3 * Noteworthy changes in release ?.? (????-??-??) [?]
6 * Noteworthy changes in release 3.8.1 (2021-09-11) [stable]
8 The generation of prototypes for yylex and yyerror in Yacc mode is
9 breaking existing grammar files. To avoid breaking too many grammars, the
10 prototypes are now generated when `-y/--yacc` is used *and* the
11 `POSIXLY_CORRECT` environment variable is defined.
13 Avoid using `-y`/`--yacc` simply to comply with Yacc's file name
14 conventions, rather, use `-o y.tab.c`. Autoconf's AC_PROG_YACC macro uses
15 `-y`. Avoid it if possible, for instance by using gnulib's gl_PROG_BISON.
17 * Noteworthy changes in release 3.8 (2021-09-07) [stable]
19 ** Backward incompatible changes
21 In conformance with the recommendations of the Graphviz team
22 (https://marc.info/?l=graphviz-devel&m=129418103126092), `-g`/`--graph`
23 now generates a *.gv file by default, instead of *.dot. A transition
26 To comply with the latest POSIX standard, in Yacc compatibility mode
27 (options `-y`/`--yacc`) Bison now generates prototypes for yyerror and
28 yylex. In some situations, this is breaking compatibility: if the user
29 has already declared these functions but with some differences (e.g., to
30 declare them as static, or to use specific attributes), the generated
31 parser will fail to compile. To disable these prototypes, #define yyerror
32 (to `yyerror`), and likewise for yylex.
34 ** Deprecated features
36 Support for the YYPRINT macro is removed. It worked only with yacc.c and
37 only for tokens. It was obsoleted by %printer, introduced in Bison 1.50
40 It has always been recommended to prefer `%define api.value.type foo` to
41 `#define YYSTYPE foo`. The latter is supported in C for compatibility
42 with Yacc, but not in C++. Warnings are now issued if `#define YYSTYPE`
43 is used in C++, and eventually support will be removed.
45 In C++ code, prefer value_type to semantic_type to denote the semantic
46 value type, which is specified by the `api.value.type` %define variable.
50 *** A skeleton for the D programming language
52 The "lalr1.d" skeleton is now officially part of Bison.
54 It was originally contributed by Oliver Mangold, based on Paolo Bonzini's
55 lalr1.java, and was improved by H. S. Teoh. Adela Vais then took over
56 maintenance and invested a lot of efforts to complete, test and document
59 It now supports all the bells and whistles of the other deterministic
60 parsers, which include: pull/push interfaces, verbose and custom error
61 messages, lookahead correction, token constructors, internationalization,
62 locations, printers, token and symbol prefixes, etc.
64 Two examples demonstrate the D parsers: a basic one (examples/d/simple),
65 and an advanced one (examples/d/calc).
67 *** Option -H, --header and directive %header
69 The option `-H`/`--header` supersedes the option `--defines`, and the
70 directive %header supersedes %defines. Both `--defines` and `%defines`
71 are, of course, maintained for backward compatibility.
75 Since version 2.4 Bison can be used to generate HTML reports. However it
76 was a two-step process: first bison must be invoked with option `--xml`,
77 and then xsltproc must be run to the convert the XML reports into HTML.
79 The new option `--html` combines these steps. The xsltproc program must
82 *** A C++ native GLR parser
84 A new version of the C++ GLR parser was added: "glr2.cc". It generates
85 "true C++11", instead of a C++ wrapper around a C parser as does the
86 existing "glr.cc" parser. As a first significant consequence, it supports
87 `%define api.value.type variant`, contrary to glr.cc.
89 It should be upward compatible in terms of interface, feature and
90 performance to "glr.cc". To try it out, simply use
94 It will eventually replace "glr.cc". However we need user feedback on
95 this skeleton. _Please_ report your results and comments about it.
99 Counterexamples now show the rule numbers, and always show ε for rules
100 with an empty right-hand side. For instance
112 *** Lookahead correction in Java
114 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
117 *** Abort parsing for memory exhaustion (C)
119 User actions may now use `YYNOMEM` (similar to `YYACCEPT` and `YYABORT`)
120 to abort the current parse with memory exhaustion.
122 *** Printing locations in debug traces (C)
124 The `YYLOCATION_PRINT(File, Loc)` macro prints a location. It is defined
125 when (i) locations are enabled, (ii) the default type for locations is
126 used, (iii) debug traces are enabled, and (iv) `YYLOCATION_PRINT` is not
129 Users may define `YYLOCATION_PRINT` to cover other cases.
133 There were no debug traces for deferred calls to user actions. They are
137 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
141 *** Reused Push Parsers
143 When a push-parser state structure is used for multiple parses, it was
144 possible for some state to leak from one run into the following one.
146 *** Fix Table Generation
148 In some very rare conditions, when there are many useless tokens, it was
149 possible to generate incorrect parsers.
152 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
156 *** Counterexample Generation
158 In some cases counterexample generation could crash. This is fixed.
160 *** Fix Table Generation
162 In some very rare conditions, when there are many useless tokens, it was
163 possible to generate incorrect parsers.
165 *** GLR parsers now support %merge together with api.value.type=union.
167 *** C++ parsers use noexcept in more places.
169 *** Generated parsers avoid some warnings about signedness issues.
171 *** C-language parsers now avoid warnings from pedantic clang.
173 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
176 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
180 *** Bug fixes in yacc.c
182 In Yacc mode, all the tokens are defined twice: once as an enum, and then
183 as a macro. YYEMPTY was missing its macro.
185 *** Bug fixes in lalr1.cc
187 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
188 even when the `parse.assert` %define variable is not enabled. It no
191 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
194 When there is a very large number of tokens, some assertions could be long
195 enough to hit arbitrary limits in Visual C++. They have been rewritten to
196 work around this limitation.
200 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
201 skeleton) used to be defined to 1. It is now defined to the version of
202 Bison as an integer (e.g., 30704 for version 3.7.4).
205 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
209 Fix concurrent build issues.
211 The bison executable is no longer linked uselessly against libreadline.
213 Fix incorrect use of yytname in glr.cc.
216 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
218 This release of Bison fixes all known bugs reported for Bison in MITRE's
219 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
220 are only about bison-the-program itself, not the generated code.
222 Although these bugs are typically irrelevant to how Bison is used, they
223 are worth fixing if only to give users peace of mind.
225 There is no known vulnerability in the generated parsers.
229 Fix concurrent build issues (introduced in Bison 3.5).
231 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
233 Fix portability issues of the test suite, and of bison itself.
235 Some unlikely crashes found by fuzzing have been fixed. This is only
236 about bison itself, not the generated parsers.
239 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
243 Crash when a token alias contains a NUL byte.
245 Portability issues with libtextstyle.
247 Portability issues of Bison itself with MSVC.
251 Improvements and fixes in the documentation.
253 More precise location about symbol type redefinitions.
256 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
258 ** Deprecated features
260 The YYPRINT macro, which works only with yacc.c and only for tokens, was
261 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
262 It is deprecated and its support will be removed eventually.
264 In conformance with the recommendations of the Graphviz team, in the next
265 version Bison the option `--graph` will generate a *.gv file by default,
266 instead of *.dot. A transition started in Bison 3.4.
270 *** Counterexample Generation
272 Contributed by Vincent Imbimbo.
274 When given `-Wcounterexamples`/`-Wcex`, bison will now output
275 counterexamples for conflicts.
277 **** Unifying Counterexamples
279 Unifying counterexamples are strings which can be parsed in two ways due
280 to the conflict. For example on a grammar that contains the usual
281 "dangling else" ambiguity:
284 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
285 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
288 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
289 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
290 Example: "if" exp "then" "if" exp "then" exp • "else" exp
293 ↳ "if" exp "then" exp
294 ↳ "if" exp "then" exp • "else" exp
295 Example: "if" exp "then" "if" exp "then" exp • "else" exp
298 ↳ "if" exp "then" exp "else" exp
299 ↳ "if" exp "then" exp •
301 When text styling is enabled, colors are used in the examples and the
302 derivations to highlight the structure of both analyses. In this case,
304 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
308 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
311 The counterexamples are "focused", in two different ways. First, they do
312 not clutter the output with all the derivations from the start symbol,
313 rather they start on the "conflicted nonterminal". They go straight to the
314 point. Second, they don't "expand" nonterminal symbols uselessly.
316 **** Nonunifying Counterexamples
318 In the case of the dangling else, Bison found an example that can be
319 parsed in two ways (therefore proving that the grammar is ambiguous).
320 When it cannot find such an example, it instead generates two examples
321 that are the same up until the dot:
324 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
325 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
326 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
331 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
332 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
333 First example: expr • ID ',' ID $end
340 Second example: expr • ID $end
346 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
350 In these cases, the parser usually doesn't have enough lookahead to
351 differentiate the two given examples.
355 Counterexamples are also included in the report when given
356 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
361 1 exp: "if" exp "then" exp • [$end, "then", "else"]
362 2 | "if" exp "then" exp • "else" exp
364 "else" shift, and go to state 8
366 "else" [reduce using rule 1 (exp)]
367 $default reduce using rule 1 (exp)
369 shift/reduce conflict on token "else":
370 1 exp: "if" exp "then" exp •
371 2 exp: "if" exp "then" exp • "else" exp
372 Example: "if" exp "then" "if" exp "then" exp • "else" exp
375 ↳ "if" exp "then" exp
376 ↳ "if" exp "then" exp • "else" exp
377 Example: "if" exp "then" "if" exp "then" exp • "else" exp
380 ↳ "if" exp "then" exp "else" exp
381 ↳ "if" exp "then" exp •
383 *** File prefix mapping
385 Contributed by Joshua Watt.
387 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
388 in the output (specifically `#line` directives and `#ifdef` header guards)
389 that begins with the prefix OLD will have it replaced with the prefix NEW,
390 similar to the `-ffile-prefix-map` in GCC. This option can be used to
391 make bison output reproducible.
397 When text styling is enabled and the terminal supports it, the warnings
398 now include hyperlinks to the documentation.
400 *** Relocatable installation
402 When installed to be relocatable (via `configure --enable-relocatable`),
403 bison will now also look for a relocated m4.
407 The `filename_type` %define variable was renamed `api.filename.type`.
410 %define filename_type "symbol"
414 %define api.filename.type {symbol}
416 (Or let `bison --update` do it for you).
418 It now defaults to `const std::string` instead of `std::string`.
420 *** Deprecated %define variable names
422 The following variables have been renamed for consistency. Backward
423 compatibility is ensured, but upgrading is recommended.
425 filename_type -> api.filename.type
426 package -> api.package
428 *** Push parsers no longer clear their state when parsing is finished
430 Previously push-parsers cleared their state when parsing was finished (on
431 success and on failure). This made it impossible to check if there were
432 parse errors, since `yynerrs` was also reset. This can be especially
433 troublesome when used in autocompletion, since a parser with error
434 recovery would suggest (irrelevant) expected tokens even if there were
437 Now the parser state can be examined when parsing is finished. The parser
438 state is reset when starting a new parse.
444 The bistromathic demonstrates %param and how to quote sources in the error
448 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
454 *** Include the generated header (yacc.c)
456 Historically, when --defines was used, bison generated a header and pasted
457 an exact copy of it into the generated parser implementation file. Since
458 Bison 3.4 it is possible to specify that the header should be `#include`d,
459 and how. For instance
461 %define api.header.include {"parse.h"}
465 %define api.header.include {<parser/parse.h>}
467 Now api.header.include defaults to `"header-basename"`, as was intended in
468 Bison 3.4, where `header-basename` is the basename of the generated
469 header. This is disabled when the generated header is `y.tab.h`, to
470 comply with Automake's ylwrap.
472 *** String aliases are faithfully propagated
474 Bison used to interpret user strings (i.e., decoding backslash escapes)
475 when reading them, and to escape them (i.e., issue non-printable
476 characters as backslash escapes, taking the locale into account) when
477 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
478 up "ciphered" as sequences of backslash escapes. This happened not only
479 in the generated sources (where the compiler will reinterpret them), but
480 also in all the generated reports (text, xml, html, dot, etc.). Reports
481 were therefore not readable when string aliases were not pure ASCII.
482 Worse yet: the output depended on the user's locale.
484 Now Bison faithfully treats the string aliases exactly the way the user
485 spelled them. This fixes all the aforementioned problems. However, now,
486 string aliases semantically equivalent but syntactically different (e.g.,
487 "A", "\x41", "\101") are considered to be different.
489 *** Crash when generating IELR
491 An old, well hidden, bug in the generation of IELR parsers was fixed.
494 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
498 In glr.cc some internal macros leaked in the user's code, and could damage
499 access to the token kinds.
502 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
506 Incorrect comments in the generated parsers.
508 Warnings in push parsers (yacc.c).
510 Incorrect display of gotos in LAC traces (lalr1.cc).
513 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
517 Some tests were fixed.
519 When token aliases contain comment delimiters:
521 %token FOO "/* foo */"
523 bison used to emit "nested" comments, which is invalid C.
526 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
530 Restored ANSI-C compliance in yacc.c.
532 GNU readline portability issues.
534 In C++, yy::parser::symbol_name is now a public member, as was intended.
538 In C++, yy::parser::symbol_type now has a public name() member function.
541 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
543 ** Backward incompatible changes
545 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
547 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
548 depend on it will now produce Yacc-like error messages (just "syntax
549 error"). It was superseded by the "%error-verbose" directive in Bison
550 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
551 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
552 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
553 parse.error verbose".
555 ** Deprecated features
557 The YYPRINT macro, which works only with yacc.c and only for tokens, was
558 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
559 It is deprecated and its support will be removed eventually.
563 *** Improved syntax error messages
565 Two new values for the %define parse.error variable offer more control to
566 the user. Available in all the skeletons (C, C++, Java).
568 **** %define parse.error detailed
570 The behavior of "%define parse.error detailed" is closely resembling that
571 of "%define parse.error verbose" with a few exceptions. First, it is safe
572 to use non-ASCII characters in token aliases (with 'verbose', the result
573 depends on the locale with which bison was run). Second, a yysymbol_name
574 function is exposed to the user, instead of the yytnamerr function and the
575 yytname table. Third, token internationalization is supported (see
578 **** %define parse.error custom
580 With this directive, the user forges and emits the syntax error message
581 herself by defining the yyreport_syntax_error function. A new type,
582 yypcontext_t, captures the circumstances of the error, and provides the
583 user with functions to get details, such as yypcontext_expected_tokens to
584 get the list of expected token kinds.
586 A possible implementation of yyreport_syntax_error is:
589 yyreport_syntax_error (const yypcontext_t *ctx)
592 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
593 fprintf (stderr, ": syntax error");
594 // Report the tokens expected at this point.
596 enum { TOKENMAX = 10 };
597 yysymbol_kind_t expected[TOKENMAX];
598 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
600 // Forward errors to yyparse.
603 for (int i = 0; i < n; ++i)
604 fprintf (stderr, "%s %s",
605 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
607 // Report the unexpected token.
609 yysymbol_kind_t lookahead = yypcontext_token (ctx);
610 if (lookahead != YYSYMBOL_YYEMPTY)
611 fprintf (stderr, " before %s", yysymbol_name (lookahead));
613 fprintf (stderr, "\n");
617 **** Token aliases internationalization
619 When the %define variable parse.error is set to `custom` or `detailed`,
620 one may specify which token aliases are to be translated using _(). For
632 In that case the user must define _() and N_(), and yysymbol_name returns
633 the translated symbol (i.e., it returns '_("variable")' rather that
634 '"variable"'). In Java, the user must provide an i18n() function.
636 *** List of expected tokens (yacc.c)
638 Push parsers may invoke yypstate_expected_tokens at any point during
639 parsing (including even before submitting the first token) to get the list
640 of possible tokens. This feature can be used to propose autocompletion
641 (see below the "bistromathic" example).
643 It makes little sense to use this feature without enabling LAC (lookahead
646 *** Returning the error token
648 When the scanner returns an invalid token or the undefined token
649 (YYUNDEF), the parser generates an error message and enters error
650 recovery. Because of that error message, most scanners that find lexical
651 errors generate an error message, and then ignore the invalid input
652 without entering the error-recovery.
654 The scanners may now return YYerror, the error token, to enter the
655 error-recovery mode without triggering an additional error message. See
656 the bistromathic for an example.
658 *** Deep overhaul of the symbol and token kinds
660 To avoid the confusion with types in programming languages, we now refer
661 to token and symbol "kinds" instead of token and symbol "types". The
662 documentation and error messages have been revised.
664 All the skeletons have been updated to use dedicated enum types rather
665 than integral types. Special symbols are now regular citizens, instead of
666 being declared in ad hoc ways.
670 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
671 LPAREN, etc. While backward compatibility is of course ensured, users are
672 nonetheless invited to replace their uses of "enum yytokentype" by
675 This type now also includes tokens that were previously hidden: YYEOF (end
676 of input), YYUNDEF (undefined token), and YYerror (error token). They
677 now have string aliases, internationalized when internationalization is
678 enabled. Therefore, by default, error messages now refer to "end of file"
679 (internationalized) rather than the cryptic "$end", or to "invalid token"
680 rather than "$undefined".
682 Therefore in most cases it is now useless to define the end-of-line token
685 %token T_EOF 0 "end of file"
687 Rather simply use "YYEOF" in your scanner.
691 The "symbol kinds" is what the parser actually uses. (Unless the
692 api.token.raw %define variable is used, the symbol kind of a terminal
693 differs from the corresponding token kind.)
695 They are now exposed as a enum, "yysymbol_kind_t".
697 This allows users to tailor the error messages the way they want, or to
698 process some symbols in a specific way in autocompletion (see the
699 bistromathic example below).
701 *** Modernize display of explanatory statements in diagnostics
703 Since Bison 2.7, output was indented four spaces for explanatory
704 statements. For example:
706 input.y:2.7-13: error: %type redeclaration for exp
707 input.y:1.7-11: previous declaration
709 Since the introduction of caret-diagnostics, it became less clear. This
710 indentation has been removed and submessages are displayed similarly as in
713 input.y:2.7-13: error: %type redeclaration for exp
714 2 | %type <float> exp
716 input.y:1.7-11: note: previous declaration
720 Contributed by Victor Morales Cayuela.
724 The token and symbol kinds are yy::parser::token_kind_type and
725 yy::parser::symbol_kind_type.
727 The symbol_type::kind() member function allows to get the kind of a
728 symbol. This can be used to write unit tests for scanners, e.g.,
730 yy::parser::symbol_type t = make_NUMBER ("123");
731 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
732 assert (t.value.as<int> () == 123);
738 In order to avoid ambiguities with "type" as in "typing", we now refer to
739 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
740 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
745 There are now examples/java: a very simple calculator, and a more complete
746 one (push-parser, location tracking, and debug traces).
748 The lexcalc example (a simple example in C based on Flex and Bison) now
749 also demonstrates location tracking.
752 A new C example, bistromathic, is a fully featured interactive calculator
753 using many Bison features: pure interface, push parser, autocompletion
754 based on the current parser state (using yypstate_expected_tokens),
755 location tracking, internationalized custom error messages, lookahead
756 correction, rich debug traces, etc.
758 It shows how to depend on the symbol kinds to tailor autocompletion. For
759 instance it recognizes the symbol kind "VARIABLE" to propose
760 autocompletion on the existing variables, rather than of the word
764 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
766 ** WARNING: Future backward-incompatibilities!
768 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
770 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
771 that still depend on it will produce Yacc-like error messages (just
772 "syntax error"). It was superseded by the "%error-verbose" directive in
773 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
774 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
775 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
776 parse.error verbose".
780 Fix portability issues of the package itself on old compilers.
782 Fix api.token.raw support in Java.
785 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
789 Error messages could quote lines containing zero-width characters (such as
790 \005) with incorrect styling. Fixes for similar issues with unexpectedly
791 short lines (e.g., the file was changed between parsing and diagnosing).
793 Some unlikely crashes found by fuzzing have been fixed. This is only
794 about bison itself, not the generated parsers.
797 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
801 Portability issues and minor cosmetic issues.
803 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
807 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
813 Fix compiler warnings.
816 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
818 ** Backward incompatible changes
820 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
821 longer treated as end-of-lines. This changes the diagnostics, and in
822 particular their locations.
824 In C++, line numbers and columns are now represented as 'int' not
825 'unsigned', so that integer overflow on positions is easily checkable via
826 'gcc -fsanitize=undefined' and the like. This affects the API for
827 positions. The default position and location classes now expose
828 'counter_type' (int), used to define line and column numbers.
830 ** Deprecated features
832 The YYPRINT macro, which works only with yacc.c and only for tokens, was
833 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
834 It is deprecated and its support will be removed eventually.
838 *** Lookahead correction in C++
840 Contributed by Adrian Vogelsgesang.
842 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
843 %define variable parse.lac.
845 *** Variable api.token.raw: Optimized token numbers (all skeletons)
847 In the generated parsers, tokens have two numbers: the "external" token
848 number as returned by yylex (which starts at 257), and the "internal"
849 symbol number (which starts at 3). Each time yylex is called, a table
850 lookup maps the external token number to the internal symbol number.
852 When the %define variable api.token.raw is set, tokens are assigned their
853 internal number, which saves one table lookup per token, and also saves
854 the generation of the mapping table.
856 The gain is typically moderate, but in extreme cases (very simple user
857 actions), a 10% improvement can be observed.
859 *** Generated parsers use better types for states
861 Stacks now use the best integral type for state numbers, instead of always
862 using 15 bits. As a result "small" parsers now have a smaller memory
863 footprint (they use 8 bits), and there is support for large automata (16
864 bits), and extra large (using int, i.e., typically 31 bits).
866 *** Generated parsers prefer signed integer types
868 Bison skeletons now prefer signed to unsigned integer types when either
869 will do, as the signed types are less error-prone and allow for better
870 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
871 portable to unusual machines where char, short and int are all the same
872 width. On non-GNU platforms this may entail including <limits.h> and (if
873 available) <stdint.h> to define integer types and constants.
875 *** A skeleton for the D programming language
877 For the last few releases, Bison has shipped a stealth experimental
878 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
879 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
882 However, because nobody has committed to improving, testing, and
883 documenting this skeleton, it is not clear that it will be supported in
886 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
887 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
889 *** Debug traces in Java
891 The Java backend no longer emits code and data for parser tracing if the
892 %define variable parse.trace is not defined.
896 *** New diagnostic: -Wdangling-alias
898 String literals, which allow for better error messages, are (too)
899 liberally accepted by Bison, which might result in silent errors. For
902 %type <exVal> cond "condition"
904 does not define "condition" as a string alias to 'cond' (nonterminal
905 symbols do not have string aliases). It is rather equivalent to
908 %token <exVal> "condition"
910 i.e., it gives the type 'exVal' to the "condition" token, which was
911 clearly not the intention.
913 Also, because string aliases need not be defined, typos such as "baz"
914 instead of "bar" will be not reported.
916 The option `-Wdangling-alias` catches these situations. On
919 %type <ival> foo "foo"
923 bison -Wdangling-alias reports
925 warning: string literal not attached to a symbol
926 | %type <ival> foo "foo"
928 warning: string literal not attached to a symbol
932 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
934 *** Better POSIX Yacc compatibility diagnostics
936 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
940 %type <ival> TOKEN1 TOKEN2 't'
947 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
948 2 | %type <ival> TOKEN1 TOKEN2 't'
950 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
951 2 | %type <ival> TOKEN1 TOKEN2 't'
953 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
954 2 | %type <ival> TOKEN1 TOKEN2 't'
957 *** Diagnostics with insertion
959 The diagnostics now display the suggestion below the underlined source.
960 Replacement for undeclared symbols are now also suggested.
967 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'?
971 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
975 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
977 *** Diagnostics about long lines
979 Quoted sources may now be truncated to fit the screen. For instance, on a
980 30-column wide terminal:
987 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
990 foo.y:1.8-10: previous declaration
993 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
996 foo.y:1.8-10: previous declaration
1002 *** Debugging glr.c and glr.cc
1004 The glr.c skeleton always had asserts to check its own behavior (not the
1005 user's). These assertions are now under the control of the parse.assert
1006 %define variable (disabled by default).
1010 Several new compiler warnings in the generated output have been avoided.
1011 Some unused features are no longer emitted. Cleaner generated code in
1016 Portability issues in the test suite.
1018 In theory, parsers using %nonassoc could crash when reporting verbose
1019 error messages. This unlikely bug has been fixed.
1021 In Java, %define api.prefix was ignored. It now behaves as expected.
1024 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
1028 In some cases, when warnings are disabled, bison could emit tons of white
1029 spaces as diagnostics.
1031 When running out of memory, bison could crash (found by fuzzing).
1033 When defining twice the EOF token, bison would crash.
1035 New warnings from recent compilers have been addressed in the generated
1036 parsers (yacc.c, glr.c, glr.cc).
1038 When lone carriage-return characters appeared in the input file,
1039 diagnostics could hang forever.
1042 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
1049 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
1051 ** Deprecated features
1053 The %pure-parser directive is deprecated in favor of '%define api.pure'
1054 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
1055 now. Note that since Bison 2.7 you are strongly encouraged to use
1056 '%define api.pure full' instead of '%define api.pure'.
1060 *** Colored diagnostics
1062 As an experimental feature, diagnostics are now colored, controlled by the
1063 new options --color and --style.
1065 To use them, install the libtextstyle library before configuring Bison.
1066 It is available from
1068 https://alpha.gnu.org/gnu/gettext/
1072 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
1074 The option --color supports the following arguments:
1075 - always, yes: Enable colors.
1076 - never, no: Disable colors.
1077 - auto, tty (default): Enable colors if the output device is a tty.
1079 To customize the styles, create a CSS file similar to
1083 .error { font-weight: 800; text-decoration: underline; }
1086 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1087 environment variable to "bison-bw.css".
1089 *** Disabling output
1091 When given -fsyntax-only, the diagnostics are reported, but no output is
1094 The name of this option is somewhat misleading as bison does more than
1095 just checking the syntax: every stage is run (including checking for
1096 conflicts for instance), except the generation of the output files.
1098 *** Include the generated header (yacc.c)
1100 Before, when --defines is used, bison generated a header, and pasted an
1101 exact copy of it into the generated parser implementation file. If the
1102 header name is not "y.tab.h", it is now #included instead of being
1105 To use an '#include' even if the header name is "y.tab.h" (which is what
1106 happens with --yacc, or when using the Autotools' ylwrap), define
1107 api.header.include to the exact argument to pass to #include. For
1110 %define api.header.include {"parse.h"}
1114 %define api.header.include {<parser/parse.h>}
1116 *** api.location.type is now supported in C (yacc.c, glr.c)
1118 The %define variable api.location.type defines the name of the type to use
1119 for locations. When defined, Bison no longer defines YYLTYPE.
1121 This can be used in programs with several parsers to factor their
1122 definition of locations: let one of them generate them, and the others
1129 In conformance with the recommendations of the Graphviz team, if %require
1130 "3.4" (or better) is specified, the option --graph generates a *.gv file
1131 by default, instead of *.dot.
1133 *** Diagnostics overhaul
1135 Column numbers were wrong with multibyte characters, which would also
1136 result in skewed diagnostics with carets. Beside, because we were
1137 indenting the quoted source with a single space, lines with tab characters
1138 were incorrectly underlined.
1140 To address these issues, and to be clearer, Bison now issues diagnostics
1141 as GCC9 does. For instance it used to display (there's a tab before the
1144 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1145 expr: expr '+' "number" { $$ = $1 + $2; }
1149 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1150 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1153 Other constructs now also have better locations, resulting in more precise
1156 *** Fix-it hints for %empty
1158 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1159 annotations, and add the missing ones.
1161 *** Generated reports
1163 The format of the reports (parse.output) was improved for readability.
1165 *** Better support for --no-line.
1167 When --no-line is used, the generated files are now cleaner: no lines are
1168 generated instead of empty lines. Together with using api.header.include,
1169 that should help people saving the generated files into version control
1170 systems get smaller diffs.
1174 A new example in C shows an simple infix calculator with a hand-written
1175 scanner (examples/c/calc).
1177 A new example in C shows a reentrant parser (capable of recursive calls)
1178 built with Flex and Bison (examples/c/reccalc).
1180 There is a new section about the history of Yaccs and Bison.
1184 A few obscure bugs were fixed, including the second oldest (known) bug in
1185 Bison: it was there when Bison was entered in the RCS version control
1186 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1190 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1194 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1198 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1202 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1203 extensions into errors. It now makes them simple warnings (-Wyacc).
1206 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1208 A new mailing list was created, Bison Announce. It is low traffic, and is
1209 only about announcing new releases and important messages (e.g., polls
1210 about major decisions to make).
1212 https://lists.gnu.org/mailman/listinfo/bison-announce
1214 ** Backward incompatible changes
1216 Support for DJGPP, which has been unmaintained and untested for years, is
1219 ** Deprecated features
1221 A new feature, --update (see below) helps adjusting existing grammars to
1224 *** Deprecated directives
1226 The %error-verbose directive is deprecated in favor of '%define
1227 parse.error verbose' since Bison 3.0, but no warning was issued.
1229 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1230 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1231 directives are slightly different, you might need to adjust your code.
1232 %name-prefix renames only symbols with external linkage, while api.prefix
1233 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1234 yytokentype, YYSTYPE, YYLTYPE, etc.
1236 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1237 {xx}' will typically have to update YY_DECL from
1239 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1243 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1245 *** Deprecated %define variable names
1247 The following variables, mostly related to parsers in Java, have been
1248 renamed for consistency. Backward compatibility is ensured, but upgrading
1251 abstract -> api.parser.abstract
1252 annotations -> api.parser.annotations
1253 extends -> api.parser.extends
1254 final -> api.parser.final
1255 implements -> api.parser.implements
1256 parser_class_name -> api.parser.class
1257 public -> api.parser.public
1258 strictfp -> api.parser.strictfp
1262 *** Generation of fix-its for IDEs/Editors
1264 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1265 bison now generates machine readable editing instructions to fix some
1266 issues. Currently, this is mostly limited to updating deprecated
1267 directives and removing duplicates. For instance:
1271 %define parser_class_name "Parser"
1272 %define api.parser.class "Parser"
1276 See the "fix-it:" lines below:
1278 $ bison -ffixit foo.y
1279 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1282 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1283 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1284 %define parser_class_name "Parser"
1285 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1286 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1287 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1288 %define api.parser.class "Parser"
1289 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1290 foo.y:2.1-34: previous definition
1291 %define parser_class_name "Parser"
1292 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1293 fix-it:"foo.y":{3:1-3:34}:""
1294 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1296 This uses the same output format as GCC and Clang.
1298 *** Updating grammar files
1300 Fixes can be applied on the fly. The previous example ends with the
1301 suggestion to re-run bison with the option -u/--update, which results in a
1302 cleaner grammar file.
1304 $ bison --update foo.y
1306 bison: file 'foo.y' was updated (backup: 'foo.y~')
1309 %define parse.error verbose
1310 %define api.parser.class {Parser}
1314 *** Bison is now relocatable
1316 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1318 A relocatable program can be moved or copied to a different location on
1319 the file system. It can also be used through mount points for network
1320 sharing. It is possible to make symbolic links to the installed and moved
1321 programs, and invoke them through the symbolic link.
1323 *** %expect and %expect-rr modifiers on individual rules
1325 One can now document (and check) which rules participate in shift/reduce
1326 and reduce/reduce conflicts. This is particularly important GLR parsers,
1327 where conflicts are a normal occurrence. For example,
1343 | argument_list ',' expression
1348 Looking at the output from -v, one can see that the shift/reduce conflict
1349 here is due to the fact that the parser does not know whether to reduce
1350 arguments to argument_list until it sees the token _after_ the following
1351 ','. By marking the rule with %expect 1 (because there is a conflict in
1352 one state), we document the source of the 1 overall shift/reduce conflict.
1354 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1355 conflicts. In this case, we mark each of the conflicting rules. For
1364 target_list '=' expr ';'
1370 | target ',' target_list
1379 | expr ',' expr_list
1387 In a statement such as
1391 the parser must reduce x to a target or an expr, but does not know which
1392 until it sees the '='. So we notate the two possible reductions to
1393 indicate that each conflicts in one rule.
1395 This feature needs user feedback, and might evolve in the future.
1397 *** C++: Actual token constructors
1399 When variants and token constructors are enabled, in addition to the
1400 type-safe named token constructors (make_ID, make_INT, etc.), we now
1401 generate genuine constructors for symbol_type.
1403 For instance with these declarations
1409 you may use these constructors:
1411 symbol_type (int token, const std::string&);
1412 symbol_type (int token, const int&);
1413 symbol_type (int token);
1415 Correct matching between token types and value types is checked via
1416 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1417 constructors are preferable, as they offer better type safety (for
1418 instance 'make_ID (42)' would not even compile), but symbol_type
1419 constructors may help when token types are discovered at run-time, e.g.,
1422 if (auto i = lookup_keyword (yytext))
1423 return yy::parser::symbol_type (i);
1425 return yy::parser::make_ID (yytext);
1428 *** C++: Variadic emplace
1430 If your application requires C++11 and you don't use symbol constructors,
1431 you may now use a variadic emplace for semantic values:
1433 %define api.value.type variant
1434 %token <std::pair<int, int>> PAIR
1438 int yylex (parser::semantic_type *lvalp)
1440 lvalp->emplace <std::pair<int, int>> (1, 2);
1441 return parser::token::PAIR;
1444 *** C++: Syntax error exceptions in GLR
1446 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1447 actions, or from the scanner.
1449 *** More POSIX Yacc compatibility warnings
1451 More Bison specific directives are now reported with -y or -Wyacc. This
1452 change was ready since the release of Bison 3.0 in September 2015. It was
1453 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1454 in numerous warnings for Bison users that use the GNU Build System.
1456 If you still experience that problem, either redefine YACC as `bison -o
1457 y.tab.c`, or pass -Wno-yacc to Bison.
1459 *** The tables yyrhs and yyphrs are back
1461 Because no Bison skeleton uses them, these tables were removed (no longer
1462 passed to the skeletons, not even computed) in 2008. However, some users
1463 have expressed interest in being able to use them in their own skeletons.
1467 *** Incorrect number of reduce/reduce conflicts
1469 On a grammar such as
1471 exp: "num" | "num" | "num"
1473 bison used to report a single RR conflict, instead of two. This is now
1474 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1475 was entered in the RCS version control system, in December 1987.
1477 Some grammar files might have to adjust their %expect-rr.
1479 *** Parser directives that were not careful enough
1481 Passing invalid arguments to %nterm, for instance character literals, used
1482 to result in unclear error messages.
1486 The examples/ directory (installed in .../share/doc/bison/examples) has
1487 been restructured per language for clarity. The examples come with a
1488 README and a Makefile. Not only can they be used to toy with Bison, they
1489 can also be starting points for your own grammars.
1491 There is now a Java example, and a simple example in C based on Flex and
1492 Bison (examples/c/lexcalc/).
1498 They now use noexcept and constexpr. Please, report missing annotations.
1500 *** Symbol Declarations
1502 The syntax of the variation directives to declare symbols was overhauled
1503 for more consistency, and also better POSIX Yacc compliance (which, for
1504 instance, allows "%type" without actually providing a type). The %nterm
1505 directive, supported by Bison since its inception, is now documented and
1506 officially supported.
1508 The syntax is now as follows:
1510 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1511 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1512 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1513 %nterm TAG? ID+ ( TAG ID+ )*
1515 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1516 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1517 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1518 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1519 one), ‘*’ (zero or more) and ‘+’ (one or more).
1522 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1526 Fix the move constructor of symbol_type.
1528 Always provide a copy constructor for symbol_type, even in modern C++.
1531 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1535 Properly support token constructors in C++ with types that include commas
1536 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1539 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1543 C++ portability issues.
1546 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1550 Several portability issues have been fixed in the build system, in the
1551 test suite, and in the generated parsers in C++.
1554 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1556 ** Backward incompatible changes
1558 Support for DJGPP, which has been unmaintained and untested for years, is
1559 obsolete. Unless there is activity to revive it, it will be removed.
1563 %printers should use yyo rather than yyoutput to denote the output stream.
1565 Variant-based symbols in C++ should use emplace() rather than build().
1567 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1571 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1573 A comment in the generated code now emphasizes that users should not
1574 depend upon non-documented implementation details, such as macros starting
1579 *** C++: Support for move semantics (lalr1.cc)
1581 The lalr1.cc skeleton now fully supports C++ move semantics, while
1582 maintaining compatibility with C++98. You may now store move-only types
1583 when using Bison's variants. For instance:
1590 %skeleton "lalr1.cc"
1591 %define api.value.type variant
1595 %token <int> INT "int";
1596 %type <std::unique_ptr<int>> int;
1597 %type <std::vector<std::unique_ptr<int>>> list;
1601 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1603 int: "int" { $$ = std::make_unique<int>($1); }
1605 *** C++: Implicit move of right-hand side values (lalr1.cc)
1607 In modern C++ (C++11 and later), you should always use 'std::move' with
1608 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1609 popped from the stack anyway. Using 'std::move' is mandatory for
1610 move-only types such as unique_ptr, and it provides a significant speedup
1611 for large types such as std::string, or std::vector, etc.
1613 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1614 by 'std::move ($n)'. The second rule in the previous grammar can be
1617 list: list int { $$ = $1; $$.emplace_back($2); }
1619 With automove enabled, the semantic values are no longer lvalues, so do
1620 not use the swap idiom:
1622 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1624 This idiom is anyway obsolete: it is preferable to move than to swap.
1626 A warning is issued when automove is enabled, and a value is used several
1629 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1630 exp: "twice" exp { $$ = $2 + $2; }
1633 Enabling api.value.automove does not require support for modern C++. The
1634 generated code is valid C++98/03, but will use copies instead of moves.
1636 The new examples/c++/variant-11.yy shows these features in action.
1638 *** C++: The implicit default semantic action is always run
1640 When variants are enabled, the default action was not run, so
1648 It now behaves like in all the other cases, as
1650 exp: "number" { $$ = $1; }
1652 possibly using std::move if automove is enabled.
1654 We do not expect backward compatibility issues. However, beware of
1655 forward compatibility issues: if you rely on default actions with
1656 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1657 generate incorrect parsers.
1659 *** C++: Renaming location.hh
1661 When both %defines and %locations are enabled, Bison generates a
1662 location.hh file. If you don't use locations outside of the parser, you
1663 may avoid its creation with:
1665 %define api.location.file none
1667 However this file is useful if, for instance, your parser builds an AST
1668 decorated with locations: you may use Bison's location independently of
1669 Bison's parser. You can now give it another name, for instance:
1671 %define api.location.file "my-location.hh"
1673 This name can have directory components, and even be absolute. The name
1674 under which the location file is included is controlled by
1675 api.location.include.
1677 This way it is possible to have several parsers share the same location
1680 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1683 %define api.namespace {foo}
1684 %define api.location.file "include/ast/loc.hh"
1685 %define api.location.include {<ast/loc.hh>}
1687 and use it in src/bar/parser.hh:
1690 %define api.namespace {bar}
1691 %code requires {#include <ast/loc.hh>}
1692 %define api.location.type {bar::location}
1694 Absolute file names are supported, so in your Makefile, passing the flag
1695 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1698 *** C++: stack.hh and position.hh are deprecated
1700 When asked to generate a header file (%defines), the lalr1.cc skeleton
1701 generates a stack.hh file. This file had no interest for users; it is now
1702 made useless: its content is included in the parser definition. It is
1703 still generated for backward compatibility.
1705 When in addition to %defines, location support is requested (%locations),
1706 the file position.hh is also generated. It is now also useless: its
1707 content is now included in location.hh.
1709 These files are no longer generated when your grammar file requires at
1710 least Bison 3.2 (%require "3.2").
1714 Portability issues on MinGW and VS2015.
1716 Portability issues in the test suite.
1718 Portability/warning issues with Flex.
1721 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1723 ** Backward incompatible changes
1725 Compiling Bison now requires a C99 compiler---as announced during the
1726 release of Bison 3.0, five years ago. Generated parsers do not require a
1729 Support for DJGPP, which has been unmaintained and untested for years, is
1730 obsolete. Unless there is activity to revive it, the next release of Bison
1731 will have it removed.
1735 *** Typed midrule actions
1737 Because their type is unknown to Bison, the values of midrule actions are
1738 not treated like the others: they don't have %printer and %destructor
1739 support. It also prevents C++ (Bison) variants to handle them properly.
1741 Typed midrule actions address these issues. Instead of:
1743 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1747 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1749 *** Reports include the type of the symbols
1751 The sections about terminal and nonterminal symbols of the '*.output' file
1752 now specify their declared type. For instance, for:
1756 the report now shows '<ival>':
1758 Terminals, with rules where they appear
1762 *** Diagnostics about useless rules
1764 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1765 of course, its rules are useless too.
1769 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1771 Previously all the useless rules were reported, including those whose
1772 left-hand side is the 'exp' nonterminal:
1774 warning: 1 nonterminal useless in grammar [-Wother]
1775 warning: 4 rules useless in grammar [-Wother]
1776 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1779 2.14-16: warning: rule useless in grammar [-Wother]
1782 3.6-16: warning: rule useless in grammar [-Wother]
1783 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1785 3.20-30: warning: rule useless in grammar [-Wother]
1786 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1788 3.34-44: warning: rule useless in grammar [-Wother]
1789 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1792 Now, rules whose left-hand side symbol is useless are no longer reported
1793 as useless. The locations of the errors have also been adjusted to point
1794 to the first use of the nonterminal as a left-hand side of a rule:
1796 warning: 1 nonterminal useless in grammar [-Wother]
1797 warning: 4 rules useless in grammar [-Wother]
1798 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1799 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1801 2.14-16: warning: rule useless in grammar [-Wother]
1805 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1807 When compiled with exceptions disabled, the generated parsers no longer
1808 uses try/catch clauses.
1810 Currently only GCC and Clang are supported.
1814 *** A demonstration of variants
1816 A new example was added (installed in .../share/doc/bison/examples),
1817 'variant.yy', which shows how to use (Bison) variants in C++.
1819 The other examples were made nicer to read.
1821 *** Some features are no longer 'experimental'
1823 The following features, mature enough, are no longer flagged as
1824 experimental in the documentation: push parsers, default %printer and
1825 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1826 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1827 semantic predicates (%?).
1831 *** GLR: Predicates support broken by #line directives
1833 Predicates (%?) in GLR such as
1836 %? {new_syntax} 'w' id new_args
1837 | %?{!new_syntax} 'w' id old_args
1839 were issued with #lines in the middle of C code.
1841 *** Printer and destructor with broken #line directives
1843 The #line directives were not properly escaped when emitting the code for
1844 %printer/%destructor, which resulted in compiler errors if there are
1845 backslashes or double-quotes in the grammar file name.
1847 *** Portability on ICC
1849 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1850 Generated parsers now work around this.
1854 There were several small fixes in the test suite and in the build system,
1855 many warnings in bison and in the generated parsers were eliminated. The
1856 documentation also received its share of minor improvements.
1858 Useless code was removed from C++ parsers, and some of the generated
1859 constructors are more 'natural'.
1862 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1866 *** C++: Fix support of 'syntax_error'
1868 One incorrect 'inline' resulted in linking errors about the constructor of
1869 the syntax_error exception.
1871 *** C++: Fix warnings
1873 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1874 warnings about yyformat being possibly null. It also warned about the
1875 deprecated implicit definition of copy constructors when there's a
1876 user-defined (copy) assignment operator.
1878 *** Location of errors
1880 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1881 ride-hand side raises a syntax error. The behavior of the default parser
1882 (yacc.c) in such a condition was undefined.
1884 Now all the parsers match the behavior of glr.c: @$ is used as the
1885 location of the error. This handles gracefully rules with and without
1888 *** Portability fixes in the test suite
1890 On some platforms, some Java and/or C++ tests were failing.
1893 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1897 *** C++ with Variants (lalr1.cc)
1899 Fix a compiler warning when no %destructor use $$.
1903 Several portability issues in tests were fixed.
1906 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1910 *** C++ with Variants (lalr1.cc)
1912 Problems with %destructor and '%define parse.assert' have been fixed.
1914 *** Named %union support (yacc.c, glr.c)
1916 Bison 3.0 introduced a regression on named %union such as
1918 %union foo { int ival; };
1920 The possibility to use a name was introduced "for Yacc compatibility".
1921 It is however not required by POSIX Yacc, and its usefulness is not clear.
1923 *** %define api.value.type union with %defines (yacc.c, glr.c)
1925 The C parsers were broken when %defines was used together with "%define
1926 api.value.type union".
1928 *** Redeclarations are reported in proper order
1936 bison used to report:
1938 foo.yy:2.10-11: error: %printer redeclaration for FOO
1941 foo.yy:3.10-11: previous declaration
1945 Now, the "previous" declaration is always the first one.
1950 Bison now installs various files in its docdir (which defaults to
1951 '/usr/local/share/doc/bison'), including the three fully blown examples
1952 extracted from the documentation:
1955 Reverse Polish Calculator, a simple introductory example.
1957 Multi-function Calc, a calculator with memory and functions and located
1960 a calculator in C++ using variant support and token constructors.
1963 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1967 *** Generated source files when errors are reported
1969 When warnings are issued and -Werror is set, bison would still generate
1970 the source files (*.c, *.h...). As a consequence, some runs of "make"
1971 could fail the first time, but not the second (as the files were generated
1974 This is fixed: bison no longer generates this source files, but, of
1975 course, still produces the various reports (*.output, *.xml, etc.).
1977 *** %empty is used in reports
1979 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1980 dot, XML and formats derived from it).
1982 *** YYERROR and variants
1984 When C++ variant support is enabled, an error triggered via YYERROR, but
1985 not caught via error recovery, resulted in a double deletion.
1988 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1992 *** Errors in caret diagnostics
1994 On some platforms, some errors could result in endless diagnostics.
1996 *** Fixes of the -Werror option
1998 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1999 diagnostics into errors instead of warnings. This is fixed.
2001 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
2002 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
2003 -Wno-error", "foo" diagnostics are now errors.
2007 As demonstrated in the documentation, one can now leave spaces between
2012 The yacc.1 man page is no longer installed if --disable-yacc was
2015 *** Fixes in the test suite
2017 Bugs and portability issues.
2020 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
2022 ** WARNING: Future backward-incompatibilities!
2024 Like other GNU packages, Bison will start using some of the C99 features
2025 for its own code, especially the definition of variables after statements.
2026 The generated C parsers still aim at C90.
2028 ** Backward incompatible changes
2030 *** Obsolete features
2032 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
2034 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
2035 use YYSTYPE and YYLTYPE.
2037 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
2038 1.875): use %lex-param, %parse-param, or %param.
2040 Missing semicolons at the end of actions are no longer added (as announced
2041 in the release 2.5).
2043 *** Use of YACC='bison -y'
2045 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
2048 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
2049 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
2050 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
2052 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
2053 implementation of Yacc, was using Bison as 'bison -y'. While it does
2054 ensure compatible output file names, it also enables warnings for
2055 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
2056 warnings for Bison extensions.
2058 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
2059 (which also generates 'y.tab.h' and 'y.output' when needed).
2060 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
2061 flags (YFLAGS, or AM_YFLAGS with Automake).
2065 *** The epilogue is no longer affected by internal #defines (glr.c)
2067 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
2068 generated code. These weren't properly undefined before the inclusion of
2069 the user epilogue, so functions such as the following were butchered by the
2070 preprocessor expansion:
2072 int yylex (YYSTYPE *yylval);
2074 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2075 identifiers for user-provided variables.
2077 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2079 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2080 locations are enabled. This is fixed.
2082 *** Warnings about useless %pure-parser/%define api.pure are restored
2084 ** Diagnostics reported by Bison
2086 Most of these features were contributed by Théophile Ranquet and Victor
2091 Version 2.7 introduced caret errors, for a prettier output. These are now
2092 activated by default. The old format can still be used by invoking Bison
2093 with -fno-caret (or -fnone).
2095 Some error messages that reproduced excerpts of the grammar are now using
2096 the caret information only. For instance on:
2103 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2104 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2108 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2109 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2113 and "bison -fno-caret" reports:
2115 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2116 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2118 *** Enhancements of the -Werror option
2120 The -Werror=CATEGORY option is now recognized, and will treat specified
2121 warnings as errors. The warnings need not have been explicitly activated
2122 using the -W option, this is similar to what GCC 4.7 does.
2124 For example, given the following command line, Bison will treat both
2125 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2126 errors (and only those):
2128 $ bison -Werror=yacc,error=conflicts-sr input.y
2130 If no categories are specified, -Werror will make all active warnings into
2131 errors. For example, the following line does the same the previous example:
2133 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2135 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2137 Note that the categories in this -Werror option may not be prefixed with
2138 "no-". However, -Wno-error[=CATEGORY] is valid.
2140 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2141 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2142 incompatibilities as warnings: "-y -Wno-error=yacc".
2144 *** The display of warnings is now richer
2146 The option that controls a given warning is now displayed:
2148 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2150 In the case of warnings treated as errors, the prefix is changed from
2151 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2152 to GCC, as [-Werror=CATEGORY].
2154 For instance, where the previous version of Bison would report (and exit
2157 bison: warnings being treated as errors
2158 input.y:1.1: warning: stray ',' treated as white space
2162 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2164 *** Deprecated constructs
2166 The new 'deprecated' warning category flags obsolete constructs whose
2167 support will be discontinued. It is enabled by default. These warnings
2168 used to be reported as 'other' warnings.
2170 *** Useless semantic types
2172 Bison now warns about useless (uninhabited) semantic types. Since
2173 semantic types are not declared to Bison (they are defined in the opaque
2174 %union structure), it is %printer/%destructor directives about useless
2175 types that trigger the warning:
2179 %printer {} <type1> <type3>
2180 %destructor {} <type2> <type4>
2182 nterm: term { $$ = $1; };
2184 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2185 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2187 *** Undefined but unused symbols
2189 Bison used to raise an error for undefined symbols that are not used in
2190 the grammar. This is now only a warning.
2193 %destructor {} symbol2
2194 %type <type> symbol3
2198 *** Useless destructors or printers
2200 Bison now warns about useless destructors or printers. In the following
2201 example, the printer for <type1>, and the destructor for <type2> are
2202 useless: all symbols of <type1> (token1) already have a printer, and all
2203 symbols of type <type2> (token2) already have a destructor.
2205 %token <type1> token1
2209 %printer {} token1 <type1> <type3>
2210 %destructor {} token2 <type2> <type4>
2214 The warnings and error messages about shift/reduce and reduce/reduce
2215 conflicts have been normalized. For instance on the following foo.y file:
2219 exp: exp '+' exp | '0' | '0';
2221 compare the previous version of bison:
2224 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2225 $ bison -Werror foo.y
2226 bison: warnings being treated as errors
2227 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2229 with the new behavior:
2232 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2233 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2234 $ bison -Werror foo.y
2235 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2236 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2238 When %expect or %expect-rr is used, such as with bar.y:
2243 exp: exp '+' exp | '0' | '0';
2248 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2249 bar.y: expected 0 shift/reduce conflicts
2250 bar.y: expected 0 reduce/reduce conflicts
2255 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2256 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2258 ** Incompatibilities with POSIX Yacc
2260 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2263 ** Additional yylex/yyparse arguments
2265 The new directive %param declares additional arguments to both yylex and
2266 yyparse. The %lex-param, %parse-param, and %param directives support one
2267 or more arguments. Instead of
2269 %lex-param {arg1_type *arg1}
2270 %lex-param {arg2_type *arg2}
2271 %parse-param {arg1_type *arg1}
2272 %parse-param {arg2_type *arg2}
2276 %param {arg1_type *arg1} {arg2_type *arg2}
2278 ** Types of values for %define variables
2280 Bison used to make no difference between '%define foo bar' and '%define
2281 foo "bar"'. The former is now called a 'keyword value', and the latter a
2282 'string value'. A third kind was added: 'code values', such as '%define
2285 Keyword variables are used for fixed value sets, e.g.,
2287 %define lr.type lalr
2289 Code variables are used for value in the target language, e.g.,
2291 %define api.value.type {struct semantic_type}
2293 String variables are used remaining cases, e.g. file names.
2295 ** Variable api.token.prefix
2297 The variable api.token.prefix changes the way tokens are identified in
2298 the generated files. This is especially useful to avoid collisions
2299 with identifiers in the target language. For instance
2301 %token FILE for ERROR
2302 %define api.token.prefix {TOK_}
2304 start: FILE for ERROR;
2306 will generate the definition of the symbols TOK_FILE, TOK_for, and
2307 TOK_ERROR in the generated sources. In particular, the scanner must
2308 use these prefixed token names, although the grammar itself still
2309 uses the short names (as in the sample rule given above).
2311 ** Variable api.value.type
2313 This new %define variable supersedes the #define macro YYSTYPE. The use
2314 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2315 using %union or %defining api.value.type results in undefined behavior.
2317 Either define api.value.type, or use "%union":
2324 %token <ival> INT "integer"
2325 %token <sval> STRING "string"
2326 %printer { fprintf (yyo, "%d", $$); } <ival>
2327 %destructor { free ($$); } <sval>
2330 yylval.ival = 42; return INT;
2331 yylval.sval = "42"; return STRING;
2333 The %define variable api.value.type supports both keyword and code values.
2335 The keyword value 'union' means that the user provides genuine types, not
2336 union member names such as "ival" and "sval" above (WARNING: will fail if
2337 -y/--yacc/%yacc is enabled).
2339 %define api.value.type union
2340 %token <int> INT "integer"
2341 %token <char *> STRING "string"
2342 %printer { fprintf (yyo, "%d", $$); } <int>
2343 %destructor { free ($$); } <char *>
2346 yylval.INT = 42; return INT;
2347 yylval.STRING = "42"; return STRING;
2349 The keyword value variant is somewhat equivalent, but for C++ special
2350 provision is made to allow classes to be used (more about this below).
2352 %define api.value.type variant
2353 %token <int> INT "integer"
2354 %token <std::string> STRING "string"
2356 Code values (in braces) denote user defined types. This is where YYSTYPE
2374 %define api.value.type {struct my_value}
2375 %token <u.ival> INT "integer"
2376 %token <u.sval> STRING "string"
2377 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2378 %destructor { free ($$); } <u.sval>
2381 yylval.u.ival = 42; return INT;
2382 yylval.u.sval = "42"; return STRING;
2384 ** Variable parse.error
2386 This variable controls the verbosity of error messages. The use of the
2387 %error-verbose directive is deprecated in favor of "%define parse.error
2390 ** Deprecated %define variable names
2392 The following variables have been renamed for consistency. Backward
2393 compatibility is ensured, but upgrading is recommended.
2395 lr.default-reductions -> lr.default-reduction
2396 lr.keep-unreachable-states -> lr.keep-unreachable-state
2397 namespace -> api.namespace
2398 stype -> api.value.type
2400 ** Semantic predicates
2402 Contributed by Paul Hilfinger.
2404 The new, experimental, semantic-predicate feature allows actions of the
2405 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2406 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2407 in GLR parsers, rather than being deferred. The result is that they allow
2408 the programmer to prune possible parses based on the values of run-time
2411 ** The directive %expect-rr is now an error in non GLR mode
2413 It used to be an error only if used in non GLR mode, _and_ if there are
2414 reduce/reduce conflicts.
2416 ** Tokens are numbered in their order of appearance
2418 Contributed by Valentin Tolmer.
2420 With '%token A B', A had a number less than the one of B. However,
2421 precedence declarations used to generate a reversed order. This is now
2422 fixed, and introducing tokens with any of %token, %left, %right,
2423 %precedence, or %nonassoc yields the same result.
2425 When mixing declarations of tokens with a literal character (e.g., 'a') or
2426 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2427 the literal characters first. For example
2431 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2432 input order is now preserved.
2434 These changes were made so that one can remove useless precedence and
2435 associativity declarations (i.e., map %nonassoc, %left or %right to
2436 %precedence, or to %token) and get exactly the same output.
2438 ** Useless precedence and associativity
2440 Contributed by Valentin Tolmer.
2442 When developing and maintaining a grammar, useless associativity and
2443 precedence directives are common. They can be a nuisance: new ambiguities
2444 arising are sometimes masked because their conflicts are resolved due to
2445 the extra precedence or associativity information. Furthermore, it can
2446 hinder the comprehension of a new grammar: one will wonder about the role
2447 of a precedence, where in fact it is useless. The following changes aim
2448 at detecting and reporting these extra directives.
2450 *** Precedence warning category
2452 A new category of warning, -Wprecedence, was introduced. It flags the
2453 useless precedence and associativity directives.
2455 *** Useless associativity
2457 Bison now warns about symbols with a declared associativity that is never
2458 used to resolve conflicts. In that case, using %precedence is sufficient;
2459 the parsing tables will remain unchanged. Solving these warnings may raise
2460 useless precedence warnings, as the symbols no longer have associativity.
2474 warning: useless associativity for '+', use %precedence [-Wprecedence]
2478 *** Useless precedence
2480 Bison now warns about symbols with a declared precedence and no declared
2481 associativity (i.e., declared with %precedence), and whose precedence is
2482 never used. In that case, the symbol can be safely declared with %token
2483 instead, without modifying the parsing tables. For example:
2487 exp: "var" '=' "number";
2491 warning: useless precedence for '=' [-Wprecedence]
2495 *** Useless precedence and associativity
2497 In case of both useless precedence and associativity, the issue is flagged
2502 exp: "var" '=' "number";
2506 warning: useless precedence and associativity for '=' [-Wprecedence]
2512 With help from Joel E. Denny and Gabriel Rassoul.
2514 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2515 marked by the new %empty directive. Using %empty on a non-empty rule is
2516 an error. The new -Wempty-rule warning reports empty rules without
2517 %empty. On the following grammar:
2527 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2530 5.8-13: error: %empty on non-empty rule
2534 ** Java skeleton improvements
2536 The constants for token names were moved to the Lexer interface. Also, it
2537 is possible to add code to the parser's constructors using "%code init"
2538 and "%define init_throws".
2539 Contributed by Paolo Bonzini.
2541 The Java skeleton now supports push parsing.
2542 Contributed by Dennis Heimbigner.
2544 ** C++ skeletons improvements
2546 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2548 Using %defines is now optional. Without it, the needed support classes
2549 are defined in the generated parser, instead of additional files (such as
2550 location.hh, position.hh and stack.hh).
2552 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2554 Both lalr1.cc and glr.cc no longer require %location.
2556 *** syntax_error exception (lalr1.cc)
2558 The C++ parser features a syntax_error exception, which can be
2559 thrown from the scanner or from user rules to raise syntax errors.
2560 This facilitates reporting errors caught in sub-functions (e.g.,
2561 rejecting too large integral literals from a conversion function
2562 used by the scanner, or rejecting invalid combinations from a
2563 factory invoked by the user actions).
2565 *** %define api.value.type variant
2567 This is based on a submission from Michiel De Wilde. With help
2568 from Théophile Ranquet.
2570 In this mode, complex C++ objects can be used as semantic values. For
2573 %token <::std::string> TEXT;
2574 %token <int> NUMBER;
2575 %token SEMICOLON ";"
2576 %type <::std::string> item;
2577 %type <::std::list<std::string>> list;
2580 list { std::cout << $1 << std::endl; }
2584 %empty { /* Generates an empty string list. */ }
2585 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2589 TEXT { std::swap ($$, $1); }
2590 | NUMBER { $$ = string_cast ($1); }
2593 *** %define api.token.constructor
2595 When variants are enabled, Bison can generate functions to build the
2596 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2597 with the semantic value (e.g., int):
2599 parser::symbol_type yylex ()
2601 parser::location_type loc = ...;
2603 return parser::make_TEXT ("Hello, world!", loc);
2605 return parser::make_NUMBER (42, loc);
2607 return parser::make_SEMICOLON (loc);
2613 There are operator- and operator-= for 'location'. Negative line/column
2614 increments can no longer underflow the resulting value.
2617 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2621 *** Fix compiler attribute portability (yacc.c)
2623 With locations enabled, __attribute__ was used unprotected.
2625 *** Fix some compiler warnings (lalr1.cc)
2628 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2632 Warnings about uninitialized yylloc in yyparse have been fixed.
2634 Restored C90 compliance (yet no report was ever made).
2636 ** Diagnostics are improved
2638 Contributed by Théophile Ranquet.
2640 *** Changes in the format of error messages
2642 This used to be the format of many error reports:
2644 input.y:2.7-12: %type redeclaration for exp
2645 input.y:1.7-12: previous declaration
2649 input.y:2.7-12: error: %type redeclaration for exp
2650 input.y:1.7-12: previous declaration
2652 *** New format for error reports: carets
2654 Caret errors have been added to Bison:
2656 input.y:2.7-12: error: %type redeclaration for exp
2659 input.y:1.7-12: previous declaration
2665 input.y:3.20-23: error: ambiguous reference: '$exp'
2666 exp: exp '+' exp { $exp = $1 + $3; };
2668 input.y:3.1-3: refers to: $exp at $$
2669 exp: exp '+' exp { $exp = $1 + $3; };
2671 input.y:3.6-8: refers to: $exp at $1
2672 exp: exp '+' exp { $exp = $1 + $3; };
2674 input.y:3.14-16: refers to: $exp at $3
2675 exp: exp '+' exp { $exp = $1 + $3; };
2678 The default behavior for now is still not to display these unless
2679 explicitly asked with -fcaret (or -fall). However, in a later release, it
2680 will be made the default behavior (but may still be deactivated with
2683 ** New value for %define variable: api.pure full
2685 The %define variable api.pure requests a pure (reentrant) parser. However,
2686 for historical reasons, using it in a location-tracking Yacc parser
2687 resulted in a yyerror function that did not take a location as a
2688 parameter. With this new value, the user may request a better pure parser,
2689 where yyerror does take a location as a parameter (in location-tracking
2692 The use of "%define api.pure true" is deprecated in favor of this new
2693 "%define api.pure full".
2695 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2697 The %define variable api.location.type defines the name of the type to use
2698 for locations. When defined, Bison no longer generates the position.hh
2699 and location.hh files, nor does the parser will include them: the user is
2700 then responsible to define her type.
2702 This can be used in programs with several parsers to factor their location
2703 and position files: let one of them generate them, and the others just use
2706 This feature was actually introduced, but not documented, in Bison 2.5,
2707 under the name "location_type" (which is maintained for backward
2710 For consistency, lalr1.java's %define variables location_type and
2711 position_type are deprecated in favor of api.location.type and
2714 ** Exception safety (lalr1.cc)
2716 The parse function now catches exceptions, uses the %destructors to
2717 release memory (the lookahead symbol and the symbols pushed on the stack)
2718 before re-throwing the exception.
2720 This feature is somewhat experimental. User feedback would be
2723 ** Graph improvements in DOT and XSLT
2725 Contributed by Théophile Ranquet.
2727 The graphical presentation of the states is more readable: their shape is
2728 now rectangular, the state number is clearly displayed, and the items are
2729 numbered and left-justified.
2731 The reductions are now explicitly represented as transitions to other
2732 diamond shaped nodes.
2734 These changes are present in both --graph output and xml2dot.xsl XSLT
2735 processing, with minor (documented) differences.
2737 ** %language is no longer an experimental feature.
2739 The introduction of this feature, in 2.4, was four years ago. The
2740 --language option and the %language directive are no longer experimental.
2744 The sections about shift/reduce and reduce/reduce conflicts resolution
2745 have been fixed and extended.
2747 Although introduced more than four years ago, XML and Graphviz reports
2748 were not properly documented.
2750 The translation of midrule actions is now described.
2753 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2755 We consider compiler warnings about Bison generated parsers to be bugs.
2756 Rather than working around them in your own project, please consider
2757 reporting them to us.
2761 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2762 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2765 Other issues in the test suite have been addressed.
2767 Null characters are correctly displayed in error messages.
2769 When possible, yylloc is correctly initialized before calling yylex. It
2770 is no longer necessary to initialize it in the %initial-action.
2773 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2775 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2778 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2782 Bugs and portability issues in the test suite have been fixed.
2784 Some errors in translations have been addressed, and --help now directs
2785 users to the appropriate place to report them.
2787 Stray Info files shipped by accident are removed.
2789 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2790 generated, are removed.
2792 All the generated headers are self-contained.
2794 ** Header guards (yacc.c, glr.c, glr.cc)
2796 In order to avoid collisions, the header guards are now
2797 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2798 For instance the header generated from
2800 %define api.prefix "calc"
2801 %defines "lib/parse.h"
2803 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2805 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2807 The compilation of pure parsers (%define api.pure) can trigger GCC
2810 input.c: In function 'yyparse':
2811 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2812 function [-Wmaybe-uninitialized]
2816 This is now fixed; pragmas to avoid these warnings are no longer needed.
2818 Warnings from clang ("equality comparison with extraneous parentheses" and
2819 "function declared 'noreturn' should not return") have also been
2823 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2827 Buffer overruns, complaints from Flex, and portability issues in the test
2828 suite have been fixed.
2830 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2832 Trailing end-of-lines in %parse-param or %lex-param would result in
2833 invalid C++. This is fixed.
2835 ** Spurious spaces and end-of-lines
2837 The generated files no longer end (nor start) with empty lines.
2840 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2842 Bison no longer executes user-specified M4 code when processing a grammar.
2846 In addition to the removal of the features announced in Bison 2.6, the
2847 next major release will remove the "Temporary hack for adding a semicolon
2848 to the user action", as announced in the release 2.5. Instead of:
2850 exp: exp "+" exp { $$ = $1 + $3 };
2854 exp: exp "+" exp { $$ = $1 + $3; };
2858 *** Type names are now properly escaped.
2860 *** glr.cc: set_debug_level and debug_level work as expected.
2862 *** Stray @ or $ in actions
2864 While Bison used to warn about stray $ or @ in action rules, it did not
2865 for other actions such as printers, destructors, or initial actions. It
2868 ** Type names in actions
2870 For consistency with rule actions, it is now possible to qualify $$ by a
2871 type-name in destructors, printers, and initial actions. For instance:
2873 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2875 will display two values for each typed and untyped symbol (provided
2876 that YYSTYPE has both "ival" and "fval" fields).
2879 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2883 The next major release of Bison will drop support for the following
2884 deprecated features. Please report disagreements to bug-bison@gnu.org.
2888 Support for generating parsers in K&R C will be removed. Parsers
2889 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2892 *** Features deprecated since Bison 1.875
2894 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2897 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2898 %lex-param, will no longer be supported.
2900 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2903 *** The generated header will be included (yacc.c)
2905 Instead of duplicating the content of the generated header (definition of
2906 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2907 as is already the case for GLR or C++ parsers. This change is deferred
2908 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2911 ** Generated Parser Headers
2913 *** Guards (yacc.c, glr.c, glr.cc)
2915 The generated headers are now guarded, as is already the case for C++
2916 parsers (lalr1.cc). For instance, with --defines=foo.h:
2921 #endif /* !YY_FOO_H */
2923 *** New declarations (yacc.c, glr.c)
2925 The generated header now declares yydebug and yyparse. Both honor
2926 --name-prefix=bar_, and yield
2928 int bar_parse (void);
2932 #define yyparse bar_parse
2935 in order to facilitate the inclusion of several parser headers inside a
2936 single compilation unit.
2938 *** Exported symbols in C++
2940 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2941 header, are removed, as they prevent the possibility of including several
2942 generated headers from a single compilation unit.
2946 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2949 ** New %define variable: api.prefix
2951 Now that the generated headers are more complete and properly protected
2952 against multiple inclusions, constant names, such as YYSTYPE are a
2953 problem. While yyparse and others are properly renamed by %name-prefix,
2954 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2955 would introduce backward compatibility issues in projects not expecting
2956 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2957 it is deprecated in favor of a new %define variable: api.prefix.
2959 The following examples compares both:
2961 %name-prefix "bar_" | %define api.prefix "bar_"
2962 %token <ival> FOO %token <ival> FOO
2963 %union { int ival; } %union { int ival; }
2969 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2970 # define BAR_FOO_H # define BAR_FOO_H
2972 /* Enabling traces. */ /* Enabling traces. */
2973 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2974 > # if defined YYDEBUG
2976 > # define BAR_DEBUG 1
2978 > # define BAR_DEBUG 0
2981 # define YYDEBUG 0 | # define BAR_DEBUG 0
2985 # if YYDEBUG | # if BAR_DEBUG
2986 extern int bar_debug; extern int bar_debug;
2989 /* Tokens. */ /* Tokens. */
2990 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2991 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2992 enum yytokentype { | enum bar_tokentype {
2997 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2998 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2999 typedef union YYSTYPE | typedef union BAR_STYPE
3002 } YYSTYPE; | } BAR_STYPE;
3003 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
3006 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
3008 int bar_parse (void); int bar_parse (void);
3010 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
3013 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
3017 The next major release will drop support for generating parsers in K&R C.
3019 ** yacc.c: YYBACKUP works as expected.
3021 ** glr.c improvements:
3023 *** Location support is eliminated when not requested:
3025 GLR parsers used to include location-related code even when locations were
3026 not requested, and therefore not even usable.
3028 *** __attribute__ is preserved:
3030 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
3031 when -std is passed to GCC).
3033 ** lalr1.java: several fixes:
3035 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
3036 first token leads to a syntax error. Some minor clean ups.
3040 *** C++11 compatibility:
3042 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
3047 The header files such as "parser.hh", "location.hh", etc. used a constant
3048 name for preprocessor guards, for instance:
3050 #ifndef BISON_LOCATION_HH
3051 # define BISON_LOCATION_HH
3053 #endif // !BISON_LOCATION_HH
3055 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
3056 case characters are converted to upper case, and series of
3057 non-alphanumerical characters are converted to an underscore.
3059 With "bison -o lang++/parser.cc", "location.hh" would now include:
3061 #ifndef YY_LANG_LOCATION_HH
3062 # define YY_LANG_LOCATION_HH
3064 #endif // !YY_LANG_LOCATION_HH
3068 The position and location constructors (and their initialize methods)
3069 accept new arguments for line and column. Several issues in the
3070 documentation were fixed.
3072 ** liby is no longer asking for "rpl_fprintf" on some platforms.
3074 ** Changes in the manual:
3076 *** %printer is documented
3078 The "%printer" directive, supported since at least Bison 1.50, is finally
3079 documented. The "mfcalc" example is extended to demonstrate it.
3081 For consistency with the C skeletons, the C++ parsers now also support
3082 "yyoutput" (as an alias to "debug_stream ()").
3084 *** Several improvements have been made:
3086 The layout for grammar excerpts was changed to a more compact scheme.
3087 Named references are motivated. The description of the automaton
3088 description file (*.output) is updated to the current format. Incorrect
3089 index entries were fixed. Some other errors were fixed.
3093 *** Conflicting prototypes with recent/modified Flex.
3095 Fixed build problems with the current, unreleased, version of Flex, and
3096 some modified versions of 2.5.35, which have modified function prototypes.
3098 *** Warnings during the build procedure have been eliminated.
3100 *** Several portability problems in the test suite have been fixed:
3102 This includes warnings with some compilers, unexpected behavior of tools
3103 such as diff, warning messages from the test suite itself, etc.
3105 *** The install-pdf target works properly:
3107 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3108 halts in the middle of its course.
3111 * Noteworthy changes in release 2.5 (2011-05-14)
3113 ** Grammar symbol names can now contain non-initial dashes:
3115 Consistently with directives (such as %error-verbose) and with
3116 %define variables (e.g. push-pull), grammar symbol names may contain
3117 dashes in any position except the beginning. This is a GNU
3118 extension over POSIX Yacc. Thus, use of this extension is reported
3119 by -Wyacc and rejected in Yacc mode (--yacc).
3121 ** Named references:
3123 Historically, Yacc and Bison have supported positional references
3124 ($n, $$) to allow access to symbol values from inside of semantic
3127 Starting from this version, Bison can also accept named references.
3128 When no ambiguity is possible, original symbol names may be used
3129 as named references:
3131 if_stmt : "if" cond_expr "then" then_stmt ';'
3132 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3134 In the more common case, explicit names may be declared:
3136 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3137 { $res = mk_if_stmt($cond, $then, $else); }
3139 Location information is also accessible using @name syntax. When
3140 accessing symbol names containing dots or dashes, explicit bracketing
3141 ($[sym.1]) must be used.
3143 These features are experimental in this version. More user feedback
3144 will help to stabilize them.
3145 Contributed by Alex Rozenman.
3147 ** IELR(1) and canonical LR(1):
3149 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3150 is, given any context-free grammar, IELR(1) generates parser tables
3151 with the full language-recognition power of canonical LR(1) but with
3152 nearly the same number of parser states as LALR(1). This reduction
3153 in parser states is often an order of magnitude. More importantly,
3154 because canonical LR(1)'s extra parser states may contain duplicate
3155 conflicts in the case of non-LR(1) grammars, the number of conflicts
3156 for IELR(1) is often an order of magnitude less as well. This can
3157 significantly reduce the complexity of developing of a grammar.
3159 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3160 place of its traditional LALR(1) parser tables, which remain the
3161 default. You can specify the type of parser tables in the grammar
3162 file with these directives:
3164 %define lr.type lalr
3165 %define lr.type ielr
3166 %define lr.type canonical-lr
3168 The default-reduction optimization in the parser tables can also be
3169 adjusted using "%define lr.default-reductions". For details on both
3170 of these features, see the new section "Tuning LR" in the Bison
3173 These features are experimental. More user feedback will help to
3176 ** LAC (Lookahead Correction) for syntax error handling
3178 Contributed by Joel E. Denny.
3180 Canonical LR, IELR, and LALR can suffer from a couple of problems
3181 upon encountering a syntax error. First, the parser might perform
3182 additional parser stack reductions before discovering the syntax
3183 error. Such reductions can perform user semantic actions that are
3184 unexpected because they are based on an invalid token, and they
3185 cause error recovery to begin in a different syntactic context than
3186 the one in which the invalid token was encountered. Second, when
3187 verbose error messages are enabled (with %error-verbose or the
3188 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3189 syntax error message can both contain invalid tokens and omit valid
3192 The culprits for the above problems are %nonassoc, default
3193 reductions in inconsistent states, and parser state merging. Thus,
3194 IELR and LALR suffer the most. Canonical LR can suffer only if
3195 %nonassoc is used or if default reductions are enabled for
3196 inconsistent states.
3198 LAC is a new mechanism within the parsing algorithm that solves
3199 these problems for canonical LR, IELR, and LALR without sacrificing
3200 %nonassoc, default reductions, or state merging. When LAC is in
3201 use, canonical LR and IELR behave almost exactly the same for both
3202 syntactically acceptable and syntactically unacceptable input.
3203 While LALR still does not support the full language-recognition
3204 power of canonical LR and IELR, LAC at least enables LALR's syntax
3205 error handling to correctly reflect LALR's language-recognition
3208 Currently, LAC is only supported for deterministic parsers in C.
3209 You can enable LAC with the following directive:
3211 %define parse.lac full
3213 See the new section "LAC" in the Bison manual for additional
3214 details including a few caveats.
3216 LAC is an experimental feature. More user feedback will help to
3219 ** %define improvements:
3221 *** Can now be invoked via the command line:
3223 Each of these command-line options
3226 --define=NAME[=VALUE]
3229 --force-define=NAME[=VALUE]
3231 is equivalent to this grammar file declaration
3233 %define NAME ["VALUE"]
3235 except that the manner in which Bison processes multiple definitions
3236 for the same NAME differs. Most importantly, -F and --force-define
3237 quietly override %define, but -D and --define do not. For further
3238 details, see the section "Bison Options" in the Bison manual.
3240 *** Variables renamed:
3242 The following %define variables
3245 lr.keep_unreachable_states
3247 have been renamed to
3250 lr.keep-unreachable-states
3252 The old names are now deprecated but will be maintained indefinitely
3253 for backward compatibility.
3255 *** Values no longer need to be quoted in the grammar file:
3257 If a %define value is an identifier, it no longer needs to be placed
3258 within quotations marks. For example,
3260 %define api.push-pull "push"
3264 %define api.push-pull push
3266 *** Unrecognized variables are now errors not warnings.
3268 *** Multiple invocations for any variable is now an error not a warning.
3270 ** Unrecognized %code qualifiers are now errors not warnings.
3272 ** Character literals not of length one:
3274 Previously, Bison quietly converted all character literals to length
3275 one. For example, without warning, Bison interpreted the operators in
3276 the following grammar to be the same token:
3282 Bison now warns when a character literal is not of length one. In
3283 some future release, Bison will start reporting an error instead.
3285 ** Destructor calls fixed for lookaheads altered in semantic actions:
3287 Previously for deterministic parsers in C, if a user semantic action
3288 altered yychar, the parser in some cases used the old yychar value to
3289 determine which destructor to call for the lookahead upon a syntax
3290 error or upon parser return. This bug has been fixed.
3292 ** C++ parsers use YYRHSLOC:
3294 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3295 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3296 to use it. If, for instance, your location structure has "first"
3297 and "last" members, instead of
3299 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3303 (Current).first = (Rhs)[1].location.first; \
3304 (Current).last = (Rhs)[N].location.last; \
3308 (Current).first = (Current).last = (Rhs)[0].location.last; \
3314 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3318 (Current).first = YYRHSLOC (Rhs, 1).first; \
3319 (Current).last = YYRHSLOC (Rhs, N).last; \
3323 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3327 ** YYLLOC_DEFAULT in C++:
3329 The default implementation of YYLLOC_DEFAULT used to be issued in
3330 the header file. It is now output in the implementation file, after
3331 the user %code sections so that its #ifndef guard does not try to
3332 override the user's YYLLOC_DEFAULT if provided.
3334 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3336 YYFAIL has existed for many years as an undocumented feature of
3337 deterministic parsers in C generated by Bison. More recently, it was
3338 a documented feature of Bison's experimental Java parsers. As
3339 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3340 semantic action now produces a deprecation warning, and Java parsers
3341 no longer implement YYFAIL at all. For further details, including a
3342 discussion of how to suppress C preprocessor warnings about YYFAIL
3343 being unused, see the Bison 2.4.2 NEWS entry.
3345 ** Temporary hack for adding a semicolon to the user action:
3347 Previously, Bison appended a semicolon to every user action for
3348 reductions when the output language defaulted to C (specifically, when
3349 neither %yacc, %language, %skeleton, or equivalent command-line
3350 options were specified). This allowed actions such as
3352 exp: exp "+" exp { $$ = $1 + $3 };
3356 exp: exp "+" exp { $$ = $1 + $3; };
3358 As a first step in removing this misfeature, Bison now issues a
3359 warning when it appends a semicolon. Moreover, in cases where Bison
3360 cannot easily determine whether a semicolon is needed (for example, an
3361 action ending with a cpp directive or a braced compound initializer),
3362 it no longer appends one. Thus, the C compiler might now complain
3363 about a missing semicolon where it did not before. Future releases of
3364 Bison will cease to append semicolons entirely.
3366 ** Verbose syntax error message fixes:
3368 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3369 specified, syntax error messages produced by the generated parser
3370 include the unexpected token as well as a list of expected tokens.
3371 The effect of %nonassoc on these verbose messages has been corrected
3372 in two ways, but a more complete fix requires LAC, described above:
3374 *** When %nonassoc is used, there can exist parser states that accept no
3375 tokens, and so the parser does not always require a lookahead token
3376 in order to detect a syntax error. Because no unexpected token or
3377 expected tokens can then be reported, the verbose syntax error
3378 message described above is suppressed, and the parser instead
3379 reports the simpler message, "syntax error". Previously, this
3380 suppression was sometimes erroneously triggered by %nonassoc when a
3381 lookahead was actually required. Now verbose messages are
3382 suppressed only when all previous lookaheads have already been
3383 shifted or discarded.
3385 *** Previously, the list of expected tokens erroneously included tokens
3386 that would actually induce a syntax error because conflicts for them
3387 were resolved with %nonassoc in the current parser state. Such
3388 tokens are now properly omitted from the list.
3390 *** Expected token lists are still often wrong due to state merging
3391 (from LALR or IELR) and default reductions, which can both add
3392 invalid tokens and subtract valid tokens. Canonical LR almost
3393 completely fixes this problem by eliminating state merging and
3394 default reductions. However, there is one minor problem left even
3395 when using canonical LR and even after the fixes above. That is,
3396 if the resolution of a conflict with %nonassoc appears in a later
3397 parser state than the one at which some syntax error is
3398 discovered, the conflicted token is still erroneously included in
3399 the expected token list. Bison's new LAC implementation,
3400 described above, eliminates this problem and the need for
3401 canonical LR. However, LAC is still experimental and is disabled
3404 ** Java skeleton fixes:
3406 *** A location handling bug has been fixed.
3408 *** The top element of each of the value stack and location stack is now
3409 cleared when popped so that it can be garbage collected.
3411 *** Parser traces now print the top element of the stack.
3413 ** -W/--warnings fixes:
3415 *** Bison now properly recognizes the "no-" versions of categories:
3417 For example, given the following command line, Bison now enables all
3418 warnings except warnings for incompatibilities with POSIX Yacc:
3420 bison -Wall,no-yacc gram.y
3422 *** Bison now treats S/R and R/R conflicts like other warnings:
3424 Previously, conflict reports were independent of Bison's normal
3425 warning system. Now, Bison recognizes the warning categories
3426 "conflicts-sr" and "conflicts-rr". This change has important
3427 consequences for the -W and --warnings command-line options. For
3430 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3431 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3432 bison -Wnone gram.y # no conflicts are reported
3433 bison -Werror gram.y # any conflict is an error
3435 However, as before, if the %expect or %expect-rr directive is
3436 specified, an unexpected number of conflicts is an error, and an
3437 expected number of conflicts is not reported, so -W and --warning
3438 then have no effect on the conflict report.
3440 *** The "none" category no longer disables a preceding "error":
3442 For example, for the following command line, Bison now reports
3443 errors instead of warnings for incompatibilities with POSIX Yacc:
3445 bison -Werror,none,yacc gram.y
3447 *** The "none" category now disables all Bison warnings:
3449 Previously, the "none" category disabled only Bison warnings for
3450 which there existed a specific -W/--warning category. However,
3451 given the following command line, Bison is now guaranteed to
3452 suppress all warnings:
3456 ** Precedence directives can now assign token number 0:
3458 Since Bison 2.3b, which restored the ability of precedence
3459 directives to assign token numbers, doing so for token number 0 has
3460 produced an assertion failure. For example:
3464 This bug has been fixed.
3467 * Noteworthy changes in release 2.4.3 (2010-08-05)
3469 ** Bison now obeys -Werror and --warnings=error for warnings about
3470 grammar rules that are useless in the parser due to conflicts.
3472 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3475 ** Failures in the test suite for GCC 4.5 have been fixed.
3477 ** Failures in the test suite for some versions of Sun Studio C++ have
3480 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3481 warnings about undefined %prec identifiers will not be converted to
3482 errors in Bison 2.5. They will remain warnings, which should be
3483 sufficient for POSIX while avoiding backward compatibility issues.
3485 ** Minor documentation fixes.
3488 * Noteworthy changes in release 2.4.2 (2010-03-20)
3490 ** Some portability problems that resulted in failures and livelocks
3491 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3492 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3493 errors should no longer cause M4 to report a broken pipe on the
3496 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3498 POSIX specifies that an error be reported for any identifier that does
3499 not appear on the LHS of a grammar rule and that is not defined by
3500 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3501 error report for the case when an identifier appears only after a
3502 %prec directive. It is now restored. However, for backward
3503 compatibility with recent Bison releases, it is only a warning for
3504 now. In Bison 2.5 and later, it will return to being an error.
3505 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3506 warning will not be converted to an error in Bison 2.5.]
3508 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3510 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3511 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3514 ** %code is now a permanent feature.
3516 A traditional Yacc prologue directive is written in the form:
3520 To provide a more flexible alternative, Bison 2.3b introduced the
3521 %code directive with the following forms for C/C++:
3524 %code requires {CODE}
3525 %code provides {CODE}
3528 These forms are now considered permanent features of Bison. See the
3529 %code entries in the section "Bison Declaration Summary" in the Bison
3530 manual for a summary of their functionality. See the section
3531 "Prologue Alternatives" for a detailed discussion including the
3532 advantages of %code over the traditional Yacc prologue directive.
3534 Bison's Java feature as a whole including its current usage of %code
3535 is still considered experimental.
3537 ** YYFAIL is deprecated and will eventually be removed.
3539 YYFAIL has existed for many years as an undocumented feature of
3540 deterministic parsers in C generated by Bison. Previously, it was
3541 documented for Bison's experimental Java parsers. YYFAIL is no longer
3542 documented for Java parsers and is formally deprecated in both cases.
3543 Users are strongly encouraged to migrate to YYERROR, which is
3546 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3547 induce a syntax error. The most obvious difference from YYERROR is
3548 that YYFAIL will automatically invoke yyerror to report the syntax
3549 error so that you don't have to. However, there are several other
3550 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3551 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3552 used. For a more detailed discussion, see:
3554 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3556 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3557 deterministic parsers in C will continue to implement it. However,
3558 because YYFAIL is already flawed, it seems futile to try to make new
3559 Bison features compatible with it. Thus, during parser generation,
3560 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3561 rule action. In a later release, YYFAIL will be disabled for
3562 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3563 be removed altogether.
3565 There exists at least one case where Bison 2.5's YYFAIL warning will
3566 be a false positive. Some projects add phony uses of YYFAIL and other
3567 Bison-defined macros for the sole purpose of suppressing C
3568 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3569 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3570 epilogue (that is, after the second "%%") in the Bison input file. In
3571 this release (2.4.2), Bison already generates its own code to suppress
3572 C preprocessor warnings for YYFAIL, so projects can remove their own
3573 phony uses of YYFAIL if compatibility with Bison releases prior to
3574 2.4.2 is not necessary.
3576 ** Internationalization.
3578 Fix a regression introduced in Bison 2.4: Under some circumstances,
3579 message translations were not installed although supported by the
3583 * Noteworthy changes in release 2.4.1 (2008-12-11)
3585 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3586 declarations have been fixed.
3588 ** Temporary hack for adding a semicolon to the user action.
3590 Bison used to prepend a trailing semicolon at the end of the user
3591 action for reductions. This allowed actions such as
3593 exp: exp "+" exp { $$ = $1 + $3 };
3597 exp: exp "+" exp { $$ = $1 + $3; };
3599 Some grammars still depend on this "feature". Bison 2.4.1 restores
3600 the previous behavior in the case of C output (specifically, when
3601 neither %language or %skeleton or equivalent command-line options
3602 are used) to leave more time for grammars depending on the old
3603 behavior to be adjusted. Future releases of Bison will disable this
3606 ** A few minor improvements to the Bison manual.
3609 * Noteworthy changes in release 2.4 (2008-11-02)
3611 ** %language is an experimental feature.
3613 We first introduced this feature in test release 2.3b as a cleaner
3614 alternative to %skeleton. Since then, we have discussed the possibility of
3615 modifying its effect on Bison's output file names. Thus, in this release,
3616 we consider %language to be an experimental feature that will likely evolve
3619 ** Forward compatibility with GNU M4 has been improved.
3621 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3625 * Noteworthy changes in release 2.3b (2008-05-27)
3627 ** The quotes around NAME that used to be required in the following directive
3630 %define NAME "VALUE"
3632 ** The directive "%pure-parser" is now deprecated in favor of:
3636 which has the same effect except that Bison is more careful to warn about
3637 unreasonable usage in the latter case.
3641 Bison can now generate an LALR(1) parser in C with a push interface. That
3642 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3643 push one token at a time to the parser using "yypush_parse", which will
3644 return to the caller after processing each token. By default, the push
3645 interface is disabled. Either of the following directives will enable it:
3647 %define api.push_pull "push" // Just push; does not require yylex.
3648 %define api.push_pull "both" // Push and pull; requires yylex.
3650 See the new section "A Push Parser" in the Bison manual for details.
3652 The current push parsing interface is experimental and may evolve. More user
3653 feedback will help to stabilize it.
3655 ** The -g and --graph options now output graphs in Graphviz DOT format,
3656 not VCG format. Like --graph, -g now also takes an optional FILE argument
3657 and thus cannot be bundled with other short options.
3661 Bison can now generate an LALR(1) parser in Java. The skeleton is
3662 "data/lalr1.java". Consider using the new %language directive instead of
3663 %skeleton to select it.
3665 See the new section "Java Parsers" in the Bison manual for details.
3667 The current Java interface is experimental and may evolve. More user
3668 feedback will help to stabilize it.
3669 Contributed by Paolo Bonzini.
3673 This new directive specifies the programming language of the generated
3674 parser, which can be C (the default), C++, or Java. Besides the skeleton
3675 that Bison uses, the directive affects the names of the generated files if
3676 the grammar file's name ends in ".y".
3678 ** XML Automaton Report
3680 Bison can now generate an XML report of the LALR(1) automaton using the new
3681 "--xml" option. The current XML schema is experimental and may evolve. More
3682 user feedback will help to stabilize it.
3683 Contributed by Wojciech Polak.
3685 ** The grammar file may now specify the name of the parser header file using
3686 %defines. For example:
3690 ** When reporting useless rules, useless nonterminals, and unused terminals,
3691 Bison now employs the terms "useless in grammar" instead of "useless",
3692 "useless in parser" instead of "never reduced", and "unused in grammar"
3693 instead of "unused".
3695 ** Unreachable State Removal
3697 Previously, Bison sometimes generated parser tables containing unreachable
3698 states. A state can become unreachable during conflict resolution if Bison
3699 disables a shift action leading to it from a predecessor state. Bison now:
3701 1. Removes unreachable states.
3703 2. Does not report any conflicts that appeared in unreachable states.
3704 WARNING: As a result, you may need to update %expect and %expect-rr
3705 directives in existing grammar files.
3707 3. For any rule used only in such states, Bison now reports the rule as
3708 "useless in parser due to conflicts".
3710 This feature can be disabled with the following directive:
3712 %define lr.keep_unreachable_states
3714 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3715 for further discussion.
3717 ** Lookahead Set Correction in the ".output" Report
3719 When instructed to generate a ".output" file including lookahead sets
3720 (using "--report=lookahead", for example), Bison now prints each reduction's
3721 lookahead set only next to the associated state's one item that (1) is
3722 associated with the same rule as the reduction and (2) has its dot at the end
3723 of its RHS. Previously, Bison also erroneously printed the lookahead set
3724 next to all of the state's other items associated with the same rule. This
3725 bug affected only the ".output" file and not the generated parser source
3728 ** --report-file=FILE is a new option to override the default ".output" file
3731 ** The "=" that used to be required in the following directives is now
3734 %file-prefix "parser"
3738 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3740 Bison 2.3a provided a new set of directives as a more flexible alternative to
3741 the traditional Yacc prologue blocks. Those have now been consolidated into
3742 a single %code directive with an optional qualifier field, which identifies
3743 the purpose of the code and thus the location(s) where Bison should generate
3746 1. "%code {CODE}" replaces "%after-header {CODE}"
3747 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3748 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3749 4. "%code top {CODE}" replaces "%before-header {CODE}"
3751 See the %code entries in section "Bison Declaration Summary" in the Bison
3752 manual for a summary of the new functionality. See the new section "Prologue
3753 Alternatives" for a detailed discussion including the advantages of %code
3754 over the traditional Yacc prologues.
3756 The prologue alternatives are experimental. More user feedback will help to
3757 determine whether they should become permanent features.
3759 ** Revised warning: unset or unused midrule values
3761 Since Bison 2.2, Bison has warned about midrule values that are set but not
3762 used within any of the actions of the parent rule. For example, Bison warns
3765 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3767 Now, Bison also warns about midrule values that are used but not set. For
3768 example, Bison warns about unset $$ in the midrule action in:
3770 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3772 However, Bison now disables both of these warnings by default since they
3773 sometimes prove to be false alarms in existing grammars employing the Yacc
3774 constructs $0 or $-N (where N is some positive integer).
3776 To enable these warnings, specify the option "--warnings=midrule-values" or
3777 "-W", which is a synonym for "--warnings=all".
3779 ** Default %destructor or %printer with "<*>" or "<>"
3781 Bison now recognizes two separate kinds of default %destructor's and
3784 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3785 %destructor/%printer for all grammar symbols for which you have formally
3786 declared semantic type tags.
3788 2. Place "<>" in a %destructor/%printer symbol list to define a default
3789 %destructor/%printer for all grammar symbols without declared semantic
3792 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3793 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3794 longer applies any %destructor to a midrule value if that midrule value is
3795 not actually ever referenced using either $$ or $n in a semantic action.
3797 The default %destructor's and %printer's are experimental. More user
3798 feedback will help to determine whether they should become permanent
3801 See the section "Freeing Discarded Symbols" in the Bison manual for further
3804 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3805 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3806 manual for a caveat concerning the treatment of literal strings.
3808 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3809 completely removed from Bison.
3812 * Noteworthy changes in release 2.3a (2006-09-13)
3814 ** Instead of %union, you can define and use your own union type
3815 YYSTYPE if your grammar contains at least one <type> tag.
3816 Your YYSTYPE need not be a macro; it can be a typedef.
3817 This change is for compatibility with other Yacc implementations,
3818 and is required by POSIX.
3820 ** Locations columns and lines start at 1.
3821 In accordance with the GNU Coding Standards and Emacs.
3823 ** You may now declare per-type and default %destructor's and %printer's:
3827 %union { char *string; }
3828 %token <string> STRING1
3829 %token <string> STRING2
3830 %type <string> string1
3831 %type <string> string2
3832 %union { char character; }
3833 %token <character> CHR
3834 %type <character> chr
3835 %destructor { free ($$); } %symbol-default
3836 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3837 %destructor { } <character>
3839 guarantees that, when the parser discards any user-defined symbol that has a
3840 semantic type tag other than "<character>", it passes its semantic value to
3841 "free". However, when the parser discards a "STRING1" or a "string1", it
3842 also prints its line number to "stdout". It performs only the second
3843 "%destructor" in this case, so it invokes "free" only once.
3845 [Although we failed to mention this here in the 2.3a release, the default
3846 %destructor's and %printer's were experimental, and they were rewritten in
3849 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3850 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3851 associating token numbers with token names. Removing the #define statements
3852 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3853 requires them. Bison still generates an enum for token names in all cases.
3855 ** Handling of traditional Yacc prologue blocks is now more consistent but
3856 potentially incompatible with previous releases of Bison.
3858 As before, you declare prologue blocks in your grammar file with the
3859 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3860 prologue blocks that you've declared before the first %union. To generate
3861 the post-prologue, Bison concatenates all prologue blocks that you've
3862 declared after the first %union.
3864 Previous releases of Bison inserted the pre-prologue into both the header
3865 file and the code file in all cases except for LALR(1) parsers in C. In the
3866 latter case, Bison inserted it only into the code file. For parsers in C++,
3867 the point of insertion was before any token definitions (which associate
3868 token numbers with names). For parsers in C, the point of insertion was
3869 after the token definitions.
3871 Now, Bison never inserts the pre-prologue into the header file. In the code
3872 file, it always inserts it before the token definitions.
3874 ** Bison now provides a more flexible alternative to the traditional Yacc
3875 prologue blocks: %before-header, %start-header, %end-header, and
3878 For example, the following declaration order in the grammar file reflects the
3879 order in which Bison will output these code blocks. However, you are free to
3880 declare these code blocks in your grammar file in whatever order is most
3884 /* Bison treats this block like a pre-prologue block: it inserts it into
3885 * the code file before the contents of the header file. It does *not*
3886 * insert it into the header file. This is a good place to put
3887 * #include's that you want at the top of your code file. A common
3888 * example is '#include "system.h"'. */
3891 /* Bison inserts this block into both the header file and the code file.
3892 * In both files, the point of insertion is before any Bison-generated
3893 * token, semantic type, location type, and class definitions. This is a
3894 * good place to define %union dependencies, for example. */
3897 /* Unlike the traditional Yacc prologue blocks, the output order for the
3898 * new %*-header blocks is not affected by their declaration position
3899 * relative to any %union in the grammar file. */
3902 /* Bison inserts this block into both the header file and the code file.
3903 * In both files, the point of insertion is after the Bison-generated
3904 * definitions. This is a good place to declare or define public
3905 * functions or data structures that depend on the Bison-generated
3909 /* Bison treats this block like a post-prologue block: it inserts it into
3910 * the code file after the contents of the header file. It does *not*
3911 * insert it into the header file. This is a good place to declare or
3912 * define internal functions or data structures that depend on the
3913 * Bison-generated definitions. */
3916 If you have multiple occurrences of any one of the above declarations, Bison
3917 will concatenate the contents in declaration order.
3919 [Although we failed to mention this here in the 2.3a release, the prologue
3920 alternatives were experimental, and they were rewritten in future versions.]
3922 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3923 The old spelling still works, but is not documented and may be removed
3924 in a future release.
3927 * Noteworthy changes in release 2.3 (2006-06-05)
3929 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3930 for compatibility with LALR(1) grammars.
3932 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3933 be to a type name that does not contain parentheses or brackets.
3936 * Noteworthy changes in release 2.2 (2006-05-19)
3938 ** The distribution terms for all Bison-generated parsers now permit
3939 using the parsers in nonfree programs. Previously, this permission
3940 was granted only for Bison-generated LALR(1) parsers in C.
3942 ** %name-prefix changes the namespace name in C++ outputs.
3944 ** The C++ parsers export their token_type.
3946 ** Bison now allows multiple %union declarations, and concatenates
3947 their contents together.
3949 ** New warning: unused values
3950 Right-hand side symbols whose values are not used are reported,
3951 if the symbols have destructors. For instance:
3953 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3957 will trigger a warning about $$ and $5 in the first rule, and $3 in
3958 the second ($1 is copied to $$ by the default rule). This example
3959 most likely contains three errors, and could be rewritten as:
3961 exp: exp "?" exp ":" exp
3962 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3964 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3967 However, if the original actions were really intended, memory leaks
3968 and all, the warnings can be suppressed by letting Bison believe the
3969 values are used, e.g.:
3971 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3972 | exp "+" exp { $$ = $1; (void) $3; }
3975 If there are midrule actions, the warning is issued if no action
3976 uses it. The following triggers no warning: $1 and $3 are used.
3978 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3980 The warning is intended to help catching lost values and memory leaks.
3981 If a value is ignored, its associated memory typically is not reclaimed.
3983 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3984 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3985 and YYERROR, for all objects on the stack, other than objects
3986 corresponding to the right-hand side of the current rule.
3988 ** %expect, %expect-rr
3989 Incorrect numbers of expected conflicts are now actual errors,
3990 instead of warnings.
3992 ** GLR, YACC parsers.
3993 The %parse-params are available in the destructors (and the
3994 experimental printers) as per the documentation.
3996 ** Bison now warns if it finds a stray "$" or "@" in an action.
3998 ** %require "VERSION"
3999 This specifies that the grammar file depends on features implemented
4000 in Bison version VERSION or higher.
4002 ** lalr1.cc: The token and value types are now class members.
4003 The tokens were defined as free form enums and cpp macros. YYSTYPE
4004 was defined as a free form union. They are now class members:
4005 tokens are enumerations of the "yy::parser::token" struct, and the
4006 semantic values have the "yy::parser::semantic_type" type.
4008 If you do not want or can update to this scheme, the directive
4009 '%define "global_tokens_and_yystype" "1"' triggers the global
4010 definition of tokens and YYSTYPE. This change is suitable both
4011 for previous releases of Bison, and this one.
4013 If you wish to update, then make sure older version of Bison will
4014 fail using '%require "2.2"'.
4016 ** DJGPP support added.
4019 * Noteworthy changes in release 2.1 (2005-09-16)
4021 ** The C++ lalr1.cc skeleton supports %lex-param.
4023 ** Bison-generated parsers now support the translation of diagnostics like
4024 "syntax error" into languages other than English. The default
4025 language is still English. For details, please see the new
4026 Internationalization section of the Bison manual. Software
4027 distributors should also see the new PACKAGING file. Thanks to
4028 Bruno Haible for this new feature.
4030 ** Wording in the Bison-generated parsers has been changed slightly to
4031 simplify translation. In particular, the message "memory exhausted"
4032 has replaced "parser stack overflow", as the old message was not
4033 always accurate for modern Bison-generated parsers.
4035 ** Destructors are now called when the parser aborts, for all symbols left
4036 behind on the stack. Also, the start symbol is now destroyed after a
4037 successful parse. In both cases, the behavior was formerly inconsistent.
4039 ** When generating verbose diagnostics, Bison-generated parsers no longer
4040 quote the literal strings associated with tokens. For example, for
4041 a syntax error associated with '%token NUM "number"' they might
4042 print 'syntax error, unexpected number' instead of 'syntax error,
4043 unexpected "number"'.
4046 * Noteworthy changes in release 2.0 (2004-12-25)
4048 ** Possibly-incompatible changes
4050 - Bison-generated parsers no longer default to using the alloca function
4051 (when available) to extend the parser stack, due to widespread
4052 problems in unchecked stack-overflow detection. You can "#define
4053 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
4054 the manual to determine safe values for YYMAXDEPTH in that case.
4056 - Error token location.
4057 During error recovery, the location of the syntax error is updated
4058 to cover the whole sequence covered by the error token: it includes
4059 the shifted symbols thrown away during the first part of the error
4060 recovery, and the lookahead rejected during the second part.
4062 - Semicolon changes:
4063 . Stray semicolons are no longer allowed at the start of a grammar.
4064 . Semicolons are now required after in-grammar declarations.
4066 - Unescaped newlines are no longer allowed in character constants or
4067 string literals. They were never portable, and GCC 3.4.0 has
4068 dropped support for them. Better diagnostics are now generated if
4069 forget a closing quote.
4071 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4075 - GLR grammars now support locations.
4077 - New directive: %initial-action.
4078 This directive allows the user to run arbitrary code (including
4079 initializing @$) from yyparse before parsing starts.
4081 - A new directive "%expect-rr N" specifies the expected number of
4082 reduce/reduce conflicts in GLR parsers.
4084 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4085 This is a GNU extension.
4087 - The option "--report=lookahead" was changed to "--report=look-ahead".
4088 [However, this was changed back after 2.3.]
4090 - Experimental %destructor support has been added to lalr1.cc.
4092 - New configure option --disable-yacc, to disable installation of the
4093 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4097 - For now, %expect-count violations are now just warnings, not errors.
4098 This is for compatibility with Bison 1.75 and earlier (when there are
4099 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4100 are too many or too few shift/reduce conflicts). However, in future
4101 versions of Bison we plan to improve the %expect machinery so that
4102 these violations will become errors again.
4104 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4105 arbitrarily limited to 16-bit counts.
4107 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4110 * Noteworthy changes in release 1.875 (2003-01-01)
4112 ** The documentation license has been upgraded to version 1.2
4113 of the GNU Free Documentation License.
4115 ** syntax error processing
4117 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4118 locations too. This fixes bugs in error-location computation.
4121 It is now possible to reclaim the memory associated to symbols
4122 discarded during error recovery. This feature is still experimental.
4125 This new directive is preferred over YYERROR_VERBOSE.
4127 - #defining yyerror to steal internal variables is discouraged.
4128 It is not guaranteed to work forever.
4130 ** POSIX conformance
4132 - Semicolons are once again optional at the end of grammar rules.
4133 This reverts to the behavior of Bison 1.33 and earlier, and improves
4134 compatibility with Yacc.
4136 - "parse error" -> "syntax error"
4137 Bison now uniformly uses the term "syntax error"; formerly, the code
4138 and manual sometimes used the term "parse error" instead. POSIX
4139 requires "syntax error" in diagnostics, and it was thought better to
4142 - The documentation now emphasizes that yylex and yyerror must be
4143 declared before use. C99 requires this.
4145 - Bison now parses C99 lexical constructs like UCNs and
4146 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4148 - File names are properly escaped in C output. E.g., foo\bar.y is
4149 output as "foo\\bar.y".
4151 - Yacc command and library now available
4152 The Bison distribution now installs a "yacc" command, as POSIX requires.
4153 Also, Bison now installs a small library liby.a containing
4154 implementations of Yacc-compatible yyerror and main functions.
4155 This library is normally not useful, but POSIX requires it.
4157 - Type clashes now generate warnings, not errors.
4159 - If the user does not define YYSTYPE as a macro, Bison now declares it
4160 using typedef instead of defining it as a macro.
4161 For consistency, YYLTYPE is also declared instead of defined.
4163 ** Other compatibility issues
4165 - %union directives can now have a tag before the "{", e.g., the
4166 directive "%union foo {...}" now generates the C code
4167 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4168 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4169 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4170 This is for compatibility with both Yacc and Bison 1.35.
4172 - ";" is output before the terminating "}" of an action, for
4173 compatibility with Bison 1.35.
4175 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4176 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4178 - "yystype" and "yyltype" are now obsolescent macros instead of being
4179 typedefs or tags; they are no longer documented and are planned to be
4180 withdrawn in a future release.
4185 Users of Bison have to decide how they handle the portability of the
4188 - "parsing stack overflow..." -> "parser stack overflow"
4189 GLR parsers now report "parser stack overflow" as per the Bison manual.
4191 ** %parse-param and %lex-param
4192 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4193 additional context to yyparse and yylex. They suffer from several
4196 - a single argument only can be added,
4197 - their types are weak (void *),
4198 - this context is not passed to ancillary functions such as yyerror,
4199 - only yacc.c parsers support them.
4201 The new %parse-param/%lex-param directives provide a more precise control.
4204 %parse-param {int *nastiness}
4205 %lex-param {int *nastiness}
4206 %parse-param {int *randomness}
4208 results in the following signatures:
4210 int yylex (int *nastiness);
4211 int yyparse (int *nastiness, int *randomness);
4213 or, if both %pure-parser and %locations are used:
4215 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4216 int yyparse (int *nastiness, int *randomness);
4218 ** Bison now warns if it detects conflicting outputs to the same file,
4219 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4220 that command outputs both code and header to foo.h.
4222 ** #line in output files
4223 - --no-line works properly.
4225 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4226 later to be built. This change originally took place a few versions
4227 ago, but nobody noticed until we recently asked someone to try
4228 building Bison with a K&R C compiler.
4231 * Noteworthy changes in release 1.75 (2002-10-14)
4233 ** Bison should now work on 64-bit hosts.
4235 ** Indonesian translation thanks to Tedi Heriyanto.
4238 Fix spurious parse errors.
4241 Some people redefine yyerror to steal yyparse' private variables.
4242 Reenable this trick until an official feature replaces it.
4245 In agreement with POSIX and with other Yaccs, leaving a default
4246 action is valid when $$ is untyped, and $1 typed:
4250 but the converse remains an error:
4254 ** Values of midrule actions
4257 foo: { ... } { $$ = $1; } ...
4259 was incorrectly rejected: $1 is defined in the second midrule
4260 action, and is equal to the $$ of the first midrule action.
4263 * Noteworthy changes in release 1.50 (2002-10-04)
4268 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4269 almost any context-free grammar, ambiguous or not. The new declarations
4270 %dprec and %merge on grammar rules allow parse-time resolution of
4271 ambiguities. Contributed by Paul Hilfinger.
4273 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4274 like the Alpha, so please stick to 32-bit hosts for now.
4277 When not in Yacc compatibility mode, when the output file was not
4278 specified, running "bison foo/bar.y" created "foo/bar.c". It
4279 now creates "bar.c".
4282 The undefined token was systematically mapped to 2 which prevented
4283 the use of 2 by the user. This is no longer the case.
4285 ** Unknown token numbers
4286 If yylex returned an out of range value, yyparse could die. This is
4290 According to POSIX, the error token must be 256.
4291 Bison extends this requirement by making it a preference: *if* the
4292 user specified that one of her tokens is numbered 256, then error
4293 will be mapped onto another number.
4295 ** Verbose error messages
4296 They no longer report "..., expecting error or..." for states where
4297 error recovery is possible.
4300 Defaults to "$end" instead of "$".
4302 ** Error recovery now conforms to documentation and to POSIX
4303 When a Bison-generated parser encounters a syntax error, it now pops
4304 the stack until it finds a state that allows shifting the error
4305 token. Formerly, it popped the stack until it found a state that
4306 allowed some non-error action other than a default reduction on the
4307 error token. The new behavior has long been the documented behavior,
4308 and has long been required by POSIX. For more details, please see
4309 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4310 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4313 Popped tokens and nonterminals are now reported.
4316 Larger grammars are now supported (larger token numbers, larger grammar
4317 size (= sum of the LHS and RHS lengths), larger LALR tables).
4318 Formerly, many of these numbers ran afoul of 16-bit limits;
4319 now these limits are 32 bits on most hosts.
4321 ** Explicit initial rule
4322 Bison used to play hacks with the initial rule, which the user does
4323 not write. It is now explicit, and visible in the reports and
4327 Before, Bison reported the useless rules, but, although not used,
4328 included them in the parsers. They are now actually removed.
4330 ** Useless rules, useless nonterminals
4331 They are now reported, as a warning, with their locations.
4333 ** Rules never reduced
4334 Rules that can never be reduced because of conflicts are now
4337 ** Incorrect "Token not used"
4338 On a grammar such as
4340 %token useless useful
4342 exp: '0' %prec useful;
4344 where a token was used to set the precedence of the last rule,
4345 bison reported both "useful" and "useless" as useless tokens.
4347 ** Revert the C++ namespace changes introduced in 1.31
4348 as they caused too many portability hassles.
4350 ** Default locations
4351 By an accident of design, the default computation of @$ was
4352 performed after another default computation was performed: @$ = @1.
4353 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4354 the computation of @$.
4356 ** Token end-of-file
4357 The token end of file may be specified by the user, in which case,
4358 the user symbol is used in the reports, the graphs, and the verbose
4359 error messages instead of "$end", which remains being the default.
4363 %token MYEOF 0 "end of file"
4366 This old option, which has been broken for ages, is removed.
4369 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4370 Croatian, thanks to Denis Lackovic.
4372 ** Incorrect token definitions
4375 bison used to output
4378 ** Token definitions as enums
4379 Tokens are output both as the traditional #define's, and, provided
4380 the compiler supports ANSI C or is a C++ compiler, as enums.
4381 This lets debuggers display names instead of integers.
4384 In addition to --verbose, bison supports --report=THINGS, which
4385 produces additional information:
4387 complete the core item sets with their closure
4388 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4389 explicitly associate lookahead tokens to items
4391 describe shift/reduce conflicts solving.
4392 Bison used to systematically output this information on top of
4393 the report. Solved conflicts are now attached to their states.
4396 Previous versions don't complain when there is a type clash on
4397 the default action if the rule has a midrule action, such as in:
4405 ** GNU M4 is now required when using Bison.
4408 * Noteworthy changes in release 1.35 (2002-03-25)
4411 Some projects use Bison's C parser with C++ compilers, and define
4412 YYSTYPE as a class. The recent adjustment of C parsers for data
4413 alignment and 64 bit architectures made this impossible.
4415 Because for the time being no real solution for C++ parser
4416 generation exists, kludges were implemented in the parser to
4417 maintain this use. In the future, when Bison has C++ parsers, this
4418 kludge will be disabled.
4420 This kludge also addresses some C++ problems when the stack was
4424 * Noteworthy changes in release 1.34 (2002-03-12)
4426 ** File name clashes are detected
4427 $ bison foo.y -d -o foo.x
4428 fatal error: header and parser would both be named "foo.x"
4430 ** A missing ";" at the end of a rule triggers a warning
4431 In accordance with POSIX, and in agreement with other
4432 Yacc implementations, Bison will mandate this semicolon in the near
4433 future. This eases the implementation of a Bison parser of Bison
4434 grammars by making this grammar LALR(1) instead of LR(2). To
4435 facilitate the transition, this release introduces a warning.
4437 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4438 many portability hassles.
4440 ** DJGPP support added.
4442 ** Fix test suite portability problems.
4445 * Noteworthy changes in release 1.33 (2002-02-07)
4448 Groff could not be compiled for the definition of size_t was lacking
4449 under some conditions.
4455 * Noteworthy changes in release 1.32 (2002-01-23)
4457 ** Fix Yacc output file names
4459 ** Portability fixes
4461 ** Italian, Dutch translations
4464 * Noteworthy changes in release 1.31 (2002-01-14)
4468 ** GNU Gettext and %expect
4469 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4470 Bison dies on incorrect %expectations, we fear there will be
4471 too many bug reports for Gettext, so _for the time being_, %expect
4472 does not trigger an error when the input file is named "plural.y".
4474 ** Use of alloca in parsers
4475 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4476 malloc exclusively. Since 1.29, but was not NEWS'ed.
4478 alloca is used only when compiled with GCC, to avoid portability
4481 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4483 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4484 (as POSIX requires) instead of being undefined.
4487 Bison has always permitted actions such as { $$ = $1 }: it adds the
4488 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4489 is no longer output: one has to write { $$ = $1; }.
4491 ** Better C++ compliance
4492 The output parsers try to respect C++ namespaces.
4493 [This turned out to be a failed experiment, and it was reverted later.]
4496 Fixed bugs when reporting useless nonterminals.
4499 The parsers work properly on 64 bit hosts.
4502 Some calls to strerror resulted in scrambled or missing error messages.
4505 When the number of shift/reduce conflicts is correct, don't issue
4508 ** The verbose report includes the rule line numbers.
4510 ** Rule line numbers are fixed in traces.
4512 ** Swedish translation
4515 Verbose parse error messages from the parsers are better looking.
4516 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4517 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4519 ** Fixed parser memory leaks.
4520 When the generated parser was using malloc to extend its stacks, the
4521 previous allocations were not freed.
4523 ** Fixed verbose output file.
4524 Some newlines were missing.
4525 Some conflicts in state descriptions were missing.
4527 ** Fixed conflict report.
4528 Option -v was needed to get the result.
4532 Mismatches are errors, not warnings.
4534 ** Fixed incorrect processing of some invalid input.
4536 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4538 ** Fixed some typos in the documentation.
4540 ** %token MY_EOF 0 is supported.
4541 Before, MY_EOF was silently renumbered as 257.
4543 ** doc/refcard.tex is updated.
4545 ** %output, %file-prefix, %name-prefix.
4549 New, aliasing "--output-file".
4552 * Noteworthy changes in release 1.30 (2001-10-26)
4554 ** "--defines" and "--graph" have now an optional argument which is the
4555 output file name. "-d" and "-g" do not change; they do not take any
4558 ** "%source_extension" and "%header_extension" are removed, failed
4561 ** Portability fixes.
4564 * Noteworthy changes in release 1.29 (2001-09-07)
4566 ** The output file does not define const, as this caused problems when used
4567 with common autoconfiguration schemes. If you still use ancient compilers
4568 that lack const, compile with the equivalent of the C compiler option
4569 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4571 ** Added "-g" and "--graph".
4573 ** The Bison manual is now distributed under the terms of the GNU FDL.
4575 ** The input and the output files has automatically a similar extension.
4577 ** Russian translation added.
4579 ** NLS support updated; should hopefully be less troublesome.
4581 ** Added the old Bison reference card.
4583 ** Added "--locations" and "%locations".
4585 ** Added "-S" and "--skeleton".
4587 ** "%raw", "-r", "--raw" is disabled.
4589 ** Special characters are escaped when output. This solves the problems
4590 of the #line lines with path names including backslashes.
4593 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4594 "%debug", "%source_extension" and "%header_extension".
4597 Automatic location tracking.
4600 * Noteworthy changes in release 1.28 (1999-07-06)
4602 ** Should compile better now with K&R compilers.
4606 ** Fixed a problem with escaping the double quote character.
4608 ** There is now a FAQ.
4611 * Noteworthy changes in release 1.27
4613 ** The make rule which prevented bison.simple from being created on
4614 some systems has been fixed.
4617 * Noteworthy changes in release 1.26
4619 ** Bison now uses Automake.
4621 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4623 ** Token numbers now start at 257 as previously documented, not 258.
4625 ** Bison honors the TMPDIR environment variable.
4627 ** A couple of buffer overruns have been fixed.
4629 ** Problems when closing files should now be reported.
4631 ** Generated parsers should now work even on operating systems which do
4632 not provide alloca().
4635 * Noteworthy changes in release 1.25 (1995-10-16)
4637 ** Errors in the input grammar are not fatal; Bison keeps reading
4638 the grammar file, and reports all the errors found in it.
4640 ** Tokens can now be specified as multiple-character strings: for
4641 example, you could use "<=" for a token which looks like <=, instead
4642 of choosing a name like LESSEQ.
4644 ** The %token_table declaration says to write a table of tokens (names
4645 and numbers) into the parser file. The yylex function can use this
4646 table to recognize multiple-character string tokens, or for other
4649 ** The %no_lines declaration says not to generate any #line preprocessor
4650 directives in the parser file.
4652 ** The %raw declaration says to use internal Bison token numbers, not
4653 Yacc-compatible token numbers, when token names are defined as macros.
4655 ** The --no-parser option produces the parser tables without including
4656 the parser engine; a project can now use its own parser engine.
4657 The actions go into a separate file called NAME.act, in the form of
4658 a switch statement body.
4661 * Noteworthy changes in release 1.23
4663 The user can define YYPARSE_PARAM as the name of an argument to be
4664 passed into yyparse. The argument should have type void *. It should
4665 actually point to an object. Grammar actions can access the variable
4666 by casting it to the proper pointer type.
4668 Line numbers in output file corrected.
4671 * Noteworthy changes in release 1.22
4673 --help option added.
4676 * Noteworthy changes in release 1.20
4678 Output file does not redefine const for C++.
4682 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4683 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4684 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4685 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4686 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4687 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4688 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4689 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4690 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4691 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4692 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4693 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4694 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4695 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4696 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4697 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4698 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4699 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4700 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4701 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4702 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4703 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4704 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4705 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4706 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4707 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4708 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE CVE
4709 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4710 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4711 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4712 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4713 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4714 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4715 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4716 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4717 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4718 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4719 LocalWords: Vais xsltproc YYNOMEM YYLOCATION signedness YYBISON MITRE's
4720 LocalWords: libreadline YYMALLOC YYFREE MSVC redefinitions POSIXLY
4723 ispell-dictionary: "american"
4728 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4730 This file is part of Bison, the GNU Parser Generator.
4732 Permission is granted to copy, distribute and/or modify this document
4733 under the terms of the GNU Free Documentation License, Version 1.3 or
4734 any later version published by the Free Software Foundation; with no
4735 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4736 Texts. A copy of the license is included in the "GNU Free
4737 Documentation License" file as part of this distribution.