These features are experimental. More user feedback will help to
stabilize them.
+** LAC (lookahead correction) for syntax error handling:
+
+ 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 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 (with %error-verbose or `#define
+ YYERROR_VERBOSE'), 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, and parser state merging. Thus,
+ IELR and LALR suffer the most. Canonical LR can suffer only if
+ %nonassoc is used or if default reductions are enabled for
+ inconsistent states.
+
+ LAC is a new mechanism within the parsing algorithm that completely
+ solves these problems for canonical LR, IELR, and LALR without
+ sacrificing %nonassoc, default reductions, or state mering. When
+ LAC is in use, canonical LR and IELR behave 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.
+
+ Currently, LAC is only supported for deterministic parsers in C.
+ You can enable LAC with the following directive:
+
+ %define parse.lac full
+
+ See the documentation for `%define parse.lac' in the section `Bison
+ Declaration Summary' in the Bison manual for additional details.
+
+ LAC is an experimental feature. More user feedback will help to
+ stabilize it.
+
** Unrecognized %code qualifiers are now an error not a warning.
** %define improvements.
** Verbose syntax error message fixes:
- When %error-verbose or `#define YYERROR_VERBOSE' is specified, syntax
- error messages produced by the generated parser include the unexpected
- token as well as a list of expected tokens. The effect of %nonassoc
- on these verbose messages has been corrected in two ways, but
- additional fixes are still being implemented:
+ When %error-verbose or `#define YYERROR_VERBOSE' is specified,
+ syntax error messages produced by the generated parser include the
+ unexpected token as well as a list of expected tokens. The effect
+ of %nonassoc on these verbose messages has been corrected in two
+ ways, but a complete fix requires LAC, described above:
*** When %nonassoc is used, there can exist parser states that accept no
tokens, and so the parser does not always require a lookahead token
tokens are now properly omitted from the list.
*** Expected token lists are still often wrong due to state merging
- (from LALR or IELR) and default reductions, which can both add and
- subtract valid tokens. Canonical LR almost completely fixes this
- problem by eliminating state merging and default reductions.
- However, there is one minor problem left even when using canonical
- LR and even after the fixes above. That is, if the resolution of a
- conflict with %nonassoc appears in a later parser state than the one
- at which some syntax error is discovered, the conflicted token is
- still erroneously included in the expected token list. We are
- currently working on a fix to eliminate this problem and to
- eliminate the need for canonical LR.
+ (from LALR or IELR) and default reductions, which can both add
+ invalid tokens and subtract valid tokens. Canonical LR almost
+ completely fixes this problem by eliminating state merging and
+ default reductions. However, there is one minor problem left even
+ when using canonical LR and even after the fixes above. That is,
+ if the resolution of a conflict with %nonassoc appears in a later
+ parser state than the one at which some syntax error is
+ discovered, the conflicted token is still erroneously included in
+ the expected token list. Bison's new LAC implementation,
+ described above, eliminates this problem and the need for
+ canonical LR. However, LAC is still experimental and is disabled
+ by default.
** Destructor calls fixed for lookaheads altered in semantic actions.