X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/1d0f55cc843d6dfca01e2c13fd49ee2c119c6631..4c38b19e2650ca8b79b0d72a9995605ca12d9875:/NEWS diff --git a/NEWS b/NEWS index 80b49da6..e3510938 100644 --- a/NEWS +++ b/NEWS @@ -3,6 +3,31 @@ Bison News * Changes in version 2.5 (????-??-??): +** Named References Support + + 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) Support IELR(1) is a minimal LR(1) parser table generation algorithm. That @@ -20,9 +45,9 @@ Bison News default. You can specify the type of parser tables in the grammar file with these directives: - %define lr.type "LALR" - %define lr.type "IELR" - %define lr.type "canonical LR" + %define lr.type lalr + %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 @@ -33,19 +58,117 @@ Bison News These features are experimental. More user feedback will help to stabilize them. -** %define can now be invoked via the command line. +** 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. + +*** Unrecognized variables are now an error not a warning. + +*** Multiple invocations for any variable is now an error not a warning. + +*** Can now be invoked via the command line. - Each of these bison command-line options + Each of these command-line options - -D NAME=VALUE - --define=NAME=VALUE + -D NAME[=VALUE] + --define=NAME[=VALUE] + + -F NAME[=VALUE] + --force-define=NAME[=VALUE] is equivalent to this grammar file declaration - %define NAME "VALUE" + %define NAME ["VALUE"] + + 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. + +*** Variables renamed. + + The following %define variables + + api.push_pull + lr.keep_unreachable_states + + have been renamed to + + api.push-pull + lr.keep-unreachable-states + + The old names are now deprecated but will be maintained indefinitely + for backward compatibility. + +*** Values no longer need to be quoted in grammar file. - for any NAME and VALUE. Omitting `=VALUE' on the command line is - equivalent to omitting `"VALUE"' in the declaration. + If a %define value is an identifier, it no longer needs to be placed + within quotations marks. For example, + + %define api.push-pull "push" + + can be rewritten as + + %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. ** Temporary hack for adding a semicolon to the user action. @@ -68,7 +191,219 @@ Bison News about a missing semicolon where it did not before. Future releases of Bison will cease to append semicolons entirely. -* Changes in version 2.4.2 (????-??-??): +** 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 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. + + 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. + +* 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: + + %{CODE%} + + To provide a more flexible alternative, Bison 2.3b introduced the + %code directive with the following forms for C/C++: + + %code {CODE} + %code requires {CODE} + %code provides {CODE} + %code top {CODE} + + These forms are now considered permanent features of Bison. See the + %code entries in the section "Bison Declaration Summary" in the Bison + manual for a summary of their functionality. See the section + "Prologue Alternatives" for a detailed discussion including the + advantages of %code over the traditional Yacc prologue directive. + + 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, + message translations were not installed although supported by the + host system. * Changes in version 2.4.1 (2008-12-11): @@ -1118,7 +1453,8 @@ End: ----- Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, -2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc. +2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, +Inc. This file is part of Bison, the GNU Parser Generator.