OCaml解析器报“unterminated action”错：新手用ocamllex/ocamlyacc遇编译问题

Hey there! As someone who’s stumbled through ocamlyacc’s quirky error messages when starting out, that Error (with mark at =) almost always ties to a syntax misstep right around an equals sign in your parser definition. Let’s walk through the most likely issues to check first:

Common Causes & Fixes #

• Using = instead of : for grammar rules
  Ocamlyacc requires grammar rules to use a colon (:) to separate the nonterminal from its productions, not an equals sign. This is one of the most common rookie mistakes.
  ❌ Wrong:

  expr = NUM { $1 };
  

  ✅ Correct:

  expr : NUM { $1 };
  

• Syntax errors in OCaml action code
  If the OCaml code inside the curly braces { ... } has a mistake (like a malformed assignment, missing semicolon, or invalid variable), ocamlyacc might flag the nearest = as the error point. For example:
  ❌ Wrong (malformed assignment):

  expr : expr PLUS expr { let res = $1 + $3; res };
  

  ✅ Correct:

  expr : expr PLUS expr { let res = $1 + $3 in res };
  

• Misplaced punctuation in rule definitions
  Missing semicolons at the end of a rule, or incorrect spacing around symbols that include = (like if you’re parsing assignment operators but haven’t defined them properly in your lexer) can also trigger this error. Double-check that all rules end with ; and that any = tokens are properly declared in your .mll file.

Next Steps #

To pinpoint the exact issue, could you share the specific lines from your parser.mly file where the error is marked? Even just 5-10 lines around the problematic = would help narrow things down quickly.

内容的提问来源于stack exchange，提问作者denbuttigieg