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5.8.3 LAC

Canonical LR, IELR, and LALR can suffer from a couple of problems upon encountering a syntax error. First, the parser might perform additional parser stack reductions before discovering the syntax error. Such reductions can perform user semantic actions that are unexpected because they are based on an invalid token, and they cause error recovery to begin in a different syntactic context than the one in which the invalid token was encountered. Second, when verbose error messages are enabled (see Error Reporting), the expected token list in the syntax error message can both contain invalid tokens and omit valid tokens.

The culprits for the above problems are %nonassoc, default reductions in inconsistent states (see Default Reductions), and parser state merging. Because IELR and LALR merge parser states, they suffer the most. Canonical LR can suffer only if %nonassoc is used or if default reductions are enabled for inconsistent states.

LAC (Lookahead Correction) is a new mechanism within the parsing algorithm that solves these problems for canonical LR, IELR, and LALR without sacrificing %nonassoc, default reductions, or state merging. You can enable LAC with the %define parse.lac directive.

— Directive: %define parse.lac value

Enable LAC to improve syntax error handling.

(This feature is experimental. More user feedback will help to stabilize it. Moreover, it is currently only available for deterministic parsers in C.)

Conceptually, the LAC mechanism is straight-forward. Whenever the parser fetches a new token from the scanner so that it can determine the next parser action, it immediately suspends normal parsing and performs an exploratory parse using a temporary copy of the normal parser state stack. During this exploratory parse, the parser does not perform user semantic actions. If the exploratory parse reaches a shift action, normal parsing then resumes on the normal parser stacks. If the exploratory parse reaches an error instead, the parser reports a syntax error. If verbose syntax error messages are enabled, the parser must then discover the list of expected tokens, so it performs a separate exploratory parse for each token in the grammar.

There is one subtlety about the use of LAC. That is, when in a consistent parser state with a default reduction, the parser will not attempt to fetch a token from the scanner because no lookahead is needed to determine the next parser action. Thus, whether default reductions are enabled in consistent states (see Default Reductions) affects how soon the parser detects a syntax error: immediately when it reaches an erroneous token or when it eventually needs that token as a lookahead to determine the next parser action. The latter behavior is probably more intuitive, so Bison currently provides no way to achieve the former behavior while default reductions are enabled in consistent states.

Thus, when LAC is in use, for some fixed decision of whether to enable default reductions in consistent states, canonical LR and IELR behave almost exactly the same for both syntactically acceptable and syntactically unacceptable input. While LALR still does not support the full language-recognition power of canonical LR and IELR, LAC at least enables LALR's syntax error handling to correctly reflect LALR's language-recognition power.

There are a few caveats to consider when using LAC:

While the LAC algorithm shares techniques that have been recognized in the parser community for years, for the publication that introduces LAC, see Denny 2010 May.