X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/dac8cc0d148c553fe38e840d144b231b7269a49b..4c2a6e42ba8b6bc4e04985f5ef3ec8926048d4b1:/NEWS diff --git a/NEWS b/NEWS index 82044f25..3f102577 100644 --- a/NEWS +++ b/NEWS @@ -3,13 +3,46 @@ Bison News * Changes in version 2.5 (????-??-??): -** IELR(1) and Canonical LR(1) Support +** Grammar symbol names can now contain non-initial dashes: + + Consistently with directives (such as %error-verbose) and with + %define variables (e.g. push-pull), grammar symbol names may contain + dashes in any position except the beginning. This is a GNU + extension over POSIX Yacc. Thus, use of this extension is reported + by -Wyacc and rejected in Yacc mode (--yacc). + +** Named references: + + Historically, Yacc and Bison have supported positional references + ($n, $$) to allow access to symbol values from inside of semantic + actions code. + + Starting from this version, Bison can also accept named references. + When no ambiguity is possible, original symbol names may be used + as named references: + + if_stmt : 'if' cond_expr 'then' then_stmt ';' + { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); } + + In the more common case, explicit names may be declared: + + stmt[res] : 'if' expr[cond] 'then' stmt[then] 'else' stmt[else] ';' + { $res = mk_if_stmt($cond, $then, $else); } + + Location information is also accessible using @name syntax. When + accessing symbol names containing dots or dashes, explicit bracketing + ($[sym.1]) must be used. + + These features are experimental in this version. More user feedback + will help to stabilize them. + +** 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 @@ -24,20 +57,58 @@ Bison News %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. -** %define improvements. +** LAC (Lookahead Correction) for syntax error handling: -*** Multiple invocations for any variable is now an error not a warning. + 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 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 the + obsolete `#define YYERROR_VERBOSE'), the expected token list in the + syntax error message can both contain invalid tokens and omit valid + tokens. -*** Can now be invoked via the command line. + 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 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 + 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 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. + +** %define improvements: + +*** Can now be invoked via the command line: Each of these command-line options @@ -56,7 +127,7 @@ 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 @@ -71,7 +142,7 @@ 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, @@ -82,15 +153,86 @@ 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. -** Temporary hack for adding a semicolon to the user action. +** Unrecognized %code qualifiers are now errors not warnings. + +** 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 + a documented feature of Bison's experimental Java parsers. As + promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a + semantic action now produces a deprecation warning, and Java parsers + no longer implement YYFAIL at all. For further details, including a + 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: Previously, Bison appended a semicolon to every user action for reductions when the output language defaulted to C (specifically, when @@ -111,32 +253,97 @@ 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. - -* Changes in version 2.4.2 (????-??-??): +** Verbose syntax error message fixes: + + 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 + 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. + +** Java skeleton fixes: + +*** A location handling bug has been fixed. + +*** The top element of each of the value stack and location stack is now + cleared when popped so that it can be garbage collected. + +* 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. + +** Failures in the test suite for GCC 4.5 have been fixed. + +** Failures in the test suite for some versions of Sun Studio C++ have + been fixed. + +** Contrary to Bison 2.4.2's NEWS entry, it has been decided that + warnings about undefined %prec identifiers will not be converted to + 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 + in the test suite on some versions of at least Solaris, AIX, HP-UX, + RHEL4, and Tru64 have been addressed. As a result, fatal Bison + errors should no longer cause M4 to report a broken pipe on the + affected platforms. + +** `%prec IDENTIFIER' requires IDENTIFIER to be defined separately. + + POSIX specifies that an error be reported for any identifier that does + not appear on the LHS of a grammar rule and that is not defined by + %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this + error report for the case when an identifier appears only after a + %prec directive. It is now restored. However, for backward + compatibility with recent Bison releases, it is only a warning for + now. In Bison 2.5 and later, it will return to being an error. + [Between the 2.4.2 and 2.4.3 releases, it was decided that this + warning will not be converted to an error in Bison 2.5.] ** Detection of GNU M4 1.4.6 or newer during configure is improved. +** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS, + YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now + avoided. + ** %code is now a permanent feature. A traditional Yacc prologue directive is written in the form: @@ -160,6 +367,45 @@ Bison News Bison's Java feature as a whole including its current usage of %code is still considered experimental. +** YYFAIL is deprecated and will eventually be removed. + + YYFAIL has existed for many years as an undocumented feature of + deterministic parsers in C generated by Bison. Previously, it was + documented for Bison's experimental Java parsers. YYFAIL is no longer + documented for Java parsers and is formally deprecated in both cases. + Users are strongly encouraged to migrate to YYERROR, which is + specified by POSIX. + + Like YYERROR, you can invoke YYFAIL from a semantic action in order to + induce a syntax error. The most obvious difference from YYERROR is + that YYFAIL will automatically invoke yyerror to report the syntax + error so that you don't have to. However, there are several other + subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from + inherent flaws when %error-verbose or `#define YYERROR_VERBOSE' is + used. For a more detailed discussion, see: + + http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html + + The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but + deterministic parsers in C will continue to implement it. However, + because YYFAIL is already flawed, it seems futile to try to make new + Bison features compatible with it. Thus, during parser generation, + Bison 2.5 will produce a warning whenever it discovers YYFAIL in a + rule action. In a later release, YYFAIL will be disabled for + %error-verbose and `#define YYERROR_VERBOSE'. Eventually, YYFAIL will + be removed altogether. + + There exists at least one case where Bison 2.5's YYFAIL warning will + be a false positive. Some projects add phony uses of YYFAIL and other + Bison-defined macros for the sole purpose of suppressing C + preprocessor warnings (from GCC cpp's -Wunused-macros, for example). + To avoid Bison's future warning, such YYFAIL uses can be moved to the + epilogue (that is, after the second `%%') in the Bison input file. In + this release (2.4.2), Bison already generates its own code to suppress + C preprocessor warnings for YYFAIL, so projects can remove their own + phony uses of YYFAIL if compatibility with Bison releases prior to + 2.4.2 is not necessary. + ** Internationalization. Fix a regression introduced in Bison 2.4: Under some circumstances, @@ -1213,8 +1459,7 @@ End: ----- -Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, -2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc. +Copyright (C) 1995-2011 Free Software Foundation, Inc. This file is part of Bison, the GNU Parser Generator.