custom_mem:
caml_alloc_custom(ops, size, used, max)
returns a fresh custom block, with room for size bytes of user data, and whose associated operations
are given by ops (a pointer to a struct custom_operations, usually statically allocated as a C
global variable).
The two parameters used and max are used to control the speed of garbage collection when
the finalized object contains pointers to out-of-heap resources. Generally speaking, the OCaml
incremental major collector adjusts its speed relative to the allocation rate of the program. The
faster the program allocates, the harder the GC works in order to reclaim quickly unreachable
blocks and avoid having large amount of “floating garbage” (unreferenced objects that the GC has
not yet collected).
Normally, the allocation rate is measured by counting the in-heap size of allocated blocks.
However, it often happens that finalized objects contain pointers to out-of-heap memory blocks and
other resources (such as file descriptors, X Windows bitmaps, etc.). For those blocks, the in-heap
size of blocks is not a good measure of the quantity of resources allocated by the program.
The two arguments used and max give the GC an idea of how much out-of-heap resources are
consumed by the finalized block being allocated: you give the amount of resources allocated to this
object as parameter used, and the maximum amount that you want to see in floating garbage as
parameter max. The units are arbitrary: the GC cares only about the ratio used/max.
For instance, if you are allocating a finalized block holding an X Windows bitmap of w by h pixels,
and you’d rather not have more than 1 mega-pixels of unreclaimed bitmaps, specify used = w ∗ h
and max = 1000000.
Another way to describe the effect of the used and max parameters is in terms of full GC cycles.
If you allocate many custom blocks with used/max = 1/N , the GC will then do one full cycle
(examining every object in the heap and calling finalization functions on those that are unreachable)
every N allocations. For instance, if used = 1 and max = 1000, the GC will do one full cycle at
least every 1000 allocations of custom blocks.
If your finalized blocks contain no pointers to out-of-heap resources, or if the previous discussion
made little sense to you, just take used = 0 and max = 1. But if you later find that the finalization
functions are not called “often enough”, consider increasing the used/max ratio.

caml_alloc_custom_mem(ops, size, used)

Use this function when your custom block holds only out-of-heap memory (memory allocated with
malloc or caml_stat_alloc) and no other resources. used should be the number of bytes of out-
of-heap memory that are held by your custom block. This function works like caml_alloc_custom
438

except that the max parameter is under the control of the user (via the custom_major_ratio,
custom_minor_ratio, and custom_minor_max_size parameters) and proportional to the heap
sizes. It has been available since OCaml 4.08.0.

22.9.3 Accessing custom blocks

The data part of a custom block v can be accessed via the pointer Data_custom_val(v). This
pointer has type void * and should be cast to the actual type of the data stored in the custom
block.
The contents of custom blocks are not scanned by the garbage collector, and must therefore
not contain any pointer inside the OCaml heap. In other terms, never store an OCaml value in a
custom block, and do not use Field, Store_field nor caml_modify to access the data part of a
custom block. Conversely, any C data structure (not containing heap pointers) can be stored in a
custom block.

22.9.4 Writing custom serialization and deserialization functions

The following functions, defined in <caml/intext.h>, are provided to write and read back the
contents of custom blocks in a portable way. Those functions handle endianness conversions when
e.g. data is written on a little-endian machine and read back on a big-endian machine.
Chapter 22. Interfacing C with OCaml 439

Function Action
caml_serialize_int_1 Write a 1-byte integer
caml_serialize_int_2 Write a 2-byte integer
caml_serialize_int_4 Write a 4-byte integer
caml_serialize_int_8 Write a 8-byte integer
caml_serialize_float_4 Write a 4-byte float
caml_serialize_float_8 Write a 8-byte float
caml_serialize_block_1 Write an array of 1-byte quantities
caml_serialize_block_2 Write an array of 2-byte quantities
caml_serialize_block_4 Write an array of 4-byte quantities
caml_serialize_block_8 Write an array of 8-byte quantities
caml_deserialize_uint_1 Read an unsigned 1-byte integer
caml_deserialize_sint_1 Read a signed 1-byte integer
caml_deserialize_uint_2 Read an unsigned 2-byte integer
caml_deserialize_sint_2 Read a signed 2-byte integer
caml_deserialize_uint_4 Read an unsigned 4-byte integer
caml_deserialize_sint_4 Read a signed 4-byte integer
caml_deserialize_uint_8 Read an unsigned 8-byte integer
caml_deserialize_sint_8 Read a signed 8-byte integer
caml_deserialize_float_4 Read a 4-byte float
caml_deserialize_float_8 Read an 8-byte float
caml_deserialize_block_1 Read an array of 1-byte quantities
caml_deserialize_block_2 Read an array of 2-byte quantities
caml_deserialize_block_4 Read an array of 4-byte quantities
caml_deserialize_block_8 Read an array of 8-byte quantities
caml_deserialize_error Signal an error during deserialization; input_value or
Marshal.from_... raise a Failure exception after clean-
ing up their internal data structures

Serialization functions are attached to the custom blocks to which they apply. Obviously,
deserialization functions cannot be attached this way, since the custom block does not exist yet when
deserialization begins! Thus, the struct custom_operations that contain deserialization functions
must be registered with the deserializer in advance, using the register_custom_operations
function declared in <caml/custom.h>. Deserialization proceeds by reading the identifier off
the input stream, allocating a custom block of the size specified in the input stream, searching
the registered struct custom_operation blocks for one with the same identifier, and calling its
deserialize function to fill the data part of the custom block.

22.9.5 Choosing identifiers

Identifiers in struct custom_operations must be chosen carefully, since they must identify uniquely
the data structure for serialization and deserialization operations. In particular, consider including
a version number in the identifier; this way, the format of the data can be changed later, yet
backward-compatible deserialisation functions can be provided.
Identifiers starting with _ (an underscore character) are reserved for the OCaml run-
time system; do not use them for your custom data. We recommend to use a URL
440

(http://mymachine.mydomain.com/mylibrary/version-number) or a Java-style package name

(com.mydomain.mymachine.mylibrary.version-number) as identifiers, to minimize the risk of
identifier collision.

22.9.6 Finalized blocks

Custom blocks generalize the finalized blocks that were present in OCaml prior to version 3.00. For
backwards compatibility, the format of custom blocks is compatible with that of finalized blocks, and
the caml_alloc_final function is still available to allocate a custom block with a given finalization
function, but default comparison, hashing and serialization functions. (In particular, the finalization
function must not access the OCaml runtime.)
caml_alloc_final(n, f, used, max) returns a fresh custom block of size n+1 words, with
finalization function f. The first word is reserved for storing the custom operations; the other n
words are available for your data. The two parameters used and max are used to control the speed
of garbage collection, as described for caml_alloc_custom.

22.10 Advanced topic: Bigarrays and the OCaml-C interface

This section explains how C stub code that interfaces C or Fortran code with OCaml code can use
Bigarrays.

22.10.1 Include file

The include file <caml/bigarray.h> must be included in the C stub file. It declares the functions,
constants and macros discussed below.

22.10.2 Accessing an OCaml bigarray from C or Fortran

If v is a OCaml value representing a Bigarray, the expression Caml_ba_data_val(v) returns a
pointer to the data part of the array. This pointer is of type void * and can be cast to the
appropriate C type for the array (e.g. double [], char [][10], etc).
Various characteristics of the OCaml Bigarray can be consulted from C as follows:

C expression Returns
Caml_ba_array_val(v)->num_dims number of dimensions
Caml_ba_array_val(v)->dim[i] i-th dimension
Caml_ba_array_val(v)->flags & CAML_BA_KIND_MASK kind of array elements

The kind of array elements is one of the following constants:

Chapter 22. Interfacing C with OCaml 441

Constant Element kind

CAML_BA_FLOAT16 16-bit half-precision floats
CAML_BA_FLOAT32 32-bit single-precision floats
CAML_BA_FLOAT64 64-bit double-precision floats
CAML_BA_SINT8 8-bit signed integers
CAML_BA_UINT8 8-bit unsigned integers
CAML_BA_SINT16 16-bit signed integers
CAML_BA_UINT16 16-bit unsigned integers
CAML_BA_INT32 32-bit signed integers
CAML_BA_INT64 64-bit signed integers
CAML_BA_CAML_INT 31- or 63-bit signed integers
CAML_BA_NATIVE_INT 32- or 64-bit (platform-native) integers
CAML_BA_COMPLEX32 32-bit single-precision complex numbers
CAML_BA_COMPLEX64 64-bit double-precision complex numbers
CAML_BA_CHAR 8-bit characters

Warning: Caml_ba_array_val(v) must always be dereferenced immediately and not stored

anywhere, including local variables. It resolves to a derived pointer: it is not a valid OCaml value
but points to a memory region managed by the GC. For this reason this value must not be stored
in any memory location that could be live cross a GC.
The following example shows the passing of a two-dimensional Bigarray to a C function and a
Fortran function.

extern void my_c_function(double * data, int dimx, int dimy);

extern void my_fortran_function_(double * data, int * dimx, int * dimy);

CAMLprim value caml_stub(value bigarray)

{
int dimx = Caml_ba_array_val(bigarray)->dim[0];
int dimy = Caml_ba_array_val(bigarray)->dim[1];
/* C passes scalar parameters by value */
my_c_function(Caml_ba_data_val(bigarray), dimx, dimy);
/* Fortran passes all parameters by reference */
my_fortran_function_(Caml_ba_data_val(bigarray), &dimx, &dimy);
return Val_unit;
}

22.10.3 Wrapping a C or Fortran array as an OCaml Bigarray

A pointer p to an already-allocated C or Fortran array can be wrapped and returned to OCaml as a
Bigarray using the caml_ba_alloc or caml_ba_alloc_dims functions.

• caml_ba_alloc(kind | layout, numdims, p, dims)

Return an OCaml Bigarray wrapping the data pointed to by p. kind is the kind of array
elements (one of the CAML_BA_ kind constants above). layout is CAML_BA_C_LAYOUT for an
array with C layout and CAML_BA_FORTRAN_LAYOUT for an array with Fortran layout. numdims
442

is the number of dimensions in the array. dims is an array of numdims long integers, giving
the sizes of the array in each dimension.

• caml_ba_alloc_dims(kind | layout, numdims, p, (long) dim 1 , (long) dim 2 , …,

(long) dim numdims )
Same as caml_ba_alloc, but the sizes of the array in each dimension are listed as extra
arguments in the function call, rather than being passed as an array.

The following example illustrates how statically-allocated C and Fortran arrays can be made available
to OCaml.

extern long my_c_array[100][200];

extern float my_fortran_array_[300][400];

CAMLprim value caml_get_c_array(value unit)

{
long dims[2];
dims[0] = 100; dims[1] = 200;
return caml_ba_alloc(CAML_BA_NATIVE_INT | CAML_BA_C_LAYOUT,
2, my_c_array, dims);
}

CAMLprim value caml_get_fortran_array(value unit)

{
return caml_ba_alloc_dims(CAML_BA_FLOAT32 | CAML_BA_FORTRAN_LAYOUT,
2, my_fortran_array_, 300L, 400L);
}

22.11 Advanced topic: cheaper C call

This section describe how to make calling C functions cheaper.
Note: This only applies to the native compiler. So whenever you use any of these methods, you
have to provide an alternative byte-code stub that ignores all the special annotations.

22.11.1 Passing unboxed values

We said earlier that all OCaml objects are represented by the C type value, and one has to use
macros such as Int_val to decode data from the value type. It is however possible to tell the
OCaml native-code compiler to do this for us and pass arguments unboxed to the C function.
Similarly it is possible to tell OCaml to expect the result unboxed and box it for us.
The motivation is that, by letting ‘ocamlopt‘ deal with boxing, it can often decide to suppress it
entirely.
For instance let’s consider this example:

external foo : float -> float -> float = "foo"

Chapter 22. Interfacing C with OCaml 443

let f a b =
let len = Array.length a in
assert (Array.length b = len);
let res = Array.make len 0. in
for i = 0 to len - 1 do
res.(i) <- foo a.(i) b.(i)
done

Float arrays are unboxed in OCaml, however the C function foo expect its arguments as boxed
floats and returns a boxed float. Hence the OCaml compiler has no choice but to box a.(i) and
b.(i) and unbox the result of foo. This results in the allocation of 3 * len temporary float values.
Now if we annotate the arguments and result with [@unboxed], the native-code compiler will
be able to avoid all these allocations:

external foo
: (float [@unboxed])
-> (float [@unboxed])
-> (float [@unboxed])
= "foo_byte" "foo"

In this case the C functions must look like:

CAMLprim double foo(double a, double b)

{
...
}

CAMLprim value foo_byte(value a, value b)

{
return caml_copy_double(foo(Double_val(a), Double_val(b)))
}

For convenience, when all arguments and the result are annotated with [@unboxed], it is possible
to put the attribute only once on the declaration itself. So we can also write instead:

external foo : float -> float -> float = "foo_byte" "foo" [@@unboxed]

The following table summarize what OCaml types can be unboxed, and what C types should be
used in correspondence:

OCaml type C type

float double
int32 int32_t
int64 int64_t
nativeint intnat

Similarly, it is possible to pass untagged OCaml integers between OCaml and C. This is done by
annotating the arguments and/or result with [@untagged]:
444

external f : string -> (int [@untagged]) = "f_byte" "f"

The corresponding C type must be intnat.

Note: Do not use the C int type in correspondence with (int [@untagged]). This is because
they often differ in size.
It is possible to annotate with [@untagged] any immediate type, i.e. types that are represented
like int. This includes bool, char, any variant type with only constant constructors. Note: this
does not include Unix.file_descr which is not represented as an integer on all platforms.

22.11.2 Direct C call

In order to be able to run the garbage collector in the middle of a C function, the OCaml native-code
compiler generates some bookkeeping code around C calls. Technically it wraps every C call with
the C function caml_c_call which is part of the OCaml runtime.
For small functions that are called repeatedly, this indirection can have a big impact on
performances. However this is not needed if we know that the C function doesn’t allocate, doesn’t
raise exceptions, and doesn’t release the domain lock (see section 22.12.2). We can instruct the
OCaml native-code compiler of this fact by annotating the external declaration with the attribute
[@@noalloc]:

external bar : int -> int -> int = "foo" [@@noalloc]

In this case calling bar from OCaml is as cheap as calling any other OCaml function, except for
the fact that the OCaml compiler can’t inline C functions...

22.11.3 Example: calling C library functions without indirection

Using these attributes, it is possible to call C library functions with no indirection. For instance
many math functions are defined this way in the OCaml standard library:

external sqrt : float -> float = "caml_sqrt_float" "sqrt"

[@@unboxed] [@@noalloc]
(** Square root. *)

external exp : float -> float = "caml_exp_float" "exp" [@@unboxed] [@@noalloc]

(** Exponential. *)

external log : float -> float = "caml_log_float" "log" [@@unboxed] [@@noalloc]

(** Natural logarithm. *)

22.12 Advanced topic: multithreading

Using multiple threads (shared-memory concurrency) in a mixed OCaml/C application requires
special precautions, which are described in this section.
Chapter 22. Interfacing C with OCaml 445

22.12.1 Registering threads created from C

Callbacks from C to OCaml are possible only if the calling thread is known to the OCaml run-time
system. Threads created from OCaml (through the Thread.create function of the system threads
library) are automatically known to the run-time system. If the application creates additional threads
from C and wishes to callback into OCaml code from these threads, it must first register them with
the run-time system. The following functions are declared in the include file <caml/threads.h>.

• caml_c_thread_register() registers the calling thread with the OCaml run-time system.
Returns 1 on success, 0 on error. Registering an already-registered thread does nothing and
returns 0.

• caml_c_thread_unregister() must be called before the thread terminates, to unregister it

from the OCaml run-time system. Returns 1 on success, 0 on error. If the calling thread was
not previously registered, does nothing and returns 0.

22.12.2 Parallel execution of long-running C code with systhreads

Domains are the unit of parallelism for OCaml programs. When using the systhreads library,
multiple threads might be attached to the same domain. However, at any time, at most one of those
thread can be executing OCaml code or C code that uses the OCaml run-time system by domain.
Technically, this is enforced by a “domain lock” that any thread must hold while executing such
code within a domain.
When OCaml calls the C code implementing a primitive, the domain lock is held, therefore the
C code has full access to the facilities of the run-time system. However, no other thread in the same
domain can execute OCaml code concurrently with the C code of the primitive. See also chapter 9.6
for the behaviour with multiple domains.
If a C primitive runs for a long time or performs potentially blocking input-output operations, it
can explicitly release the domain lock, enabling other OCaml threads in the same domain to run
concurrently with its operations. The C code must re-acquire the domain lock before returning to
OCaml. This is achieved with the following functions, declared in the include file <caml/threads.h>.

• caml_release_runtime_system() The calling thread releases the domain lock and other
OCaml resources, enabling other threads to run OCaml code in parallel with the execution of
the calling thread.

• caml_acquire_runtime_system() The calling thread re-acquires the domain lock and other
OCaml resources. It may block until no other thread in the same domain uses the OCaml
run-time system.

These functions poll for pending signals by calling asynchronous callbacks (section 22.5.3) before
releasing and after acquiring the lock. They can therefore execute arbitrary OCaml code including
raising an asynchronous exception.
After caml_release_runtime_system() was called and until caml_acquire_runtime_system()
is called, the C code must not access any OCaml data, nor call any function of the run-time system,
nor call back into OCaml code. Consequently, arguments provided by OCaml to the C primitive
must be copied into C data structures before calling caml_release_runtime_system(), and results
446

to be returned to OCaml must be encoded as OCaml values after caml_acquire_runtime_system()

returns.
Example: the following C primitive invokes rmdir to delete a directory. The rmdir function can
block for some time, so we choose to release the OCaml run-time system while it is running.

#include <caml/memory.h>
#include <caml/threads.h>
#include <caml/unixsupport.h>

CAMLprim value stub_rmdir(value vpath)

{
CAMLparam1(vpath);

/* Raise Unix.Unix_error(Unix.ENOENT, ...) if the OCaml string

contains embedded null bytes */
caml_unix_check_path(vpath, "rmdir");

/* Copy the string argument to a C string, allocated outside the

OCaml heap. */
char_os *path = caml_stat_strdup_to_os(String_val(vpath));
/* Release the OCaml run-time system */
caml_release_runtime_system();

/* Delete the directory */

#ifdef _WIN32
int ret = _wrmdir(path);
#else
int ret = rmdir(path);
#endif

/* Free the copy of the string, which we might as well do before

acquiring the runtime system to benefit from parallelism. */
caml_stat_free(path);
/* Re-acquire the OCaml run-time system */
caml_acquire_runtime_system();

/* Raise Unix.Unix_error(...) if rmdir failed */

if (ret == -1) caml_uerror("rmdir", vpath);

/* Return to OCaml */
CAMLreturn(Val_unit);
}

The macro Caml_state evaluates to the domain state variable, and checks in debug mode that
the domain lock is held. Such a check is also placed in normal mode at key entry points of the
C API; this is why calling some of the runtime functions and macros without correctly owning
Chapter 22. Interfacing C with OCaml 447

the domain lock can result in a fatal error: no domain lock held. The variant Caml_state_opt
does not perform any check but evaluates to NULL when the domain lock is not held. This lets you
determine whether a thread belonging to a domain currently holds its domain lock, for various
purposes.
Callbacks from C to OCaml must be performed while holding the domain lock to the OCaml
run-time system. This is naturally the case if the callback is performed by a C primitive that did
not release the run-time system. If the C primitive released the run-time system previously, or the
callback is performed from other C code that was not invoked from OCaml (e.g. an event loop in a
GUI application), the run-time system must be acquired before the callback and released after:
caml_acquire_runtime_system();
/* Resolve OCaml function vfun to be invoked */
/* Build OCaml argument varg to the callback */
vres = callback(vfun, varg);
/* Copy relevant parts of result vres to C data structures */
caml_release_runtime_system();
Note: the acquire and release functions described above were introduced in OCaml 3.12.
Older code uses the following historical names, declared in <caml/signals.h>:
• caml_enter_blocking_section as an alias for caml_release_runtime_system
• caml_leave_blocking_section as an alias for caml_acquire_runtime_system
Intuition: a “blocking section” is a piece of C code that does not use the OCaml run-time system,
typically a blocking input/output operation.

22.13 Advanced topic: interfacing with Windows Unicode APIs

This section contains some general guidelines for writing C stubs that use Windows Unicode APIs.
The OCaml system under Windows can be configured at build time in one of two modes:

• legacy mode: All path names, environment variables, command line arguments, etc. on the
OCaml side are assumed to be encoded using the current 8-bit code page of the system.
• Unicode mode: All path names, environment variables, command line arguments, etc. on
the OCaml side are assumed to be encoded using UTF-8.

In what follows, we say that a string has the OCaml encoding if it is encoded in UTF-8 when in
Unicode mode, in the current code page in legacy mode, or is an arbitrary string under Unix. A
string has the platform encoding if it is encoded in UTF-16 under Windows or is an arbitrary string
under Unix.
From the point of view of the writer of C stubs, the challenges of interacting with Windows
Unicode APIs are twofold:

• The Windows API uses the UTF-16 encoding to support Unicode. The runtime system
performs the necessary conversions so that the OCaml programmer only needs to deal with
the OCaml encoding. C stubs that call Windows Unicode APIs need to use specific runtime
functions to perform the necessary conversions in a compatible way.
448

• When writing stubs that need to be compiled under both Windows and Unix, the stubs need
to be written in a way that allow the necessary conversions under Windows but that also work
under Unix, where typically nothing particular needs to be done to support Unicode.

The native C character type under Windows is WCHAR, two bytes wide, while under Unix it is
char, one byte wide. A type char_os is defined in <caml/misc.h> that stands for the concrete C
character type of each platform. Strings in the platform encoding are of type char_os *.
The following functions are exposed to help write compatible C stubs. To use them, you need to
include both <caml/misc.h> and <caml/osdeps.h>.

• char_os* caml_stat_strdup_to_os(const char *) copies the argument while translating

from OCaml encoding to the platform encoding. This function is typically used to convert the
char * underlying an OCaml string before passing it to an operating system API that takes
a Unicode argument. Under Unix, it is equivalent to caml_stat_strdup.
Note: For maximum backwards compatibility in Unicode mode, if the argument is not a valid
UTF-8 string, this function will fall back to assuming that it is encoded in the current code
page.

• char* caml_stat_strdup_of_os(const char_os *) copies the argument while trans-

lating from the platform encoding to the OCaml encoding. It is the inverse of
caml_stat_strdup_to_os. This function is typically used to convert a string obtained from
the operating system before passing it on to OCaml code. Under Unix, it is equivalent to
caml_stat_strdup.

• value caml_copy_string_of_os(char_os *) allocates an OCaml string with contents equal

to the argument string converted to the OCaml encoding. This function is essentially equiva-
lent to caml_stat_strdup_of_os followed by caml_copy_string, except that it avoids the
allocation of the intermediate string returned by caml_stat_strdup_of_os. Under Unix, it
is equivalent to caml_copy_string.

Note: The strings returned by caml_stat_strdup_to_os and caml_stat_strdup_of_os are

allocated using caml_stat_alloc, so they need to be deallocated using caml_stat_free when they
are no longer needed.

Example We want to bind the function getenv in a way that works both under Unix and
Windows. Under Unix this function has the prototype:

char *getenv(const char *);

While the Unicode version under Windows has the prototype:

WCHAR *_wgetenv(const WCHAR *);

In terms of char_os, both functions take an argument of type char_os * and return a result of
the same type. We begin by choosing the right implementation of the function to bind:
Chapter 22. Interfacing C with OCaml 449

#ifdef _WIN32
#define getenv_os _wgetenv
#else
#define getenv_os getenv
#endif

The rest of the binding is the same for both platforms:

#include <caml/mlvalues.h>
#include <caml/misc.h>
#include <caml/alloc.h>
#include <caml/fail.h>
#include <caml/osdeps.h>
#include <stdlib.h>

CAMLprim value stub_getenv(value var_name)

{
CAMLparam1(var_name);
CAMLlocal1(var_value);
char_os *var_name_os, *var_value_os;

var_name_os = caml_stat_strdup_to_os(String_val(var_name));
var_value_os = getenv_os(var_name_os);
caml_stat_free(var_name_os);

if (var_value_os == NULL)
caml_raise_not_found();

var_value = caml_copy_string_of_os(var_value_os);

CAMLreturn(var_value);
}

22.14 Building mixed C/OCaml libraries: ocamlmklib

The ocamlmklib command facilitates the construction of libraries containing both OCaml code and
C code, and usable both in static linking and dynamic linking modes. This command is available
under Windows since Objective Caml 3.11 and under other operating systems since Objective Caml
3.03.
The ocamlmklib command takes three kinds of arguments:
• OCaml source files and object files (.cmo, .cmx, .ml) comprising the OCaml part of the
library;
• C object files (.o, .a, respectively, .obj, .lib) comprising the C part of the library;
• Support libraries for the C part (-llib).
450

It generates the following outputs:

• An OCaml bytecode library .cma incorporating the .cmo and .ml OCaml files given as
arguments, and automatically referencing the C library generated with the C object files.

• An OCaml native-code library .cmxa incorporating the .cmx and .ml OCaml files given as
arguments, and automatically referencing the C library generated with the C object files.

• If dynamic linking is supported on the target platform, a .so (respectively, .dll) shared
library built from the C object files given as arguments, and automatically referencing the
support libraries.

• A C static library .a(respectively, .lib) built from the C object files.

In addition, the following options are recognized:

-cclib, -ccopt, -I, -linkall

These options are passed as is to ocamlc or ocamlopt. See the documentation of these
commands.

-rpath, -R, -Wl,-rpath, -Wl,-R

These options are passed as is to the C compiler. Refer to the documentation of the C compiler.

-custom
Force the construction of a statically linked library only, even if dynamic linking is supported.

-failsafe
Fall back to building a statically linked library if a problem occurs while building the shared
library (e.g. some of the support libraries are not available as shared libraries).

-Ldir
Add dir to the search path for support libraries (-llib).

-ocamlc cmd
Use cmd instead of ocamlc to call the bytecode compiler.

-ocamlopt cmd
Use cmd instead of ocamlopt to call the native-code compiler.

-o output
Set the name of the generated OCaml library. ocamlmklib will generate output.cma and/or
output.cmxa. If not specified, defaults to a.

-oc outputc
Set the name of the generated C library. ocamlmklib will generate liboutputc.so (if shared
libraries are supported) and liboutputc.a. If not specified, defaults to the output name given
with -o.
Chapter 22. Interfacing C with OCaml 451

Example Consider an OCaml interface to the standard libz C library for reading and writing
compressed files. Assume this library resides in /usr/local/zlib. This interface is composed of
an OCaml part zip.cmo/zip.cmx and a C part zipstubs.o containing the stub code around the
libz entry points. The following command builds the OCaml libraries zip.cma and zip.cmxa, as
well as the companion C libraries dllzip.so and libzip.a:
ocamlmklib -o zip zip.cmo zip.cmx zipstubs.o -lz -L/usr/local/zlib
If shared libraries are supported, this performs the following commands:
ocamlc -a -o zip.cma zip.cmo -dllib -lzip \
-cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -cclib -lzip \
-cclib -lzip -cclib -lz -ccopt -L/usr/local/zlib
gcc -shared -o dllzip.so zipstubs.o -lz -L/usr/local/zlib
ar rc libzip.a zipstubs.o
Note: This example is on a Unix system. The exact command lines may be different on other
systems.
If shared libraries are not supported, the following commands are performed instead:
ocamlc -a -custom -o zip.cma zip.cmo -cclib -lzip \
-cclib -lz -ccopt -L/usr/local/zlib
ocamlopt -a -o zip.cmxa zip.cmx -lzip \
-cclib -lz -ccopt -L/usr/local/zlib
ar rc libzip.a zipstubs.o
Instead of building simultaneously the bytecode library, the native-code library and the C
libraries, ocamlmklib can be called three times to build each separately. Thus,
ocamlmklib -o zip zip.cmo -lz -L/usr/local/zlib
builds the bytecode library zip.cma, and
ocamlmklib -o zip zip.cmx -lz -L/usr/local/zlib
builds the native-code library zip.cmxa, and
ocamlmklib -o zip zipstubs.o -lz -L/usr/local/zlib
builds the C libraries dllzip.so and libzip.a. Notice that the support libraries (-lz) and the
corresponding options (-L/usr/local/zlib) must be given on all three invocations of ocamlmklib,
because they are needed at different times depending on whether shared libraries are supported.

22.15 Cautionary words: the internal runtime API

Not all header available in the caml/ directory were described in previous sections. All those
unmentioned headers are part of the internal runtime API, for which there is no stability guarantee.
If you really need access to this internal runtime API, this section provides some guidelines that
may help you to write code that might not break on every new version of OCaml.
452

Note Programmers which come to rely on the internal API for a use-case which they find realistic
and useful are encouraged to open a request for improvement on the bug tracker.

22.15.1 Internal variables and CAML_INTERNALS

Since OCaml 4.04, it is possible to get access to every part of the internal runtime API by defining
the CAML_INTERNALS macro before loading caml header files. If this macro is not defined, parts of
the internal runtime API are hidden.
If you are using internal C variables, do not redefine them by hand. You should import those
variables by including the corresponding header files. The representation of those variables has
already changed once in OCaml 4.10, and is still under evolution. If your code relies on such internal
and brittle properties, it will be broken at some point in time.
For instance, rather than redefining caml_young_limit:

extern int caml_young_limit;

which breaks in OCaml ≥ 4.10, you should include the minor_gc header:

#include <caml/minor_gc.h>

22.15.2 OCaml version macros

Finally, if including the right headers is not enough, or if you need to support version older than
OCaml 4.04, the header file caml/version.h should help you to define your own compatibility layer.
This file provides few macros defining the current OCaml version. In particular, the OCAML_VERSION
macro describes the current version, its format is MmmPP. For example, if you need some specific
handling for versions older than 4.10.0, you could write

#include <caml/version.h>
#if OCAML_VERSION >= 41000
...
#else
...
#endif
Chapter 23

Optimisation with Flambda

23.1 Overview
Flambda is the term used to describe a series of optimisation passes provided by the native code
compilers as of OCaml 4.03.
Flambda aims to make it easier to write idiomatic OCaml code without incurring performance
penalties.
To use the Flambda optimisers it is necessary to pass the -flambda option to the OCaml
configure script. (There is no support for a single compiler that can operate in both Flambda and
non-Flambda modes.) Code compiled with Flambda cannot be linked into the same program as
code compiled without Flambda. Attempting to do this will result in a compiler error.
Whether or not a particular ocamlopt uses Flambda may be determined by invoking it with the
-config option and looking for any line starting with “flambda:”. If such a line is present and says
“true”, then Flambda is supported, otherwise it is not.
Flambda provides full optimisation across different compilation units, so long as the .cmx files for
the dependencies of the unit currently being compiled are available. (A compilation unit corresponds
to a single .ml source file.) However it does not yet act entirely as a whole-program compiler: for
example, elimination of dead code across a complete set of compilation units is not supported.
Optimisation with Flambda is not currently supported when generating bytecode.
Flambda should not in general affect the semantics of existing programs. Two exceptions to
this rule are: possible elimination of pure code that is being benchmarked (see section 23.14) and
changes in behaviour of code using unsafe operations (see section 23.15).
Flambda does not yet optimise array or string bounds checks. Neither does it take hints for
optimisation from any assertions written by the user in the code.
Consult the Glossary at the end of this chapter for definitions of technical terms used below.

23.2 Command-line flags

The Flambda optimisers provide a variety of command-line flags that may be used to control their
behaviour. Detailed descriptions of each flag are given in the referenced sections. Those sections
also describe any arguments which the particular flags take.
Commonly-used options:

453
454

-O2 Perform more optimisation than usual. Compilation times may be lengthened. (This flag is
an abbreviation for a certain set of parameters described in section 23.5.)

-O3 Perform even more optimisation than usual, possibly including unrolling of recursive functions.
Compilation times may be significantly lengthened.

-Oclassic
Make inlining decisions at the point of definition of a function rather than at the call site(s).
This mirrors the behaviour of OCaml compilers not using Flambda. Compared to compilation
using the new Flambda inlining heuristics (for example at -O2) it produces smaller .cmx files,
shorter compilation times and code that probably runs rather slower. When using -Oclassic,
only the following options described in this section are relevant: -inlining-report and
-inline. If any other of the options described in this section are used, the behaviour is
undefined and may cause an error in future versions of the compiler.

-inlining-report
Emit .inlining files (one per round of optimisation) showing all of the inliner’s decisions.

Less commonly-used options:

-remove-unused-arguments
Remove unused function arguments even when the argument is not specialised. This may
have a small performance penalty. See section 23.10.3.

-unbox-closures
Pass free variables via specialised arguments rather than closures (an optimisation for reducing
allocation). See section 23.9.3. This may have a small performance penalty.

Advanced options, only needed for detailed tuning:

-inline
The behaviour depends on whether -Oclassic is used.

• When not in -Oclassic mode, -inline limits the total size of functions considered for
inlining during any speculative inlining search. (See section 23.3.10.) Note that this
parameter does not control the assessment as to whether any particular function may be
inlined. Raising it to excessive amounts will not necessarily cause more functions to be
inlined.
• When in -Oclassic mode, -inline behaves as in previous versions of the compiler: it
is the maximum size of function to be considered for inlining. See section 23.3.2.

-inline-toplevel
The equivalent of -inline but used when speculative inlining starts at toplevel. See section
23.3.10. Not used in -Oclassic mode.

-inline-branch-factor
Controls how the inliner assesses whether a code path is likely to be hot or cold. See section
23.3.9.
Chapter 23. Optimisation with Flambda 455

-inline-alloc-cost, -inline-branch-cost, -inline-call-cost

Controls how the inliner assesses the runtime performance penalties associated with various
operations. See section 23.3.9.

-inline-indirect-cost, -inline-prim-cost
Likewise.

-inline-lifting-benefit
Controls inlining of functors at toplevel. See section 23.3.9.

-inline-max-depth
The maximum depth of any speculative inlining search. See section 23.3.10.

-inline-max-unroll
The maximum depth of any unrolling of recursive functions during any speculative inlining
search. See section 23.3.10.

-no-unbox-free-vars-of-closures
Do not unbox closure variables. See section 23.9.1.

-no-unbox-specialised-args
Do not unbox arguments to which functions have been specialised. See section 23.9.2.

-rounds
How many rounds of optimisation to perform. See section 23.2.1.

-unbox-closures-factor
Scaling factor for benefit calculation when using -unbox-closures. See section 23.9.3.

Notes

• The set of command line flags relating to optimisation should typically be specified to be the
same across an entire project. Flambda does not currently record the requested flags in the
.cmx files. As such, inlining of functions from previously-compiled units will subject their
code to the optimisation parameters of the unit currently being compiled, rather than those
specified when they were previously compiled. It is hoped to rectify this deficiency in the
future.

• Flambda-specific flags do not affect linking with the exception of affecting the optimisation of
code in the startup file (containing generated functions such as currying helpers). Typically
such optimisation will not be significant, so eliding such flags at link time might be reasonable.

• Flambda-specific flags are silently accepted even when the -flambda option was not provided
to the configure script. (There is no means provided to change this behaviour.) This is
intended to make it more straightforward to run benchmarks with and without the Flambda
optimisers in effect.

• Some of the Flambda flags may be subject to change in future releases.

456

23.2.1 Specification of optimisation parameters by round

Flambda operates in rounds: one round consists of a certain sequence of transformations that
may then be repeated in order to achieve more satisfactory results. The number of rounds can be
set manually using the -rounds parameter (although this is not necessary when using predefined
optimisation levels such as with -O2 and -O3). For high optimisation the number of rounds might
be set at 3 or 4.
Command-line flags that may apply per round, for example those with -cost in the name,
accept arguments of the form:
n | round=n[,...]

• If the first form is used, with a single integer specified, the value will apply to all rounds.

• If the second form is used, zero-based round integers specify values which are to be used only
for those rounds.

The flags -Oclassic, -O2 and -O3 are applied before all other flags, meaning that certain
parameters may be overridden without having to specify every parameter usually invoked by the
given optimisation level.

23.3 Inlining
Inlining refers to the copying of the code of a function to a place where the function is called.
The code of the function will be surrounded by bindings of its parameters to the corresponding
arguments.
The aims of inlining are:
• to reduce the runtime overhead caused by function calls (including setting up for such calls
and returning afterwards);

• to reduce instruction cache misses by expressing frequently-taken paths through the program
using fewer machine instructions; and

• to reduce the amount of allocation (especially of closures).

These goals are often reached not just by inlining itself but also by other optimisations that the
compiler is able to perform as a result of inlining.
When a recursive call to a function (within the definition of that function or another in the same
mutually-recursive group) is inlined, the procedure is also known as unrolling. This is somewhat
akin to loop peeling. For example, given the following code:
let rec fact x =
if x = 0 then
1
else
x * fact (x - 1)

let n = fact 4
Chapter 23. Optimisation with Flambda 457

unrolling once at the call site fact 4 produces (with the body of fact unchanged):
let n =
if 4 = 0 then
1
else
4 * fact (4 - 1)
This simplifies to:
let n = 4 * fact 3
Flambda provides significantly enhanced inlining capabilities relative to previous versions of the
compiler.

23.3.1 Aside: when inlining is performed

Inlining is performed together with all of the other Flambda optimisation passes, that is to say, after
closure conversion. This has three particular advantages over a potentially more straightforward
implementation prior to closure conversion:
• It permits higher-order inlining, for example when a non-inlinable function always returns the
same function yet with different environments of definition. Not all such cases are supported
yet, but it is intended that such support will be improved in future.

• It is easier to integrate with cross-module optimisation, since imported information about

other modules is already in the correct intermediate language.

• It becomes more straightforward to optimise closure allocations since the layout of closures does
not have to be estimated in any way: it is known. Similarly, it becomes more straightforward
to control which variables end up in which closures, helping to avoid closure bloat.

23.3.2 Classic inlining heuristic

In -Oclassic mode the behaviour of the Flambda inliner mimics previous versions of the compiler.
(Code may still be subject to further optimisations not performed by previous versions of the
compiler: functors may be inlined, constants are lifted and unused code is eliminated all as described
elsewhere in this chapter. See sections 23.3.5, 23.8.1 and 23.10. At the definition site of a function,
the body of the function is measured. It will then be marked as eligible for inlining (and hence
inlined at every direct call site) if:
• the measured size (in unspecified units) is smaller than that of a function call plus the argument
of the -inline command-line flag; and

• the function is not recursive.

Non-Flambda versions of the compiler cannot inline functions that contain a definition of another
function. However -Oclassic does permit this. Further, non-Flambda versions also cannot inline
functions that are only themselves exposed as a result of a previous pass of inlining, but again this
is permitted by -Oclassic. For example:
458

module M : sig
val i : int
end = struct
let f x =
let g y = x + y in
g
let h = f 3
let i = h 4 (* h is correctly discovered to be g and inlined *)
end
All of this contrasts with the normal Flambda mode, that is to say without -Oclassic, where:
• the inlining decision is made at the call site; and
• recursive functions can be handled, by specialisation (see below).
The Flambda mode is described in the next section.

23.3.3 Overview of “Flambda” inlining heuristics

The Flambda inlining heuristics, used whenever the compiler is configured for Flambda and -
Oclassic was not specified, make inlining decisions at call sites. This helps in situations where the
context is important. For example:
let f b x =
if b then
x
else
... big expression ...

let g x = f true x
In this case, we would like to inline f into g, because a conditional jump can be eliminated and
the code size should reduce. If the inlining decision has been made after the declaration of f without
seeing the use, its size would have probably made it ineligible for inlining; but at the call site, its
final size can be known. Further, this function should probably not be inlined systematically: if b is
unknown, or indeed false, there is little benefit to trade off against a large increase in code size.
In the existing non-Flambda inliner this isn’t a great problem because chains of inlining were cut
off fairly quickly. However it has led to excessive use of overly-large inlining parameters such as
-inline 10000.
In more detail, at each call site the following procedure is followed:
• Determine whether it is clear that inlining would be beneficial without, for the moment, doing
any inlining within the function itself. (The exact assessment of benefit is described below.) If
so, the function is inlined.
• If inlining the function is not clearly beneficial, then inlining will be performed speculatively
inside the function itself. The search for speculative inlining possibilities is controlled by two
parameters: the inlining threshold and the inlining depth. (These are described in more detail
below.)
Chapter 23. Optimisation with Flambda 459

– If such speculation shows that performing some inlining inside the function would be
beneficial, then such inlining is performed and the resulting function inlined at the
original call site.
– Otherwise, nothing happens.

Inlining within recursive functions of calls to other functions in the same mutually-recursive group
is kept in check by an unrolling depth, described below. This ensures that functions are not
unrolled to excess. (Unrolling is only enabled if -O3 optimisation level is selected and/or the
-inline-max-unroll flag is passed with an argument greater than zero.)

23.3.4 Handling of specific language constructs

23.3.5 Functors
There is nothing particular about functors that inhibits inlining compared to normal functions. To
the inliner, these both look the same, except that functors are marked as such.
Applications of functors at toplevel are biased in favour of inlining. (This bias may be adjusted:
see the documentation for -inline-lifting-benefit below.)
Applications of functors not at toplevel, for example in a local module inside some other
expression, are treated by the inliner identically to normal function calls.

23.3.6 First-class modules

The inliner will be able to consider inlining a call to a function in a first class module if it knows
which particular function is going to be called. The presence of the first-class module record that
wraps the set of functions in the module does not per se inhibit inlining.

23.3.7 Objects
Method calls to objects are not at present inlined by Flambda.

23.3.8 Inlining reports

If the -inlining-report option is provided to the compiler then a file will be emitted corresponding
to each round of optimisation. For the OCaml source file basename.ml the files are named base-
name.round.inlining.org, with round a zero-based integer. Inside the files, which are formatted
as “org mode”, will be found English prose describing the decisions that the inliner took.

23.3.9 Assessment of inlining benefit

Inlining typically results in an increase in code size, which if left unchecked, may not only lead to
grossly large executables and excessive compilation times but also a decrease in performance due to
worse locality. As such, the Flambda inliner trades off the change in code size against the expected
runtime performance benefit, with the benefit being computed based on the number of operations
that the compiler observes may be removed as a result of inlining.
For example given the following code:
460

let f b x =
if b then
x
else
... big expression ...

let g x = f true x
it would be observed that inlining of f would remove:
• one direct call;
• one conditional branch.
Formally, an estimate of runtime performance benefit is computed by first summing the cost of
the operations that are known to be removed as a result of the inlining and subsequent simplification
of the inlined body. The individual costs for the various kinds of operations may be adjusted using
the various -inline-...-cost flags as follows. Costs are specified as integers. All of these flags
accept a single argument describing such integers using the conventions detailed in section 23.2.1.
-inline-alloc-cost
The cost of an allocation.
-inline-branch-cost
The cost of a branch.
-inline-call-cost
The cost of a direct function call.
-inline-indirect-cost
The cost of an indirect function call.
-inline-prim-cost
The cost of a primitive. Primitives encompass operations including arithmetic and memory
access.
(Default values are described in section 23.5 below.)
The initial benefit value is then scaled by a factor that attempts to compensate for the fact that
the current point in the code, if under some number of conditional branches, may be cold. (Flambda
does not currently compute hot and cold paths.) The factor—the estimated probability that the
inliner really is on a hot path—is calculated as (1+f 1
)d
, where f is set by -inline-branch-factor
and d is the nesting depth of branches at the current point. As the inliner descends into more
deeply-nested branches, the benefit of inlining thus lessens.
The resulting benefit value is known as the estimated benefit.
The change in code size is also estimated: morally speaking it should be the change in machine
code size, but since that is not available to the inliner, an approximation is used.
If the estimated benefit exceeds the increase in code size then the inlined version of the function
will be kept. Otherwise the function will not be inlined.
Applications of functors at toplevel will be given an additional benefit (which may be controlled
by the -inline-lifting-benefit flag) to bias inlining in such situations towards keeping the
inlined version.
Chapter 23. Optimisation with Flambda 461

23.3.10 Control of speculation

As described above, there are three parameters that restrict the search for inlining opportunities
during speculation:
• the inlining threshold;
• the inlining depth;
• the unrolling depth.
These parameters are ultimately bounded by the arguments provided to the corresponding command-
line flags (or their default values):
• -inline (or, if the call site that triggered speculation is at toplevel, -inline-toplevel);
• -inline-max-depth;
• -inline-max-unroll.
Note in particular that -inline does not have the meaning that it has in the previous compiler
or in -Oclassic mode. In both of those situations -inline was effectively some kind of basic
assessment of inlining benefit. However in Flambda inlining mode it corresponds to a constraint on
the search; the assessment of benefit is independent, as described above.
When speculation starts the inlining threshold starts at the value set by -inline (or -inline-
toplevel if appropriate, see above). Upon making a speculative inlining decision the threshold is
reduced by the code size of the function being inlined. If the threshold becomes exhausted, at or
below zero, no further speculation will be performed.
The inlining depth starts at zero and is increased by one every time the inliner descends into
another function. It is then decreased by one every time the inliner leaves such function. If the depth
exceeds the value set by -inline-max-depth then speculation stops. This parameter is intended as
a general backstop for situations where the inlining threshold does not control the search sufficiently.
The unrolling depth applies to calls within the same mutually-recursive group of functions. Each
time an inlining of such a call is performed the depth is incremented by one when examining the
resulting body. If the depth reaches the limit set by -inline-max-unroll then speculation stops.

23.4 Specialisation
The inliner may discover a call site to a recursive function where something is known about the
arguments: for example, they may be equal to some other variables currently in scope. In this
situation it may be beneficial to specialise the function to those arguments. This is done by
copying the declaration of the function (and any others involved in any same mutually-recursive
declaration) and noting the extra information about the arguments. The arguments augmented by
this information are known as specialised arguments. In order to try to ensure that specialisation is
not performed uselessly, arguments are only specialised if it can be shown that they are invariant:
in other words, during the execution of the recursive function(s) themselves, the arguments never
change.
Unless overridden by an attribute (see below), specialisation of a function will not be attempted
if:
462

• the compiler is in -Oclassic mode;

• the function is not obviously recursive;

• the function is not closed.

The compiler can prove invariance of function arguments across multiple functions within a
recursive group (although this has some limitations, as shown by the example below).
It should be noted that the unboxing of closures pass (see below) can introduce specialised
arguments on non-recursive functions. (No other place in the compiler currently does this.)

Example: the well-known List.iter function This function might be written like so:

let rec iter f l =

match l with
| [] -> ()
| h :: t ->
f h;
iter f t

and used like this:

let print_int x =
print_endline (Int.to_string x)

let run xs =
iter print_int (List.rev xs)

The argument f to iter is invariant so the function may be specialised:

let run xs =
let rec iter' f l =
(* The compiler knows: f holds the same value as foo throughout iter'. *)
match l with
| [] -> ()
| h :: t ->
f h;
iter' f t
in
iter' print_int (List.rev xs)

The compiler notes down that for the function iter', the argument f is specialised to the
constant closure print_int. This means that the body of iter' may be simplified:

let run xs =
let rec iter' f l =
(* The compiler knows: f holds the same value as foo throughout iter'. *)
match l with
| [] -> ()
Chapter 23. Optimisation with Flambda 463

| h :: t ->
print_int h; (* this is now a direct call *)
iter' f t
in
iter' print_int (List.rev xs)
The call to print_int can indeed be inlined:
let run xs =
let rec iter' f l =
(* The compiler knows: f holds the same value as foo throughout iter'. *)
match l with
| [] -> ()
| h :: t ->
print_endline (Int.to_string h);
iter' f t
in
iter' print_int (List.rev xs)
The unused specialised argument f may now be removed, leaving:
let run xs =
let rec iter' l =
match l with
| [] -> ()
| h :: t ->
print_endline (Int.to_string h);
iter' t
in
iter' (List.rev xs)

Aside on invariant parameters. The compiler cannot currently detect invariance in cases such
as the following.
let rec iter_swap f g l =
match l with
| [] -> ()
| 0 :: t ->
iter_swap g f l
| h :: t ->
f h;
iter_swap f g t

23.4.1 Assessment of specialisation benefit

The benefit of specialisation is assessed in a similar way as for inlining. Specialised argument
information may mean that the body of the function being specialised can be simplified: the
removed operations are accumulated into a benefit. This, together with the size of the duplicated
(specialised) function declaration, is then assessed against the size of the call to the original function.
464

23.5 Default settings of parameters

The default settings (when not using -Oclassic) are for one round of optimisation using the
following parameters.

Parameter Setting
-inline 10
-inline-branch-factor 0.1
-inline-alloc-cost 7
-inline-branch-cost 5
-inline-call-cost 5
-inline-indirect-cost 4
-inline-prim-cost 3
-inline-lifting-benefit 1300
-inline-toplevel 160
-inline-max-depth 1
-inline-max-unroll 0
-unbox-closures-factor 10

23.5.1 Settings at -O2 optimisation level

When -O2 is specified two rounds of optimisation are performed. The first round uses the default
parameters (see above). The second uses the following parameters.

Parameter Setting
-inline 25
-inline-branch-factor Same as default
-inline-alloc-cost Double the default
-inline-branch-cost Double the default
-inline-call-cost Double the default
-inline-indirect-cost Double the default
-inline-prim-cost Double the default
-inline-lifting-benefit Same as default
-inline-toplevel 400
-inline-max-depth 2
-inline-max-unroll Same as default
-unbox-closures-factor Same as default

23.5.2 Settings at -O3 optimisation level

When -O3 is specified three rounds of optimisation are performed. The first two rounds are as for
-O2. The third round uses the following parameters.
Chapter 23. Optimisation with Flambda 465

Parameter Setting
-inline 50
-inline-branch-factor Same as default
-inline-alloc-cost Triple the default
-inline-branch-cost Triple the default
-inline-call-cost Triple the default
-inline-indirect-cost Triple the default
-inline-prim-cost Triple the default
-inline-lifting-benefit Same as default
-inline-toplevel 800
-inline-max-depth 3
-inline-max-unroll 1
-unbox-closures-factor Same as default

23.6 Manual control of inlining and specialisation

Should the inliner prove recalcitrant and refuse to inline a particular function, or if the observed
inlining decisions are not to the programmer’s satisfaction for some other reason, inlining behaviour
can be dictated by the programmer directly in the source code. One example where this might be
appropriate is when the programmer, but not the compiler, knows that a particular function call is
on a cold code path. It might be desirable to prevent inlining of the function so that the code size
along the hot path is kept smaller, so as to increase locality.
The inliner is directed using attributes. For non-recursive functions (and one-step unrolling of
recursive functions, although @unroll is more clear for this purpose) the following are supported:

@@inline always or @@inline never

Attached to a declaration of a function or functor, these direct the inliner to either always
or never inline, irrespective of the size/benefit calculation. (If the function is recursive then
the body is substituted and no special action is taken for the recursive call site(s).) @@inline
with no argument is equivalent to @@inline always.

@inlined always or @inlined never

Attached to a function application, these direct the inliner likewise. These attributes at
call sites override any other attribute that may be present on the corresponding declaration.
@inlined with no argument is equivalent to @inlined always. @@inlined hint is equivalent
to @@inline always except that it will not trigger warning 55 if the function application
cannot be inlined.

For recursive functions the relevant attributes are:

@@specialise always or @@specialise never

Attached to a declaration of a function or functor, this directs the inliner to either always
or never specialise the function so long as it has appropriate contextual knowledge, irre-
spective of the size/benefit calculation. @@specialise with no argument is equivalent to
@@specialise always.
466

@specialised always or @specialised never

Attached to a function application, this directs the inliner likewise. This attribute at a call site
overrides any other attribute that may be present on the corresponding declaration. (Note that
the function will still only be specialised if there exist one or more invariant parameters whose
values are known.) @specialised with no argument is equivalent to @specialised always.

@unrolled n
This attribute is attached to a function application and always takes an integer argument.
Each time the inliner sees the attribute it behaves as follows:

• If n is zero or less, nothing happens.

• Otherwise the function being called is substituted at the call site with its body having
been rewritten such that any recursive calls to that function or any others in the same
mutually-recursive group are annotated with the attribute unrolled(n − 1). Inlining
may continue on that body.

As such, n behaves as the “maximum depth of unrolling”.

A compiler warning will be emitted if it was found impossible to obey an annotation from an
@inlined or @specialised attribute.

Example showing correct placement of attributes

module F (M : sig type t end) = struct
let[@inline never] bar x =
x * 3

let foo x =
(bar [@inlined]) (42 + x)
end [@@inline never]

module X = F [@inlined] (struct type t = int end)

23.7 Simplification
Simplification, which is run in conjunction with inlining, propagates information (known as ap-
proximations) about which variables hold what values at runtime. Certain relationships between
variables and symbols are also tracked: for example, some variable may be known to always hold
the same value as some other variable; or perhaps some variable may be known to always hold the
value pointed to by some symbol.
The propagation can help to eliminate allocations in cases such as:
let f x y =
...
let p = x, y in
...
... (fst p) ... (snd p) ...
Chapter 23. Optimisation with Flambda 467

The projections from p may be replaced by uses of the variables x and y, potentially meaning
that p becomes unused.
The propagation performed by the simplification pass is also important for discovering which
functions flow to indirect call sites. This can enable the transformation of such call sites into direct
call sites, which makes them eligible for an inlining transformation.
Note that no information is propagated about the contents of strings, even in safe-string
mode, because it cannot yet be guaranteed that they are immutable throughout a given program.

23.8 Other code motion transformations

23.8.1 Lifting of constants
Expressions found to be constant will be lifted to symbol bindings—that is to say, they will be
statically allocated in the object file—when they evaluate to boxed values. Such constants may be
straightforward numeric constants, such as the floating-point number 42.0, or more complicated
values such as constant closures.
Lifting of constants to toplevel reduces allocation at runtime.
The compiler aims to share constants lifted to toplevel such that there are no duplicate definitions.
However if .cmx files are hidden from the compiler then maximal sharing may not be possible.

Notes about float arrays The following language semantics apply specifically to constant float
arrays. (By “constant float array” is meant an array consisting entirely of floating point numbers
that are known at compile time. A common case is a literal such as [| 42.0; 43.0; |].

• Constant float arrays at the toplevel are mutable and never shared. (That is to say, for each
such definition there is a distinct symbol in the data section of the object file pointing at the
array.)

• Constant float arrays not at toplevel are mutable and are created each time the expression is
evaluated. This can be thought of as an operation that takes an immutable array (which in
the source code has no associated name; let us call it the initialising array) and duplicates it
into a fresh mutable array.

– If the array is of size four or less, the expression will create a fresh block and write the
values into it one by one. There is no reference to the initialising array as a whole.
– Otherwise, the initialising array is lifted out and subject to the normal constant sharing
procedure; creation of the array consists of bulk copying the initialising array into a fresh
value on the OCaml heap.

23.8.2 Lifting of toplevel let bindings

Toplevel let-expressions may be lifted to symbol bindings to ensure that the corresponding bound
variables are not captured by closures. If the defining expression of a given binding is found to be
constant, it is bound as such (the technical term is a let-symbol binding).
Otherwise, the symbol is bound to a (statically-allocated) preallocated block containing one field.
At runtime, the defining expression will be evaluated and the first field of the block filled with the
468

resulting value. This initialise-symbol binding causes one extra indirection but ensures, by virtue
of the symbol’s address being known at compile time, that uses of the value are not captured by
closures.
It should be noted that the blocks corresponding to initialise-symbol bindings are kept alive
forever, by virtue of them occurring in a static table of GC roots within the object file. This
extended lifetime of expressions may on occasion be surprising. If it is desired to create some
non-constant value (for example when writing GC tests) that does not have this extended lifetime,
then it may be created and used inside a function, with the application point of that function
(perhaps at toplevel)—or indeed the function declaration itself—marked as to never be inlined. This
technique prevents lifting of the definition of the value in question (assuming of course that it is not
constant).

23.9 Unboxing transformations

The transformations in this section relate to the splitting apart of boxed (that is to say, non-
immediate) values. They are largely intended to reduce allocation, which tends to result in a runtime
performance profile with lower variance and smaller tails.

23.9.1 Unboxing of closure variables

This transformation is enabled unless -no-unbox-free-vars-of-closures is provided.
Variables that appear in closure environments may themselves be boxed values. As such, they
may be split into further closure variables, each of which corresponds to some projection from the
original closure variable(s). This transformation is called unboxing of closure variables or unboxing
of free variables of closures. It is only applied when there is reasonable certainty that there are no
uses of the boxed free variable itself within the corresponding function bodies.

Example: In the following code, the compiler observes that the closure returned from the function
f contains a variable pair (free in the body of f) that may be split into two separate variables.

let f x0 x1 =
let pair = x0, x1 in
Printf.printf "foo\n";
fun y ->
fst pair + snd pair + y

After some simplification one obtains:

let f x0 x1 =
let pair_0 = x0 in
let pair_1 = x1 in
Printf.printf "foo\n";
fun y ->
pair_0 + pair_1 + y

and then:
Chapter 23. Optimisation with Flambda 469

let f x0 x1 =
Printf.printf "foo\n";
fun y ->
x0 + x1 + y

The allocation of the pair has been eliminated.

This transformation does not operate if it would cause the closure to contain more than twice as
many closure variables as it did beforehand.

23.9.2 Unboxing of specialised arguments

This transformation is enabled unless -no-unbox-specialised-args is provided.
It may become the case during compilation that one or more invariant arguments to a function
become specialised to a particular value. When such values are themselves boxed the corresponding
specialised arguments may be split into more specialised arguments corresponding to the projections
out of the boxed value that occur within the function body. This transformation is called unboxing of
specialised arguments. It is only applied when there is reasonable certainty that the boxed argument
itself is unused within the function.
If the function in question is involved in a recursive group then unboxing of specialised arguments
may be immediately replicated across the group based on the dataflow between invariant arguments.

Example: Having been given the following code, the compiler will inline loop into f, and then
observe inv being invariant and always the pair formed by adding 42 and 43 to the argument x of
the function f.

let rec loop inv xs =

match xs with
| [] -> fst inv + snd inv
| x::xs -> x + loop2 xs inv
and loop2 ys inv =
match ys with
| [] -> 4
| y::ys -> y - loop inv ys

let f x =
Printf.printf "%d\n" (loop (x + 42, x + 43) [1; 2; 3])

Since the functions have sufficiently few arguments, more specialised arguments will be added.
After some simplification one obtains:

let f x =
let rec loop' xs inv_0 inv_1 =
match xs with
| [] -> inv_0 + inv_1
| x::xs -> x + loop2' xs inv_0 inv_1
and loop2' ys inv_0 inv_1 =
match ys with
470

| [] -> 4
| y::ys -> y - loop' ys inv_0 inv_1
in
Printf.printf "%d\n" (loop' [1; 2; 3] (x + 42) (x + 43))

The allocation of the pair within f has been removed. (Since the two closures for loop' and
loop2' are constant they will also be lifted to toplevel with no runtime allocation penalty. This
would also happen without having run the transformation to unbox specialise arguments.)
The transformation to unbox specialised arguments never introduces extra allocation.
The transformation will not unbox arguments if it would result in the original function having
sufficiently many arguments so as to inhibit tail-call optimisation.
The transformation is implemented by creating a wrapper function that accepts the original ar-
guments. Meanwhile, the original function is renamed and extra arguments are added corresponding
to the unboxed specialised arguments; this new function is called from the wrapper. The wrapper
will then be inlined at direct call sites. Indeed, all call sites will be direct unless -unbox-closures
is being used, since they will have been generated by the compiler when originally specialising the
function. (In the case of -unbox-closures other functions may appear with specialised arguments;
in this case there may be indirect calls and these will incur a small penalty owing to having to
bounce through the wrapper. The technique of direct call surrogates used for -unbox-closures is
not used by the transformation to unbox specialised arguments.)

23.9.3 Unboxing of closures

This transformation is not enabled by default. It may be enabled using the -unbox-closures flag.
The transformation replaces closure variables by specialised arguments. The aim is to cause
more closures to become closed. It is particularly applicable, as a means of reducing allocation,
where the function concerned cannot be inlined or specialised. For example, some non-recursive
function might be too large to inline; or some recursive function might offer no opportunities for
specialisation perhaps because its only argument is one of type unit.
At present there may be a small penalty in terms of actual runtime performance when this
transformation is enabled, although more stable performance may be obtained due to reduced
allocation. It is recommended that developers experiment to determine whether the option is
beneficial for their code. (It is expected that in the future it will be possible for the performance
degradation to be removed.)

Simple example: In the following code (which might typically occur when g is too large to
inline) the value of x would usually be communicated to the application of the + function via the
closure of g.

let f x =
let g y =
x + y
in
(g [@inlined never]) 42
Chapter 23. Optimisation with Flambda 471

Unboxing of the closure causes the value for x inside g to be passed as an argument to g rather
than through its closure. This means that the closure of g becomes constant and may be lifted to
toplevel, eliminating the runtime allocation.
The transformation is implemented by adding a new wrapper function in the manner of that
used when unboxing specialised arguments. The closure variables are still free in the wrapper, but
the intention is that when the wrapper is inlined at direct call sites, the relevant values are passed
directly to the main function via the new specialised arguments.
Adding such a wrapper will penalise indirect calls to the function (which might exist in arbitrary
places; remember that this transformation is not for example applied only on functions the compiler
has produced as a result of specialisation) since such calls will bounce through the wrapper. To
mitigate this, if a function is small enough when weighed up against the number of free variables
being removed, it will be duplicated by the transformation to obtain two versions: the original (used
for indirect calls, since we can do no better) and the wrapper/rewritten function pair as described
in the previous paragraph. The wrapper/rewritten function pair will only be used at direct call
sites of the function. (The wrapper in this case is known as a direct call surrogate, since it takes the
place of another function—the unchanged version used for indirect calls—at direct call sites.)
The -unbox-closures-factor command line flag, which takes an integer, may be used to adjust
the point at which a function is deemed large enough to be ineligible for duplication. The benefit of
duplication is scaled by the integer before being evaluated against the size.

Harder example: In the following code, there are two closure variables that would typically
cause closure allocations. One is called fv and occurs inside the function baz; the other is called z
and occurs inside the function bar. In this toy (yet sophisticated) example we again use an attribute
to simulate the typical situation where the first argument of baz is too large to inline.

let foo c =
let rec bar zs fv =
match zs with
| [] -> []
| z::zs ->
let rec baz f = function
| [] -> []
| a::l -> let r = fv + ((f [@inlined never]) a) in r :: baz f l
in
(map2 (fun y -> z + y) [z; 2; 3; 4]) @ bar zs fv
in
Printf.printf "%d" (List.length (bar [1; 2; 3; 4] c))

The code resulting from applying -O3 -unbox-closures to this code passes the free variables
via function arguments in order to eliminate all closure allocation in this example (aside from any
that might be performed inside printf).
472

23.10 Removal of unused code and values

23.10.1 Removal of redundant let expressions
The simplification pass removes unused let bindings so long as their corresponding defining
expressions have “no effects”. See the section “Treatment of effects” below for the precise definition
of this term.

23.10.2 Removal of redundant program constructs

This transformation is analogous to the removal of let-expressions whose defining expressions have
no effects. It operates instead on symbol bindings, removing those that have no effects.

23.10.3 Removal of unused arguments

This transformation is only enabled by default for specialised arguments. It may be enabled for all
arguments using the -remove-unused-arguments flag.
The pass analyses functions to determine which arguments are unused. Removal is effected by
creating a wrapper function, which will be inlined at every direct call site, that accepts the original
arguments and then discards the unused ones before calling the original function. As a consequence,
this transformation may be detrimental if the original function is usually indirectly called, since
such calls will now bounce through the wrapper. (The technique of direct call surrogates used to
reduce this penalty during unboxing of closure variables (see above) does not yet apply to the pass
that removes unused arguments.)

23.10.4 Removal of unused closure variables

This transformation performs an analysis across the whole compilation unit to determine whether
there exist closure variables that are never used. Such closure variables are then eliminated. (Note
that this has to be a whole-unit analysis because a projection of a closure variable from some
particular closure may have propagated to an arbitrary location within the code due to inlining.)

23.11 Other code transformations

23.11.1 Transformation of non-escaping references into mutable variables
Flambda performs a simple analysis analogous to that performed elsewhere in the compiler that
can transform refs into mutable variables that may then be held in registers (or on the stack as
appropriate) rather than being allocated on the OCaml heap. This only happens so long as the
reference concerned can be shown to not escape from its defining scope.

23.11.2 Substitution of closure variables for specialised arguments

This transformation discovers closure variables that are known to be equal to specialised arguments.
Such closure variables are replaced by the specialised arguments; the closure variables may then be
removed by the “removal of unused closure variables” pass (see below).
Chapter 23. Optimisation with Flambda 473

23.12 Treatment of effects

The Flambda optimisers classify expressions in order to determine whether an expression:

• does not need to be evaluated at all; and/or

• may be duplicated.

This is done by forming judgements on the effects and the coeffects that might be performed
were the expression to be executed. Effects talk about how the expression might affect the world;
coeffects talk about how the world might affect the expression.
Effects are classified as follows:

No effects:
The expression does not change the observable state of the world. For example, it must not
write to any mutable storage, call arbitrary external functions or change control flow (e.g. by
raising an exception). Note that allocation is not classed as having “no effects” (see below).

• It is assumed in the compiler that expressions with no effects, whose results are not
used, may be eliminated. (This typically happens where the expression in question is
the defining expression of a let; in such cases the let-expression will be eliminated.) It
is further assumed that such expressions with no effects may be duplicated (and thus
possibly executed more than once).
• Exceptions arising from allocation points, for example “out of memory” or exceptions
propagated from finalizers or signal handlers, are treated as “effects out of the ether” and
thus ignored for our determination here of effectfulness. The same goes for floating point
operations that may cause hardware traps on some platforms.

Only generative effects:

The expression does not change the observable state of the world save for possibly affecting
the state of the garbage collector by performing an allocation. Expressions that only have
generative effects and whose results are unused may be eliminated by the compiler. However,
unlike expressions with “no effects”, such expressions will never be eligible for duplication.

Arbitrary effects:
All other expressions.

There is a single classification for coeffects:

No coeffects:
The expression does not observe the effects (in the sense described above) of other expressions.
For example, it must not read from any mutable storage or call arbitrary external functions.

It is assumed in the compiler that, subject to data dependencies, expressions with neither effects
nor coeffects may be reordered with respect to other expressions.
474

23.13 Compilation of statically-allocated modules

Compilation of modules that are able to be statically allocated (for example, the module corre-
sponding to an entire compilation unit, as opposed to a first class module dependent on values
computed at runtime) initially follows the strategy used for bytecode. A sequence of let-bindings,
which may be interspersed with arbitrary effects, surrounds a record creation that becomes the
module block. The Flambda-specific transformation follows: these bindings are lifted to toplevel
symbols, as described above.

23.14 Inhibition of optimisation

Especially when writing benchmarking suites that run non-side-effecting algorithms in loops, it may
be found that the optimiser entirely elides the code being benchmarked. This behaviour can be
prevented by using the Sys.opaque_identity function (which indeed behaves as a normal OCaml
function and does not possess any “magic” semantics). The documentation of the Sys module
should be consulted for further details.

23.15 Use of unsafe operations

The behaviour of the Flambda simplification pass means that certain unsafe operations, which may
without Flambda or when using previous versions of the compiler be safe, must not be used. This
specifically refers to functions found in the Obj module.
In particular, it is forbidden to change any value (for example using Obj.set_field or Obj.set_
tag) that is not mutable. (Values returned from C stubs are always treated as mutable.) The
compiler will emit warning 59 if it detects such a write—but it cannot warn in all cases. Here is an
example of code that will trigger the warning:

let f x =
let a = 42, x in
(Obj.magic a : int ref) := 1;
fst a

The reason this is unsafe is because the simplification pass believes that fst a holds the value
42; and indeed it must, unless type soundness has been broken via unsafe operations.
If it must be the case that code has to be written that triggers warning 59, but the code is
known to actually be correct (for some definition of correct), then Sys.opaque_identity may be
used to wrap the value before unsafe operations are performed upon it. Great care must be taken
when doing this to ensure that the opacity is added at the correct place. It must be emphasised that
this use of Sys.opaque_identity is only for exceptional cases. It should not be used in normal
code or to try to guide the optimiser.
As an example, this code will return the integer 1:

let f x =
let a = Sys.opaque_identity (42, x) in
(Obj.magic a : int ref) := 1;
fst a
Chapter 23. Optimisation with Flambda 475

However the following code will still return 42:

let f x =
let a = 42, x in
Sys.opaque_identity (Obj.magic a : int ref) := 1;
fst a
High levels of inlining performed by Flambda may expose bugs in code thought previously to
be correct. Take care, for example, not to add type annotations that claim some mutable value is
always immediate if it might be possible for an unsafe operation to update it to a boxed value.

23.16 Glossary
The following terminology is used in this chapter of the manual.

Call site
See direct call site and indirect call site below.
Closed function
A function whose body has no free variables except its parameters and any to which are bound
other functions within the same (possibly mutually-recursive) declaration.
Closure
The runtime representation of a function. This includes pointers to the code of the function
together with the values of any variables that are used in the body of the function but actually
defined outside of the function, in the enclosing scope. The values of such variables, collectively
known as the environment, are required because the function may be invoked from a place
where the original bindings of such variables are no longer in scope. A group of possibly
mutually-recursive functions defined using let rec all share a single closure. (Note to developers:
in the Flambda source code a closure always corresponds to a single function; a set of closures
refers to a group of such.)
Closure variable
A member of the environment held within the closure of a given function.
Constant
Some entity (typically an expression) the value of which is known by the compiler at compile
time. Constantness may be explicit from the source code or inferred by the Flambda optimisers.
Constant closure
A closure that is statically allocated in an object file. It is almost always the case that the
environment portion of such a closure is empty.
Defining expression
The expression e in let x = e in e'.
Direct call site
A place in a program’s code where a function is called and it is known at compile time which
function it will always be.
476

Indirect call site

A place in a program’s code where a function is called but is not known to be a direct call site.

Program
A collection of symbol bindings forming the definition of a single compilation unit (i.e. .cmx
file).

Specialised argument
An argument to a function that is known to always hold a particular value at runtime. These
are introduced by the inliner when specialising recursive functions; and the unbox-closures
pass. (See section 23.4.)

Symbol
A name referencing a particular place in an object file or executable image. At that particular
place will be some constant value. Symbols may be examined using operating system-specific
tools (for example objdump on Linux).

Symbol binding
Analogous to a let-expression but working at the level of symbols defined in the object file.
The address of a symbol is fixed, but it may be bound to both constant and non-constant
expressions.

Toplevel
An expression in the current program which is not enclosed within any function declaration.

Variable
A named entity to which some OCaml value is bound by a let expression, pattern-matching
construction, or similar.
Chapter 24

Fuzzing with afl-fuzz

24.1 Overview
American fuzzy lop (“afl-fuzz”) is a fuzzer, a tool for testing software by providing randomly-generated
inputs, searching for those inputs which cause the program to crash.
Unlike most fuzzers, afl-fuzz observes the internal behaviour of the program being tested, and
adjusts the test cases it generates to trigger unexplored execution paths. As a result, test cases
generated by afl-fuzz cover more of the possible behaviours of the tested program than other fuzzers.
This requires that programs to be tested are instrumented to communicate with afl-fuzz. The
native-code compiler “ocamlopt” can generate such instrumentation, allowing afl-fuzz to be used
against programs written in OCaml.
For more information on afl-fuzz, see the website at http://lcamtuf.coredump.cx/afl/

24.2 Generating instrumentation

The instrumentation that afl-fuzz requires is not generated by default, and must be explicitly
enabled, by passing the -afl-instrument option to ocamlopt.
To fuzz a large system without modifying build tools, OCaml’s configure script also accepts the
afl-instrument option. If OCaml is configured with afl-instrument, then all programs compiled
by ocamlopt will be instrumented.

24.2.1 Advanced options

In rare cases, it is useful to control the amount of instrumentation generated. By passing the
-afl-inst-ratio N argument to ocamlopt with N less than 100, instrumentation can be generated
for only N% of branches. (See the afl-fuzz documentation on the parameter AFL_INST_RATIO for
the precise effect of this).

24.3 Example
As an example, we fuzz-test the following program, readline.ml:

477
478

let _ =
let s = read_line () in
match Array.to_list (Array.init (String.length s) (String.get s)) with
['s'; 'e'; 'c'; 'r'; 'e'; 't'; ' '; 'c'; 'o'; 'd'; 'e'] -> failwith "uh oh"
| _ -> ()

There is a single input (the string “secret code”) which causes this program to crash, but finding
it by blind random search is infeasible.
Instead, we compile with afl-fuzz instrumentation enabled:

ocamlopt -afl-instrument readline.ml -o readline

Next, we run the program under afl-fuzz:

mkdir input
echo asdf > input/testcase
mkdir output
afl-fuzz -m none -i input -o output ./readline

By inspecting instrumentation output, the fuzzer finds the crashing input quickly.
Note: To fuzz-test an OCaml program with afl-fuzz, passing the option -m none is required to
disable afl-fuzz’s default 50MB virtual memory limit.
Chapter 25

Runtime tracing with runtime events

This chapter describes the runtime events tracing system which enables continuous extraction of
performance information from the OCaml runtime with very low overhead. The system and interfaces
are low-level and tightly coupled to the runtime implementation, it is intended for end-users to rely
on tooling to consume and visualise data of interest.
Data emitted includes:
• Event times of garbage collector and runtime phases
• Minor and major heap sizings and utilization
• Allocation and promotion rates between heaps
Additional events can be declared and consumed, providing higher-level monitoring capabilities
to OCaml libraries. They are referred to as custom events.

25.1 Overview
There are three main classes of events emitted by the runtime events system:

Spans Events spanning over a duration in time. For example, the runtime events tracing system
emits a span event that starts when a minor collection begins in the OCaml garbage collector
and ends when the collection is completed. Spans can contain other spans, e.g other span
events may be emitted that begin after a minor collection has begun and end before it does.
Lifecycle events Events that occur at a moment in time. For example, when a domain terminates,
a corresponding lifecycle event is emitted.
Counters Events that include a measurement of some quantity of interest. For example, the
number of words promoted from the minor to the major heap during the last minor garbage
collection is emitted as a counter event.

The runtime events tracing system is designed to be used in different contexts:

Self monitoring OCaml programs and libraries can install their own callbacks to listen for runtime
events and react to them programmatically, for example, to export events to disk or over the
network.

479
480

External monitoring An external process can consume the runtime events of an OCaml program
whose runtime tracing system has been enabled by setting the corresponding environment
variable.

The runtime events tracing system logs events to a ring buffer. Consequently, old events are
being overwritten by new events. Consumers can either continuously consume events or choose to
only do so in response to some circumstance, e.g if a particular query or operation takes longer than
expected to complete.

25.2 Architecture
The runtime tracing system conceptually consists of two parts: 1) the event sources which emit
events and 2) the events transport that ingests and transports these events.

25.2.1 Event sources

Event sources are event-emitting points. There are a number of such sources in the OCaml runtime
system. These are further split into two sets: 1) event sources that are always available and 2) event
sources that are only available in the instrumented runtime. Event sources in the instrumented
runtime are primarily of interest to developers of the OCaml runtime and garbage collector and, at
present, only consist of major heap allocation size counter events.
The full set of events emitted by event sources and their documentation can be found in
section ??.

25.2.2 Events transport

The events transport part of the system ingests events emitted by the event sources and makes
them available to consumers.

25.2.3 Ring buffers

Events are transported using a data structure known as a ring buffer. This data structure consists
of two pointers into a linear backing array, the tail pointer points to a location where new events
can be written and the head pointer points to the oldest event in the buffer that can be read. When
insufficient space is available in the backing array to write new events, the head pointer is advanced
and the oldest events are overwritten by new ones.
The ring buffer implementation used in runtime events can be written by at most one producer
at a time but can be read simultaneously by multiple consumers without coordination from the
producer. There is a unique ring buffer for every running domain and, on domain termination,
ring buffers may be re-used for newly spawned domains. Ring buffers are only allocated (including
for the main domain) when runtime events are enabled. The ring buffers themselves are stored in
a memory-mapped file with the processes identifier as the name and the extension .events, this
enables them to be read from outside the main OCaml process. See Runtime_events[??] for more
information.
Chapter 25. Runtime tracing with runtime events 481

25.2.4 Consumption APIs

The runtime event tracing system provides both OCaml and C APIs which are cursor-based and
polling-driven. The high-level process for consuming events is as follows:

1. A cursor is created via Runtime_events.create_cursor for either the current process or an

external process (specified by a path and PID).

2. Runtime_events.Callbacks.create is called to register a callback function to receive the

events.

3. The cursor is polled via Runtime_events.read_poll using the callbacks created in the
previous step. For each matching event in the ring buffers, the provided callback functions are
called. In addition to the emitted events, callbacks are given the emitter domain’s ID and the
emission timestamp.

25.3 Usage
25.3.1 With OCaml APIs
We start with a simple example that prints the name, begin and end times of events emitted by the
runtime event tracing system:

let runtime_begin _ ts phase =

Printf.printf "Begin\t%s\t%Ld\n"
(Runtime_events.runtime_phase_name phase)
(Runtime_events.Timestamp.to_int64 ts)

let runtime_end _ ts phase =

Printf.printf "End\t%s\t%Ld\n"
(Runtime_events.runtime_phase_name phase)
(Runtime_events.Timestamp.to_int64 ts)

let () =
Runtime_events.start ();
let cursor = Runtime_events.create_cursor None in
let callbacks = Runtime_events.Callbacks.create ~runtime_begin ~runtime_end ()
in
while true do
let list_ref = ref [] in (* for later fake GC work *)
for _ = 1 to 100 do
(* here we do some fake GC work *)
list_ref := [];
for _ = 1 to 10 do
list_ref := (Sys.opaque_identity(ref 42)) :: !list_ref
done;
Gc.full_major ();
482

done;
ignore(Runtime_events.read_poll cursor callbacks None);
Unix.sleep 1
done

The next step is to compile and link the program with the runtime_events library. This can be
done as follows:

ocamlopt -I +runtime_events -I +unix unix.cmxa runtime_events.cmxa

example.ml -o example

When using the dune build system, this example can be built as follows:

(executable
(name example)
(modules example)
(libraries unix runtime_events))

Running the compiled binary of the example gives an output similar to:

Begin explicit_gc_full_major 24086187297852

Begin stw_leader 24086187298594
Begin minor 24086187299404
Begin minor_global_roots 24086187299807
End minor_global_roots 24086187331461
Begin minor_remembered_set 24086187331631
Begin minor_finalizers_oldify 24086187544312
End minor_finalizers_oldify 24086187544704
Begin minor_remembered_set_promote 24086187544879
End minor_remembered_set_promote 24086187606414
End minor_remembered_set 24086187606584
Begin minor_finalizers_admin 24086187606854
End minor_finalizers_admin 24086187607152
Begin minor_local_roots 24086187607329
Begin minor_local_roots_promote 24086187609699
End minor_local_roots_promote 24086187610539
End minor_local_roots 24086187610709
End minor 24086187611746
Begin minor_clear 24086187612238
End minor_clear 24086187612580
End stw_leader 24086187613209
...

This is an example of self-monitoring, where a program explicitly starts listening to runtime

events and monitors itself.
Chapter 25. Runtime tracing with runtime events 483

For external monitoring, a program does not need to be aware of the existence of runtime events.
Runtime events can be controlled via the environment variable OCAML_RUNTIME_EVENTS_START
which, when set, will cause the runtime tracing system to be started at program initialization.
We could remove Runtime_events.start (); from the previous example and, instead, call the
program as below to produce the same result:

OCAML_RUNTIME_EVENTS_START=1 ./example

25.3.2 Environment variables

Environment variables can be used to control different aspects of the runtime event tracing system.
The following environment variables are available:

• OCAML_RUNTIME_EVENTS_START if set will cause the runtime events system to be

started as part of the OCaml runtime initialization.

• OCAML_RUNTIME_EVENTS_DIR sets the directory where the .events files containing

the runtime event tracing system’s ring buffers will be located. If not present the program’s
working directory will be used.

• OCAML_RUNTIME_EVENTS_PRESERVE if set will make the OCaml runtime preserve

the runtime events ring buffer files past the termination of the OCaml program. This can be
useful for monitoring very short running programs. If not set, the .events files of the OCaml
program will be deleted at program termination.

The size of the runtime events ring buffers can be configured via OCAMLRUNPARAM, see
section 15.2 for more information.

25.3.3 Building with the instrumented runtime

Some events are only emitted by the instrumented runtime. To receive them, the OCaml program
needs to be compiled and linked against the instrumented runtime. For our example program from
earlier, this is achieved as follows:

ocamlopt -runtime-variant i -I +runtime_events -I +unix unix.cmxa runtime_events.cmxa example.m

o example

And for dune:

(executable
(name example)
(modules example)
(flags "-runtime-variant=i")
(libraries unix runtime_events))
484

25.3.4 With tooling

Programmatic access to events is intended primarily for writers of observability libraries and tooling
for end-users. The flexible API enables use of the performance data from runtime events for logging
and monitoring purposes.
In this section we cover several utilities in the runtime_events_tools package which provide
simple ways of extracting and summarising data from runtime events. The trace utility in particular
produces similar data to the previous ’eventlog’ instrumentation system available in OCaml 4.12 to
4.14.
First, install runtime_events_tools in an OCaml 5.0+ opam switch:

opam install runtime_events_tools

This should install the olly tool in your path. You can now generate runtime traces for programs
compiled with OCaml 5.0+ using the trace subcommand:

olly trace trace.json 'your_program.exe .. args ..'

Runtime tracing data will be generated in the json Trace Event Format to trace.json. This can
then be loaded into the Chrome tracing viewer or into Perfetto to visualize the collected trace.

25.3.5 Measuring GC latency

The olly utility also includes a latency subcommand which consumes runtime events data and on
program completion emits a parseable histogram summary of pause durations. It can be run as
follows:

olly latency 'your_program.exe .. args ..'

This should produce an output similar to the following:

GC latency profile:
#[Mean (ms): 2.46, Stddev (ms): 3.87]
#[Min (ms): 0.01, max (ms): 9.17]

Percentile Latency (ms)

25.0000 0.01
50.0000 0.23
60.0000 0.23
70.0000 0.45
75.0000 0.45
80.0000 0.45
85.0000 0.45
90.0000 9.17
95.0000 9.17
96.0000 9.17
97.0000 9.17
Chapter 25. Runtime tracing with runtime events 485

98.0000 9.17
99.0000 9.17
99.9000 9.17
99.9900 9.17
99.9990 9.17
99.9999 9.17
100.0000 9.17

25.4 Custom events

25.4.1 Overview
The runtime events system supports defining custom events. Identified by a globally unique name,
they are emitted with payloads of built-in (unit, int, span) and user-defined types. To understand
the manipulation of custom events, it is useful to know how they are transported and stored: their
representation consists of a name string (in fact, an index into an array of all custom names) and an
arbitrary byte sequence. Custom event types can be registered by providing encoding and decoding
functions to and from a byte sequence, via the function Runtime_events.Type.register.
Defining a new custom event (whether its payload is of a built-in type or a custom type) is
done via another registration function, Runtime_events.User.register. This function records
the association between the custom event’s name, its type, and a tag. The tag is then used when
emitting or consuming custom events; it acts as a shorthand for the event name.
Once an event is registered, values for that event are emitted using Runtime_events.User.write.
To summarize, for a user to emit and consume custom events using a custom type they need to:

1. register the custom type

2. extend the Runtime_events.User.tag variant with a new constructor
3. register the new custom event, binding together the custom event name, the new tag, and the
new custom type
4. emit instances of the event using Runtime_events.User.write
5. indicate a callback which should receive all events of the custom type using Runtime_
events.Callbacks.add_user_event to register it inside a Runtime_events.Callbacks.t
object to be used when polling (see section 25.3.1 above).
6. (Optional, if there are distinct events registered with the same custom type:) In that callback,
pattern-match on the event tag in order to discriminate between different event tags of the
same custom type.

Note that if the emitter and the consumer are different programs, both must perform steps 1 to
3 to register custom events and custom event types (if any). Note that the tag values need not be
the same in both programs; the only values that should match are the names.
Unregistered events for user-defined types are currently ignored. As a fallback, unregistered,
custom events of a built-in type are available, but are all tagged UNK (unknown).
There are thus three cases for the consumer when an event is received:
486

• event is registered: payload and tag are available.

• event is not registered and has a built-in event type (unit, int, span): only the payload is
available.

• event is not registered and has a custom event type: event is dropped.

Note that event consumers bind callbacks to event types, so they can work as generic listeners
interpreting payloads coming from events that were not registered. Because this only works for
events of built-in types, it can be useful to emit events in pairs: an event of a custom type with
an event of a built-in type, enabling the design of a specialized consumer while staying compatible
with generic tracing tools.

25.4.2 Tracing custom events: an example

The following program uses the Runtime_events.User and Runtime_events.Type modules to
declare two custom events providing span and int values. The tag extensible variant is extended
with CustomSpan and CustomInt.

type Runtime_events.User.tag += CustomSpan | CustomInt

let count_span =
Runtime_events.User.register "count.span" CustomSpan
Runtime_events.Type.span

let count_value =
Runtime_events.User.register "count.value" CustomInt
Runtime_events.Type.int

let count () =
Runtime_events.User.write count_span Begin;
for i = 1 to 5 do
Runtime_events.User.write count_value i
done;
Runtime_events.User.write count_span End

let () =
Runtime_events.start ();
for _ = 1 to 3 do
count ()
done

On the consumer side, one can use the provided event tag and type to match on the relevant
events. For the sake of completeness, we assume that the consumer is a different program, and
repeat the definition of the new tag constructors, as well as the registering of the custom events.

type Runtime_events.User.tag += CustomSpan | CustomInt

Chapter 25. Runtime tracing with runtime events 487

let count_span =
Runtime_events.User.register "count.span" CustomSpan
Runtime_events.Type.span

let count_value =
Runtime_events.User.register "count.value" CustomInt
Runtime_events.Type.int

let span_event_handler domain_id ts event value =

(* we're only interested in our CustomSpan event *)
match Runtime_events.User.tag event, value with
| CustomSpan, Runtime_events.Type.Begin -> Printf.printf "> count begin\n"
| CustomSpan, End -> Printf.printf "< count end\n"
| _ -> ()

let int_event_handler domain_id ts event value =

(* we're only interested in our CustomInt event *)
match Runtime_events.User.tag event with
| CustomInt -> Printf.printf "| count %d\n" value
| _ -> ()

let () =
let open Runtime_events in
let cursor = create_cursor None in
let callbacks =
Callbacks.create ()
|> Callbacks.add_user_event Type.span span_event_handler
|> Callbacks.add_user_event Type.int int_event_handler
in
for _ = 0 to 100 do
ignore(read_poll cursor callbacks None)
done

Giving the following output:

> count begin
| count 1
| count 2
| count 3
| count 4
| count 5
< count end
> count begin
| count 1
| count 2
488

[...]
Chapter 26

The “Tail Modulo Constructor”

program transformation

(Introduced in OCaml 4.14)

Note: this feature is considered experimental, and its interface may evolve, with user feedback,
in the next few releases of the language.
Consider this natural implementation of the List.map function:
let rec map f l =
match l with
| [] -> []
| x :: xs ->
let y = f x in
y :: map f xs
A well-known limitation of this implementation is that the recursive call, map f xs, is not in
tail position. The runtime needs to remember to continue with y :: r after the call returns a value
r, therefore this function consumes some amount of call-stack space on each recursive call. The
stack usage of map f li is proportional to the length of li. This is a correctness issue for large
lists on systems configured with limited stack space – the dreaded Stack_overflow exception.
# let with_stack_limit stack_limit f =
let old_gc_settings = Gc.get () in
Gc.set { old_gc_settings with stack_limit };
Fun.protect ~finally:(fun () -> Gc.set old_gc_settings) f
;;
val with_stack_limit : int -> (unit -> 'a) -> 'a = <fun>

# with_stack_limit 20_000 (fun () ->

List.length (map Fun.id (List.init 1_000_000 Fun.id))
);;
Stack overflow during evaluation (looping recursion?).
In this implementation of map, the recursive call happens in a position that is not a tail position
in the program, but within a datatype constructor application that is itself in tail position. We

489
490

say that those positions, that are composed of tail positions and constructor applications, are tail
modulo constructor (TMC) positions – we sometimes write tail modulo cons for brevity.
It is possible to rewrite programs such that tail modulo cons positions become tail positions;
after this transformation, the implementation of map above becomes tail-recursive, in the sense
that it only consumes a constant amount of stack space. The OCaml compiler implements this
transformation on demand, using the [@tail_mod_cons] or [@ocaml.tail_mod_cons] attribute
on the function to transform.
let[@tail_mod_cons] rec map f l =
match l with
| [] -> []
| x :: xs ->
let y = f x in
y :: map f xs
# List.length (map Fun.id (List.init 1_000_000 Fun.id));;
- : int = 1000000
This transformation only improves calls in tail-modulo-cons position, it does not improve
recursive calls that do not fit in this fragment:
(∗ does ∗not∗ work: addition is not a data constructor ∗)
let[@tail_mod_cons] rec length l =
match l with
| [] -> 0
| _ :: xs -> 1 + length xs
Warning 71 [unused -tmc - attribute ]: This function is marked @tail_mod_cons
but is never applied in TMC position .
It is of course possible to use the [@tail_mod_cons] transformation on functions that contain
some recursive calls in tail-modulo-cons position, and some calls in other, arbitrary positions. Only
the tail calls and tail-modulo-cons calls will happen in constant stack space.

General design This feature is provided as an explicit program transformation, not an implicit
optimization. It is annotation-driven: the user is expected to express their intent by adding
annotations in the program using attributes, and will be asked to do so in any ambiguous situation.
We expect it to be used mostly by advanced OCaml users needing to get some guarantees on
the stack-consumption behavior of their programs. Our recommendation is to use the [@tailcall]
annotation on all callsites that should not consume any stack space. [@tail_mod_cons] extends the
set of functions on which calls can be annotated to be tail calls, helping establish stack-consumption
guarantees in more cases.

Performance A standard approach to get a tail-recursive version of List.map is to use an

accumulator to collect output elements, and reverse it at the end of the traversal.
let rec map f l = map_aux f [] l
and map_aux f acc l =
match l with
Chapter 26. The “Tail Modulo Constructor” program transformation 491

| [] -> List.rev acc

| x :: xs ->
let y = f x in
map_aux f (y :: acc) xs
This version is tail-recursive, but it is measurably slower than the simple, non-tail-recursive ver-
sion. In contrast, the tail-mod-cons transformation provides an implementation that has comparable
performance to the original version, even on small inputs.

Evaluation order Beware that the tail-modulo-cons transformation has an effect on evaluation
order: the constructor argument that is transformed into tail-position will always be evaluated last.
Consider the following example:
type 'a two_headed_list =
| Nil
| Consnoc of 'a * 'a two_headed_list * 'a

let[@tail_mod_cons] rec map f = function

| Nil -> Nil
| Consnoc (front, body, rear) ->
Consnoc (f front, map f body, f rear)
Due to the [@tail_mod_cons] transformation, the calls to f front and f rear will be evaluated
before map f body. In particular, this is likely to be different from the evaluation order of the
unannotated version. (The evaluation order of constructor arguments is unspecified in OCaml, but
many implementations typically use left-to-right or right-to-left.)
This effect on evaluation order is one of the reasons why the tail-modulo-cons transformation
has to be explicitly requested by the user, instead of being applied as an automatic optimization.

Why tail-modulo-cons? Other program transformations, in particular a transformation to

continuation-passing style (CPS), can make all functions tail-recursive, instead of targeting only a
small fragment. Some reasons to provide builtin support for the less-general tail-mod-cons are as
follows:

• The tail-mod-cons transformation preserves the performance of the original, non-tail-recursive

version, while a continuation-passing-style transformation incurs a measurable constant-factor
overhead.

• The tail-mod-cons transformation cannot be expressed as a source-to-source transformation of

OCaml programs, as it relies on mutable state in type-unsafe ways. In contrast, continuation-
passing-style versions can be written by hand, possibly using a convenient monadic notation.

Note: OCaml call stack size In OCaml 4.x and earlier, bytecode programs respect the
stack_limit runtime parameter configuration (as set using Gc.set in the example above), or the
l setting of the OCAMLRUNPARAM variable. Native programs ignore these settings and only respect
the operating system native stack limit, as set by ulimit on Unix systems. Most operating systems
run with a relatively low stack size limit by default, so stack overflow on non-tail-recursive functions
are a common programming bug.
492

Starting from OCaml 5.0, native code does not use the native system stack for OCaml function
calls anymore, so it is not affected by the operating system native stack size; both native and
bytecode programs respect the OCaml runtime’s own limit. The runtime limit is set to a much
higher default than most operating system native stacks, with a limit of at least 512MiB, so stack
overflow should be much less common in practice. There is still a stack limit by default, as it
remains useful to quickly catch bugs with looping non-tail-recursive functions. Without a stack
limit, one has to wait for the whole memory to be consumed by the stack for the program to crash,
which can take a long time and make the system unresponsive.
This means that the tail modulo constructor transformation is less important on OCaml 5:
it does improve performance noticeably in some cases, but it is not necessary for basic correctness
for most use-cases.

26.1 Disambiguation
It may happen that several arguments of a constructor are recursive calls to a tail-modulo-cons
function. The transformation can only turn one of these calls into a tail call. The compiler will not
make an implicit choice, but ask the user to provide an explicit disambiguation.
Consider this type of syntactic expressions (assuming some pre-existing type var of expression
variables):
type var (∗ some pre−existing type of variables ∗)

type exp =
| Var of var
| Let of binding * exp
and binding = var * exp
Consider a map function on variables. The direct definition has two recursive calls inside
arguments of the Let constructor, so it gets rejected as ambiguous.
let[@tail_mod_cons] rec map_vars f exp =
match exp with
| Var v -> Var (f v)
| Let ((v, def), body) ->
Let ((f v, map_vars f def), map_vars f body)
Error : [ @tail_mod_cons ]: this constructor application may be TMC - transformed
in several different ways. Please disambiguate by adding an explicit
[ @tailcall ] attribute to the call that should be made tail -recursive ,
or a [ @tailcall false] attribute on calls that should not be
transformed .
This call could be annotated .
This call could be annotated .
To disambiguate, the user should add a [@tailcall] attribute to the recursive call that should
be transformed to tail position:
let[@tail_mod_cons] rec map_vars f exp =
match exp with
Chapter 26. The “Tail Modulo Constructor” program transformation 493

| Var v -> Var (f v)

| Let ((v, def), body) ->
Let ((f v, map_vars f def), (map_vars[@tailcall]) f body)
Be aware that the resulting function is not tail-recursive, the recursive call on def will consume
stack space. However, expression trees tend to be right-leaning (lots of Let in sequence, rather than
nested inside each other), so putting the call on body in tail position is an interesting improvement
over the naive definition: it gives bounded stack space consumption if we assume a bound on the
nesting depth of Let constructs.
One would also get an error when using conflicting annotations, asking for two of the constructor
arguments to be put in tail position:
let[@tail_mod_cons] rec map_vars f exp =
match exp with
| Var v -> Var (f v)
| Let ((v, def), body) ->
Let ((f v, (map_vars[@tailcall]) f def), (map_vars[@tailcall]) f body)
Error : [ @tail_mod_cons ]: this constructor application may be TMC - transformed
in several different ways. Only one of the arguments may become a TMC
call , but several arguments contain calls that are explicitly marked
as tail - recursive . Please fix the conflict by reviewing and fixing the
conflicting annotations .
This call is explicitly annotated .
This call is explicitly annotated .

26.2 Danger: getting out of tail-mod-cons

Due to the nature of the tail-mod-cons transformation (see Section 26.3 for a presentation of
transformation):

• Calls from a tail-mod-cons function to another tail-mod-cons function declared in the same
recursive-binding group are transformed into tail calls, as soon as they occur in tail position
or tail-modulo-cons position in the source function.

• Calls from a function not annotated tail-mod-cons to a tail-mod-cons function or, conversely,
from a tail-mod-cons function to a non-tail-mod-cons function are transformed into non-tail
calls, even if they syntactically appear in tail position in the source program.

The fact that calls in tail position in the source program may become non-tail calls if they go
from a tail-mod-cons to a non-tail-mod-cons function is surprising, and the transformation will warn
about them.
For example:
let[@tail_mod_cons] rec flatten = function
| [] -> []
| xs :: xss ->
let rec append_flatten xs xss =
match xs with
494

| [] -> flatten xss

| x :: xs -> x :: append_flatten xs xss
in append_flatten xs xss

Warning 71 [unused -tmc - attribute ]: This function is marked @tail_mod_cons

but is never applied in TMC position .
Warning 72 [tmc -breaks - tailcall ]: This call
is in tail -modulo -cons position in a TMC function ,
but the function called is not itself specialized for TMC ,
so the call will not be transformed into a tail call.
Please either mark the called function with the [ @tail_mod_cons ]
attribute , or mark this call with the [ @tailcall false] attribute
to make its non - tailness explicit .
Here the append_flatten helper is not annotated with [@tail_mod_cons], so the calls
append_flatten xs xss and flatten xss will not be tail calls. The correct fix here is to annotate
append_flatten to be tail-mod-cons.
let[@tail_mod_cons] rec flatten = function
| [] -> []
| xs :: xss ->
let[@tail_mod_cons] rec append_flatten xs xss =
match xs with
| [] -> flatten xss
| x :: xs -> x :: append_flatten xs xss
in append_flatten xs xss
The same warning occurs when append_flatten is a non-tail-mod-cons function of the same
recursive group; using the tail-mod-cons transformation is a property of individual functions, not
whole recursive groups.
let[@tail_mod_cons] rec flatten = function
| [] -> []
| xs :: xss -> append_flatten xs xss

and append_flatten xs xss =

match xs with
| [] -> flatten xss
| x :: xs -> x :: append_flatten xs xss

Warning 71 [unused -tmc - attribute ]: This function is marked @tail_mod_cons

but is never applied in TMC position .
Warning 72 [tmc -breaks - tailcall ]: This call
is in tail -modulo -cons position in a TMC function ,
but the function called is not itself specialized for TMC ,
so the call will not be transformed into a tail call.
Please either mark the called function with the [ @tail_mod_cons ]
attribute , or mark this call with the [ @tailcall false] attribute
to make its non - tailness explicit .
Chapter 26. The “Tail Modulo Constructor” program transformation 495

Again, the fix is to specialize append_flatten as well:

let[@tail_mod_cons] rec flatten = function
| [] -> []
| xs :: xss -> append_flatten xs xss

and[@tail_mod_cons] append_flatten xs xss =

match xs with
| [] -> flatten xss
| x :: xs -> x :: append_flatten xs xss
Non-recursive functions can also be annotated [@tail_mod_cons]; this is typically useful for
local bindings to recursive functions.
Incorrect version:
let[@tail_mod_cons] rec map_vars f exp =
let self exp = map_vars f exp in
match exp with
| Var v -> Var (f v)
| Let ((v, def), body) ->
Let ((f v, self def), (self[@tailcall]) body)

Warning 51 [wrong -tailcall - expectation ]: expected tailcall

Warning 51 [wrong -tailcall - expectation ]: expected tailcall

Warning 71 [unused -tmc - attribute ]: This function is marked @tail_mod_cons
but is never applied in TMC position .
Recommended fix:
let[@tail_mod_cons] rec map_vars f exp =
let[@tail_mod_cons] self exp = map_vars f exp in
match exp with
| Var v -> Var (f v)
| Let ((v, def), body) ->
Let ((f v, self def), (self[@tailcall]) body)
In other cases, there is either no benefit in making the called function tail-mod-cons, or it is not
possible: for example, it is a function parameter (the transformation only works with direct calls to
known functions).
For example, consider a substitution function on binary trees:
type 'a tree = Leaf of 'a | Node of 'a tree * 'a tree

let[@tail_mod_cons] rec bind (f : 'a -> 'a tree) (t : 'a tree) : 'a tree =
match t with
| Leaf v -> f v
| Node (left, right) ->
Node (bind f left, (bind[@tailcall]) f right)
Warning 72 [tmc -breaks - tailcall ]: This call
496

is in tail -modulo -cons position in a TMC function ,

but the function called is not itself specialized for TMC ,
so the call will not be transformed into a tail call.
Please either mark the called function with the [ @tail_mod_cons ]
attribute , or mark this call with the [ @tailcall false] attribute
to make its non - tailness explicit .
Here f is a function parameter, not a direct call, and the current implementation is strictly
first-order, it does not support tail-mod-cons arguments. In this case, the user should indicate that
they realize this call to f v is not, in fact, in tail position, by using (f[@tailcall false]) v.
type 'a tree = Leaf of 'a | Node of 'a tree * 'a tree

let[@tail_mod_cons] rec bind (f : 'a -> 'a tree) (t : 'a tree) : 'a tree =
match t with
| Leaf v -> (f[@tailcall false]) v
| Node (left, right) ->
Node (bind f left, (bind[@tailcall]) f right)

26.3 Details on the transformation

To use this advanced feature, it helps to be aware that the function transformation produces a
specialized function in destination-passing-style.
Recall our map example:
let rec map f l =
match l with
| [] -> []
| x :: xs ->
let y = f x in
y :: map f xs
Below is a description of the transformed program in pseudo-OCaml notation: some operations
are not expressible in OCaml source code. (The transformation in fact happens on the Lambda
intermediate representation of the OCaml compiler.)

let rec map f l =

match l with
| [] -> []
| x :: xs ->
let y = f x in
let dst = y ::{mutable} Hole in
map_dps f xs dst 1;
dst

and map_dps f l dst idx =

match l with
| [] -> dst.idx <- []
Chapter 26. The “Tail Modulo Constructor” program transformation 497

| x :: xs ->
let y = f x in
let dst' = y ::{mutable} Hole in
dst.idx <- dst';
map_dps f xs dst' 1

The source version of map gets transformed into two functions, a direct-style version that is also
called map, and a destination-passing-style version (DPS) called map_dps. The destination-passing-
style version does not return a result directly, instead it writes it into a memory location specified
by two additional function parameters, dst (a memory block) and i (a position within the memory
block).
The source call y :: map f xs gets transformed into the creation of a mutable block
y ::{mutable} Hole, whose second parameter is an un-initialized hole. The block is then passed
to map_dps as a destination parameter (with offset 1).
Notice that map does not call itself recursively, it calls map_dps. Then, map_dps calls itself
recursively, in a tail-recursive way.
The call from map to map_dps is not a tail call (this is something that we could improve in the
future); but this call happens only once when invoking map f l, with all list elements after the first
one processed in constant stack by map_dps.
This explains the “getting out of tail-mod-cons” subtleties. Consider our previous example
involving mutual recursion between flatten and append_flatten.
let[@tail_mod_cons] rec flatten l =
match l with
| [] -> []
| xs :: xss ->
append_flatten xs xss
The call to append_flatten, which syntactically appears in tail position, gets transformed
differently depending on whether the function has a destination-passing-style version available, that
is, whether it is itself annotated [@tail_mod_cons]:
(* if append_flatten_dps exists *)
and flatten_dps l dst i =
match l with
| [] -> dst.i <- []
| xs :: xss ->
append_flatten_dps xs xss dst i

(* if append_flatten_dps does not exist *)

and rec flatten_dps l dst i =
match l with
| [] -> dst.i <- []
| xs :: xss ->
dst.i <- append_flatten xs xss
If append_flatten does not have a destination-passing-style version, the call gets transformed
to a non-tail call.
498

26.4 Current limitations

Purely syntactic criterion Just like tail calls in general, the notion of tail-modulo-constructor
position is purely syntactic; some simple refactoring will move calls out of tail-modulo-constructor
position.
(∗ works as expected ∗)
let[@tail_mod_cons] rec map f li =
match li with
| [] -> []
| x :: xs ->
let y = f x in
y ::
(∗ this call is in TMC position ∗)
map f xs
(∗ not optimizable anymore ∗)
let[@tail_mod_cons] rec map f li =
match li with
| [] -> []
| x :: xs ->
let y = f x in
let ys =
(* this call is not in TMC position anymore *)
map f xs in
y :: ys
Warning 71 [unused -tmc - attribute ]: This function is marked @tail_mod_cons
but is never applied in TMC position .

Local, first-order transformation When a function gets transformed with tail-mod-cons, two
definitions are generated, one providing a direct-style interface and one providing the destination-
passing-style version. However, not all calls to this function in tail-modulo-cons position will use
the destination-passing-style version and become tail calls:

• The transformation is local: only tail-mod-cons calls to foo within the same compilation unit
as foo become tail calls.

• The transformation is first-order: only direct calls to known tail-mod-cons functions become
tail calls when in tail-mod-cons position, never calls to function parameters.

Consider the call Option.map foo x for example: even if foo is called in tail-mod-cons position
within the definition of Option.map, that call will never become a tail call. (This would be the case
even if the call to Option.map was inside the Option module.)
In general this limitation is not a problem for recursive functions: the first call from an outside
module or a higher-order function will consume stack space, but further recursive calls in tail-mod-
cons position will get optimized. For example, if List.map is defined as a tail-mod-cons function,
calls from outside the List module will not become tail calls when in tail positions, but the recursive
Chapter 26. The “Tail Modulo Constructor” program transformation 499

calls within the definition of List.map are in tail-modulo-cons positions and do become tail calls:
processing the first element of the list will consume stack space, but all further elements are handled
in constant space.
These limitations may be an issue in more complex situations where mutual recursion happens
between functions, with some functions not annotated tail-mod-cons, or defined across different
modules, or called indirectly, for example through function parameters.

Non-exact calls to tupled functions OCaml performs an implicit optimization for “tupled”
functions, which take a single parameter that is a tuple: let f (x, y, z) = .... Direct calls to
these functions with a tuple literal argument (like f (a, b, c)) will call the “tupled” function by
passing the parameters directly, instead of building a tuple of them. Other calls, either indirect
calls or calls passing a more complex tuple value (like let t = (a, b, c) in f t) are compiled
as “inexact” calls that go through a wrapper.
The [@tail_mod_cons] transformation supports tupled functions, but will only optimize
“exact” calls in tail position; direct calls to something other than a tuple literal will not
become tail calls. The user can manually unpack a tuple to force a call to be “exact”:
let (x, y, z) = t in f (x, y, z). If there is any doubt as to whether a call can be
tail-mod-cons-optimized or not, one can use the [@tailcall] attribute on the called function,
which will warn if the transformation is not possible.
let rec map (f, l) =
match l with
| [] -> []
| x :: xs ->
let y = f x in
let args = (f, xs) in
(∗ this inexact call cannot be tail−optimized, so a warning will be raised ∗)
y :: (map[@tailcall]) args
Warning 51 [wrong -tailcall - expectation ]: expected tailcall
500
Chapter 27

Runtime detection of data races with

ThreadSanitizer

27.1 Overview and usage

OCaml, since version 5.0, allows shared-memory parallelism and thus mutation of data shared
between multiple threads. This creates the possibility of data races, i.e., unordered accesses to
the same memory location with at least one of them being a write. In OCaml, data races are
easy to introduce, and the behaviour of programs with data races can be unintuitive — the
observed behaviours cannot be explained by simply interleaving operations from different concurrent
threads. More information about data races and their consequences can be found in section 9.4 and
Chapter 10.
To help detect data races, OCaml supports ThreadSantizer (TSan), a dynamic data race detector
that has been successfully used in languages such as C/C++, Swift, etc. TSan support for OCaml
is available since OCaml 5.2.
To use TSan, you must configure the compiler with --enable-tsan. You can also install an
opam switch with the TSan feature enabled as follows:

opam switch create <YOUR-SWITCH-NAME-HERE> ocaml-option-tsan

TSan support for OCaml is currently available for the x86_64 architecture, on FreeBSD, Linux
and macOS, for the arm64 architecture on Linux and macOS, and for the POWER, riscv and s390x
architectures on Linux.
Building OCaml with TSan support requires GCC or Clang. Minimal supported versions are
GCC 11 and Clang 14. Note that TSan data race reports with GCC 11 are known to result in poor
stack trace reporting (no line numbers), which is fixed in GCC 12.
A TSan-enabled compiler differs from a regular compiler in the following way: all programs
compiled by ocamlopt are instrumented with calls to the TSan runtime, and TSan will detect data
races encountered during execution.
For instance, consider the following program:
let a = ref 0 and b = ref 0

let d1 () =

501
502

a := 1;
!b

let d2 () =
b := 1;
!a

let () =
let h = Domain.spawn d2 in
let r1 = d1 () in
let r2 = Domain.join h in
assert (not (r1 = 0 && r2 = 0))
This program has data races. The memory locations a and b are read and written concurrently
by multiple domains d1 and d2. a and b are “non-atomic” locations according to the memory model
(see Chapter 10), and there is no synchronization between accesses to them. Hence, there are two
data races here corresponding to the two memory locations a and b.
When you compile and run this program with ocamlopt, you may observe data race reports on
the standard error, such as:

==================
WARNING: ThreadSanitizer: data race (pid=3808831)
Write of size 8 at 0x8febe0 by thread T1 (mutexes: write M90):
#0 camlSimple_race.d2_274 simple_race.ml:8 (simple_race.exe+0x420a72)
#1 camlDomain.body_706 stdlib/domain.ml:211 (simple_race.exe+0x440f2f)
#2 caml_start_program <null> (simple_race.exe+0x47cf37)
#3 caml_callback_exn runtime/callback.c:197 (simple_race.exe+0x445f7b)
#4 domain_thread_func runtime/domain.c:1167 (simple_race.exe+0x44a113)

Previous read of size 8 at 0x8febe0 by main thread (mutexes: write M86):

#0 camlSimple_race.d1_271 simple_race.ml:5 (simple_race.exe+0x420a22)
#1 camlSimple_race.entry simple_race.ml:13 (simple_race.exe+0x420d16)
#2 caml_program <null> (simple_race.exe+0x41ffb9)
#3 caml_start_program <null> (simple_race.exe+0x47cf37)
[...]

WARNING: ThreadSanitizer: data race (pid=3808831)

Read of size 8 at 0x8febf0 by thread T1 (mutexes: write M90):
#0 camlSimple_race.d2_274 simple_race.ml:9 (simple_race.exe+0x420a92)
#1 camlDomain.body_706 stdlib/domain.ml:211 (simple_race.exe+0x440f2f)
#2 caml_start_program <null> (simple_race.exe+0x47cf37)
#3 caml_callback_exn runtime/callback.c:197 (simple_race.exe+0x445f7b)
#4 domain_thread_func runtime/domain.c:1167 (simple_race.exe+0x44a113)

Previous write of size 8 at 0x8febf0 by main thread (mutexes: write M86):

#0 camlSimple_race.d1_271 simple_race.ml:4 (simple_race.exe+0x420a01)
Chapter 27. Runtime detection of data races with ThreadSanitizer 503

#1 camlSimple_race.entry simple_race.ml:13 (simple_race.exe+0x420d16)

#2 caml_program <null> (simple_race.exe+0x41ffb9)
#3 caml_start_program <null> (simple_race.exe+0x47cf37)
[...]

==================
ThreadSanitizer: reported 2 warnings

For each detected data race, TSan reports the location of the conflicting accesses, their nature
(read, write, atomic read, etc.), and the associated stack trace.
If you run the above program several times, the output may vary: sometimes TSan will report
two data races, sometimes one, and sometimes none. This is due to the combination of two factors:

• First, TSan reports only the data races encountered during execution, i.e., conflicting, un-
ordered memory accesses that are effectively performed.
• In addition, in this program, depending on executions, there may be no such memory accesses:
if d1 returns before d2 has finished spawning, then all memory accesses originating from d1
may happen-before the ones originating from d2, since spawning a domain involves inter-thread
synchronization. In that case, the accesses are considered to be ordered and no data race is
reported.

This example illustrates the fact that data races can sometimes be hidden by unrelated synchro-
nizing operations.

27.2 Performance implications

TSan instrumentation imposes a non-negligible cost at runtime. Empirically, this cost has been
observed to cause a slowdown which can range from 2x to 7x. One of the main factors of high
slowdowns is frequent access to mutable data. In contrast, the initialising writes to and reads from
immutable memory locations are not instrumented. TSan also allocates very large amounts of
virtual memory, although it uses only a fraction of it. The memory consumption is increased by a
factor between 4 and 7.

27.3 False negatives and false positives

As illustrated by the previous example, TSan will only report the data races encountered during
execution. Another important caveat is that TSan remembers only a finite number of memory
accesses per memory location. At the time of writing, this number is 4. Data races involving a
forgotten access will not be detected. Lastly, the documentation of TSan states that there is a tiny
probability to miss a race if two threads access the same location at the same time. TSan may
overlook data races only in these three specific cases.
For data races between two memory accesses made from OCaml code, TSan does not produce
false positives; that is, TSan will not emit spurious reports.
When mixing OCaml and C code, through the use of C primitives, the very notion of false positive
becomes less clear, as it involves two memory models – OCaml and C11. However, TSan should
504

behave mostly as one would expect: non-atomic reads and writes in C will race with non-atomic reads
and writes in OCaml, and C atomics will not race with OCaml atomics. There is one theoretical
possibility of false positive: if a value is initialized from C without using caml_initialize (which
is allowed under the condition that the GC does not run between the allocation and the write, see
Chapter 22) and a conflicting access is made later by another thread. This does not constitute a
data race, but TSan may report it as such.

27.4 Runtime options

TSan supports a number of configuration options at runtime using the TSAN_OPTIONS environment
variable. TSAN_OPTIONS should contain one or more options separated by spaces. See the documen-
tation of TSan flags and the documentation of flags common to all sanitizers for more information.
Notably, TSAN_OPTIONS allows suppressing some data races from TSan reports. Suppressing data
race reports is useful for intentional races or libraries that cannot be fixed.
For example, to suppress reports originating from functions in the OCaml module My_module,
one can run

TSAN_OPTIONS="suppressions=suppr.txt" ./my_instrumented_program

with suppr.txt a file containing:

race_top:^camlMy_module

(Note that this depends on the format of OCaml symbols in the executable. Some builders, like
Dune, might result in different formats. You should adapt this example to the symbols effectively
present in your stack traces.)
The TSAN_OPTIONS variable also allows for increasing the “history size”, e.g.:

TSAN_OPTIONS="history_size=7" ./my_instrumented_program

TSan’s history records events such as function entry and exit, and is used to reconstruct stack
traces. Increasing the history size can sometimes be necessary to obtain the second stack trace,
but it also increases memory consumption. This setting does not change the number of memory
accesses remembered per memory location.
Another useful runtime option is exitcode=0, which still reports data races but does not change
the exit code. This can be useful if TSan complains about data races in programs that you don’t
care about and the non-zero exit code disturbs your workflow.

27.5 Guidelines for linking

As a general rule, OCaml programs instrumented with TSan should only be linked with OCaml or
C objects also instrumented with TSan. Doing otherwise may result in crashes. The only exception
to this rule are C libraries that do not call into the OCaml runtime system in any way, i.e., do not
allocate, raise exceptions, call back into OCaml code, etc. Examples include the libc or system
libraries. Data races in non-instrumented libraries will not be reported.
Chapter 27. Runtime detection of data races with ThreadSanitizer 505

C code interacting with OCaml should always be built through the ocamlopt command, which
will pass the required instrumentation flags to the C compiler. The CAMLno_tsan qualifier can be
used to prevent functions from being instrumented:

CAMLno_tsan void f(int arg)

{
/* This function will not be instrumented. */
...
}

Races from non-instrumented functions will not be reported. CAMLno_tsan should only be used
by experts. It can be used to reduce the performance overhead in certain corner cases, or to suppress
some known alarms. For the latter, using a suppressions file with TSAN_OPTIONS should be preferred
when possible, as it allows for finer-grained control, and qualifying a function f with CAMLno_tsan
results in missing entries in TSan’s stack traces when a data race happens in a transitive callee of f.
There is no way to disable instrumentation in OCaml code.

27.6 Changes in the delivery of signals

TSan intercepts all signals and passes them down to the instrumented program. This overlay from
TSan is not always transparent for the program. Synchronous signals such as SIGSEV, SIGILL,
SIGBUS, etc. will be passed down immediately, whereas asynchronous signals such as SIGINT will be
delayed until the next call to the TSan runtime (e.g. until the next access to mutable data). This
limitation of TSan can have surprising effects: for instance, pure, recursive functions that do not
allocate cannot be interrupted until they terminate.
506
 Part IV

The OCaml library

507
Chapter 28

The core library

This chapter describes the OCaml core library, which is composed of declarations for built-in types
and exceptions, plus the module Stdlib that provides basic operations on these built-in types. The
Stdlib module is special in two ways:

• It is automatically linked with the user’s object code files by the ocamlc command (chapter 13).

• It is automatically “opened” when a compilation starts, or when the toplevel system is launched.
Hence, it is possible to use unqualified identifiers to refer to the functions provided by the
Stdlib module, without adding a open Stdlib directive.

Conventions
The declarations of the built-in types and the components of module Stdlib are printed one by
one in typewriter font, followed by a short comment. All library modules and the components they
provide are indexed at the end of this report.

28.1 Built-in types and predefined exceptions

The following built-in types and predefined exceptions are always defined in the compilation
environment, but are not part of any module. As a consequence, they can only be referred by their
short names.

Built-in types
type int
The type of integer numbers.

type char
The type of characters.

type bytes
The type of (writable) byte sequences.

509
510

type string
The type of (read-only) character strings.

type float
The type of floating-point numbers.

type bool = false | true

The type of booleans (truth values).

type unit = ()
The type of the unit value.

type exn
The type of exception values.

type 'a array

The type of arrays whose elements have type 'a.

type 'a list = [] | :: of 'a * 'a list

The type of lists whose elements have type 'a.

type 'a option = None | Some of 'a

The type of optional values of type 'a.

type int32
The type of signed 32-bit integers. Literals for 32-bit integers are suffixed by l. See the
Int32[29.28] module.

type int64
The type of signed 64-bit integers. Literals for 64-bit integers are suffixed by L. See the
Int64[29.29] module.

type nativeint
The type of signed, platform-native integers (32 bits on 32-bit processors, 64 bits on 64-bit
processors). Literals for native integers are suffixed by n. See the Nativeint[29.38] module.

type ('a, 'b, 'c, 'd, 'e, 'f) format6

The type of format strings. 'a is the type of the parameters of the format, 'f is the result
type for the printf-style functions, 'b is the type of the first argument given to %a and %t
printing functions (see module Printf[29.44]), 'c is the result type of these functions, and
also the type of the argument transmitted to the first argument of kprintf-style functions,
'd is the result type for the scanf-style functions (see module Scanf[29.48]), and 'e is the
type of the receiver function for the scanf-style functions.

type 'a lazy_t

This type is used to implement the Lazy[29.30] module. It should not be used directly.
Chapter 28. The core library 511

Predefined exceptions
exception Match_failure of (string * int * int)
Exception raised when none of the cases of a pattern-matching apply. The arguments are the
location of the match keyword in the source code (file name, line number, column number).

exception Assert_failure of (string * int * int)

Exception raised when an assertion fails. The arguments are the location of the assert
keyword in the source code (file name, line number, column number).

exception Invalid_argument of string

Exception raised by library functions to signal that the given arguments do not make sense.
The string gives some information to the programmer. As a general rule, this exception
should not be caught, it denotes a programming error and the code should be modified not to
trigger it.

exception Failure of string

Exception raised by library functions to signal that they are undefined on the given arguments.
The string is meant to give some information to the programmer; you must not pattern
match on the string literal because it may change in future versions (use Failure _ instead).

exception Not_found
Exception raised by search functions when the desired object could not be found.

exception Out_of_memory
Exception raised by the garbage collector when there is insufficient memory to complete the
computation. (Not reliable for allocations on the minor heap.)

exception Stack_overflow
Exception raised by the bytecode interpreter when the evaluation stack reaches its maximal
size. This often indicates infinite or excessively deep recursion in the user’s program. Before
4.10, it was not fully implemented by the native-code compiler.

exception Sys_error of string

Exception raised by the input/output functions to report an operating system error. The
string is meant to give some information to the programmer; you must not pattern match on
the string literal because it may change in future versions (use Sys_error _ instead).

exception End_of_file
Exception raised by input functions to signal that the end of file has been reached.

exception Division_by_zero
Exception raised by integer division and remainder operations when their second argument is
zero.
512

exception Sys_blocked_io
A special case of Sys_error raised when no I/O is possible on a non-blocking I/O channel.

exception Undefined_recursive_module of (string * int * int)

Exception raised when an ill-founded recursive module definition is evaluated. (See
section 12.2.) The arguments are the location of the definition in the source code (file name,
line number, column number).

28.2 Module Stdlib : The OCaml Standard library.

This module is automatically opened at the beginning of each compilation. All components of this
module can therefore be referred by their short name, without prefixing them by Stdlib.
In particular, it provides the basic operations over the built-in types (numbers, booleans, byte
sequences, strings, exceptions, references, lists, arrays, input-output channels, . . .) and the standard
library modules[28.2].

Exceptions
val raise : exn -> 'a
Raise the given exception value

val raise_notrace : exn -> 'a

A faster version raise which does not record the backtrace.
Since: 4.02

val invalid_arg : string -> 'a

Raise exception Invalid_argument with the given string.

val failwith : string -> 'a

Raise exception Failure with the given string.

exception Exit
The Exit exception is not raised by any library function. It is provided for use in your
programs.

exception Match_failure of (string * int * int)

Exception raised when none of the cases of a pattern-matching apply. The arguments are the
location of the match keyword in the source code (file name, line number, column number).

exception Assert_failure of (string * int * int)

Exception raised when an assertion fails. The arguments are the location of the assert
keyword in the source code (file name, line number, column number).
Chapter 28. The core library 513

exception Invalid_argument of string

Exception raised by library functions to signal that the given arguments do not make sense.
The string gives some information to the programmer. As a general rule, this exception
should not be caught, it denotes a programming error and the code should be modified not to
trigger it.

exception Failure of string

Exception raised by library functions to signal that they are undefined on the given arguments.
The string is meant to give some information to the programmer; you must not pattern match
on the string literal because it may change in future versions (use Failure _ instead).

exception Not_found
Exception raised by search functions when the desired object could not be found.

exception Out_of_memory
Exception raised by the garbage collector when there is insufficient memory to complete the
computation. (Not reliable for allocations on the minor heap.)

exception Stack_overflow
Exception raised by the bytecode interpreter when the evaluation stack reaches its maximal
size. This often indicates infinite or excessively deep recursion in the user’s program.
Before 4.10, it was not fully implemented by the native-code compiler.

exception Sys_error of string

Exception raised by the input/output functions to report an operating system error. The
string is meant to give some information to the programmer; you must not pattern match on
the string literal because it may change in future versions (use Sys_error _ instead).

exception End_of_file
Exception raised by input functions to signal that the end of file has been reached.

exception Division_by_zero
Exception raised by integer division and remainder operations when their second argument is
zero.

exception Sys_blocked_io
A special case of Sys_error raised when no I/O is possible on a non-blocking I/O channel.

exception Undefined_recursive_module of (string * int * int)

Exception raised when an ill-founded recursive module definition is evaluated. The arguments
are the location of the definition in the source code (file name, line number, column number).
514

Comparisons
val (=) : 'a -> 'a -> bool
e1 = e2 tests for structural equality of e1 and e2. Mutable structures (e.g. references and
arrays) are equal if and only if their current contents are structurally equal, even if the two
mutable objects are not the same physical object. Equality between functional values raises
Invalid_argument. Equality between cyclic data structures may not terminate.
Left-associative operator, see Ocaml_operators[29.61] for more information.

val (<>) : 'a -> 'a -> bool

Negation of (=)[28.2]. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val (<) : 'a -> 'a -> bool

See (>=)[28.2]. Left-associative operator, see Ocaml_operators[29.61] for more information.

val (>) : 'a -> 'a -> bool

See (>=)[28.2]. Left-associative operator, see Ocaml_operators[29.61] for more information.

val (<=) : 'a -> 'a -> bool

See (>=)[28.2]. Left-associative operator, see Ocaml_operators[29.61] for more information.

val (>=) : 'a -> 'a -> bool

Structural ordering functions. These functions coincide with the usual orderings over integers,
characters, strings, byte sequences and floating-point numbers, and extend them to a total
ordering over all types. The ordering is compatible with ( = ). As in the case of ( = ),
mutable structures are compared by contents. Comparison between functional values raises
Invalid_argument. Comparison between cyclic structures may not terminate.
Left-associative operator, see Ocaml_operators[29.61] for more information.

val compare : 'a -> 'a -> int

compare x y returns 0 if x is equal to y, a negative integer if x is less than y, and a positive
integer if x is greater than y. The ordering implemented by compare is compatible with the
comparison predicates =, < and > defined above, with one difference on the treatment of the
float value nan[28.2]. Namely, the comparison predicates treat nan as different from any other
float value, including itself; while compare treats nan as equal to itself and less than any other
float value. This treatment of nan ensures that compare defines a total ordering relation.
compare applied to functional values may raise Invalid_argument. compare applied to
cyclic structures may not terminate.
The compare function can be used as the comparison function required by the
Set.Make[29.50] and Map.Make[29.34] functors, as well as the List.sort[29.32] and
Array.sort[29.2] functions.

val min : 'a -> 'a -> 'a

Chapter 28. The core library 515

Return the smaller of the two arguments. The result is unspecified if one of the arguments
contains the float value nan.

val max : 'a -> 'a -> 'a

Return the greater of the two arguments. The result is unspecified if one of the arguments
contains the float value nan.

val (==) : 'a -> 'a -> bool

e1 == e2 tests for physical equality of e1 and e2. On mutable types such as references,
arrays, byte sequences, records with mutable fields and objects with mutable instance
variables, e1 == e2 is true if and only if physical modification of e1 also affects e2. On
non-mutable types, the behavior of ( == ) is implementation-dependent; however, it is
guaranteed that e1 == e2 implies compare e1 e2 = 0. Left-associative operator, see
Ocaml_operators[29.61] for more information.

val (!=) : 'a -> 'a -> bool

Negation of (==)[28.2]. Left-associative operator, see Ocaml_operators[29.61] for more
information.

Boolean operations
val not : bool -> bool
The boolean negation.

val (&&) : bool -> bool -> bool

The boolean ’and’. Evaluation is sequential, left-to-right: in e1 && e2, e1 is evaluated first,
and if it returns false, e2 is not evaluated at all. Right-associative operator, see
Ocaml_operators[29.61] for more information.

val (||) : bool -> bool -> bool

The boolean ’or’. Evaluation is sequential, left-to-right: in e1 || e2, e1 is evaluated first,
and if it returns true, e2 is not evaluated at all. Right-associative operator, see
Ocaml_operators[29.61] for more information.

Debugging
val __LOC__ : string
__LOC__ returns the location at which this expression appears in the file currently being
parsed by the compiler, with the standard error format of OCaml: ”File %S, line %d,
characters %d-%d”.
Since: 4.02

val __FILE__ : string

516

__FILE__ returns the name of the file currently being parsed by the compiler.
Since: 4.02

val __LINE__ : int

__LINE__ returns the line number at which this expression appears in the file currently being
parsed by the compiler.
Since: 4.02

val __MODULE__ : string

__MODULE__ returns the module name of the file being parsed by the compiler.
Since: 4.02

val __POS__ : string * int * int * int

__POS__ returns a tuple (file,lnum,cnum,enum), corresponding to the location at which
this expression appears in the file currently being parsed by the compiler. file is the current
filename, lnum the line number, cnum the character position in the line and enum the last
character position in the line.
Since: 4.02

val __FUNCTION__ : string

__FUNCTION__ returns the name of the current function or method, including any enclosing
modules or classes.
Since: 4.12

val __LOC_OF__ : 'a -> string * 'a

__LOC_OF__ expr returns a pair (loc, expr) where loc is the location of expr in the file
currently being parsed by the compiler, with the standard error format of OCaml: ”File %S,
line %d, characters %d-%d”.
Since: 4.02

val __LINE_OF__ : 'a -> int * 'a

__LINE_OF__ expr returns a pair (line, expr), where line is the line number at which
the expression expr appears in the file currently being parsed by the compiler.
Since: 4.02

val __POS_OF__ : 'a -> (string * int * int * int) * 'a

__POS_OF__ expr returns a pair (loc,expr), where loc is a tuple (file,lnum,cnum,enum)
corresponding to the location at which the expression expr appears in the file currently being
parsed by the compiler. file is the current filename, lnum the line number, cnum the
character position in the line and enum the last character position in the line.
Since: 4.02
Chapter 28. The core library 517

Composition operators
val (|>) : 'a -> ('a -> 'b) -> 'b
Reverse-application operator: x |> f |> g is exactly equivalent to g (f (x)).
Left-associative operator, see Ocaml_operators[29.61] for more information.
Since: 4.01

val (@@) : ('a -> 'b) -> 'a -> 'b

Application operator: g @@ f @@ x is exactly equivalent to g (f (x)). Right-associative
operator, see Ocaml_operators[29.61] for more information.
Since: 4.01

Integer arithmetic
Integers are Sys.int_size bits wide. All operations are taken modulo 2Sys.int_size . They do not
fail on overflow.
val (~-) : int -> int
Unary negation. You can also write - e instead of ~- e. Unary operator, see
Ocaml_operators[29.61] for more information.

val (~+) : int -> int

Unary addition. You can also write + e instead of ~+ e. Unary operator, see
Ocaml_operators[29.61] for more information.
Since: 3.12

val succ : int -> int

succ x is x + 1.

val pred : int -> int

pred x is x - 1.

val (+) : int -> int -> int

Integer addition. Left-associative operator, see Ocaml_operators[29.61] for more information.

val (-) : int -> int -> int

Integer subtraction. Left-associative operator, , see Ocaml_operators[29.61] for more
information.

val ( * ) : int -> int -> int

Integer multiplication. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val (/) : int -> int -> int

518

Integer division. Integer division rounds the real quotient of its arguments towards zero.
More precisely, if x >= 0 and y > 0, x / y is the greatest integer less than or equal to the
real quotient of x by y. Moreover, (- x) / y = x / (- y) = - (x / y). Left-associative
operator, see Ocaml_operators[29.61] for more information.
Raises Division_by_zero if the second argument is 0.

val (mod) : int -> int -> int

Integer remainder. If y is not zero, the result of x mod y satisfies the following properties: x
= (x / y) * y + x mod y and abs(x mod y) <= abs(y) - 1. If y = 0, x mod y raises
Division_by_zero. Note that x mod y is negative only if x < 0. Left-associative operator,
see Ocaml_operators[29.61] for more information.
Raises Division_by_zero if y is zero.

val abs : int -> int

abs x is the absolute value of x. On min_int this is min_int itself and thus remains negative.

val max_int : int

The greatest representable integer.

val min_int : int

The smallest representable integer.

Bitwise operations
val (land) : int -> int -> int
Bitwise logical and. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val (lor) : int -> int -> int

Bitwise logical or. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val (lxor) : int -> int -> int

Bitwise logical exclusive or. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val lnot : int -> int

Bitwise logical negation.

val (lsl) : int -> int -> int

n lsl m shifts n to the left by m bits. The result is unspecified if m < 0 or m > Sys.int_size.
Right-associative operator, see Ocaml_operators[29.61] for more information.

val (lsr) : int -> int -> int

Chapter 28. The core library 519

n lsr m shifts n to the right by m bits. This is a logical shift: zeroes are inserted regardless of
the sign of n. The result is unspecified if m < 0 or m > Sys.int_size. Right-associative
operator, see Ocaml_operators[29.61] for more information.

val (asr) : int -> int -> int

n asr m shifts n to the right by m bits. This is an arithmetic shift: the sign bit of n is
replicated. The result is unspecified if m < 0 or m > Sys.int_size. Right-associative
operator, see Ocaml_operators[29.61] for more information.

Floating-point arithmetic
OCaml’s floating-point numbers follow the IEEE 754 standard, using double precision (64 bits)
numbers. Floating-point operations never raise an exception on overflow, underflow, division by zero,
etc. Instead, special IEEE numbers are returned as appropriate, such as infinity for 1.0 /. 0.0,
neg_infinity for -1.0 /. 0.0, and nan (’not a number’) for 0.0 /. 0.0. These special numbers
then propagate through floating-point computations as expected: for instance, 1.0 /. infinity is
0.0, basic arithmetic operations (+., -., *., /.) with nan as an argument return nan, . . .
val (~-.) : float -> float
Unary negation. You can also write -. e instead of ~-. e. Unary operator, see
Ocaml_operators[29.61] for more information.

val (~+.) : float -> float

Unary addition. You can also write +. e instead of ~+. e. Unary operator, see
Ocaml_operators[29.61] for more information.
Since: 3.12

val (+.) : float -> float -> float

Floating-point addition. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val (-.) : float -> float -> float

Floating-point subtraction. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val ( *. ) : float -> float -> float

Floating-point multiplication. Left-associative operator, see Ocaml_operators[29.61] for
more information.

val (/.) : float -> float -> float

Floating-point division. Left-associative operator, see Ocaml_operators[29.61] for more
information.

val ( ** ) : float -> float -> float

520

Exponentiation. Right-associative operator, see Ocaml_operators[29.61] for more

information.

val sqrt : float -> float

Square root.

val exp : float -> float

Exponential.

val log : float -> float

Natural logarithm.

val log10 : float -> float

Base 10 logarithm.

val expm1 : float -> float

expm1 x computes exp x -. 1.0, giving numerically-accurate results even if x is close to
0.0.
Since: 3.12

val log1p : float -> float

log1p x computes log(1.0 +. x) (natural logarithm), giving numerically-accurate results
even if x is close to 0.0.
Since: 3.12

val cos : float -> float

Cosine. Argument is in radians.

val sin : float -> float

Sine. Argument is in radians.

val tan : float -> float

Tangent. Argument is in radians.

val acos : float -> float

Arc cosine. The argument must fall within the range [-1.0, 1.0]. Result is in radians and
is between 0.0 and pi.

val asin : float -> float

Arc sine. The argument must fall within the range [-1.0, 1.0]. Result is in radians and is
between -pi/2 and pi/2.

val atan : float -> float

Arc tangent. Result is in radians and is between -pi/2 and pi/2.
Chapter 28. The core library 521

val atan2 : float -> float -> float

atan2 y x returns the arc tangent of y /. x. The signs of x and y are used to determine the
quadrant of the result. Result is in radians and is between -pi and pi.

val hypot : float -> float -> float

hypot x y returns sqrt(x *. x + y *. y), that is, the length of the hypotenuse of a
right-angled triangle with sides of length x and y, or, equivalently, the distance of the point
(x,y) to origin. If one of x or y is infinite, returns infinity even if the other is nan.
Since: 4.00

val cosh : float -> float

Hyperbolic cosine. Argument is in radians.

val sinh : float -> float

Hyperbolic sine. Argument is in radians.

val tanh : float -> float

Hyperbolic tangent. Argument is in radians.

val acosh : float -> float

Hyperbolic arc cosine. The argument must fall within the range [1.0, inf]. Result is in
radians and is between 0.0 and inf.
Since: 4.13

val asinh : float -> float

Hyperbolic arc sine. The argument and result range over the entire real line. Result is in
radians.
Since: 4.13

val atanh : float -> float

Hyperbolic arc tangent. The argument must fall within the range [-1.0, 1.0]. Result is in
radians and ranges over the entire real line.
Since: 4.13

val ceil : float -> float

Round above to an integer value. ceil f returns the least integer value greater than or equal
to f. The result is returned as a float.

val floor : float -> float

Round below to an integer value. floor f returns the greatest integer value less than or
equal to f. The result is returned as a float.

val abs_float : float -> float

522

abs_float f returns the absolute value of f.

val copysign : float -> float -> float

copysign x y returns a float whose absolute value is that of x and whose sign is that of y. If
x is nan, returns nan. If y is nan, returns either x or -. x, but it is not specified which.
Since: 4.00

val mod_float : float -> float -> float

mod_float a b returns the remainder of a with respect to b. The returned value is a -. n
*. b, where n is the quotient a /. b rounded towards zero to an integer.

val frexp : float -> float * int

frexp f returns the pair of the significant and the exponent of f. When f is zero, the
significant x and the exponent n of f are equal to zero. When f is non-zero, they are defined
by f = x *. 2 ** n and 0.5 <= x < 1.0.

val ldexp : float -> int -> float

ldexp x n returns x *. 2 ** n.

val modf : float -> float * float

modf f returns the pair of the fractional and integral part of f.

val float : int -> float

Same as float_of_int[28.2].

val float_of_int : int -> float

Convert an integer to floating-point.

val truncate : float -> int

Same as int_of_float[28.2].

val int_of_float : float -> int

Truncate the given floating-point number to an integer. The result is unspecified if the
argument is nan or falls outside the range of representable integers.

val infinity : float

Positive infinity.

val neg_infinity : float

Negative infinity.

val nan : float

Chapter 28. The core library 523

A special floating-point value denoting the result of an undefined operation such as 0.0 /.
0.0. Stands for ’not a number’. Any floating-point operation with nan as argument returns
nan as result, unless otherwise specified in IEEE 754 standard. As for floating-point
comparisons, =, <, <=, > and >= return false and <> returns true if one or both of their
arguments is nan.
nan is a quiet NaN since 5.1; it was a signaling NaN before.
val max_float : float
The largest positive finite value of type float.
val min_float : float
The smallest positive, non-zero, non-denormalized value of type float.
val epsilon_float : float
The difference between 1.0 and the smallest exactly representable floating-point number
greater than 1.0.
type fpclass =
| FP_normal
Normal number, none of the below
| FP_subnormal
Number very close to 0.0, has reduced precision
| FP_zero
Number is 0.0 or -0.0
| FP_infinite
Number is positive or negative infinity
| FP_nan
Not a number: result of an undefined operation
The five classes of floating-point numbers, as determined by the classify_float[28.2]
function.
val classify_float : float -> fpclass
Return the class of the given floating-point number: normal, subnormal, zero, infinite, or not
a number.

String operations
More string operations are provided in module String[28.2].
val (^) : string -> string -> string
String concatenation. Right-associative operator, see Ocaml_operators[29.61] for more
information.
Raises Invalid_argument if the result is longer then than Sys.max_string_length[29.56]
bytes.
524

Character operations
More character operations are provided in module Char[28.2].
val int_of_char : char -> int
Return the ASCII code of the argument.

val char_of_int : int -> char

Return the character with the given ASCII code.
Raises Invalid_argument if the argument is outside the range 0–255.

Unit operations
val ignore : 'a -> unit
Discard the value of its argument and return (). For instance, ignore(f x) discards the
result of the side-effecting function f. It is equivalent to f x; (), except that the latter may
generate a compiler warning; writing ignore(f x) instead avoids the warning.

String conversion functions

val string_of_bool : bool -> string
Return the string representation of a boolean. As the returned values may be shared, the user
should not modify them directly.

val bool_of_string_opt : string -> bool option

Convert the given string to a boolean.
Return None if the string is not "true" or "false".
Since: 4.05

val bool_of_string : string -> bool

Same as bool_of_string_opt[28.2], but raise Invalid_argument "bool_of_string"
instead of returning None.

val string_of_int : int -> string

Return the string representation of an integer, in decimal.

val int_of_string_opt : string -> int option

Convert the given string to an integer. The string is read in decimal (by default, or if the
string begins with 0u), in hexadecimal (if it begins with 0x or 0X), in octal (if it begins with
0o or 0O), or in binary (if it begins with 0b or 0B).
The 0u prefix reads the input as an unsigned integer in the range [0, 2*max_int+1]. If the
input exceeds max_int[28.2] it is converted to the signed integer min_int + input -
max_int - 1.
Chapter 28. The core library 525

The _ (underscore) character can appear anywhere in the string and is ignored.
Return None if the given string is not a valid representation of an integer, or if the integer
represented exceeds the range of integers representable in type int.
Since: 4.05

val int_of_string : string -> int

Same as int_of_string_opt[28.2], but raise Failure "int_of_string" instead of
returning None.

val string_of_float : float -> string

Return a string representation of a floating-point number.
This conversion can involve a loss of precision. For greater control over the manner in which
the number is printed, see Printf[28.2].

val float_of_string_opt : string -> float option

Convert the given string to a float. The string is read in decimal (by default) or in
hexadecimal (marked by 0x or 0X).
The format of decimal floating-point numbers is [-] dd.ddd (e|E) [+|-] dd , where d
stands for a decimal digit.
The format of hexadecimal floating-point numbers is [-] 0(x|X) hh.hhh (p|P) [+|-] dd
, where h stands for an hexadecimal digit and d for a decimal digit.
In both cases, at least one of the integer and fractional parts must be given; the exponent
part is optional.
The _ (underscore) character can appear anywhere in the string and is ignored.
Depending on the execution platforms, other representations of floating-point numbers can be
accepted, but should not be relied upon.
Return None if the given string is not a valid representation of a float.
Since: 4.05

val float_of_string : string -> float

Same as float_of_string_opt[28.2], but raise Failure "float_of_string" instead of
returning None.

Pair operations
val fst : 'a * 'b -> 'a
Return the first component of a pair.

val snd : 'a * 'b -> 'b

Return the second component of a pair.
526

List operations
More list operations are provided in module List[28.2].
val (@) : 'a list -> 'a list -> 'a list
l0 @ l1 appends l1 to l0. Same function as List.append[29.32]. Right-associative operator,
see Ocaml_operators[29.61] for more information.
Since: 5.1 this function is tail-recursive.

Input/output
Note: all input/output functions can raise Sys_error when the system calls they invoke fail.
type in_channel
The type of input channel.

type out_channel
The type of output channel.

val stdin : in_channel

The standard input for the process.

val stdout : out_channel

The standard output for the process.

val stderr : out_channel

The standard error output for the process.

Output functions on standard output

val print_char : char -> unit
Print a character on standard output.

val print_string : string -> unit

Print a string on standard output.

val print_bytes : bytes -> unit

Print a byte sequence on standard output.
Since: 4.02

val print_int : int -> unit

Print an integer, in decimal, on standard output.

val print_float : float -> unit

Chapter 28. The core library 527

Print a floating-point number, in decimal, on standard output.

The conversion of the number to a string uses string_of_float[28.2] and can involve a loss
of precision.

val print_endline : string -> unit

Print a string, followed by a newline character, on standard output and flush standard output.

val print_newline : unit -> unit

Print a newline character on standard output, and flush standard output. This can be used to
simulate line buffering of standard output.

Output functions on standard error

val prerr_char : char -> unit
Print a character on standard error.

val prerr_string : string -> unit

Print a string on standard error.

val prerr_bytes : bytes -> unit

Print a byte sequence on standard error.
Since: 4.02

val prerr_int : int -> unit

Print an integer, in decimal, on standard error.

val prerr_float : float -> unit

Print a floating-point number, in decimal, on standard error.
The conversion of the number to a string uses string_of_float[28.2] and can involve a loss
of precision.

val prerr_endline : string -> unit

Print a string, followed by a newline character on standard error and flush standard error.

val prerr_newline : unit -> unit

Print a newline character on standard error, and flush standard error.
528

Input functions on standard input

val read_line : unit -> string
Flush standard output, then read characters from standard input until a newline character is
encountered.
Return the string of all characters read, without the newline character at the end.
Raises End_of_file if the end of the file is reached at the beginning of line.

val read_int_opt : unit -> int option

Flush standard output, then read one line from standard input and convert it to an integer.
Return None if the line read is not a valid representation of an integer.
Since: 4.05

val read_int : unit -> int

Same as read_int_opt[28.2], but raise Failure "int_of_string" instead of returning None.

val read_float_opt : unit -> float option

Flush standard output, then read one line from standard input and convert it to a
floating-point number.
Return None if the line read is not a valid representation of a floating-point number.
Since: 4.05

val read_float : unit -> float

Same as read_float_opt[28.2], but raise Failure "float_of_string" instead of returning
None.

General output functions

type open_flag =
| Open_rdonly
open for reading.
| Open_wronly
open for writing.
| Open_append
open for appending: always write at end of file.
| Open_creat
create the file if it does not exist.
| Open_trunc
empty the file if it already exists.
Chapter 28. The core library 529

| Open_excl
fail if Open_creat and the file already exists.
| Open_binary
open in binary mode (no conversion).
| Open_text
open in text mode (may perform conversions).
| Open_nonblock
open in non-blocking mode.
Opening modes for open_out_gen[28.2] and open_in_gen[28.2].

val open_out : string -> out_channel

Open the named file for writing, and return a new output channel on that file, positioned at
the beginning of the file. The file is truncated to zero length if it already exists. It is created
if it does not already exists.

val open_out_bin : string -> out_channel

Same as open_out[28.2], but the file is opened in binary mode, so that no translation takes
place during writes. On operating systems that do not distinguish between text mode and
binary mode, this function behaves like open_out[28.2].

val open_out_gen : open_flag list -> int -> string -> out_channel
open_out_gen mode perm filename opens the named file for writing, as described above.
The extra argument mode specifies the opening mode. The extra argument perm specifies the
file permissions, in case the file must be created. open_out[28.2] and open_out_bin[28.2] are
special cases of this function.

val flush : out_channel -> unit

Flush the buffer associated with the given output channel, performing all pending writes on
that channel. Interactive programs must be careful about flushing standard output and
standard error at the right time.

val flush_all : unit -> unit

Flush all open output channels; ignore errors.

val output_char : out_channel -> char -> unit

Write the character on the given output channel.

val output_string : out_channel -> string -> unit

Write the string on the given output channel.

val output_bytes : out_channel -> bytes -> unit

530

Write the byte sequence on the given output channel.

Since: 4.02

val output : out_channel -> bytes -> int -> int -> unit
output oc buf pos len writes len characters from byte sequence buf, starting at offset
pos, to the given output channel oc.
Raises Invalid_argument if pos and len do not designate a valid range of buf.

val output_substring : out_channel -> string -> int -> int -> unit
Same as output but take a string as argument instead of a byte sequence.
Since: 4.02

val output_byte : out_channel -> int -> unit

Write one 8-bit integer (as the single character with that code) on the given output channel.
The given integer is taken modulo 256.

val output_binary_int : out_channel -> int -> unit

Write one integer in binary format (4 bytes, big-endian) on the given output channel. The
given integer is taken modulo 232 . The only reliable way to read it back is through the
input_binary_int[28.2] function. The format is compatible across all machines for a given
version of OCaml.

val output_value : out_channel -> 'a -> unit

Write the representation of a structured value of any type to a channel. Circularities and
sharing inside the value are detected and preserved. The object can be read back, by the
function input_value[28.2]. See the description of module Marshal[28.2] for more
information. output_value[28.2] is equivalent to Marshal.to_channel[29.35] with an empty
list of flags.

val seek_out : out_channel -> int -> unit

seek_out chan pos sets the current writing position to pos for channel chan. This works
only for regular files. On files of other kinds (such as terminals, pipes and sockets), the
behavior is unspecified.

val pos_out : out_channel -> int

Return the current writing position for the given channel. Does not work on channels opened
with the Open_append flag (returns unspecified results). For files opened in text mode under
Windows, the returned position is approximate (owing to end-of-line conversion); in
particular, saving the current position with pos_out, then going back to this position using
seek_out will not work. For this programming idiom to work reliably and portably, the file
must be opened in binary mode.

val out_channel_length : out_channel -> int

Chapter 28. The core library 531

Return the size (number of characters) of the regular file on which the given channel is
opened. If the channel is opened on a file that is not a regular file, the result is meaningless.

val close_out : out_channel -> unit

Close the given channel, flushing all buffered write operations. Output functions raise a
Sys_error exception when they are applied to a closed output channel, except close_out
and flush, which do nothing when applied to an already closed channel. Note that
close_out may raise Sys_error if the operating system signals an error when flushing or
closing.

val close_out_noerr : out_channel -> unit

Same as close_out, but ignore all errors.

val set_binary_mode_out : out_channel -> bool -> unit

set_binary_mode_out oc true sets the channel oc to binary mode: no translations take
place during output. set_binary_mode_out oc false sets the channel oc to text mode:
depending on the operating system, some translations may take place during output. For
instance, under Windows, end-of-lines will be translated from \n to \r\n. This function has
no effect under operating systems that do not distinguish between text mode and binary
mode.

General input functions

val open_in : string -> in_channel
Open the named file for reading, and return a new input channel on that file, positioned at
the beginning of the file.

val open_in_bin : string -> in_channel

Same as open_in[28.2], but the file is opened in binary mode, so that no translation takes
place during reads. On operating systems that do not distinguish between text mode and
binary mode, this function behaves like open_in[28.2].

val open_in_gen : open_flag list -> int -> string -> in_channel
open_in_gen mode perm filename opens the named file for reading, as described above.
The extra arguments mode and perm specify the opening mode and file permissions.
open_in[28.2] and open_in_bin[28.2] are special cases of this function.

val input_char : in_channel -> char

Read one character from the given input channel.
Raises End_of_file if there are no more characters to read.

val input_line : in_channel -> string

532

Read characters from the given input channel, until a newline character is encountered.
Return the string of all characters read, without the newline character at the end.
Raises End_of_file if the end of the file is reached at the beginning of line.

val input : in_channel -> bytes -> int -> int -> int
input ic buf pos len reads up to len characters from the given channel ic, storing them
in byte sequence buf, starting at character number pos. It returns the actual number of
characters read, between 0 and len (inclusive). A return value of 0 means that the end of file
was reached. A return value between 0 and len exclusive means that not all requested len
characters were read, either because no more characters were available at that time, or
because the implementation found it convenient to do a partial read; input must be called
again to read the remaining characters, if desired. (See also really_input[28.2] for reading
exactly len characters.) Exception Invalid_argument "input" is raised if pos and len do
not designate a valid range of buf.

val really_input : in_channel -> bytes -> int -> int -> unit
really_input ic buf pos len reads len characters from channel ic, storing them in byte
sequence buf, starting at character number pos.
Raises

• End_of_file if the end of file is reached before len characters have been read.
• Invalid_argument if pos and len do not designate a valid range of buf.

val really_input_string : in_channel -> int -> string

really_input_string ic len reads len characters from channel ic and returns them in a
new string.
Since: 4.02
Raises End_of_file if the end of file is reached before len characters have been read.

val input_byte : in_channel -> int

Same as input_char[28.2], but return the 8-bit integer representing the character.
Raises End_of_file if the end of file was reached.

val input_binary_int : in_channel -> int

Read an integer encoded in binary format (4 bytes, big-endian) from the given input channel.
See output_binary_int[28.2].
Raises End_of_file if the end of file was reached while reading the integer.

val input_value : in_channel -> 'a

Read the representation of a structured value, as produced by output_value[28.2], and
return the corresponding value. This function is identical to Marshal.from_channel[29.35];
see the description of module Marshal[28.2] for more information, in particular concerning
the lack of type safety.
Chapter 28. The core library 533

val seek_in : in_channel -> int -> unit

seek_in chan pos sets the current reading position to pos for channel chan. This works
only for regular files. On files of other kinds, the behavior is unspecified.

val pos_in : in_channel -> int

Return the current reading position for the given channel. For files opened in text mode
under Windows, the returned position is approximate (owing to end-of-line conversion); in
particular, saving the current position with pos_in, then going back to this position using
seek_in will not work. For this programming idiom to work reliably and portably, the file
must be opened in binary mode.

val in_channel_length : in_channel -> int

Return the size (number of characters) of the regular file on which the given channel is
opened. If the channel is opened on a file that is not a regular file, the result is meaningless.
The returned size does not take into account the end-of-line translations that can be
performed when reading from a channel opened in text mode.

val close_in : in_channel -> unit

Close the given channel. Input functions raise a Sys_error exception when they are applied
to a closed input channel, except close_in, which does nothing when applied to an already
closed channel.

val close_in_noerr : in_channel -> unit

Same as close_in, but ignore all errors.

val set_binary_mode_in : in_channel -> bool -> unit

set_binary_mode_in ic true sets the channel ic to binary mode: no translations take
place during input. set_binary_mode_out ic false sets the channel ic to text mode:
depending on the operating system, some translations may take place during input. For
instance, under Windows, end-of-lines will be translated from \r\n to \n. This function has
no effect under operating systems that do not distinguish between text mode and binary
mode.

Operations on large files

module LargeFile :
sig
val seek_out : out_channel -> int64 -> unit
val pos_out : out_channel -> int64
val out_channel_length : out_channel -> int64
val seek_in : in_channel -> int64 -> unit
val pos_in : in_channel -> int64
val in_channel_length : in_channel -> int64
534

end

Operations on large files. This sub-module provides 64-bit variants of the channel functions
that manipulate file positions and file sizes. By representing positions and sizes by 64-bit
integers (type int64) instead of regular integers (type int), these alternate functions allow
operating on files whose sizes are greater than max_int.

References
type 'a ref =
{ mutable contents : 'a ;
}
The type of references (mutable indirection cells) containing a value of type 'a.

val ref : 'a -> 'a ref

Return a fresh reference containing the given value.

val (!) : 'a ref -> 'a

!r returns the current contents of reference r. Equivalent to fun r -> r.contents. Unary
operator, see Ocaml_operators[29.61] for more information.

val (:=) : 'a ref -> 'a -> unit

r := a stores the value of a in reference r. Equivalent to fun r v -> r.contents <- v.
Right-associative operator, see Ocaml_operators[29.61] for more information.

val incr : int ref -> unit

Increment the integer contained in the given reference. Equivalent to fun r -> r := succ
!r.

val decr : int ref -> unit

Decrement the integer contained in the given reference. Equivalent to fun r -> r := pred
!r.

Result type
type ('a, 'b) result =
| Ok of 'a
| Error of 'b
Since: 4.03
Chapter 28. The core library 535

Operations on format strings

Format strings are character strings with special lexical conventions that defines the functionality
of formatted input/output functions. Format strings are used to read data with formatted input
functions from module Scanf[28.2] and to print data with formatted output functions from modules
Printf[28.2] and Format[28.2].
Format strings are made of three kinds of entities:

• conversions specifications, introduced by the special character '%' followed by one or more
characters specifying what kind of argument to read or print,

• formatting indications, introduced by the special character '@' followed by one or more
characters specifying how to read or print the argument,

• plain characters that are regular characters with usual lexical conventions. Plain characters
specify string literals to be read in the input or printed in the output.

There is an additional lexical rule to escape the special characters '%' and '@' in format strings:
if a special character follows a '%' character, it is treated as a plain character. In other words, "%%"
is considered as a plain '%' and "%@" as a plain '@'.
For more information about conversion specifications and formatting indications available, read
the documentation of modules Scanf[28.2], Printf[28.2] and Format[28.2].
Format strings have a general and highly polymorphic type ('a, 'b, 'c, 'd, 'e, 'f)
format6. The two simplified types, format and format4 below are included for backward
compatibility with earlier releases of OCaml.
The meaning of format string type parameters is as follows:

• 'a is the type of the parameters of the format for formatted output functions (printf-style
functions); 'a is the type of the values read by the format for formatted input functions
(scanf-style functions).

• 'b is the type of input source for formatted input functions and the type of output target
for formatted output functions. For printf-style functions from module Printf[28.2], 'b is
typically out_channel; for printf-style functions from module Format[28.2], 'b is typically
Format.formatter[29.22]; for scanf-style functions from module Scanf[28.2], 'b is typically
Scanf.Scanning.in_channel[29.48].

Type argument 'b is also the type of the first argument given to user’s defined printing functions
for %a and %t conversions, and user’s defined reading functions for %r conversion.

• 'c is the type of the result of the %a and %t printing functions, and also the type of the
argument transmitted to the first argument of kprintf-style functions or to the kscanf-style
functions.

• 'd is the type of parameters for the scanf-style functions.

• 'e is the type of the receiver function for the scanf-style functions.
536

• 'f is the final result type of a formatted input/output function invocation: for the printf-style
functions, it is typically unit; for the scanf-style functions, it is typically the result type of
the receiver function.

type ('a, 'b, 'c, 'd, 'e, 'f) format6 = ('a, 'b, 'c, 'd, 'e, 'f) CamlinternalFormatBasics.forma
type ('a, 'b, 'c, 'd) format4 = ('a, 'b, 'c, 'c, 'c, 'd) format6
type ('a, 'b, 'c) format = ('a, 'b, 'c, 'c) format4
val string_of_format : ('a, 'b, 'c, 'd, 'e, 'f) format6 -> string
Converts a format string into a string.

val format_of_string :
('a, 'b, 'c, 'd, 'e, 'f) format6 ->
('a, 'b, 'c, 'd, 'e, 'f) format6
format_of_string s returns a format string read from the string literal s. Note:
format_of_string can not convert a string argument that is not a literal. If you need this
functionality, use the more general Scanf.format_from_string[29.48] function.

val (^^) :
('a, 'b, 'c, 'd, 'e, 'f) format6 ->
('f, 'b, 'c, 'e, 'g, 'h) format6 ->
('a, 'b, 'c, 'd, 'g, 'h) format6
f1 ^^ f2 catenates format strings f1 and f2. The result is a format string that behaves as
the concatenation of format strings f1 and f2: in case of formatted output, it accepts
arguments from f1, then arguments from f2; in case of formatted input, it returns results
from f1, then results from f2. Right-associative operator, see Ocaml_operators[29.61] for
more information.

Program termination
val exit : int -> 'a
Terminate the process, returning the given status code to the operating system: usually 0 to
indicate no errors, and a small positive integer to indicate failure. All open output channels
are flushed with flush_all. The callbacks registered with Domain.at_exit[29.14] are called
followed by those registered with at_exit[28.2].
An implicit exit 0 is performed each time a program terminates normally. An implicit exit
2 is performed if the program terminates early because of an uncaught exception.

val at_exit : (unit -> unit) -> unit

Register the given function to be called at program termination time. The functions
registered with at_exit will be called when the program does any of the following:

• executes exit[28.2]
• terminates, either normally or because of an uncaught exception
Chapter 28. The core library 537

• executes the C function caml_shutdown. The functions are called in ’last in, first out’
order: the function most recently added with at_exit is called first.

Standard library modules

module Arg :
Arg
module Array :
Array
module ArrayLabels :
ArrayLabels
module Atomic :
Atomic
module Bigarray :
Bigarray
module Bool :
Bool
module Buffer :
Buffer
module Bytes :
Bytes
module BytesLabels :
BytesLabels
module Callback :
Callback
module Char :
Char
module Complex :
Complex
module Condition :
Condition
module Digest :
Digest
module Domain :
Domain
Alert unstable. The Domain interface may change in incompatible ways in the future.

module Dynarray :
Dynarray
module Effect :
Effect
538

Alert unstable. The Effect interface may change in incompatible ways in the future.

module Either :
Either
module Ephemeron :
Ephemeron
module Filename :
Filename
module Float :
Float
module Format :
Format
module Fun :
Fun
module Gc :
Gc
module Hashtbl :
Hashtbl
module In_channel :
In_channel
module Int :
Int
module Int32 :
Int32
module Int64 :
Int64
module Lazy :
Lazy
module Lexing :
Lexing
module List :
List
module ListLabels :
ListLabels
module Map :
Map
module Marshal :
Marshal
module MoreLabels :
MoreLabels
module Mutex :
Chapter 28. The core library 539

Mutex
module Nativeint :
Nativeint
module Obj :
Obj
module Oo :
Oo
module Option :
Option
module Out_channel :
Out_channel
module Parsing :
Parsing
module Printexc :
Printexc
module Printf :
Printf
module Queue :
Queue
module Random :
Random
module Result :
Result
module Scanf :
Scanf
module Semaphore :
Semaphore
module Seq :
Seq
module Set :
Set
module Stack :
Stack
module StdLabels :
StdLabels
module String :
String
module StringLabels :
StringLabels
module Sys :
Sys
540

module Type :
Type
module Uchar :
Uchar
module Unit :
Unit
module Weak :
Weak
Chapter 29

The standard library

This chapter describes the functions provided by the OCaml standard library. The modules from the
standard library are automatically linked with the user’s object code files by the ocamlc command.
Hence, these modules can be used in standalone programs without having to add any .cmo file on
the command line for the linking phase. Similarly, in interactive use, these globals can be used in
toplevel phrases without having to load any .cmo file in memory.
Unlike the core Stdlib module, submodules are not automatically “opened” when compilation
starts, or when the toplevel system is launched. Hence it is necessary to use qualified identifiers to
refer to the functions provided by these modules, or to add open directives.

Conventions
For easy reference, the modules are listed below in alphabetical order of module names. For each
module, the declarations from its signature are printed one by one in typewriter font, followed by a
short comment. All modules and the identifiers they export are indexed at the end of this report.

Overview
Here is a short listing, by theme, of the standard library modules.

541
542

Data structures:
String p. 880 string operations
Bytes p. 595 operations on byte sequences
Array p. 548 array operations
List p. 756 list operations
StdLabels p. 879 labelized versions of the above 4 modules
Unit p. 911 unit values
Bool p. 589 boolean values
Char p. 623 character operations
Uchar p. 909 Unicode characters
Int p. 739 integer values
Option p. 818 option values
Result p. 844 result values
Effect p. 646 effect handlers
Either p. 649 either values
Hashtbl p. 722 hash tables and hash functions
Random p. 839 pseudo-random number generator
Set p. 869 sets over ordered types
Map p. 774 association tables over ordered types
MoreLabels p. 785 labelized versions of Hashtbl, Set, and Map
Oo p. 817 useful functions on objects
Stack p. 877 last-in first-out stacks
Queue p. 835 first-in first-out queues
Buffer p. 590 buffers that grow on demand
Dynarray p. 635 dynamic arrays: arrays that grow on demand
Seq p. 856 functional iterators
Lazy p. 751 delayed evaluation
Weak p. 912 references that don’t prevent objects from being garbage-collected
Atomic p. 564 atomic references (for compatibility with concurrent runtimes)
Ephemeron p. 651 ephemerons and weak hash tables
Bigarray p. 567 large, multi-dimensional, numerical arrays

Arithmetic:
Complex p. 624 complex numbers
Float p. 661 floating-point numbers
Int32 p. 742 operations on 32-bit integers
Int64 p. 746 operations on 64-bit integers
Nativeint p. 813 operations on platform-native integers
Chapter 29. The standard library 543

input/output:
In_channel p. 734 input channels
Out_channel p. 819 output channels
Format p. 683 pretty printing with automatic indentation and line breaking
Marshal p. 782 marshaling of data structures
Printf p. 832 formatting printing functions
Scanf p. 846 formatted input functions
Digest p. 631 MD5 message digest

Parsing:
Lexing p. 753 the run-time library for lexers generated by ocamllex
Parsing p. 824 the run-time library for parsers generated by ocamlyacc

System interface:
Arg p. 543 parsing of command line arguments
Callback p. 623 registering OCaml functions to be called from C
Filename p. 657 operations on file names
Gc p. 712 memory management control and statistics
Printexc p. 825 a catch-all exception handler
Sys p. 898 system interface

Multicore interface:
Domain p. 628 domain spawn and join
Mutex p. 812 mutual exclusion locks
Condition p. 626 condition variables
Semaphore p. 875 semaphores
Effect p. 646 deep and shallow effect handlers

Misc:
Fun p. 711 function values
Type p. 906 type introspection

29.1 Module Arg : Parsing of command line arguments.

This module provides a general mechanism for extracting options and arguments from the command
line to the program. For example:

let usage_msg = "append [-verbose] <file1> [<file2>] ... -o <output>"

let verbose = ref false
let input_files = ref []
let output_file = ref ""
544

let anon_fun filename =

input_files := filename::!input_files

let speclist =
[("-verbose", Arg.Set verbose, "Output debug information");
("-o", Arg.Set_string output_file, "Set output file name")]

let () =
Arg.parse speclist anon_fun usage_msg;
(* Main functionality here *)

Syntax of command lines: A keyword is a character string starting with a -. An option

is a keyword alone or followed by an argument. The types of keywords are: Unit, Bool, Set,
Clear, String, Set_string, Int, Set_int, Float, Set_float, Tuple, Symbol, Rest, Rest_all
and Expand.
Unit, Set and Clear keywords take no argument.
A Rest or Rest_all keyword takes the remainder of the command line as arguments. (More
explanations below.)
Every other keyword takes the following word on the command line as argument. For compatibility
with GNU getopt_long, keyword=arg is also allowed. Arguments not preceded by a keyword are
called anonymous arguments.
Examples (cmd is assumed to be the command name):
• cmd -flag (a unit option)

• cmd -int 1 (an int option with argument 1)

• cmd -string foobar (a string option with argument "foobar")

• cmd -float 12.34 (a float option with argument 12.34)

• cmd a b c (three anonymous arguments: "a", "b", and "c")

• cmd a b -- c d (two anonymous arguments and a rest option with two arguments)
Rest takes a function that is called repeatedly for each remaining command line argument.
Rest_all takes a function that is called once, with the list of all remaining arguments.
Note that if no arguments follow a Rest keyword then the function is not called at all whereas
the function for a Rest_all keyword is called with an empty list.
Alert unsynchronized_access. The Arg module relies on a mutable global state, parsing
functions should only be called from a single domain.

type spec =
| Unit of (unit -> unit)
Call the function with unit argument
| Bool of (bool -> unit)
Call the function with a bool argument
Chapter 29. The standard library 545

| Set of bool ref

Set the reference to true
| Clear of bool ref
Set the reference to false
| String of (string -> unit)
Call the function with a string argument
| Set_string of string ref
Set the reference to the string argument
| Int of (int -> unit)
Call the function with an int argument
| Set_int of int ref
Set the reference to the int argument
| Float of (float -> unit)
Call the function with a float argument
| Set_float of float ref
Set the reference to the float argument
| Tuple of spec list
Take several arguments according to the spec list
| Symbol of string list * (string -> unit)
Take one of the symbols as argument and call the function with the symbol
| Rest of (string -> unit)
Stop interpreting keywords and call the function with each remaining argument
| Rest_all of (string list -> unit)
Stop interpreting keywords and call the function with all remaining arguments
| Expand of (string -> string array)
If the remaining arguments to process are of the form ["-foo"; "arg"] @ rest
where ”foo” is registered as Expand f, then the arguments f "arg" @ rest are
processed. Only allowed in parse_and_expand_argv_dynamic.
The concrete type describing the behavior associated with a keyword.

type key = string

type doc = string
type usage_msg = string
type anon_fun = string -> unit
val parse : (key * spec * doc) list -> anon_fun -> usage_msg -> unit
546

Arg.parse speclist anon_fun usage_msg parses the command line. speclist is a list of
triples (key, spec, doc). key is the option keyword, it must start with a '-' character.
spec gives the option type and the function to call when this option is found on the
command line. doc is a one-line description of this option. anon_fun is called on anonymous
arguments. The functions in spec and anon_fun are called in the same order as their
arguments appear on the command line.
If an error occurs, Arg.parse exits the program, after printing to standard error an error
message as follows:

• The reason for the error: unknown option, invalid or missing argument, etc.
• usage_msg
• The list of options, each followed by the corresponding doc string. Beware: options that
have an empty doc string will not be included in the list.

For the user to be able to specify anonymous arguments starting with a -, include for
example ("-", String anon_fun, doc) in speclist.
By default, parse recognizes two unit options, -help and --help, which will print to
standard output usage_msg and the list of options, and exit the program. You can override
this behaviour by specifying your own -help and --help options in speclist.

val parse_dynamic :
(key * spec * doc) list ref ->
anon_fun -> usage_msg -> unit
Same as Arg.parse[29.1], except that the speclist argument is a reference and may be
updated during the parsing. A typical use for this feature is to parse command lines of the
form:

• command subcommand options where the list of options depends on the value of the
subcommand argument.

Since: 4.01

val parse_argv :
?current:int ref ->
string array ->
(key * spec * doc) list -> anon_fun -> usage_msg -> unit
Arg.parse_argv ~current args speclist anon_fun usage_msg parses the array args as
if it were the command line. It uses and updates the value of ~current (if given), or
Arg.current[29.1]. You must set it before calling parse_argv. The initial value of current
is the index of the program name (argument 0) in the array. If an error occurs,
Arg.parse_argv raises Arg.Bad[29.1] with the error message as argument. If option -help or
--help is given, Arg.parse_argv raises Arg.Help[29.1] with the help message as argument.

val parse_argv_dynamic :
?current:int ref ->
Chapter 29. The standard library 547

string array ->

(key * spec * doc) list ref ->
anon_fun -> string -> unit
Same as Arg.parse_argv[29.1], except that the speclist argument is a reference and may
be updated during the parsing. See Arg.parse_dynamic[29.1].
Since: 4.01

val parse_and_expand_argv_dynamic :
int ref ->
string array ref ->
(key * spec * doc) list ref ->
anon_fun -> string -> unit
Same as Arg.parse_argv_dynamic[29.1], except that the argv argument is a reference and
may be updated during the parsing of Expand arguments. See
Arg.parse_argv_dynamic[29.1].
Since: 4.05

val parse_expand : (key * spec * doc) list -> anon_fun -> usage_msg -> unit
Same as Arg.parse[29.1], except that the Expand arguments are allowed and the
Arg.current[29.1] reference is not updated.
Since: 4.05

exception Help of string

Raised by Arg.parse_argv when the user asks for help.

exception Bad of string

Functions in spec or anon_fun can raise Arg.Bad with an error message to reject invalid
arguments. Arg.Bad is also raised by Arg.parse_argv[29.1] in case of an error.

val usage : (key * spec * doc) list -> usage_msg -> unit
Arg.usage speclist usage_msg prints to standard error an error message that includes the
list of valid options. This is the same message that Arg.parse[29.1] prints in case of error.
speclist and usage_msg are the same as for Arg.parse[29.1].

val usage_string : (key * spec * doc) list -> usage_msg -> string
Returns the message that would have been printed by Arg.usage[29.1], if provided with the
same parameters.

val align :
?limit:int ->
(key * spec * doc) list -> (key * spec * doc) list
Align the documentation strings by inserting spaces at the first alignment separator (tab or, if
tab is not found, space), according to the length of the keyword. Use a alignment separator as
the first character in a doc string if you want to align the whole string. The doc strings
corresponding to Symbol arguments are aligned on the next line.
548

val current : int ref

Position (in Sys.argv[29.56]) of the argument being processed. You can change this value,
e.g. to force Arg.parse[29.1] to skip some arguments. Arg.parse[29.1] uses the initial value
of Arg.current[29.1] as the index of argument 0 (the program name) and starts parsing
arguments at the next element.

val read_arg : string -> string array

Arg.read_arg file reads newline-terminated command line arguments from file file.
Since: 4.05

val read_arg0 : string -> string array

Identical to Arg.read_arg[29.1] but assumes null character terminated command line
arguments.
Since: 4.05

val write_arg : string -> string array -> unit

Arg.write_arg file args writes the arguments args newline-terminated into the file file.
If any of the arguments in args contains a newline, use Arg.write_arg0[29.1] instead.
Since: 4.05

val write_arg0 : string -> string array -> unit

Identical to Arg.write_arg[29.1] but uses the null character for terminator instead of
newline.
Since: 4.05

29.2 Module Array : Array operations.

The labeled version of this module can be used as described in the StdLabels[29.53] module.

type 'a t = 'a array

An alias for the type of arrays.

val length : 'a array -> int

Return the length (number of elements) of the given array.

val get : 'a array -> int -> 'a

get a n returns the element number n of array a. The first element has number 0. The last
element has number length a - 1. You can also write a.(n) instead of get a n.
Raises Invalid_argument if n is outside the range 0 to (length a - 1).

val set : 'a array -> int -> 'a -> unit
Chapter 29. The standard library 549

set a n x modifies array a in place, replacing element number n with x. You can also write
a.(n) <- x instead of set a n x.
Raises Invalid_argument if n is outside the range 0 to length a - 1.

val make : int -> 'a -> 'a array

make n x returns a fresh array of length n, initialized with x. All the elements of this new
array are initially physically equal to x (in the sense of the == predicate). Consequently, if x
is mutable, it is shared among all elements of the array, and modifying x through one of the
array entries will modify all other entries at the same time.
Raises Invalid_argument if n < 0 or n > Sys.max_array_length. If the value of x is a
floating-point number, then the maximum size is only Sys.max_array_length / 2.

val create_float : int -> float array

create_float n returns a fresh float array of length n, with uninitialized data.
Since: 4.03

val init : int -> (int -> 'a) -> 'a array
init n f returns a fresh array of length n, with element number i initialized to the result of
f i. In other terms, init n f tabulates the results of f applied in order to the integers 0 to
n-1.
Raises Invalid_argument if n < 0 or n > Sys.max_array_length. If the return type of f
is float, then the maximum size is only Sys.max_array_length / 2.

val make_matrix : int -> int -> 'a -> 'a array array
make_matrix dimx dimy e returns a two-dimensional array (an array of arrays) with first
dimension dimx and second dimension dimy. All the elements of this new matrix are initially
physically equal to e. The element (x,y) of a matrix m is accessed with the notation
m.(x).(y).
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_array_length[29.56]. If the value of e is a floating-point number, then the
maximum size is only Sys.max_array_length / 2.

val init_matrix : int -> int -> (int -> int -> 'a) -> 'a array array
init_matrix dimx dimy f returns a two-dimensional array (an array of arrays) with first
dimension dimx and second dimension dimy, where the element at index (x,y) is initialized
with f x y. The element (x,y) of a matrix m is accessed with the notation m.(x).(y).
Since: 5.2
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_array_length[29.56]. If the return type of f is float, then the maximum size is
only Sys.max_array_length / 2.

val append : 'a array -> 'a array -> 'a array
550

append v1 v2 returns a fresh array containing the concatenation of the arrays v1 and v2.
Raises Invalid_argument if length v1 + length v2 > Sys.max_array_length.

val concat : 'a array list -> 'a array

Same as Array.append[29.2], but concatenates a list of arrays.

val sub : 'a array -> int -> int -> 'a array
sub a pos len returns a fresh array of length len, containing the elements number pos to
pos + len - 1 of array a.
Raises Invalid_argument if pos and len do not designate a valid subarray of a; that is, if
pos < 0, or len < 0, or pos + len > length a.

val copy : 'a array -> 'a array

copy a returns a copy of a, that is, a fresh array containing the same elements as a.

val fill : 'a array -> int -> int -> 'a -> unit
fill a pos len x modifies the array a in place, storing x in elements number pos to pos +
len - 1.
Raises Invalid_argument if pos and len do not designate a valid subarray of a.

val blit : 'a array -> int -> 'a array -> int -> int -> unit
blit src src_pos dst dst_pos len copies len elements from array src, starting at
element number src_pos, to array dst, starting at element number dst_pos. It works
correctly even if src and dst are the same array, and the source and destination chunks
overlap.
Raises Invalid_argument if src_pos and len do not designate a valid subarray of src, or if
dst_pos and len do not designate a valid subarray of dst.

val to_list : 'a array -> 'a list

to_list a returns the list of all the elements of a.

val of_list : 'a list -> 'a array

of_list l returns a fresh array containing the elements of l.
Raises Invalid_argument if the length of l is greater than Sys.max_array_length.

Iterators
val iter : ('a -> unit) -> 'a array -> unit
iter f a applies function f in turn to all the elements of a. It is equivalent to f a.(0); f
a.(1); ...; f a.(length a - 1); ().

val iteri : (int -> 'a -> unit) -> 'a array -> unit
Chapter 29. The standard library 551

Same as Array.iter[29.2], but the function is applied to the index of the element as first
argument, and the element itself as second argument.

val map : ('a -> 'b) -> 'a array -> 'b array
map f a applies function f to all the elements of a, and builds an array with the results
returned by f: [| f a.(0); f a.(1); ...; f a.(length a - 1) |].

val map_inplace : ('a -> 'a) -> 'a array -> unit
map_inplace f a applies function f to all elements of a, and updates their values in place.
Since: 5.1

val mapi : (int -> 'a -> 'b) -> 'a array -> 'b array
Same as Array.map[29.2], but the function is applied to the index of the element as first
argument, and the element itself as second argument.

val mapi_inplace : (int -> 'a -> 'a) -> 'a array -> unit
Same as Array.map_inplace[29.2], but the function is applied to the index of the element as
first argument, and the element itself as second argument.
Since: 5.1

val fold_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a array -> 'acc
fold_left f init a computes f (... (f (f init a.(0)) a.(1)) ...) a.(n-1), where
n is the length of the array a.

val fold_left_map :
('acc -> 'a -> 'acc * 'b) -> 'acc -> 'a array -> 'acc * 'b array
fold_left_map is a combination of Array.fold_left[29.2] and Array.map[29.2] that
threads an accumulator through calls to f.
Since: 4.13

val fold_right : ('a -> 'acc -> 'acc) -> 'a array -> 'acc -> 'acc
fold_right f a init computes f a.(0) (f a.(1) ( ... (f a.(n-1) init) ...)),
where n is the length of the array a.

Iterators on two arrays

val iter2 : ('a -> 'b -> unit) -> 'a array -> 'b array -> unit
iter2 f a b applies function f to all the elements of a and b.
Since: 4.03 (4.05 in ArrayLabels)
Raises Invalid_argument if the arrays are not the same size.

val map2 : ('a -> 'b -> 'c) -> 'a array -> 'b array -> 'c array
552

map2 f a b applies function f to all the elements of a and b, and builds an array with the
results returned by f: [| f a.(0) b.(0); ...; f a.(length a - 1) b.(length b -
1)|].
Since: 4.03 (4.05 in ArrayLabels)
Raises Invalid_argument if the arrays are not the same size.

Array scanning
val for_all : ('a -> bool) -> 'a array -> bool
for_all f [|a1; ...; an|] checks if all elements of the array satisfy the predicate f. That
is, it returns (f a1) && (f a2) && ... && (f an).
Since: 4.03

val exists : ('a -> bool) -> 'a array -> bool
exists f [|a1; ...; an|] checks if at least one element of the array satisfies the predicate
f. That is, it returns (f a1) || (f a2) || ... || (f an).
Since: 4.03

val for_all2 : ('a -> 'b -> bool) -> 'a array -> 'b array -> bool
Same as Array.for_all[29.2], but for a two-argument predicate.
Since: 4.11
Raises Invalid_argument if the two arrays have different lengths.

val exists2 : ('a -> 'b -> bool) -> 'a array -> 'b array -> bool
Same as Array.exists[29.2], but for a two-argument predicate.
Since: 4.11
Raises Invalid_argument if the two arrays have different lengths.

val mem : 'a -> 'a array -> bool

mem a set is true if and only if a is structurally equal to an element of set (i.e. there is an x
in set such that compare a x = 0).
Since: 4.03

val memq : 'a -> 'a array -> bool

Same as Array.mem[29.2], but uses physical equality instead of structural equality to compare
array elements.
Since: 4.03

val find_opt : ('a -> bool) -> 'a array -> 'a option
Chapter 29. The standard library 553

find_opt f a returns the first element of the array a that satisfies the predicate f, or None if
there is no value that satisfies f in the array a.
Since: 4.13

val find_index : ('a -> bool) -> 'a array -> int option
find_index f a returns Some i, where i is the index of the first element of the array a that
satisfies f x, if there is such an element.
It returns None if there is no such element.
Since: 5.1

val find_map : ('a -> 'b option) -> 'a array -> 'b option
find_map f a applies f to the elements of a in order, and returns the first result of the form
Some v, or None if none exist.
Since: 4.13

val find_mapi : (int -> 'a -> 'b option) -> 'a array -> 'b option
Same as find_map, but the predicate is applied to the index of the element as first argument
(counting from 0), and the element itself as second argument.
Since: 5.1

Arrays of pairs
val split : ('a * 'b) array -> 'a array * 'b array
split [|(a1,b1); ...; (an,bn)|] is ([|a1; ...; an|], [|b1; ...; bn|]).
Since: 4.13

val combine : 'a array -> 'b array -> ('a * 'b) array
combine [|a1; ...; an|] [|b1; ...; bn|] is [|(a1,b1); ...; (an,bn)|]. Raise
Invalid_argument if the two arrays have different lengths.
Since: 4.13

Sorting and shuffling

val sort : ('a -> 'a -> int) -> 'a array -> unit
Sort an array in increasing order according to a comparison function. The comparison
function must return 0 if its arguments compare as equal, a positive integer if the first is
greater, and a negative integer if the first is smaller (see below for a complete specification).
For example, compare[28.2] is a suitable comparison function. After calling sort, the array is
sorted in place in increasing order. sort is guaranteed to run in constant heap space and (at
most) logarithmic stack space.
The current implementation uses Heap Sort. It runs in constant stack space.
554

Specification of the comparison function: Let a be the array and cmp the comparison function.
The following must be true for all x, y, z in a :
• cmp x y > 0 if and only if cmp y x < 0
• if cmp x y ≥ 0 and cmp y z ≥ 0 then cmp x z ≥ 0
When sort returns, a contains the same elements as before, reordered in such a way that for
all i and j valid indices of a :
• cmp a.(i) a.(j) ≥ 0 if i ≥ j

val stable_sort : ('a -> 'a -> int) -> 'a array -> unit
Same as Array.sort[29.2], but the sorting algorithm is stable (i.e. elements that compare
equal are kept in their original order) and not guaranteed to run in constant heap space.
The current implementation uses Merge Sort. It uses a temporary array of length n/2, where
n is the length of the array. It is usually faster than the current implementation of
Array.sort[29.2].

val fast_sort : ('a -> 'a -> int) -> 'a array -> unit
Same as Array.sort[29.2] or Array.stable_sort[29.2], whichever is faster on typical input.

val shuffle : rand:(int -> int) -> 'a array -> unit
shuffle rand a randomly permutes a’s element using rand for randomness. The
distribution of permutations is uniform.
rand must be such that a call to rand n returns a uniformly distributed random number in
the range [0;n-1]. Random.int[29.46] can be used for this (do not forget to initialize[29.46]
the generator).
Since: 5.2

Arrays and Sequences

val to_seq : 'a array -> 'a Seq.t
Iterate on the array, in increasing order. Modifications of the array during iteration will be
reflected in the sequence.
Since: 4.07

val to_seqi : 'a array -> (int * 'a) Seq.t

Iterate on the array, in increasing order, yielding indices along elements. Modifications of the
array during iteration will be reflected in the sequence.
Since: 4.07

val of_seq : 'a Seq.t -> 'a array

Create an array from the generator
Since: 4.07
Chapter 29. The standard library 555

Arrays and concurrency safety

Care must be taken when concurrently accessing arrays from multiple domains: accessing an array
will never crash a program, but unsynchronized accesses might yield surprising (non-sequentially-
consistent) results.

Atomicity
Every array operation that accesses more than one array element is not atomic. This includes
iteration, scanning, sorting, splitting and combining arrays.
For example, consider the following program:
let size = 100_000_000
let a = Array.make size 1
let d1 = Domain.spawn (fun () ->
Array.iteri (fun i x -> a.(i) <- x + 1) a
)
let d2 = Domain.spawn (fun () ->
Array.iteri (fun i x -> a.(i) <- 2 * x + 1) a
)
let () = Domain.join d1; Domain.join d2

After executing this code, each field of the array a is either 2, 3, 4 or 5. If atomicity is required,
then the user must implement their own synchronization (for example, using Mutex.t[29.37]).

Data races
If two domains only access disjoint parts of the array, then the observed behaviour is the equivalent
to some sequential interleaving of the operations from the two domains.
A data race is said to occur when two domains access the same array element without syn-
chronization and at least one of the accesses is a write. In the absence of data races, the observed
behaviour is equivalent to some sequential interleaving of the operations from different domains.
Whenever possible, data races should be avoided by using synchronization to mediate the accesses
to the array elements.
Indeed, in the presence of data races, programs will not crash but the observed behaviour may
not be equivalent to any sequential interleaving of operations from different domains. Nevertheless,
even in the presence of data races, a read operation will return the value of some prior write to that
location (with a few exceptions for float arrays).

Float arrays
Float arrays have two supplementary caveats in the presence of data races.
First, the blit operation might copy an array byte-by-byte. Data races between such a blit
operation and another operation might produce surprising values due to tearing: partial writes
interleaved with other operations can create float values that would not exist with a sequential
execution.
For instance, at the end of
556

let zeros = Array.make size 0.

let max_floats = Array.make size Float.max_float
let res = Array.copy zeros
let d1 = Domain.spawn (fun () -> Array.blit zeros 0 res 0 size)
let d2 = Domain.spawn (fun () -> Array.blit max_floats 0 res 0 size)
let () = Domain.join d1; Domain.join d2

the res array might contain values that are neither 0. nor max_float.
Second, on 32-bit architectures, getting or setting a field involves two separate memory accesses.
In the presence of data races, the user may observe tearing on any operation.

29.3 Module ArrayLabels : Array operations.

The labeled version of this module can be used as described in the StdLabels[29.53] module.

type 'a t = 'a array

An alias for the type of arrays.

val length : 'a array -> int

Return the length (number of elements) of the given array.

val get : 'a array -> int -> 'a

get a n returns the element number n of array a. The first element has number 0. The last
element has number length a - 1. You can also write a.(n) instead of get a n.
Raises Invalid_argument if n is outside the range 0 to (length a - 1).

val set : 'a array -> int -> 'a -> unit
set a n x modifies array a in place, replacing element number n with x. You can also write
a.(n) <- x instead of set a n x.
Raises Invalid_argument if n is outside the range 0 to length a - 1.

val make : int -> 'a -> 'a array

make n x returns a fresh array of length n, initialized with x. All the elements of this new
array are initially physically equal to x (in the sense of the == predicate). Consequently, if x
is mutable, it is shared among all elements of the array, and modifying x through one of the
array entries will modify all other entries at the same time.
Raises Invalid_argument if n < 0 or n > Sys.max_array_length. If the value of x is a
floating-point number, then the maximum size is only Sys.max_array_length / 2.

val create_float : int -> float array

create_float n returns a fresh float array of length n, with uninitialized data.
Since: 4.03
Chapter 29. The standard library 557

val init : int -> f:(int -> 'a) -> 'a array
init n ~f returns a fresh array of length n, with element number i initialized to the result
of f i. In other terms, init n ~f tabulates the results of f applied in order to the integers 0
to n-1.
Raises Invalid_argument if n < 0 or n > Sys.max_array_length. If the return type of f
is float, then the maximum size is only Sys.max_array_length / 2.

val make_matrix : dimx:int -> dimy:int -> 'a -> 'a array array
make_matrix ~dimx ~dimy e returns a two-dimensional array (an array of arrays) with first
dimension dimx and second dimension dimy. All the elements of this new matrix are initially
physically equal to e. The element (x,y) of a matrix m is accessed with the notation
m.(x).(y).
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_array_length[29.56]. If the value of e is a floating-point number, then the
maximum size is only Sys.max_array_length / 2.

val init_matrix :
dimx:int -> dimy:int -> f:(int -> int -> 'a) -> 'a array array
init_matrix ~dimx ~dimy ~f returns a two-dimensional array (an array of arrays) with first
dimension dimx and second dimension dimy, where the element at index (x,y) is initialized
with f x y. The element (x,y) of a matrix m is accessed with the notation m.(x).(y).
Since: 5.2
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_array_length[29.56]. If the return type of f is float, then the maximum size is
only Sys.max_array_length / 2.

val append : 'a array -> 'a array -> 'a array
append v1 v2 returns a fresh array containing the concatenation of the arrays v1 and v2.
Raises Invalid_argument if length v1 + length v2 > Sys.max_array_length.

val concat : 'a array list -> 'a array

Same as ArrayLabels.append[29.3], but concatenates a list of arrays.

val sub : 'a array -> pos:int -> len:int -> 'a array
sub a ~pos ~len returns a fresh array of length len, containing the elements number pos to
pos + len - 1 of array a.
Raises Invalid_argument if pos and len do not designate a valid subarray of a; that is, if
pos < 0, or len < 0, or pos + len > length a.

val copy : 'a array -> 'a array

copy a returns a copy of a, that is, a fresh array containing the same elements as a.

val fill : 'a array -> pos:int -> len:int -> 'a -> unit
558

fill a ~pos ~len x modifies the array a in place, storing x in elements number pos to pos
+ len - 1.
Raises Invalid_argument if pos and len do not designate a valid subarray of a.

val blit :
src:'a array -> src_pos:int -> dst:'a array -> dst_pos:int -> len:int -> unit
blit ~src ~src_pos ~dst ~dst_pos ~len copies len elements from array src, starting at
element number src_pos, to array dst, starting at element number dst_pos. It works
correctly even if src and dst are the same array, and the source and destination chunks
overlap.
Raises Invalid_argument if src_pos and len do not designate a valid subarray of src, or if
dst_pos and len do not designate a valid subarray of dst.

val to_list : 'a array -> 'a list

to_list a returns the list of all the elements of a.

val of_list : 'a list -> 'a array

of_list l returns a fresh array containing the elements of l.
Raises Invalid_argument if the length of l is greater than Sys.max_array_length.

Iterators
val iter : f:('a -> unit) -> 'a array -> unit
iter ~f a applies function f in turn to all the elements of a. It is equivalent to f a.(0); f
a.(1); ...; f a.(length a - 1); ().

val iteri : f:(int -> 'a -> unit) -> 'a array -> unit
Same as ArrayLabels.iter[29.3], but the function is applied to the index of the element as
first argument, and the element itself as second argument.

val map : f:('a -> 'b) -> 'a array -> 'b array
map ~f a applies function f to all the elements of a, and builds an array with the results
returned by f: [| f a.(0); f a.(1); ...; f a.(length a - 1) |].

val map_inplace : f:('a -> 'a) -> 'a array -> unit
map_inplace ~f a applies function f to all elements of a, and updates their values in place.
Since: 5.1

val mapi : f:(int -> 'a -> 'b) -> 'a array -> 'b array
Same as ArrayLabels.map[29.3], but the function is applied to the index of the element as
first argument, and the element itself as second argument.

val mapi_inplace : f:(int -> 'a -> 'a) -> 'a array -> unit
Chapter 29. The standard library 559

Same as ArrayLabels.map_inplace[29.3], but the function is applied to the index of the

element as first argument, and the element itself as second argument.
Since: 5.1

val fold_left : f:('acc -> 'a -> 'acc) -> init:'acc -> 'a array -> 'acc
fold_left ~f ~init a computes f (... (f (f init a.(0)) a.(1)) ...) a.(n-1),
where n is the length of the array a.

val fold_left_map :
f:('acc -> 'a -> 'acc * 'b) -> init:'acc -> 'a array -> 'acc * 'b array
fold_left_map is a combination of ArrayLabels.fold_left[29.3] and
ArrayLabels.map[29.3] that threads an accumulator through calls to f.
Since: 4.13

val fold_right : f:('a -> 'acc -> 'acc) -> 'a array -> init:'acc -> 'acc
fold_right ~f a ~init computes f a.(0) (f a.(1) ( ... (f a.(n-1) init) ...)),
where n is the length of the array a.

Iterators on two arrays

val iter2 : f:('a -> 'b -> unit) -> 'a array -> 'b array -> unit
iter2 ~f a b applies function f to all the elements of a and b.
Since: 4.05
Raises Invalid_argument if the arrays are not the same size.

val map2 : f:('a -> 'b -> 'c) -> 'a array -> 'b array -> 'c array
map2 ~f a b applies function f to all the elements of a and b, and builds an array with the
results returned by f: [| f a.(0) b.(0); ...; f a.(length a - 1) b.(length b -
1)|].
Since: 4.05
Raises Invalid_argument if the arrays are not the same size.

Array scanning
val for_all : f:('a -> bool) -> 'a array -> bool
for_all ~f [|a1; ...; an|] checks if all elements of the array satisfy the predicate f.
That is, it returns (f a1) && (f a2) && ... && (f an).
Since: 4.03

val exists : f:('a -> bool) -> 'a array -> bool
560

exists ~f [|a1; ...; an|] checks if at least one element of the array satisfies the
predicate f. That is, it returns (f a1) || (f a2) || ... || (f an).
Since: 4.03

val for_all2 : f:('a -> 'b -> bool) -> 'a array -> 'b array -> bool
Same as ArrayLabels.for_all[29.3], but for a two-argument predicate.
Since: 4.11
Raises Invalid_argument if the two arrays have different lengths.

val exists2 : f:('a -> 'b -> bool) -> 'a array -> 'b array -> bool
Same as ArrayLabels.exists[29.3], but for a two-argument predicate.
Since: 4.11
Raises Invalid_argument if the two arrays have different lengths.

val mem : 'a -> set:'a array -> bool

mem a ~set is true if and only if a is structurally equal to an element of set (i.e. there is an
x in set such that compare a x = 0).
Since: 4.03

val memq : 'a -> set:'a array -> bool

Same as ArrayLabels.mem[29.3], but uses physical equality instead of structural equality to
compare array elements.
Since: 4.03

val find_opt : f:('a -> bool) -> 'a array -> 'a option
find_opt ~f a returns the first element of the array a that satisfies the predicate f, or None
if there is no value that satisfies f in the array a.
Since: 4.13

val find_index : f:('a -> bool) -> 'a array -> int option
find_index ~f a returns Some i, where i is the index of the first element of the array a
that satisfies f x, if there is such an element.
It returns None if there is no such element.
Since: 5.1

val find_map : f:('a -> 'b option) -> 'a array -> 'b option
find_map ~f a applies f to the elements of a in order, and returns the first result of the
form Some v, or None if none exist.
Since: 4.13

val find_mapi : f:(int -> 'a -> 'b option) -> 'a array -> 'b option
Chapter 29. The standard library 561

Same as find_map, but the predicate is applied to the index of the element as first argument
(counting from 0), and the element itself as second argument.
Since: 5.1

Arrays of pairs
val split : ('a * 'b) array -> 'a array * 'b array
split [|(a1,b1); ...; (an,bn)|] is ([|a1; ...; an|], [|b1; ...; bn|]).
Since: 4.13

val combine : 'a array -> 'b array -> ('a * 'b) array
combine [|a1; ...; an|] [|b1; ...; bn|] is [|(a1,b1); ...; (an,bn)|]. Raise
Invalid_argument if the two arrays have different lengths.
Since: 4.13

Sorting and shuffling

val sort : cmp:('a -> 'a -> int) -> 'a array -> unit
Sort an array in increasing order according to a comparison function. The comparison
function must return 0 if its arguments compare as equal, a positive integer if the first is
greater, and a negative integer if the first is smaller (see below for a complete specification).
For example, compare[28.2] is a suitable comparison function. After calling sort, the array is
sorted in place in increasing order. sort is guaranteed to run in constant heap space and (at
most) logarithmic stack space.
The current implementation uses Heap Sort. It runs in constant stack space.
Specification of the comparison function: Let a be the array and cmp the comparison function.
The following must be true for all x, y, z in a :
• cmp x y > 0 if and only if cmp y x < 0
• if cmp x y ≥ 0 and cmp y z ≥ 0 then cmp x z ≥ 0
When sort returns, a contains the same elements as before, reordered in such a way that for
all i and j valid indices of a :
• cmp a.(i) a.(j) ≥ 0 if i ≥ j

val stable_sort : cmp:('a -> 'a -> int) -> 'a array -> unit
Same as ArrayLabels.sort[29.3], but the sorting algorithm is stable (i.e. elements that
compare equal are kept in their original order) and not guaranteed to run in constant heap
space.
The current implementation uses Merge Sort. It uses a temporary array of length n/2, where
n is the length of the array. It is usually faster than the current implementation of
ArrayLabels.sort[29.3].
562

val fast_sort : cmp:('a -> 'a -> int) -> 'a array -> unit
Same as ArrayLabels.sort[29.3] or ArrayLabels.stable_sort[29.3], whichever is faster on
typical input.

val shuffle : rand:(int -> int) -> 'a array -> unit
shuffle ~rand a randomly permutes a’s element using rand for randomness. The
distribution of permutations is uniform.
rand must be such that a call to rand n returns a uniformly distributed random number in
the range [0;n-1]. Random.int[29.46] can be used for this (do not forget to initialize[29.46]
the generator).
Since: 5.2

Arrays and Sequences

val to_seq : 'a array -> 'a Seq.t
Iterate on the array, in increasing order. Modifications of the array during iteration will be
reflected in the sequence.
Since: 4.07

val to_seqi : 'a array -> (int * 'a) Seq.t

Iterate on the array, in increasing order, yielding indices along elements. Modifications of the
array during iteration will be reflected in the sequence.
Since: 4.07

val of_seq : 'a Seq.t -> 'a array

Create an array from the generator
Since: 4.07

Arrays and concurrency safety

Care must be taken when concurrently accessing arrays from multiple domains: accessing an array
will never crash a program, but unsynchronized accesses might yield surprising (non-sequentially-
consistent) results.

Atomicity
Every array operation that accesses more than one array element is not atomic. This includes
iteration, scanning, sorting, splitting and combining arrays.
For example, consider the following program:
let size = 100_000_000
let a = ArrayLabels.make size 1
let d1 = Domain.spawn (fun () ->
Chapter 29. The standard library 563

ArrayLabels.iteri ~f:(fun i x -> a.(i) <- x + 1) a

)
let d2 = Domain.spawn (fun () ->
ArrayLabels.iteri ~f:(fun i x -> a.(i) <- 2 * x + 1) a
)
let () = Domain.join d1; Domain.join d2

After executing this code, each field of the array a is either 2, 3, 4 or 5. If atomicity is required,
then the user must implement their own synchronization (for example, using Mutex.t[29.37]).

Data races
If two domains only access disjoint parts of the array, then the observed behaviour is the equivalent
to some sequential interleaving of the operations from the two domains.
A data race is said to occur when two domains access the same array element without syn-
chronization and at least one of the accesses is a write. In the absence of data races, the observed
behaviour is equivalent to some sequential interleaving of the operations from different domains.
Whenever possible, data races should be avoided by using synchronization to mediate the accesses
to the array elements.
Indeed, in the presence of data races, programs will not crash but the observed behaviour may
not be equivalent to any sequential interleaving of operations from different domains. Nevertheless,
even in the presence of data races, a read operation will return the value of some prior write to that
location (with a few exceptions for float arrays).

Float arrays
Float arrays have two supplementary caveats in the presence of data races.
First, the blit operation might copy an array byte-by-byte. Data races between such a blit
operation and another operation might produce surprising values due to tearing: partial writes
interleaved with other operations can create float values that would not exist with a sequential
execution.
For instance, at the end of
let zeros = Array.make size 0.
let max_floats = Array.make size Float.max_float
let res = Array.copy zeros
let d1 = Domain.spawn (fun () -> Array.blit zeros 0 res 0 size)
let d2 = Domain.spawn (fun () -> Array.blit max_floats 0 res 0 size)
let () = Domain.join d1; Domain.join d2

the res array might contain values that are neither 0. nor max_float.
Second, on 32-bit architectures, getting or setting a field involves two separate memory accesses.
In the presence of data races, the user may observe tearing on any operation.
564

29.4 Module Atomic : Atomic references.

See the examples[29.45] below. See ’Memory model: The hard bits’ chapter in the manual.
Since: 4.12

type !'a t
An atomic (mutable) reference to a value of type 'a.

val make : 'a -> 'a t

Create an atomic reference.

val make_contended : 'a -> 'a t

Create an atomic reference that is alone on a cache line. It occupies 4-16x the memory of one
allocated with make v.
The primary purpose is to prevent false-sharing and the resulting performance degradation.
When a CPU performs an atomic operation, it temporarily takes ownership of an entire cache
line that contains the atomic reference. If multiple atomic references share the same cache
line, modifying these disjoint memory regions simultaneously becomes impossible, which can
create a bottleneck. Hence, as a general guideline, if an atomic reference is experiencing
contention, assigning it its own cache line may enhance performance.

val get : 'a t -> 'a

Get the current value of the atomic reference.

val set : 'a t -> 'a -> unit

Set a new value for the atomic reference.

val exchange : 'a t -> 'a -> 'a

Set a new value for the atomic reference, and return the current value.

val compare_and_set : 'a t -> 'a -> 'a -> bool

compare_and_set r seen v sets the new value of r to v only if its current value is
physically equal to seen – the comparison and the set occur atomically. Returns true if the
comparison succeeded (so the set happened) and false otherwise.

val fetch_and_add : int t -> int -> int

fetch_and_add r n atomically increments the value of r by n, and returns the current value
(before the increment).

val incr : int t -> unit

incr r atomically increments the value of r by 1.

val decr : int t -> unit

decr r atomically decrements the value of r by 1.
Chapter 29. The standard library 565

Examples
Basic Thread Coordination
A basic use case is to have global counters that are updated in a thread-safe way, for example
to keep some sorts of metrics over IOs performed by the program. Another basic use case is to
coordinate the termination of threads in a given program, for example when one thread finds an
answer, or when the program is shut down by the user.
Here, for example, we’re going to try to find a number whose hash satisfies a basic property. To
do that, we’ll run multiple threads which will try random numbers until they find one that works.
Of course the output below is a sample run and will change every time the program is run.

(* use for termination *)

let stop_all_threads = Atomic.make false

(* total number of individual attempts to find a number *)

let num_attempts = Atomic.make 0

(* find a number that satisfies [p], by... trying random numbers

until one fits. *)
let find_number_where (p:int -> bool) =
let rand = Random.State.make_self_init() in
while not (Atomic.get stop_all_threads) do

let n = Random.State.full_int rand max_int in

ignore (Atomic.fetch_and_add num_attempts 1 : int);

if p (Hashtbl.hash n) then (
Printf.printf "found %d (hash=%d)\n%!" n (Hashtbl.hash n);
Atomic.set stop_all_threads true; (* signal all threads to stop *)
)
done;;

(* run multiple domains to search for a [n] where [hash n <= 100] *)
let () =
let criterion n = n <= 100 in
let threads =
Array.init 8
(fun _ -> Domain.spawn (fun () -> find_number_where criterion))
in
Array.iter Domain.join threads;
Printf.printf "total number of attempts: %d\n%!"
(Atomic.get num_attempts) ;;

- : unit = ()
566

found 1651745641680046833 (hash=33)

total number of attempts: 30230350

Treiber Stack
Another example is a basic Treiber stack[https://en.wikipedia.org/wiki/Treiber_stack] (a
thread-safe stack) that can be safely shared between threads.
Note how both push and pop are recursive, because they attempt to swap the new stack (with
one more, or one fewer, element) with the old stack. This is optimistic concurrency: each iteration
of, say, push stack x gets the old stack l, and hopes that by the time it tries to replace l with
x::l, nobody else has had time to modify the list. If the compare_and_set fails it means we were
too optimistic, and must try again.

type 'a stack = 'a list Atomic.t

let rec push (stack: _ stack) elt : unit =

let cur = Atomic.get stack in
let success = Atomic.compare_and_set stack cur (elt :: cur) in
if not success then
push stack elt

let rec pop (stack: _ stack) : _ option =

let cur = Atomic.get stack in
match cur with
| [] -> None
| x :: tail ->
let success = Atomic.compare_and_set stack cur tail in
if success then Some x
else pop stack

# let st = Atomic.make []
# push st 1
- : unit = ()
# push st 2
- : unit = ()
# pop st
- : int option = Some 2
# pop st
- : int option = Some 1
# pop st
- : int option = None
Chapter 29. The standard library 567

29.5 Module Bigarray : Large, multi-dimensional, numerical ar-

rays.
This module implements multi-dimensional arrays of integers and floating-point numbers, thereafter
referred to as ’Bigarrays’, to distinguish them from the standard OCaml arrays described in
Array[29.2].
The implementation allows efficient sharing of large numerical arrays between OCaml code and
C or Fortran numerical libraries.
The main differences between ’Bigarrays’ and standard OCaml arrays are as follows:

• Bigarrays are not limited in size, unlike OCaml arrays. (Normal float arrays are limited
to 2,097,151 elements on a 32-bit platform, and normal arrays of other types to 4,194,303
elements.)

• Bigarrays are multi-dimensional. Any number of dimensions between 0 and 16 is supported.

In contrast, OCaml arrays are mono-dimensional and require encoding multi-dimensional
arrays as arrays of arrays.

• Bigarrays can only contain integers and floating-point numbers, while OCaml arrays can
contain arbitrary OCaml data types.

• Bigarrays provide more space-efficient storage of integer and floating-point elements than
normal OCaml arrays, in particular because they support ’small’ types such as single-precision
floats and 8 and 16-bit integers, in addition to the standard OCaml types of double-precision
floats and 32 and 64-bit integers.

• The memory layout of Bigarrays is entirely compatible with that of arrays in C and Fortran,
allowing large arrays to be passed back and forth between OCaml code and C / Fortran code
with no data copying at all.

• Bigarrays support interesting high-level operations that normal arrays do not provide efficiently,
such as extracting sub-arrays and ’slicing’ a multi-dimensional array along certain dimensions,
all without any copying.

Users of this module are encouraged to do open Bigarray in their source, then refer to array
types and operations via short dot notation, e.g. Array1.t or Array2.sub.
Bigarrays support all the OCaml ad-hoc polymorphic operations:

• comparisons (=, <>, <=, etc, as well as compare[28.2]);

• hashing (module Hash);

• and structured input-output (the functions from the Marshal[29.35] module, as well as
output_value[28.2] and input_value[28.2]).
568

Element kinds
Bigarrays can contain elements of the following kinds:

• IEEE half precision (16 bits) floating-point numbers (Bigarray.float16_elt[29.5]),

• IEEE single precision (32 bits) floating-point numbers (Bigarray.float32_elt[29.5]),

• IEEE double precision (64 bits) floating-point numbers (Bigarray.float64_elt[29.5]),

• IEEE single precision (2 * 32 bits) floating-point complex numbers (Bigarray.complex32_

elt[29.5]),

• IEEE double precision (2 * 64 bits) floating-point complex numbers (Bigarray.complex64_

elt[29.5]),

• 8-bit integers (signed or unsigned) (Bigarray.int8_signed_elt[29.5] or Bigarray.int8_

unsigned_elt[29.5]),

• 16-bit integers (signed or unsigned) (Bigarray.int16_signed_elt[29.5] or Bigarray.int16_

unsigned_elt[29.5]),

• OCaml integers (signed, 31 bits on 32-bit architectures, 63 bits on 64-bit architectures)

(Bigarray.int_elt[29.5]),

• 32-bit signed integers (Bigarray.int32_elt[29.5]),

• 64-bit signed integers (Bigarray.int64_elt[29.5]),

• platform-native signed integers (32 bits on 32-bit architectures, 64 bits on 64-bit architectures)
(Bigarray.nativeint_elt[29.5]).

Each element kind is represented at the type level by one of the *_elt types defined below
(defined with a single constructor instead of abstract types for technical injectivity reasons).
type float16_elt =
| Float16_elt
type float32_elt =
| Float32_elt
type float64_elt =
| Float64_elt
type int8_signed_elt =
| Int8_signed_elt
type int8_unsigned_elt =
| Int8_unsigned_elt
type int16_signed_elt =
| Int16_signed_elt
type int16_unsigned_elt =
| Int16_unsigned_elt
Chapter 29. The standard library 569

type int32_elt =
| Int32_elt
type int64_elt =
| Int64_elt
type int_elt =
| Int_elt
type nativeint_elt =
| Nativeint_elt
type complex32_elt =
| Complex32_elt
type complex64_elt =
| Complex64_elt
type ('a, 'b) kind =
| Float32 : (float, float32_elt) kind
| Float64 : (float, float64_elt) kind
| Int8_signed : (int, int8_signed_elt) kind
| Int8_unsigned : (int, int8_unsigned_elt) kind
| Int16_signed : (int, int16_signed_elt) kind
| Int16_unsigned : (int, int16_unsigned_elt) kind
| Int32 : (int32, int32_elt) kind
| Int64 : (int64, int64_elt) kind
| Int : (int, int_elt) kind
| Nativeint : (nativeint, nativeint_elt) kind
| Complex32 : (Complex.t, complex32_elt) kind
| Complex64 : (Complex.t, complex64_elt) kind
| Char : (char, int8_unsigned_elt) kind
| Float16 : (float, float16_elt) kind
To each element kind is associated an OCaml type, which is the type of OCaml values that
can be stored in the Bigarray or read back from it. This type is not necessarily the same as
the type of the array elements proper: for instance, a Bigarray whose elements are of kind
float32_elt contains 32-bit single precision floats, but reading or writing one of its elements
from OCaml uses the OCaml type float, which is 64-bit double precision floats.
The GADT type ('a, 'b) kind captures this association of an OCaml type 'a for values
read or written in the Bigarray, and of an element kind 'b which represents the actual
contents of the Bigarray. Its constructors list all possible associations of OCaml types with
element kinds, and are re-exported below for backward-compatibility reasons.
Using a generalized algebraic datatype (GADT) here allows writing well-typed polymorphic
functions whose return type depend on the argument type, such as:

let zero : type a b. (a, b) kind -> a = function

| Float32 -> 0.0 | Complex32 -> Complex.zero
| Float64 -> 0.0 | Complex64 -> Complex.zero
| Float16 -> 0.0
570

| Int8_signed -> 0 | Int8_unsigned -> 0

| Int16_signed -> 0 | Int16_unsigned -> 0
| Int32 -> 0l | Int64 -> 0L
| Int -> 0 | Nativeint -> 0n
| Char -> '\000'

Since: 5.2 Constructor Float16 for the GADT.

val float16 : (float, float16_elt) kind

See Bigarray.char[29.5].
Since: 5.2

val float32 : (float, float32_elt) kind

See Bigarray.char[29.5].

val float64 : (float, float64_elt) kind

See Bigarray.char[29.5].

val complex32 : (Complex.t, complex32_elt) kind

See Bigarray.char[29.5].

val complex64 : (Complex.t, complex64_elt) kind

See Bigarray.char[29.5].

val int8_signed : (int, int8_signed_elt) kind

See Bigarray.char[29.5].

val int8_unsigned : (int, int8_unsigned_elt) kind

See Bigarray.char[29.5].

val int16_signed : (int, int16_signed_elt) kind

See Bigarray.char[29.5].

val int16_unsigned : (int, int16_unsigned_elt) kind

See Bigarray.char[29.5].

val int : (int, int_elt) kind

See Bigarray.char[29.5] and [29.5].
Beware that this is a bigarray containing OCaml integers (signed, 31 bits on 32-bit
architectures, 63 bits on 64-bit architectures), which does not match the C int type.

val int32 : (int32, int32_elt) kind

See Bigarray.char[29.5].
Chapter 29. The standard library 571

val int64 : (int64, int64_elt) kind

See Bigarray.char[29.5].

val nativeint : (nativeint, nativeint_elt) kind

See Bigarray.char[29.5].

val char : (char, int8_unsigned_elt) kind

As shown by the types of the values above, Bigarrays of kind float16_elt, float32_elt
and float64_elt are accessed using the OCaml type float. Bigarrays of complex kinds
complex32_elt, complex64_elt are accessed with the OCaml type Complex.t[29.12].
Bigarrays of integer kinds are accessed using the smallest OCaml integer type large enough to
represent the array elements: int for 8- and 16-bit integer Bigarrays, as well as
OCaml-integer Bigarrays; int32 for 32-bit integer Bigarrays; int64 for 64-bit integer
Bigarrays; and nativeint for platform-native integer Bigarrays. Finally, Bigarrays of kind
int8_unsigned_elt can also be accessed as arrays of characters instead of arrays of small
integers, by using the kind value char instead of int8_unsigned.

val kind_size_in_bytes : ('a, 'b) kind -> int

kind_size_in_bytes k is the number of bytes used to store an element of type k.
Since: 4.03

Array layouts
type c_layout =
| C_layout_typ
See Bigarray.fortran_layout[29.5].

type fortran_layout =
| Fortran_layout_typ
To facilitate interoperability with existing C and Fortran code, this library supports two
different memory layouts for Bigarrays, one compatible with the C conventions, the other
compatible with the Fortran conventions.
In the C-style layout, array indices start at 0, and multi-dimensional arrays are laid out in
row-major format. That is, for a two-dimensional array, all elements of row 0 are contiguous
in memory, followed by all elements of row 1, etc. In other terms, the array elements at (x,y)
and (x, y+1) are adjacent in memory.
In the Fortran-style layout, array indices start at 1, and multi-dimensional arrays are laid out
in column-major format. That is, for a two-dimensional array, all elements of column 0 are
contiguous in memory, followed by all elements of column 1, etc. In other terms, the array
elements at (x,y) and (x+1, y) are adjacent in memory.
Each layout style is identified at the type level by the phantom types
Bigarray.c_layout[29.5] and Bigarray.fortran_layout[29.5] respectively.
572

Supported layouts
The GADT type 'a layout represents one of the two supported memory layouts: C-style or
Fortran-style. Its constructors are re-exported as values below for backward-compatibility reasons.
type 'a layout =
| C_layout : c_layout layout
| Fortran_layout : fortran_layout layout
val c_layout : c_layout layout
val fortran_layout : fortran_layout layout

Generic arrays (of arbitrarily many dimensions)

module Genarray :
sig
type (!'a, !'b, !'c) t
The type Genarray.t is the type of Bigarrays with variable numbers of dimensions. Any
number of dimensions between 0 and 16 is supported.
The three type parameters to Genarray.t identify the array element kind and layout, as
follows:
• the first parameter, 'a, is the OCaml type for accessing array elements (float, int,
int32, int64, nativeint);
• the second parameter, 'b, is the actual kind of array elements (float32_elt,
float64_elt, int8_signed_elt, int8_unsigned_elt, etc);
• the third parameter, 'c, identifies the array layout (c_layout or fortran_layout).
For instance, (float, float32_elt, fortran_layout) Genarray.t is the type of
generic Bigarrays containing 32-bit floats in Fortran layout; reads and writes in this
array use the OCaml type float.

val create :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> int array -> ('a, 'b, 'c) t
Genarray.create kind layout dimensions returns a new Bigarray whose element
kind is determined by the parameter kind (one of float32, float64, int8_signed, etc)
and whose layout is determined by the parameter layout (one of c_layout or
fortran_layout). The dimensions parameter is an array of integers that indicate the
size of the Bigarray in each dimension. The length of dimensions determines the
number of dimensions of the Bigarray.
For instance, Genarray.create int32 c_layout [|4;6;8|] returns a fresh Bigarray
of 32-bit integers, in C layout, having three dimensions, the three dimensions being 4, 6
and 8 respectively.
Bigarrays returned by Genarray.create are not initialized: the initial values of array
elements is unspecified.
Chapter 29. The standard library 573

Genarray.create raises Invalid_argument if the number of dimensions is not in the

range 0 to 16 inclusive, or if one of the dimensions is negative.

val init :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout ->
int array -> (int array -> 'a) -> ('a, 'b, 'c) t
Genarray.init kind layout dimensions f returns a new Bigarray b whose element
kind is determined by the parameter kind (one of float32, float64, int8_signed, etc)
and whose layout is determined by the parameter layout (one of c_layout or
fortran_layout). The dimensions parameter is an array of integers that indicate the
size of the Bigarray in each dimension. The length of dimensions determines the
number of dimensions of the Bigarray.
Each element Genarray.get b i is initialized to the result of f i. In other words,
Genarray.init kind layout dimensions f tabulates the results of f applied to the
indices of a new Bigarray whose layout is described by kind, layout and dimensions.
The index array i may be shared and mutated between calls to f.
For instance, Genarray.init int c_layout [|2; 1; 3|] (Array.fold_left (+)
0) returns a fresh Bigarray of integers, in C layout, having three dimensions (2, 1, 3,
respectively), with the element values 0, 1, 2, 1, 2, 3.
Genarray.init raises Invalid_argument if the number of dimensions is not in the
range 0 to 16 inclusive, or if one of the dimensions is negative.
Since: 4.12

val num_dims : ('a, 'b, 'c) t -> int

Return the number of dimensions of the given Bigarray.

val dims : ('a, 'b, 'c) t -> int array

Genarray.dims a returns all dimensions of the Bigarray a, as an array of integers of
length Genarray.num_dims a.

val nth_dim : ('a, 'b, 'c) t -> int -> int

Genarray.nth_dim a n returns the n-th dimension of the Bigarray a. The first
dimension corresponds to n = 0; the second dimension corresponds to n = 1; the last
dimension, to n = Genarray.num_dims a - 1.
Raises Invalid_argument if n is less than 0 or greater or equal than
Genarray.num_dims a.

val kind : ('a, 'b, 'c) t -> ('a, 'b) Bigarray.kind

Return the kind of the given Bigarray.

val layout : ('a, 'b, 'c) t -> 'c Bigarray.layout

574

Return the layout of the given Bigarray.

val change_layout : ('a, 'b, 'c) t ->

'd Bigarray.layout -> ('a, 'b, 'd) t
Genarray.change_layout a layout returns a Bigarray with the specified layout,
sharing the data with a (and hence having the same dimensions as a). No copying of
elements is involved: the new array and the original array share the same storage space.
The dimensions are reversed, such that get v [| a; b |] in C layout becomes get v
[| b+1; a+1 |] in Fortran layout.
Since: 4.04

val size_in_bytes : ('a, 'b, 'c) t -> int

size_in_bytes a is the number of elements in a multiplied by a’s
Bigarray.kind_size_in_bytes[29.5].
Since: 4.03

val get : ('a, 'b, 'c) t -> int array -> 'a
Read an element of a generic Bigarray. Genarray.get a [|i1; ...; iN|] returns the
element of a whose coordinates are i1 in the first dimension, i2 in the second dimension,
. . ., iN in the N-th dimension.
If a has C layout, the coordinates must be greater or equal than 0 and strictly less than
the corresponding dimensions of a. If a has Fortran layout, the coordinates must be
greater or equal than 1 and less or equal than the corresponding dimensions of a.
If N > 3, alternate syntax is provided: you can write a.{i1, i2, ..., iN} instead of
Genarray.get a [|i1; ...; iN|]. (The syntax a.{...} with one, two or three
coordinates is reserved for accessing one-, two- and three-dimensional arrays as described
below.)
Raises Invalid_argument if the array a does not have exactly N dimensions, or if the
coordinates are outside the array bounds.

val set : ('a, 'b, 'c) t -> int array -> 'a -> unit
Assign an element of a generic Bigarray. Genarray.set a [|i1; ...; iN|] v stores
the value v in the element of a whose coordinates are i1 in the first dimension, i2 in the
second dimension, . . ., iN in the N-th dimension.
The array a must have exactly N dimensions, and all coordinates must lie inside the
array bounds, as described for Genarray.get; otherwise, Invalid_argument is raised.
If N > 3, alternate syntax is provided: you can write a.{i1, i2, ..., iN} <- v
instead of Genarray.set a [|i1; ...; iN|] v. (The syntax a.{...} <- v with one,
two or three coordinates is reserved for updating one-, two- and three-dimensional arrays
as described below.)

val sub_left :
('a, 'b, Bigarray.c_layout) t ->
int -> int -> ('a, 'b, Bigarray.c_layout) t
Chapter 29. The standard library 575

Extract a sub-array of the given Bigarray by restricting the first (left-most) dimension.
Genarray.sub_left a ofs len returns a Bigarray with the same number of dimensions
as a, and the same dimensions as a, except the first dimension, which corresponds to the
interval [ofs ... ofs + len - 1] of the first dimension of a. No copying of elements
is involved: the sub-array and the original array share the same storage space. In other
terms, the element at coordinates [|i1; ...; iN|] of the sub-array is identical to the
element at coordinates [|i1+ofs; ...; iN|] of the original array a.
Genarray.sub_left applies only to Bigarrays in C layout.
Raises Invalid_argument if ofs and len do not designate a valid sub-array of a, that
is, if ofs < 0, or len < 0, or ofs + len > Genarray.nth_dim a 0.

val sub_right :
('a, 'b, Bigarray.fortran_layout) t ->
int -> int -> ('a, 'b, Bigarray.fortran_layout) t
Extract a sub-array of the given Bigarray by restricting the last (right-most) dimension.
Genarray.sub_right a ofs len returns a Bigarray with the same number of
dimensions as a, and the same dimensions as a, except the last dimension, which
corresponds to the interval [ofs ... ofs + len - 1] of the last dimension of a. No
copying of elements is involved: the sub-array and the original array share the same
storage space. In other terms, the element at coordinates [|i1; ...; iN|] of the
sub-array is identical to the element at coordinates [|i1; ...; iN+ofs|] of the
original array a.
Genarray.sub_right applies only to Bigarrays in Fortran layout.
Raises Invalid_argument if ofs and len do not designate a valid sub-array of a, that
is, if ofs < 1, or len < 0, or ofs + len > Genarray.nth_dim a
(Genarray.num_dims a - 1).

val slice_left :
('a, 'b, Bigarray.c_layout) t ->
int array -> ('a, 'b, Bigarray.c_layout) t
Extract a sub-array of lower dimension from the given Bigarray by fixing one or several
of the first (left-most) coordinates. Genarray.slice_left a [|i1; ... ; iM|]
returns the ’slice’ of a obtained by setting the first M coordinates to i1, . . ., iM. If a has
N dimensions, the slice has dimension N - M, and the element at coordinates [|j1; ...;
j(N-M)|] in the slice is identical to the element at coordinates [|i1; ...; iM; j1;
...; j(N-M)|] in the original array a. No copying of elements is involved: the slice and
the original array share the same storage space.
Genarray.slice_left applies only to Bigarrays in C layout.
Raises Invalid_argument if M >= N, or if [|i1; ... ; iM|] is outside the bounds of
a.

val slice_right :
('a, 'b, Bigarray.fortran_layout) t ->
int array -> ('a, 'b, Bigarray.fortran_layout) t
576

Extract a sub-array of lower dimension from the given Bigarray by fixing one or several
of the last (right-most) coordinates. Genarray.slice_right a [|i1; ... ; iM|]
returns the ’slice’ of a obtained by setting the last M coordinates to i1, . . ., iM. If a has N
dimensions, the slice has dimension N - M, and the element at coordinates [|j1; ...;
j(N-M)|] in the slice is identical to the element at coordinates [|j1; ...; j(N-M);
i1; ...; iM|] in the original array a. No copying of elements is involved: the slice and
the original array share the same storage space.
Genarray.slice_right applies only to Bigarrays in Fortran layout.
Raises Invalid_argument if M >= N, or if [|i1; ... ; iM|] is outside the bounds of
a.

val blit : ('a, 'b, 'c) t -> ('a, 'b, 'c) t -> unit
Copy all elements of a Bigarray in another Bigarray. Genarray.blit src dst copies all
elements of src into dst. Both arrays src and dst must have the same number of
dimensions and equal dimensions. Copying a sub-array of src to a sub-array of dst can
be achieved by applying Genarray.blit to sub-array or slices of src and dst.

val fill : ('a, 'b, 'c) t -> 'a -> unit

Set all elements of a Bigarray to a given value. Genarray.fill a v stores the value v
in all elements of the Bigarray a. Setting only some elements of a to v can be achieved
by applying Genarray.fill to a sub-array or a slice of a.

end

Zero-dimensional arrays
module Array0 :
sig
type (!'a, !'b, !'c) t
The type of zero-dimensional Bigarrays whose elements have OCaml type 'a,
representation kind 'b, and memory layout 'c.

val create : ('a, 'b) Bigarray.kind ->

'c Bigarray.layout -> ('a, 'b, 'c) t
Array0.create kind layout returns a new Bigarray of zero dimension. kind and
layout determine the array element kind and the array layout as described for
Bigarray.Genarray.create[29.5].

val init :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> 'a -> ('a, 'b, 'c) t
Chapter 29. The standard library 577

Array0.init kind layout v behaves like Array0.create kind layout except that
the element is additionally initialized to the value v.
Since: 4.12

val kind : ('a, 'b, 'c) t -> ('a, 'b) Bigarray.kind

Return the kind of the given Bigarray.

val layout : ('a, 'b, 'c) t -> 'c Bigarray.layout

Return the layout of the given Bigarray.

val change_layout : ('a, 'b, 'c) t ->

'd Bigarray.layout -> ('a, 'b, 'd) t
Array0.change_layout a layout returns a Bigarray with the specified layout,
sharing the data with a. No copying of elements is involved: the new array and the
original array share the same storage space.
Since: 4.06

val size_in_bytes : ('a, 'b, 'c) t -> int

size_in_bytes a is a’s Bigarray.kind_size_in_bytes[29.5].

val get : ('a, 'b, 'c) t -> 'a

Array0.get a returns the only element in a.

val set : ('a, 'b, 'c) t -> 'a -> unit

Array0.set a x v stores the value v in a.

val blit : ('a, 'b, 'c) t -> ('a, 'b, 'c) t -> unit
Copy the first Bigarray to the second Bigarray. See Bigarray.Genarray.blit[29.5] for
more details.

val fill : ('a, 'b, 'c) t -> 'a -> unit

Fill the given Bigarray with the given value. See Bigarray.Genarray.fill[29.5] for
more details.

val of_value :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> 'a -> ('a, 'b, 'c) t
Build a zero-dimensional Bigarray initialized from the given value.

end
578

Zero-dimensional arrays. The Array0 structure provides operations similar to those of

Bigarray.Genarray[29.5], but specialized to the case of zero-dimensional arrays that only
contain a single scalar value. Statically knowing the number of dimensions of the array allows
faster operations, and more precise static type-checking.
Since: 4.05

One-dimensional arrays
module Array1 :
sig
type (!'a, !'b, !'c) t
The type of one-dimensional Bigarrays whose elements have OCaml type 'a,
representation kind 'b, and memory layout 'c.

val create :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> int -> ('a, 'b, 'c) t
Array1.create kind layout dim returns a new Bigarray of one dimension, whose size
is dim. kind and layout determine the array element kind and the array layout as
described for Bigarray.Genarray.create[29.5].

val init :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> int -> (int -> 'a) -> ('a, 'b, 'c) t
Array1.init kind layout dim f returns a new Bigarray b of one dimension, whose
size is dim. kind and layout determine the array element kind and the array layout as
described for Bigarray.Genarray.create[29.5].
Each element Array1.get b i of the array is initialized to the result of f i.
In other words, Array1.init kind layout dimensions f tabulates the results of f
applied to the indices of a new Bigarray whose layout is described by kind, layout and
dim.
Since: 4.12

val dim : ('a, 'b, 'c) t -> int

Return the size (dimension) of the given one-dimensional Bigarray.

val kind : ('a, 'b, 'c) t -> ('a, 'b) Bigarray.kind

Return the kind of the given Bigarray.

val layout : ('a, 'b, 'c) t -> 'c Bigarray.layout

Return the layout of the given Bigarray.
Chapter 29. The standard library 579

val change_layout : ('a, 'b, 'c) t ->

'd Bigarray.layout -> ('a, 'b, 'd) t
Array1.change_layout a layout returns a Bigarray with the specified layout,
sharing the data with a (and hence having the same dimension as a). No copying of
elements is involved: the new array and the original array share the same storage space.
Since: 4.06

val size_in_bytes : ('a, 'b, 'c) t -> int

size_in_bytes a is the number of elements in a multiplied by a’s
Bigarray.kind_size_in_bytes[29.5].
Since: 4.03

val get : ('a, 'b, 'c) t -> int -> 'a

Array1.get a x, or alternatively a.{x}, returns the element of a at index x. x must be
greater or equal than 0 and strictly less than Array1.dim a if a has C layout. If a has
Fortran layout, x must be greater or equal than 1 and less or equal than Array1.dim a.
Otherwise, Invalid_argument is raised.

val set : ('a, 'b, 'c) t -> int -> 'a -> unit
Array1.set a x v, also written a.{x} <- v, stores the value v at index x in a. x must
be inside the bounds of a as described in Bigarray.Array1.get[29.5]; otherwise,
Invalid_argument is raised.

val sub : ('a, 'b, 'c) t ->

int -> int -> ('a, 'b, 'c) t
Extract a sub-array of the given one-dimensional Bigarray. See
Bigarray.Genarray.sub_left[29.5] for more details.

val slice : ('a, 'b, 'c) t -> int -> ('a, 'b, 'c) Bigarray.Array0.t
Extract a scalar (zero-dimensional slice) of the given one-dimensional Bigarray. The
integer parameter is the index of the scalar to extract. See
Bigarray.Genarray.slice_left[29.5] and Bigarray.Genarray.slice_right[29.5] for
more details.
Since: 4.05

val blit : ('a, 'b, 'c) t -> ('a, 'b, 'c) t -> unit
Copy the first Bigarray to the second Bigarray. See Bigarray.Genarray.blit[29.5] for
more details.

val fill : ('a, 'b, 'c) t -> 'a -> unit

Fill the given Bigarray with the given value. See Bigarray.Genarray.fill[29.5] for
more details.
580

val of_array :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> 'a array -> ('a, 'b, 'c) t
Build a one-dimensional Bigarray initialized from the given array.

val unsafe_get : ('a, 'b, 'c) t -> int -> 'a

Like Bigarray.Array1.get[29.5], but bounds checking is not always performed. Use
with caution and only when the program logic guarantees that the access is within
bounds.

val unsafe_set : ('a, 'b, 'c) t -> int -> 'a -> unit
Like Bigarray.Array1.set[29.5], but bounds checking is not always performed. Use
with caution and only when the program logic guarantees that the access is within
bounds.

end

One-dimensional arrays. The Array1 structure provides operations similar to those of

Bigarray.Genarray[29.5], but specialized to the case of one-dimensional arrays. (The
Bigarray.Array2[29.5] and Bigarray.Array3[29.5] structures below provide operations
specialized for two- and three-dimensional arrays.) Statically knowing the number of
dimensions of the array allows faster operations, and more precise static type-checking.

Two-dimensional arrays
module Array2 :
sig
type (!'a, !'b, !'c) t
The type of two-dimensional Bigarrays whose elements have OCaml type 'a,
representation kind 'b, and memory layout 'c.

val create :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> int -> int -> ('a, 'b, 'c) t
Array2.create kind layout dim1 dim2 returns a new Bigarray of two dimensions,
whose size is dim1 in the first dimension and dim2 in the second dimension. kind and
layout determine the array element kind and the array layout as described for
Bigarray.Genarray.create[29.5].

val init :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout ->
int -> int -> (int -> int -> 'a) -> ('a, 'b, 'c) t
Chapter 29. The standard library 581

Array2.init kind layout dim1 dim2 f returns a new Bigarray b of two dimensions,
whose size is dim2 in the first dimension and dim2 in the second dimension. kind and
layout determine the array element kind and the array layout as described for
Bigarray.Genarray.create[29.5].
Each element Array2.get b i j of the array is initialized to the result of f i j.
In other words, Array2.init kind layout dim1 dim2 f tabulates the results of f
applied to the indices of a new Bigarray whose layout is described by kind, layout,
dim1 and dim2.
Since: 4.12

val dim1 : ('a, 'b, 'c) t -> int

Return the first dimension of the given two-dimensional Bigarray.

val dim2 : ('a, 'b, 'c) t -> int

Return the second dimension of the given two-dimensional Bigarray.

val kind : ('a, 'b, 'c) t -> ('a, 'b) Bigarray.kind

Return the kind of the given Bigarray.

val layout : ('a, 'b, 'c) t -> 'c Bigarray.layout

Return the layout of the given Bigarray.

val change_layout : ('a, 'b, 'c) t ->

'd Bigarray.layout -> ('a, 'b, 'd) t
Array2.change_layout a layout returns a Bigarray with the specified layout,
sharing the data with a (and hence having the same dimensions as a). No copying of
elements is involved: the new array and the original array share the same storage space.
The dimensions are reversed, such that get v [| a; b |] in C layout becomes get v
[| b+1; a+1 |] in Fortran layout.
Since: 4.06

val size_in_bytes : ('a, 'b, 'c) t -> int

size_in_bytes a is the number of elements in a multiplied by a’s
Bigarray.kind_size_in_bytes[29.5].
Since: 4.03

val get : ('a, 'b, 'c) t -> int -> int -> 'a
Array2.get a x y, also written a.{x,y}, returns the element of a at coordinates (x, y).
x and y must be within the bounds of a, as described for Bigarray.Genarray.get[29.5];
otherwise, Invalid_argument is raised.

val set : ('a, 'b, 'c) t -> int -> int -> 'a -> unit
582

Array2.set a x y v, or alternatively a.{x,y} <- v, stores the value v at coordinates

(x, y) in a. x and y must be within the bounds of a, as described for
Bigarray.Genarray.set[29.5]; otherwise, Invalid_argument is raised.

val sub_left :
('a, 'b, Bigarray.c_layout) t ->
int -> int -> ('a, 'b, Bigarray.c_layout) t
Extract a two-dimensional sub-array of the given two-dimensional Bigarray by
restricting the first dimension. See Bigarray.Genarray.sub_left[29.5] for more details.
Array2.sub_left applies only to arrays with C layout.

val sub_right :
('a, 'b, Bigarray.fortran_layout) t ->
int -> int -> ('a, 'b, Bigarray.fortran_layout) t
Extract a two-dimensional sub-array of the given two-dimensional Bigarray by
restricting the second dimension. See Bigarray.Genarray.sub_right[29.5] for more
details. Array2.sub_right applies only to arrays with Fortran layout.

val slice_left :
('a, 'b, Bigarray.c_layout) t ->
int -> ('a, 'b, Bigarray.c_layout) Bigarray.Array1.t
Extract a row (one-dimensional slice) of the given two-dimensional Bigarray. The integer
parameter is the index of the row to extract. See Bigarray.Genarray.slice_left[29.5]
for more details. Array2.slice_left applies only to arrays with C layout.

val slice_right :
('a, 'b, Bigarray.fortran_layout) t ->
int -> ('a, 'b, Bigarray.fortran_layout) Bigarray.Array1.t
Extract a column (one-dimensional slice) of the given two-dimensional Bigarray. The
integer parameter is the index of the column to extract. See
Bigarray.Genarray.slice_right[29.5] for more details. Array2.slice_right applies
only to arrays with Fortran layout.

val blit : ('a, 'b, 'c) t -> ('a, 'b, 'c) t -> unit
Copy the first Bigarray to the second Bigarray. See Bigarray.Genarray.blit[29.5] for
more details.

val fill : ('a, 'b, 'c) t -> 'a -> unit

Fill the given Bigarray with the given value. See Bigarray.Genarray.fill[29.5] for
more details.

val of_array :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> 'a array array -> ('a, 'b, 'c) t
Chapter 29. The standard library 583

Build a two-dimensional Bigarray initialized from the given array of arrays.

val unsafe_get : ('a, 'b, 'c) t -> int -> int -> 'a
Like Bigarray.Array2.get[29.5], but bounds checking is not always performed.

val unsafe_set : ('a, 'b, 'c) t -> int -> int -> 'a -> unit
Like Bigarray.Array2.set[29.5], but bounds checking is not always performed.

end
Two-dimensional arrays. The Array2 structure provides operations similar to those of
Bigarray.Genarray[29.5], but specialized to the case of two-dimensional arrays.

Three-dimensional arrays
module Array3 :
sig
type (!'a, !'b, !'c) t
The type of three-dimensional Bigarrays whose elements have OCaml type 'a,
representation kind 'b, and memory layout 'c.

val create :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> int -> int -> int -> ('a, 'b, 'c) t
Array3.create kind layout dim1 dim2 dim3 returns a new Bigarray of three
dimensions, whose size is dim1 in the first dimension, dim2 in the second dimension, and
dim3 in the third. kind and layout determine the array element kind and the array
layout as described for Bigarray.Genarray.create[29.5].

val init :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout ->
int ->
int -> int -> (int -> int -> int -> 'a) -> ('a, 'b, 'c) t
Array3.init kind layout dim1 dim2 dim3 f returns a new Bigarray b of three
dimensions, whose size is dim1 in the first dimension, dim2 in the second dimension, and
dim3 in the third. kind and layout determine the array element kind and the array
layout as described for Bigarray.Genarray.create[29.5].
Each element Array3.get b i j k of the array is initialized to the result of f i j k.
In other words, Array3.init kind layout dim1 dim2 dim3 f tabulates the results of
f applied to the indices of a new Bigarray whose layout is described by kind, layout,
dim1, dim2 and dim3.
Since: 4.12
584

val dim1 : ('a, 'b, 'c) t -> int

Return the first dimension of the given three-dimensional Bigarray.

val dim2 : ('a, 'b, 'c) t -> int

Return the second dimension of the given three-dimensional Bigarray.

val dim3 : ('a, 'b, 'c) t -> int

Return the third dimension of the given three-dimensional Bigarray.

val kind : ('a, 'b, 'c) t -> ('a, 'b) Bigarray.kind

Return the kind of the given Bigarray.

val layout : ('a, 'b, 'c) t -> 'c Bigarray.layout

Return the layout of the given Bigarray.

val change_layout : ('a, 'b, 'c) t ->

'd Bigarray.layout -> ('a, 'b, 'd) t
Array3.change_layout a layout returns a Bigarray with the specified layout,
sharing the data with a (and hence having the same dimensions as a). No copying of
elements is involved: the new array and the original array share the same storage space.
The dimensions are reversed, such that get v [| a; b; c |] in C layout becomes get
v [| c+1; b+1; a+1 |] in Fortran layout.
Since: 4.06

val size_in_bytes : ('a, 'b, 'c) t -> int

size_in_bytes a is the number of elements in a multiplied by a’s
Bigarray.kind_size_in_bytes[29.5].
Since: 4.03

val get : ('a, 'b, 'c) t -> int -> int -> int -> 'a
Array3.get a x y z, also written a.{x,y,z}, returns the element of a at coordinates
(x, y, z). x, y and z must be within the bounds of a, as described for
Bigarray.Genarray.get[29.5]; otherwise, Invalid_argument is raised.

val set : ('a, 'b, 'c) t -> int -> int -> int -> 'a -> unit
Array3.set a x y v, or alternatively a.{x,y,z} <- v, stores the value v at
coordinates (x, y, z) in a. x, y and z must be within the bounds of a, as described for
Bigarray.Genarray.set[29.5]; otherwise, Invalid_argument is raised.

val sub_left :
('a, 'b, Bigarray.c_layout) t ->
int -> int -> ('a, 'b, Bigarray.c_layout) t
Chapter 29. The standard library 585

Extract a three-dimensional sub-array of the given three-dimensional Bigarray by

restricting the first dimension. See Bigarray.Genarray.sub_left[29.5] for more details.
Array3.sub_left applies only to arrays with C layout.

val sub_right :
('a, 'b, Bigarray.fortran_layout) t ->
int -> int -> ('a, 'b, Bigarray.fortran_layout) t
Extract a three-dimensional sub-array of the given three-dimensional Bigarray by
restricting the second dimension. See Bigarray.Genarray.sub_right[29.5] for more
details. Array3.sub_right applies only to arrays with Fortran layout.

val slice_left_1 :
('a, 'b, Bigarray.c_layout) t ->
int -> int -> ('a, 'b, Bigarray.c_layout) Bigarray.Array1.t
Extract a one-dimensional slice of the given three-dimensional Bigarray by fixing the
first two coordinates. The integer parameters are the coordinates of the slice to extract.
See Bigarray.Genarray.slice_left[29.5] for more details. Array3.slice_left_1
applies only to arrays with C layout.

val slice_right_1 :
('a, 'b, Bigarray.fortran_layout) t ->
int -> int -> ('a, 'b, Bigarray.fortran_layout) Bigarray.Array1.t
Extract a one-dimensional slice of the given three-dimensional Bigarray by fixing the last
two coordinates. The integer parameters are the coordinates of the slice to extract. See
Bigarray.Genarray.slice_right[29.5] for more details. Array3.slice_right_1
applies only to arrays with Fortran layout.

val slice_left_2 :
('a, 'b, Bigarray.c_layout) t ->
int -> ('a, 'b, Bigarray.c_layout) Bigarray.Array2.t
Extract a two-dimensional slice of the given three-dimensional Bigarray by fixing the
first coordinate. The integer parameter is the first coordinate of the slice to extract. See
Bigarray.Genarray.slice_left[29.5] for more details. Array3.slice_left_2 applies
only to arrays with C layout.

val slice_right_2 :
('a, 'b, Bigarray.fortran_layout) t ->
int -> ('a, 'b, Bigarray.fortran_layout) Bigarray.Array2.t
Extract a two-dimensional slice of the given three-dimensional Bigarray by fixing the last
coordinate. The integer parameter is the coordinate of the slice to extract. See
Bigarray.Genarray.slice_right[29.5] for more details. Array3.slice_right_2
applies only to arrays with Fortran layout.

val blit : ('a, 'b, 'c) t -> ('a, 'b, 'c) t -> unit
586

Copy the first Bigarray to the second Bigarray. See Bigarray.Genarray.blit[29.5] for
more details.

val fill : ('a, 'b, 'c) t -> 'a -> unit

Fill the given Bigarray with the given value. See Bigarray.Genarray.fill[29.5] for
more details.

val of_array :
('a, 'b) Bigarray.kind ->
'c Bigarray.layout -> 'a array array array -> ('a, 'b, 'c) t
Build a three-dimensional Bigarray initialized from the given array of arrays of arrays.

val unsafe_get : ('a, 'b, 'c) t -> int -> int -> int -> 'a
Like Bigarray.Array3.get[29.5], but bounds checking is not always performed.

val unsafe_set : ('a, 'b, 'c) t -> int -> int -> int -> 'a -> unit
Like Bigarray.Array3.set[29.5], but bounds checking is not always performed.

end

Three-dimensional arrays. The Array3 structure provides operations similar to those of

Bigarray.Genarray[29.5], but specialized to the case of three-dimensional arrays.

Coercions between generic Bigarrays and fixed-dimension Bigarrays

val genarray_of_array0 : ('a, 'b, 'c) Array0.t -> ('a, 'b, 'c) Genarray.t
Return the generic Bigarray corresponding to the given zero-dimensional Bigarray.
Since: 4.05

val genarray_of_array1 : ('a, 'b, 'c) Array1.t -> ('a, 'b, 'c) Genarray.t
Return the generic Bigarray corresponding to the given one-dimensional Bigarray.

val genarray_of_array2 : ('a, 'b, 'c) Array2.t -> ('a, 'b, 'c) Genarray.t
Return the generic Bigarray corresponding to the given two-dimensional Bigarray.

val genarray_of_array3 : ('a, 'b, 'c) Array3.t -> ('a, 'b, 'c) Genarray.t
Return the generic Bigarray corresponding to the given three-dimensional Bigarray.

val array0_of_genarray : ('a, 'b, 'c) Genarray.t -> ('a, 'b, 'c) Array0.t
Return the zero-dimensional Bigarray corresponding to the given generic Bigarray.
Since: 4.05
Raises Invalid_argument if the generic Bigarray does not have exactly zero dimension.
Chapter 29. The standard library 587

val array1_of_genarray : ('a, 'b, 'c) Genarray.t -> ('a, 'b, 'c) Array1.t
Return the one-dimensional Bigarray corresponding to the given generic Bigarray.
Raises Invalid_argument if the generic Bigarray does not have exactly one dimension.

val array2_of_genarray : ('a, 'b, 'c) Genarray.t -> ('a, 'b, 'c) Array2.t
Return the two-dimensional Bigarray corresponding to the given generic Bigarray.
Raises Invalid_argument if the generic Bigarray does not have exactly two dimensions.

val array3_of_genarray : ('a, 'b, 'c) Genarray.t -> ('a, 'b, 'c) Array3.t
Return the three-dimensional Bigarray corresponding to the given generic Bigarray.
Raises Invalid_argument if the generic Bigarray does not have exactly three dimensions.

Re-shaping Bigarrays
val reshape :
('a, 'b, 'c) Genarray.t ->
int array -> ('a, 'b, 'c) Genarray.t
reshape b [|d1;...;dN|] converts the Bigarray b to a N-dimensional array of dimensions
d1. . .dN. The returned array and the original array b share their data and have the same
layout. For instance, assuming that b is a one-dimensional array of dimension 12, reshape b
[|3;4|] returns a two-dimensional array b' of dimensions 3 and 4. If b has C layout, the
element (x,y) of b' corresponds to the element x * 3 + y of b. If b has Fortran layout, the
element (x,y) of b' corresponds to the element x + (y - 1) * 4 of b. The returned
Bigarray must have exactly the same number of elements as the original Bigarray b. That is,
the product of the dimensions of b must be equal to i1 * ... * iN. Otherwise,
Invalid_argument is raised.

val reshape_0 : ('a, 'b, 'c) Genarray.t -> ('a, 'b, 'c) Array0.t
Specialized version of Bigarray.reshape[29.5] for reshaping to zero-dimensional arrays.
Since: 4.05

val reshape_1 : ('a, 'b, 'c) Genarray.t -> int -> ('a, 'b, 'c) Array1.t
Specialized version of Bigarray.reshape[29.5] for reshaping to one-dimensional arrays.

val reshape_2 :
('a, 'b, 'c) Genarray.t ->
int -> int -> ('a, 'b, 'c) Array2.t
Specialized version of Bigarray.reshape[29.5] for reshaping to two-dimensional arrays.

val reshape_3 :
('a, 'b, 'c) Genarray.t ->
int -> int -> int -> ('a, 'b, 'c) Array3.t
Specialized version of Bigarray.reshape[29.5] for reshaping to three-dimensional arrays.
588

Bigarrays and concurrency safety

Care must be taken when concurrently accessing bigarrays from multiple domains: accessing a
bigarray will never crash a program, but unsynchronized accesses might yield surprising (non-
sequentially-consistent) results.

Atomicity
Every bigarray operation that accesses more than one array element is not atomic. This includes
slicing, bliting, and filling bigarrays.
For example, consider the following program:
open Bigarray
let size = 100_000_000
let a = Array1.init Int C_layout size (fun _ -> 1)
let update f a () =
for i = 0 to size - 1 do a.{i} <- f a.{i} done
let d1 = Domain.spawn (update (fun x -> x + 1) a)
let d2 = Domain.spawn (update (fun x -> 2 * x + 1) a)
let () = Domain.join d1; Domain.join d2

After executing this code, each field of the bigarray a is either 2, 3, 4 or 5. If atomicity is required,
then the user must implement their own synchronization (for example, using Mutex.t[29.37]).

Data races
If two domains only access disjoint parts of the bigarray, then the observed behaviour is the equivalent
to some sequential interleaving of the operations from the two domains.
A data race is said to occur when two domains access the same bigarray element without
synchronization and at least one of the accesses is a write. In the absence of data races, the observed
behaviour is equivalent to some sequential interleaving of the operations from different domains.
Whenever possible, data races should be avoided by using synchronization to mediate the accesses
to the bigarray elements.
Indeed, in the presence of data races, programs will not crash but the observed behaviour may
not be equivalent to any sequential interleaving of operations from different domains.

Tearing
Bigarrays have a distinct caveat in the presence of data races: concurrent bigarray operations might
produce surprising values due to tearing. More precisely, the interleaving of partial writes and reads
might create values that would not exist with a sequential execution. For instance, at the end of
let res = Array1.init Complex64 c_layout size (fun _ -> Complex.zero)
let d1 = Domain.spawn (fun () -> Array1.fill res Complex.one)
let d2 = Domain.spawn (fun () -> Array1.fill res Complex.i)
let () = Domain.join d1; Domain.join d2
Chapter 29. The standard library 589

the res bigarray might contain values that are neither Complex.i nor Complex.one (for instance
1 + i).

29.6 Module Bool : Boolean values.

Since: 4.08

Booleans
type t = bool =
| false
| true
The type of booleans (truth values).
The constructors false and true are included here so that they have paths, but they are not
intended to be used in user-defined data types.

val not : bool -> bool

not b is the boolean negation of b.

val (&&) : bool -> bool -> bool

e0 && e1 is the lazy boolean conjunction of expressions e0 and e1. If e0 evaluates to false,
e1 is not evaluated. Right-associative operator at precedence level 3/11.

val (||) : bool -> bool -> bool

e0 || e1 is the lazy boolean disjunction of expressions e0 and e1. If e0 evaluates to true,
e1 is not evaluated. Right-associative operator at precedence level 2/11.

Predicates and comparisons

val equal : bool -> bool -> bool
equal b0 b1 is true if and only if b0 and b1 are both true or both false.

val compare : bool -> bool -> int

compare b0 b1 is a total order on boolean values. false is smaller than true.

Converting
val to_int : bool -> int
to_int b is 0 if b is false and 1 if b is true.

val to_float : bool -> float

590

to_float b is 0. if b is false and 1. if b is true.

val to_string : bool -> string

to_string b is "true" if b is true and "false" if b is false.

val seeded_hash : int -> bool -> int

A seeded hash function for booleans, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.1

val hash : bool -> int

An unseeded hash function for booleans, with the same output value as Hashtbl.hash[29.25].
This function allows this module to be passed as argument to the functor
Hashtbl.Make[29.25].
Since: 5.1

29.7 Module Buffer : Extensible buffers.

This module implements buffers that automatically expand as necessary. It provides accumulative
concatenation of strings in linear time (instead of quadratic time when strings are concatenated
pairwise). For example:

let concat_strings ss =
let b = Buffer.create 16 in
List.iter (Buffer.add_string b) ss;
Buffer.contents b

Alert unsynchronized_access. Unsynchronized accesses to buffers are a programming error.

Unsynchronized accesses
Unsynchronized accesses to a buffer may lead to an invalid buffer state. Thus, concurrent
accesses to a buffer must be synchronized (for instance with a Mutex.t[29.37]).
type t
The abstract type of buffers.

val create : int -> t

create n returns a fresh buffer, initially empty. The n parameter is the initial size of the
internal byte sequence that holds the buffer contents. That byte sequence is automatically
reallocated when more than n characters are stored in the buffer, but shrinks back to n
characters when reset is called. For best performance, n should be of the same order of
magnitude as the number of characters that are expected to be stored in the buffer (for
Chapter 29. The standard library 591

instance, 80 for a buffer that holds one output line). Nothing bad will happen if the buffer
grows beyond that limit, however. In doubt, take n = 16 for instance. If n is not between 1
and Sys.max_string_length[29.56], it will be clipped to that interval.

val contents : t -> string

Return a copy of the current contents of the buffer. The buffer itself is unchanged.

val to_bytes : t -> bytes

Return a copy of the current contents of the buffer. The buffer itself is unchanged.
Since: 4.02

val sub : t -> int -> int -> string

Buffer.sub b off len returns a copy of len bytes from the current contents of the buffer b,
starting at offset off.
Raises Invalid_argument if off and len do not designate a valid range of b.

val blit : t -> int -> bytes -> int -> int -> unit
Buffer.blit src srcoff dst dstoff len copies len characters from the current contents
of the buffer src, starting at offset srcoff to dst, starting at character dstoff.
Since: 3.11.2
Raises Invalid_argument if srcoff and len do not designate a valid range of src, or if
dstoff and len do not designate a valid range of dst.

val nth : t -> int -> char

Get the n-th character of the buffer.
Raises Invalid_argument if index out of bounds

val length : t -> int

Return the number of characters currently contained in the buffer.

val clear : t -> unit

Empty the buffer.

val reset : t -> unit

Empty the buffer and deallocate the internal byte sequence holding the buffer contents,
replacing it with the initial internal byte sequence of length n that was allocated by
Buffer.create[29.7] n. For long-lived buffers that may have grown a lot, reset allows faster
reclamation of the space used by the buffer.

val output_buffer : out_channel -> t -> unit

output_buffer oc b writes the current contents of buffer b on the output channel oc.

val truncate : t -> int -> unit

592

truncate b len truncates the length of b to len Note: the internal byte sequence is not
shortened.
Since: 4.05
Raises Invalid_argument if len < 0 or len > length b.

Appending
Note: all add_* operations can raise Failure if the internal byte sequence of the buffer would need
to grow beyond Sys.max_string_length[29.56].
val add_char : t -> char -> unit
add_char b c appends the character c at the end of buffer b.

val add_utf_8_uchar : t -> Uchar.t -> unit

add_utf_8_uchar b u appends the UTF-8[https://tools.ietf.org/html/rfc3629]
encoding of u at the end of buffer b.
Since: 4.06

val add_utf_16le_uchar : t -> Uchar.t -> unit

add_utf_16le_uchar b u appends the
UTF-16LE[https://tools.ietf.org/html/rfc2781] encoding of u at the end of buffer b.
Since: 4.06

val add_utf_16be_uchar : t -> Uchar.t -> unit

add_utf_16be_uchar b u appends the
UTF-16BE[https://tools.ietf.org/html/rfc2781] encoding of u at the end of buffer b.
Since: 4.06

val add_string : t -> string -> unit

add_string b s appends the string s at the end of buffer b.

val add_bytes : t -> bytes -> unit

add_bytes b s appends the byte sequence s at the end of buffer b.
Since: 4.02

val add_substring : t -> string -> int -> int -> unit
add_substring b s ofs len takes len characters from offset ofs in string s and appends
them at the end of buffer b.
Raises Invalid_argument if ofs and len do not designate a valid range of s.

val add_subbytes : t -> bytes -> int -> int -> unit
Chapter 29. The standard library 593

add_subbytes b s ofs len takes len characters from offset ofs in byte sequence s and
appends them at the end of buffer b.
Since: 4.02
Raises Invalid_argument if ofs and len do not designate a valid range of s.

val add_substitute : t -> (string -> string) -> string -> unit
add_substitute b f s appends the string pattern s at the end of buffer b with substitution.
The substitution process looks for variable references in the pattern and substitutes each
variable reference with its value, as obtained by applying the mapping f to the variable name.
Inside the string pattern, a variable reference is a non-escaped $ immediately followed by a
variable name, which is one of the following:

• a non empty sequence of alphanumeric or _ characters,

• an arbitrary sequence of characters enclosed by a pair of matching parentheses or curly
brackets. An escaped $ character is a $ that immediately follows a backslash character;
the two characters together stand for a plain $.

val add_buffer : t -> t -> unit

add_buffer b1 b2 appends the current contents of buffer b2 at the end of buffer b1. b2 is
not modified.

val add_channel : t -> in_channel -> int -> unit

add_channel b ic n reads at most n characters from the input channel ic and stores them
at the end of buffer b.
Raises

• End_of_file if the channel contains fewer than n characters. In this case, the
characters are still added to the buffer, so as to avoid loss of data.
• Invalid_argument if len < 0 or len > Sys.max_string_length.

Buffers and Sequences

val to_seq : t -> char Seq.t
Iterate on the buffer, in increasing order.
The behavior is not specified if the buffer is modified during iteration.
Since: 4.07

val to_seqi : t -> (int * char) Seq.t

Iterate on the buffer, in increasing order, yielding indices along chars.
The behavior is not specified if the buffer is modified during iteration.
Since: 4.07
594

val add_seq : t -> char Seq.t -> unit

Add chars to the buffer
Since: 4.07

val of_seq : char Seq.t -> t

Create a buffer from the generator
Since: 4.07

Binary encoding of integers

The functions in this section append binary encodings of integers to buffers.
Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are
stored first. Big-endian is also known as network byte order. Native-endian encoding is either
little-endian or big-endian depending on Sys.big_endian[29.56].
32-bit and 64-bit integers are represented by the int32 and int64 types, which can be interpreted
either as signed or unsigned numbers.
8-bit and 16-bit integers are represented by the int type, which has more bits than the binary
encoding. Functions that encode these values truncate their inputs to their least significant bytes.
val add_uint8 : t -> int -> unit
add_uint8 b i appends a binary unsigned 8-bit integer i to b.
Since: 4.08

val add_int8 : t -> int -> unit

add_int8 b i appends a binary signed 8-bit integer i to b.
Since: 4.08

val add_uint16_ne : t -> int -> unit

add_uint16_ne b i appends a binary native-endian unsigned 16-bit integer i to b.
Since: 4.08

val add_uint16_be : t -> int -> unit

add_uint16_be b i appends a binary big-endian unsigned 16-bit integer i to b.
Since: 4.08

val add_uint16_le : t -> int -> unit

add_uint16_le b i appends a binary little-endian unsigned 16-bit integer i to b.
Since: 4.08

val add_int16_ne : t -> int -> unit

add_int16_ne b i appends a binary native-endian signed 16-bit integer i to b.
Since: 4.08
Chapter 29. The standard library 595

val add_int16_be : t -> int -> unit

add_int16_be b i appends a binary big-endian signed 16-bit integer i to b.
Since: 4.08

val add_int16_le : t -> int -> unit

add_int16_le b i appends a binary little-endian signed 16-bit integer i to b.
Since: 4.08

val add_int32_ne : t -> int32 -> unit

add_int32_ne b i appends a binary native-endian 32-bit integer i to b.
Since: 4.08

val add_int32_be : t -> int32 -> unit

add_int32_be b i appends a binary big-endian 32-bit integer i to b.
Since: 4.08

val add_int32_le : t -> int32 -> unit

add_int32_le b i appends a binary little-endian 32-bit integer i to b.
Since: 4.08

val add_int64_ne : t -> int64 -> unit

add_int64_ne b i appends a binary native-endian 64-bit integer i to b.
Since: 4.08

val add_int64_be : t -> int64 -> unit

add_int64_be b i appends a binary big-endian 64-bit integer i to b.
Since: 4.08

val add_int64_le : t -> int64 -> unit

add_int64_ne b i appends a binary little-endian 64-bit integer i to b.
Since: 4.08

29.8 Module Bytes : Byte sequence operations.

A byte sequence is a mutable data structure that contains a fixed-length sequence of bytes. Each
byte can be indexed in constant time for reading or writing.
Given a byte sequence s of length l, we can access each of the l bytes of s via its index in
the sequence. Indexes start at 0, and we will call an index valid in s if it falls within the range
[0...l-1] (inclusive). A position is the point between two bytes or at the beginning or end of the
sequence. We call a position valid in s if it falls within the range [0...l] (inclusive). Note that
the byte at index n is between positions n and n+1.
596

Two parameters start and len are said to designate a valid range of s if len >= 0 and start
and start+len are valid positions in s.
Byte sequences can be modified in place, for instance via the set and blit functions described
below. See also strings (module String[29.54]), which are almost the same data structure, but
cannot be modified in place.
Bytes are represented by the OCaml type char.
The labeled version of this module can be used as described in the StdLabels[29.53] module.
Since: 4.02

val length : bytes -> int

Return the length (number of bytes) of the argument.

val get : bytes -> int -> char

get s n returns the byte at index n in argument s.
Raises Invalid_argument if n is not a valid index in s.

val set : bytes -> int -> char -> unit

set s n c modifies s in place, replacing the byte at index n with c.
Raises Invalid_argument if n is not a valid index in s.

val create : int -> bytes

create n returns a new byte sequence of length n. The sequence is uninitialized and contains
arbitrary bytes.
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val make : int -> char -> bytes

make n c returns a new byte sequence of length n, filled with the byte c.
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val init : int -> (int -> char) -> bytes

init n f returns a fresh byte sequence of length n, with character i initialized to the result
of f i (in increasing index order).
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val empty : bytes

A byte sequence of size 0.

val copy : bytes -> bytes

Return a new byte sequence that contains the same bytes as the argument.

val of_string : string -> bytes

Return a new byte sequence that contains the same bytes as the given string.
Chapter 29. The standard library 597

val to_string : bytes -> string

Return a new string that contains the same bytes as the given byte sequence.

val sub : bytes -> int -> int -> bytes

sub s pos len returns a new byte sequence of length len, containing the subsequence of s
that starts at position pos and has length len.
Raises Invalid_argument if pos and len do not designate a valid range of s.

val sub_string : bytes -> int -> int -> string

Same as Bytes.sub[29.8] but return a string instead of a byte sequence.

val extend : bytes -> int -> int -> bytes

extend s left right returns a new byte sequence that contains the bytes of s, with left
uninitialized bytes prepended and right uninitialized bytes appended to it. If left or right
is negative, then bytes are removed (instead of appended) from the corresponding side of s.
Since: 4.05 in BytesLabels
Raises Invalid_argument if the result length is negative or longer than
Sys.max_string_length[29.56] bytes.

val fill : bytes -> int -> int -> char -> unit
fill s pos len c modifies s in place, replacing len characters with c, starting at pos.
Raises Invalid_argument if pos and len do not designate a valid range of s.

val blit : bytes -> int -> bytes -> int -> int -> unit
blit src src_pos dst dst_pos len copies len bytes from byte sequence src, starting at
index src_pos, to byte sequence dst, starting at index dst_pos. It works correctly even if
src and dst are the same byte sequence, and the source and destination intervals overlap.
Raises Invalid_argument if src_pos and len do not designate a valid range of src, or if
dst_pos and len do not designate a valid range of dst.

val blit_string : string -> int -> bytes -> int -> int -> unit
blit_string src src_pos dst dst_pos len copies len bytes from string src, starting at
index src_pos, to byte sequence dst, starting at index dst_pos.
Since: 4.05 in BytesLabels
Raises Invalid_argument if src_pos and len do not designate a valid range of src, or if
dst_pos and len do not designate a valid range of dst.

val concat : bytes -> bytes list -> bytes

concat sep sl concatenates the list of byte sequences sl, inserting the separator byte
sequence sep between each, and returns the result as a new byte sequence.
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.
598

val cat : bytes -> bytes -> bytes

cat s1 s2 concatenates s1 and s2 and returns the result as a new byte sequence.
Since: 4.05 in BytesLabels
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val iter : (char -> unit) -> bytes -> unit

iter f s applies function f in turn to all the bytes of s. It is equivalent to f (get s 0); f
(get s 1); ...; f (get s (length s - 1)); ().

val iteri : (int -> char -> unit) -> bytes -> unit
Same as Bytes.iter[29.8], but the function is applied to the index of the byte as first
argument and the byte itself as second argument.

val map : (char -> char) -> bytes -> bytes

map f s applies function f in turn to all the bytes of s (in increasing index order) and stores
the resulting bytes in a new sequence that is returned as the result.

val mapi : (int -> char -> char) -> bytes -> bytes
mapi f s calls f with each character of s and its index (in increasing index order) and stores
the resulting bytes in a new sequence that is returned as the result.

val fold_left : ('acc -> char -> 'acc) -> 'acc -> bytes -> 'acc
fold_left f x s computes f (... (f (f x (get s 0)) (get s 1)) ...) (get s
(n-1)), where n is the length of s.
Since: 4.13

val fold_right : (char -> 'acc -> 'acc) -> bytes -> 'acc -> 'acc
fold_right f s x computes f (get s 0) (f (get s 1) ( ... (f (get s (n-1)) x)
...)), where n is the length of s.
Since: 4.13

val for_all : (char -> bool) -> bytes -> bool

for_all p s checks if all characters in s satisfy the predicate p.
Since: 4.13

val exists : (char -> bool) -> bytes -> bool

exists p s checks if at least one character of s satisfies the predicate p.
Since: 4.13

val trim : bytes -> bytes

Return a copy of the argument, without leading and trailing whitespace. The bytes regarded
as whitespace are the ASCII characters ' ', '\012', '\n', '\r', and '\t'.
Chapter 29. The standard library 599

val escaped : bytes -> bytes

Return a copy of the argument, with special characters represented by escape sequences,
following the lexical conventions of OCaml. All characters outside the ASCII printable range
(32..126) are escaped, as well as backslash and double-quote.
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val index : bytes -> char -> int

index s c returns the index of the first occurrence of byte c in s.
Raises Not_found if c does not occur in s.

val index_opt : bytes -> char -> int option

index_opt s c returns the index of the first occurrence of byte c in s or None if c does not
occur in s.
Since: 4.05

val rindex : bytes -> char -> int

rindex s c returns the index of the last occurrence of byte c in s.
Raises Not_found if c does not occur in s.

val rindex_opt : bytes -> char -> int option

rindex_opt s c returns the index of the last occurrence of byte c in s or None if c does not
occur in s.
Since: 4.05

val index_from : bytes -> int -> char -> int

index_from s i c returns the index of the first occurrence of byte c in s after position i.
index s c is equivalent to index_from s 0 c.
Raises

• Invalid_argument if i is not a valid position in s.

• Not_found if c does not occur in s after position i.

val index_from_opt : bytes -> int -> char -> int option
index_from_opt s i c returns the index of the first occurrence of byte c in s after position
i or None if c does not occur in s after position i. index_opt s c is equivalent to
index_from_opt s 0 c.
Since: 4.05
Raises Invalid_argument if i is not a valid position in s.

val rindex_from : bytes -> int -> char -> int

600

rindex_from s i c returns the index of the last occurrence of byte c in s before position
i+1. rindex s c is equivalent to rindex_from s (length s - 1) c.
Raises

• Invalid_argument if i+1 is not a valid position in s.

• Not_found if c does not occur in s before position i+1.

val rindex_from_opt : bytes -> int -> char -> int option
rindex_from_opt s i c returns the index of the last occurrence of byte c in s before
position i+1 or None if c does not occur in s before position i+1. rindex_opt s c is
equivalent to rindex_from s (length s - 1) c.
Since: 4.05
Raises Invalid_argument if i+1 is not a valid position in s.

val contains : bytes -> char -> bool

contains s c tests if byte c appears in s.

val contains_from : bytes -> int -> char -> bool

contains_from s start c tests if byte c appears in s after position start. contains s c
is equivalent to contains_from s 0 c.
Raises Invalid_argument if start is not a valid position in s.

val rcontains_from : bytes -> int -> char -> bool

rcontains_from s stop c tests if byte c appears in s before position stop+1.
Raises Invalid_argument if stop < 0 or stop+1 is not a valid position in s.

val uppercase_ascii : bytes -> bytes

Return a copy of the argument, with all lowercase letters translated to uppercase, using the
US-ASCII character set.
Since: 4.03 (4.05 in BytesLabels)

val lowercase_ascii : bytes -> bytes

Return a copy of the argument, with all uppercase letters translated to lowercase, using the
US-ASCII character set.
Since: 4.03 (4.05 in BytesLabels)

val capitalize_ascii : bytes -> bytes

Return a copy of the argument, with the first character set to uppercase, using the US-ASCII
character set.
Since: 4.03 (4.05 in BytesLabels)

val uncapitalize_ascii : bytes -> bytes

Chapter 29. The standard library 601

Return a copy of the argument, with the first character set to lowercase, using the US-ASCII
character set.
Since: 4.03 (4.05 in BytesLabels)

type t = bytes
An alias for the type of byte sequences.

val compare : t -> t -> int

The comparison function for byte sequences, with the same specification as compare[28.2].
Along with the type t, this function compare allows the module Bytes to be passed as
argument to the functors Set.Make[29.50] and Map.Make[29.34].

val equal : t -> t -> bool

The equality function for byte sequences.
Since: 4.03 (4.05 in BytesLabels)

val starts_with : prefix:bytes -> bytes -> bool

starts_with ~prefix s is true if and only if s starts with prefix.
Since: 4.13

val ends_with : suffix:bytes -> bytes -> bool

ends_with ~suffix s is true if and only if s ends with suffix.
Since: 4.13

Unsafe conversions (for advanced users)

This section describes unsafe, low-level conversion functions between bytes and string. They
do not copy the internal data; used improperly, they can break the immutability invariant on
strings. They are available for expert library authors, but for most purposes you should use the
always-correct Bytes.to_string[29.8] and Bytes.of_string[29.8] instead.
val unsafe_to_string : bytes -> string
Unsafely convert a byte sequence into a string.
To reason about the use of unsafe_to_string, it is convenient to consider an ”ownership”
discipline. A piece of code that manipulates some data ”owns” it; there are several disjoint
ownership modes, including:

• Unique ownership: the data may be accessed and mutated

• Shared ownership: the data has several owners, that may only access it, not mutate it.

Unique ownership is linear: passing the data to another piece of code means giving up
ownership (we cannot write the data again). A unique owner may decide to make the data
shared (giving up mutation rights on it), but shared data may not become uniquely-owned
again.
602

unsafe_to_string s can only be used when the caller owns the byte sequence s – either
uniquely or as shared immutable data. The caller gives up ownership of s, and gains
ownership of the returned string.
There are two valid use-cases that respect this ownership discipline:
1. Creating a string by initializing and mutating a byte sequence that is never changed after
initialization is performed.

let string_init len f : string =

let s = Bytes.create len in
for i = 0 to len - 1 do Bytes.set s i (f i) done;
Bytes.unsafe_to_string s

This function is safe because the byte sequence s will never be accessed or mutated after
unsafe_to_string is called. The string_init code gives up ownership of s, and returns
the ownership of the resulting string to its caller.
Note that it would be unsafe if s was passed as an additional parameter to the function f as
it could escape this way and be mutated in the future – string_init would give up
ownership of s to pass it to f, and could not call unsafe_to_string safely.
We have provided the String.init[29.54], String.map[29.54] and String.mapi[29.54]
functions to cover most cases of building new strings. You should prefer those over
to_string or unsafe_to_string whenever applicable.
2. Temporarily giving ownership of a byte sequence to a function that expects a uniquely
owned string and returns ownership back, so that we can mutate the sequence again after the
call ended.

let bytes_length (s : bytes) =

String.length (Bytes.unsafe_to_string s)

In this use-case, we do not promise that s will never be mutated after the call to
bytes_length s. The String.length[29.54] function temporarily borrows unique ownership
of the byte sequence (and sees it as a string), but returns this ownership back to the caller,
which may assume that s is still a valid byte sequence after the call. Note that this is only
correct because we know that String.length[29.54] does not capture its argument – it could
escape by a side-channel such as a memoization combinator.
The caller may not mutate s while the string is borrowed (it has temporarily given up
ownership). This affects concurrent programs, but also higher-order functions: if
String.length[29.54] returned a closure to be called later, s should not be mutated until
this closure is fully applied and returns ownership.

val unsafe_of_string : string -> bytes

Unsafely convert a shared string to a byte sequence that should not be mutated.
Chapter 29. The standard library 603

The same ownership discipline that makes unsafe_to_string correct applies to

unsafe_of_string: you may use it if you were the owner of the string value, and you will
own the return bytes in the same mode.
In practice, unique ownership of string values is extremely difficult to reason about correctly.
You should always assume strings are shared, never uniquely owned.
For example, string literals are implicitly shared by the compiler, so you never uniquely own
them.

let incorrect = Bytes.unsafe_of_string "hello"

let s = Bytes.of_string "hello"

The first declaration is incorrect, because the string literal "hello" could be shared by the
compiler with other parts of the program, and mutating incorrect is a bug. You must
always use the second version, which performs a copy and is thus correct.
Assuming unique ownership of strings that are not string literals, but are (partly) built from
string literals, is also incorrect. For example, mutating unsafe_of_string ("foo" ^ s)
could mutate the shared string "foo" – assuming a rope-like representation of strings. More
generally, functions operating on strings will assume shared ownership, they do not preserve
unique ownership. It is thus incorrect to assume unique ownership of the result of
unsafe_of_string.
The only case we have reasonable confidence is safe is if the produced bytes is shared – used
as an immutable byte sequence. This is possibly useful for incremental migration of low-level
programs that manipulate immutable sequences of bytes (for example
Marshal.from_bytes[29.35]) and previously used the string type for this purpose.

val split_on_char : char -> bytes -> bytes list

split_on_char sep s returns the list of all (possibly empty) subsequences of s that are
delimited by the sep character. If s is empty, the result is the singleton list [empty].
The function’s output is specified by the following invariants:

• The list is not empty.

• Concatenating its elements using sep as a separator returns a byte sequence equal to the
input (Bytes.concat (Bytes.make 1 sep) (Bytes.split_on_char sep s) = s).
• No byte sequence in the result contains the sep character.

Since: 4.13

Iterators
val to_seq : t -> char Seq.t
Iterate on the string, in increasing index order. Modifications of the string during iteration
will be reflected in the sequence.
Since: 4.07
604

val to_seqi : t -> (int * char) Seq.t

Iterate on the string, in increasing order, yielding indices along chars
Since: 4.07

val of_seq : char Seq.t -> t

Create a string from the generator
Since: 4.07

UTF codecs and validations

UTF-8
val get_utf_8_uchar : t -> int -> Uchar.utf_decode
get_utf_8_uchar b i decodes an UTF-8 character at index i in b.

val set_utf_8_uchar : t -> int -> Uchar.t -> int

set_utf_8_uchar b i u UTF-8 encodes u at index i in b and returns the number of bytes n
that were written starting at i. If n is 0 there was not enough space to encode u at i and b
was left untouched. Otherwise a new character can be encoded at i + n.

val is_valid_utf_8 : t -> bool

is_valid_utf_8 b is true if and only if b contains valid UTF-8 data.

UTF-16BE
val get_utf_16be_uchar : t -> int -> Uchar.utf_decode
get_utf_16be_uchar b i decodes an UTF-16BE character at index i in b.

val set_utf_16be_uchar : t -> int -> Uchar.t -> int

set_utf_16be_uchar b i u UTF-16BE encodes u at index i in b and returns the number of
bytes n that were written starting at i. If n is 0 there was not enough space to encode u at i
and b was left untouched. Otherwise a new character can be encoded at i + n.

val is_valid_utf_16be : t -> bool

is_valid_utf_16be b is true if and only if b contains valid UTF-16BE data.

UTF-16LE
val get_utf_16le_uchar : t -> int -> Uchar.utf_decode
get_utf_16le_uchar b i decodes an UTF-16LE character at index i in b.

val set_utf_16le_uchar : t -> int -> Uchar.t -> int

Chapter 29. The standard library 605

set_utf_16le_uchar b i u UTF-16LE encodes u at index i in b and returns the number of

bytes n that were written starting at i. If n is 0 there was not enough space to encode u at i
and b was left untouched. Otherwise a new character can be encoded at i + n.

val is_valid_utf_16le : t -> bool

is_valid_utf_16le b is true if and only if b contains valid UTF-16LE data.

Binary encoding/decoding of integers

The functions in this section binary encode and decode integers to and from byte sequences.
All following functions raise Invalid_argument if the space needed at index i to decode or
encode the integer is not available.
Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are
stored first. Big-endian is also known as network byte order. Native-endian encoding is either
little-endian or big-endian depending on Sys.big_endian[29.56].
32-bit and 64-bit integers are represented by the int32 and int64 types, which can be interpreted
either as signed or unsigned numbers.
8-bit and 16-bit integers are represented by the int type, which has more bits than the binary
encoding. These extra bits are handled as follows:

• Functions that decode signed (resp. unsigned) 8-bit or 16-bit integers represented by int
values sign-extend (resp. zero-extend) their result.

• Functions that encode 8-bit or 16-bit integers represented by int values truncate their input
to their least significant bytes.

val get_uint8 : bytes -> int -> int

get_uint8 b i is b’s unsigned 8-bit integer starting at byte index i.
Since: 4.08

val get_int8 : bytes -> int -> int

get_int8 b i is b’s signed 8-bit integer starting at byte index i.
Since: 4.08

val get_uint16_ne : bytes -> int -> int

get_uint16_ne b i is b’s native-endian unsigned 16-bit integer starting at byte index i.
Since: 4.08

val get_uint16_be : bytes -> int -> int

get_uint16_be b i is b’s big-endian unsigned 16-bit integer starting at byte index i.
Since: 4.08

val get_uint16_le : bytes -> int -> int

606

get_uint16_le b i is b’s little-endian unsigned 16-bit integer starting at byte index i.

Since: 4.08

val get_int16_ne : bytes -> int -> int

get_int16_ne b i is b’s native-endian signed 16-bit integer starting at byte index i.
Since: 4.08

val get_int16_be : bytes -> int -> int

get_int16_be b i is b’s big-endian signed 16-bit integer starting at byte index i.
Since: 4.08

val get_int16_le : bytes -> int -> int

get_int16_le b i is b’s little-endian signed 16-bit integer starting at byte index i.
Since: 4.08

val get_int32_ne : bytes -> int -> int32

get_int32_ne b i is b’s native-endian 32-bit integer starting at byte index i.
Since: 4.08

val get_int32_be : bytes -> int -> int32

get_int32_be b i is b’s big-endian 32-bit integer starting at byte index i.
Since: 4.08

val get_int32_le : bytes -> int -> int32

get_int32_le b i is b’s little-endian 32-bit integer starting at byte index i.
Since: 4.08

val get_int64_ne : bytes -> int -> int64

get_int64_ne b i is b’s native-endian 64-bit integer starting at byte index i.
Since: 4.08

val get_int64_be : bytes -> int -> int64

get_int64_be b i is b’s big-endian 64-bit integer starting at byte index i.
Since: 4.08

val get_int64_le : bytes -> int -> int64

get_int64_le b i is b’s little-endian 64-bit integer starting at byte index i.
Since: 4.08

val set_uint8 : bytes -> int -> int -> unit

Chapter 29. The standard library 607

set_uint8 b i v sets b’s unsigned 8-bit integer starting at byte index i to v.

Since: 4.08

val set_int8 : bytes -> int -> int -> unit

set_int8 b i v sets b’s signed 8-bit integer starting at byte index i to v.
Since: 4.08

val set_uint16_ne : bytes -> int -> int -> unit

set_uint16_ne b i v sets b’s native-endian unsigned 16-bit integer starting at byte index i
to v.
Since: 4.08

val set_uint16_be : bytes -> int -> int -> unit

set_uint16_be b i v sets b’s big-endian unsigned 16-bit integer starting at byte index i to
v.
Since: 4.08

val set_uint16_le : bytes -> int -> int -> unit

set_uint16_le b i v sets b’s little-endian unsigned 16-bit integer starting at byte index i
to v.
Since: 4.08

val set_int16_ne : bytes -> int -> int -> unit

set_int16_ne b i v sets b’s native-endian signed 16-bit integer starting at byte index i to
v.
Since: 4.08

val set_int16_be : bytes -> int -> int -> unit

set_int16_be b i v sets b’s big-endian signed 16-bit integer starting at byte index i to v.
Since: 4.08

val set_int16_le : bytes -> int -> int -> unit

set_int16_le b i v sets b’s little-endian signed 16-bit integer starting at byte index i to v.
Since: 4.08

val set_int32_ne : bytes -> int -> int32 -> unit

set_int32_ne b i v sets b’s native-endian 32-bit integer starting at byte index i to v.
Since: 4.08

val set_int32_be : bytes -> int -> int32 -> unit

set_int32_be b i v sets b’s big-endian 32-bit integer starting at byte index i to v.
Since: 4.08
608

val set_int32_le : bytes -> int -> int32 -> unit

set_int32_le b i v sets b’s little-endian 32-bit integer starting at byte index i to v.
Since: 4.08

val set_int64_ne : bytes -> int -> int64 -> unit

set_int64_ne b i v sets b’s native-endian 64-bit integer starting at byte index i to v.
Since: 4.08

val set_int64_be : bytes -> int -> int64 -> unit

set_int64_be b i v sets b’s big-endian 64-bit integer starting at byte index i to v.
Since: 4.08

val set_int64_le : bytes -> int -> int64 -> unit

set_int64_le b i v sets b’s little-endian 64-bit integer starting at byte index i to v.
Since: 4.08

Byte sequences and concurrency safety

Care must be taken when concurrently accessing byte sequences from multiple domains: accessing
a byte sequence will never crash a program, but unsynchronized accesses might yield surprising
(non-sequentially-consistent) results.

Atomicity
Every byte sequence operation that accesses more than one byte is not atomic. This includes
iteration and scanning.
For example, consider the following program:
let size = 100_000_000
let b = Bytes.make size ' '
let update b f () =
Bytes.iteri (fun i x -> Bytes.set b i (Char.chr (f (Char.code x)))) b
let d1 = Domain.spawn (update b (fun x -> x + 1))
let d2 = Domain.spawn (update b (fun x -> 2 * x + 1))
let () = Domain.join d1; Domain.join d2

the bytes sequence b may contain a non-deterministic mixture of '!', 'A', 'B', and 'C' values.
After executing this code, each byte of the sequence b is either '!', 'A', 'B', or 'C'. If
atomicity is required, then the user must implement their own synchronization (for example, using
Mutex.t[29.37]).
Chapter 29. The standard library 609

Data races
If two domains only access disjoint parts of a byte sequence, then the observed behaviour is the
equivalent to some sequential interleaving of the operations from the two domains.
A data race is said to occur when two domains access the same byte without synchronization
and at least one of the accesses is a write. In the absence of data races, the observed behaviour is
equivalent to some sequential interleaving of the operations from different domains.
Whenever possible, data races should be avoided by using synchronization to mediate the accesses
to the elements of the sequence.
Indeed, in the presence of data races, programs will not crash but the observed behaviour may
not be equivalent to any sequential interleaving of operations from different domains. Nevertheless,
even in the presence of data races, a read operation will return the value of some prior write to that
location.

Mixed-size accesses
Another subtle point is that if a data race involves mixed-size writes and reads to the same location,
the order in which those writes and reads are observed by domains is not specified. For instance,
the following code write sequentially a 32-bit integer and a char to the same index

let b = Bytes.make 10 '\000'

let d1 = Domain.spawn (fun () -> Bytes.set_int32_ne b 0 100; b.[0] <- 'd' )

In this situation, a domain that observes the write of ’d’ to b.0 is not guaranteed to also observe
the write to indices 1, 2, or 3.

29.9 Module BytesLabels : Byte sequence operations.

A byte sequence is a mutable data structure that contains a fixed-length sequence of bytes. Each
byte can be indexed in constant time for reading or writing.
Given a byte sequence s of length l, we can access each of the l bytes of s via its index in
the sequence. Indexes start at 0, and we will call an index valid in s if it falls within the range
[0...l-1] (inclusive). A position is the point between two bytes or at the beginning or end of the
sequence. We call a position valid in s if it falls within the range [0...l] (inclusive). Note that
the byte at index n is between positions n and n+1.
Two parameters start and len are said to designate a valid range of s if len >= 0 and start
and start+len are valid positions in s.
Byte sequences can be modified in place, for instance via the set and blit functions described
below. See also strings (module String[29.54]), which are almost the same data structure, but
cannot be modified in place.
Bytes are represented by the OCaml type char.
The labeled version of this module can be used as described in the StdLabels[29.53] module.
Since: 4.02

val length : bytes -> int

610

Return the length (number of bytes) of the argument.

val get : bytes -> int -> char

get s n returns the byte at index n in argument s.
Raises Invalid_argument if n is not a valid index in s.

val set : bytes -> int -> char -> unit

set s n c modifies s in place, replacing the byte at index n with c.
Raises Invalid_argument if n is not a valid index in s.

val create : int -> bytes

create n returns a new byte sequence of length n. The sequence is uninitialized and contains
arbitrary bytes.
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val make : int -> char -> bytes

make n c returns a new byte sequence of length n, filled with the byte c.
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val init : int -> f:(int -> char) -> bytes

init n f returns a fresh byte sequence of length n, with character i initialized to the result
of f i (in increasing index order).
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val empty : bytes

A byte sequence of size 0.

val copy : bytes -> bytes

Return a new byte sequence that contains the same bytes as the argument.

val of_string : string -> bytes

Return a new byte sequence that contains the same bytes as the given string.

val to_string : bytes -> string

Return a new string that contains the same bytes as the given byte sequence.

val sub : bytes -> pos:int -> len:int -> bytes

sub s ~pos ~len returns a new byte sequence of length len, containing the subsequence of
s that starts at position pos and has length len.
Raises Invalid_argument if pos and len do not designate a valid range of s.

val sub_string : bytes -> pos:int -> len:int -> string

Same as BytesLabels.sub[29.9] but return a string instead of a byte sequence.
Chapter 29. The standard library 611

val extend : bytes -> left:int -> right:int -> bytes

extend s ~left ~right returns a new byte sequence that contains the bytes of s, with left
uninitialized bytes prepended and right uninitialized bytes appended to it. If left or right
is negative, then bytes are removed (instead of appended) from the corresponding side of s.
Since: 4.05 in BytesLabels
Raises Invalid_argument if the result length is negative or longer than
Sys.max_string_length[29.56] bytes.

val fill : bytes -> pos:int -> len:int -> char -> unit
fill s ~pos ~len c modifies s in place, replacing len characters with c, starting at pos.
Raises Invalid_argument if pos and len do not designate a valid range of s.

val blit :
src:bytes -> src_pos:int -> dst:bytes -> dst_pos:int -> len:int -> unit
blit ~src ~src_pos ~dst ~dst_pos ~len copies len bytes from byte sequence src,
starting at index src_pos, to byte sequence dst, starting at index dst_pos. It works
correctly even if src and dst are the same byte sequence, and the source and destination
intervals overlap.
Raises Invalid_argument if src_pos and len do not designate a valid range of src, or if
dst_pos and len do not designate a valid range of dst.

val blit_string :
src:string -> src_pos:int -> dst:bytes -> dst_pos:int -> len:int -> unit
blit_string ~src ~src_pos ~dst ~dst_pos ~len copies len bytes from string src,
starting at index src_pos, to byte sequence dst, starting at index dst_pos.
Since: 4.05 in BytesLabels
Raises Invalid_argument if src_pos and len do not designate a valid range of src, or if
dst_pos and len do not designate a valid range of dst.

val concat : sep:bytes -> bytes list -> bytes

concat ~sep sl concatenates the list of byte sequences sl, inserting the separator byte
sequence sep between each, and returns the result as a new byte sequence.
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val cat : bytes -> bytes -> bytes

cat s1 s2 concatenates s1 and s2 and returns the result as a new byte sequence.
Since: 4.05 in BytesLabels
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val iter : f:(char -> unit) -> bytes -> unit

iter ~f s applies function f in turn to all the bytes of s. It is equivalent to f (get s 0);
f (get s 1); ...; f (get s (length s - 1)); ().
612

val iteri : f:(int -> char -> unit) -> bytes -> unit
Same as BytesLabels.iter[29.9], but the function is applied to the index of the byte as first
argument and the byte itself as second argument.

val map : f:(char -> char) -> bytes -> bytes

map ~f s applies function f in turn to all the bytes of s (in increasing index order) and
stores the resulting bytes in a new sequence that is returned as the result.

val mapi : f:(int -> char -> char) -> bytes -> bytes
mapi ~f s calls f with each character of s and its index (in increasing index order) and
stores the resulting bytes in a new sequence that is returned as the result.

val fold_left : f:('acc -> char -> 'acc) -> init:'acc -> bytes -> 'acc
fold_left f x s computes f (... (f (f x (get s 0)) (get s 1)) ...) (get s
(n-1)), where n is the length of s.
Since: 4.13

val fold_right : f:(char -> 'acc -> 'acc) -> bytes -> init:'acc -> 'acc
fold_right f s x computes f (get s 0) (f (get s 1) ( ... (f (get s (n-1)) x)
...)), where n is the length of s.
Since: 4.13

val for_all : f:(char -> bool) -> bytes -> bool

for_all p s checks if all characters in s satisfy the predicate p.
Since: 4.13

val exists : f:(char -> bool) -> bytes -> bool

exists p s checks if at least one character of s satisfies the predicate p.
Since: 4.13

val trim : bytes -> bytes

Return a copy of the argument, without leading and trailing whitespace. The bytes regarded
as whitespace are the ASCII characters ' ', '\012', '\n', '\r', and '\t'.

val escaped : bytes -> bytes

Return a copy of the argument, with special characters represented by escape sequences,
following the lexical conventions of OCaml. All characters outside the ASCII printable range
(32..126) are escaped, as well as backslash and double-quote.
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val index : bytes -> char -> int

index s c returns the index of the first occurrence of byte c in s.
Raises Not_found if c does not occur in s.
Chapter 29. The standard library 613

val index_opt : bytes -> char -> int option

index_opt s c returns the index of the first occurrence of byte c in s or None if c does not
occur in s.
Since: 4.05

val rindex : bytes -> char -> int

rindex s c returns the index of the last occurrence of byte c in s.
Raises Not_found if c does not occur in s.

val rindex_opt : bytes -> char -> int option

rindex_opt s c returns the index of the last occurrence of byte c in s or None if c does not
occur in s.
Since: 4.05

val index_from : bytes -> int -> char -> int

index_from s i c returns the index of the first occurrence of byte c in s after position i.
index s c is equivalent to index_from s 0 c.
Raises
• Invalid_argument if i is not a valid position in s.
• Not_found if c does not occur in s after position i.

val index_from_opt : bytes -> int -> char -> int option
index_from_opt s i c returns the index of the first occurrence of byte c in s after position
i or None if c does not occur in s after position i. index_opt s c is equivalent to
index_from_opt s 0 c.
Since: 4.05
Raises Invalid_argument if i is not a valid position in s.

val rindex_from : bytes -> int -> char -> int

rindex_from s i c returns the index of the last occurrence of byte c in s before position
i+1. rindex s c is equivalent to rindex_from s (length s - 1) c.
Raises
• Invalid_argument if i+1 is not a valid position in s.
• Not_found if c does not occur in s before position i+1.

val rindex_from_opt : bytes -> int -> char -> int option
rindex_from_opt s i c returns the index of the last occurrence of byte c in s before
position i+1 or None if c does not occur in s before position i+1. rindex_opt s c is
equivalent to rindex_from s (length s - 1) c.
Since: 4.05
Raises Invalid_argument if i+1 is not a valid position in s.
614

val contains : bytes -> char -> bool

contains s c tests if byte c appears in s.

val contains_from : bytes -> int -> char -> bool

contains_from s start c tests if byte c appears in s after position start. contains s c
is equivalent to contains_from s 0 c.
Raises Invalid_argument if start is not a valid position in s.

val rcontains_from : bytes -> int -> char -> bool

rcontains_from s stop c tests if byte c appears in s before position stop+1.
Raises Invalid_argument if stop < 0 or stop+1 is not a valid position in s.

val uppercase_ascii : bytes -> bytes

Return a copy of the argument, with all lowercase letters translated to uppercase, using the
US-ASCII character set.
Since: 4.05

val lowercase_ascii : bytes -> bytes

Return a copy of the argument, with all uppercase letters translated to lowercase, using the
US-ASCII character set.
Since: 4.05

val capitalize_ascii : bytes -> bytes

Return a copy of the argument, with the first character set to uppercase, using the US-ASCII
character set.
Since: 4.05

val uncapitalize_ascii : bytes -> bytes

Return a copy of the argument, with the first character set to lowercase, using the US-ASCII
character set.
Since: 4.05

type t = bytes
An alias for the type of byte sequences.

val compare : t -> t -> int

The comparison function for byte sequences, with the same specification as compare[28.2].
Along with the type t, this function compare allows the module Bytes to be passed as
argument to the functors Set.Make[29.50] and Map.Make[29.34].

val equal : t -> t -> bool

The equality function for byte sequences.
Since: 4.05
Chapter 29. The standard library 615

val starts_with : prefix:bytes -> bytes -> bool

starts_with ~prefix s is true if and only if s starts with prefix.
Since: 4.13

val ends_with : suffix:bytes -> bytes -> bool

ends_with ~suffix s is true if and only if s ends with suffix.
Since: 4.13

Unsafe conversions (for advanced users)

This section describes unsafe, low-level conversion functions between bytes and string. They
do not copy the internal data; used improperly, they can break the immutability invariant on
strings. They are available for expert library authors, but for most purposes you should use the
always-correct BytesLabels.to_string[29.9] and BytesLabels.of_string[29.9] instead.
val unsafe_to_string : bytes -> string
Unsafely convert a byte sequence into a string.
To reason about the use of unsafe_to_string, it is convenient to consider an ”ownership”
discipline. A piece of code that manipulates some data ”owns” it; there are several disjoint
ownership modes, including:

• Unique ownership: the data may be accessed and mutated

• Shared ownership: the data has several owners, that may only access it, not mutate it.

Unique ownership is linear: passing the data to another piece of code means giving up
ownership (we cannot write the data again). A unique owner may decide to make the data
shared (giving up mutation rights on it), but shared data may not become uniquely-owned
again.
unsafe_to_string s can only be used when the caller owns the byte sequence s – either
uniquely or as shared immutable data. The caller gives up ownership of s, and gains
ownership of the returned string.
There are two valid use-cases that respect this ownership discipline:
1. Creating a string by initializing and mutating a byte sequence that is never changed after
initialization is performed.

let string_init len f : string =

let s = Bytes.create len in
for i = 0 to len - 1 do Bytes.set s i (f i) done;
Bytes.unsafe_to_string s

This function is safe because the byte sequence s will never be accessed or mutated after
unsafe_to_string is called. The string_init code gives up ownership of s, and returns
the ownership of the resulting string to its caller.
616

Note that it would be unsafe if s was passed as an additional parameter to the function f as
it could escape this way and be mutated in the future – string_init would give up
ownership of s to pass it to f, and could not call unsafe_to_string safely.
We have provided the String.init[29.54], String.map[29.54] and String.mapi[29.54]
functions to cover most cases of building new strings. You should prefer those over
to_string or unsafe_to_string whenever applicable.
2. Temporarily giving ownership of a byte sequence to a function that expects a uniquely
owned string and returns ownership back, so that we can mutate the sequence again after the
call ended.

let bytes_length (s : bytes) =

String.length (Bytes.unsafe_to_string s)

In this use-case, we do not promise that s will never be mutated after the call to
bytes_length s. The String.length[29.54] function temporarily borrows unique ownership
of the byte sequence (and sees it as a string), but returns this ownership back to the caller,
which may assume that s is still a valid byte sequence after the call. Note that this is only
correct because we know that String.length[29.54] does not capture its argument – it could
escape by a side-channel such as a memoization combinator.
The caller may not mutate s while the string is borrowed (it has temporarily given up
ownership). This affects concurrent programs, but also higher-order functions: if
String.length[29.54] returned a closure to be called later, s should not be mutated until
this closure is fully applied and returns ownership.

val unsafe_of_string : string -> bytes

Unsafely convert a shared string to a byte sequence that should not be mutated.
The same ownership discipline that makes unsafe_to_string correct applies to
unsafe_of_string: you may use it if you were the owner of the string value, and you will
own the return bytes in the same mode.
In practice, unique ownership of string values is extremely difficult to reason about correctly.
You should always assume strings are shared, never uniquely owned.
For example, string literals are implicitly shared by the compiler, so you never uniquely own
them.

let incorrect = Bytes.unsafe_of_string "hello"

let s = Bytes.of_string "hello"

The first declaration is incorrect, because the string literal "hello" could be shared by the
compiler with other parts of the program, and mutating incorrect is a bug. You must
always use the second version, which performs a copy and is thus correct.
Assuming unique ownership of strings that are not string literals, but are (partly) built from
string literals, is also incorrect. For example, mutating unsafe_of_string ("foo" ^ s)
Chapter 29. The standard library 617

could mutate the shared string "foo" – assuming a rope-like representation of strings. More
generally, functions operating on strings will assume shared ownership, they do not preserve
unique ownership. It is thus incorrect to assume unique ownership of the result of
unsafe_of_string.
The only case we have reasonable confidence is safe is if the produced bytes is shared – used
as an immutable byte sequence. This is possibly useful for incremental migration of low-level
programs that manipulate immutable sequences of bytes (for example
Marshal.from_bytes[29.35]) and previously used the string type for this purpose.

val split_on_char : sep:char -> bytes -> bytes list

split_on_char sep s returns the list of all (possibly empty) subsequences of s that are
delimited by the sep character. If s is empty, the result is the singleton list [empty].
The function’s output is specified by the following invariants:

• The list is not empty.

• Concatenating its elements using sep as a separator returns a byte sequence equal to the
input (Bytes.concat (Bytes.make 1 sep) (Bytes.split_on_char sep s) = s).
• No byte sequence in the result contains the sep character.

Since: 4.13

Iterators
val to_seq : t -> char Seq.t
Iterate on the string, in increasing index order. Modifications of the string during iteration
will be reflected in the sequence.
Since: 4.07

val to_seqi : t -> (int * char) Seq.t

Iterate on the string, in increasing order, yielding indices along chars
Since: 4.07

val of_seq : char Seq.t -> t

Create a string from the generator
Since: 4.07

UTF codecs and validations

UTF-8
val get_utf_8_uchar : t -> int -> Uchar.utf_decode
get_utf_8_uchar b i decodes an UTF-8 character at index i in b.
618

val set_utf_8_uchar : t -> int -> Uchar.t -> int

set_utf_8_uchar b i u UTF-8 encodes u at index i in b and returns the number of bytes n
that were written starting at i. If n is 0 there was not enough space to encode u at i and b
was left untouched. Otherwise a new character can be encoded at i + n.

val is_valid_utf_8 : t -> bool

is_valid_utf_8 b is true if and only if b contains valid UTF-8 data.

UTF-16BE
val get_utf_16be_uchar : t -> int -> Uchar.utf_decode
get_utf_16be_uchar b i decodes an UTF-16BE character at index i in b.

val set_utf_16be_uchar : t -> int -> Uchar.t -> int

set_utf_16be_uchar b i u UTF-16BE encodes u at index i in b and returns the number of
bytes n that were written starting at i. If n is 0 there was not enough space to encode u at i
and b was left untouched. Otherwise a new character can be encoded at i + n.

val is_valid_utf_16be : t -> bool

is_valid_utf_16be b is true if and only if b contains valid UTF-16BE data.

UTF-16LE
val get_utf_16le_uchar : t -> int -> Uchar.utf_decode
get_utf_16le_uchar b i decodes an UTF-16LE character at index i in b.

val set_utf_16le_uchar : t -> int -> Uchar.t -> int

set_utf_16le_uchar b i u UTF-16LE encodes u at index i in b and returns the number of
bytes n that were written starting at i. If n is 0 there was not enough space to encode u at i
and b was left untouched. Otherwise a new character can be encoded at i + n.

val is_valid_utf_16le : t -> bool

is_valid_utf_16le b is true if and only if b contains valid UTF-16LE data.

Binary encoding/decoding of integers

The functions in this section binary encode and decode integers to and from byte sequences.
All following functions raise Invalid_argument if the space needed at index i to decode or
encode the integer is not available.
Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are
stored first. Big-endian is also known as network byte order. Native-endian encoding is either
little-endian or big-endian depending on Sys.big_endian[29.56].
Chapter 29. The standard library 619

32-bit and 64-bit integers are represented by the int32 and int64 types, which can be interpreted
either as signed or unsigned numbers.
8-bit and 16-bit integers are represented by the int type, which has more bits than the binary
encoding. These extra bits are handled as follows:

• Functions that decode signed (resp. unsigned) 8-bit or 16-bit integers represented by int
values sign-extend (resp. zero-extend) their result.

• Functions that encode 8-bit or 16-bit integers represented by int values truncate their input
to their least significant bytes.

val get_uint8 : bytes -> int -> int

get_uint8 b i is b’s unsigned 8-bit integer starting at byte index i.
Since: 4.08

val get_int8 : bytes -> int -> int

get_int8 b i is b’s signed 8-bit integer starting at byte index i.
Since: 4.08

val get_uint16_ne : bytes -> int -> int

get_uint16_ne b i is b’s native-endian unsigned 16-bit integer starting at byte index i.
Since: 4.08

val get_uint16_be : bytes -> int -> int

get_uint16_be b i is b’s big-endian unsigned 16-bit integer starting at byte index i.
Since: 4.08

val get_uint16_le : bytes -> int -> int

get_uint16_le b i is b’s little-endian unsigned 16-bit integer starting at byte index i.
Since: 4.08

val get_int16_ne : bytes -> int -> int

get_int16_ne b i is b’s native-endian signed 16-bit integer starting at byte index i.
Since: 4.08

val get_int16_be : bytes -> int -> int

get_int16_be b i is b’s big-endian signed 16-bit integer starting at byte index i.
Since: 4.08

val get_int16_le : bytes -> int -> int

get_int16_le b i is b’s little-endian signed 16-bit integer starting at byte index i.
Since: 4.08
620

val get_int32_ne : bytes -> int -> int32

get_int32_ne b i is b’s native-endian 32-bit integer starting at byte index i.
Since: 4.08

val get_int32_be : bytes -> int -> int32

get_int32_be b i is b’s big-endian 32-bit integer starting at byte index i.
Since: 4.08

val get_int32_le : bytes -> int -> int32

get_int32_le b i is b’s little-endian 32-bit integer starting at byte index i.
Since: 4.08

val get_int64_ne : bytes -> int -> int64

get_int64_ne b i is b’s native-endian 64-bit integer starting at byte index i.
Since: 4.08

val get_int64_be : bytes -> int -> int64

get_int64_be b i is b’s big-endian 64-bit integer starting at byte index i.
Since: 4.08

val get_int64_le : bytes -> int -> int64

get_int64_le b i is b’s little-endian 64-bit integer starting at byte index i.
Since: 4.08

val set_uint8 : bytes -> int -> int -> unit

set_uint8 b i v sets b’s unsigned 8-bit integer starting at byte index i to v.
Since: 4.08

val set_int8 : bytes -> int -> int -> unit

set_int8 b i v sets b’s signed 8-bit integer starting at byte index i to v.
Since: 4.08

val set_uint16_ne : bytes -> int -> int -> unit

set_uint16_ne b i v sets b’s native-endian unsigned 16-bit integer starting at byte index i
to v.
Since: 4.08

val set_uint16_be : bytes -> int -> int -> unit

set_uint16_be b i v sets b’s big-endian unsigned 16-bit integer starting at byte index i to
v.
Since: 4.08
Chapter 29. The standard library 621

val set_uint16_le : bytes -> int -> int -> unit

set_uint16_le b i v sets b’s little-endian unsigned 16-bit integer starting at byte index i
to v.
Since: 4.08

val set_int16_ne : bytes -> int -> int -> unit

set_int16_ne b i v sets b’s native-endian signed 16-bit integer starting at byte index i to
v.
Since: 4.08

val set_int16_be : bytes -> int -> int -> unit

set_int16_be b i v sets b’s big-endian signed 16-bit integer starting at byte index i to v.
Since: 4.08

val set_int16_le : bytes -> int -> int -> unit

set_int16_le b i v sets b’s little-endian signed 16-bit integer starting at byte index i to v.
Since: 4.08

val set_int32_ne : bytes -> int -> int32 -> unit

set_int32_ne b i v sets b’s native-endian 32-bit integer starting at byte index i to v.
Since: 4.08

val set_int32_be : bytes -> int -> int32 -> unit

set_int32_be b i v sets b’s big-endian 32-bit integer starting at byte index i to v.
Since: 4.08

val set_int32_le : bytes -> int -> int32 -> unit

set_int32_le b i v sets b’s little-endian 32-bit integer starting at byte index i to v.
Since: 4.08

val set_int64_ne : bytes -> int -> int64 -> unit

set_int64_ne b i v sets b’s native-endian 64-bit integer starting at byte index i to v.
Since: 4.08

val set_int64_be : bytes -> int -> int64 -> unit

set_int64_be b i v sets b’s big-endian 64-bit integer starting at byte index i to v.
Since: 4.08

val set_int64_le : bytes -> int -> int64 -> unit

set_int64_le b i v sets b’s little-endian 64-bit integer starting at byte index i to v.
Since: 4.08
622

Byte sequences and concurrency safety

Care must be taken when concurrently accessing byte sequences from multiple domains: accessing
a byte sequence will never crash a program, but unsynchronized accesses might yield surprising
(non-sequentially-consistent) results.

Atomicity
Every byte sequence operation that accesses more than one byte is not atomic. This includes
iteration and scanning.
For example, consider the following program:
let size = 100_000_000
let b = Bytes.make size ' '
let update b f () =
Bytes.iteri (fun i x -> Bytes.set b i (Char.chr (f (Char.code x)))) b
let d1 = Domain.spawn (update b (fun x -> x + 1))
let d2 = Domain.spawn (update b (fun x -> 2 * x + 1))
let () = Domain.join d1; Domain.join d2

the bytes sequence b may contain a non-deterministic mixture of '!', 'A', 'B', and 'C' values.
After executing this code, each byte of the sequence b is either '!', 'A', 'B', or 'C'. If
atomicity is required, then the user must implement their own synchronization (for example, using
Mutex.t[29.37]).

Data races
If two domains only access disjoint parts of a byte sequence, then the observed behaviour is the
equivalent to some sequential interleaving of the operations from the two domains.
A data race is said to occur when two domains access the same byte without synchronization
and at least one of the accesses is a write. In the absence of data races, the observed behaviour is
equivalent to some sequential interleaving of the operations from different domains.
Whenever possible, data races should be avoided by using synchronization to mediate the accesses
to the elements of the sequence.
Indeed, in the presence of data races, programs will not crash but the observed behaviour may
not be equivalent to any sequential interleaving of operations from different domains. Nevertheless,
even in the presence of data races, a read operation will return the value of some prior write to that
location.

Mixed-size accesses
Another subtle point is that if a data race involves mixed-size writes and reads to the same location,
the order in which those writes and reads are observed by domains is not specified. For instance,
the following code write sequentially a 32-bit integer and a char to the same index

let b = Bytes.make 10 '\000'

let d1 = Domain.spawn (fun () -> Bytes.set_int32_ne b 0 100; b.[0] <- 'd' )
Chapter 29. The standard library 623

In this situation, a domain that observes the write of ’d’ to b.0 is not guaranteed to also observe
the write to indices 1, 2, or 3.

29.10 Module Callback : Registering OCaml values with the C

runtime.
This module allows OCaml values to be registered with the C runtime under a symbolic name, so
that C code can later call back registered OCaml functions, or raise registered OCaml exceptions.

val register : string -> 'a -> unit

Callback.register n v registers the value v under the name n. C code can later retrieve a
handle to v by calling caml_named_value(n).

val register_exception : string -> exn -> unit

Callback.register_exception n exn registers the exception contained in the exception
value exn under the name n. C code can later retrieve a handle to the exception by calling
caml_named_value(n). The exception value thus obtained is suitable for passing as first
argument to raise_constant or raise_with_arg.

29.11 Module Char : Character operations.

val code : char -> int

Return the ASCII code of the argument.

val chr : int -> char

Return the character with the given ASCII code.
Raises Invalid_argument if the argument is outside the range 0–255.

val escaped : char -> string

Return a string representing the given character, with special characters escaped following the
lexical conventions of OCaml. All characters outside the ASCII printable range (32..126) are
escaped, as well as backslash, double-quote, and single-quote.

val lowercase_ascii : char -> char

Convert the given character to its equivalent lowercase character, using the US-ASCII
character set.
Since: 4.03

val uppercase_ascii : char -> char

624

Convert the given character to its equivalent uppercase character, using the US-ASCII
character set.
Since: 4.03

type t = char
An alias for the type of characters.

val compare : t -> t -> int

The comparison function for characters, with the same specification as compare[28.2]. Along
with the type t, this function compare allows the module Char to be passed as argument to
the functors Set.Make[29.50] and Map.Make[29.34].

val equal : t -> t -> bool

The equal function for chars.
Since: 4.03

val seeded_hash : int -> t -> int

A seeded hash function for characters, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.1

val hash : t -> int

An unseeded hash function for characters, with the same output value as
Hashtbl.hash[29.25]. This function allows this module to be passed as argument to the
functor Hashtbl.Make[29.25].
Since: 5.1

29.12 Module Complex : Complex numbers.

This module provides arithmetic operations on complex numbers. Complex numbers are represented
by their real and imaginary parts (cartesian representation). Each part is represented by a double-
precision floating-point number (type float).

type t =
{ re : float ;
im : float ;
}
The type of complex numbers. re is the real part and im the imaginary part.

val zero : t
The complex number 0.
Chapter 29. The standard library 625

val one : t
The complex number 1.

val i : t
The complex number i.

val neg : t -> t

Unary negation.

val conj : t -> t

Conjugate: given the complex x + i.y, returns x - i.y.

val add : t -> t -> t

Addition

val sub : t -> t -> t

Subtraction

val mul : t -> t -> t

Multiplication

val inv : t -> t

Multiplicative inverse (1/z).

val div : t -> t -> t

Division

val sqrt : t -> t

Square root. The result x + i.y is such that x > 0 or x = 0 and y >= 0. This function has
a discontinuity along the negative real axis.

val norm2 : t -> float

Norm squared: given x + i.y, returns x^2 + y^2.

val norm : t -> float

Norm: given x + i.y, returns sqrt(x^2 + y^2).

val arg : t -> float

Argument. The argument of a complex number is the angle in the complex plane between the
positive real axis and a line passing through zero and the number. This angle ranges from
-pi to pi. This function has a discontinuity along the negative real axis.

val polar : float -> float -> t

polar norm arg returns the complex having norm norm and argument arg.
626

val exp : t -> t

Exponentiation. exp z returns e to the z power.

val log : t -> t

Natural logarithm (in base e).

val pow : t -> t -> t

Power function. pow z1 z2 returns z1 to the z2 power.

29.13 Module Condition : Condition variables.

Condition variables are useful when several threads wish to access a shared data structure that is
protected by a mutex (a mutual exclusion lock).
A condition variable is a communication channel. On the receiver side, one or more threads can
indicate that they wish to wait for a certain property to become true. On the sender side, a thread
can signal that this property has become true, causing one (or more) waiting threads to be woken
up.
For instance, in the implementation of a queue data structure, if a thread that wishes to extract
an element finds that the queue is currently empty, then this thread waits for the queue to become
nonempty. A thread that inserts an element into the queue signals that the queue has become
nonempty. A condition variable is used for this purpose. This communication channel conveys the
information that the property ”the queue is nonempty” is true, or more accurately, may be true.
(We explain below why the receiver of a signal cannot be certain that the property holds.)
To continue the example of the queue, assuming that the queue has a fixed maximum capacity,
then a thread that wishes to insert an element may find that the queue is full. Then, this thread
must wait for the queue to become not full, and a thread that extracts an element of the queue
signals that the queue has become not full. Another condition variable is used for this purpose.
In short, a condition variable c is used to convey the information that a certain property P
about a shared data structure D, protected by a mutex m, may be true.
Condition variables provide an efficient alternative to busy-waiting. When one wishes to wait
for the property P to be true, instead of writing a busy-waiting loop:

Mutex.lock m;
while not P do
Mutex.unlock m; Mutex.lock m
done;
<update the data structure>;
Mutex.unlock m

one uses Condition.wait[29.13] in the body of the loop, as follows:

Mutex.lock m;
while not P do
Chapter 29. The standard library 627

Condition.wait c m
done;
<update the data structure>;
Mutex.unlock m

The busy-waiting loop is inefficient because the waiting thread consumes processing time and
creates contention of the mutex m. Calling Condition.wait[29.13] allows the waiting thread to be
suspended, so it does not consume any computing resources while waiting.
With a condition variable c, exactly one mutex m is associated. This association is implicit:
the mutex m is not explicitly passed as an argument to Condition.create[29.13]. It is up to the
programmer to know, for each condition variable c, which is the associated mutex m.
With a mutex m, several condition variables can be associated. In the example of the bounded
queue, one condition variable is used to indicate that the queue is nonempty, and another condition
variable is used to indicate that the queue is not full.
With a condition variable c, exactly one logical property P should be associated. Examples
of such properties include ”the queue is nonempty” and ”the queue is not full”. It is up to the
programmer to keep track, for each condition variable, of the corresponding property P. A signal is
sent on the condition variable c as an indication that the property P is true, or may be true. On
the receiving end, however, a thread that is woken up cannot assume that P is true; after a call to
Condition.wait[29.13] terminates, one must explicitly test whether P is true. There are several
reasons why this is so. One reason is that, between the moment when the signal is sent and the
moment when a waiting thread receives the signal and is scheduled, the property P may be falsified
by some other thread that is able to acquire the mutex m and alter the data structure D. Another
reason is that spurious wakeups may occur: a waiting thread can be woken up even if no signal was
sent.
Here is a complete example, where a mutex protects a sequential unbounded queue, and where
a condition variable is used to signal that the queue is nonempty.

type 'a safe_queue =

{ queue : 'a Queue.t; mutex : Mutex.t; nonempty : Condition.t }

let create () =
{ queue = Queue.create(); mutex = Mutex.create();
nonempty = Condition.create() }

let add v q =
Mutex.lock q.mutex;
let was_empty = Queue.is_empty q.queue in
Queue.add v q.queue;
if was_empty then Condition.broadcast q.nonempty;
Mutex.unlock q.mutex

let take q =
Mutex.lock q.mutex;
while Queue.is_empty q.queue do Condition.wait q.nonempty q.mutex done;
628

let v = Queue.take q.queue in (* cannot fail since queue is nonempty *)

Mutex.unlock q.mutex;
v

Because the call to Condition.broadcast[29.13] takes place inside the critical section, the
following property holds whenever the mutex is unlocked: if the queue is nonempty, then no thread
is waiting, or, in other words, if some thread is waiting, then the queue must be empty. This is a
desirable property: if a thread that attempts to execute a take operation could remain suspended
even though the queue is nonempty, that would be a problematic situation, known as a deadlock.

type t
The type of condition variables.

val create : unit -> t

create() creates and returns a new condition variable. This condition variable should be
associated (in the programmer’s mind) with a certain mutex m and with a certain property P
of the data structure that is protected by the mutex m.

val wait : t -> Mutex.t -> unit

The call wait c m is permitted only if m is the mutex associated with the condition variable
c, and only if m is currently locked. This call atomically unlocks the mutex m and suspends
the current thread on the condition variable c. This thread can later be woken up after the
condition variable c has been signaled via Condition.signal[29.13] or
Condition.broadcast[29.13]; however, it can also be woken up for no reason. The mutex m
is locked again before wait returns. One cannot assume that the property P associated with
the condition variable c holds when wait returns; one must explicitly test whether P holds
after calling wait.

val signal : t -> unit

signal c wakes up one of the threads waiting on the condition variable c, if there is one. If
there is none, this call has no effect.
It is recommended to call signal c inside a critical section, that is, while the mutex m
associated with c is locked.

val broadcast : t -> unit

broadcast c wakes up all threads waiting on the condition variable c. If there are none, this
call has no effect.
It is recommended to call broadcast c inside a critical section, that is, while the mutex m
associated with c is locked.

29.14 Module Domain

Alert unstable. The Domain interface may change in incompatible ways in the future.
Chapter 29. The standard library 629

Domains.
See ’Parallel programming’ chapter in the manual.
type !'a t
A domain of type 'a t runs independently, eventually producing a result of type ’a, or an
exception

val spawn : (unit -> 'a) -> 'a t

spawn f creates a new domain that runs in parallel with the current domain.
Raises Failure if the program has insufficient resources to create another domain.

val join : 'a t -> 'a

join d blocks until domain d runs to completion. If d results in a value, then that is returned
by join d. If d raises an uncaught exception, then that is re-raised by join d.

type id = private int

Domains have unique integer identifiers

val get_id : 'a t -> id

get_id d returns the identifier of the domain d

val self : unit -> id

self () is the identifier of the currently running domain

val before_first_spawn : (unit -> unit) -> unit

before_first_spawn f registers f to be called before the first domain is spawned by the
program. The functions registered with before_first_spawn are called on the main (initial)
domain. The functions registered with before_first_spawn are called in ’first in, first out’
order: the oldest function added with before_first_spawn is called first.
Raises Invalid_argument if the program has already spawned a domain.

val at_exit : (unit -> unit) -> unit

at_exit f registers f to be called when the current domain exits. Note that at_exit
callbacks are domain-local and only apply to the calling domain. The registered functions are
called in ’last in, first out’ order: the function most recently added with at_exit is called
first. An example:

let temp_file_key = Domain.DLS.new_key (fun _ ->

let tmp = snd (Filename.open_temp_file "" "") in
Domain.at_exit (fun () -> close_out_noerr tmp);
tmp)

The snippet above creates a key that when retrieved for the first time will open a temporary
file and register an at_exit callback to close it, thus guaranteeing the descriptor is not leaked
in case the current domain exits.
630

val cpu_relax : unit -> unit

If busy-waiting, calling cpu_relax () between iterations will improve performance on some
CPU architectures

val is_main_domain : unit -> bool

is_main_domain () returns true if called from the initial domain.

val recommended_domain_count : unit -> int

The recommended maximum number of domains which should be running simultaneously
(including domains already running).
The value returned is at least 1.

val self_index : unit -> int

The index of the current domain. It is an integer unique among currently-running domains, in
the interval 0; N-1 where N is the peak number of domains running simultaneously so far.
The index of a terminated domain may be reused for a new domain. Use (Domain.self ()
:> int) instead for an identifier unique among all domains ever created by the program.
Since: 5.3

module DLS :
sig

Domain-local Storage
type 'a key
Type of a DLS key

val new_key : ?split_from_parent:('a -> 'a) -> (unit -> 'a) -> 'a key
new_key f returns a new key bound to initialiser f for accessing domain-local variables.
If split_from_parent is not provided, the value for a new domain will be computed
on-demand by the new domain: the first get call will call the initializer f and store that
value.
Warning. f may be called several times if another call to get occurs during
initialization on the same domain. Only the ’first’ value computed will be used, the
other now-useless values will be discarded. Your initialization function should support
this situation, or contain logic to detect this case and fail.
If split_from_parent is provided, spawning a domain will derive the child value (for
this key) from the parent value. This computation happens in the parent domain and it
always happens, regardless of whether the child domain will use it. If the splitting
function is expensive or requires child-side computation, consider using 'a Lazy.t key:

let init () = ...

Chapter 29. The standard library 631

let split_from_parent parent_value =

... parent-side computation ...;
lazy (
... child-side computation ...
)

let key = Domain.DLS.new_key ~split_from_parent init

let get () = Lazy.force (Domain.DLS.get key)

In this case a part of the computation happens on the child domain; in particular, it can
access parent_value concurrently with the parent domain, which may require explicit
synchronization to avoid data races.

val get : 'a key -> 'a

get k returns v if a value v is associated to the key k on the calling domain’s
domain-local state. Sets k’s value with its initialiser and returns it otherwise.

val set : 'a key -> 'a -> unit

set k v updates the calling domain’s domain-local state to associate the key k with
value v. It overwrites any previous values associated to k, which cannot be restored later.

end

29.15 Module Digest : Message digest.

This module provides functions to compute ’digests’, also known as ’hashes’, of arbitrary-length
strings or files. The supported hashing algorithms are BLAKE2 and MD5.

Basic functions
The functions in this section use the MD5 hash function to produce 128-bit digests (16 bytes). MD5
is not cryptographically secure. Hence, these functions should not be used for security-sensitive
applications. The BLAKE2 functions below are cryptographically secure.
type t = string
The type of digests: 16-byte strings.

val compare : t -> t -> int

The comparison function for 16-byte digests, with the same specification as compare[28.2] and
the implementation shared with String.compare[29.54]. Along with the type t, this function
compare allows the module Digest to be passed as argument to the functors Set.Make[29.50]
and Map.Make[29.34].
Since: 4.00
632

val equal : t -> t -> bool

The equal function for 16-byte digests.
Since: 4.03

val string : string -> t

Return the digest of the given string.

val bytes : bytes -> t

Return the digest of the given byte sequence.
Since: 4.02

val substring : string -> int -> int -> t

Digest.substring s ofs len returns the digest of the substring of s starting at index ofs
and containing len characters.

val subbytes : bytes -> int -> int -> t

Digest.subbytes s ofs len returns the digest of the subsequence of s starting at index
ofs and containing len bytes.
Since: 4.02

val channel : in_channel -> int -> t

If len is nonnegative, Digest.channel ic len reads len characters from channel ic and
returns their digest, or raises End_of_file if end-of-file is reached before len characters are
read. If len is negative, Digest.channel ic len reads all characters from ic until
end-of-file is reached and return their digest.

val file : string -> t

Return the digest of the file whose name is given.

val output : out_channel -> t -> unit

Write a digest on the given output channel.

val input : in_channel -> t

Read a digest from the given input channel.

val to_hex : t -> string

Return the printable hexadecimal representation of the given digest.
Raises Invalid_argument if the argument is not exactly 16 bytes.

val of_hex : string -> t

Convert a hexadecimal representation back into the corresponding digest.
Since: 5.2
Raises Invalid_argument if the argument is not exactly 32 hexadecimal characters.
Chapter 29. The standard library 633

val from_hex : string -> t

Same function as Digest.of_hex[29.15].
Since: 4.00

Generic interface
module type S =
sig
type t = string
The type of digests.

val hash_length : int

The length of digests, in bytes.

val compare : t -> t -> int

Compare two digests, with the same specification as compare[28.2].

val equal : t -> t -> bool

Test two digests for equality.

val string : string -> t

Return the digest of the given string.

val bytes : bytes -> t

Return the digest of the given byte sequence.

val substring : string -> int -> int -> t

substring s ofs len returns the digest of the substring of s starting at index ofs and
containing len characters.

val subbytes : bytes -> int -> int -> t

subbytes s ofs len returns the digest of the subsequence of s starting at index ofs
and containing len bytes.

val channel : in_channel -> int -> t

Read characters from the channel and return their digest. See Digest.channel[29.15]
for the full specification.

val file : string -> t

Return the digest of the file whose name is given.
634

val output : out_channel -> t -> unit

Write a digest on the given output channel.

val input : in_channel -> t

Read a digest from the given input channel.

val to_hex : t -> string

Return the printable hexadecimal representation of the given digest.
Raises Invalid_argument if the length of the argument is not hash_length,

val of_hex : string -> t

Convert a hexadecimal representation back into the corresponding digest.
Raises Invalid_argument if the length of the argument is not 2 * hash_length, or if
the arguments contains non-hexadecimal characters.

end

The signature for a hash function that produces digests of length hash_length from
character strings, byte arrays, and files.
Since: 5.2

Specific hash functions

module BLAKE128 :
S
BLAKE128 is the BLAKE2b hash function producing 128-bit (16-byte) digests. It is
cryptographically secure. However, the small size of the digests enables brute-force attacks in
2{^64} attempts.
Since: 5.2

module BLAKE256 :
S
BLAKE256 is the BLAKE2b hash function producing 256-bit (32-byte) digests. It is
cryptographically secure, and the digests are large enough to thwart brute-force attacks.
Since: 5.2

module BLAKE512 :
S
BLAKE512 is the BLAKE2b hash function producing 512-bit (64-byte) digests. It is
cryptographically secure, and the digests are large enough to thwart brute-force attacks.
Since: 5.2
Chapter 29. The standard library 635

module MD5 :
S
MD5 is the MD5 hash function. It produces 128-bit (16-byte) digests and is not
cryptographically secure at all. It should be used only for compatibility with earlier designs
that mandate the use of MD5.
Since: 5.2

29.16 Module Dynarray : Dynamic arrays.

The Array[29.2] module provide arrays of fixed length. Dynarray[29.16] provides arrays whose
length can change over time, by adding or removing elements at the end of the array.
This is typically used to accumulate elements whose number is not known in advance or changes
during computation, while also providing fast access to elements at arbitrary indices.

let dynarray_of_list li =
let arr = Dynarray.create () in
List.iter (fun v -> Dynarray.add_last arr v) li;
arr

The Buffer[29.7] module provides similar features, but it is specialized for accumulating
characters into a dynamically-resized string.
The Stack[29.52] module provides a last-in first-out data structure that can be easily implemented
on top of dynamic arrays.
Since: 5.2
Alert unsynchronized_access. Unsynchronized accesses to dynamic arrays are a programming
error.

Unsynchronized accesses
Concurrent accesses to dynamic arrays must be synchronized (for instance with a Mutex.t[29.37]).
Unsynchronized accesses to a dynamic array are a programming error that may lead to an invalid
dynamic array state, on which some operations would fail with an Invalid_argument exception.

Dynamic arrays
type !'a t
A dynamic array containing values of type 'a.
A dynamic array a provides constant-time get and set operations on indices between 0 and
Dynarray.length a - 1 included. Its Dynarray.length[29.16] may change over time by
adding or removing elements to the end of the array.
We say that an index into a dynarray a is valid if it is in 0 .. length a - 1 and invalid
otherwise.

val create : unit -> 'a t

636

create () is a new, empty array.

val make : int -> 'a -> 'a t

make n x is a new array of length n, filled with x.
Raises Invalid_argument if n < 0 or n > Sys.max_array_length.

val init : int -> (int -> 'a) -> 'a t

init n f is a new array a of length n, such that get a i is f i. In other words, the
elements of a are f 0, then f 1, then f 2. . . and f (n - 1) last, evaluated in that order.
This is similar to Array.init[29.2].
Raises Invalid_argument if n < 0 or n > Sys.max_array_length.

val get : 'a t -> int -> 'a

get a i is the i-th element of a, starting with index 0.
Raises Invalid_argument if the index is invalid

val set : 'a t -> int -> 'a -> unit

set a i x sets the i-th element of a to be x.
i must be a valid index. set does not add new elements to the array – see
Dynarray.add_last[29.16] to add an element.
Raises Invalid_argument if the index is invalid.

val length : 'a t -> int

length a is the number of elements in the array.

val is_empty : 'a t -> bool

is_empty a is true if a is empty, that is, if length a = 0.

val get_last : 'a t -> 'a

get_last a is the element of a at index length a - 1.
Raises Invalid_argument if a is empty.

val find_last : 'a t -> 'a option

find_last a is None if a is empty and Some (get_last a) otherwise.

val copy : 'a t -> 'a t

copy a is a shallow copy of a, a new array containing the same elements as a.
Chapter 29. The standard library 637

Adding elements
Note: all operations adding elements raise Invalid_argument if the length needs to grow beyond
Sys.max_array_length[29.56].
val add_last : 'a t -> 'a -> unit
add_last a x adds the element x at the end of the array a.

val append_array : 'a t -> 'a array -> unit

append_array a b adds all elements of b at the end of a, in the order they appear in b.
For example:

let a = Dynarray.of_list [1;2] in

Dynarray.append_array a [|3; 4|];
assert (Dynarray.to_list a = [1; 2; 3; 4])

val append_list : 'a t -> 'a list -> unit

Like Dynarray.append_array[29.16] but with a list.

val append : 'a t -> 'a t -> unit

append a b is like append_array a b, but b is itself a dynamic array instead of a fixed-size
array.
Warning: append a a is a programming error because it iterates on a and adds elements to it
at the same time – see the Iteration[29.16] section below. It fails with Invalid_argument. If
you really want to append a copy of a to itself, you can use Dynarray.append_array a
(Dynarray.to_array a) which copies a into a temporary array.

val append_seq : 'a t -> 'a Seq.t -> unit

Like Dynarray.append_array[29.16] but with a sequence.
Warning: append_seq a (to_seq_reentrant a) simultaneously traverses a and adds
element to it; the ordering of those operations is unspecified, and may result in an infinite
loop – the new elements may in turn be produced by to_seq_reentrant a and get added
again and again.

val append_iter : 'a t -> (('a -> unit) -> 'x -> unit) -> 'x -> unit
append_iter a iter x adds each element of x to the end of a. This is iter (add_last a)
x.
For example, append_iter a List.iter [1;2;3] would add elements 1, 2, and then 3 at
the end of a. append_iter a Queue.iter q adds elements from the queue q.

val blit :
src:'a t ->
src_pos:int -> dst:'a t -> dst_pos:int -> len:int -> unit
638

blit ~src ~src_pos ~dst ~dst_pos ~len copies len elements from a source dynarray
src, starting at index src_pos, to a destination dynarray dst, starting at index dst_pos. It
works correctly even if src and dst are the same array, and the source and destination
chunks overlap.
Unlike Array.blit[29.2], Dynarray.blit[29.16] can extend the destination array with new
elements: it is valid to call blit even when dst_pos + len is larger than length dst. The
only requirement is that dst_pos must be at most length dst (included), so that there is no
gap between the current elements and the blit region.
Raises Invalid_argument if src_pos and len do not designate a valid subarray of src, or if
dst_pos is strictly below 0 or strictly above length dst.

Removing elements
val pop_last_opt : 'a t -> 'a option
pop_last_opt a removes and returns the last element of a, or None if the array is empty.

val pop_last : 'a t -> 'a

pop_last a removes and returns the last element of a.
Raises Not_found on an empty array.

val remove_last : 'a t -> unit

remove_last a removes the last element of a, if any. It does nothing if a is empty.

val truncate : 'a t -> int -> unit

truncate a n truncates a to have at most n elements.
It removes elements whose index is greater or equal to n. It does nothing if n >= length a.
truncate a n is equivalent to:

if n < 0 then invalid_argument "...";

while length a > n do
remove_last a
done

Raises Invalid_argument if n < 0.

val clear : 'a t -> unit

clear a is truncate a 0, it removes all the elements of a.
Chapter 29. The standard library 639

Iteration
The iteration functions traverse the elements of a dynamic array. Traversals of a are computed in
increasing index order: from the element of index 0 to the element of index length a - 1.
It is a programming error to change the length of an array (by adding or removing elements)
during an iteration on the array. Any iteration function will fail with Invalid_argument if it detects
such a length change.
val iter : ('a -> unit) -> 'a t -> unit
iter f a calls f on each element of a.

val iteri : (int -> 'a -> unit) -> 'a t -> unit
iteri f a calls f i x for each x at index i in a.

val map : ('a -> 'b) -> 'a t -> 'b t

map f a is a new array of elements of the form f x for each element x of a.
For example, if the elements of a are x0, x1, x2, then the elements of b are f x0, f x1, f x2.

val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
mapi f a is a new array of elements of the form f i x for each element x of a at index i.
For example, if the elements of a are x0, x1, x2, then the elements of b are f 0 x0, f 1 x1, f
2 x2.

val fold_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a t -> 'acc
fold_left f acc a folds f over a in order, starting with accumulator acc.
For example, if the elements of a are x0, x1, then fold f acc a is

let acc = f acc x0 in

let acc = f acc x1 in
acc

val fold_right : ('a -> 'acc -> 'acc) -> 'a t -> 'acc -> 'acc
fold_right f a acc computes f x0 (f x1 (... (f xn acc) ...)) where x0, x1, . . ., xn
are the elements of a.

val filter : ('a -> bool) -> 'a t -> 'a t

filter f a is a new array of all the elements of a that satisfy f. In other words, it is an
array b such that, for each element x in a in order, x is added to b if f x is true.
For example, filter (fun x -> x >= 0) a is a new array of all non-negative elements of a,
in order.

val filter_map : ('a -> 'b option) -> 'a t -> 'b t
filter_map f a is a new array of elements y such that f x is Some y for an element x of a.
In others words, it is an array b such that, for each element x of a in order:
640

• if f x = Some y, then y is added to b,

• if f x = None, then no element is added to b.

For example, filter_map int_of_string_opt inputs returns a new array of integers read
from the strings in inputs, ignoring strings that cannot be converted to integers.

Dynarray scanning
val exists : ('a -> bool) -> 'a t -> bool
exists f a is true if some element of a satisfies f.
For example, if the elements of a are x0, x1, x2, then exists f a is f x0 || f x1 || f x2.

val for_all : ('a -> bool) -> 'a t -> bool

for_all f a is true if all elements of a satisfy f. This includes the case where a is empty.
For example, if the elements of a are x0, x1, then exists f a is f x0 && f x1 && f x2.

val mem : 'a -> 'a t -> bool

mem a set is true if and only if a is structurally equal to an element of set (i.e. there is an x
in set such that compare a x = 0).
Since: 5.3

val memq : 'a -> 'a t -> bool

Same as Dynarray.mem[29.16], but uses physical equality instead of structural equality to
compare array elements.
Since: 5.3

val find_opt : ('a -> bool) -> 'a t -> 'a option
find_opt f a returns the first element of the array a that satisfies the predicate f, or None if
there is no value that satisfies f in the array a.
Since: 5.3

val find_index : ('a -> bool) -> 'a t -> int option
find_index f a returns Some i, where i is the index of the first element of the array a that
satisfies f x, if there is such an element.
It returns None if there is no such element.
Since: 5.3

val find_map : ('a -> 'b option) -> 'a t -> 'b option
find_map f a applies f to the elements of a in order, and returns the first result of the form
Some v, or None if none exist.
Since: 5.3
Chapter 29. The standard library 641

val find_mapi : (int -> 'a -> 'b option) -> 'a t -> 'b option
Same as find_map, but the predicate is applied to the index of the element as first argument
(counting from 0), and the element itself as second argument.
Since: 5.3

Comparison functions
Comparison functions iterate over their arguments; it is a programming error to change their length
during the iteration, see the Iteration[29.16] section above.
val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool
equal eq a b holds when a and b have the same length, and for all indices i we have eq
(get a i) (get b i).
Since: 5.3

val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int
Provided the function cmp defines a preorder on elements, compare cmp a b compares first a
and b by their length, and then, if equal, by their elements according to the lexicographic
preorder.
For more details on comparison functions, see Array.sort[29.2].
Since: 5.3

Conversions to other data structures

Note: the of_* functions raise Invalid_argument if the length needs to grow beyond Sys.max_
array_length[29.56].
The to_* functions, except those specifically marked ”reentrant”, iterate on their dynarray
argument. In particular it is a programming error if the length of the dynarray changes during their
execution, and the conversion functions raise Invalid_argument if they observe such a change.
val of_array : 'a array -> 'a t
of_array arr returns a dynamic array corresponding to the fixed-sized array a. Operates in
O(n) time by making a copy.

val to_array : 'a t -> 'a array

to_array a returns a fixed-sized array corresponding to the dynamic array a. This always
allocate a new array and copies elements into it.

val of_list : 'a list -> 'a t

of_list l is the array containing the elements of l in the same order.

val to_list : 'a t -> 'a list

to_list a is a list with the elements contained in the array a.
642

val of_seq : 'a Seq.t -> 'a t

of_seq seq is an array containing the same elements as seq.
It traverses seq once and will terminate only if seq is finite.

val to_seq : 'a t -> 'a Seq.t

to_seq a is the sequence of elements get a 0, get a 1. . . get a (length a - 1).

val to_seq_reentrant : 'a t -> 'a Seq.t

to_seq_reentrant a is a reentrant variant of Dynarray.to_seq[29.16], in the sense that one
may still access its elements after the length of a has changed.
Demanding the i-th element of the resulting sequence (which can happen zero, one or several
times) will access the i-th element of a at the time of the demand. The sequence stops if a
has less than i elements at this point.

val to_seq_rev : 'a t -> 'a Seq.t

to_seq_rev a is the sequence of elements get a (l - 1), get a (l - 2). . . get a 0,
where l is length a at the time to_seq_rev is invoked.

val to_seq_rev_reentrant : 'a t -> 'a Seq.t

to_seq_rev_reentrant a is a reentrant variant of Dynarray.to_seq_rev[29.16], in the
sense that one may still access its elements after the length of a has changed.
Elements that have been removed from the array by the time they are demanded in the
sequence are skipped.

Advanced topics for performance

Backing array, capacity
Internally, a dynamic array uses a backing array (a fixed-size array as provided by the Array[29.2]
module) whose length is greater or equal to the length of the dynamic array. We define the capacity
of a dynamic array as the length of its backing array.
The capacity of a dynamic array is relevant in advanced scenarios, when reasoning about the
performance of dynamic array programs:
• The memory usage of a dynamic array is proportional to its capacity, rather than its length.
• When there is no empty space left at the end of the backing array, adding elements requires
allocating a new, larger backing array.
The implementation uses a standard exponential reallocation strategy which guarantees amortized
constant-time operation; in particular, the total capacity of all backing arrays allocated over the
lifetime of a dynamic array is at worst proportional to the total number of elements added.
In other words, users need not care about capacity and reallocations, and they will get reasonable
behavior by default. However, in some performance-sensitive scenarios the functions below can help
control memory usage or guarantee an optimal number of reallocations.
val capacity : 'a t -> int
Chapter 29. The standard library 643

capacity a is the length of a’s backing array.

val ensure_capacity : 'a t -> int -> unit

ensure_capacity a n makes sure that the capacity of a is at least n.
Raises Invalid_argument if the requested capacity is outside the range 0 ..
Sys.max_array_length.
An example would be to reimplement Dynarray.of_array[29.16] without using
Dynarray.init[29.16]:

let of_array arr =

let a = Dynarray.create () in
Dynarray.ensure_capacity a (Array.length arr);
Array.iter (fun v -> add_last a v) arr

Using ensure_capacity guarantees that at most one reallocation will take place, instead of
possibly several.
Without this ensure_capacity hint, the number of resizes would be logarithmic in the
length of arr, creating a constant-factor slowdown noticeable when arr is large.

val ensure_extra_capacity : 'a t -> int -> unit

ensure_extra_capacity a n is ensure_capacity a (length a + n), it makes sure that
a has room for n extra items.
Raises Invalid_argument if the total requested capacity is outside the range 0 ..
Sys.max_array_length.
A use case would be to implement Dynarray.append_array[29.16]:

let append_array a arr =

ensure_extra_capacity a (Array.length arr);
Array.iter (fun v -> add_last a v) arr

val fit_capacity : 'a t -> unit

fit_capacity a reallocates a backing array if necessary, so that the resulting capacity is
exactly length a, with no additional empty space at the end. This can be useful to make
sure there is no memory wasted on a long-lived array.
Note that calling fit_capacity breaks the amortized complexity guarantees provided by the
default reallocation strategy. Calling it repeatedly on an array may have quadratic
complexity, both in time and in total number of words allocated.
If you know that a dynamic array has reached its final length, which will remain fixed in the
future, it is sufficient to call to_array and only keep the resulting fixed-size array.
fit_capacity is useful when you need to keep a dynamic array for eventual future resizes.
644

val set_capacity : 'a t -> int -> unit

set_capacity a n reallocates a backing array if necessary, so that the resulting capacity is
exactly n. In particular, all elements of index n or greater are removed.
Like Dynarray.fit_capacity[29.16], this function breaks the amortized complexity
guarantees provided by the reallocation strategy. Calling it repeatedly on an array may have
quadratic complexity, both in time and in total number of words allocated.
This is an advanced function; in particular, Dynarray.ensure_capacity[29.16] should be
preferred to increase the capacity, as it preserves those amortized guarantees.
Raises Invalid_argument if n < 0.

val reset : 'a t -> unit

reset a clears a and replaces its backing array by an empty array.
It is equivalent to set_capacity a 0 or clear a; fit_capacity a.

No leaks: preservation of memory liveness

The user-provided values reachable from a dynamic array a are exactly the elements in the indices
0 to length a - 1. In particular, no user-provided values are ”leaked” by being present in the
backing array at index length a or later.

Code examples
Min-heaps for mutable priority queues
We can use dynamic arrays to implement a mutable priority queue. A priority queue provides a
function to add elements, and a function to extract the minimum element – according to some
comparison function.

(* We present our priority queues as a functor

parametrized on the comparison function. *)
module Heap (Elem : Map.OrderedType) : sig
type t
val create : unit -> t
val add : t -> Elem.t -> unit
val pop_min : t -> Elem.t option
end = struct

(* Our priority queues are implemented using the standard "min heap"
data structure, a dynamic array representing a binary tree. *)
type t = Elem.t Dynarray.t
let create = Dynarray.create

(* The node of index [i] has as children the nodes of index [2 * i + 1]

and [2 * i + 2] -- if they are valid indices in the dynarray. *)
Chapter 29. The standard library 645

let left_child i = 2 * i + 1
let right_child i = 2 * i + 2
let parent_node i = (i - 1) / 2

(* We use indexing operators for convenient notations. *)

let ( .!() ) = Dynarray.get
let ( .!()<- ) = Dynarray.set

(* Auxiliary functions to compare and swap two elements

in the dynamic array. *)
let order h i j =
Elem.compare h.!(i) h.!(j)

let swap h i j =
let v = h.!(i) in
h.!(i) <- h.!(j);
h.!(j) <- v

(* We say that a heap respects the "heap ordering" if the value of

each node is smaller than the value of its children. The
algorithm manipulates arrays that respect the heap algorithm,
except for one node whose value may be too small or too large.

The auxiliary functions [heap_up] and [heap_down] take

such a misplaced value, and move it "up" (respectively: "down")
the tree by permuting it with its parent value (respectively:
a child value) until the heap ordering is restored. *)

let rec heap_up h i =

if i = 0 then () else
let parent = parent_node i in
if order h i parent < 0 then
(swap h i parent; heap_up h parent)

and heap_down h ~len i =

let left, right = left_child i, right_child i in
if left >= len then () (* no child, stop *) else
let smallest =
if right >= len then left (* no right child *) else
if order h left right < 0 then left else right
in
if order h i smallest > 0 then
(swap h i smallest; heap_down h ~len smallest)

let add h s =
646

let i = Dynarray.length h in
Dynarray.add_last h s;
heap_up h i

let pop_min h =
if Dynarray.is_empty h then None
else begin
(* Standard trick: swap the 'best' value at index 0
with the last value of the array. *)
let last = Dynarray.length h - 1 in
swap h 0 last;
(* At this point [pop_last] returns the 'best' value,
and leaves a heap with one misplaced element at index [0]. *)
let best = Dynarray.pop_last h in
(* Restore the heap ordering -- does nothing if the heap is empty. *)
heap_down h ~len:last 0;
Some best
end
end

The production code from which this example was inspired includes logic to free the backing
array when the heap becomes empty, only in the case where the capacity is above a certain threshold.
This can be done by calling the following function from pop:

let shrink h =
if Dynarray.length h = 0 && Dynarray.capacity h > 1 lsl 18 then
Dynarray.reset h

The Heap functor can be used to implement a sorting function, by adding all elements into a
priority queue and then extracting them in order.

let heap_sort (type a) cmp li =

let module Heap = Heap(struct type t = a let compare = cmp end) in
let heap = Heap.create () in
List.iter (Heap.add heap) li;
List.map (fun _ -> Heap.pop_min heap |> Option.get) li

29.17 Module Effect

Alert unstable. The Effect interface may change in incompatible ways in the future.

Effects.
See ’Language extensions/Effect handlers’ section in the manual.
Chapter 29. The standard library 647

type 'a t = 'a eff = ..

The type of effects.

exception Unhandled : 'a t -> exn

Unhandled e is raised when effect e is performed and there is no handler for it.

exception Continuation_already_resumed
Exception raised when a continuation is continued or discontinued more than once.

val perform : 'a t -> 'a

perform e performs an effect e.
Raises Unhandled if there is no handler for e.

module Deep :
sig

Deep handlers
type ('a, 'b) continuation = ('a, 'b) continuation
('a,'b) continuation is a delimited continuation that expects a 'a value and returns
a 'b value.

val continue : ('a, 'b) continuation -> 'a -> 'b

continue k x resumes the continuation k by passing x to k.
Raises Continuation_already_resumed if the continuation has already been resumed.

val discontinue : ('a, 'b) continuation -> exn -> 'b

discontinue k e resumes the continuation k by raising the exception e in k.
Raises Continuation_already_resumed if the continuation has already been resumed.

val discontinue_with_backtrace :
('a, 'b) continuation ->
exn -> Printexc.raw_backtrace -> 'b
discontinue_with_backtrace k e bt resumes the continuation k by raising the
exception e in k using bt as the origin for the exception.
Raises Continuation_already_resumed if the continuation has already been resumed.

type ('a, 'b) handler =

{ retc : 'a -> 'b ;
exnc : exn -> 'b ;
effc : 'c. 'c Effect.t -> (('c, 'b) continuation -> 'b) option ;
}
648

('a,'b) handler is a handler record with three fields – retc is the value handler, exnc
handles exceptions, and effc handles the effects performed by the computation enclosed
by the handler.

val match_with : ('c -> 'a) -> 'c -> ('a, 'b) handler -> 'b
match_with f v h runs the computation f v in the handler h.

type 'a effect_handler =

{ effc : 'b. 'b Effect.t -> (('b, 'a) continuation -> 'a) option ;
}
'a effect_handler is a deep handler with an identity value handler fun x -> x and
an exception handler that raises any exception fun e -> raise e.

val try_with : ('b -> 'a) -> 'b -> 'a effect_handler -> 'a
try_with f v h runs the computation f v under the handler h.

val get_callstack : ('a, 'b) continuation -> int -> Printexc.raw_backtrace

get_callstack c n returns a description of the top of the call stack on the
continuation c, with at most n entries.

end

module Shallow :
sig
type ('a, 'b) continuation
('a,'b) continuation is a delimited continuation that expects a 'a value and returns
a 'b value.

val fiber : ('a -> 'b) -> ('a, 'b) continuation

fiber f constructs a continuation that runs the computation f.

type ('a, 'b) handler =

{ retc : 'a -> 'b ;
exnc : exn -> 'b ;
effc : 'c. 'c Effect.t -> (('c, 'a) continuation -> 'b) option ;
}
('a,'b) handler is a handler record with three fields – retc is the value handler, exnc
handles exceptions, and effc handles the effects performed by the computation enclosed
by the handler.

val continue_with : ('c, 'a) continuation ->

'c -> ('a, 'b) handler -> 'b
Chapter 29. The standard library 649

continue_with k v h resumes the continuation k with value v with the handler h.

Raises Continuation_already_resumed if the continuation has already been resumed.

val discontinue_with :
('c, 'a) continuation ->
exn -> ('a, 'b) handler -> 'b
discontinue_with k e h resumes the continuation k by raising the exception e with
the handler h.
Raises Continuation_already_resumed if the continuation has already been resumed.

val discontinue_with_backtrace :
('a, 'b) continuation ->
exn -> Printexc.raw_backtrace -> ('b, 'c) handler -> 'c
discontinue_with k e bt h resumes the continuation k by raising the exception e
with the handler h using the raw backtrace bt as the origin of the exception.
Raises Continuation_already_resumed if the continuation has already been resumed.

val get_callstack : ('a, 'b) continuation -> int -> Printexc.raw_backtrace

get_callstack c n returns a description of the top of the call stack on the
continuation c, with at most n entries.

end

29.18 Module Either : Either type.

Either is the simplest and most generic sum/variant type: a value of ('a, 'b) Either.t is either
a Left (v : 'a) or a Right (v : 'b).
It is a natural choice in the API of generic functions where values could fall in two different
cases, possibly at different types, without assigning a specific meaning to what each case should be.
For example:
List.partition_map:
('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list
If you are looking for a parametrized type where one alternative means success and the other
means failure, you should use the more specific type Result.t[29.47].
Since: 4.12

type ('a, 'b) t =

| Left of 'a
| Right of 'b
A value of ('a, 'b) Either.t contains either a value of 'a or a value of 'b

val left : 'a -> ('a, 'b) t

650

left v is Left v.

val right : 'b -> ('a, 'b) t

right v is Right v.

val is_left : ('a, 'b) t -> bool

is_left (Left v) is true, is_left (Right v) is false.

val is_right : ('a, 'b) t -> bool

is_right (Left v) is false, is_right (Right v) is true.

val find_left : ('a, 'b) t -> 'a option

find_left (Left v) is Some v, find_left (Right _) is None

val find_right : ('a, 'b) t -> 'b option

find_right (Right v) is Some v, find_right (Left _) is None

val map_left : ('a1 -> 'a2) -> ('a1, 'b) t -> ('a2, 'b) t
map_left f e is Left (f v) if e is Left v and e if e is Right _.

val map_right : ('b1 -> 'b2) -> ('a, 'b1) t -> ('a, 'b2) t
map_right f e is Right (f v) if e is Right v and e if e is Left _.

val map :
left:('a1 -> 'a2) ->
right:('b1 -> 'b2) -> ('a1, 'b1) t -> ('a2, 'b2) t
map ~left ~right (Left v) is Left (left v), map ~left ~right (Right v) is Right
(right v).

val fold : left:('a -> 'c) -> right:('b -> 'c) -> ('a, 'b) t -> 'c
fold ~left ~right (Left v) is left v, and fold ~left ~right (Right v) is right v.

val iter : left:('a -> unit) -> right:('b -> unit) -> ('a, 'b) t -> unit
iter ~left ~right (Left v) is left v, and iter ~left ~right (Right v) is right v.

val for_all : left:('a -> bool) -> right:('b -> bool) -> ('a, 'b) t -> bool
for_all ~left ~right (Left v) is left v, and for_all ~left ~right (Right v) is
right v.

val equal :
left:('a -> 'a -> bool) ->
right:('b -> 'b -> bool) -> ('a, 'b) t -> ('a, 'b) t -> bool
equal ~left ~right e0 e1 tests equality of e0 and e1 using left and right to
respectively compare values wrapped by Left _ and Right _.
Chapter 29. The standard library 651

val compare :
left:('a -> 'a -> int) ->
right:('b -> 'b -> int) -> ('a, 'b) t -> ('a, 'b) t -> int
compare ~left ~right e0 e1 totally orders e0 and e1 using left and right to
respectively compare values wrapped by Left _ and Right _. Left _ values are smaller
than Right _ values.

29.19 Module Ephemeron : Ephemerons and weak hash tables.

Ephemerons and weak hash tables are useful when one wants to cache or memorize the computation
of a function, as long as the arguments and the function are used, without creating memory leaks
by continuously keeping old computation results that are not useful anymore because one argument
or the function is freed. An implementation using Hashtbl.t[29.25] is not suitable because all
associations would keep the arguments and the result in memory.
Ephemerons can also be used for ”adding” a field to an arbitrary boxed OCaml value: you can
attach some information to a value created by an external library without memory leaks.
Ephemerons hold some keys and one or no data. They are all boxed OCaml values. The keys of
an ephemeron have the same behavior as weak pointers according to the garbage collector. In fact
OCaml weak pointers are implemented as ephemerons without data.
The keys and data of an ephemeron are said to be full if they point to a value, or empty if the
value has never been set, has been unset, or was erased by the GC. In the function that accesses the
keys or data these two states are represented by the option type.
The data is considered by the garbage collector alive if all the full keys are alive and if the
ephemeron is alive. When one of the keys is not considered alive anymore by the GC, the data is
emptied from the ephemeron. The data could be alive for another reason and in that case the GC
will not free it, but the ephemeron will not hold the data anymore.
The ephemerons complicate the notion of liveness of values, because it is not anymore an
equivalence with the reachability from root value by usual pointers (not weak and not ephemerons).
With ephemerons the notion of liveness is constructed by the least fixpoint of: A value is alive if:

• it is a root value

• it is reachable from alive value by usual pointers

• it is the data of an alive ephemeron with all its full keys alive

Notes:

• All the types defined in this module cannot be marshaled using output_value[28.2] or the
functions of the Marshal[29.35] module.

Ephemerons are defined in a language agnostic way in this paper: B. Hayes, Ephemerons: A
New Finalization Mechanism, OOPSLA’97
Since: 4.03
Alert unsynchronized_access. Unsynchronized accesses to weak hash tables are a program-
ming error.
652

Unsynchronized accesses
Unsynchronized accesses to a weak hash table may lead to an invalid weak hash table state.
Thus, concurrent accesses to a buffer must be synchronized (for instance with a Mutex.t[29.37]).
module type S =
sig
Propose the same interface as usual hash table. However since the bindings are weak, even if
mem h k is true, a subsequent find h k may raise Not_found because the garbage collector
can run between the two.
type key
type !'a t
val create : int -> 'a t
val clear : 'a t -> unit
val reset : 'a t -> unit
val copy : 'a t -> 'a t
val add : 'a t -> key -> 'a -> unit
val remove : 'a t -> key -> unit
val find : 'a t -> key -> 'a
val find_opt : 'a t -> key -> 'a option
val find_all : 'a t -> key -> 'a list
val replace : 'a t -> key -> 'a -> unit
val mem : 'a t -> key -> bool
val length : 'a t -> int
val stats : 'a t -> Hashtbl.statistics
val add_seq : 'a t -> (key * 'a) Seq.t -> unit
val replace_seq : 'a t -> (key * 'a) Seq.t -> unit
val of_seq : (key * 'a) Seq.t -> 'a t
val clean : 'a t -> unit
remove all dead bindings. Done automatically during automatic resizing.

val stats_alive : 'a t -> Hashtbl.statistics

same as Hashtbl.SeededS.stats[29.25] but only count the alive bindings

end
The output signature of the functors Ephemeron.K1.Make[29.19] and
Ephemeron.K2.Make[29.19]. These hash tables are weak in the keys. If all the keys of a
binding are alive the binding is kept, but if one of the keys of the binding is dead then the
binding is removed.

module type SeededS =

sig
Chapter 29. The standard library 653

type key
type !'a t
val create : ?random:bool -> int -> 'a t
val clear : 'a t -> unit
val reset : 'a t -> unit
val copy : 'a t -> 'a t
val add : 'a t -> key -> 'a -> unit
val remove : 'a t -> key -> unit
val find : 'a t -> key -> 'a
val find_opt : 'a t -> key -> 'a option
val find_all : 'a t -> key -> 'a list
val replace : 'a t -> key -> 'a -> unit
val mem : 'a t -> key -> bool
val length : 'a t -> int
val stats : 'a t -> Hashtbl.statistics
val add_seq : 'a t -> (key * 'a) Seq.t -> unit
val replace_seq : 'a t -> (key * 'a) Seq.t -> unit
val of_seq : (key * 'a) Seq.t -> 'a t
val clean : 'a t -> unit
remove all dead bindings. Done automatically during automatic resizing.

val stats_alive : 'a t -> Hashtbl.statistics

same as Hashtbl.SeededS.stats[29.25] but only count the alive bindings

end
The output signature of the functors Ephemeron.K1.MakeSeeded[29.19] and
Ephemeron.K2.MakeSeeded[29.19].

module K1 :
sig
type ('k, 'd) t
an ephemeron with one key

val make : 'k -> 'd -> ('k, 'd) t

Ephemeron.K1.make k d creates an ephemeron with key k and data d.

val query : ('k, 'd) t -> 'k -> 'd option

Ephemeron.K1.query eph key returns Some x (where x is the ephemeron’s data) if key
is physically equal to eph’s key, and None if eph is empty or key is not equal to eph’s key.
654

module Make :
functor (H : Hashtbl.HashedType) -> Ephemeron.S with type key = H.t
Functor building an implementation of a weak hash table

module MakeSeeded :
functor (H : Hashtbl.SeededHashedType) -> Ephemeron.SeededS with type key =
H.t
Functor building an implementation of a weak hash table. The seed is similar to the one
of Hashtbl.MakeSeeded[29.25].

module Bucket :
sig

type ('k, 'd) t

A bucket is a mutable ”list” of ephemerons.

val make : unit -> ('k, 'd) t

Create a new bucket.

val add : ('k, 'd) t -> 'k -> 'd -> unit
Add an ephemeron to the bucket.

val remove : ('k, 'd) t -> 'k -> unit

remove b k removes from b the most-recently added ephemeron with key k, or does
nothing if there is no such ephemeron.

val find : ('k, 'd) t -> 'k -> 'd option

Returns the data of the most-recently added ephemeron with the given key, or None
if there is no such ephemeron.

val length : ('k, 'd) t -> int

Returns an upper bound on the length of the bucket.

val clear : ('k, 'd) t -> unit

Remove all ephemerons from the bucket.

end

end

Ephemerons with one key.

module K2 :
sig
type ('k1, 'k2, 'd) t
Chapter 29. The standard library 655

an ephemeron with two keys

val make : 'k1 -> 'k2 -> 'd -> ('k1, 'k2, 'd) t
Same as Ephemeron.K1.make[29.19]

val query : ('k1, 'k2, 'd) t -> 'k1 -> 'k2 -> 'd option
Same as Ephemeron.K1.query[29.19]

module Make :
functor (H1 : Hashtbl.HashedType) -> functor (H2 : Hashtbl.HashedType) ->
Ephemeron.S with type key = H1.t * H2.t
Functor building an implementation of a weak hash table

module MakeSeeded :
functor (H1 : Hashtbl.SeededHashedType) -> functor (H2 : Hashtbl.SeededHashedType)
-> Ephemeron.SeededS with type key = H1.t * H2.t
Functor building an implementation of a weak hash table. The seed is similar to the one
of Hashtbl.MakeSeeded[29.25].

module Bucket :
sig

type ('k1, 'k2, 'd) t

A bucket is a mutable ”list” of ephemerons.

val make : unit -> ('k1, 'k2, 'd) t

Create a new bucket.

val add : ('k1, 'k2, 'd) t -> 'k1 -> 'k2 -> 'd -> unit
Add an ephemeron to the bucket.

val remove : ('k1, 'k2, 'd) t -> 'k1 -> 'k2 -> unit
remove b k1 k2 removes from b the most-recently added ephemeron with keys k1
and k2, or does nothing if there is no such ephemeron.

val find : ('k1, 'k2, 'd) t -> 'k1 -> 'k2 -> 'd option
Returns the data of the most-recently added ephemeron with the given keys, or
None if there is no such ephemeron.

val length : ('k1, 'k2, 'd) t -> int

Returns an upper bound on the length of the bucket.

val clear : ('k1, 'k2, 'd) t -> unit

656

Remove all ephemerons from the bucket.

end

end

Ephemerons with two keys.

module Kn :
sig
type ('k, 'd) t
an ephemeron with an arbitrary number of keys of the same type

val make : 'k array -> 'd -> ('k, 'd) t

Same as Ephemeron.K1.make[29.19]

val query : ('k, 'd) t -> 'k array -> 'd option
Same as Ephemeron.K1.query[29.19]

module Make :
functor (H : Hashtbl.HashedType) -> Ephemeron.S with type key = H.t array
Functor building an implementation of a weak hash table

module MakeSeeded :
functor (H : Hashtbl.SeededHashedType) -> Ephemeron.SeededS with type key =
H.t array
Functor building an implementation of a weak hash table. The seed is similar to the one
of Hashtbl.MakeSeeded[29.25].

module Bucket :
sig

type ('k, 'd) t

A bucket is a mutable ”list” of ephemerons.

val make : unit -> ('k, 'd) t

Create a new bucket.

val add : ('k, 'd) t -> 'k array -> 'd -> unit
Add an ephemeron to the bucket.

val remove : ('k, 'd) t -> 'k array -> unit

remove b k removes from b the most-recently added ephemeron with keys k, or
does nothing if there is no such ephemeron.
Chapter 29. The standard library 657

val find : ('k, 'd) t -> 'k array -> 'd option
Returns the data of the most-recently added ephemeron with the given keys, or
None if there is no such ephemeron.

val length : ('k, 'd) t -> int

Returns an upper bound on the length of the bucket.

val clear : ('k, 'd) t -> unit

Remove all ephemerons from the bucket.

end

end

Ephemerons with arbitrary number of keys of the same type.

29.20 Module Filename : Operations on file names.

val current_dir_name : string

The conventional name for the current directory (e.g. . in Unix).

val parent_dir_name : string

The conventional name for the parent of the current directory (e.g. .. in Unix).

val dir_sep : string

The directory separator (e.g. / in Unix).
Since: 3.11.2

val concat : string -> string -> string

concat dir file returns a file name that designates file file in directory dir.

val is_relative : string -> bool

Return true if the file name is relative to the current directory, false if it is absolute (i.e. in
Unix, starts with /).

val is_implicit : string -> bool

Return true if the file name is relative and does not start with an explicit reference to the
current directory (./ or ../ in Unix), false if it starts with an explicit reference to the root
directory or the current directory.

val check_suffix : string -> string -> bool

658

check_suffix name suff returns true if the filename name ends with the suffix suff.
Under Windows ports (including Cygwin), comparison is case-insensitive, relying on
String.lowercase_ascii. Note that this does not match exactly the interpretation of
case-insensitive filename equivalence from Windows.

val chop_suffix : string -> string -> string

chop_suffix name suff removes the suffix suff from the filename name.
Raises Invalid_argument if name does not end with the suffix suff.

val chop_suffix_opt : suffix:string -> string -> string option

chop_suffix_opt ~suffix filename removes the suffix from the filename if possible, or
returns None if the filename does not end with the suffix.
Under Windows ports (including Cygwin), comparison is case-insensitive, relying on
String.lowercase_ascii. Note that this does not match exactly the interpretation of
case-insensitive filename equivalence from Windows.
Since: 4.08

val extension : string -> string

extension name is the shortest suffix ext of name0 where:

• name0 is the longest suffix of name that does not contain a directory separator;
• ext starts with a period;
• ext is preceded by at least one non-period character in name0.

If such a suffix does not exist, extension name is the empty string.
Since: 4.04

val remove_extension : string -> string

Return the given file name without its extension, as defined in Filename.extension[29.20].
If the extension is empty, the function returns the given file name.
The following invariant holds for any file name s:
remove_extension s ^ extension s = s
Since: 4.04

val chop_extension : string -> string

Same as Filename.remove_extension[29.20], but raise Invalid_argument if the given
name has an empty extension.

val basename : string -> string

Chapter 29. The standard library 659

Split a file name into directory name / base file name. If name is a valid file name, then
concat (dirname name) (basename name) returns a file name which is equivalent to name.
Moreover, after setting the current directory to dirname name (with Sys.chdir[29.56]),
references to basename name (which is a relative file name) designate the same file as name
before the call to Sys.chdir[29.56].
This function conforms to the specification of POSIX.1-2008 for the basename utility.

val dirname : string -> string

See Filename.basename[29.20]. This function conforms to the specification of POSIX.1-2008
for the dirname utility.

val null : string

null is "/dev/null" on POSIX and "NUL" on Windows. It represents a file on the OS that
discards all writes and returns end of file on reads.
Since: 4.10

val temp_file : ?temp_dir:string -> string -> string -> string

temp_file prefix suffix returns the name of a fresh temporary file in the temporary
directory. The base name of the temporary file is formed by concatenating prefix, then a
suitably chosen integer number, then suffix. The optional argument temp_dir indicates the
temporary directory to use, defaulting to the current result of
Filename.get_temp_dir_name[29.20]. The temporary file is created empty, with permissions
0o600 (readable and writable only by the file owner). The file is guaranteed to be different
from any other file that existed when temp_file was called.
Before 3.11.2 no ?temp_dir optional argument
Raises Sys_error if the file could not be created.

val open_temp_file :
?mode:open_flag list ->
?perms:int ->
?temp_dir:string -> string -> string -> string * out_channel
Same as Filename.temp_file[29.20], but returns both the name of a fresh temporary file,
and an output channel opened (atomically) on this file. This function is more secure than
temp_file: there is no risk that the temporary file will be modified (e.g. replaced by a
symbolic link) before the program opens it. The optional argument mode is a list of additional
flags to control the opening of the file. It can contain one or several of Open_append,
Open_binary, and Open_text. The default is [Open_text] (open in text mode). The file is
created with permissions perms (defaults to readable and writable only by the file owner,
0o600).
Before 4.03 no ?perms optional argument
Before 3.11.2 no ?temp_dir optional argument
Raises Sys_error if the file could not be opened.
660

val temp_dir : ?temp_dir:string -> ?perms:int -> string -> string -> string
temp_dir prefix suffix creates and returns the name of a fresh temporary directory with
permissions perms (defaults to 0o700) inside temp_dir. The base name of the temporary
directory is formed by concatenating prefix, then a suitably chosen integer number, then
suffix. The optional argument temp_dir indicates the temporary directory to use,
defaulting to the current result of Filename.get_temp_dir_name[29.20]. The temporary
directory is created empty, with permissions 0o700 (readable, writable, and searchable only
by the file owner). The directory is guaranteed to be different from any other directory that
existed when temp_dir was called.
If temp_dir does not exist, this function does not create it. Instead, it raises Sys_error.
Since: 5.1
Raises Sys_error if the directory could not be created.

val get_temp_dir_name : unit -> string

The name of the temporary directory: Under Unix, the value of the TMPDIR environment
variable, or ”/tmp” if the variable is not set. Under Windows, the value of the TEMP
environment variable, or ”.” if the variable is not set. The temporary directory can be changed
with Filename.set_temp_dir_name[29.20].
Since: 4.00

val set_temp_dir_name : string -> unit

Change the temporary directory returned by Filename.get_temp_dir_name[29.20] and used
by Filename.temp_file[29.20] and Filename.open_temp_file[29.20]. The temporary
directory is a domain-local value which is inherited by child domains.
Since: 4.00

val quote : string -> string

Return a quoted version of a file name, suitable for use as one argument in a command line,
escaping all meta-characters. Warning: under Windows, the output is only suitable for use
with programs that follow the standard Windows quoting conventions.

val quote_command :
string ->
?stdin:string -> ?stdout:string -> ?stderr:string -> string list -> string
quote_command cmd args returns a quoted command line, suitable for use as an argument
to Sys.command[29.56], Unix.system[31.1], and the Unix.open_process[31.1] functions.
The string cmd is the command to call. The list args is the list of arguments to pass to this
command. It can be empty.
The optional arguments ?stdin and ?stdout and ?stderr are file names used to redirect the
standard input, the standard output, or the standard error of the command. If ~stdin:f is
given, a redirection < f is performed and the standard input of the command reads from file
f. If ~stdout:f is given, a redirection > f is performed and the standard output of the
Chapter 29. The standard library 661

command is written to file f. If ~stderr:f is given, a redirection 2> f is performed and the
standard error of the command is written to file f. If both ~stdout:f and ~stderr:f are
given, with the exact same file name f, a 2>&1 redirection is performed so that the standard
output and the standard error of the command are interleaved and redirected to the same file
f.
Under Unix and Cygwin, the command, the arguments, and the redirections if any are quoted
using Filename.quote[29.20], then concatenated. Under Win32, additional quoting is
performed as required by the cmd.exe shell that is called by Sys.command[29.56].
Since: 4.10
Raises Failure if the command cannot be escaped on the current platform.

29.21 Module Float : Floating-point arithmetic.

OCaml’s floating-point numbers follow the IEEE 754 standard, using double precision (64 bits)
numbers. Floating-point operations never raise an exception on overflow, underflow, division by zero,
etc. Instead, special IEEE numbers are returned as appropriate, such as infinity for 1.0 /. 0.0,
neg_infinity for -1.0 /. 0.0, and nan (’not a number’) for 0.0 /. 0.0. These special numbers
then propagate through floating-point computations as expected: for instance, 1.0 /. infinity is
0.0, basic arithmetic operations (+., -., *., /.) with nan as an argument return nan, . . .
Since: 4.07

val zero : float

The floating point 0.
Since: 4.08

val one : float

The floating-point 1.
Since: 4.08

val minus_one : float

The floating-point -1.
Since: 4.08

val neg : float -> float

Unary negation.

val add : float -> float -> float

Floating-point addition.

val sub : float -> float -> float

Floating-point subtraction.
662

val mul : float -> float -> float

Floating-point multiplication.

val div : float -> float -> float

Floating-point division.

val fma : float -> float -> float -> float

fma x y z returns x * y + z, with a best effort for computing this expression with a single
rounding, using either hardware instructions (providing full IEEE compliance) or a software
emulation.
On 64-bit Cygwin, 64-bit mingw-w64 and MSVC 2017 and earlier, this function may be
emulated owing to known bugs on limitations on these platforms. Note: since software
emulation of the fma is costly, make sure that you are using hardware fma support if
performance matters.
Since: 4.08

val rem : float -> float -> float

rem a b returns the remainder of a with respect to b. The returned value is a -. n *. b,
where n is the quotient a /. b rounded towards zero to an integer.

val succ : float -> float

succ x returns the floating point number right after x i.e., the smallest floating-point number
greater than x. See also Float.next_after[29.21].
Since: 4.08

val pred : float -> float

pred x returns the floating-point number right before x i.e., the greatest floating-point
number smaller than x. See also Float.next_after[29.21].
Since: 4.08

val abs : float -> float

abs f returns the absolute value of f.

val infinity : float

Positive infinity.

val neg_infinity : float

Negative infinity.

val nan : float

Chapter 29. The standard library 663

A special floating-point value denoting the result of an undefined operation such as 0.0 /.
0.0. Stands for ’not a number’. Any floating-point operation with nan as argument returns
nan as result, unless otherwise specified in IEEE 754 standard. As for floating-point
comparisons, =, <, <=, > and >= return false and <> returns true if one or both of their
arguments is nan.
nan is quiet_nan since 5.1; it was a signaling NaN before.

val signaling_nan : float

Signaling NaN. The corresponding signals do not raise OCaml exception, but the value can be
useful for interoperability with C libraries.
Since: 5.1

val quiet_nan : float

Quiet NaN.
Since: 5.1

val pi : float
The constant pi.

val max_float : float

The largest positive finite value of type float.

val min_float : float

The smallest positive, non-zero, non-denormalized value of type float.

val epsilon : float

The difference between 1.0 and the smallest exactly representable floating-point number
greater than 1.0.

val is_finite : float -> bool

is_finite x is true if and only if x is finite i.e., not infinite and not Float.nan[29.21].
Since: 4.08

val is_infinite : float -> bool

is_infinite x is true if and only if x is Float.infinity[29.21] or
Float.neg_infinity[29.21].
Since: 4.08

val is_nan : float -> bool

is_nan x is true if and only if x is not a number (see Float.nan[29.21]).
Since: 4.08

val is_integer : float -> bool

664

is_integer x is true if and only if x is an integer.

Since: 4.08

val of_int : int -> float

Convert an integer to floating-point.

val to_int : float -> int

Truncate the given floating-point number to an integer. The result is unspecified if the
argument is nan or falls outside the range of representable integers.

val of_string : string -> float

Convert the given string to a float. The string is read in decimal (by default) or in
hexadecimal (marked by 0x or 0X). The format of decimal floating-point numbers is [-]
dd.ddd (e|E) [+|-] dd , where d stands for a decimal digit. The format of hexadecimal
floating-point numbers is [-] 0(x|X) hh.hhh (p|P) [+|-] dd , where h stands for an
hexadecimal digit and d for a decimal digit. In both cases, at least one of the integer and
fractional parts must be given; the exponent part is optional. The _ (underscore) character
can appear anywhere in the string and is ignored. Depending on the execution platforms,
other representations of floating-point numbers can be accepted, but should not be relied
upon.
Raises Failure if the given string is not a valid representation of a float.

val of_string_opt : string -> float option

Same as of_string, but returns None instead of raising.

val to_string : float -> string

Return a string representation of a floating-point number.
This conversion can involve a loss of precision. For greater control over the manner in which
the number is printed, see Printf[29.44].
This function is an alias for string_of_float[28.2].

type fpclass = fpclass =

| FP_normal
Normal number, none of the below
| FP_subnormal
Number very close to 0.0, has reduced precision
| FP_zero
Number is 0.0 or -0.0
| FP_infinite
Number is positive or negative infinity
| FP_nan
Chapter 29. The standard library 665

Not a number: result of an undefined operation

The five classes of floating-point numbers, as determined by the
Float.classify_float[29.21] function.

val classify_float : float -> fpclass

Return the class of the given floating-point number: normal, subnormal, zero, infinite, or not
a number.

val pow : float -> float -> float

Exponentiation.

val sqrt : float -> float

Square root.

val cbrt : float -> float

Cube root.
Since: 4.13

val exp : float -> float

Exponential.

val exp2 : float -> float

Base 2 exponential function.
Since: 4.13

val log : float -> float

Natural logarithm.

val log10 : float -> float

Base 10 logarithm.

val log2 : float -> float

Base 2 logarithm.
Since: 4.13

val expm1 : float -> float

expm1 x computes exp x -. 1.0, giving numerically-accurate results even if x is close to
0.0.

val log1p : float -> float

log1p x computes log(1.0 +. x) (natural logarithm), giving numerically-accurate results
even if x is close to 0.0.
666

val cos : float -> float

Cosine. Argument is in radians.

val sin : float -> float

Sine. Argument is in radians.

val tan : float -> float

Tangent. Argument is in radians.

val acos : float -> float

Arc cosine. The argument must fall within the range [-1.0, 1.0]. Result is in radians and
is between 0.0 and pi.

val asin : float -> float

Arc sine. The argument must fall within the range [-1.0, 1.0]. Result is in radians and is
between -pi/2 and pi/2.

val atan : float -> float

Arc tangent. Result is in radians and is between -pi/2 and pi/2.

val atan2 : float -> float -> float

atan2 y x returns the arc tangent of y /. x. The signs of x and y are used to determine the
quadrant of the result. Result is in radians and is between -pi and pi.

val hypot : float -> float -> float

hypot x y returns sqrt(x *. x +. y *. y), that is, the length of the hypotenuse of a
right-angled triangle with sides of length x and y, or, equivalently, the distance of the point
(x,y) to origin. If one of x or y is infinite, returns infinity even if the other is nan.

val cosh : float -> float

Hyperbolic cosine. Argument is in radians.

val sinh : float -> float

Hyperbolic sine. Argument is in radians.

val tanh : float -> float

Hyperbolic tangent. Argument is in radians.

val acosh : float -> float

Hyperbolic arc cosine. The argument must fall within the range [1.0, inf]. Result is in
radians and is between 0.0 and inf.
Since: 4.13

val asinh : float -> float

Chapter 29. The standard library 667

Hyperbolic arc sine. The argument and result range over the entire real line. Result is in
radians.
Since: 4.13

val atanh : float -> float

Hyperbolic arc tangent. The argument must fall within the range [-1.0, 1.0]. Result is in
radians and ranges over the entire real line.
Since: 4.13

val erf : float -> float

Error function. The argument ranges over the entire real line. The result is always within
[-1.0, 1.0].
Since: 4.13

val erfc : float -> float

Complementary error function (erfc x = 1 - erf x). The argument ranges over the entire
real line. The result is always within [0.0, 2.0].
Since: 4.13

val trunc : float -> float

trunc x rounds x to the nearest integer whose absolute value is less than or equal to x.
Since: 4.08

val round : float -> float

round x rounds x to the nearest integer with ties (fractional values of 0.5) rounded away
from zero, regardless of the current rounding direction. If x is an integer, +0., -0., nan, or
infinite, x itself is returned.
On 64-bit mingw-w64, this function may be emulated owing to a bug in the C runtime library
(CRT) on this platform.
Since: 4.08

val ceil : float -> float

Round above to an integer value. ceil f returns the least integer value greater than or equal
to f. The result is returned as a float.

val floor : float -> float

Round below to an integer value. floor f returns the greatest integer value less than or
equal to f. The result is returned as a float.

val next_after : float -> float -> float

668

next_after x y returns the next representable floating-point value following x in the

direction of y. More precisely, if y is greater (resp. less) than x, it returns the smallest (resp.
largest) representable number greater (resp. less) than x. If x equals y, the function returns y.
If x or y is nan, a nan is returned. Note that next_after max_float infinity =
infinity and that next_after 0. infinity is the smallest denormalized positive number.
If x is the smallest denormalized positive number, next_after x 0. = 0.
Since: 4.08

val copy_sign : float -> float -> float

copy_sign x y returns a float whose absolute value is that of x and whose sign is that of y.
If x is nan, returns nan. If y is nan, returns either x or -. x, but it is not specified which.

val sign_bit : float -> bool

sign_bit x is true if and only if the sign bit of x is set. For example sign_bit 1. and
signbit 0. are false while sign_bit (-1.) and sign_bit (-0.) are true.
Since: 4.08

val frexp : float -> float * int

frexp f returns the pair of the significant and the exponent of f. When f is zero, the
significant x and the exponent n of f are equal to zero. When f is non-zero, they are defined
by f = x *. 2 ** n and 0.5 <= x < 1.0.

val ldexp : float -> int -> float

ldexp x n returns x *. 2 ** n.

val modf : float -> float * float

modf f returns the pair of the fractional and integral part of f.

type t = float
An alias for the type of floating-point numbers.

val compare : t -> t -> int

compare x y returns 0 if x is equal to y, a negative integer if x is less than y, and a positive
integer if x is greater than y. compare treats nan as equal to itself and less than any other
float value. This treatment of nan ensures that compare defines a total ordering relation.

val equal : t -> t -> bool

The equal function for floating-point numbers, compared using Float.compare[29.21].

val min : t -> t -> t

min x y returns the minimum of x and y. It returns nan when x or y is nan. Moreover min
(-0.) (+0.) = -0.
Since: 4.08
Chapter 29. The standard library 669

val max : float -> float -> float

max x y returns the maximum of x and y. It returns nan when x or y is nan. Moreover max
(-0.) (+0.) = +0.
Since: 4.08

val min_max : float -> float -> float * float

min_max x y is (min x y, max x y), just more efficient.
Since: 4.08

val min_num : t -> t -> t

min_num x y returns the minimum of x and y treating nan as missing values. If both x and y
are nan, nan is returned. Moreover min_num (-0.) (+0.) = -0.
Since: 4.08

val max_num : t -> t -> t

max_num x y returns the maximum of x and y treating nan as missing values. If both x and
y are nan nan is returned. Moreover max_num (-0.) (+0.) = +0.
Since: 4.08

val min_max_num : float -> float -> float * float

min_max_num x y is (min_num x y, max_num x y), just more efficient. Note that in
particular min_max_num x nan = (x, x) and min_max_num nan y = (y, y).
Since: 4.08

val seeded_hash : int -> t -> int

A seeded hash function for floats, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.1

val hash : t -> int

An unseeded hash function for floats, with the same output value as Hashtbl.hash[29.25].
This function allows this module to be passed as argument to the functor
Hashtbl.Make[29.25].

module Array :
sig
type t = floatarray
The type of float arrays with packed representation.
Since: 4.08

val length : t -> int

670

Return the length (number of elements) of the given floatarray.

val get : t -> int -> float

get a n returns the element number n of floatarray a.
Raises Invalid_argument if n is outside the range 0 to (length a - 1).

val set : t -> int -> float -> unit

set a n x modifies floatarray a in place, replacing element number n with x.
Raises Invalid_argument if n is outside the range 0 to (length a - 1).

val make : int -> float -> t

make n x returns a fresh floatarray of length n, initialized with x.
Raises Invalid_argument if n < 0 or n > Sys.max_floatarray_length.

val create : int -> t

create n returns a fresh floatarray of length n, with uninitialized data.
Raises Invalid_argument if n < 0 or n > Sys.max_floatarray_length.

val init : int -> (int -> float) -> t

init n f returns a fresh floatarray of length n, with element number i initialized to the
result of f i. In other terms, init n f tabulates the results of f applied to the integers
0 to n-1.
Raises Invalid_argument if n < 0 or n > Sys.max_floatarray_length.

val make_matrix : int -> int -> float -> t array

make_matrix dimx dimy e returns a two-dimensional array (an array of arrays) with
first dimension dimx and second dimension dimy, where all elements are initialized with
e.
Since: 5.2
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_floatarray_length[29.56].

val init_matrix : int -> int -> (int -> int -> float) -> t array
init_matrix dimx dimy f returns a two-dimensional array (an array of arrays) with
first dimension dimx and second dimension dimy, where the element at index (x,y) is
initialized with f x y.
Since: 5.2
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_floatarray_length[29.56].

val append : t -> t -> t

Chapter 29. The standard library 671

append v1 v2 returns a fresh floatarray containing the concatenation of the floatarrays

v1 and v2.
Raises Invalid_argument if length v1 + length v2 >
Sys.max_floatarray_length.

val concat : t list -> t

Same as Float.Array.append[29.21], but concatenates a list of floatarrays.

val sub : t -> int -> int -> t

sub a pos len returns a fresh floatarray of length len, containing the elements number
pos to pos + len - 1 of floatarray a.
Raises Invalid_argument if pos and len do not designate a valid subarray of a; that
is, if pos < 0, or len < 0, or pos + len > length a.

val copy : t -> t

copy a returns a copy of a, that is, a fresh floatarray containing the same elements as a.

val fill : t -> int -> int -> float -> unit
fill a pos len x modifies the floatarray a in place, storing x in elements number pos
to pos + len - 1.
Raises Invalid_argument if pos and len do not designate a valid subarray of a.

val blit : t -> int -> t -> int -> int -> unit
blit src src_pos dst dst_pos len copies len elements from floatarray src, starting
at element number src_pos, to floatarray dst, starting at element number dst_pos. It
works correctly even if src and dst are the same floatarray, and the source and
destination chunks overlap.
Raises Invalid_argument if src_pos and len do not designate a valid subarray of src,
or if dst_pos and len do not designate a valid subarray of dst.

val to_list : t -> float list

to_list a returns the list of all the elements of a.

val of_list : float list -> t

of_list l returns a fresh floatarray containing the elements of l.
Raises Invalid_argument if the length of l is greater than
Sys.max_floatarray_length.
672

Iterators
val iter : (float -> unit) -> t -> unit
iter f a applies function f in turn to all the elements of a. It is equivalent to f a.(0);
f a.(1); ...; f a.(length a - 1); ().

val iteri : (int -> float -> unit) -> t -> unit
Same as Float.Array.iter[29.21], but the function is applied with the index of the
element as first argument, and the element itself as second argument.

val map : (float -> float) -> t -> t

map f a applies function f to all the elements of a, and builds a floatarray with the
results returned by f.

val map_inplace : (float -> float) -> t -> unit

map_inplace f a applies function f to all elements of a, and updates their values in
place.
Since: 5.1

val mapi : (int -> float -> float) -> t -> t

Same as Float.Array.map[29.21], but the function is applied to the index of the
element as first argument, and the element itself as second argument.

val mapi_inplace : (int -> float -> float) -> t -> unit
Same as Float.Array.map_inplace[29.21], but the function is applied to the index of
the element as first argument, and the element itself as second argument.
Since: 5.1

val fold_left : ('acc -> float -> 'acc) -> 'acc -> t -> 'acc
fold_left f x init computes f (... (f (f x init.(0)) init.(1)) ...)
init.(n-1), where n is the length of the floatarray init.

val fold_right : (float -> 'acc -> 'acc) -> t -> 'acc -> 'acc
fold_right f a init computes f a.(0) (f a.(1) ( ... (f a.(n-1) init)
...)), where n is the length of the floatarray a.

Iterators on two arrays

val iter2 : (float -> float -> unit) -> t -> t -> unit
Array.iter2 f a b applies function f to all the elements of a and b.
Raises Invalid_argument if the floatarrays are not the same size.
Chapter 29. The standard library 673

val map2 : (float -> float -> float) -> t -> t -> t
map2 f a b applies function f to all the elements of a and b, and builds a floatarray
with the results returned by f: [| f a.(0) b.(0); ...; f a.(length a - 1)
b.(length b - 1)|].
Raises Invalid_argument if the floatarrays are not the same size.

Array scanning
val for_all : (float -> bool) -> t -> bool
for_all f [|a1; ...; an|] checks if all elements of the floatarray satisfy the
predicate f. That is, it returns (f a1) && (f a2) && ... && (f an).

val exists : (float -> bool) -> t -> bool

exists f [|a1; ...; an|] checks if at least one element of the floatarray satisfies the
predicate f. That is, it returns (f a1) || (f a2) || ... || (f an).

val mem : float -> t -> bool

mem a set is true if and only if there is an element of set that is structurally equal to a,
i.e. there is an x in set such that compare a x = 0.

val mem_ieee : float -> t -> bool

Same as Float.Array.mem[29.21], but uses IEEE equality instead of structural equality.

Array searching
val find_opt : (float -> bool) -> t -> float option
val find_index : (float -> bool) -> t -> int option
find_index f a returns Some i, where i is the index of the first element of the array a
that satisfies f x, if there is such an element.
It returns None if there is no such element.
Since: 5.1

val find_map : (float -> 'a option) -> t -> 'a option
val find_mapi : (int -> float -> 'a option) -> t -> 'a option
Same as find_map, but the predicate is applied to the index of the element as first
argument (counting from 0), and the element itself as second argument.
Since: 5.1
674

Sorting and shuffling

val sort : (float -> float -> int) -> t -> unit
Sort a floatarray in increasing order according to a comparison function. The
comparison function must return 0 if its arguments compare as equal, a positive integer
if the first is greater, and a negative integer if the first is smaller (see below for a
complete specification). For example, compare[28.2] is a suitable comparison function.
After calling sort, the array is sorted in place in increasing order. sort is guaranteed to
run in constant heap space and (at most) logarithmic stack space.
The current implementation uses Heap Sort. It runs in constant stack space.
Specification of the comparison function: Let a be the floatarray and cmp the
comparison function. The following must be true for all x, y, z in a :
• cmp x y > 0 if and only if cmp y x < 0
• if cmp x y ≥ 0 and cmp y z ≥ 0 then cmp x z ≥ 0
When sort returns, a contains the same elements as before, reordered in such a way
that for all i and j valid indices of a :
• cmp a.(i) a.(j) ≥ 0 if i ≥ j

val stable_sort : (float -> float -> int) -> t -> unit
Same as Float.Array.sort[29.21], but the sorting algorithm is stable (i.e. elements
that compare equal are kept in their original order) and not guaranteed to run in
constant heap space.
The current implementation uses Merge Sort. It uses a temporary floatarray of length
n/2, where n is the length of the floatarray. It is usually faster than the current
implementation of Float.Array.sort[29.21].

val fast_sort : (float -> float -> int) -> t -> unit
Same as Float.Array.sort[29.21] or Float.Array.stable_sort[29.21], whichever is
faster on typical input.

val shuffle : rand:(int -> int) -> t -> unit

shuffle rand a randomly permutes a’s elements using rand for randomness. The
distribution of permutations is uniform.
rand must be such that a call to rand n returns a uniformly distributed random number
in the range [0;n-1]. Random.int[29.46] can be used for this (do not forget to
initialize[29.46] the generator).
Since: 5.2

Float arrays and Sequences

val to_seq : t -> float Seq.t
Chapter 29. The standard library 675

Iterate on the floatarray, in increasing order. Modifications of the floatarray during

iteration will be reflected in the sequence.

val to_seqi : t -> (int * float) Seq.t

Iterate on the floatarray, in increasing order, yielding indices along elements.
Modifications of the floatarray during iteration will be reflected in the sequence.

val of_seq : float Seq.t -> t

Create an array from the generator.

val map_to_array : (float -> 'a) -> t -> 'a array

map_to_array f a applies function f to all the elements of a, and builds an array with
the results returned by f: [| f a.(0); f a.(1); ...; f a.(length a - 1) |].

val map_from_array : ('a -> float) -> 'a array -> t

map_from_array f a applies function f to all the elements of a, and builds a floatarray
with the results returned by f.

Arrays and concurrency safety

Care must be taken when concurrently accessing float arrays from multiple domains: accessing
an array will never crash a program, but unsynchronized accesses might yield surprising
(non-sequentially-consistent) results.

Atomicity

Every float array operation that accesses more than one array element is not atomic. This
includes iteration, scanning, sorting, splitting and combining arrays.
For example, consider the following program:
let size = 100_000_000
let a = Float.Array.make size 1.
let update a f () =
Float.Array.iteri (fun i x -> Float.Array.set a i (f x)) a
let d1 = Domain.spawn (update a (fun x -> x +. 1.))
let d2 = Domain.spawn (update a (fun x -> 2. *. x +. 1.))
let () = Domain.join d1; Domain.join d2

After executing this code, each field of the float array a is either 2., 3., 4. or 5.. If atomicity
is required, then the user must implement their own synchronization (for example, using
Mutex.t[29.37]).
676

Data races

If two domains only access disjoint parts of the array, then the observed behaviour is the
equivalent to some sequential interleaving of the operations from the two domains.
A data race is said to occur when two domains access the same array element without
synchronization and at least one of the accesses is a write. In the absence of data races,
the observed behaviour is equivalent to some sequential interleaving of the operations from
different domains.
Whenever possible, data races should be avoided by using synchronization to mediate the
accesses to the array elements.
Indeed, in the presence of data races, programs will not crash but the observed behaviour
may not be equivalent to any sequential interleaving of operations from different domains.
Nevertheless, even in the presence of data races, a read operation will return the value of some
prior write to that location with a few exceptions.

Tearing

Float arrays have two supplementary caveats in the presence of data races.
First, the blit operation might copy an array byte-by-byte. Data races between such a blit
operation and another operation might produce surprising values due to tearing: partial writes
interleaved with other operations can create float values that would not exist with a sequential
execution.
For instance, at the end of
let zeros = Float.Array.make size 0.
let max_floats = Float.Array.make size Float.max_float
let res = Float.Array.copy zeros
let d1 = Domain.spawn (fun () -> Float.Array.blit zeros 0 res 0 size)
let d2 = Domain.spawn (fun () -> Float.Array.blit max_floats 0 res 0 size)
let () = Domain.join d1; Domain.join d2

the res float array might contain values that are neither 0. nor max_float.
Second, on 32-bit architectures, getting or setting a field involves two separate memory accesses.
In the presence of data races, the user may observe tearing on any operation.
end
Float arrays with packed representation.

module ArrayLabels :
sig
type t = floatarray
The type of float arrays with packed representation.
Since: 4.08
Chapter 29. The standard library 677

val length : t -> int

Return the length (number of elements) of the given floatarray.

val get : t -> int -> float

get a n returns the element number n of floatarray a.
Raises Invalid_argument if n is outside the range 0 to (length a - 1).

val set : t -> int -> float -> unit

set a n x modifies floatarray a in place, replacing element number n with x.
Raises Invalid_argument if n is outside the range 0 to (length a - 1).

val make : int -> float -> t

make n x returns a fresh floatarray of length n, initialized with x.
Raises Invalid_argument if n < 0 or n > Sys.max_floatarray_length.

val create : int -> t

create n returns a fresh floatarray of length n, with uninitialized data.
Raises Invalid_argument if n < 0 or n > Sys.max_floatarray_length.

val init : int -> f:(int -> float) -> t

init n ~f returns a fresh floatarray of length n, with element number i initialized to
the result of f i. In other terms, init n ~f tabulates the results of f applied to the
integers 0 to n-1.
Raises Invalid_argument if n < 0 or n > Sys.max_floatarray_length.

val make_matrix : dimx:int -> dimy:int -> float -> t array

make_matrix ~dimx ~dimy e returns a two-dimensional array (an array of arrays) with
first dimension dimx and second dimension dimy, where all elements are initialized with
e.
Since: 5.2
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_floatarray_length[29.56].

val init_matrix : dimx:int -> dimy:int -> f:(int -> int -> float) -> t array
init_matrix ~dimx ~dimy ~f returns a two-dimensional array (an array of arrays)
with first dimension dimx and second dimension dimy, where the element at index (x,y)
is initialized with f x y.
Since: 5.2
Raises Invalid_argument if dimx or dimy is negative or greater than
Sys.max_floatarray_length[29.56].
678

val append : t -> t -> t

append v1 v2 returns a fresh floatarray containing the concatenation of the floatarrays
v1 and v2.
Raises Invalid_argument if length v1 + length v2 >
Sys.max_floatarray_length.

val concat : t list -> t

Same as Float.ArrayLabels.append[29.21], but concatenates a list of floatarrays.

val sub : t -> pos:int -> len:int -> t

sub a ~pos ~len returns a fresh floatarray of length len, containing the elements
number pos to pos + len - 1 of floatarray a.
Raises Invalid_argument if pos and len do not designate a valid subarray of a; that
is, if pos < 0, or len < 0, or pos + len > length a.

val copy : t -> t

copy a returns a copy of a, that is, a fresh floatarray containing the same elements as a.

val fill : t -> pos:int -> len:int -> float -> unit
fill a ~pos ~len x modifies the floatarray a in place, storing x in elements number
pos to pos + len - 1.
Raises Invalid_argument if pos and len do not designate a valid subarray of a.

val blit : src:t ->

src_pos:int -> dst:t -> dst_pos:int -> len:int -> unit
blit ~src ~src_pos ~dst ~dst_pos ~len copies len elements from floatarray src,
starting at element number src_pos, to floatarray dst, starting at element number
dst_pos. It works correctly even if src and dst are the same floatarray, and the source
and destination chunks overlap.
Raises Invalid_argument if src_pos and len do not designate a valid subarray of src,
or if dst_pos and len do not designate a valid subarray of dst.

val to_list : t -> float list

to_list a returns the list of all the elements of a.

val of_list : float list -> t

of_list l returns a fresh floatarray containing the elements of l.
Raises Invalid_argument if the length of l is greater than
Sys.max_floatarray_length.
Chapter 29. The standard library 679

Iterators
val iter : f:(float -> unit) -> t -> unit
iter ~f a applies function f in turn to all the elements of a. It is equivalent to f
a.(0); f a.(1); ...; f a.(length a - 1); ().

val iteri : f:(int -> float -> unit) -> t -> unit
Same as Float.ArrayLabels.iter[29.21], but the function is applied with the index of
the element as first argument, and the element itself as second argument.

val map : f:(float -> float) -> t -> t

map ~f a applies function f to all the elements of a, and builds a floatarray with the
results returned by f.

val map_inplace : f:(float -> float) -> t -> unit

map_inplace f a applies function f to all elements of a, and updates their values in
place.
Since: 5.1

val mapi : f:(int -> float -> float) -> t -> t

Same as Float.ArrayLabels.map[29.21], but the function is applied to the index of the
element as first argument, and the element itself as second argument.

val mapi_inplace : f:(int -> float -> float) -> t -> unit
Same as Float.ArrayLabels.map_inplace[29.21], but the function is applied to the
index of the element as first argument, and the element itself as second argument.
Since: 5.1

val fold_left : f:('acc -> float -> 'acc) -> init:'acc -> t -> 'acc
fold_left ~f x ~init computes f (... (f (f x init.(0)) init.(1)) ...)
init.(n-1), where n is the length of the floatarray init.

val fold_right : f:(float -> 'acc -> 'acc) -> t -> init:'acc -> 'acc
fold_right f a init computes f a.(0) (f a.(1) ( ... (f a.(n-1) init)
...)), where n is the length of the floatarray a.

Iterators on two arrays

val iter2 : f:(float -> float -> unit) ->
t -> t -> unit
680

Array.iter2 ~f a b applies function f to all the elements of a and b.

Raises Invalid_argument if the floatarrays are not the same size.

val map2 : f:(float -> float -> float) ->

t -> t -> t
map2 ~f a b applies function f to all the elements of a and b, and builds a floatarray
with the results returned by f: [| f a.(0) b.(0); ...; f a.(length a - 1)
b.(length b - 1)|].
Raises Invalid_argument if the floatarrays are not the same size.

Array scanning
val for_all : f:(float -> bool) -> t -> bool
for_all ~f [|a1; ...; an|] checks if all elements of the floatarray satisfy the
predicate f. That is, it returns (f a1) && (f a2) && ... && (f an).

val exists : f:(float -> bool) -> t -> bool

exists f [|a1; ...; an|] checks if at least one element of the floatarray satisfies the
predicate f. That is, it returns (f a1) || (f a2) || ... || (f an).

val mem : float -> set:t -> bool

mem a ~set is true if and only if there is an element of set that is structurally equal to
a, i.e. there is an x in set such that compare a x = 0.

val mem_ieee : float -> set:t -> bool

Same as Float.ArrayLabels.mem[29.21], but uses IEEE equality instead of structural
equality.

Array searching
val find_opt : f:(float -> bool) -> t -> float option
val find_index : f:(float -> bool) -> t -> int option
find_index ~f a returns Some i, where i is the index of the first element of the array
a that satisfies f x, if there is such an element.
It returns None if there is no such element.
Since: 5.1

val find_map : f:(float -> 'a option) -> t -> 'a option
val find_mapi : f:(int -> float -> 'a option) -> t -> 'a option
Same as find_map, but the predicate is applied to the index of the element as first
argument (counting from 0), and the element itself as second argument.
Since: 5.1
Chapter 29. The standard library 681

Sorting and shuffling

val sort : cmp:(float -> float -> int) -> t -> unit
Sort a floatarray in increasing order according to a comparison function. The
comparison function must return 0 if its arguments compare as equal, a positive integer
if the first is greater, and a negative integer if the first is smaller (see below for a
complete specification). For example, compare[28.2] is a suitable comparison function.
After calling sort, the array is sorted in place in increasing order. sort is guaranteed to
run in constant heap space and (at most) logarithmic stack space.
The current implementation uses Heap Sort. It runs in constant stack space.
Specification of the comparison function: Let a be the floatarray and cmp the
comparison function. The following must be true for all x, y, z in a :
• cmp x y > 0 if and only if cmp y x < 0
• if cmp x y ≥ 0 and cmp y z ≥ 0 then cmp x z ≥ 0
When sort returns, a contains the same elements as before, reordered in such a way
that for all i and j valid indices of a :
• cmp a.(i) a.(j) ≥ 0 if i ≥ j

val stable_sort : cmp:(float -> float -> int) -> t -> unit
Same as Float.ArrayLabels.sort[29.21], but the sorting algorithm is stable (i.e.
elements that compare equal are kept in their original order) and not guaranteed to run
in constant heap space.
The current implementation uses Merge Sort. It uses a temporary floatarray of length
n/2, where n is the length of the floatarray. It is usually faster than the current
implementation of Float.ArrayLabels.sort[29.21].

val fast_sort : cmp:(float -> float -> int) -> t -> unit
Same as Float.ArrayLabels.sort[29.21] or Float.ArrayLabels.stable_sort[29.21],
whichever is faster on typical input.

val shuffle : rand:(int -> int) -> t -> unit

shuffle ~rand a randomly permutes a’s elements using rand for randomness. The
distribution of permutations is uniform.
rand must be such that a call to rand n returns a uniformly distributed random number
in the range [0;n-1]. Random.int[29.46] can be used for this (do not forget to
initialize[29.46] the generator).
Since: 5.2

Float arrays and Sequences

val to_seq : t -> float Seq.t
682

Iterate on the floatarray, in increasing order. Modifications of the floatarray during

iteration will be reflected in the sequence.

val to_seqi : t -> (int * float) Seq.t

Iterate on the floatarray, in increasing order, yielding indices along elements.
Modifications of the floatarray during iteration will be reflected in the sequence.

val of_seq : float Seq.t -> t

Create an array from the generator.

val map_to_array : f:(float -> 'a) -> t -> 'a array

map_to_array ~f a applies function f to all the elements of a, and builds an array with
the results returned by f: [| f a.(0); f a.(1); ...; f a.(length a - 1) |].

val map_from_array : f:('a -> float) -> 'a array -> t

map_from_array ~f a applies function f to all the elements of a, and builds a
floatarray with the results returned by f.

Arrays and concurrency safety

Care must be taken when concurrently accessing float arrays from multiple domains: accessing
an array will never crash a program, but unsynchronized accesses might yield surprising
(non-sequentially-consistent) results.

Atomicity

Every float array operation that accesses more than one array element is not atomic. This
includes iteration, scanning, sorting, splitting and combining arrays.
For example, consider the following program:
let size = 100_000_000
let a = Float.ArrayLabels.make size 1.
let update a f () =
Float.ArrayLabels.iteri ~f:(fun i x -> Float.Array.set a i (f x)) a
let d1 = Domain.spawn (update a (fun x -> x +. 1.))
let d2 = Domain.spawn (update a (fun x -> 2. *. x +. 1.))
let () = Domain.join d1; Domain.join d2

After executing this code, each field of the float array a is either 2., 3., 4. or 5.. If atomicity
is required, then the user must implement their own synchronization (for example, using
Mutex.t[29.37]).
Chapter 29. The standard library 683

Data races

If two domains only access disjoint parts of the array, then the observed behaviour is the
equivalent to some sequential interleaving of the operations from the two domains.
A data race is said to occur when two domains access the same array element without
synchronization and at least one of the accesses is a write. In the absence of data races,
the observed behaviour is equivalent to some sequential interleaving of the operations from
different domains.
Whenever possible, data races should be avoided by using synchronization to mediate the
accesses to the array elements.
Indeed, in the presence of data races, programs will not crash but the observed behaviour
may not be equivalent to any sequential interleaving of operations from different domains.
Nevertheless, even in the presence of data races, a read operation will return the value of some
prior write to that location with a few exceptions.

Tearing

Float arrays have two supplementary caveats in the presence of data races.
First, the blit operation might copy an array byte-by-byte. Data races between such a blit
operation and another operation might produce surprising values due to tearing: partial writes
interleaved with other operations can create float values that would not exist with a sequential
execution.
For instance, at the end of
let zeros = Float.Array.make size 0.
let max_floats = Float.Array.make size Float.max_float
let res = Float.Array.copy zeros
let d1 = Domain.spawn (fun () -> Float.Array.blit zeros 0 res 0 size)
let d2 = Domain.spawn (fun () -> Float.Array.blit max_floats 0 res 0 size)
let () = Domain.join d1; Domain.join d2

the res float array might contain values that are neither 0. nor max_float.
Second, on 32-bit architectures, getting or setting a field involves two separate memory accesses.
In the presence of data races, the user may observe tearing on any operation.
end
Float arrays with packed representation (labeled functions).

29.22 Module Format : Pretty-printing.

If you are new to this module, see the examples[29.45] below.
This module implements a pretty-printing facility to format values within ’pretty-printing
boxes’[29.22] and ’semantic tags’[29.22] combined with a set of printf-like functions[29.22]. The
684

pretty-printer splits lines at specified break hints[29.22], and indents lines according to the box
structure. Similarly, semantic tags[29.22] can be used to decouple text presentation from its contents.
This pretty-printing facility is implemented as an overlay on top of abstract formatters[29.22]
which provide basic output functions. Some formatters are predefined, notably:
• Format.std_formatter[29.22] outputs to stdout[28.2]
• Format.err_formatter[29.22] outputs to stderr[28.2]
Most functions in the Format[29.22] module come in two variants: a short version that operates on
the current domain’s standard formatter as obtained using Format.get_std_formatter[29.22] and
the generic version prefixed by pp_ that takes a formatter as its first argument. For the version that
operates on the current domain’s standard formatter, the call to Format.get_std_formatter[29.22]
is delayed until the last argument is received.
More formatters can be created with Format.formatter_of_out_channel[29.22],
Format.formatter_of_buffer[29.22], Format.formatter_of_symbolic_output_buffer[29.22]
or using custom formatters[29.22].
Warning: Since formatters[29.22] contain mutable state, it is not thread-safe to use the same
formatter on multiple domains in parallel without synchronization.
If multiple domains write to the same output channel using the predefined formatters (as obtained
by Format.get_std_formatter[29.22] or Format.get_err_formatter[29.22]), the output from the
domains will be interleaved with each other at points where the formatters are flushed, such as with
Format.print_flush[29.22]. This synchronization is not performed by formatters obtained from
Format.formatter_of_out_channel[29.22] (on the standard out channels or others).

Introduction
You may consider this module as providing an extension to the printf facility to provide automatic
line splitting. The addition of pretty-printing annotations to your regular printf format strings
gives you fancy indentation and line breaks. Pretty-printing annotations are described below in the
documentation of the function Format.fprintf[29.22].
You may also use the explicit pretty-printing box management and printing functions provided
by this module. This style is more basic but more verbose than the concise fprintf format strings.
For instance, the sequence open_box 0; print_string "x ="; print_space (); print_int
1; close_box (); print_newline () that prints x = 1 within a pretty-printing box, can be
abbreviated as printf "@[%s@ %i@]@." "x =" 1, or even shorter printf "@[x =@ %i@]@." 1.
Rule of thumb for casual users of this library:
• use simple pretty-printing boxes (as obtained by open_box 0);
• use simple break hints as obtained by print_cut () that outputs a simple break hint, or by
print_space () that outputs a space indicating a break hint;
• once a pretty-printing box is open, display its material with basic printing functions (e. g.
print_int and print_string);
• when the material for a pretty-printing box has been printed, call close_box () to close the
box;
Chapter 29. The standard library 685

• at the end of pretty-printing, flush the pretty-printer to display all the remaining material,
e.g. evaluate print_newline ().
The behavior of pretty-printing commands is unspecified if there is no open pretty-printing box.
Each box opened by one of the open_ functions below must be closed using close_box for proper
formatting. Otherwise, some of the material printed in the boxes may not be output, or may be
formatted incorrectly.
In case of interactive use, each phrase is executed in the initial state of the standard pretty-printer:
after each phrase execution, the interactive system closes all open pretty-printing boxes, flushes all
pending text, and resets the standard pretty-printer.
Warning: mixing calls to pretty-printing functions of this module with calls to Stdlib[28.2] low
level output functions is error prone.
The pretty-printing functions output material that is delayed in the pretty-printer queue
and stacks in order to compute proper line splitting. In contrast, basic I/O output functions
write directly in their output device. As a consequence, the output of a basic I/O function
may appear before the output of a pretty-printing function that has been called before. For in-
stance, Stdlib.print_string "<"; Format.print_string "PRETTY"; Stdlib.print_string
">"; Format.print_string "TEXT"; leads to output <>PRETTYTEXT.

Formatters
type formatter
Abstract data corresponding to a pretty-printer (also called a formatter) and all its machinery.
See also [29.22].

Pretty-printing boxes
The pretty-printing engine uses the concepts of pretty-printing box and break hint to drive indentation
and line splitting behavior of the pretty-printer.
Each different pretty-printing box kind introduces a specific line splitting policy:

• within an horizontal box, break hints never split the line (but the line may be split in a box
nested deeper),

• within a vertical box, break hints always split the line,

• within an horizontal/vertical box, if the box fits on the current line then break hints never
split the line, otherwise break hint always split the line,

• within a compacting box, a break hint never splits the line, unless there is no more room on
the current line.

Note that line splitting policy is box specific: the policy of a box does not rule the policy of
inner boxes. For instance, if a vertical box is nested in an horizontal box, all break hints within the
vertical box will split the line.
Moreover, opening a box after the maximum indentation limit[29.22] splits the line whether or
not the box would end up fitting on the line.
686

val pp_open_box : formatter -> int -> unit

val open_box : int -> unit
pp_open_box ppf d opens a new compacting pretty-printing box with offset d in the
formatter ppf.
Within this box, the pretty-printer prints as much as possible material on every line.
A break hint splits the line if there is no more room on the line to print the remainder of the
box.
Within this box, the pretty-printer emphasizes the box structure: if a structural box does not
fit fully on a simple line, a break hint also splits the line if the splitting “moves to the left”
(i.e. the new line gets an indentation smaller than the one of the current line).
This box is the general purpose pretty-printing box.
If the pretty-printer splits the line in the box, offset d is added to the current indentation.

val pp_close_box : formatter -> unit -> unit

val close_box : unit -> unit
Closes the most recently open pretty-printing box.

val pp_open_hbox : formatter -> unit -> unit

val open_hbox : unit -> unit
pp_open_hbox ppf () opens a new ’horizontal’ pretty-printing box.
This box prints material on a single line.
Break hints in a horizontal box never split the line. (Line splitting may still occur inside
boxes nested deeper).

val pp_open_vbox : formatter -> int -> unit

val open_vbox : int -> unit
pp_open_vbox ppf d opens a new ’vertical’ pretty-printing box with offset d.
This box prints material on as many lines as break hints in the box.
Every break hint in a vertical box splits the line.
If the pretty-printer splits the line in the box, d is added to the current indentation.

val pp_open_hvbox : formatter -> int -> unit

val open_hvbox : int -> unit
pp_open_hvbox ppf d opens a new ’horizontal/vertical’ pretty-printing box with offset d.
This box behaves as an horizontal box if it fits on a single line, otherwise it behaves as a
vertical box.
If the pretty-printer splits the line in the box, d is added to the current indentation.

val pp_open_hovbox : formatter -> int -> unit

val open_hovbox : int -> unit
Chapter 29. The standard library 687

pp_open_hovbox ppf d opens a new ’horizontal-or-vertical’ pretty-printing box with offset d.

This box prints material as much as possible on every line.
A break hint splits the line if there is no more room on the line to print the remainder of the
box.
If the pretty-printer splits the line in the box, d is added to the current indentation.

Formatting functions
val pp_print_string : formatter -> string -> unit
val print_string : string -> unit
pp_print_string ppf s prints s in the current pretty-printing box.

val pp_print_substring : pos:int -> len:int -> formatter -> string -> unit
val print_substring : pos:int -> len:int -> string -> unit
pp_print_substring ~pos ~len ppf s prints the substring of s that starts at position pos
and stops at position pos+len in the current pretty-printing box.
Since: 5.3

val pp_print_bytes : formatter -> bytes -> unit

val print_bytes : bytes -> unit
pp_print_bytes ppf b prints b in the current pretty-printing box.
Since: 4.13

val pp_print_as : formatter -> int -> string -> unit

val print_as : int -> string -> unit
pp_print_as ppf len s prints s in the current pretty-printing box. The pretty-printer
formats s as if it were of length len.

val pp_print_substring_as :
pos:int -> len:int -> formatter -> int -> string -> unit
val print_substring_as : pos:int -> len:int -> int -> string -> unit
pp_print_substring_as ~first ~len ppf len_as s prints the substring of s that starts
at position pos and stop at position pos+len in the current pretty-printing box as if it were
of length len_as.
Since: 5.1

val pp_print_int : formatter -> int -> unit

val print_int : int -> unit
Print an integer in the current pretty-printing box.

val pp_print_float : formatter -> float -> unit

val print_float : float -> unit
688

Print a floating point number in the current pretty-printing box.

val pp_print_char : formatter -> char -> unit

val print_char : char -> unit
Print a character in the current pretty-printing box.

val pp_print_bool : formatter -> bool -> unit

val print_bool : bool -> unit
Print a boolean in the current pretty-printing box.

val pp_print_nothing : formatter -> unit -> unit

Print nothing.
Since: 5.2

Break hints
A ’break hint’ tells the pretty-printer to output some space or split the line whichever way is more
appropriate to the current pretty-printing box splitting rules.
Break hints are used to separate printing items and are mandatory to let the pretty-printer
correctly split lines and indent items.
Simple break hints are:
• the ’space’: output a space or split the line if appropriate,

• the ’cut’: split the line if appropriate.

Note: the notions of space and line splitting are abstract for the pretty-printing engine, since
those notions can be completely redefined by the programmer. However, in the pretty-printer default
setting, “output a space” simply means printing a space character (ASCII code 32) and “split the
line” means printing a newline character (ASCII code 10).
val pp_print_space : formatter -> unit -> unit
val print_space : unit -> unit
pp_print_space ppf () emits a ’space’ break hint: the pretty-printer may split the line at
this point, otherwise it prints one space.
pp_print_space ppf () is equivalent to pp_print_break ppf 1 0.

val pp_print_cut : formatter -> unit -> unit

val print_cut : unit -> unit
pp_print_cut ppf () emits a ’cut’ break hint: the pretty-printer may split the line at this
point, otherwise it prints nothing.
pp_print_cut ppf () is equivalent to pp_print_break ppf 0 0.

val pp_print_break : formatter -> int -> int -> unit

val print_break : int -> int -> unit
Chapter 29. The standard library 689

pp_print_break ppf nspaces offset emits a ’full’ break hint: the pretty-printer may split
the line at this point, otherwise it prints nspaces spaces.
If the pretty-printer splits the line, offset is added to the current indentation.

val pp_print_custom_break :
formatter ->
fits:string * int * string -> breaks:string * int * string -> unit
pp_print_custom_break ppf ~fits:(s1, n, s2) ~breaks:(s3, m, s4) emits a custom
break hint: the pretty-printer may split the line at this point.
If it does not split the line, then the s1 is emitted, then n spaces, then s2.
If it splits the line, then it emits the s3 string, then an indent (according to the box rules),
then an offset of m spaces, then the s4 string.
While n and m are handled by formatter_out_functions.out_indent, the strings will be
handled by formatter_out_functions.out_string. This allows for a custom formatter
that handles indentation distinctly, for example, outputs <br/> tags or &nbsp; entities.
The custom break is useful if you want to change which visible (non-whitespace) characters
are printed in case of break or no break. For example, when printing a list [a; b; c] , you
might want to add a trailing semicolon when it is printed vertically:

[
a;
b;
c;
]

You can do this as follows:

printf "@[<v 0>[@;<0 2>@[<v 0>a;@,b;@,c@]%t]@]@\n"

(pp_print_custom_break ~fits:("", 0, "") ~breaks:(";", 0, ""))

Since: 4.08

val pp_force_newline : formatter -> unit -> unit

val force_newline : unit -> unit
Force a new line in the current pretty-printing box.
The pretty-printer must split the line at this point,
Not the normal way of pretty-printing, since imperative line splitting may interfere with
current line counters and box size calculation. Using break hints within an enclosing vertical
box is a better alternative.

val pp_print_if_newline : formatter -> unit -> unit

val print_if_newline : unit -> unit
690

Execute the next formatting command if the preceding line has just been split. Otherwise,
ignore the next formatting command.

Pretty-printing termination
val pp_print_flush : formatter -> unit -> unit
val print_flush : unit -> unit
End of pretty-printing: resets the pretty-printer to initial state.
All open pretty-printing boxes are closed, all pending text is printed. In addition, the
pretty-printer low level output device is flushed to ensure that all pending text is really
displayed.
Note: never use print_flush in the normal course of a pretty-printing routine, since the
pretty-printer uses a complex buffering machinery to properly indent the output; manually
flushing those buffers at random would conflict with the pretty-printer strategy and result to
poor rendering.
Only consider using print_flush when displaying all pending material is mandatory (for
instance in case of interactive use when you want the user to read some text) and when
resetting the pretty-printer state will not disturb further pretty-printing.
Warning: If the output device of the pretty-printer is an output channel, repeated calls to
print_flush means repeated calls to flush[28.2] to flush the out channel; these explicit flush
calls could foil the buffering strategy of output channels and could dramatically impact
efficiency.

val pp_print_newline : formatter -> unit -> unit

val print_newline : unit -> unit
End of pretty-printing: resets the pretty-printer to initial state.
All open pretty-printing boxes are closed, all pending text is printed.
Equivalent to Format.print_flush[29.22] with a new line emitted on the pretty-printer
low-level output device immediately before the device is flushed. See corresponding words of
caution for Format.print_flush[29.22].
Note: this is not the normal way to output a new line; the preferred method is using break
hints within a vertical pretty-printing box.

Margin
val pp_infinity : int
pp_infinity is the maximal size of the margin. Its exact value is implementation dependent
but is guaranteed to be greater than 109 .
Since: 5.2
Chapter 29. The standard library 691

val pp_set_margin : formatter -> int -> unit

val set_margin : int -> unit
pp_set_margin ppf d sets the right margin to d (in characters): the pretty-printer splits
lines that overflow the right margin according to the break hints given. Setting the margin to
d means that the formatting engine aims at printing at most d-1 characters per line. Nothing
happens if d is smaller than 2. If d >= Format.pp_infinity[29.22], the right margin is set
to Format.pp_infinity[29.22] - 1. If d is less than the current maximum indentation limit,
the maximum indentation limit is decreased while trying to preserve a minimal ratio
max_indent/margin>=50% and if possible the current difference margin - max_indent.
See also Format.pp_set_geometry[29.22].

val pp_get_margin : formatter -> unit -> int

val get_margin : unit -> int
Returns the position of the right margin.

Maximum indentation limit

val pp_set_max_indent : formatter -> int -> unit
val set_max_indent : int -> unit
pp_set_max_indent ppf d sets the maximum indentation limit of lines to d (in charac-
ters): once this limit is reached, new pretty-printing boxes are rejected to the left, unless
the enclosing box fully fits on the current line. As an illustration,
set_margin 10; set_max_indent 5; printf "@[123456@[7@]89A@]@."
yields

123456
789A

because the nested box "@[7@]" is opened after the maximum indentation limit (7>5)
and its parent box does not fit on the current line. Either decreasing the length of the
parent box to make it fit on a line:
printf "@[123456@[7@]89@]@."
or opening an intermediary box before the maximum indentation limit which fits on the
current line
printf "@[123@[456@[7@]89@]A@]@."
avoids the rejection to the left of the inner boxes and print respectively "123456789" and
"123456789A" . Note also that vertical boxes never fit on a line whereas horizontal boxes
always fully fit on the current line. Opening a box may split a line whereas the contents may
have fit. If this behavior is problematic, it can be curtailed by setting the maximum
indentation limit to margin - 1. Note that setting the maximum indentation limit to
margin is invalid.
692

Nothing happens if d is smaller than 2.

If d is greater than the current margin, it is ignored, and the current maximum indentation
limit is kept.
See also Format.pp_set_geometry[29.22].

val pp_get_max_indent : formatter -> unit -> int

val get_max_indent : unit -> int
Return the maximum indentation limit (in characters).

Geometry
Geometric functions can be used to manipulate simultaneously the coupled variables, margin and
maximum indentation limit.
type geometry =
{ max_indent : int ;
margin : int ;
}
Since: 4.08

val check_geometry : geometry -> bool

Check if the formatter geometry is valid: 1 < max_indent < margin <
Format.pp_infinity[29.22]
Since: 4.08

val pp_set_geometry : formatter -> max_indent:int -> margin:int -> unit

val set_geometry : max_indent:int -> margin:int -> unit
val pp_safe_set_geometry : formatter -> max_indent:int -> margin:int -> unit
val safe_set_geometry : max_indent:int -> margin:int -> unit
pp_set_geometry ppf ~max_indent ~margin sets both the margin and maximum
indentation limit for ppf.
When 1 < max_indent < margin < Format.pp_infinity[29.22], pp_set_geometry ppf
~max_indent ~margin is equivalent to pp_set_margin ppf margin; pp_set_max_indent
ppf max_indent; and avoids the subtly incorrect pp_set_max_indent ppf max_indent;
pp_set_margin ppf margin;
Outside of this domain, pp_set_geometry raises an invalid argument exception whereas
pp_safe_set_geometry does nothing.
Since: 4.08

val pp_update_geometry : formatter -> (geometry -> geometry) -> unit

Chapter 29. The standard library 693

pp_update_geometry ppf (fun geo -> { geo with ... }) lets you update a formatter’s
geometry in a way that is robust to extension of the geometry record with new fields.
Raises an invalid argument exception if the returned geometry does not satisfy
Format.check_geometry[29.22].
Since: 4.11

val update_geometry : (geometry -> geometry) -> unit

val pp_get_geometry : formatter -> unit -> geometry
val get_geometry : unit -> geometry
Return the current geometry of the formatter
Since: 4.08

Maximum formatting depth

The maximum formatting depth is the maximum number of pretty-printing boxes simultaneously
open.
Material inside boxes nested deeper is printed as an ellipsis (more precisely as the text returned
by Format.get_ellipsis_text[29.22] ()).
val pp_set_max_boxes : formatter -> int -> unit
val set_max_boxes : int -> unit
pp_set_max_boxes ppf max sets the maximum number of pretty-printing boxes
simultaneously open.
Material inside boxes nested deeper is printed as an ellipsis (more precisely as the text
returned by Format.get_ellipsis_text[29.22] ()).
Nothing happens if max is smaller than 2.

val pp_get_max_boxes : formatter -> unit -> int

val get_max_boxes : unit -> int
Returns the maximum number of pretty-printing boxes allowed before ellipsis.

val pp_over_max_boxes : formatter -> unit -> bool

val over_max_boxes : unit -> bool
Tests if the maximum number of pretty-printing boxes allowed have already been opened.

Tabulation boxes
A tabulation box prints material on lines divided into cells of fixed length. A tabulation box provides
a simple way to display vertical columns of left adjusted text.
This box features command set_tab to define cell boundaries, and command print_tab to
move from cell to cell and split the line when there is no more cells to print on the line.
694

Note: printing within tabulation box is line directed, so arbitrary line splitting inside a tabulation
box leads to poor rendering. Yet, controlled use of tabulation boxes allows simple printing of columns
within module Format[29.22].
val pp_open_tbox : formatter -> unit -> unit
val open_tbox : unit -> unit
open_tbox () opens a new tabulation box.
This box prints lines separated into cells of fixed width.
Inside a tabulation box, special tabulation markers defines points of interest on the line (for
instance to delimit cell boundaries). Function Format.set_tab[29.22] sets a tabulation
marker at insertion point.
A tabulation box features specific tabulation breaks to move to next tabulation marker or split
the line. Function Format.print_tbreak[29.22] prints a tabulation break.

val pp_close_tbox : formatter -> unit -> unit

val close_tbox : unit -> unit
Closes the most recently opened tabulation box.

val pp_set_tab : formatter -> unit -> unit

val set_tab : unit -> unit
Sets a tabulation marker at current insertion point.

val pp_print_tab : formatter -> unit -> unit

val print_tab : unit -> unit
print_tab () emits a ’next’ tabulation break hint: if not already set on a tabulation marker,
the insertion point moves to the first tabulation marker on the right, or the pretty-printer
splits the line and insertion point moves to the leftmost tabulation marker.
It is equivalent to print_tbreak 0 0.

val pp_print_tbreak : formatter -> int -> int -> unit

val print_tbreak : int -> int -> unit
print_tbreak nspaces offset emits a ’full’ tabulation break hint.
If not already set on a tabulation marker, the insertion point moves to the first tabulation
marker on the right and the pretty-printer prints nspaces spaces.
If there is no next tabulation marker on the right, the pretty-printer splits the line at this
point, then insertion point moves to the leftmost tabulation marker of the box.
If the pretty-printer splits the line, offset is added to the current indentation.
Chapter 29. The standard library 695

Ellipsis
val pp_set_ellipsis_text : formatter -> string -> unit
val set_ellipsis_text : string -> unit
Set the text of the ellipsis printed when too many pretty-printing boxes are open (a single dot,
., by default).

val pp_get_ellipsis_text : formatter -> unit -> string

val get_ellipsis_text : unit -> string
Return the text of the ellipsis.

Semantic tags
type stag = ..
Semantic tags (or simply tags) are user’s defined annotations to associate user’s specific
operations to printed entities.
Common usage of semantic tags is text decoration to get specific font or text size
rendering for a display device, or marking delimitation of entities (e.g. HTML or TeX
elements or terminal escape sequences). More sophisticated usage of semantic tags
could handle dynamic modification of the pretty-printer behavior to properly print the
material within some specific tags. For instance, we can define an RGB tag like so:

type stag += RGB of {r:int;g:int;b:int}

In order to properly delimit printed entities, a semantic tag must be opened before and closed
after the entity. Semantic tags must be properly nested like parentheses using
Format.pp_open_stag[29.22] and Format.pp_close_stag[29.22].
Tag specific operations occur any time a tag is opened or closed, At each occurrence, two
kinds of operations are performed tag-marking and tag-printing:

• The tag-marking operation is the simpler tag specific operation: it simply writes a tag
specific string into the output device of the formatter. Tag-marking does not interfere
with line-splitting computation.
• The tag-printing operation is the more involved tag specific operation: it can print
arbitrary material to the formatter. Tag-printing is tightly linked to the current
pretty-printer operations.

Roughly speaking, tag-marking is commonly used to get a better rendering of texts in the
rendering device, while tag-printing allows fine tuning of printing routines to print the same
entity differently according to the semantic tags (i.e. print additional material or even omit
parts of the output).
More precisely: when a semantic tag is opened or closed then both and successive
’tag-printing’ and ’tag-marking’ operations occur:
696

• Tag-printing a semantic tag means calling the formatter specific function

print_open_stag (resp. print_close_stag) with the name of the tag as argument:
that tag-printing function can then print any regular material to the formatter (so that
this material is enqueued as usual in the formatter queue for further line splitting
computation).
• Tag-marking a semantic tag means calling the formatter specific function
mark_open_stag (resp. mark_close_stag) with the name of the tag as argument: that
tag-marking function can then return the ’tag-opening marker’ (resp. ‘tag-closing
marker’) for direct output into the output device of the formatter.
Being written directly into the output device of the formatter, semantic tag marker strings
are not considered as part of the printing material that drives line splitting (in other words,
the length of the strings corresponding to tag markers is considered as zero for line splitting).
Thus, semantic tag handling is in some sense transparent to pretty-printing and does not
interfere with usual indentation. Hence, a single pretty-printing routine can output both
simple ’verbatim’ material or richer decorated output depending on the treatment of tags. By
default, tags are not active, hence the output is not decorated with tag information. Once
set_tags is set to true, the pretty-printer engine honors tags and decorates the output
accordingly.
Default tag-marking functions behave the HTML way: string tags[29.22] are enclosed in ”<”
and ”>” while other tags are ignored; hence, opening marker for tag string "t" is "<t>" and
closing marker is "</t>".
Default tag-printing functions just do nothing.
Tag-marking and tag-printing functions are user definable and can be set by calling
Format.set_formatter_stag_functions[29.22].
Semantic tag operations may be set on or off with Format.set_tags[29.22]. Tag-marking
operations may be set on or off with Format.set_mark_tags[29.22]. Tag-printing operations
may be set on or off with Format.set_print_tags[29.22].
Since: 4.08

type tag = string

type stag +=
| String_tag of tag
String_tag s is a string tag s. String tags can be inserted either by explicitly using
the constructor String_tag or by using the dedicated format syntax "@{<s> ... @}".
Since: 4.08
val pp_open_stag : formatter -> stag -> unit
val open_stag : stag -> unit
pp_open_stag ppf t opens the semantic tag named t.
The print_open_stag tag-printing function of the formatter is called with t as argument;
then the opening tag marker for t, as given by mark_open_stag t, is written into the output
device of the formatter.
Chapter 29. The standard library 697

Since: 4.08

val pp_close_stag : formatter -> unit -> unit

val close_stag : unit -> unit
pp_close_stag ppf () closes the most recently opened semantic tag t.
The closing tag marker, as given by mark_close_stag t, is written into the output device of
the formatter; then the print_close_stag tag-printing function of the formatter is called
with t as argument.
Since: 4.08

val pp_set_tags : formatter -> bool -> unit

val set_tags : bool -> unit
pp_set_tags ppf b turns on or off the treatment of semantic tags (default is off).

val pp_set_print_tags : formatter -> bool -> unit

val set_print_tags : bool -> unit
pp_set_print_tags ppf b turns on or off the tag-printing operations.

val pp_set_mark_tags : formatter -> bool -> unit

val set_mark_tags : bool -> unit
pp_set_mark_tags ppf b turns on or off the tag-marking operations.

val pp_get_print_tags : formatter -> unit -> bool

val get_print_tags : unit -> bool
Return the current status of tag-printing operations.

val pp_get_mark_tags : formatter -> unit -> bool

val get_mark_tags : unit -> bool
Return the current status of tag-marking operations.

val pp_set_formatter_out_channel : formatter -> out_channel -> unit

Redirecting the standard formatter output

val set_formatter_out_channel : out_channel -> unit

Redirect the standard pretty-printer output to the given channel. (All the output functions of
the standard formatter are set to the default output functions printing to the given channel.)
set_formatter_out_channel is equivalent to
Format.pp_set_formatter_out_channel[29.22] std_formatter.
698

val pp_set_formatter_output_functions :
formatter -> (string -> int -> int -> unit) -> (unit -> unit) -> unit
val set_formatter_output_functions :
(string -> int -> int -> unit) -> (unit -> unit) -> unit
pp_set_formatter_output_functions ppf out flush redirects the standard
pretty-printer output functions to the functions out and flush.
The out function performs all the pretty-printer string output. It is called with a string s, a
start position p, and a number of characters n; it is supposed to output characters p to p + n
- 1 of s.
The flush function is called whenever the pretty-printer is flushed (via conversion %!, or
pretty-printing indications @? or @., or using low level functions print_flush or
print_newline).

val pp_get_formatter_output_functions :
formatter -> unit -> (string -> int -> int -> unit) * (unit -> unit)
val get_formatter_output_functions :
unit -> (string -> int -> int -> unit) * (unit -> unit)
Return the current output functions of the standard pretty-printer.

Redefining formatter output

The Format module is versatile enough to let you completely redefine the meaning of pretty-printing
output: you may provide your own functions to define how to handle indentation, line splitting, and
even printing of all the characters that have to be printed!

Redefining output functions

type formatter_out_functions =
{ out_string : string -> int -> int -> unit ;
out_flush : unit -> unit ;
out_newline : unit -> unit ;
out_spaces : int -> unit ;
out_indent : int -> unit ;
Since: 4.06
}
The set of output functions specific to a formatter:

• the out_string function performs all the pretty-printer string output. It is called with
a string s, a start position p, and a number of characters n; it is supposed to output
characters p to p + n - 1 of s.
• the out_flush function flushes the pretty-printer output device.
• out_newline is called to open a new line when the pretty-printer splits the line.
Chapter 29. The standard library 699

• the out_spaces function outputs spaces when a break hint leads to spaces instead of a
line split. It is called with the number of spaces to output.
• the out_indent function performs new line indentation when the pretty-printer splits
the line. It is called with the indentation value of the new line.

By default:

• fields out_string and out_flush are output device specific; (e.g. output_string[28.2]
and flush[28.2] for a out_channel[28.2] device, or Buffer.add_substring and
ignore[28.2] for a Buffer.t output device),
• field out_newline is equivalent to out_string "\n" 0 1;
• fields out_spaces and out_indent are equivalent to out_string (String.make n '
') 0 n.

Since: 4.01

val pp_set_formatter_out_functions :
formatter -> formatter_out_functions -> unit
val set_formatter_out_functions : formatter_out_functions -> unit
pp_set_formatter_out_functions ppf out_funs Set all the pretty-printer output
functions of ppf to those of argument out_funs,
This way, you can change the meaning of indentation (which can be something else than just
printing space characters) and the meaning of new lines opening (which can be connected to
any other action needed by the application at hand).
Reasonable defaults for functions out_spaces and out_newline are respectively
out_funs.out_string (String.make n ' ') 0 n and out_funs.out_string "\n" 0 1.
Since: 4.01

val pp_get_formatter_out_functions :
formatter -> unit -> formatter_out_functions
val get_formatter_out_functions : unit -> formatter_out_functions
Return the current output functions of the pretty-printer, including line splitting and
indentation functions. Useful to record the current setting and restore it afterwards.
Since: 4.01

Redefining semantic tag operations

type formatter_stag_functions =
{ mark_open_stag : stag -> string ;
mark_close_stag : stag -> string ;
print_open_stag : stag -> unit ;
print_close_stag : stag -> unit ;
}
700

The semantic tag handling functions specific to a formatter: mark versions are the
’tag-marking’ functions that associate a string marker to a tag in order for the pretty-printing
engine to write those markers as 0 length tokens in the output device of the formatter. print
versions are the ’tag-printing’ functions that can perform regular printing when a tag is closed
or opened.
Since: 4.08

val pp_set_formatter_stag_functions :
formatter -> formatter_stag_functions -> unit
val set_formatter_stag_functions : formatter_stag_functions -> unit
pp_set_formatter_stag_functions ppf tag_funs changes the meaning of opening and
closing semantic tag operations to use the functions in tag_funs when printing on ppf.
When opening a semantic tag with name t, the string t is passed to the opening tag-marking
function (the mark_open_stag field of the record tag_funs), that must return the opening
tag marker for that name. When the next call to close_stag () happens, the semantic tag
name t is sent back to the closing tag-marking function (the mark_close_stag field of record
tag_funs), that must return a closing tag marker for that name.
The print_ field of the record contains the tag-printing functions that are called at tag
opening and tag closing time, to output regular material in the pretty-printer queue.
Since: 4.08

val pp_get_formatter_stag_functions :
formatter -> unit -> formatter_stag_functions
val get_formatter_stag_functions : unit -> formatter_stag_functions
Return the current semantic tag operation functions of the standard pretty-printer.
Since: 4.08

Defining formatters
Defining new formatters permits unrelated output of material in parallel on several output devices.
All the parameters of a formatter are local to the formatter: right margin, maximum indentation
limit, maximum number of pretty-printing boxes simultaneously open, ellipsis, and so on, are specific
to each formatter and may be fixed independently.
For instance, given a Buffer.t[29.7] buffer b, Format.formatter_of_buffer[29.22] b returns
a new formatter using buffer b as its output device. Similarly, given a out_channel[28.2] output
channel oc, Format.formatter_of_out_channel[29.22] oc returns a new formatter using channel
oc as its output device.
Alternatively, given out_funs, a complete set of output functions for a formatter, then
Format.formatter_of_out_functions[29.22] out_funs computes a new formatter using those
functions for output.
val formatter_of_out_channel : out_channel -> formatter
formatter_of_out_channel oc returns a new formatter writing to the corresponding
output channel oc.
Chapter 29. The standard library 701

val synchronized_formatter_of_out_channel :
out_channel -> formatter Domain.DLS.key
synchronized_formatter_of_out_channel oc returns the key to the domain-local state
that holds the domain-local formatter for writing to the corresponding output channel oc.
When the formatter is used with multiple domains, the output from the domains will be
interleaved with each other at points where the formatter is flushed, such as with
Format.print_flush[29.22].
Alert unstable

val std_formatter : formatter

The initial domain’s standard formatter to write to standard output.
It is defined as Format.formatter_of_out_channel[29.22] stdout[28.2].

val get_std_formatter : unit -> formatter

get_std_formatter () returns the current domain’s standard formatter used to write to
standard output.
Since: 5.0

val err_formatter : formatter

The initial domain’s formatter to write to standard error.
It is defined as Format.formatter_of_out_channel[29.22] stderr[28.2].

val get_err_formatter : unit -> formatter

get_err_formatter () returns the current domain’s formatter used to write to standard
error.
Since: 5.0

val formatter_of_buffer : Buffer.t -> formatter

formatter_of_buffer b returns a new formatter writing to buffer b. At the end of
pretty-printing, the formatter must be flushed using Format.pp_print_flush[29.22] or
Format.pp_print_newline[29.22], to print all the pending material into the buffer.

val stdbuf : Buffer.t

The initial domain’s string buffer in which str_formatter writes.

val get_stdbuf : unit -> Buffer.t

get_stdbuf () returns the current domain’s string buffer in which the current domain’s
string formatter writes.
Since: 5.0

val str_formatter : formatter

702

The initial domain’s formatter to output to the Format.stdbuf[29.22] string buffer.

str_formatter is defined as Format.formatter_of_buffer[29.22] Format.stdbuf[29.22].

val get_str_formatter : unit -> formatter

The current domain’s formatter to output to the current domains string buffer.
Since: 5.0

val flush_str_formatter : unit -> string

Returns the material printed with str_formatter of the current domain, flushes the
formatter and resets the corresponding buffer.

val make_formatter :
(string -> int -> int -> unit) -> (unit -> unit) -> formatter
make_formatter out flush returns a new formatter that outputs with function out, and
flushes with function flush.
For instance,

make_formatter
(Stdlib.output_substring oc)
(fun () -> Stdlib.flush oc)

returns a formatter to the out_channel[28.2] oc.

val make_synchronized_formatter :
(string -> int -> int -> unit) ->
(unit -> unit) -> formatter Domain.DLS.key
make_synchronized_formatter out flush returns the key to the domain-local state that
holds the domain-local formatter that outputs with function out, and flushes with function
flush.
When the formatter is used with multiple domains, the output from the domains will be
interleaved with each other at points where the formatter is flushed, such as with
Format.print_flush[29.22].
Since: 5.0
Alert unstable

val formatter_of_out_functions : formatter_out_functions -> formatter

formatter_of_out_functions out_funs returns a new formatter that writes with the set
of output functions out_funs.
See definition of type Format.formatter_out_functions[29.22] for the meaning of
argument out_funs.
Since: 4.06
Chapter 29. The standard library 703

Symbolic pretty-printing
Symbolic pretty-printing is pretty-printing using a symbolic formatter, i.e. a formatter that outputs
symbolic pretty-printing items.
When using a symbolic formatter, all regular pretty-printing activities occur but output material
is symbolic and stored in a buffer of output items. At the end of pretty-printing, flushing the output
buffer allows post-processing of symbolic output before performing low level output operations.
In practice, first define a symbolic output buffer b using:
• let sob = make_symbolic_output_buffer (). Then define a symbolic formatter with:

• let ppf = formatter_of_symbolic_output_buffer sob

Use symbolic formatter ppf as usual, and retrieve symbolic items at end of pretty-printing by
flushing symbolic output buffer sob with:
• flush_symbolic_output_buffer sob.

type symbolic_output_item =
| Output_flush
symbolic flush command
| Output_newline
symbolic newline command
| Output_string of string
Output_string s: symbolic output for string s
| Output_spaces of int
Output_spaces n: symbolic command to output n spaces
| Output_indent of int
Output_indent i: symbolic indentation of size i
Items produced by symbolic pretty-printers
Since: 4.06

type symbolic_output_buffer
The output buffer of a symbolic pretty-printer.
Since: 4.06

val make_symbolic_output_buffer : unit -> symbolic_output_buffer

make_symbolic_output_buffer () returns a fresh buffer for symbolic output.
Since: 4.06

val clear_symbolic_output_buffer : symbolic_output_buffer -> unit

clear_symbolic_output_buffer sob resets buffer sob.
Since: 4.06
704

val get_symbolic_output_buffer :
symbolic_output_buffer -> symbolic_output_item list
get_symbolic_output_buffer sob returns the contents of buffer sob.
Since: 4.06

val flush_symbolic_output_buffer :
symbolic_output_buffer -> symbolic_output_item list
flush_symbolic_output_buffer sob returns the contents of buffer sob and resets buffer
sob. flush_symbolic_output_buffer sob is equivalent to let items =
get_symbolic_output_buffer sob in clear_symbolic_output_buffer sob; items
Since: 4.06

val add_symbolic_output_item :
symbolic_output_buffer -> symbolic_output_item -> unit
add_symbolic_output_item sob itm adds item itm to buffer sob.
Since: 4.06

val formatter_of_symbolic_output_buffer : symbolic_output_buffer -> formatter

formatter_of_symbolic_output_buffer sob returns a symbolic formatter that outputs to
symbolic_output_buffer sob.
Since: 4.06

Convenience formatting functions.

val pp_print_iter :
?pp_sep:(formatter -> unit -> unit) ->
(('a -> unit) -> 'b -> unit) ->
(formatter -> 'a -> unit) -> formatter -> 'b -> unit
pp_print_iter ~pp_sep iter pp_v ppf v formats on ppf the iterations of iter over a
collection v of values using pp_v. Iterations are separated by pp_sep (defaults to
Format.pp_print_cut[29.22]).
Since: 5.1

val pp_print_list :
?pp_sep:(formatter -> unit -> unit) ->
(formatter -> 'a -> unit) -> formatter -> 'a list -> unit
pp_print_list ?pp_sep pp_v ppf l prints items of list l, using pp_v to print each item,
and calling pp_sep between items (pp_sep defaults to Format.pp_print_cut[29.22]). Does
nothing on empty lists.
Since: 4.02
Chapter 29. The standard library 705

val pp_print_array :
?pp_sep:(formatter -> unit -> unit) ->
(formatter -> 'a -> unit) -> formatter -> 'a array -> unit
pp_print_array ?pp_sep pp_v ppf a prints items of array a, using pp_v to print each
item, and calling pp_sep between items (pp_sep defaults to Format.pp_print_cut[29.22]).
Does nothing on empty arrays.
If a is mutated after pp_print_array is called, the printed values may not be what is
expected because Format can delay the printing. This can be avoided by flushing ppf.
Since: 5.1

val pp_print_seq :
?pp_sep:(formatter -> unit -> unit) ->
(formatter -> 'a -> unit) ->
formatter -> 'a Seq.t -> unit
pp_print_seq ?pp_sep pp_v ppf s prints items of sequence s, using pp_v to print each
item, and calling pp_sep between items (pp_sep defaults to Format.pp_print_cut[29.22].
Does nothing on empty sequences.
This function does not terminate on infinite sequences.
Since: 4.12

val pp_print_text : formatter -> string -> unit

pp_print_text ppf s prints s with spaces and newlines respectively printed using
Format.pp_print_space[29.22] and Format.pp_force_newline[29.22].
Since: 4.02

val pp_print_option :
?none:(formatter -> unit -> unit) ->
(formatter -> 'a -> unit) -> formatter -> 'a option -> unit
pp_print_option ?none pp_v ppf o prints o on ppf using pp_v if o is Some v and none if
it is None. none prints nothing by default.
Since: 4.08

val pp_print_result :
ok:(formatter -> 'a -> unit) ->
error:(formatter -> 'e -> unit) ->
formatter -> ('a, 'e) result -> unit
pp_print_result ~ok ~error ppf r prints r on ppf using ok if r is Ok _ and error if r is
Error _.
Since: 4.08

val pp_print_either :
left:(formatter -> 'a -> unit) ->
right:(formatter -> 'b -> unit) ->
formatter -> ('a, 'b) Either.t -> unit
706

pp_print_either ~left ~right ppf e prints e on ppf using left if e is Either.Left

_ and right if e is Either.Right _.
Since: 4.13

Formatted pretty-printing
Module Format provides a complete set of printf like functions for pretty-printing using format
string specifications.
Specific annotations may be added in the format strings to give pretty-printing commands to
the pretty-printing engine.
Those annotations are introduced in the format strings using the @ character. For instance, @
means a space break, @, means a cut, @[ opens a new box, and @] closes the last open box.
val fprintf : formatter -> ('a, formatter, unit) format -> 'a
fprintf ff fmt arg1 ... argN formats the arguments arg1 to argN according to the format
string fmt, and outputs the resulting string on the formatter ff.
The format string fmt is a character string which contains three types of objects: plain characters
and conversion specifications as specified in the Printf[29.44] module, and pretty-printing indications
specific to the Format module.
The pretty-printing indication characters are introduced by a @ character, and their meanings
are:

• @[: open a pretty-printing box. The type and offset of the box may be optionally specified
with the following syntax: the < character, followed by an optional box type indication, then
an optional integer offset, and the closing > character. Pretty-printing box type is one of h,
v, hv, b, or hov. ’h’ stands for an ’horizontal’ pretty-printing box, ’v’ stands for a ’vertical’
pretty-printing box, ’hv’ stands for an ’horizontal/vertical’ pretty-printing box, ’b’ stands for
an ’horizontal-or-vertical’ pretty-printing box demonstrating indentation, ’hov’ stands a simple
’horizontal-or-vertical’ pretty-printing box. For instance, @[<hov 2> opens an ’horizontal-or-
vertical’ pretty-printing box with indentation 2 as obtained with open_hovbox 2. For more
details about pretty-printing boxes, see the various box opening functions open_*box.

• @]: close the most recently opened pretty-printing box.

• @,: output a ’cut’ break hint, as with print_cut ().

• @ : output a ’space’ break hint, as with print_space ().

• @;: output a ’full’ break hint as with print_break. The nspaces and offset parameters
of the break hint may be optionally specified with the following syntax: the < character,
followed by an integer nspaces value, then an integer offset, and a closing > character. If
no parameters are provided, the full break defaults to a ’space’ break hint.

• @.: flush the pretty-printer and split the line, as with print_newline ().

• @<n>: print the following item as if it were of length n. Hence, printf "@<0>%s" arg prints
arg as a zero length string. If @<n> is not followed by a conversion specification, then the
following character of the format is printed as if it were of length n.
Chapter 29. The standard library 707

• @{: open a semantic tag. The name of the tag may be optionally specified with the following
syntax: the < character, followed by an optional string specification, and the closing > character.
The string specification is any character string that does not contain the closing character
'>'. If omitted, the tag name defaults to the empty string. For more details about semantic
tags, see the functions Format.open_stag[29.22] and Format.close_stag[29.22].

• @}: close the most recently opened semantic tag.

• @?: flush the pretty-printer as with print_flush (). This is equivalent to the conversion %!.

• @\n: force a newline, as with force_newline (), not the normal way of pretty-printing, you
should prefer using break hints inside a vertical pretty-printing box.

Note: To prevent the interpretation of a @ character as a pretty-printing indication, escape it

with a % character. Old quotation mode @@ is deprecated since it is not compatible with formatted
input interpretation of character '@'.
Example: printf "@[%s@ %d@]@." "x =" 1 is equivalent to open_box (); print_string
"x ="; print_space (); print_int 1; close_box (); print_newline (). It prints x = 1
within a pretty-printing ’horizontal-or-vertical’ box.
val printf : ('a, formatter, unit) format -> 'a
Same as fprintf above, but output on get_std_formatter ().
It is defined similarly to fun fmt -> fprintf (get_std_formatter ()) fmt but delays
calling get_std_formatter until after the final argument required by the format is received.
When used with multiple domains, the output from the domains will be interleaved with each
other at points where the formatter is flushed, such as with Format.print_flush[29.22].

val eprintf : ('a, formatter, unit) format -> 'a

Same as fprintf above, but output on get_err_formatter ().
It is defined similarly to fun fmt -> fprintf (get_err_formatter ()) fmt but delays
calling get_err_formatter until after the final argument required by the format is received.
When used with multiple domains, the output from the domains will be interleaved with each
other at points where the formatter is flushed, such as with Format.print_flush[29.22].

val sprintf : ('a, unit, string) format -> 'a

Same as printf above, but instead of printing on a formatter, returns a string containing the
result of formatting the arguments. Note that the pretty-printer queue is flushed at the end of
each call to sprintf. Note that if your format string contains a %a, you should use asprintf.
In case of multiple and related calls to sprintf to output material on a single string, you
should consider using fprintf with the predefined formatter str_formatter and call
flush_str_formatter () to get the final result.
Alternatively, you can use Format.fprintf with a formatter writing to a buffer of your own:
flushing the formatter and the buffer at the end of pretty-printing returns the desired string.

val asprintf : ('a, formatter, unit, string) format4 -> 'a

708

Same as printf above, but instead of printing on a formatter, returns a string containing the
result of formatting the arguments. The type of asprintf is general enough to interact nicely
with %a conversions.
Since: 4.01

val dprintf : ('a, formatter, unit, formatter -> unit) format4 -> 'a
Same as Format.fprintf[29.22], except the formatter is the last argument. dprintf "..."
a b c is a function of type formatter -> unit which can be given to a format specifier %t.
This can be used as a replacement for Format.asprintf[29.22] to delay formatting
decisions. Using the string returned by Format.asprintf[29.22] in a formatting context
forces formatting decisions to be taken in isolation, and the final string may be created
prematurely. Format.dprintf[29.22] allows delay of formatting decisions until the final
formatting context is known. For example:

let t = Format.dprintf "%i@ %i@ %i" 1 2 3 in

...
Format.printf "@[<v>%t@]" t

Since: 4.08

val ifprintf : formatter -> ('a, formatter, unit) format -> 'a
Same as fprintf above, but does not print anything. Useful to ignore some material when
conditionally printing.
Since: 3.10

Formatted Pretty-Printing with continuations.

val kfprintf :
(formatter -> 'a) ->
formatter -> ('b, formatter, unit, 'a) format4 -> 'b
Same as fprintf above, but instead of returning immediately, passes the formatter to its first
argument at the end of printing.

val kdprintf :
((formatter -> unit) -> 'a) ->
('b, formatter, unit, 'a) format4 -> 'b
Same as Format.dprintf[29.22] above, but instead of returning immediately, passes the
suspended printer to its first argument at the end of printing.
Since: 4.08

val ikfprintf :
(formatter -> 'a) ->
formatter -> ('b, formatter, unit, 'a) format4 -> 'b
Chapter 29. The standard library 709

Same as kfprintf above, but does not print anything. Useful to ignore some material when
conditionally printing.
Since: 3.12

val ksprintf : (string -> 'a) -> ('b, unit, string, 'a) format4 -> 'b
Same as sprintf above, but instead of returning the string, passes it to the first argument.

val kasprintf : (string -> 'a) -> ('b, formatter, unit, 'a) format4 -> 'b
Same as asprintf above, but instead of returning the string, passes it to the first argument.
Since: 4.03

Examples
A few warmup examples to get an idea of how Format is used.
We have a list l of pairs (int * bool), which the toplevel prints for us:
# let l = List.init 20 (fun n -> n, n mod 2 = 0)
val l : (int * bool) list =
[(0, true); (1, false); (2, true); (3, false); (4, true); (5, false);
(6, true); (7, false); (8, true); (9, false); (10, true); (11, false);
(12, true); (13, false); (14, true); (15, false); (16, true); (17, false);
(18, true); (19, false)]

If we want to print it ourself without the toplevel magic, we can try this:

# let pp_pair out (x,y) = Format.fprintf out "(%d, %b)" x y

val pp_pair : Format.formatter -> int * bool -> unit = <fun>
# Format.printf "l: [@[<hov>%a@]]@."
Format.(pp_print_list ~pp_sep:(fun out () -> fprintf out ";@ ") pp_pair) l
l: [(0, true); (1, false); (2, true); (3, false); (4, true); (5, false);
(6, true); (7, false); (8, true); (9, false); (10, true); (11, false);
(12, true); (13, false); (14, true); (15, false); (16, true);
(17, false); (18, true); (19, false)]

What this does, briefly, is:

• pp_pair prints a pair bool*int surrounded in ”(” ”)”. It takes a formatter (into which
formatting happens), and the pair itself. When printing is done it returns ().

• Format.printf "l = [@[<hov>%a@]]@." ... l is like printf, but with additional format-
ting instructions (denoted with ”@”). The pair ”@[<hov>” and ”@]” is a ”horizontal-or-vertical
box”.

• ”@.” ends formatting with a newline. It is similar to ”\n” but is also aware of the
Format.formatter’s state. Do not use ”\n” with Format.
710

• ”%a” is a formatting instruction, like ”%d” or ”%s” for printf. However, where ”%d” prints
an integer and ”%s” prints a string, ”%a” takes a printer (of type Format.formatter ->
'a -> unit) and a value (of type 'a) and applies the printer to the value. This is key to
compositionality of printers.

• We build a list printer using Format.pp_print_list ~pp_sep:(...) pp_pair. pp_print_

list takes an element printer and returns a list printer. The ?pp_sep optional argument, if
provided, is called in between each element to print a separator.

• Here, for a separator, we use (fun out () -> Format.fprintf out ";@ "). It prints ”;”,
and then ”@ ” which is a breaking space (either it prints ” ”, or it prints a newline if the box
is about to overflow). This ”@ ” is responsible for the list printing splitting into several lines.

If we omit ”@ ”, we get an ugly single-line print:

# Format.printf "l: [@[<hov>%a@]]@."
Format.(pp_print_list ~pp_sep:(fun out () -> fprintf out "; ") pp_pair) l
l: [(0, true); (1, false); (2, true); (* ... *); (18, true); (19, false)]
- : unit = ()

Generally, it is good practice to define custom printers for important types in your program. If,
for example, you were to define basic geometry types like so:

type point = {
x: float;
y: float;
}

type rectangle = {
ll: point; (* lower left *)
ur: point; (* upper right *)
}

For debugging purpose, or to display information in logs, or on the console, it would be convenient
to define printers for these types. Here is an example of to do it. Note that ”%.3f” is a float
printer up to 3 digits of precision after the dot; ”%f” would print as many digits as required, which
is somewhat verbose; ”%h” is an hexadecimal float printer.

let pp_point out (p:point) =

Format.fprintf out "{ @[x=%.3f;@ y=%.3f@] }" p.x p.y

let pp_rectangle out (r:rectangle) =

Format.fprintf out "{ @[ll=%a;@ ur=%a@] }"
pp_point r.ll pp_point r.ur

In the .mli file, we could have:

Chapter 29. The standard library 711

val pp_point : Format.formatter -> point -> unit

val pp_rectangle : Format.formatter -> rectangle -> unit

These printers can now be used with ”%a” inside other printers.
# Format.printf "some rectangle: %a@."
(Format.pp_print_option pp_rectangle)
(Some {ll={x=1.; y=2.}; ur={x=42.; y=500.12345}})
some rectangle: { l={ x=1.000; y=2.000 }; ur={ x=42.000; y=500.123 } }

# Format.printf "no rectangle: %a@."

(Format.pp_option pp_rectangle)
None
no rectangle:

See how we combine pp_print_option (option printer) and our newly defined rectangle printer,
like we did with pp_print_list earlier.
For a more extensive tutorial, see
”Using the Format module”[https://caml.inria.fr/resources/doc/guides/format.en.
html].
A final note: the Format module is a starting point. The OCaml ecosystem has libraries that
makes formatting easier and more expressive, with more combinators, more concise names, etc. An
example of such a library is Fmt[https://erratique.ch/software/fmt].
Automatic deriving of pretty-printers from type definitions is also possible, using
https://github.com/ocaml-ppx/ppx_deriving[ppx_deriving.show] or similar ppx derivers.

29.23 Module Fun : Function manipulation.

Since: 4.08

Combinators
val id : 'a -> 'a
id is the identity function. For any argument x, id x is x.

val const : 'a -> 'b -> 'a

const c is a function that always returns the value c. For any argument x, (const c) x is c.

val compose : ('b -> 'c) -> ('a -> 'b) -> 'a -> 'c
compose f g is a function composition of applying g then f. For any arguments f, g, and x,
compose f g x is f (g x).
Since: 5.2
712

val flip : ('a -> 'b -> 'c) -> 'b -> 'a -> 'c
flip f reverses the argument order of the binary function f. For any arguments x and y,
(flip f) x y is f y x.

val negate : ('a -> bool) -> 'a -> bool

negate p is the negation of the predicate function p. For any argument x, (negate p) x is
not (p x).

Exception handling
val protect : finally:(unit -> unit) -> (unit -> 'a) -> 'a
protect ~finally work invokes work () and then finally () before work () returns
with its value or an exception. In the latter case the exception is re-raised after finally ().
If finally () raises an exception, then the exception Fun.Finally_raised[29.23] is raised
instead.
protect can be used to enforce local invariants whether work () returns normally or raises
an exception. However, it does not protect against unexpected exceptions raised inside
finally () such as Out_of_memory[28.2], Stack_overflow[28.2], or asynchronous
exceptions raised by signal handlers (e.g. Sys.Break[29.56]).
Note: It is a programming error if other kinds of exceptions are raised by finally, as any
exception raised in work () will be lost in the event of a Fun.Finally_raised[29.23]
exception. Therefore, one should make sure to handle those inside the finally.

exception Finally_raised of exn

Finally_raised exn is raised by protect ~finally work when finally raises an
exception exn. This exception denotes either an unexpected exception or a programming
error. As a general rule, one should not catch a Finally_raised exception except as part of
a catch-all handler.

29.24 Module Gc : Memory management control and statistics;

finalised values.

type stat =
{ minor_words : float ;
Number of words allocated in the minor heap since the program was started.
promoted_words : float ;
Number of words allocated in the minor heap that survived a minor collection and were
moved to the major heap since the program was started.
major_words : float ;
Chapter 29. The standard library 713

Number of words allocated in the major heap, including the promoted words, since the
program was started.
minor_collections : int ;
Number of minor collections since the program was started.
major_collections : int ;
Number of major collection cycles completed since the program was started.
heap_words : int ;
Total size of the major heap, in words.
heap_chunks : int ;
Number of contiguous pieces of memory that make up the major heap. This metric is
currently not available in OCaml 5: the field value is always 0.
live_words : int ;
Number of words of live data in the major heap, including the header words.
Note that ”live” words refers to every word in the major heap that isn’t currently
known to be collectable, which includes words that have become unreachable by the
program after the start of the previous gc cycle. It is typically much simpler and more
predictable to call Gc.full_major[29.24] (or Gc.compact[29.24]) then computing gc
stats, as then ”live” words has the simple meaning of ”reachable by the program”. One
caveat is that a single call to Gc.full_major[29.24] will not reclaim values that have a
finaliser from Gc.finalise[29.24] (this does not apply to Gc.finalise_last[29.24]). If
this caveat matters, simply call Gc.full_major[29.24] twice instead of once.
live_blocks : int ;
Number of live blocks in the major heap.
See live_words for a caveat about what ”live” means.
free_words : int ;
Number of words in the free list.
free_blocks : int ;
Number of blocks in the free list. This metric is currently not available in OCaml 5:
the field value is always 0.
largest_free : int ;
Size (in words) of the largest block in the free list. This metric is currently not
available in OCaml 5: the field value is always 0.
fragments : int ;
Number of wasted words due to fragmentation. These are 1-words free blocks placed
between two live blocks. They are not available for allocation.
compactions : int ;
Number of heap compactions since the program was started.
714

top_heap_words : int ;
Maximum size reached by the major heap, in words.
stack_size : int ;
Current size of the stack, in words. This metric is currently not available in OCaml 5:
the field value is always 0.
Since: 3.12
forced_major_collections : int ;
Number of forced full major collections completed since the program was started.
Since: 4.12
}
The memory management counters are returned in a stat record. These counters give values
for the whole program.
The total amount of memory allocated by the program since it was started is (in words)
minor_words + major_words - promoted_words. Multiply by the word size (4 on a 32-bit
machine, 8 on a 64-bit machine) to get the number of bytes.

type control =
{ minor_heap_size : int ;
The size (in words) of the minor heap. Changing this parameter will trigger a minor
collection. The total size of the minor heap used by this program is the sum of the heap
sizes of the active domains. Default: 256k.
major_heap_increment : int ;
How much to add to the major heap when increasing it. If this number is less than or
equal to 1000, it is a percentage of the current heap size (i.e. setting it to 100 will
double the heap size at each increase). If it is more than 1000, it is a fixed number of
words that will be added to the heap.
This field is currently not available in OCaml 5: the field value is always 0.
space_overhead : int ;
The major GC speed is computed from this parameter. This is the memory that will
be ”wasted” because the GC does not immediately collect unreachable blocks. It is
expressed as a percentage of the memory used for live data. The GC will work more
(use more CPU time and collect blocks more eagerly) if space_overhead is smaller.
Default: 120.
verbose : int ;
This value controls the GC messages on standard error output. It is a sum of some of
the following flags, to print messages on the corresponding events:

• 0x001 Start and end of major GC cycle.

• 0x002 Minor collection and major GC slice.
• 0x004 Growing and shrinking of the heap.
Chapter 29. The standard library 715

• 0x008 Resizing of stacks and memory manager tables.

• 0x010 Heap compaction.
• 0x020 Change of GC parameters.
• 0x040 Computation of major GC slice size.
• 0x080 Calling of finalisation functions.
• 0x100 Bytecode executable and shared library search at start-up.
• 0x200 Computation of compaction-triggering condition.
• 0x400 Output GC statistics at program exit. Default: 0.

max_overhead : int ;
Heap compaction is triggered when the estimated amount of ”wasted” memory is more
than max_overhead percent of the amount of live data. If max_overhead is set to 0,
heap compaction is triggered at the end of each major GC cycle (this setting is intended
for testing purposes only). If max_overhead >= 1000000, compaction is never
triggered.
This field is currently not available in OCaml 5: the field value is always 0.
stack_limit : int ;
The maximum size of the fiber stacks (in words). Default: 128M.
allocation_policy : int ;
The policy used for allocating in the major heap.
This field is currently not available in OCaml 5: the field value is always 0.
Prior to OCaml 5.0, possible values were 0, 1 and 2.

• 0 was the next-fit policy

• 1 was the first-fit policy (since OCaml 3.11)

• 2 was the best-fit policy (since OCaml 4.10)

Since: 3.11
window_size : int ;
The size of the window used by the major GC for smoothing out variations in its
workload. This is an integer between 1 and 50.
Since: 4.03
This field is currently not available in OCaml 5: the field value is always [0].
custom_major_ratio : int ;
716

Target ratio of floating garbage to major heap size for out-of-heap memory held by
custom values located in the major heap. The GC speed is adjusted to try to use this
much memory for dead values that are not yet collected. Expressed as a percentage of
major heap size. The default value keeps the out-of-heap floating garbage about the
same size as the in-heap overhead. Note: this only applies to values allocated with
caml_alloc_custom_mem (e.g. bigarrays). Default: 44.
Since: 4.08
custom_minor_ratio : int ;
Bound on floating garbage for out-of-heap memory held by custom values in the minor
heap. A minor GC is triggered when this much memory is held by custom values located
in the minor heap. Expressed as a percentage of minor heap size. Note: this only applies
to values allocated with caml_alloc_custom_mem (e.g. bigarrays). Default: 100.
Since: 4.08
custom_minor_max_size : int ;
Maximum amount of out-of-heap memory for each custom value allocated in the minor
heap. Custom values that hold more than this many bytes are allocated on the major
heap. Note: this only applies to values allocated with caml_alloc_custom_mem (e.g.
bigarrays). Default: 70000 bytes.
Since: 4.08
}
The GC parameters are given as a control record. Note that these parameters can also be
initialised by setting the OCAMLRUNPARAM environment variable. See the documentation
of ocamlrun.

val stat : unit -> stat

Return the current values of the memory management counters in a stat record that
represents the program’s total memory stats. The heap_chunks, free_blocks,
largest_free, and stack_size metrics are currently not available in OCaml 5: their
returned field values are therefore 0. This function causes a full major collection.

val quick_stat : unit -> stat

Returns a record with the current values of the memory management counters like stat.
Unlike stat, quick_stat does not perform a full major collection, and hence, is much faster.
However, quick_stat reports the counters sampled at the last minor collection or at the end
of the last major collection cycle (whichever is the latest). Hence, the memory stats returned
by quick_stat are not instantaneously accurate.

val counters : unit -> float * float * float

Return (minor_words, promoted_words, major_words) for the current domain or
potentially previous domains. This function is as fast as quick_stat.

val minor_words : unit -> float

Chapter 29. The standard library 717

Number of words allocated in the minor heap by this domain or potentially previous domains.
This number is accurate in byte-code programs, but only an approximation in programs
compiled to native code.
In native code this function does not allocate.
Since: 4.04

val get : unit -> control

Return the current values of the GC parameters in a control record.
The major_heap_increment, max_overhead, allocation_policy, and window_size fields
are currently not available in OCaml 5: their returned field values are therefore 0.
Alert unsynchronized_access. GC parameters are a mutable global state.

val set : control -> unit

set r changes the GC parameters according to the control record r. The normal usage is:
Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
The major_heap_increment, max_overhead, allocation_policy, and window_size fields
are currently not available in OCaml 5: setting them therefore has no effect.
Alert unsynchronized_access. GC parameters are a mutable global state.

val minor : unit -> unit

Trigger a minor collection.

val major_slice : int -> int

major_slice n Do a minor collection and a slice of major collection. n is the size of the slice:
the GC will do enough work to free (on average) n words of memory. If n = 0, the GC will
try to do enough work to ensure that the next automatic slice has no work to do. This
function returns an unspecified integer (currently: 0).

val major : unit -> unit

Do a minor collection and finish the current major collection cycle.

val full_major : unit -> unit

Do a minor collection, finish the current major collection cycle, and perform a complete new
cycle. This will collect all currently unreachable blocks.

val compact : unit -> unit

Perform a full major collection and compact the heap. Note that heap compaction is a
lengthy operation.

val print_stat : out_channel -> unit

Print the current values of the memory management counters (in human-readable form) of
the total program into the channel argument.
718

val allocated_bytes : unit -> float

Return the number of bytes allocated by this domain and potentially a previous domain. It is
returned as a float to avoid overflow problems with int on 32-bit machines.

val get_minor_free : unit -> int

Return the current size of the free space inside the minor heap of this domain.
Since: 4.03

val finalise : ('a -> unit) -> 'a -> unit

finalise f v registers f as a finalisation function for v. v must be heap-allocated. f will be
called with v as argument at some point between the first time v becomes unreachable
(including through weak pointers) and the time v is collected by the GC. Several functions can
be registered for the same value, or even several instances of the same function. Each instance
will be called once (or never, if the program terminates before v becomes unreachable).
The GC will call the finalisation functions in the order of deallocation. When several values
become unreachable at the same time (i.e. during the same GC cycle), the finalisation
functions will be called in the reverse order of the corresponding calls to finalise. If
finalise is called in the same order as the values are allocated, that means each value is
finalised before the values it depends upon. Of course, this becomes false if additional
dependencies are introduced by assignments.
In the presence of multiple OCaml threads it should be assumed that any particular finaliser
may be executed in any of the threads.
Anything reachable from the closure of finalisation functions is considered reachable, so the
following code will not work as expected:

• let v = ... in Gc.finalise (fun _ -> ...v...) v

Instead you should make sure that v is not in the closure of the finalisation function by
writing:

• let f = fun x -> ... let v = ... in Gc.finalise f v

The f function can use all features of OCaml, including assignments that make the value
reachable again. It can also loop forever (in this case, the other finalisation functions will not
be called during the execution of f, unless it calls finalise_release). It can call finalise
on v or other values to register other functions or even itself. It can raise an exception; in this
case the exception will interrupt whatever the program was doing when the function was
called.
finalise will raise Invalid_argument if v is not guaranteed to be heap-allocated. Some
examples of values that are not heap-allocated are integers, constant constructors, booleans,
the empty array, the empty list, the unit value. The exact list of what is heap-allocated or not
is implementation-dependent. Some constant values can be heap-allocated but never
deallocated during the lifetime of the program, for example a list of integer constants; this is
also implementation-dependent. Note that values of types float are sometimes allocated and
sometimes not, so finalising them is unsafe, and finalise will also raise Invalid_argument
Chapter 29. The standard library 719

for them. Values of type 'a Lazy.t (for any 'a) are like float in this respect, except that
the compiler sometimes optimizes them in a way that prevents finalise from detecting
them. In this case, it will not raise Invalid_argument, but you should still avoid calling
finalise on lazy values.
The results of calling String.make[29.54], Bytes.make[29.8], Bytes.create[29.8],
Array.make[29.2], and ref[28.2] are guaranteed to be heap-allocated and non-constant except
when the length argument is 0.

val finalise_last : (unit -> unit) -> 'a -> unit

same as Gc.finalise[29.24] except the value is not given as argument. So you can’t use the
given value for the computation of the finalisation function. The benefit is that the function
is called after the value is unreachable for the last time instead of the first time. So contrary
to Gc.finalise[29.24] the value will never be reachable again or used again. In particular
every weak pointer and ephemeron that contained this value as key or data is unset before
running the finalisation function. Moreover the finalisation functions attached with
Gc.finalise[29.24] are always called before the finalisation functions attached with
Gc.finalise_last[29.24].
Since: 4.04

val finalise_release : unit -> unit

A finalisation function may call finalise_release to tell the GC that it can launch the next
finalisation function without waiting for the current one to return.

type alarm
An alarm is a piece of data that calls a user function at the end of major GC cycle. The
following functions are provided to create and delete alarms.

val create_alarm : (unit -> unit) -> alarm

create_alarm f will arrange for f to be called at the end of major GC cycles, not caused by
f itself, starting with the current cycle or the next one. f will run on the same domain that
created the alarm, until the domain exits or delete_alarm is called. A value of type alarm is
returned that you can use to call delete_alarm.
It is not guaranteed that the Gc alarm runs at the end of every major GC cycle, but it is
guaranteed that it will run eventually.
As an example, here is a crude way to interrupt a function if the memory consumption of the
program exceeds a given limit in MB, suitable for use in the toplevel:

let run_with_memory_limit (limit : int) (f : unit -> 'a) : 'a =

let limit_memory () =
let mem = Gc.(quick_stat ()).heap_words in
if mem / (1024 * 1024) > limit / (Sys.word_size / 8) then
raise Out_of_memory
in
720

let alarm = Gc.create_alarm limit_memory in

Fun.protect f ~finally:(fun () -> Gc.delete_alarm alarm ; Gc.compact ())

val delete_alarm : alarm -> unit

delete_alarm a will stop the calls to the function associated to a. Calling delete_alarm a
again has no effect.

val eventlog_pause : unit -> unit

Deprecated. Use Runtime_events.pause instead.

val eventlog_resume : unit -> unit

Deprecated. Use Runtime_events.resume instead.

module Memprof :
sig
type t
the type of a profile

type allocation_source =
| Normal
| Marshal
| Custom
type allocation = private
{ n_samples : int ;
The number of samples in this block (≥ 1).
size : int ;
The size of the block, in words, excluding the header.
source : allocation_source ;
The cause of the allocation.
callstack : Printexc.raw_backtrace ;
The callstack for the allocation.
}
The type of metadata associated with allocations. This is the type of records passed to
the callback triggered by the sampling of an allocation.

type ('minor, 'major) tracker =

{ alloc_minor : allocation -> 'minor option ;
alloc_major : allocation -> 'major option ;
promote : 'minor -> 'major option ;
Chapter 29. The standard library 721

dealloc_minor : 'minor -> unit ;

dealloc_major : 'major -> unit ;
}
A ('minor, 'major) tracker describes how memprof should track sampled blocks
over their lifetime, keeping a user-defined piece of metadata for each of them: 'minor is
the type of metadata to keep for minor blocks, and 'major the type of metadata for
major blocks.
The member functions in a tracker are called callbacks.
If an allocation or promotion callback raises an exception or returns None, memprof
stops tracking the corresponding block.

val null_tracker : ('minor, 'major) tracker

Default callbacks simply return None or ()

val start :
sampling_rate:float ->
?callstack_size:int -> ('minor, 'major) tracker -> t
Start a profile with the given parameters. Raises an exception if a profile is already
sampling in the current domain.
Sampling begins immediately. The parameter sampling_rate is the sampling rate in
samples per word (including headers). Usually, with cheap callbacks, a rate of 1e-4 has
no visible effect on performance, and 1e-3 causes the program to run a few percent
slower. 0.0 ≤ sampling_rate ≤ 1.0.
The parameter callstack_size is the length of the callstack recorded at every sample.
Its default is max_int.
The parameter tracker determines how to track sampled blocks over their lifetime in
the minor and major heap.
Sampling and running callbacks are temporarily disabled on the current thread when
calling a callback, so callbacks do not need to be re-entrant if the program is
single-threaded and single-domain. However, if threads or multiple domains are used, it
is possible that several callbacks will run in parallel. In this case, callback functions
must be re-entrant.
Note that a callback may be postponed slightly after the actual event. The callstack
passed to an allocation callback always accurately reflects the allocation, but the
program state may have evolved between the allocation and the call to the callback.
If a new thread or domain is created when the current domain is sampling for a profile,
the child thread or domain joins that profile (using the same sampling_rate,
callstack_size, and tracker callbacks).
An allocation callback is always run by the thread which allocated the block. If the
thread exits or the profile is stopped before the callback is called, the allocation callback
is not called and the block is not tracked.
722

Each subsequent callback is generally run by the domain which allocated the block. If
the domain terminates or the profile is stopped before the callback is called, the callback
may be run by a different domain.
Different domains may sample for different profiles simultaneously.

val stop : unit -> unit

Stop sampling for the current profile. Fails if no profile is sampling in the current
domain. Stops sampling in all threads and domains sharing the profile.
Promotion and deallocation callbacks from a profile may run after stop is called, until
discard is applied to the profile.
A profile is implicitly stopped (but not discarded) if all domains and threads sampling
for it are terminated.

val discard : t -> unit

Discards all profiling state for a stopped profile, which prevents any more callbacks for it.
Raises an exception if called on a profile which has not been stopped.

end

Memprof is a profiling engine which randomly samples allocated memory words. Every
allocated word has a probability of being sampled equal to a configurable sampling rate.
Once a block is sampled, it becomes tracked. A tracked block triggers a user-defined callback
as soon as it is allocated, promoted or deallocated.
Since blocks are composed of several words, a block can potentially be sampled several times.
If a block is sampled several times, then each of the callbacks is called once for each event of
this block: the multiplicity is given in the n_samples field of the allocation structure.
This engine makes it possible to implement a low-overhead memory profiler as an OCaml
library.
Note: this API is EXPERIMENTAL. It may change without prior notice.

29.25 Module Hashtbl : Hash tables and hash functions.

Hash tables are hashed association tables, with in-place modification. Because most operations on a
hash table modify their input, they’re more commonly used in imperative code. The lookup of the
value associated with a key (see Hashtbl.find[29.25], Hashtbl.find_opt[29.25]) is normally very
fast, often faster than the equivalent lookup in Map[29.34].
The functors Hashtbl.Make[29.25] and Hashtbl.MakeSeeded[29.25] can be used when perfor-
mance or flexibility are key. The user provides custom equality and hash functions for the key type,
and obtains a custom hash table type for this particular type of key.
Warning a hash table is only as good as the hash function. A bad hash function will turn the
table into a degenerate association list, with linear time lookup instead of constant time lookup.
Chapter 29. The standard library 723

The polymorphic Hashtbl.t[29.25] hash table is useful in simpler cases or in interactive envi-
ronments. It uses the polymorphic Hashtbl.hash[29.25] function defined in the OCaml runtime (at
the time of writing, it’s SipHash), as well as the polymorphic equality (=).
See the examples section[29.45].
Alert unsynchronized_access. Unsynchronized accesses to hash tables are a programming
error.

Unsynchronized accesses
Unsynchronized accesses to a hash table may lead to an invalid hash table state. Thus, concurrent
accesses to a hash tables must be synchronized (for instance with a Mutex.t[29.37]).

Generic interface
type (!'a, !'b) t
The type of hash tables from type 'a to type 'b.

val create : ?random:bool -> int -> ('a, 'b) t

Hashtbl.create n creates a new, empty hash table, with initial size greater or equal to the
suggested size n. For best results, n should be on the order of the expected number of
elements that will be in the table. The table grows as needed, so n is just an initial guess. If n
is very small or negative then it is disregarded and a small default size is used.
The optional ~random parameter (a boolean) controls whether the internal organization of the
hash table is randomized at each execution of Hashtbl.create or deterministic over all
executions.
A hash table that is created with ~random set to false uses a fixed hash function
(Hashtbl.hash[29.25]) to distribute keys among buckets. As a consequence, collisions
between keys happen deterministically. In Web-facing applications or other security-sensitive
applications, the deterministic collision patterns can be exploited by a malicious user to
create a denial-of-service attack: the attacker sends input crafted to create many collisions in
the table, slowing the application down.
A hash table that is created with ~random set to true uses the seeded hash function
Hashtbl.seeded_hash[29.25] with a seed that is randomly chosen at hash table creation
time. In effect, the hash function used is randomly selected among 2^{30} different hash
functions. All these hash functions have different collision patterns, rendering ineffective the
denial-of-service attack described above. However, because of randomization, enumerating all
elements of the hash table using Hashtbl.fold[29.25] or Hashtbl.iter[29.25] is no longer
deterministic: elements are enumerated in different orders at different runs of the program.
If no ~random parameter is given, hash tables are created in non-random mode by default.
This default can be changed either programmatically by calling Hashtbl.randomize[29.25] or
by setting the R flag in the OCAMLRUNPARAM environment variable.
Before 4.00 the ~random parameter was not present and all hash tables were created in
non-randomized mode.

val clear : ('a, 'b) t -> unit

724

Empty a hash table. Use reset instead of clear to shrink the size of the bucket table to its
initial size.

val reset : ('a, 'b) t -> unit

Empty a hash table and shrink the size of the bucket table to its initial size.
Since: 4.00

val copy : ('a, 'b) t -> ('a, 'b) t

Return a copy of the given hashtable.

val add : ('a, 'b) t -> 'a -> 'b -> unit
Hashtbl.add tbl key data adds a binding of key to data in table tbl.
Warning: Previous bindings for key are not removed, but simply hidden. That is, after
performing Hashtbl.remove[29.25] tbl key, the previous binding for key, if any, is restored.
(Same behavior as with association lists.)
If you desire the classic behavior of replacing elements, see Hashtbl.replace[29.25].

val find : ('a, 'b) t -> 'a -> 'b

Hashtbl.find tbl x returns the current binding of x in tbl, or raises Not_found if no such
binding exists.

val find_opt : ('a, 'b) t -> 'a -> 'b option

Hashtbl.find_opt tbl x returns the current binding of x in tbl, or None if no such binding
exists.
Since: 4.05

val find_all : ('a, 'b) t -> 'a -> 'b list

Hashtbl.find_all tbl x returns the list of all data associated with x in tbl. The current
binding is returned first, then the previous bindings, in reverse order of introduction in the
table.

val mem : ('a, 'b) t -> 'a -> bool

Hashtbl.mem tbl x checks if x is bound in tbl.

val remove : ('a, 'b) t -> 'a -> unit

Hashtbl.remove tbl x removes the current binding of x in tbl, restoring the previous
binding if it exists. It does nothing if x is not bound in tbl.

val replace : ('a, 'b) t -> 'a -> 'b -> unit
Hashtbl.replace tbl key data replaces the current binding of key in tbl by a binding of
key to data. If key is unbound in tbl, a binding of key to data is added to tbl. This is
functionally equivalent to Hashtbl.remove[29.25] tbl key followed by Hashtbl.add[29.25]
tbl key data.
Chapter 29. The standard library 725

val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit
Hashtbl.iter f tbl applies f to all bindings in table tbl. f receives the key as first
argument, and the associated value as second argument. Each binding is presented exactly
once to f.
The order in which the bindings are passed to f is unspecified. However, if the table contains
several bindings for the same key, they are passed to f in reverse order of introduction, that
is, the most recent binding is passed first.
If the hash table was created in non-randomized mode, the order in which the bindings are
enumerated is reproducible between successive runs of the program, and even between minor
versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.
The behavior is not specified if the hash table is modified by f during the iteration.

val filter_map_inplace : ('a -> 'b -> 'b option) -> ('a, 'b) t -> unit
Hashtbl.filter_map_inplace f tbl applies f to all bindings in table tbl and update each
binding depending on the result of f. If f returns None, the binding is discarded. If it returns
Some new_val, the binding is update to associate the key to new_val.
Other comments for Hashtbl.iter[29.25] apply as well.
Since: 4.03

val fold : ('a -> 'b -> 'acc -> 'acc) -> ('a, 'b) t -> 'acc -> 'acc
Hashtbl.fold f tbl init computes (f kN dN ... (f k1 d1 init)...), where k1 ...
kN are the keys of all bindings in tbl, and d1 ... dN are the associated values. Each binding
is presented exactly once to f.
The order in which the bindings are passed to f is unspecified. However, if the table contains
several bindings for the same key, they are passed to f in reverse order of introduction, that
is, the most recent binding is passed first.
If the hash table was created in non-randomized mode, the order in which the bindings are
enumerated is reproducible between successive runs of the program, and even between minor
versions of OCaml. For randomized hash tables, the order of enumeration is entirely random.
The behavior is not specified if the hash table is modified by f during the iteration.

val length : ('a, 'b) t -> int

Hashtbl.length tbl returns the number of bindings in tbl. It takes constant time.
Multiple bindings are counted once each, so Hashtbl.length gives the number of times
Hashtbl.iter calls its first argument.

val randomize : unit -> unit

After a call to Hashtbl.randomize(), hash tables are created in randomized mode by
default: Hashtbl.create[29.25] returns randomized hash tables, unless the ~random:false
optional parameter is given. The same effect can be achieved by setting the R parameter in
the OCAMLRUNPARAM environment variable.
726

It is recommended that applications or Web frameworks that need to protect themselves

against the denial-of-service attack described in Hashtbl.create[29.25] call
Hashtbl.randomize() at initialization time before any domains are created.
Note that once Hashtbl.randomize() was called, there is no way to revert to the
non-randomized default behavior of Hashtbl.create[29.25]. This is intentional.
Non-randomized hash tables can still be created using Hashtbl.create ~random:false.
Since: 4.00

val is_randomized : unit -> bool

Return true if the tables are currently created in randomized mode by default, false
otherwise.
Since: 4.03

val rebuild : ?random:bool -> ('a, 'b) t -> ('a, 'b) t

Return a copy of the given hashtable. Unlike Hashtbl.copy[29.25], Hashtbl.rebuild[29.25]
h re-hashes all the (key, value) entries of the original table h. The returned hash table is
randomized if h was randomized, or the optional random parameter is true, or if the default is
to create randomized hash tables; see Hashtbl.create[29.25] for more information.
Hashtbl.rebuild[29.25] can safely be used to import a hash table built by an old version of
the Hashtbl[29.25] module, then marshaled to persistent storage. After unmarshaling, apply
Hashtbl.rebuild[29.25] to produce a hash table for the current version of the
Hashtbl[29.25] module.
Since: 4.12

type statistics =
{ num_bindings : int ;
Number of bindings present in the table. Same value as returned by
Hashtbl.length[29.25].
num_buckets : int ;
Number of buckets in the table.
max_bucket_length : int ;
Maximal number of bindings per bucket.
bucket_histogram : int array ;
Histogram of bucket sizes. This array histo has length max_bucket_length + 1. The
value of histo.(i) is the number of buckets whose size is i.
}
Since: 4.00

val stats : ('a, 'b) t -> statistics

Hashtbl.stats tbl returns statistics about the table tbl: number of buckets, size of the
biggest bucket, distribution of buckets by size.
Since: 4.00
Chapter 29. The standard library 727

Hash tables and Sequences

val to_seq : ('a, 'b) t -> ('a * 'b) Seq.t
Iterate on the whole table. The order in which the bindings appear in the sequence is
unspecified. However, if the table contains several bindings for the same key, they appear in
reversed order of introduction, that is, the most recent binding appears first.
The behavior is not specified if the hash table is modified during the iteration.
Since: 4.07

val to_seq_keys : ('a, 'b) t -> 'a Seq.t

Same as Seq.map fst (to_seq m)
Since: 4.07

val to_seq_values : ('a, 'b) t -> 'b Seq.t

Same as Seq.map snd (to_seq m)
Since: 4.07

val add_seq : ('a, 'b) t -> ('a * 'b) Seq.t -> unit
Add the given bindings to the table, using Hashtbl.add[29.25]
Since: 4.07

val replace_seq : ('a, 'b) t -> ('a * 'b) Seq.t -> unit
Add the given bindings to the table, using Hashtbl.replace[29.25]
Since: 4.07

val of_seq : ('a * 'b) Seq.t -> ('a, 'b) t

Build a table from the given bindings. The bindings are added in the same order they appear
in the sequence, using Hashtbl.replace_seq[29.25], which means that if two pairs have the
same key, only the latest one will appear in the table.
Since: 4.07

Functorial interface
The functorial interface allows the use of specific comparison and hash functions, either for per-
formance/security concerns, or because keys are not hashable/comparable with the polymorphic
builtins.
For instance, one might want to specialize a table for integer keys:

module IntHash =
struct
type t = int
let equal i j = i=j
728

let hash i = i land max_int

end

module IntHashtbl = Hashtbl.Make(IntHash)

let h = IntHashtbl.create 17 in
IntHashtbl.add h 12 "hello"

This creates a new module IntHashtbl, with a new type 'a IntHashtbl.t of tables from int
to 'a. In this example, h contains string values so its type is string IntHashtbl.t.
Note that the new type 'a IntHashtbl.t is not compatible with the type ('a,'b) Hashtbl.t
of the generic interface. For example, Hashtbl.length h would not type-check, you must use
IntHashtbl.length.
module type HashedType =
sig
type t
The type of the hashtable keys.

val equal : t -> t -> bool

The equality predicate used to compare keys.

val hash : t -> int

A hashing function on keys. It must be such that if two keys are equal according to
equal, then they have identical hash values as computed by hash. Examples: suitable
(equal, hash) pairs for arbitrary key types include
• ((=), Hashtbl.HashedType.hash[29.25]) for comparing objects by structure
(provided objects do not contain floats)
• ((fun x y -> compare x y = 0), Hashtbl.HashedType.hash[29.25]) for
comparing objects by structure and handling nan[28.2] correctly
• ((==), Hashtbl.HashedType.hash[29.25]) for comparing objects by physical
equality (e.g. for mutable or cyclic objects).

end
The input signature of the functor Hashtbl.Make[29.25].

module type S =
sig
type key
type !'a t
val create : int -> 'a t
val clear : 'a t -> unit
val reset : 'a t -> unit
Chapter 29. The standard library 729

Since: 4.00

val copy : 'a t -> 'a t

val add : 'a t -> key -> 'a -> unit
val remove : 'a t -> key -> unit
val find : 'a t -> key -> 'a
val find_opt : 'a t -> key -> 'a option
Since: 4.05

val find_all : 'a t -> key -> 'a list

val replace : 'a t -> key -> 'a -> unit
val mem : 'a t -> key -> bool
val iter : (key -> 'a -> unit) -> 'a t -> unit
val filter_map_inplace : (key -> 'a -> 'a option) -> 'a t -> unit
Since: 4.03

val fold : (key -> 'a -> 'acc -> 'acc) -> 'a t -> 'acc -> 'acc
val length : 'a t -> int
val stats : 'a t -> Hashtbl.statistics
Since: 4.00

val to_seq : 'a t -> (key * 'a) Seq.t

Since: 4.07

val to_seq_keys : 'a t -> key Seq.t

Since: 4.07

val to_seq_values : 'a t -> 'a Seq.t

Since: 4.07

val add_seq : 'a t -> (key * 'a) Seq.t -> unit

Since: 4.07

val replace_seq : 'a t -> (key * 'a) Seq.t -> unit

Since: 4.07

val of_seq : (key * 'a) Seq.t -> 'a t

Since: 4.07
730

end
The output signature of the functor Hashtbl.Make[29.25].

module Make :
functor (H : HashedType) -> S with type key = H.t
Functor building an implementation of the hashtable structure. The functor Hashtbl.Make
returns a structure containing a type key of keys and a type 'a t of hash tables associating
data of type 'a to keys of type key. The operations perform similarly to those of the generic
interface, but use the hashing and equality functions specified in the functor argument H
instead of generic equality and hashing. Since the hash function is not seeded, the create
operation of the result structure always returns non-randomized hash tables.

module type SeededHashedType =

sig
type t
The type of the hashtable keys.

val equal : t -> t -> bool

The equality predicate used to compare keys.

val seeded_hash : int -> t -> int

A seeded hashing function on keys. The first argument is the seed. It must be the case
that if equal x y is true, then seeded_hash seed x = seeded_hash seed y for any
value of seed. A suitable choice for seeded_hash is the function
Hashtbl.seeded_hash[29.25] below.

end
The input signature of the functor Hashtbl.MakeSeeded[29.25].
Since: 4.00

module type SeededS =

sig
type key
type !'a t
val create : ?random:bool -> int -> 'a t
val clear : 'a t -> unit
val reset : 'a t -> unit
val copy : 'a t -> 'a t
val add : 'a t -> key -> 'a -> unit
val remove : 'a t -> key -> unit
val find : 'a t -> key -> 'a
val find_opt : 'a t -> key -> 'a option
Chapter 29. The standard library 731

Since: 4.05

val find_all : 'a t -> key -> 'a list

val replace : 'a t -> key -> 'a -> unit
val mem : 'a t -> key -> bool
val iter : (key -> 'a -> unit) -> 'a t -> unit
val filter_map_inplace : (key -> 'a -> 'a option) -> 'a t -> unit
Since: 4.03

val fold : (key -> 'a -> 'acc -> 'acc) ->
'a t -> 'acc -> 'acc
val length : 'a t -> int
val stats : 'a t -> Hashtbl.statistics
val to_seq : 'a t -> (key * 'a) Seq.t
Since: 4.07

val to_seq_keys : 'a t -> key Seq.t

Since: 4.07

val to_seq_values : 'a t -> 'a Seq.t

Since: 4.07

val add_seq : 'a t -> (key * 'a) Seq.t -> unit

Since: 4.07

val replace_seq : 'a t -> (key * 'a) Seq.t -> unit

Since: 4.07

val of_seq : (key * 'a) Seq.t -> 'a t

Since: 4.07

end

The output signature of the functor Hashtbl.MakeSeeded[29.25].

Since: 4.00

module MakeSeeded :
functor (H : SeededHashedType) -> SeededS with type key = H.t
732

Functor building an implementation of the hashtable structure. The functor

Hashtbl.MakeSeeded returns a structure containing a type key of keys and a type 'a t of
hash tables associating data of type 'a to keys of type key. The operations perform similarly
to those of the generic interface, but use the seeded hashing and equality functions specified in
the functor argument H instead of generic equality and hashing. The create operation of the
result structure supports the ~random optional parameter and returns randomized hash tables
if ~random:true is passed or if randomization is globally on (see Hashtbl.randomize[29.25]).
Since: 4.00

The polymorphic hash functions

val hash : 'a -> int
Hashtbl.hash x associates a nonnegative integer to any value of any type. It is guaranteed
that if x = y or Stdlib.compare x y = 0, then hash x = hash y. Moreover, hash always
terminates, even on cyclic structures.

val seeded_hash : int -> 'a -> int

A variant of Hashtbl.hash[29.25] that is further parameterized by an integer seed.
Since: 4.00

val hash_param : int -> int -> 'a -> int

Hashtbl.hash_param meaningful total x computes a hash value for x, with the same
properties as for hash. The two extra integer parameters meaningful and total give more
precise control over hashing. Hashing performs a breadth-first, left-to-right traversal of the
structure x, stopping after meaningful meaningful nodes were encountered, or total nodes
(meaningful or not) were encountered. If total as specified by the user exceeds a certain
value, currently 256, then it is capped to that value. Meaningful nodes are: integers;
floating-point numbers; strings; characters; booleans; and constant constructors. Larger
values of meaningful and total means that more nodes are taken into account to compute
the final hash value, and therefore collisions are less likely to happen. However, hashing takes
longer. The parameters meaningful and total govern the tradeoff between accuracy and
speed. As default choices, Hashtbl.hash[29.25] and Hashtbl.seeded_hash[29.25] take
meaningful = 10 and total = 100.

val seeded_hash_param : int -> int -> int -> 'a -> int
A variant of Hashtbl.hash_param[29.25] that is further parameterized by an integer seed.
Usage: Hashtbl.seeded_hash_param meaningful total seed x.
Since: 4.00

Examples
Basic Example
Chapter 29. The standard library 733

(* 0...99 *)
let seq = Seq.ints 0 |> Seq.take 100

(* build from Seq.t *)

# let tbl =
seq
|> Seq.map (fun x -> x, string_of_int x)
|> Hashtbl.of_seq
val tbl : (int, string) Hashtbl.t = <abstr>

# Hashtbl.length tbl
- : int = 100

# Hashtbl.find_opt tbl 32
- : string option = Some "32"

# Hashtbl.find_opt tbl 166

- : string option = None

# Hashtbl.replace tbl 166 "one six six"

- : unit = ()

# Hashtbl.find_opt tbl 166

- : string option = Some "one six six"

# Hashtbl.length tbl
- : int = 101

Counting Elements
Given a sequence of elements (here, a Seq.t[29.49]), we want to count how many times each distinct
element occurs in the sequence. A simple way to do this, assuming the elements are comparable
and hashable, is to use a hash table that maps elements to their number of occurrences.
Here we illustrate that principle using a sequence of (ascii) characters (type char). We use a
custom Char_tbl specialized for char.

# module Char_tbl = Hashtbl.Make(struct

type t = char
let equal = Char.equal
let hash = Hashtbl.hash
end)

(* count distinct occurrences of chars in [seq] *)

# let count_chars (seq : char Seq.t) : _ list =
734

let counts = Char_tbl.create 16 in

Seq.iter
(fun c ->
let count_c =
Char_tbl.find_opt counts c
|> Option.value ~default:0
in
Char_tbl.replace counts c (count_c + 1))
seq;
(* turn into a list *)
Char_tbl.fold (fun c n l -> (c,n) :: l) counts []
|> List.sort (fun (c1,_)(c2,_) -> Char.compare c1 c2)
val count_chars : Char_tbl.key Seq.t -> (Char.t * int) list = <fun>

(* basic seq from a string *)

# let seq = String.to_seq "hello world, and all the camels in it!"
val seq : char Seq.t = <fun>

# count_chars seq
- : (Char.t * int) list =
[(' ', 7); ('!', 1); (',', 1); ('a', 3); ('c', 1); ('d', 2); ('e', 3);
('h', 2); ('i', 2); ('l', 6); ('m', 1); ('n', 2); ('o', 2); ('r', 1);
('s', 1); ('t', 2); ('w', 1)]

(* "abcabcabc..." *)
# let seq2 =
Seq.cycle (String.to_seq "abc") |> Seq.take 31
val seq2 : char Seq.t = <fun>

# String.of_seq seq2
- : String.t = "abcabcabcabcabcabcabcabcabcabca"

# count_chars seq2
- : (Char.t * int) list = [('a', 11); ('b', 10); ('c', 10)]

29.26 Module In_channel : Input channels.

This module provides functions for working with input channels.
See the example section[29.45] below.
Since: 4.14
Chapter 29. The standard library 735

Channels
type t = in_channel
The type of input channel.

type open_flag = open_flag =

| Open_rdonly
open for reading.
| Open_wronly
open for writing.
| Open_append
open for appending: always write at end of file.
| Open_creat
create the file if it does not exist.
| Open_trunc
empty the file if it already exists.
| Open_excl
fail if Open_creat and the file already exists.
| Open_binary
open in binary mode (no conversion).
| Open_text
open in text mode (may perform conversions).
| Open_nonblock
open in non-blocking mode.
Opening modes for In_channel.open_gen[29.26].

val stdin : t
The standard input for the process.

val open_bin : string -> t

Open the named file for reading, and return a new input channel on that file, positioned at
the beginning of the file.

val open_text : string -> t

Same as In_channel.open_bin[29.26], but the file is opened in text mode, so that newline
translation takes place during reads. On operating systems that do not distinguish between
text mode and binary mode, this function behaves like In_channel.open_bin[29.26].

val open_gen : open_flag list -> int -> string -> t

736

open_gen mode perm filename opens the named file for reading, as described above. The
extra arguments mode and perm specify the opening mode and file permissions.
In_channel.open_text[29.26] and In_channel.open_bin[29.26] are special cases of this
function.

val with_open_bin : string -> (t -> 'a) -> 'a

with_open_bin fn f opens a channel ic on file fn and returns f ic. After f returns, either
with a value or by raising an exception, ic is guaranteed to be closed.

val with_open_text : string -> (t -> 'a) -> 'a

Like In_channel.with_open_bin[29.26], but the channel is opened in text mode (see
In_channel.open_text[29.26]).

val with_open_gen : open_flag list -> int -> string -> (t -> 'a) -> 'a
Like In_channel.with_open_bin[29.26], but can specify the opening mode and file
permission, in case the file must be created (see In_channel.open_gen[29.26]).

val close : t -> unit

Close the given channel. Input functions raise a Sys_error exception when they are applied
to a closed input channel, except In_channel.close[29.26], which does nothing when applied
to an already closed channel.

val close_noerr : t -> unit

Same as In_channel.close[29.26], but ignore all errors.

Input
val input_char : t -> char option
Read one character from the given input channel. Returns None if there are no more
characters to read.

val input_byte : t -> int option

Same as In_channel.input_char[29.26], but return the 8-bit integer representing the
character. Returns None if the end of file was reached.

val input_line : t -> string option

input_line ic reads characters from ic until a newline or the end of file is reached. Returns
the string of all characters read, without the newline (if any). Returns None if the end of the
file has been reached. In particular, this will be the case if the last line of input is empty.
A newline is the character \n unless the file is open in text mode and Sys.win32[29.56] is
true in which case it is the sequence of characters \r\n.

val really_input_string : t -> int -> string option

Chapter 29. The standard library 737

really_input_string ic len reads len characters from channel ic and returns them in a
new string. Returns None if the end of file is reached before len characters have been read.
If the same channel is read concurrently by multiple threads, the returned string is not
guaranteed to contain contiguous characters from the input.

val input_all : t -> string

input_all ic reads all remaining data from ic.
If the same channel is read concurrently by multiple threads, the returned string is not
guaranteed to contain contiguous characters from the input.

val input_lines : t -> string list

input_lines ic reads lines using In_channel.input_line[29.26] until the end of file is
reached. It returns the list of all lines read, in the order they were read. The newline
characters that terminate lines are not included in the returned strings. Empty lines produce
empty strings.
Since: 5.1

Advanced input
val input : t -> bytes -> int -> int -> int
input ic buf pos len reads up to len characters from the given channel ic, storing them
in byte sequence buf, starting at character number pos. It returns the actual number of
characters read, between 0 and len (inclusive). A return value of 0 means that the end of file
was reached.
Use In_channel.really_input[29.26] to read exactly len characters.
Raises Invalid_argument if pos and len do not designate a valid range of buf.

val input_bigarray :
t ->
('a, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
Bigarray.Array1.t -> int -> int -> int
Same as In_channel.input[29.26], but read the data into a bigarray.
Since: 5.2

val really_input : t -> bytes -> int -> int -> unit option
really_input ic buf pos len reads len characters from channel ic, storing them in byte
sequence buf, starting at character number pos.
Returns None if the end of file is reached before len characters have been read.
If the same channel is read concurrently by multiple threads, the bytes read by
really_input are not guaranteed to be contiguous.
Raises Invalid_argument if pos and len do not designate a valid range of buf.
738

val really_input_bigarray :
t ->
('a, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
Bigarray.Array1.t -> int -> int -> unit option
Same as In_channel.really_input[29.26], but read the data into a bigarray.
Since: 5.2

val fold_lines : ('acc -> string -> 'acc) -> 'acc -> t -> 'acc
fold_lines f init ic reads lines from ic using In_channel.input_line[29.26] until the
end of file is reached, and successively passes each line to function f in the style of a fold.
More precisely, if lines l1, ..., lN are read, fold_lines f init ic computes f (... (f
(f init l1) l2) ...) lN. If f has no side effects, this is equivalent to List.fold_left f
init (In_channel.input_lines ic), but is more efficient since it does not construct the
list of all lines read.
Since: 5.1

Seeking
val seek : t -> int64 -> unit
seek chan pos sets the current reading position to pos for channel chan. This works only
for regular files. On files of other kinds, the behavior is unspecified.

val pos : t -> int64

Return the current reading position for the given channel. For files opened in text mode
under Windows, the returned position is approximate (owing to end-of-line conversion); in
particular, saving the current position with In_channel.pos[29.26], then going back to this
position using In_channel.seek[29.26] will not work. For this programming idiom to work
reliably and portably, the file must be opened in binary mode.

Attributes
val length : t -> int64
Return the size (number of characters) of the regular file on which the given channel is
opened. If the channel is opened on a file that is not a regular file, the result is meaningless.
The returned size does not take into account the end-of-line translations that can be
performed when reading from a channel opened in text mode.

val set_binary_mode : t -> bool -> unit

set_binary_mode ic true sets the channel ic to binary mode: no translations take place
during input.
set_binary_mode ic false sets the channel ic to text mode: depending on the operating
system, some translations may take place during input. For instance, under Windows,
end-of-lines will be translated from \r\n to \n.
Chapter 29. The standard library 739

This function has no effect under operating systems that do not distinguish between text
mode and binary mode.

val is_binary_mode : t -> bool

is_binary_mode ic returns whether the channel ic is in binary mode (see
In_channel.set_binary_mode[29.26]).
Since: 5.2

val isatty : t -> bool

isatty ic is true if ic refers to a terminal or console window, false otherwise.
Since: 5.1

Examples
Reading the contents of a file:

let read_file file = In_channel.with_open_bin file In_channel.input_all

Reading a line from stdin:

let user_input () = In_channel.input_line In_channel.stdin

29.27 Module Int : Integer values.

Integers are Sys.int_size[29.56] bits wide and use two’s complement representation. All operations
are taken modulo 2Sys.int_size . They do not fail on overflow.
Since: 4.08

Integers
type t = int
The type for integer values.

val zero : int

zero is the integer 0.

val one : int

one is the integer 1.

val minus_one : int

minus_one is the integer -1.
740

val neg : int -> int

neg x is ~-x.

val add : int -> int -> int

add x y is the addition x + y.

val sub : int -> int -> int

sub x y is the subtraction x - y.

val mul : int -> int -> int

mul x y is the multiplication x * y.

val div : int -> int -> int

div x y is the division x / y. See (/)[28.2] for details.

val rem : int -> int -> int

rem x y is the remainder x mod y. See (mod)[28.2] for details.

val succ : int -> int

succ x is add x 1.

val pred : int -> int

pred x is sub x 1.

val abs : int -> int

abs x is the absolute value of x. That is x if x is positive and neg x if x is negative.
Warning. This may be negative if the argument is Int.min_int[29.27].

val max_int : int

max_int is the greatest representable integer, 2Sys.int_size−1 -1.

val min_int : int

min_int is the smallest representable integer, -2Sys.int_size−1 .

val logand : int -> int -> int

logand x y is the bitwise logical and of x and y.

val logor : int -> int -> int

logor x y is the bitwise logical or of x and y.

val logxor : int -> int -> int

logxor x y is the bitwise logical exclusive or of x and y.

val lognot : int -> int

Chapter 29. The standard library 741

lognot x is the bitwise logical negation of x.

val shift_left : int -> int -> int

shift_left x n shifts x to the left by n bits. The result is unspecified if n < 0 or n >
Sys.int_size[29.56].

val shift_right : int -> int -> int

shift_right x n shifts x to the right by n bits. This is an arithmetic shift: the sign bit of x
is replicated and inserted in the vacated bits. The result is unspecified if n < 0 or n >
Sys.int_size[29.56].

val shift_right_logical : int -> int -> int

shift_right x n shifts x to the right by n bits. This is a logical shift: zeroes are inserted in
the vacated bits regardless of the sign of x. The result is unspecified if n < 0 or n >
Sys.int_size[29.56].

Predicates and comparisons

val equal : int -> int -> bool
equal x y is true if and only if x = y.

val compare : int -> int -> int

compare x y is compare[28.2] x y but more efficient.

val min : int -> int -> int

Return the smaller of the two arguments.
Since: 4.13

val max : int -> int -> int

Return the greater of the two arguments.
Since: 4.13

Converting
val to_float : int -> float
to_float x is x as a floating point number.

val of_float : float -> int

of_float x truncates x to an integer. The result is unspecified if the argument is nan or falls
outside the range of representable integers.

val to_string : int -> string

742

to_string x is the written representation of x in decimal.

val seeded_hash : int -> int -> int

A seeded hash function for ints, with the same output value as Hashtbl.seeded_hash[29.25].
This function allows this module to be passed as argument to the functor
Hashtbl.MakeSeeded[29.25].
Since: 5.1

val hash : int -> int

An unseeded hash function for ints, with the same output value as Hashtbl.hash[29.25]. This
function allows this module to be passed as argument to the functor Hashtbl.Make[29.25].
Since: 5.1

29.28 Module Int32 : 32-bit integers.

This module provides operations on the type int32 of signed 32-bit integers. Unlike the built-in
int type, the type int32 is guaranteed to be exactly 32-bit wide on all platforms. All arithmetic
operations over int32 are taken modulo 232 .
Performance notice: values of type int32 occupy more memory space than values of type int,
and arithmetic operations on int32 are generally slower than those on int. Use int32 only when
the application requires exact 32-bit arithmetic.
Literals for 32-bit integers are suffixed by l:

let zero: int32 = 0l

let one: int32 = 1l
let m_one: int32 = -1l

val zero : int32

The 32-bit integer 0.

val one : int32

The 32-bit integer 1.

val minus_one : int32

The 32-bit integer -1.

val neg : int32 -> int32

Unary negation.

val add : int32 -> int32 -> int32

Addition.
Chapter 29. The standard library 743

val sub : int32 -> int32 -> int32

Subtraction.

val mul : int32 -> int32 -> int32

Multiplication.

val div : int32 -> int32 -> int32

Integer division. This division rounds the real quotient of its arguments towards zero, as
specified for (/)[28.2].
Raises Division_by_zero if the second argument is zero.

val unsigned_div : int32 -> int32 -> int32

Same as Int32.div[29.28], except that arguments and result are interpreted as unsigned
32-bit integers.
Since: 4.08

val rem : int32 -> int32 -> int32

Integer remainder. If y is not zero, the result of Int32.rem x y satisfies the following
property: x = Int32.add (Int32.mul (Int32.div x y) y) (Int32.rem x y). If y = 0,
Int32.rem x y raises Division_by_zero.

val unsigned_rem : int32 -> int32 -> int32

Same as Int32.rem[29.28], except that arguments and result are interpreted as unsigned
32-bit integers.
Since: 4.08

val succ : int32 -> int32

Successor. Int32.succ x is Int32.add x Int32.one.

val pred : int32 -> int32

Predecessor. Int32.pred x is Int32.sub x Int32.one.

val abs : int32 -> int32

abs x is the absolute value of x. On min_int this is min_int itself and thus remains negative.

val max_int : int32

The greatest representable 32-bit integer, 231 - 1.

val min_int : int32

The smallest representable 32-bit integer, -231 .

val logand : int32 -> int32 -> int32

Bitwise logical and.
744

val logor : int32 -> int32 -> int32

Bitwise logical or.

val logxor : int32 -> int32 -> int32

Bitwise logical exclusive or.

val lognot : int32 -> int32

Bitwise logical negation.

val shift_left : int32 -> int -> int32

Int32.shift_left x y shifts x to the left by y bits. The result is unspecified if y < 0 or y
>= 32.

val shift_right : int32 -> int -> int32

Int32.shift_right x y shifts x to the right by y bits. This is an arithmetic shift: the sign
bit of x is replicated and inserted in the vacated bits. The result is unspecified if y < 0 or y
>= 32.

val shift_right_logical : int32 -> int -> int32

Int32.shift_right_logical x y shifts x to the right by y bits. This is a logical shift:
zeroes are inserted in the vacated bits regardless of the sign of x. The result is unspecified if y
< 0 or y >= 32.

val of_int : int -> int32

Convert the given integer (type int) to a 32-bit integer (type int32). On 64-bit platforms,
the argument is taken modulo 232 .

val to_int : int32 -> int

Convert the given 32-bit integer (type int32) to an integer (type int). On 32-bit platforms,
the 32-bit integer is taken modulo 231 , i.e. the high-order bit is lost during the conversion.
On 64-bit platforms, the conversion is exact.

val unsigned_to_int : int32 -> int option

Same as Int32.to_int[29.28], but interprets the argument as an unsigned integer. Returns
None if the unsigned value of the argument cannot fit into an int.
Since: 4.08

val of_float : float -> int32

Convert the given floating-point number to a 32-bit integer, discarding the fractional part
(truncate towards 0). If the truncated floating-point number is outside the range
[Int32.min_int[29.28], Int32.max_int[29.28]], no exception is raised, and an unspecified,
platform-dependent integer is returned.

val to_float : int32 -> float

Chapter 29. The standard library 745

Convert the given 32-bit integer to a floating-point number.

val of_string : string -> int32

Convert the given string to a 32-bit integer. The string is read in decimal (by default, or if
the string begins with 0u) or in hexadecimal, octal or binary if the string begins with 0x, 0o
or 0b respectively.
The 0u prefix reads the input as an unsigned integer in the range [0, 2*Int32.max_int+1].
If the input exceeds Int32.max_int[29.28] it is converted to the signed integer
Int32.min_int + input - Int32.max_int - 1.
The _ (underscore) character can appear anywhere in the string and is ignored.
Raises Failure if the given string is not a valid representation of an integer, or if the integer
represented exceeds the range of integers representable in type int32.

val of_string_opt : string -> int32 option

Same as of_string, but return None instead of raising.
Since: 4.05

val to_string : int32 -> string

Return the string representation of its argument, in signed decimal.

val bits_of_float : float -> int32

Return the internal representation of the given float according to the IEEE 754 floating-point
’single format’ bit layout. Bit 31 of the result represents the sign of the float; bits 30 to 23
represent the (biased) exponent; bits 22 to 0 represent the mantissa.

val float_of_bits : int32 -> float

Return the floating-point number whose internal representation, according to the IEEE 754
floating-point ’single format’ bit layout, is the given int32.

type t = int32
An alias for the type of 32-bit integers.

val compare : t -> t -> int

The comparison function for 32-bit integers, with the same specification as compare[28.2].
Along with the type t, this function compare allows the module Int32 to be passed as
argument to the functors Set.Make[29.50] and Map.Make[29.34].

val unsigned_compare : t -> t -> int

Same as Int32.compare[29.28], except that arguments are interpreted as unsigned 32-bit
integers.
Since: 4.08

val equal : t -> t -> bool

746

The equal function for int32s.

Since: 4.03

val min : t -> t -> t

Return the smaller of the two arguments.
Since: 4.13

val max : t -> t -> t

Return the greater of the two arguments.
Since: 4.13

val seeded_hash : int -> t -> int

A seeded hash function for 32-bit ints, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.1

val hash : t -> int

An unseeded hash function for 32-bit ints, with the same output value as
Hashtbl.hash[29.25]. This function allows this module to be passed as argument to the
functor Hashtbl.Make[29.25].
Since: 5.1

29.29 Module Int64 : 64-bit integers.

This module provides operations on the type int64 of signed 64-bit integers. Unlike the built-in
int type, the type int64 is guaranteed to be exactly 64-bit wide on all platforms. All arithmetic
operations over int64 are taken modulo 264
Performance notice: values of type int64 occupy more memory space than values of type int,
and arithmetic operations on int64 are generally slower than those on int. Use int64 only when
the application requires exact 64-bit arithmetic.
Literals for 64-bit integers are suffixed by L:

let zero: int64 = 0L

let one: int64 = 1L
let m_one: int64 = -1L

val zero : int64

The 64-bit integer 0.

val one : int64

Chapter 29. The standard library 747

The 64-bit integer 1.

val minus_one : int64

The 64-bit integer -1.

val neg : int64 -> int64

Unary negation.

val add : int64 -> int64 -> int64

Addition.

val sub : int64 -> int64 -> int64

Subtraction.

val mul : int64 -> int64 -> int64

Multiplication.

val div : int64 -> int64 -> int64

Integer division.
Raises Division_by_zero if the second argument is zero. This division rounds the real
quotient of its arguments towards zero, as specified for (/)[28.2].

val unsigned_div : int64 -> int64 -> int64

Same as Int64.div[29.29], except that arguments and result are interpreted as unsigned
64-bit integers.
Since: 4.08

val rem : int64 -> int64 -> int64

Integer remainder. If y is not zero, the result of Int64.rem x y satisfies the following
property: x = Int64.add (Int64.mul (Int64.div x y) y) (Int64.rem x y). If y = 0,
Int64.rem x y raises Division_by_zero.

val unsigned_rem : int64 -> int64 -> int64

Same as Int64.rem[29.29], except that arguments and result are interpreted as unsigned
64-bit integers.
Since: 4.08

val succ : int64 -> int64

Successor. Int64.succ x is Int64.add x Int64.one.

val pred : int64 -> int64

Predecessor. Int64.pred x is Int64.sub x Int64.one.

val abs : int64 -> int64

748

abs x is the absolute value of x. On min_int this is min_int itself and thus remains negative.

val max_int : int64

The greatest representable 64-bit integer, 263 - 1.

val min_int : int64

The smallest representable 64-bit integer, -263 .

val logand : int64 -> int64 -> int64

Bitwise logical and.

val logor : int64 -> int64 -> int64

Bitwise logical or.

val logxor : int64 -> int64 -> int64

Bitwise logical exclusive or.

val lognot : int64 -> int64

Bitwise logical negation.

val shift_left : int64 -> int -> int64

Int64.shift_left x y shifts x to the left by y bits. The result is unspecified if y < 0 or y
>= 64.

val shift_right : int64 -> int -> int64

Int64.shift_right x y shifts x to the right by y bits. This is an arithmetic shift: the sign
bit of x is replicated and inserted in the vacated bits. The result is unspecified if y < 0 or y
>= 64.

val shift_right_logical : int64 -> int -> int64

Int64.shift_right_logical x y shifts x to the right by y bits. This is a logical shift:
zeroes are inserted in the vacated bits regardless of the sign of x. The result is unspecified if y
< 0 or y >= 64.

val of_int : int -> int64

Convert the given integer (type int) to a 64-bit integer (type int64).

val to_int : int64 -> int

Convert the given 64-bit integer (type int64) to an integer (type int). On 64-bit platforms,
the 64-bit integer is taken modulo 263 , i.e. the high-order bit is lost during the conversion.
On 32-bit platforms, the 64-bit integer is taken modulo 231 , i.e. the top 33 bits are lost during
the conversion.

val unsigned_to_int : int64 -> int option

Chapter 29. The standard library 749

Same as Int64.to_int[29.29], but interprets the argument as an unsigned integer. Returns

None if the unsigned value of the argument cannot fit into an int.
Since: 4.08

val of_float : float -> int64

Convert the given floating-point number to a 64-bit integer, discarding the fractional part
(truncate towards 0). If the truncated floating-point number is outside the range
[Int64.min_int[29.29], Int64.max_int[29.29]], no exception is raised, and an unspecified,
platform-dependent integer is returned.

val to_float : int64 -> float

Convert the given 64-bit integer to a floating-point number.

val of_int32 : int32 -> int64

Convert the given 32-bit integer (type int32) to a 64-bit integer (type int64).

val to_int32 : int64 -> int32

Convert the given 64-bit integer (type int64) to a 32-bit integer (type int32). The 64-bit
integer is taken modulo 232 , i.e. the top 32 bits are lost during the conversion.

val of_nativeint : nativeint -> int64

Convert the given native integer (type nativeint) to a 64-bit integer (type int64).

val to_nativeint : int64 -> nativeint

Convert the given 64-bit integer (type int64) to a native integer. On 32-bit platforms, the
64-bit integer is taken modulo 232 . On 64-bit platforms, the conversion is exact.

val of_string : string -> int64

Convert the given string to a 64-bit integer. The string is read in decimal (by default, or if
the string begins with 0u) or in hexadecimal, octal or binary if the string begins with 0x, 0o
or 0b respectively.
The 0u prefix reads the input as an unsigned integer in the range [0, 2*Int64.max_int+1].
If the input exceeds Int64.max_int[29.29] it is converted to the signed integer
Int64.min_int + input - Int64.max_int - 1.
The _ (underscore) character can appear anywhere in the string and is ignored.
Raises Failure if the given string is not a valid representation of an integer, or if the integer
represented exceeds the range of integers representable in type int64.

val of_string_opt : string -> int64 option

Same as of_string, but return None instead of raising.
Since: 4.05

val to_string : int64 -> string

750

Return the string representation of its argument, in decimal.

val bits_of_float : float -> int64

Return the internal representation of the given float according to the IEEE 754 floating-point
’double format’ bit layout. Bit 63 of the result represents the sign of the float; bits 62 to 52
represent the (biased) exponent; bits 51 to 0 represent the mantissa.

val float_of_bits : int64 -> float

Return the floating-point number whose internal representation, according to the IEEE 754
floating-point ’double format’ bit layout, is the given int64.

type t = int64
An alias for the type of 64-bit integers.

val compare : t -> t -> int

The comparison function for 64-bit integers, with the same specification as compare[28.2].
Along with the type t, this function compare allows the module Int64 to be passed as
argument to the functors Set.Make[29.50] and Map.Make[29.34].

val unsigned_compare : t -> t -> int

Same as Int64.compare[29.29], except that arguments are interpreted as unsigned 64-bit
integers.
Since: 4.08

val equal : t -> t -> bool

The equal function for int64s.
Since: 4.03

val min : t -> t -> t

Return the smaller of the two arguments.
Since: 4.13

val max : t -> t -> t

Return the greater of the two arguments.
Since: 4.13

val seeded_hash : int -> t -> int

A seeded hash function for 64-bit ints, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.1

val hash : t -> int

Chapter 29. The standard library 751

An unseeded hash function for 64-bit ints, with the same output value as
Hashtbl.hash[29.25]. This function allows this module to be passed as argument to the
functor Hashtbl.Make[29.25].
Since: 5.1

29.30 Module Lazy : Deferred computations.

type 'a t = 'a CamlinternalLazy.t

A value of type 'a Lazy.t is a deferred computation, called a suspension, that has a result of
type 'a. The special expression syntax lazy (expr) makes a suspension of the computation
of expr, without computing expr itself yet. ”Forcing” the suspension will then compute expr
and return its result. Matching a suspension with the special pattern syntax lazy(pattern)
also computes the underlying expression and tries to bind it to pattern:

let lazy_option_map f x =
match x with
| lazy (Some x) -> Some (Lazy.force f x)
| _ -> None

Note: If lazy patterns appear in multiple cases in a pattern-matching, lazy expressions may
be forced even outside of the case ultimately selected by the pattern matching. In the
example above, the suspension x is always computed.
Note: lazy_t is the built-in type constructor used by the compiler for the lazy keyword.
You should not use it directly. Always use Lazy.t instead.
Note: Lazy.force is not concurrency-safe. If you use this module with multiple fibers,
systhreads or domains, then you will need to add some locks. The module however ensures
memory-safety, and hence, concurrently accessing this module will not lead to a crash but the
behaviour is unspecified.
Note: if the program is compiled with the -rectypes option, ill-founded recursive definitions
of the form let rec x = lazy x or let rec x = lazy(lazy(...(lazy x))) are accepted
by the type-checker and lead, when forced, to ill-formed values that trigger infinite loops in
the garbage collector and other parts of the run-time system. Without the -rectypes option,
such ill-founded recursive definitions are rejected by the type-checker.

exception Undefined
Raised when forcing a suspension concurrently from multiple fibers, systhreads or domains, or
when the suspension tries to force itself recursively.

val force : 'a t -> 'a

752

force x forces the suspension x and returns its result. If x has already been forced,
Lazy.force x returns the same value again without recomputing it. If it raised an exception,
the same exception is raised again.
Raises Undefined (see Lazy.Undefined[29.30]).

Iterators
val map : ('a -> 'b) -> 'a t -> 'b t
map f x returns a suspension that, when forced, forces x and applies f to its value.
It is equivalent to lazy (f (Lazy.force x)).
Since: 4.13

Reasoning on already-forced suspensions

val is_val : 'a t -> bool
is_val x returns true if x has already been forced and did not raise an exception.
Since: 4.00

val from_val : 'a -> 'a t

from_val v evaluates v first (as any function would) and returns an already-forced
suspension of its result. It is the same as let x = v in lazy x, but uses dynamic tests to
optimize suspension creation in some cases.
Since: 4.00

val map_val : ('a -> 'b) -> 'a t -> 'b t

map_val f x applies f directly if x is already forced, otherwise it behaves as map f x.
When x is already forced, this behavior saves the construction of a suspension, but on the
other hand it performs more work eagerly that may not be useful if you never force the
function result.
If f raises an exception, it will be raised immediately when is_val x, or raised only when
forcing the thunk otherwise.
If map_val f x does not raise an exception, then is_val (map_val f x) is equal to is_val
x.
Since: 4.13

Advanced
The following definitions are for advanced uses only; they require familiarity with the lazy compilation
scheme to be used appropriately.
val from_fun : (unit -> 'a) -> 'a t
Chapter 29. The standard library 753

from_fun f is the same as lazy (f ()) but slightly more efficient.

It should only be used if the function f is already defined. In particular it is always less
efficient to write from_fun (fun () -> expr) than lazy expr.
Since: 4.00

val force_val : 'a t -> 'a

force_val x forces the suspension x and returns its result. If x has already been forced,
force_val x returns the same value again without recomputing it.
If the computation of x raises an exception, it is unspecified whether force_val x raises the
same exception or Lazy.Undefined[29.30].
Raises

• Undefined if the forcing of x tries to force x itself recursively.

• Undefined (see Lazy.Undefined[29.30]).

29.31 Module Lexing : The run-time library for lexers generated

by ocamllex.

Positions
type position =
{ pos_fname : string ;
pos_lnum : int ;
pos_bol : int ;
pos_cnum : int ;
}
A value of type position describes a point in a source file. pos_fname is the file name;
pos_lnum is the line number; pos_bol is the offset of the beginning of the line (number of
characters between the beginning of the lexbuf and the beginning of the line); pos_cnum is
the offset of the position (number of characters between the beginning of the lexbuf and the
position). The difference between pos_cnum and pos_bol is the character offset within the
line (i.e. the column number, assuming each character is one column wide).
See the documentation of type lexbuf for information about how the lexing engine will
manage positions.

val dummy_pos : position

A value of type position, guaranteed to be different from any valid position.
754

Lexer buffers
type lexbuf =
{ refill_buff : lexbuf -> unit ;
mutable lex_buffer : bytes ;
mutable lex_buffer_len : int ;
mutable lex_abs_pos : int ;
mutable lex_start_pos : int ;
mutable lex_curr_pos : int ;
mutable lex_last_pos : int ;
mutable lex_last_action : int ;
mutable lex_eof_reached : bool ;
mutable lex_mem : int array ;
mutable lex_start_p : position ;
mutable lex_curr_p : position ;
}
The type of lexer buffers. A lexer buffer is the argument passed to the scanning functions
defined by the generated scanners. The lexer buffer holds the current state of the scanner,
plus a function to refill the buffer from the input.
Lexers can optionally maintain the lex_curr_p and lex_start_p position fields. This
”position tracking” mode is the default, and it corresponds to passing ~with_position:true
to functions that create lexer buffers. In this mode, the lexing engine and lexer actions are
co-responsible for properly updating the position fields, as described in the next paragraph.
When the mode is explicitly disabled (with ~with_position:false), the lexing engine will
not touch the position fields and the lexer actions should be careful not to do it either; the
lex_curr_p and lex_start_p field will then always hold the dummy_pos invalid position.
Not tracking positions avoids allocations and memory writes and can significantly improve
the performance of the lexer in contexts where lex_start_p and lex_curr_p are not needed.
Position tracking mode works as follows. At each token, the lexing engine will copy
lex_curr_p to lex_start_p, then change the pos_cnum field of lex_curr_p by updating it
with the number of characters read since the start of the lexbuf. The other fields are left
unchanged by the lexing engine. In order to keep them accurate, they must be initialised
before the first use of the lexbuf, and updated by the relevant lexer actions (i.e. at each end
of line – see also new_line).

val from_channel : ?with_positions:bool -> in_channel -> lexbuf

Create a lexer buffer on the given input channel. Lexing.from_channel inchan returns a
lexer buffer which reads from the input channel inchan, at the current reading position.

val from_string : ?with_positions:bool -> string -> lexbuf

Create a lexer buffer which reads from the given string. Reading starts from the first character
in the string. An end-of-input condition is generated when the end of the string is reached.

val from_function : ?with_positions:bool -> (bytes -> int -> int) -> lexbuf
Chapter 29. The standard library 755

Create a lexer buffer with the given function as its reading method. When the scanner needs
more characters, it will call the given function, giving it a byte sequence s and a byte count n.
The function should put n bytes or fewer in s, starting at index 0, and return the number of
bytes provided. A return value of 0 means end of input.

val set_position : lexbuf -> position -> unit

Set the initial tracked input position for lexbuf to a custom value. Ignores pos_fname. See
Lexing.set_filename[29.31] for changing this field.
Since: 4.11

val set_filename : lexbuf -> string -> unit

Set filename in the initial tracked position to file in lexbuf.
Since: 4.11

val with_positions : lexbuf -> bool

Tell whether the lexer buffer keeps track of position fields lex_curr_p / lex_start_p, as
determined by the corresponding optional argument for functions that create lexer buffers
(whose default value is true).
When with_positions is false, lexer actions should not modify position fields. Doing it
nevertheless could re-enable the with_position mode and degrade performances.

Functions for lexer semantic actions

The following functions can be called from the semantic actions of lexer definitions (the ML code
enclosed in braces that computes the value returned by lexing functions). They give access to the
character string matched by the regular expression associated with the semantic action. These
functions must be applied to the argument lexbuf, which, in the code generated by ocamllex, is
bound to the lexer buffer passed to the parsing function.
val lexeme : lexbuf -> string
Lexing.lexeme lexbuf returns the string matched by the regular expression.

val lexeme_char : lexbuf -> int -> char

Lexing.lexeme_char lexbuf i returns character number i in the matched string.

val lexeme_start : lexbuf -> int

Lexing.lexeme_start lexbuf returns the offset in the input stream of the first character of
the matched string. The first character of the stream has offset 0.

val lexeme_end : lexbuf -> int

Lexing.lexeme_end lexbuf returns the offset in the input stream of the character following
the last character of the matched string. The first character of the stream has offset 0.

val lexeme_start_p : lexbuf -> position

756

Like lexeme_start, but return a complete position instead of an offset. When position
tracking is disabled, the function returns dummy_pos.

val lexeme_end_p : lexbuf -> position

Like lexeme_end, but return a complete position instead of an offset. When position
tracking is disabled, the function returns dummy_pos.

val new_line : lexbuf -> unit

Update the lex_curr_p field of the lexbuf to reflect the start of a new line. You can call this
function in the semantic action of the rule that matches the end-of-line character. The
function does nothing when position tracking is disabled.
Since: 3.11

Miscellaneous functions
val flush_input : lexbuf -> unit
Discard the contents of the buffer and reset the current position to 0. The next use of the
lexbuf will trigger a refill.

29.32 Module List : List operations.

Some functions are flagged as not tail-recursive. A tail-recursive function uses constant stack space,
while a non-tail-recursive function uses stack space proportional to the length of its list argument,
which can be a problem with very long lists. When the function takes several list arguments, an
approximate formula giving stack usage (in some unspecified constant unit) is shown in parentheses.
The above considerations can usually be ignored if your lists are not longer than about 10000
elements.
The labeled version of this module can be used as described in the StdLabels[29.53] module.

type 'a t = 'a list =

| []
| (::) of 'a * 'a list
An alias for the type of lists.

val length : 'a list -> int

Return the length (number of elements) of the given list.

val compare_lengths : 'a list -> 'b list -> int

Compare the lengths of two lists. compare_lengths l1 l2 is equivalent to compare
(length l1) (length l2), except that the computation stops after reaching the end of the
shortest list.
Since: 4.05
Chapter 29. The standard library 757

val compare_length_with : 'a list -> int -> int

Compare the length of a list to an integer. compare_length_with l len is equivalent to
compare (length l) len, except that the computation stops after at most len iterations on
the list.
Since: 4.05

val is_empty : 'a list -> bool

is_empty l is true if and only if l has no elements. It is equivalent to
compare_length_with l 0 = 0.
Since: 5.1

val cons : 'a -> 'a list -> 'a list

cons x xs is x :: xs
Since: 4.03 (4.05 in ListLabels)

val hd : 'a list -> 'a

Return the first element of the given list.
Raises Failure if the list is empty.

val tl : 'a list -> 'a list

Return the given list without its first element.
Raises Failure if the list is empty.

val nth : 'a list -> int -> 'a

Return the n-th element of the given list. The first element (head of the list) is at position 0.
Raises

• Failure if the list is too short.

• Invalid_argument if n is negative.

val nth_opt : 'a list -> int -> 'a option

Return the n-th element of the given list. The first element (head of the list) is at position 0.
Return None if the list is too short.
Since: 4.05
Raises Invalid_argument if n is negative.

val rev : 'a list -> 'a list

List reversal.

val init : int -> (int -> 'a) -> 'a list
758

init len f is [f 0; f 1; ...; f (len-1)], evaluated left to right.

Since: 4.06
Raises Invalid_argument if len < 0.

val append : 'a list -> 'a list -> 'a list
append l0 l1 appends l1 to l0. Same function as the infix operator @.
Since: 5.1 this function is tail-recursive.

val rev_append : 'a list -> 'a list -> 'a list
rev_append l1 l2 reverses l1 and concatenates it with l2. This is equivalent to
(List.rev[29.32] l1) @ l2.

val concat : 'a list list -> 'a list

Concatenate a list of lists. The elements of the argument are all concatenated together (in the
same order) to give the result. Not tail-recursive (length of the argument + length of the
longest sub-list).

val flatten : 'a list list -> 'a list

Same as List.concat[29.32]. Not tail-recursive (length of the argument + length of the
longest sub-list).

Comparison
val equal : ('a -> 'a -> bool) -> 'a list -> 'a list -> bool
equal eq [a1; ...; an] [b1; ..; bm] holds when the two input lists have the same
length, and for each pair of elements ai, bi at the same position we have eq ai bi.
Note: the eq function may be called even if the lists have different length. If you know your
equality function is costly, you may want to check List.compare_lengths[29.32] first.
Since: 4.12

val compare : ('a -> 'a -> int) -> 'a list -> 'a list -> int
compare cmp [a1; ...; an] [b1; ...; bm] performs a lexicographic comparison of the
two input lists, using the same 'a -> 'a -> int interface as compare[28.2]:

• a1 :: l1 is smaller than a2 :: l2 (negative result) if a1 is smaller than a2, or if they

are equal (0 result) and l1 is smaller than l2
• the empty list [] is strictly smaller than non-empty lists

Note: the cmp function will be called even if the lists have different lengths.
Since: 4.12
Chapter 29. The standard library 759

Iterators
val iter : ('a -> unit) -> 'a list -> unit
iter f [a1; ...; an] applies function f in turn to [a1; ...; an]. It is equivalent to f
a1; f a2; ...; f an.
val iteri : (int -> 'a -> unit) -> 'a list -> unit
Same as List.iter[29.32], but the function is applied to the index of the element as first
argument (counting from 0), and the element itself as second argument.
Since: 4.00
val map : ('a -> 'b) -> 'a list -> 'b list
map f [a1; ...; an] applies function f to a1, ..., an, and builds the list [f a1; ...;
f an] with the results returned by f.
val mapi : (int -> 'a -> 'b) -> 'a list -> 'b list
Same as List.map[29.32], but the function is applied to the index of the element as first
argument (counting from 0), and the element itself as second argument.
Since: 4.00
val rev_map : ('a -> 'b) -> 'a list -> 'b list
rev_map f l gives the same result as List.rev[29.32] (List.map[29.32] f l), but is more
efficient.
val filter_map : ('a -> 'b option) -> 'a list -> 'b list
filter_map f l applies f to every element of l, filters out the None elements and returns
the list of the arguments of the Some elements.
Since: 4.08
val concat_map : ('a -> 'b list) -> 'a list -> 'b list
concat_map f l gives the same result as List.concat[29.32] (List.map[29.32] f l).
Tail-recursive.
Since: 4.10
val fold_left_map :
('acc -> 'a -> 'acc * 'b) -> 'acc -> 'a list -> 'acc * 'b list
fold_left_map is a combination of fold_left and map that threads an accumulator through
calls to f.
Since: 4.11
val fold_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a list -> 'acc
fold_left f init [b1; ...; bn] is f (... (f (f init b1) b2) ...) bn.
val fold_right : ('a -> 'acc -> 'acc) -> 'a list -> 'acc -> 'acc
fold_right f [a1; ...; an] init is f a1 (f a2 (... (f an init) ...)). Not
tail-recursive.
760

Iterators on two lists

val iter2 : ('a -> 'b -> unit) -> 'a list -> 'b list -> unit
iter2 f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn.
Raises Invalid_argument if the two lists are determined to have different lengths.

val map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
map2 f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn].
Raises Invalid_argument if the two lists are determined to have different lengths.

val rev_map2 : ('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
rev_map2 f l1 l2 gives the same result as List.rev[29.32] (List.map2[29.32] f l1 l2),
but is more efficient.

val fold_left2 :
('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a list -> 'b list -> 'acc
fold_left2 f init [a1; ...; an] [b1; ...; bn] is f (... (f (f init a1 b1) a2
b2) ...) an bn.
Raises Invalid_argument if the two lists are determined to have different lengths.

val fold_right2 :
('a -> 'b -> 'acc -> 'acc) -> 'a list -> 'b list -> 'acc -> 'acc
fold_right2 f [a1; ...; an] [b1; ...; bn] init is f a1 b1 (f a2 b2 (... (f an
bn init) ...)).
Raises Invalid_argument if the two lists are determined to have different lengths. Not
tail-recursive.

List scanning
val for_all : ('a -> bool) -> 'a list -> bool
for_all f [a1; ...; an] checks if all elements of the list satisfy the predicate f. That is,
it returns (f a1) && (f a2) && ... && (f an) for a non-empty list and true if the list is
empty.

val exists : ('a -> bool) -> 'a list -> bool
exists f [a1; ...; an] checks if at least one element of the list satisfies the predicate f.
That is, it returns (f a1) || (f a2) || ... || (f an) for a non-empty list and false if
the list is empty.

val for_all2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as List.for_all[29.32], but for a two-argument predicate.
Raises Invalid_argument if the two lists are determined to have different lengths.
Chapter 29. The standard library 761

val exists2 : ('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as List.exists[29.32], but for a two-argument predicate.
Raises Invalid_argument if the two lists are determined to have different lengths.

val mem : 'a -> 'a list -> bool

mem a set is true if and only if a is equal to an element of set.

val memq : 'a -> 'a list -> bool

Same as List.mem[29.32], but uses physical equality instead of structural equality to compare
list elements.

List searching
val find : ('a -> bool) -> 'a list -> 'a
find f l returns the first element of the list l that satisfies the predicate f.
Raises Not_found if there is no value that satisfies f in the list l.

val find_opt : ('a -> bool) -> 'a list -> 'a option
find f l returns the first element of the list l that satisfies the predicate f. Returns None if
there is no value that satisfies f in the list l.
Since: 4.05

val find_index : ('a -> bool) -> 'a list -> int option
find_index f xs returns Some i, where i is the index of the first element of the list xs that
satisfies f x, if there is such an element.
It returns None if there is no such element.
Since: 5.1

val find_map : ('a -> 'b option) -> 'a list -> 'b option
find_map f l applies f to the elements of l in order, and returns the first result of the form
Some v, or None if none exist.
Since: 4.10

val find_mapi : (int -> 'a -> 'b option) -> 'a list -> 'b option
Same as find_map, but the predicate is applied to the index of the element as first argument
(counting from 0), and the element itself as second argument.
Since: 5.1

val filter : ('a -> bool) -> 'a list -> 'a list
filter f l returns all the elements of the list l that satisfy the predicate f. The order of
the elements in the input list is preserved.
762

val find_all : ('a -> bool) -> 'a list -> 'a list
find_all is another name for List.filter[29.32].

val filteri : (int -> 'a -> bool) -> 'a list -> 'a list
Same as List.filter[29.32], but the predicate is applied to the index of the element as first
argument (counting from 0), and the element itself as second argument.
Since: 4.11

List manipulation
val take : int -> 'a list -> 'a list
take n l returns the prefix of l of length n, or a copy of l if n > length l.
n must be nonnegative.
Since: 5.3
Raises Invalid_argument if n is negative.

val drop : int -> 'a list -> 'a list

drop n l returns the suffix of l after n elements, or [] if n > length l.
n must be nonnegative.
Since: 5.3
Raises Invalid_argument if n is negative.

val take_while : ('a -> bool) -> 'a list -> 'a list
take_while p l is the longest (possibly empty) prefix of l containing only elements that
satisfy p.
Since: 5.3

val drop_while : ('a -> bool) -> 'a list -> 'a list
drop_while p l is the longest (possibly empty) suffix of l starting at the first element that
does not satisfy p.
Since: 5.3

val partition : ('a -> bool) -> 'a list -> 'a list * 'a list
partition f l returns a pair of lists (l1, l2), where l1 is the list of all the elements of l
that satisfy the predicate f, and l2 is the list of all the elements of l that do not satisfy f.
The order of the elements in the input list is preserved.

val partition_map : ('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list
partition_map f l returns a pair of lists (l1, l2) such that, for each element x of the
input list l:
Chapter 29. The standard library 763

• if f x is Left y1, then y1 is in l1, and

• if f x is Right y2, then y2 is in l2.

The output elements are included in l1 and l2 in the same relative order as the
corresponding input elements in l.
In particular, partition_map (fun x -> if f x then Left x else Right x) l is
equivalent to partition f l.
Since: 4.12

Association lists
val assoc : 'a -> ('a * 'b) list -> 'b
assoc a l returns the value associated with key a in the list of pairs l. That is, assoc a [
...; (a,b); ...] = b if (a,b) is the leftmost binding of a in list l.
Raises Not_found if there is no value associated with a in the list l.

val assoc_opt : 'a -> ('a * 'b) list -> 'b option
assoc_opt a l returns the value associated with key a in the list of pairs l. That is,
assoc_opt a [ ...; (a,b); ...] = Some b if (a,b) is the leftmost binding of a in list l.
Returns None if there is no value associated with a in the list l.
Since: 4.05

val assq : 'a -> ('a * 'b) list -> 'b

Same as List.assoc[29.32], but uses physical equality instead of structural equality to
compare keys.

val assq_opt : 'a -> ('a * 'b) list -> 'b option
Same as List.assoc_opt[29.32], but uses physical equality instead of structural equality to
compare keys.
Since: 4.05

val mem_assoc : 'a -> ('a * 'b) list -> bool

Same as List.assoc[29.32], but simply return true if a binding exists, and false if no
bindings exist for the given key.

val mem_assq : 'a -> ('a * 'b) list -> bool

Same as List.mem_assoc[29.32], but uses physical equality instead of structural equality to
compare keys.

val remove_assoc : 'a -> ('a * 'b) list -> ('a * 'b) list
remove_assoc a l returns the list of pairs l without the first pair with key a, if any. Not
tail-recursive.
764

val remove_assq : 'a -> ('a * 'b) list -> ('a * 'b) list
Same as List.remove_assoc[29.32], but uses physical equality instead of structural equality
to compare keys. Not tail-recursive.

Lists of pairs
val split : ('a * 'b) list -> 'a list * 'b list
Transform a list of pairs into a pair of lists: split [(a1,b1); ...; (an,bn)] is ([a1;
...; an], [b1; ...; bn]). Not tail-recursive.

val combine : 'a list -> 'b list -> ('a * 'b) list
Transform a pair of lists into a list of pairs: combine [a1; ...; an] [b1; ...; bn] is
[(a1,b1); ...; (an,bn)].
Raises Invalid_argument if the two lists have different lengths. Not tail-recursive.

Sorting
val sort : ('a -> 'a -> int) -> 'a list -> 'a list
Sort a list in increasing order according to a comparison function. The comparison function
must return 0 if its arguments compare as equal, a positive integer if the first is greater, and a
negative integer if the first is smaller (see Array.sort for a complete specification). For
example, compare[28.2] is a suitable comparison function. The resulting list is sorted in
increasing order. List.sort[29.32] is guaranteed to run in constant heap space (in addition
to the size of the result list) and logarithmic stack space.
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic
stack space.

val stable_sort : ('a -> 'a -> int) -> 'a list -> 'a list
Same as List.sort[29.32], but the sorting algorithm is guaranteed to be stable (i.e. elements
that compare equal are kept in their original order).
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic
stack space.

val fast_sort : ('a -> 'a -> int) -> 'a list -> 'a list
Same as List.sort[29.32] or List.stable_sort[29.32], whichever is faster on typical input.

val sort_uniq : ('a -> 'a -> int) -> 'a list -> 'a list
Same as List.sort[29.32], but also remove duplicates.
Since: 4.02 (4.03 in ListLabels)

val merge : ('a -> 'a -> int) -> 'a list -> 'a list -> 'a list
Chapter 29. The standard library 765

Merge two lists: Assuming that l1 and l2 are sorted according to the comparison function
cmp, merge cmp l1 l2 will return a sorted list containing all the elements of l1 and l2. If
several elements compare equal, the elements of l1 will be before the elements of l2. Not
tail-recursive (sum of the lengths of the arguments).

Lists and Sequences

val to_seq : 'a list -> 'a Seq.t
Iterate on the list.
Since: 4.07

val of_seq : 'a Seq.t -> 'a list

Create a list from a sequence.
Since: 4.07

29.33 Module ListLabels : List operations.

Some functions are flagged as not tail-recursive. A tail-recursive function uses constant stack space,
while a non-tail-recursive function uses stack space proportional to the length of its list argument,
which can be a problem with very long lists. When the function takes several list arguments, an
approximate formula giving stack usage (in some unspecified constant unit) is shown in parentheses.
The above considerations can usually be ignored if your lists are not longer than about 10000
elements.
The labeled version of this module can be used as described in the StdLabels[29.53] module.

type 'a t = 'a list =

| []
| (::) of 'a * 'a list
An alias for the type of lists.

val length : 'a list -> int

Return the length (number of elements) of the given list.

val compare_lengths : 'a list -> 'b list -> int

Compare the lengths of two lists. compare_lengths l1 l2 is equivalent to compare
(length l1) (length l2), except that the computation stops after reaching the end of the
shortest list.
Since: 4.05

val compare_length_with : 'a list -> len:int -> int

766

Compare the length of a list to an integer. compare_length_with l len is equivalent to

compare (length l) len, except that the computation stops after at most len iterations on
the list.
Since: 4.05

val is_empty : 'a list -> bool

is_empty l is true if and only if l has no elements. It is equivalent to
compare_length_with l 0 = 0.
Since: 5.1

val cons : 'a -> 'a list -> 'a list

cons x xs is x :: xs
Since: 4.05

val hd : 'a list -> 'a

Return the first element of the given list.
Raises Failure if the list is empty.

val tl : 'a list -> 'a list

Return the given list without its first element.
Raises Failure if the list is empty.

val nth : 'a list -> int -> 'a

Return the n-th element of the given list. The first element (head of the list) is at position 0.
Raises

• Failure if the list is too short.

• Invalid_argument if n is negative.

val nth_opt : 'a list -> int -> 'a option

Return the n-th element of the given list. The first element (head of the list) is at position 0.
Return None if the list is too short.
Since: 4.05
Raises Invalid_argument if n is negative.

val rev : 'a list -> 'a list

List reversal.

val init : len:int -> f:(int -> 'a) -> 'a list
init ~len ~f is [f 0; f 1; ...; f (len-1)], evaluated left to right.
Since: 4.06
Raises Invalid_argument if len < 0.
Chapter 29. The standard library 767

val append : 'a list -> 'a list -> 'a list
append l0 l1 appends l1 to l0. Same function as the infix operator @.
Since: 5.1 this function is tail-recursive.

val rev_append : 'a list -> 'a list -> 'a list
rev_append l1 l2 reverses l1 and concatenates it with l2. This is equivalent to
(ListLabels.rev[29.33] l1) @ l2.

val concat : 'a list list -> 'a list

Concatenate a list of lists. The elements of the argument are all concatenated together (in the
same order) to give the result. Not tail-recursive (length of the argument + length of the
longest sub-list).

val flatten : 'a list list -> 'a list

Same as ListLabels.concat[29.33]. Not tail-recursive (length of the argument + length of
the longest sub-list).

Comparison
val equal : eq:('a -> 'a -> bool) -> 'a list -> 'a list -> bool
equal eq [a1; ...; an] [b1; ..; bm] holds when the two input lists have the same
length, and for each pair of elements ai, bi at the same position we have eq ai bi.
Note: the eq function may be called even if the lists have different length. If you know your
equality function is costly, you may want to check ListLabels.compare_lengths[29.33] first.
Since: 4.12

val compare : cmp:('a -> 'a -> int) -> 'a list -> 'a list -> int
compare cmp [a1; ...; an] [b1; ...; bm] performs a lexicographic comparison of the
two input lists, using the same 'a -> 'a -> int interface as compare[28.2]:

• a1 :: l1 is smaller than a2 :: l2 (negative result) if a1 is smaller than a2, or if they

are equal (0 result) and l1 is smaller than l2
• the empty list [] is strictly smaller than non-empty lists

Note: the cmp function will be called even if the lists have different lengths.
Since: 4.12
768

Iterators
val iter : f:('a -> unit) -> 'a list -> unit
iter ~f [a1; ...; an] applies function f in turn to [a1; ...; an]. It is equivalent to f
a1; f a2; ...; f an.
val iteri : f:(int -> 'a -> unit) -> 'a list -> unit
Same as ListLabels.iter[29.33], but the function is applied to the index of the element as
first argument (counting from 0), and the element itself as second argument.
Since: 4.00
val map : f:('a -> 'b) -> 'a list -> 'b list
map ~f [a1; ...; an] applies function f to a1, ..., an, and builds the list [f a1; ...;
f an] with the results returned by f.
val mapi : f:(int -> 'a -> 'b) -> 'a list -> 'b list
Same as ListLabels.map[29.33], but the function is applied to the index of the element as
first argument (counting from 0), and the element itself as second argument.
Since: 4.00
val rev_map : f:('a -> 'b) -> 'a list -> 'b list
rev_map ~f l gives the same result as ListLabels.rev[29.33] (ListLabels.map[29.33] f
l), but is more efficient.
val filter_map : f:('a -> 'b option) -> 'a list -> 'b list
filter_map ~f l applies f to every element of l, filters out the None elements and returns
the list of the arguments of the Some elements.
Since: 4.08
val concat_map : f:('a -> 'b list) -> 'a list -> 'b list
concat_map ~f l gives the same result as ListLabels.concat[29.33]
(ListLabels.map[29.33] f l). Tail-recursive.
Since: 4.10
val fold_left_map :
f:('acc -> 'a -> 'acc * 'b) -> init:'acc -> 'a list -> 'acc * 'b list
fold_left_map is a combination of fold_left and map that threads an accumulator through
calls to f.
Since: 4.11
val fold_left : f:('acc -> 'a -> 'acc) -> init:'acc -> 'a list -> 'acc
fold_left ~f ~init [b1; ...; bn] is f (... (f (f init b1) b2) ...) bn.
val fold_right : f:('a -> 'acc -> 'acc) -> 'a list -> init:'acc -> 'acc
fold_right ~f [a1; ...; an] ~init is f a1 (f a2 (... (f an init) ...)). Not
tail-recursive.
Chapter 29. The standard library 769

Iterators on two lists

val iter2 : f:('a -> 'b -> unit) -> 'a list -> 'b list -> unit
iter2 ~f [a1; ...; an] [b1; ...; bn] calls in turn f a1 b1; ...; f an bn.
Raises Invalid_argument if the two lists are determined to have different lengths.

val map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
map2 ~f [a1; ...; an] [b1; ...; bn] is [f a1 b1; ...; f an bn].
Raises Invalid_argument if the two lists are determined to have different lengths.

val rev_map2 : f:('a -> 'b -> 'c) -> 'a list -> 'b list -> 'c list
rev_map2 ~f l1 l2 gives the same result as ListLabels.rev[29.33]
(ListLabels.map2[29.33] f l1 l2), but is more efficient.

val fold_left2 :
f:('acc -> 'a -> 'b -> 'acc) -> init:'acc -> 'a list -> 'b list -> 'acc
fold_left2 ~f ~init [a1; ...; an] [b1; ...; bn] is f (... (f (f init a1 b1) a2
b2) ...) an bn.
Raises Invalid_argument if the two lists are determined to have different lengths.

val fold_right2 :
f:('a -> 'b -> 'acc -> 'acc) -> 'a list -> 'b list -> init:'acc -> 'acc
fold_right2 ~f [a1; ...; an] [b1; ...; bn] ~init is f a1 b1 (f a2 b2 (... (f
an bn init) ...)).
Raises Invalid_argument if the two lists are determined to have different lengths. Not
tail-recursive.

List scanning
val for_all : f:('a -> bool) -> 'a list -> bool
for_all ~f [a1; ...; an] checks if all elements of the list satisfy the predicate f. That is,
it returns (f a1) && (f a2) && ... && (f an) for a non-empty list and true if the list is
empty.

val exists : f:('a -> bool) -> 'a list -> bool
exists ~f [a1; ...; an] checks if at least one element of the list satisfies the predicate f.
That is, it returns (f a1) || (f a2) || ... || (f an) for a non-empty list and false if
the list is empty.

val for_all2 : f:('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as ListLabels.for_all[29.33], but for a two-argument predicate.
Raises Invalid_argument if the two lists are determined to have different lengths.
770

val exists2 : f:('a -> 'b -> bool) -> 'a list -> 'b list -> bool
Same as ListLabels.exists[29.33], but for a two-argument predicate.
Raises Invalid_argument if the two lists are determined to have different lengths.

val mem : 'a -> set:'a list -> bool

mem a ~set is true if and only if a is equal to an element of set.

val memq : 'a -> set:'a list -> bool

Same as ListLabels.mem[29.33], but uses physical equality instead of structural equality to
compare list elements.

List searching
val find : f:('a -> bool) -> 'a list -> 'a
find ~f l returns the first element of the list l that satisfies the predicate f.
Raises Not_found if there is no value that satisfies f in the list l.

val find_opt : f:('a -> bool) -> 'a list -> 'a option
find ~f l returns the first element of the list l that satisfies the predicate f. Returns None
if there is no value that satisfies f in the list l.
Since: 4.05

val find_index : f:('a -> bool) -> 'a list -> int option
find_index ~f xs returns Some i, where i is the index of the first element of the list xs
that satisfies f x, if there is such an element.
It returns None if there is no such element.
Since: 5.1

val find_map : f:('a -> 'b option) -> 'a list -> 'b option
find_map ~f l applies f to the elements of l in order, and returns the first result of the
form Some v, or None if none exist.
Since: 4.10

val find_mapi : f:(int -> 'a -> 'b option) -> 'a list -> 'b option
Same as find_map, but the predicate is applied to the index of the element as first argument
(counting from 0), and the element itself as second argument.
Since: 5.1

val filter : f:('a -> bool) -> 'a list -> 'a list
filter ~f l returns all the elements of the list l that satisfy the predicate f. The order of
the elements in the input list is preserved.
Chapter 29. The standard library 771

val find_all : f:('a -> bool) -> 'a list -> 'a list
find_all is another name for ListLabels.filter[29.33].

val filteri : f:(int -> 'a -> bool) -> 'a list -> 'a list
Same as ListLabels.filter[29.33], but the predicate is applied to the index of the element
as first argument (counting from 0), and the element itself as second argument.
Since: 4.11

List manipulation
val take : int -> 'a list -> 'a list
take n l returns the prefix of l of length n, or a copy of l if n > length l.
n must be nonnegative.
Since: 5.3
Raises Invalid_argument if n is negative.

val drop : int -> 'a list -> 'a list

drop n l returns the suffix of l after n elements, or [] if n > length l.
n must be nonnegative.
Since: 5.3
Raises Invalid_argument if n is negative.

val take_while : f:('a -> bool) -> 'a list -> 'a list
take_while p l is the longest (possibly empty) prefix of l containing only elements that
satisfy p.
Since: 5.3

val drop_while : f:('a -> bool) -> 'a list -> 'a list
drop_while p l is the longest (possibly empty) suffix of l starting at the first element that
does not satisfy p.
Since: 5.3

val partition : f:('a -> bool) -> 'a list -> 'a list * 'a list
partition ~f l returns a pair of lists (l1, l2), where l1 is the list of all the elements of l
that satisfy the predicate f, and l2 is the list of all the elements of l that do not satisfy f.
The order of the elements in the input list is preserved.

val partition_map :
f:('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list
partition_map f l returns a pair of lists (l1, l2) such that, for each element x of the
input list l:
772

• if f x is Left y1, then y1 is in l1, and

• if f x is Right y2, then y2 is in l2.

The output elements are included in l1 and l2 in the same relative order as the
corresponding input elements in l.
In particular, partition_map (fun x -> if f x then Left x else Right x) l is
equivalent to partition f l.
Since: 4.12

Association lists
val assoc : 'a -> ('a * 'b) list -> 'b
assoc a l returns the value associated with key a in the list of pairs l. That is, assoc a [
...; (a,b); ...] = b if (a,b) is the leftmost binding of a in list l.
Raises Not_found if there is no value associated with a in the list l.

val assoc_opt : 'a -> ('a * 'b) list -> 'b option
assoc_opt a l returns the value associated with key a in the list of pairs l. That is,
assoc_opt a [ ...; (a,b); ...] = Some b if (a,b) is the leftmost binding of a in list l.
Returns None if there is no value associated with a in the list l.
Since: 4.05

val assq : 'a -> ('a * 'b) list -> 'b

Same as ListLabels.assoc[29.33], but uses physical equality instead of structural equality
to compare keys.

val assq_opt : 'a -> ('a * 'b) list -> 'b option
Same as ListLabels.assoc_opt[29.33], but uses physical equality instead of structural
equality to compare keys.
Since: 4.05

val mem_assoc : 'a -> map:('a * 'b) list -> bool

Same as ListLabels.assoc[29.33], but simply return true if a binding exists, and false if
no bindings exist for the given key.

val mem_assq : 'a -> map:('a * 'b) list -> bool

Same as ListLabels.mem_assoc[29.33], but uses physical equality instead of structural
equality to compare keys.

val remove_assoc : 'a -> ('a * 'b) list -> ('a * 'b) list
remove_assoc a l returns the list of pairs l without the first pair with key a, if any. Not
tail-recursive.
Chapter 29. The standard library 773

val remove_assq : 'a -> ('a * 'b) list -> ('a * 'b) list
Same as ListLabels.remove_assoc[29.33], but uses physical equality instead of structural
equality to compare keys. Not tail-recursive.

Lists of pairs
val split : ('a * 'b) list -> 'a list * 'b list
Transform a list of pairs into a pair of lists: split [(a1,b1); ...; (an,bn)] is ([a1;
...; an], [b1; ...; bn]). Not tail-recursive.

val combine : 'a list -> 'b list -> ('a * 'b) list
Transform a pair of lists into a list of pairs: combine [a1; ...; an] [b1; ...; bn] is
[(a1,b1); ...; (an,bn)].
Raises Invalid_argument if the two lists have different lengths. Not tail-recursive.

Sorting
val sort : cmp:('a -> 'a -> int) -> 'a list -> 'a list
Sort a list in increasing order according to a comparison function. The comparison function
must return 0 if its arguments compare as equal, a positive integer if the first is greater, and a
negative integer if the first is smaller (see Array.sort for a complete specification). For
example, compare[28.2] is a suitable comparison function. The resulting list is sorted in
increasing order. ListLabels.sort[29.33] is guaranteed to run in constant heap space (in
addition to the size of the result list) and logarithmic stack space.
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic
stack space.

val stable_sort : cmp:('a -> 'a -> int) -> 'a list -> 'a list
Same as ListLabels.sort[29.33], but the sorting algorithm is guaranteed to be stable (i.e.
elements that compare equal are kept in their original order).
The current implementation uses Merge Sort. It runs in constant heap space and logarithmic
stack space.

val fast_sort : cmp:('a -> 'a -> int) -> 'a list -> 'a list
Same as ListLabels.sort[29.33] or ListLabels.stable_sort[29.33], whichever is faster on
typical input.

val sort_uniq : cmp:('a -> 'a -> int) -> 'a list -> 'a list
Same as ListLabels.sort[29.33], but also remove duplicates.
Since: 4.03

val merge : cmp:('a -> 'a -> int) -> 'a list -> 'a list -> 'a list
774

Merge two lists: Assuming that l1 and l2 are sorted according to the comparison function
cmp, merge ~cmp l1 l2 will return a sorted list containing all the elements of l1 and l2. If
several elements compare equal, the elements of l1 will be before the elements of l2. Not
tail-recursive (sum of the lengths of the arguments).

Lists and Sequences

val to_seq : 'a list -> 'a Seq.t
Iterate on the list.
Since: 4.07

val of_seq : 'a Seq.t -> 'a list

Create a list from a sequence.
Since: 4.07

29.34 Module Map : Association tables over ordered types.

This module implements applicative association tables, also known as finite maps or dictionaries,
given a total ordering function over the keys. All operations over maps are purely applicative (no
side-effects). The implementation uses balanced binary trees, and therefore searching and insertion
take time logarithmic in the size of the map.
For instance:

module IntPairs =
struct
type t = int * int
let compare (x0,y0) (x1,y1) =
match Stdlib.compare x0 x1 with
0 -> Stdlib.compare y0 y1
| c -> c
end

module PairsMap = Map.Make(IntPairs)

let m = PairsMap.(empty |> add (0,1) "hello" |> add (1,0) "world")

This creates a new module PairsMap, with a new type 'a PairsMap.t of maps from int * int
to 'a. In this example, m contains string values so its type is string PairsMap.t.

module type OrderedType =

sig
type t
Chapter 29. The standard library 775

The type of the map keys.

val compare : t -> t -> int

A total ordering function over the keys. This is a two-argument function f such that f
e1 e2 is zero if the keys e1 and e2 are equal, f e1 e2 is strictly negative if e1 is smaller
than e2, and f e1 e2 is strictly positive if e1 is greater than e2. Example: a suitable
ordering function is the generic structural comparison function compare[28.2].

end

Input signature of the functor Map.Make[29.34].

module type S =
sig

Maps
type key
The type of the map keys.

type !+'a t
The type of maps from type key to type 'a.

val empty : 'a t

The empty map.

val add : key -> 'a -> 'a t -> 'a t

add key data m returns a map containing the same bindings as m, plus a binding of key
to data. If key was already bound in m to a value that is physically equal to data, m is
returned unchanged (the result of the function is then physically equal to m). Otherwise,
the previous binding of key in m disappears.
Before 4.03 Physical equality was not ensured.

val add_to_list : key -> 'a -> 'a list t -> 'a list t
add_to_list key data m is m with key mapped to l such that l is data :: Map.find
key m if key was bound in m and [v] otherwise.
Since: 5.1

val update : key -> ('a option -> 'a option) -> 'a t -> 'a t
776

update key f m returns a map containing the same bindings as m, except for the
binding of key. Depending on the value of y where y is f (find_opt key m), the
binding of key is added, removed or updated. If y is None, the binding is removed if it
exists; otherwise, if y is Some z then key is associated to z in the resulting map. If key
was already bound in m to a value that is physically equal to z, m is returned unchanged
(the result of the function is then physically equal to m).
Since: 4.06

val singleton : key -> 'a -> 'a t

singleton x y returns the one-element map that contains a binding y for x.
Since: 3.12

val remove : key -> 'a t -> 'a t

remove x m returns a map containing the same bindings as m, except for x which is
unbound in the returned map. If x was not in m, m is returned unchanged (the result of
the function is then physically equal to m).
Before 4.03 Physical equality was not ensured.

val merge :
(key -> 'a option -> 'b option -> 'c option) ->
'a t -> 'b t -> 'c t
merge f m1 m2 computes a map whose keys are a subset of the keys of m1 and of m2.
The presence of each such binding, and the corresponding value, is determined with the
function f. In terms of the find_opt operation, we have find_opt x (merge f m1 m2)
= f x (find_opt x m1) (find_opt x m2) for any key x, provided that f x None
None = None.
Since: 3.12

val union : (key -> 'a -> 'a -> 'a option) ->
'a t -> 'a t -> 'a t
union f m1 m2 computes a map whose keys are a subset of the keys of m1 and of m2.
When the same binding is defined in both arguments, the function f is used to combine
them. This is a special case of merge: union f m1 m2 is equivalent to merge f' m1 m2,
where
• f' _key None None = None
• f' _key (Some v) None = Some v
• f' _key None (Some v) = Some v
• f' key (Some v1) (Some v2) = f key v1 v2
Since: 4.03

val cardinal : 'a t -> int

Return the number of bindings of a map.
Since: 3.12
Chapter 29. The standard library 777

Bindings
val bindings : 'a t -> (key * 'a) list
Return the list of all bindings of the given map. The returned list is sorted in increasing
order of keys with respect to the ordering Ord.compare, where Ord is the argument
given to Map.Make[29.34].
Since: 3.12

val min_binding : 'a t -> key * 'a

Return the binding with the smallest key in a given map (with respect to the
Ord.compare ordering), or raise Not_found if the map is empty.
Since: 3.12

val min_binding_opt : 'a t -> (key * 'a) option

Return the binding with the smallest key in the given map (with respect to the
Ord.compare ordering), or None if the map is empty.
Since: 4.05

val max_binding : 'a t -> key * 'a

Same as Map.S.min_binding[29.34], but returns the binding with the largest key in the
given map.
Since: 3.12

val max_binding_opt : 'a t -> (key * 'a) option

Same as Map.S.min_binding_opt[29.34], but returns the binding with the largest key in
the given map.
Since: 4.05

val choose : 'a t -> key * 'a

Return one binding of the given map, or raise Not_found if the map is empty. Which
binding is chosen is unspecified, but equal bindings will be chosen for equal maps.
Since: 3.12

val choose_opt : 'a t -> (key * 'a) option

Return one binding of the given map, or None if the map is empty. Which binding is
chosen is unspecified, but equal bindings will be chosen for equal maps.
Since: 4.05
778

Searching
val find : key -> 'a t -> 'a
find x m returns the current value of x in m, or raises Not_found if no binding for x
exists.

val find_opt : key -> 'a t -> 'a option

find_opt x m returns Some v if the current value of x in m is v, or None if no binding
for x exists.
Since: 4.05

val find_first : (key -> bool) -> 'a t -> key * 'a
find_first f m, where f is a monotonically increasing function, returns the binding of
m with the lowest key k such that f k, or raises Not_found if no such key exists.
For example, find_first (fun k -> Ord.compare k x >= 0) m will return the first
binding k, v of m where Ord.compare k x >= 0 (intuitively: k >= x), or raise
Not_found if x is greater than any element of m.
Since: 4.05

val find_first_opt : (key -> bool) -> 'a t -> (key * 'a) option
find_first_opt f m, where f is a monotonically increasing function, returns an option
containing the binding of m with the lowest key k such that f k, or None if no such key
exists.
Since: 4.05

val find_last : (key -> bool) -> 'a t -> key * 'a
find_last f m, where f is a monotonically decreasing function, returns the binding of
m with the highest key k such that f k, or raises Not_found if no such key exists.
Since: 4.05

val find_last_opt : (key -> bool) -> 'a t -> (key * 'a) option
find_last_opt f m, where f is a monotonically decreasing function, returns an option
containing the binding of m with the highest key k such that f k, or None if no such key
exists.
Since: 4.05

Traversing
val iter : (key -> 'a -> unit) -> 'a t -> unit
iter f m applies f to all bindings in map m. f receives the key as first argument, and
the associated value as second argument. The bindings are passed to f in increasing
order with respect to the ordering over the type of the keys.
Chapter 29. The standard library 779

val fold : (key -> 'a -> 'acc -> 'acc) -> 'a t -> 'acc -> 'acc
fold f m init computes (f kN dN ... (f k1 d1 init)...), where k1 ... kN are
the keys of all bindings in m (in increasing order), and d1 ... dN are the associated
data.

Transforming
val map : ('a -> 'b) -> 'a t -> 'b t
map f m returns a map with same domain as m, where the associated value a of all
bindings of m has been replaced by the result of the application of f to a. The bindings
are passed to f in increasing order with respect to the ordering over the type of the keys.

val mapi : (key -> 'a -> 'b) -> 'a t -> 'b t
Same as Map.S.map[29.34], but the function receives as arguments both the key and the
associated value for each binding of the map.

val filter : (key -> 'a -> bool) -> 'a t -> 'a t
filter f m returns the map with all the bindings in m that satisfy predicate p. If every
binding in m satisfies f, m is returned unchanged (the result of the function is then
physically equal to m)
Before 4.03 Physical equality was not ensured.
Since: 3.12

val filter_map : (key -> 'a -> 'b option) -> 'a t -> 'b t
filter_map f m applies the function f to every binding of m, and builds a map from the
results. For each binding (k, v) in the input map:
• if f k v is None then k is not in the result,
• if f k v is Some v' then the binding (k, v') is in the output map.
For example, the following function on maps whose values are lists

filter_map
(fun _k li -> match li with [] -> None | _::tl -> Some tl)
m

drops all bindings of m whose value is an empty list, and pops the first element of each
value that is non-empty.
Since: 4.11

val partition : (key -> 'a -> bool) -> 'a t -> 'a t * 'a t
780

partition f m returns a pair of maps (m1, m2), where m1 contains all the bindings of
m that satisfy the predicate f, and m2 is the map with all the bindings of m that do not
satisfy f.
Since: 3.12

val split : key -> 'a t -> 'a t * 'a option * 'a t
split x m returns a triple (l, data, r), where l is the map with all the bindings of m
whose key is strictly less than x; r is the map with all the bindings of m whose key is
strictly greater than x; data is None if m contains no binding for x, or Some v if m binds
v to x.
Since: 3.12

Predicates and comparisons

val is_empty : 'a t -> bool
Test whether a map is empty or not.

val mem : key -> 'a t -> bool

mem x m returns true if m contains a binding for x, and false otherwise.

val equal : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool
equal cmp m1 m2 tests whether the maps m1 and m2 are equal, that is, contain equal
keys and associate them with equal data. cmp is the equality predicate used to compare
the data associated with the keys.

val compare : ('a -> 'a -> int) -> 'a t -> 'a t -> int
Total ordering between maps. The first argument is a total ordering used to compare
data associated with equal keys in the two maps.

val for_all : (key -> 'a -> bool) -> 'a t -> bool
for_all f m checks if all the bindings of the map satisfy the predicate f.
Since: 3.12

val exists : (key -> 'a -> bool) -> 'a t -> bool
exists f m checks if at least one binding of the map satisfies the predicate f.
Since: 3.12
Chapter 29. The standard library 781

Converting
val to_list : 'a t -> (key * 'a) list
to_list m is Map.S.bindings[29.34] m.
Since: 5.1

val of_list : (key * 'a) list -> 'a t

of_list bs adds the bindings of bs to the empty map, in list order (if a key is bound
twice in bs the last one takes over).
Since: 5.1

val to_seq : 'a t -> (key * 'a) Seq.t

Iterate on the whole map, in ascending order of keys
Since: 4.07

val to_rev_seq : 'a t -> (key * 'a) Seq.t

Iterate on the whole map, in descending order of keys
Since: 4.12

val to_seq_from : key -> 'a t -> (key * 'a) Seq.t

to_seq_from k m iterates on a subset of the bindings of m, in ascending order of keys,
from key k or above.
Since: 4.07

val add_seq : (key * 'a) Seq.t -> 'a t -> 'a t

Add the given bindings to the map, in order.
Since: 4.07

val of_seq : (key * 'a) Seq.t -> 'a t

Build a map from the given bindings
Since: 4.07

end

Output signature of the functor Map.Make[29.34].

module Make :
functor (Ord : OrderedType) -> S with type key = Ord.t
Functor building an implementation of the map structure given a totally ordered type.
782

29.35 Module Marshal : Marshaling of data structures.

This module provides functions to encode arbitrary data structures as sequences of bytes, which can
then be written on a file or sent over a pipe or network connection. The bytes can then be read
back later, possibly in another process, and decoded back into a data structure. The format for the
byte sequences is compatible across all machines for a given version of OCaml.
Warning: marshaling is currently not type-safe. The type of marshaled data is not transmitted
along the value of the data, making it impossible to check that the data read back possesses the type
expected by the context. In particular, the result type of the Marshal.from_* functions is given as
'a, but this is misleading: the returned OCaml value does not possess type 'a for all 'a; it has one,
unique type which cannot be determined at compile-time. The programmer should explicitly give
the expected type of the returned value, using the following syntax:

• (Marshal.from_channel chan : type). Anything can happen at run-time if the object in

the file does not belong to the given type.

Values of extensible variant types, for example exceptions (of extensible type exn), returned by
the unmarshaller should not be pattern-matched over through match ... with or try ... with,
because unmarshalling does not preserve the information required for matching their constructors.
Structural equalities with other extensible variant values does not work either. Most other uses
such as Printexc.to_string, will still work as expected.
The representation of marshaled values is not human-readable, and uses bytes that are not
printable characters. Therefore, input and output channels used in conjunction with Marshal.to_
channel and Marshal.from_channel must be opened in binary mode, using e.g. open_out_bin
or open_in_bin; channels opened in text mode will cause unmarshaling errors on platforms where
text channels behave differently than binary channels, e.g. Windows.

type extern_flags =
| No_sharing
Don’t preserve sharing
| Closures
Send function closures
| Compat_32
Ensure 32-bit compatibility
The flags to the Marshal.to_* functions below.
val to_channel : out_channel -> 'a -> extern_flags list -> unit
Marshal.to_channel chan v flags writes the representation of v on channel chan. The
flags argument is a possibly empty list of flags that governs the marshaling behavior with
respect to sharing, functional values, and compatibility between 32- and 64-bit platforms.
If flags does not contain Marshal.No_sharing, circularities and sharing inside the value v
are detected and preserved in the sequence of bytes produced. In particular, this guarantees
that marshaling always terminates. Sharing between values marshaled by successive calls to
Marshal.to_channel is neither detected nor preserved, though. If flags contains
Chapter 29. The standard library 783

Marshal.No_sharing, sharing is ignored. This results in faster marshaling if v contains no

shared substructures, but may cause slower marshaling and larger byte representations if v
actually contains sharing, or even non-termination if v contains cycles.
If flags does not contain Marshal.Closures, marshaling fails when it encounters a
functional value inside v: only ’pure’ data structures, containing neither functions nor objects,
can safely be transmitted between different programs. If flags contains Marshal.Closures,
functional values will be marshaled as a the position in the code of the program together with
the values corresponding to the free variables captured in the closure. In this case, the output
of marshaling can only be read back in processes that run exactly the same program, with
exactly the same compiled code. (This is checked at un-marshaling time, using an MD5 digest
of the code transmitted along with the code position.)
The exact definition of which free variables are captured in a closure is not specified and can
vary between bytecode and native code (and according to optimization flags). In particular, a
function value accessing a global reference may or may not include the reference in its closure.
If it does, unmarshaling the corresponding closure will create a new reference, different from
the global one.
If flags contains Marshal.Compat_32, marshaling fails when it encounters an integer value
outside the range -230 , 230 -1 of integers that are representable on a 32-bit platform. This
ensures that marshaled data generated on a 64-bit platform can be safely read back on a
32-bit platform. If flags does not contain Marshal.Compat_32, integer values outside the
range -230 , 230 -1 are marshaled, and can be read back on a 64-bit platform, but will cause an
error at un-marshaling time when read back on a 32-bit platform. The Mashal.Compat_32
flag only matters when marshaling is performed on a 64-bit platform; it has no effect if
marshaling is performed on a 32-bit platform.
Raises Failure if chan is not in binary mode.

val to_bytes : 'a -> extern_flags list -> bytes

Marshal.to_bytes v flags returns a byte sequence containing the representation of v. The
flags argument has the same meaning as for Marshal.to_channel[29.35].
Since: 4.02

val to_string : 'a -> extern_flags list -> string

Same as to_bytes but return the result as a string instead of a byte sequence.

val to_buffer : bytes -> int -> int -> 'a -> extern_flags list -> int
Marshal.to_buffer buff ofs len v flags marshals the value v, storing its byte
representation in the sequence buff, starting at index ofs, and writing at most len bytes. It
returns the number of bytes actually written to the sequence. If the byte representation of v
does not fit in len characters, the exception Failure is raised.

val from_channel : in_channel -> 'a

Marshal.from_channel chan reads from channel chan the byte representation of a
structured value, as produced by one of the Marshal.to_* functions, and reconstructs and
returns the corresponding value.
784

Raises

• End_of_file if chan is already at the end of the file.

• Failure if the end of the file is reached during unmarshalling itself or if chan is not in
binary mode.

val from_bytes : bytes -> int -> 'a

Marshal.from_bytes buff ofs unmarshals a structured value like
Marshal.from_channel[29.35] does, except that the byte representation is not read from a
channel, but taken from the byte sequence buff, starting at position ofs. The byte sequence
is not mutated.
Since: 4.02

val from_string : string -> int -> 'a

Same as from_bytes but take a string as argument instead of a byte sequence.

val header_size : int

The bytes representing a marshaled value are composed of a fixed-size header and a
variable-sized data part, whose size can be determined from the header.
Marshal.header_size[29.35] is the size, in bytes, of the header. Marshal.data_size[29.35]
buff ofs is the size, in bytes, of the data part, assuming a valid header is stored in buff
starting at position ofs. Finally, Marshal.total_size[29.35] buff ofs is the total size, in
bytes, of the marshaled value. Both Marshal.data_size[29.35] and
Marshal.total_size[29.35] raise Failure if buff, ofs does not contain a valid header.
To read the byte representation of a marshaled value into a byte sequence, the program needs
to read first Marshal.header_size[29.35] bytes into the sequence, then determine the length
of the remainder of the representation using Marshal.data_size[29.35], make sure the
sequence is large enough to hold the remaining data, then read it, and finally call
Marshal.from_bytes[29.35] to unmarshal the value.

val data_size : bytes -> int -> int

See Marshal.header_size[29.35].

val total_size : bytes -> int -> int

See Marshal.header_size[29.35].

Marshal and domain safety

Care must be taken when marshaling a mutable value that may be modified by a different domain.
Mutating a value that is being marshaled (i.e., turned into a sequence of bytes) is a programming
error and might result in surprising values (when unmarshaling) due to tearing, since marshaling
involves byte-per-byte copy.
Chapter 29. The standard library 785

29.36 Module MoreLabels : Extra labeled libraries.

This meta-module provides labelized versions of the MoreLabels.Hashtbl[29.36],
MoreLabels.Map[29.36] and MoreLabels.Set[29.36] modules.
This module is intended to be used through open MoreLabels which replaces
MoreLabels.Hashtbl[29.36], MoreLabels.Map[29.36], and MoreLabels.Set[29.36] with
their labeled counterparts.
For example:

open MoreLabels

Hashtbl.iter ~f:(fun ~key ~data -> g key data) table

module Hashtbl :
sig

Hash tables and hash functions.

Hash tables are hashed association tables, with in-place modification. Because most op-
erations on a hash table modify their input, they’re more commonly used in imperative
code. The lookup of the value associated with a key (see MoreLabels.Hashtbl.find[29.36],
MoreLabels.Hashtbl.find_opt[29.36]) is normally very fast, often faster than the equivalent
lookup in MoreLabels.Map[29.36].
The functors MoreLabels.Hashtbl.Make[29.36] and MoreLabels.Hashtbl.MakeSeeded[29.36]
can be used when performance or flexibility are key. The user provides custom equality and
hash functions for the key type, and obtains a custom hash table type for this particular type
of key.
Warning a hash table is only as good as the hash function. A bad hash function will turn
the table into a degenerate association list, with linear time lookup instead of constant time
lookup.
The polymorphic MoreLabels.Hashtbl.t[29.36] hash table is useful in simpler cases or in
interactive environments. It uses the polymorphic MoreLabels.Hashtbl.hash[29.36] function
defined in the OCaml runtime (at the time of writing, it’s SipHash), as well as the polymorphic
equality (=).
See the examples section[29.45].
Unsynchronized accesses
Unsynchronized accesses to a hash table may lead to an invalid hash table state. Thus,
concurrent accesses to a hash tables must be synchronized (for instance with a Mutex.t[29.37]).

Generic interface
type ('a, 'b) t = ('a, 'b) Hashtbl.t
786

The type of hash tables from type 'a to type 'b.

val create : ?random:bool -> int -> ('a, 'b) t

Hashtbl.create n creates a new, empty hash table, with initial size greater or equal to
the suggested size n. For best results, n should be on the order of the expected number
of elements that will be in the table. The table grows as needed, so n is just an initial
guess. If n is very small or negative then it is disregarded and a small default size is used.
The optional ~random parameter (a boolean) controls whether the internal organization
of the hash table is randomized at each execution of Hashtbl.create or deterministic
over all executions.
A hash table that is created with ~random set to false uses a fixed hash function
(MoreLabels.Hashtbl.hash[29.36]) to distribute keys among buckets. As a
consequence, collisions between keys happen deterministically. In Web-facing
applications or other security-sensitive applications, the deterministic collision patterns
can be exploited by a malicious user to create a denial-of-service attack: the attacker
sends input crafted to create many collisions in the table, slowing the application down.
A hash table that is created with ~random set to true uses the seeded hash function
MoreLabels.Hashtbl.seeded_hash[29.36] with a seed that is randomly chosen at hash
table creation time. In effect, the hash function used is randomly selected among 2^{30}
different hash functions. All these hash functions have different collision patterns,
rendering ineffective the denial-of-service attack described above. However, because of
randomization, enumerating all elements of the hash table using
MoreLabels.Hashtbl.fold[29.36] or MoreLabels.Hashtbl.iter[29.36] is no longer
deterministic: elements are enumerated in different orders at different runs of the
program.
If no ~random parameter is given, hash tables are created in non-random mode by
default. This default can be changed either programmatically by calling
MoreLabels.Hashtbl.randomize[29.36] or by setting the R flag in the OCAMLRUNPARAM
environment variable.
Before 4.00 the ~random parameter was not present and all hash tables were created in
non-randomized mode.

val clear : ('a, 'b) t -> unit

Empty a hash table. Use reset instead of clear to shrink the size of the bucket table
to its initial size.

val reset : ('a, 'b) t -> unit

Empty a hash table and shrink the size of the bucket table to its initial size.
Since: 4.00

val copy : ('a, 'b) t -> ('a, 'b) t

Return a copy of the given hashtable.
Chapter 29. The standard library 787

val add : ('a, 'b) t -> key:'a -> data:'b -> unit
Hashtbl.add tbl ~key ~data adds a binding of key to data in table tbl.
Warning: Previous bindings for key are not removed, but simply hidden. That is, after
performing MoreLabels.Hashtbl.remove[29.36] tbl key, the previous binding for key,
if any, is restored. (Same behavior as with association lists.)
If you desire the classic behavior of replacing elements, see
MoreLabels.Hashtbl.replace[29.36].

val find : ('a, 'b) t -> 'a -> 'b

Hashtbl.find tbl x returns the current binding of x in tbl, or raises Not_found if no
such binding exists.

val find_opt : ('a, 'b) t -> 'a -> 'b option

Hashtbl.find_opt tbl x returns the current binding of x in tbl, or None if no such
binding exists.
Since: 4.05

val find_all : ('a, 'b) t -> 'a -> 'b list

Hashtbl.find_all tbl x returns the list of all data associated with x in tbl. The
current binding is returned first, then the previous bindings, in reverse order of
introduction in the table.

val mem : ('a, 'b) t -> 'a -> bool

Hashtbl.mem tbl x checks if x is bound in tbl.

val remove : ('a, 'b) t -> 'a -> unit

Hashtbl.remove tbl x removes the current binding of x in tbl, restoring the previous
binding if it exists. It does nothing if x is not bound in tbl.

val replace : ('a, 'b) t -> key:'a -> data:'b -> unit
Hashtbl.replace tbl ~key ~data replaces the current binding of key in tbl by a
binding of key to data. If key is unbound in tbl, a binding of key to data is added to
tbl. This is functionally equivalent to MoreLabels.Hashtbl.remove[29.36] tbl key
followed by MoreLabels.Hashtbl.add[29.36] tbl key data.

val iter : f:(key:'a -> data:'b -> unit) -> ('a, 'b) t -> unit
Hashtbl.iter ~f tbl applies f to all bindings in table tbl. f receives the key as first
argument, and the associated value as second argument. Each binding is presented
exactly once to f.
The order in which the bindings are passed to f is unspecified. However, if the table
contains several bindings for the same key, they are passed to f in reverse order of
introduction, that is, the most recent binding is passed first.
788

If the hash table was created in non-randomized mode, the order in which the bindings
are enumerated is reproducible between successive runs of the program, and even
between minor versions of OCaml. For randomized hash tables, the order of enumeration
is entirely random.
The behavior is not specified if the hash table is modified by f during the iteration.

val filter_map_inplace :
f:(key:'a -> data:'b -> 'b option) -> ('a, 'b) t -> unit
Hashtbl.filter_map_inplace ~f tbl applies f to all bindings in table tbl and
update each binding depending on the result of f. If f returns None, the binding is
discarded. If it returns Some new_val, the binding is update to associate the key to
new_val.
Other comments for MoreLabels.Hashtbl.iter[29.36] apply as well.
Since: 4.03

val fold :
f:(key:'a -> data:'b -> 'acc -> 'acc) ->
('a, 'b) t -> init:'acc -> 'acc
Hashtbl.fold ~f tbl ~init computes (f kN dN ... (f k1 d1 init)...), where
k1 ... kN are the keys of all bindings in tbl, and d1 ... dN are the associated values.
Each binding is presented exactly once to f.
The order in which the bindings are passed to f is unspecified. However, if the table
contains several bindings for the same key, they are passed to f in reverse order of
introduction, that is, the most recent binding is passed first.
If the hash table was created in non-randomized mode, the order in which the bindings
are enumerated is reproducible between successive runs of the program, and even
between minor versions of OCaml. For randomized hash tables, the order of enumeration
is entirely random.
The behavior is not specified if the hash table is modified by f during the iteration.

val length : ('a, 'b) t -> int

Hashtbl.length tbl returns the number of bindings in tbl. It takes constant time.
Multiple bindings are counted once each, so Hashtbl.length gives the number of times
Hashtbl.iter calls its first argument.

val randomize : unit -> unit

After a call to Hashtbl.randomize(), hash tables are created in randomized mode by
default: MoreLabels.Hashtbl.create[29.36] returns randomized hash tables, unless the
~random:false optional parameter is given. The same effect can be achieved by setting
the R parameter in the OCAMLRUNPARAM environment variable.
It is recommended that applications or Web frameworks that need to protect themselves
against the denial-of-service attack described in MoreLabels.Hashtbl.create[29.36]
call Hashtbl.randomize() at initialization time before any domains are created.
Chapter 29. The standard library 789

Note that once Hashtbl.randomize() was called, there is no way to revert to the
non-randomized default behavior of MoreLabels.Hashtbl.create[29.36]. This is
intentional. Non-randomized hash tables can still be created using Hashtbl.create
~random:false.
Since: 4.00

val is_randomized : unit -> bool

Return true if the tables are currently created in randomized mode by default, false
otherwise.
Since: 4.03

val rebuild : ?random:bool ->

('a, 'b) t -> ('a, 'b) t
Return a copy of the given hashtable. Unlike MoreLabels.Hashtbl.copy[29.36],
MoreLabels.Hashtbl.rebuild[29.36] h re-hashes all the (key, value) entries of the
original table h. The returned hash table is randomized if h was randomized, or the
optional random parameter is true, or if the default is to create randomized hash tables;
see MoreLabels.Hashtbl.create[29.36] for more information.
MoreLabels.Hashtbl.rebuild[29.36] can safely be used to import a hash table built by
an old version of the MoreLabels.Hashtbl[29.36] module, then marshaled to persistent
storage. After unmarshaling, apply MoreLabels.Hashtbl.rebuild[29.36] to produce a
hash table for the current version of the MoreLabels.Hashtbl[29.36] module.
Since: 4.12

type statistics = Hashtbl.statistics =

{ num_bindings : int ;
Number of bindings present in the table. Same value as returned by
MoreLabels.Hashtbl.length[29.36].
num_buckets : int ;
Number of buckets in the table.
max_bucket_length : int ;
Maximal number of bindings per bucket.
bucket_histogram : int array ;
Histogram of bucket sizes. This array histo has length max_bucket_length +
1. The value of histo.(i) is the number of buckets whose size is i.
}
Since: 4.00

val stats : ('a, 'b) t -> statistics

790

Hashtbl.stats tbl returns statistics about the table tbl: number of buckets, size of
the biggest bucket, distribution of buckets by size.
Since: 4.00

Hash tables and Sequences

val to_seq : ('a, 'b) t -> ('a * 'b) Seq.t
Iterate on the whole table. The order in which the bindings appear in the sequence is
unspecified. However, if the table contains several bindings for the same key, they
appear in reversed order of introduction, that is, the most recent binding appears first.
The behavior is not specified if the hash table is modified during the iteration.
Since: 4.07

val to_seq_keys : ('a, 'b) t -> 'a Seq.t

Same as Seq.map fst (to_seq m)
Since: 4.07

val to_seq_values : ('a, 'b) t -> 'b Seq.t

Same as Seq.map snd (to_seq m)
Since: 4.07

val add_seq : ('a, 'b) t -> ('a * 'b) Seq.t -> unit
Add the given bindings to the table, using MoreLabels.Hashtbl.add[29.36]
Since: 4.07

val replace_seq : ('a, 'b) t -> ('a * 'b) Seq.t -> unit
Add the given bindings to the table, using MoreLabels.Hashtbl.replace[29.36]
Since: 4.07

val of_seq : ('a * 'b) Seq.t -> ('a, 'b) t

Build a table from the given bindings. The bindings are added in the same order they
appear in the sequence, using MoreLabels.Hashtbl.replace_seq[29.36], which means
that if two pairs have the same key, only the latest one will appear in the table.
Since: 4.07
Chapter 29. The standard library 791

Functorial interface

The functorial interface allows the use of specific comparison and hash functions, either
for performance/security concerns, or because keys are not hashable/comparable with the
polymorphic builtins.
For instance, one might want to specialize a table for integer keys:

module IntHash =
struct
type t = int
let equal i j = i=j
let hash i = i land max_int
end

module IntHashtbl = Hashtbl.Make(IntHash)

let h = IntHashtbl.create 17 in
IntHashtbl.add h 12 "hello"

This creates a new module IntHashtbl, with a new type 'a IntHashtbl.t of tables from
int to 'a. In this example, h contains string values so its type is string IntHashtbl.t.
Note that the new type 'a IntHashtbl.t is not compatible with the type ('a,'b) Hashtbl.t
of the generic interface. For example, Hashtbl.length h would not type-check, you must use
IntHashtbl.length.
module type HashedType =
sig

type t
The type of the hashtable keys.

val equal : t -> t -> bool

The equality predicate used to compare keys.

val hash : t -> int

A hashing function on keys. It must be such that if two keys are equal according to
equal, then they have identical hash values as computed by hash. Examples:
suitable (equal, hash) pairs for arbitrary key types include
• ((=), MoreLabels.Hashtbl.HashedType.hash[29.36]) for comparing objects by
structure (provided objects do not contain floats)
• ((fun x y -> compare x y = 0),
MoreLabels.Hashtbl.HashedType.hash[29.36]) for comparing objects by
structure and handling nan[28.2] correctly
• ((==), MoreLabels.Hashtbl.HashedType.hash[29.36]) for comparing objects
by physical equality (e.g. for mutable or cyclic objects).
792

end

The input signature of the functor MoreLabels.Hashtbl.Make[29.36].

module type S =
sig

type key
type !'a t
val create : int -> 'a t
val clear : 'a t -> unit
val reset : 'a t -> unit
Since: 4.00

val copy : 'a t -> 'a t

val add : 'a t -> key:key -> data:'a -> unit
val remove : 'a t -> key -> unit
val find : 'a t -> key -> 'a
val find_opt : 'a t -> key -> 'a option
Since: 4.05

val find_all : 'a t -> key -> 'a list

val replace : 'a t -> key:key -> data:'a -> unit
val mem : 'a t -> key -> bool
val iter : f:(key:key -> data:'a -> unit) ->
'a t -> unit
val filter_map_inplace :
f:(key:key -> data:'a -> 'a option) ->
'a t -> unit
Since: 4.03

val fold :
f:(key:key -> data:'a -> 'acc -> 'acc) ->
'a t -> init:'acc -> 'acc
val length : 'a t -> int
val stats : 'a t -> MoreLabels.Hashtbl.statistics
Since: 4.00

val to_seq : 'a t -> (key * 'a) Seq.t

Since: 4.07

val to_seq_keys : 'a t -> key Seq.t

Since: 4.07
Chapter 29. The standard library 793

val to_seq_values : 'a t -> 'a Seq.t

Since: 4.07

val add_seq : 'a t ->

(key * 'a) Seq.t -> unit
Since: 4.07

val replace_seq : 'a t ->

(key * 'a) Seq.t -> unit
Since: 4.07

val of_seq : (key * 'a) Seq.t -> 'a t

Since: 4.07

end

The output signature of the functor MoreLabels.Hashtbl.Make[29.36].

module Make :
functor (H : HashedType) -> S with type key = H.t and type 'a t = 'a
Hashtbl.Make(H).t
Functor building an implementation of the hashtable structure. The functor
Hashtbl.Make returns a structure containing a type key of keys and a type 'a t of
hash tables associating data of type 'a to keys of type key. The operations perform
similarly to those of the generic interface, but use the hashing and equality functions
specified in the functor argument H instead of generic equality and hashing. Since the
hash function is not seeded, the create operation of the result structure always returns
non-randomized hash tables.

module type SeededHashedType =

sig

type t
The type of the hashtable keys.

val equal : t ->

t -> bool
The equality predicate used to compare keys.

val seeded_hash : int -> t -> int

A seeded hashing function on keys. The first argument is the seed. It must be the
case that if equal x y is true, then seeded_hash seed x = seeded_hash seed y
for any value of seed. A suitable choice for seeded_hash is the function
MoreLabels.Hashtbl.seeded_hash[29.36] below.
794

end

The input signature of the functor MoreLabels.Hashtbl.MakeSeeded[29.36].

Since: 4.00

module type SeededS =

sig

type key
type !'a t
val create : ?random:bool -> int -> 'a t
val clear : 'a t -> unit
val reset : 'a t -> unit
val copy : 'a t -> 'a t
val add : 'a t ->
key:key -> data:'a -> unit
val remove : 'a t -> key -> unit
val find : 'a t -> key -> 'a
val find_opt : 'a t ->
key -> 'a option
Since: 4.05

val find_all : 'a t -> key -> 'a list

val replace : 'a t ->
key:key -> data:'a -> unit
val mem : 'a t -> key -> bool
val iter : f:(key:key -> data:'a -> unit) ->
'a t -> unit
val filter_map_inplace :
f:(key:key -> data:'a -> 'a option) ->
'a t -> unit
Since: 4.03

val fold :
f:(key:key -> data:'a -> 'acc -> 'acc) ->
'a t -> init:'acc -> 'acc
val length : 'a t -> int
val stats : 'a t -> MoreLabels.Hashtbl.statistics
val to_seq : 'a t ->
(key * 'a) Seq.t
Since: 4.07
Chapter 29. The standard library 795

val to_seq_keys : 'a t ->

key Seq.t
Since: 4.07

val to_seq_values : 'a t -> 'a Seq.t

Since: 4.07

val add_seq : 'a t ->

(key * 'a) Seq.t -> unit
Since: 4.07

val replace_seq : 'a t ->

(key * 'a) Seq.t -> unit
Since: 4.07

val of_seq : (key * 'a) Seq.t ->

'a t
Since: 4.07

end

The output signature of the functor MoreLabels.Hashtbl.MakeSeeded[29.36].

Since: 4.00

module MakeSeeded :
functor (H : SeededHashedType) -> SeededS with type key = H.t and type 'a t
= 'a Hashtbl.MakeSeeded(H).t
Functor building an implementation of the hashtable structure. The functor
Hashtbl.MakeSeeded returns a structure containing a type key of keys and a type 'a t
of hash tables associating data of type 'a to keys of type key. The operations perform
similarly to those of the generic interface, but use the seeded hashing and equality
functions specified in the functor argument H instead of generic equality and hashing.
The create operation of the result structure supports the ~random optional parameter
and returns randomized hash tables if ~random:true is passed or if randomization is
globally on (see MoreLabels.Hashtbl.randomize[29.36]).
Since: 4.00

The polymorphic hash functions

val hash : 'a -> int
Hashtbl.hash x associates a nonnegative integer to any value of any type. It is
guaranteed that if x = y or Stdlib.compare x y = 0, then hash x = hash y.
Moreover, hash always terminates, even on cyclic structures.
796

val seeded_hash : int -> 'a -> int

A variant of MoreLabels.Hashtbl.hash[29.36] that is further parameterized by an
integer seed.
Since: 4.00

val hash_param : int -> int -> 'a -> int

Hashtbl.hash_param meaningful total x computes a hash value for x, with the
same properties as for hash. The two extra integer parameters meaningful and total
give more precise control over hashing. Hashing performs a breadth-first, left-to-right
traversal of the structure x, stopping after meaningful meaningful nodes were
encountered, or total nodes (meaningful or not) were encountered. If total as specified
by the user exceeds a certain value, currently 256, then it is capped to that value.
Meaningful nodes are: integers; floating-point numbers; strings; characters; booleans;
and constant constructors. Larger values of meaningful and total means that more
nodes are taken into account to compute the final hash value, and therefore collisions are
less likely to happen. However, hashing takes longer. The parameters meaningful and
total govern the tradeoff between accuracy and speed. As default choices,
MoreLabels.Hashtbl.hash[29.36] and MoreLabels.Hashtbl.seeded_hash[29.36] take
meaningful = 10 and total = 100.

val seeded_hash_param : int -> int -> int -> 'a -> int
A variant of MoreLabels.Hashtbl.hash_param[29.36] that is further parameterized by
an integer seed. Usage: Hashtbl.seeded_hash_param meaningful total seed x.
Since: 4.00

Examples

Basic Example

(* 0...99 *)
let seq = Seq.ints 0 |> Seq.take 100

(* build from Seq.t *)

# let tbl =
seq
|> Seq.map (fun x -> x, string_of_int x)
|> Hashtbl.of_seq
val tbl : (int, string) Hashtbl.t = <abstr>

# Hashtbl.length tbl
- : int = 100
Chapter 29. The standard library 797

# Hashtbl.find_opt tbl 32
- : string option = Some "32"

# Hashtbl.find_opt tbl 166

- : string option = None

# Hashtbl.replace tbl 166 "one six six"

- : unit = ()

# Hashtbl.find_opt tbl 166

- : string option = Some "one six six"

# Hashtbl.length tbl
- : int = 101

Counting Elements

Given a sequence of elements (here, a Seq.t[29.49]), we want to count how many times each
distinct element occurs in the sequence. A simple way to do this, assuming the elements
are comparable and hashable, is to use a hash table that maps elements to their number of
occurrences.
Here we illustrate that principle using a sequence of (ascii) characters (type char). We use a
custom Char_tbl specialized for char.

# module Char_tbl = Hashtbl.Make(struct

type t = char
let equal = Char.equal
let hash = Hashtbl.hash
end)

(* count distinct occurrences of chars in [seq] *)

# let count_chars (seq : char Seq.t) : _ list =
let counts = Char_tbl.create 16 in
Seq.iter
(fun c ->
let count_c =
Char_tbl.find_opt counts c
|> Option.value ~default:0
in
Char_tbl.replace counts c (count_c + 1))
seq;
(* turn into a list *)
Char_tbl.fold (fun c n l -> (c,n) :: l) counts []
798

|> List.sort (fun (c1,_)(c2,_) -> Char.compare c1 c2)

val count_chars : Char_tbl.key Seq.t -> (Char.t * int) list = <fun>

(* basic seq from a string *)

# let seq = String.to_seq "hello world, and all the camels in it!"
val seq : char Seq.t = <fun>

# count_chars seq
- : (Char.t * int) list =
[(' ', 7); ('!', 1); (',', 1); ('a', 3); ('c', 1); ('d', 2); ('e', 3);
('h', 2); ('i', 2); ('l', 6); ('m', 1); ('n', 2); ('o', 2); ('r', 1);
('s', 1); ('t', 2); ('w', 1)]

(* "abcabcabc..." *)
# let seq2 =
Seq.cycle (String.to_seq "abc") |> Seq.take 31
val seq2 : char Seq.t = <fun>

# String.of_seq seq2
- : String.t = "abcabcabcabcabcabcabcabcabcabca"

# count_chars seq2
- : (Char.t * int) list = [('a', 11); ('b', 10); ('c', 10)]

end

module Map :
sig
Association tables over ordered types.
This module implements applicative association tables, also known as finite maps or dictionaries,
given a total ordering function over the keys. All operations over maps are purely applicative
(no side-effects). The implementation uses balanced binary trees, and therefore searching and
insertion take time logarithmic in the size of the map.
For instance:

module IntPairs =
struct
type t = int * int
let compare (x0,y0) (x1,y1) =
match Stdlib.compare x0 x1 with
0 -> Stdlib.compare y0 y1
| c -> c
end
Chapter 29. The standard library 799

module PairsMap = Map.Make(IntPairs)

let m = PairsMap.(empty |> add (0,1) "hello" |> add (1,0) "world")

This creates a new module PairsMap, with a new type 'a PairsMap.t of maps from int *
int to 'a. In this example, m contains string values so its type is string PairsMap.t.
module type OrderedType =
sig

type t
The type of the map keys.

val compare : t -> t -> int

A total ordering function over the keys. This is a two-argument function f such that
f e1 e2 is zero if the keys e1 and e2 are equal, f e1 e2 is strictly negative if e1 is
smaller than e2, and f e1 e2 is strictly positive if e1 is greater than e2. Example:
a suitable ordering function is the generic structural comparison function
compare[28.2].

end

Input signature of the functor MoreLabels.Map.Make[29.36].

module type S =
sig

Maps
type key
The type of the map keys.

type !+'a t
The type of maps from type key to type 'a.

val empty : 'a t

The empty map.

val add : key:key ->

data:'a -> 'a t -> 'a t
add ~key ~data m returns a map containing the same bindings as m, plus a binding
of key to data. If key was already bound in m to a value that is physically equal to
data, m is returned unchanged (the result of the function is then physically equal to
m). Otherwise, the previous binding of key in m disappears.
Before 4.03 Physical equality was not ensured.
800

val add_to_list : key:key ->

data:'a -> 'a list t -> 'a list t
add_to_list ~key ~data m is m with key mapped to l such that l is data ::
Map.find key m if key was bound in m and [v] otherwise.
Since: 5.1

val update : key:key ->

f:('a option -> 'a option) -> 'a t -> 'a t
update ~key ~f m returns a map containing the same bindings as m, except for the
binding of key. Depending on the value of y where y is f (find_opt key m), the
binding of key is added, removed or updated. If y is None, the binding is removed if
it exists; otherwise, if y is Some z then key is associated to z in the resulting map.
If key was already bound in m to a value that is physically equal to z, m is returned
unchanged (the result of the function is then physically equal to m).
Since: 4.06

val singleton : key -> 'a -> 'a t

singleton x y returns the one-element map that contains a binding y for x.
Since: 3.12

val remove : key -> 'a t -> 'a t

remove x m returns a map containing the same bindings as m, except for x which is
unbound in the returned map. If x was not in m, m is returned unchanged (the result
of the function is then physically equal to m).
Before 4.03 Physical equality was not ensured.

val merge :
f:(key -> 'a option -> 'b option -> 'c option) ->
'a t -> 'b t -> 'c t
merge ~f m1 m2 computes a map whose keys are a subset of the keys of m1 and of
m2. The presence of each such binding, and the corresponding value, is determined
with the function f. In terms of the find_opt operation, we have find_opt x
(merge f m1 m2) = f x (find_opt x m1) (find_opt x m2) for any key x,
provided that f x None None = None.
Since: 3.12

val union : f:(key -> 'a -> 'a -> 'a option) ->
'a t -> 'a t -> 'a t
union ~f m1 m2 computes a map whose keys are a subset of the keys of m1 and of
m2. When the same binding is defined in both arguments, the function f is used to
combine them. This is a special case of merge: union f m1 m2 is equivalent to
merge f' m1 m2, where
• f' _key None None = None
• f' _key (Some v) None = Some v
• f' _key None (Some v) = Some v
Chapter 29. The standard library 801

• f' key (Some v1) (Some v2) = f key v1 v2

Since: 4.03

val cardinal : 'a t -> int

Return the number of bindings of a map.
Since: 3.12

Bindings
val bindings : 'a t -> (key * 'a) list
Return the list of all bindings of the given map. The returned list is sorted in
increasing order of keys with respect to the ordering Ord.compare, where Ord is the
argument given to MoreLabels.Map.Make[29.36].
Since: 3.12

val min_binding : 'a t -> key * 'a

Return the binding with the smallest key in a given map (with respect to the
Ord.compare ordering), or raise Not_found if the map is empty.
Since: 3.12

val min_binding_opt : 'a t -> (key * 'a) option

Return the binding with the smallest key in the given map (with respect to the
Ord.compare ordering), or None if the map is empty.
Since: 4.05

val max_binding : 'a t -> key * 'a

Same as MoreLabels.Map.S.min_binding[29.36], but returns the binding with the
largest key in the given map.
Since: 3.12

val max_binding_opt : 'a t -> (key * 'a) option

Same as MoreLabels.Map.S.min_binding_opt[29.36], but returns the binding with
the largest key in the given map.
Since: 4.05

val choose : 'a t -> key * 'a

Return one binding of the given map, or raise Not_found if the map is empty.
Which binding is chosen is unspecified, but equal bindings will be chosen for equal
maps.
Since: 3.12

val choose_opt : 'a t -> (key * 'a) option

Return one binding of the given map, or None if the map is empty. Which binding is
chosen is unspecified, but equal bindings will be chosen for equal maps.
Since: 4.05
802

Searching
val find : key -> 'a t -> 'a
find x m returns the current value of x in m, or raises Not_found if no binding for x
exists.

val find_opt : key -> 'a t -> 'a option

find_opt x m returns Some v if the current value of x in m is v, or None if no
binding for x exists.
Since: 4.05

val find_first : f:(key -> bool) ->

'a t -> key * 'a
find_first ~f m, where f is a monotonically increasing function, returns the
binding of m with the lowest key k such that f k, or raises Not_found if no such key
exists.
For example, find_first (fun k -> Ord.compare k x >= 0) m will return the
first binding k, v of m where Ord.compare k x >= 0 (intuitively: k >= x), or raise
Not_found if x is greater than any element of m.
Since: 4.05

val find_first_opt : f:(key -> bool) ->

'a t -> (key * 'a) option
find_first_opt ~f m, where f is a monotonically increasing function, returns an
option containing the binding of m with the lowest key k such that f k, or None if
no such key exists.
Since: 4.05

val find_last : f:(key -> bool) ->

'a t -> key * 'a
find_last ~f m, where f is a monotonically decreasing function, returns the
binding of m with the highest key k such that f k, or raises Not_found if no such
key exists.
Since: 4.05

val find_last_opt : f:(key -> bool) ->

'a t -> (key * 'a) option
find_last_opt ~f m, where f is a monotonically decreasing function, returns an
option containing the binding of m with the highest key k such that f k, or None if
no such key exists.
Since: 4.05

Traversing
val iter : f:(key:key -> data:'a -> unit) ->
'a t -> unit
Chapter 29. The standard library 803

iter ~f m applies f to all bindings in map m. f receives the key as first argument,
and the associated value as second argument. The bindings are passed to f in
increasing order with respect to the ordering over the type of the keys.

val fold :
f:(key:key -> data:'a -> 'acc -> 'acc) ->
'a t -> init:'acc -> 'acc
fold ~f m ~init computes (f kN dN ... (f k1 d1 init)...), where k1 ...
kN are the keys of all bindings in m (in increasing order), and d1 ... dN are the
associated data.

Transforming
val map : f:('a -> 'b) -> 'a t -> 'b t
map ~f m returns a map with same domain as m, where the associated value a of all
bindings of m has been replaced by the result of the application of f to a. The
bindings are passed to f in increasing order with respect to the ordering over the
type of the keys.

val mapi : f:(key -> 'a -> 'b) ->

'a t -> 'b t
Same as MoreLabels.Map.S.map[29.36], but the function receives as arguments
both the key and the associated value for each binding of the map.

val filter : f:(key -> 'a -> bool) ->

'a t -> 'a t
filter ~f m returns the map with all the bindings in m that satisfy predicate p. If
every binding in m satisfies f, m is returned unchanged (the result of the function is
then physically equal to m)
Before 4.03 Physical equality was not ensured.
Since: 3.12

val filter_map : f:(key -> 'a -> 'b option) ->

'a t -> 'b t
filter_map ~f m applies the function f to every binding of m, and builds a map
from the results. For each binding (k, v) in the input map:
• if f k v is None then k is not in the result,
• if f k v is Some v' then the binding (k, v') is in the output map.
For example, the following function on maps whose values are lists

filter_map
(fun _k li -> match li with [] -> None | _::tl -> Some tl)
m
804

drops all bindings of m whose value is an empty list, and pops the first element of
each value that is non-empty.
Since: 4.11

val partition : f:(key -> 'a -> bool) ->

'a t -> 'a t * 'a t
partition ~f m returns a pair of maps (m1, m2), where m1 contains all the
bindings of m that satisfy the predicate f, and m2 is the map with all the bindings of
m that do not satisfy f.
Since: 3.12

val split : key ->

'a t ->
'a t * 'a option * 'a t
split x m returns a triple (l, data, r), where l is the map with all the bindings
of m whose key is strictly less than x; r is the map with all the bindings of m whose
key is strictly greater than x; data is None if m contains no binding for x, or Some v
if m binds v to x.
Since: 3.12

Predicates and comparisons

val is_empty : 'a t -> bool
Test whether a map is empty or not.

val mem : key -> 'a t -> bool

mem x m returns true if m contains a binding for x, and false otherwise.

val equal : cmp:('a -> 'a -> bool) ->

'a t -> 'a t -> bool
equal ~cmp m1 m2 tests whether the maps m1 and m2 are equal, that is, contain
equal keys and associate them with equal data. cmp is the equality predicate used to
compare the data associated with the keys.

val compare : cmp:('a -> 'a -> int) ->

'a t -> 'a t -> int
Total ordering between maps. The first argument is a total ordering used to
compare data associated with equal keys in the two maps.

val for_all : f:(key -> 'a -> bool) -> 'a t -> bool
for_all ~f m checks if all the bindings of the map satisfy the predicate f.
Since: 3.12

val exists : f:(key -> 'a -> bool) -> 'a t -> bool
exists ~f m checks if at least one binding of the map satisfies the predicate f.
Since: 3.12
Chapter 29. The standard library 805

Converting
val to_list : 'a t -> (key * 'a) list
to_list m is MoreLabels.Map.S.bindings[29.36] m.
Since: 5.1

val of_list : (key * 'a) list -> 'a t

of_list bs adds the bindings of bs to the empty map, in list order (if a key is
bound twice in bs the last one takes over).
Since: 5.1

val to_seq : 'a t -> (key * 'a) Seq.t

Iterate on the whole map, in ascending order of keys
Since: 4.07

val to_rev_seq : 'a t -> (key * 'a) Seq.t

Iterate on the whole map, in descending order of keys
Since: 4.12

val to_seq_from : key ->

'a t -> (key * 'a) Seq.t
to_seq_from k m iterates on a subset of the bindings of m, in ascending order of
keys, from key k or above.
Since: 4.07

val add_seq : (key * 'a) Seq.t ->

'a t -> 'a t
Add the given bindings to the map, in order.
Since: 4.07

val of_seq : (key * 'a) Seq.t -> 'a t

Build a map from the given bindings
Since: 4.07
end

Output signature of the functor MoreLabels.Map.Make[29.36].

module Make :
functor (Ord : OrderedType) -> S with type key = Ord.t and type 'a t = 'a
Map.Make(Ord).t
Functor building an implementation of the map structure given a totally ordered type.

end
module Set :
sig
806

Sets over ordered types.

This module implements the set data structure, given a total ordering function over the set
elements. All operations over sets are purely applicative (no side-effects). The implementation
uses balanced binary trees, and is therefore reasonably efficient: insertion and membership
take time logarithmic in the size of the set, for instance.
The MoreLabels.Set.Make[29.36] functor constructs implementations for any type,
given a compare function. For instance:

module IntPairs =
struct
type t = int * int
let compare (x0,y0) (x1,y1) =
match Stdlib.compare x0 x1 with
0 -> Stdlib.compare y0 y1
| c -> c
end

module PairsSet = Set.Make(IntPairs)

let m = PairsSet.(empty |> add (2,3) |> add (5,7) |> add (11,13))

This creates a new module PairsSet, with a new type PairsSet.t of sets of int * int.
module type OrderedType =
sig

type t
The type of the set elements.

val compare : t -> t -> int

A total ordering function over the set elements. This is a two-argument function f
such that f e1 e2 is zero if the elements e1 and e2 are equal, f e1 e2 is strictly
negative if e1 is smaller than e2, and f e1 e2 is strictly positive if e1 is greater
than e2. Example: a suitable ordering function is the generic structural comparison
function compare[28.2].

end

Input signature of the functor MoreLabels.Set.Make[29.36].

module type S =
sig
Chapter 29. The standard library 807

Sets
type elt
The type of the set elements.

type t
The type of sets.

val empty : t
The empty set.

val add : elt -> t -> t

add x s returns a set containing all elements of s, plus x. If x was already in s, s is
returned unchanged (the result of the function is then physically equal to s).
Before 4.03 Physical equality was not ensured.

val singleton : elt -> t

singleton x returns the one-element set containing only x.

val remove : elt -> t -> t

remove x s returns a set containing all elements of s, except x. If x was not in s, s
is returned unchanged (the result of the function is then physically equal to s).
Before 4.03 Physical equality was not ensured.

val union : t -> t -> t

Set union.

val inter : t -> t -> t

Set intersection.

val disjoint : t -> t -> bool

Test if two sets are disjoint.
Since: 4.08

val diff : t -> t -> t

Set difference: diff s1 s2 contains the elements of s1 that are not in s2.

val cardinal : t -> int

Return the number of elements of a set.

Elements
val elements : t -> elt list
Return the list of all elements of the given set. The returned list is sorted in
increasing order with respect to the ordering Ord.compare, where Ord is the
argument given to MoreLabels.Set.Make[29.36].
808

val min_elt : t -> elt

Return the smallest element of the given set (with respect to the Ord.compare
ordering), or raise Not_found if the set is empty.
val min_elt_opt : t -> elt option
Return the smallest element of the given set (with respect to the Ord.compare
ordering), or None if the set is empty.
Since: 4.05
val max_elt : t -> elt
Same as MoreLabels.Set.S.min_elt[29.36], but returns the largest element of the
given set.
val max_elt_opt : t -> elt option
Same as MoreLabels.Set.S.min_elt_opt[29.36], but returns the largest element of
the given set.
Since: 4.05
val choose : t -> elt
Return one element of the given set, or raise Not_found if the set is empty. Which
element is chosen is unspecified, but equal elements will be chosen for equal sets.
val choose_opt : t -> elt option
Return one element of the given set, or None if the set is empty. Which element is
chosen is unspecified, but equal elements will be chosen for equal sets.
Since: 4.05

Searching
val find : elt -> t -> elt
find x s returns the element of s equal to x (according to Ord.compare), or raise
Not_found if no such element exists.
Since: 4.01
val find_opt : elt -> t -> elt option
find_opt x s returns the element of s equal to x (according to Ord.compare), or
None if no such element exists.
Since: 4.05
val find_first : f:(elt -> bool) ->
t -> elt
find_first ~f s, where f is a monotonically increasing function, returns the
lowest element e of s such that f e, or raises Not_found if no such element exists.
For example, find_first (fun e -> Ord.compare e x >= 0) s will return the
first element e of s where Ord.compare e x >= 0 (intuitively: e >= x), or raise
Not_found if x is greater than any element of s.
Since: 4.05
Chapter 29. The standard library 809

val find_first_opt : f:(elt -> bool) ->

t -> elt option
find_first_opt ~f s, where f is a monotonically increasing function, returns an
option containing the lowest element e of s such that f e, or None if no such
element exists.
Since: 4.05

val find_last : f:(elt -> bool) ->

t -> elt
find_last ~f s, where f is a monotonically decreasing function, returns the
highest element e of s such that f e, or raises Not_found if no such element exists.
Since: 4.05

val find_last_opt : f:(elt -> bool) ->

t -> elt option
find_last_opt ~f s, where f is a monotonically decreasing function, returns an
option containing the highest element e of s such that f e, or None if no such
element exists.
Since: 4.05

Traversing
val iter : f:(elt -> unit) -> t -> unit
iter ~f s applies f in turn to all elements of s. The elements of s are presented to
f in increasing order with respect to the ordering over the type of the elements.

val fold : f:(elt -> 'acc -> 'acc) ->

t -> init:'acc -> 'acc
fold ~f s init computes (f xN ... (f x2 (f x1 init))...), where x1 ...
xN are the elements of s, in increasing order.

Transforming
val map : f:(elt -> elt) ->
t -> t
map ~f s is the set whose elements are f a0,f a1. . . f aN, where a0,a1. . .aN are
the elements of s.
The elements are passed to f in increasing order with respect to the ordering over
the type of the elements.
If no element of s is changed by f, s is returned unchanged. (If each output of f is
physically equal to its input, the returned set is physically equal to s.)
Since: 4.04

val filter : f:(elt -> bool) -> t -> t

810

filter ~f s returns the set of all elements in s that satisfy predicate f. If f

satisfies every element in s, s is returned unchanged (the result of the function is
then physically equal to s).
Before 4.03 Physical equality was not ensured.

val filter_map : f:(elt -> elt option) ->

t -> t
filter_map ~f s returns the set of all v such that f x = Some v for some element
x of s.
For example,
filter_map (fun n -> if n mod 2 = 0 then Some (n / 2) else None) s
is the set of halves of the even elements of s.
If no element of s is changed or dropped by f (if f x = Some x for each element x),
then s is returned unchanged: the result of the function is then physically equal to s.
Since: 4.11

val partition : f:(elt -> bool) ->

t -> t * t
partition ~f s returns a pair of sets (s1, s2), where s1 is the set of all the
elements of s that satisfy the predicate f, and s2 is the set of all the elements of s
that do not satisfy f.

val split : elt ->

t -> t * bool * t
split x s returns a triple (l, present, r), where l is the set of elements of s
that are strictly less than x; r is the set of elements of s that are strictly greater
than x; present is false if s contains no element equal to x, or true if s contains
an element equal to x.

Predicates and comparisons

val is_empty : t -> bool
Test whether a set is empty or not.

val mem : elt -> t -> bool

mem x s tests whether x belongs to the set s.

val equal : t -> t -> bool

equal s1 s2 tests whether the sets s1 and s2 are equal, that is, contain equal
elements.

val compare : t -> t -> int

Total ordering between sets. Can be used as the ordering function for doing sets of
sets.
Chapter 29. The standard library 811

val subset : t -> t -> bool

subset s1 s2 tests whether the set s1 is a subset of the set s2.

val for_all : f:(elt -> bool) -> t -> bool

for_all ~f s checks if all elements of the set satisfy the predicate f.

val exists : f:(elt -> bool) -> t -> bool

exists ~f s checks if at least one element of the set satisfies the predicate f.

Converting
val to_list : t -> elt list
to_list s is MoreLabels.Set.S.elements[29.36] s.
Since: 5.1

val of_list : elt list -> t

of_list l creates a set from a list of elements. This is usually more efficient than
folding add over the list, except perhaps for lists with many duplicated elements.
Since: 4.02

val to_seq_from : elt ->

t -> elt Seq.t
to_seq_from x s iterates on a subset of the elements of s in ascending order, from
x or above.
Since: 4.07

val to_seq : t -> elt Seq.t

Iterate on the whole set, in ascending order
Since: 4.07

val to_rev_seq : t -> elt Seq.t

Iterate on the whole set, in descending order
Since: 4.12

val add_seq : elt Seq.t -> t -> t

Add the given elements to the set, in order.
Since: 4.07

val of_seq : elt Seq.t -> t

Build a set from the given bindings
Since: 4.07

end

Output signature of the functor MoreLabels.Set.Make[29.36].

812

module Make :
functor (Ord : OrderedType) -> S with type elt = Ord.t and type t =
Set.Make(Ord).t
Functor building an implementation of the set structure given a totally ordered type.

end

29.37 Module Mutex : Locks for mutual exclusion.

Mutexes (mutual-exclusion locks) are used to implement critical sections and protect shared mutable
data structures against concurrent accesses. The typical use is (if m is the mutex associated with
the data structure D):

Mutex.lock m;
(* Critical section that operates over D *);
Mutex.unlock m

type t
The type of mutexes.

val create : unit -> t

Return a new mutex.

val lock : t -> unit

Lock the given mutex. Only one thread can have the mutex locked at any time. A thread
that attempts to lock a mutex already locked by another thread will suspend until the other
thread unlocks the mutex.
Before 4.12 Sys_error was not raised for recursive locking (platform-dependent behaviour)
Raises Sys_error if the mutex is already locked by the thread calling Mutex.lock[29.37].

val try_lock : t -> bool

Same as Mutex.lock[29.37], but does not suspend the calling thread if the mutex is already
locked: just return false immediately in that case. If the mutex is unlocked, lock it and
return true.

val unlock : t -> unit

Unlock the given mutex. Other threads suspended trying to lock the mutex will restart. The
mutex must have been previously locked by the thread that calls Mutex.unlock[29.37].
Before 4.12 Sys_error was not raised when unlocking an unlocked mutex or when
unlocking a mutex from a different thread.
Raises Sys_error if the mutex is unlocked or was locked by another thread.
Chapter 29. The standard library 813

val protect : t -> (unit -> 'a) -> 'a

protect mutex f runs f() in a critical section where mutex is locked (using
Mutex.lock[29.37]); it then takes care of releasing mutex, whether f() returned a value or
raised an exception.
The unlocking operation is guaranteed to always takes place, even in the event an
asynchronous exception (e.g. Sys.Break[29.56]) is raised in some signal handler.
Since: 5.1

29.38 Module Nativeint : Processor-native integers.

This module provides operations on the type nativeint of signed 32-bit integers (on 32-bit platforms)
or signed 64-bit integers (on 64-bit platforms). This integer type has exactly the same width as that
of a pointer type in the C compiler. All arithmetic operations over nativeint are taken modulo
232 or 264 depending on the word size of the architecture.
Performance notice: values of type nativeint occupy more memory space than values of type
int, and arithmetic operations on nativeint are generally slower than those on int. Use nativeint
only when the application requires the extra bit of precision over the int type.
Literals for native integers are suffixed by n:

let zero: nativeint = 0n

let one: nativeint = 1n
let m_one: nativeint = -1n

val zero : nativeint

The native integer 0.

val one : nativeint

The native integer 1.

val minus_one : nativeint

The native integer -1.

val neg : nativeint -> nativeint

Unary negation.

val add : nativeint -> nativeint -> nativeint

Addition.

val sub : nativeint -> nativeint -> nativeint

Subtraction.

val mul : nativeint -> nativeint -> nativeint

814

Multiplication.

val div : nativeint -> nativeint -> nativeint

Integer division. This division rounds the real quotient of its arguments towards zero, as
specified for (/)[28.2].
Raises Division_by_zero if the second argument is zero.

val unsigned_div : nativeint -> nativeint -> nativeint

Same as Nativeint.div[29.38], except that arguments and result are interpreted as unsigned
native integers.
Since: 4.08

val rem : nativeint -> nativeint -> nativeint

Integer remainder. If y is not zero, the result of Nativeint.rem x y satisfies the following
properties: Nativeint.zero <= Nativeint.rem x y < Nativeint.abs y and x =
Nativeint.add (Nativeint.mul (Nativeint.div x y) y) (Nativeint.rem x y). If y
= 0, Nativeint.rem x y raises Division_by_zero.

val unsigned_rem : nativeint -> nativeint -> nativeint

Same as Nativeint.rem[29.38], except that arguments and result are interpreted as unsigned
native integers.
Since: 4.08

val succ : nativeint -> nativeint

Successor. Nativeint.succ x is Nativeint.add x Nativeint.one.

val pred : nativeint -> nativeint

Predecessor. Nativeint.pred x is Nativeint.sub x Nativeint.one.

val abs : nativeint -> nativeint

abs x is the absolute value of x. On min_int this is min_int itself and thus remains negative.

val size : int

The size in bits of a native integer. This is equal to 32 on a 32-bit platform and to 64 on a
64-bit platform.

val max_int : nativeint

The greatest representable native integer, either 231 - 1 on a 32-bit platform, or 263 - 1 on a
64-bit platform.

val min_int : nativeint

The smallest representable native integer, either -231 on a 32-bit platform, or -263 on a 64-bit
platform.
Chapter 29. The standard library 815

val logand : nativeint -> nativeint -> nativeint

Bitwise logical and.

val logor : nativeint -> nativeint -> nativeint

Bitwise logical or.

val logxor : nativeint -> nativeint -> nativeint

Bitwise logical exclusive or.

val lognot : nativeint -> nativeint

Bitwise logical negation.

val shift_left : nativeint -> int -> nativeint

Nativeint.shift_left x y shifts x to the left by y bits. The result is unspecified if y < 0
or y >= bitsize, where bitsize is 32 on a 32-bit platform and 64 on a 64-bit platform.

val shift_right : nativeint -> int -> nativeint

Nativeint.shift_right x y shifts x to the right by y bits. This is an arithmetic shift: the
sign bit of x is replicated and inserted in the vacated bits. The result is unspecified if y < 0
or y >= bitsize.

val shift_right_logical : nativeint -> int -> nativeint

Nativeint.shift_right_logical x y shifts x to the right by y bits. This is a logical shift:
zeroes are inserted in the vacated bits regardless of the sign of x. The result is unspecified if y
< 0 or y >= bitsize.

val of_int : int -> nativeint

Convert the given integer (type int) to a native integer (type nativeint).

val to_int : nativeint -> int

Convert the given native integer (type nativeint) to an integer (type int). The high-order
bit is lost during the conversion.

val unsigned_to_int : nativeint -> int option

Same as Nativeint.to_int[29.38], but interprets the argument as an unsigned integer.
Returns None if the unsigned value of the argument cannot fit into an int.
Since: 4.08

val of_float : float -> nativeint

Convert the given floating-point number to a native integer, discarding the fractional part
(truncate towards 0). If the truncated floating-point number is outside the range
[Nativeint.min_int[29.38], Nativeint.max_int[29.38]], no exception is raised, and an
unspecified, platform-dependent integer is returned.
816

val to_float : nativeint -> float

Convert the given native integer to a floating-point number.

val of_int32 : int32 -> nativeint

Convert the given 32-bit integer (type int32) to a native integer.

val to_int32 : nativeint -> int32

Convert the given native integer to a 32-bit integer (type int32). On 64-bit platforms, the
64-bit native integer is taken modulo 232 , i.e. the top 32 bits are lost. On 32-bit platforms,
the conversion is exact.

val of_string : string -> nativeint

Convert the given string to a native integer. The string is read in decimal (by default, or if
the string begins with 0u) or in hexadecimal, octal or binary if the string begins with 0x, 0o
or 0b respectively.
The 0u prefix reads the input as an unsigned integer in the range [0,
2*Nativeint.max_int+1]. If the input exceeds Nativeint.max_int[29.38] it is converted to
the signed integer Int64.min_int + input - Nativeint.max_int - 1.
Raises Failure if the given string is not a valid representation of an integer, or if the integer
represented exceeds the range of integers representable in type nativeint.

val of_string_opt : string -> nativeint option

Same as of_string, but return None instead of raising.
Since: 4.05

val to_string : nativeint -> string

Return the string representation of its argument, in decimal.

type t = nativeint
An alias for the type of native integers.

val compare : t -> t -> int

The comparison function for native integers, with the same specification as compare[28.2].
Along with the type t, this function compare allows the module Nativeint to be passed as
argument to the functors Set.Make[29.50] and Map.Make[29.34].

val unsigned_compare : t -> t -> int

Same as Nativeint.compare[29.38], except that arguments are interpreted as unsigned
native integers.
Since: 4.08

val equal : t -> t -> bool

Chapter 29. The standard library 817

The equal function for native ints.

Since: 4.03

val min : t -> t -> t

Return the smaller of the two arguments.
Since: 4.13

val max : t -> t -> t

Return the greater of the two arguments.
Since: 4.13

val seeded_hash : int -> t -> int

A seeded hash function for native ints, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.1

val hash : t -> int

An unseeded hash function for native ints, with the same output value as
Hashtbl.hash[29.25]. This function allows this module to be passed as argument to the
functor Hashtbl.Make[29.25].
Since: 5.1

29.39 Module Oo : Operations on objects

val copy : (< .. > as 'a) -> 'a

Oo.copy o returns a copy of object o, that is a fresh object with the same methods and
instance variables as o.
Alert unsynchronized_access. Unsynchronized accesses to mutable objects are a
programming error.

val id : < .. > -> int

Return an integer identifying this object, unique for the current execution of the program.
The generic comparison and hashing functions are based on this integer. When an object is
obtained by unmarshaling, the id is refreshed, and thus different from the original object. As
a consequence, the internal invariants of data structures such as hash table or sets containing
objects are broken after unmarshaling the data structures.
818

29.40 Module Option : Option values.

Option values explicitly indicate the presence or absence of a value.
Since: 4.08

Options
type 'a t = 'a option =
| None
| Some of 'a
The type for option values. Either None or a value Some v.

val none : 'a option

none is None.

val some : 'a -> 'a option

some v is Some v.

val value : 'a option -> default:'a -> 'a

value o ~default is v if o is Some v and default otherwise.

val get : 'a option -> 'a

get o is v if o is Some v and raise otherwise.
Raises Invalid_argument if o is None.

val bind : 'a option -> ('a -> 'b option) -> 'b option
bind o f is f v if o is Some v and None if o is None.

val join : 'a option option -> 'a option

join oo is Some v if oo is Some (Some v) and None otherwise.

val map : ('a -> 'b) -> 'a option -> 'b option
map f o is None if o is None and Some (f v) if o is Some v.

val fold : none:'a -> some:('b -> 'a) -> 'b option -> 'a
fold ~none ~some o is none if o is None and some v if o is Some v.

val iter : ('a -> unit) -> 'a option -> unit
iter f o is f v if o is Some v and () otherwise.
Chapter 29. The standard library 819

Predicates and comparisons

val is_none : 'a option -> bool
is_none o is true if and only if o is None.

val is_some : 'a option -> bool

is_some o is true if and only if o is Some o.

val equal : ('a -> 'a -> bool) -> 'a option -> 'a option -> bool
equal eq o0 o1 is true if and only if o0 and o1 are both None or if they are Some v0 and
Some v1 and eq v0 v1 is true.

val compare : ('a -> 'a -> int) -> 'a option -> 'a option -> int
compare cmp o0 o1 is a total order on options using cmp to compare values wrapped by
Some _. None is smaller than Some _ values.

Converting
val to_result : none:'e -> 'a option -> ('a, 'e) result
to_result ~none o is Ok v if o is Some v and Error none otherwise.

val to_list : 'a option -> 'a list

to_list o is [] if o is None and [v] if o is Some v.

val to_seq : 'a option -> 'a Seq.t

to_seq o is o as a sequence. None is the empty sequence and Some v is the singleton
sequence containing v.

29.41 Module Out_channel : Output channels.

This module provides functions for working with output channels.
See the example section[29.45] below.
Since: 4.14

Channels
type t = out_channel
The type of output channel.

type open_flag = open_flag =

| Open_rdonly
open for reading.
820

| Open_wronly
open for writing.
| Open_append
open for appending: always write at end of file.
| Open_creat
create the file if it does not exist.
| Open_trunc
empty the file if it already exists.
| Open_excl
fail if Open_creat and the file already exists.
| Open_binary
open in binary mode (no conversion).
| Open_text
open in text mode (may perform conversions).
| Open_nonblock
open in non-blocking mode.
Opening modes for Out_channel.open_gen[29.41].

val stdout : t
The standard output for the process.

val stderr : t
The standard error output for the process.

val open_bin : string -> t

Open the named file for writing, and return a new output channel on that file, positioned at
the beginning of the file. The file is truncated to zero length if it already exists. It is created
if it does not already exists.

val open_text : string -> t

Same as Out_channel.open_bin[29.41], but the file is opened in text mode, so that newline
translation takes place during writes. On operating systems that do not distinguish between
text mode and binary mode, this function behaves like Out_channel.open_bin[29.41].

val open_gen : open_flag list -> int -> string -> t

open_gen mode perm filename opens the named file for writing, as described above. The
extra argument mode specifies the opening mode. The extra argument perm specifies the file
permissions, in case the file must be created. Out_channel.open_text[29.41] and
Out_channel.open_bin[29.41] are special cases of this function.
Chapter 29. The standard library 821

val with_open_bin : string -> (t -> 'a) -> 'a

with_open_bin fn f opens a channel oc on file fn and returns f oc. After f returns, either
with a value or by raising an exception, oc is guaranteed to be closed.

val with_open_text : string -> (t -> 'a) -> 'a

Like Out_channel.with_open_bin[29.41], but the channel is opened in text mode (see
Out_channel.open_text[29.41]).

val with_open_gen : open_flag list -> int -> string -> (t -> 'a) -> 'a
Like Out_channel.with_open_bin[29.41], but can specify the opening mode and file
permission, in case the file must be created (see Out_channel.open_gen[29.41]).

val close : t -> unit

Close the given channel, flushing all buffered write operations. Output functions raise a
Sys_error exception when they are applied to a closed output channel, except
Out_channel.close[29.41] and Out_channel.flush[29.41], which do nothing when applied
to an already closed channel. Note that Out_channel.close[29.41] may raise Sys_error if
the operating system signals an error when flushing or closing.

val close_noerr : t -> unit

Same as Out_channel.close[29.41], but ignore all errors.

Output
val output_char : t -> char -> unit
Write the character on the given output channel.

val output_byte : t -> int -> unit

Write one 8-bit integer (as the single character with that code) on the given output channel.
The given integer is taken modulo 256.

val output_string : t -> string -> unit

Write the string on the given output channel.

val output_bytes : t -> bytes -> unit

Write the byte sequence on the given output channel.

Advanced output
val output : t -> bytes -> int -> int -> unit
output oc buf pos len writes len characters from byte sequence buf, starting at offset
pos, to the given output channel oc.
Raises Invalid_argument if pos and len do not designate a valid range of buf.
822

val output_substring : t -> string -> int -> int -> unit
Same as Out_channel.output[29.41] but take a string as argument instead of a byte
sequence.

val output_bigarray :
t ->
('a, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
Bigarray.Array1.t -> int -> int -> unit
Same as Out_channel.output[29.41] but take the data from a bigarray.
Since: 5.2

Flushing
val flush : t -> unit
Flush the buffer associated with the given output channel, performing all pending writes on
that channel. Interactive programs must be careful about flushing standard output and
standard error at the right time.

val flush_all : unit -> unit

Flush all open output channels; ignore errors.

Seeking
val seek : t -> int64 -> unit
seek chan pos sets the current writing position to pos for channel chan. This works only
for regular files. On files of other kinds (such as terminals, pipes and sockets), the behavior is
unspecified.

val pos : t -> int64

Return the current writing position for the given channel. Does not work on channels opened
with the Open_append flag (returns unspecified results).
For files opened in text mode under Windows, the returned position is approximate (owing to
end-of-line conversion); in particular, saving the current position with
Out_channel.pos[29.41], then going back to this position using Out_channel.seek[29.41]
will not work. For this programming idiom to work reliably and portably, the file must be
opened in binary mode.

Attributes
val length : t -> int64
Return the size (number of characters) of the regular file on which the given channel is
opened. If the channel is opened on a file that is not a regular file, the result is meaningless.
Chapter 29. The standard library 823

val set_binary_mode : t -> bool -> unit

set_binary_mode oc true sets the channel oc to binary mode: no translations take place
during output.
set_binary_mode oc false sets the channel oc to text mode: depending on the operating
system, some translations may take place during output. For instance, under Windows,
end-of-lines will be translated from \n to \r\n.
This function has no effect under operating systems that do not distinguish between text
mode and binary mode.

val is_binary_mode : t -> bool

is_binary_mode oc returns whether the channel oc is in binary mode (see
Out_channel.set_binary_mode[29.41]).
Since: 5.2

val set_buffered : t -> bool -> unit

set_buffered oc true sets the channel oc to buffered mode. In this mode, data output on
oc will be buffered until either the internal buffer is full or the function
Out_channel.flush[29.41] or Out_channel.flush_all[29.41] is called, at which point it will
be sent to the output device.
set_buffered oc false sets the channel oc to unbuffered mode. In this mode, data output
on oc will be sent to the output device immediately.
All channels are open in buffered mode by default.

val is_buffered : t -> bool

is_buffered oc returns whether the channel oc is buffered (see
Out_channel.set_buffered[29.41]).

val isatty : t -> bool

isatty oc is true if oc refers to a terminal or console window, false otherwise.
Since: 5.1

Examples
Writing the contents of a file:

let write_file file s =

Out_channel.with_open_bin file
(fun oc -> Out_channel.output_string oc s))
824

29.42 Module Parsing : The run-time library for parsers generated

by ocamlyacc.

val symbol_start : unit -> int

symbol_start and Parsing.symbol_end[29.42] are to be called in the action part of a
grammar rule only. They return the offset of the string that matches the left-hand side of the
rule: symbol_start() returns the offset of the first character; symbol_end() returns the
offset after the last character. The first character in a file is at offset 0.

val symbol_end : unit -> int

See Parsing.symbol_start[29.42].

val rhs_start : int -> int

Same as Parsing.symbol_start[29.42] and Parsing.symbol_end[29.42], but return the
offset of the string matching the nth item on the right-hand side of the rule, where n is the
integer parameter to rhs_start and rhs_end. n is 1 for the leftmost item.

val rhs_end : int -> int

See Parsing.rhs_start[29.42].

val symbol_start_pos : unit -> Lexing.position

Same as symbol_start, but return a position instead of an offset.

val symbol_end_pos : unit -> Lexing.position

Same as symbol_end, but return a position instead of an offset.

val rhs_start_pos : int -> Lexing.position

Same as rhs_start, but return a position instead of an offset.

val rhs_end_pos : int -> Lexing.position

Same as rhs_end, but return a position instead of an offset.

val clear_parser : unit -> unit

Empty the parser stack. Call it just after a parsing function has returned, to remove all
pointers from the parser stack to structures that were built by semantic actions during
parsing. This is optional, but lowers the memory requirements of the programs.

exception Parse_error
Raised when a parser encounters a syntax error. Can also be raised from the action part of a
grammar rule, to initiate error recovery.

val set_trace : bool -> bool

Chapter 29. The standard library 825

Control debugging support for ocamlyacc-generated parsers. After Parsing.set_trace

true, the pushdown automaton that executes the parsers prints a trace of its actions (reading
a token, shifting a state, reducing by a rule) on standard output. Parsing.set_trace false
turns this debugging trace off. The boolean returned is the previous state of the trace flag.
Since: 3.11

29.43 Module Printexc : Facilities for printing exceptions and

inspecting current call stack.

type t = exn = ..
The type of exception values.

val to_string : exn -> string

Printexc.to_string e returns a string representation of the exception e.

val to_string_default : exn -> string

Printexc.to_string_default e returns a string representation of the exception e, ignoring
all registered exception printers.
Since: 4.09

val print : ('a -> 'b) -> 'a -> 'b

Printexc.print fn x applies fn to x and returns the result. If the evaluation of fn x raises
any exception, the name of the exception is printed on standard error output, and the
exception is raised again. The typical use is to catch and report exceptions that escape a
function application.

val catch : ('a -> 'b) -> 'a -> 'b

Deprecated. This function is no longer needed.Printexc.catch fn x is similar to
Printexc.print[29.43], but aborts the program with exit code 2 after printing the uncaught
exception. This function is deprecated: the runtime system is now able to print uncaught
exceptions as precisely as Printexc.catch does. Moreover, calling Printexc.catch makes it
harder to track the location of the exception using the debugger or the stack backtrace facility.
So, do not use Printexc.catch in new code.

val print_backtrace : out_channel -> unit

Printexc.print_backtrace oc prints an exception backtrace on the output channel oc.
The backtrace lists the program locations where the most-recently raised exception was raised
and where it was propagated through function calls.
If the call is not inside an exception handler, the returned backtrace is unspecified. If the call
is after some exception-catching code (before in the handler, or in a when-guard during the
826

matching of the exception handler), the backtrace may correspond to a later exception than
the handled one.
Since: 3.11

val get_backtrace : unit -> string

Printexc.get_backtrace () returns a string containing the same exception backtrace that
Printexc.print_backtrace would print. Same restriction usage than
Printexc.print_backtrace[29.43].
Since: 3.11

val record_backtrace : bool -> unit

Printexc.record_backtrace b turns recording of exception backtraces on (if b = true) or
off (if b = false). Initially, backtraces are not recorded, unless the b flag is given to the
program through the OCAMLRUNPARAM variable.
Since: 3.11

val backtrace_status : unit -> bool

Printexc.backtrace_status() returns true if exception backtraces are currently recorded,
false if not.
Since: 3.11

val register_printer : (exn -> string option) -> unit

Printexc.register_printer fn registers fn as an exception printer. The printer should
return None or raise an exception if it does not know how to convert the passed exception,
and Some s with s the resulting string if it can convert the passed exception. Exceptions
raised by the printer are ignored.
When converting an exception into a string, the printers will be invoked in the reverse order
of their registrations, until a printer returns a Some s value (if no such printer exists, the
runtime will use a generic printer).
When using this mechanism, one should be aware that an exception backtrace is attached to
the thread that saw it raised, rather than to the exception itself. Practically, it means that
the code related to fn should not use the backtrace if it has itself raised an exception before.
Since: 3.11.2

val use_printers : exn -> string option

Printexc.use_printers e returns None if there are no registered printers and Some s with
s the resulting string otherwise.
Since: 4.09
Chapter 29. The standard library 827

Raw backtraces
type raw_backtrace
The type raw_backtrace stores a backtrace in a low-level format, which can be converted to
usable form using raw_backtrace_entries and backtrace_slots_of_raw_entry below.
Converting backtraces to backtrace_slots is slower than capturing the backtraces. If an
application processes many backtraces, it can be useful to use raw_backtrace to avoid or
delay conversion.
Raw backtraces cannot be marshalled. If you need marshalling, you should use the array
returned by the backtrace_slots function of the next section.
Since: 4.01

type raw_backtrace_entry = private int

A raw_backtrace_entry is an element of a raw_backtrace.
Each raw_backtrace_entry is an opaque integer, whose value is not stable between different
programs, or even between different runs of the same binary.
A raw_backtrace_entry can be converted to a usable form using
backtrace_slots_of_raw_entry below. Note that, due to inlining, a single
raw_backtrace_entry may convert to several backtrace_slots. Since the values of a
raw_backtrace_entry are not stable, they cannot be marshalled. If they are to be converted,
the conversion must be done by the process that generated them.
Again due to inlining, there may be multiple distinct raw_backtrace_entry values that
convert to equal backtrace_slots. However, if two raw_backtrace_entrys are equal as
integers, then they represent the same backtrace_slots.
Since: 4.12

val raw_backtrace_entries : raw_backtrace -> raw_backtrace_entry array

Since: 4.12

val get_raw_backtrace : unit -> raw_backtrace

Printexc.get_raw_backtrace () returns the same exception backtrace that
Printexc.print_backtrace would print, but in a raw format. Same restriction usage than
Printexc.print_backtrace[29.43].
Since: 4.01

val print_raw_backtrace : out_channel -> raw_backtrace -> unit

Print a raw backtrace in the same format Printexc.print_backtrace uses.
Since: 4.01

val raw_backtrace_to_string : raw_backtrace -> string

Return a string from a raw backtrace, in the same format Printexc.get_backtrace uses.
Since: 4.01
828

val raise_with_backtrace : exn -> raw_backtrace -> 'a

Reraise the exception using the given raw_backtrace for the origin of the exception
Since: 4.05

Current call stack

val get_callstack : int -> raw_backtrace
Printexc.get_callstack n returns a description of the top of the call stack on the current
program point (for the current thread), with at most n entries. (Note: this function is not
related to exceptions at all, despite being part of the Printexc module.)
Since: 4.01

Uncaught exceptions
val default_uncaught_exception_handler : exn -> raw_backtrace -> unit
Printexc.default_uncaught_exception_handler prints the exception and backtrace on
standard error output.
Since: 4.11

val set_uncaught_exception_handler : (exn -> raw_backtrace -> unit) -> unit

Printexc.set_uncaught_exception_handler fn registers fn as the handler for uncaught
exceptions. The default handler is
Printexc.default_uncaught_exception_handler[29.43].
Note that when fn is called all the functions registered with at_exit[28.2] have already been
called. Because of this you must make sure any output channel fn writes on is flushed.
Also note that exceptions raised by user code in the interactive toplevel are not passed to this
function as they are caught by the toplevel itself.
If fn raises an exception, both the exceptions passed to fn and raised by fn will be printed
with their respective backtrace.
Since: 4.02

Manipulation of backtrace information

These functions are used to traverse the slots of a raw backtrace and extract information from them
in a programmer-friendly format.
type backtrace_slot
The abstract type backtrace_slot represents a single slot of a backtrace.
Since: 4.02

val backtrace_slots : raw_backtrace -> backtrace_slot array option

Chapter 29. The standard library 829

Returns the slots of a raw backtrace, or None if none of them contain useful information.
In the return array, the slot at index 0 corresponds to the most recent function call, raise, or
primitive get_backtrace call in the trace.
Some possible reasons for returning None are as follow:

• none of the slots in the trace come from modules compiled with debug information (-g)
• the program is a bytecode program that has not been linked with debug information
enabled (ocamlc -g)

Since: 4.02

val backtrace_slots_of_raw_entry :
raw_backtrace_entry -> backtrace_slot array option
Returns the slots of a single raw backtrace entry, or None if this entry lacks debug information.
Slots are returned in the same order as backtrace_slots: the slot at index 0 is the most
recent call, raise, or primitive, and subsequent slots represent callers.
Since: 4.12

type location =
{ filename : string ;
line_number : int ;
start_char : int ;
end_char : int ;
end_line : int ;
Since: 5.2
end_col : int ;
Since: 5.2
}
The type of location information found in backtraces. start_char and end_char are
positions relative to the beginning of line_number. end_col is relative to the beginning of
end_line.
Since: 4.02

module Slot :
sig
type t = Printexc.backtrace_slot
val is_raise : t -> bool
is_raise slot is true when slot refers to a raising point in the code, and false
when it comes from a simple function call.
Since: 4.02

val is_inline : t -> bool

830

is_inline slot is true when slot refers to a call that got inlined by the compiler,
and false when it comes from any other context.
Since: 4.04

val location : t -> Printexc.location option

location slot returns the location information of the slot, if available, and None
otherwise.
Some possible reasons for failing to return a location are as follow:
• the slot corresponds to a compiler-inserted raise
• the slot corresponds to a part of the program that has not been compiled with
debug information (-g)
Since: 4.02

val name : t -> string option

name slot returns the name of the function or definition enclosing the location referred
to by the slot.
name slot returns None if the name is unavailable, which may happen for the same
reasons as location returning None.
Since: 4.11

val format : int -> t -> string option

format pos slot returns the string representation of slot as
raw_backtrace_to_string would format it, assuming it is the pos-th element of the
backtrace: the 0-th element is pretty-printed differently than the others.
Whole-backtrace printing functions also skip some uninformative slots; in that case,
format pos slot returns None.
Since: 4.02

end

Since: 4.02

Raw backtrace slots

type raw_backtrace_slot
This type is used to iterate over the slots of a raw_backtrace. For most purposes,
backtrace_slots_of_raw_entry is easier to use.
Like raw_backtrace_entry, values of this type are process-specific and must absolutely not
be marshalled, and are unsafe to use for this reason (marshalling them may not fail, but
un-marshalling and using the result will result in undefined behavior).
Chapter 29. The standard library 831

Elements of this type can still be compared and hashed: when two elements are equal, then
they represent the same source location (the converse is not necessarily true in presence of
inlining, for example).
Since: 4.02

val raw_backtrace_length : raw_backtrace -> int

raw_backtrace_length bckt returns the number of slots in the backtrace bckt.
Since: 4.02

val get_raw_backtrace_slot : raw_backtrace -> int -> raw_backtrace_slot

get_raw_backtrace_slot bckt pos returns the slot in position pos in the backtrace bckt.
Since: 4.02

val convert_raw_backtrace_slot : raw_backtrace_slot -> backtrace_slot

Extracts the user-friendly backtrace_slot from a low-level raw_backtrace_slot.
Since: 4.02

val get_raw_backtrace_next_slot :
raw_backtrace_slot -> raw_backtrace_slot option
get_raw_backtrace_next_slot slot returns the next slot inlined, if any.
Sample code to iterate over all frames (inlined and non-inlined):

(* Iterate over inlined frames *)

let rec iter_raw_backtrace_slot f slot =
f slot;
match get_raw_backtrace_next_slot slot with
| None -> ()
| Some slot' -> iter_raw_backtrace_slot f slot'

(* Iterate over stack frames *)

let iter_raw_backtrace f bt =
for i = 0 to raw_backtrace_length bt - 1 do
iter_raw_backtrace_slot f (get_raw_backtrace_slot bt i)
done

Since: 4.04

Exception slots
val exn_slot_id : exn -> int
Printexc.exn_slot_id returns an integer which uniquely identifies the constructor used to
create the exception value exn (in the current runtime).
Since: 4.02
832

val exn_slot_name : exn -> string

Printexc.exn_slot_name exn returns the internal name of the constructor used to create
the exception value exn.
Since: 4.02

29.44 Module Printf : Formatted output functions.

val fprintf : out_channel -> ('a, out_channel, unit) format -> 'a
fprintf outchan format arg1 ... argN formats the arguments arg1 to argN according
to the format string format, and outputs the resulting string on the channel outchan.
The format string is a character string which contains two types of objects: plain characters,
which are simply copied to the output channel, and conversion specifications, each of which
causes conversion and printing of arguments.
Conversion specifications have the following form:
% [flags] [width] [.precision] type
In short, a conversion specification consists in the % character, followed by optional modifiers
and a type which is made of one or two characters.
The types and their meanings are:

• d, i: convert an integer argument to signed decimal. The flag # adds underscores to

large values for readability.
• u, n, l, L, or N: convert an integer argument to unsigned decimal. Warning: n, l, L, and
N are used for scanf, and should not be used for printf. The flag # adds underscores to
large values for readability.
• x: convert an integer argument to unsigned hexadecimal, using lowercase letters. The
flag # adds a 0x prefix to non zero values.
• X: convert an integer argument to unsigned hexadecimal, using uppercase letters. The
flag # adds a 0X prefix to non zero values.
• o: convert an integer argument to unsigned octal. The flag # adds a 0 prefix to non zero
values.
• s: insert a string argument.
• S: convert a string argument to OCaml syntax (double quotes, escapes).
• c: insert a character argument.
• C: convert a character argument to OCaml syntax (single quotes, escapes).
• f: convert a floating-point argument to decimal notation, in the style dddd.ddd.
• F: convert a floating-point argument to OCaml syntax (dddd. or dddd.ddd or d.ddd
e+-dd). Converts to hexadecimal with the # flag (see h).
Chapter 29. The standard library 833

• e or E: convert a floating-point argument to decimal notation, in the style d.ddd e+-dd

(mantissa and exponent).
• g or G: convert a floating-point argument to decimal notation, in style f or e, E
(whichever is more compact). Moreover, any trailing zeros are removed from the
fractional part of the result and the decimal-point character is removed if there is no
fractional part remaining.
• h or H: convert a floating-point argument to hexadecimal notation, in the style 0xh.hhhh
p+-dd (hexadecimal mantissa, exponent in decimal and denotes a power of 2).
• B: convert a boolean argument to the string true or false
• b: convert a boolean argument (deprecated; do not use in new programs).
• ld, li, lu, lx, lX, lo: convert an int32 argument to the format specified by the second
letter (decimal, hexadecimal, etc).
• nd, ni, nu, nx, nX, no: convert a nativeint argument to the format specified by the
second letter.
• Ld, Li, Lu, Lx, LX, Lo: convert an int64 argument to the format specified by the second
letter.
• a: user-defined printer. Take two arguments and apply the first one to outchan (the
current output channel) and to the second argument. The first argument must therefore
have type out_channel -> 'b -> unit and the second 'b. The output produced by
the function is inserted in the output of fprintf at the current point.
• t: same as %a, but take only one argument (with type out_channel -> unit) and
apply it to outchan.
• { fmt %}: convert a format string argument to its type digest. The argument must have
the same type as the internal format string fmt.
• ( fmt %): format string substitution. Take a format string argument and substitute it
to the internal format string fmt to print following arguments. The argument must have
the same type as the internal format string fmt.
• !: take no argument and flush the output.
• %: take no argument and output one % character.
• @: take no argument and output one @ character.
• ,: take no argument and output nothing: a no-op delimiter for conversion specifications.

The optional flags are:

• -: left-justify the output (default is right justification).

• 0: for numerical conversions, pad with zeroes instead of spaces.
• +: for signed numerical conversions, prefix number with a + sign if positive.
• space: for signed numerical conversions, prefix number with a space if positive.
• #: request an alternate formatting style for the integer types and the floating-point type
F.
834

The optional width is an integer indicating the minimal width of the result. For instance, %6d
prints an integer, prefixing it with spaces to fill at least 6 characters.
The optional precision is a dot . followed by an integer indicating how many digits follow
the decimal point in the %f, %e, %E, %h, and %H conversions or the maximum number of
significant digits to appear for the %F, %g and %G conversions. For instance, %.4f prints a
float with 4 fractional digits.
The integer in a width or precision can also be specified as *, in which case an extra integer
argument is taken to specify the corresponding width or precision. This integer argument
precedes immediately the argument to print. For instance, %.*f prints a float with as many
fractional digits as the value of the argument given before the float.

val printf : ('a, out_channel, unit) format -> 'a

Same as Printf.fprintf[29.44], but output on stdout.

val eprintf : ('a, out_channel, unit) format -> 'a

Same as Printf.fprintf[29.44], but output on stderr.

val sprintf : ('a, unit, string) format -> 'a

Same as Printf.fprintf[29.44], but instead of printing on an output channel, return a
string containing the result of formatting the arguments.

val bprintf : Buffer.t -> ('a, Buffer.t, unit) format -> 'a
Same as Printf.fprintf[29.44], but instead of printing on an output channel, append the
formatted arguments to the given extensible buffer (see module Buffer[29.7]).

val ifprintf : 'b -> ('a, 'b, 'c, unit) format4 -> 'a
Same as Printf.fprintf[29.44], but does not print anything. Useful to ignore some material
when conditionally printing.
Since: 3.10

val ibprintf : Buffer.t -> ('a, Buffer.t, unit) format -> 'a
Same as Printf.bprintf[29.44], but does not print anything. Useful to ignore some material
when conditionally printing.
Since: 4.11
Formatted output functions with continuations.
val kfprintf :
(out_channel -> 'd) ->
out_channel -> ('a, out_channel, unit, 'd) format4 -> 'a
Same as fprintf, but instead of returning immediately, passes the out channel to its first
argument at the end of printing.
Since: 3.09

val ikfprintf : ('b -> 'd) -> 'b -> ('a, 'b, 'c, 'd) format4 -> 'a
Chapter 29. The standard library 835

Same as kfprintf above, but does not print anything. Useful to ignore some material when
conditionally printing.
Since: 4.01

val ksprintf : (string -> 'd) -> ('a, unit, string, 'd) format4 -> 'a
Same as sprintf above, but instead of returning the string, passes it to the first argument.
Since: 3.09

val kbprintf :
(Buffer.t -> 'd) ->
Buffer.t -> ('a, Buffer.t, unit, 'd) format4 -> 'a
Same as bprintf, but instead of returning immediately, passes the buffer to its first argument
at the end of printing.
Since: 3.10

val ikbprintf :
(Buffer.t -> 'd) ->
Buffer.t -> ('a, Buffer.t, unit, 'd) format4 -> 'a
Same as kbprintf above, but does not print anything. Useful to ignore some material when
conditionally printing.
Since: 4.11

Deprecated
val kprintf : (string -> 'b) -> ('a, unit, string, 'b) format4 -> 'a
Deprecated. Use Printf.ksprintf instead.A deprecated synonym for ksprintf.

29.45 Module Queue : First-in first-out queues.

This module implements queues (FIFOs), with in-place modification. See the example section[29.45]
below.
Alert unsynchronized_access. Unsynchronized accesses to queues are a programming error.

Unsynchronized accesses
Unsynchronized accesses to a queue may lead to an invalid queue state. Thus, concurrent
accesses to queues must be synchronized (for instance with a Mutex.t[29.37]).
type !'a t
The type of queues containing elements of type 'a.

exception Empty
Raised when Queue.take[29.45] or Queue.peek[29.45] is applied to an empty queue.

val create : unit -> 'a t

836

Return a new queue, initially empty.

val add : 'a -> 'a t -> unit

add x q adds the element x at the end of the queue q.

val push : 'a -> 'a t -> unit

push is a synonym for add.

val take : 'a t -> 'a

take q removes and returns the first element in queue q, or raises Queue.Empty[29.45] if the
queue is empty.

val take_opt : 'a t -> 'a option

take_opt q removes and returns the first element in queue q, or returns None if the queue is
empty.
Since: 4.08

val pop : 'a t -> 'a

pop is a synonym for take.

val peek : 'a t -> 'a

peek q returns the first element in queue q, without removing it from the queue, or raises
Queue.Empty[29.45] if the queue is empty.

val peek_opt : 'a t -> 'a option

peek_opt q returns the first element in queue q, without removing it from the queue, or
returns None if the queue is empty.
Since: 4.08

val top : 'a t -> 'a

top is a synonym for peek.

val drop : 'a t -> unit

drop q removes the first element in queue q, or raises Queue.Empty[29.45] if the queue is
empty.
Since: 5.3

val clear : 'a t -> unit

Discard all elements from a queue.

val copy : 'a t -> 'a t

Return a copy of the given queue.

val is_empty : 'a t -> bool

Chapter 29. The standard library 837

Return true if the given queue is empty, false otherwise.

val length : 'a t -> int

Return the number of elements in a queue.

val iter : ('a -> unit) -> 'a t -> unit

iter f q applies f in turn to all elements of q, from the least recently entered to the most
recently entered. The queue itself is unchanged.

val fold : ('acc -> 'a -> 'acc) -> 'acc -> 'a t -> 'acc
fold f accu q is equivalent to List.fold_left f accu l, where l is the list of q’s
elements. The queue remains unchanged.

val transfer : 'a t -> 'a t -> unit

transfer q1 q2 adds all of q1’s elements at the end of the queue q2, then clears q1. It is
equivalent to the sequence iter (fun x -> add x q2) q1; clear q1, but runs in constant
time.

Iterators
val to_seq : 'a t -> 'a Seq.t
Iterate on the queue, in front-to-back order. The behavior is not specified if the queue is
modified during the iteration.
Since: 4.07

val add_seq : 'a t -> 'a Seq.t -> unit

Add the elements from a sequence to the end of the queue.
Since: 4.07

val of_seq : 'a Seq.t -> 'a t

Create a queue from a sequence.
Since: 4.07

Examples
Basic Example
A basic example:

# let q = Queue.create ()
val q : '_weak1 Queue.t = <abstr>
838

# Queue.push 1 q; Queue.push 2 q; Queue.push 3 q

- : unit = ()

# Queue.length q
- : int = 3

# Queue.pop q
- : int = 1

# Queue.pop q
- : int = 2

# Queue.pop q
- : int = 3

# Queue.pop q
Exception: Stdlib.Queue.Empty.

Search Through a Graph

For a more elaborate example, a classic algorithmic use of queues is to implement a BFS (breadth-first
search) through a graph.

type graph = {
edges: (int, int list) Hashtbl.t
}

(* Search in graph [g] using BFS, starting from node [start].

It returns the first node that satisfies [p], or [None] if
no node reachable from [start] satisfies [p].
*)
let search_for ~(g:graph) ~(start:int) (p:int -> bool) : int option =
let to_explore = Queue.create() in
let explored = Hashtbl.create 16 in

Queue.push start to_explore;

let rec loop () =
if Queue.is_empty to_explore then None
else
(* node to explore *)
let node = Queue.pop to_explore in
explore_node node

and explore_node node =

Chapter 29. The standard library 839

if not (Hashtbl.mem explored node) then (

if p node then Some node (* found *)
else (
Hashtbl.add explored node ();
let children =
Hashtbl.find_opt g.edges node
|> Option.value ~default:[]
in
List.iter (fun child -> Queue.push child to_explore) children;
loop()
)
) else loop()
in
loop()

(* a sample graph *)
let my_graph: graph =
let edges =
List.to_seq [
1, [2;3];
2, [10; 11];
3, [4;5];
5, [100];
11, [0; 20];
]
|> Hashtbl.of_seq
in {edges}

# search_for ~g:my_graph ~start:1 (fun x -> x = 30)

- : int option = None

# search_for ~g:my_graph ~start:1 (fun x -> x >= 15)

- : int option = Some 20

# search_for ~g:my_graph ~start:1 (fun x -> x >= 50)

- : int option = Some 100

29.46 Module Random : Pseudo-random number generators

(PRNG).
With multiple domains, each domain has its own generator that evolves independently of the
generators of other domains. When a domain is created, its generator is initialized by splitting the
state of the generator associated with the parent domain.
840

In contrast, all threads within a domain share the same domain-local generator. Independent
generators can be created with the Random.split[29.46] function and used with the functions from
the Random.State[29.46] module.
Before 5.0 Random value generation used a different algorithm. This affects all the functions
in this module which return random values.

Basic functions
val init : int -> unit
Initialize the domain-local generator, using the argument as a seed. The same seed will
always yield the same sequence of numbers.

val full_init : int array -> unit

Same as Random.init[29.46] but takes more data as seed.

val self_init : unit -> unit

Initialize the domain-local generator with a random seed chosen in a system-dependent way.
If /dev/urandom is available on the host machine, it is used to provide a highly random
initial seed. Otherwise, a less random seed is computed from system parameters (current
time, process IDs, domain-local state).

val bits : unit -> int

Return 30 random bits in a nonnegative integer.

val int : int -> int

Random.int bound returns a random integer between 0 (inclusive) and bound (exclusive).
bound must be greater than 0 and less than 230 .
Raises Invalid_argument if bound ≤ 0 or bound ≥ 230 .

val full_int : int -> int

Random.full_int bound returns a random integer between 0 (inclusive) and bound
(exclusive). bound may be any positive integer.
If bound is less than 231 , then Random.full_int bound yields identical output across
systems with varying int sizes.
If bound is less than 230 , then Random.full_int bound is equal to Random.int[29.46] bound.
If bound is at least 230 (on 64-bit systems, or non-standard environments such as JavaScript),
then Random.full_int returns a value whereas Random.int[29.46] raises
Invalid_argument[28.2].
Since: 4.13
Raises Invalid_argument if bound ≤ 0.

val int_in_range : min:int -> max:int -> int

Chapter 29. The standard library 841

Random.int_in_range ~min ~max returns a random integer between min (inclusive) and max
(inclusive). Both min and max are allowed to be negative; min must be less than or equal to
max.
If both bounds fit in 32-bit signed integers (that is, if -231 ≤ min and max < 231 ), then
int_in_range yields identical output across systems with varying int sizes.
Since: 5.2
Raises Invalid_argument if min > max.

val int32 : Int32.t -> Int32.t

Random.int32 bound returns a random integer between 0 (inclusive) and bound (exclusive).
bound must be greater than 0.
Raises Invalid_argument if bound ≤ 0.

val int32_in_range : min:int32 -> max:int32 -> int32

Random.int32_in_range ~min ~max returns a random integer between min (inclusive) and
max (inclusive). Both min and max are allowed to be negative; min must be less than or equal
to max.
Since: 5.2
Raises Invalid_argument if min > max.

val nativeint : Nativeint.t -> Nativeint.t

Random.nativeint bound returns a random integer between 0 (inclusive) and bound
(exclusive). bound must be greater than 0.
Raises Invalid_argument if bound ≤ 0.

val nativeint_in_range : min:nativeint -> max:nativeint -> nativeint

Random.nativeint_in_range ~min ~max returns a random integer between min (inclusive)
and max (inclusive). Both min and max are allowed to be negative; min must be less than or
equal to max.
Since: 5.2
Raises Invalid_argument if min > max.

val int64 : Int64.t -> Int64.t

Random.int64 bound returns a random integer between 0 (inclusive) and bound (exclusive).
bound must be greater than 0.
Raises Invalid_argument if bound ≤ 0.

val int64_in_range : min:int64 -> max:int64 -> int64

Random.int64_in_range ~min ~max returns a random integer between min (inclusive) and
max (inclusive). Both min and max are allowed to be negative; min must be less than or equal
to max.
Since: 5.2
Raises Invalid_argument if min > max.
842

val float : float -> float

Random.float bound returns a random floating-point number between 0 and bound
(inclusive). If bound is negative, the result is negative or zero. If bound is 0, the result is 0.

val bool : unit -> bool

Random.bool () returns true or false with probability 0.5 each.

val bits32 : unit -> Int32.t

Random.bits32 () returns 32 random bits as an integer between Int32.min_int[29.28] and
Int32.max_int[29.28].
Since: 4.14

val bits64 : unit -> Int64.t

Random.bits64 () returns 64 random bits as an integer between Int64.min_int[29.29] and
Int64.max_int[29.29].
Since: 4.14

val nativebits : unit -> Nativeint.t

Random.nativebits () returns 32 or 64 random bits (depending on the bit width of the
platform) as an integer between Nativeint.min_int[29.38] and Nativeint.max_int[29.38].
Since: 4.14

Advanced functions
The functions from module Random.State[29.46] manipulate the current state of the random
generator explicitly. This allows using one or several deterministic PRNGs, even in a multi-threaded
program, without interference from other parts of the program.
module State :
sig
type t
The type of PRNG states.

val make : int array -> t

Create a new state and initialize it with the given seed.

val make_self_init : unit -> t

Create a new state and initialize it with a random seed chosen in a system-dependent
way. The seed is obtained as described in Random.self_init[29.46].

val copy : t -> t

Return a copy of the given state.
Chapter 29. The standard library 843

val bits : t -> int

val int : t -> int -> int
val full_int : t -> int -> int
val int_in_range : t -> min:int -> max:int -> int
val int32 : t -> Int32.t -> Int32.t
val int32_in_range : t -> min:int32 -> max:int32 -> int32
val nativeint : t -> Nativeint.t -> Nativeint.t
val nativeint_in_range : t -> min:nativeint -> max:nativeint -> nativeint
val int64 : t -> Int64.t -> Int64.t
val int64_in_range : t -> min:int64 -> max:int64 -> int64
val float : t -> float -> float
val bool : t -> bool
val bits32 : t -> Int32.t
val bits64 : t -> Int64.t
val nativebits : t -> Nativeint.t
These functions are the same as the basic functions, except that they use (and update)
the given PRNG state instead of the default one.

val split : t -> t

Draw a fresh PRNG state from the given PRNG state. (The given PRNG state is
modified.) The new PRNG is statistically independent from the given PRNG. Data can
be drawn from both PRNGs, in any order, without risk of correlation. Both PRNGs can
be split later, arbitrarily many times.
Since: 5.0

val to_binary_string : t -> string

Serializes the PRNG state into an immutable sequence of bytes. See
Random.State.of_binary_string[29.46] for deserialization.
The string type is intended here for serialization only, the encoding is not
human-readable and may not be printable.
Note that the serialization format may differ across OCaml versions.
Since: 5.1

val of_binary_string : string -> t

Deserializes a byte sequence obtained by calling
Random.State.to_binary_string[29.46]. The resulting PRNG state will produce the
same random numbers as the state that was passed as input to
Random.State.to_binary_string[29.46].
Since: 5.1
Raises Failure if the input is not in the expected format.
844

Note that the serialization format may differ across OCaml versions.
Unlike the functions provided by the Marshal[29.35] module, this function either
produces a valid state or fails cleanly with a Failure exception. It can be safely used on
user-provided, untrusted inputs.

end

val get_state : unit -> State.t

get_state() returns a fresh copy of the current state of the domain-local generator (which is
used by the basic functions).

val set_state : State.t -> unit

set_state s updates the current state of the domain-local generator (which is used by the
basic functions) by copying the state s into it.

val split : unit -> State.t

Draw a fresh PRNG state from the current state of the domain-local generator used by the
default functions. (The state of the domain-local generator is modified.) See
Random.State.split[29.46].
Since: 5.0

29.47 Module Result : Result values.

Result values handle computation results and errors in an explicit and declarative manner without
resorting to exceptions.
Since: 4.08

Results
type ('a, 'e) t = ('a, 'e) result =
| Ok of 'a
| Error of 'e
The type for result values. Either a value Ok v or an error Error e.

val ok : 'a -> ('a, 'e) result

ok v is Ok v.

val error : 'e -> ('a, 'e) result

error e is Error e.

val value : ('a, 'e) result -> default:'a -> 'a

value r ~default is v if r is Ok v and default otherwise.
Chapter 29. The standard library 845

val get_ok : ('a, 'e) result -> 'a

get_ok r is v if r is Ok v and raise otherwise.
Raises Invalid_argument if r is Error _.

val get_error : ('a, 'e) result -> 'e

get_error r is e if r is Error e and raise otherwise.
Raises Invalid_argument if r is Ok _.

val bind : ('a, 'e) result ->

('a -> ('b, 'e) result) -> ('b, 'e) result
bind r f is f v if r is Ok v and r if r is Error _.

val join : (('a, 'e) result, 'e) result -> ('a, 'e) result
join rr is r if rr is Ok r and rr if rr is Error _.

val map : ('a -> 'b) -> ('a, 'e) result -> ('b, 'e) result
map f r is Ok (f v) if r is Ok v and r if r is Error _.

val map_error : ('e -> 'f) -> ('a, 'e) result -> ('a, 'f) result
map_error f r is Error (f e) if r is Error e and r if r is Ok _.

val fold : ok:('a -> 'c) -> error:('e -> 'c) -> ('a, 'e) result -> 'c
fold ~ok ~error r is ok v if r is Ok v and error e if r is Error e.

val iter : ('a -> unit) -> ('a, 'e) result -> unit
iter f r is f v if r is Ok v and () otherwise.

val iter_error : ('e -> unit) -> ('a, 'e) result -> unit
iter_error f r is f e if r is Error e and () otherwise.

Predicates and comparisons

val is_ok : ('a, 'e) result -> bool
is_ok r is true if and only if r is Ok _.

val is_error : ('a, 'e) result -> bool

is_error r is true if and only if r is Error _.

val equal :
ok:('a -> 'a -> bool) ->
error:('e -> 'e -> bool) ->
('a, 'e) result -> ('a, 'e) result -> bool
846

equal ~ok ~error r0 r1 tests equality of r0 and r1 using ok and error to respectively
compare values wrapped by Ok _ and Error _.

val compare :
ok:('a -> 'a -> int) ->
error:('e -> 'e -> int) ->
('a, 'e) result -> ('a, 'e) result -> int
compare ~ok ~error r0 r1 totally orders r0 and r1 using ok and error to respectively
compare values wrapped by Ok _ and Error _. Ok _ values are smaller than Error
_ values.

Converting
val to_option : ('a, 'e) result -> 'a option
to_option r is r as an option, mapping Ok v to Some v and Error _ to None.

val to_list : ('a, 'e) result -> 'a list

to_list r is [v] if r is Ok v and [] otherwise.

val to_seq : ('a, 'e) result -> 'a Seq.t

to_seq r is r as a sequence. Ok v is the singleton sequence containing v and Error _ is the
empty sequence.

29.48 Module Scanf : Formatted input functions.

Alert unsynchronized_access. Unsynchronized accesses to Scanning.in_channel are a program-
ming error.

Introduction
Functional input with format strings
The module Scanf[29.48] provides formatted input functions or scanners.
The formatted input functions can read from any kind of input, including strings, files, or
anything that can return characters. The more general source of characters is named a formatted
input channel (or scanning buffer) and has type Scanf.Scanning.in_channel[29.48]. The more
general formatted input function reads from any scanning buffer and is named bscanf.
Generally speaking, the formatted input functions have 3 arguments:

• the first argument is a source of characters for the input,

• the second argument is a format string that specifies the values to read,

• the third argument is a receiver function that is applied to the values read.
Chapter 29. The standard library 847

Hence, a typical call to the formatted input function Scanf.bscanf[29.48] is bscanf ic fmt f,
where:

• ic is a source of characters (typically a formatted input channel with type Scanf.Scanning.in_

channel[29.48]),

• fmt is a format string (the same format strings as those used to print material with module
Printf[29.44] or Format[29.22]),

• f is a function that has as many arguments as the number of values to read in the input
according to fmt.

A simple example
As suggested above, the expression bscanf ic "%d" f reads a decimal integer n from the source of
characters ic and returns f n.
For instance,

• if we use stdin as the source of characters (Scanf.Scanning.stdin[29.48] is the predefined

formatted input channel that reads from standard input),

• if we define the receiver f as let f x = x + 1,

then bscanf Scanning.stdin "%d" f reads an integer n from the standard input and returns f
n (that is n + 1). Thus, if we evaluate bscanf stdin "%d" f, and then enter 41 at the keyboard,
the result we get is 42.

Formatted input as a functional feature

The OCaml scanning facility is reminiscent of the corresponding C feature. However, it is also
largely different, simpler, and yet more powerful: the formatted input functions are higher-order
functionals and the parameter passing mechanism is just the regular function application not the
variable assignment based mechanism which is typical for formatted input in imperative languages;
the OCaml format strings also feature useful additions to easily define complex tokens; as expected
within a functional programming language, the formatted input functions also support polymorphism,
in particular arbitrary interaction with polymorphic user-defined scanners. Furthermore, the OCaml
formatted input facility is fully type-checked at compile time.
Unsynchronized accesses
Unsynchronized accesses to a Scanf.Scanning.in_channel[29.48] may lead to an invalid
Scanf.Scanning.in_channel[29.48] state. Thus, concurrent accesses to Scanf.Scanning.in_
channel[29.48]s must be synchronized (for instance with a Mutex.t[29.37]).

Formatted input channel

module Scanning :
sig
type in_channel
848

The notion of input channel for the Scanf[29.48] module: those channels provide all the
machinery necessary to read from any source of characters, including a in_channel[28.2]
value. A Scanf.Scanning.in_channel value is also called a formatted input channel or
equivalently a scanning buffer. The type Scanf.Scanning.scanbuf[29.48] below is an
alias for Scanning.in_channel. Note that a Scanning.in_channel is not
concurrency-safe: concurrent use may produce arbitrary values or exceptions.
Since: 3.12

type scanbuf = in_channel

The type of scanning buffers. A scanning buffer is the source from which a formatted
input function gets characters. The scanning buffer holds the current state of the scan,
plus a function to get the next char from the input, and a token buffer to store the
string matched so far.
Note: a scanning action may often require to examine one character in advance; when
this ’lookahead’ character does not belong to the token read, it is stored back in the
scanning buffer and becomes the next character yet to be read.

val stdin : in_channel

The standard input notion for the Scanf[29.48] module. Scanning.stdin is the
Scanf.Scanning.in_channel[29.48] formatted input channel attached to stdin[28.2].
Note: in the interactive system, when input is read from stdin[28.2], the newline
character that triggers evaluation is part of the input; thus, the scanning specifications
must properly skip this additional newline character (for instance, simply add a '\n' as
the last character of the format string).
Since: 3.12

type file_name = string

A convenient alias to designate a file name.
Since: 4.00

val open_in : file_name -> in_channel

Scanning.open_in fname returns a Scanf.Scanning.in_channel[29.48] formatted
input channel for bufferized reading in text mode from file fname.
Note: open_in returns a formatted input channel that efficiently reads characters in
large chunks; in contrast, from_channel below returns formatted input channels that
must read one character at a time, leading to a much slower scanning rate.
Since: 3.12

val open_in_bin : file_name -> in_channel

Scanning.open_in_bin fname returns a Scanf.Scanning.in_channel[29.48]
formatted input channel for bufferized reading in binary mode from file fname.
Since: 3.12
Chapter 29. The standard library 849

val close_in : in_channel -> unit

Closes the in_channel[28.2] associated with the given
Scanf.Scanning.in_channel[29.48] formatted input channel.
Since: 3.12

val from_file : file_name -> in_channel

An alias for Scanf.Scanning.open_in[29.48] above.

val from_file_bin : string -> in_channel

An alias for Scanf.Scanning.open_in_bin[29.48] above.

val from_string : string -> in_channel

Scanning.from_string s returns a Scanf.Scanning.in_channel[29.48] formatted
input channel which reads from the given string. Reading starts from the first character
in the string. The end-of-input condition is set when the end of the string is reached.

val from_function : (unit -> char) -> in_channel

Scanning.from_function f returns a Scanf.Scanning.in_channel[29.48] formatted
input channel with the given function as its reading method.
When scanning needs one more character, the given function is called.
When the function has no more character to provide, it must signal an end-of-input
condition by raising the exception End_of_file.

val from_channel : in_channel -> in_channel

Scanning.from_channel ic returns a Scanf.Scanning.in_channel[29.48] formatted
input channel which reads from the regular in_channel[28.2] input channel ic
argument. Reading starts at current reading position of ic.

val end_of_input : in_channel -> bool

Scanning.end_of_input ic tests the end-of-input condition of the given
Scanf.Scanning.in_channel[29.48] formatted input channel.

val beginning_of_input : in_channel -> bool

Scanning.beginning_of_input ic tests the beginning of input condition of the given
Scanf.Scanning.in_channel[29.48] formatted input channel.

val name_of_input : in_channel -> string

Scanning.name_of_input ic returns the name of the character source for the given
Scanf.Scanning.in_channel[29.48] formatted input channel.
Since: 3.09

end
850

Type of formatted input functions

type ('a, 'b, 'c, 'd) scanner = ('a, Scanning.in_channel, 'b, 'c, 'a -> 'd, 'd) format6 -> 'c
The type of formatted input scanners: ('a, 'b, 'c, 'd) scanner is the type of a
formatted input function that reads from some formatted input channel according to some
format string; more precisely, if scan is some formatted input function, then scan ic fmt f
applies f to all the arguments specified by format string fmt, when scan has read those
arguments from the Scanf.Scanning.in_channel[29.48] formatted input channel ic.
For instance, the Scanf.scanf[29.48] function below has type ('a, 'b, 'c, 'd) scanner,
since it is a formatted input function that reads from Scanf.Scanning.stdin[29.48]: scanf
fmt f applies f to the arguments specified by fmt, reading those arguments from stdin[28.2]
as expected.
If the format fmt has some %r indications, the corresponding formatted input functions must
be provided before receiver function f. For instance, if read_elem is an input function for
values of type t, then bscanf ic "%r;" read_elem f reads a value v of type t followed by
a ';' character, and returns f v.
Since: 3.10

type ('a, 'b, 'c, 'd) scanner_opt = ('a, Scanning.in_channel, 'b, 'c, 'a -> 'd option, 'd) form
'c
exception Scan_failure of string
When the input can not be read according to the format string specification, formatted input
functions typically raise exception Scan_failure.

The general formatted input function

val bscanf : Scanning.in_channel -> ('a, 'b, 'c, 'd) scanner
bscanf ic fmt r1 ... rN f reads characters from the Scanf.Scanning.in_channel[29.48]
formatted input channel ic and converts them to values according to format string fmt. As a final
step, receiver function f is applied to the values read and gives the result of the bscanf call.
For instance, if f is the function fun s i -> i + 1, then Scanf.sscanf "x = 1" "%s = %i"
f returns 2.
Arguments r1 to rN are user-defined input functions that read the argument corresponding to
the %r conversions specified in the format string.
val bscanf_opt : Scanning.in_channel -> ('a, 'b, 'c, 'd) scanner_opt
Same as Scanf.bscanf[29.48], but returns None in case of scanning failure.
Since: 5.0

Format string description

The format string is a character string which contains three types of objects:
Chapter 29. The standard library 851

• plain characters, which are simply matched with the characters of the input (with a special
case for space and line feed, see [29.48]),

• conversion specifications, each of which causes reading and conversion of one argument for the
function f (see [29.48]),

• scanning indications to specify boundaries of tokens (see scanning [29.48]).

The space character in format strings

As mentioned above, a plain character in the format string is just matched with the next character
of the input; however, two characters are special exceptions to this rule: the space character (' '
or ASCII code 32) and the line feed character ('\n' or ASCII code 10). A space does not match
a single space character, but any amount of ’whitespace’ in the input. More precisely, a space
inside the format string matches any number of tab, space, line feed and carriage return characters.
Similarly, a line feed character in the format string matches either a single line feed or a carriage
return followed by a line feed.
Matching any amount of whitespace, a space in the format string also matches no amount of
whitespace at all; hence, the call bscanf ib "Price = %d $" (fun p -> p) succeeds and returns
1 when reading an input with various whitespace in it, such as Price = 1 $, Price = 1 $, or even
Price=1$.

Conversion specifications in format strings

Conversion specifications consist in the % character, followed by an optional flag, an optional field
width, and followed by one or two conversion characters.
The conversion characters and their meanings are:

• d: reads an optionally signed decimal integer (0-9+).

• i: reads an optionally signed integer (usual input conventions for decimal (0-9+), hexadecimal
(0x[0-9a-f]+ and 0X[0-9A-F]+), octal (0o[0-7]+), and binary (0b[0-1]+) notations are
understood).

• u: reads an unsigned decimal integer.

• x or X: reads an unsigned hexadecimal integer ([0-9a-fA-F]+).

• o: reads an unsigned octal integer ([0-7]+).

• s: reads a string argument that spreads as much as possible, until the following bounding
condition holds:

– a whitespace has been found (see [29.48]),

– a scanning indication (see scanning [29.48]) has been encountered,
– the end-of-input has been reached.

Hence, this conversion always succeeds: it returns an empty string if the bounding condition
holds when the scan begins.
852

• S: reads a delimited string argument (delimiters and special escaped characters follow the
lexical conventions of OCaml).

• c: reads a single character. To test the current input character without reading it, specify
a null field width, i.e. use specification %0c. Raise Invalid_argument, if the field width
specification is greater than 1.

• C: reads a single delimited character (delimiters and special escaped characters follow the
lexical conventions of OCaml).

• f, e, E, g, G: reads an optionally signed floating-point number in decimal notation, in the style

dddd.ddd e/E+-dd.

• h, H: reads an optionally signed floating-point number in hexadecimal notation.

• F: reads a floating point number according to the lexical conventions of OCaml (hence the
decimal point is mandatory if the exponent part is not mentioned).

• B: reads a boolean argument (true or false).

• b: reads a boolean argument (for backward compatibility; do not use in new programs).

• ld, li, lu, lx, lX, lo: reads an int32 argument to the format specified by the second letter
for regular integers.

• nd, ni, nu, nx, nX, no: reads a nativeint argument to the format specified by the second
letter for regular integers.

• Ld, Li, Lu, Lx, LX, Lo: reads an int64 argument to the format specified by the second letter
for regular integers.

• [ range ]: reads characters that matches one of the characters mentioned in the range of
characters range (or not mentioned in it, if the range starts with ^). Reads a string that can
be empty, if the next input character does not match the range. The set of characters from
c1 to c2 (inclusively) is denoted by c1-c2. Hence, %[0-9] returns a string representing a
decimal number or an empty string if no decimal digit is found; similarly, %[0-9a-f] returns
a string of hexadecimal digits. If a closing bracket appears in a range, it must occur as the
first character of the range (or just after the ^ in case of range negation); hence []] matches
a ] character and [^]] matches any character that is not ]. Use %% and %@ to include a % or
a @ in a range.

• r: user-defined reader. Takes the next ri formatted input function and applies it to the
scanning buffer ib to read the next argument. The input function ri must therefore have
type Scanning.in_channel -> 'a and the argument read has type 'a.

• { fmt %}: reads a format string argument. The format string read must have the same type
as the format string specification fmt. For instance, "%{ %i %}" reads any format string
that can read a value of type int; hence, if s is the string "fmt:\"number is %u\"", then
Scanf.sscanf s "fmt: %{%i%}" succeeds and returns the format string "number is %u".
Chapter 29. The standard library 853

• ( fmt %): scanning sub-format substitution. Reads a format string rf in the input, then goes
on scanning with rf instead of scanning with fmt. The format string rf must have the same
type as the format string specification fmt that it replaces. For instance, "%( %i %)" reads
any format string that can read a value of type int. The conversion returns the format string
read rf, and then a value read using rf. Hence, if s is the string "\"%4d\"1234.00", then
Scanf.sscanf s "%(%i%)" (fun fmt i -> fmt, i) evaluates to ("%4d", 1234). This be-
haviour is not mere format substitution, since the conversion returns the format string read as
additional argument. If you need pure format substitution, use special flag _ to discard the
extraneous argument: conversion %_( fmt %) reads a format string rf and then behaves the
same as format string rf. Hence, if s is the string "\"%4d\"1234.00", then Scanf.sscanf s
"%_(%i%)" is simply equivalent to Scanf.sscanf "1234.00" "%4d".

• l: returns the number of lines read so far.

• n: returns the number of characters read so far.

• N or L: returns the number of tokens read so far.

• !: matches the end of input condition.

• %: matches one % character in the input.

• @: matches one @ character in the input.

• ,: does nothing.

Following the % character that introduces a conversion, there may be the special flag _: the
conversion that follows occurs as usual, but the resulting value is discarded. For instance, if f is
the function fun i -> i + 1, and s is the string "x = 1", then Scanf.sscanf s "%_s = %i" f
returns 2.
The field width is composed of an optional integer literal indicating the maximal width of the
token to read. For instance, %6d reads an integer, having at most 6 decimal digits; %4f reads a float
with at most 4 characters; and %8[\000-\255] returns the next 8 characters (or all the characters
still available, if fewer than 8 characters are available in the input).
Notes:

• as mentioned above, a %s conversion always succeeds, even if there is nothing to read in the
input: in this case, it simply returns "".

• in addition to the relevant digits, '_' characters may appear inside numbers (this is reminiscent
to the usual OCaml lexical conventions). If stricter scanning is desired, use the range conversion
facility instead of the number conversions.

• the scanf facility is not intended for heavy duty lexical analysis and parsing. If it appears
not expressive enough for your needs, several alternative exists: regular expressions (module
Str[32.1]), stream parsers, ocamllex-generated lexers, ocamlyacc-generated parsers.
854

Scanning indications in format strings

Scanning indications appear just after the string conversions %s and %[ range ] to delimit the end
of the token. A scanning indication is introduced by a @ character, followed by some plain character
c. It means that the string token should end just before the next matching c (which is skipped). If
no c character is encountered, the string token spreads as much as possible. For instance, "%s@\t"
reads a string up to the next tab character or to the end of input. If a @ character appears anywhere
else in the format string, it is treated as a plain character.
Note:

• As usual in format strings, % and @ characters must be escaped using %% and %@; this rule
still holds within range specifications and scanning indications. For instance, format "%s@%%"
reads a string up to the next % character, and format "%s@%@" reads a string up to the next @.

• The scanning indications introduce slight differences in the syntax of Scanf[29.48] format
strings, compared to those used for the Printf[29.44] module. However, the scanning indica-
tions are similar to those used in the Format[29.22] module; hence, when producing formatted
text to be scanned by Scanf.bscanf[29.48], it is wise to use printing functions from the
Format[29.22] module (or, if you need to use functions from Printf[29.44], banish or carefully
double check the format strings that contain '@' characters).

Exceptions during scanning

Scanners may raise the following exceptions when the input cannot be read according to the format
string:

• Raise Scanf.Scan_failure[29.48] if the input does not match the format.

• Raise Failure if a conversion to a number is not possible.

• Raise End_of_file if the end of input is encountered while some more characters are needed
to read the current conversion specification.

• Raise Invalid_argument if the format string is invalid.

Note:

• as a consequence, scanning a %s conversion never raises exception End_of_file: if the end of

input is reached the conversion succeeds and simply returns the characters read so far, or ""
if none were ever read.

Specialised formatted input functions

val sscanf : string -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf[29.48], but reads from the given string.

val sscanf_opt : string -> ('a, 'b, 'c, 'd) scanner_opt

Chapter 29. The standard library 855

Same as Scanf.sscanf[29.48], but returns None in case of scanning failure.

Since: 5.0

val scanf : ('a, 'b, 'c, 'd) scanner

Same as Scanf.bscanf[29.48], but reads from the predefined formatted input channel
Scanf.Scanning.stdin[29.48] that is connected to stdin[28.2].

val scanf_opt : ('a, 'b, 'c, 'd) scanner_opt

Same as Scanf.scanf[29.48], but returns None in case of scanning failure.
Since: 5.0

val kscanf :
Scanning.in_channel ->
(Scanning.in_channel -> exn -> 'd) -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf[29.48], but takes an additional function argument ef that is called in
case of error: if the scanning process or some conversion fails, the scanning function aborts
and calls the error handling function ef with the formatted input channel and the exception
that aborted the scanning process as arguments.

val ksscanf :
string ->
(Scanning.in_channel -> exn -> 'd) -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.kscanf[29.48] but reads from the given string.
Since: 4.02

Reading format strings from input

val bscanf_format :
Scanning.in_channel ->
('a, 'b, 'c, 'd, 'e, 'f) format6 ->
(('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g
bscanf_format ic fmt f reads a format string token from the formatted input channel ic,
according to the given format string fmt, and applies f to the resulting format string value.
Since: 3.09
Raises Scan_failure if the format string value read does not have the same type as fmt.

val sscanf_format :
string ->
('a, 'b, 'c, 'd, 'e, 'f) format6 ->
(('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g
Same as Scanf.bscanf_format[29.48], but reads from the given string.
Since: 3.09
856

val format_from_string :
string ->
('a, 'b, 'c, 'd, 'e, 'f) format6 ->
('a, 'b, 'c, 'd, 'e, 'f) format6
format_from_string s fmt converts a string argument to a format string, according to the
given format string fmt.
Since: 3.10
Raises Scan_failure if s, considered as a format string, does not have the same type as fmt.

val unescaped : string -> string

unescaped s return a copy of s with escape sequences (according to the lexical conventions of
OCaml) replaced by their corresponding special characters. More precisely, Scanf.unescaped
has the following property: for all string s, Scanf.unescaped (String.escaped s) = s.
Always return a copy of the argument, even if there is no escape sequence in the argument.
Since: 4.00
Raises Scan_failure if s is not properly escaped (i.e. s has invalid escape sequences or
special characters that are not properly escaped). For instance, Scanf.unescaped "\"" will
fail.

29.49 Module Seq : Sequences.

A sequence of type 'a Seq.t can be thought of as a delayed list, that is, a list whose elements are
computed only when they are demanded by a consumer. This allows sequences to be produced and
transformed lazily (one element at a time) rather than eagerly (all elements at once). This also
allows constructing conceptually infinite sequences.
The type 'a Seq.t is defined as a synonym for unit -> 'a Seq.node. This is a function type:
therefore, it is opaque. The consumer can query a sequence in order to request the next element (if
there is one), but cannot otherwise inspect the sequence in any way.
Because it is opaque, the type 'a Seq.t does not reveal whether a sequence is:

• persistent, which means that the sequence can be used as many times as desired, producing
the same elements every time, just like an immutable list; or

• ephemeral, which means that the sequence is not persistent. Querying an ephemeral sequence
might have an observable side effect, such as incrementing a mutable counter. As a common
special case, an ephemeral sequence can be affine, which means that it must be queried at
most once.

It also does not reveal whether the elements of the sequence are:

• pre-computed and stored in memory, which means that querying the sequence is cheap;

• computed when first demanded and then stored in memory, which means that querying
the sequence once can be expensive, but querying the same sequence again is cheap; or
Chapter 29. The standard library 857

• re-computed every time they are demanded, which may or may not be cheap.

It is up to the programmer to keep these distinctions in mind so as to understand the time and
space requirements of sequences.
For the sake of simplicity, most of the documentation that follows is written under the implicit
assumption that the sequences at hand are persistent. We normally do not point out when or
how many times each function is invoked, because that would be too verbose. For instance, in the
description of map, we write: ”if xs is the sequence x0; x1; ... then map f xs is the sequence f
x0; f x1; ...”. If we wished to be more explicit, we could point out that the transformation takes
place on demand: that is, the elements of map f xs are computed only when they are demanded.
In other words, the definition let ys = map f xs terminates immediately and does not invoke f.
The function call f x0 takes place only when the first element of ys is demanded, via the function
call ys(). Furthermore, calling ys() twice causes f x0 to be called twice as well. If one wishes for
f to be applied at most once to each element of xs, even in scenarios where ys is queried more than
once, then one should use let ys = memoize (map f xs).
As a general rule, the functions that build sequences, such as map, filter, scan, take, etc.,
produce sequences whose elements are computed only on demand. The functions that eagerly
consume sequences, such as is_empty, find, length, iter, fold_left, etc., are the functions that
force computation to take place.
When possible, we recommend using sequences rather than dispensers (functions of type unit
-> 'a option that produce elements upon demand). Whereas sequences can be persistent or
ephemeral, dispensers are always ephemeral, and are typically more difficult to work with than
sequences. Two conversion functions, Seq.to_dispenser[29.49] and Seq.of_dispenser[29.49], are
provided.
Since: 4.07

type 'a t = unit -> 'a node

A sequence xs of type 'a t is a delayed list of elements of type 'a. Such a sequence is
queried by performing a function application xs(). This function application returns a node,
allowing the caller to determine whether the sequence is empty or nonempty, and in the latter
case, to obtain its head and tail.

type 'a node =

| Nil
| Cons of 'a * 'a t
A node is either Nil, which means that the sequence is empty, or Cons (x, xs), which
means that x is the first element of the sequence and that xs is the remainder of the sequence.

Consuming sequences
The functions in this section consume their argument, a sequence, either partially or completely:

• is_empty and uncons consume the sequence down to depth 1. That is, they demand the first
argument of the sequence, if there is one.
858

• iter, fold_left, length, etc., consume the sequence all the way to its end. They terminate
only if the sequence is finite.

• for_all, exists, find, etc. consume the sequence down to a certain depth, which is a priori
unpredictable.

Similarly, among the functions that consume two sequences, one can distinguish two groups:

• iter2 and fold_left2 consume both sequences all the way to the end, provided the sequences
have the same length.

• for_all2, exists2, equal, compare consume the sequences down to a certain depth, which
is a priori unpredictable.

The functions that consume two sequences can be applied to two sequences of distinct lengths:
in that case, the excess elements in the longer sequence are ignored. (It may be the case that one
excess element is demanded, even though this element is not used.)
None of the functions in this section is lazy. These functions are consumers: they force some
computation to take place.
val is_empty : 'a t -> bool
is_empty xs determines whether the sequence xs is empty.
It is recommended that the sequence xs be persistent. Indeed, is_empty xs demands the
head of the sequence xs, so, if xs is ephemeral, it may be the case that xs cannot be used any
more after this call has taken place.
Since: 4.14

val uncons : 'a t -> ('a * 'a t) option

If xs is empty, then uncons xs is None.
If xs is nonempty, then uncons xs is Some (x, ys) where x is the head of the sequence and
ys its tail.
Since: 4.14

val length : 'a t -> int

length xs is the length of the sequence xs.
The sequence xs must be finite.
Since: 4.14

val iter : ('a -> unit) -> 'a t -> unit

iter f xs invokes f x successively for every element x of the sequence xs, from left to right.
It terminates only if the sequence xs is finite.

val fold_left : ('acc -> 'a -> 'acc) -> 'acc -> 'a t -> 'acc
Chapter 29. The standard library 859

fold_left f _ xs invokes f _ x successively for every element x of the sequence xs, from
left to right.
An accumulator of type 'a is threaded through the calls to f.
It terminates only if the sequence xs is finite.

val iteri : (int -> 'a -> unit) -> 'a t -> unit
iteri f xs invokes f i x successively for every element x located at index i in the sequence
xs.
It terminates only if the sequence xs is finite.
iteri f xs is equivalent to iter (fun (i, x) -> f i x) (zip (ints 0) xs).
Since: 4.14

val fold_lefti : ('acc -> int -> 'a -> 'acc) -> 'acc -> 'a t -> 'acc
fold_lefti f _ xs invokes f _ i x successively for every element x located at index i of
the sequence xs.
An accumulator of type 'b is threaded through the calls to f.
It terminates only if the sequence xs is finite.
fold_lefti f accu xs is equivalent to fold_left (fun accu (i, x) -> f accu i x)
accu (zip (ints 0) xs).
Since: 4.14

val for_all : ('a -> bool) -> 'a t -> bool

for_all p xs determines whether all elements x of the sequence xs satisfy p x.
The sequence xs must be finite.
Since: 4.14

val exists : ('a -> bool) -> 'a t -> bool

exists xs p determines whether at least one element x of the sequence xs satisfies p x.
The sequence xs must be finite.
Since: 4.14

val find : ('a -> bool) -> 'a t -> 'a option
find p xs returns Some x, where x is the first element of the sequence xs that satisfies p x,
if there is such an element.
It returns None if there is no such element.
The sequence xs must be finite.
Since: 4.14

val find_index : ('a -> bool) -> 'a t -> int option
860

find_index p xs returns Some i, where i is the index of the first element of the sequence
xs that satisfies p x, if there is such an element.
It returns None if there is no such element.
The sequence xs must be finite.
Since: 5.1

val find_map : ('a -> 'b option) -> 'a t -> 'b option
find_map f xs returns Some y, where x is the first element of the sequence xs such that f x
= Some _, if there is such an element, and where y is defined by f x = Some y.
It returns None if there is no such element.
The sequence xs must be finite.
Since: 4.14

val find_mapi : (int -> 'a -> 'b option) -> 'a t -> 'b option
Same as find_map, but the predicate is applied to the index of the element as first argument
(counting from 0), and the element itself as second argument.
The sequence xs must be finite.
Since: 5.1

val iter2 : ('a -> 'b -> unit) -> 'a t -> 'b t -> unit
iter2 f xs ys invokes f x y successively for every pair (x, y) of elements drawn
synchronously from the sequences xs and ys.
If the sequences xs and ys have different lengths, then iteration stops as soon as one sequence
is exhausted; the excess elements in the other sequence are ignored.
Iteration terminates only if at least one of the sequences xs and ys is finite.
iter2 f xs ys is equivalent to iter (fun (x, y) -> f x y) (zip xs ys).
Since: 4.14

val fold_left2 : ('acc -> 'a -> 'b -> 'acc) -> 'acc -> 'a t -> 'b t -> 'acc
fold_left2 f _ xs ys invokes f _ x y successively for every pair (x, y) of elements
drawn synchronously from the sequences xs and ys.
An accumulator of type 'a is threaded through the calls to f.
If the sequences xs and ys have different lengths, then iteration stops as soon as one sequence
is exhausted; the excess elements in the other sequence are ignored.
Iteration terminates only if at least one of the sequences xs and ys is finite.
fold_left2 f accu xs ys is equivalent to fold_left (fun accu (x, y) -> f accu x
y) (zip xs ys).
Since: 4.14

val for_all2 : ('a -> 'b -> bool) -> 'a t -> 'b t -> bool
Chapter 29. The standard library 861

for_all2 p xs ys determines whether all pairs (x, y) of elements drawn synchronously

from the sequences xs and ys satisfy p x y.
If the sequences xs and ys have different lengths, then iteration stops as soon as one sequence
is exhausted; the excess elements in the other sequence are ignored. In particular, if xs or ys
is empty, then for_all2 p xs ys is true. This is where for_all2 and equal differ: equal
eq xs ys can be true only if xs and ys have the same length.
At least one of the sequences xs and ys must be finite.
for_all2 p xs ys is equivalent to for_all (fun b -> b) (map2 p xs ys).
Since: 4.14

val exists2 : ('a -> 'b -> bool) -> 'a t -> 'b t -> bool
exists2 p xs ys determines whether some pair (x, y) of elements drawn synchronously
from the sequences xs and ys satisfies p x y.
If the sequences xs and ys have different lengths, then iteration must stop as soon as one
sequence is exhausted; the excess elements in the other sequence are ignored.
At least one of the sequences xs and ys must be finite.
exists2 p xs ys is equivalent to exists (fun b -> b) (map2 p xs ys).
Since: 4.14

val equal : ('a -> 'b -> bool) -> 'a t -> 'b t -> bool
Provided the function eq defines an equality on elements, equal eq xs ys determines
whether the sequences xs and ys are pointwise equal.
At least one of the sequences xs and ys must be finite.
Since: 4.14

val compare : ('a -> 'b -> int) -> 'a t -> 'b t -> int
Provided the function cmp defines a preorder on elements, compare cmp xs ys compares the
sequences xs and ys according to the lexicographic preorder.
For more details on comparison functions, see Array.sort[29.2].
At least one of the sequences xs and ys must be finite.
Since: 4.14

Constructing sequences
The functions in this section are lazy: that is, they return sequences whose elements are computed
only when demanded.
val empty : 'a t
empty is the empty sequence. It has no elements. Its length is 0.

val return : 'a -> 'a t

862

return x is the sequence whose sole element is x. Its length is 1.

val cons : 'a -> 'a t -> 'a t

cons x xs is the sequence that begins with the element x, followed with the sequence xs.
Writing cons (f()) xs causes the function call f() to take place immediately. For this call
to be delayed until the sequence is queried, one must instead write (fun () -> Cons(f(),
xs)).
Since: 4.11

val init : int -> (int -> 'a) -> 'a t

init n f is the sequence f 0; f 1; ...; f (n-1).
n must be nonnegative.
If desired, the infinite sequence f 0; f 1; ... can be defined as map f (ints 0).
Since: 4.14
Raises Invalid_argument if n is negative.

val unfold : ('b -> ('a * 'b) option) -> 'b -> 'a t
unfold constructs a sequence out of a step function and an initial state.
If f u is None then unfold f u is the empty sequence. If f u is Some (x, u') then unfold
f u is the nonempty sequence cons x (unfold f u').
For example, unfold (function [] -> None | h :: t -> Some (h, t)) l is equivalent
to List.to_seq l.
Since: 4.11

val repeat : 'a -> 'a t

repeat x is the infinite sequence where the element x is repeated indefinitely.
repeat x is equivalent to cycle (return x).
Since: 4.14

val forever : (unit -> 'a) -> 'a t

forever f is an infinite sequence where every element is produced (on demand) by the
function call f().
For instance, forever Random.bool is an infinite sequence of random bits.
forever f is equivalent to map f (repeat ()).
Since: 4.14

val cycle : 'a t -> 'a t

Chapter 29. The standard library 863

cycle xs is the infinite sequence that consists of an infinite number of repetitions of the
sequence xs.
If xs is an empty sequence, then cycle xs is empty as well.
Consuming (a prefix of) the sequence cycle xs once can cause the sequence xs to be
consumed more than once. Therefore, xs must be persistent.
Since: 4.14

val iterate : ('a -> 'a) -> 'a -> 'a t

iterate f x is the infinite sequence whose elements are x, f x, f (f x), and so on.
In other words, it is the orbit of the function f, starting at x.
Since: 4.14

Transforming sequences
The functions in this section are lazy: that is, they return sequences whose elements are computed
only when demanded.
val map : ('a -> 'b) -> 'a t -> 'b t
map f xs is the image of the sequence xs through the transformation f.
If xs is the sequence x0; x1; ... then map f xs is the sequence f x0; f x1; ....

val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
mapi is analogous to map, but applies the function f to an index and an element.
mapi f xs is equivalent to map2 f (ints 0) xs.
Since: 4.14

val filter : ('a -> bool) -> 'a t -> 'a t

filter p xs is the sequence of the elements x of xs that satisfy p x.
In other words, filter p xs is the sequence xs, deprived of the elements x such that p x is
false.

val filter_map : ('a -> 'b option) -> 'a t -> 'b t
filter_map f xs is the sequence of the elements y such that f x = Some y, where x ranges
over xs.
filter_map f xs is equivalent to map Option.get (filter Option.is_some (map f
xs)).

val scan : ('b -> 'a -> 'b) -> 'b -> 'a t -> 'b t
If xs is a sequence [x0; x1; x2; ...], then scan f a0 xs is a sequence of accumulators
[a0; a1; a2; ...] where a1 is f a0 x0, a2 is f a1 x1, and so on.
864

Thus, scan f a0 xs is conceptually related to fold_left f a0 xs. However, instead of

performing an eager iteration and immediately returning the final accumulator, it returns a
sequence of accumulators.
For instance, scan (+) 0 transforms a sequence of integers into the sequence of its partial
sums.
If xs has length n then scan f a0 xs has length n+1.
Since: 4.14

val take : int -> 'a t -> 'a t

take n xs is the sequence of the first n elements of xs.
If xs has fewer than n elements, then take n xs is equivalent to xs.
n must be nonnegative.
Since: 4.14
Raises Invalid_argument if n is negative.

val drop : int -> 'a t -> 'a t

drop n xs is the sequence xs, deprived of its first n elements.
If xs has fewer than n elements, then drop n xs is empty.
n must be nonnegative.
drop is lazy: the first n+1 elements of the sequence xs are demanded only when the first
element of drop n xs is demanded.
Since: 4.14
Raises Invalid_argument if n is negative.

val take_while : ('a -> bool) -> 'a t -> 'a t

take_while p xs is the longest prefix of the sequence xs where every element x satisfies p x.
Since: 4.14

val drop_while : ('a -> bool) -> 'a t -> 'a t

drop_while p xs is the sequence xs, deprived of the prefix take_while p xs.
Since: 4.14

val group : ('a -> 'a -> bool) -> 'a t -> 'a t t
Provided the function eq defines an equality on elements, group eq xs is the sequence of the
maximal runs of adjacent duplicate elements of the sequence xs.
Every element of group eq xs is a nonempty sequence of equal elements.
The concatenation concat (group eq xs) is equal to xs.
Consuming group eq xs, and consuming the sequences that it contains, can cause xs to be
consumed more than once. Therefore, xs must be persistent.
Since: 4.14
Chapter 29. The standard library 865

val memoize : 'a t -> 'a t

The sequence memoize xs has the same elements as the sequence xs.
Regardless of whether xs is ephemeral or persistent, memoize xs is persistent: even if it is
queried several times, xs is queried at most once.
The construction of the sequence memoize xs internally relies on suspensions provided by the
module Lazy[29.30]. These suspensions are not thread-safe. Therefore, the sequence memoize
xs must not be queried by multiple threads concurrently.
Since: 4.14

exception Forced_twice
This exception is raised when a sequence returned by Seq.once[29.49] (or a suffix of it) is
queried more than once.
Since: 4.14

val once : 'a t -> 'a t

The sequence once xs has the same elements as the sequence xs.
Regardless of whether xs is ephemeral or persistent, once xs is an ephemeral sequence: it
can be queried at most once. If it (or a suffix of it) is queried more than once, then the
exception Forced_twice is raised. This can be useful, while debugging or testing, to ensure
that a sequence is consumed at most once.
Since: 4.14
Raises Forced_twice if once xs, or a suffix of it, is queried more than once.

val transpose : 'a t t -> 'a t t

If xss is a matrix (a sequence of rows), then transpose xss is the sequence of the columns
of the matrix xss.
The rows of the matrix xss are not required to have the same length.
The matrix xss is not required to be finite (in either direction).
The matrix xss must be persistent.
Since: 4.14

Combining sequences
val append : 'a t -> 'a t -> 'a t
append xs ys is the concatenation of the sequences xs and ys.
Its elements are the elements of xs, followed by the elements of ys.
Since: 4.11

val concat : 'a t t -> 'a t

866

If xss is a sequence of sequences, then concat xss is its concatenation.

If xss is the sequence xs0; xs1; ... then concat xss is the sequence xs0 @ xs1 @ ....
Since: 4.13

val flat_map : ('a -> 'b t) -> 'a t -> 'b t

flat_map f xs is equivalent to concat (map f xs).

val concat_map : ('a -> 'b t) -> 'a t -> 'b t

concat_map f xs is equivalent to concat (map f xs).
concat_map is an alias for flat_map.
Since: 4.13

val zip : 'a t -> 'b t -> ('a * 'b) t

zip xs ys is the sequence of pairs (x, y) drawn synchronously from the sequences xs and
ys.
If the sequences xs and ys have different lengths, then the sequence ends as soon as one
sequence is exhausted; the excess elements in the other sequence are ignored.
zip xs ys is equivalent to map2 (fun a b -> (a, b)) xs ys.
Since: 4.14

val map2 : ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t
map2 f xs ys is the sequence of the elements f x y, where the pairs (x, y) are drawn
synchronously from the sequences xs and ys.
If the sequences xs and ys have different lengths, then the sequence ends as soon as one
sequence is exhausted; the excess elements in the other sequence are ignored.
map2 f xs ys is equivalent to map (fun (x, y) -> f x y) (zip xs ys).
Since: 4.14

val interleave : 'a t -> 'a t -> 'a t

interleave xs ys is the sequence that begins with the first element of xs, continues with
the first element of ys, and so on.
When one of the sequences xs and ys is exhausted, interleave xs ys continues with the
rest of the other sequence.
Since: 4.14

val sorted_merge : ('a -> 'a -> int) -> 'a t -> 'a t -> 'a t
If the sequences xs and ys are sorted according to the total preorder cmp, then sorted_merge
cmp xs ys is the sorted sequence obtained by merging the sequences xs and ys.
For more details on comparison functions, see Array.sort[29.2].
Since: 4.14
Chapter 29. The standard library 867

val product : 'a t -> 'b t -> ('a * 'b) t

product xs ys is the Cartesian product of the sequences xs and ys.
For every element x of xs and for every element y of ys, the pair (x, y) appears once as an
element of product xs ys.
The order in which the pairs appear is unspecified.
The sequences xs and ys are not required to be finite.
The sequences xs and ys must be persistent.
Since: 4.14

val map_product : ('a -> 'b -> 'c) -> 'a t -> 'b t -> 'c t
The sequence map_product f xs ys is the image through f of the Cartesian product of the
sequences xs and ys.
For every element x of xs and for every element y of ys, the element f x y appears once as
an element of map_product f xs ys.
The order in which these elements appear is unspecified.
The sequences xs and ys are not required to be finite.
The sequences xs and ys must be persistent.
map_product f xs ys is equivalent to map (fun (x, y) -> f x y) (product xs ys).
Since: 4.14

Splitting a sequence into two sequences

val unzip : ('a * 'b) t -> 'a t * 'b t
unzip transforms a sequence of pairs into a pair of sequences.
unzip xs is equivalent to (map fst xs, map snd xs).
Querying either of the sequences returned by unzip xs causes xs to be queried. Therefore,
querying both of them causes xs to be queried twice. Thus, xs must be persistent and cheap.
If that is not the case, use unzip (memoize xs).
Since: 4.14

val split : ('a * 'b) t -> 'a t * 'b t

split is an alias for unzip.
Since: 4.14

val partition_map : ('a -> ('b, 'c) Either.t) -> 'a t -> 'b t * 'c t
partition_map f xs returns a pair of sequences (ys, zs), where:

• ys is the sequence of the elements y such that f x = Left y, where x ranges over xs;

• zs is the sequence of the elements z such that f x = Right z, where x ranges over xs.
868

partition_map f xs is equivalent to a pair of filter_map Either.find_left (map f xs)

and filter_map Either.find_right (map f xs).
Querying either of the sequences returned by partition_map f xs causes xs to be queried.
Therefore, querying both of them causes xs to be queried twice. Thus, xs must be persistent
and cheap. If that is not the case, use partition_map f (memoize xs).
Since: 4.14

val partition : ('a -> bool) -> 'a t -> 'a t * 'a t
partition p xs returns a pair of the subsequence of the elements of xs that satisfy p and
the subsequence of the elements of xs that do not satisfy p.
partition p xs is equivalent to filter p xs, filter (fun x -> not (p x)) xs.
Consuming both of the sequences returned by partition p xs causes xs to be consumed
twice and causes the function f to be applied twice to each element of the list. Therefore, f
should be pure and cheap. Furthermore, xs should be persistent and cheap. If that is not the
case, use partition p (memoize xs).
Since: 4.14

Converting between sequences and dispensers

A dispenser is a representation of a sequence as a function of type unit -> 'a option. Every
time this function is invoked, it returns the next element of the sequence. When there are no more
elements, it returns None. A dispenser has mutable internal state, therefore is ephemeral: the
sequence that it represents can be consumed at most once.
val of_dispenser : (unit -> 'a option) -> 'a t
of_dispenser it is the sequence of the elements produced by the dispenser it. It is an
ephemeral sequence: it can be consumed at most once. If a persistent sequence is needed, use
memoize (of_dispenser it).
Since: 4.14

val to_dispenser : 'a t -> unit -> 'a option

to_dispenser xs is a fresh dispenser on the sequence xs.
This dispenser has mutable internal state, which is not protected by a lock; so, it must not be
used by several threads concurrently.
Since: 4.14

Sequences of integers
val ints : int -> int t
ints i is the infinite sequence of the integers beginning at i and counting up.
Since: 4.14
Chapter 29. The standard library 869

29.50 Module Set : Sets over ordered types.

This module implements the set data structure, given a total ordering function over the set elements.
All operations over sets are purely applicative (no side-effects). The implementation uses balanced
binary trees, and is therefore reasonably efficient: insertion and membership take time logarithmic
in the size of the set, for instance.
The Set.Make[29.50] functor constructs implementations for any type, given a compare function.
For instance:

module IntPairs =
struct
type t = int * int
let compare (x0,y0) (x1,y1) =
match Stdlib.compare x0 x1 with
0 -> Stdlib.compare y0 y1
| c -> c
end

module PairsSet = Set.Make(IntPairs)

let m = PairsSet.(empty |> add (2,3) |> add (5,7) |> add (11,13))

This creates a new module PairsSet, with a new type PairsSet.t of sets of int * int.

module type OrderedType =

sig
type t
The type of the set elements.

val compare : t -> t -> int

A total ordering function over the set elements. This is a two-argument function f such
that f e1 e2 is zero if the elements e1 and e2 are equal, f e1 e2 is strictly negative if
e1 is smaller than e2, and f e1 e2 is strictly positive if e1 is greater than e2. Example:
a suitable ordering function is the generic structural comparison function compare[28.2].

end

Input signature of the functor Set.Make[29.50].

module type S =
sig
870

Sets
type elt
The type of the set elements.

type t
The type of sets.

val empty : t
The empty set.

val add : elt -> t -> t

add x s returns a set containing all elements of s, plus x. If x was already in s, s is
returned unchanged (the result of the function is then physically equal to s).
Before 4.03 Physical equality was not ensured.

val singleton : elt -> t

singleton x returns the one-element set containing only x.

val remove : elt -> t -> t

remove x s returns a set containing all elements of s, except x. If x was not in s, s is
returned unchanged (the result of the function is then physically equal to s).
Before 4.03 Physical equality was not ensured.

val union : t -> t -> t

Set union.

val inter : t -> t -> t

Set intersection.

val disjoint : t -> t -> bool

Test if two sets are disjoint.
Since: 4.08

val diff : t -> t -> t

Set difference: diff s1 s2 contains the elements of s1 that are not in s2.

val cardinal : t -> int

Return the number of elements of a set.
Chapter 29. The standard library 871

Elements
val elements : t -> elt list
Return the list of all elements of the given set. The returned list is sorted in increasing
order with respect to the ordering Ord.compare, where Ord is the argument given to
Set.Make[29.50].

val min_elt : t -> elt

Return the smallest element of the given set (with respect to the Ord.compare ordering),
or raise Not_found if the set is empty.

val min_elt_opt : t -> elt option

Return the smallest element of the given set (with respect to the Ord.compare ordering),
or None if the set is empty.
Since: 4.05

val max_elt : t -> elt

Same as Set.S.min_elt[29.50], but returns the largest element of the given set.

val max_elt_opt : t -> elt option

Same as Set.S.min_elt_opt[29.50], but returns the largest element of the given set.
Since: 4.05

val choose : t -> elt

Return one element of the given set, or raise Not_found if the set is empty. Which
element is chosen is unspecified, but equal elements will be chosen for equal sets.

val choose_opt : t -> elt option

Return one element of the given set, or None if the set is empty. Which element is
chosen is unspecified, but equal elements will be chosen for equal sets.
Since: 4.05

Searching
val find : elt -> t -> elt
find x s returns the element of s equal to x (according to Ord.compare), or raise
Not_found if no such element exists.
Since: 4.01

val find_opt : elt -> t -> elt option

872

find_opt x s returns the element of s equal to x (according to Ord.compare), or None

if no such element exists.
Since: 4.05

val find_first : (elt -> bool) -> t -> elt

find_first f s, where f is a monotonically increasing function, returns the lowest
element e of s such that f e, or raises Not_found if no such element exists.
For example, find_first (fun e -> Ord.compare e x >= 0) s will return the first
element e of s where Ord.compare e x >= 0 (intuitively: e >= x), or raise Not_found
if x is greater than any element of s.
Since: 4.05

val find_first_opt : (elt -> bool) -> t -> elt option

find_first_opt f s, where f is a monotonically increasing function, returns an option
containing the lowest element e of s such that f e, or None if no such element exists.
Since: 4.05

val find_last : (elt -> bool) -> t -> elt

find_last f s, where f is a monotonically decreasing function, returns the highest
element e of s such that f e, or raises Not_found if no such element exists.
Since: 4.05

val find_last_opt : (elt -> bool) -> t -> elt option

find_last_opt f s, where f is a monotonically decreasing function, returns an option
containing the highest element e of s such that f e, or None if no such element exists.
Since: 4.05

Traversing
val iter : (elt -> unit) -> t -> unit
iter f s applies f in turn to all elements of s. The elements of s are presented to f in
increasing order with respect to the ordering over the type of the elements.

val fold : (elt -> 'acc -> 'acc) -> t -> 'acc -> 'acc
fold f s init computes (f xN ... (f x2 (f x1 init))...), where x1 ... xN are
the elements of s, in increasing order.
Chapter 29. The standard library 873

Transforming
val map : (elt -> elt) -> t -> t
map f s is the set whose elements are f a0,f a1. . . f aN, where a0,a1. . .aN are the
elements of s.
The elements are passed to f in increasing order with respect to the ordering over the
type of the elements.
If no element of s is changed by f, s is returned unchanged. (If each output of f is
physically equal to its input, the returned set is physically equal to s.)
Since: 4.04

val filter : (elt -> bool) -> t -> t

filter f s returns the set of all elements in s that satisfy predicate f. If f satisfies
every element in s, s is returned unchanged (the result of the function is then physically
equal to s).
Before 4.03 Physical equality was not ensured.

val filter_map : (elt -> elt option) -> t -> t

filter_map f s returns the set of all v such that f x = Some v for some element x of
s.
For example,
filter_map (fun n -> if n mod 2 = 0 then Some (n / 2) else None) s
is the set of halves of the even elements of s.
If no element of s is changed or dropped by f (if f x = Some x for each element x),
then s is returned unchanged: the result of the function is then physically equal to s.
Since: 4.11

val partition : (elt -> bool) -> t -> t * t

partition f s returns a pair of sets (s1, s2), where s1 is the set of all the elements
of s that satisfy the predicate f, and s2 is the set of all the elements of s that do not
satisfy f.

val split : elt -> t -> t * bool * t

split x s returns a triple (l, present, r), where l is the set of elements of s that
are strictly less than x; r is the set of elements of s that are strictly greater than x;
present is false if s contains no element equal to x, or true if s contains an element
equal to x.
874

Predicates and comparisons

val is_empty : t -> bool
Test whether a set is empty or not.

val mem : elt -> t -> bool

mem x s tests whether x belongs to the set s.

val equal : t -> t -> bool

equal s1 s2 tests whether the sets s1 and s2 are equal, that is, contain equal elements.

val compare : t -> t -> int

Total ordering between sets. Can be used as the ordering function for doing sets of sets.

val subset : t -> t -> bool

subset s1 s2 tests whether the set s1 is a subset of the set s2.

val for_all : (elt -> bool) -> t -> bool

for_all f s checks if all elements of the set satisfy the predicate f.

val exists : (elt -> bool) -> t -> bool

exists f s checks if at least one element of the set satisfies the predicate f.

Converting
val to_list : t -> elt list
to_list s is Set.S.elements[29.50] s.
Since: 5.1

val of_list : elt list -> t

of_list l creates a set from a list of elements. This is usually more efficient than
folding add over the list, except perhaps for lists with many duplicated elements.
Since: 4.02

val to_seq_from : elt -> t -> elt Seq.t

to_seq_from x s iterates on a subset of the elements of s in ascending order, from x or
above.
Since: 4.07

val to_seq : t -> elt Seq.t

Chapter 29. The standard library 875

Iterate on the whole set, in ascending order

Since: 4.07

val to_rev_seq : t -> elt Seq.t

Iterate on the whole set, in descending order
Since: 4.12

val add_seq : elt Seq.t -> t -> t

Add the given elements to the set, in order.
Since: 4.07

val of_seq : elt Seq.t -> t

Build a set from the given bindings
Since: 4.07

end
Output signature of the functor Set.Make[29.50].

module Make :
functor (Ord : OrderedType) -> S with type elt = Ord.t
Functor building an implementation of the set structure given a totally ordered type.

29.51 Module Semaphore : Semaphores

A semaphore is a thread synchronization device that can be used to control access to a shared
resource.
Two flavors of semaphores are provided: counting semaphores and binary semaphores.
Since: 4.12

Counting semaphores
A counting semaphore is a counter that can be accessed concurrently by several threads. The typical
use is to synchronize producers and consumers of a resource by counting how many units of the
resource are available.
The two basic operations on semaphores are:
• ”release” (also called ”V”, ”post”, ”up”, and ”signal”), which increments the value of the
counter. This corresponds to producing one more unit of the shared resource and making it
available to others.
• ”acquire” (also called ”P”, ”wait”, ”down”, and ”pend”), which waits until the counter is
greater than zero and decrements it. This corresponds to consuming one unit of the shared
resource.
876

module Counting :
sig
type t
The type of counting semaphores.

val make : int -> t

make n returns a new counting semaphore, with initial value n. The initial value n must
be nonnegative.
Raises Invalid_argument if n < 0

val release : t -> unit

release s increments the value of semaphore s. If other threads are waiting on s, one
of them is restarted. If the current value of s is equal to max_int, the value of the
semaphore is unchanged and a Sys_error exception is raised to signal overflow.
Raises Sys_error if the value of the semaphore would overflow max_int

val acquire : t -> unit

acquire s blocks the calling thread until the value of semaphore s is not zero, then
atomically decrements the value of s and returns.

val try_acquire : t -> bool

try_acquire s immediately returns false if the value of semaphore s is zero.
Otherwise, the value of s is atomically decremented and try_acquire s returns true.

val get_value : t -> int

get_value s returns the current value of semaphore s. The current value can be
modified at any time by concurrent Semaphore.Counting.release[29.51] and
Semaphore.Counting.acquire[29.51] operations. Hence, the get_value operation is
racy, and its result should only be used for debugging or informational messages.

end

Binary semaphores
Binary semaphores are a variant of counting semaphores where semaphores can only take two values,
0 and 1.
A binary semaphore can be used to control access to a single shared resource, with value 1
meaning ”resource is available” and value 0 meaning ”resource is unavailable”.
The ”release” operation of a binary semaphore sets its value to 1, and ”acquire” waits until the
value is 1 and sets it to 0.
A binary semaphore can be used instead of a mutex (see module Mutex[29.37]) when the mutex
discipline (of unlocking the mutex from the thread that locked it) is too restrictive. The ”acquire”
Chapter 29. The standard library 877

operation corresponds to locking the mutex, and the ”release” operation to unlocking it, but ”release”
can be performed in a thread different than the one that performed the ”acquire”. Likewise, it is
safe to release a binary semaphore that is already available.
module Binary :
sig
type t
The type of binary semaphores.

val make : bool -> t

make b returns a new binary semaphore. If b is true, the initial value of the semaphore
is 1, meaning ”available”. If b is false, the initial value of the semaphore is 0, meaning
”unavailable”.

val release : t -> unit

release s sets the value of semaphore s to 1, putting it in the ”available” state. If
other threads are waiting on s, one of them is restarted.

val acquire : t -> unit

acquire s blocks the calling thread until the semaphore s has value 1 (is available),
then atomically sets it to 0 and returns.

val try_acquire : t -> bool

try_acquire s immediately returns false if the semaphore s has value 0. If s has
value 1, its value is atomically set to 0 and try_acquire s returns true.

end

29.52 Module Stack : Last-in first-out stacks.

This module implements stacks (LIFOs), with in-place modification.
Alert unsynchronized_access. Unsynchronized accesses to stacks are a programming error.

Unsynchronized accesses
Unsynchronized accesses to a stack may lead to an invalid queue state. Thus, concurrent accesses
to stacks must be synchronized (for instance with a Mutex.t[29.37]).
type !'a t
The type of stacks containing elements of type 'a.

exception Empty
Raised when Stack.pop[29.52] or Stack.top[29.52] is applied to an empty stack.
878

val create : unit -> 'a t

Return a new stack, initially empty.

val push : 'a -> 'a t -> unit

push x s adds the element x at the top of stack s.

val pop : 'a t -> 'a

pop s removes and returns the topmost element in stack s, or raises Stack.Empty[29.52] if
the stack is empty.

val pop_opt : 'a t -> 'a option

pop_opt s removes and returns the topmost element in stack s, or returns None if the stack
is empty.
Since: 4.08

val drop : 'a t -> unit

drop s removes the topmost element in stack s, or raises Stack.Empty[29.52] if the stack is
empty.
Since: 5.1

val top : 'a t -> 'a

top s returns the topmost element in stack s, or raises Stack.Empty[29.52] if the stack is
empty.

val top_opt : 'a t -> 'a option

top_opt s returns the topmost element in stack s, or None if the stack is empty.
Since: 4.08

val clear : 'a t -> unit

Discard all elements from a stack.

val copy : 'a t -> 'a t

Return a copy of the given stack.

val is_empty : 'a t -> bool

Return true if the given stack is empty, false otherwise.

val length : 'a t -> int

Return the number of elements in a stack. Time complexity O(1)

val iter : ('a -> unit) -> 'a t -> unit

iter f s applies f in turn to all elements of s, from the element at the top of the stack to
the element at the bottom of the stack. The stack itself is unchanged.
Chapter 29. The standard library 879

val fold : ('acc -> 'a -> 'acc) -> 'acc -> 'a t -> 'acc
fold f accu s is (f (... (f (f accu x1) x2) ...) xn) where x1 is the top of the
stack, x2 the second element, and xn the bottom element. The stack is unchanged.
Since: 4.03

Stacks and Sequences

val to_seq : 'a t -> 'a Seq.t
Iterate on the stack, top to bottom. It is safe to modify the stack during iteration.
Since: 4.07

val add_seq : 'a t -> 'a Seq.t -> unit

Add the elements from the sequence on the top of the stack.
Since: 4.07

val of_seq : 'a Seq.t -> 'a t

Create a stack from the sequence.
Since: 4.07

29.53 Module StdLabels : Standard labeled libraries.

This meta-module provides versions of the StdLabels.Array[29.53], StdLabels.Bytes[29.53],
StdLabels.List[29.53] and StdLabels.String[29.53] modules where function arguments are sys-
tematically labeled. It is intended to be opened at the top of source files, as shown below.

open StdLabels

let to_upper = String.map ~f:Char.uppercase_ascii

let seq len = List.init ~f:(fun i -> i) ~len
let everything = Array.create_matrix ~dimx:42 ~dimy:42 42

module Array :
ArrayLabels
module Bytes :
BytesLabels
module List :
ListLabels
module String :
StringLabels
880

29.54 Module String : Strings.

A string s of length n is an indexable and immutable sequence of n bytes. For historical reasons
these bytes are referred to as characters.
The semantics of string functions is defined in terms of indices and positions. These are depicted
and described as follows.

positions 0 1 2 3 4 n-1 n
+---+---+---+---+ +-----+
indices | 0 | 1 | 2 | 3 | ... | n-1 |
+---+---+---+---+ +-----+

• An index i of s is an integer in the range [0;n-1]. It represents the ith byte (character) of s
which can be accessed using the constant time string indexing operator s.[i].
• A position i of s is an integer in the range [0;n]. It represents either the point at the beginning
of the string, or the point between two indices, or the point at the end of the string. The ith
byte index is between position i and i+1.

Two integers start and len are said to define a valid substring of s if len >= 0 and start,
start+len are positions of s.
Unicode text. Strings being arbitrary sequences of bytes, they can hold any kind of textual
encoding. However the recommended encoding for storing Unicode text in OCaml strings is UTF-8.
This is the encoding used by Unicode escapes in string literals. For example the string "\u{1F42B}"
is the UTF-8 encoding of the Unicode character U+1F42B.
Past mutability. Before OCaml 4.02, strings used to be modifiable in place like Bytes.t[29.8]
mutable sequences of bytes. OCaml 4 had various compiler flags and configuration options to
support the transition period from mutable to immutable strings. Those options are no longer
available, and strings are now always immutable.
The labeled version of this module can be used as described in the StdLabels[29.53] module.

Strings
type t = string
The type for strings.

val make : int -> char -> string

make n c is a string of length n with each index holding the character c.
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val init : int -> (int -> char) -> string

init n f is a string of length n with index i holding the character f i (called in increasing
index order).
Since: 4.02
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].
Chapter 29. The standard library 881

val empty : string

The empty string.
Since: 4.13

val length : string -> int

length s is the length (number of bytes/characters) of s.

val get : string -> int -> char

get s i is the character at index i in s. This is the same as writing s.[i].
Raises Invalid_argument if i not an index of s.

val of_bytes : bytes -> string

Return a new string that contains the same bytes as the given byte sequence.
Since: 4.13

val to_bytes : string -> bytes

Return a new byte sequence that contains the same bytes as the given string.
Since: 4.13

val blit : string -> int -> bytes -> int -> int -> unit
Same as Bytes.blit_string[29.8] which should be preferred.

Concatenating
Note. The (^)[28.2] binary operator concatenates two strings.
val concat : string -> string list -> string
concat sep ss concatenates the list of strings ss, inserting the separator string sep between
each.
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val cat : string -> string -> string

cat s1 s2 concatenates s1 and s2 (s1 ^ s2).
Since: 4.13
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.
882

Predicates and comparisons

val equal : t -> t -> bool
equal s0 s1 is true if and only if s0 and s1 are character-wise equal.
Since: 4.03 (4.05 in StringLabels)

val compare : t -> t -> int

compare s0 s1 sorts s0 and s1 in lexicographical order. compare behaves like compare[28.2]
on strings but may be more efficient.

val starts_with : prefix:string -> string -> bool

starts_with ~prefix s is true if and only if s starts with prefix.
Since: 4.13

val ends_with : suffix:string -> string -> bool

ends_with ~suffix s is true if and only if s ends with suffix.
Since: 4.13

val contains_from : string -> int -> char -> bool

contains_from s start c is true if and only if c appears in s after position start.
Raises Invalid_argument if start is not a valid position in s.

val rcontains_from : string -> int -> char -> bool

rcontains_from s stop c is true if and only if c appears in s before position stop+1.
Raises Invalid_argument if stop < 0 or stop+1 is not a valid position in s.

val contains : string -> char -> bool

contains s c is String.contains_from[29.54] s 0 c.

Extracting substrings
val sub : string -> int -> int -> string
sub s pos len is a string of length len, containing the substring of s that starts at position
pos and has length len.
Raises Invalid_argument if pos and len do not designate a valid substring of s.

val split_on_char : char -> string -> string list

split_on_char sep s is the list of all (possibly empty) substrings of s that are delimited by
the character sep. If s is empty, the result is the singleton list [""].
The function’s result is specified by the following invariants:

• The list is not empty.

Chapter 29. The standard library 883

• Concatenating its elements using sep as a separator returns a string equal to the input
(concat (make 1 sep) (split_on_char sep s) = s).
• No string in the result contains the sep character.

Since: 4.04 (4.05 in StringLabels)

Transforming
val map : (char -> char) -> string -> string
map f s is the string resulting from applying f to all the characters of s in increasing order.
Since: 4.00

val mapi : (int -> char -> char) -> string -> string
mapi f s is like String.map[29.54] but the index of the character is also passed to f.
Since: 4.02

val fold_left : ('acc -> char -> 'acc) -> 'acc -> string -> 'acc
fold_left f x s computes f (... (f (f x s.[0]) s.[1]) ...) s.[n-1], where n is
the length of the string s.
Since: 4.13

val fold_right : (char -> 'acc -> 'acc) -> string -> 'acc -> 'acc
fold_right f s x computes f s.[0] (f s.[1] ( ... (f s.[n-1] x) ...)), where n is
the length of the string s.
Since: 4.13

val for_all : (char -> bool) -> string -> bool

for_all p s checks if all characters in s satisfy the predicate p.
Since: 4.13

val exists : (char -> bool) -> string -> bool

exists p s checks if at least one character of s satisfies the predicate p.
Since: 4.13

val trim : string -> string

trim s is s without leading and trailing whitespace. Whitespace characters are: ' ', '\x0C'
(form feed), '\n', '\r', and '\t'.
Since: 4.00

val escaped : string -> string

884

escaped s is s with special characters represented by escape sequences, following the lexical
conventions of OCaml.
All characters outside the US-ASCII printable range [0x20;0x7E] are escaped, as well as
backslash (0x2F) and double-quote (0x22).
The function Scanf.unescaped[29.48] is a left inverse of escaped, i.e. Scanf.unescaped
(escaped s) = s for any string s (unless escaped s fails).
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val uppercase_ascii : string -> string

uppercase_ascii s is s with all lowercase letters translated to uppercase, using the
US-ASCII character set.
Since: 4.03 (4.05 in StringLabels)

val lowercase_ascii : string -> string

lowercase_ascii s is s with all uppercase letters translated to lowercase, using the
US-ASCII character set.
Since: 4.03 (4.05 in StringLabels)

val capitalize_ascii : string -> string

capitalize_ascii s is s with the first character set to uppercase, using the US-ASCII
character set.
Since: 4.03 (4.05 in StringLabels)

val uncapitalize_ascii : string -> string

uncapitalize_ascii s is s with the first character set to lowercase, using the US-ASCII
character set.
Since: 4.03 (4.05 in StringLabels)

Traversing
val iter : (char -> unit) -> string -> unit
iter f s applies function f in turn to all the characters of s. It is equivalent to f s.[0]; f
s.[1]; ...; f s.[length s - 1]; ().

val iteri : (int -> char -> unit) -> string -> unit
iteri is like String.iter[29.54], but the function is also given the corresponding character
index.
Since: 4.00
Chapter 29. The standard library 885

Searching
val index_from : string -> int -> char -> int
index_from s i c is the index of the first occurrence of c in s after position i.
Raises

• Not_found if c does not occur in s after position i.

• Invalid_argument if i is not a valid position in s.

val index_from_opt : string -> int -> char -> int option
index_from_opt s i c is the index of the first occurrence of c in s after position i (if any).
Since: 4.05
Raises Invalid_argument if i is not a valid position in s.

val rindex_from : string -> int -> char -> int

rindex_from s i c is the index of the last occurrence of c in s before position i+1.
Raises

• Not_found if c does not occur in s before position i+1.

• Invalid_argument if i+1 is not a valid position in s.

val rindex_from_opt : string -> int -> char -> int option
rindex_from_opt s i c is the index of the last occurrence of c in s before position i+1 (if
any).
Since: 4.05
Raises Invalid_argument if i+1 is not a valid position in s.

val index : string -> char -> int

index s c is String.index_from[29.54] s 0 c.

val index_opt : string -> char -> int option

index_opt s c is String.index_from_opt[29.54] s 0 c.
Since: 4.05

val rindex : string -> char -> int

rindex s c is String.rindex_from[29.54] s (length s - 1) c.

val rindex_opt : string -> char -> int option

rindex_opt s c is String.rindex_from_opt[29.54] s (length s - 1) c.
Since: 4.05
886

Strings and Sequences

val to_seq : t -> char Seq.t
to_seq s is a sequence made of the string’s characters in increasing order.
Since: 4.07

val to_seqi : t -> (int * char) Seq.t

to_seqi s is like String.to_seq[29.54] but also tuples the corresponding index.
Since: 4.07

val of_seq : char Seq.t -> t

of_seq s is a string made of the sequence’s characters.
Since: 4.07

UTF decoding and validations

UTF-8
val get_utf_8_uchar : t -> int -> Uchar.utf_decode
get_utf_8_uchar b i decodes an UTF-8 character at index i in b.

val is_valid_utf_8 : t -> bool

is_valid_utf_8 b is true if and only if b contains valid UTF-8 data.

UTF-16BE
val get_utf_16be_uchar : t -> int -> Uchar.utf_decode
get_utf_16be_uchar b i decodes an UTF-16BE character at index i in b.

val is_valid_utf_16be : t -> bool

is_valid_utf_16be b is true if and only if b contains valid UTF-16BE data.

UTF-16LE
val get_utf_16le_uchar : t -> int -> Uchar.utf_decode
get_utf_16le_uchar b i decodes an UTF-16LE character at index i in b.

val is_valid_utf_16le : t -> bool

is_valid_utf_16le b is true if and only if b contains valid UTF-16LE data.
Chapter 29. The standard library 887

Binary decoding of integers

The functions in this section binary decode integers from strings.
All following functions raise Invalid_argument if the characters needed at index i to decode
the integer are not available.
Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are
stored first. Big-endian is also known as network byte order. Native-endian encoding is either
little-endian or big-endian depending on Sys.big_endian[29.56].
32-bit and 64-bit integers are represented by the int32 and int64 types, which can be interpreted
either as signed or unsigned numbers.
8-bit and 16-bit integers are represented by the int type, which has more bits than the binary
encoding. These extra bits are sign-extended (or zero-extended) for functions which decode 8-bit or
16-bit integers and represented them with int values.
val get_uint8 : string -> int -> int
get_uint8 b i is b’s unsigned 8-bit integer starting at character index i.
Since: 4.13

val get_int8 : string -> int -> int

get_int8 b i is b’s signed 8-bit integer starting at character index i.
Since: 4.13

val get_uint16_ne : string -> int -> int

get_uint16_ne b i is b’s native-endian unsigned 16-bit integer starting at character index i.
Since: 4.13

val get_uint16_be : string -> int -> int

get_uint16_be b i is b’s big-endian unsigned 16-bit integer starting at character index i.
Since: 4.13

val get_uint16_le : string -> int -> int

get_uint16_le b i is b’s little-endian unsigned 16-bit integer starting at character index i.
Since: 4.13

val get_int16_ne : string -> int -> int

get_int16_ne b i is b’s native-endian signed 16-bit integer starting at character index i.
Since: 4.13

val get_int16_be : string -> int -> int

get_int16_be b i is b’s big-endian signed 16-bit integer starting at character index i.
Since: 4.13

val get_int16_le : string -> int -> int

888

get_int16_le b i is b’s little-endian signed 16-bit integer starting at character index i.

Since: 4.13

val get_int32_ne : string -> int -> int32

get_int32_ne b i is b’s native-endian 32-bit integer starting at character index i.
Since: 4.13

val hash : t -> int

An unseeded hash function for strings, with the same output value as Hashtbl.hash[29.25].
This function allows this module to be passed as argument to the functor
Hashtbl.Make[29.25].
Since: 5.0

val seeded_hash : int -> t -> int

A seeded hash function for strings, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.0

val get_int32_be : string -> int -> int32

get_int32_be b i is b’s big-endian 32-bit integer starting at character index i.
Since: 4.13

val get_int32_le : string -> int -> int32

get_int32_le b i is b’s little-endian 32-bit integer starting at character index i.
Since: 4.13

val get_int64_ne : string -> int -> int64

get_int64_ne b i is b’s native-endian 64-bit integer starting at character index i.
Since: 4.13

val get_int64_be : string -> int -> int64

get_int64_be b i is b’s big-endian 64-bit integer starting at character index i.
Since: 4.13

val get_int64_le : string -> int -> int64

get_int64_le b i is b’s little-endian 64-bit integer starting at character index i.
Since: 4.13
Chapter 29. The standard library 889

29.55 Module StringLabels : Strings.

A string s of length n is an indexable and immutable sequence of n bytes. For historical reasons
these bytes are referred to as characters.
The semantics of string functions is defined in terms of indices and positions. These are depicted
and described as follows.

positions 0 1 2 3 4 n-1 n
+---+---+---+---+ +-----+
indices | 0 | 1 | 2 | 3 | ... | n-1 |
+---+---+---+---+ +-----+

• An index i of s is an integer in the range [0;n-1]. It represents the ith byte (character) of s
which can be accessed using the constant time string indexing operator s.[i].
• A position i of s is an integer in the range [0;n]. It represents either the point at the beginning
of the string, or the point between two indices, or the point at the end of the string. The ith
byte index is between position i and i+1.

Two integers start and len are said to define a valid substring of s if len >= 0 and start,
start+len are positions of s.
Unicode text. Strings being arbitrary sequences of bytes, they can hold any kind of textual
encoding. However the recommended encoding for storing Unicode text in OCaml strings is UTF-8.
This is the encoding used by Unicode escapes in string literals. For example the string "\u{1F42B}"
is the UTF-8 encoding of the Unicode character U+1F42B.
Past mutability. Before OCaml 4.02, strings used to be modifiable in place like Bytes.t[29.8]
mutable sequences of bytes. OCaml 4 had various compiler flags and configuration options to
support the transition period from mutable to immutable strings. Those options are no longer
available, and strings are now always immutable.
The labeled version of this module can be used as described in the StdLabels[29.53] module.

Strings
type t = string
The type for strings.

val make : int -> char -> string

make n c is a string of length n with each index holding the character c.
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].

val init : int -> f:(int -> char) -> string

init n ~f is a string of length n with index i holding the character f i (called in increasing
index order).
Since: 4.02
Raises Invalid_argument if n < 0 or n > Sys.max_string_length[29.56].
890

val empty : string

The empty string.
Since: 4.13

val length : string -> int

length s is the length (number of bytes/characters) of s.

val get : string -> int -> char

get s i is the character at index i in s. This is the same as writing s.[i].
Raises Invalid_argument if i not an index of s.

val of_bytes : bytes -> string

Return a new string that contains the same bytes as the given byte sequence.
Since: 4.13

val to_bytes : string -> bytes

Return a new byte sequence that contains the same bytes as the given string.
Since: 4.13

val blit :
src:string -> src_pos:int -> dst:bytes -> dst_pos:int -> len:int -> unit
Same as Bytes.blit_string[29.8] which should be preferred.

Concatenating
Note. The (^)[28.2] binary operator concatenates two strings.
val concat : sep:string -> string list -> string
concat ~sep ss concatenates the list of strings ss, inserting the separator string sep
between each.
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val cat : string -> string -> string

cat s1 s2 concatenates s1 and s2 (s1 ^ s2).
Since: 4.13
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.
Chapter 29. The standard library 891

Predicates and comparisons

val equal : t -> t -> bool
equal s0 s1 is true if and only if s0 and s1 are character-wise equal.
Since: 4.05

val compare : t -> t -> int

compare s0 s1 sorts s0 and s1 in lexicographical order. compare behaves like compare[28.2]
on strings but may be more efficient.

val starts_with : prefix:string -> string -> bool

starts_with ~prefix s is true if and only if s starts with prefix.
Since: 4.13

val ends_with : suffix:string -> string -> bool

ends_with ~suffix s is true if and only if s ends with suffix.
Since: 4.13

val contains_from : string -> int -> char -> bool

contains_from s start c is true if and only if c appears in s after position start.
Raises Invalid_argument if start is not a valid position in s.

val rcontains_from : string -> int -> char -> bool

rcontains_from s stop c is true if and only if c appears in s before position stop+1.
Raises Invalid_argument if stop < 0 or stop+1 is not a valid position in s.

val contains : string -> char -> bool

contains s c is String.contains_from[29.54] s 0 c.

Extracting substrings
val sub : string -> pos:int -> len:int -> string
sub s ~pos ~len is a string of length len, containing the substring of s that starts at
position pos and has length len.
Raises Invalid_argument if pos and len do not designate a valid substring of s.

val split_on_char : sep:char -> string -> string list

split_on_char ~sep s is the list of all (possibly empty) substrings of s that are delimited
by the character sep. If s is empty, the result is the singleton list [""].
The function’s result is specified by the following invariants:

• The list is not empty.

892

• Concatenating its elements using sep as a separator returns a string equal to the input
(concat (make 1 sep) (split_on_char sep s) = s).
• No string in the result contains the sep character.

Since: 4.05

Transforming
val map : f:(char -> char) -> string -> string
map f s is the string resulting from applying f to all the characters of s in increasing order.
Since: 4.00

val mapi : f:(int -> char -> char) -> string -> string
mapi ~f s is like StringLabels.map[29.55] but the index of the character is also passed to f.
Since: 4.02

val fold_left : f:('acc -> char -> 'acc) -> init:'acc -> string -> 'acc
fold_left f x s computes f (... (f (f x s.[0]) s.[1]) ...) s.[n-1], where n is
the length of the string s.
Since: 4.13

val fold_right : f:(char -> 'acc -> 'acc) -> string -> init:'acc -> 'acc
fold_right f s x computes f s.[0] (f s.[1] ( ... (f s.[n-1] x) ...)), where n is
the length of the string s.
Since: 4.13

val for_all : f:(char -> bool) -> string -> bool

for_all p s checks if all characters in s satisfy the predicate p.
Since: 4.13

val exists : f:(char -> bool) -> string -> bool

exists p s checks if at least one character of s satisfies the predicate p.
Since: 4.13

val trim : string -> string

trim s is s without leading and trailing whitespace. Whitespace characters are: ' ', '\x0C'
(form feed), '\n', '\r', and '\t'.
Since: 4.00

val escaped : string -> string

Chapter 29. The standard library 893

escaped s is s with special characters represented by escape sequences, following the lexical
conventions of OCaml.
All characters outside the US-ASCII printable range [0x20;0x7E] are escaped, as well as
backslash (0x2F) and double-quote (0x22).
The function Scanf.unescaped[29.48] is a left inverse of escaped, i.e. Scanf.unescaped
(escaped s) = s for any string s (unless escaped s fails).
Raises Invalid_argument if the result is longer than Sys.max_string_length[29.56] bytes.

val uppercase_ascii : string -> string

uppercase_ascii s is s with all lowercase letters translated to uppercase, using the
US-ASCII character set.
Since: 4.05

val lowercase_ascii : string -> string

lowercase_ascii s is s with all uppercase letters translated to lowercase, using the
US-ASCII character set.
Since: 4.05

val capitalize_ascii : string -> string

capitalize_ascii s is s with the first character set to uppercase, using the US-ASCII
character set.
Since: 4.05

val uncapitalize_ascii : string -> string

uncapitalize_ascii s is s with the first character set to lowercase, using the US-ASCII
character set.
Since: 4.05

Traversing
val iter : f:(char -> unit) -> string -> unit
iter ~f s applies function f in turn to all the characters of s. It is equivalent to f s.[0];
f s.[1]; ...; f s.[length s - 1]; ().

val iteri : f:(int -> char -> unit) -> string -> unit
iteri is like StringLabels.iter[29.55], but the function is also given the corresponding
character index.
Since: 4.00
894

Searching
val index_from : string -> int -> char -> int
index_from s i c is the index of the first occurrence of c in s after position i.
Raises

• Not_found if c does not occur in s after position i.

• Invalid_argument if i is not a valid position in s.

val index_from_opt : string -> int -> char -> int option
index_from_opt s i c is the index of the first occurrence of c in s after position i (if any).
Since: 4.05
Raises Invalid_argument if i is not a valid position in s.

val rindex_from : string -> int -> char -> int

rindex_from s i c is the index of the last occurrence of c in s before position i+1.
Raises

• Not_found if c does not occur in s before position i+1.

• Invalid_argument if i+1 is not a valid position in s.

val rindex_from_opt : string -> int -> char -> int option
rindex_from_opt s i c is the index of the last occurrence of c in s before position i+1 (if
any).
Since: 4.05
Raises Invalid_argument if i+1 is not a valid position in s.

val index : string -> char -> int

index s c is String.index_from[29.54] s 0 c.

val index_opt : string -> char -> int option

index_opt s c is String.index_from_opt[29.54] s 0 c.
Since: 4.05

val rindex : string -> char -> int

rindex s c is String.rindex_from[29.54] s (length s - 1) c.

val rindex_opt : string -> char -> int option

rindex_opt s c is String.rindex_from_opt[29.54] s (length s - 1) c.
Since: 4.05
Chapter 29. The standard library 895

Strings and Sequences

val to_seq : t -> char Seq.t
to_seq s is a sequence made of the string’s characters in increasing order.
Since: 4.07

val to_seqi : t -> (int * char) Seq.t

to_seqi s is like StringLabels.to_seq[29.55] but also tuples the corresponding index.
Since: 4.07

val of_seq : char Seq.t -> t

of_seq s is a string made of the sequence’s characters.
Since: 4.07

UTF decoding and validations

UTF-8
val get_utf_8_uchar : t -> int -> Uchar.utf_decode
get_utf_8_uchar b i decodes an UTF-8 character at index i in b.

val is_valid_utf_8 : t -> bool

is_valid_utf_8 b is true if and only if b contains valid UTF-8 data.

UTF-16BE
val get_utf_16be_uchar : t -> int -> Uchar.utf_decode
get_utf_16be_uchar b i decodes an UTF-16BE character at index i in b.

val is_valid_utf_16be : t -> bool

is_valid_utf_16be b is true if and only if b contains valid UTF-16BE data.

UTF-16LE
val get_utf_16le_uchar : t -> int -> Uchar.utf_decode
get_utf_16le_uchar b i decodes an UTF-16LE character at index i in b.

val is_valid_utf_16le : t -> bool

is_valid_utf_16le b is true if and only if b contains valid UTF-16LE data.
896

Binary decoding of integers

The functions in this section binary decode integers from strings.
All following functions raise Invalid_argument if the characters needed at index i to decode
the integer are not available.
Little-endian (resp. big-endian) encoding means that least (resp. most) significant bytes are
stored first. Big-endian is also known as network byte order. Native-endian encoding is either
little-endian or big-endian depending on Sys.big_endian[29.56].
32-bit and 64-bit integers are represented by the int32 and int64 types, which can be interpreted
either as signed or unsigned numbers.
8-bit and 16-bit integers are represented by the int type, which has more bits than the binary
encoding. These extra bits are sign-extended (or zero-extended) for functions which decode 8-bit or
16-bit integers and represented them with int values.
val get_uint8 : string -> int -> int
get_uint8 b i is b’s unsigned 8-bit integer starting at character index i.
Since: 4.13

val get_int8 : string -> int -> int

get_int8 b i is b’s signed 8-bit integer starting at character index i.
Since: 4.13

val get_uint16_ne : string -> int -> int

get_uint16_ne b i is b’s native-endian unsigned 16-bit integer starting at character index i.
Since: 4.13

val get_uint16_be : string -> int -> int

get_uint16_be b i is b’s big-endian unsigned 16-bit integer starting at character index i.
Since: 4.13

val get_uint16_le : string -> int -> int

get_uint16_le b i is b’s little-endian unsigned 16-bit integer starting at character index i.
Since: 4.13

val get_int16_ne : string -> int -> int

get_int16_ne b i is b’s native-endian signed 16-bit integer starting at character index i.
Since: 4.13

val get_int16_be : string -> int -> int

get_int16_be b i is b’s big-endian signed 16-bit integer starting at character index i.
Since: 4.13

val get_int16_le : string -> int -> int

Chapter 29. The standard library 897

get_int16_le b i is b’s little-endian signed 16-bit integer starting at character index i.

Since: 4.13

val get_int32_ne : string -> int -> int32

get_int32_ne b i is b’s native-endian 32-bit integer starting at character index i.
Since: 4.13

val hash : t -> int

An unseeded hash function for strings, with the same output value as Hashtbl.hash[29.25].
This function allows this module to be passed as argument to the functor
Hashtbl.Make[29.25].
Since: 5.0

val seeded_hash : int -> t -> int

A seeded hash function for strings, with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as argument to
the functor Hashtbl.MakeSeeded[29.25].
Since: 5.0

val get_int32_be : string -> int -> int32

get_int32_be b i is b’s big-endian 32-bit integer starting at character index i.
Since: 4.13

val get_int32_le : string -> int -> int32

get_int32_le b i is b’s little-endian 32-bit integer starting at character index i.
Since: 4.13

val get_int64_ne : string -> int -> int64

get_int64_ne b i is b’s native-endian 64-bit integer starting at character index i.
Since: 4.13

val get_int64_be : string -> int -> int64

get_int64_be b i is b’s big-endian 64-bit integer starting at character index i.
Since: 4.13

val get_int64_le : string -> int -> int64

get_int64_le b i is b’s little-endian 64-bit integer starting at character index i.
Since: 4.13
898

29.56 Module Sys : System interface.

Every function in this module raises Sys_error with an informative message when the underlying
system call signal an error.

val argv : string array

The command line arguments given to the process. The first element is the command name
used to invoke the program. The following elements are the command-line arguments given to
the program.

val executable_name : string

The name of the file containing the executable currently running. This name may be absolute
or relative to the current directory, depending on the platform and whether the program was
compiled to bytecode or a native executable.

val file_exists : string -> bool

Test if a file with the given name exists.

val is_directory : string -> bool

Returns true if the given name refers to a directory, false if it refers to another kind of file.
Since: 3.10
Raises Sys_error if no file exists with the given name.

val is_regular_file : string -> bool

Returns true if the given name refers to a regular file, false if it refers to another kind of file.
Since: 5.1
Raises Sys_error if no file exists with the given name.

val remove : string -> unit

Remove the given file name from the file system.

val rename : string -> string -> unit

Rename a file or directory. rename oldpath newpath renames the file or directory called
oldpath, giving it newpath as its new name, moving it between (parent) directories if needed.
If a file named newpath already exists, its contents will be replaced with those of oldpath.
Depending on the operating system, the metadata (permissions, owner, etc) of newpath can
either be preserved or be replaced by those of oldpath.
Since: 4.06 concerning the ”replace existing file” behavior

val getenv : string -> string

Return the value associated to a variable in the process environment.
Raises Not_found if the variable is unbound.
Chapter 29. The standard library 899

val getenv_opt : string -> string option

Return the value associated to a variable in the process environment or None if the variable is
unbound.
Since: 4.05

val command : string -> int

Execute the given shell command and return its exit code.
The argument of Sys.command[29.56] is generally the name of a command followed by zero,
one or several arguments, separated by whitespace. The given argument is interpreted by a
shell: either the Windows shell cmd.exe for the Win32 ports of OCaml, or the POSIX shell
sh for other ports. It can contain shell builtin commands such as echo, and also special
characters such as file redirections > and <, which will be honored by the shell.
Conversely, whitespace or special shell characters occurring in command names or in their
arguments must be quoted or escaped so that the shell does not interpret them. The quoting
rules vary between the POSIX shell and the Windows shell. The
Filename.quote_command[29.20] performs the appropriate quoting given a command name, a
list of arguments, and optional file redirections.

val time : unit -> float

Return the processor time, in seconds, used by the program since the beginning of execution.

val chdir : string -> unit

Change the current working directory of the process.

val mkdir : string -> int -> unit

Create a directory with the given permissions.
Since: 4.12

val rmdir : string -> unit

Remove an empty directory.
Since: 4.12

val getcwd : unit -> string

Return the current working directory of the process.

val readdir : string -> string array

Return the names of all files present in the given directory. Names denoting the current
directory and the parent directory ("." and ".." in Unix) are not returned. Each string in
the result is a file name rather than a complete path. There is no guarantee that the name
strings in the resulting array will appear in any specific order; they are not, in particular,
guaranteed to appear in alphabetical order.

val interactive : bool ref

900

This reference is initially set to false in standalone programs and to true if the code is
being executed under the interactive toplevel system ocaml.
Alert unsynchronized_access. The interactive status is a mutable global state.

val os_type : string

Operating system currently executing the OCaml program. One of

• "Unix" (for all Unix versions, including Linux and Mac OS X),
• "Win32" (for MS-Windows, OCaml compiled with MSVC++ or MinGW-w64),
• "Cygwin" (for MS-Windows, OCaml compiled with Cygwin).

type backend_type =
| Native
| Bytecode
| Other of string
Currently, the official distribution only supports Native and Bytecode, but it can be other
backends with alternative compilers, for example, javascript.
Since: 4.04

val backend_type : backend_type

Backend type currently executing the OCaml program.
Since: 4.04

val unix : bool

True if Sys.os_type = "Unix".
Since: 4.01

val win32 : bool

True if Sys.os_type = "Win32".
Since: 4.01

val cygwin : bool

True if Sys.os_type = "Cygwin".
Since: 4.01

val word_size : int

Size of one word on the machine currently executing the OCaml program, in bits: 32 or 64.

val int_size : int

Size of int, in bits. It is 31 (resp. 63) when using OCaml on a 32-bit (resp. 64-bit) platform.
It may differ for other implementations, e.g. it can be 32 bits when compiling to JavaScript.
Since: 4.03
Chapter 29. The standard library 901

val big_endian : bool

Whether the machine currently executing the Caml program is big-endian.
Since: 4.00

val max_string_length : int

Maximum length of strings and byte sequences.

val max_array_length : int

Maximum length of a normal array (i.e. any array whose elements are not of type float).
The maximum length of a float array is max_floatarray_length if OCaml was configured
with --enable-flat-float-array and max_array_length if configured with
--disable-flat-float-array.

val max_floatarray_length : int

Maximum length of a floatarray. This is also the maximum length of a float array when
OCaml is configured with --enable-flat-float-array.

val runtime_variant : unit -> string

Return the name of the runtime variant the program is running on. This is normally the
argument given to -runtime-variant at compile time, but for byte-code it can be changed
after compilation.
Since: 4.03

val runtime_parameters : unit -> string

Return the value of the runtime parameters, in the same format as the contents of the
OCAMLRUNPARAM environment variable.
Since: 4.03

val poll_actions : unit -> unit

Run any pending runtime actions, such as minor collections, major GC slices, signal handlers,
finalizers, or memprof callbacks.
Since: 5.3

Signal handling
type signal_behavior =
| Signal_default
| Signal_ignore
| Signal_handle of (int -> unit)
What to do when receiving a signal:

• Signal_default: take the default behavior (usually: abort the program)

• Signal_ignore: ignore the signal
902

• Signal_handle f: call function f, giving it the signal number as argument.

val signal : int -> signal_behavior -> signal_behavior

Set the behavior of the system on receipt of a given signal. The first argument is the signal
number. Return the behavior previously associated with the signal. If the signal number is
invalid (or not available on your system), an Invalid_argument exception is raised.

val set_signal : int -> signal_behavior -> unit

Same as Sys.signal[29.56] but return value is ignored.

Signal numbers for the standard POSIX signals.

val sigabrt : int
Abnormal termination

val sigalrm : int

Timeout

val sigfpe : int

Arithmetic exception

val sighup : int

Hangup on controlling terminal

val sigill : int

Invalid hardware instruction

val sigint : int

Interactive interrupt (ctrl-C)

val sigkill : int

Termination (cannot be ignored)

val sigpipe : int

Broken pipe

val sigquit : int

Interactive termination

val sigsegv : int

Invalid memory reference

val sigterm : int

Chapter 29. The standard library 903

Termination

val sigusr1 : int

Application-defined signal 1

val sigusr2 : int

Application-defined signal 2

val sigchld : int

Child process terminated

val sigcont : int

Continue

val sigstop : int

Stop

val sigtstp : int

Interactive stop

val sigttin : int

Terminal read from background process

val sigttou : int

Terminal write from background process

val sigvtalrm : int

Timeout in virtual time

val sigprof : int

Profiling interrupt

val sigbus : int

Bus error
Since: 4.03

val sigpoll : int

Pollable event
Since: 4.03

val sigsys : int

Bad argument to routine
Since: 4.03
904

val sigtrap : int

Trace/breakpoint trap
Since: 4.03

val sigurg : int

Urgent condition on socket
Since: 4.03

val sigxcpu : int

Timeout in cpu time
Since: 4.03

val sigxfsz : int

File size limit exceeded
Since: 4.03

exception Break
Exception raised on interactive interrupt if Sys.catch_break[29.56] is enabled.

val catch_break : bool -> unit

catch_break governs whether interactive interrupt (ctrl-C) terminates the program or raises
the Break exception. Call catch_break true to enable raising Break, and catch_break
false to let the system terminate the program on user interrupt.
Inside multi-threaded programs, the Break exception will arise in any one of the active
threads, and will keep arising on further interactive interrupt until all threads are terminated.
Use signal masks from Thread.sigmask to direct the interrupt towards a specific thread.

val ocaml_version : string

ocaml_version is the version of OCaml. It is a string of the form
"major.minor[.patchlevel][(+|~)additional-info]", where major, minor, and
patchlevel are integers, and additional-info is an arbitrary string. The [.patchlevel]
part was absent before version 3.08.0 and became mandatory from 3.08.0 onwards. The
[(+|~)additional-info] part may be absent.

val development_version : bool

true if this is a development version, false otherwise.
Since: 4.14

type extra_prefix =
| Plus
| Tilde
type extra_info = extra_prefix * string
Chapter 29. The standard library 905

Since: 4.14

type ocaml_release_info =
{ major : int ;
minor : int ;
patchlevel : int ;
extra : extra_info option ;
}
Since: 4.14

val ocaml_release : ocaml_release_info

ocaml_release is the version of OCaml.
Since: 4.14

val enable_runtime_warnings : bool -> unit

Control whether the OCaml runtime system can emit warnings on stderr. Currently, the only
supported warning is triggered when a channel created by open_* functions is finalized
without being closed. Runtime warnings are disabled by default.
Since: 4.03
Alert unsynchronized_access. The status of runtime warnings is a mutable global state.

val runtime_warnings_enabled : unit -> bool

Return whether runtime warnings are currently enabled.
Since: 4.03
Alert unsynchronized_access. The status of runtime warnings is a mutable global state.

Optimization
val opaque_identity : 'a -> 'a
For the purposes of optimization, opaque_identity behaves like an unknown (and thus
possibly side-effecting) function.
At runtime, opaque_identity disappears altogether. However, it does prevent the argument
from being garbage collected until the location where the call would have occurred.
A typical use of this function is to prevent pure computations from being optimized
away in benchmarking loops. For example:

for _round = 1 to 100_000 do

ignore (Sys.opaque_identity (my_pure_computation ()))
done

Since: 4.03
906

module Immediate64 :
sig

This module allows to define a type t with the immediate64 attribute. This attribute means
that the type is immediate on 64 bit architectures. On other architectures, it might or might
not be immediate.
module type Non_immediate =
sig

type t
end

module type Immediate =

sig

type t
end

module Make :
functor (Immediate : Immediate) -> functor (Non_immediate : Non_immediate) ->
sig

type t
type 'a repr =
| Immediate : Sys.Immediate64.Immediate.t repr
| Non_immediate : Sys.Immediate64.Non_immediate.t repr
val repr : t repr
end

end

29.57 Module Type : Type introspection.

Since: 5.1

Type equality witness

type (_, _) eq =
| Equal : ('a, 'a) eq
The purpose of eq is to represent type equalities that may not otherwise be known by the
type checker (e.g. because they may depend on dynamic data).
A value of type (a, b) eq represents the fact that types a and b are equal.
Chapter 29. The standard library 907

If one has a value eq : (a, b) eq that proves types a and b are equal, one can use it
to convert a value of type a to a value of type b by pattern matching on Equal:

let cast (type a) (type b) (Equal : (a, b) Type.eq) (a : a) : b = a

At runtime, this function simply returns its second argument unchanged.

Type identifiers
module Id :
sig

Type identifiers
type !'a t
The type for identifiers for type 'a.

val make : unit -> 'a t

make () is a new type identifier.

val uid : 'a t -> int

uid id is a runtime unique identifier for id.

val provably_equal : 'a t -> 'b t -> ('a, 'b) Type.eq option
provably_equal i0 i1 is Some Equal if identifier i0 is equal to i1 and None otherwise.

Example

The following shows how type identifiers can be used to implement a simple heterogeneous
key-value dictionary. In contrast to Map[28.2] values whose keys map to a single,
homogeneous type of values, this dictionary can associate a different type of value to
each key.

(** Heterogeneous dictionaries. *)

module Dict : sig
type t
(** The type for dictionaries. *)

type 'a key

(** The type for keys binding values of type ['a]. *)
908

val key : unit -> 'a key

(** [key ()] is a new dictionary key. *)

val empty : t
(** [empty] is the empty dictionary. *)

val add : 'a key -> 'a -> t -> t

(** [add k v d] is [d] with [k] bound to [v]. *)

val remove : 'a key -> t -> t

(** [remove k d] is [d] with the last binding of [k] removed. *)

val find : 'a key -> t -> 'a option

(** [find k d] is the binding of [k] in [d], if any. *)
end = struct
type 'a key = 'a Type.Id.t
type binding = B : 'a key * 'a -> binding
type t = (int * binding) list

let key () = Type.Id.make ()

let empty = []
let add k v d = (Type.Id.uid k, B (k, v)) :: d
let remove k d = List.remove_assoc (Type.Id.uid k) d
let find : type a. a key -> t -> a option = fun k d ->
match List.assoc_opt (Type.Id.uid k) d with
| None -> None
| Some (B (k', v)) ->
match Type.Id.provably_equal k k' with
| Some Type.Equal -> Some v
| None -> assert false
end

end

Type identifiers.
A type identifier is a value that denotes a type. Given two type identifiers, they can be tested
for equality[29.57] to prove they denote the same type. Note that:

• Unequal identifiers do not imply unequal types: a given type can be denoted by more
than one identifier.
• Type identifiers can be marshalled, but they get a new, distinct, identity on
unmarshalling, so the equalities are lost.

See an example[29.57] of use.

Chapter 29. The standard library 909

29.58 Module Uchar : Unicode characters.

Since: 4.03

type t
The type for Unicode characters.
A value of this type represents a Unicode scalar
value[http://unicode.org/glossary/#unicode_scalar_value] which is an integer in the
ranges 0x0000. . .0xD7FF or 0xE000. . .0x10FFFF.

val min : t
min is U+0000.

val max : t
max is U+10FFFF.

val bom : t
bom is U+FEFF, the byte order mark[http://unicode.org/glossary/#byte_order_mark]
(BOM) character.
Since: 4.06

val rep : t
rep is U+FFFD, the
replacement[http://unicode.org/glossary/#replacement_character] character.
Since: 4.06

val succ : t -> t

succ u is the scalar value after u in the set of Unicode scalar values.
Raises Invalid_argument if u is Uchar.max[29.58].

val pred : t -> t

pred u is the scalar value before u in the set of Unicode scalar values.
Raises Invalid_argument if u is Uchar.min[29.58].

val is_valid : int -> bool

is_valid n is true if and only if n is a Unicode scalar value (i.e. in the ranges
0x0000. . .0xD7FF or 0xE000. . .0x10FFFF).

val of_int : int -> t

of_int i is i as a Unicode character.
Raises Invalid_argument if i does not satisfy Uchar.is_valid[29.58].

val to_int : t -> int

910

to_int u is u as an integer.

val is_char : t -> bool

is_char u is true if and only if u is a latin1 OCaml character.

val of_char : char -> t

of_char c is c as a Unicode character.

val to_char : t -> char

to_char u is u as an OCaml latin1 character.
Raises Invalid_argument if u does not satisfy Uchar.is_char[29.58].

val equal : t -> t -> bool

equal u u' is u = u'.

val compare : t -> t -> int

compare u u' is Stdlib.compare u u'.

val seeded_hash : int -> t -> int

seeded_hash seed u A seeded hash function with the same output value as
Hashtbl.seeded_hash[29.25]. This function allows this module to be passed as an argument
to the functor Hashtbl.MakeSeeded[29.25].
Since: 5.3

val hash : t -> int

An unseeded hash function with the same output value as Hashtbl.hash[29.25]. This
function allows this module to be passed as an argument to the functor Hashtbl.Make[29.25].
Since: 5.3

UTF codecs tools

type utf_decode
The type for UTF decode results. Values of this type represent the result of a Unicode
Transformation Format decoding attempt.

val utf_decode_is_valid : utf_decode -> bool

utf_decode_is_valid d is true if and only if d holds a valid decode.

val utf_decode_uchar : utf_decode -> t

utf_decode_uchar d is the Unicode character decoded by d if utf_decode_is_valid d is
true and Uchar.rep[29.58] otherwise.

val utf_decode_length : utf_decode -> int

Chapter 29. The standard library 911

utf_decode_length d is the number of elements from the source that were consumed by the
decode d. This is always strictly positive and smaller or equal to 4. The kind of source
elements depends on the actual decoder; for the decoders of the standard library this function
always returns a length in bytes.

val utf_decode : int -> t -> utf_decode

utf_decode n u is a valid UTF decode for u that consumed n elements from the source for
decoding. n must be positive and smaller or equal to 4 (this is not checked by the module).

val utf_decode_invalid : int -> utf_decode

utf_decode_invalid n is an invalid UTF decode that consumed n elements from the source
to error. n must be positive and smaller or equal to 4 (this is not checked by the module).
The resulting decode has Uchar.rep[29.58] as the decoded Unicode character.

val utf_8_byte_length : t -> int

utf_8_byte_length u is the number of bytes needed to encode u in UTF-8.

val utf_16_byte_length : t -> int

utf_16_byte_length u is the number of bytes needed to encode u in UTF-16.

29.59 Module Unit : Unit values.

Since: 4.08

The unit type

type t = unit =
| ()
The unit type.
The constructor () is included here so that it has a path, but it is not intended to be used in
user-defined data types.

val equal : t -> t -> bool

equal u1 u2 is true.

val compare : t -> t -> int

compare u1 u2 is 0.

val to_string : t -> string

to_string b is "()".
912

29.60 Module Weak : Arrays of weak pointers and hash sets of weak
pointers.

Low-level functions
type !'a t
The type of arrays of weak pointers (weak arrays). A weak pointer is a value that the garbage
collector may erase whenever the value is not used any more (through normal pointers) by the
program. Note that finalisation functions are run before the weak pointers are erased, because
the finalisation functions can make values alive again (before 4.03 the finalisation functions
were run after).
A weak pointer is said to be full if it points to a value, empty if the value was erased by the
GC.
Notes:

• Integers are not allocated and cannot be stored in weak arrays.

• Weak arrays cannot be marshaled using output_value[28.2] nor the functions of the
Marshal[29.35] module.

val create : int -> 'a t

Weak.create n returns a new weak array of length n. All the pointers are initially empty.
Raises Invalid_argument if n is not comprised between zero and
Obj.Ephemeron.max_ephe_length[??] (limits included).

val length : 'a t -> int

Weak.length ar returns the length (number of elements) of ar.

val set : 'a t -> int -> 'a option -> unit
Weak.set ar n (Some el) sets the nth cell of ar to be a (full) pointer to el; Weak.set ar
n None sets the nth cell of ar to empty.
Raises Invalid_argument if n is not in the range 0 to Weak.length[29.60] ar - 1.

val get : 'a t -> int -> 'a option

Weak.get ar n returns None if the nth cell of ar is empty, Some x (where x is the value) if it
is full.
Raises Invalid_argument if n is not in the range 0 to Weak.length[29.60] ar - 1.

val get_copy : 'a t -> int -> 'a option

Weak.get_copy ar n returns None if the nth cell of ar is empty, Some x (where x is a
(shallow) copy of the value) if it is full. In addition to pitfalls with mutable values, the
interesting difference with get is that get_copy does not prevent the incremental GC from
erasing the value in its current cycle (get may delay the erasure to the next GC cycle).
Chapter 29. The standard library 913

Raises Invalid_argument if n is not in the range 0 to Weak.length[29.60] ar - 1.

If the element is a custom block it is not copied.

val check : 'a t -> int -> bool

Weak.check ar n returns true if the nth cell of ar is full, false if it is empty. Note that
even if Weak.check ar n returns true, a subsequent Weak.get[29.60] ar n can return None.
Raises Invalid_argument if n is not in the range 0 to Weak.length[29.60] ar - 1.

val fill : 'a t -> int -> int -> 'a option -> unit
Weak.fill ar ofs len el sets to el all pointers of ar from ofs to ofs + len - 1.
Raises Invalid_argument if ofs and len do not designate a valid subarray of ar.

val blit : 'a t -> int -> 'a t -> int -> int -> unit
Weak.blit ar1 off1 ar2 off2 len copies len weak pointers from ar1 (starting at off1)
to ar2 (starting at off2). It works correctly even if ar1 and ar2 are the same.
Raises Invalid_argument if off1 and len do not designate a valid subarray of ar1, or if
off2 and len do not designate a valid subarray of ar2.

Weak hash sets

A weak hash set is a hashed set of values. Each value may magically disappear from the set when it is
not used by the rest of the program any more. This is normally used to share data structures without
inducing memory leaks. Weak hash sets are defined on values from a Hashtbl.HashedType[29.25]
module; the equal relation and hash function are taken from that module. We will say that v is an
instance of x if equal x v is true.
The equal relation must be able to work on a shallow copy of the values and give the same
result as with the values themselves.
Unsynchronized accesses
Unsynchronized accesses to weak hash sets are a programming error. Unsynchronized accesses
to a weak hash set may lead to an invalid weak hash set state. Thus, concurrent accesses to weak
hash sets must be synchronized (for instance with a Mutex.t[29.37]).
module type S =
sig
type data
The type of the elements stored in the table.

type t
The type of tables that contain elements of type data. Note that weak hash sets cannot
be marshaled using output_value[28.2] or the functions of the Marshal[29.35] module.

val create : int -> t

914

create n creates a new empty weak hash set, of initial size n. The table will grow as
needed.

val clear : t -> unit

Remove all elements from the table.

val merge : t -> data -> data

merge t x returns an instance of x found in t if any, or else adds x to t and return x.

val add : t -> data -> unit

add t x adds x to t. If there is already an instance of x in t, it is unspecified which one
will be returned by subsequent calls to find and merge.

val remove : t -> data -> unit

remove t x removes from t one instance of x. Does nothing if there is no instance of x
in t.

val find : t -> data -> data

find t x returns an instance of x found in t.
Raises Not_found if there is no such element.

val find_opt : t -> data -> data option

find_opt t x returns an instance of x found in t or None if there is no such element.
Since: 4.05

val find_all : t -> data -> data list

find_all t x returns a list of all the instances of x found in t.

val mem : t -> data -> bool

mem t x returns true if there is at least one instance of x in t, false otherwise.

val iter : (data -> unit) -> t -> unit

iter f t calls f on each element of t, in some unspecified order. It is not specified
what happens if f tries to change t itself.

val fold : (data -> 'acc -> 'acc) -> t -> 'acc -> 'acc
fold f t init computes (f d1 (... (f dN init))) where d1 ... dN are the
elements of t in some unspecified order. It is not specified what happens if f tries to
change t itself.

val count : t -> int

Chapter 29. The standard library 915

Count the number of elements in the table. count t gives the same result as fold (fun
_ n -> n+1) t 0 but does not delay the deallocation of the dead elements.

val stats : t -> int * int * int * int * int * int

Return statistics on the table. The numbers are, in order: table length, number of
entries, sum of bucket lengths, smallest bucket length, median bucket length, biggest
bucket length.

end

The output signature of the functor Weak.Make[29.60].

module Make :
functor (H : Hashtbl.HashedType) -> S with type data = H.t
Functor building an implementation of the weak hash set structure. H.equal can’t be the
physical equality, since only shallow copies of the elements in the set are given to it.

29.61 Ocaml_operators : Precedence level and associativity of

operators
The following table lists the precedence level of all operator classes from the highest to the lowest
precedence. A few other syntactic constructions are also listed as references.
Operator class Associativity
! . . .˜. . . –
. · · · () . · · · [] . · · ·{} –
#… left
function application left
- -. –
∗ ∗ . . . lsl lsr asr right
∗ . . . /…%…mod land lor lxor left
+…-… left
:: right
@…^… right
=…<…>…|…&…$…!= left
& && right
or || right
, –
<- := right
if –
; right
916
Chapter 30

The compiler front-end

This chapter describes the OCaml front-end, which declares the abstract syntax tree used by the
compiler, provides a way to parse, print and pretty-print OCaml code, and ultimately allows one to
write abstract syntax tree preprocessors invoked via the -ppx flag (see chapters 13 and 16).
It is important to note that the exported front-end interface follows the evolution of the OCaml
language and implementation, and thus does not provide any backwards compatibility guarantees.
The front-end is a part of compiler-libs library. Programs that use the compiler-libs library
should be built as follows:

ocamlfind ocamlc other options -package compiler-libs.common other files

ocamlfind ocamlopt other options -package compiler-libs.common other files

Use of the ocamlfind utility is recommended. However, if this is not possible, an alternative
method may be used:

ocamlc other options -I +compiler-libs ocamlcommon.cma other files

ocamlopt other options -I +compiler-libs ocamlcommon.cmxa other files

For interactive use of the compiler-libs library, start ocaml and type
#load "compiler-libs/ocamlcommon.cma";;.

30.1 Module Ast_mapper : The interface of a -ppx rewriter

A -ppx rewriter is a program that accepts a serialized abstract syntax tree and outputs another,
possibly modified, abstract syntax tree. This module encapsulates the interface between the compiler
and the -ppx rewriters, handling such details as the serialization format, forwarding of command-line
flags, and storing state.
Ast_mapper.mapper[30.1] enables AST rewriting using open recursion. A typical mapper would
be based on Ast_mapper.default_mapper[30.1], a deep identity mapper, and will fall back on it
for handling the syntax it does not modify. For example:

open Asttypes
open Parsetree
open Ast_mapper

917
918

let test_mapper argv =

{ default_mapper with
expr = fun mapper expr ->
match expr with
| { pexp_desc = Pexp_extension ({ txt = "test" }, PStr [])} ->
Ast_helper.Exp.constant (Pconst_integer ("42", None))
| other -> default_mapper.expr mapper other; }

let () =
register "ppx_test" test_mapper
This -ppx rewriter, which replaces [%test] in expressions with the constant 42, can be compiled
using ocamlc -o ppx_test -I +compiler-libs ocamlcommon.cma ppx_test.ml.
Warning: this module is unstable and part of compiler-libs[30].

A generic Parsetree mapper

type mapper =
{ attribute : mapper -> Parsetree.attribute -> Parsetree.attribute ;
attributes : mapper -> Parsetree.attribute list -> Parsetree.attribute list ;
binding_op : mapper -> Parsetree.binding_op -> Parsetree.binding_op ;
case : mapper -> Parsetree.case -> Parsetree.case ;
cases : mapper -> Parsetree.case list -> Parsetree.case list ;
class_declaration : mapper ->
Parsetree.class_declaration -> Parsetree.class_declaration ;
class_description : mapper ->
Parsetree.class_description -> Parsetree.class_description ;
class_expr : mapper -> Parsetree.class_expr -> Parsetree.class_expr ;
class_field : mapper -> Parsetree.class_field -> Parsetree.class_field ;
class_signature : mapper -> Parsetree.class_signature -> Parsetree.class_signature ;
class_structure : mapper -> Parsetree.class_structure -> Parsetree.class_structure ;
class_type : mapper -> Parsetree.class_type -> Parsetree.class_type ;
class_type_declaration : mapper ->
Parsetree.class_type_declaration -> Parsetree.class_type_declaration ;
class_type_field : mapper -> Parsetree.class_type_field -> Parsetree.class_type_field ;
constant : mapper -> Parsetree.constant -> Parsetree.constant ;
constructor_declaration : mapper ->
Parsetree.constructor_declaration -> Parsetree.constructor_declaration ;
directive_argument : mapper ->
Parsetree.directive_argument -> Parsetree.directive_argument ;
expr : mapper -> Parsetree.expression -> Parsetree.expression ;
extension : mapper -> Parsetree.extension -> Parsetree.extension ;
extension_constructor : mapper ->
Parsetree.extension_constructor -> Parsetree.extension_constructor ;
Chapter 30. The compiler front-end 919

include_declaration : mapper ->

Parsetree.include_declaration -> Parsetree.include_declaration ;
include_description : mapper ->
Parsetree.include_description -> Parsetree.include_description ;
label_declaration : mapper ->
Parsetree.label_declaration -> Parsetree.label_declaration ;
location : mapper -> Location.t -> Location.t ;
module_binding : mapper -> Parsetree.module_binding -> Parsetree.module_binding ;
module_declaration : mapper ->
Parsetree.module_declaration -> Parsetree.module_declaration ;
module_substitution : mapper ->
Parsetree.module_substitution -> Parsetree.module_substitution ;
module_expr : mapper -> Parsetree.module_expr -> Parsetree.module_expr ;
module_type : mapper -> Parsetree.module_type -> Parsetree.module_type ;
module_type_declaration : mapper ->
Parsetree.module_type_declaration -> Parsetree.module_type_declaration ;
open_declaration : mapper -> Parsetree.open_declaration -> Parsetree.open_declaration ;
open_description : mapper -> Parsetree.open_description -> Parsetree.open_description ;
pat : mapper -> Parsetree.pattern -> Parsetree.pattern ;
payload : mapper -> Parsetree.payload -> Parsetree.payload ;
signature : mapper -> Parsetree.signature -> Parsetree.signature ;
signature_item : mapper -> Parsetree.signature_item -> Parsetree.signature_item ;
structure : mapper -> Parsetree.structure -> Parsetree.structure ;
structure_item : mapper -> Parsetree.structure_item -> Parsetree.structure_item ;
toplevel_directive : mapper ->
Parsetree.toplevel_directive -> Parsetree.toplevel_directive ;
toplevel_phrase : mapper -> Parsetree.toplevel_phrase -> Parsetree.toplevel_phrase ;
typ : mapper -> Parsetree.core_type -> Parsetree.core_type ;
type_declaration : mapper -> Parsetree.type_declaration -> Parsetree.type_declaration ;
type_extension : mapper -> Parsetree.type_extension -> Parsetree.type_extension ;
type_exception : mapper -> Parsetree.type_exception -> Parsetree.type_exception ;
type_kind : mapper -> Parsetree.type_kind -> Parsetree.type_kind ;
value_binding : mapper -> Parsetree.value_binding -> Parsetree.value_binding ;
value_description : mapper ->
Parsetree.value_description -> Parsetree.value_description ;
with_constraint : mapper -> Parsetree.with_constraint -> Parsetree.with_constraint ;
}
A mapper record implements one ”method” per syntactic category, using an open recursion
style: each method takes as its first argument the mapper to be applied to children in the
syntax tree.

val default_mapper : mapper

A default mapper, which implements a ”deep identity” mapping.
920

Apply mappers to compilation units

val tool_name : unit -> string
Can be used within a ppx preprocessor to know which tool is calling it "ocamlc", "ocamlopt",
"ocamldoc", "ocamldep", "ocaml", . . . Some global variables that reflect command-line
options are automatically synchronized between the calling tool and the ppx preprocessor:
Clflags.include_dirs[??], Clflags.hidden_include_dirs[??], Load_path[??],
Clflags.open_modules[??], Clflags.for_package[??], Clflags.debug[??].

val apply : source:string -> target:string -> mapper -> unit

Apply a mapper (parametrized by the unit name) to a dumped parsetree found in the source
file and put the result in the target file. The structure or signature field of the mapper is
applied to the implementation or interface.

val run_main : (string list -> mapper) -> unit

Entry point to call to implement a standalone -ppx rewriter from a mapper, parametrized by
the command line arguments. The current unit name can be obtained from
Location.input_name[30.3]. This function implements proper error reporting for uncaught
exceptions.

Registration API
val register_function : (string -> (string list -> mapper) -> unit) ref
val register : string -> (string list -> mapper) -> unit
Apply the register_function. The default behavior is to run the mapper immediately,
taking arguments from the process command line. This is to support a scenario where a
mapper is linked as a stand-alone executable.
It is possible to overwrite the register_function to define ”-ppx drivers”, which combine
several mappers in a single process. Typically, a driver starts by defining
register_function to a custom implementation, then lets ppx rewriters (linked statically or
dynamically) register themselves, and then run all or some of them. It is also possible to have
-ppx drivers apply rewriters to only specific parts of an AST.
The first argument to register is a symbolic name to be used by the ppx driver.

Convenience functions to write mappers

val map_opt : ('a -> 'b) -> 'a option -> 'b option
val extension_of_error : Location.error -> Parsetree.extension
Encode an error into an ’ocaml.error’ extension node which can be inserted in a generated
Parsetree. The compiler will be responsible for reporting the error.

val attribute_of_warning : Location.t -> string -> Parsetree.attribute

Encode a warning message into an ’ocaml.ppwarning’ attribute which can be inserted in a
generated Parsetree. The compiler will be responsible for reporting the warning.
Chapter 30. The compiler front-end 921

Helper functions to call external mappers

val add_ppx_context_str :
tool_name:string -> Parsetree.structure -> Parsetree.structure
Extract information from the current environment and encode it into an attribute which is
prepended to the list of structure items in order to pass the information to an external
processor.

val add_ppx_context_sig :
tool_name:string -> Parsetree.signature -> Parsetree.signature
Same as add_ppx_context_str, but for signatures.

val drop_ppx_context_str :
restore:bool -> Parsetree.structure -> Parsetree.structure
Drop the ocaml.ppx.context attribute from a structure. If restore is true, also restore the
associated data in the current process.

val drop_ppx_context_sig :
restore:bool -> Parsetree.signature -> Parsetree.signature
Same as drop_ppx_context_str, but for signatures.

Cookies
Cookies are used to pass information from a ppx processor to a further invocation of itself, when
called from the OCaml toplevel (or other tools that support cookies).
val set_cookie : string -> Parsetree.expression -> unit
val get_cookie : string -> Parsetree.expression option

30.2 Module Asttypes : Auxiliary AST types used by parsetree

and typedtree.
Warning: this module is unstable and part of compiler-libs[30].

type constant =
| Const_int of int
| Const_char of char
| Const_string of string * Location.t * string option
| Const_float of string
| Const_int32 of int32
| Const_int64 of int64
| Const_nativeint of nativeint
type rec_flag =
| Nonrecursive
922

| Recursive
type direction_flag =
| Upto
| Downto
type private_flag =
| Private
| Public
type mutable_flag =
| Immutable
| Mutable
type virtual_flag =
| Virtual
| Concrete
type override_flag =
| Override
| Fresh
type closed_flag =
| Closed
| Open
type label = string
type arg_label =
| Nolabel
| Labelled of string
label:T -> ...
| Optional of string
?label:T -> ...
type 'a loc = 'a Location.loc =
{ txt : 'a ;
loc : Location.t ;
}
type variance =
| Covariant
| Contravariant
| NoVariance
type injectivity =
| Injective
| NoInjectivity
val string_of_label : arg_label -> string
Chapter 30. The compiler front-end 923

30.3 Module Location : Source code locations (ranges of positions),

used in parsetree.
Warning: this module is unstable and part of compiler-libs[30].

type t = Warnings.loc =
{ loc_start : Lexing.position ;
loc_end : Lexing.position ;
loc_ghost : bool ;
}
Note on the use of Lexing.position in this module. If pos_fname = "", then use !input_name
instead. If pos_lnum = -1, then pos_bol = 0. Use pos_cnum and re-parse the file to get the line
and character numbers. Else all fields are correct.
val none : t
An arbitrary value of type t; describes an empty ghost range.

val is_none : t -> bool

True for Location.none, false any other location

val in_file : string -> t

Return an empty ghost range located in a given file.

val init : Lexing.lexbuf -> string -> unit

Set the file name and line number of the lexbuf to be the start of the named file.

val curr : Lexing.lexbuf -> t

Get the location of the current token from the lexbuf.

val symbol_rloc : unit -> t

val symbol_gloc : unit -> t
val rhs_loc : int -> t
rhs_loc n returns the location of the symbol at position n, starting at 1, in the current
parser rule.

val rhs_interval : int -> int -> t

val get_pos_info : Lexing.position -> string * int * int
file, line, char

type 'a loc =

{ txt : 'a ;
loc : t ;
}
val mknoloc : 'a -> 'a loc
val mkloc : 'a -> t -> 'a loc
924

Input info
val input_name : string ref
val input_lexbuf : Lexing.lexbuf option ref
val input_phrase_buffer : Buffer.t option ref

Toplevel-specific functions
val echo_eof : unit -> unit
val reset : unit -> unit

Rewriting path
val rewrite_absolute_path : string -> string
rewrite_absolute_path path rewrites path to honor the BUILD_PATH_PREFIX_MAP
variable if it is set. It does not check whether path is absolute or not. The result is as follows:

• If BUILD_PATH_PREFIX_MAP is not set, just return path.

• otherwise, rewrite using the mapping (and if there are no matching prefixes that will just
return path).

See
the BUILD_PATH_PREFIX_MAP
spec[https://reproducible-builds.org/specs/build-path-prefix-map/]

val rewrite_find_first_existing : string -> string option

rewrite_find_first_existing path uses a BUILD_PATH_PREFIX_MAP mapping and
tries to find a source in mapping that maps to a result that exists in the file system. There
are the following return values:

• None, means either

– BUILD_PATH_PREFIX_MAP is not set and path does not exists, or
– no source prefixes of path in the mapping were found,
• Some target, means target exists and either
– BUILD_PATH_PREFIX_MAP is not set and target = path, or
– target is the first file (in priority order) that path mapped to that exists in the file
system.
• Not_found raised, means some source prefixes in the map were found that matched
path, but none of them existed in the file system. The caller should catch this and issue
an appropriate error message.

See
the BUILD_PATH_PREFIX_MAP
spec[https://reproducible-builds.org/specs/build-path-prefix-map/]
Chapter 30. The compiler front-end 925

val rewrite_find_all_existing_dirs : string -> string list

rewrite_find_all_existing_dirs dir accumulates a list of existing directories, dirs, that
are the result of mapping a potentially abstract directory, dir, over all the mapping pairs in
the BUILD_PATH_PREFIX_MAP environment variable, if any. The list dirs will be in
priority order (head as highest priority).
The possible results are:
• [], means either
– BUILD_PATH_PREFIX_MAP is not set and dir is not an existing directory, or
– if set, then there were no matching prefixes of dir.
• Some dirs, means dirs are the directories found. Either
– BUILD_PATH_PREFIX_MAP is not set and dirs = [dir], or
– it was set and dirs are the mapped existing directories.
• Not_found raised, means some source prefixes in the map were found that matched dir,
but none of mapping results were existing directories (possibly due to misconfiguration).
The caller should catch this and issue an appropriate error message.
See
the BUILD_PATH_PREFIX_MAP
spec[https://reproducible-builds.org/specs/build-path-prefix-map/]

val absolute_path : string -> string

absolute_path path first makes an absolute path, s from path, prepending the current
working directory if path was relative. Then s is rewritten using rewrite_absolute_path.
Finally the result is normalized by eliminating instances of '.' or '..'.

Printing locations
val show_filename : string -> string
In -absname mode, return the absolute path for this filename. Otherwise, returns the filename
unchanged.

val print_filename : Format.formatter -> string -> unit

val print_loc : Format.formatter -> t -> unit
val print_locs : Format.formatter -> t list -> unit
val separate_new_message : Format.formatter -> unit
module Doc :
sig
val separate_new_message : unit Format_doc.printer
val filename : string Format_doc.printer
val quoted_filename : string Format_doc.printer
val loc : Location.t Format_doc.printer
val locs : Location.t list Format_doc.printer
926

end

Toplevel-specific location highlighting

val highlight_terminfo : Lexing.lexbuf -> Format.formatter -> t list -> unit

Reporting errors and warnings

The type of reports and report printers
type msg = Format_doc.t loc
val msg : ?loc:t ->
('a, Format_doc.formatter, unit, msg) format4 -> 'a
type report_kind =
| Report_error
| Report_warning of string
| Report_warning_as_error of string
| Report_alert of string
| Report_alert_as_error of string
type report =
{ kind : report_kind ;
main : msg ;
sub : msg list ;
footnote : Format_doc.t option ;
}
type report_printer =
{ pp : report_printer -> Format.formatter -> report -> unit ;
pp_report_kind : report_printer ->
report -> Format.formatter -> report_kind -> unit ;
pp_main_loc : report_printer ->
report -> Format.formatter -> t -> unit ;
pp_main_txt : report_printer ->
report -> Format.formatter -> Format_doc.t -> unit ;
pp_submsgs : report_printer ->
report -> Format.formatter -> msg list -> unit ;
pp_submsg : report_printer ->
report -> Format.formatter -> msg -> unit ;
pp_submsg_loc : report_printer ->
report -> Format.formatter -> t -> unit ;
pp_submsg_txt : report_printer ->
report -> Format.formatter -> Format_doc.t -> unit ;
}
A printer for reports, defined using open-recursion. The goal is to make it easy to define new
printers by re-using code from existing ones.
Chapter 30. The compiler front-end 927

Report printers used in the compiler

val batch_mode_printer : report_printer
val terminfo_toplevel_printer : Lexing.lexbuf -> report_printer
val best_toplevel_printer : unit -> report_printer
Detects the terminal capabilities and selects an adequate printer

Printing a report
val print_report : Format.formatter -> report -> unit
Display an error or warning report.

val report_printer : (unit -> report_printer) ref

Hook for redefining the printer of reports.
The hook is a unit -> report_printer and not simply a report_printer: this is useful so
that it can detect the type of the output (a file, a terminal, . . .) and select a printer
accordingly.

val default_report_printer : unit -> report_printer

Original report printer for use in hooks.

Reporting warnings
Converting a Warnings.t into a report
val report_warning : t -> Warnings.t -> report option
report_warning loc w produces a report for the given warning w, or None if the warning is
not to be printed.

val warning_reporter : (t -> Warnings.t -> report option) ref

Hook for intercepting warnings.

val default_warning_reporter : t -> Warnings.t -> report option

Original warning reporter for use in hooks.

Printing warnings
val formatter_for_warnings : Format.formatter ref
val print_warning : t -> Format.formatter -> Warnings.t -> unit
Prints a warning. This is simply the composition of report_warning and print_report.

val prerr_warning : t -> Warnings.t -> unit

Same as print_warning, but uses !formatter_for_warnings as output formatter.
928

Reporting alerts
Converting an Alert.t into a report
val report_alert : t -> Warnings.alert -> report option
report_alert loc w produces a report for the given alert w, or None if the alert is not to be
printed.

val alert_reporter : (t -> Warnings.alert -> report option) ref

Hook for intercepting alerts.

val default_alert_reporter : t -> Warnings.alert -> report option

Original alert reporter for use in hooks.

Printing alerts
val print_alert : t -> Format.formatter -> Warnings.alert -> unit
Prints an alert. This is simply the composition of report_alert and print_report.

val prerr_alert : t -> Warnings.alert -> unit

Same as print_alert, but uses !formatter_for_warnings as output formatter.

val deprecated : ?def:t -> ?use:t -> t -> string -> unit
Prints a deprecation alert.

val alert : ?def:t ->

?use:t -> kind:string -> t -> string -> unit
Prints an arbitrary alert.

val auto_include_alert : string -> unit

Prints an alert that -I +lib has been automatically added to the load path

val deprecated_script_alert : string -> unit

deprecated_script_alert command prints an alert that command foo has been deprecated
in favour of command ./foo

Reporting errors
type error = report
An error is a report which report_kind must be Report_error.
Chapter 30. The compiler front-end 929

type delayed_msg = unit -> Format_doc.t option

val error :
?loc:t ->
?sub:msg list ->
?footnote:delayed_msg -> string -> error
val errorf :
?loc:t ->
?sub:msg list ->
?footnote:delayed_msg ->
('a, Format_doc.formatter, unit, error) format4 -> 'a
val error_of_printer :
?loc:t ->
?sub:msg list ->
?footnote:delayed_msg ->
(Format_doc.formatter -> 'a -> unit) -> 'a -> error
val error_of_printer_file :
(Format_doc.formatter -> 'a -> unit) -> 'a -> error

Automatically reporting errors for raised exceptions

val register_error_of_exn : (exn -> error option) -> unit
Each compiler module which defines a custom type of exception which can surface as a
user-visible error should register a ”printer” for this exception using
register_error_of_exn. The result of the printer is an error value containing a location, a
message, and optionally sub-messages (each of them being located as well).

val error_of_exn : exn -> [ `Already_displayed | `Ok of error ] option

exception Error of error
Raising Error e signals an error e; the exception will be caught and the error will be printed.

exception Already_displayed_error
Raising Already_displayed_error signals an error which has already been printed. The
exception will be caught, but nothing will be printed

val raise_errorf :
?loc:t ->
?sub:msg list ->
?footnote:delayed_msg ->
('a, Format_doc.formatter, unit, 'b) format4 -> 'a
val report_exception : Format.formatter -> exn -> unit
Reraise the exception if it is unknown.
930

30.4 Module Longident : Long identifiers, used in parsetree.

Warning: this module is unstable and part of compiler-libs[30].
To print a longident, see Pprintast.longident[30.7], using Format.asprintf[29.22] to convert
to a string.

type t =
| Lident of string
| Ldot of t * string
| Lapply of t * t
val flatten : t -> string list
val unflatten : string list -> t option
For a non-empty list l, unflatten l is Some lid where lid is the long identifier created by
concatenating the elements of l with Ldot. unflatten [] is None.

val last : t -> string

val parse : string -> t
Deprecated. this function may misparse its input, use ”Parse.longident” or
”Longident.unflatten”This function is broken on identifiers that are not just
”Word.Word.word”; for example, it returns incorrect results on infix operators and extended
module paths.
If you want to generate long identifiers that are a list of dot-separated identifiers, the function
Longident.unflatten[30.4] is safer and faster. Longident.unflatten[30.4] is available
since OCaml 4.06.0.
If you want to parse any identifier correctly, use the long-identifiers functions from the
Parse[30.5] module, in particular Parse.longident[30.5]. They are available since OCaml
4.11, and also provide proper input-location support.

30.5 Module Parse : Entry points in the parser

Warning: this module is unstable and part of compiler-libs[30].

val implementation : Lexing.lexbuf -> Parsetree.structure

val interface : Lexing.lexbuf -> Parsetree.signature
val toplevel_phrase : Lexing.lexbuf -> Parsetree.toplevel_phrase
val use_file : Lexing.lexbuf -> Parsetree.toplevel_phrase list
val core_type : Lexing.lexbuf -> Parsetree.core_type
val expression : Lexing.lexbuf -> Parsetree.expression
val pattern : Lexing.lexbuf -> Parsetree.pattern
val module_type : Lexing.lexbuf -> Parsetree.module_type
val module_expr : Lexing.lexbuf -> Parsetree.module_expr
Chapter 30. The compiler front-end 931

The functions below can be used to parse Longident safely.

val longident : Lexing.lexbuf -> Longident.t
The function longident is guaranteed to parse all subclasses of Longident.t[30.4] used in
OCaml: values, constructors, simple or extended module paths, and types or module types.
However, this function accepts inputs which are not accepted by the compiler, because they
combine functor applications and infix operators. In valid OCaml syntax, only value-level
identifiers may end with infix operators Foo.( + ). Moreover, in value-level identifiers the
module path Foo must be simple (M.N rather than F(X)): functor applications may only
appear in type-level identifiers. As a consequence, a path such as F(X).( + ) is not a valid
OCaml identifier; but it is accepted by this function.

The next functions are specialized to a subclass of Longident.t[30.4]

val val_ident : Lexing.lexbuf -> Longident.t
This function parses a syntactically valid path for a value. For instance, x, M.x, and (+.) are
valid. Contrarily, M.A, F(X).x, and true are rejected.
Longident for OCaml’s value cannot contain functor application. The last component of the
Longident.t[30.4] is not capitalized, but can be an operator A.Path.To.(.%.%.(;..)<-)

val constr_ident : Lexing.lexbuf -> Longident.t

This function parses a syntactically valid path for a variant constructor. For instance, A, M.A
and M.(::) are valid, but both M.a and F(X).A are rejected.
Longident for OCaml’s variant constructors cannot contain functor application. The last
component of the Longident.t[30.4] is capitalized, or it may be one the special constructors:
true,false,(),[],(::). Among those special constructors, only (::) can be prefixed by a
module path (A.B.C.(::)).

val simple_module_path : Lexing.lexbuf -> Longident.t

This function parses a syntactically valid path for a module. For instance, A, and M.A are
valid, but both M.a and F(X).A are rejected.
Longident for OCaml’s module cannot contain functor application. The last component of the
Longident.t[30.4] is capitalized.

val extended_module_path : Lexing.lexbuf -> Longident.t

This function parse syntactically valid path for an extended module. For instance, A.B and
F(A).B are valid. Contrarily, (.%()) or [] are both rejected.
The last component of the Longident.t[30.4] is capitalized.

val type_ident : Lexing.lexbuf -> Longident.t

This function parse syntactically valid path for a type or a module type. For instance, A, t,
M.t and F(X).t are valid. Contrarily, (.%()) or [] are both rejected.
In path for type and module types, only operators and special constructors are rejected.
932

30.6 Module Parsetree : Abstract syntax tree produced by parsing

Warning: this module is unstable and part of compiler-libs[30].

type constant =
{ pconst_desc : constant_desc ;
pconst_loc : Location.t ;
}
type constant_desc =
| Pconst_integer of string * char option
Integer constants such as 3 3l 3L 3n.
Suffixes [g-z][G-Z] are accepted by the parser. Suffixes except 'l', 'L' and 'n' are
rejected by the typechecker
| Pconst_char of char
Character such as 'c'.
| Pconst_string of string * Location.t * string option
Constant string such as "constant" or {delim|other constant|delim}.
The location span the content of the string, without the delimiters.
| Pconst_float of string * char option
Float constant such as 3.4, 2e5 or 1.4e-4.
Suffixes g-zG-Z are accepted by the parser. Suffixes are rejected by the typechecker.
type location_stack = Location.t list

Extension points
type attribute =
{ attr_name : string Asttypes.loc ;
attr_payload : payload ;
attr_loc : Location.t ;
}
Attributes such as [@id ARG] and [@@id ARG].
Metadata containers passed around within the AST. The compiler ignores unknown
attributes.

type extension = string Asttypes.loc * payload

Extension points such as [%id ARG] and [%%id ARG].
Sub-language placeholder – rejected by the typechecker.

type attributes = attribute list

type payload =
| PStr of structure
| PSig of signature
Chapter 30. The compiler front-end 933

: SIG in an attribute or an extension point

| PTyp of core_type
: T in an attribute or an extension point
| PPat of pattern * expression option
? P or ? P when E, in an attribute or an extension point

Core language
Type expressions
type core_type =
{ ptyp_desc : core_type_desc ;
ptyp_loc : Location.t ;
ptyp_loc_stack : location_stack ;
ptyp_attributes : attributes ;
... [@id1] [@id2]
}
type core_type_desc =
| Ptyp_any
_
| Ptyp_var of string
A type variable such as 'a
| Ptyp_arrow of Asttypes.arg_label * core_type * core_type
Ptyp_arrow(lbl, T1, T2) represents:

• T1 -> T2 when lbl is Nolabel[??],

• ~l:T1 -> T2 when lbl is Labelled[??],
• ?l:T1 -> T2 when lbl is Optional[??].

| Ptyp_tuple of core_type list

Ptyp_tuple([T1 ; ... ; Tn]) represents a product type T1 * ... * Tn.
Invariant: n >= 2.
| Ptyp_constr of Longident.t Asttypes.loc * core_type list
Ptyp_constr(lident, l) represents:

• tconstr when l=[],

• T tconstr when l=[T],
• (T1, ..., Tn) tconstr when l=[T1 ; ... ; Tn].

| Ptyp_object of object_field list * Asttypes.closed_flag

934

Ptyp_object([ l1:T1; ...; ln:Tn ], flag) represents:

• < l1:T1; ...; ln:Tn > when flag is Closed[??],

• < l1:T1; ...; ln:Tn; .. > when flag is Open[??].

| Ptyp_class of Longident.t Asttypes.loc * core_type list

Ptyp_class(tconstr, l) represents:

• #tconstr when l=[],

• T #tconstr when l=[T],
• (T1, ..., Tn) #tconstr when l=[T1 ; ... ; Tn].

| Ptyp_alias of core_type * string Asttypes.loc

T as 'a.
| Ptyp_variant of row_field list * Asttypes.closed_flag * Asttypes.label list option
Ptyp_variant([`A;`B], flag, labels) represents:

• [ `A|`B ] when flag is Closed[??], and labels is None,

• [> `A|`B ] when flag is Open[??], and labels is None,
• [< `A|`B ] when flag is Closed[??], and labels is Some [],
• [< `A|`B > `X `Y ] when flag is Closed[??], and labels is Some ["X";"Y"].

| Ptyp_poly of string Asttypes.loc list * core_type

'a1 ... 'an. T
Can only appear in the following context:

• As the Parsetree.core_type[30.6] of a Ppat_constraint[??]

node corresponding to a constraint on a let-binding:
let x : 'a1 ... 'an. T = e ...

• Under Cfk_virtual[??] for methods (not values).

• As the Parsetree.core_type[30.6] of a Pctf_method[??] node.

• As the Parsetree.core_type[30.6] of a Pexp_poly[??] node.

• As the pld_type[??] field of a Parsetree.label_declaration[30.6].

• As a Parsetree.core_type[30.6] of a Ptyp_object[??] node.

• As the pval_type[??] field of a Parsetree.value_description[30.6].

| Ptyp_package of package_type
(module S).
Chapter 30. The compiler front-end 935

| Ptyp_open of Longident.t Asttypes.loc * core_type

M.(T)
| Ptyp_extension of extension
[%id].
type package_type = Longident.t Asttypes.loc *
(Longident.t Asttypes.loc * core_type) list
As Parsetree.package_type[30.6] typed values:

• (S, []) represents (module S),

• (S, [(t1, T1) ; ... ; (tn, Tn)]) represents (module S with type t1 = T1 and
... and tn = Tn).

type row_field =
{ prf_desc : row_field_desc ;
prf_loc : Location.t ;
prf_attributes : attributes ;
}
type row_field_desc =
| Rtag of Asttypes.label Asttypes.loc * bool * core_type list
Rtag(`A, b, l) represents:

• `A when b is true and l is [],

• `A of T when b is false and l is [T],
• `A of T1 & .. & Tn when b is false and l is [T1;...Tn],
• `A of & T1 & .. & Tn when b is true and l is [T1;...Tn].

• The bool field is true if the tag contains a constant (empty) constructor.
• & occurs when several types are used for the same constructor (see 4.2 in the
manual)

| Rinherit of core_type
[ | t ]
type object_field =
{ pof_desc : object_field_desc ;
pof_loc : Location.t ;
pof_attributes : attributes ;
}
type object_field_desc =
| Otag of Asttypes.label Asttypes.loc * core_type
| Oinherit of core_type
936

Patterns
type pattern =
{ ppat_desc : pattern_desc ;
ppat_loc : Location.t ;
ppat_loc_stack : location_stack ;
ppat_attributes : attributes ;
... [@id1] [@id2]
}
type pattern_desc =
| Ppat_any
The pattern _.
| Ppat_var of string Asttypes.loc
A variable pattern such as x
| Ppat_alias of pattern * string Asttypes.loc
An alias pattern such as P as 'a
| Ppat_constant of constant
Patterns such as 1, 'a', "true", 1.0, 1l, 1L, 1n
| Ppat_interval of constant * constant
Patterns such as 'a'..'z'.
Other forms of interval are recognized by the parser but rejected by the type-checker.
| Ppat_tuple of pattern list
Patterns (P1, ..., Pn).
Invariant: n >= 2
| Ppat_construct of Longident.t Asttypes.loc
* (string Asttypes.loc list * pattern) option
Ppat_construct(C, args) represents:

• C when args is None,

• C P when args is Some ([], P)
• C (P1, ..., Pn) when args is Some ([], Ppat_tuple [P1; ...; Pn])
• C (type a b) P when args is Some ([a; b], P)

| Ppat_variant of Asttypes.label * pattern option

Ppat_variant(`A, pat) represents:

• `A when pat is None,

• `A P when pat is Some P

| Ppat_record of (Longident.t Asttypes.loc * pattern) list * Asttypes.closed_flag

Chapter 30. The compiler front-end 937

Ppat_record([(l1, P1) ; ... ; (ln, Pn)], flag) represents:

• { l1=P1; ...; ln=Pn } when flag is Closed[??]

• { l1=P1; ...; ln=Pn; _} when flag is Open[??]

Invariant: n > 0
| Ppat_array of pattern list
Pattern [| P1; ...; Pn |]
| Ppat_or of pattern * pattern
Pattern P1 | P2
| Ppat_constraint of pattern * core_type
Pattern (P : T)
| Ppat_type of Longident.t Asttypes.loc
Pattern #tconst
| Ppat_lazy of pattern
Pattern lazy P
| Ppat_unpack of string option Asttypes.loc
Ppat_unpack(s) represents:

• (module P) when s is Some "P"

• (module _) when s is None

Note: (module P : S) is represented as Ppat_constraint(Ppat_unpack(Some "P"),

Ptyp_package S)
| Ppat_exception of pattern
Pattern exception P
| Ppat_effect of pattern * pattern
| Ppat_extension of extension
Pattern [%id]
| Ppat_open of Longident.t Asttypes.loc * pattern
Pattern M.(P)

Value expressions
type expression =
{ pexp_desc : expression_desc ;
pexp_loc : Location.t ;
pexp_loc_stack : location_stack ;
pexp_attributes : attributes ;
... [@id1] [@id2]
938

}
type expression_desc =
| Pexp_ident of Longident.t Asttypes.loc
Identifiers such as x and M.x
| Pexp_constant of constant
Expressions constant such as 1, 'a', "true", 1.0, 1l, 1L, 1n
| Pexp_let of Asttypes.rec_flag * value_binding list * expression
Pexp_let(flag, [(P1,E1) ; ... ; (Pn,En)], E) represents:

• let P1 = E1 and ... and Pn = EN in E when flag is Nonrecursive[??],

• let rec P1 = E1 and ... and Pn = EN in E when flag is Recursive[??].

| Pexp_function of function_param list * type_constraint option

* function_body
Pexp_function ([P1; ...; Pn], C, body) represents any construct involving fun
or function, including:

• fun P1 ... Pn -> E when body = Pfunction_body E

• fun P1 ... Pn -> function p1 -> e1 | ... | pm -> em when body =
Pfunction_cases [ p1 -> e1; ...; pm -> em ]

C represents a type constraint or coercion placed immediately before the arrow, e.g. fun
P1 ... Pn : ty -> ... when C = Some (Pconstraint ty).
A function must have parameters. Pexp_function (params, _, body) must have
non-empty params or a Pfunction_cases _ body.
| Pexp_apply of expression * (Asttypes.arg_label * expression) list
Pexp_apply(E0, [(l1, E1) ; ... ; (ln, En)]) represents E0 ~l1:E1 ...
~ln:En
li can be Nolabel[??] (non labeled argument), Labelled[??] (labelled arguments) or
Optional[??] (optional argument).
Invariant: n > 0
| Pexp_match of expression * case list
match E0 with P1 -> E1 | ... | Pn -> En
| Pexp_try of expression * case list
try E0 with P1 -> E1 | ... | Pn -> En
| Pexp_tuple of expression list
Expressions (E1, ..., En)
Invariant: n >= 2
| Pexp_construct of Longident.t Asttypes.loc * expression option
Chapter 30. The compiler front-end 939

Pexp_construct(C, exp) represents:

• C when exp is None,

• C E when exp is Some E,
• C (E1, ..., En) when exp is Some (Pexp_tuple[E1;...;En])

| Pexp_variant of Asttypes.label * expression option

Pexp_variant(`A, exp) represents

• `A when exp is None

• `A E when exp is Some E

| Pexp_record of (Longident.t Asttypes.loc * expression) list

* expression option
Pexp_record([(l1,P1) ; ... ; (ln,Pn)], exp0) represents

• { l1=P1; ...; ln=Pn } when exp0 is None

• { E0 with l1=P1; ...; ln=Pn } when exp0 is Some E0

Invariant: n > 0
| Pexp_field of expression * Longident.t Asttypes.loc
E.l
| Pexp_setfield of expression * Longident.t Asttypes.loc * expression
E1.l <- E2
| Pexp_array of expression list
[| E1; ...; En |]
| Pexp_ifthenelse of expression * expression * expression option
if E1 then E2 else E3
| Pexp_sequence of expression * expression
E1; E2
| Pexp_while of expression * expression
while E1 do E2 done
| Pexp_for of pattern * expression * expression
* Asttypes.direction_flag * expression
Pexp_for(i, E1, E2, direction, E3) represents:

• for i = E1 to E2 do E3 done when direction is Upto[??]

• for i = E1 downto E2 do E3 done when direction is Downto[??]

| Pexp_constraint of expression * core_type

(E : T)
940

| Pexp_coerce of expression * core_type option * core_type

Pexp_coerce(E, from, T) represents

• (E :> T) when from is None,

• (E : T0 :> T) when from is Some T0.

| Pexp_send of expression * Asttypes.label Asttypes.loc

E # m
| Pexp_new of Longident.t Asttypes.loc
new M.c
| Pexp_setinstvar of Asttypes.label Asttypes.loc * expression
x <- 2
| Pexp_override of (Asttypes.label Asttypes.loc * expression) list
{< x1 = E1; ...; xn = En >}
| Pexp_letmodule of string option Asttypes.loc * module_expr * expression
let module M = ME in E
| Pexp_letexception of extension_constructor * expression
let exception C in E
| Pexp_assert of expression
assert E.
Note: assert false is treated in a special way by the type-checker.
| Pexp_lazy of expression
lazy E
| Pexp_poly of expression * core_type option
Used for method bodies.
Can only be used as the expression under Cfk_concrete[??] for methods (not values).
| Pexp_object of class_structure
object ... end
| Pexp_newtype of string Asttypes.loc * expression
fun (type t) -> E
| Pexp_pack of module_expr
(module ME).
(module ME : S) is represented as Pexp_constraint(Pexp_pack ME, Ptyp_package
S)
| Pexp_open of open_declaration * expression
- M.(E)
Chapter 30. The compiler front-end 941

• let open M in E
• let open! M in E

| Pexp_letop of letop
- let* P = E0 in E1

• let* P0 = E00 and* P1 = E01 in E1

| Pexp_extension of extension
[%id]
| Pexp_unreachable
.
type case =
{ pc_lhs : pattern ;
pc_guard : expression option ;
pc_rhs : expression ;
}
Values of type Parsetree.case[30.6] represents (P -> E) or (P when E0 -> E)

type letop =
{ let_ : binding_op ;
ands : binding_op list ;
body : expression ;
}
type binding_op =
{ pbop_op : string Asttypes.loc ;
pbop_pat : pattern ;
pbop_exp : expression ;
pbop_loc : Location.t ;
}
type function_param_desc =
| Pparam_val of Asttypes.arg_label * expression option * pattern
Pparam_val (lbl, exp0, P) represents the parameter:

• P when lbl is Nolabel[??] and exp0 is None

• ~l:P when lbl is Labelled l[??] and exp0 is None
• ?l:P when lbl is Optional l[??] and exp0 is None
• ?l:(P = E0) when lbl is Optional l[??] and exp0 is Some E0

Note: If E0 is provided, only Optional[??] is allowed.

| Pparam_newtype of string Asttypes.loc
942

Pparam_newtype x represents the parameter (type x). x carries the location of the
identifier, whereas the pparam_loc on the enclosing function_param node is the
location of the (type x) as a whole.
Multiple parameters (type a b c) are represented as multiple Pparam_newtype nodes,
let’s say:
[ { pparam_kind = Pparam_newtype a; pparam_loc = loc1 };
{ pparam_kind = Pparam_newtype b; pparam_loc = loc2 };
{ pparam_kind = Pparam_newtype c; pparam_loc = loc3 };
]

Here, the first loc loc1 is the location of (type a b c), and the subsequent locs loc2
and loc3 are the same as loc1, except marked as ghost locations. The locations on a,
b, c, correspond to the variables a, b, and c in the source code.
type function_param =
{ pparam_loc : Location.t ;
pparam_desc : function_param_desc ;
}
type function_body =
| Pfunction_body of expression
| Pfunction_cases of case list * Location.t * attributes
In Pfunction_cases (_, loc, attrs), the location extends from the start of the
function keyword to the end of the last case. The compiler will only use
typechecking-related attributes from attrs, e.g. enabling or disabling a warning.
See the comment on Pexp_function[??].

type type_constraint =
| Pconstraint of core_type
| Pcoerce of core_type option * core_type
See the comment on Pexp_function[??].

Value descriptions
type value_description =
{ pval_name : string Asttypes.loc ;
pval_type : core_type ;
pval_prim : string list ;
pval_attributes : attributes ;
... [@@id1] [@@id2]
pval_loc : Location.t ;
}
Values of type Parsetree.value_description[30.6] represents:
• val x: T, when pval_prim[??] is []
Chapter 30. The compiler front-end 943

• external x: T = "s1" ... "sn" when pval_prim[??] is ["s1";..."sn"]

Type declarations
type type_declaration =
{ ptype_name : string Asttypes.loc ;
ptype_params : (core_type * (Asttypes.variance * Asttypes.injectivity)) list ;
('a1,...'an) t
ptype_cstrs : (core_type * core_type * Location.t) list ;
... constraint T1=T1' ... constraint Tn=Tn'
ptype_kind : type_kind ;
ptype_private : Asttypes.private_flag ;
for = private ...
ptype_manifest : core_type option ;
represents = T
ptype_attributes : attributes ;
... [@@id1] [@@id2]
ptype_loc : Location.t ;
}
Here are type declarations and their representation, for various ptype_kind[??] and
ptype_manifest[??] values:

• type t when type_kind is Ptype_abstract[??], and manifest is None,

• type t = T0 when type_kind is Ptype_abstract[??], and manifest is Some T0,
• type t = C of T | ... when type_kind is Ptype_variant[??], and manifest is
None,
• type t = T0 = C of T | ... when type_kind is Ptype_variant[??], and manifest
is Some T0,
• type t = {l: T; ...} when type_kind is Ptype_record[??], and manifest is None,
• type t = T0 = {l : T; ...} when type_kind is Ptype_record[??], and manifest
is Some T0,
• type t = .. when type_kind is Ptype_open[??], and manifest is None.

type type_kind =
| Ptype_abstract
| Ptype_variant of constructor_declaration list
| Ptype_record of label_declaration list
Invariant: non-empty list
944

| Ptype_open
type label_declaration =
{ pld_name : string Asttypes.loc ;
pld_mutable : Asttypes.mutable_flag ;
pld_type : core_type ;
pld_loc : Location.t ;
pld_attributes : attributes ;
l : T [@id1] [@id2]
}
- { ...; l: T; ... } when pld_mutable[??] is Immutable[??],
• { ...; mutable l: T; ... } when pld_mutable[??] is Mutable[??].
Note: T can be a Ptyp_poly[??].

type constructor_declaration =
{ pcd_name : string Asttypes.loc ;
pcd_vars : string Asttypes.loc list ;
pcd_args : constructor_arguments ;
pcd_res : core_type option ;
pcd_loc : Location.t ;
pcd_attributes : attributes ;
C of ... [@id1] [@id2]
}
type constructor_arguments =
| Pcstr_tuple of core_type list
| Pcstr_record of label_declaration list
Values of type Parsetree.constructor_declaration[30.6] represents the constructor
arguments of:
• C of T1 * ... * Tn when res = None, and args = Pcstr_tuple [T1; ... ;
Tn],
• C: T0 when res = Some T0, and args = Pcstr_tuple [],
• C: T1 * ... * Tn -> T0 when res = Some T0, and args = Pcstr_tuple [T1;
... ; Tn],
• C of {...} when res = None, and args = Pcstr_record [...],
• C: {...} -> T0 when res = Some T0, and args = Pcstr_record [...].

type type_extension =
{ ptyext_path : Longident.t Asttypes.loc ;
ptyext_params : (core_type * (Asttypes.variance * Asttypes.injectivity)) list ;
ptyext_constructors : extension_constructor list ;
ptyext_private : Asttypes.private_flag ;
ptyext_loc : Location.t ;
ptyext_attributes : attributes ;
Chapter 30. The compiler front-end 945

. . . @@id1 @@id2
}
Definition of new extensions constructors for the extensive sum type t (type t += ...).

type extension_constructor =
{ pext_name : string Asttypes.loc ;
pext_kind : extension_constructor_kind ;
pext_loc : Location.t ;
pext_attributes : attributes ;
C of ... [@id1] [@id2]
}
type type_exception =
{ ptyexn_constructor : extension_constructor ;
ptyexn_loc : Location.t ;
ptyexn_attributes : attributes ;
... [@@id1] [@@id2]
}
Definition of a new exception (exception E).

type extension_constructor_kind =
| Pext_decl of string Asttypes.loc list * constructor_arguments
* core_type option
Pext_decl(existentials, c_args, t_opt) describes a new extension constructor.
It can be:

• C of T1 * ... * Tn when:
– existentials is [],
– c_args is [T1; ...; Tn],
– t_opt is None
• C: T0 when
– existentials is [],
– c_args is [],
– t_opt is Some T0.
• C: T1 * ... * Tn -> T0 when
– existentials is [],
– c_args is [T1; ...; Tn],
– t_opt is Some T0.
• C: 'a... . T1 * ... * Tn -> T0 when
– existentials is ['a;...],
– c_args is [T1; ... ; Tn],
946

– t_opt is Some T0.

| Pext_rebind of Longident.t Asttypes.loc

Pext_rebind(D) re-export the constructor D with the new name C

Class language
Type expressions for the class language
type class_type =
{ pcty_desc : class_type_desc ;
pcty_loc : Location.t ;
pcty_attributes : attributes ;
... [@id1] [@id2]
}
type class_type_desc =
| Pcty_constr of Longident.t Asttypes.loc * core_type list
-c

• ['a1, ..., 'an] c

| Pcty_signature of class_signature
object ... end
| Pcty_arrow of Asttypes.arg_label * core_type * class_type
Pcty_arrow(lbl, T, CT) represents:

• T -> CT when lbl is Nolabel[??],

• ~l:T -> CT when lbl is Labelled l[??],
• ?l:T -> CT when lbl is Optional l[??].

| Pcty_extension of extension
%id
| Pcty_open of open_description * class_type
let open M in CT
type class_signature =
{ pcsig_self : core_type ;
pcsig_fields : class_type_field list ;
}
Values of type class_signature represents:

• object('selfpat) ... end

• object ... end when pcsig_self[??] is Ptyp_any[??]
Chapter 30. The compiler front-end 947

type class_type_field =
{ pctf_desc : class_type_field_desc ;
pctf_loc : Location.t ;
pctf_attributes : attributes ;
... [@@id1] [@@id2]
}
type class_type_field_desc =
| Pctf_inherit of class_type
inherit CT
| Pctf_val of (Asttypes.label Asttypes.loc * Asttypes.mutable_flag *
Asttypes.virtual_flag * core_type)
val x: T
| Pctf_method of (Asttypes.label Asttypes.loc * Asttypes.private_flag *
Asttypes.virtual_flag * core_type)
method x: T
Note: T can be a Ptyp_poly[??].
| Pctf_constraint of (core_type * core_type)
constraint T1 = T2
| Pctf_attribute of attribute
[@@@id]
| Pctf_extension of extension
[%%id]
type 'a class_infos =
{ pci_virt : Asttypes.virtual_flag ;
pci_params : (core_type * (Asttypes.variance * Asttypes.injectivity)) list ;
pci_name : string Asttypes.loc ;
pci_expr : 'a ;
pci_loc : Location.t ;
pci_attributes : attributes ;
... [@@id1] [@@id2]
}
Values of type class_expr class_infos represents:
• class c = ...
• class ['a1,...,'an] c = ...
• class virtual c = ...
They are also used for ”class type” declaration.

type class_description = class_type class_infos

type class_type_declaration = class_type class_infos
948

Value expressions for the class language

type class_expr =
{ pcl_desc : class_expr_desc ;
pcl_loc : Location.t ;
pcl_attributes : attributes ;
... [@id1] [@id2]
}
type class_expr_desc =
| Pcl_constr of Longident.t Asttypes.loc * core_type list
c and ['a1, ..., 'an] c
| Pcl_structure of class_structure
object ... end
| Pcl_fun of Asttypes.arg_label * expression option * pattern
* class_expr
Pcl_fun(lbl, exp0, P, CE) represents:

• fun P -> CE when lbl is Nolabel[??] and exp0 is None,

• fun ~l:P -> CE when lbl is Labelled l[??] and exp0 is None,
• fun ?l:P -> CE when lbl is Optional l[??] and exp0 is None,
• fun ?l:(P = E0) -> CE when lbl is Optional l[??] and exp0 is Some E0.

| Pcl_apply of class_expr * (Asttypes.arg_label * expression) list

Pcl_apply(CE, [(l1,E1) ; ... ; (ln,En)]) represents CE ~l1:E1 ... ~ln:En.
li can be empty (non labeled argument) or start with ? (optional argument).
Invariant: n > 0
| Pcl_let of Asttypes.rec_flag * value_binding list * class_expr
Pcl_let(rec, [(P1, E1); ... ; (Pn, En)], CE) represents:

• let P1 = E1 and ... and Pn = EN in CE when rec is Nonrecursive[??],

• let rec P1 = E1 and ... and Pn = EN in CE when rec is Recursive[??].

| Pcl_constraint of class_expr * class_type

(CE : CT)
| Pcl_extension of extension
[%id]
| Pcl_open of open_description * class_expr
let open M in CE
Chapter 30. The compiler front-end 949

type class_structure =
{ pcstr_self : pattern ;
pcstr_fields : class_field list ;
}
Values of type Parsetree.class_structure[30.6] represents:

• object(selfpat) ... end

• object ... end when pcstr_self[??] is Ppat_any[??]

type class_field =
{ pcf_desc : class_field_desc ;
pcf_loc : Location.t ;
pcf_attributes : attributes ;
... [@@id1] [@@id2]
}
type class_field_desc =
| Pcf_inherit of Asttypes.override_flag * class_expr * string Asttypes.loc option
Pcf_inherit(flag, CE, s) represents:

• inherit CE when flag is Fresh[??] and s is None,

• inherit CE as x when flag is Fresh[??] and s is Some x,
• inherit! CE when flag is Override[??] and s is None,
• inherit! CE as x when flag is Override[??] and s is Some x

| Pcf_val of (Asttypes.label Asttypes.loc * Asttypes.mutable_flag *

class_field_kind)
Pcf_val(x,flag, kind) represents:

• val x = E when flag is Immutable[??] and kind is Cfk_concrete(Fresh,

E)[??]
• val virtual x: T when flag is Immutable[??] and kind is Cfk_virtual(T)[??]
• val mutable x = E when flag is Mutable[??] and kind is
Cfk_concrete(Fresh, E)[??]
• val mutable virtual x: T when flag is Mutable[??] and kind is
Cfk_virtual(T)[??]

| Pcf_method of (Asttypes.label Asttypes.loc * Asttypes.private_flag *

class_field_kind)
- method x = E (E can be a Pexp_poly[??])

• method virtual x: T (T can be a Ptyp_poly[??])

| Pcf_constraint of (core_type * core_type)

950

constraint T1 = T2
| Pcf_initializer of expression
initializer E
| Pcf_attribute of attribute
[@@@id]
| Pcf_extension of extension
[%%id]
type class_field_kind =
| Cfk_virtual of core_type
| Cfk_concrete of Asttypes.override_flag * expression
type class_declaration = class_expr class_infos

Module language
Type expressions for the module language
type module_type =
{ pmty_desc : module_type_desc ;
pmty_loc : Location.t ;
pmty_attributes : attributes ;
... [@id1] [@id2]
}
type module_type_desc =
| Pmty_ident of Longident.t Asttypes.loc
Pmty_ident(S) represents S
| Pmty_signature of signature
sig ... end
| Pmty_functor of functor_parameter * module_type
functor(X : MT1) -> MT2
| Pmty_with of module_type * with_constraint list
MT with ...
| Pmty_typeof of module_expr
module type of ME
| Pmty_extension of extension
[%id]
| Pmty_alias of Longident.t Asttypes.loc
(module M)
Chapter 30. The compiler front-end 951

type functor_parameter =
| Unit
()
| Named of string option Asttypes.loc * module_type
Named(name, MT) represents:

• (X : MT) when name is Some X,

• (_ : MT) when name is None

type signature = signature_item list

type signature_item =
{ psig_desc : signature_item_desc ;
psig_loc : Location.t ;
}
type signature_item_desc =
| Psig_value of value_description
- val x: T

• external x: T = "s1" ... "sn"

| Psig_type of Asttypes.rec_flag * type_declaration list

type t1 = ... and ... and tn = ...
| Psig_typesubst of type_declaration list
type t1 := ... and ... and tn := ...
| Psig_typext of type_extension
type t1 += ...
| Psig_exception of type_exception
exception C of T
| Psig_module of module_declaration
module X = M and module X : MT
| Psig_modsubst of module_substitution
module X := M
| Psig_recmodule of module_declaration list
module rec X1 : MT1 and ... and Xn : MTn
| Psig_modtype of module_type_declaration
module type S = MT and module type S
| Psig_modtypesubst of module_type_declaration
module type S := ...
| Psig_open of open_description
952

open X
| Psig_include of include_description
include MT
| Psig_class of class_description list
class c1 : ... and ... and cn : ...
| Psig_class_type of class_type_declaration list
class type ct1 = ... and ... and ctn = ...
| Psig_attribute of attribute
[@@@id]
| Psig_extension of extension * attributes
[%%id]
type module_declaration =
{ pmd_name : string option Asttypes.loc ;
pmd_type : module_type ;
pmd_attributes : attributes ;
... [@@id1] [@@id2]
pmd_loc : Location.t ;
}
Values of type module_declaration represents S : MT

type module_substitution =
{ pms_name : string Asttypes.loc ;
pms_manifest : Longident.t Asttypes.loc ;
pms_attributes : attributes ;
... [@@id1] [@@id2]
pms_loc : Location.t ;
}
Values of type module_substitution represents S := M

type module_type_declaration =
{ pmtd_name : string Asttypes.loc ;
pmtd_type : module_type option ;
pmtd_attributes : attributes ;
... [@@id1] [@@id2]
pmtd_loc : Location.t ;
}
Values of type module_type_declaration represents:

• S = MT,
Chapter 30. The compiler front-end 953

• S for abstract module type declaration, when pmtd_type[??] is None.

type 'a open_infos =

{ popen_expr : 'a ;
popen_override : Asttypes.override_flag ;
popen_loc : Location.t ;
popen_attributes : attributes ;
}
Values of type 'a open_infos represents:

• open! X when popen_override[??] is Override[??] (silences the ”used identifier

shadowing” warning)
• open X when popen_override[??] is Fresh[??]

type open_description = Longident.t Asttypes.loc open_infos

Values of type open_description represents:

• open M.N
• open M(N).O

type open_declaration = module_expr open_infos

Values of type open_declaration represents:

• open M.N
• open M(N).O
• open struct ... end

type 'a include_infos =

{ pincl_mod : 'a ;
pincl_loc : Location.t ;
pincl_attributes : attributes ;
}
type include_description = module_type include_infos
Values of type include_description represents include MT

type include_declaration = module_expr include_infos

Values of type include_declaration represents include ME

type with_constraint =
| Pwith_type of Longident.t Asttypes.loc * type_declaration
with type X.t = ...
Note: the last component of the longident must match the name of the
type_declaration.
954

| Pwith_module of Longident.t Asttypes.loc * Longident.t Asttypes.loc

with module X.Y = Z
| Pwith_modtype of Longident.t Asttypes.loc * module_type
with module type X.Y = Z
| Pwith_modtypesubst of Longident.t Asttypes.loc * module_type
with module type X.Y := sig end
| Pwith_typesubst of Longident.t Asttypes.loc * type_declaration
with type X.t := ..., same format as [Pwith_type]
| Pwith_modsubst of Longident.t Asttypes.loc * Longident.t Asttypes.loc
with module X.Y := Z

Value expressions for the module language

type module_expr =
{ pmod_desc : module_expr_desc ;
pmod_loc : Location.t ;
pmod_attributes : attributes ;
... [@id1] [@id2]
}
type module_expr_desc =
| Pmod_ident of Longident.t Asttypes.loc
X
| Pmod_structure of structure
struct ... end
| Pmod_functor of functor_parameter * module_expr
functor(X : MT1) -> ME
| Pmod_apply of module_expr * module_expr
ME1(ME2)
| Pmod_apply_unit of module_expr
ME1()
| Pmod_constraint of module_expr * module_type
(ME : MT)
| Pmod_unpack of expression
(val E)
| Pmod_extension of extension
[%id]
Chapter 30. The compiler front-end 955

type structure = structure_item list

type structure_item =
{ pstr_desc : structure_item_desc ;
pstr_loc : Location.t ;
}
type structure_item_desc =
| Pstr_eval of expression * attributes
E
| Pstr_value of Asttypes.rec_flag * value_binding list
Pstr_value(rec, [(P1, E1 ; ... ; (Pn, En))]) represents:

• let P1 = E1 and ... and Pn = EN when rec is Nonrecursive[??],

• let rec P1 = E1 and ... and Pn = EN when rec is Recursive[??].

| Pstr_primitive of value_description
- val x: T

• external x: T = "s1" ... "sn"

| Pstr_type of Asttypes.rec_flag * type_declaration list

type t1 = ... and ... and tn = ...
| Pstr_typext of type_extension
type t1 += ...
| Pstr_exception of type_exception
- exception C of T

• exception C = M.X

| Pstr_module of module_binding
module X = ME
| Pstr_recmodule of module_binding list
module rec X1 = ME1 and ... and Xn = MEn
| Pstr_modtype of module_type_declaration
module type S = MT
| Pstr_open of open_declaration
open X
| Pstr_class of class_declaration list
class c1 = ... and ... and cn = ...
| Pstr_class_type of class_type_declaration list
class type ct1 = ... and ... and ctn = ...
956

| Pstr_include of include_declaration
include ME
| Pstr_attribute of attribute
[@@@id]
| Pstr_extension of extension * attributes
[%%id]
type value_constraint =
| Pvc_constraint of { locally_abstract_univars : string Asttypes.loc list ;
typ : core_type ;
}
| Pvc_coercion of { ground : core_type option ;
coercion : core_type ;
}
- Pvc_constraint { locally_abstract_univars=[]; typ} is a simple type
constraint on a value binding: let x : typ

• More generally, in Pvc_constraint { locally_abstract_univars; typ}

locally_abstract_univars is the list of locally abstract type variables in let
x: type a ... . typ
• Pvc_coercion { ground=None; coercion } represents let x :> typ
• Pvc_coercion { ground=Some g; coercion } represents let x : g :> typ

type value_binding =
{ pvb_pat : pattern ;
pvb_expr : expression ;
pvb_constraint : value_constraint option ;
pvb_attributes : attributes ;
pvb_loc : Location.t ;
}
let pat : type_constraint = exp

type module_binding =
{ pmb_name : string option Asttypes.loc ;
pmb_expr : module_expr ;
pmb_attributes : attributes ;
pmb_loc : Location.t ;
}
Values of type module_binding represents module X = ME
Chapter 30. The compiler front-end 957

Toplevel
Toplevel phrases
type toplevel_phrase =
| Ptop_def of structure
| Ptop_dir of toplevel_directive
#use, #load . . .
type toplevel_directive =
{ pdir_name : string Asttypes.loc ;
pdir_arg : directive_argument option ;
pdir_loc : Location.t ;
}
type directive_argument =
{ pdira_desc : directive_argument_desc ;
pdira_loc : Location.t ;
}
type directive_argument_desc =
| Pdir_string of string
| Pdir_int of string * char option
| Pdir_ident of Longident.t
| Pdir_bool of bool

30.7 Module Pprintast : Pretty-printers for Parsetree[30.6]

Warning: this module is unstable and part of compiler-libs[30].

type space_formatter = (unit, Format.formatter, unit) format

val longident : Format.formatter -> Longident.t -> unit
val constr : Format.formatter -> Longident.t -> unit
val expression : Format.formatter -> Parsetree.expression -> unit
val string_of_expression : Parsetree.expression -> string
val pattern : Format.formatter -> Parsetree.pattern -> unit
val core_type : Format.formatter -> Parsetree.core_type -> unit
val signature : Format.formatter -> Parsetree.signature -> unit
val structure : Format.formatter -> Parsetree.structure -> unit
val string_of_structure : Parsetree.structure -> string
val module_expr : Format.formatter -> Parsetree.module_expr -> unit
val toplevel_phrase : Format.formatter -> Parsetree.toplevel_phrase -> unit
val top_phrase : Format.formatter -> Parsetree.toplevel_phrase -> unit
val class_field : Format.formatter -> Parsetree.class_field -> unit
958

val class_type_field : Format.formatter -> Parsetree.class_type_field -> unit

val class_expr : Format.formatter -> Parsetree.class_expr -> unit
val class_type : Format.formatter -> Parsetree.class_type -> unit
val module_type : Format.formatter -> Parsetree.module_type -> unit
val structure_item : Format.formatter -> Parsetree.structure_item -> unit
val signature_item : Format.formatter -> Parsetree.signature_item -> unit
val binding : Format.formatter -> Parsetree.value_binding -> unit
val payload : Format.formatter -> Parsetree.payload -> unit
val tyvar_of_name : string -> string
Turn a type variable name into a valid identifier, taking care of the special treatment required
for the single quote character in second position, or for keywords by escaping them with \#.
No-op on ”_”.

val tyvar : Format.formatter -> string -> unit

Print a type variable name as a valid identifier, taking care of the special treatment required
for the single quote character in second position, or for keywords by escaping them with \#.
No-op on ”_”.

module Doc :
sig
val longident : Longident.t Format_doc.printer
val constr : Longident.t Format_doc.printer
val tyvar : string Format_doc.printer
val nominal_exp : Parsetree.expression -> Format_doc.t option
Returns a format document if the expression reads nicely as the subject of a sentence in
a error message.

end

Format_doc[??] functions for error messages

Chapter 31

The unix library: Unix system calls

The unix library makes many Unix system calls and system-related library functions available to
OCaml programs. This chapter describes briefly the functions provided. Refer to sections 2 and 3
of the Unix manual for more details on the behavior of these functions.
Not all functions are provided by all Unix variants. If some functions are not available, they will
raise Invalid_arg when called.
Programs that use the unix library must be linked as follows:

ocamlc other options -I +unix unix.cma other files

ocamlopt other options -I +unix unix.cmxa other files

For interactive use of the unix library, do:

ocamlmktop -o mytop -I +unix unix.cma

./mytop

or (if dynamic linking of C libraries is supported on your platform), start ocaml and type
# #directory "+unix";;
# #load "unix.cma";;

Windows:
A fairly complete emulation of the Unix system calls is provided in the Windows version
of OCaml. The end of this chapter gives more information on the functions that are not
supported under Windows.

31.1 Module Unix : Interface to the Unix system.

To use the labeled version of this module, add module Unix = UnixLabels in your implementation.
Note: all the functions of this module (except Unix.error_message[31.1] and Unix.handle_
unix_error[31.1]) are liable to raise the Unix.Unix_error[31.1] exception whenever the underlying
system call signals an error.

959
960

Error report
type error =
| E2BIG
Argument list too long
| EACCES
Permission denied
| EAGAIN
Resource temporarily unavailable; try again
| EBADF
Bad file descriptor
| EBUSY
Resource unavailable
| ECHILD
No child process
| EDEADLK
Resource deadlock would occur
| EDOM
Domain error for math functions, etc.
| EEXIST
File exists
| EFAULT
Bad address
| EFBIG
File too large
| EINTR
Function interrupted by signal
| EINVAL
Invalid argument
| EIO
Hardware I/O error
| EISDIR
Is a directory
| EMFILE
Too many open files by the process
Chapter 31. The unix library: Unix system calls 961

| EMLINK
Too many links
| ENAMETOOLONG
Filename too long
| ENFILE
Too many open files in the system
| ENODEV
No such device
| ENOENT
No such file or directory
| ENOEXEC
Not an executable file
| ENOLCK
No locks available
| ENOMEM
Not enough memory
| ENOSPC
No space left on device
| ENOSYS
Function not supported
| ENOTDIR
Not a directory
| ENOTEMPTY
Directory not empty
| ENOTTY
Inappropriate I/O control operation
| ENXIO
No such device or address
| EPERM
Operation not permitted
| EPIPE
Broken pipe
| ERANGE
Result too large
962

| EROFS
Read-only file system
| ESPIPE
Invalid seek e.g. on a pipe
| ESRCH
No such process
| EXDEV
Invalid link
| EWOULDBLOCK
Operation would block
| EINPROGRESS
Operation now in progress
| EALREADY
Operation already in progress
| ENOTSOCK
Socket operation on non-socket
| EDESTADDRREQ
Destination address required
| EMSGSIZE
Message too long
| EPROTOTYPE
Protocol wrong type for socket
| ENOPROTOOPT
Protocol not available
| EPROTONOSUPPORT
Protocol not supported
| ESOCKTNOSUPPORT
Socket type not supported
| EOPNOTSUPP
Operation not supported on socket
| EPFNOSUPPORT
Protocol family not supported
| EAFNOSUPPORT
Address family not supported by protocol family
Chapter 31. The unix library: Unix system calls 963

| EADDRINUSE
Address already in use
| EADDRNOTAVAIL
Can’t assign requested address
| ENETDOWN
Network is down
| ENETUNREACH
Network is unreachable
| ENETRESET
Network dropped connection on reset
| ECONNABORTED
Software caused connection abort
| ECONNRESET
Connection reset by peer
| ENOBUFS
No buffer space available
| EISCONN
Socket is already connected
| ENOTCONN
Socket is not connected
| ESHUTDOWN
Can’t send after socket shutdown
| ETOOMANYREFS
Too many references: can’t splice
| ETIMEDOUT
Connection timed out
| ECONNREFUSED
Connection refused
| EHOSTDOWN
Host is down
| EHOSTUNREACH
No route to host
| ELOOP
Too many levels of symbolic links
964

| EOVERFLOW
File size or position not representable
| EUNKNOWNERR of int
Unknown error
The type of error codes. Errors defined in the POSIX standard and additional errors from
UNIX98 and BSD. All other errors are mapped to EUNKNOWNERR.

exception Unix_error of error * string * string

Raised by the system calls below when an error is encountered. The first component is the
error code; the second component is the function name; the third component is the string
parameter to the function, if it has one, or the empty string otherwise.
UnixLabels.Unix_error[??] and Unix.Unix_error[31.1] are the same, and catching one
will catch the other.

val error_message : error -> string

Return a string describing the given error code.

val handle_unix_error : ('a -> 'b) -> 'a -> 'b

handle_unix_error f x applies f to x and returns the result. If the exception
Unix.Unix_error[31.1] is raised, it prints a message describing the error and exits with code
2.

Access to the process environment

val environment : unit -> string array
Return the process environment, as an array of strings with the format “variable=value”. The
returned array is empty if the process has special privileges.

val unsafe_environment : unit -> string array

Return the process environment, as an array of strings with the format “variable=value”.
Unlike Unix.environment[31.1], this function returns a populated array even if the process
has special privileges. See the documentation for Unix.unsafe_getenv[31.1] for more details.
Since: 4.06 (4.12 in UnixLabels)

val getenv : string -> string

Return the value associated to a variable in the process environment, unless the process has
special privileges.
Raises Not_found if the variable is unbound or the process has special privileges.
This function is identical to Sys.getenv[29.56].

val unsafe_getenv : string -> string

Chapter 31. The unix library: Unix system calls 965

Return the value associated to a variable in the process environment.

Unlike Unix.getenv[31.1], this function returns the value even if the process has special
privileges. It is considered unsafe because the programmer of a setuid or setgid program must
be careful to avoid using maliciously crafted environment variables in the search path for
executables, the locations for temporary files or logs, and the like.
Since: 4.06
Raises Not_found if the variable is unbound.

val putenv : string -> string -> unit

putenv name value sets the value associated to a variable in the process environment. name
is the name of the environment variable, and value its new associated value.

Process handling
type process_status =
| WEXITED of int
The process terminated normally by exit; the argument is the return code.
| WSIGNALED of int
The process was killed by a signal; the argument is the signal number.
| WSTOPPED of int
The process was stopped by a signal; the argument is the signal number.
The termination status of a process. See module Sys[29.56] for the definitions of the standard
signal numbers. Note that they are not the numbers used by the OS.
On Windows: only WEXITED is used (as there are no inter-process signals) but with specific
return codes to indicate special termination causes. Look for NTSTATUS values in the Windows
documentation to decode such error return codes. In particular, STATUS_ACCESS_VIOLATION
error code is the 32-bit 0xC0000005: as Int32.of_int 0xC0000005 is -1073741819,
WEXITED -1073741819 is the Windows equivalent of WSIGNALED Sys.sigsegv.

type wait_flag =
| WNOHANG
Do not block if no child has died yet, but immediately return with a pid equal to 0.
| WUNTRACED
Report also the children that receive stop signals.
Flags for Unix.waitpid[31.1].

val execv : string -> string array -> 'a

execv prog args execute the program in file prog, with the arguments args, and the
current process environment. Note that the first argument, args.(0), is by convention the
966

filename of the program being executed, just like Sys.argv.(0). These execv* functions
never return: on success, the current program is replaced by the new one.
On Windows: the CRT simply spawns a new process and exits the current one. This will
have unwanted consequences if e.g. another process is waiting on the current one. Using
Unix.create_process[31.1] or one of the open_process_* functions instead is
recommended.
Raises Unix_error on failure

val execve : string -> string array -> string array -> 'a
Same as Unix.execv[31.1], except that the third argument provides the environment to the
program executed.

val execvp : string -> string array -> 'a

Same as Unix.execv[31.1], except that the program is searched in the path.

val execvpe : string -> string array -> string array -> 'a
Same as Unix.execve[31.1], except that the program is searched in the path.

val fork : unit -> int

Fork a new process. The returned integer is 0 for the child process, the pid of the child
process for the parent process. It fails if the OCaml process is multi-core (any domain has
been spawned). In addition, if any thread from the Thread module has been spawned, then
the child process might be in a corrupted state.
Raises

• Invalid_argument on Windows. Use Unix.create_process[31.1] or threads instead.

• Failure if any domain has been spawned.

val wait : unit -> int * process_status

Wait until one of the children processes die, and return its pid and termination status.
Raises Invalid_argument on Windows. Use Unix.waitpid[31.1] instead.

val waitpid : wait_flag list -> int -> int * process_status

Same as Unix.wait[31.1], but waits for the child process whose pid is given. A pid of -1
means wait for any child. A pid of 0 means wait for any child in the same process group as
the current process. Negative pid arguments represent process groups. The list of options
indicates whether waitpid should return immediately without waiting, and whether it should
report stopped children.
On Windows: can only wait for a given PID, not any child process.

val system : string -> process_status

Chapter 31. The unix library: Unix system calls 967

Execute the given command, wait until it terminates, and return its termination status. The
string is interpreted by the shell /bin/sh (or the command interpreter cmd.exe on Windows)
and therefore can contain redirections, quotes, variables, etc. To properly quote whitespace
and shell special characters occurring in file names or command arguments, the use of
Filename.quote_command[29.20] is recommended. The result WEXITED 127 indicates that
the shell couldn’t be executed.

val _exit : int -> 'a

Terminate the calling process immediately, returning the given status code to the operating
system: usually 0 to indicate no errors, and a small positive integer to indicate failure. Unlike
exit[28.2], Unix._exit[31.1] performs no finalization whatsoever: functions registered with
at_exit[28.2] are not called, input/output channels are not flushed, and the C run-time
system is not finalized either.
The typical use of Unix._exit[31.1] is after a Unix.fork[31.1] operation, when the child
process runs into a fatal error and must exit. In this case, it is preferable to not perform any
finalization action in the child process, as these actions could interfere with similar actions
performed by the parent process. For example, output channels should not be flushed by the
child process, as the parent process may flush them again later, resulting in duplicate output.
Since: 4.12

val getpid : unit -> int

Return the pid of the process.

val getppid : unit -> int

Return the pid of the parent process.
Raises Invalid_argument on Windows (because it is meaningless)

val nice : int -> int

Change the process priority. The integer argument is added to the “nice” value. (Higher
values of the “nice” value mean lower priorities.) Return the new nice value.
Raises Invalid_argument on Windows

Basic file input/output

type file_descr
The abstract type of file descriptors.

val stdin : file_descr

File descriptor for standard input.

val stdout : file_descr

File descriptor for standard output.
968

val stderr : file_descr

File descriptor for standard error.

type open_flag =
| O_RDONLY
Open for reading
| O_WRONLY
Open for writing
| O_RDWR
Open for reading and writing
| O_NONBLOCK
Open in non-blocking mode
| O_APPEND
Open for append
| O_CREAT
Create if nonexistent
| O_TRUNC
Truncate to 0 length if existing
| O_EXCL
Fail if existing
| O_NOCTTY
Don’t make this dev a controlling tty
| O_DSYNC
Writes complete as ‘Synchronised I/O data integrity completion’
| O_SYNC
Writes complete as ‘Synchronised I/O file integrity completion’
| O_RSYNC
Reads complete as writes (depending on O_SYNC/O_DSYNC)
| O_SHARE_DELETE
Windows only: allow the file to be deleted while still open
| O_CLOEXEC
Set the close-on-exec flag on the descriptor returned by Unix.openfile[31.1]. See
Unix.set_close_on_exec[31.1] for more information.
| O_KEEPEXEC
Clear the close-on-exec flag. This is currently the default.
Chapter 31. The unix library: Unix system calls 969

The flags to Unix.openfile[31.1].

type file_perm = int

The type of file access rights, e.g. 0o640 is read and write for user, read for group, none for
others

val openfile : string -> open_flag list -> file_perm -> file_descr
Open the named file with the given flags. Third argument is the permissions to give to the
file if it is created (see Unix.umask[31.1]). Return a file descriptor on the named file.

val close : file_descr -> unit

Close a file descriptor.

val fsync : file_descr -> unit

Flush file buffers to disk.
Since: 4.08 (4.12 in UnixLabels)

val read : file_descr -> bytes -> int -> int -> int
read fd buf pos len reads len bytes from descriptor fd, storing them in byte sequence
buf, starting at position pos in buf. Return the number of bytes actually read.

val read_bigarray :
file_descr ->
('a, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
Bigarray.Array1.t -> int -> int -> int
Same as Unix.read[31.1], but read the data into a bigarray.
Since: 5.2

val write : file_descr -> bytes -> int -> int -> int
write fd buf pos len writes len bytes to descriptor fd, taking them from byte sequence
buf, starting at position pos in buff. Return the number of bytes actually written. write
repeats the writing operation until all bytes have been written or an error occurs.

val write_bigarray :
file_descr ->
('a, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
Bigarray.Array1.t -> int -> int -> int
Same as Unix.write[31.1], but take the data from a bigarray.
Since: 5.2

val single_write : file_descr -> bytes -> int -> int -> int
Same as Unix.write[31.1], but attempts to write only once. Thus, if an error occurs,
single_write guarantees that no data has been written.
970

val write_substring : file_descr -> string -> int -> int -> int
Same as Unix.write[31.1], but take the data from a string instead of a byte sequence.
Since: 4.02

val single_write_substring : file_descr -> string -> int -> int -> int
Same as Unix.single_write[31.1], but take the data from a string instead of a byte
sequence.
Since: 4.02

val single_write_bigarray :
file_descr ->
('a, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
Bigarray.Array1.t -> int -> int -> int
Same as Unix.single_write[31.1], but take the data from a bigarray.
Since: 5.2

Interfacing with the standard input/output library

val in_channel_of_descr : file_descr -> in_channel
Create an input channel reading from the given descriptor. The channel is initially in binary
mode; use set_binary_mode_in ic false if text mode is desired. Text mode is supported
only if the descriptor refers to a file or pipe, but is not supported if it refers to a socket.
On Windows: set_binary_mode_in[28.2] always fails on channels created with this function.
Beware that input channels are buffered, so more characters may have been read from the
descriptor than those accessed using channel functions. Channels also keep a copy of the
current position in the file.
Closing the channel ic returned by in_channel_of_descr fd using close_in ic also closes
the underlying descriptor fd. It is incorrect to close both the channel ic and the descriptor
fd.
If several channels are created on the same descriptor, one of the channels must be closed, but
not the others. Consider for example a descriptor s connected to a socket and two channels ic
= in_channel_of_descr s and oc = out_channel_of_descr s. The recommended closing
protocol is to perform close_out oc, which flushes buffered output to the socket then closes
the socket. The ic channel must not be closed and will be collected by the GC eventually.

val out_channel_of_descr : file_descr -> out_channel

Create an output channel writing on the given descriptor. The channel is initially in binary
mode; use set_binary_mode_out oc false if text mode is desired. Text mode is supported
only if the descriptor refers to a file or pipe, but is not supported if it refers to a socket.
On Windows: set_binary_mode_out[28.2] always fails on channels created with this
function.
Chapter 31. The unix library: Unix system calls 971

Beware that output channels are buffered, so you may have to call flush[28.2] to ensure that
all data has been sent to the descriptor. Channels also keep a copy of the current position in
the file.
Closing the channel oc returned by out_channel_of_descr fd using close_out oc also
closes the underlying descriptor fd. It is incorrect to close both the channel ic and the
descriptor fd.
See Unix.in_channel_of_descr[31.1] for a discussion of the closing protocol when several
channels are created on the same descriptor.

val descr_of_in_channel : in_channel -> file_descr

Return the descriptor corresponding to an input channel.

val descr_of_out_channel : out_channel -> file_descr

Return the descriptor corresponding to an output channel.

Seeking and truncating

type seek_command =
| SEEK_SET
indicates positions relative to the beginning of the file
| SEEK_CUR
indicates positions relative to the current position
| SEEK_END
indicates positions relative to the end of the file
Positioning modes for Unix.lseek[31.1].

val lseek : file_descr -> int -> seek_command -> int

Set the current position for a file descriptor, and return the resulting offset (from the
beginning of the file).

val truncate : string -> int -> unit

Truncates the named file to the given size.

val ftruncate : file_descr -> int -> unit

Truncates the file corresponding to the given descriptor to the given size.
972

File status
type file_kind =
| S_REG
Regular file
| S_DIR
Directory
| S_CHR
Character device
| S_BLK
Block device
| S_LNK
Symbolic link
| S_FIFO
Named pipe
| S_SOCK
Socket
type stats =
{ st_dev : int ;
Device number
st_ino : int ;
Inode number
st_kind : file_kind ;
Kind of the file
st_perm : file_perm ;
Access rights
st_nlink : int ;
Number of links
st_uid : int ;
User id of the owner
st_gid : int ;
Group ID of the file’s group
st_rdev : int ;
Device ID (if special file)
st_size : int ;
Size in bytes
Chapter 31. The unix library: Unix system calls 973

st_atime : float ;
Last access time
st_mtime : float ;
Last modification time
st_ctime : float ;
Last status change time
}
The information returned by the Unix.stat[31.1] calls.

val stat : string -> stats

Return the information for the named file.

val lstat : string -> stats

Same as Unix.stat[31.1], but in case the file is a symbolic link, return the information for
the link itself.

val fstat : file_descr -> stats

Return the information for the file associated with the given descriptor.

val isatty : file_descr -> bool

Return true if the given file descriptor refers to a terminal or console window, false
otherwise.

File operations on large files

module LargeFile :
sig
val lseek : Unix.file_descr -> int64 -> Unix.seek_command -> int64
See lseek.

val truncate : string -> int64 -> unit

See truncate.

val ftruncate : Unix.file_descr -> int64 -> unit

See ftruncate.

type stats =
{ st_dev : int ;
Device number
st_ino : int ;
974

Inode number
st_kind : Unix.file_kind ;
Kind of the file
st_perm : Unix.file_perm ;
Access rights
st_nlink : int ;
Number of links
st_uid : int ;
User id of the owner
st_gid : int ;
Group ID of the file’s group
st_rdev : int ;
Device ID (if special file)
st_size : int64 ;
Size in bytes
st_atime : float ;
Last access time
st_mtime : float ;
Last modification time
st_ctime : float ;
Last status change time
}
val stat : string -> stats
val lstat : string -> stats
val fstat : Unix.file_descr -> stats
end

File operations on large files. This sub-module provides 64-bit variants of the functions
Unix.LargeFile.lseek[31.1] (for positioning a file descriptor),
Unix.LargeFile.truncate[31.1] and Unix.LargeFile.ftruncate[31.1] (for changing the
size of a file), and Unix.LargeFile.stat[31.1], Unix.LargeFile.lstat[31.1] and
Unix.LargeFile.fstat[31.1] (for obtaining information on files). These alternate functions
represent positions and sizes by 64-bit integers (type int64) instead of regular integers (type
int), thus allowing operating on files whose sizes are greater than max_int.
Chapter 31. The unix library: Unix system calls 975

Mapping files into memory

val map_file :
file_descr ->
?pos:int64 ->
('a, 'b) Bigarray.kind ->
'c Bigarray.layout ->
bool -> int array -> ('a, 'b, 'c) Bigarray.Genarray.t
Memory mapping of a file as a Bigarray. map_file fd kind layout shared dims returns a
Bigarray of kind kind, layout layout, and dimensions as specified in dims. The data
contained in this Bigarray are the contents of the file referred to by the file descriptor fd (as
opened previously with Unix.openfile[31.1], for example). The optional pos parameter is
the byte offset in the file of the data being mapped; it defaults to 0 (map from the beginning
of the file).
If shared is true, all modifications performed on the array are reflected in the file. This
requires that fd be opened with write permissions. If shared is false, modifications
performed on the array are done in memory only, using copy-on-write of the modified pages;
the underlying file is not affected.
Unix.map_file[31.1] is much more efficient than reading the whole file in a Bigarray,
modifying that Bigarray, and writing it afterwards.
To adjust automatically the dimensions of the Bigarray to the actual size of the file, the major
dimension (that is, the first dimension for an array with C layout, and the last dimension for
an array with Fortran layout) can be given as -1. Unix.map_file[31.1] then determines the
major dimension from the size of the file. The file must contain an integral number of
sub-arrays as determined by the non-major dimensions, otherwise Failure is raised.
If all dimensions of the Bigarray are given, the file size is matched against the size of the
Bigarray. If the file is larger than the Bigarray, only the initial portion of the file is mapped
to the Bigarray. If the file is smaller than the big array, the file is automatically grown to the
size of the Bigarray. This requires write permissions on fd.
Array accesses are bounds-checked, but the bounds are determined by the initial call to
map_file. Therefore, you should make sure no other process modifies the mapped file while
you’re accessing it, or a SIGBUS signal may be raised. This happens, for instance, if the file is
shrunk.
Invalid_argument or Failure may be raised in cases where argument validation fails.
Since: 4.06

Operations on file names

val unlink : string -> unit
Removes the named file.
If the named file is a directory, raises:
976

• EPERM on POSIX compliant system

• EISDIR on Linux ≥ 2.1.132
• EACCESS on Windows

val rename : string -> string -> unit

rename src dst changes the name of a file from src to dst, moving it between directories if
needed. If dst already exists, its contents will be replaced with those of src. Depending on
the operating system, the metadata (permissions, owner, etc) of dst can either be preserved
or be replaced by those of src.

val link : ?follow:bool -> string -> string -> unit

link ?follow src dst creates a hard link named dst to the file named src.
Raises

• ENOSYS On Unix if ~follow:_ is requested, but linkat is unavailable.

• ENOSYS On Windows if ~follow:false is requested.

val realpath : string -> string

realpath p is an absolute pathname for p obtained by resolving all extra / characters,
relative path segments and symbolic links.
Since: 4.13

File permissions and ownership

type access_permission =
| R_OK
Read permission
| W_OK
Write permission
| X_OK
Execution permission
| F_OK
File exists
Flags for the Unix.access[31.1] call.

val chmod : string -> file_perm -> unit

Change the permissions of the named file.

val fchmod : file_descr -> file_perm -> unit

Chapter 31. The unix library: Unix system calls 977

Change the permissions of an opened file.

Raises Invalid_argument on Windows

val chown : string -> int -> int -> unit

Change the owner uid and owner gid of the named file.
Raises Invalid_argument on Windows

val fchown : file_descr -> int -> int -> unit

Change the owner uid and owner gid of an opened file.
Raises Invalid_argument on Windows

val umask : file_perm -> file_perm

Set the process’s file mode creation mask, and return the previous mask.
Raises Invalid_argument on Windows

val access : string -> access_permission list -> unit

Check that the process has the given permissions over the named file.
On Windows: execute permission X_OK cannot be tested, just tests for read permission
instead.
Raises Unix_error otherwise.

Operations on file descriptors

val dup : ?cloexec:bool -> file_descr -> file_descr
Return a new file descriptor referencing the same file as the given descriptor. See
Unix.set_close_on_exec[31.1] for documentation on the cloexec optional argument.

val dup2 : ?cloexec:bool -> file_descr -> file_descr -> unit

dup2 src dst duplicates src to dst, closing dst if already opened. See
Unix.set_close_on_exec[31.1] for documentation on the cloexec optional argument.

val set_nonblock : file_descr -> unit

Set the “non-blocking” flag on the given descriptor. When the non-blocking flag is set,
reading on a descriptor on which there is temporarily no data available raises the EAGAIN or
EWOULDBLOCK error instead of blocking; writing on a descriptor on which there is temporarily
no room for writing also raises EAGAIN or EWOULDBLOCK.

val clear_nonblock : file_descr -> unit

Clear the “non-blocking” flag on the given descriptor. See Unix.set_nonblock[31.1].

val set_close_on_exec : file_descr -> unit

978

Set the “close-on-exec” flag on the given descriptor. A descriptor with the close-on-exec flag is
automatically closed when the current process starts another program with one of the exec,
create_process and open_process functions.
It is often a security hole to leak file descriptors opened on, say, a private file to an external
program: the program, then, gets access to the private file and can do bad things with it.
Hence, it is highly recommended to set all file descriptors “close-on-exec”, except in the very
few cases where a file descriptor actually needs to be transmitted to another program.
The best way to set a file descriptor “close-on-exec” is to create it in this state. To this end,
the openfile function has O_CLOEXEC and O_KEEPEXEC flags to enforce “close-on-exec” mode
or “keep-on-exec” mode, respectively. All other operations in the Unix module that create file
descriptors have an optional argument ?cloexec:bool to indicate whether the file descriptor
should be created in “close-on-exec” mode (by writing ~cloexec:true) or in “keep-on-exec”
mode (by writing ~cloexec:false). For historical reasons, the default file descriptor creation
mode is “keep-on-exec”, if no cloexec optional argument is given. This is not a safe default,
hence it is highly recommended to pass explicit cloexec arguments to operations that create
file descriptors.
The cloexec optional arguments and the O_KEEPEXEC flag were introduced in OCaml 4.05.
Earlier, the common practice was to create file descriptors in the default, “keep-on-exec”
mode, then call set_close_on_exec on those freshly-created file descriptors. This is not as
safe as creating the file descriptor in “close-on-exec” mode because, in multithreaded
programs, a window of vulnerability exists between the time when the file descriptor is
created and the time set_close_on_exec completes. If another thread spawns another
program during this window, the descriptor will leak, as it is still in the “keep-on-exec” mode.
Regarding the atomicity guarantees given by ~cloexec:true or by the use of the O_CLOEXEC
flag: on all platforms it is guaranteed that a concurrently-executing Caml thread cannot leak
the descriptor by starting a new process. On Linux, this guarantee extends to
concurrently-executing C threads. As of Feb 2017, other operating systems lack the necessary
system calls and still expose a window of vulnerability during which a C thread can see the
newly-created file descriptor in “keep-on-exec” mode.

val clear_close_on_exec : file_descr -> unit

Clear the “close-on-exec” flag on the given descriptor. See Unix.set_close_on_exec[31.1].

Directories
val mkdir : string -> file_perm -> unit
Create a directory with the given permissions (see Unix.umask[31.1]).

val rmdir : string -> unit

Remove an empty directory.

val chdir : string -> unit

Change the process working directory.
Chapter 31. The unix library: Unix system calls 979

val getcwd : unit -> string

Return the name of the current working directory.

val chroot : string -> unit

Change the process root directory.
Raises Invalid_argument on Windows

type dir_handle
The type of descriptors over opened directories.

val opendir : string -> dir_handle

Open a descriptor on a directory

val readdir : dir_handle -> string

Return the next entry in a directory.
Raises End_of_file when the end of the directory has been reached.

val rewinddir : dir_handle -> unit

Reposition the descriptor to the beginning of the directory

val closedir : dir_handle -> unit

Close a directory descriptor.

Pipes and redirections

val pipe : ?cloexec:bool -> unit -> file_descr * file_descr
Create a pipe. The first component of the result is opened for reading, that’s the exit to the
pipe. The second component is opened for writing, that’s the entrance to the pipe. See
Unix.set_close_on_exec[31.1] for documentation on the cloexec optional argument.

val mkfifo : string -> file_perm -> unit

Create a named pipe with the given permissions (see Unix.umask[31.1]).
Raises Invalid_argument on Windows

High-level process and redirection management

val create_process :
string ->
string array -> file_descr -> file_descr -> file_descr -> int
980

create_process prog args stdin stdout stderr creates a new process that executes the
program in file prog, with arguments args. Note that the first argument, args.(0), is by
convention the filename of the program being executed, just like Sys.argv.(0). The pid of
the new process is returned immediately; the new process executes concurrently with the
current process. The standard input and outputs of the new process are connected to the
descriptors stdin, stdout and stderr. Passing e.g. Unix.stdout[31.1] for stdout prevents
the redirection and causes the new process to have the same standard output as the current
process. The executable file prog is searched in the path. The new process has the same
environment as the current process.

val create_process_env :
string ->
string array ->
string array -> file_descr -> file_descr -> file_descr -> int
create_process_env prog args env stdin stdout stderr works as
Unix.create_process[31.1], except that the extra argument env specifies the environment
passed to the program.

val open_process_in : string -> in_channel

High-level pipe and process management. This function runs the given command in parallel
with the program. The standard output of the command is redirected to a pipe, which can be
read via the returned input channel. The command is interpreted by the shell /bin/sh (or
cmd.exe on Windows), cf. Unix.system[31.1]. The Filename.quote_command[29.20]
function can be used to quote the command and its arguments as appropriate for the shell
being used. If the command does not need to be run through the shell,
Unix.open_process_args_in[31.1] can be used as a more robust and more efficient
alternative to Unix.open_process_in[31.1].

val open_process_out : string -> out_channel

Same as Unix.open_process_in[31.1], but redirect the standard input of the command to a
pipe. Data written to the returned output channel is sent to the standard input of the
command. Warning: writes on output channels are buffered, hence be careful to call
flush[28.2] at the right times to ensure correct synchronization. If the command does not
need to be run through the shell, Unix.open_process_args_out[31.1] can be used instead of
Unix.open_process_out[31.1].

val open_process : string -> in_channel * out_channel

Same as Unix.open_process_out[31.1], but redirects both the standard input and standard
output of the command to pipes connected to the two returned channels. The input channel
is connected to the output of the command, and the output channel to the input of the
command. If the command does not need to be run through the shell,
Unix.open_process_args[31.1] can be used instead of Unix.open_process[31.1].

val open_process_full :
string ->
string array -> in_channel * out_channel * in_channel
Chapter 31. The unix library: Unix system calls 981

Similar to Unix.open_process[31.1], but the second argument specifies the environment

passed to the command. The result is a triple of channels connected respectively to the
standard output, standard input, and standard error of the command. If the command does
not need to be run through the shell, Unix.open_process_args_full[31.1] can be used
instead of Unix.open_process_full[31.1].

val open_process_args : string -> string array -> in_channel * out_channel

open_process_args prog args runs the program prog with arguments args. Note that the
first argument, args.(0), is by convention the filename of the program being executed, just
like Sys.argv.(0). The new process executes concurrently with the current process. The
standard input and output of the new process are redirected to pipes, which can be
respectively read and written via the returned channels. The input channel is connected to
the output of the program, and the output channel to the input of the program.
Warning: writes on output channels are buffered, hence be careful to call flush[28.2] at the
right times to ensure correct synchronization.
The executable file prog is searched for in the path. This behaviour changed in 4.12;
previously prog was looked up only in the current directory.
The new process has the same environment as the current process.
Since: 4.08

val open_process_args_in : string -> string array -> in_channel

Same as Unix.open_process_args[31.1], but redirects only the standard output of the new
process.
Since: 4.08

val open_process_args_out : string -> string array -> out_channel

Same as Unix.open_process_args[31.1], but redirects only the standard input of the new
process.
Since: 4.08

val open_process_args_full :
string ->
string array ->
string array -> in_channel * out_channel * in_channel
Similar to Unix.open_process_args[31.1], but the third argument specifies the environment
passed to the new process. The result is a triple of channels connected respectively to the
standard output, standard input, and standard error of the program.
Since: 4.08

val process_in_pid : in_channel -> int

Return the pid of a process opened via Unix.open_process_args_in[31.1] or the pid of the
shell opened via Unix.open_process_in[31.1].
Since: 4.08 (4.12 in UnixLabels)
982

val process_out_pid : out_channel -> int

Return the pid of a process opened via Unix.open_process_args_out[31.1] or the pid of the
shell opened via Unix.open_process_out[31.1].
Since: 4.08 (4.12 in UnixLabels)

val process_pid : in_channel * out_channel -> int

Return the pid of a process opened via Unix.open_process_args[31.1] or the pid of the shell
opened via Unix.open_process_args[31.1].
Since: 4.08 (4.12 in UnixLabels)

val process_full_pid : in_channel * out_channel * in_channel -> int

Return the pid of a process opened via Unix.open_process_args_full[31.1] or the pid of
the shell opened via Unix.open_process_full[31.1].
Since: 4.08 (4.12 in UnixLabels)

val close_process_in : in_channel -> process_status

Close channels opened by Unix.open_process_in[31.1], wait for the associated command to
terminate, and return its termination status.

val close_process_out : out_channel -> process_status

Close channels opened by Unix.open_process_out[31.1], wait for the associated command
to terminate, and return its termination status.

val close_process : in_channel * out_channel -> process_status

Close channels opened by Unix.open_process[31.1], wait for the associated command to
terminate, and return its termination status.

val close_process_full :
in_channel * out_channel * in_channel ->
process_status
Close channels opened by Unix.open_process_full[31.1], wait for the associated command
to terminate, and return its termination status.

Symbolic links
val symlink : ?to_dir:bool -> string -> string -> unit
symlink ?to_dir src dst creates the file dst as a symbolic link to the file src. On
Windows, ~to_dir indicates if the symbolic link points to a directory or a file; if omitted,
symlink examines src using stat and picks appropriately, if src does not exist then false
is assumed (for this reason, it is recommended that the ~to_dir parameter be specified in
new code). On Unix, ~to_dir is ignored.
Windows symbolic links are available in Windows Vista onwards. There are some important
differences between Windows symlinks and their POSIX counterparts.
Chapter 31. The unix library: Unix system calls 983

Windows symbolic links come in two flavours: directory and regular, which designate whether
the symbolic link points to a directory or a file. The type must be correct - a directory
symlink which actually points to a file cannot be selected with chdir and a file symlink which
actually points to a directory cannot be read or written (note that Cygwin’s emulation layer
ignores this distinction).
When symbolic links are created to existing targets, this distinction doesn’t matter and
symlink will automatically create the correct kind of symbolic link. The distinction matters
when a symbolic link is created to a non-existent target.
The other caveat is that by default symbolic links are a privileged operation. Administrators
will always need to be running elevated (or with UAC disabled) and by default normal user
accounts need to be granted the SeCreateSymbolicLinkPrivilege via Local Security Policy
(secpol.msc) or via Active Directory.
Unix.has_symlink[31.1] can be used to check that a process is able to create symbolic links.

val has_symlink : unit -> bool

Returns true if the user is able to create symbolic links. On Windows, this indicates that the
user not only has the SeCreateSymbolicLinkPrivilege but is also running elevated, if necessary.
On other platforms, this is simply indicates that the symlink system call is available.
Since: 4.03

val readlink : string -> string

Read the contents of a symbolic link.

Polling
val select :
file_descr list ->
file_descr list ->
file_descr list ->
float -> file_descr list * file_descr list * file_descr list
Wait until some input/output operations become possible on some channels. The three list
arguments are, respectively, a set of descriptors to check for reading (first argument), for
writing (second argument), or for exceptional conditions (third argument). The fourth
argument is the maximal timeout, in seconds; a negative fourth argument means no timeout
(unbounded wait). The result is composed of three sets of descriptors: those ready for reading
(first component), ready for writing (second component), and over which an exceptional
condition is pending (third component).

Locking
type lock_command =
| F_ULOCK
984

Unlock a region
| F_LOCK
Lock a region for writing, and block if already locked
| F_TLOCK
Lock a region for writing, or fail if already locked
| F_TEST
Test a region for other process locks
| F_RLOCK
Lock a region for reading, and block if already locked
| F_TRLOCK
Lock a region for reading, or fail if already locked
Commands for Unix.lockf[31.1].

val lockf : file_descr -> lock_command -> int -> unit

lockf fd mode len puts a lock on a region of the file opened as fd. The region starts at the
current read/write position for fd (as set by Unix.lseek[31.1]), and extends len bytes
forward if len is positive, len bytes backwards if len is negative, or to the end of the file if
len is zero. A write lock prevents any other process from acquiring a read or write lock on
the region. A read lock prevents any other process from acquiring a write lock on the region,
but lets other processes acquire read locks on it.
The F_LOCK and F_TLOCK commands attempts to put a write lock on the specified region.
The F_RLOCK and F_TRLOCK commands attempts to put a read lock on the specified region. If
one or several locks put by another process prevent the current process from acquiring the
lock, F_LOCK and F_RLOCK block until these locks are removed, while F_TLOCK and F_TRLOCK
fail immediately with an exception. The F_ULOCK removes whatever locks the current process
has on the specified region. Finally, the F_TEST command tests whether a write lock can be
acquired on the specified region, without actually putting a lock. It returns immediately if
successful, or fails otherwise.
What happens when a process tries to lock a region of a file that is already locked by the
same process depends on the OS. On POSIX-compliant systems, the second lock operation
succeeds and may ”promote” the older lock from read lock to write lock. On Windows, the
second lock operation will block or fail.

Signals
Note: installation of signal handlers is performed via the functions Sys.signal[29.56] and Sys.set_
signal[29.56].
val kill : int -> int -> unit
kill pid signal sends signal number signal to the process with id pid.
On Windows: only the Sys.sigkill[29.56] signal is emulated.
Chapter 31. The unix library: Unix system calls 985

type sigprocmask_command =
| SIG_SETMASK
| SIG_BLOCK
| SIG_UNBLOCK
val sigprocmask : sigprocmask_command -> int list -> int list
sigprocmask mode sigs changes the set of blocked signals. If mode is SIG_SETMASK, blocked
signals are set to those in the list sigs. If mode is SIG_BLOCK, the signals in sigs are added
to the set of blocked signals. If mode is SIG_UNBLOCK, the signals in sigs are removed from
the set of blocked signals. sigprocmask returns the set of previously blocked signals.
When the systhreads version of the Thread module is loaded, this function redirects to
Thread.sigmask. I.e., sigprocmask only changes the mask of the current thread.
Raises Invalid_argument on Windows (no inter-process signals on Windows)

val sigpending : unit -> int list

Return the set of blocked signals that are currently pending.
Raises Invalid_argument on Windows (no inter-process signals on Windows)

val sigsuspend : int list -> unit

sigsuspend sigs atomically sets the blocked signals to sigs and waits for a non-ignored,
non-blocked signal to be delivered. On return, the blocked signals are reset to their initial
value.
Raises Invalid_argument on Windows (no inter-process signals on Windows)

val pause : unit -> unit

Wait until a non-ignored, non-blocked signal is delivered.
Raises Invalid_argument on Windows (no inter-process signals on Windows)

Time functions
type process_times =
{ tms_utime : float ;
User time for the process
tms_stime : float ;
System time for the process
tms_cutime : float ;
User time for the children processes
tms_cstime : float ;
System time for the children processes
}
The execution times (CPU times) of a process.
986

type tm =
{ tm_sec : int ;
Seconds 0..60
tm_min : int ;
Minutes 0..59
tm_hour : int ;
Hours 0..23
tm_mday : int ;
Day of month 1..31
tm_mon : int ;
Month of year 0..11
tm_year : int ;
Year - 1900
tm_wday : int ;
Day of week (Sunday is 0)
tm_yday : int ;
Day of year 0..365
tm_isdst : bool ;
Daylight time savings in effect
}
The type representing wallclock time and calendar date.

val time : unit -> float

Return the current time since 00:00:00 GMT, Jan. 1, 1970, in seconds.

val gettimeofday : unit -> float

Same as Unix.time[31.1], but with resolution better than 1 second.

val gmtime : float -> tm

Convert a time in seconds, as returned by Unix.time[31.1], into a date and a time. Assumes
UTC (Coordinated Universal Time), also known as GMT. To perform the inverse conversion,
set the TZ environment variable to ”UTC”, use Unix.mktime[31.1], and then restore the
original value of TZ.

val localtime : float -> tm

Convert a time in seconds, as returned by Unix.time[31.1], into a date and a time. Assumes
the local time zone. The function performing the inverse conversion is Unix.mktime[31.1].

val mktime : tm -> float * tm

Chapter 31. The unix library: Unix system calls 987

Convert a date and time, specified by the tm argument, into a time in seconds, as returned by
Unix.time[31.1]. The tm_isdst, tm_wday and tm_yday fields of tm are ignored. Also return
a normalized copy of the given tm record, with the tm_wday, tm_yday, and tm_isdst fields
recomputed from the other fields, and the other fields normalized (so that, e.g., 40 October is
changed into 9 November). The tm argument is interpreted in the local time zone.

val alarm : int -> int

Schedule a SIGALRM signal after the given number of seconds.
Raises Invalid_argument on Windows

val sleep : int -> unit

Stop execution for the given number of seconds.

val sleepf : float -> unit

Stop execution for the given number of seconds. Like sleep, but fractions of seconds are
supported.
Since: 4.03 (4.12 in UnixLabels)

val times : unit -> process_times

Return the execution times of the process.
On Windows: partially implemented, will not report timings for child processes.

val utimes : string -> float -> float -> unit

Set the last access time (second arg) and last modification time (third arg) for a file. Times
are expressed in seconds from 00:00:00 GMT, Jan. 1, 1970. If both times are 0.0, the access
and last modification times are both set to the current time.

type interval_timer =
| ITIMER_REAL
decrements in real time, and sends the signal SIGALRM when expired.
| ITIMER_VIRTUAL
decrements in process virtual time, and sends SIGVTALRM when expired.
| ITIMER_PROF
(for profiling) decrements both when the process is running and when the system is
running on behalf of the process; it sends SIGPROF when expired.
The three kinds of interval timers.

type interval_timer_status =
{ it_interval : float ;
Period
it_value : float ;
988

Current value of the timer

}
The type describing the status of an interval timer

val getitimer : interval_timer -> interval_timer_status

Return the current status of the given interval timer.
Raises Invalid_argument on Windows

val setitimer :
interval_timer ->
interval_timer_status -> interval_timer_status
setitimer t s sets the interval timer t and returns its previous status. The s argument is
interpreted as follows: s.it_value, if nonzero, is the time to the next timer expiration;
s.it_interval, if nonzero, specifies a value to be used in reloading it_value when the
timer expires. Setting s.it_value to zero disables the timer. Setting s.it_interval to zero
causes the timer to be disabled after its next expiration.
Raises Invalid_argument on Windows

User id, group id

val getuid : unit -> int
Return the user id of the user executing the process.
On Windows: always returns 1.

val geteuid : unit -> int

Return the effective user id under which the process runs.
On Windows: always returns 1.

val setuid : int -> unit

Set the real user id and effective user id for the process.
Raises Invalid_argument on Windows

val getgid : unit -> int

Return the group id of the user executing the process.
On Windows: always returns 1.

val getegid : unit -> int

Return the effective group id under which the process runs.
On Windows: always returns 1.

val setgid : int -> unit

Chapter 31. The unix library: Unix system calls 989

Set the real group id and effective group id for the process.
Raises Invalid_argument on Windows

val getgroups : unit -> int array

Return the list of groups to which the user executing the process belongs.
On Windows: always returns [|1|].

val setgroups : int array -> unit

setgroups groups sets the supplementary group IDs for the calling process. Appropriate
privileges are required.
Raises Invalid_argument on Windows

val initgroups : string -> int -> unit

initgroups user group initializes the group access list by reading the group database
/etc/group and using all groups of which user is a member. The additional group group is
also added to the list.
Raises Invalid_argument on Windows

type passwd_entry =
{ pw_name : string ;
pw_passwd : string ;
pw_uid : int ;
pw_gid : int ;
pw_gecos : string ;
pw_dir : string ;
pw_shell : string ;
}
Structure of entries in the passwd database.

type group_entry =
{ gr_name : string ;
gr_passwd : string ;
gr_gid : int ;
gr_mem : string array ;
}
Structure of entries in the groups database.

val getlogin : unit -> string

Return the login name of the user executing the process.

val getpwnam : string -> passwd_entry

Find an entry in passwd with the given name.
Raises Not_found if no such entry exists, or always on Windows.
990

val getgrnam : string -> group_entry

Find an entry in group with the given name.
Raises Not_found if no such entry exists, or always on Windows.

val getpwuid : int -> passwd_entry

Find an entry in passwd with the given user id.
Raises Not_found if no such entry exists, or always on Windows.

val getgrgid : int -> group_entry

Find an entry in group with the given group id.
Raises Not_found if no such entry exists, or always on Windows.

Internet addresses
type inet_addr
The abstract type of Internet addresses.

val inet_addr_of_string : string -> inet_addr

Conversion from the printable representation of an Internet address to its internal
representation. The argument string consists of 4 numbers separated by periods
(XXX.YYY.ZZZ.TTT) for IPv4 addresses, and up to 8 numbers separated by colons for IPv6
addresses.
Raises Failure when given a string that does not match these formats.

val string_of_inet_addr : inet_addr -> string

Return the printable representation of the given Internet address. See
Unix.inet_addr_of_string[31.1] for a description of the printable representation.

val inet_addr_any : inet_addr

A special IPv4 address, for use only with bind, representing all the Internet addresses that
the host machine possesses.

val inet_addr_loopback : inet_addr

A special IPv4 address representing the host machine (127.0.0.1).

val inet6_addr_any : inet_addr

A special IPv6 address, for use only with bind, representing all the Internet addresses that
the host machine possesses.

val inet6_addr_loopback : inet_addr

A special IPv6 address representing the host machine (::1).

val is_inet6_addr : inet_addr -> bool

Whether the given inet_addr is an IPv6 address.
Since: 4.12
Chapter 31. The unix library: Unix system calls 991

Sockets
type socket_domain =
| PF_UNIX
Unix domain
| PF_INET
Internet domain (IPv4)
| PF_INET6
Internet domain (IPv6)
The type of socket domains. Not all platforms support IPv6 sockets (type PF_INET6).
On Windows: PF_UNIX supported since 4.14.0 on Windows 10 1803 and later.

type socket_type =
| SOCK_STREAM
Stream socket
| SOCK_DGRAM
Datagram socket
| SOCK_RAW
Raw socket
| SOCK_SEQPACKET
Sequenced packets socket
The type of socket kinds, specifying the semantics of communications. SOCK_SEQPACKET is
included for completeness, but is rarely supported by the OS, and needs system calls that are
not available in this library.

type sockaddr =
| ADDR_UNIX of string
| ADDR_INET of inet_addr * int
The type of socket addresses. ADDR_UNIX name is a socket address in the Unix domain; name
is a file name in the file system. ADDR_INET(addr,port) is a socket address in the Internet
domain; addr is the Internet address of the machine, and port is the port number.

val socket :
?cloexec:bool ->
socket_domain -> socket_type -> int -> file_descr
Create a new socket in the given domain, and with the given kind. The third argument is the
protocol type; 0 selects the default protocol for that kind of sockets. See
Unix.set_close_on_exec[31.1] for documentation on the cloexec optional argument.

val domain_of_sockaddr : sockaddr -> socket_domain

Return the socket domain adequate for the given socket address.
992

val socketpair :
?cloexec:bool ->
socket_domain ->
socket_type -> int -> file_descr * file_descr
Create a pair of unnamed sockets, connected together. See Unix.set_close_on_exec[31.1]
for documentation on the cloexec optional argument.

val accept : ?cloexec:bool -> file_descr -> file_descr * sockaddr

Accept connections on the given socket. The returned descriptor is a socket connected to the
client; the returned address is the address of the connecting client. See
Unix.set_close_on_exec[31.1] for documentation on the cloexec optional argument.

val bind : file_descr -> sockaddr -> unit

Bind a socket to an address.

val connect : file_descr -> sockaddr -> unit

Connect a socket to an address.

val listen : file_descr -> int -> unit

Set up a socket for receiving connection requests. The integer argument is the maximal
number of pending requests.

type shutdown_command =
| SHUTDOWN_RECEIVE
Close for receiving
| SHUTDOWN_SEND
Close for sending
| SHUTDOWN_ALL
Close both
The type of commands for shutdown.

val shutdown : file_descr -> shutdown_command -> unit

Shutdown a socket connection. SHUTDOWN_SEND as second argument causes reads on the other
end of the connection to return an end-of-file condition. SHUTDOWN_RECEIVE causes writes on
the other end of the connection to return a closed pipe condition (SIGPIPE signal).

val getsockname : file_descr -> sockaddr

Return the address of the given socket.

val getpeername : file_descr -> sockaddr

Return the address of the host connected to the given socket.
Chapter 31. The unix library: Unix system calls 993

type msg_flag =
| MSG_OOB
| MSG_DONTROUTE
| MSG_PEEK
The flags for Unix.recv[31.1], Unix.recvfrom[31.1], Unix.send[31.1] and
Unix.sendto[31.1].

val recv : file_descr -> bytes -> int -> int -> msg_flag list -> int
Receive data from a connected socket.

val recvfrom :
file_descr ->
bytes -> int -> int -> msg_flag list -> int * sockaddr
Receive data from an unconnected socket.

val send : file_descr -> bytes -> int -> int -> msg_flag list -> int
Send data over a connected socket.

val send_substring :
file_descr -> string -> int -> int -> msg_flag list -> int
Same as send, but take the data from a string instead of a byte sequence.
Since: 4.02

val sendto :
file_descr ->
bytes -> int -> int -> msg_flag list -> sockaddr -> int
Send data over an unconnected socket.

val sendto_substring :
file_descr ->
string -> int -> int -> msg_flag list -> sockaddr -> int
Same as sendto, but take the data from a string instead of a byte sequence.
Since: 4.02

Socket options
type socket_bool_option =
| SO_DEBUG
Record debugging information
| SO_BROADCAST
Permit sending of broadcast messages
| SO_REUSEADDR
994

Allow reuse of local addresses for bind

| SO_KEEPALIVE
Keep connection active
| SO_DONTROUTE
Bypass the standard routing algorithms
| SO_OOBINLINE
Leave out-of-band data in line
| SO_ACCEPTCONN
Report whether socket listening is enabled
| TCP_NODELAY
Control the Nagle algorithm for TCP sockets
| IPV6_ONLY
Forbid binding an IPv6 socket to an IPv4 address
| SO_REUSEPORT
Allow reuse of address and port bindings.
Since: 4.12.
The socket options that can be consulted with Unix.getsockopt[31.1] and modified with
Unix.setsockopt[31.1]. These options have a boolean (true/false) value.

type socket_int_option =
| SO_SNDBUF
Size of send buffer
| SO_RCVBUF
Size of received buffer
| SO_ERROR
Deprecated. Use Unix.getsockopt_error instead.Deprecated. Use
Unix.getsockopt_error[31.1] instead.
| SO_TYPE
Report the socket type
| SO_RCVLOWAT
Minimum number of bytes to process for input operations
| SO_SNDLOWAT
Minimum number of bytes to process for output operations
The socket options that can be consulted with Unix.getsockopt_int[31.1] and modified
with Unix.setsockopt_int[31.1]. These options have an integer value.
Chapter 31. The unix library: Unix system calls 995

type socket_optint_option =
| SO_LINGER
Whether to linger on closed connections that have data present, and for how long (in
seconds)
The socket options that can be consulted with Unix.getsockopt_optint[31.1] and modified
with Unix.setsockopt_optint[31.1]. These options have a value of type int option, with
None meaning “disabled”.

type socket_float_option =
| SO_RCVTIMEO
Timeout for input operations
| SO_SNDTIMEO
Timeout for output operations
The socket options that can be consulted with Unix.getsockopt_float[31.1] and modified
with Unix.setsockopt_float[31.1]. These options have a floating-point value representing a
time in seconds. The value 0 means infinite timeout.

val getsockopt : file_descr -> socket_bool_option -> bool

Return the current status of a boolean-valued option in the given socket.

val setsockopt : file_descr -> socket_bool_option -> bool -> unit

Set or clear a boolean-valued option in the given socket.

val getsockopt_int : file_descr -> socket_int_option -> int

Same as Unix.getsockopt[31.1] for an integer-valued socket option.

val setsockopt_int : file_descr -> socket_int_option -> int -> unit

Same as Unix.setsockopt[31.1] for an integer-valued socket option.

val getsockopt_optint : file_descr -> socket_optint_option -> int option

Same as Unix.getsockopt[31.1] for a socket option whose value is an int option.

val setsockopt_optint :
file_descr -> socket_optint_option -> int option -> unit
Same as Unix.setsockopt[31.1] for a socket option whose value is an int option.

val getsockopt_float : file_descr -> socket_float_option -> float

Same as Unix.getsockopt[31.1] for a socket option whose value is a floating-point number.

val setsockopt_float : file_descr -> socket_float_option -> float -> unit

Same as Unix.setsockopt[31.1] for a socket option whose value is a floating-point number.

val getsockopt_error : file_descr -> error option

Return the error condition associated with the given socket, and clear it.
996

High-level network connection functions

val open_connection : sockaddr -> in_channel * out_channel
Connect to a server at the given address. Return a pair of buffered channels connected to the
server. Remember to call flush[28.2] on the output channel at the right times to ensure
correct synchronization.
The two channels returned by open_connection share a descriptor to a socket. Therefore,
when the connection is over, you should call close_out[28.2] on the output channel, which
will also close the underlying socket. Do not call close_in[28.2] on the input channel; it will
be collected by the GC eventually.

val shutdown_connection : in_channel -> unit

“Shut down” a connection established with Unix.open_connection[31.1]; that is, transmit
an end-of-file condition to the server reading on the other side of the connection. This does
not close the socket and the channels used by the connection. See
Unix.open_connection[31.1] for how to close them once the connection is over.

val establish_server :
(in_channel -> out_channel -> unit) -> sockaddr -> unit
Establish a server on the given address. The function given as first argument is called for each
connection with two buffered channels connected to the client. A new process is created for
each connection. The function Unix.establish_server[31.1] never returns normally.
The two channels given to the function share a descriptor to a socket. The function does not
need to close the channels, since this occurs automatically when the function returns. If the
function prefers explicit closing, it should close the output channel using close_out[28.2] and
leave the input channel unclosed, for reasons explained in Unix.in_channel_of_descr[31.1].
Raises Invalid_argument on Windows. Use threads instead.

Host and protocol databases

type host_entry =
{ h_name : string ;
h_aliases : string array ;
h_addrtype : socket_domain ;
h_addr_list : inet_addr array ;
}
Structure of entries in the hosts database.

type protocol_entry =
{ p_name : string ;
p_aliases : string array ;
p_proto : int ;
}
Chapter 31. The unix library: Unix system calls 997

Structure of entries in the protocols database.

type service_entry =
{ s_name : string ;
s_aliases : string array ;
s_port : int ;
s_proto : string ;
}
Structure of entries in the services database.

val gethostname : unit -> string

Return the name of the local host.

val gethostbyname : string -> host_entry

Find an entry in hosts with the given name.
Raises Not_found if no such entry exists.

val gethostbyaddr : inet_addr -> host_entry

Find an entry in hosts with the given address.
Raises Not_found if no such entry exists.

val getprotobyname : string -> protocol_entry

Find an entry in protocols with the given name.
Raises Not_found if no such entry exists.

val getprotobynumber : int -> protocol_entry

Find an entry in protocols with the given protocol number.
Raises Not_found if no such entry exists.

val getservbyname : string -> string -> service_entry

Find an entry in services with the given name.
Raises Not_found if no such entry exists.

val getservbyport : int -> string -> service_entry

Find an entry in services with the given service number.
Raises Not_found if no such entry exists.

type addr_info =
{ ai_family : socket_domain ;
Socket domain
ai_socktype : socket_type ;
Socket type
998

ai_protocol : int ;
Socket protocol number
ai_addr : sockaddr ;
Address
ai_canonname : string ;
Canonical host name
}
Address information returned by Unix.getaddrinfo[31.1].

type getaddrinfo_option =
| AI_FAMILY of socket_domain
Impose the given socket domain
| AI_SOCKTYPE of socket_type
Impose the given socket type
| AI_PROTOCOL of int
Impose the given protocol
| AI_NUMERICHOST
Do not call name resolver, expect numeric IP address
| AI_CANONNAME
Fill the ai_canonname field of the result
| AI_PASSIVE
Set address to “any” address for use with Unix.bind[31.1]
Options to Unix.getaddrinfo[31.1].

val getaddrinfo :
string -> string -> getaddrinfo_option list -> addr_info list
getaddrinfo host service opts returns a list of Unix.addr_info[31.1] records describing
socket parameters and addresses suitable for communicating with the given host and service.
The empty list is returned if the host or service names are unknown, or the constraints
expressed in opts cannot be satisfied.
host is either a host name or the string representation of an IP address. host can be given as
the empty string; in this case, the “any” address or the “loopback” address are used,
depending whether opts contains AI_PASSIVE. service is either a service name or the string
representation of a port number. service can be given as the empty string; in this case, the
port field of the returned addresses is set to 0. opts is a possibly empty list of options that
allows the caller to force a particular socket domain (e.g. IPv6 only or IPv4 only) or a
particular socket type (e.g. TCP only or UDP only).

type name_info =
{ ni_hostname : string ;
Chapter 31. The unix library: Unix system calls 999

Name or IP address of host

ni_service : string ;
Name of service or port number
}
Host and service information returned by Unix.getnameinfo[31.1].

type getnameinfo_option =
| NI_NOFQDN
Do not qualify local host names
| NI_NUMERICHOST
Always return host as IP address
| NI_NAMEREQD
Fail if host name cannot be determined
| NI_NUMERICSERV
Always return service as port number
| NI_DGRAM
Consider the service as UDP-based instead of the default TCP
Options to Unix.getnameinfo[31.1].

val getnameinfo : sockaddr -> getnameinfo_option list -> name_info

getnameinfo addr opts returns the host name and service name corresponding to the
socket address addr. opts is a possibly empty list of options that governs how these names
are obtained.
Raises Not_found if an error occurs.

Terminal interface
The following functions implement the POSIX standard terminal interface. They provide control
over asynchronous communication ports and pseudo-terminals. Refer to the termios man page for
a complete description.
type terminal_io =
{ mutable c_ignbrk : bool ;
Ignore the break condition.
mutable c_brkint : bool ;
Signal interrupt on break condition.
mutable c_ignpar : bool ;
Ignore characters with parity errors.
mutable c_parmrk : bool ;
1000

Mark parity errors.

mutable c_inpck : bool ;
Enable parity check on input.
mutable c_istrip : bool ;
Strip 8th bit on input characters.
mutable c_inlcr : bool ;
Map NL to CR on input.
mutable c_igncr : bool ;
Ignore CR on input.
mutable c_icrnl : bool ;
Map CR to NL on input.
mutable c_ixon : bool ;
Recognize XON/XOFF characters on input.
mutable c_ixoff : bool ;
Emit XON/XOFF chars to control input flow.
mutable c_opost : bool ;
Enable output processing.
mutable c_obaud : int ;
Output baud rate (0 means close connection).
mutable c_ibaud : int ;
Input baud rate.
mutable c_csize : int ;
Number of bits per character (5-8).
mutable c_cstopb : int ;
Number of stop bits (1-2).
mutable c_cread : bool ;
Reception is enabled.
mutable c_parenb : bool ;
Enable parity generation and detection.
mutable c_parodd : bool ;
Specify odd parity instead of even.
mutable c_hupcl : bool ;
Hang up on last close.
mutable c_clocal : bool ;
Chapter 31. The unix library: Unix system calls 1001

Ignore modem status lines.

mutable c_isig : bool ;
Generate signal on INTR, QUIT, SUSP.
mutable c_icanon : bool ;
Enable canonical processing (line buffering and editing)
mutable c_noflsh : bool ;
Disable flush after INTR, QUIT, SUSP.
mutable c_echo : bool ;
Echo input characters.
mutable c_echoe : bool ;
Echo ERASE (to erase previous character).
mutable c_echok : bool ;
Echo KILL (to erase the current line).
mutable c_echonl : bool ;
Echo NL even if c_echo is not set.
mutable c_vintr : char ;
Interrupt character (usually ctrl-C).
mutable c_vquit : char ;
Quit character (usually ctrl-\).
mutable c_verase : char ;
Erase character (usually DEL or ctrl-H).
mutable c_vkill : char ;
Kill line character (usually ctrl-U).
mutable c_veof : char ;
End-of-file character (usually ctrl-D).
mutable c_veol : char ;
Alternate end-of-line char. (usually none).
mutable c_vmin : int ;
Minimum number of characters to read before the read request is satisfied.
mutable c_vtime : int ;
Maximum read wait (in 0.1s units).
mutable c_vstart : char ;
Start character (usually ctrl-Q).
mutable c_vstop : char ;
1002

Stop character (usually ctrl-S).

}
val tcgetattr : file_descr -> terminal_io
Return the status of the terminal referred to by the given file descriptor.
Raises Invalid_argument on Windows

type setattr_when =
| TCSANOW
| TCSADRAIN
| TCSAFLUSH
val tcsetattr : file_descr -> setattr_when -> terminal_io -> unit
Set the status of the terminal referred to by the given file descriptor. The second argument
indicates when the status change takes place: immediately (TCSANOW), when all pending
output has been transmitted (TCSADRAIN), or after flushing all input that has been received
but not read (TCSAFLUSH). TCSADRAIN is recommended when changing the output parameters;
TCSAFLUSH, when changing the input parameters.
Raises Invalid_argument on Windows

val tcsendbreak : file_descr -> int -> unit

Send a break condition on the given file descriptor. The second argument is the duration of
the break, in 0.1s units; 0 means standard duration (0.25s).
Raises Invalid_argument on Windows

val tcdrain : file_descr -> unit

Waits until all output written on the given file descriptor has been transmitted.
Raises Invalid_argument on Windows

type flush_queue =
| TCIFLUSH
| TCOFLUSH
| TCIOFLUSH
val tcflush : file_descr -> flush_queue -> unit
Discard data written on the given file descriptor but not yet transmitted, or data received but
not yet read, depending on the second argument: TCIFLUSH flushes data received but not
read, TCOFLUSH flushes data written but not transmitted, and TCIOFLUSH flushes both.
Raises Invalid_argument on Windows

type flow_action =
| TCOOFF
| TCOON
| TCIOFF
| TCION
val tcflow : file_descr -> flow_action -> unit
Chapter 31. The unix library: Unix system calls 1003

Suspend or restart reception or transmission of data on the given file descriptor, depending on
the second argument: TCOOFF suspends output, TCOON restarts output, TCIOFF transmits a
STOP character to suspend input, and TCION transmits a START character to restart input.
Raises Invalid_argument on Windows

val setsid : unit -> int

Put the calling process in a new session and detach it from its controlling terminal.
Raises Invalid_argument on Windows

31.2 Module UnixLabels: labelized version of the interface

This module is identical to Unix (31.1), and only differs by the addition of labels. You may see
these labels directly by looking at unixLabels.mli, or by using the ocamlbrowser tool.
1004

Windows:
The Cygwin port of OCaml fully implements all functions from the Unix module. The
native Win32 ports implement a subset of them. Below is a list of the functions that are not
implemented, or only partially implemented, by the Win32 ports. Functions not mentioned
are fully implemented and behave as described previously in this chapter.
Chapter 31. The unix library: Unix system calls 1005

Functions Comment
fork not implemented, use create_process or
threads
wait not implemented, use waitpid
waitpid can only wait for a given PID, not any child
process
getppid not implemented (meaningless under Windows)
nice not implemented
truncate, ftruncate implemented (since 4.10.0)
link implemented (since 3.02)
fchmod not implemented
chown, fchown not implemented (make no sense on a DOS file
system)
umask not implemented
access execute permission X_OK cannot be tested, it just
tests for read permission instead
chroot not implemented
mkfifo not implemented
symlink, readlink implemented (since 4.03.0)
kill partially implemented (since 4.00.0): only the
sigkill signal is implemented
sigprocmask, sigpending, sigsuspend not implemented (no inter-process signals on
Windows
pause not implemented (no inter-process signals in Win-
dows)
alarm not implemented
times partially implemented, will not report timings
for child processes
getitimer, setitimer not implemented
getuid, geteuid, getgid, getegid always return 1
setuid, setgid, setgroups, initgroups not implemented
getgroups always returns [|1|] (since 2.00)
getpwnam, getpwuid always raise Not_found
getgrnam, getgrgid always raise Not_found
type socket_domain PF_INET is fully supported; PF_INET6 is fully
supported (since 4.01.0); PF_UNIX is supported
since 4.14.0, but only works on Windows 10 1803
and later.
establish_server not implemented; use threads
terminal functions (tc*) not implemented
setsid not implemented
1006
Chapter 32

The str library: regular expressions

and string processing

The str library provides high-level string processing functions, some based on regular expressions. It
is intended to support the kind of file processing that is usually performed with scripting languages
such as awk, perl or sed.
Programs that use the str library must be linked as follows:

ocamlc other options -I +str str.cma other files

ocamlopt other options -I +str str.cmxa other files

For interactive use of the str library, do:

ocamlmktop -o mytop str.cma

./mytop

or (if dynamic linking of C libraries is supported on your platform), start ocaml and type
# #directory "+str";;
# #load "str.cma";;

32.1 Module Str : Regular expressions and high-level string pro-

cessing

Regular expressions
The Str[32.1] library provides regular expressions on sequences of bytes. It is, in general, unsuitable
to match Unicode characters.
type regexp
The type of compiled regular expressions.

val regexp : string -> regexp

Compile a regular expression. The following constructs are recognized:

1007
1008

• . Matches any character except newline.

• * (postfix) Matches the preceding expression zero, one or several times
• + (postfix) Matches the preceding expression one or several times
• ? (postfix) Matches the preceding expression once or not at all
• [..] Character set. Ranges are denoted with -, as in [a-z]. An initial ^, as in [^0-9],
complements the set. To include a ] character in a set, make it the first character of the
set. To include a - character in a set, make it the first or the last character of the set.
• ^ Matches at beginning of line: either at the beginning of the matched string, or just
after a ’\n’ character.
• $ Matches at end of line: either at the end of the matched string, or just before a ’\n’
character.
• \| (infix) Alternative between two expressions.
• \(..\) Grouping and naming of the enclosed expression.
• \1 The text matched by the first \(...\) expression (\2 for the second expression,
and so on up to \9).
• \b Matches word boundaries.
• \ Quotes special characters. The special characters are $^\.*+?[].

In regular expressions you will often use backslash characters; it’s easier to use a quoted
string literal {|...|} to avoid having to escape backslashes.
For example, the following expression:
let r = Str.regexp {|hello \([A-Za-z]+\)|} in
Str.replace_first r {|\1|} "hello world"
returns the string "world".
If you want a regular expression that matches a literal backslash character, you need to
double it: Str.regexp {|\\|}.
You can use regular string literals "..." too, however you will have to escape backslashes.
The example above can be rewritten with a regular string literal as:
let r = Str.regexp "hello \\([A-Za-z]+\\)" in
Str.replace_first r "\\1" "hello world"
And the regular expression for matching a backslash becomes a quadruple backslash:
Str.regexp "\\\\".

val regexp_case_fold : string -> regexp

Same as regexp, but the compiled expression will match text in a case-insensitive way:
uppercase and lowercase letters will be considered equivalent.

val quote : string -> string

Str.quote s returns a regexp string that matches exactly s and nothing else.

val regexp_string : string -> regexp

Chapter 32. The str library: regular expressions and string processing 1009

Str.regexp_string s returns a regular expression that matches exactly s and nothing else.

val regexp_string_case_fold : string -> regexp

Str.regexp_string_case_fold is similar to Str.regexp_string[32.1], but the regexp
matches in a case-insensitive way.

String matching and searching

val string_match : regexp -> string -> int -> bool
string_match r s start tests whether a substring of s that starts at position start
matches the regular expression r. The first character of a string has position 0, as usual.

val search_forward : regexp -> string -> int -> int

search_forward r s start searches the string s for a substring matching the regular
expression r. The search starts at position start and proceeds towards the end of the string.
Return the position of the first character of the matched substring.
Raises Not_found if no substring matches.

val search_backward : regexp -> string -> int -> int

search_backward r s last searches the string s for a substring matching the regular
expression r. The search first considers substrings that start at position last and proceeds
towards the beginning of string. Return the position of the first character of the matched
substring.
Raises Not_found if no substring matches.

val string_partial_match : regexp -> string -> int -> bool

Similar to Str.string_match[32.1], but also returns true if the argument string is a prefix of
a string that matches. This includes the case of a true complete match.

val matched_string : string -> string

matched_string s returns the substring of s that was matched by the last call to one of the
following matching or searching functions:

• Str.string_match[32.1]
• Str.search_forward[32.1]
• Str.search_backward[32.1]
• Str.string_partial_match[32.1]
• Str.global_substitute[32.1]
• Str.substitute_first[32.1]

provided that none of the following functions was called in between:

• Str.global_replace[32.1]
1010

• Str.replace_first[32.1]
• Str.split[32.1]
• Str.bounded_split[32.1]
• Str.split_delim[32.1]
• Str.bounded_split_delim[32.1]
• Str.full_split[32.1]
• Str.bounded_full_split[32.1]

Note: in the case of global_substitute and substitute_first, a call to matched_string

is only valid within the subst argument, not after global_substitute or
substitute_first returns.
The user must make sure that the parameter s is the same string that was passed to the
matching or searching function.

val match_beginning : unit -> int

match_beginning() returns the position of the first character of the substring that was
matched by the last call to a matching or searching function (see Str.matched_string[32.1]
for details).

val match_end : unit -> int

match_end() returns the position of the character following the last character of the
substring that was matched by the last call to a matching or searching function (see
Str.matched_string[32.1] for details).

val matched_group : int -> string -> string

matched_group n s returns the substring of s that was matched by the nth group \(...\)
of the regular expression that was matched by the last call to a matching or searching
function (see Str.matched_string[32.1] for details). When n is 0, it returns the substring
matched by the whole regular expression. The user must make sure that the parameter s is
the same string that was passed to the matching or searching function.
Raises Not_found if the nth group of the regular expression was not matched. This can
happen with groups inside alternatives \|, options ? or repetitions *. For instance, the empty
string will match \(a\)*, but matched_group 1 "" will raise Not_found because the first
group itself was not matched.

val group_beginning : int -> int

group_beginning n returns the position of the first character of the substring that was
matched by the nth group of the regular expression that was matched by the last call to a
matching or searching function (see Str.matched_string[32.1] for details).
Raises

• Not_found if the nth group of the regular expression was not matched.
• Invalid_argument if there are fewer than n groups in the regular expression.
Chapter 32. The str library: regular expressions and string processing 1011

val group_end : int -> int

group_end n returns the position of the character following the last character of substring
that was matched by the nth group of the regular expression that was matched by the last
call to a matching or searching function (see Str.matched_string[32.1] for details).
Raises

• Not_found if the nth group of the regular expression was not matched.
• Invalid_argument if there are fewer than n groups in the regular expression.

Replacement
val global_replace : regexp -> string -> string -> string
global_replace regexp templ s returns a string identical to s, except that all substrings
of s that match regexp have been replaced by templ. The replacement template templ can
contain \1, \2, etc; these sequences will be replaced by the text matched by the
corresponding group in the regular expression. \0 stands for the text matched by the whole
regular expression.

val replace_first : regexp -> string -> string -> string

Same as Str.global_replace[32.1], except that only the first substring matching the regular
expression is replaced.

val global_substitute : regexp -> (string -> string) -> string -> string
global_substitute regexp subst s returns a string identical to s, except that all
substrings of s that match regexp have been replaced by the result of function subst. The
function subst is called once for each matching substring, and receives s (the whole text) as
argument.

val substitute_first : regexp -> (string -> string) -> string -> string
Same as Str.global_substitute[32.1], except that only the first substring matching the
regular expression is replaced.

val replace_matched : string -> string -> string

replace_matched repl s returns the replacement text repl in which \1, \2, etc. have been
replaced by the text matched by the corresponding groups in the regular expression that was
matched by the last call to a matching or searching function (see Str.matched_string[32.1]
for details). s must be the same string that was passed to the matching or searching function.

Splitting
val split : regexp -> string -> string list
1012

split r s splits s into substrings, taking as delimiters the substrings that match r, and
returns the list of substrings. For instance, split (regexp "[ \t]+") s splits s into
blank-separated words. An occurrence of the delimiter at the beginning or at the end of the
string is ignored.

val bounded_split : regexp -> string -> int -> string list
Same as Str.split[32.1], but splits into at most n substrings, where n is the extra integer
parameter.

val split_delim : regexp -> string -> string list

Same as Str.split[32.1] but occurrences of the delimiter at the beginning and at the end of
the string are recognized and returned as empty strings in the result. For instance,
split_delim (regexp " ") " abc " returns [""; "abc"; ""], while split with the
same arguments returns ["abc"].

val bounded_split_delim : regexp -> string -> int -> string list
Same as Str.bounded_split[32.1], but occurrences of the delimiter at the beginning and at
the end of the string are recognized and returned as empty strings in the result.

type split_result =
| Text of string
| Delim of string
val full_split : regexp -> string -> split_result list
Same as Str.split_delim[32.1], but returns the delimiters as well as the substrings
contained between delimiters. The former are tagged Delim in the result list; the latter are
tagged Text. For instance, full_split (regexp "[{}]") "{ab}" returns [Delim "{";
Text "ab"; Delim "}"].

val bounded_full_split : regexp -> string -> int -> split_result list
Same as Str.bounded_split_delim[32.1], but returns the delimiters as well as the
substrings contained between delimiters. The former are tagged Delim in the result list; the
latter are tagged Text.

Extracting substrings
val string_before : string -> int -> string
string_before s n returns the substring of all characters of s that precede position n
(excluding the character at position n).

val string_after : string -> int -> string

string_after s n returns the substring of all characters of s that follow position n
(including the character at position n).

val first_chars : string -> int -> string

Chapter 32. The str library: regular expressions and string processing 1013

first_chars s n returns the first n characters of s. This is the same function as

Str.string_before[32.1].

val last_chars : string -> int -> string

last_chars s n returns the last n characters of s.
1014
Chapter 33

The runtime_events library

The runtime_events library provides an API for consuming runtime tracing and metrics information
from the runtime. See chapter 25 for more information.
Programs that use runtime_events must be linked as follows:

ocamlc -I +runtime_events other options unix.cma runtime_events.cma other files

ocamlopt -I +runtime_events other options unix.cmxa runtime_events.cmxa other files

Compilation units that use the runtime_events library must also be compiled with the
-I +runtime_events option (see chapter 13).

33.1 Module Runtime_events : Runtime events - ring buffer-based

runtime tracing
This module enables users to enable and subscribe to tracing events from the Garbage Collector
and other parts of the OCaml runtime. This can be useful for diagnostic or performance monitoring
purposes. This module can be used to subscribe to events for the current process or external
processes asynchronously.
When enabled (either via setting the OCAML_RUNTIME_EVENTS_START environment
variable or calling Runtime_events.start) a file with the pid of the process and extension .events
will be created. By default this is in the current directory but can be over-ridden by the OCAML_
RUNTIME_EVENTS_DIR environment variable. Each domain maintains its own ring buffer in a
section of the larger file into which it emits events.
There is additionally a set of C APIs in runtime_events.h that can enable zero-impact monitoring
of the current process or bindings for other languages.
The runtime events system’s behaviour can be controlled by the following environment variables:

• OCAML_RUNTIME_EVENTS_START if set will cause the runtime events system to be

started as part of the OCaml runtime initialization.

• OCAML_RUNTIME_EVENTS_DIR sets the directory where the runtime events ring buffers
will be located. If not present the program’s working directory will be used.

1015
1016

• OCAML_RUNTIME_EVENTS_PRESERVE if set will prevent the OCaml runtime from

removing its ring buffers when it terminates. This can help if monitoring very short running
programs.

type runtime_counter =
| EV_C_FORCE_MINOR_ALLOC_SMALL
Triggering of a minor collection due to a full minor heap.
Since: 5.0
| EV_C_FORCE_MINOR_MAKE_VECT
Triggering of a minor collection due to Array.make.
Since: 5.0
| EV_C_FORCE_MINOR_SET_MINOR_HEAP_SIZE
Triggering of a minor collection due to Gc.minor_heap_size.
Since: 5.0
| EV_C_FORCE_MINOR_MEMPROF
Triggering of a minor collection during memprof young sampling.
Since: 5.3
| EV_C_MINOR_PROMOTED
Total words promoted from the minor heap to the major in the last minor collection.
Since: 5.0
| EV_C_MINOR_ALLOCATED
Total bytes allocated in the minor heap in the last minor collection.
Since: 5.0
| EV_C_REQUEST_MAJOR_ALLOC_SHR
Major slice requested due to allocation in major heap.
Since: 5.0
| EV_C_REQUEST_MAJOR_ADJUST_GC_SPEED
Major slice requested by caml_adjust_gc_speed.
Since: 5.0
| EV_C_REQUEST_MINOR_REALLOC_REF_TABLE
Triggering of a minor collection due to ref table reallocation.
Since: 5.0
| EV_C_REQUEST_MINOR_REALLOC_EPHE_REF_TABLE
Triggering of a minor collection due to ephe_ref table reallocation.
Since: 5.0
Chapter 33. The runtime_events library 1017

| EV_C_REQUEST_MINOR_REALLOC_CUSTOM_TABLE
Triggering of a minor collection due to custom table reallocation.
Since: 5.0
| EV_C_MAJOR_HEAP_POOL_WORDS
Total words in a Domain’s major heap pools. This is the sum of unallocated and live
words in each pool.
Since: 5.1
| EV_C_MAJOR_HEAP_POOL_LIVE_WORDS
Current live words in a Domain’s major heap pools.
Since: 5.1
| EV_C_MAJOR_HEAP_LARGE_WORDS
Total words of a Domain’s major heap large allocations. A large allocation is an
allocation larger than the largest sized pool.
Since: 5.1
| EV_C_MAJOR_HEAP_POOL_FRAG_WORDS
Words in a Domain’s major heap pools lost to fragmentation. This is due to there not
being a pool with the exact size of an allocation and a larger sized pool needing to be
used.
Since: 5.1
| EV_C_MAJOR_HEAP_POOL_LIVE_BLOCKS
Live blocks of a Domain’s major heap pools.
Since: 5.1
| EV_C_MAJOR_HEAP_LARGE_BLOCKS
Live blocks of a Domain’s major heap large allocations.
Since: 5.1
| EV_C_MAJOR_HEAP_WORDS
Major heap size in words of a Domain.
Since: 5.3
| EV_C_MAJOR_ALLOCATED_WORDS
Allocations to the major heap of this Domain in words, since the last major slice.
Since: 5.3
| EV_C_MAJOR_ALLOCATED_WORK
The amount of major GC ’work’ needing to be done as a result of allocations to the
major heap of this Domain in words, since the last major slice.
Since: 5.3
| EV_C_MAJOR_DEPENDENT_WORK
1018

The amount of major GC ’work’ needing to be done as a result of dependent

allocations to the major heap of this Domain in words, since the last major slice.
Dependent memory is non-heap memory that depends on heap memory being collected
in order to be freed.
Since: 5.3
| EV_C_MAJOR_EXTRA_WORK
The amount of major GC ’work’ needing to be done as a result of extra non-memory
resources that are dependent on heap memory being collected in order to be freed.
Since: 5.3
| EV_C_MAJOR_WORK_COUNTER
The global amount of major GC ’work’ done by all domains since the program began.
Since: 5.3
| EV_C_MAJOR_ALLOC_COUNTER
The global words of major GC allocations done by all domains since the program
began.
Since: 5.3
| EV_C_MAJOR_SLICE_TARGET
The target amount of global ’work’ that should be done by all domains at the end of
the major slice (see EV_C_MAJOR_SLICE_COUNTER).
Since: 5.3
| EV_C_MAJOR_SLICE_BUDGET
The budget in ’work’ that a domain has to do during the major slice.
Since: 5.3
The type for counter events emitted by the runtime. Counter events are used to measure a
quantity at a point in time or record the occurence of an event. In the latter case their value
will be one.

type runtime_phase =
| EV_EXPLICIT_GC_SET
Event spanning a call to Gc.set.
Since: 5.0
| EV_EXPLICIT_GC_STAT
Event spanning a call to Gc.stat.
Since: 5.0
| EV_EXPLICIT_GC_MINOR
Event spanning a call to Gc.minor, which forces a minor collection.
Since: 5.0
Chapter 33. The runtime_events library 1019

| EV_EXPLICIT_GC_MAJOR
Event spanning a call to Gc.major, which forces a major collection.
Since: 5.0
| EV_EXPLICIT_GC_FULL_MAJOR
Event spanning a call to Gc.full_major, which forces a full major collection.
Since: 5.0
| EV_EXPLICIT_GC_COMPACT
Event spanning a call to Gc.compact, which triggers a compaction.
Since: 5.0
| EV_MAJOR
Event spanning any major GC work.
Since: 5.0
| EV_MAJOR_SWEEP
Event spanning the sweeping work of a major GC.
Since: 5.0
| EV_MAJOR_MARK_ROOTS
Event spanning the marking of roots in a major GC.
Since: 5.0
| EV_MAJOR_MEMPROF_ROOTS
Event spanning the marking of memprof roots in a major GC.
Since: 5.3
| EV_MAJOR_MARK
Event spanning the marking of the heap in a major GC.
Since: 5.0
| EV_MINOR
Event spanning any minor GC work.
Since: 5.0
| EV_MINOR_LOCAL_ROOTS
Event spanning the scanning and major allocation of local roots during a minor GC.
Since: 5.0
| EV_MINOR_MEMPROF_ROOTS
Event spanning the scanning and promotion of memprof roots in a minor GC.
Since: 5.3
| EV_MINOR_MEMPROF_CLEAN
1020

Event spanning cleaning and updating of memprof structures at the end of a minor GC.
Since: 5.3
| EV_MINOR_FINALIZED
Event spanning the running of finalisers for dead custom blocks at the end of a minor
GC.
Since: 5.0
| EV_EXPLICIT_GC_MAJOR_SLICE
Event spanning a call to Gc.major_slice.
Since: 5.0
| EV_FINALISE_UPDATE_FIRST
Event spanning time spent in the first phase of finalisation at the end of a major GC
cycle.
Since: 5.0
| EV_FINALISE_UPDATE_LAST
Event spanning time spent in the last phase of finalisation at the end of a major GC
cycle.
Since: 5.0
| EV_INTERRUPT_REMOTE
Event spanning work triggered by an interrupt from another domain. This is usually a
stop-the-world request.
Since: 5.0
| EV_MAJOR_EPHE_MARK
Event spanning the marking of ephemeron tables in a major GC.
Since: 5.0
| EV_MAJOR_EPHE_SWEEP
Event spanning the sweeping of ephemeron tables in a major GC.
Since: 5.0
| EV_MAJOR_FINISH_MARKING
Event spanning work done at the end of marking in a major GC.
Since: 5.0
| EV_MAJOR_GC_CYCLE_DOMAINS
Event spanning work done at the end of a major GC cycle. This includes a minor
collection.
Since: 5.0
| EV_MAJOR_GC_PHASE_CHANGE
Chapter 33. The runtime_events library 1021

Event spanning the change of phase in the major GC which involves a global barrier.
Since: 5.0
| EV_MAJOR_GC_STW
Event spanning the stop-the-world phase done at the end of a major GC cycle.
Since: 5.0
| EV_MAJOR_MARK_OPPORTUNISTIC
Event spanning the work done during opportunistic marking in a major GC.
Since: 5.0
| EV_MAJOR_SLICE
Event spanning the work done during a major slice in a major GC.
Since: 5.0
| EV_MAJOR_FINISH_CYCLE
Event spanning attempts to drive all domains to the end of a major GC cycle.
Since: 5.0
| EV_MINOR_CLEAR
Event spanning the cleaning of the minor heap and supporting structures at the end of
a minor GC.
Since: 5.0
| EV_MINOR_FINALIZERS_OLDIFY
Event spanning the promotion of finalisers during a minor GC.
Since: 5.0
| EV_MINOR_GLOBAL_ROOTS
Event spanning the scanning and major allocation of global roots during a minor GC.
Since: 5.0
| EV_MINOR_LEAVE_BARRIER
Event spanning the time spent in the barrier at the end of a minor GC, waiting for all
domains to finish their work.
Since: 5.0
| EV_STW_API_BARRIER
Event spanning the time spent waiting for all other domains to reach the
stop-the-world entry barrier.
Since: 5.0
| EV_STW_HANDLER
Event spanning the time spent in the stop-the-world handler, including time spent in
the stop-the-world callback itself.
Since: 5.0
1022

| EV_STW_LEADER
Event spanning the time spent as the leader of a stop-the-world.
Since: 5.0
| EV_MAJOR_FINISH_SWEEPING
Event spanning the time spent finishing sweeping when forced to as part of domain
termination.
Since: 5.0
| EV_MAJOR_MEMPROF_CLEAN
Event spanning the time spent cleaning memprof structures at the end of a major GC.
Since: 5.3
| EV_MINOR_FINALIZERS_ADMIN
Event spanning finalisers book-keeping at the end of a minor GC.
Since: 5.0
| EV_MINOR_REMEMBERED_SET
Event spanning the scanning and major allocation of remembered sets during a minor
GC.
Since: 5.0
| EV_MINOR_REMEMBERED_SET_PROMOTE
Event spanning the promotion of blocks in the remembered set and global roots during
a minor GC.
Since: 5.0
| EV_MINOR_LOCAL_ROOTS_PROMOTE
Event spanning the promotion of local roots during a minor GC.
Since: 5.0
| EV_DOMAIN_CONDITION_WAIT
Event spanning waiting in Condition.wait.
Since: 5.0
| EV_DOMAIN_RESIZE_HEAP_RESERVATION
Event spanning resizing the domain heap reservation, as a result of minor heap size
changes.
Since: 5.0
| EV_COMPACT
Event spanning compaction of the heap during a call to Gc.compact.
Since: 5.2
| EV_COMPACT_EVACUATE
Chapter 33. The runtime_events library 1023

Event spanning evacuating major GC pools during a compaction.

Since: 5.2
| EV_COMPACT_FORWARD
Event spanning the walking of the heap to update changed pointers after an evacuation
during a compaction.
Since: 5.2
| EV_COMPACT_RELEASE
Event spanning releasing the evacuated pools at the end of a compaction.
Since: 5.2
The type for span events emitted by the runtime.

type lifecycle =
| EV_RING_START
Event indicating that the Runtime_events ring buffer has been started. Includes the
PID of the process as an argument.
Since: 5.0
| EV_RING_STOP
Event indicating that the Runtime_events ring buffer has been stopped.
Since: 5.0
| EV_RING_PAUSE
Event indicating that the Runtime_events ring buffer has been paused.
Since: 5.0
| EV_RING_RESUME
Event indicating that the Runtime_events ring buffer has been resumed.
Since: 5.0
| EV_FORK_PARENT
Event indicating that a fork has occurred and the current domain is the parent.
Includes the PID of the child as an argument.
Since: 5.0
| EV_FORK_CHILD
Event indicating that a fork has occurred and the current domain is the child.
Since: 5.0
| EV_DOMAIN_SPAWN
Event indicating that a new domain has been spawned. Includes the PID of the new
domain as an argument.
Since: 5.0
1024

| EV_DOMAIN_TERMINATE
Event indicating that a domain has terminated. Includes the PID of the domain as an
argument.
Since: 5.0
Lifecycle events for Runtime_events and domains.

val lifecycle_name : lifecycle -> string

Return a string representation of a given lifecycle event type.

val runtime_phase_name : runtime_phase -> string

Return a string representation of a given runtime phase event type.

val runtime_counter_name : runtime_counter -> string

Return a string representation of a given runtime counter type.

type cursor
Type of the cursor used when consuming.

module Timestamp :
sig
type t
Type for the int64 timestamp to allow for future changes.

val to_int64 : t -> int64

end

module Type :
sig
type 'a t
The type for a user event content type.

val unit : unit t

An event that has no data associated with it.

type span =
| Begin
| End
val span : span t
An event that has a beginning and an end.

val int : int t

Chapter 33. The runtime_events library 1025

An event containing an integer value.

val register :
encode:(bytes -> 'a -> int) ->
decode:(bytes -> int -> 'a) -> 'a t
Registers a custom type by providing an encoder and a decoder. The encoder writes the
value in the provided buffer and returns the number of bytes written. The decoder gets a
slice of the buffer of specified length, and returns the decoded value.
The maximum value length is 1024 bytes.

end

module User :
sig

User events is a way for libraries to provide runtime events that can be consumed by other
tools. These events can carry known data types or custom values. The current maximum
number of user events is 8192.
type tag = ..
The type for a user event tag. Tags are used to discriminate between user events of the
same type.

type 'value t
The type for a user event. User events describe their tag, carried data type and an
unique string-based name.

val register : string ->

tag ->
'value Runtime_events.Type.t -> 'value t
register name tag ty registers a new event with an unique name, carrying a tag and
values of type ty.

val write : 'value t -> 'value -> unit

write t v emits value v for event t.

val name : 'a t -> string

name t is the unique identifying name of event t.

val tag : 'a t -> tag

tag t is the associated tag of event t, when it is known. An event can be unknown if it
was not registered in the consumer program.
1026

end
module Callbacks :
sig
type t
Type of callbacks.

val create :
?runtime_begin:(int ->
Runtime_events.Timestamp.t ->
Runtime_events.runtime_phase -> unit) ->
?runtime_end:(int ->
Runtime_events.Timestamp.t ->
Runtime_events.runtime_phase -> unit) ->
?runtime_counter:(int ->
Runtime_events.Timestamp.t ->
Runtime_events.runtime_counter -> int -> unit) ->
?alloc:(int -> Runtime_events.Timestamp.t -> int array -> unit) ->
?lifecycle:(int ->
Runtime_events.Timestamp.t ->
Runtime_events.lifecycle -> int option -> unit) ->
?lost_events:(int -> int -> unit) -> unit -> t
Create a Callback that optionally subscribes to one or more runtime events. The first
int supplied to callbacks is the ring buffer index. Each domain owns a single ring buffer
for the duration of the domain’s existence. After a domain terminates, a newly spawned
domain may take ownership of the ring buffer. A runtime_begin callback is called when
the runtime enters a new phase (e.g a runtime_begin with EV_MINOR is called at the
start of a minor GC). A runtime_end callback is called when the runtime leaves a
certain phase. The runtime_counter callback is called when a counter is emitted by the
runtime. lifecycle callbacks are called when the ring undergoes a change in lifecycle
and a consumer may need to respond. alloc callbacks are currently only called on the
instrumented runtime. lost_events callbacks are called if the consumer code detects
some unconsumed events have been overwritten.

val add_user_event :
'a Runtime_events.Type.t ->
(int -> Runtime_events.Timestamp.t -> 'a Runtime_events.User.t -> 'a -> unit) ->
t -> t
add_user_event ty callback t extends t to additionally subscribe to user events of
type ty. When such an event happens, callback is called with the corresponding event
and payload.

end
val start : unit -> unit
Chapter 33. The runtime_events library 1027

start () will start the collection of events in the runtime if not already started.
Events can be consumed by creating a cursor with create_cursor and providing a set of
callbacks to be called for each type of event.

val path : unit -> string option

If runtime events are being collected, path () returns Some p where p is a path to the
runtime events file. Otherwise, it returns None.

val pause : unit -> unit

pause () will pause the collection of events in the runtime. Traces are collected if the
program has called Runtime_events.start () or the
OCAML_RUNTIME_EVENTS_START environment variable has been set.

val resume : unit -> unit

resume () will resume the collection of events in the runtime. Traces are collected if the
program has called Runtime_events.start () or the
OCAML_RUNTIME_EVENTS_START environment variable has been set.

val create_cursor : (string * int) option -> cursor

create_cursor path_pid creates a cursor to read from an runtime_events. Cursors can be
created for runtime_events in and out of process. A runtime_events ring-buffer may have
multiple cursors reading from it at any point in time and a program may have multiple
cursors open concurrently (for example if multiple consumers want different sets of events). If
path_pid is None then a cursor is created for the current process. Otherwise the pair
contains a string path to the directory that contains the pid.events file and int pid for the
runtime_events of an external process to monitor.

val free_cursor : cursor -> unit

Free a previously created runtime_events cursor.

val read_poll : cursor -> Callbacks.t -> int option -> int
read_poll cursor callbacks max_option calls the corresponding functions on callbacks
for up to max_option events read off cursor’s runtime_events and returns the number of
events read.
1028
Chapter 34

The threads library

The threads library allows concurrent programming in OCaml. It provides multiple threads of
control (also called lightweight processes) that execute concurrently in the same memory space.
Threads communicate by in-place modification of shared data structures, or by sending and receiving
data on communication channels.
The threads library is implemented on top of the threading facilities provided by the operating
system: POSIX 1003.1c threads for Linux, MacOS, and other Unix-like systems; Win32 threads for
Windows. Only one thread at a time is allowed to run OCaml code on a particular domain 9.5.1.
Hence, opportunities for parallelism are limited to the parts of the program that run system or
C library code. However, threads provide concurrency and can be used to structure programs as
several communicating processes. Threads also efficiently support concurrent, overlapping I/O
operations.
Programs that use threads must be linked as follows:

ocamlc -I +unix -I +threads other options unix.cma threads.cma other files

ocamlopt -I +unix -I +threads other options unix.cmxa threads.cmxa other files

Compilation units that use the threads library must also be compiled with the -I +threads
option (see chapter 13).

34.1 Module Thread : Lightweight threads for Posix 1003.1c and

Win32.

type t
The type of thread handles.

Thread creation and termination

val create : ('a -> 'b) -> 'a -> t
Thread.create funct arg creates a new thread of control, in which the function application
funct arg is executed concurrently with the other threads of the domain. The application of

1029
1030

Thread.create returns the handle of the newly created thread. The new thread terminates
when the application funct arg returns, either normally or by raising the Thread.Exit[34.1]
exception or by raising any other uncaught exception. In the last case, the uncaught
exception is printed on standard error, but not propagated back to the parent thread.
Similarly, the result of the application funct arg is discarded and not directly accessible to
the parent thread.
See also Domain.spawn[29.14] if you want parallel execution instead.

val self : unit -> t

Return the handle for the thread currently executing.

val id : t -> int

Return the identifier of the given thread. A thread identifier is an integer that identifies
uniquely the thread. It can be used to build data structures indexed by threads.

exception Exit
Exception raised by user code to initiate termination of the current thread. In a thread
created by Thread.create[34.1] funct arg, if the Thread.Exit[34.1] exception reaches the
top of the application funct arg, it has the effect of terminating the current thread silently.
In other contexts, there is no implicit handling of the Thread.Exit[34.1] exception.

val exit : unit -> unit

Deprecated. Use ’raise Thread.Exit’ instead.Raise the Thread.Exit[34.1] exception. In a
thread created by Thread.create[34.1], this will cause the thread to terminate prematurely,
unless the thread function handles the exception itself. Fun.protect[29.23] finalizers and
catch-all exception handlers will be executed.
To make it clear that an exception is raised and will trigger finalizers and catch-all exception
handlers, it is recommended to write raise Thread.Exit instead of Thread.exit ().
Before 5.0 A different implementation was used, not based on raising an exception, and not
running finalizers and catch-all handlers. The previous implementation had a different
behavior when called outside of a thread created by Thread.create.

Suspending threads
val delay : float -> unit
delay d suspends the execution of the calling thread for d seconds. The other program
threads continue to run during this time.

val join : t -> unit

join th suspends the execution of the calling thread until the thread th has terminated.

val yield : unit -> unit

Re-schedule the calling thread without suspending it. This function can be used to give
scheduling hints, telling the scheduler that now is a good time to switch to other threads.
Chapter 34. The threads library 1031

Waiting for file descriptors or processes

The functions below are leftovers from an earlier, VM-based threading system. The Unix[31.1]
module provides equivalent functionality, in a more general and more standard-conformant manner.
It is recommended to use Unix[31.1] functions directly.
val wait_timed_read : Unix.file_descr -> float -> bool
Deprecated. Use Unix.select instead.See Thread.wait_timed_write[34.1].

val wait_timed_write : Unix.file_descr -> float -> bool

Deprecated. Use Unix.select instead.Suspend the execution of the calling thread until at least
one character or EOF is available for reading (wait_timed_read) or one character can be
written without blocking (wait_timed_write) on the given Unix file descriptor. Wait for at
most the amount of time given as second argument (in seconds). Return true if the file
descriptor is ready for input/output and false if the timeout expired. The same functionality
can be achieved with Unix.select[31.1].

val select :
Unix.file_descr list ->
Unix.file_descr list ->
Unix.file_descr list ->
float -> Unix.file_descr list * Unix.file_descr list * Unix.file_descr list
Deprecated. Use Unix.select instead.Same function as Unix.select[31.1]. Suspend the
execution of the calling thread until input/output becomes possible on the given Unix file
descriptors. The arguments and results have the same meaning as for Unix.select[31.1].

val wait_pid : int -> int * Unix.process_status

Deprecated. Use Unix.waitpid instead.Same function as Unix.waitpid[31.1]. wait_pid p
suspends the execution of the calling thread until the process specified by the process
identifier p terminates. Returns the pid of the child caught and its termination status, as per
Unix.wait[31.1].

Management of signals
Signal handling follows the POSIX thread model: signals generated by a thread are delivered to
that thread; signals generated externally are delivered to one of the threads that does not block it.
Each thread possesses a set of blocked signals, which can be modified using Thread.sigmask[34.1].
This set is inherited at thread creation time. Per-thread signal masks are supported only by the
system thread library under Unix, but not under Win32, nor by the VM thread library.
val sigmask : Unix.sigprocmask_command -> int list -> int list
sigmask cmd sigs changes the set of blocked signals for the calling thread. If cmd is
SIG_SETMASK, blocked signals are set to those in the list sigs. If cmd is SIG_BLOCK, the
signals in sigs are added to the set of blocked signals. If cmd is SIG_UNBLOCK, the signals in
sigs are removed from the set of blocked signals. sigmask returns the set of previously
blocked signals for the thread.
1032

val wait_signal : int list -> int

wait_signal sigs suspends the execution of the calling thread until the process receives one
of the signals specified in the list sigs. It then returns the number of the signal received.
Signal handlers attached to the signals in sigs will not be invoked. The signals sigs are
expected to be blocked before calling wait_signal.

Uncaught exceptions
val default_uncaught_exception_handler : exn -> unit
Thread.default_uncaught_exception_handler will print the thread’s id, exception and
backtrace (if available).

val set_uncaught_exception_handler : (exn -> unit) -> unit

Thread.set_uncaught_exception_handler fn registers fn as the handler for uncaught
exceptions.
If the newly set uncaught exception handler raise an exception,
Thread.default_uncaught_exception_handler[34.1] will be called.

34.2 Module Event : First-class synchronous communication.

This module implements synchronous inter-thread communications over channels. As in John
Reppy’s Concurrent ML system, the communication events are first-class values: they can be built
and combined independently before being offered for communication.

type 'a channel

The type of communication channels carrying values of type 'a.

val new_channel : unit -> 'a channel

Return a new channel.

type +'a event

The type of communication events returning a result of type 'a.

val send : 'a channel -> 'a -> unit event

send ch v returns the event consisting in sending the value v over the channel ch. The result
value of this event is ().

val receive : 'a channel -> 'a event

receive ch returns the event consisting in receiving a value from the channel ch. The result
value of this event is the value received.

val always : 'a -> 'a event

Chapter 34. The threads library 1033

always v returns an event that is always ready for synchronization. The result value of this
event is v.

val choose : 'a event list -> 'a event

choose evl returns the event that is the alternative of all the events in the list evl.

val wrap : 'a event -> ('a -> 'b) -> 'b event
wrap ev fn returns the event that performs the same communications as ev, then applies the
post-processing function fn on the return value.

val wrap_abort : 'a event -> (unit -> unit) -> 'a event
wrap_abort ev fn returns the event that performs the same communications as ev, but if it
is not selected the function fn is called after the synchronization.

val guard : (unit -> 'a event) -> 'a event

guard fn returns the event that, when synchronized, computes fn() and behaves as the
resulting event. This enables computing events with side-effects at the time of the
synchronization operation.

val sync : 'a event -> 'a

’Synchronize’ on an event: offer all the communication possibilities specified in the event to
the outside world, and block until one of the communications succeed. The result value of
that communication is returned.

val select : 'a event list -> 'a

’Synchronize’ on an alternative of events. select evl is shorthand for sync(choose evl).

val poll : 'a event -> 'a option

Non-blocking version of Event.sync[34.2]: offer all the communication possibilities specified
in the event to the outside world, and if one can take place immediately, perform it and
return Some r where r is the result value of that communication. Otherwise, return None
without blocking.
1034
Chapter 35

The dynlink library: dynamic loading

and linking of object files

The dynlink library supports type-safe dynamic loading and linking of bytecode object files (.cmo
and .cma files) in a running bytecode program, or of native plugins (usually .cmxs files) in a running
native program. Type safety is ensured by limiting the set of modules from the running program
that the loaded object file can access, and checking that the running program and the loaded object
file have been compiled against the same interfaces for these modules. In native code, there are also
some compatibility checks on the implementations (to avoid errors with cross-module optimizations);
it might be useful to hide .cmx files when building native plugins so that they remain independent
of the implementation of modules in the main program.
Programs that use the dynlink library simply need to include the dynlink library directory with
-I +dynlink and link dynlink.cma or dynlink.cmxa with their object files and other libraries.
Note: in order to insure that the dynamically-loaded modules have access to all the libraries
that are visible to the main program (and not just to the parts of those libraries that are actually
used in the main program), programs using the dynlink library should be linked with -linkall.

35.1 Module Dynlink : Dynamic loading of .cmo, .cma and .cmxs

files.

val is_native : bool

true if the program is native, false if the program is bytecode.

Dynamic loading of compiled files

val loadfile : string -> unit
In bytecode: load the given bytecode object file (.cmo file) or bytecode library file (.cma file),
and link it with the running program. In native code: load the given OCaml plugin file
(usually .cmxs), and link it with the running program.

1035
1036

All toplevel expressions in the loaded compilation units are evaluated. No facilities are
provided to access value names defined by the unit. Therefore, the unit must itself register its
entry points with the main program (or a previously-loaded library) e.g. by modifying tables
of functions.
An exception will be raised if the given library defines toplevel modules whose names clash
with modules existing either in the main program or a shared library previously loaded with
loadfile. Modules from shared libraries previously loaded with loadfile_private are not
included in this restriction.
The compilation units loaded by this function are added to the ”allowed units” list (see
Dynlink.set_allowed_units[35.1]).

val loadfile_private : string -> unit

Same as loadfile, except that the compilation units just loaded are hidden (cannot be
referenced) from other modules dynamically loaded afterwards.
An exception will be raised if the given library defines toplevel modules whose names clash
with modules existing in either the main program or a shared library previously loaded with
loadfile. Modules from shared libraries previously loaded with loadfile_private are not
included in this restriction.
An exception will also be raised if the given library defines toplevel modules whose name
matches that of an interface depended on by a module existing in either the main program or
a shared library previously loaded with loadfile. This applies even if such dependency is
only a ”module alias” dependency (i.e. just on the name rather than the contents of the
interface).
The compilation units loaded by this function are not added to the ”allowed units” list (see
Dynlink.set_allowed_units[35.1]) since they cannot be referenced from other compilation
units.

val adapt_filename : string -> string

In bytecode, the identity function. In native code, replace the last extension with .cmxs.

Access control
val set_allowed_units : string list -> unit
Set the list of compilation units that may be referenced from units that are dynamically
loaded in the future to be exactly the given value.
Initially all compilation units composing the program currently running are available for
reference from dynamically-linked units. set_allowed_units can be used to restrict access
to a subset of these units, e.g. to the units that compose the API for dynamically-linked code,
and prevent access to all other units, e.g. private, internal modules of the running program.
Note that Dynlink.loadfile[35.1] changes the allowed-units list.

val allow_only : string list -> unit

Chapter 35. The dynlink library: dynamic loading and linking of object files 1037

allow_only units sets the list of allowed units to be the intersection of the existing allowed
units and the given list of units. As such it can never increase the set of allowed units.

val prohibit : string list -> unit

prohibit units prohibits dynamically-linked units from referencing the units named in list
units by removing such units from the allowed units list. This can be used to prevent access
to selected units, e.g. private, internal modules of the running program.

val main_program_units : unit -> string list

Return the list of compilation units that form the main program (i.e. are not dynamically
linked).

val public_dynamically_loaded_units : unit -> string list

Return the list of compilation units that have been dynamically loaded via loadfile (and not
via loadfile_private). Note that compilation units loaded dynamically cannot be unloaded.

val all_units : unit -> string list

Return the list of compilation units that form the main program together with those that
have been dynamically loaded via loadfile (and not via loadfile_private).

val allow_unsafe_modules : bool -> unit

Govern whether unsafe object files are allowed to be dynamically linked. A compilation unit
is ’unsafe’ if it contains declarations of external functions, which can break type safety. By
default, dynamic linking of unsafe object files is not allowed. In native code, this function
does nothing; object files with external functions are always allowed to be dynamically linked.

Error reporting
type linking_error = private
| Undefined_global of string
| Unavailable_primitive of string
| Uninitialized_global of string
type error = private
| Not_a_bytecode_file of string
| Inconsistent_import of string
| Unavailable_unit of string
| Unsafe_file
| Linking_error of string * linking_error
| Corrupted_interface of string
| Cannot_open_dynamic_library of exn
| Library's_module_initializers_failed of exn
| Inconsistent_implementation of string
| Module_already_loaded of string
| Private_library_cannot_implement_interface of string
exception Error of error
1038

Errors in dynamic linking are reported by raising the Error exception with a description of
the error. A common case is the dynamic library not being found on the system: this is
reported via Cannot_open_dynamic_library (the enclosed exception may be
platform-specific).

val error_message : error -> string

Convert an error description to a printable message.
Chapter 36

Recently removed or moved libraries

(Graphics, Bigarray, Num, LablTk)

This chapter describes three libraries which were formerly part of the OCaml distribution (Graphics,
Num, and LablTk), and a library which has now become part of OCaml’s standard library, and is
documented there (Bigarray).

36.1 The Graphics Library

Since OCaml 4.09, the graphics library is distributed as an external package. Its new home is:
https://github.com/ocaml/graphics
If you are using the opam package manager, you should install the corresponding graphics
package:

opam install graphics

Before OCaml 4.09, this package simply ensures that the graphics library was installed by the
compiler, and starting from OCaml 4.09 this package effectively provides the graphics library.

36.2 The Bigarray Library

As of OCaml 4.07, the bigarray library has been integrated into OCaml’s standard library.
The bigarray functionality may now be found in the standard library Bigarray module, except
for the map_file function which is now part of the Unix library. The documentation has been
integrated into the documentation for the standard library.
The legacy bigarray library bundled with the compiler is a compatibility library with exactly
the same interface as before, i.e. with map_file included.
We strongly recommend that you port your code to use the standard library version instead, as
the changes required are minimal.
If you choose to use the compatibility library, you must link your programs as follows:
ocamlc other options bigarray.cma other files
ocamlopt other options bigarray.cmxa other files

1039
1040

For interactive use of the bigarray compatibility library, do:

ocamlmktop -o mytop bigarray.cma

./mytop

or (if dynamic linking of C libraries is supported on your platform), start ocaml and type
#load "bigarray.cma";;.

36.3 The Num Library

The num library implements integer arithmetic and rational arithmetic in arbitrary precision. It
was split off the core OCaml distribution starting with the 4.06.0 release, and can now be found at
https://github.com/ocaml/num.
New applications that need arbitrary-precision arithmetic should use the Zarith library (https:
//github.com/ocaml/Zarith) instead of the Num library, and older applications that already use
Num are encouraged to switch to Zarith. Zarith delivers much better performance than Num and
has a nicer API.

36.4 The Labltk Library and OCamlBrowser

Since OCaml version 4.02, the OCamlBrowser tool and the Labltk library are distributed separately
from the OCaml compiler. The project is now hosted at https://github.com/garrigue/labltk.
Part V

Indexes

1041
INDEX TO THE LIBRARY 1043

Index to the library

( * ), 517 __LOC_OF__, 516

( ** ), 519 __MODULE__, 516
( *. ), 519 __POS__, 516
(&&), 515, 589 __POS_OF__, 516
(@), 526 _exit, 967
(@@), 517
(!), 534 abs, 518, 662, 740, 743, 747, 814
(!=), 515 abs_float, 521
(:=), 534 absolute_path, 925
(=), 514 accept, 992
(==), 515 access, 977
(^), 523 access_permission, 976
(^^), 536 acos, 520, 666
(>), 514 acosh, 521, 666
(>=), 514 acquire, 876, 877
(<), 514 adapt_filename, 1036
(<=), 514 add, 625, 652–656, 661, 724, 729, 730, 740, 742,
(<>), 514 747, 775, 787, 792, 794, 799, 807, 813,
(-), 517 836, 870, 914
(-.), 519 add_buffer, 593
(|>), 517 add_bytes, 592
(||), 515, 589 add_channel, 593
(+), 517 add_char, 592
(+.), 519 add_int16_be, 595
(/), 517 add_int16_le, 595
(/.), 519 add_int16_ne, 594
(~-), 517 add_int32_be, 595
(~-.), 519 add_int32_le, 595
(~+), 517 add_int32_ne, 595
(~+.), 519 add_int64_be, 595
(asr), 519 add_int64_le, 595
(land), 518 add_int64_ne, 595
(lor), 518 add_int8, 594
(lsl), 518 add_last, 637
(lsr), 518 add_ppx_context_sig, 921
(lxor), 518 add_ppx_context_str, 921
(mod), 518 add_seq, 594, 652, 653, 727, 729, 731, 781,
__FILE__, 515 790, 793, 795, 805, 811, 837, 875, 879
__FUNCTION__, 516 add_string, 592
__LINE__, 516 add_subbytes, 592
__LINE_OF__, 516 add_substitute, 593
__LOC__, 515 add_substring, 592
1044

add_symbolic_output_item, 704 asin, 520, 666

add_to_list, 775, 800 asinh, 521, 666
add_uint16_be, 594 asprintf, 707
add_uint16_le, 594 Assert_failure, 512
add_uint16_ne, 594 Assert_failure, 191, 511
add_uint8, 594 assoc, 763, 772
add_user_event, 1026 assoc_opt, 763, 772
add_utf_16be_uchar, 592 assq, 763, 772
add_utf_16le_uchar, 592 assq_opt, 763, 772
add_utf_8_uchar, 592 Ast_mapper, 917
addr_info, 998 Asttypes, 921
alarm, 719, 987 at_exit, 536, 629
alert, 928 atan, 520, 666
alert_reporter, 928 atan2, 521, 666
align, 547 atanh, 521, 667
all_units, 1037 Atomic, 537, 564
allocated_bytes, 718 attribute, 932
allocation, 720 attribute_of_warning, 920
allocation_source, 720 attributes, 932
allow_only, 1036 auto_include_alert, 928
allow_unsafe_modules, 1037
Already_displayed_error, 929 backend_type, 900
always, 1032 backtrace_slot, 828
anon_fun, 545 backtrace_slots, 828
append, 549, 557, 637, 670, 678, 758, 767, 865 backtrace_slots_of_raw_entry, 829
append_array, 637 backtrace_status, 826
append_iter, 637 Bad, 547
append_list, 637 basename, 658
append_seq, 637 batch_mode_printer, 927
apply, 920 before_first_spawn, 629
Arg, 537, 543 beginning_of_input, 849
arg, 625 best_toplevel_printer, 927
arg_label, 922 big_endian, 901
argv, 898 Bigarray, 537, 567
Array, 537, 548, 669, 879 Binary, 877
array, 510 bind, 818, 845, 992
Array0, 576 binding, 958
array0_of_genarray, 586 binding_op, 941
Array1, 578 bindings, 777, 801
array1_of_genarray, 587 bits, 840, 843
Array2, 580 bits_of_float, 745, 750
array2_of_genarray, 587 bits32, 842, 843
Array3, 583 bits64, 842, 843
array3_of_genarray, 587 BLAKE128, 634
ArrayLabels, 537, 556, 676 BLAKE256, 634
INDEX TO THE LIBRARY 1045

BLAKE512, 634 chmod, 976

blit, 550, 558, 576, 577, 579, 582, 585, 591, choose, 777, 801, 808, 871, 1033
597, 611, 637, 671, 678, 881, 890, 913 choose_opt, 777, 801, 808, 871
blit_string, 597, 611 chop_extension, 658
bom, 909 chop_suffix, 658
Bool, 537, 589 chop_suffix_opt, 658
bool, 510, 842, 843 chown, 977
bool_of_string, 524 chr, 623
bool_of_string_opt, 524 chroot, 979
bounded_full_split, 1012 class_declaration, 950
bounded_split, 1012 class_description, 947
bounded_split_delim, 1012 class_expr, 948, 958
bprintf, 834 class_expr_desc, 949
Break, 904 class_field, 949, 957
broadcast, 628 class_field_desc, 950
bscanf, 850 class_field_kind, 950
bscanf_format, 855 class_infos, 947
bscanf_opt, 850 class_signature, 946
Bucket, 654–656 class_structure, 949
Buffer, 537, 590 class_type, 946, 958
Bytes, 537, 595, 879 class_type_declaration, 947
bytes, 509, 632, 633 class_type_desc, 946
BytesLabels, 537, 609 class_type_field, 947, 958
class_type_field_desc, 947
c_layout, 571, 572 classify_float, 523, 665
Callback, 537, 623 clean, 652, 653
Callbacks, 1026 clear, 591, 638, 652–655, 657, 723, 728, 730,
capacity, 642 786, 792, 794, 836, 878, 914
capitalize_ascii, 600, 614, 884, 893 clear_close_on_exec, 978
cardinal, 776, 801, 807, 870 clear_nonblock, 977
case, 941 clear_parser, 824
cat, 598, 611, 881, 890 clear_symbolic_output_buffer, 703
catch, 825 close, 736, 821, 969
catch_break, 904 close_box, 686
cbrt, 665 close_in, 533, 849
ceil, 521, 667 close_in_noerr, 533
change_layout, 574, 577, 579, 581, 584 close_noerr, 736, 821
channel, 632, 633, 1032 close_out, 531
Char, 537, 623 close_out_noerr, 531
char, 509, 571 close_process, 982
char_of_int, 524 close_process_full, 982
chdir, 899, 978 close_process_in, 982
check, 913 close_process_out, 982
check_geometry, 692 close_stag, 697
check_suffix, 657 close_tbox, 694
1046

closed_flag, 922 copy_sign, 668

closedir, 979 copysign, 522
code, 623 core_type, 930, 933, 957
combine, 553, 561, 764, 773 core_type_desc, 935
command, 899 cos, 520, 666
compact, 717 cosh, 521, 666
compare, 514, 589, 601, 614, 624, 631, 633, count, 914
641, 651, 668, 741, 745, 750, 758, 767, counters, 716
775, 780, 799, 804, 806, 810, 816, 819, Counting, 876
846, 861, 869, 874, 882, 891, 910, 911 cpu_relax, 630
compare_and_set, 564 create, 572, 576, 578, 580, 583, 590, 596, 610,
compare_length_with, 757, 765 628, 635, 652, 653, 670, 677, 723, 728,
compare_lengths, 756, 765 730, 786, 792, 794, 812, 835, 878, 912,
Complex, 537, 624 913, 1026, 1029
complex32, 570 create_alarm, 719
complex32_elt, 569 create_cursor, 1027
complex64, 570 create_float, 549, 556
complex64_elt, 569 create_process, 979
compose, 711 create_process_env, 980
concat, 550, 557, 597, 611, 657, 671, 678, 758, curr, 923
767, 865, 881, 890 current, 548
concat_map, 759, 768, 866 current_dir_name, 657
Condition, 537, 626 cursor, 1024
conj, 625 cycle, 862
connect, 992 cygwin, 900
cons, 757, 766, 862
const, 711 data, 913
constant, 921, 932 data_size, 784
constant_desc, 932 decr, 534, 564
constr, 957, 958 Deep, 647
constr_ident, 931 default_alert_reporter, 928
constructor_arguments, 944 default_mapper, 919
constructor_declaration, 944 default_report_printer, 927
contains, 600, 614, 882, 891 default_uncaught_exception_handler,
contains_from, 600, 614, 882, 891 828, 1032
contents, 591 default_warning_reporter, 927
continuation, 647, 648 delay, 1030
Continuation_already_resumed, 647 delayed_msg, 929
continue, 647 delete_alarm, 720
continue_with, 648 deprecated, 928
control, 716 deprecated_script_alert, 928
convert_raw_backtrace_slot, 831 descr_of_in_channel, 971
copy, 550, 557, 596, 610, 636, 652, 653, 671, descr_of_out_channel, 971
678, 724, 729, 730, 786, 792, 794, 817, development_version, 904
836, 842, 878 diff, 807, 870
INDEX TO THE LIBRARY 1047

Digest, 537, 631 enable_runtime_warnings, 905

dim, 578 End_of_file, 513
dim1, 581, 584 end_of_input, 849
dim2, 581, 584 End_of_file, 511
dim3, 584 ends_with, 601, 615, 882, 891
dims, 573 ensure_capacity, 643
dir_handle, 979 ensure_extra_capacity, 643
dir_sep, 657 environment, 964
direction_flag, 922 Ephemeron, 538, 651
directive_argument, 957 eprintf, 707, 834
directive_argument_desc, 957 epsilon, 663
dirname, 659 epsilon_float, 523
discard, 722 eq, 906
discontinue, 647 equal, 589, 601, 614, 624, 632, 633, 641, 650,
discontinue_with, 649 668, 728, 730, 741, 745, 750, 758, 767,
discontinue_with_backtrace, 647, 649 780, 791, 793, 804, 810, 816, 819, 845,
disjoint, 807, 870 861, 874, 882, 891, 910, 911
div, 625, 662, 740, 743, 747, 814 erf, 667
Division_by_zero, 513 erfc, 667
Division_by_zero, 511 err_formatter, 701
DLS, 630 Error, 929, 1037
Doc, 925, 958 error, 844, 928, 929, 964, 1037
doc, 545 error_message, 964, 1038
Domain, 537, 628 error_of_exn, 929
domain_of_sockaddr, 991 error_of_printer, 929
dprintf, 708 error_of_printer_file, 929
drop, 762, 771, 836, 864, 878 errorf, 929
drop_ppx_context_sig, 921 escaped, 599, 612, 623, 883, 892
drop_ppx_context_str, 921 establish_server, 996
drop_while, 762, 771, 864 Event, 1032
dummy_pos, 753 event, 1032
dup, 977 eventlog_pause, 720
dup2, 977 eventlog_resume, 720
Dynarray, 537, 635 exchange, 564
Dynlink, 1035 executable_name, 898
execv, 965
echo_eof, 924 execve, 966
Effect, 537, 646 execvp, 966
effect_handler, 648 execvpe, 966
Either, 538, 649 exists, 552, 559, 598, 612, 640, 673, 680, 760,
elements, 807, 871 769, 780, 804, 811, 859, 874, 883, 892
elt, 807, 870 exists2, 552, 560, 761, 770, 861
Empty, 835, 877 Exit, 512, 1030
empty, 596, 610, 775, 799, 807, 861, 870, 881, exit, 536, 1030
890 exn, 510
1048

exn_slot_id, 831 778, 787, 792, 794, 802, 808, 859, 871,
exn_slot_name, 832 914
exp, 520, 626, 665 find_all, 652, 653, 724, 729, 731, 762, 771,
exp2, 665 787, 792, 794, 914
expm1, 520, 665 find_first, 778, 802, 808, 872
expression, 930, 938, 957 find_first_opt, 778, 802, 809, 872
expression_desc, 941 find_index, 553, 560, 640, 673, 680, 761, 770,
extend, 597, 611 859
extended_module_path, 931 find_last, 636, 778, 802, 809, 872
extension, 658, 932 find_last_opt, 778, 802, 809, 872
extension_constructor, 945 find_left, 650
extension_constructor_kind, 946 find_map, 553, 560, 640, 673, 680, 761, 770,
extension_of_error, 920 860
extern_flags, 782 find_mapi, 553, 560, 641, 673, 680, 761, 770,
extra_info, 904 860
extra_prefix, 904 find_opt, 552, 560, 640, 652, 653, 673, 680,
724, 729, 730, 761, 770, 778, 787, 792,
Failure, 511, 513 794, 802, 808, 871, 914
failwith, 512 find_right, 650
fast_sort, 554, 562, 674, 681, 764, 773 first_chars, 1012
fchmod, 976 fit_capacity, 643
fchown, 977 flat_map, 866
fetch_and_add, 564 flatten, 758, 767, 930
fiber, 648 flip, 712
file, 632, 633 Float, 538, 661
file_descr, 967 float, 510, 522, 842, 843
file_exists, 898 float_of_bits, 745, 750
file_kind, 972 float_of_int, 522
file_name, 848 float_of_string, 525
file_perm, 969 float_of_string_opt, 525
Filename, 538, 657 float16, 570
filename, 925 float16_elt, 568
fill, 550, 557, 576, 577, 579, 582, 586, 597, float32, 570
611, 671, 678, 913 float32_elt, 568
filter, 639, 761, 770, 779, 803, 809, 863, 873 float64, 570
filter_map, 639, 759, 768, 779, 803, 810, 863, float64_elt, 568
873 floor, 521, 667
filter_map_inplace, 725, 729, 731, 788, 792, flow_action, 1002
794 flush, 529, 822
filteri, 762, 771 flush_all, 529, 822
finalise, 718 flush_input, 756
finalise_last, 719 flush_queue, 1002
finalise_release, 719 flush_str_formatter, 702
Finally_raised, 712 flush_symbolic_output_buffer, 704
find, 652–655, 657, 724, 729, 730, 761, 770, fma, 662
INDEX TO THE LIBRARY 1049

fold, 650, 725, 729, 731, 779, 788, 792, 794, from_fun, 752
803, 809, 818, 837, 845, 872, 879, 914 from_function, 754, 849
fold_left, 551, 559, 598, 612, 639, 672, 679, from_hex, 633
759, 768, 858, 883, 892 from_string, 754, 784, 849
fold_left_map, 551, 559, 759, 768 from_val, 752
fold_left2, 760, 769, 860 fst, 525
fold_lefti, 859 fstat, 973, 974
fold_lines, 738 fsync, 969
fold_right, 551, 559, 598, 612, 639, 672, 679, ftruncate, 971, 973
759, 768, 883, 892 full_init, 840
fold_right2, 760, 769 full_int, 840, 843
for_all, 552, 559, 598, 612, 640, 650, 673, full_major, 717
680, 760, 769, 780, 804, 811, 859, 874, full_split, 1012
883, 892 Fun, 538, 711
for_all2, 552, 560, 760, 769, 860 function_body, 942
force, 169, 751 function_param, 942
force_newline, 689 function_param_desc, 942
force_val, 753 functor_parameter, 951
Forced_twice, 865
forever, 862 Gc, 538, 712
fork, 966 Genarray, 572
Format, 538, 683 genarray_of_array0, 586
format, 536, 830 genarray_of_array1, 586
format_from_string, 856 genarray_of_array2, 586
format_of_string, 536 genarray_of_array3, 586
format4, 510, 536 geometry, 692
format6, 536 get, 548, 556, 564, 574, 577, 579, 581, 584,
formatter, 685 596, 610, 631, 636, 670, 677, 717, 818,
formatter_for_warnings, 927 881, 890, 912
formatter_of_buffer, 701 get_backtrace, 826
formatter_of_out_channel, 700 get_callstack, 648, 649, 828
formatter_of_out_functions, 702 get_cookie, 921
formatter_of_symbolic_output_buffer, get_copy, 912
704 get_ellipsis_text, 695
formatter_out_functions, 698 get_err_formatter, 701
formatter_stag_functions, 699 get_error, 845
fortran_layout, 571, 572 get_formatter_out_functions, 699
fpclass, 523, 665 get_formatter_output_functions, 698
fprintf, 706, 832 get_formatter_stag_functions, 700
free_cursor, 1027 get_geometry, 693
frexp, 522, 668 get_id, 629
from_bytes, 784 get_int16_be, 606, 619, 887, 896
from_channel, 754, 783, 849 get_int16_le, 606, 619, 887, 896
from_file, 849 get_int16_ne, 606, 619, 887, 896
from_file_bin, 849 get_int32_be, 606, 620, 888, 897
1050

get_int32_le, 606, 620, 888, 897 gethostname, 997

get_int32_ne, 606, 620, 888, 897 getitimer, 988
get_int64_be, 606, 620, 888, 897 getlogin, 989
get_int64_le, 606, 620, 888, 897 getnameinfo, 999
get_int64_ne, 606, 620, 888, 897 getnameinfo_option, 999
get_int8, 605, 619, 887, 896 getpeername, 992
get_last, 636 getpid, 967
get_margin, 691 getppid, 967
get_mark_tags, 697 getprotobyname, 997
get_max_boxes, 693 getprotobynumber, 997
get_max_indent, 692 getpwnam, 989
get_minor_free, 718 getpwuid, 990
get_ok, 845 getservbyname, 997
get_pos_info, 923 getservbyport, 997
get_print_tags, 697 getsockname, 992
get_raw_backtrace, 827 getsockopt, 995
get_raw_backtrace_next_slot, 831 getsockopt_error, 995
get_raw_backtrace_slot, 831 getsockopt_float, 995
get_state, 844 getsockopt_int, 995
get_std_formatter, 701 getsockopt_optint, 995
get_stdbuf, 701 gettimeofday, 986
get_str_formatter, 702 getuid, 988
get_symbolic_output_buffer, 704 global_replace, 1011
get_temp_dir_name, 660 global_substitute, 1011
get_uint16_be, 605, 619, 887, 896 gmtime, 986
get_uint16_le, 605, 619, 887, 896 group, 864
get_uint16_ne, 605, 619, 887, 896 group_beginning, 1010
get_uint8, 605, 619, 887, 896 group_end, 1011
get_utf_16be_uchar, 604, 618, 886, 895 group_entry, 989
get_utf_16le_uchar, 604, 618, 886, 895 guard, 1033
get_utf_8_uchar, 604, 617, 886, 895
get_value, 876 handle_unix_error, 964
getaddrinfo, 998 handler, 647, 648
getaddrinfo_option, 998 has_symlink, 983
getcwd, 899, 979 hash, 590, 624, 669, 728, 732, 742, 746, 750,
getegid, 988 791, 795, 817, 888, 897, 910
getenv, 898, 964 hash_length, 633
getenv_opt, 899 hash_param, 732, 796
geteuid, 988 HashedType, 728, 791
getgid, 988 Hashtbl, 538, 722, 785
getgrgid, 990 hd, 757, 766
getgrnam, 990 header_size, 784
getgroups, 989 Help, 547
gethostbyaddr, 997 highlight_terminfo, 926
gethostbyname, 997 host_entry, 996
INDEX TO THE LIBRARY 1051

hypot, 521, 666 input_lexbuf, 924

input_line, 531, 736
i, 625 input_lines, 737
ibprintf, 834 input_name, 924
Id, 907 input_phrase_buffer, 924
id, 629, 711, 817, 1030 input_value, 532
ifprintf, 708, 834 Int, 538, 739
ignore, 524 int, 509, 570, 840, 843, 1024
ikbprintf, 835 int_elt, 569
ikfprintf, 708, 834 int_in_range, 840, 843
Immediate, 906 int_of_char, 524
Immediate64, 906 int_of_float, 522
implementation, 930 int_of_string, 525
In_channel, 538, 734 int_of_string_opt, 524
in_channel, 526, 847 int_size, 900
in_channel_length, 533 int16_signed, 570
in_channel_of_descr, 970 int16_signed_elt, 568
in_file, 923 int16_unsigned, 570
include_declaration, 953 int16_unsigned_elt, 568
include_description, 953 Int32, 538, 742
include_infos, 953 int32, 510, 570, 841, 843
incr, 534, 564 int32_elt, 569
index, 599, 612, 885, 894 int32_in_range, 841, 843
index_from, 599, 613, 885, 894 Int64, 538, 746
index_from_opt, 599, 613, 885, 894 int64, 510, 571, 841, 843
index_opt, 599, 613, 885, 894 int64_elt, 569
inet_addr, 990 int64_in_range, 841, 843
inet_addr_any, 990 int8_signed, 570
inet_addr_loopback, 990 int8_signed_elt, 568
inet_addr_of_string, 990 int8_unsigned, 570
inet6_addr_any, 990 int8_unsigned_elt, 568
inet6_addr_loopback, 990 inter, 807, 870
infinity, 522, 662 interactive, 899
init, 549, 557, 573, 576, 578, 580, 583, 596, interface, 930
610, 636, 670, 677, 757, 766, 840, 862, interleave, 866
880, 889, 923 interval_timer, 987
init_matrix, 549, 557, 670, 677 interval_timer_status, 988
initgroups, 989 ints, 868
injectivity, 922 inv, 625
input, 532, 632, 634, 737 invalid_arg, 512
input_all, 737 Invalid_argument, 513
input_bigarray, 737 Invalid_argument, 511
input_binary_int, 532 is_binary_mode, 739, 823
input_byte, 532, 736 is_buffered, 823
input_char, 531, 736 is_char, 910
1052

is_directory, 898 key, 545, 630, 652, 653, 728, 730, 775, 792,
is_empty, 636, 757, 766, 780, 804, 810, 836, 794, 799
858, 874, 878 kfprintf, 708, 834
is_error, 845 kill, 984
is_finite, 663 kind, 569, 573, 577, 578, 581, 584
is_implicit, 657 kind_size_in_bytes, 571
is_inet6_addr, 990 Kn, 656
is_infinite, 663 kprintf, 835
is_inline, 829 kscanf, 855
is_integer, 663 ksprintf, 709, 835
is_left, 650 ksscanf, 855
is_main_domain, 630
is_nan, 663 label, 922
is_native, 1035 label_declaration, 944
is_none, 819, 923 LargeFile, 533, 973
is_ok, 845 last, 930
is_raise, 829 last_chars, 1013
is_randomized, 726, 789 layout, 572, 573, 577, 578, 581, 584
is_regular_file, 898 Lazy, 538, 751
is_relative, 657 Lazy (module), 169
is_right, 650 lazy_t, 510
is_some, 819 ldexp, 522, 668
is_val, 752 left, 649
is_valid, 909 length, 548, 556, 591, 596, 609, 636, 652–655,
is_valid_utf_16be, 604, 618, 886, 895 657, 669, 677, 725, 729, 731, 738, 756,
is_valid_utf_16le, 605, 618, 886, 895 765, 788, 792, 794, 822, 837, 858, 878,
is_valid_utf_8, 604, 618, 886, 895 881, 890, 912
isatty, 739, 823, 973 letop, 941
iter, 550, 558, 598, 611, 639, 650, 672, 679, lexbuf, 754
725, 729, 731, 759, 768, 778, 787, 792, lexeme, 755
794, 802, 809, 818, 837, 845, 858, 872, lexeme_char, 755
878, 884, 893, 914 lexeme_end, 755
iter_error, 845 lexeme_end_p, 756
iter2, 551, 559, 672, 679, 760, 769, 860 lexeme_start, 755
iterate, 863 lexeme_start_p, 755
iteri, 550, 558, 598, 612, 639, 672, 679, 759, Lexing, 538, 753
768, 859, 884, 893 lifecycle, 1024
lifecycle_name, 1024
join, 629, 818, 845, 1030 link, 976
linking_error, 1037
K1, 653 List, 538, 756, 879
K2, 654 list, 510
kasprintf, 709 listen, 992
kbprintf, 835 ListLabels, 538, 765
kdprintf, 708 lnot, 518
INDEX TO THE LIBRARY 1053

loadfile, 1035 map_from_array, 675, 682

loadfile_private, 1036 map_inplace, 551, 558, 672, 679
loc, 922, 923, 925 map_left, 650
localtime, 986 map_opt, 920
Location, 923 map_product, 867
location, 829, 830 map_right, 650
location_stack, 932 map_to_array, 675, 682
lock, 812 map_val, 752
lock_command, 984 map2, 551, 559, 673, 680, 760, 769, 866
lockf, 984 mapi, 551, 558, 598, 612, 639, 672, 679, 759,
locs, 925 768, 779, 803, 863, 883, 892
log, 520, 626, 665 mapi_inplace, 551, 558, 672, 679
log10, 520, 665 mapper, 919
log1p, 520, 665 Marshal, 538, 782
log2, 665 match_beginning, 1010
logand, 740, 743, 748, 815 match_end, 1010
lognot, 740, 744, 748, 815 Match_failure, 512
logor, 740, 744, 748, 815 match_with, 648
logxor, 740, 744, 748, 815 matched_group, 1010
Longident, 930 matched_string, 1009
longident, 931, 957, 958 Match_failure, 175, 178, 181, 511
lowercase_ascii, 600, 614, 623, 884, 893 max, 515, 669, 741, 746, 750, 817, 909
lseek, 971, 973 max_array_length, 901
lstat, 973, 974 max_binding, 777, 801
max_binding_opt, 777, 801
main_program_units, 1037 max_elt, 808, 871
major, 717 max_elt_opt, 808, 871
major_slice, 717 max_float, 523, 663
Make, 654–656, 730, 781, 793, 805, 812, 875, max_floatarray_length, 901
906, 915 max_int, 518, 740, 743, 748, 814
make, 549, 556, 564, 596, 610, 636, 653–656, max_num, 669
670, 677, 842, 876, 877, 880, 889, 907 max_string_length, 901
make_contended, 564 MD5, 635
make_formatter, 702 mem, 552, 560, 640, 652, 653, 673, 680, 724,
make_matrix, 549, 557, 670, 677 729, 731, 761, 770, 780, 787, 792, 794,
make_self_init, 842 804, 810, 874, 914
make_symbolic_output_buffer, 703 mem_assoc, 763, 772
make_synchronized_formatter, 702 mem_assq, 763, 772
MakeSeeded, 654–656, 731, 795 mem_ieee, 673, 680
Map, 538, 774, 798 memoize, 865
map, 551, 558, 598, 612, 639, 650, 672, 679, Memprof, 720
752, 759, 768, 779, 803, 809, 818, 845, memq, 552, 560, 640, 761, 770
863, 873, 883, 892 merge, 764, 773, 776, 800, 914
map_error, 845 min, 514, 668, 741, 746, 750, 817, 909
map_file, 975 min_binding, 777, 801
1054

min_binding_opt, 777, 801 new_channel, 1032

min_elt, 808, 871 new_key, 630
min_elt_opt, 808, 871 new_line, 756
min_float, 523, 663 next_after, 667
min_int, 518, 740, 743, 748, 814 nice, 967
min_max, 669 node, 857
min_max_num, 669 nominal_exp, 958
min_num, 669 Non_immediate, 906
minor, 717 none, 818, 923
minor_words, 716 norm, 625
minus_one, 661, 739, 742, 747, 813 norm2, 625
mkdir, 899, 978 not, 515, 589
mkfifo, 979 Not_found, 513
mkloc, 923 Not_found, 511
mknoloc, 923 nth, 591, 757, 766
mktime, 986 nth_dim, 573
mod_float, 522 nth_opt, 757, 766
modf, 522, 668 null, 659
module_binding, 956 null_tracker, 721
module_declaration, 952 num_dims, 573
module_expr, 930, 954, 957
module_expr_desc, 955 Obj, 539
module_substitution, 952 object_field, 935
module_type, 930, 950, 958 object_field_desc, 935
module_type_declaration, 952 Ocaml_operators, 915
module_type_desc, 951 ocaml_release, 905
MoreLabels, 538, 785 ocaml_release_info, 905
msg, 926 ocaml_version, 904
msg_flag, 993 of_array, 580, 582, 586, 641
mul, 625, 662, 740, 743, 747, 813 of_binary_string, 843
mutable_flag, 922 of_bytes, 881, 890
Mutex, 538, 812 of_char, 910
of_dispenser, 868
name, 830, 1025 of_float, 741, 744, 749, 815
name_info, 999 of_hex, 632, 634
name_of_input, 849 of_int, 664, 744, 748, 815, 909
nan, 522, 662 of_int32, 749, 816
nativebits, 842, 843 of_list, 550, 558, 641, 671, 678, 781, 805,
Nativeint, 539, 813 811, 874
nativeint, 510, 571, 841, 843 of_nativeint, 749
nativeint_elt, 569 of_seq, 554, 562, 594, 604, 617, 642, 652, 653,
nativeint_in_range, 841, 843 675, 682, 727, 729, 731, 765, 774, 781,
neg, 625, 661, 740, 742, 747, 813 790, 793, 795, 805, 811, 837, 875, 879,
neg_infinity, 522, 662 886, 895
negate, 712 of_string, 596, 610, 664, 745, 749, 816
INDEX TO THE LIBRARY 1055

of_string_opt, 664, 745, 749, 816 out_channel_length, 530, 533

of_value, 577 out_channel_of_descr, 970
ok, 844 Out_of_memory, 513
once, 865 Out_of_memory, 511
one, 625, 661, 739, 742, 746, 813 output, 530, 632, 634, 821
Oo, 539, 817 output_bigarray, 822
opaque_identity, 905 output_binary_int, 530
open_bin, 735, 820 output_buffer, 591
open_box, 686 output_byte, 530, 821
open_connection, 996 output_bytes, 529, 821
open_declaration, 953 output_char, 529, 821
open_description, 953 output_string, 529, 821
open_flag, 529, 735, 820, 968 output_substring, 530, 822
open_gen, 735, 820 output_value, 530
open_hbox, 686 over_max_boxes, 693
open_hovbox, 686 override_flag, 922
open_hvbox, 686
open_in, 531, 848 package_type, 935
open_in_bin, 531, 848 parent_dir_name, 657
open_in_gen, 531 Parse, 930
open_infos, 953 parse, 545, 930
open_out, 529 parse_and_expand_argv_dynamic, 547
open_out_bin, 529 parse_argv, 546
open_out_gen, 529 parse_argv_dynamic, 547
open_process, 980 parse_dynamic, 546
open_process_args, 981 Parse_error, 824
open_process_args_full, 981 parse_expand, 547
open_process_args_in, 981 Parsetree, 932
open_process_args_out, 981 Parsing, 539, 824
open_process_full, 980 partition, 762, 771, 779, 804, 810, 868, 873
open_process_in, 980 partition_map, 762, 771, 867
open_process_out, 980 passwd_entry, 989
open_stag, 696 path, 1027
open_tbox, 694 pattern, 930, 936, 957
open_temp_file, 659 pattern_desc, 937
open_text, 735, 820 pause, 985, 1027
open_vbox, 686 payload, 933, 958
opendir, 979 peek, 836
openfile, 969 peek_opt, 836
Option, 539, 818 perform, 647
option, 510 pi, 663
OrderedType, 774, 799, 806, 869 pipe, 979
os_type, 900 polar, 625
Out_channel, 539, 819 poll, 1033
out_channel, 526 poll_actions, 901
1056

pop, 836, 878 pp_print_iter, 704

pop_last, 638 pp_print_list, 704
pop_last_opt, 638 pp_print_newline, 690
pop_opt, 878 pp_print_nothing, 688
pos, 738, 822 pp_print_option, 705
pos_in, 533 pp_print_result, 705
pos_out, 530, 533 pp_print_seq, 705
position, 753 pp_print_space, 688
pow, 626, 665 pp_print_string, 687
pp_close_box, 686 pp_print_substring, 687
pp_close_stag, 697 pp_print_substring_as, 687
pp_close_tbox, 694 pp_print_tab, 694
pp_force_newline, 689 pp_print_tbreak, 694
pp_get_ellipsis_text, 695 pp_print_text, 705
pp_get_formatter_out_functions, 699 pp_safe_set_geometry, 692
pp_get_formatter_output_functions, 698 pp_set_ellipsis_text, 695
pp_get_formatter_stag_functions, 700 pp_set_formatter_out_channel, 697
pp_get_geometry, 693 pp_set_formatter_out_functions, 699
pp_get_margin, 691 pp_set_formatter_output_functions, 698
pp_get_mark_tags, 697 pp_set_formatter_stag_functions, 700
pp_get_max_boxes, 693 pp_set_geometry, 692
pp_get_max_indent, 692 pp_set_margin, 691
pp_get_print_tags, 697 pp_set_mark_tags, 697
pp_infinity, 690 pp_set_max_boxes, 693
pp_open_box, 686 pp_set_max_indent, 691
pp_open_hbox, 686 pp_set_print_tags, 697
pp_open_hovbox, 686 pp_set_tab, 694
pp_open_hvbox, 686 pp_set_tags, 697
pp_open_stag, 696 pp_update_geometry, 692
pp_open_tbox, 694 Pprintast, 957
pp_open_vbox, 686 pred, 517, 662, 740, 743, 747, 814, 909
pp_over_max_boxes, 693 prerr_alert, 928
pp_print_array, 705 prerr_bytes, 527
pp_print_as, 687 prerr_char, 527
pp_print_bool, 688 prerr_endline, 527
pp_print_break, 688 prerr_float, 527
pp_print_bytes, 687 prerr_int, 527
pp_print_char, 688 prerr_newline, 527
pp_print_custom_break, 689 prerr_string, 527
pp_print_cut, 688 prerr_warning, 927
pp_print_either, 705 print, 825
pp_print_float, 687 print_alert, 928
pp_print_flush, 690 print_as, 687
pp_print_if_newline, 689 print_backtrace, 825
pp_print_int, 687 print_bool, 688
INDEX TO THE LIBRARY 1057

print_break, 688 quiet_nan, 663

print_bytes, 526, 687 quote, 660, 1008
print_char, 526, 688 quote_command, 660
print_cut, 688 quoted_filename, 925
print_endline, 527
print_filename, 925 raise, 512
print_float, 526, 687 raise_errorf, 929
print_flush, 690 raise_notrace, 512
print_if_newline, 689 raise_with_backtrace, 828
print_int, 526, 687 Random, 539, 839
print_loc, 925 randomize, 725, 788
print_locs, 925 raw_backtrace, 827
print_newline, 527, 690 raw_backtrace_entries, 827
print_raw_backtrace, 827 raw_backtrace_entry, 827
print_report, 927 raw_backtrace_length, 831
print_space, 688 raw_backtrace_slot, 830
print_stat, 717 raw_backtrace_to_string, 827
print_string, 526, 687 rcontains_from, 600, 614, 882, 891
print_substring, 687 read, 969
print_substring_as, 687 read_arg, 548
print_tab, 694 read_arg0, 548
print_tbreak, 694 read_bigarray, 969
print_warning, 927 read_float, 528
Printexc, 539, 825 read_float_opt, 528
Printf, 539, 832 read_int, 528
printf, 707, 834 read_int_opt, 528
private_flag, 922 read_line, 528
process_full_pid, 982 read_poll, 1027
process_in_pid, 981 readdir, 899, 979
process_out_pid, 982 readlink, 983
process_pid, 982 really_input, 532, 737
process_status, 965 really_input_bigarray, 738
process_times, 985 really_input_string, 532, 736
product, 867 realpath, 976
prohibit, 1037 rebuild, 726, 789
protect, 712, 813 rec_flag, 922
protocol_entry, 996 receive, 1032
provably_equal, 907 recommended_domain_count, 630
public_dynamically_loaded_units, 1037 record_backtrace, 826
push, 836, 878 recv, 993
putenv, 965 recvfrom, 993
ref, 534
query, 653, 655, 656 regexp, 1007
Queue, 539, 835 regexp_case_fold, 1008
quick_stat, 716 regexp_string, 1008
1058

regexp_string_case_fold, 1009 rewrite_absolute_path, 924

register, 623, 920, 1025 rewrite_find_all_existing_dirs, 925
register_error_of_exn, 929 rewrite_find_first_existing, 924
register_exception, 623 rhs_end, 824
register_function, 920 rhs_end_pos, 824
register_printer, 826 rhs_interval, 923
release, 876, 877 rhs_loc, 923
rem, 662, 740, 743, 747, 814 rhs_start, 824
remove, 652–656, 724, 729, 730, 776, 787, 792, rhs_start_pos, 824
794, 800, 807, 870, 898, 914 right, 650
remove_assoc, 763, 772 rindex, 599, 613, 885, 894
remove_assq, 764, 773 rindex_from, 599, 613, 885, 894
remove_extension, 658 rindex_from_opt, 600, 613, 885, 894
remove_last, 638 rindex_opt, 599, 613, 885, 894
rename, 898, 976 rmdir, 899, 978
rep, 909 round, 667
repeat, 862 row_field, 935
replace, 652, 653, 724, 729, 731, 787, 792, 794 row_field_desc, 935
replace_first, 1011 run_main, 920
replace_matched, 1011 runtime_counter, 1018
replace_seq, 652, 653, 727, 729, 731, 790, runtime_counter_name, 1024
793, 795 Runtime_events, 1015
report, 926 runtime_parameters, 901
report_alert, 928 runtime_phase, 1023
report_exception, 929 runtime_phase_name, 1024
report_kind, 926 runtime_variant, 901
report_printer, 926, 927 runtime_warnings_enabled, 905
report_warning, 927
repr, 906 S, 633, 652, 728, 775, 792, 799, 806, 869, 913
reset, 591, 644, 652, 653, 724, 728, 730, 786, safe_set_geometry, 692
792, 794, 924 scan, 863
reshape, 587 Scan_failure, 850
reshape_0, 587 scanbuf, 848
reshape_1, 587 Scanf, 539, 846
reshape_2, 587 scanf, 855
reshape_3, 587 scanf_opt, 855
Result, 539, 844 scanner, 850
result, 534 scanner_opt, 850
resume, 1027 Scanning, 847
return, 861 search_backward, 1009
rev, 757, 766 search_forward, 1009
rev_append, 758, 767 seeded_hash, 590, 624, 669, 730, 732, 742,
rev_map, 759, 768 746, 750, 793, 796, 817, 888, 897, 910
rev_map2, 760, 769 seeded_hash_param, 732, 796
rewinddir, 979 SeededHashedType, 730, 793
INDEX TO THE LIBRARY 1059

SeededS, 652, 730, 794 set_margin, 691

seek, 738, 822 set_mark_tags, 697
seek_command, 971 set_max_boxes, 693
seek_in, 533 set_max_indent, 691
seek_out, 530, 533 set_nonblock, 977
select, 983, 1031, 1033 set_position, 755
self, 629, 1030 set_print_tags, 697
self_index, 630 set_signal, 902
self_init, 840 set_state, 844
Semaphore, 539, 875 set_tab, 694
send, 993, 1032 set_tags, 697
send_substring, 993 set_temp_dir_name, 660
sendto, 993 set_trace, 824
sendto_substring, 993 set_uint16_be, 607, 620
separate_new_message, 925 set_uint16_le, 607, 621
Seq, 539, 856 set_uint16_ne, 607, 620
service_entry, 997 set_uint8, 606, 620
Set, 539, 805, 869 set_uncaught_exception_handler, 828,
set, 548, 556, 564, 574, 577, 579, 581, 584, 1032
596, 610, 631, 636, 670, 677, 717, 912 set_utf_16be_uchar, 604, 618
set_allowed_units, 1036 set_utf_16le_uchar, 604, 618
set_binary_mode, 738, 823 set_utf_8_uchar, 604, 618
set_binary_mode_in, 533 setattr_when, 1002
set_binary_mode_out, 531 setgid, 988
set_buffered, 823 setgroups, 989
set_capacity, 644 setitimer, 988
set_close_on_exec, 977 setsid, 1003
set_cookie, 921 setsockopt, 995
set_ellipsis_text, 695 setsockopt_float, 995
set_filename, 755 setsockopt_int, 995
set_formatter_out_channel, 697 setsockopt_optint, 995
set_formatter_out_functions, 699 setuid, 988
set_formatter_output_functions, 698 Shallow, 648
set_formatter_stag_functions, 700 shift_left, 741, 744, 748, 815
set_geometry, 692 shift_right, 741, 744, 748, 815
set_int16_be, 607, 621 shift_right_logical, 741, 744, 748, 815
set_int16_le, 607, 621 show_filename, 925
set_int16_ne, 607, 621 shuffle, 554, 562, 674, 681
set_int32_be, 607, 621 shutdown, 992
set_int32_le, 608, 621 shutdown_command, 992
set_int32_ne, 607, 621 shutdown_connection, 996
set_int64_be, 608, 621 sigabrt, 902
set_int64_le, 608, 621 sigalrm, 902
set_int64_ne, 608, 621 sigbus, 903
set_int8, 607, 620 sigchld, 903
1060

sigcont, 903 sleep, 987

sigfpe, 902 sleepf, 987
sighup, 902 slice, 579
sigill, 902 slice_left, 575, 582
sigint, 902 slice_left_1, 585
sigkill, 902 slice_left_2, 585
sigmask, 1031 slice_right, 575, 582
sign_bit, 668 slice_right_1, 585
signal, 628, 902 slice_right_2, 585
signal_behavior, 901 Slot, 829
signaling_nan, 663 snd, 525
signature, 951, 957 sockaddr, 991
signature_item, 951, 958 socket, 991
signature_item_desc, 952 socket_bool_option, 994
sigpending, 985 socket_domain, 991
sigpipe, 902 socket_float_option, 995
sigpoll, 903 socket_int_option, 994
sigprocmask, 985 socket_optint_option, 995
sigprocmask_command, 985 socket_type, 991
sigprof, 903 socketpair, 992
sigquit, 902 some, 818
sigsegv, 902 sort, 553, 561, 674, 681, 764, 773
sigstop, 903 sort_uniq, 764, 773
sigsuspend, 985 sorted_merge, 866
sigsys, 903 space_formatter, 957
sigterm, 902 span, 1024
sigtrap, 904 spawn, 629
sigtstp, 903 spec, 545
sigttin, 903 split, 553, 561, 764, 773, 780, 804, 810, 843,
sigttou, 903 844, 867, 873, 1011
sigurg, 904 split_delim, 1012
sigusr1, 903 split_on_char, 603, 617, 882, 891
sigusr2, 903 split_result, 1012
sigvtalrm, 903 sprintf, 707, 834
sigxcpu, 904 sqrt, 520, 625, 665
sigxfsz, 904 sscanf, 854
simple_module_path, 931 sscanf_format, 855
sin, 520, 666 sscanf_opt, 854
single_write, 969 stable_sort, 554, 561, 674, 681, 764, 773
single_write_bigarray, 970 Stack, 539, 877
single_write_substring, 970 Stack_overflow, 513
singleton, 776, 800, 807, 870 Stack_overflow, 511
sinh, 521, 666 stag, 695
size, 814 start, 721, 1026
size_in_bytes, 574, 577, 579, 581, 584 starts_with, 601, 615, 882, 891
INDEX TO THE LIBRARY 1061

stat, 714, 716, 973, 974 symbol_end_pos, 824

State, 842 symbol_gloc, 923
statistics, 726, 789 symbol_rloc, 923
stats, 652, 653, 726, 729, 731, 789, 792, 794, symbol_start, 824
915, 973, 974 symbol_start_pos, 824
stats_alive, 652, 653 symbolic_output_buffer, 703
std_formatter, 701 symbolic_output_item, 703
stdbuf, 701 symlink, 982
stderr, 526, 820, 968 sync, 1033
stdin, 526, 735, 848, 967 synchronized_formatter_of_out_channel,
StdLabels, 539, 879 701
Stdlib, 512 Sys, 539, 898
stdout, 526, 820, 967 Sys_blocked_io, 513
stop, 722 Sys_error, 513
Str, 1007 Sys_blocked_io, 512
str_formatter, 701 Sys_error, 511
String, 539, 879, 880 system, 966
string, 510, 632, 633
string_after, 1012 t, 548, 556, 564, 572, 576, 578, 580, 583, 589,
string_before, 1012 590, 601, 614, 624, 628, 629, 631, 633,
string_match, 1009 635, 647, 649, 652–656, 668, 669, 676,
string_of_bool, 524 720, 723, 728, 730, 735, 739, 745, 750,
string_of_expression, 957 751, 756, 765, 774, 775, 785, 791–794,
string_of_float, 525 799, 806, 807, 812, 816, 818, 819, 825,
string_of_format, 536 829, 835, 842, 844, 857, 869, 870, 876,
string_of_inet_addr, 990 877, 880, 889, 906, 907, 909, 911–913,
string_of_int, 524 923, 930, 1024–1026, 1029
string_of_label, 922 tag, 696, 1025
string_of_structure, 957 take, 762, 771, 836, 864
string_partial_match, 1009 take_opt, 836
StringLabels, 539, 889 take_while, 762, 771, 864
structure, 955, 957 tan, 520, 666
structure_item, 955, 958 tanh, 521, 666
structure_item_desc, 956 tcdrain, 1002
sub, 550, 557, 579, 591, 597, 610, 625, 661, tcflow, 1002
671, 678, 740, 743, 747, 813, 882, 891 tcflush, 1002
sub_left, 574, 582, 584 tcgetattr, 1002
sub_right, 575, 582, 585 tcsendbreak, 1002
sub_string, 597, 610 tcsetattr, 1002
subbytes, 632, 633 temp_dir, 660
subset, 811, 874 temp_file, 659
substitute_first, 1011 terminal_io, 1002
substring, 632, 633 terminfo_toplevel_printer, 927
succ, 517, 662, 740, 743, 747, 814, 909 Thread, 1029
symbol_end, 824 time, 899, 986
1062

times, 987 transfer, 837

Timestamp, 1024 transpose, 865
tl, 757, 766 trim, 598, 612, 883, 892
tm, 986 trunc, 667
to_array, 641 truncate, 522, 591, 638, 971, 973
to_binary_string, 843 try_acquire, 876, 877
to_buffer, 783 try_lock, 812
to_bytes, 591, 783, 881, 890 try_with, 648
to_channel, 782 Type, 540, 906, 1024
to_char, 910 type_constraint, 942
to_dispenser, 868 type_declaration, 943
to_float, 589, 741, 744, 749, 816 type_exception, 945
to_hex, 632, 634 type_extension, 945
to_int, 589, 664, 744, 748, 815, 909 type_ident, 931
to_int32, 749, 816 type_kind, 944
to_int64, 1024 tyvar, 958
to_list, 550, 558, 641, 671, 678, 781, 805, tyvar_of_name, 958
811, 819, 846, 874
to_nativeint, 749 Uchar, 540, 909
to_option, 846 uid, 907
to_result, 819 umask, 977
to_rev_seq, 781, 805, 811, 875 uncapitalize_ascii, 600, 614, 884, 893
to_seq, 554, 562, 593, 603, 617, 642, 674, 681, uncons, 858
727, 729, 731, 765, 774, 781, 790, 792, Undefined, 751
794, 805, 811, 819, 837, 846, 874, 879, Undefined_recursive_module, 513
886, 895 Undefined_recursive_module, 512
to_seq_from, 781, 805, 811, 874 unescaped, 856
to_seq_keys, 727, 729, 731, 790, 792, 795 unflatten, 930
to_seq_reentrant, 642 unfold, 862
to_seq_rev, 642 Unhandled, 647
to_seq_rev_reentrant, 642 union, 776, 800, 807, 870
to_seq_values, 727, 729, 731, 790, 793, 795 Unit, 540, 911
to_seqi, 554, 562, 593, 604, 617, 675, 682, unit, 510, 1024
886, 895 Unix, 959
to_string, 590, 597, 610, 664, 741, 745, 749, unix, 900
783, 816, 825, 911 Unix_error, 964
to_string_default, 825 UnixLabels (module), 1003
tool_name, 920 unlink, 975
top, 836, 878 unlock, 812
top_opt, 878 unsafe_environment, 964
top_phrase, 957 unsafe_get, 580, 583, 586
toplevel_directive, 957 unsafe_getenv, 964
toplevel_phrase, 930, 957 unsafe_of_string, 602, 616
total_size, 784 unsafe_set, 580, 583, 586
tracker, 721 unsafe_to_string, 601, 615
INDEX TO THE LIBRARY 1063

unsigned_compare, 745, 750, 816 word_size, 900

unsigned_div, 743, 747, 814 wrap, 1033
unsigned_rem, 743, 747, 814 wrap_abort, 1033
unsigned_to_int, 744, 748, 815 write, 969, 1025
unzip, 867 write_arg, 548
update, 775, 800 write_arg0, 548
update_geometry, 693 write_bigarray, 969
uppercase_ascii, 600, 614, 623, 884, 893 write_substring, 970
usage, 547
usage_msg, 545 yield, 1030
usage_string, 547
zero, 624, 661, 739, 742, 746, 813
use_file, 930
zip, 866
use_printers, 826
User, 1025
utf_16_byte_length, 911
utf_8_byte_length, 911
utf_decode, 910, 911
utf_decode_invalid, 911
utf_decode_is_valid, 910
utf_decode_length, 910
utf_decode_uchar, 910
utimes, 987

val_ident, 931
value, 818, 844
value_binding, 956
value_constraint, 956
value_description, 942
variance, 922
virtual_flag, 922

wait, 628, 966

wait_flag, 965
wait_pid, 1031
wait_signal, 1032
wait_timed_read, 1031
wait_timed_write, 1031
waitpid, 966
warning_reporter, 927
Weak, 540, 912
win32, 900
with_constraint, 954
with_open_bin, 736, 821
with_open_gen, 736, 821
with_open_text, 736, 821
with_positions, 755
1064

Index of keywords

and, 171, 193, 198, 202, 203, 208, 214, 219 mutable, 193, 195, 196, 198, 201
as, 160, 161, 164, 165, 198, 201
asr, 158, 172, 186 new, 171, 187
assert, 191 nonrec, 193

begin, 163, 171, 173 object, 171, 188, 196, 198

of, 162, 193, 222
class, 202, 203, 206, 208, 209 open, 169, 203, 206, 208, 210
constraint, 193, 196, 198, 202 open!, 227
or, 171, 172, 181
do, see while, for
done, see while, for private, 196, 198, 201, 202, 216, 217
downto, see for
rec, see let, module
else, see if
end, 163, 171, 173, 196, 198, 203, 204, 208 sig, 203, 204
exception, 196, 203, 205, 208, 209 struct, 208
external, 203, 205, 208, 209
then, see if
false, 163 to, see for
for, 171, 182 true, 163
fun, 171, 172, 176, 198, 218 try, 171, 172, 183
function, 171, 172, 175 type, 193, 203, 205, 206, 208–210, 218, 222,
functor, 203, 207, 208, 210 228

if, 171, 172, 180 val, 196, 198, 201, 203, 205, 219
in, see let virtual, see val, method, class
include, 203, 207, 208, 210, 222
inherit, 196, 198, 201 when, 171, 177, 229
initializer, 198, 202 while, 182
with, 171, 203, 207, 219, 222
land, 158, 172, 186
lazy, 169, 171, 192
let, 171, 172, 178, 192, 198, 208, 209
lor, 158, 172, 186
lsl, 158, 172, 186
lsr, 158, 172, 186
lxor, 158, 172, 186

match, 171, 172, 180, 228

method, 196, 198, 201, 202
mod, 158, 172, 186
module, 192, 203, 206, 208–210, 214, 219, 222,
226
open, 192