Ino is a DSL to generate data structures for golang.

go install github.com/nihei9/ino/cmd/ino@latest

First, save your source code written in Ino to a file with the extension .ino. In this example, let's say you saved the file named example.ino.

Next, convert the source code in Ino to golang using ino command. --package flag allows you to change the package name. The default package name is main.

ino --package example

Then you will get files example.go and ino_builtin.go.

You can also use go:generate directive.

//go:generate ino --package example

Source code is Unicode text encoded in UTF-8.

Identifiers name types and variables. The form of identifiers is [A-Za-z][A-Za-z0-9]* in the regular expression. Identifiers in Ino are limited in the number of characters we can use compared to golang.

The following keywords may not be used as identifiers.

• data

Ino currently supports Int and String built-in types, corresponding to int and string of golang, respectively.

data keyword allows you to generate a sum type.

data Pen
    = BallpointPen String
    | FountainPen
    ;

The following three public APIs are available.

• type Pen interface
• func BallpointPen(String) Pen: returns a BallpointPen variant of Pen type
• func FountainPen() Pen: returns a FountainPen variant of Pen type

Now, you can check that a value of Pen is which variant using OK method and can retrieve fields using Fields method. Fields method isn't available in FountainPen because it has no fields.

bpB := BallpointPen(String("Black"))
ok := bpB.Maybe().BallpointPen().OK()            // true
ok = bpB.Maybe().FountainPen().OK()              // false
color, ok := bpB.Maybe().BallpointPen().Fields() // String("Black"), true
color, ok = bpB.Maybe().FountainPen().Fields()   // String(""), false

fp := FountainPen()
ok = fp.Maybe().FountainPen().OK()  // true
ok = fp.Maybe().BallpointPen().OK() // false

The following helpful APIs are also available.

• func NewPenCaseSet[U any](...*case_Pen[U]) (*PenCaseSet[U], error) and func (s *PenCaseSet[U]) Match(x Pen) (U, error): run pattern matching.
• func ApplyToBallpointPen[T any](x Pen, f func(String) T) (T, bool): when x is a BallpointPen variant, applies f to x's fields and retuns the result and true.
• func ApplyToFountainPen[T any](x Pen, f func() T) (T, bool): when x is a FountainPen variant, runs f and returns the result and true.

The result type of Match method and ApplyTo* function are polymorphic, so you can use an arbitrary type for the result type of the callback function.

Pattern matching:

⚠️ New*CaseSet function performs exhaustiveness checking, but the checking guarantees only the exhaustiveness of tags. In other words, the exhaustive of parameters is not guaranteed.

cases, err := NewPenCaseSet(
    CaseBallpointPen(BallpointPen(String("Black")), func(color String) string {
        return "ballpoint-"+string(color)
    }),
    CaseFountainPen(FountainPen(), func() string {
        return "fountain"
    }),
)
if err != nil {
    // When the above cases are not exhaustive pattern, an error occurs.
}

pen := BallpointPen(String("Black"))

result, err := cases.Match(pen)
if err != nil {
    // When `pen` doesn't match any of the patterns, an error occurs.
}
fmt.Print(result) // ballpoint-Black

ApplyTo*:

color2Num := func(color String) int {
    switch color {
    case "Black": return 1
    default: return 9
    }
}

bpB := BallpointPen(String("Black"))
bpR := BallpointPen(String("Red"))
fp := FountainPen()

v, ok := ApplyToBallpointPen(bpB, color2Num) // 1, true
v, ok = ApplyToBallpointPen(bpR, color2Num)  // 9, true
v, ok = ApplyToBallpointPen(fp, color2Num)   // 0, false

You can define polymorphic types using type variables.

Type variables can follow a data type name. In this example, a is a type variable.

data Option a
    = None
    | Some a
    ;

• type Option[T Eqer] interface
• func None[T Eqer]() Option[T]: returns a None variant of Option type
• func Some[T Eqer](p1 T) Option[T]: returns a Some variant of Option type

s1 := Some(Int(100))              // Option[Int]
n := None[Int]()                  // Option[Int]
s2 := Some(Some(String("Hello"))) // Option[Option[String]]

num, ok := s1.Maybe().Some().Fields()  // Int(100), true
opt, ok := s2.Maybe().Some().Fields()  // Some(String("Hello")), true
str, ok := opt.Maybe().Some().Fields() // String("Hello"), true

Pattern matching APIs and ApplyTo* functions are also available.

• func NewOptionCaseSet[T Eqer, U any](...*case_Option[T, U]) (*OptionCaseSet[T, U], error) and func (s *OptionCaseSet[T, U]) Match(x Option[T]) (U, error)
• func ApplyToNone[T Eqer, U any](x Option[T], f func() U) (U, bool)
• func ApplyToSome[T Eqer, U any](x Option[T], f func(T) U) (U, bool)

Polymorphic-type literals must be enclosed in parentheses.

data List a
    = Nil
    | Cons a (List a)
    ;