Minimal Explanatory Stack Subtrace with Boundaries

MESS-B filters Nix evaluator stack traces to their causal essence while preserving semantic boundaries between user code and library internals.

Nix error traces are nightmares. A simple "package imported as module" error produces 55 frames spanning 19KB, of which 2 contain actionable information and 0 appear in user code.

$ nix build 2>&1 | mess-b

error[E-INFINITE-RECURSION]: infinite recursion encountered

  --> /nix/store/.../nixos/common.nix:109:20
   | [boundary: module-system → user-code]

  --> .../lib/modules.nix:508:28
   | while evaluating the module argument 'stdenv'

hint: a package derivation may have been imported as a module

  (6/55 frames shown, 89% reduction)

# With cabal
cabal install mess-b

# With nix
nix profile install github:weyl-ai/mess-b

# Pipe from nix
nix build 2>&1 | mess-b

# From file
mess-b -i error.log

# Adjust frame limit
mess-b -n 5

# Verbose output
mess-b --verbose

import MessB

main :: IO ()
main = do
  rawTrace <- getContents
  case parseAndFilter rawTrace of
    Left errorMessage -> putStrLn $ "Parse error: " <> errorMessage
    Right filtered -> putStrLn $ renderCompact filtered

1. Parse - Extract structured frames from raw Nix output
2. Classify - Categorize each frame (UserCode, ModuleSystem, LibraryInternal, etc.)
3. Detect Boundaries - Find transitions between code regions (where blame transfers)
4. Filter - Retain high-value frames, boundaries, and user code
5. Hint - Generate diagnostic hints from known error patterns
6. Render - Format output with boundary annotations and statistics

Three-tier fallback:

1. Explicit role - Frame has annotation (HighConfidence)
2. Ontology lookup - Function name in known-functions table (HighConfidence)
3. Path heuristics - File path patterns (MediumConfidence)

Key invariant: User code is never misclassified as library internal.

Boundaries mark transitions between code regions:

• module-system → user-code - Blame transfers to user
• library → user-code - External code enters user space
• overlay → package-set - Overlay composition point

# Build
cabal build

# Test
cabal test

# Run property tests
cabal test --test-option=--hedgehog-tests=10000

MIT

Weyl AI - weyl.ai