%define lr.type ielr
%define lr.type canonical-lr
- The default reduction optimization in the parser tables can also be
- adjusted using `%define lr.default-reductions'. See the documentation
- for `%define lr.type' and `%define lr.default-reductions' in the
- section `Bison Declaration Summary' in the Bison manual for the
- details.
+ The default-reduction optimization in the parser tables can also be
+ adjusted using `%define lr.default-reductions'. For details on both
+ of these features, see the new section `Tuning LR' in the Bison
+ manual.
These features are experimental. More user feedback will help to
stabilize them.
-** LAC (lookahead correction) for syntax error handling:
+** 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
+ 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 (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.
+ verbose error messages are enabled (with %error-verbose or the
+ obsolete `#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,
%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.
+ LAC 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. When LAC is in
+ use, 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
%define parse.lac full
- See the documentation for `%define parse.lac' in the section `Bison
- Declaration Summary' in the Bison manual for additional details.
+ See the new section `LAC' in the Bison manual for additional
+ details including a few caveats.
LAC is an experimental feature. More user feedback will help to
stabilize it.
** 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 a complete fix requires LAC, described above:
+ When %error-verbose or the obsolete `#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 more 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
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