X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/7d424de14c0385012d328642bec6874c8e6cef17..a6552c5d2a559399b2ca2619159e3de14cff4468:/NEWS diff --git a/NEWS b/NEWS index e5ebfe4b..293e2dfd 100644 --- a/NEWS +++ b/NEWS @@ -24,6 +24,15 @@ Bison News Also, it is possible to add code to the parser's constructors using "%code init" and "%define init_throws". +** C++ skeleton improvements + + The C++ parser features a syntax_error exception, which can be + thrown from the scanner or from user rules to raise syntax errors. + This facilitates reporting errors caught in sub-functions (e.g., + rejecting too large integral literals from a conversion function + used by the scanner, or rejecting invalid combinations from a + factory invoked by the user actions). + ** Variable api.tokens.prefix The variable api.tokens.prefix changes the way tokens are identified in @@ -51,9 +60,34 @@ Bison News use of the %error-verbose directive is deprecated in favor of %define parse.error "verbose". -* Changes in version 2.5 (????-??-??): +** 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 2.5.1 (????-??-??): + +** Some portability problems in the test suite have been fixed. + +** Minor improvements have been made to the manual. + +** YYBACKUP works as expected. -** Named References Support +* Changes in version 2.5 (2011-05-14): + +** 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 @@ -63,12 +97,12 @@ Bison News When no ambiguity is possible, original symbol names may be used as named references: - if_stmt : 'if' cond_expr 'then' then_stmt ';' + 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] ';' + 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 @@ -78,13 +112,13 @@ 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, + 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, 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 @@ -99,24 +133,58 @@ 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. -** 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 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. -*** 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 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 -*** Can now be invoked via the command line. + 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 @@ -133,9 +201,9 @@ Bison News except that the manner in which Bison processes multiple definitions for the same NAME differs. Most importantly, -F and --force-define quietly override %define, but -D and --define do not. For further - details, see the section "Bison Options" in the Bison manual. + details, see the section `Bison Options' in the Bison manual. -*** Variables renamed. +*** Variables renamed: The following %define variables @@ -150,7 +218,7 @@ 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 the grammar file: If a %define value is an identifier, it no longer needs to be placed within quotations marks. For example, @@ -161,47 +229,13 @@ Bison News %define api.push-pull push -** Symbol names. - - 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). - -** 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. +*** Unrecognized variables are now errors not warnings. -** 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 - neither %yacc, %language, %skeleton, or equivalent command-line - options were specified). This allowed actions such as - - exp: exp "+" exp { $$ = $1 + $3 }; +*** Multiple invocations for any variable is now an error not a warning. - instead of +** Unrecognized %code qualifiers are now errors not warnings. - exp: exp "+" exp { $$ = $1 + $3; }; - - As a first step in removing this misfeature, Bison now issues a - warning when it appends a semicolon. Moreover, in cases where Bison - cannot easily determine whether a semicolon is needed (for example, an - action ending with a cpp directive or a braced compound initializer), - it no longer appends one. Thus, the C compiler might now complain - 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. +** 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 @@ -212,30 +246,21 @@ 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. + some future release, Bison will start reporting an error instead. - 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. +** 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 +** 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 + to use it. If, for instance, your location structure has `first' + and `last' members, instead of # define YYLLOC_DEFAULT(Current, Rhs, N) \ do \ @@ -265,14 +290,149 @@ Bison News } \ while (false) -** YYLLOC_DEFAULT in C++ +** 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. -* Changes in version 2.4.3 (????-??-??): +** 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 + neither %yacc, %language, %skeleton, or equivalent command-line + options were specified). This allowed actions such as + + exp: exp "+" exp { $$ = $1 + $3 }; + + instead of + + exp: exp "+" exp { $$ = $1 + $3; }; + + As a first step in removing this misfeature, Bison now issues a + warning when it appends a semicolon. Moreover, in cases where Bison + cannot easily determine whether a semicolon is needed (for example, an + action ending with a cpp directive or a braced compound initializer), + it no longer appends one. Thus, the C compiler might now complain + about a missing semicolon where it did not before. Future releases of + Bison will cease to append semicolons entirely. + +** 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. + +*** Parser traces now print the top element of the stack. + +** -W/--warnings fixes: + +*** Bison now properly recognizes the `no-' versions of categories: + + For example, given the following command line, Bison now enables all + warnings except warnings for incompatibilities with POSIX Yacc: + + bison -Wall,no-yacc gram.y + +*** Bison now treats S/R and R/R conflicts like other warnings: + + Previously, conflict reports were independent of Bison's normal + warning system. Now, Bison recognizes the warning categories + `conflicts-sr' and `conflicts-rr'. This change has important + consequences for the -W and --warnings command-line options. For + example: + + bison -Wno-conflicts-sr gram.y # S/R conflicts not reported + bison -Wno-conflicts-rr gram.y # R/R conflicts not reported + bison -Wnone gram.y # no conflicts are reported + bison -Werror gram.y # any conflict is an error + + However, as before, if the %expect or %expect-rr directive is + specified, an unexpected number of conflicts is an error, and an + expected number of conflicts is not reported, so -W and --warning + then have no effect on the conflict report. + +*** The `none' category no longer disables a preceding `error': + + For example, for the following command line, Bison now reports + errors instead of warnings for incompatibilities with POSIX Yacc: + + bison -Werror,none,yacc gram.y + +*** The `none' category now disables all Bison warnings: + + Previously, the `none' category disabled only Bison warnings for + which there existed a specific -W/--warning category. However, + given the following command line, Bison is now guaranteed to + suppress all warnings: + + bison -Wnone gram.y + +** Precedence directives can now assign token number 0: + + Since Bison 2.3b, which restored the ability of precedence + directives to assign token numbers, doing so for token number 0 has + produced an assertion failure. For example: + + %left END 0 + + This bug has been fixed. + +* 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. @@ -287,6 +447,8 @@ 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 @@ -745,26 +907,26 @@ Bison News if the symbols have destructors. For instance: exp: exp "?" exp ":" exp { $1 ? $1 : $3; } - | exp "+" exp - ; + | exp "+" exp + ; will trigger a warning about $$ and $5 in the first rule, and $3 in the second ($1 is copied to $$ by the default rule). This example most likely contains three errors, and could be rewritten as: exp: exp "?" exp ":" exp - { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); } - | exp "+" exp - { $$ = $1 ? $1 : $3; if ($1) free ($3); } - ; + { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); } + | exp "+" exp + { $$ = $1 ? $1 : $3; if ($1) free ($3); } + ; However, if the original actions were really intended, memory leaks and all, the warnings can be suppressed by letting Bison believe the values are used, e.g.: exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); } - | exp "+" exp { $$ = $1; (void) $3; } - ; + | exp "+" exp { $$ = $1; (void) $3; } + ; If there are mid-rule actions, the warning is issued if no action uses it. The following triggers no warning: $1 and $3 are used. @@ -1008,16 +1170,16 @@ Bison News In agreement with POSIX and with other Yaccs, leaving a default action is valid when $$ is untyped, and $1 typed: - untyped: ... typed; + untyped: ... typed; but the converse remains an error: - typed: ... untyped; + typed: ... untyped; ** Values of mid-rule actions The following code: - foo: { ... } { $$ = $1; } ... + foo: { ... } { $$ = $1; } ... was incorrectly rejected: $1 is defined in the second mid-rule action, and is equal to the $$ of the first mid-rule action. @@ -1428,9 +1590,7 @@ End: ----- -Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, -2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, -Inc. +Copyright (C) 1995-2012 Free Software Foundation, Inc. This file is part of Bison, the GNU Parser Generator.