X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/4631c34fb78bebfd3764ebe77d4f4baae757dbf6..4c38b19e2650ca8b79b0d72a9995605ca12d9875:/NEWS?ds=sidebyside diff --git a/NEWS b/NEWS index 61b14b08..e3510938 100644 --- a/NEWS +++ b/NEWS @@ -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. @@ -164,14 +204,43 @@ Bison News Bison now warns when a character literal is not of length one. In some future release, Bison will report an error instead. -** Verbose error messages fixed for nonassociative tokens. - - When %error-verbose is specified, syntax error messages produced by - the generated parser include the unexpected token as well as a list of - expected tokens. Previously, this list erroneously included tokens - that would actually induce a syntax error because conflicts for them - were resolved with %nonassoc. Such tokens are now properly omitted - from the list. +** 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 %nonassoc is used, there can exist parser states that accept no + tokens, and so the parser does not always require a lookahead token + in order to detect a syntax error. Because no unexpected token or + expected tokens can then be reported, the verbose syntax error + message described above is suppressed, and the parser instead + reports the simpler message, "syntax error". Previously, this + suppression was sometimes erroneously triggered by %nonassoc when a + lookahead was actually required. Now verbose messages are + suppressed only when all previous lookaheads have already been + shifted or discarded. + +*** Previously, the list of expected tokens erroneously included tokens + that would actually induce a syntax error because conflicts for them + were resolved with %nonassoc in the current parser state. Such + 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 + 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. @@ -222,7 +291,7 @@ Bison News the user %code sections so that its #ifndef guard does not try to override the user's YYLLOC_DEFAULT if provided. -* Changes in version 2.4.3 (????-??-??): +* Changes in version 2.4.3 (2010-08-05): ** Bison now obeys -Werror and --warnings=error for warnings about grammar rules that are useless in the parser due to conflicts. @@ -240,6 +309,8 @@ Bison News errors in Bison 2.5. They will remain warnings, which should be sufficient for POSIX while avoiding backward compatibility issues. +** Minor documentation fixes. + * Changes in version 2.4.2 (2010-03-20): ** Some portability problems that resulted in failures and livelocks