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author | Joel E. Denny <joeldenny@joeldenny.org> | 2010-12-19 22:12:32 -0500 |
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committer | Joel E. Denny <joeldenny@joeldenny.org> | 2010-12-23 21:50:22 -0500 |
commit | 4c38b19e2650ca8b79b0d72a9995605ca12d9875 (patch) | |
tree | 3a6178c7f1f41b15ed903cea009f672b1a424afa /NEWS | |
parent | 723fe7d18a058e8764beb0c50fbcfa554ee5f6c4 (diff) | |
download | bison-4c38b19e2650ca8b79b0d72a9995605ca12d9875.tar.gz |
parse.lac: document.
* NEWS (2.5): Add entry for LAC, and mention LAC in entry for
other corrections to verbose syntax error messages.
* doc/bison.texinfo (Decl Summary): Rewrite entries for
lr.default-reductions and lr.type to be clearer, to mention
%nonassoc's effect on canonical LR, and to mention LAC. Add entry
for parse.lac.
(Glossary): Add entry for LAC.
(cherry picked from commit fcf834f9ecf080784b741782f4206df1e1a2957a)
Conflicts:
doc/bison.texinfo
Diffstat (limited to 'NEWS')
-rw-r--r-- | NEWS | 72 |
1 files changed, 57 insertions, 15 deletions
@@ -58,6 +58,46 @@ Bison News 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. @@ -166,11 +206,11 @@ Bison News ** 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 @@ -189,16 +229,18 @@ Bison News 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. |