dune menhir / ocamllex generation error in CI

By Daniel Zoghalchali·Latchkey

dune runs menhir or ocamllex to generate parser/lexer code before compiling. A grammar conflict or a lexer syntax error stops generation, so the produced .ml never exists and the build fails.

What this error means

dune build fails inside a menhir or ocamllex rule with "Warning: N states have conflicts", "menhir: ... failed", or an ocamllex parse error, before the OCaml compiler runs.

File "src/parser.mly", line 42:
Error: 3 states have shift/reduce conflicts.
menhir: 1 error(s) encountered

Common causes

Grammar conflicts treated as errors

menhir reports shift/reduce or reduce/reduce conflicts; with strict settings these fail the build rather than warn.

A lexer or grammar syntax error

An ocamllex rule or menhir declaration has a syntax mistake, so generation cannot produce valid OCaml.

How to fix it

Resolve grammar conflicts

1. Run menhir with --explain to get a conflict report.
2. Add precedence declarations or refactor the ambiguous rules.
3. Rebuild until menhir reports no conflicts.

menhir --explain src/parser.mly

Fix the lexer/grammar syntax

Correct the ocamllex or menhir syntax the error points at, then let dune regenerate.

dune build src/parser.ml src/lexer.ml

How to prevent it

• Keep grammars conflict-free with explicit precedence.
• Run menhir --explain locally before pushing grammar changes.
• Regenerate parser/lexer code as part of local builds.

Frequently asked questions

Related guides

References

• GitHub Actions documentation