Yacc compatibility

grmtools supports most major Yacc features, to the extent that many Yacc grammars can be used unchanged with grmtools. In this book we assume familiarity with Yacc syntax and its major features: the Yacc manual is recommended reading.

Major differences

There are several differences between Yacc and grmtools including:

  • grmtools has no equivalent of any of the yy* functions (e.g. yyerror, yylex, yylval, yyparse and so on). This means, for example, that grammar actions cannot currently influence the lexer in any way.

  • grmtools has an entirely different approach to error recovery. The token error and the special action expressions yyerrok and yyclearin are not supported. In general, users can simply remove alternatives that consist solely of error.

  • %union can be mapped to %actiontype in grmtools, though this is rarely the best way of using a Yacc grammar in Rust. See the Grmtools Yacc variant below for the most common way of making grammars do something useful; in a limited number of cases (e.g. if you just want to build a parse tree), you may find the "Original" Yacc variant useful.


YaccKind::Grmtools is grmtools' own variant of Yacc syntax, and the one that most users will want to use. The most significant difference to "normal" Yacc is that rules are annotated with a Rust type to which all their production's actions must adhere to. Note that whilst a rule's productions must all adhere to a single type, different rules can have different types. Consider the following snippet:

R1 -> Result<i32, ()>:
     'a' { Ok(5) }
   | 'b' { Err(()) }

R2 -> u64:
   | { 0 }

Here the rule R1 has a Rust return type of Result<X, ()> (between -> and :). Both of its productions adhere to this type, the first by instantiating Ok(5) and the second Err(()). The rule R2 has a return type of u64.

“Original” Yacc

Although the name is not fully accurate (grmtools supports a slightly disjoint subset of original Yacc's input), this mode allows users to most easily test externally created Yacc files. Several sub-variants are allowed:

  • YaccKind::Original(YaccOriginalActionKind::GenericParseTree) does not execute user actions, but instead creates a generic parse tree, where elements are instances of the lrpar::parser::Node enum. This is useful for quickly testing whether a parser is accepting the intended language.

  • YaccKind::Original(YaccOriginalActionKind::NoAction) parses input and reports errors but does not execute any user actions. This is useful if you are trying to find out whether a corpus of input parses successfully against your grammar or not.

  • YaccKind::Original(YaccOriginalActionKind::UserAction) models original Yacc most closely but, in a Rust setting, is probably of little use beyond simple calculator like languages. Instead of Yacc's %union directive, users can specify %actiontype which is a Rust type to which every production's actions in the grammar must adhere to. Unless all actions happen to naturally return the same type, this quickly becomes cumbersome to use. For most use cases, YaccKind::Grmtools is a superior alternative.