X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/bb9191dd311e4c6d80f8dd12c6a7ce9254404fbc..8a4281b987577d911e418e8a37aef0c9c7121bf8:/NEWS diff --git a/NEWS b/NEWS index cacdb770..593807c7 100644 --- a/NEWS +++ b/NEWS @@ -3,42 +3,6 @@ Bison News * Changes in version ?.? (????-??-??): -** C++ parsers use YYRHSLOC - - Similarly to the C parsers, the C++ parsers now define the YYRHSLOC - macro and use it in the default YYLLOC_DEFAULT. You are encouraged - to use it. If, for instance, your location structure has "first" - and "last" members, instead of - - # define YYLLOC_DEFAULT(Current, Rhs, N) \ - do \ - if (N) \ - { \ - (Current).first = (Rhs)[1].location.first; \ - (Current).last = (Rhs)[N].location.last; \ - } \ - else \ - { \ - (Current).first = (Current).last = (Rhs)[0].location.last; \ - } \ - while (false) - - use: - - # define YYLLOC_DEFAULT(Current, Rhs, N) \ - do \ - if (N) \ - { \ - (Current).first = YYRHSLOC (Rhs, 1).first; \ - (Current).last = YYRHSLOC (Rhs, N).last; \ - } \ - else \ - { \ - (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \ - } \ - while (false) - - ** Additional yylex/yyparse arguments The new directive %param declare additional argument to both yylex @@ -87,9 +51,18 @@ Bison News use of the %error-verbose directive is deprecated in favor of %define parse.error "verbose". +** Semantic predicates + + The new, experimental, semantic-predicate feature allows actions of + the form %?{ BOOLEAN-EXPRESSION }, which cause syntax errors (as for + YYERROR) if the expression evaluates to 0, and are evaluated immediately + in GLR parsers, rather than being deferred. The result is that they + allow the programmer to prune possible parses based on the values of + runtime expressions. + * Changes in version 2.5 (????-??-??): -** Named References Support +** Named references: Historically, Yacc and Bison have supported positional references ($n, $$) to allow access to symbol values from inside of semantic @@ -114,13 +87,13 @@ Bison News These features are experimental in this version. More user feedback will help to stabilize them. -** IELR(1) and Canonical LR(1) Support +** IELR(1) and canonical LR(1): IELR(1) is a minimal LR(1) parser table generation algorithm. That is, given any context-free grammar, IELR(1) generates parser tables with the full language recognition power of canonical LR(1) but with - nearly the same number of parser states as LALR(1). This reduction in - parser states is often an order of magnitude. More importantly, + nearly the same number of parser states as LALR(1). This reduction + in parser states is often an order of magnitude. More importantly, because canonical LR(1)'s extra parser states may contain duplicate conflicts in the case of non-LR(1) grammars, the number of conflicts for IELR(1) is often an order of magnitude less as well. This can @@ -144,15 +117,57 @@ Bison News These features are experimental. More user feedback will help to stabilize them. -** Unrecognized %code qualifiers are now an error not a warning. +** LAC (lookahead correction) for syntax error handling: -** %define improvements. + 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. -*** Unrecognized variables are now an error not a warning. + 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. -*** Multiple invocations for any variable is now an error not a warning. + 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: -*** Can now be invoked via the command line. + %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. + +** Grammar symbol names can now contain dashes: + + Consistently with directives (such as %error-verbose) and variables + (e.g. push-pull), grammar symbol names may include dashes in any + position, similarly to periods and underscores. This is GNU + extension over POSIX Yacc whose use is reported by -Wyacc, and + rejected in Yacc mode (--yacc). + +** %define improvements: + +*** Can now be invoked via the command line: Each of these command-line options @@ -171,7 +186,7 @@ Bison News quietly override %define, but -D and --define do not. For further details, see the section "Bison Options" in the Bison manual. -*** Variables renamed. +*** Variables renamed: The following %define variables @@ -186,7 +201,7 @@ Bison News The old names are now deprecated but will be maintained indefinitely for backward compatibility. -*** Values no longer need to be quoted in grammar file. +*** Values no longer need to be quoted in grammar file: If a %define value is an identifier, it no longer needs to be placed within quotations marks. For example, @@ -197,15 +212,75 @@ Bison News %define api.push-pull push -** Symbol names. +*** Unrecognized variables are now errors not warnings. - Consistently with directives (such as %error-verbose) and variables - (e.g. push-pull), symbol names may include dashes in any position, - similarly to periods and underscores. This is GNU extension over - POSIX Yacc whose use is reported by -Wyacc, and rejected in Yacc - mode (--yacc). +*** Multiple invocations for any variable is now an error not a warning. + +** Unrecognized %code qualifiers are now errors not warnings. -** YYFAIL now produces warnings and Java parsers no longer implement it. +** Character literals not of length one: + + Previously, Bison quietly converted all character literals to length + one. For example, without warning, Bison interpreted the operators in + the following grammar to be the same token: + + exp: exp '++' + | exp '+' exp + ; + + Bison now warns when a character literal is not of length one. In + some future release, Bison will start reporting an error instead. + +** Destructor calls fixed for lookaheads altered in semantic actions: + + Previously for deterministic parsers in C, if a user semantic action + altered yychar, the parser in some cases used the old yychar value to + determine which destructor to call for the lookahead upon a syntax + error or upon parser return. This bug has been fixed. + +** C++ parsers use YYRHSLOC: + + Similarly to the C parsers, the C++ parsers now define the YYRHSLOC + macro and use it in the default YYLLOC_DEFAULT. You are encouraged + to use it. If, for instance, your location structure has "first" + and "last" members, instead of + + # define YYLLOC_DEFAULT(Current, Rhs, N) \ + do \ + if (N) \ + { \ + (Current).first = (Rhs)[1].location.first; \ + (Current).last = (Rhs)[N].location.last; \ + } \ + else \ + { \ + (Current).first = (Current).last = (Rhs)[0].location.last; \ + } \ + while (false) + + use: + + # define YYLLOC_DEFAULT(Current, Rhs, N) \ + do \ + if (N) \ + { \ + (Current).first = YYRHSLOC (Rhs, 1).first; \ + (Current).last = YYRHSLOC (Rhs, N).last; \ + } \ + else \ + { \ + (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \ + } \ + while (false) + +** YYLLOC_DEFAULT in C++: + + The default implementation of YYLLOC_DEFAULT used to be issued in + the header file. It is now output in the implementation file, after + the user %code sections so that its #ifndef guard does not try to + override the user's YYLLOC_DEFAULT if provided. + +** YYFAIL now produces warnings and Java parsers no longer implement it: YYFAIL has existed for many years as an undocumented feature of deterministic parsers in C generated by Bison. More recently, it was @@ -216,7 +291,7 @@ Bison News discussion of how to suppress C preprocessor warnings about YYFAIL being unused, see the Bison 2.4.2 NEWS entry. -** Temporary hack for adding a semicolon to the user action. +** Temporary hack for adding a semicolon to the user action: Previously, Bison appended a semicolon to every user action for reductions when the output language defaulted to C (specifically, when @@ -237,36 +312,48 @@ Bison News about a missing semicolon where it did not before. Future releases of Bison will cease to append semicolons entirely. -** Character literals not of length one. - - Previously, Bison quietly converted all character literals to length - one. For example, without warning, Bison interpreted the operators in - the following grammar to be the same token: - - exp: exp '++' - | exp '+' exp - ; - - 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. - -** Destructor calls fixed for lookaheads altered in semantic actions. - - Previously for deterministic parsers in C, if a user semantic action - altered yychar, the parser in some cases used the old yychar value to - determine which destructor to call for the lookahead upon a syntax - error or upon parser return. This bug has been fixed. - -* Changes in version 2.4.3 (????-??-??): +** 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. + +* 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. ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have been fixed. @@ -281,6 +368,8 @@ 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 @@ -1422,7 +1511,7 @@ End: ----- -Copyright (C) 1995-2010 Free Software Foundation, Inc. +Copyright (C) 1995-2011 Free Software Foundation, Inc. This file is part of Bison, the GNU Parser Generator.