3 * Noteworthy changes in release ?.? (????-??-??) [?]
5 The generation of prototypes for yylex and yyerror in Yacc mode is
6 breaking existing grammar files. To avoid breaking too many grammars, the
7 prototypes are now generated when `-y/--yacc` is used *and* the
8 `POSIXLY_CORRECT` environment variable is defined.
10 Avoid using `-y`/`--yacc` simply to comply with Yacc's file name
11 conventions, rather, use `-o y.tab.c`. Autoconf's AC_PROG_YACC macro uses
12 `-y`. Avoid it if possible, for instance by using gnulib's gl_PROG_BISON.
14 * Noteworthy changes in release 3.8 (2021-09-07) [stable]
16 ** Backward incompatible changes
18 In conformance with the recommendations of the Graphviz team
19 (https://marc.info/?l=graphviz-devel&m=129418103126092), `-g`/`--graph`
20 now generates a *.gv file by default, instead of *.dot. A transition
23 To comply with the latest POSIX standard, in Yacc compatibility mode
24 (options `-y`/`--yacc`) Bison now generates prototypes for yyerror and
25 yylex. In some situations, this is breaking compatibility: if the user
26 has already declared these functions but with some differences (e.g., to
27 declare them as static, or to use specific attributes), the generated
28 parser will fail to compile. To disable these prototypes, #define yyerror
29 (to `yyerror`), and likewise for yylex.
31 ** Deprecated features
33 Support for the YYPRINT macro is removed. It worked only with yacc.c and
34 only for tokens. It was obsoleted by %printer, introduced in Bison 1.50
37 It has always been recommended to prefer `%define api.value.type foo` to
38 `#define YYSTYPE foo`. The latter is supported in C for compatibility
39 with Yacc, but not in C++. Warnings are now issued if `#define YYSTYPE`
40 is used in C++, and eventually support will be removed.
42 In C++ code, prefer value_type to semantic_type to denote the semantic
43 value type, which is specified by the `api.value.type` %define variable.
47 *** A skeleton for the D programming language
49 The "lalr1.d" skeleton is now officially part of Bison.
51 It was originally contributed by Oliver Mangold, based on Paolo Bonzini's
52 lalr1.java, and was improved by H. S. Teoh. Adela Vais then took over
53 maintenance and invested a lot of efforts to complete, test and document
56 It now supports all the bells and whistles of the other deterministic
57 parsers, which include: pull/push interfaces, verbose and custom error
58 messages, lookahead correction, token constructors, internationalization,
59 locations, printers, token and symbol prefixes, etc.
61 Two examples demonstrate the D parsers: a basic one (examples/d/simple),
62 and an advanced one (examples/d/calc).
64 *** Option -H, --header and directive %header
66 The option `-H`/`--header` supersedes the option `--defines`, and the
67 directive %header supersedes %defines. Both `--defines` and `%defines`
68 are, of course, maintained for backward compatibility.
72 Since version 2.4 Bison can be used to generate HTML reports. However it
73 was a two-step process: first bison must be invoked with option `--xml`,
74 and then xsltproc must be run to the convert the XML reports into HTML.
76 The new option `--html` combines these steps. The xsltproc program must
79 *** A C++ native GLR parser
81 A new version of the C++ GLR parser was added: "glr2.cc". It generates
82 "true C++11", instead of a C++ wrapper around a C parser as does the
83 existing "glr.cc" parser. As a first significant consequence, it supports
84 `%define api.value.type variant`, contrary to glr.cc.
86 It should be upward compatible in terms of interface, feature and
87 performance to "glr.cc". To try it out, simply use
91 It will eventually replace "glr.cc". However we need user feedback on
92 this skeleton. _Please_ report your results and comments about it.
96 Counterexamples now show the rule numbers, and always show ε for rules
97 with an empty right-hand side. For instance
109 *** Lookahead correction in Java
111 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
114 *** Abort parsing for memory exhaustion (C)
116 User actions may now use `YYNOMEM` (similar to `YYACCEPT` and `YYABORT`)
117 to abort the current parse with memory exhaustion.
119 *** Printing locations in debug traces (C)
121 The `YYLOCATION_PRINT(File, Loc)` macro prints a location. It is defined
122 when (i) locations are enabled, (ii) the default type for locations is
123 used, (iii) debug traces are enabled, and (iv) `YYLOCATION_PRINT` is not
126 Users may define `YYLOCATION_PRINT` to cover other cases.
130 There were no debug traces for deferred calls to user actions. They are
134 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
138 *** Reused Push Parsers
140 When a push-parser state structure is used for multiple parses, it was
141 possible for some state to leak from one run into the following one.
143 *** Fix Table Generation
145 In some very rare conditions, when there are many useless tokens, it was
146 possible to generate incorrect parsers.
149 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
153 *** Counterexample Generation
155 In some cases counterexample generation could crash. This is fixed.
157 *** Fix Table Generation
159 In some very rare conditions, when there are many useless tokens, it was
160 possible to generate incorrect parsers.
162 *** GLR parsers now support %merge together with api.value.type=union.
164 *** C++ parsers use noexcept in more places.
166 *** Generated parsers avoid some warnings about signedness issues.
168 *** C-language parsers now avoid warnings from pedantic clang.
170 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
173 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
177 *** Bug fixes in yacc.c
179 In Yacc mode, all the tokens are defined twice: once as an enum, and then
180 as a macro. YYEMPTY was missing its macro.
182 *** Bug fixes in lalr1.cc
184 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
185 even when the `parse.assert` %define variable is not enabled. It no
188 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
191 When there is a very large number of tokens, some assertions could be long
192 enough to hit arbitrary limits in Visual C++. They have been rewritten to
193 work around this limitation.
197 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
198 skeleton) used to be defined to 1. It is now defined to the version of
199 Bison as an integer (e.g., 30704 for version 3.7.4).
202 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
206 Fix concurrent build issues.
208 The bison executable is no longer linked uselessly against libreadline.
210 Fix incorrect use of yytname in glr.cc.
213 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
215 This release of Bison fixes all known bugs reported for Bison in MITRE's
216 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
217 are only about bison-the-program itself, not the generated code.
219 Although these bugs are typically irrelevant to how Bison is used, they
220 are worth fixing if only to give users peace of mind.
222 There is no known vulnerability in the generated parsers.
226 Fix concurrent build issues (introduced in Bison 3.5).
228 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
230 Fix portability issues of the test suite, and of bison itself.
232 Some unlikely crashes found by fuzzing have been fixed. This is only
233 about bison itself, not the generated parsers.
236 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
240 Crash when a token alias contains a NUL byte.
242 Portability issues with libtextstyle.
244 Portability issues of Bison itself with MSVC.
248 Improvements and fixes in the documentation.
250 More precise location about symbol type redefinitions.
253 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
255 ** Deprecated features
257 The YYPRINT macro, which works only with yacc.c and only for tokens, was
258 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
259 It is deprecated and its support will be removed eventually.
261 In conformance with the recommendations of the Graphviz team, in the next
262 version Bison the option `--graph` will generate a *.gv file by default,
263 instead of *.dot. A transition started in Bison 3.4.
267 *** Counterexample Generation
269 Contributed by Vincent Imbimbo.
271 When given `-Wcounterexamples`/`-Wcex`, bison will now output
272 counterexamples for conflicts.
274 **** Unifying Counterexamples
276 Unifying counterexamples are strings which can be parsed in two ways due
277 to the conflict. For example on a grammar that contains the usual
278 "dangling else" ambiguity:
281 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
282 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
285 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
286 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
287 Example: "if" exp "then" "if" exp "then" exp • "else" exp
290 ↳ "if" exp "then" exp
291 ↳ "if" exp "then" exp • "else" exp
292 Example: "if" exp "then" "if" exp "then" exp • "else" exp
295 ↳ "if" exp "then" exp "else" exp
296 ↳ "if" exp "then" exp •
298 When text styling is enabled, colors are used in the examples and the
299 derivations to highlight the structure of both analyses. In this case,
301 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
305 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
308 The counterexamples are "focused", in two different ways. First, they do
309 not clutter the output with all the derivations from the start symbol,
310 rather they start on the "conflicted nonterminal". They go straight to the
311 point. Second, they don't "expand" nonterminal symbols uselessly.
313 **** Nonunifying Counterexamples
315 In the case of the dangling else, Bison found an example that can be
316 parsed in two ways (therefore proving that the grammar is ambiguous).
317 When it cannot find such an example, it instead generates two examples
318 that are the same up until the dot:
321 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
322 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
323 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
328 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
329 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
330 First example: expr • ID ',' ID $end
337 Second example: expr • ID $end
343 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
347 In these cases, the parser usually doesn't have enough lookahead to
348 differentiate the two given examples.
352 Counterexamples are also included in the report when given
353 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
358 1 exp: "if" exp "then" exp • [$end, "then", "else"]
359 2 | "if" exp "then" exp • "else" exp
361 "else" shift, and go to state 8
363 "else" [reduce using rule 1 (exp)]
364 $default reduce using rule 1 (exp)
366 shift/reduce conflict on token "else":
367 1 exp: "if" exp "then" exp •
368 2 exp: "if" exp "then" exp • "else" exp
369 Example: "if" exp "then" "if" exp "then" exp • "else" exp
372 ↳ "if" exp "then" exp
373 ↳ "if" exp "then" exp • "else" exp
374 Example: "if" exp "then" "if" exp "then" exp • "else" exp
377 ↳ "if" exp "then" exp "else" exp
378 ↳ "if" exp "then" exp •
380 *** File prefix mapping
382 Contributed by Joshua Watt.
384 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
385 in the output (specifically `#line` directives and `#ifdef` header guards)
386 that begins with the prefix OLD will have it replaced with the prefix NEW,
387 similar to the `-ffile-prefix-map` in GCC. This option can be used to
388 make bison output reproducible.
394 When text styling is enabled and the terminal supports it, the warnings
395 now include hyperlinks to the documentation.
397 *** Relocatable installation
399 When installed to be relocatable (via `configure --enable-relocatable`),
400 bison will now also look for a relocated m4.
404 The `filename_type` %define variable was renamed `api.filename.type`.
407 %define filename_type "symbol"
411 %define api.filename.type {symbol}
413 (Or let `bison --update` do it for you).
415 It now defaults to `const std::string` instead of `std::string`.
417 *** Deprecated %define variable names
419 The following variables have been renamed for consistency. Backward
420 compatibility is ensured, but upgrading is recommended.
422 filename_type -> api.filename.type
423 package -> api.package
425 *** Push parsers no longer clear their state when parsing is finished
427 Previously push-parsers cleared their state when parsing was finished (on
428 success and on failure). This made it impossible to check if there were
429 parse errors, since `yynerrs` was also reset. This can be especially
430 troublesome when used in autocompletion, since a parser with error
431 recovery would suggest (irrelevant) expected tokens even if there were
434 Now the parser state can be examined when parsing is finished. The parser
435 state is reset when starting a new parse.
441 The bistromathic demonstrates %param and how to quote sources in the error
445 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
451 *** Include the generated header (yacc.c)
453 Historically, when --defines was used, bison generated a header and pasted
454 an exact copy of it into the generated parser implementation file. Since
455 Bison 3.4 it is possible to specify that the header should be `#include`d,
456 and how. For instance
458 %define api.header.include {"parse.h"}
462 %define api.header.include {<parser/parse.h>}
464 Now api.header.include defaults to `"header-basename"`, as was intended in
465 Bison 3.4, where `header-basename` is the basename of the generated
466 header. This is disabled when the generated header is `y.tab.h`, to
467 comply with Automake's ylwrap.
469 *** String aliases are faithfully propagated
471 Bison used to interpret user strings (i.e., decoding backslash escapes)
472 when reading them, and to escape them (i.e., issue non-printable
473 characters as backslash escapes, taking the locale into account) when
474 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
475 up "ciphered" as sequences of backslash escapes. This happened not only
476 in the generated sources (where the compiler will reinterpret them), but
477 also in all the generated reports (text, xml, html, dot, etc.). Reports
478 were therefore not readable when string aliases were not pure ASCII.
479 Worse yet: the output depended on the user's locale.
481 Now Bison faithfully treats the string aliases exactly the way the user
482 spelled them. This fixes all the aforementioned problems. However, now,
483 string aliases semantically equivalent but syntactically different (e.g.,
484 "A", "\x41", "\101") are considered to be different.
486 *** Crash when generating IELR
488 An old, well hidden, bug in the generation of IELR parsers was fixed.
491 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
495 In glr.cc some internal macros leaked in the user's code, and could damage
496 access to the token kinds.
499 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
503 Incorrect comments in the generated parsers.
505 Warnings in push parsers (yacc.c).
507 Incorrect display of gotos in LAC traces (lalr1.cc).
510 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
514 Some tests were fixed.
516 When token aliases contain comment delimiters:
518 %token FOO "/* foo */"
520 bison used to emit "nested" comments, which is invalid C.
523 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
527 Restored ANSI-C compliance in yacc.c.
529 GNU readline portability issues.
531 In C++, yy::parser::symbol_name is now a public member, as was intended.
535 In C++, yy::parser::symbol_type now has a public name() member function.
538 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
540 ** Backward incompatible changes
542 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
544 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
545 depend on it will now produce Yacc-like error messages (just "syntax
546 error"). It was superseded by the "%error-verbose" directive in Bison
547 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
548 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
549 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
550 parse.error verbose".
552 ** Deprecated features
554 The YYPRINT macro, which works only with yacc.c and only for tokens, was
555 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
556 It is deprecated and its support will be removed eventually.
560 *** Improved syntax error messages
562 Two new values for the %define parse.error variable offer more control to
563 the user. Available in all the skeletons (C, C++, Java).
565 **** %define parse.error detailed
567 The behavior of "%define parse.error detailed" is closely resembling that
568 of "%define parse.error verbose" with a few exceptions. First, it is safe
569 to use non-ASCII characters in token aliases (with 'verbose', the result
570 depends on the locale with which bison was run). Second, a yysymbol_name
571 function is exposed to the user, instead of the yytnamerr function and the
572 yytname table. Third, token internationalization is supported (see
575 **** %define parse.error custom
577 With this directive, the user forges and emits the syntax error message
578 herself by defining the yyreport_syntax_error function. A new type,
579 yypcontext_t, captures the circumstances of the error, and provides the
580 user with functions to get details, such as yypcontext_expected_tokens to
581 get the list of expected token kinds.
583 A possible implementation of yyreport_syntax_error is:
586 yyreport_syntax_error (const yypcontext_t *ctx)
589 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
590 fprintf (stderr, ": syntax error");
591 // Report the tokens expected at this point.
593 enum { TOKENMAX = 10 };
594 yysymbol_kind_t expected[TOKENMAX];
595 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
597 // Forward errors to yyparse.
600 for (int i = 0; i < n; ++i)
601 fprintf (stderr, "%s %s",
602 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
604 // Report the unexpected token.
606 yysymbol_kind_t lookahead = yypcontext_token (ctx);
607 if (lookahead != YYSYMBOL_YYEMPTY)
608 fprintf (stderr, " before %s", yysymbol_name (lookahead));
610 fprintf (stderr, "\n");
614 **** Token aliases internationalization
616 When the %define variable parse.error is set to `custom` or `detailed`,
617 one may specify which token aliases are to be translated using _(). For
629 In that case the user must define _() and N_(), and yysymbol_name returns
630 the translated symbol (i.e., it returns '_("variable")' rather that
631 '"variable"'). In Java, the user must provide an i18n() function.
633 *** List of expected tokens (yacc.c)
635 Push parsers may invoke yypstate_expected_tokens at any point during
636 parsing (including even before submitting the first token) to get the list
637 of possible tokens. This feature can be used to propose autocompletion
638 (see below the "bistromathic" example).
640 It makes little sense to use this feature without enabling LAC (lookahead
643 *** Returning the error token
645 When the scanner returns an invalid token or the undefined token
646 (YYUNDEF), the parser generates an error message and enters error
647 recovery. Because of that error message, most scanners that find lexical
648 errors generate an error message, and then ignore the invalid input
649 without entering the error-recovery.
651 The scanners may now return YYerror, the error token, to enter the
652 error-recovery mode without triggering an additional error message. See
653 the bistromathic for an example.
655 *** Deep overhaul of the symbol and token kinds
657 To avoid the confusion with types in programming languages, we now refer
658 to token and symbol "kinds" instead of token and symbol "types". The
659 documentation and error messages have been revised.
661 All the skeletons have been updated to use dedicated enum types rather
662 than integral types. Special symbols are now regular citizens, instead of
663 being declared in ad hoc ways.
667 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
668 LPAREN, etc. While backward compatibility is of course ensured, users are
669 nonetheless invited to replace their uses of "enum yytokentype" by
672 This type now also includes tokens that were previously hidden: YYEOF (end
673 of input), YYUNDEF (undefined token), and YYerror (error token). They
674 now have string aliases, internationalized when internationalization is
675 enabled. Therefore, by default, error messages now refer to "end of file"
676 (internationalized) rather than the cryptic "$end", or to "invalid token"
677 rather than "$undefined".
679 Therefore in most cases it is now useless to define the end-of-line token
682 %token T_EOF 0 "end of file"
684 Rather simply use "YYEOF" in your scanner.
688 The "symbol kinds" is what the parser actually uses. (Unless the
689 api.token.raw %define variable is used, the symbol kind of a terminal
690 differs from the corresponding token kind.)
692 They are now exposed as a enum, "yysymbol_kind_t".
694 This allows users to tailor the error messages the way they want, or to
695 process some symbols in a specific way in autocompletion (see the
696 bistromathic example below).
698 *** Modernize display of explanatory statements in diagnostics
700 Since Bison 2.7, output was indented four spaces for explanatory
701 statements. For example:
703 input.y:2.7-13: error: %type redeclaration for exp
704 input.y:1.7-11: previous declaration
706 Since the introduction of caret-diagnostics, it became less clear. This
707 indentation has been removed and submessages are displayed similarly as in
710 input.y:2.7-13: error: %type redeclaration for exp
711 2 | %type <float> exp
713 input.y:1.7-11: note: previous declaration
717 Contributed by Victor Morales Cayuela.
721 The token and symbol kinds are yy::parser::token_kind_type and
722 yy::parser::symbol_kind_type.
724 The symbol_type::kind() member function allows to get the kind of a
725 symbol. This can be used to write unit tests for scanners, e.g.,
727 yy::parser::symbol_type t = make_NUMBER ("123");
728 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
729 assert (t.value.as<int> () == 123);
735 In order to avoid ambiguities with "type" as in "typing", we now refer to
736 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
737 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
742 There are now examples/java: a very simple calculator, and a more complete
743 one (push-parser, location tracking, and debug traces).
745 The lexcalc example (a simple example in C based on Flex and Bison) now
746 also demonstrates location tracking.
749 A new C example, bistromathic, is a fully featured interactive calculator
750 using many Bison features: pure interface, push parser, autocompletion
751 based on the current parser state (using yypstate_expected_tokens),
752 location tracking, internationalized custom error messages, lookahead
753 correction, rich debug traces, etc.
755 It shows how to depend on the symbol kinds to tailor autocompletion. For
756 instance it recognizes the symbol kind "VARIABLE" to propose
757 autocompletion on the existing variables, rather than of the word
761 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
763 ** WARNING: Future backward-incompatibilities!
765 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
767 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
768 that still depend on it will produce Yacc-like error messages (just
769 "syntax error"). It was superseded by the "%error-verbose" directive in
770 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
771 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
772 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
773 parse.error verbose".
777 Fix portability issues of the package itself on old compilers.
779 Fix api.token.raw support in Java.
782 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
786 Error messages could quote lines containing zero-width characters (such as
787 \005) with incorrect styling. Fixes for similar issues with unexpectedly
788 short lines (e.g., the file was changed between parsing and diagnosing).
790 Some unlikely crashes found by fuzzing have been fixed. This is only
791 about bison itself, not the generated parsers.
794 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
798 Portability issues and minor cosmetic issues.
800 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
804 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
810 Fix compiler warnings.
813 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
815 ** Backward incompatible changes
817 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
818 longer treated as end-of-lines. This changes the diagnostics, and in
819 particular their locations.
821 In C++, line numbers and columns are now represented as 'int' not
822 'unsigned', so that integer overflow on positions is easily checkable via
823 'gcc -fsanitize=undefined' and the like. This affects the API for
824 positions. The default position and location classes now expose
825 'counter_type' (int), used to define line and column numbers.
827 ** Deprecated features
829 The YYPRINT macro, which works only with yacc.c and only for tokens, was
830 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
831 It is deprecated and its support will be removed eventually.
835 *** Lookahead correction in C++
837 Contributed by Adrian Vogelsgesang.
839 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
840 %define variable parse.lac.
842 *** Variable api.token.raw: Optimized token numbers (all skeletons)
844 In the generated parsers, tokens have two numbers: the "external" token
845 number as returned by yylex (which starts at 257), and the "internal"
846 symbol number (which starts at 3). Each time yylex is called, a table
847 lookup maps the external token number to the internal symbol number.
849 When the %define variable api.token.raw is set, tokens are assigned their
850 internal number, which saves one table lookup per token, and also saves
851 the generation of the mapping table.
853 The gain is typically moderate, but in extreme cases (very simple user
854 actions), a 10% improvement can be observed.
856 *** Generated parsers use better types for states
858 Stacks now use the best integral type for state numbers, instead of always
859 using 15 bits. As a result "small" parsers now have a smaller memory
860 footprint (they use 8 bits), and there is support for large automata (16
861 bits), and extra large (using int, i.e., typically 31 bits).
863 *** Generated parsers prefer signed integer types
865 Bison skeletons now prefer signed to unsigned integer types when either
866 will do, as the signed types are less error-prone and allow for better
867 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
868 portable to unusual machines where char, short and int are all the same
869 width. On non-GNU platforms this may entail including <limits.h> and (if
870 available) <stdint.h> to define integer types and constants.
872 *** A skeleton for the D programming language
874 For the last few releases, Bison has shipped a stealth experimental
875 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
876 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
879 However, because nobody has committed to improving, testing, and
880 documenting this skeleton, it is not clear that it will be supported in
883 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
884 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
886 *** Debug traces in Java
888 The Java backend no longer emits code and data for parser tracing if the
889 %define variable parse.trace is not defined.
893 *** New diagnostic: -Wdangling-alias
895 String literals, which allow for better error messages, are (too)
896 liberally accepted by Bison, which might result in silent errors. For
899 %type <exVal> cond "condition"
901 does not define "condition" as a string alias to 'cond' (nonterminal
902 symbols do not have string aliases). It is rather equivalent to
905 %token <exVal> "condition"
907 i.e., it gives the type 'exVal' to the "condition" token, which was
908 clearly not the intention.
910 Also, because string aliases need not be defined, typos such as "baz"
911 instead of "bar" will be not reported.
913 The option `-Wdangling-alias` catches these situations. On
916 %type <ival> foo "foo"
920 bison -Wdangling-alias reports
922 warning: string literal not attached to a symbol
923 | %type <ival> foo "foo"
925 warning: string literal not attached to a symbol
929 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
931 *** Better POSIX Yacc compatibility diagnostics
933 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
937 %type <ival> TOKEN1 TOKEN2 't'
944 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
945 2 | %type <ival> TOKEN1 TOKEN2 't'
947 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
948 2 | %type <ival> TOKEN1 TOKEN2 't'
950 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
951 2 | %type <ival> TOKEN1 TOKEN2 't'
954 *** Diagnostics with insertion
956 The diagnostics now display the suggestion below the underlined source.
957 Replacement for undeclared symbols are now also suggested.
964 foo.y:2.7-9: error: symbol 'lis' is used, but is not defined as a token and has no rules; did you mean 'list'?
968 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
972 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
974 *** Diagnostics about long lines
976 Quoted sources may now be truncated to fit the screen. For instance, on a
977 30-column wide terminal:
984 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
987 foo.y:1.8-10: previous declaration
990 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
993 foo.y:1.8-10: previous declaration
999 *** Debugging glr.c and glr.cc
1001 The glr.c skeleton always had asserts to check its own behavior (not the
1002 user's). These assertions are now under the control of the parse.assert
1003 %define variable (disabled by default).
1007 Several new compiler warnings in the generated output have been avoided.
1008 Some unused features are no longer emitted. Cleaner generated code in
1013 Portability issues in the test suite.
1015 In theory, parsers using %nonassoc could crash when reporting verbose
1016 error messages. This unlikely bug has been fixed.
1018 In Java, %define api.prefix was ignored. It now behaves as expected.
1021 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
1025 In some cases, when warnings are disabled, bison could emit tons of white
1026 spaces as diagnostics.
1028 When running out of memory, bison could crash (found by fuzzing).
1030 When defining twice the EOF token, bison would crash.
1032 New warnings from recent compilers have been addressed in the generated
1033 parsers (yacc.c, glr.c, glr.cc).
1035 When lone carriage-return characters appeared in the input file,
1036 diagnostics could hang forever.
1039 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
1046 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
1048 ** Deprecated features
1050 The %pure-parser directive is deprecated in favor of '%define api.pure'
1051 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
1052 now. Note that since Bison 2.7 you are strongly encouraged to use
1053 '%define api.pure full' instead of '%define api.pure'.
1057 *** Colored diagnostics
1059 As an experimental feature, diagnostics are now colored, controlled by the
1060 new options --color and --style.
1062 To use them, install the libtextstyle library before configuring Bison.
1063 It is available from
1065 https://alpha.gnu.org/gnu/gettext/
1069 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
1071 The option --color supports the following arguments:
1072 - always, yes: Enable colors.
1073 - never, no: Disable colors.
1074 - auto, tty (default): Enable colors if the output device is a tty.
1076 To customize the styles, create a CSS file similar to
1080 .error { font-weight: 800; text-decoration: underline; }
1083 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1084 environment variable to "bison-bw.css".
1086 *** Disabling output
1088 When given -fsyntax-only, the diagnostics are reported, but no output is
1091 The name of this option is somewhat misleading as bison does more than
1092 just checking the syntax: every stage is run (including checking for
1093 conflicts for instance), except the generation of the output files.
1095 *** Include the generated header (yacc.c)
1097 Before, when --defines is used, bison generated a header, and pasted an
1098 exact copy of it into the generated parser implementation file. If the
1099 header name is not "y.tab.h", it is now #included instead of being
1102 To use an '#include' even if the header name is "y.tab.h" (which is what
1103 happens with --yacc, or when using the Autotools' ylwrap), define
1104 api.header.include to the exact argument to pass to #include. For
1107 %define api.header.include {"parse.h"}
1111 %define api.header.include {<parser/parse.h>}
1113 *** api.location.type is now supported in C (yacc.c, glr.c)
1115 The %define variable api.location.type defines the name of the type to use
1116 for locations. When defined, Bison no longer defines YYLTYPE.
1118 This can be used in programs with several parsers to factor their
1119 definition of locations: let one of them generate them, and the others
1126 In conformance with the recommendations of the Graphviz team, if %require
1127 "3.4" (or better) is specified, the option --graph generates a *.gv file
1128 by default, instead of *.dot.
1130 *** Diagnostics overhaul
1132 Column numbers were wrong with multibyte characters, which would also
1133 result in skewed diagnostics with carets. Beside, because we were
1134 indenting the quoted source with a single space, lines with tab characters
1135 were incorrectly underlined.
1137 To address these issues, and to be clearer, Bison now issues diagnostics
1138 as GCC9 does. For instance it used to display (there's a tab before the
1141 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1142 expr: expr '+' "number" { $$ = $1 + $2; }
1146 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1147 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1150 Other constructs now also have better locations, resulting in more precise
1153 *** Fix-it hints for %empty
1155 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1156 annotations, and add the missing ones.
1158 *** Generated reports
1160 The format of the reports (parse.output) was improved for readability.
1162 *** Better support for --no-line.
1164 When --no-line is used, the generated files are now cleaner: no lines are
1165 generated instead of empty lines. Together with using api.header.include,
1166 that should help people saving the generated files into version control
1167 systems get smaller diffs.
1171 A new example in C shows an simple infix calculator with a hand-written
1172 scanner (examples/c/calc).
1174 A new example in C shows a reentrant parser (capable of recursive calls)
1175 built with Flex and Bison (examples/c/reccalc).
1177 There is a new section about the history of Yaccs and Bison.
1181 A few obscure bugs were fixed, including the second oldest (known) bug in
1182 Bison: it was there when Bison was entered in the RCS version control
1183 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1187 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1191 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1195 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1199 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1200 extensions into errors. It now makes them simple warnings (-Wyacc).
1203 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1205 A new mailing list was created, Bison Announce. It is low traffic, and is
1206 only about announcing new releases and important messages (e.g., polls
1207 about major decisions to make).
1209 https://lists.gnu.org/mailman/listinfo/bison-announce
1211 ** Backward incompatible changes
1213 Support for DJGPP, which has been unmaintained and untested for years, is
1216 ** Deprecated features
1218 A new feature, --update (see below) helps adjusting existing grammars to
1221 *** Deprecated directives
1223 The %error-verbose directive is deprecated in favor of '%define
1224 parse.error verbose' since Bison 3.0, but no warning was issued.
1226 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1227 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1228 directives are slightly different, you might need to adjust your code.
1229 %name-prefix renames only symbols with external linkage, while api.prefix
1230 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1231 yytokentype, YYSTYPE, YYLTYPE, etc.
1233 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1234 {xx}' will typically have to update YY_DECL from
1236 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1240 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1242 *** Deprecated %define variable names
1244 The following variables, mostly related to parsers in Java, have been
1245 renamed for consistency. Backward compatibility is ensured, but upgrading
1248 abstract -> api.parser.abstract
1249 annotations -> api.parser.annotations
1250 extends -> api.parser.extends
1251 final -> api.parser.final
1252 implements -> api.parser.implements
1253 parser_class_name -> api.parser.class
1254 public -> api.parser.public
1255 strictfp -> api.parser.strictfp
1259 *** Generation of fix-its for IDEs/Editors
1261 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1262 bison now generates machine readable editing instructions to fix some
1263 issues. Currently, this is mostly limited to updating deprecated
1264 directives and removing duplicates. For instance:
1268 %define parser_class_name "Parser"
1269 %define api.parser.class "Parser"
1273 See the "fix-it:" lines below:
1275 $ bison -ffixit foo.y
1276 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1279 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1280 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1281 %define parser_class_name "Parser"
1282 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1283 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1284 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1285 %define api.parser.class "Parser"
1286 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1287 foo.y:2.1-34: previous definition
1288 %define parser_class_name "Parser"
1289 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1290 fix-it:"foo.y":{3:1-3:34}:""
1291 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1293 This uses the same output format as GCC and Clang.
1295 *** Updating grammar files
1297 Fixes can be applied on the fly. The previous example ends with the
1298 suggestion to re-run bison with the option -u/--update, which results in a
1299 cleaner grammar file.
1301 $ bison --update foo.y
1303 bison: file 'foo.y' was updated (backup: 'foo.y~')
1306 %define parse.error verbose
1307 %define api.parser.class {Parser}
1311 *** Bison is now relocatable
1313 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1315 A relocatable program can be moved or copied to a different location on
1316 the file system. It can also be used through mount points for network
1317 sharing. It is possible to make symbolic links to the installed and moved
1318 programs, and invoke them through the symbolic link.
1320 *** %expect and %expect-rr modifiers on individual rules
1322 One can now document (and check) which rules participate in shift/reduce
1323 and reduce/reduce conflicts. This is particularly important GLR parsers,
1324 where conflicts are a normal occurrence. For example,
1340 | argument_list ',' expression
1345 Looking at the output from -v, one can see that the shift/reduce conflict
1346 here is due to the fact that the parser does not know whether to reduce
1347 arguments to argument_list until it sees the token _after_ the following
1348 ','. By marking the rule with %expect 1 (because there is a conflict in
1349 one state), we document the source of the 1 overall shift/reduce conflict.
1351 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1352 conflicts. In this case, we mark each of the conflicting rules. For
1361 target_list '=' expr ';'
1367 | target ',' target_list
1376 | expr ',' expr_list
1384 In a statement such as
1388 the parser must reduce x to a target or an expr, but does not know which
1389 until it sees the '='. So we notate the two possible reductions to
1390 indicate that each conflicts in one rule.
1392 This feature needs user feedback, and might evolve in the future.
1394 *** C++: Actual token constructors
1396 When variants and token constructors are enabled, in addition to the
1397 type-safe named token constructors (make_ID, make_INT, etc.), we now
1398 generate genuine constructors for symbol_type.
1400 For instance with these declarations
1406 you may use these constructors:
1408 symbol_type (int token, const std::string&);
1409 symbol_type (int token, const int&);
1410 symbol_type (int token);
1412 Correct matching between token types and value types is checked via
1413 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1414 constructors are preferable, as they offer better type safety (for
1415 instance 'make_ID (42)' would not even compile), but symbol_type
1416 constructors may help when token types are discovered at run-time, e.g.,
1419 if (auto i = lookup_keyword (yytext))
1420 return yy::parser::symbol_type (i);
1422 return yy::parser::make_ID (yytext);
1425 *** C++: Variadic emplace
1427 If your application requires C++11 and you don't use symbol constructors,
1428 you may now use a variadic emplace for semantic values:
1430 %define api.value.type variant
1431 %token <std::pair<int, int>> PAIR
1435 int yylex (parser::semantic_type *lvalp)
1437 lvalp->emplace <std::pair<int, int>> (1, 2);
1438 return parser::token::PAIR;
1441 *** C++: Syntax error exceptions in GLR
1443 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1444 actions, or from the scanner.
1446 *** More POSIX Yacc compatibility warnings
1448 More Bison specific directives are now reported with -y or -Wyacc. This
1449 change was ready since the release of Bison 3.0 in September 2015. It was
1450 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1451 in numerous warnings for Bison users that use the GNU Build System.
1453 If you still experience that problem, either redefine YACC as `bison -o
1454 y.tab.c`, or pass -Wno-yacc to Bison.
1456 *** The tables yyrhs and yyphrs are back
1458 Because no Bison skeleton uses them, these tables were removed (no longer
1459 passed to the skeletons, not even computed) in 2008. However, some users
1460 have expressed interest in being able to use them in their own skeletons.
1464 *** Incorrect number of reduce/reduce conflicts
1466 On a grammar such as
1468 exp: "num" | "num" | "num"
1470 bison used to report a single RR conflict, instead of two. This is now
1471 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1472 was entered in the RCS version control system, in December 1987.
1474 Some grammar files might have to adjust their %expect-rr.
1476 *** Parser directives that were not careful enough
1478 Passing invalid arguments to %nterm, for instance character literals, used
1479 to result in unclear error messages.
1483 The examples/ directory (installed in .../share/doc/bison/examples) has
1484 been restructured per language for clarity. The examples come with a
1485 README and a Makefile. Not only can they be used to toy with Bison, they
1486 can also be starting points for your own grammars.
1488 There is now a Java example, and a simple example in C based on Flex and
1489 Bison (examples/c/lexcalc/).
1495 They now use noexcept and constexpr. Please, report missing annotations.
1497 *** Symbol Declarations
1499 The syntax of the variation directives to declare symbols was overhauled
1500 for more consistency, and also better POSIX Yacc compliance (which, for
1501 instance, allows "%type" without actually providing a type). The %nterm
1502 directive, supported by Bison since its inception, is now documented and
1503 officially supported.
1505 The syntax is now as follows:
1507 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1508 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1509 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1510 %nterm TAG? ID+ ( TAG ID+ )*
1512 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1513 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1514 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1515 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1516 one), ‘*’ (zero or more) and ‘+’ (one or more).
1519 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1523 Fix the move constructor of symbol_type.
1525 Always provide a copy constructor for symbol_type, even in modern C++.
1528 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1532 Properly support token constructors in C++ with types that include commas
1533 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1536 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1540 C++ portability issues.
1543 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1547 Several portability issues have been fixed in the build system, in the
1548 test suite, and in the generated parsers in C++.
1551 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1553 ** Backward incompatible changes
1555 Support for DJGPP, which has been unmaintained and untested for years, is
1556 obsolete. Unless there is activity to revive it, it will be removed.
1560 %printers should use yyo rather than yyoutput to denote the output stream.
1562 Variant-based symbols in C++ should use emplace() rather than build().
1564 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1568 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1570 A comment in the generated code now emphasizes that users should not
1571 depend upon non-documented implementation details, such as macros starting
1576 *** C++: Support for move semantics (lalr1.cc)
1578 The lalr1.cc skeleton now fully supports C++ move semantics, while
1579 maintaining compatibility with C++98. You may now store move-only types
1580 when using Bison's variants. For instance:
1587 %skeleton "lalr1.cc"
1588 %define api.value.type variant
1592 %token <int> INT "int";
1593 %type <std::unique_ptr<int>> int;
1594 %type <std::vector<std::unique_ptr<int>>> list;
1598 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1600 int: "int" { $$ = std::make_unique<int>($1); }
1602 *** C++: Implicit move of right-hand side values (lalr1.cc)
1604 In modern C++ (C++11 and later), you should always use 'std::move' with
1605 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1606 popped from the stack anyway. Using 'std::move' is mandatory for
1607 move-only types such as unique_ptr, and it provides a significant speedup
1608 for large types such as std::string, or std::vector, etc.
1610 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1611 by 'std::move ($n)'. The second rule in the previous grammar can be
1614 list: list int { $$ = $1; $$.emplace_back($2); }
1616 With automove enabled, the semantic values are no longer lvalues, so do
1617 not use the swap idiom:
1619 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1621 This idiom is anyway obsolete: it is preferable to move than to swap.
1623 A warning is issued when automove is enabled, and a value is used several
1626 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1627 exp: "twice" exp { $$ = $2 + $2; }
1630 Enabling api.value.automove does not require support for modern C++. The
1631 generated code is valid C++98/03, but will use copies instead of moves.
1633 The new examples/c++/variant-11.yy shows these features in action.
1635 *** C++: The implicit default semantic action is always run
1637 When variants are enabled, the default action was not run, so
1645 It now behaves like in all the other cases, as
1647 exp: "number" { $$ = $1; }
1649 possibly using std::move if automove is enabled.
1651 We do not expect backward compatibility issues. However, beware of
1652 forward compatibility issues: if you rely on default actions with
1653 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1654 generate incorrect parsers.
1656 *** C++: Renaming location.hh
1658 When both %defines and %locations are enabled, Bison generates a
1659 location.hh file. If you don't use locations outside of the parser, you
1660 may avoid its creation with:
1662 %define api.location.file none
1664 However this file is useful if, for instance, your parser builds an AST
1665 decorated with locations: you may use Bison's location independently of
1666 Bison's parser. You can now give it another name, for instance:
1668 %define api.location.file "my-location.hh"
1670 This name can have directory components, and even be absolute. The name
1671 under which the location file is included is controlled by
1672 api.location.include.
1674 This way it is possible to have several parsers share the same location
1677 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1680 %define api.namespace {foo}
1681 %define api.location.file "include/ast/loc.hh"
1682 %define api.location.include {<ast/loc.hh>}
1684 and use it in src/bar/parser.hh:
1687 %define api.namespace {bar}
1688 %code requires {#include <ast/loc.hh>}
1689 %define api.location.type {bar::location}
1691 Absolute file names are supported, so in your Makefile, passing the flag
1692 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1695 *** C++: stack.hh and position.hh are deprecated
1697 When asked to generate a header file (%defines), the lalr1.cc skeleton
1698 generates a stack.hh file. This file had no interest for users; it is now
1699 made useless: its content is included in the parser definition. It is
1700 still generated for backward compatibility.
1702 When in addition to %defines, location support is requested (%locations),
1703 the file position.hh is also generated. It is now also useless: its
1704 content is now included in location.hh.
1706 These files are no longer generated when your grammar file requires at
1707 least Bison 3.2 (%require "3.2").
1711 Portability issues on MinGW and VS2015.
1713 Portability issues in the test suite.
1715 Portability/warning issues with Flex.
1718 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1720 ** Backward incompatible changes
1722 Compiling Bison now requires a C99 compiler---as announced during the
1723 release of Bison 3.0, five years ago. Generated parsers do not require a
1726 Support for DJGPP, which has been unmaintained and untested for years, is
1727 obsolete. Unless there is activity to revive it, the next release of Bison
1728 will have it removed.
1732 *** Typed midrule actions
1734 Because their type is unknown to Bison, the values of midrule actions are
1735 not treated like the others: they don't have %printer and %destructor
1736 support. It also prevents C++ (Bison) variants to handle them properly.
1738 Typed midrule actions address these issues. Instead of:
1740 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1744 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1746 *** Reports include the type of the symbols
1748 The sections about terminal and nonterminal symbols of the '*.output' file
1749 now specify their declared type. For instance, for:
1753 the report now shows '<ival>':
1755 Terminals, with rules where they appear
1759 *** Diagnostics about useless rules
1761 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1762 of course, its rules are useless too.
1766 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1768 Previously all the useless rules were reported, including those whose
1769 left-hand side is the 'exp' nonterminal:
1771 warning: 1 nonterminal useless in grammar [-Wother]
1772 warning: 4 rules useless in grammar [-Wother]
1773 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1776 2.14-16: warning: rule useless in grammar [-Wother]
1779 3.6-16: warning: rule useless in grammar [-Wother]
1780 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1782 3.20-30: warning: rule useless in grammar [-Wother]
1783 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1785 3.34-44: warning: rule useless in grammar [-Wother]
1786 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1789 Now, rules whose left-hand side symbol is useless are no longer reported
1790 as useless. The locations of the errors have also been adjusted to point
1791 to the first use of the nonterminal as a left-hand side of a rule:
1793 warning: 1 nonterminal useless in grammar [-Wother]
1794 warning: 4 rules useless in grammar [-Wother]
1795 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1796 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1798 2.14-16: warning: rule useless in grammar [-Wother]
1802 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1804 When compiled with exceptions disabled, the generated parsers no longer
1805 uses try/catch clauses.
1807 Currently only GCC and Clang are supported.
1811 *** A demonstration of variants
1813 A new example was added (installed in .../share/doc/bison/examples),
1814 'variant.yy', which shows how to use (Bison) variants in C++.
1816 The other examples were made nicer to read.
1818 *** Some features are no longer 'experimental'
1820 The following features, mature enough, are no longer flagged as
1821 experimental in the documentation: push parsers, default %printer and
1822 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1823 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1824 semantic predicates (%?).
1828 *** GLR: Predicates support broken by #line directives
1830 Predicates (%?) in GLR such as
1833 %? {new_syntax} 'w' id new_args
1834 | %?{!new_syntax} 'w' id old_args
1836 were issued with #lines in the middle of C code.
1838 *** Printer and destructor with broken #line directives
1840 The #line directives were not properly escaped when emitting the code for
1841 %printer/%destructor, which resulted in compiler errors if there are
1842 backslashes or double-quotes in the grammar file name.
1844 *** Portability on ICC
1846 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1847 Generated parsers now work around this.
1851 There were several small fixes in the test suite and in the build system,
1852 many warnings in bison and in the generated parsers were eliminated. The
1853 documentation also received its share of minor improvements.
1855 Useless code was removed from C++ parsers, and some of the generated
1856 constructors are more 'natural'.
1859 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1863 *** C++: Fix support of 'syntax_error'
1865 One incorrect 'inline' resulted in linking errors about the constructor of
1866 the syntax_error exception.
1868 *** C++: Fix warnings
1870 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1871 warnings about yyformat being possibly null. It also warned about the
1872 deprecated implicit definition of copy constructors when there's a
1873 user-defined (copy) assignment operator.
1875 *** Location of errors
1877 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1878 ride-hand side raises a syntax error. The behavior of the default parser
1879 (yacc.c) in such a condition was undefined.
1881 Now all the parsers match the behavior of glr.c: @$ is used as the
1882 location of the error. This handles gracefully rules with and without
1885 *** Portability fixes in the test suite
1887 On some platforms, some Java and/or C++ tests were failing.
1890 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1894 *** C++ with Variants (lalr1.cc)
1896 Fix a compiler warning when no %destructor use $$.
1900 Several portability issues in tests were fixed.
1903 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1907 *** C++ with Variants (lalr1.cc)
1909 Problems with %destructor and '%define parse.assert' have been fixed.
1911 *** Named %union support (yacc.c, glr.c)
1913 Bison 3.0 introduced a regression on named %union such as
1915 %union foo { int ival; };
1917 The possibility to use a name was introduced "for Yacc compatibility".
1918 It is however not required by POSIX Yacc, and its usefulness is not clear.
1920 *** %define api.value.type union with %defines (yacc.c, glr.c)
1922 The C parsers were broken when %defines was used together with "%define
1923 api.value.type union".
1925 *** Redeclarations are reported in proper order
1933 bison used to report:
1935 foo.yy:2.10-11: error: %printer redeclaration for FOO
1938 foo.yy:3.10-11: previous declaration
1942 Now, the "previous" declaration is always the first one.
1947 Bison now installs various files in its docdir (which defaults to
1948 '/usr/local/share/doc/bison'), including the three fully blown examples
1949 extracted from the documentation:
1952 Reverse Polish Calculator, a simple introductory example.
1954 Multi-function Calc, a calculator with memory and functions and located
1957 a calculator in C++ using variant support and token constructors.
1960 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1964 *** Generated source files when errors are reported
1966 When warnings are issued and -Werror is set, bison would still generate
1967 the source files (*.c, *.h...). As a consequence, some runs of "make"
1968 could fail the first time, but not the second (as the files were generated
1971 This is fixed: bison no longer generates this source files, but, of
1972 course, still produces the various reports (*.output, *.xml, etc.).
1974 *** %empty is used in reports
1976 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1977 dot, XML and formats derived from it).
1979 *** YYERROR and variants
1981 When C++ variant support is enabled, an error triggered via YYERROR, but
1982 not caught via error recovery, resulted in a double deletion.
1985 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1989 *** Errors in caret diagnostics
1991 On some platforms, some errors could result in endless diagnostics.
1993 *** Fixes of the -Werror option
1995 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1996 diagnostics into errors instead of warnings. This is fixed.
1998 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1999 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
2000 -Wno-error", "foo" diagnostics are now errors.
2004 As demonstrated in the documentation, one can now leave spaces between
2009 The yacc.1 man page is no longer installed if --disable-yacc was
2012 *** Fixes in the test suite
2014 Bugs and portability issues.
2017 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
2019 ** WARNING: Future backward-incompatibilities!
2021 Like other GNU packages, Bison will start using some of the C99 features
2022 for its own code, especially the definition of variables after statements.
2023 The generated C parsers still aim at C90.
2025 ** Backward incompatible changes
2027 *** Obsolete features
2029 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
2031 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
2032 use YYSTYPE and YYLTYPE.
2034 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
2035 1.875): use %lex-param, %parse-param, or %param.
2037 Missing semicolons at the end of actions are no longer added (as announced
2038 in the release 2.5).
2040 *** Use of YACC='bison -y'
2042 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
2045 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
2046 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
2047 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
2049 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
2050 implementation of Yacc, was using Bison as 'bison -y'. While it does
2051 ensure compatible output file names, it also enables warnings for
2052 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
2053 warnings for Bison extensions.
2055 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
2056 (which also generates 'y.tab.h' and 'y.output' when needed).
2057 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
2058 flags (YFLAGS, or AM_YFLAGS with Automake).
2062 *** The epilogue is no longer affected by internal #defines (glr.c)
2064 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
2065 generated code. These weren't properly undefined before the inclusion of
2066 the user epilogue, so functions such as the following were butchered by the
2067 preprocessor expansion:
2069 int yylex (YYSTYPE *yylval);
2071 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2072 identifiers for user-provided variables.
2074 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2076 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2077 locations are enabled. This is fixed.
2079 *** Warnings about useless %pure-parser/%define api.pure are restored
2081 ** Diagnostics reported by Bison
2083 Most of these features were contributed by Théophile Ranquet and Victor
2088 Version 2.7 introduced caret errors, for a prettier output. These are now
2089 activated by default. The old format can still be used by invoking Bison
2090 with -fno-caret (or -fnone).
2092 Some error messages that reproduced excerpts of the grammar are now using
2093 the caret information only. For instance on:
2100 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2101 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2105 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2106 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2110 and "bison -fno-caret" reports:
2112 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2113 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2115 *** Enhancements of the -Werror option
2117 The -Werror=CATEGORY option is now recognized, and will treat specified
2118 warnings as errors. The warnings need not have been explicitly activated
2119 using the -W option, this is similar to what GCC 4.7 does.
2121 For example, given the following command line, Bison will treat both
2122 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2123 errors (and only those):
2125 $ bison -Werror=yacc,error=conflicts-sr input.y
2127 If no categories are specified, -Werror will make all active warnings into
2128 errors. For example, the following line does the same the previous example:
2130 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2132 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2134 Note that the categories in this -Werror option may not be prefixed with
2135 "no-". However, -Wno-error[=CATEGORY] is valid.
2137 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2138 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2139 incompatibilities as warnings: "-y -Wno-error=yacc".
2141 *** The display of warnings is now richer
2143 The option that controls a given warning is now displayed:
2145 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2147 In the case of warnings treated as errors, the prefix is changed from
2148 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2149 to GCC, as [-Werror=CATEGORY].
2151 For instance, where the previous version of Bison would report (and exit
2154 bison: warnings being treated as errors
2155 input.y:1.1: warning: stray ',' treated as white space
2159 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2161 *** Deprecated constructs
2163 The new 'deprecated' warning category flags obsolete constructs whose
2164 support will be discontinued. It is enabled by default. These warnings
2165 used to be reported as 'other' warnings.
2167 *** Useless semantic types
2169 Bison now warns about useless (uninhabited) semantic types. Since
2170 semantic types are not declared to Bison (they are defined in the opaque
2171 %union structure), it is %printer/%destructor directives about useless
2172 types that trigger the warning:
2176 %printer {} <type1> <type3>
2177 %destructor {} <type2> <type4>
2179 nterm: term { $$ = $1; };
2181 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2182 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2184 *** Undefined but unused symbols
2186 Bison used to raise an error for undefined symbols that are not used in
2187 the grammar. This is now only a warning.
2190 %destructor {} symbol2
2191 %type <type> symbol3
2195 *** Useless destructors or printers
2197 Bison now warns about useless destructors or printers. In the following
2198 example, the printer for <type1>, and the destructor for <type2> are
2199 useless: all symbols of <type1> (token1) already have a printer, and all
2200 symbols of type <type2> (token2) already have a destructor.
2202 %token <type1> token1
2206 %printer {} token1 <type1> <type3>
2207 %destructor {} token2 <type2> <type4>
2211 The warnings and error messages about shift/reduce and reduce/reduce
2212 conflicts have been normalized. For instance on the following foo.y file:
2216 exp: exp '+' exp | '0' | '0';
2218 compare the previous version of bison:
2221 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2222 $ bison -Werror foo.y
2223 bison: warnings being treated as errors
2224 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2226 with the new behavior:
2229 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2230 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2231 $ bison -Werror foo.y
2232 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2233 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2235 When %expect or %expect-rr is used, such as with bar.y:
2240 exp: exp '+' exp | '0' | '0';
2245 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2246 bar.y: expected 0 shift/reduce conflicts
2247 bar.y: expected 0 reduce/reduce conflicts
2252 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2253 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2255 ** Incompatibilities with POSIX Yacc
2257 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2260 ** Additional yylex/yyparse arguments
2262 The new directive %param declares additional arguments to both yylex and
2263 yyparse. The %lex-param, %parse-param, and %param directives support one
2264 or more arguments. Instead of
2266 %lex-param {arg1_type *arg1}
2267 %lex-param {arg2_type *arg2}
2268 %parse-param {arg1_type *arg1}
2269 %parse-param {arg2_type *arg2}
2273 %param {arg1_type *arg1} {arg2_type *arg2}
2275 ** Types of values for %define variables
2277 Bison used to make no difference between '%define foo bar' and '%define
2278 foo "bar"'. The former is now called a 'keyword value', and the latter a
2279 'string value'. A third kind was added: 'code values', such as '%define
2282 Keyword variables are used for fixed value sets, e.g.,
2284 %define lr.type lalr
2286 Code variables are used for value in the target language, e.g.,
2288 %define api.value.type {struct semantic_type}
2290 String variables are used remaining cases, e.g. file names.
2292 ** Variable api.token.prefix
2294 The variable api.token.prefix changes the way tokens are identified in
2295 the generated files. This is especially useful to avoid collisions
2296 with identifiers in the target language. For instance
2298 %token FILE for ERROR
2299 %define api.token.prefix {TOK_}
2301 start: FILE for ERROR;
2303 will generate the definition of the symbols TOK_FILE, TOK_for, and
2304 TOK_ERROR in the generated sources. In particular, the scanner must
2305 use these prefixed token names, although the grammar itself still
2306 uses the short names (as in the sample rule given above).
2308 ** Variable api.value.type
2310 This new %define variable supersedes the #define macro YYSTYPE. The use
2311 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2312 using %union or %defining api.value.type results in undefined behavior.
2314 Either define api.value.type, or use "%union":
2321 %token <ival> INT "integer"
2322 %token <sval> STRING "string"
2323 %printer { fprintf (yyo, "%d", $$); } <ival>
2324 %destructor { free ($$); } <sval>
2327 yylval.ival = 42; return INT;
2328 yylval.sval = "42"; return STRING;
2330 The %define variable api.value.type supports both keyword and code values.
2332 The keyword value 'union' means that the user provides genuine types, not
2333 union member names such as "ival" and "sval" above (WARNING: will fail if
2334 -y/--yacc/%yacc is enabled).
2336 %define api.value.type union
2337 %token <int> INT "integer"
2338 %token <char *> STRING "string"
2339 %printer { fprintf (yyo, "%d", $$); } <int>
2340 %destructor { free ($$); } <char *>
2343 yylval.INT = 42; return INT;
2344 yylval.STRING = "42"; return STRING;
2346 The keyword value variant is somewhat equivalent, but for C++ special
2347 provision is made to allow classes to be used (more about this below).
2349 %define api.value.type variant
2350 %token <int> INT "integer"
2351 %token <std::string> STRING "string"
2353 Code values (in braces) denote user defined types. This is where YYSTYPE
2371 %define api.value.type {struct my_value}
2372 %token <u.ival> INT "integer"
2373 %token <u.sval> STRING "string"
2374 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2375 %destructor { free ($$); } <u.sval>
2378 yylval.u.ival = 42; return INT;
2379 yylval.u.sval = "42"; return STRING;
2381 ** Variable parse.error
2383 This variable controls the verbosity of error messages. The use of the
2384 %error-verbose directive is deprecated in favor of "%define parse.error
2387 ** Deprecated %define variable names
2389 The following variables have been renamed for consistency. Backward
2390 compatibility is ensured, but upgrading is recommended.
2392 lr.default-reductions -> lr.default-reduction
2393 lr.keep-unreachable-states -> lr.keep-unreachable-state
2394 namespace -> api.namespace
2395 stype -> api.value.type
2397 ** Semantic predicates
2399 Contributed by Paul Hilfinger.
2401 The new, experimental, semantic-predicate feature allows actions of the
2402 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2403 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2404 in GLR parsers, rather than being deferred. The result is that they allow
2405 the programmer to prune possible parses based on the values of run-time
2408 ** The directive %expect-rr is now an error in non GLR mode
2410 It used to be an error only if used in non GLR mode, _and_ if there are
2411 reduce/reduce conflicts.
2413 ** Tokens are numbered in their order of appearance
2415 Contributed by Valentin Tolmer.
2417 With '%token A B', A had a number less than the one of B. However,
2418 precedence declarations used to generate a reversed order. This is now
2419 fixed, and introducing tokens with any of %token, %left, %right,
2420 %precedence, or %nonassoc yields the same result.
2422 When mixing declarations of tokens with a literal character (e.g., 'a') or
2423 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2424 the literal characters first. For example
2428 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2429 input order is now preserved.
2431 These changes were made so that one can remove useless precedence and
2432 associativity declarations (i.e., map %nonassoc, %left or %right to
2433 %precedence, or to %token) and get exactly the same output.
2435 ** Useless precedence and associativity
2437 Contributed by Valentin Tolmer.
2439 When developing and maintaining a grammar, useless associativity and
2440 precedence directives are common. They can be a nuisance: new ambiguities
2441 arising are sometimes masked because their conflicts are resolved due to
2442 the extra precedence or associativity information. Furthermore, it can
2443 hinder the comprehension of a new grammar: one will wonder about the role
2444 of a precedence, where in fact it is useless. The following changes aim
2445 at detecting and reporting these extra directives.
2447 *** Precedence warning category
2449 A new category of warning, -Wprecedence, was introduced. It flags the
2450 useless precedence and associativity directives.
2452 *** Useless associativity
2454 Bison now warns about symbols with a declared associativity that is never
2455 used to resolve conflicts. In that case, using %precedence is sufficient;
2456 the parsing tables will remain unchanged. Solving these warnings may raise
2457 useless precedence warnings, as the symbols no longer have associativity.
2471 warning: useless associativity for '+', use %precedence [-Wprecedence]
2475 *** Useless precedence
2477 Bison now warns about symbols with a declared precedence and no declared
2478 associativity (i.e., declared with %precedence), and whose precedence is
2479 never used. In that case, the symbol can be safely declared with %token
2480 instead, without modifying the parsing tables. For example:
2484 exp: "var" '=' "number";
2488 warning: useless precedence for '=' [-Wprecedence]
2492 *** Useless precedence and associativity
2494 In case of both useless precedence and associativity, the issue is flagged
2499 exp: "var" '=' "number";
2503 warning: useless precedence and associativity for '=' [-Wprecedence]
2509 With help from Joel E. Denny and Gabriel Rassoul.
2511 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2512 marked by the new %empty directive. Using %empty on a non-empty rule is
2513 an error. The new -Wempty-rule warning reports empty rules without
2514 %empty. On the following grammar:
2524 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2527 5.8-13: error: %empty on non-empty rule
2531 ** Java skeleton improvements
2533 The constants for token names were moved to the Lexer interface. Also, it
2534 is possible to add code to the parser's constructors using "%code init"
2535 and "%define init_throws".
2536 Contributed by Paolo Bonzini.
2538 The Java skeleton now supports push parsing.
2539 Contributed by Dennis Heimbigner.
2541 ** C++ skeletons improvements
2543 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2545 Using %defines is now optional. Without it, the needed support classes
2546 are defined in the generated parser, instead of additional files (such as
2547 location.hh, position.hh and stack.hh).
2549 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2551 Both lalr1.cc and glr.cc no longer require %location.
2553 *** syntax_error exception (lalr1.cc)
2555 The C++ parser features a syntax_error exception, which can be
2556 thrown from the scanner or from user rules to raise syntax errors.
2557 This facilitates reporting errors caught in sub-functions (e.g.,
2558 rejecting too large integral literals from a conversion function
2559 used by the scanner, or rejecting invalid combinations from a
2560 factory invoked by the user actions).
2562 *** %define api.value.type variant
2564 This is based on a submission from Michiel De Wilde. With help
2565 from Théophile Ranquet.
2567 In this mode, complex C++ objects can be used as semantic values. For
2570 %token <::std::string> TEXT;
2571 %token <int> NUMBER;
2572 %token SEMICOLON ";"
2573 %type <::std::string> item;
2574 %type <::std::list<std::string>> list;
2577 list { std::cout << $1 << std::endl; }
2581 %empty { /* Generates an empty string list. */ }
2582 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2586 TEXT { std::swap ($$, $1); }
2587 | NUMBER { $$ = string_cast ($1); }
2590 *** %define api.token.constructor
2592 When variants are enabled, Bison can generate functions to build the
2593 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2594 with the semantic value (e.g., int):
2596 parser::symbol_type yylex ()
2598 parser::location_type loc = ...;
2600 return parser::make_TEXT ("Hello, world!", loc);
2602 return parser::make_NUMBER (42, loc);
2604 return parser::make_SEMICOLON (loc);
2610 There are operator- and operator-= for 'location'. Negative line/column
2611 increments can no longer underflow the resulting value.
2614 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2618 *** Fix compiler attribute portability (yacc.c)
2620 With locations enabled, __attribute__ was used unprotected.
2622 *** Fix some compiler warnings (lalr1.cc)
2625 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2629 Warnings about uninitialized yylloc in yyparse have been fixed.
2631 Restored C90 compliance (yet no report was ever made).
2633 ** Diagnostics are improved
2635 Contributed by Théophile Ranquet.
2637 *** Changes in the format of error messages
2639 This used to be the format of many error reports:
2641 input.y:2.7-12: %type redeclaration for exp
2642 input.y:1.7-12: previous declaration
2646 input.y:2.7-12: error: %type redeclaration for exp
2647 input.y:1.7-12: previous declaration
2649 *** New format for error reports: carets
2651 Caret errors have been added to Bison:
2653 input.y:2.7-12: error: %type redeclaration for exp
2656 input.y:1.7-12: previous declaration
2662 input.y:3.20-23: error: ambiguous reference: '$exp'
2663 exp: exp '+' exp { $exp = $1 + $3; };
2665 input.y:3.1-3: refers to: $exp at $$
2666 exp: exp '+' exp { $exp = $1 + $3; };
2668 input.y:3.6-8: refers to: $exp at $1
2669 exp: exp '+' exp { $exp = $1 + $3; };
2671 input.y:3.14-16: refers to: $exp at $3
2672 exp: exp '+' exp { $exp = $1 + $3; };
2675 The default behavior for now is still not to display these unless
2676 explicitly asked with -fcaret (or -fall). However, in a later release, it
2677 will be made the default behavior (but may still be deactivated with
2680 ** New value for %define variable: api.pure full
2682 The %define variable api.pure requests a pure (reentrant) parser. However,
2683 for historical reasons, using it in a location-tracking Yacc parser
2684 resulted in a yyerror function that did not take a location as a
2685 parameter. With this new value, the user may request a better pure parser,
2686 where yyerror does take a location as a parameter (in location-tracking
2689 The use of "%define api.pure true" is deprecated in favor of this new
2690 "%define api.pure full".
2692 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2694 The %define variable api.location.type defines the name of the type to use
2695 for locations. When defined, Bison no longer generates the position.hh
2696 and location.hh files, nor does the parser will include them: the user is
2697 then responsible to define her type.
2699 This can be used in programs with several parsers to factor their location
2700 and position files: let one of them generate them, and the others just use
2703 This feature was actually introduced, but not documented, in Bison 2.5,
2704 under the name "location_type" (which is maintained for backward
2707 For consistency, lalr1.java's %define variables location_type and
2708 position_type are deprecated in favor of api.location.type and
2711 ** Exception safety (lalr1.cc)
2713 The parse function now catches exceptions, uses the %destructors to
2714 release memory (the lookahead symbol and the symbols pushed on the stack)
2715 before re-throwing the exception.
2717 This feature is somewhat experimental. User feedback would be
2720 ** Graph improvements in DOT and XSLT
2722 Contributed by Théophile Ranquet.
2724 The graphical presentation of the states is more readable: their shape is
2725 now rectangular, the state number is clearly displayed, and the items are
2726 numbered and left-justified.
2728 The reductions are now explicitly represented as transitions to other
2729 diamond shaped nodes.
2731 These changes are present in both --graph output and xml2dot.xsl XSLT
2732 processing, with minor (documented) differences.
2734 ** %language is no longer an experimental feature.
2736 The introduction of this feature, in 2.4, was four years ago. The
2737 --language option and the %language directive are no longer experimental.
2741 The sections about shift/reduce and reduce/reduce conflicts resolution
2742 have been fixed and extended.
2744 Although introduced more than four years ago, XML and Graphviz reports
2745 were not properly documented.
2747 The translation of midrule actions is now described.
2750 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2752 We consider compiler warnings about Bison generated parsers to be bugs.
2753 Rather than working around them in your own project, please consider
2754 reporting them to us.
2758 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2759 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2762 Other issues in the test suite have been addressed.
2764 Null characters are correctly displayed in error messages.
2766 When possible, yylloc is correctly initialized before calling yylex. It
2767 is no longer necessary to initialize it in the %initial-action.
2770 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2772 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2775 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2779 Bugs and portability issues in the test suite have been fixed.
2781 Some errors in translations have been addressed, and --help now directs
2782 users to the appropriate place to report them.
2784 Stray Info files shipped by accident are removed.
2786 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2787 generated, are removed.
2789 All the generated headers are self-contained.
2791 ** Header guards (yacc.c, glr.c, glr.cc)
2793 In order to avoid collisions, the header guards are now
2794 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2795 For instance the header generated from
2797 %define api.prefix "calc"
2798 %defines "lib/parse.h"
2800 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2802 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2804 The compilation of pure parsers (%define api.pure) can trigger GCC
2807 input.c: In function 'yyparse':
2808 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2809 function [-Wmaybe-uninitialized]
2813 This is now fixed; pragmas to avoid these warnings are no longer needed.
2815 Warnings from clang ("equality comparison with extraneous parentheses" and
2816 "function declared 'noreturn' should not return") have also been
2820 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2824 Buffer overruns, complaints from Flex, and portability issues in the test
2825 suite have been fixed.
2827 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2829 Trailing end-of-lines in %parse-param or %lex-param would result in
2830 invalid C++. This is fixed.
2832 ** Spurious spaces and end-of-lines
2834 The generated files no longer end (nor start) with empty lines.
2837 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2839 Bison no longer executes user-specified M4 code when processing a grammar.
2843 In addition to the removal of the features announced in Bison 2.6, the
2844 next major release will remove the "Temporary hack for adding a semicolon
2845 to the user action", as announced in the release 2.5. Instead of:
2847 exp: exp "+" exp { $$ = $1 + $3 };
2851 exp: exp "+" exp { $$ = $1 + $3; };
2855 *** Type names are now properly escaped.
2857 *** glr.cc: set_debug_level and debug_level work as expected.
2859 *** Stray @ or $ in actions
2861 While Bison used to warn about stray $ or @ in action rules, it did not
2862 for other actions such as printers, destructors, or initial actions. It
2865 ** Type names in actions
2867 For consistency with rule actions, it is now possible to qualify $$ by a
2868 type-name in destructors, printers, and initial actions. For instance:
2870 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2872 will display two values for each typed and untyped symbol (provided
2873 that YYSTYPE has both "ival" and "fval" fields).
2876 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2880 The next major release of Bison will drop support for the following
2881 deprecated features. Please report disagreements to bug-bison@gnu.org.
2885 Support for generating parsers in K&R C will be removed. Parsers
2886 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2889 *** Features deprecated since Bison 1.875
2891 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2894 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2895 %lex-param, will no longer be supported.
2897 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2900 *** The generated header will be included (yacc.c)
2902 Instead of duplicating the content of the generated header (definition of
2903 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2904 as is already the case for GLR or C++ parsers. This change is deferred
2905 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2908 ** Generated Parser Headers
2910 *** Guards (yacc.c, glr.c, glr.cc)
2912 The generated headers are now guarded, as is already the case for C++
2913 parsers (lalr1.cc). For instance, with --defines=foo.h:
2918 #endif /* !YY_FOO_H */
2920 *** New declarations (yacc.c, glr.c)
2922 The generated header now declares yydebug and yyparse. Both honor
2923 --name-prefix=bar_, and yield
2925 int bar_parse (void);
2929 #define yyparse bar_parse
2932 in order to facilitate the inclusion of several parser headers inside a
2933 single compilation unit.
2935 *** Exported symbols in C++
2937 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2938 header, are removed, as they prevent the possibility of including several
2939 generated headers from a single compilation unit.
2943 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2946 ** New %define variable: api.prefix
2948 Now that the generated headers are more complete and properly protected
2949 against multiple inclusions, constant names, such as YYSTYPE are a
2950 problem. While yyparse and others are properly renamed by %name-prefix,
2951 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2952 would introduce backward compatibility issues in projects not expecting
2953 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2954 it is deprecated in favor of a new %define variable: api.prefix.
2956 The following examples compares both:
2958 %name-prefix "bar_" | %define api.prefix "bar_"
2959 %token <ival> FOO %token <ival> FOO
2960 %union { int ival; } %union { int ival; }
2966 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2967 # define BAR_FOO_H # define BAR_FOO_H
2969 /* Enabling traces. */ /* Enabling traces. */
2970 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2971 > # if defined YYDEBUG
2973 > # define BAR_DEBUG 1
2975 > # define BAR_DEBUG 0
2978 # define YYDEBUG 0 | # define BAR_DEBUG 0
2982 # if YYDEBUG | # if BAR_DEBUG
2983 extern int bar_debug; extern int bar_debug;
2986 /* Tokens. */ /* Tokens. */
2987 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2988 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2989 enum yytokentype { | enum bar_tokentype {
2994 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2995 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2996 typedef union YYSTYPE | typedef union BAR_STYPE
2999 } YYSTYPE; | } BAR_STYPE;
3000 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
3003 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
3005 int bar_parse (void); int bar_parse (void);
3007 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
3010 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
3014 The next major release will drop support for generating parsers in K&R C.
3016 ** yacc.c: YYBACKUP works as expected.
3018 ** glr.c improvements:
3020 *** Location support is eliminated when not requested:
3022 GLR parsers used to include location-related code even when locations were
3023 not requested, and therefore not even usable.
3025 *** __attribute__ is preserved:
3027 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
3028 when -std is passed to GCC).
3030 ** lalr1.java: several fixes:
3032 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
3033 first token leads to a syntax error. Some minor clean ups.
3037 *** C++11 compatibility:
3039 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
3044 The header files such as "parser.hh", "location.hh", etc. used a constant
3045 name for preprocessor guards, for instance:
3047 #ifndef BISON_LOCATION_HH
3048 # define BISON_LOCATION_HH
3050 #endif // !BISON_LOCATION_HH
3052 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
3053 case characters are converted to upper case, and series of
3054 non-alphanumerical characters are converted to an underscore.
3056 With "bison -o lang++/parser.cc", "location.hh" would now include:
3058 #ifndef YY_LANG_LOCATION_HH
3059 # define YY_LANG_LOCATION_HH
3061 #endif // !YY_LANG_LOCATION_HH
3065 The position and location constructors (and their initialize methods)
3066 accept new arguments for line and column. Several issues in the
3067 documentation were fixed.
3069 ** liby is no longer asking for "rpl_fprintf" on some platforms.
3071 ** Changes in the manual:
3073 *** %printer is documented
3075 The "%printer" directive, supported since at least Bison 1.50, is finally
3076 documented. The "mfcalc" example is extended to demonstrate it.
3078 For consistency with the C skeletons, the C++ parsers now also support
3079 "yyoutput" (as an alias to "debug_stream ()").
3081 *** Several improvements have been made:
3083 The layout for grammar excerpts was changed to a more compact scheme.
3084 Named references are motivated. The description of the automaton
3085 description file (*.output) is updated to the current format. Incorrect
3086 index entries were fixed. Some other errors were fixed.
3090 *** Conflicting prototypes with recent/modified Flex.
3092 Fixed build problems with the current, unreleased, version of Flex, and
3093 some modified versions of 2.5.35, which have modified function prototypes.
3095 *** Warnings during the build procedure have been eliminated.
3097 *** Several portability problems in the test suite have been fixed:
3099 This includes warnings with some compilers, unexpected behavior of tools
3100 such as diff, warning messages from the test suite itself, etc.
3102 *** The install-pdf target works properly:
3104 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3105 halts in the middle of its course.
3108 * Noteworthy changes in release 2.5 (2011-05-14)
3110 ** Grammar symbol names can now contain non-initial dashes:
3112 Consistently with directives (such as %error-verbose) and with
3113 %define variables (e.g. push-pull), grammar symbol names may contain
3114 dashes in any position except the beginning. This is a GNU
3115 extension over POSIX Yacc. Thus, use of this extension is reported
3116 by -Wyacc and rejected in Yacc mode (--yacc).
3118 ** Named references:
3120 Historically, Yacc and Bison have supported positional references
3121 ($n, $$) to allow access to symbol values from inside of semantic
3124 Starting from this version, Bison can also accept named references.
3125 When no ambiguity is possible, original symbol names may be used
3126 as named references:
3128 if_stmt : "if" cond_expr "then" then_stmt ';'
3129 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3131 In the more common case, explicit names may be declared:
3133 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3134 { $res = mk_if_stmt($cond, $then, $else); }
3136 Location information is also accessible using @name syntax. When
3137 accessing symbol names containing dots or dashes, explicit bracketing
3138 ($[sym.1]) must be used.
3140 These features are experimental in this version. More user feedback
3141 will help to stabilize them.
3142 Contributed by Alex Rozenman.
3144 ** IELR(1) and canonical LR(1):
3146 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3147 is, given any context-free grammar, IELR(1) generates parser tables
3148 with the full language-recognition power of canonical LR(1) but with
3149 nearly the same number of parser states as LALR(1). This reduction
3150 in parser states is often an order of magnitude. More importantly,
3151 because canonical LR(1)'s extra parser states may contain duplicate
3152 conflicts in the case of non-LR(1) grammars, the number of conflicts
3153 for IELR(1) is often an order of magnitude less as well. This can
3154 significantly reduce the complexity of developing of a grammar.
3156 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3157 place of its traditional LALR(1) parser tables, which remain the
3158 default. You can specify the type of parser tables in the grammar
3159 file with these directives:
3161 %define lr.type lalr
3162 %define lr.type ielr
3163 %define lr.type canonical-lr
3165 The default-reduction optimization in the parser tables can also be
3166 adjusted using "%define lr.default-reductions". For details on both
3167 of these features, see the new section "Tuning LR" in the Bison
3170 These features are experimental. More user feedback will help to
3173 ** LAC (Lookahead Correction) for syntax error handling
3175 Contributed by Joel E. Denny.
3177 Canonical LR, IELR, and LALR can suffer from a couple of problems
3178 upon encountering a syntax error. First, the parser might perform
3179 additional parser stack reductions before discovering the syntax
3180 error. Such reductions can perform user semantic actions that are
3181 unexpected because they are based on an invalid token, and they
3182 cause error recovery to begin in a different syntactic context than
3183 the one in which the invalid token was encountered. Second, when
3184 verbose error messages are enabled (with %error-verbose or the
3185 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3186 syntax error message can both contain invalid tokens and omit valid
3189 The culprits for the above problems are %nonassoc, default
3190 reductions in inconsistent states, and parser state merging. Thus,
3191 IELR and LALR suffer the most. Canonical LR can suffer only if
3192 %nonassoc is used or if default reductions are enabled for
3193 inconsistent states.
3195 LAC is a new mechanism within the parsing algorithm that solves
3196 these problems for canonical LR, IELR, and LALR without sacrificing
3197 %nonassoc, default reductions, or state merging. When LAC is in
3198 use, canonical LR and IELR behave almost exactly the same for both
3199 syntactically acceptable and syntactically unacceptable input.
3200 While LALR still does not support the full language-recognition
3201 power of canonical LR and IELR, LAC at least enables LALR's syntax
3202 error handling to correctly reflect LALR's language-recognition
3205 Currently, LAC is only supported for deterministic parsers in C.
3206 You can enable LAC with the following directive:
3208 %define parse.lac full
3210 See the new section "LAC" in the Bison manual for additional
3211 details including a few caveats.
3213 LAC is an experimental feature. More user feedback will help to
3216 ** %define improvements:
3218 *** Can now be invoked via the command line:
3220 Each of these command-line options
3223 --define=NAME[=VALUE]
3226 --force-define=NAME[=VALUE]
3228 is equivalent to this grammar file declaration
3230 %define NAME ["VALUE"]
3232 except that the manner in which Bison processes multiple definitions
3233 for the same NAME differs. Most importantly, -F and --force-define
3234 quietly override %define, but -D and --define do not. For further
3235 details, see the section "Bison Options" in the Bison manual.
3237 *** Variables renamed:
3239 The following %define variables
3242 lr.keep_unreachable_states
3244 have been renamed to
3247 lr.keep-unreachable-states
3249 The old names are now deprecated but will be maintained indefinitely
3250 for backward compatibility.
3252 *** Values no longer need to be quoted in the grammar file:
3254 If a %define value is an identifier, it no longer needs to be placed
3255 within quotations marks. For example,
3257 %define api.push-pull "push"
3261 %define api.push-pull push
3263 *** Unrecognized variables are now errors not warnings.
3265 *** Multiple invocations for any variable is now an error not a warning.
3267 ** Unrecognized %code qualifiers are now errors not warnings.
3269 ** Character literals not of length one:
3271 Previously, Bison quietly converted all character literals to length
3272 one. For example, without warning, Bison interpreted the operators in
3273 the following grammar to be the same token:
3279 Bison now warns when a character literal is not of length one. In
3280 some future release, Bison will start reporting an error instead.
3282 ** Destructor calls fixed for lookaheads altered in semantic actions:
3284 Previously for deterministic parsers in C, if a user semantic action
3285 altered yychar, the parser in some cases used the old yychar value to
3286 determine which destructor to call for the lookahead upon a syntax
3287 error or upon parser return. This bug has been fixed.
3289 ** C++ parsers use YYRHSLOC:
3291 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3292 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3293 to use it. If, for instance, your location structure has "first"
3294 and "last" members, instead of
3296 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3300 (Current).first = (Rhs)[1].location.first; \
3301 (Current).last = (Rhs)[N].location.last; \
3305 (Current).first = (Current).last = (Rhs)[0].location.last; \
3311 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3315 (Current).first = YYRHSLOC (Rhs, 1).first; \
3316 (Current).last = YYRHSLOC (Rhs, N).last; \
3320 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3324 ** YYLLOC_DEFAULT in C++:
3326 The default implementation of YYLLOC_DEFAULT used to be issued in
3327 the header file. It is now output in the implementation file, after
3328 the user %code sections so that its #ifndef guard does not try to
3329 override the user's YYLLOC_DEFAULT if provided.
3331 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3333 YYFAIL has existed for many years as an undocumented feature of
3334 deterministic parsers in C generated by Bison. More recently, it was
3335 a documented feature of Bison's experimental Java parsers. As
3336 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3337 semantic action now produces a deprecation warning, and Java parsers
3338 no longer implement YYFAIL at all. For further details, including a
3339 discussion of how to suppress C preprocessor warnings about YYFAIL
3340 being unused, see the Bison 2.4.2 NEWS entry.
3342 ** Temporary hack for adding a semicolon to the user action:
3344 Previously, Bison appended a semicolon to every user action for
3345 reductions when the output language defaulted to C (specifically, when
3346 neither %yacc, %language, %skeleton, or equivalent command-line
3347 options were specified). This allowed actions such as
3349 exp: exp "+" exp { $$ = $1 + $3 };
3353 exp: exp "+" exp { $$ = $1 + $3; };
3355 As a first step in removing this misfeature, Bison now issues a
3356 warning when it appends a semicolon. Moreover, in cases where Bison
3357 cannot easily determine whether a semicolon is needed (for example, an
3358 action ending with a cpp directive or a braced compound initializer),
3359 it no longer appends one. Thus, the C compiler might now complain
3360 about a missing semicolon where it did not before. Future releases of
3361 Bison will cease to append semicolons entirely.
3363 ** Verbose syntax error message fixes:
3365 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3366 specified, syntax error messages produced by the generated parser
3367 include the unexpected token as well as a list of expected tokens.
3368 The effect of %nonassoc on these verbose messages has been corrected
3369 in two ways, but a more complete fix requires LAC, described above:
3371 *** When %nonassoc is used, there can exist parser states that accept no
3372 tokens, and so the parser does not always require a lookahead token
3373 in order to detect a syntax error. Because no unexpected token or
3374 expected tokens can then be reported, the verbose syntax error
3375 message described above is suppressed, and the parser instead
3376 reports the simpler message, "syntax error". Previously, this
3377 suppression was sometimes erroneously triggered by %nonassoc when a
3378 lookahead was actually required. Now verbose messages are
3379 suppressed only when all previous lookaheads have already been
3380 shifted or discarded.
3382 *** Previously, the list of expected tokens erroneously included tokens
3383 that would actually induce a syntax error because conflicts for them
3384 were resolved with %nonassoc in the current parser state. Such
3385 tokens are now properly omitted from the list.
3387 *** Expected token lists are still often wrong due to state merging
3388 (from LALR or IELR) and default reductions, which can both add
3389 invalid tokens and subtract valid tokens. Canonical LR almost
3390 completely fixes this problem by eliminating state merging and
3391 default reductions. However, there is one minor problem left even
3392 when using canonical LR and even after the fixes above. That is,
3393 if the resolution of a conflict with %nonassoc appears in a later
3394 parser state than the one at which some syntax error is
3395 discovered, the conflicted token is still erroneously included in
3396 the expected token list. Bison's new LAC implementation,
3397 described above, eliminates this problem and the need for
3398 canonical LR. However, LAC is still experimental and is disabled
3401 ** Java skeleton fixes:
3403 *** A location handling bug has been fixed.
3405 *** The top element of each of the value stack and location stack is now
3406 cleared when popped so that it can be garbage collected.
3408 *** Parser traces now print the top element of the stack.
3410 ** -W/--warnings fixes:
3412 *** Bison now properly recognizes the "no-" versions of categories:
3414 For example, given the following command line, Bison now enables all
3415 warnings except warnings for incompatibilities with POSIX Yacc:
3417 bison -Wall,no-yacc gram.y
3419 *** Bison now treats S/R and R/R conflicts like other warnings:
3421 Previously, conflict reports were independent of Bison's normal
3422 warning system. Now, Bison recognizes the warning categories
3423 "conflicts-sr" and "conflicts-rr". This change has important
3424 consequences for the -W and --warnings command-line options. For
3427 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3428 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3429 bison -Wnone gram.y # no conflicts are reported
3430 bison -Werror gram.y # any conflict is an error
3432 However, as before, if the %expect or %expect-rr directive is
3433 specified, an unexpected number of conflicts is an error, and an
3434 expected number of conflicts is not reported, so -W and --warning
3435 then have no effect on the conflict report.
3437 *** The "none" category no longer disables a preceding "error":
3439 For example, for the following command line, Bison now reports
3440 errors instead of warnings for incompatibilities with POSIX Yacc:
3442 bison -Werror,none,yacc gram.y
3444 *** The "none" category now disables all Bison warnings:
3446 Previously, the "none" category disabled only Bison warnings for
3447 which there existed a specific -W/--warning category. However,
3448 given the following command line, Bison is now guaranteed to
3449 suppress all warnings:
3453 ** Precedence directives can now assign token number 0:
3455 Since Bison 2.3b, which restored the ability of precedence
3456 directives to assign token numbers, doing so for token number 0 has
3457 produced an assertion failure. For example:
3461 This bug has been fixed.
3464 * Noteworthy changes in release 2.4.3 (2010-08-05)
3466 ** Bison now obeys -Werror and --warnings=error for warnings about
3467 grammar rules that are useless in the parser due to conflicts.
3469 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3472 ** Failures in the test suite for GCC 4.5 have been fixed.
3474 ** Failures in the test suite for some versions of Sun Studio C++ have
3477 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3478 warnings about undefined %prec identifiers will not be converted to
3479 errors in Bison 2.5. They will remain warnings, which should be
3480 sufficient for POSIX while avoiding backward compatibility issues.
3482 ** Minor documentation fixes.
3485 * Noteworthy changes in release 2.4.2 (2010-03-20)
3487 ** Some portability problems that resulted in failures and livelocks
3488 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3489 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3490 errors should no longer cause M4 to report a broken pipe on the
3493 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3495 POSIX specifies that an error be reported for any identifier that does
3496 not appear on the LHS of a grammar rule and that is not defined by
3497 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3498 error report for the case when an identifier appears only after a
3499 %prec directive. It is now restored. However, for backward
3500 compatibility with recent Bison releases, it is only a warning for
3501 now. In Bison 2.5 and later, it will return to being an error.
3502 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3503 warning will not be converted to an error in Bison 2.5.]
3505 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3507 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3508 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3511 ** %code is now a permanent feature.
3513 A traditional Yacc prologue directive is written in the form:
3517 To provide a more flexible alternative, Bison 2.3b introduced the
3518 %code directive with the following forms for C/C++:
3521 %code requires {CODE}
3522 %code provides {CODE}
3525 These forms are now considered permanent features of Bison. See the
3526 %code entries in the section "Bison Declaration Summary" in the Bison
3527 manual for a summary of their functionality. See the section
3528 "Prologue Alternatives" for a detailed discussion including the
3529 advantages of %code over the traditional Yacc prologue directive.
3531 Bison's Java feature as a whole including its current usage of %code
3532 is still considered experimental.
3534 ** YYFAIL is deprecated and will eventually be removed.
3536 YYFAIL has existed for many years as an undocumented feature of
3537 deterministic parsers in C generated by Bison. Previously, it was
3538 documented for Bison's experimental Java parsers. YYFAIL is no longer
3539 documented for Java parsers and is formally deprecated in both cases.
3540 Users are strongly encouraged to migrate to YYERROR, which is
3543 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3544 induce a syntax error. The most obvious difference from YYERROR is
3545 that YYFAIL will automatically invoke yyerror to report the syntax
3546 error so that you don't have to. However, there are several other
3547 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3548 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3549 used. For a more detailed discussion, see:
3551 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3553 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3554 deterministic parsers in C will continue to implement it. However,
3555 because YYFAIL is already flawed, it seems futile to try to make new
3556 Bison features compatible with it. Thus, during parser generation,
3557 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3558 rule action. In a later release, YYFAIL will be disabled for
3559 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3560 be removed altogether.
3562 There exists at least one case where Bison 2.5's YYFAIL warning will
3563 be a false positive. Some projects add phony uses of YYFAIL and other
3564 Bison-defined macros for the sole purpose of suppressing C
3565 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3566 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3567 epilogue (that is, after the second "%%") in the Bison input file. In
3568 this release (2.4.2), Bison already generates its own code to suppress
3569 C preprocessor warnings for YYFAIL, so projects can remove their own
3570 phony uses of YYFAIL if compatibility with Bison releases prior to
3571 2.4.2 is not necessary.
3573 ** Internationalization.
3575 Fix a regression introduced in Bison 2.4: Under some circumstances,
3576 message translations were not installed although supported by the
3580 * Noteworthy changes in release 2.4.1 (2008-12-11)
3582 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3583 declarations have been fixed.
3585 ** Temporary hack for adding a semicolon to the user action.
3587 Bison used to prepend a trailing semicolon at the end of the user
3588 action for reductions. This allowed actions such as
3590 exp: exp "+" exp { $$ = $1 + $3 };
3594 exp: exp "+" exp { $$ = $1 + $3; };
3596 Some grammars still depend on this "feature". Bison 2.4.1 restores
3597 the previous behavior in the case of C output (specifically, when
3598 neither %language or %skeleton or equivalent command-line options
3599 are used) to leave more time for grammars depending on the old
3600 behavior to be adjusted. Future releases of Bison will disable this
3603 ** A few minor improvements to the Bison manual.
3606 * Noteworthy changes in release 2.4 (2008-11-02)
3608 ** %language is an experimental feature.
3610 We first introduced this feature in test release 2.3b as a cleaner
3611 alternative to %skeleton. Since then, we have discussed the possibility of
3612 modifying its effect on Bison's output file names. Thus, in this release,
3613 we consider %language to be an experimental feature that will likely evolve
3616 ** Forward compatibility with GNU M4 has been improved.
3618 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3622 * Noteworthy changes in release 2.3b (2008-05-27)
3624 ** The quotes around NAME that used to be required in the following directive
3627 %define NAME "VALUE"
3629 ** The directive "%pure-parser" is now deprecated in favor of:
3633 which has the same effect except that Bison is more careful to warn about
3634 unreasonable usage in the latter case.
3638 Bison can now generate an LALR(1) parser in C with a push interface. That
3639 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3640 push one token at a time to the parser using "yypush_parse", which will
3641 return to the caller after processing each token. By default, the push
3642 interface is disabled. Either of the following directives will enable it:
3644 %define api.push_pull "push" // Just push; does not require yylex.
3645 %define api.push_pull "both" // Push and pull; requires yylex.
3647 See the new section "A Push Parser" in the Bison manual for details.
3649 The current push parsing interface is experimental and may evolve. More user
3650 feedback will help to stabilize it.
3652 ** The -g and --graph options now output graphs in Graphviz DOT format,
3653 not VCG format. Like --graph, -g now also takes an optional FILE argument
3654 and thus cannot be bundled with other short options.
3658 Bison can now generate an LALR(1) parser in Java. The skeleton is
3659 "data/lalr1.java". Consider using the new %language directive instead of
3660 %skeleton to select it.
3662 See the new section "Java Parsers" in the Bison manual for details.
3664 The current Java interface is experimental and may evolve. More user
3665 feedback will help to stabilize it.
3666 Contributed by Paolo Bonzini.
3670 This new directive specifies the programming language of the generated
3671 parser, which can be C (the default), C++, or Java. Besides the skeleton
3672 that Bison uses, the directive affects the names of the generated files if
3673 the grammar file's name ends in ".y".
3675 ** XML Automaton Report
3677 Bison can now generate an XML report of the LALR(1) automaton using the new
3678 "--xml" option. The current XML schema is experimental and may evolve. More
3679 user feedback will help to stabilize it.
3680 Contributed by Wojciech Polak.
3682 ** The grammar file may now specify the name of the parser header file using
3683 %defines. For example:
3687 ** When reporting useless rules, useless nonterminals, and unused terminals,
3688 Bison now employs the terms "useless in grammar" instead of "useless",
3689 "useless in parser" instead of "never reduced", and "unused in grammar"
3690 instead of "unused".
3692 ** Unreachable State Removal
3694 Previously, Bison sometimes generated parser tables containing unreachable
3695 states. A state can become unreachable during conflict resolution if Bison
3696 disables a shift action leading to it from a predecessor state. Bison now:
3698 1. Removes unreachable states.
3700 2. Does not report any conflicts that appeared in unreachable states.
3701 WARNING: As a result, you may need to update %expect and %expect-rr
3702 directives in existing grammar files.
3704 3. For any rule used only in such states, Bison now reports the rule as
3705 "useless in parser due to conflicts".
3707 This feature can be disabled with the following directive:
3709 %define lr.keep_unreachable_states
3711 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3712 for further discussion.
3714 ** Lookahead Set Correction in the ".output" Report
3716 When instructed to generate a ".output" file including lookahead sets
3717 (using "--report=lookahead", for example), Bison now prints each reduction's
3718 lookahead set only next to the associated state's one item that (1) is
3719 associated with the same rule as the reduction and (2) has its dot at the end
3720 of its RHS. Previously, Bison also erroneously printed the lookahead set
3721 next to all of the state's other items associated with the same rule. This
3722 bug affected only the ".output" file and not the generated parser source
3725 ** --report-file=FILE is a new option to override the default ".output" file
3728 ** The "=" that used to be required in the following directives is now
3731 %file-prefix "parser"
3735 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3737 Bison 2.3a provided a new set of directives as a more flexible alternative to
3738 the traditional Yacc prologue blocks. Those have now been consolidated into
3739 a single %code directive with an optional qualifier field, which identifies
3740 the purpose of the code and thus the location(s) where Bison should generate
3743 1. "%code {CODE}" replaces "%after-header {CODE}"
3744 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3745 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3746 4. "%code top {CODE}" replaces "%before-header {CODE}"
3748 See the %code entries in section "Bison Declaration Summary" in the Bison
3749 manual for a summary of the new functionality. See the new section "Prologue
3750 Alternatives" for a detailed discussion including the advantages of %code
3751 over the traditional Yacc prologues.
3753 The prologue alternatives are experimental. More user feedback will help to
3754 determine whether they should become permanent features.
3756 ** Revised warning: unset or unused midrule values
3758 Since Bison 2.2, Bison has warned about midrule values that are set but not
3759 used within any of the actions of the parent rule. For example, Bison warns
3762 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3764 Now, Bison also warns about midrule values that are used but not set. For
3765 example, Bison warns about unset $$ in the midrule action in:
3767 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3769 However, Bison now disables both of these warnings by default since they
3770 sometimes prove to be false alarms in existing grammars employing the Yacc
3771 constructs $0 or $-N (where N is some positive integer).
3773 To enable these warnings, specify the option "--warnings=midrule-values" or
3774 "-W", which is a synonym for "--warnings=all".
3776 ** Default %destructor or %printer with "<*>" or "<>"
3778 Bison now recognizes two separate kinds of default %destructor's and
3781 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3782 %destructor/%printer for all grammar symbols for which you have formally
3783 declared semantic type tags.
3785 2. Place "<>" in a %destructor/%printer symbol list to define a default
3786 %destructor/%printer for all grammar symbols without declared semantic
3789 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3790 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3791 longer applies any %destructor to a midrule value if that midrule value is
3792 not actually ever referenced using either $$ or $n in a semantic action.
3794 The default %destructor's and %printer's are experimental. More user
3795 feedback will help to determine whether they should become permanent
3798 See the section "Freeing Discarded Symbols" in the Bison manual for further
3801 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3802 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3803 manual for a caveat concerning the treatment of literal strings.
3805 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3806 completely removed from Bison.
3809 * Noteworthy changes in release 2.3a (2006-09-13)
3811 ** Instead of %union, you can define and use your own union type
3812 YYSTYPE if your grammar contains at least one <type> tag.
3813 Your YYSTYPE need not be a macro; it can be a typedef.
3814 This change is for compatibility with other Yacc implementations,
3815 and is required by POSIX.
3817 ** Locations columns and lines start at 1.
3818 In accordance with the GNU Coding Standards and Emacs.
3820 ** You may now declare per-type and default %destructor's and %printer's:
3824 %union { char *string; }
3825 %token <string> STRING1
3826 %token <string> STRING2
3827 %type <string> string1
3828 %type <string> string2
3829 %union { char character; }
3830 %token <character> CHR
3831 %type <character> chr
3832 %destructor { free ($$); } %symbol-default
3833 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3834 %destructor { } <character>
3836 guarantees that, when the parser discards any user-defined symbol that has a
3837 semantic type tag other than "<character>", it passes its semantic value to
3838 "free". However, when the parser discards a "STRING1" or a "string1", it
3839 also prints its line number to "stdout". It performs only the second
3840 "%destructor" in this case, so it invokes "free" only once.
3842 [Although we failed to mention this here in the 2.3a release, the default
3843 %destructor's and %printer's were experimental, and they were rewritten in
3846 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3847 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3848 associating token numbers with token names. Removing the #define statements
3849 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3850 requires them. Bison still generates an enum for token names in all cases.
3852 ** Handling of traditional Yacc prologue blocks is now more consistent but
3853 potentially incompatible with previous releases of Bison.
3855 As before, you declare prologue blocks in your grammar file with the
3856 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3857 prologue blocks that you've declared before the first %union. To generate
3858 the post-prologue, Bison concatenates all prologue blocks that you've
3859 declared after the first %union.
3861 Previous releases of Bison inserted the pre-prologue into both the header
3862 file and the code file in all cases except for LALR(1) parsers in C. In the
3863 latter case, Bison inserted it only into the code file. For parsers in C++,
3864 the point of insertion was before any token definitions (which associate
3865 token numbers with names). For parsers in C, the point of insertion was
3866 after the token definitions.
3868 Now, Bison never inserts the pre-prologue into the header file. In the code
3869 file, it always inserts it before the token definitions.
3871 ** Bison now provides a more flexible alternative to the traditional Yacc
3872 prologue blocks: %before-header, %start-header, %end-header, and
3875 For example, the following declaration order in the grammar file reflects the
3876 order in which Bison will output these code blocks. However, you are free to
3877 declare these code blocks in your grammar file in whatever order is most
3881 /* Bison treats this block like a pre-prologue block: it inserts it into
3882 * the code file before the contents of the header file. It does *not*
3883 * insert it into the header file. This is a good place to put
3884 * #include's that you want at the top of your code file. A common
3885 * example is '#include "system.h"'. */
3888 /* Bison inserts this block into both the header file and the code file.
3889 * In both files, the point of insertion is before any Bison-generated
3890 * token, semantic type, location type, and class definitions. This is a
3891 * good place to define %union dependencies, for example. */
3894 /* Unlike the traditional Yacc prologue blocks, the output order for the
3895 * new %*-header blocks is not affected by their declaration position
3896 * relative to any %union in the grammar file. */
3899 /* Bison inserts this block into both the header file and the code file.
3900 * In both files, the point of insertion is after the Bison-generated
3901 * definitions. This is a good place to declare or define public
3902 * functions or data structures that depend on the Bison-generated
3906 /* Bison treats this block like a post-prologue block: it inserts it into
3907 * the code file after the contents of the header file. It does *not*
3908 * insert it into the header file. This is a good place to declare or
3909 * define internal functions or data structures that depend on the
3910 * Bison-generated definitions. */
3913 If you have multiple occurrences of any one of the above declarations, Bison
3914 will concatenate the contents in declaration order.
3916 [Although we failed to mention this here in the 2.3a release, the prologue
3917 alternatives were experimental, and they were rewritten in future versions.]
3919 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3920 The old spelling still works, but is not documented and may be removed
3921 in a future release.
3924 * Noteworthy changes in release 2.3 (2006-06-05)
3926 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3927 for compatibility with LALR(1) grammars.
3929 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3930 be to a type name that does not contain parentheses or brackets.
3933 * Noteworthy changes in release 2.2 (2006-05-19)
3935 ** The distribution terms for all Bison-generated parsers now permit
3936 using the parsers in nonfree programs. Previously, this permission
3937 was granted only for Bison-generated LALR(1) parsers in C.
3939 ** %name-prefix changes the namespace name in C++ outputs.
3941 ** The C++ parsers export their token_type.
3943 ** Bison now allows multiple %union declarations, and concatenates
3944 their contents together.
3946 ** New warning: unused values
3947 Right-hand side symbols whose values are not used are reported,
3948 if the symbols have destructors. For instance:
3950 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3954 will trigger a warning about $$ and $5 in the first rule, and $3 in
3955 the second ($1 is copied to $$ by the default rule). This example
3956 most likely contains three errors, and could be rewritten as:
3958 exp: exp "?" exp ":" exp
3959 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3961 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3964 However, if the original actions were really intended, memory leaks
3965 and all, the warnings can be suppressed by letting Bison believe the
3966 values are used, e.g.:
3968 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3969 | exp "+" exp { $$ = $1; (void) $3; }
3972 If there are midrule actions, the warning is issued if no action
3973 uses it. The following triggers no warning: $1 and $3 are used.
3975 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3977 The warning is intended to help catching lost values and memory leaks.
3978 If a value is ignored, its associated memory typically is not reclaimed.
3980 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3981 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3982 and YYERROR, for all objects on the stack, other than objects
3983 corresponding to the right-hand side of the current rule.
3985 ** %expect, %expect-rr
3986 Incorrect numbers of expected conflicts are now actual errors,
3987 instead of warnings.
3989 ** GLR, YACC parsers.
3990 The %parse-params are available in the destructors (and the
3991 experimental printers) as per the documentation.
3993 ** Bison now warns if it finds a stray "$" or "@" in an action.
3995 ** %require "VERSION"
3996 This specifies that the grammar file depends on features implemented
3997 in Bison version VERSION or higher.
3999 ** lalr1.cc: The token and value types are now class members.
4000 The tokens were defined as free form enums and cpp macros. YYSTYPE
4001 was defined as a free form union. They are now class members:
4002 tokens are enumerations of the "yy::parser::token" struct, and the
4003 semantic values have the "yy::parser::semantic_type" type.
4005 If you do not want or can update to this scheme, the directive
4006 '%define "global_tokens_and_yystype" "1"' triggers the global
4007 definition of tokens and YYSTYPE. This change is suitable both
4008 for previous releases of Bison, and this one.
4010 If you wish to update, then make sure older version of Bison will
4011 fail using '%require "2.2"'.
4013 ** DJGPP support added.
4016 * Noteworthy changes in release 2.1 (2005-09-16)
4018 ** The C++ lalr1.cc skeleton supports %lex-param.
4020 ** Bison-generated parsers now support the translation of diagnostics like
4021 "syntax error" into languages other than English. The default
4022 language is still English. For details, please see the new
4023 Internationalization section of the Bison manual. Software
4024 distributors should also see the new PACKAGING file. Thanks to
4025 Bruno Haible for this new feature.
4027 ** Wording in the Bison-generated parsers has been changed slightly to
4028 simplify translation. In particular, the message "memory exhausted"
4029 has replaced "parser stack overflow", as the old message was not
4030 always accurate for modern Bison-generated parsers.
4032 ** Destructors are now called when the parser aborts, for all symbols left
4033 behind on the stack. Also, the start symbol is now destroyed after a
4034 successful parse. In both cases, the behavior was formerly inconsistent.
4036 ** When generating verbose diagnostics, Bison-generated parsers no longer
4037 quote the literal strings associated with tokens. For example, for
4038 a syntax error associated with '%token NUM "number"' they might
4039 print 'syntax error, unexpected number' instead of 'syntax error,
4040 unexpected "number"'.
4043 * Noteworthy changes in release 2.0 (2004-12-25)
4045 ** Possibly-incompatible changes
4047 - Bison-generated parsers no longer default to using the alloca function
4048 (when available) to extend the parser stack, due to widespread
4049 problems in unchecked stack-overflow detection. You can "#define
4050 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
4051 the manual to determine safe values for YYMAXDEPTH in that case.
4053 - Error token location.
4054 During error recovery, the location of the syntax error is updated
4055 to cover the whole sequence covered by the error token: it includes
4056 the shifted symbols thrown away during the first part of the error
4057 recovery, and the lookahead rejected during the second part.
4059 - Semicolon changes:
4060 . Stray semicolons are no longer allowed at the start of a grammar.
4061 . Semicolons are now required after in-grammar declarations.
4063 - Unescaped newlines are no longer allowed in character constants or
4064 string literals. They were never portable, and GCC 3.4.0 has
4065 dropped support for them. Better diagnostics are now generated if
4066 forget a closing quote.
4068 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4072 - GLR grammars now support locations.
4074 - New directive: %initial-action.
4075 This directive allows the user to run arbitrary code (including
4076 initializing @$) from yyparse before parsing starts.
4078 - A new directive "%expect-rr N" specifies the expected number of
4079 reduce/reduce conflicts in GLR parsers.
4081 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4082 This is a GNU extension.
4084 - The option "--report=lookahead" was changed to "--report=look-ahead".
4085 [However, this was changed back after 2.3.]
4087 - Experimental %destructor support has been added to lalr1.cc.
4089 - New configure option --disable-yacc, to disable installation of the
4090 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4094 - For now, %expect-count violations are now just warnings, not errors.
4095 This is for compatibility with Bison 1.75 and earlier (when there are
4096 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4097 are too many or too few shift/reduce conflicts). However, in future
4098 versions of Bison we plan to improve the %expect machinery so that
4099 these violations will become errors again.
4101 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4102 arbitrarily limited to 16-bit counts.
4104 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4107 * Noteworthy changes in release 1.875 (2003-01-01)
4109 ** The documentation license has been upgraded to version 1.2
4110 of the GNU Free Documentation License.
4112 ** syntax error processing
4114 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4115 locations too. This fixes bugs in error-location computation.
4118 It is now possible to reclaim the memory associated to symbols
4119 discarded during error recovery. This feature is still experimental.
4122 This new directive is preferred over YYERROR_VERBOSE.
4124 - #defining yyerror to steal internal variables is discouraged.
4125 It is not guaranteed to work forever.
4127 ** POSIX conformance
4129 - Semicolons are once again optional at the end of grammar rules.
4130 This reverts to the behavior of Bison 1.33 and earlier, and improves
4131 compatibility with Yacc.
4133 - "parse error" -> "syntax error"
4134 Bison now uniformly uses the term "syntax error"; formerly, the code
4135 and manual sometimes used the term "parse error" instead. POSIX
4136 requires "syntax error" in diagnostics, and it was thought better to
4139 - The documentation now emphasizes that yylex and yyerror must be
4140 declared before use. C99 requires this.
4142 - Bison now parses C99 lexical constructs like UCNs and
4143 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4145 - File names are properly escaped in C output. E.g., foo\bar.y is
4146 output as "foo\\bar.y".
4148 - Yacc command and library now available
4149 The Bison distribution now installs a "yacc" command, as POSIX requires.
4150 Also, Bison now installs a small library liby.a containing
4151 implementations of Yacc-compatible yyerror and main functions.
4152 This library is normally not useful, but POSIX requires it.
4154 - Type clashes now generate warnings, not errors.
4156 - If the user does not define YYSTYPE as a macro, Bison now declares it
4157 using typedef instead of defining it as a macro.
4158 For consistency, YYLTYPE is also declared instead of defined.
4160 ** Other compatibility issues
4162 - %union directives can now have a tag before the "{", e.g., the
4163 directive "%union foo {...}" now generates the C code
4164 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4165 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4166 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4167 This is for compatibility with both Yacc and Bison 1.35.
4169 - ";" is output before the terminating "}" of an action, for
4170 compatibility with Bison 1.35.
4172 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4173 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4175 - "yystype" and "yyltype" are now obsolescent macros instead of being
4176 typedefs or tags; they are no longer documented and are planned to be
4177 withdrawn in a future release.
4182 Users of Bison have to decide how they handle the portability of the
4185 - "parsing stack overflow..." -> "parser stack overflow"
4186 GLR parsers now report "parser stack overflow" as per the Bison manual.
4188 ** %parse-param and %lex-param
4189 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4190 additional context to yyparse and yylex. They suffer from several
4193 - a single argument only can be added,
4194 - their types are weak (void *),
4195 - this context is not passed to ancillary functions such as yyerror,
4196 - only yacc.c parsers support them.
4198 The new %parse-param/%lex-param directives provide a more precise control.
4201 %parse-param {int *nastiness}
4202 %lex-param {int *nastiness}
4203 %parse-param {int *randomness}
4205 results in the following signatures:
4207 int yylex (int *nastiness);
4208 int yyparse (int *nastiness, int *randomness);
4210 or, if both %pure-parser and %locations are used:
4212 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4213 int yyparse (int *nastiness, int *randomness);
4215 ** Bison now warns if it detects conflicting outputs to the same file,
4216 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4217 that command outputs both code and header to foo.h.
4219 ** #line in output files
4220 - --no-line works properly.
4222 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4223 later to be built. This change originally took place a few versions
4224 ago, but nobody noticed until we recently asked someone to try
4225 building Bison with a K&R C compiler.
4228 * Noteworthy changes in release 1.75 (2002-10-14)
4230 ** Bison should now work on 64-bit hosts.
4232 ** Indonesian translation thanks to Tedi Heriyanto.
4235 Fix spurious parse errors.
4238 Some people redefine yyerror to steal yyparse' private variables.
4239 Reenable this trick until an official feature replaces it.
4242 In agreement with POSIX and with other Yaccs, leaving a default
4243 action is valid when $$ is untyped, and $1 typed:
4247 but the converse remains an error:
4251 ** Values of midrule actions
4254 foo: { ... } { $$ = $1; } ...
4256 was incorrectly rejected: $1 is defined in the second midrule
4257 action, and is equal to the $$ of the first midrule action.
4260 * Noteworthy changes in release 1.50 (2002-10-04)
4265 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4266 almost any context-free grammar, ambiguous or not. The new declarations
4267 %dprec and %merge on grammar rules allow parse-time resolution of
4268 ambiguities. Contributed by Paul Hilfinger.
4270 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4271 like the Alpha, so please stick to 32-bit hosts for now.
4274 When not in Yacc compatibility mode, when the output file was not
4275 specified, running "bison foo/bar.y" created "foo/bar.c". It
4276 now creates "bar.c".
4279 The undefined token was systematically mapped to 2 which prevented
4280 the use of 2 by the user. This is no longer the case.
4282 ** Unknown token numbers
4283 If yylex returned an out of range value, yyparse could die. This is
4287 According to POSIX, the error token must be 256.
4288 Bison extends this requirement by making it a preference: *if* the
4289 user specified that one of her tokens is numbered 256, then error
4290 will be mapped onto another number.
4292 ** Verbose error messages
4293 They no longer report "..., expecting error or..." for states where
4294 error recovery is possible.
4297 Defaults to "$end" instead of "$".
4299 ** Error recovery now conforms to documentation and to POSIX
4300 When a Bison-generated parser encounters a syntax error, it now pops
4301 the stack until it finds a state that allows shifting the error
4302 token. Formerly, it popped the stack until it found a state that
4303 allowed some non-error action other than a default reduction on the
4304 error token. The new behavior has long been the documented behavior,
4305 and has long been required by POSIX. For more details, please see
4306 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4307 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4310 Popped tokens and nonterminals are now reported.
4313 Larger grammars are now supported (larger token numbers, larger grammar
4314 size (= sum of the LHS and RHS lengths), larger LALR tables).
4315 Formerly, many of these numbers ran afoul of 16-bit limits;
4316 now these limits are 32 bits on most hosts.
4318 ** Explicit initial rule
4319 Bison used to play hacks with the initial rule, which the user does
4320 not write. It is now explicit, and visible in the reports and
4324 Before, Bison reported the useless rules, but, although not used,
4325 included them in the parsers. They are now actually removed.
4327 ** Useless rules, useless nonterminals
4328 They are now reported, as a warning, with their locations.
4330 ** Rules never reduced
4331 Rules that can never be reduced because of conflicts are now
4334 ** Incorrect "Token not used"
4335 On a grammar such as
4337 %token useless useful
4339 exp: '0' %prec useful;
4341 where a token was used to set the precedence of the last rule,
4342 bison reported both "useful" and "useless" as useless tokens.
4344 ** Revert the C++ namespace changes introduced in 1.31
4345 as they caused too many portability hassles.
4347 ** Default locations
4348 By an accident of design, the default computation of @$ was
4349 performed after another default computation was performed: @$ = @1.
4350 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4351 the computation of @$.
4353 ** Token end-of-file
4354 The token end of file may be specified by the user, in which case,
4355 the user symbol is used in the reports, the graphs, and the verbose
4356 error messages instead of "$end", which remains being the default.
4360 %token MYEOF 0 "end of file"
4363 This old option, which has been broken for ages, is removed.
4366 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4367 Croatian, thanks to Denis Lackovic.
4369 ** Incorrect token definitions
4372 bison used to output
4375 ** Token definitions as enums
4376 Tokens are output both as the traditional #define's, and, provided
4377 the compiler supports ANSI C or is a C++ compiler, as enums.
4378 This lets debuggers display names instead of integers.
4381 In addition to --verbose, bison supports --report=THINGS, which
4382 produces additional information:
4384 complete the core item sets with their closure
4385 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4386 explicitly associate lookahead tokens to items
4388 describe shift/reduce conflicts solving.
4389 Bison used to systematically output this information on top of
4390 the report. Solved conflicts are now attached to their states.
4393 Previous versions don't complain when there is a type clash on
4394 the default action if the rule has a midrule action, such as in:
4402 ** GNU M4 is now required when using Bison.
4405 * Noteworthy changes in release 1.35 (2002-03-25)
4408 Some projects use Bison's C parser with C++ compilers, and define
4409 YYSTYPE as a class. The recent adjustment of C parsers for data
4410 alignment and 64 bit architectures made this impossible.
4412 Because for the time being no real solution for C++ parser
4413 generation exists, kludges were implemented in the parser to
4414 maintain this use. In the future, when Bison has C++ parsers, this
4415 kludge will be disabled.
4417 This kludge also addresses some C++ problems when the stack was
4421 * Noteworthy changes in release 1.34 (2002-03-12)
4423 ** File name clashes are detected
4424 $ bison foo.y -d -o foo.x
4425 fatal error: header and parser would both be named "foo.x"
4427 ** A missing ";" at the end of a rule triggers a warning
4428 In accordance with POSIX, and in agreement with other
4429 Yacc implementations, Bison will mandate this semicolon in the near
4430 future. This eases the implementation of a Bison parser of Bison
4431 grammars by making this grammar LALR(1) instead of LR(2). To
4432 facilitate the transition, this release introduces a warning.
4434 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4435 many portability hassles.
4437 ** DJGPP support added.
4439 ** Fix test suite portability problems.
4442 * Noteworthy changes in release 1.33 (2002-02-07)
4445 Groff could not be compiled for the definition of size_t was lacking
4446 under some conditions.
4452 * Noteworthy changes in release 1.32 (2002-01-23)
4454 ** Fix Yacc output file names
4456 ** Portability fixes
4458 ** Italian, Dutch translations
4461 * Noteworthy changes in release 1.31 (2002-01-14)
4465 ** GNU Gettext and %expect
4466 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4467 Bison dies on incorrect %expectations, we fear there will be
4468 too many bug reports for Gettext, so _for the time being_, %expect
4469 does not trigger an error when the input file is named "plural.y".
4471 ** Use of alloca in parsers
4472 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4473 malloc exclusively. Since 1.29, but was not NEWS'ed.
4475 alloca is used only when compiled with GCC, to avoid portability
4478 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4480 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4481 (as POSIX requires) instead of being undefined.
4484 Bison has always permitted actions such as { $$ = $1 }: it adds the
4485 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4486 is no longer output: one has to write { $$ = $1; }.
4488 ** Better C++ compliance
4489 The output parsers try to respect C++ namespaces.
4490 [This turned out to be a failed experiment, and it was reverted later.]
4493 Fixed bugs when reporting useless nonterminals.
4496 The parsers work properly on 64 bit hosts.
4499 Some calls to strerror resulted in scrambled or missing error messages.
4502 When the number of shift/reduce conflicts is correct, don't issue
4505 ** The verbose report includes the rule line numbers.
4507 ** Rule line numbers are fixed in traces.
4509 ** Swedish translation
4512 Verbose parse error messages from the parsers are better looking.
4513 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4514 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4516 ** Fixed parser memory leaks.
4517 When the generated parser was using malloc to extend its stacks, the
4518 previous allocations were not freed.
4520 ** Fixed verbose output file.
4521 Some newlines were missing.
4522 Some conflicts in state descriptions were missing.
4524 ** Fixed conflict report.
4525 Option -v was needed to get the result.
4529 Mismatches are errors, not warnings.
4531 ** Fixed incorrect processing of some invalid input.
4533 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4535 ** Fixed some typos in the documentation.
4537 ** %token MY_EOF 0 is supported.
4538 Before, MY_EOF was silently renumbered as 257.
4540 ** doc/refcard.tex is updated.
4542 ** %output, %file-prefix, %name-prefix.
4546 New, aliasing "--output-file".
4549 * Noteworthy changes in release 1.30 (2001-10-26)
4551 ** "--defines" and "--graph" have now an optional argument which is the
4552 output file name. "-d" and "-g" do not change; they do not take any
4555 ** "%source_extension" and "%header_extension" are removed, failed
4558 ** Portability fixes.
4561 * Noteworthy changes in release 1.29 (2001-09-07)
4563 ** The output file does not define const, as this caused problems when used
4564 with common autoconfiguration schemes. If you still use ancient compilers
4565 that lack const, compile with the equivalent of the C compiler option
4566 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4568 ** Added "-g" and "--graph".
4570 ** The Bison manual is now distributed under the terms of the GNU FDL.
4572 ** The input and the output files has automatically a similar extension.
4574 ** Russian translation added.
4576 ** NLS support updated; should hopefully be less troublesome.
4578 ** Added the old Bison reference card.
4580 ** Added "--locations" and "%locations".
4582 ** Added "-S" and "--skeleton".
4584 ** "%raw", "-r", "--raw" is disabled.
4586 ** Special characters are escaped when output. This solves the problems
4587 of the #line lines with path names including backslashes.
4590 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4591 "%debug", "%source_extension" and "%header_extension".
4594 Automatic location tracking.
4597 * Noteworthy changes in release 1.28 (1999-07-06)
4599 ** Should compile better now with K&R compilers.
4603 ** Fixed a problem with escaping the double quote character.
4605 ** There is now a FAQ.
4608 * Noteworthy changes in release 1.27
4610 ** The make rule which prevented bison.simple from being created on
4611 some systems has been fixed.
4614 * Noteworthy changes in release 1.26
4616 ** Bison now uses Automake.
4618 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4620 ** Token numbers now start at 257 as previously documented, not 258.
4622 ** Bison honors the TMPDIR environment variable.
4624 ** A couple of buffer overruns have been fixed.
4626 ** Problems when closing files should now be reported.
4628 ** Generated parsers should now work even on operating systems which do
4629 not provide alloca().
4632 * Noteworthy changes in release 1.25 (1995-10-16)
4634 ** Errors in the input grammar are not fatal; Bison keeps reading
4635 the grammar file, and reports all the errors found in it.
4637 ** Tokens can now be specified as multiple-character strings: for
4638 example, you could use "<=" for a token which looks like <=, instead
4639 of choosing a name like LESSEQ.
4641 ** The %token_table declaration says to write a table of tokens (names
4642 and numbers) into the parser file. The yylex function can use this
4643 table to recognize multiple-character string tokens, or for other
4646 ** The %no_lines declaration says not to generate any #line preprocessor
4647 directives in the parser file.
4649 ** The %raw declaration says to use internal Bison token numbers, not
4650 Yacc-compatible token numbers, when token names are defined as macros.
4652 ** The --no-parser option produces the parser tables without including
4653 the parser engine; a project can now use its own parser engine.
4654 The actions go into a separate file called NAME.act, in the form of
4655 a switch statement body.
4658 * Noteworthy changes in release 1.23
4660 The user can define YYPARSE_PARAM as the name of an argument to be
4661 passed into yyparse. The argument should have type void *. It should
4662 actually point to an object. Grammar actions can access the variable
4663 by casting it to the proper pointer type.
4665 Line numbers in output file corrected.
4668 * Noteworthy changes in release 1.22
4670 --help option added.
4673 * Noteworthy changes in release 1.20
4675 Output file does not redefine const for C++.
4679 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4680 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4681 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4682 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4683 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4684 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4685 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4686 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4687 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4688 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4689 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4690 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4691 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4692 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4693 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4694 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4695 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4696 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4697 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4698 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4699 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4700 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4701 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4702 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4703 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4704 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4705 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE CVE
4706 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4707 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4708 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4709 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4710 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4711 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4712 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4713 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4714 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4715 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4716 LocalWords: Vais xsltproc YYNOMEM YYLOCATION signedness YYBISON MITRE's
4717 LocalWords: libreadline YYMALLOC YYFREE MSVC redefinitions POSIXLY
4720 ispell-dictionary: "american"
4725 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4727 This file is part of Bison, the GNU Parser Generator.
4729 Permission is granted to copy, distribute and/or modify this document
4730 under the terms of the GNU Free Documentation License, Version 1.3 or
4731 any later version published by the Free Software Foundation; with no
4732 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4733 Texts. A copy of the license is included in the "GNU Free
4734 Documentation License" file as part of this distribution.