3 * Noteworthy changes in release ?.? (????-??-??) [?]
5 Portability issues in the test suite and in C++ skeletons.
7 * Noteworthy changes in release 3.7.90 (2021-08-13) [beta]
9 ** Backward incompatible changes
11 In conformance with the recommendations of the Graphviz team
12 (https://marc.info/?l=graphviz-devel&m=129418103126092), `-g`/`--graph`
13 now generates a *.gv file by default, instead of *.dot. A transition
16 To comply with the latest POSIX standard, in Yacc compatibility mode
17 (options `-y`/`--yacc`) Bison now generates prototypes for yyerror and
18 yylex. In some situations, this is breaking compatibility: if the user
19 has already declared these functions but with some differences (e.g., to
20 declare them as static, or to use specific attributes), the generated
21 parser will fail to compile. To disable these prototypes, #define yyerror
22 (to `yyerror`), and likewise for yylex.
24 ** Deprecated features
26 Support for the YYPRINT macro is removed. It worked only with yacc.c and
27 only for tokens. It was obsoleted by %printer, introduced in Bison 1.50
30 It has always been recommended to prefer `%define api.value.type foo` to
31 `#define YYSTYPE foo`. The latter is supported in C for compatibility
32 with Yacc, but not in C++. Warnings are now issued if `#define YYSTYPE`
33 is used in C++, and eventually support will be removed.
35 In C++ code, prefer value_type to semantic_type to denote the semantic
36 value type, which is specified by the `api.value.type` %define variable.
40 *** A skeleton for the D programming language
42 The "lalr1.d" skeleton is now officially part of Bison.
44 It was originally contributed by Oliver Mangold, based on Paolo Bonzini's
45 lalr1.java, and was improved by H. S. Teoh. Adela Vais then took over
46 maintenance and invested a lot of efforts to complete, test and document
49 It now supports all the bells and whistles of the other deterministic
50 parsers, which include: pull/push interfaces, verbose and custom error
51 messages, lookahead correction, token constructors, internationalization,
52 locations, printers, token and symbol prefixes, etc.
54 Two examples demonstrate the D parsers: a basic one (examples/d/simple),
55 and an advanced one (examples/d/calc).
57 *** Option -H, --header and directive %header
59 The option `-H`/`--header` supersedes the option `--defines`, and the
60 directive %header supersedes %defines. Both `--defines` and `%defines`
61 are, of course, maintained for backward compatibility.
65 Since version 2.4 Bison can be used to generate HTML reports. However it
66 was a two-step process: first bison must be invoked with option `--xml`,
67 and then xsltproc must be run to the convert the XML reports into HTML.
69 The new option `--html` combines these steps. The xsltproc program must
72 *** A C++ native GLR parser
74 A new version of the C++ GLR parser was added: "glr2.cc". It generates
75 "true C++11", instead of a C++ wrapper around a C parser as does the
76 existing "glr.cc" parser. As a first significant consequence, it supports
77 `%define api.value.type variant`, contrary to glr.cc.
79 It should be upward compatible in terms of interface, feature and
80 performance to "glr.cc". To try it out, simply use
84 It will eventually replace "glr.cc". However we need user feedback on
85 this skeleton. _Please_ report your results and comments about it.
89 Counterexamples now show the rule numbers, and always show ε for rules
90 with an empty right-hand side. For instance
102 *** Lookahead correction in Java
104 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
107 *** Abort parsing for memory exhaustion (C)
109 User actions may now use `YYNOMEM` (similar to `YYACCEPT` and `YYABORT`)
110 to abort the current parse with memory exhaustion.
112 *** Printing locations in debug traces (C)
114 The `YYLOCATION_PRINT(File, Loc)` macro prints a location. It is defined
115 when (i) locations are enabled, (ii) the default type for locations is
116 used, (iii) debug traces are enabled, and (iv) `YYLOCATION_PRINT` is not
119 Users may define `YYLOCATION_PRINT` to cover other cases.
123 There were no debug traces for deferred calls to user actions. They are
127 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
131 *** Reused Push Parsers
133 When a push-parser state structure is used for multiple parses, it was
134 possible for some state to leak from one run into the following one.
136 *** Fix Table Generation
138 In some very rare conditions, when there are many useless tokens, it was
139 possible to generate incorrect parsers.
142 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
146 *** Counterexample Generation
148 In some cases counterexample generation could crash. This is fixed.
150 *** Fix Table Generation
152 In some very rare conditions, when there are many useless tokens, it was
153 possible to generate incorrect parsers.
155 *** GLR parsers now support %merge together with api.value.type=union.
157 *** C++ parsers use noexcept in more places.
159 *** Generated parsers avoid some warnings about signedness issues.
161 *** C-language parsers now avoid warnings from pedantic clang.
163 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
166 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
170 *** Bug fixes in yacc.c
172 In Yacc mode, all the tokens are defined twice: once as an enum, and then
173 as a macro. YYEMPTY was missing its macro.
175 *** Bug fixes in lalr1.cc
177 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
178 even when the `parse.assert` %define variable is not enabled. It no
181 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
184 When there is a very large number of tokens, some assertions could be long
185 enough to hit arbitrary limits in Visual C++. They have been rewritten to
186 work around this limitation.
190 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
191 skeleton) used to be defined to 1. It is now defined to the version of
192 Bison as an integer (e.g., 30704 for version 3.7.4).
195 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
199 Fix concurrent build issues.
201 The bison executable is no longer linked uselessly against libreadline.
203 Fix incorrect use of yytname in glr.cc.
206 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
208 This release of Bison fixes all known bugs reported for Bison in MITRE's
209 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
210 are only about bison-the-program itself, not the generated code.
212 Although these bugs are typically irrelevant to how Bison is used, they
213 are worth fixing if only to give users peace of mind.
215 There is no known vulnerability in the generated parsers.
219 Fix concurrent build issues (introduced in Bison 3.5).
221 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
223 Fix portability issues of the test suite, and of bison itself.
225 Some unlikely crashes found by fuzzing have been fixed. This is only
226 about bison itself, not the generated parsers.
229 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
233 Crash when a token alias contains a NUL byte.
235 Portability issues with libtextstyle.
237 Portability issues of Bison itself with MSVC.
241 Improvements and fixes in the documentation.
243 More precise location about symbol type redefinitions.
246 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
248 ** Deprecated features
250 The YYPRINT macro, which works only with yacc.c and only for tokens, was
251 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
252 It is deprecated and its support will be removed eventually.
254 In conformance with the recommendations of the Graphviz team, in the next
255 version Bison the option `--graph` will generate a *.gv file by default,
256 instead of *.dot. A transition started in Bison 3.4.
260 *** Counterexample Generation
262 Contributed by Vincent Imbimbo.
264 When given `-Wcounterexamples`/`-Wcex`, bison will now output
265 counterexamples for conflicts.
267 **** Unifying Counterexamples
269 Unifying counterexamples are strings which can be parsed in two ways due
270 to the conflict. For example on a grammar that contains the usual
271 "dangling else" ambiguity:
274 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
275 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
278 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
279 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
280 Example: "if" exp "then" "if" exp "then" exp • "else" exp
283 ↳ "if" exp "then" exp
284 ↳ "if" exp "then" exp • "else" exp
285 Example: "if" exp "then" "if" exp "then" exp • "else" exp
288 ↳ "if" exp "then" exp "else" exp
289 ↳ "if" exp "then" exp •
291 When text styling is enabled, colors are used in the examples and the
292 derivations to highlight the structure of both analyses. In this case,
294 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
298 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
301 The counterexamples are "focused", in two different ways. First, they do
302 not clutter the output with all the derivations from the start symbol,
303 rather they start on the "conflicted nonterminal". They go straight to the
304 point. Second, they don't "expand" nonterminal symbols uselessly.
306 **** Nonunifying Counterexamples
308 In the case of the dangling else, Bison found an example that can be
309 parsed in two ways (therefore proving that the grammar is ambiguous).
310 When it cannot find such an example, it instead generates two examples
311 that are the same up until the dot:
314 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
315 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
316 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
321 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
322 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
323 First example: expr • ID ',' ID $end
330 Second example: expr • ID $end
336 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
340 In these cases, the parser usually doesn't have enough lookahead to
341 differentiate the two given examples.
345 Counterexamples are also included in the report when given
346 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
351 1 exp: "if" exp "then" exp • [$end, "then", "else"]
352 2 | "if" exp "then" exp • "else" exp
354 "else" shift, and go to state 8
356 "else" [reduce using rule 1 (exp)]
357 $default reduce using rule 1 (exp)
359 shift/reduce conflict on token "else":
360 1 exp: "if" exp "then" exp •
361 2 exp: "if" exp "then" exp • "else" exp
362 Example: "if" exp "then" "if" exp "then" exp • "else" exp
365 ↳ "if" exp "then" exp
366 ↳ "if" exp "then" exp • "else" exp
367 Example: "if" exp "then" "if" exp "then" exp • "else" exp
370 ↳ "if" exp "then" exp "else" exp
371 ↳ "if" exp "then" exp •
373 *** File prefix mapping
375 Contributed by Joshua Watt.
377 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
378 in the output (specifically `#line` directives and `#ifdef` header guards)
379 that begins with the prefix OLD will have it replaced with the prefix NEW,
380 similar to the `-ffile-prefix-map` in GCC. This option can be used to
381 make bison output reproducible.
387 When text styling is enabled and the terminal supports it, the warnings
388 now include hyperlinks to the documentation.
390 *** Relocatable installation
392 When installed to be relocatable (via `configure --enable-relocatable`),
393 bison will now also look for a relocated m4.
397 The `filename_type` %define variable was renamed `api.filename.type`.
400 %define filename_type "symbol"
404 %define api.filename.type {symbol}
406 (Or let `bison --update` do it for you).
408 It now defaults to `const std::string` instead of `std::string`.
410 *** Deprecated %define variable names
412 The following variables have been renamed for consistency. Backward
413 compatibility is ensured, but upgrading is recommended.
415 filename_type -> api.filename.type
416 package -> api.package
418 *** Push parsers no longer clear their state when parsing is finished
420 Previously push-parsers cleared their state when parsing was finished (on
421 success and on failure). This made it impossible to check if there were
422 parse errors, since `yynerrs` was also reset. This can be especially
423 troublesome when used in autocompletion, since a parser with error
424 recovery would suggest (irrelevant) expected tokens even if there were
427 Now the parser state can be examined when parsing is finished. The parser
428 state is reset when starting a new parse.
434 The bistromathic demonstrates %param and how to quote sources in the error
438 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
444 *** Include the generated header (yacc.c)
446 Historically, when --defines was used, bison generated a header and pasted
447 an exact copy of it into the generated parser implementation file. Since
448 Bison 3.4 it is possible to specify that the header should be `#include`d,
449 and how. For instance
451 %define api.header.include {"parse.h"}
455 %define api.header.include {<parser/parse.h>}
457 Now api.header.include defaults to `"header-basename"`, as was intended in
458 Bison 3.4, where `header-basename` is the basename of the generated
459 header. This is disabled when the generated header is `y.tab.h`, to
460 comply with Automake's ylwrap.
462 *** String aliases are faithfully propagated
464 Bison used to interpret user strings (i.e., decoding backslash escapes)
465 when reading them, and to escape them (i.e., issue non-printable
466 characters as backslash escapes, taking the locale into account) when
467 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
468 up "ciphered" as sequences of backslash escapes. This happened not only
469 in the generated sources (where the compiler will reinterpret them), but
470 also in all the generated reports (text, xml, html, dot, etc.). Reports
471 were therefore not readable when string aliases were not pure ASCII.
472 Worse yet: the output depended on the user's locale.
474 Now Bison faithfully treats the string aliases exactly the way the user
475 spelled them. This fixes all the aforementioned problems. However, now,
476 string aliases semantically equivalent but syntactically different (e.g.,
477 "A", "\x41", "\101") are considered to be different.
479 *** Crash when generating IELR
481 An old, well hidden, bug in the generation of IELR parsers was fixed.
484 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
488 In glr.cc some internal macros leaked in the user's code, and could damage
489 access to the token kinds.
492 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
496 Incorrect comments in the generated parsers.
498 Warnings in push parsers (yacc.c).
500 Incorrect display of gotos in LAC traces (lalr1.cc).
503 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
507 Some tests were fixed.
509 When token aliases contain comment delimiters:
511 %token FOO "/* foo */"
513 bison used to emit "nested" comments, which is invalid C.
516 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
520 Restored ANSI-C compliance in yacc.c.
522 GNU readline portability issues.
524 In C++, yy::parser::symbol_name is now a public member, as was intended.
528 In C++, yy::parser::symbol_type now has a public name() member function.
531 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
533 ** Backward incompatible changes
535 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
537 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
538 depend on it will now produce Yacc-like error messages (just "syntax
539 error"). It was superseded by the "%error-verbose" directive in Bison
540 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
541 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
542 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
543 parse.error verbose".
545 ** Deprecated features
547 The YYPRINT macro, which works only with yacc.c and only for tokens, was
548 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
549 It is deprecated and its support will be removed eventually.
553 *** Improved syntax error messages
555 Two new values for the %define parse.error variable offer more control to
556 the user. Available in all the skeletons (C, C++, Java).
558 **** %define parse.error detailed
560 The behavior of "%define parse.error detailed" is closely resembling that
561 of "%define parse.error verbose" with a few exceptions. First, it is safe
562 to use non-ASCII characters in token aliases (with 'verbose', the result
563 depends on the locale with which bison was run). Second, a yysymbol_name
564 function is exposed to the user, instead of the yytnamerr function and the
565 yytname table. Third, token internationalization is supported (see
568 **** %define parse.error custom
570 With this directive, the user forges and emits the syntax error message
571 herself by defining the yyreport_syntax_error function. A new type,
572 yypcontext_t, captures the circumstances of the error, and provides the
573 user with functions to get details, such as yypcontext_expected_tokens to
574 get the list of expected token kinds.
576 A possible implementation of yyreport_syntax_error is:
579 yyreport_syntax_error (const yypcontext_t *ctx)
582 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
583 fprintf (stderr, ": syntax error");
584 // Report the tokens expected at this point.
586 enum { TOKENMAX = 10 };
587 yysymbol_kind_t expected[TOKENMAX];
588 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
590 // Forward errors to yyparse.
593 for (int i = 0; i < n; ++i)
594 fprintf (stderr, "%s %s",
595 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
597 // Report the unexpected token.
599 yysymbol_kind_t lookahead = yypcontext_token (ctx);
600 if (lookahead != YYSYMBOL_YYEMPTY)
601 fprintf (stderr, " before %s", yysymbol_name (lookahead));
603 fprintf (stderr, "\n");
607 **** Token aliases internationalization
609 When the %define variable parse.error is set to `custom` or `detailed`,
610 one may specify which token aliases are to be translated using _(). For
622 In that case the user must define _() and N_(), and yysymbol_name returns
623 the translated symbol (i.e., it returns '_("variable")' rather that
624 '"variable"'). In Java, the user must provide an i18n() function.
626 *** List of expected tokens (yacc.c)
628 Push parsers may invoke yypstate_expected_tokens at any point during
629 parsing (including even before submitting the first token) to get the list
630 of possible tokens. This feature can be used to propose autocompletion
631 (see below the "bistromathic" example).
633 It makes little sense to use this feature without enabling LAC (lookahead
636 *** Returning the error token
638 When the scanner returns an invalid token or the undefined token
639 (YYUNDEF), the parser generates an error message and enters error
640 recovery. Because of that error message, most scanners that find lexical
641 errors generate an error message, and then ignore the invalid input
642 without entering the error-recovery.
644 The scanners may now return YYerror, the error token, to enter the
645 error-recovery mode without triggering an additional error message. See
646 the bistromathic for an example.
648 *** Deep overhaul of the symbol and token kinds
650 To avoid the confusion with types in programming languages, we now refer
651 to token and symbol "kinds" instead of token and symbol "types". The
652 documentation and error messages have been revised.
654 All the skeletons have been updated to use dedicated enum types rather
655 than integral types. Special symbols are now regular citizens, instead of
656 being declared in ad hoc ways.
660 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
661 LPAREN, etc. While backward compatibility is of course ensured, users are
662 nonetheless invited to replace their uses of "enum yytokentype" by
665 This type now also includes tokens that were previously hidden: YYEOF (end
666 of input), YYUNDEF (undefined token), and YYerror (error token). They
667 now have string aliases, internationalized when internationalization is
668 enabled. Therefore, by default, error messages now refer to "end of file"
669 (internationalized) rather than the cryptic "$end", or to "invalid token"
670 rather than "$undefined".
672 Therefore in most cases it is now useless to define the end-of-line token
675 %token T_EOF 0 "end of file"
677 Rather simply use "YYEOF" in your scanner.
681 The "symbol kinds" is what the parser actually uses. (Unless the
682 api.token.raw %define variable is used, the symbol kind of a terminal
683 differs from the corresponding token kind.)
685 They are now exposed as a enum, "yysymbol_kind_t".
687 This allows users to tailor the error messages the way they want, or to
688 process some symbols in a specific way in autocompletion (see the
689 bistromathic example below).
691 *** Modernize display of explanatory statements in diagnostics
693 Since Bison 2.7, output was indented four spaces for explanatory
694 statements. For example:
696 input.y:2.7-13: error: %type redeclaration for exp
697 input.y:1.7-11: previous declaration
699 Since the introduction of caret-diagnostics, it became less clear. This
700 indentation has been removed and submessages are displayed similarly as in
703 input.y:2.7-13: error: %type redeclaration for exp
704 2 | %type <float> exp
706 input.y:1.7-11: note: previous declaration
710 Contributed by Victor Morales Cayuela.
714 The token and symbol kinds are yy::parser::token_kind_type and
715 yy::parser::symbol_kind_type.
717 The symbol_type::kind() member function allows to get the kind of a
718 symbol. This can be used to write unit tests for scanners, e.g.,
720 yy::parser::symbol_type t = make_NUMBER ("123");
721 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
722 assert (t.value.as<int> () == 123);
728 In order to avoid ambiguities with "type" as in "typing", we now refer to
729 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
730 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
735 There are now examples/java: a very simple calculator, and a more complete
736 one (push-parser, location tracking, and debug traces).
738 The lexcalc example (a simple example in C based on Flex and Bison) now
739 also demonstrates location tracking.
742 A new C example, bistromathic, is a fully featured interactive calculator
743 using many Bison features: pure interface, push parser, autocompletion
744 based on the current parser state (using yypstate_expected_tokens),
745 location tracking, internationalized custom error messages, lookahead
746 correction, rich debug traces, etc.
748 It shows how to depend on the symbol kinds to tailor autocompletion. For
749 instance it recognizes the symbol kind "VARIABLE" to propose
750 autocompletion on the existing variables, rather than of the word
754 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
756 ** WARNING: Future backward-incompatibilities!
758 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
760 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
761 that still depend on it will produce Yacc-like error messages (just
762 "syntax error"). It was superseded by the "%error-verbose" directive in
763 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
764 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
765 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
766 parse.error verbose".
770 Fix portability issues of the package itself on old compilers.
772 Fix api.token.raw support in Java.
775 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
779 Error messages could quote lines containing zero-width characters (such as
780 \005) with incorrect styling. Fixes for similar issues with unexpectedly
781 short lines (e.g., the file was changed between parsing and diagnosing).
783 Some unlikely crashes found by fuzzing have been fixed. This is only
784 about bison itself, not the generated parsers.
787 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
791 Portability issues and minor cosmetic issues.
793 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
797 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
803 Fix compiler warnings.
806 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
808 ** Backward incompatible changes
810 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
811 longer treated as end-of-lines. This changes the diagnostics, and in
812 particular their locations.
814 In C++, line numbers and columns are now represented as 'int' not
815 'unsigned', so that integer overflow on positions is easily checkable via
816 'gcc -fsanitize=undefined' and the like. This affects the API for
817 positions. The default position and location classes now expose
818 'counter_type' (int), used to define line and column numbers.
820 ** Deprecated features
822 The YYPRINT macro, which works only with yacc.c and only for tokens, was
823 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
824 It is deprecated and its support will be removed eventually.
828 *** Lookahead correction in C++
830 Contributed by Adrian Vogelsgesang.
832 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
833 %define variable parse.lac.
835 *** Variable api.token.raw: Optimized token numbers (all skeletons)
837 In the generated parsers, tokens have two numbers: the "external" token
838 number as returned by yylex (which starts at 257), and the "internal"
839 symbol number (which starts at 3). Each time yylex is called, a table
840 lookup maps the external token number to the internal symbol number.
842 When the %define variable api.token.raw is set, tokens are assigned their
843 internal number, which saves one table lookup per token, and also saves
844 the generation of the mapping table.
846 The gain is typically moderate, but in extreme cases (very simple user
847 actions), a 10% improvement can be observed.
849 *** Generated parsers use better types for states
851 Stacks now use the best integral type for state numbers, instead of always
852 using 15 bits. As a result "small" parsers now have a smaller memory
853 footprint (they use 8 bits), and there is support for large automata (16
854 bits), and extra large (using int, i.e., typically 31 bits).
856 *** Generated parsers prefer signed integer types
858 Bison skeletons now prefer signed to unsigned integer types when either
859 will do, as the signed types are less error-prone and allow for better
860 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
861 portable to unusual machines where char, short and int are all the same
862 width. On non-GNU platforms this may entail including <limits.h> and (if
863 available) <stdint.h> to define integer types and constants.
865 *** A skeleton for the D programming language
867 For the last few releases, Bison has shipped a stealth experimental
868 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
869 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
872 However, because nobody has committed to improving, testing, and
873 documenting this skeleton, it is not clear that it will be supported in
876 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
877 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
879 *** Debug traces in Java
881 The Java backend no longer emits code and data for parser tracing if the
882 %define variable parse.trace is not defined.
886 *** New diagnostic: -Wdangling-alias
888 String literals, which allow for better error messages, are (too)
889 liberally accepted by Bison, which might result in silent errors. For
892 %type <exVal> cond "condition"
894 does not define "condition" as a string alias to 'cond' (nonterminal
895 symbols do not have string aliases). It is rather equivalent to
898 %token <exVal> "condition"
900 i.e., it gives the type 'exVal' to the "condition" token, which was
901 clearly not the intention.
903 Also, because string aliases need not be defined, typos such as "baz"
904 instead of "bar" will be not reported.
906 The option `-Wdangling-alias` catches these situations. On
909 %type <ival> foo "foo"
913 bison -Wdangling-alias reports
915 warning: string literal not attached to a symbol
916 | %type <ival> foo "foo"
918 warning: string literal not attached to a symbol
922 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
924 *** Better POSIX Yacc compatibility diagnostics
926 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
930 %type <ival> TOKEN1 TOKEN2 't'
937 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
938 2 | %type <ival> TOKEN1 TOKEN2 't'
940 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
941 2 | %type <ival> TOKEN1 TOKEN2 't'
943 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
944 2 | %type <ival> TOKEN1 TOKEN2 't'
947 *** Diagnostics with insertion
949 The diagnostics now display the suggestion below the underlined source.
950 Replacement for undeclared symbols are now also suggested.
957 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'?
961 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
965 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
967 *** Diagnostics about long lines
969 Quoted sources may now be truncated to fit the screen. For instance, on a
970 30-column wide terminal:
977 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
980 foo.y:1.8-10: previous declaration
983 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
986 foo.y:1.8-10: previous declaration
992 *** Debugging glr.c and glr.cc
994 The glr.c skeleton always had asserts to check its own behavior (not the
995 user's). These assertions are now under the control of the parse.assert
996 %define variable (disabled by default).
1000 Several new compiler warnings in the generated output have been avoided.
1001 Some unused features are no longer emitted. Cleaner generated code in
1006 Portability issues in the test suite.
1008 In theory, parsers using %nonassoc could crash when reporting verbose
1009 error messages. This unlikely bug has been fixed.
1011 In Java, %define api.prefix was ignored. It now behaves as expected.
1014 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
1018 In some cases, when warnings are disabled, bison could emit tons of white
1019 spaces as diagnostics.
1021 When running out of memory, bison could crash (found by fuzzing).
1023 When defining twice the EOF token, bison would crash.
1025 New warnings from recent compilers have been addressed in the generated
1026 parsers (yacc.c, glr.c, glr.cc).
1028 When lone carriage-return characters appeared in the input file,
1029 diagnostics could hang forever.
1032 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
1039 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
1041 ** Deprecated features
1043 The %pure-parser directive is deprecated in favor of '%define api.pure'
1044 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
1045 now. Note that since Bison 2.7 you are strongly encouraged to use
1046 '%define api.pure full' instead of '%define api.pure'.
1050 *** Colored diagnostics
1052 As an experimental feature, diagnostics are now colored, controlled by the
1053 new options --color and --style.
1055 To use them, install the libtextstyle library before configuring Bison.
1056 It is available from
1058 https://alpha.gnu.org/gnu/gettext/
1062 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
1064 The option --color supports the following arguments:
1065 - always, yes: Enable colors.
1066 - never, no: Disable colors.
1067 - auto, tty (default): Enable colors if the output device is a tty.
1069 To customize the styles, create a CSS file similar to
1073 .error { font-weight: 800; text-decoration: underline; }
1076 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1077 environment variable to "bison-bw.css".
1079 *** Disabling output
1081 When given -fsyntax-only, the diagnostics are reported, but no output is
1084 The name of this option is somewhat misleading as bison does more than
1085 just checking the syntax: every stage is run (including checking for
1086 conflicts for instance), except the generation of the output files.
1088 *** Include the generated header (yacc.c)
1090 Before, when --defines is used, bison generated a header, and pasted an
1091 exact copy of it into the generated parser implementation file. If the
1092 header name is not "y.tab.h", it is now #included instead of being
1095 To use an '#include' even if the header name is "y.tab.h" (which is what
1096 happens with --yacc, or when using the Autotools' ylwrap), define
1097 api.header.include to the exact argument to pass to #include. For
1100 %define api.header.include {"parse.h"}
1104 %define api.header.include {<parser/parse.h>}
1106 *** api.location.type is now supported in C (yacc.c, glr.c)
1108 The %define variable api.location.type defines the name of the type to use
1109 for locations. When defined, Bison no longer defines YYLTYPE.
1111 This can be used in programs with several parsers to factor their
1112 definition of locations: let one of them generate them, and the others
1119 In conformance with the recommendations of the Graphviz team, if %require
1120 "3.4" (or better) is specified, the option --graph generates a *.gv file
1121 by default, instead of *.dot.
1123 *** Diagnostics overhaul
1125 Column numbers were wrong with multibyte characters, which would also
1126 result in skewed diagnostics with carets. Beside, because we were
1127 indenting the quoted source with a single space, lines with tab characters
1128 were incorrectly underlined.
1130 To address these issues, and to be clearer, Bison now issues diagnostics
1131 as GCC9 does. For instance it used to display (there's a tab before the
1134 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1135 expr: expr '+' "number" { $$ = $1 + $2; }
1139 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1140 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1143 Other constructs now also have better locations, resulting in more precise
1146 *** Fix-it hints for %empty
1148 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1149 annotations, and add the missing ones.
1151 *** Generated reports
1153 The format of the reports (parse.output) was improved for readability.
1155 *** Better support for --no-line.
1157 When --no-line is used, the generated files are now cleaner: no lines are
1158 generated instead of empty lines. Together with using api.header.include,
1159 that should help people saving the generated files into version control
1160 systems get smaller diffs.
1164 A new example in C shows an simple infix calculator with a hand-written
1165 scanner (examples/c/calc).
1167 A new example in C shows a reentrant parser (capable of recursive calls)
1168 built with Flex and Bison (examples/c/reccalc).
1170 There is a new section about the history of Yaccs and Bison.
1174 A few obscure bugs were fixed, including the second oldest (known) bug in
1175 Bison: it was there when Bison was entered in the RCS version control
1176 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1180 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1184 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1188 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1192 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1193 extensions into errors. It now makes them simple warnings (-Wyacc).
1196 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1198 A new mailing list was created, Bison Announce. It is low traffic, and is
1199 only about announcing new releases and important messages (e.g., polls
1200 about major decisions to make).
1202 https://lists.gnu.org/mailman/listinfo/bison-announce
1204 ** Backward incompatible changes
1206 Support for DJGPP, which has been unmaintained and untested for years, is
1209 ** Deprecated features
1211 A new feature, --update (see below) helps adjusting existing grammars to
1214 *** Deprecated directives
1216 The %error-verbose directive is deprecated in favor of '%define
1217 parse.error verbose' since Bison 3.0, but no warning was issued.
1219 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1220 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1221 directives are slightly different, you might need to adjust your code.
1222 %name-prefix renames only symbols with external linkage, while api.prefix
1223 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1224 yytokentype, YYSTYPE, YYLTYPE, etc.
1226 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1227 {xx}' will typically have to update YY_DECL from
1229 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1233 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1235 *** Deprecated %define variable names
1237 The following variables, mostly related to parsers in Java, have been
1238 renamed for consistency. Backward compatibility is ensured, but upgrading
1241 abstract -> api.parser.abstract
1242 annotations -> api.parser.annotations
1243 extends -> api.parser.extends
1244 final -> api.parser.final
1245 implements -> api.parser.implements
1246 parser_class_name -> api.parser.class
1247 public -> api.parser.public
1248 strictfp -> api.parser.strictfp
1252 *** Generation of fix-its for IDEs/Editors
1254 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1255 bison now generates machine readable editing instructions to fix some
1256 issues. Currently, this is mostly limited to updating deprecated
1257 directives and removing duplicates. For instance:
1261 %define parser_class_name "Parser"
1262 %define api.parser.class "Parser"
1266 See the "fix-it:" lines below:
1268 $ bison -ffixit foo.y
1269 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1272 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1273 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1274 %define parser_class_name "Parser"
1275 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1276 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1277 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1278 %define api.parser.class "Parser"
1279 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1280 foo.y:2.1-34: previous definition
1281 %define parser_class_name "Parser"
1282 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1283 fix-it:"foo.y":{3:1-3:34}:""
1284 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1286 This uses the same output format as GCC and Clang.
1288 *** Updating grammar files
1290 Fixes can be applied on the fly. The previous example ends with the
1291 suggestion to re-run bison with the option -u/--update, which results in a
1292 cleaner grammar file.
1294 $ bison --update foo.y
1296 bison: file 'foo.y' was updated (backup: 'foo.y~')
1299 %define parse.error verbose
1300 %define api.parser.class {Parser}
1304 *** Bison is now relocatable
1306 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1308 A relocatable program can be moved or copied to a different location on
1309 the file system. It can also be used through mount points for network
1310 sharing. It is possible to make symbolic links to the installed and moved
1311 programs, and invoke them through the symbolic link.
1313 *** %expect and %expect-rr modifiers on individual rules
1315 One can now document (and check) which rules participate in shift/reduce
1316 and reduce/reduce conflicts. This is particularly important GLR parsers,
1317 where conflicts are a normal occurrence. For example,
1333 | argument_list ',' expression
1338 Looking at the output from -v, one can see that the shift/reduce conflict
1339 here is due to the fact that the parser does not know whether to reduce
1340 arguments to argument_list until it sees the token _after_ the following
1341 ','. By marking the rule with %expect 1 (because there is a conflict in
1342 one state), we document the source of the 1 overall shift/reduce conflict.
1344 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1345 conflicts. In this case, we mark each of the conflicting rules. For
1354 target_list '=' expr ';'
1360 | target ',' target_list
1369 | expr ',' expr_list
1377 In a statement such as
1381 the parser must reduce x to a target or an expr, but does not know which
1382 until it sees the '='. So we notate the two possible reductions to
1383 indicate that each conflicts in one rule.
1385 This feature needs user feedback, and might evolve in the future.
1387 *** C++: Actual token constructors
1389 When variants and token constructors are enabled, in addition to the
1390 type-safe named token constructors (make_ID, make_INT, etc.), we now
1391 generate genuine constructors for symbol_type.
1393 For instance with these declarations
1399 you may use these constructors:
1401 symbol_type (int token, const std::string&);
1402 symbol_type (int token, const int&);
1403 symbol_type (int token);
1405 Correct matching between token types and value types is checked via
1406 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1407 constructors are preferable, as they offer better type safety (for
1408 instance 'make_ID (42)' would not even compile), but symbol_type
1409 constructors may help when token types are discovered at run-time, e.g.,
1412 if (auto i = lookup_keyword (yytext))
1413 return yy::parser::symbol_type (i);
1415 return yy::parser::make_ID (yytext);
1418 *** C++: Variadic emplace
1420 If your application requires C++11 and you don't use symbol constructors,
1421 you may now use a variadic emplace for semantic values:
1423 %define api.value.type variant
1424 %token <std::pair<int, int>> PAIR
1428 int yylex (parser::semantic_type *lvalp)
1430 lvalp->emplace <std::pair<int, int>> (1, 2);
1431 return parser::token::PAIR;
1434 *** C++: Syntax error exceptions in GLR
1436 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1437 actions, or from the scanner.
1439 *** More POSIX Yacc compatibility warnings
1441 More Bison specific directives are now reported with -y or -Wyacc. This
1442 change was ready since the release of Bison 3.0 in September 2015. It was
1443 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1444 in numerous warnings for Bison users that use the GNU Build System.
1446 If you still experience that problem, either redefine YACC as `bison -o
1447 y.tab.c`, or pass -Wno-yacc to Bison.
1449 *** The tables yyrhs and yyphrs are back
1451 Because no Bison skeleton uses them, these tables were removed (no longer
1452 passed to the skeletons, not even computed) in 2008. However, some users
1453 have expressed interest in being able to use them in their own skeletons.
1457 *** Incorrect number of reduce/reduce conflicts
1459 On a grammar such as
1461 exp: "num" | "num" | "num"
1463 bison used to report a single RR conflict, instead of two. This is now
1464 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1465 was entered in the RCS version control system, in December 1987.
1467 Some grammar files might have to adjust their %expect-rr.
1469 *** Parser directives that were not careful enough
1471 Passing invalid arguments to %nterm, for instance character literals, used
1472 to result in unclear error messages.
1476 The examples/ directory (installed in .../share/doc/bison/examples) has
1477 been restructured per language for clarity. The examples come with a
1478 README and a Makefile. Not only can they be used to toy with Bison, they
1479 can also be starting points for your own grammars.
1481 There is now a Java example, and a simple example in C based on Flex and
1482 Bison (examples/c/lexcalc/).
1488 They now use noexcept and constexpr. Please, report missing annotations.
1490 *** Symbol Declarations
1492 The syntax of the variation directives to declare symbols was overhauled
1493 for more consistency, and also better POSIX Yacc compliance (which, for
1494 instance, allows "%type" without actually providing a type). The %nterm
1495 directive, supported by Bison since its inception, is now documented and
1496 officially supported.
1498 The syntax is now as follows:
1500 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1501 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1502 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1503 %nterm TAG? ID+ ( TAG ID+ )*
1505 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1506 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1507 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1508 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1509 one), ‘*’ (zero or more) and ‘+’ (one or more).
1512 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1516 Fix the move constructor of symbol_type.
1518 Always provide a copy constructor for symbol_type, even in modern C++.
1521 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1525 Properly support token constructors in C++ with types that include commas
1526 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1529 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1533 C++ portability issues.
1536 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1540 Several portability issues have been fixed in the build system, in the
1541 test suite, and in the generated parsers in C++.
1544 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1546 ** Backward incompatible changes
1548 Support for DJGPP, which has been unmaintained and untested for years, is
1549 obsolete. Unless there is activity to revive it, it will be removed.
1553 %printers should use yyo rather than yyoutput to denote the output stream.
1555 Variant-based symbols in C++ should use emplace() rather than build().
1557 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1561 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1563 A comment in the generated code now emphasizes that users should not
1564 depend upon non-documented implementation details, such as macros starting
1569 *** C++: Support for move semantics (lalr1.cc)
1571 The lalr1.cc skeleton now fully supports C++ move semantics, while
1572 maintaining compatibility with C++98. You may now store move-only types
1573 when using Bison's variants. For instance:
1580 %skeleton "lalr1.cc"
1581 %define api.value.type variant
1585 %token <int> INT "int";
1586 %type <std::unique_ptr<int>> int;
1587 %type <std::vector<std::unique_ptr<int>>> list;
1591 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1593 int: "int" { $$ = std::make_unique<int>($1); }
1595 *** C++: Implicit move of right-hand side values (lalr1.cc)
1597 In modern C++ (C++11 and later), you should always use 'std::move' with
1598 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1599 popped from the stack anyway. Using 'std::move' is mandatory for
1600 move-only types such as unique_ptr, and it provides a significant speedup
1601 for large types such as std::string, or std::vector, etc.
1603 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1604 by 'std::move ($n)'. The second rule in the previous grammar can be
1607 list: list int { $$ = $1; $$.emplace_back($2); }
1609 With automove enabled, the semantic values are no longer lvalues, so do
1610 not use the swap idiom:
1612 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1614 This idiom is anyway obsolete: it is preferable to move than to swap.
1616 A warning is issued when automove is enabled, and a value is used several
1619 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1620 exp: "twice" exp { $$ = $2 + $2; }
1623 Enabling api.value.automove does not require support for modern C++. The
1624 generated code is valid C++98/03, but will use copies instead of moves.
1626 The new examples/c++/variant-11.yy shows these features in action.
1628 *** C++: The implicit default semantic action is always run
1630 When variants are enabled, the default action was not run, so
1638 It now behaves like in all the other cases, as
1640 exp: "number" { $$ = $1; }
1642 possibly using std::move if automove is enabled.
1644 We do not expect backward compatibility issues. However, beware of
1645 forward compatibility issues: if you rely on default actions with
1646 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1647 generate incorrect parsers.
1649 *** C++: Renaming location.hh
1651 When both %defines and %locations are enabled, Bison generates a
1652 location.hh file. If you don't use locations outside of the parser, you
1653 may avoid its creation with:
1655 %define api.location.file none
1657 However this file is useful if, for instance, your parser builds an AST
1658 decorated with locations: you may use Bison's location independently of
1659 Bison's parser. You can now give it another name, for instance:
1661 %define api.location.file "my-location.hh"
1663 This name can have directory components, and even be absolute. The name
1664 under which the location file is included is controlled by
1665 api.location.include.
1667 This way it is possible to have several parsers share the same location
1670 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1673 %define api.namespace {foo}
1674 %define api.location.file "include/ast/loc.hh"
1675 %define api.location.include {<ast/loc.hh>}
1677 and use it in src/bar/parser.hh:
1680 %define api.namespace {bar}
1681 %code requires {#include <ast/loc.hh>}
1682 %define api.location.type {bar::location}
1684 Absolute file names are supported, so in your Makefile, passing the flag
1685 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1688 *** C++: stack.hh and position.hh are deprecated
1690 When asked to generate a header file (%defines), the lalr1.cc skeleton
1691 generates a stack.hh file. This file had no interest for users; it is now
1692 made useless: its content is included in the parser definition. It is
1693 still generated for backward compatibility.
1695 When in addition to %defines, location support is requested (%locations),
1696 the file position.hh is also generated. It is now also useless: its
1697 content is now included in location.hh.
1699 These files are no longer generated when your grammar file requires at
1700 least Bison 3.2 (%require "3.2").
1704 Portability issues on MinGW and VS2015.
1706 Portability issues in the test suite.
1708 Portability/warning issues with Flex.
1711 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1713 ** Backward incompatible changes
1715 Compiling Bison now requires a C99 compiler---as announced during the
1716 release of Bison 3.0, five years ago. Generated parsers do not require a
1719 Support for DJGPP, which has been unmaintained and untested for years, is
1720 obsolete. Unless there is activity to revive it, the next release of Bison
1721 will have it removed.
1725 *** Typed midrule actions
1727 Because their type is unknown to Bison, the values of midrule actions are
1728 not treated like the others: they don't have %printer and %destructor
1729 support. It also prevents C++ (Bison) variants to handle them properly.
1731 Typed midrule actions address these issues. Instead of:
1733 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1737 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1739 *** Reports include the type of the symbols
1741 The sections about terminal and nonterminal symbols of the '*.output' file
1742 now specify their declared type. For instance, for:
1746 the report now shows '<ival>':
1748 Terminals, with rules where they appear
1752 *** Diagnostics about useless rules
1754 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1755 of course, its rules are useless too.
1759 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1761 Previously all the useless rules were reported, including those whose
1762 left-hand side is the 'exp' nonterminal:
1764 warning: 1 nonterminal useless in grammar [-Wother]
1765 warning: 4 rules useless in grammar [-Wother]
1766 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1769 2.14-16: warning: rule useless in grammar [-Wother]
1772 3.6-16: warning: rule useless in grammar [-Wother]
1773 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1775 3.20-30: warning: rule useless in grammar [-Wother]
1776 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1778 3.34-44: warning: rule useless in grammar [-Wother]
1779 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1782 Now, rules whose left-hand side symbol is useless are no longer reported
1783 as useless. The locations of the errors have also been adjusted to point
1784 to the first use of the nonterminal as a left-hand side of a rule:
1786 warning: 1 nonterminal useless in grammar [-Wother]
1787 warning: 4 rules useless in grammar [-Wother]
1788 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1789 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1791 2.14-16: warning: rule useless in grammar [-Wother]
1795 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1797 When compiled with exceptions disabled, the generated parsers no longer
1798 uses try/catch clauses.
1800 Currently only GCC and Clang are supported.
1804 *** A demonstration of variants
1806 A new example was added (installed in .../share/doc/bison/examples),
1807 'variant.yy', which shows how to use (Bison) variants in C++.
1809 The other examples were made nicer to read.
1811 *** Some features are no longer 'experimental'
1813 The following features, mature enough, are no longer flagged as
1814 experimental in the documentation: push parsers, default %printer and
1815 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1816 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1817 semantic predicates (%?).
1821 *** GLR: Predicates support broken by #line directives
1823 Predicates (%?) in GLR such as
1826 %? {new_syntax} 'w' id new_args
1827 | %?{!new_syntax} 'w' id old_args
1829 were issued with #lines in the middle of C code.
1831 *** Printer and destructor with broken #line directives
1833 The #line directives were not properly escaped when emitting the code for
1834 %printer/%destructor, which resulted in compiler errors if there are
1835 backslashes or double-quotes in the grammar file name.
1837 *** Portability on ICC
1839 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1840 Generated parsers now work around this.
1844 There were several small fixes in the test suite and in the build system,
1845 many warnings in bison and in the generated parsers were eliminated. The
1846 documentation also received its share of minor improvements.
1848 Useless code was removed from C++ parsers, and some of the generated
1849 constructors are more 'natural'.
1852 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1856 *** C++: Fix support of 'syntax_error'
1858 One incorrect 'inline' resulted in linking errors about the constructor of
1859 the syntax_error exception.
1861 *** C++: Fix warnings
1863 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1864 warnings about yyformat being possibly null. It also warned about the
1865 deprecated implicit definition of copy constructors when there's a
1866 user-defined (copy) assignment operator.
1868 *** Location of errors
1870 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1871 ride-hand side raises a syntax error. The behavior of the default parser
1872 (yacc.c) in such a condition was undefined.
1874 Now all the parsers match the behavior of glr.c: @$ is used as the
1875 location of the error. This handles gracefully rules with and without
1878 *** Portability fixes in the test suite
1880 On some platforms, some Java and/or C++ tests were failing.
1883 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1887 *** C++ with Variants (lalr1.cc)
1889 Fix a compiler warning when no %destructor use $$.
1893 Several portability issues in tests were fixed.
1896 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1900 *** C++ with Variants (lalr1.cc)
1902 Problems with %destructor and '%define parse.assert' have been fixed.
1904 *** Named %union support (yacc.c, glr.c)
1906 Bison 3.0 introduced a regression on named %union such as
1908 %union foo { int ival; };
1910 The possibility to use a name was introduced "for Yacc compatibility".
1911 It is however not required by POSIX Yacc, and its usefulness is not clear.
1913 *** %define api.value.type union with %defines (yacc.c, glr.c)
1915 The C parsers were broken when %defines was used together with "%define
1916 api.value.type union".
1918 *** Redeclarations are reported in proper order
1926 bison used to report:
1928 foo.yy:2.10-11: error: %printer redeclaration for FOO
1931 foo.yy:3.10-11: previous declaration
1935 Now, the "previous" declaration is always the first one.
1940 Bison now installs various files in its docdir (which defaults to
1941 '/usr/local/share/doc/bison'), including the three fully blown examples
1942 extracted from the documentation:
1945 Reverse Polish Calculator, a simple introductory example.
1947 Multi-function Calc, a calculator with memory and functions and located
1950 a calculator in C++ using variant support and token constructors.
1953 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1957 *** Generated source files when errors are reported
1959 When warnings are issued and -Werror is set, bison would still generate
1960 the source files (*.c, *.h...). As a consequence, some runs of "make"
1961 could fail the first time, but not the second (as the files were generated
1964 This is fixed: bison no longer generates this source files, but, of
1965 course, still produces the various reports (*.output, *.xml, etc.).
1967 *** %empty is used in reports
1969 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1970 dot, XML and formats derived from it).
1972 *** YYERROR and variants
1974 When C++ variant support is enabled, an error triggered via YYERROR, but
1975 not caught via error recovery, resulted in a double deletion.
1978 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1982 *** Errors in caret diagnostics
1984 On some platforms, some errors could result in endless diagnostics.
1986 *** Fixes of the -Werror option
1988 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1989 diagnostics into errors instead of warnings. This is fixed.
1991 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1992 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1993 -Wno-error", "foo" diagnostics are now errors.
1997 As demonstrated in the documentation, one can now leave spaces between
2002 The yacc.1 man page is no longer installed if --disable-yacc was
2005 *** Fixes in the test suite
2007 Bugs and portability issues.
2010 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
2012 ** WARNING: Future backward-incompatibilities!
2014 Like other GNU packages, Bison will start using some of the C99 features
2015 for its own code, especially the definition of variables after statements.
2016 The generated C parsers still aim at C90.
2018 ** Backward incompatible changes
2020 *** Obsolete features
2022 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
2024 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
2025 use YYSTYPE and YYLTYPE.
2027 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
2028 1.875): use %lex-param, %parse-param, or %param.
2030 Missing semicolons at the end of actions are no longer added (as announced
2031 in the release 2.5).
2033 *** Use of YACC='bison -y'
2035 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
2038 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
2039 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
2040 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
2042 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
2043 implementation of Yacc, was using Bison as 'bison -y'. While it does
2044 ensure compatible output file names, it also enables warnings for
2045 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
2046 warnings for Bison extensions.
2048 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
2049 (which also generates 'y.tab.h' and 'y.output' when needed).
2050 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
2051 flags (YFLAGS, or AM_YFLAGS with Automake).
2055 *** The epilogue is no longer affected by internal #defines (glr.c)
2057 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
2058 generated code. These weren't properly undefined before the inclusion of
2059 the user epilogue, so functions such as the following were butchered by the
2060 preprocessor expansion:
2062 int yylex (YYSTYPE *yylval);
2064 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2065 identifiers for user-provided variables.
2067 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2069 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2070 locations are enabled. This is fixed.
2072 *** Warnings about useless %pure-parser/%define api.pure are restored
2074 ** Diagnostics reported by Bison
2076 Most of these features were contributed by Théophile Ranquet and Victor
2081 Version 2.7 introduced caret errors, for a prettier output. These are now
2082 activated by default. The old format can still be used by invoking Bison
2083 with -fno-caret (or -fnone).
2085 Some error messages that reproduced excerpts of the grammar are now using
2086 the caret information only. For instance on:
2093 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2094 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2098 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2099 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2103 and "bison -fno-caret" reports:
2105 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2106 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2108 *** Enhancements of the -Werror option
2110 The -Werror=CATEGORY option is now recognized, and will treat specified
2111 warnings as errors. The warnings need not have been explicitly activated
2112 using the -W option, this is similar to what GCC 4.7 does.
2114 For example, given the following command line, Bison will treat both
2115 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2116 errors (and only those):
2118 $ bison -Werror=yacc,error=conflicts-sr input.y
2120 If no categories are specified, -Werror will make all active warnings into
2121 errors. For example, the following line does the same the previous example:
2123 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2125 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2127 Note that the categories in this -Werror option may not be prefixed with
2128 "no-". However, -Wno-error[=CATEGORY] is valid.
2130 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2131 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2132 incompatibilities as warnings: "-y -Wno-error=yacc".
2134 *** The display of warnings is now richer
2136 The option that controls a given warning is now displayed:
2138 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2140 In the case of warnings treated as errors, the prefix is changed from
2141 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2142 to GCC, as [-Werror=CATEGORY].
2144 For instance, where the previous version of Bison would report (and exit
2147 bison: warnings being treated as errors
2148 input.y:1.1: warning: stray ',' treated as white space
2152 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2154 *** Deprecated constructs
2156 The new 'deprecated' warning category flags obsolete constructs whose
2157 support will be discontinued. It is enabled by default. These warnings
2158 used to be reported as 'other' warnings.
2160 *** Useless semantic types
2162 Bison now warns about useless (uninhabited) semantic types. Since
2163 semantic types are not declared to Bison (they are defined in the opaque
2164 %union structure), it is %printer/%destructor directives about useless
2165 types that trigger the warning:
2169 %printer {} <type1> <type3>
2170 %destructor {} <type2> <type4>
2172 nterm: term { $$ = $1; };
2174 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2175 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2177 *** Undefined but unused symbols
2179 Bison used to raise an error for undefined symbols that are not used in
2180 the grammar. This is now only a warning.
2183 %destructor {} symbol2
2184 %type <type> symbol3
2188 *** Useless destructors or printers
2190 Bison now warns about useless destructors or printers. In the following
2191 example, the printer for <type1>, and the destructor for <type2> are
2192 useless: all symbols of <type1> (token1) already have a printer, and all
2193 symbols of type <type2> (token2) already have a destructor.
2195 %token <type1> token1
2199 %printer {} token1 <type1> <type3>
2200 %destructor {} token2 <type2> <type4>
2204 The warnings and error messages about shift/reduce and reduce/reduce
2205 conflicts have been normalized. For instance on the following foo.y file:
2209 exp: exp '+' exp | '0' | '0';
2211 compare the previous version of bison:
2214 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2215 $ bison -Werror foo.y
2216 bison: warnings being treated as errors
2217 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2219 with the new behavior:
2222 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2223 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2224 $ bison -Werror foo.y
2225 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2226 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2228 When %expect or %expect-rr is used, such as with bar.y:
2233 exp: exp '+' exp | '0' | '0';
2238 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2239 bar.y: expected 0 shift/reduce conflicts
2240 bar.y: expected 0 reduce/reduce conflicts
2245 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2246 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2248 ** Incompatibilities with POSIX Yacc
2250 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2253 ** Additional yylex/yyparse arguments
2255 The new directive %param declares additional arguments to both yylex and
2256 yyparse. The %lex-param, %parse-param, and %param directives support one
2257 or more arguments. Instead of
2259 %lex-param {arg1_type *arg1}
2260 %lex-param {arg2_type *arg2}
2261 %parse-param {arg1_type *arg1}
2262 %parse-param {arg2_type *arg2}
2266 %param {arg1_type *arg1} {arg2_type *arg2}
2268 ** Types of values for %define variables
2270 Bison used to make no difference between '%define foo bar' and '%define
2271 foo "bar"'. The former is now called a 'keyword value', and the latter a
2272 'string value'. A third kind was added: 'code values', such as '%define
2275 Keyword variables are used for fixed value sets, e.g.,
2277 %define lr.type lalr
2279 Code variables are used for value in the target language, e.g.,
2281 %define api.value.type {struct semantic_type}
2283 String variables are used remaining cases, e.g. file names.
2285 ** Variable api.token.prefix
2287 The variable api.token.prefix changes the way tokens are identified in
2288 the generated files. This is especially useful to avoid collisions
2289 with identifiers in the target language. For instance
2291 %token FILE for ERROR
2292 %define api.token.prefix {TOK_}
2294 start: FILE for ERROR;
2296 will generate the definition of the symbols TOK_FILE, TOK_for, and
2297 TOK_ERROR in the generated sources. In particular, the scanner must
2298 use these prefixed token names, although the grammar itself still
2299 uses the short names (as in the sample rule given above).
2301 ** Variable api.value.type
2303 This new %define variable supersedes the #define macro YYSTYPE. The use
2304 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2305 using %union or %defining api.value.type results in undefined behavior.
2307 Either define api.value.type, or use "%union":
2314 %token <ival> INT "integer"
2315 %token <sval> STRING "string"
2316 %printer { fprintf (yyo, "%d", $$); } <ival>
2317 %destructor { free ($$); } <sval>
2320 yylval.ival = 42; return INT;
2321 yylval.sval = "42"; return STRING;
2323 The %define variable api.value.type supports both keyword and code values.
2325 The keyword value 'union' means that the user provides genuine types, not
2326 union member names such as "ival" and "sval" above (WARNING: will fail if
2327 -y/--yacc/%yacc is enabled).
2329 %define api.value.type union
2330 %token <int> INT "integer"
2331 %token <char *> STRING "string"
2332 %printer { fprintf (yyo, "%d", $$); } <int>
2333 %destructor { free ($$); } <char *>
2336 yylval.INT = 42; return INT;
2337 yylval.STRING = "42"; return STRING;
2339 The keyword value variant is somewhat equivalent, but for C++ special
2340 provision is made to allow classes to be used (more about this below).
2342 %define api.value.type variant
2343 %token <int> INT "integer"
2344 %token <std::string> STRING "string"
2346 Code values (in braces) denote user defined types. This is where YYSTYPE
2364 %define api.value.type {struct my_value}
2365 %token <u.ival> INT "integer"
2366 %token <u.sval> STRING "string"
2367 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2368 %destructor { free ($$); } <u.sval>
2371 yylval.u.ival = 42; return INT;
2372 yylval.u.sval = "42"; return STRING;
2374 ** Variable parse.error
2376 This variable controls the verbosity of error messages. The use of the
2377 %error-verbose directive is deprecated in favor of "%define parse.error
2380 ** Deprecated %define variable names
2382 The following variables have been renamed for consistency. Backward
2383 compatibility is ensured, but upgrading is recommended.
2385 lr.default-reductions -> lr.default-reduction
2386 lr.keep-unreachable-states -> lr.keep-unreachable-state
2387 namespace -> api.namespace
2388 stype -> api.value.type
2390 ** Semantic predicates
2392 Contributed by Paul Hilfinger.
2394 The new, experimental, semantic-predicate feature allows actions of the
2395 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2396 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2397 in GLR parsers, rather than being deferred. The result is that they allow
2398 the programmer to prune possible parses based on the values of run-time
2401 ** The directive %expect-rr is now an error in non GLR mode
2403 It used to be an error only if used in non GLR mode, _and_ if there are
2404 reduce/reduce conflicts.
2406 ** Tokens are numbered in their order of appearance
2408 Contributed by Valentin Tolmer.
2410 With '%token A B', A had a number less than the one of B. However,
2411 precedence declarations used to generate a reversed order. This is now
2412 fixed, and introducing tokens with any of %token, %left, %right,
2413 %precedence, or %nonassoc yields the same result.
2415 When mixing declarations of tokens with a literal character (e.g., 'a') or
2416 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2417 the literal characters first. For example
2421 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2422 input order is now preserved.
2424 These changes were made so that one can remove useless precedence and
2425 associativity declarations (i.e., map %nonassoc, %left or %right to
2426 %precedence, or to %token) and get exactly the same output.
2428 ** Useless precedence and associativity
2430 Contributed by Valentin Tolmer.
2432 When developing and maintaining a grammar, useless associativity and
2433 precedence directives are common. They can be a nuisance: new ambiguities
2434 arising are sometimes masked because their conflicts are resolved due to
2435 the extra precedence or associativity information. Furthermore, it can
2436 hinder the comprehension of a new grammar: one will wonder about the role
2437 of a precedence, where in fact it is useless. The following changes aim
2438 at detecting and reporting these extra directives.
2440 *** Precedence warning category
2442 A new category of warning, -Wprecedence, was introduced. It flags the
2443 useless precedence and associativity directives.
2445 *** Useless associativity
2447 Bison now warns about symbols with a declared associativity that is never
2448 used to resolve conflicts. In that case, using %precedence is sufficient;
2449 the parsing tables will remain unchanged. Solving these warnings may raise
2450 useless precedence warnings, as the symbols no longer have associativity.
2464 warning: useless associativity for '+', use %precedence [-Wprecedence]
2468 *** Useless precedence
2470 Bison now warns about symbols with a declared precedence and no declared
2471 associativity (i.e., declared with %precedence), and whose precedence is
2472 never used. In that case, the symbol can be safely declared with %token
2473 instead, without modifying the parsing tables. For example:
2477 exp: "var" '=' "number";
2481 warning: useless precedence for '=' [-Wprecedence]
2485 *** Useless precedence and associativity
2487 In case of both useless precedence and associativity, the issue is flagged
2492 exp: "var" '=' "number";
2496 warning: useless precedence and associativity for '=' [-Wprecedence]
2502 With help from Joel E. Denny and Gabriel Rassoul.
2504 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2505 marked by the new %empty directive. Using %empty on a non-empty rule is
2506 an error. The new -Wempty-rule warning reports empty rules without
2507 %empty. On the following grammar:
2517 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2520 5.8-13: error: %empty on non-empty rule
2524 ** Java skeleton improvements
2526 The constants for token names were moved to the Lexer interface. Also, it
2527 is possible to add code to the parser's constructors using "%code init"
2528 and "%define init_throws".
2529 Contributed by Paolo Bonzini.
2531 The Java skeleton now supports push parsing.
2532 Contributed by Dennis Heimbigner.
2534 ** C++ skeletons improvements
2536 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2538 Using %defines is now optional. Without it, the needed support classes
2539 are defined in the generated parser, instead of additional files (such as
2540 location.hh, position.hh and stack.hh).
2542 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2544 Both lalr1.cc and glr.cc no longer require %location.
2546 *** syntax_error exception (lalr1.cc)
2548 The C++ parser features a syntax_error exception, which can be
2549 thrown from the scanner or from user rules to raise syntax errors.
2550 This facilitates reporting errors caught in sub-functions (e.g.,
2551 rejecting too large integral literals from a conversion function
2552 used by the scanner, or rejecting invalid combinations from a
2553 factory invoked by the user actions).
2555 *** %define api.value.type variant
2557 This is based on a submission from Michiel De Wilde. With help
2558 from Théophile Ranquet.
2560 In this mode, complex C++ objects can be used as semantic values. For
2563 %token <::std::string> TEXT;
2564 %token <int> NUMBER;
2565 %token SEMICOLON ";"
2566 %type <::std::string> item;
2567 %type <::std::list<std::string>> list;
2570 list { std::cout << $1 << std::endl; }
2574 %empty { /* Generates an empty string list. */ }
2575 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2579 TEXT { std::swap ($$, $1); }
2580 | NUMBER { $$ = string_cast ($1); }
2583 *** %define api.token.constructor
2585 When variants are enabled, Bison can generate functions to build the
2586 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2587 with the semantic value (e.g., int):
2589 parser::symbol_type yylex ()
2591 parser::location_type loc = ...;
2593 return parser::make_TEXT ("Hello, world!", loc);
2595 return parser::make_NUMBER (42, loc);
2597 return parser::make_SEMICOLON (loc);
2603 There are operator- and operator-= for 'location'. Negative line/column
2604 increments can no longer underflow the resulting value.
2607 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2611 *** Fix compiler attribute portability (yacc.c)
2613 With locations enabled, __attribute__ was used unprotected.
2615 *** Fix some compiler warnings (lalr1.cc)
2618 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2622 Warnings about uninitialized yylloc in yyparse have been fixed.
2624 Restored C90 compliance (yet no report was ever made).
2626 ** Diagnostics are improved
2628 Contributed by Théophile Ranquet.
2630 *** Changes in the format of error messages
2632 This used to be the format of many error reports:
2634 input.y:2.7-12: %type redeclaration for exp
2635 input.y:1.7-12: previous declaration
2639 input.y:2.7-12: error: %type redeclaration for exp
2640 input.y:1.7-12: previous declaration
2642 *** New format for error reports: carets
2644 Caret errors have been added to Bison:
2646 input.y:2.7-12: error: %type redeclaration for exp
2649 input.y:1.7-12: previous declaration
2655 input.y:3.20-23: error: ambiguous reference: '$exp'
2656 exp: exp '+' exp { $exp = $1 + $3; };
2658 input.y:3.1-3: refers to: $exp at $$
2659 exp: exp '+' exp { $exp = $1 + $3; };
2661 input.y:3.6-8: refers to: $exp at $1
2662 exp: exp '+' exp { $exp = $1 + $3; };
2664 input.y:3.14-16: refers to: $exp at $3
2665 exp: exp '+' exp { $exp = $1 + $3; };
2668 The default behavior for now is still not to display these unless
2669 explicitly asked with -fcaret (or -fall). However, in a later release, it
2670 will be made the default behavior (but may still be deactivated with
2673 ** New value for %define variable: api.pure full
2675 The %define variable api.pure requests a pure (reentrant) parser. However,
2676 for historical reasons, using it in a location-tracking Yacc parser
2677 resulted in a yyerror function that did not take a location as a
2678 parameter. With this new value, the user may request a better pure parser,
2679 where yyerror does take a location as a parameter (in location-tracking
2682 The use of "%define api.pure true" is deprecated in favor of this new
2683 "%define api.pure full".
2685 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2687 The %define variable api.location.type defines the name of the type to use
2688 for locations. When defined, Bison no longer generates the position.hh
2689 and location.hh files, nor does the parser will include them: the user is
2690 then responsible to define her type.
2692 This can be used in programs with several parsers to factor their location
2693 and position files: let one of them generate them, and the others just use
2696 This feature was actually introduced, but not documented, in Bison 2.5,
2697 under the name "location_type" (which is maintained for backward
2700 For consistency, lalr1.java's %define variables location_type and
2701 position_type are deprecated in favor of api.location.type and
2704 ** Exception safety (lalr1.cc)
2706 The parse function now catches exceptions, uses the %destructors to
2707 release memory (the lookahead symbol and the symbols pushed on the stack)
2708 before re-throwing the exception.
2710 This feature is somewhat experimental. User feedback would be
2713 ** Graph improvements in DOT and XSLT
2715 Contributed by Théophile Ranquet.
2717 The graphical presentation of the states is more readable: their shape is
2718 now rectangular, the state number is clearly displayed, and the items are
2719 numbered and left-justified.
2721 The reductions are now explicitly represented as transitions to other
2722 diamond shaped nodes.
2724 These changes are present in both --graph output and xml2dot.xsl XSLT
2725 processing, with minor (documented) differences.
2727 ** %language is no longer an experimental feature.
2729 The introduction of this feature, in 2.4, was four years ago. The
2730 --language option and the %language directive are no longer experimental.
2734 The sections about shift/reduce and reduce/reduce conflicts resolution
2735 have been fixed and extended.
2737 Although introduced more than four years ago, XML and Graphviz reports
2738 were not properly documented.
2740 The translation of midrule actions is now described.
2743 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2745 We consider compiler warnings about Bison generated parsers to be bugs.
2746 Rather than working around them in your own project, please consider
2747 reporting them to us.
2751 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2752 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2755 Other issues in the test suite have been addressed.
2757 Null characters are correctly displayed in error messages.
2759 When possible, yylloc is correctly initialized before calling yylex. It
2760 is no longer necessary to initialize it in the %initial-action.
2763 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2765 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2768 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2772 Bugs and portability issues in the test suite have been fixed.
2774 Some errors in translations have been addressed, and --help now directs
2775 users to the appropriate place to report them.
2777 Stray Info files shipped by accident are removed.
2779 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2780 generated, are removed.
2782 All the generated headers are self-contained.
2784 ** Header guards (yacc.c, glr.c, glr.cc)
2786 In order to avoid collisions, the header guards are now
2787 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2788 For instance the header generated from
2790 %define api.prefix "calc"
2791 %defines "lib/parse.h"
2793 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2795 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2797 The compilation of pure parsers (%define api.pure) can trigger GCC
2800 input.c: In function 'yyparse':
2801 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2802 function [-Wmaybe-uninitialized]
2806 This is now fixed; pragmas to avoid these warnings are no longer needed.
2808 Warnings from clang ("equality comparison with extraneous parentheses" and
2809 "function declared 'noreturn' should not return") have also been
2813 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2817 Buffer overruns, complaints from Flex, and portability issues in the test
2818 suite have been fixed.
2820 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2822 Trailing end-of-lines in %parse-param or %lex-param would result in
2823 invalid C++. This is fixed.
2825 ** Spurious spaces and end-of-lines
2827 The generated files no longer end (nor start) with empty lines.
2830 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2832 Bison no longer executes user-specified M4 code when processing a grammar.
2836 In addition to the removal of the features announced in Bison 2.6, the
2837 next major release will remove the "Temporary hack for adding a semicolon
2838 to the user action", as announced in the release 2.5. Instead of:
2840 exp: exp "+" exp { $$ = $1 + $3 };
2844 exp: exp "+" exp { $$ = $1 + $3; };
2848 *** Type names are now properly escaped.
2850 *** glr.cc: set_debug_level and debug_level work as expected.
2852 *** Stray @ or $ in actions
2854 While Bison used to warn about stray $ or @ in action rules, it did not
2855 for other actions such as printers, destructors, or initial actions. It
2858 ** Type names in actions
2860 For consistency with rule actions, it is now possible to qualify $$ by a
2861 type-name in destructors, printers, and initial actions. For instance:
2863 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2865 will display two values for each typed and untyped symbol (provided
2866 that YYSTYPE has both "ival" and "fval" fields).
2869 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2873 The next major release of Bison will drop support for the following
2874 deprecated features. Please report disagreements to bug-bison@gnu.org.
2878 Support for generating parsers in K&R C will be removed. Parsers
2879 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2882 *** Features deprecated since Bison 1.875
2884 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2887 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2888 %lex-param, will no longer be supported.
2890 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2893 *** The generated header will be included (yacc.c)
2895 Instead of duplicating the content of the generated header (definition of
2896 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2897 as is already the case for GLR or C++ parsers. This change is deferred
2898 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2901 ** Generated Parser Headers
2903 *** Guards (yacc.c, glr.c, glr.cc)
2905 The generated headers are now guarded, as is already the case for C++
2906 parsers (lalr1.cc). For instance, with --defines=foo.h:
2911 #endif /* !YY_FOO_H */
2913 *** New declarations (yacc.c, glr.c)
2915 The generated header now declares yydebug and yyparse. Both honor
2916 --name-prefix=bar_, and yield
2918 int bar_parse (void);
2922 #define yyparse bar_parse
2925 in order to facilitate the inclusion of several parser headers inside a
2926 single compilation unit.
2928 *** Exported symbols in C++
2930 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2931 header, are removed, as they prevent the possibility of including several
2932 generated headers from a single compilation unit.
2936 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2939 ** New %define variable: api.prefix
2941 Now that the generated headers are more complete and properly protected
2942 against multiple inclusions, constant names, such as YYSTYPE are a
2943 problem. While yyparse and others are properly renamed by %name-prefix,
2944 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2945 would introduce backward compatibility issues in projects not expecting
2946 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2947 it is deprecated in favor of a new %define variable: api.prefix.
2949 The following examples compares both:
2951 %name-prefix "bar_" | %define api.prefix "bar_"
2952 %token <ival> FOO %token <ival> FOO
2953 %union { int ival; } %union { int ival; }
2959 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2960 # define BAR_FOO_H # define BAR_FOO_H
2962 /* Enabling traces. */ /* Enabling traces. */
2963 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2964 > # if defined YYDEBUG
2966 > # define BAR_DEBUG 1
2968 > # define BAR_DEBUG 0
2971 # define YYDEBUG 0 | # define BAR_DEBUG 0
2975 # if YYDEBUG | # if BAR_DEBUG
2976 extern int bar_debug; extern int bar_debug;
2979 /* Tokens. */ /* Tokens. */
2980 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2981 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2982 enum yytokentype { | enum bar_tokentype {
2987 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2988 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2989 typedef union YYSTYPE | typedef union BAR_STYPE
2992 } YYSTYPE; | } BAR_STYPE;
2993 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2996 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2998 int bar_parse (void); int bar_parse (void);
3000 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
3003 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
3007 The next major release will drop support for generating parsers in K&R C.
3009 ** yacc.c: YYBACKUP works as expected.
3011 ** glr.c improvements:
3013 *** Location support is eliminated when not requested:
3015 GLR parsers used to include location-related code even when locations were
3016 not requested, and therefore not even usable.
3018 *** __attribute__ is preserved:
3020 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
3021 when -std is passed to GCC).
3023 ** lalr1.java: several fixes:
3025 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
3026 first token leads to a syntax error. Some minor clean ups.
3030 *** C++11 compatibility:
3032 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
3037 The header files such as "parser.hh", "location.hh", etc. used a constant
3038 name for preprocessor guards, for instance:
3040 #ifndef BISON_LOCATION_HH
3041 # define BISON_LOCATION_HH
3043 #endif // !BISON_LOCATION_HH
3045 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
3046 case characters are converted to upper case, and series of
3047 non-alphanumerical characters are converted to an underscore.
3049 With "bison -o lang++/parser.cc", "location.hh" would now include:
3051 #ifndef YY_LANG_LOCATION_HH
3052 # define YY_LANG_LOCATION_HH
3054 #endif // !YY_LANG_LOCATION_HH
3058 The position and location constructors (and their initialize methods)
3059 accept new arguments for line and column. Several issues in the
3060 documentation were fixed.
3062 ** liby is no longer asking for "rpl_fprintf" on some platforms.
3064 ** Changes in the manual:
3066 *** %printer is documented
3068 The "%printer" directive, supported since at least Bison 1.50, is finally
3069 documented. The "mfcalc" example is extended to demonstrate it.
3071 For consistency with the C skeletons, the C++ parsers now also support
3072 "yyoutput" (as an alias to "debug_stream ()").
3074 *** Several improvements have been made:
3076 The layout for grammar excerpts was changed to a more compact scheme.
3077 Named references are motivated. The description of the automaton
3078 description file (*.output) is updated to the current format. Incorrect
3079 index entries were fixed. Some other errors were fixed.
3083 *** Conflicting prototypes with recent/modified Flex.
3085 Fixed build problems with the current, unreleased, version of Flex, and
3086 some modified versions of 2.5.35, which have modified function prototypes.
3088 *** Warnings during the build procedure have been eliminated.
3090 *** Several portability problems in the test suite have been fixed:
3092 This includes warnings with some compilers, unexpected behavior of tools
3093 such as diff, warning messages from the test suite itself, etc.
3095 *** The install-pdf target works properly:
3097 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3098 halts in the middle of its course.
3101 * Noteworthy changes in release 2.5 (2011-05-14)
3103 ** Grammar symbol names can now contain non-initial dashes:
3105 Consistently with directives (such as %error-verbose) and with
3106 %define variables (e.g. push-pull), grammar symbol names may contain
3107 dashes in any position except the beginning. This is a GNU
3108 extension over POSIX Yacc. Thus, use of this extension is reported
3109 by -Wyacc and rejected in Yacc mode (--yacc).
3111 ** Named references:
3113 Historically, Yacc and Bison have supported positional references
3114 ($n, $$) to allow access to symbol values from inside of semantic
3117 Starting from this version, Bison can also accept named references.
3118 When no ambiguity is possible, original symbol names may be used
3119 as named references:
3121 if_stmt : "if" cond_expr "then" then_stmt ';'
3122 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3124 In the more common case, explicit names may be declared:
3126 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3127 { $res = mk_if_stmt($cond, $then, $else); }
3129 Location information is also accessible using @name syntax. When
3130 accessing symbol names containing dots or dashes, explicit bracketing
3131 ($[sym.1]) must be used.
3133 These features are experimental in this version. More user feedback
3134 will help to stabilize them.
3135 Contributed by Alex Rozenman.
3137 ** IELR(1) and canonical LR(1):
3139 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3140 is, given any context-free grammar, IELR(1) generates parser tables
3141 with the full language-recognition power of canonical LR(1) but with
3142 nearly the same number of parser states as LALR(1). This reduction
3143 in parser states is often an order of magnitude. More importantly,
3144 because canonical LR(1)'s extra parser states may contain duplicate
3145 conflicts in the case of non-LR(1) grammars, the number of conflicts
3146 for IELR(1) is often an order of magnitude less as well. This can
3147 significantly reduce the complexity of developing of a grammar.
3149 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3150 place of its traditional LALR(1) parser tables, which remain the
3151 default. You can specify the type of parser tables in the grammar
3152 file with these directives:
3154 %define lr.type lalr
3155 %define lr.type ielr
3156 %define lr.type canonical-lr
3158 The default-reduction optimization in the parser tables can also be
3159 adjusted using "%define lr.default-reductions". For details on both
3160 of these features, see the new section "Tuning LR" in the Bison
3163 These features are experimental. More user feedback will help to
3166 ** LAC (Lookahead Correction) for syntax error handling
3168 Contributed by Joel E. Denny.
3170 Canonical LR, IELR, and LALR can suffer from a couple of problems
3171 upon encountering a syntax error. First, the parser might perform
3172 additional parser stack reductions before discovering the syntax
3173 error. Such reductions can perform user semantic actions that are
3174 unexpected because they are based on an invalid token, and they
3175 cause error recovery to begin in a different syntactic context than
3176 the one in which the invalid token was encountered. Second, when
3177 verbose error messages are enabled (with %error-verbose or the
3178 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3179 syntax error message can both contain invalid tokens and omit valid
3182 The culprits for the above problems are %nonassoc, default
3183 reductions in inconsistent states, and parser state merging. Thus,
3184 IELR and LALR suffer the most. Canonical LR can suffer only if
3185 %nonassoc is used or if default reductions are enabled for
3186 inconsistent states.
3188 LAC is a new mechanism within the parsing algorithm that solves
3189 these problems for canonical LR, IELR, and LALR without sacrificing
3190 %nonassoc, default reductions, or state merging. When LAC is in
3191 use, canonical LR and IELR behave almost exactly the same for both
3192 syntactically acceptable and syntactically unacceptable input.
3193 While LALR still does not support the full language-recognition
3194 power of canonical LR and IELR, LAC at least enables LALR's syntax
3195 error handling to correctly reflect LALR's language-recognition
3198 Currently, LAC is only supported for deterministic parsers in C.
3199 You can enable LAC with the following directive:
3201 %define parse.lac full
3203 See the new section "LAC" in the Bison manual for additional
3204 details including a few caveats.
3206 LAC is an experimental feature. More user feedback will help to
3209 ** %define improvements:
3211 *** Can now be invoked via the command line:
3213 Each of these command-line options
3216 --define=NAME[=VALUE]
3219 --force-define=NAME[=VALUE]
3221 is equivalent to this grammar file declaration
3223 %define NAME ["VALUE"]
3225 except that the manner in which Bison processes multiple definitions
3226 for the same NAME differs. Most importantly, -F and --force-define
3227 quietly override %define, but -D and --define do not. For further
3228 details, see the section "Bison Options" in the Bison manual.
3230 *** Variables renamed:
3232 The following %define variables
3235 lr.keep_unreachable_states
3237 have been renamed to
3240 lr.keep-unreachable-states
3242 The old names are now deprecated but will be maintained indefinitely
3243 for backward compatibility.
3245 *** Values no longer need to be quoted in the grammar file:
3247 If a %define value is an identifier, it no longer needs to be placed
3248 within quotations marks. For example,
3250 %define api.push-pull "push"
3254 %define api.push-pull push
3256 *** Unrecognized variables are now errors not warnings.
3258 *** Multiple invocations for any variable is now an error not a warning.
3260 ** Unrecognized %code qualifiers are now errors not warnings.
3262 ** Character literals not of length one:
3264 Previously, Bison quietly converted all character literals to length
3265 one. For example, without warning, Bison interpreted the operators in
3266 the following grammar to be the same token:
3272 Bison now warns when a character literal is not of length one. In
3273 some future release, Bison will start reporting an error instead.
3275 ** Destructor calls fixed for lookaheads altered in semantic actions:
3277 Previously for deterministic parsers in C, if a user semantic action
3278 altered yychar, the parser in some cases used the old yychar value to
3279 determine which destructor to call for the lookahead upon a syntax
3280 error or upon parser return. This bug has been fixed.
3282 ** C++ parsers use YYRHSLOC:
3284 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3285 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3286 to use it. If, for instance, your location structure has "first"
3287 and "last" members, instead of
3289 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3293 (Current).first = (Rhs)[1].location.first; \
3294 (Current).last = (Rhs)[N].location.last; \
3298 (Current).first = (Current).last = (Rhs)[0].location.last; \
3304 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3308 (Current).first = YYRHSLOC (Rhs, 1).first; \
3309 (Current).last = YYRHSLOC (Rhs, N).last; \
3313 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3317 ** YYLLOC_DEFAULT in C++:
3319 The default implementation of YYLLOC_DEFAULT used to be issued in
3320 the header file. It is now output in the implementation file, after
3321 the user %code sections so that its #ifndef guard does not try to
3322 override the user's YYLLOC_DEFAULT if provided.
3324 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3326 YYFAIL has existed for many years as an undocumented feature of
3327 deterministic parsers in C generated by Bison. More recently, it was
3328 a documented feature of Bison's experimental Java parsers. As
3329 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3330 semantic action now produces a deprecation warning, and Java parsers
3331 no longer implement YYFAIL at all. For further details, including a
3332 discussion of how to suppress C preprocessor warnings about YYFAIL
3333 being unused, see the Bison 2.4.2 NEWS entry.
3335 ** Temporary hack for adding a semicolon to the user action:
3337 Previously, Bison appended a semicolon to every user action for
3338 reductions when the output language defaulted to C (specifically, when
3339 neither %yacc, %language, %skeleton, or equivalent command-line
3340 options were specified). This allowed actions such as
3342 exp: exp "+" exp { $$ = $1 + $3 };
3346 exp: exp "+" exp { $$ = $1 + $3; };
3348 As a first step in removing this misfeature, Bison now issues a
3349 warning when it appends a semicolon. Moreover, in cases where Bison
3350 cannot easily determine whether a semicolon is needed (for example, an
3351 action ending with a cpp directive or a braced compound initializer),
3352 it no longer appends one. Thus, the C compiler might now complain
3353 about a missing semicolon where it did not before. Future releases of
3354 Bison will cease to append semicolons entirely.
3356 ** Verbose syntax error message fixes:
3358 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3359 specified, syntax error messages produced by the generated parser
3360 include the unexpected token as well as a list of expected tokens.
3361 The effect of %nonassoc on these verbose messages has been corrected
3362 in two ways, but a more complete fix requires LAC, described above:
3364 *** When %nonassoc is used, there can exist parser states that accept no
3365 tokens, and so the parser does not always require a lookahead token
3366 in order to detect a syntax error. Because no unexpected token or
3367 expected tokens can then be reported, the verbose syntax error
3368 message described above is suppressed, and the parser instead
3369 reports the simpler message, "syntax error". Previously, this
3370 suppression was sometimes erroneously triggered by %nonassoc when a
3371 lookahead was actually required. Now verbose messages are
3372 suppressed only when all previous lookaheads have already been
3373 shifted or discarded.
3375 *** Previously, the list of expected tokens erroneously included tokens
3376 that would actually induce a syntax error because conflicts for them
3377 were resolved with %nonassoc in the current parser state. Such
3378 tokens are now properly omitted from the list.
3380 *** Expected token lists are still often wrong due to state merging
3381 (from LALR or IELR) and default reductions, which can both add
3382 invalid tokens and subtract valid tokens. Canonical LR almost
3383 completely fixes this problem by eliminating state merging and
3384 default reductions. However, there is one minor problem left even
3385 when using canonical LR and even after the fixes above. That is,
3386 if the resolution of a conflict with %nonassoc appears in a later
3387 parser state than the one at which some syntax error is
3388 discovered, the conflicted token is still erroneously included in
3389 the expected token list. Bison's new LAC implementation,
3390 described above, eliminates this problem and the need for
3391 canonical LR. However, LAC is still experimental and is disabled
3394 ** Java skeleton fixes:
3396 *** A location handling bug has been fixed.
3398 *** The top element of each of the value stack and location stack is now
3399 cleared when popped so that it can be garbage collected.
3401 *** Parser traces now print the top element of the stack.
3403 ** -W/--warnings fixes:
3405 *** Bison now properly recognizes the "no-" versions of categories:
3407 For example, given the following command line, Bison now enables all
3408 warnings except warnings for incompatibilities with POSIX Yacc:
3410 bison -Wall,no-yacc gram.y
3412 *** Bison now treats S/R and R/R conflicts like other warnings:
3414 Previously, conflict reports were independent of Bison's normal
3415 warning system. Now, Bison recognizes the warning categories
3416 "conflicts-sr" and "conflicts-rr". This change has important
3417 consequences for the -W and --warnings command-line options. For
3420 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3421 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3422 bison -Wnone gram.y # no conflicts are reported
3423 bison -Werror gram.y # any conflict is an error
3425 However, as before, if the %expect or %expect-rr directive is
3426 specified, an unexpected number of conflicts is an error, and an
3427 expected number of conflicts is not reported, so -W and --warning
3428 then have no effect on the conflict report.
3430 *** The "none" category no longer disables a preceding "error":
3432 For example, for the following command line, Bison now reports
3433 errors instead of warnings for incompatibilities with POSIX Yacc:
3435 bison -Werror,none,yacc gram.y
3437 *** The "none" category now disables all Bison warnings:
3439 Previously, the "none" category disabled only Bison warnings for
3440 which there existed a specific -W/--warning category. However,
3441 given the following command line, Bison is now guaranteed to
3442 suppress all warnings:
3446 ** Precedence directives can now assign token number 0:
3448 Since Bison 2.3b, which restored the ability of precedence
3449 directives to assign token numbers, doing so for token number 0 has
3450 produced an assertion failure. For example:
3454 This bug has been fixed.
3457 * Noteworthy changes in release 2.4.3 (2010-08-05)
3459 ** Bison now obeys -Werror and --warnings=error for warnings about
3460 grammar rules that are useless in the parser due to conflicts.
3462 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3465 ** Failures in the test suite for GCC 4.5 have been fixed.
3467 ** Failures in the test suite for some versions of Sun Studio C++ have
3470 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3471 warnings about undefined %prec identifiers will not be converted to
3472 errors in Bison 2.5. They will remain warnings, which should be
3473 sufficient for POSIX while avoiding backward compatibility issues.
3475 ** Minor documentation fixes.
3478 * Noteworthy changes in release 2.4.2 (2010-03-20)
3480 ** Some portability problems that resulted in failures and livelocks
3481 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3482 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3483 errors should no longer cause M4 to report a broken pipe on the
3486 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3488 POSIX specifies that an error be reported for any identifier that does
3489 not appear on the LHS of a grammar rule and that is not defined by
3490 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3491 error report for the case when an identifier appears only after a
3492 %prec directive. It is now restored. However, for backward
3493 compatibility with recent Bison releases, it is only a warning for
3494 now. In Bison 2.5 and later, it will return to being an error.
3495 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3496 warning will not be converted to an error in Bison 2.5.]
3498 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3500 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3501 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3504 ** %code is now a permanent feature.
3506 A traditional Yacc prologue directive is written in the form:
3510 To provide a more flexible alternative, Bison 2.3b introduced the
3511 %code directive with the following forms for C/C++:
3514 %code requires {CODE}
3515 %code provides {CODE}
3518 These forms are now considered permanent features of Bison. See the
3519 %code entries in the section "Bison Declaration Summary" in the Bison
3520 manual for a summary of their functionality. See the section
3521 "Prologue Alternatives" for a detailed discussion including the
3522 advantages of %code over the traditional Yacc prologue directive.
3524 Bison's Java feature as a whole including its current usage of %code
3525 is still considered experimental.
3527 ** YYFAIL is deprecated and will eventually be removed.
3529 YYFAIL has existed for many years as an undocumented feature of
3530 deterministic parsers in C generated by Bison. Previously, it was
3531 documented for Bison's experimental Java parsers. YYFAIL is no longer
3532 documented for Java parsers and is formally deprecated in both cases.
3533 Users are strongly encouraged to migrate to YYERROR, which is
3536 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3537 induce a syntax error. The most obvious difference from YYERROR is
3538 that YYFAIL will automatically invoke yyerror to report the syntax
3539 error so that you don't have to. However, there are several other
3540 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3541 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3542 used. For a more detailed discussion, see:
3544 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3546 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3547 deterministic parsers in C will continue to implement it. However,
3548 because YYFAIL is already flawed, it seems futile to try to make new
3549 Bison features compatible with it. Thus, during parser generation,
3550 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3551 rule action. In a later release, YYFAIL will be disabled for
3552 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3553 be removed altogether.
3555 There exists at least one case where Bison 2.5's YYFAIL warning will
3556 be a false positive. Some projects add phony uses of YYFAIL and other
3557 Bison-defined macros for the sole purpose of suppressing C
3558 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3559 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3560 epilogue (that is, after the second "%%") in the Bison input file. In
3561 this release (2.4.2), Bison already generates its own code to suppress
3562 C preprocessor warnings for YYFAIL, so projects can remove their own
3563 phony uses of YYFAIL if compatibility with Bison releases prior to
3564 2.4.2 is not necessary.
3566 ** Internationalization.
3568 Fix a regression introduced in Bison 2.4: Under some circumstances,
3569 message translations were not installed although supported by the
3573 * Noteworthy changes in release 2.4.1 (2008-12-11)
3575 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3576 declarations have been fixed.
3578 ** Temporary hack for adding a semicolon to the user action.
3580 Bison used to prepend a trailing semicolon at the end of the user
3581 action for reductions. This allowed actions such as
3583 exp: exp "+" exp { $$ = $1 + $3 };
3587 exp: exp "+" exp { $$ = $1 + $3; };
3589 Some grammars still depend on this "feature". Bison 2.4.1 restores
3590 the previous behavior in the case of C output (specifically, when
3591 neither %language or %skeleton or equivalent command-line options
3592 are used) to leave more time for grammars depending on the old
3593 behavior to be adjusted. Future releases of Bison will disable this
3596 ** A few minor improvements to the Bison manual.
3599 * Noteworthy changes in release 2.4 (2008-11-02)
3601 ** %language is an experimental feature.
3603 We first introduced this feature in test release 2.3b as a cleaner
3604 alternative to %skeleton. Since then, we have discussed the possibility of
3605 modifying its effect on Bison's output file names. Thus, in this release,
3606 we consider %language to be an experimental feature that will likely evolve
3609 ** Forward compatibility with GNU M4 has been improved.
3611 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3615 * Noteworthy changes in release 2.3b (2008-05-27)
3617 ** The quotes around NAME that used to be required in the following directive
3620 %define NAME "VALUE"
3622 ** The directive "%pure-parser" is now deprecated in favor of:
3626 which has the same effect except that Bison is more careful to warn about
3627 unreasonable usage in the latter case.
3631 Bison can now generate an LALR(1) parser in C with a push interface. That
3632 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3633 push one token at a time to the parser using "yypush_parse", which will
3634 return to the caller after processing each token. By default, the push
3635 interface is disabled. Either of the following directives will enable it:
3637 %define api.push_pull "push" // Just push; does not require yylex.
3638 %define api.push_pull "both" // Push and pull; requires yylex.
3640 See the new section "A Push Parser" in the Bison manual for details.
3642 The current push parsing interface is experimental and may evolve. More user
3643 feedback will help to stabilize it.
3645 ** The -g and --graph options now output graphs in Graphviz DOT format,
3646 not VCG format. Like --graph, -g now also takes an optional FILE argument
3647 and thus cannot be bundled with other short options.
3651 Bison can now generate an LALR(1) parser in Java. The skeleton is
3652 "data/lalr1.java". Consider using the new %language directive instead of
3653 %skeleton to select it.
3655 See the new section "Java Parsers" in the Bison manual for details.
3657 The current Java interface is experimental and may evolve. More user
3658 feedback will help to stabilize it.
3659 Contributed by Paolo Bonzini.
3663 This new directive specifies the programming language of the generated
3664 parser, which can be C (the default), C++, or Java. Besides the skeleton
3665 that Bison uses, the directive affects the names of the generated files if
3666 the grammar file's name ends in ".y".
3668 ** XML Automaton Report
3670 Bison can now generate an XML report of the LALR(1) automaton using the new
3671 "--xml" option. The current XML schema is experimental and may evolve. More
3672 user feedback will help to stabilize it.
3673 Contributed by Wojciech Polak.
3675 ** The grammar file may now specify the name of the parser header file using
3676 %defines. For example:
3680 ** When reporting useless rules, useless nonterminals, and unused terminals,
3681 Bison now employs the terms "useless in grammar" instead of "useless",
3682 "useless in parser" instead of "never reduced", and "unused in grammar"
3683 instead of "unused".
3685 ** Unreachable State Removal
3687 Previously, Bison sometimes generated parser tables containing unreachable
3688 states. A state can become unreachable during conflict resolution if Bison
3689 disables a shift action leading to it from a predecessor state. Bison now:
3691 1. Removes unreachable states.
3693 2. Does not report any conflicts that appeared in unreachable states.
3694 WARNING: As a result, you may need to update %expect and %expect-rr
3695 directives in existing grammar files.
3697 3. For any rule used only in such states, Bison now reports the rule as
3698 "useless in parser due to conflicts".
3700 This feature can be disabled with the following directive:
3702 %define lr.keep_unreachable_states
3704 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3705 for further discussion.
3707 ** Lookahead Set Correction in the ".output" Report
3709 When instructed to generate a ".output" file including lookahead sets
3710 (using "--report=lookahead", for example), Bison now prints each reduction's
3711 lookahead set only next to the associated state's one item that (1) is
3712 associated with the same rule as the reduction and (2) has its dot at the end
3713 of its RHS. Previously, Bison also erroneously printed the lookahead set
3714 next to all of the state's other items associated with the same rule. This
3715 bug affected only the ".output" file and not the generated parser source
3718 ** --report-file=FILE is a new option to override the default ".output" file
3721 ** The "=" that used to be required in the following directives is now
3724 %file-prefix "parser"
3728 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3730 Bison 2.3a provided a new set of directives as a more flexible alternative to
3731 the traditional Yacc prologue blocks. Those have now been consolidated into
3732 a single %code directive with an optional qualifier field, which identifies
3733 the purpose of the code and thus the location(s) where Bison should generate
3736 1. "%code {CODE}" replaces "%after-header {CODE}"
3737 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3738 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3739 4. "%code top {CODE}" replaces "%before-header {CODE}"
3741 See the %code entries in section "Bison Declaration Summary" in the Bison
3742 manual for a summary of the new functionality. See the new section "Prologue
3743 Alternatives" for a detailed discussion including the advantages of %code
3744 over the traditional Yacc prologues.
3746 The prologue alternatives are experimental. More user feedback will help to
3747 determine whether they should become permanent features.
3749 ** Revised warning: unset or unused midrule values
3751 Since Bison 2.2, Bison has warned about midrule values that are set but not
3752 used within any of the actions of the parent rule. For example, Bison warns
3755 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3757 Now, Bison also warns about midrule values that are used but not set. For
3758 example, Bison warns about unset $$ in the midrule action in:
3760 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3762 However, Bison now disables both of these warnings by default since they
3763 sometimes prove to be false alarms in existing grammars employing the Yacc
3764 constructs $0 or $-N (where N is some positive integer).
3766 To enable these warnings, specify the option "--warnings=midrule-values" or
3767 "-W", which is a synonym for "--warnings=all".
3769 ** Default %destructor or %printer with "<*>" or "<>"
3771 Bison now recognizes two separate kinds of default %destructor's and
3774 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3775 %destructor/%printer for all grammar symbols for which you have formally
3776 declared semantic type tags.
3778 2. Place "<>" in a %destructor/%printer symbol list to define a default
3779 %destructor/%printer for all grammar symbols without declared semantic
3782 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3783 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3784 longer applies any %destructor to a midrule value if that midrule value is
3785 not actually ever referenced using either $$ or $n in a semantic action.
3787 The default %destructor's and %printer's are experimental. More user
3788 feedback will help to determine whether they should become permanent
3791 See the section "Freeing Discarded Symbols" in the Bison manual for further
3794 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3795 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3796 manual for a caveat concerning the treatment of literal strings.
3798 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3799 completely removed from Bison.
3802 * Noteworthy changes in release 2.3a (2006-09-13)
3804 ** Instead of %union, you can define and use your own union type
3805 YYSTYPE if your grammar contains at least one <type> tag.
3806 Your YYSTYPE need not be a macro; it can be a typedef.
3807 This change is for compatibility with other Yacc implementations,
3808 and is required by POSIX.
3810 ** Locations columns and lines start at 1.
3811 In accordance with the GNU Coding Standards and Emacs.
3813 ** You may now declare per-type and default %destructor's and %printer's:
3817 %union { char *string; }
3818 %token <string> STRING1
3819 %token <string> STRING2
3820 %type <string> string1
3821 %type <string> string2
3822 %union { char character; }
3823 %token <character> CHR
3824 %type <character> chr
3825 %destructor { free ($$); } %symbol-default
3826 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3827 %destructor { } <character>
3829 guarantees that, when the parser discards any user-defined symbol that has a
3830 semantic type tag other than "<character>", it passes its semantic value to
3831 "free". However, when the parser discards a "STRING1" or a "string1", it
3832 also prints its line number to "stdout". It performs only the second
3833 "%destructor" in this case, so it invokes "free" only once.
3835 [Although we failed to mention this here in the 2.3a release, the default
3836 %destructor's and %printer's were experimental, and they were rewritten in
3839 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3840 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3841 associating token numbers with token names. Removing the #define statements
3842 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3843 requires them. Bison still generates an enum for token names in all cases.
3845 ** Handling of traditional Yacc prologue blocks is now more consistent but
3846 potentially incompatible with previous releases of Bison.
3848 As before, you declare prologue blocks in your grammar file with the
3849 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3850 prologue blocks that you've declared before the first %union. To generate
3851 the post-prologue, Bison concatenates all prologue blocks that you've
3852 declared after the first %union.
3854 Previous releases of Bison inserted the pre-prologue into both the header
3855 file and the code file in all cases except for LALR(1) parsers in C. In the
3856 latter case, Bison inserted it only into the code file. For parsers in C++,
3857 the point of insertion was before any token definitions (which associate
3858 token numbers with names). For parsers in C, the point of insertion was
3859 after the token definitions.
3861 Now, Bison never inserts the pre-prologue into the header file. In the code
3862 file, it always inserts it before the token definitions.
3864 ** Bison now provides a more flexible alternative to the traditional Yacc
3865 prologue blocks: %before-header, %start-header, %end-header, and
3868 For example, the following declaration order in the grammar file reflects the
3869 order in which Bison will output these code blocks. However, you are free to
3870 declare these code blocks in your grammar file in whatever order is most
3874 /* Bison treats this block like a pre-prologue block: it inserts it into
3875 * the code file before the contents of the header file. It does *not*
3876 * insert it into the header file. This is a good place to put
3877 * #include's that you want at the top of your code file. A common
3878 * example is '#include "system.h"'. */
3881 /* Bison inserts this block into both the header file and the code file.
3882 * In both files, the point of insertion is before any Bison-generated
3883 * token, semantic type, location type, and class definitions. This is a
3884 * good place to define %union dependencies, for example. */
3887 /* Unlike the traditional Yacc prologue blocks, the output order for the
3888 * new %*-header blocks is not affected by their declaration position
3889 * relative to any %union in the grammar file. */
3892 /* Bison inserts this block into both the header file and the code file.
3893 * In both files, the point of insertion is after the Bison-generated
3894 * definitions. This is a good place to declare or define public
3895 * functions or data structures that depend on the Bison-generated
3899 /* Bison treats this block like a post-prologue block: it inserts it into
3900 * the code file after the contents of the header file. It does *not*
3901 * insert it into the header file. This is a good place to declare or
3902 * define internal functions or data structures that depend on the
3903 * Bison-generated definitions. */
3906 If you have multiple occurrences of any one of the above declarations, Bison
3907 will concatenate the contents in declaration order.
3909 [Although we failed to mention this here in the 2.3a release, the prologue
3910 alternatives were experimental, and they were rewritten in future versions.]
3912 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3913 The old spelling still works, but is not documented and may be removed
3914 in a future release.
3917 * Noteworthy changes in release 2.3 (2006-06-05)
3919 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3920 for compatibility with LALR(1) grammars.
3922 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3923 be to a type name that does not contain parentheses or brackets.
3926 * Noteworthy changes in release 2.2 (2006-05-19)
3928 ** The distribution terms for all Bison-generated parsers now permit
3929 using the parsers in nonfree programs. Previously, this permission
3930 was granted only for Bison-generated LALR(1) parsers in C.
3932 ** %name-prefix changes the namespace name in C++ outputs.
3934 ** The C++ parsers export their token_type.
3936 ** Bison now allows multiple %union declarations, and concatenates
3937 their contents together.
3939 ** New warning: unused values
3940 Right-hand side symbols whose values are not used are reported,
3941 if the symbols have destructors. For instance:
3943 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3947 will trigger a warning about $$ and $5 in the first rule, and $3 in
3948 the second ($1 is copied to $$ by the default rule). This example
3949 most likely contains three errors, and could be rewritten as:
3951 exp: exp "?" exp ":" exp
3952 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3954 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3957 However, if the original actions were really intended, memory leaks
3958 and all, the warnings can be suppressed by letting Bison believe the
3959 values are used, e.g.:
3961 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3962 | exp "+" exp { $$ = $1; (void) $3; }
3965 If there are midrule actions, the warning is issued if no action
3966 uses it. The following triggers no warning: $1 and $3 are used.
3968 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3970 The warning is intended to help catching lost values and memory leaks.
3971 If a value is ignored, its associated memory typically is not reclaimed.
3973 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3974 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3975 and YYERROR, for all objects on the stack, other than objects
3976 corresponding to the right-hand side of the current rule.
3978 ** %expect, %expect-rr
3979 Incorrect numbers of expected conflicts are now actual errors,
3980 instead of warnings.
3982 ** GLR, YACC parsers.
3983 The %parse-params are available in the destructors (and the
3984 experimental printers) as per the documentation.
3986 ** Bison now warns if it finds a stray "$" or "@" in an action.
3988 ** %require "VERSION"
3989 This specifies that the grammar file depends on features implemented
3990 in Bison version VERSION or higher.
3992 ** lalr1.cc: The token and value types are now class members.
3993 The tokens were defined as free form enums and cpp macros. YYSTYPE
3994 was defined as a free form union. They are now class members:
3995 tokens are enumerations of the "yy::parser::token" struct, and the
3996 semantic values have the "yy::parser::semantic_type" type.
3998 If you do not want or can update to this scheme, the directive
3999 '%define "global_tokens_and_yystype" "1"' triggers the global
4000 definition of tokens and YYSTYPE. This change is suitable both
4001 for previous releases of Bison, and this one.
4003 If you wish to update, then make sure older version of Bison will
4004 fail using '%require "2.2"'.
4006 ** DJGPP support added.
4009 * Noteworthy changes in release 2.1 (2005-09-16)
4011 ** The C++ lalr1.cc skeleton supports %lex-param.
4013 ** Bison-generated parsers now support the translation of diagnostics like
4014 "syntax error" into languages other than English. The default
4015 language is still English. For details, please see the new
4016 Internationalization section of the Bison manual. Software
4017 distributors should also see the new PACKAGING file. Thanks to
4018 Bruno Haible for this new feature.
4020 ** Wording in the Bison-generated parsers has been changed slightly to
4021 simplify translation. In particular, the message "memory exhausted"
4022 has replaced "parser stack overflow", as the old message was not
4023 always accurate for modern Bison-generated parsers.
4025 ** Destructors are now called when the parser aborts, for all symbols left
4026 behind on the stack. Also, the start symbol is now destroyed after a
4027 successful parse. In both cases, the behavior was formerly inconsistent.
4029 ** When generating verbose diagnostics, Bison-generated parsers no longer
4030 quote the literal strings associated with tokens. For example, for
4031 a syntax error associated with '%token NUM "number"' they might
4032 print 'syntax error, unexpected number' instead of 'syntax error,
4033 unexpected "number"'.
4036 * Noteworthy changes in release 2.0 (2004-12-25)
4038 ** Possibly-incompatible changes
4040 - Bison-generated parsers no longer default to using the alloca function
4041 (when available) to extend the parser stack, due to widespread
4042 problems in unchecked stack-overflow detection. You can "#define
4043 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
4044 the manual to determine safe values for YYMAXDEPTH in that case.
4046 - Error token location.
4047 During error recovery, the location of the syntax error is updated
4048 to cover the whole sequence covered by the error token: it includes
4049 the shifted symbols thrown away during the first part of the error
4050 recovery, and the lookahead rejected during the second part.
4052 - Semicolon changes:
4053 . Stray semicolons are no longer allowed at the start of a grammar.
4054 . Semicolons are now required after in-grammar declarations.
4056 - Unescaped newlines are no longer allowed in character constants or
4057 string literals. They were never portable, and GCC 3.4.0 has
4058 dropped support for them. Better diagnostics are now generated if
4059 forget a closing quote.
4061 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4065 - GLR grammars now support locations.
4067 - New directive: %initial-action.
4068 This directive allows the user to run arbitrary code (including
4069 initializing @$) from yyparse before parsing starts.
4071 - A new directive "%expect-rr N" specifies the expected number of
4072 reduce/reduce conflicts in GLR parsers.
4074 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4075 This is a GNU extension.
4077 - The option "--report=lookahead" was changed to "--report=look-ahead".
4078 [However, this was changed back after 2.3.]
4080 - Experimental %destructor support has been added to lalr1.cc.
4082 - New configure option --disable-yacc, to disable installation of the
4083 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4087 - For now, %expect-count violations are now just warnings, not errors.
4088 This is for compatibility with Bison 1.75 and earlier (when there are
4089 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4090 are too many or too few shift/reduce conflicts). However, in future
4091 versions of Bison we plan to improve the %expect machinery so that
4092 these violations will become errors again.
4094 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4095 arbitrarily limited to 16-bit counts.
4097 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4100 * Noteworthy changes in release 1.875 (2003-01-01)
4102 ** The documentation license has been upgraded to version 1.2
4103 of the GNU Free Documentation License.
4105 ** syntax error processing
4107 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4108 locations too. This fixes bugs in error-location computation.
4111 It is now possible to reclaim the memory associated to symbols
4112 discarded during error recovery. This feature is still experimental.
4115 This new directive is preferred over YYERROR_VERBOSE.
4117 - #defining yyerror to steal internal variables is discouraged.
4118 It is not guaranteed to work forever.
4120 ** POSIX conformance
4122 - Semicolons are once again optional at the end of grammar rules.
4123 This reverts to the behavior of Bison 1.33 and earlier, and improves
4124 compatibility with Yacc.
4126 - "parse error" -> "syntax error"
4127 Bison now uniformly uses the term "syntax error"; formerly, the code
4128 and manual sometimes used the term "parse error" instead. POSIX
4129 requires "syntax error" in diagnostics, and it was thought better to
4132 - The documentation now emphasizes that yylex and yyerror must be
4133 declared before use. C99 requires this.
4135 - Bison now parses C99 lexical constructs like UCNs and
4136 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4138 - File names are properly escaped in C output. E.g., foo\bar.y is
4139 output as "foo\\bar.y".
4141 - Yacc command and library now available
4142 The Bison distribution now installs a "yacc" command, as POSIX requires.
4143 Also, Bison now installs a small library liby.a containing
4144 implementations of Yacc-compatible yyerror and main functions.
4145 This library is normally not useful, but POSIX requires it.
4147 - Type clashes now generate warnings, not errors.
4149 - If the user does not define YYSTYPE as a macro, Bison now declares it
4150 using typedef instead of defining it as a macro.
4151 For consistency, YYLTYPE is also declared instead of defined.
4153 ** Other compatibility issues
4155 - %union directives can now have a tag before the "{", e.g., the
4156 directive "%union foo {...}" now generates the C code
4157 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4158 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4159 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4160 This is for compatibility with both Yacc and Bison 1.35.
4162 - ";" is output before the terminating "}" of an action, for
4163 compatibility with Bison 1.35.
4165 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4166 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4168 - "yystype" and "yyltype" are now obsolescent macros instead of being
4169 typedefs or tags; they are no longer documented and are planned to be
4170 withdrawn in a future release.
4175 Users of Bison have to decide how they handle the portability of the
4178 - "parsing stack overflow..." -> "parser stack overflow"
4179 GLR parsers now report "parser stack overflow" as per the Bison manual.
4181 ** %parse-param and %lex-param
4182 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4183 additional context to yyparse and yylex. They suffer from several
4186 - a single argument only can be added,
4187 - their types are weak (void *),
4188 - this context is not passed to ancillary functions such as yyerror,
4189 - only yacc.c parsers support them.
4191 The new %parse-param/%lex-param directives provide a more precise control.
4194 %parse-param {int *nastiness}
4195 %lex-param {int *nastiness}
4196 %parse-param {int *randomness}
4198 results in the following signatures:
4200 int yylex (int *nastiness);
4201 int yyparse (int *nastiness, int *randomness);
4203 or, if both %pure-parser and %locations are used:
4205 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4206 int yyparse (int *nastiness, int *randomness);
4208 ** Bison now warns if it detects conflicting outputs to the same file,
4209 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4210 that command outputs both code and header to foo.h.
4212 ** #line in output files
4213 - --no-line works properly.
4215 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4216 later to be built. This change originally took place a few versions
4217 ago, but nobody noticed until we recently asked someone to try
4218 building Bison with a K&R C compiler.
4221 * Noteworthy changes in release 1.75 (2002-10-14)
4223 ** Bison should now work on 64-bit hosts.
4225 ** Indonesian translation thanks to Tedi Heriyanto.
4228 Fix spurious parse errors.
4231 Some people redefine yyerror to steal yyparse' private variables.
4232 Reenable this trick until an official feature replaces it.
4235 In agreement with POSIX and with other Yaccs, leaving a default
4236 action is valid when $$ is untyped, and $1 typed:
4240 but the converse remains an error:
4244 ** Values of midrule actions
4247 foo: { ... } { $$ = $1; } ...
4249 was incorrectly rejected: $1 is defined in the second midrule
4250 action, and is equal to the $$ of the first midrule action.
4253 * Noteworthy changes in release 1.50 (2002-10-04)
4258 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4259 almost any context-free grammar, ambiguous or not. The new declarations
4260 %dprec and %merge on grammar rules allow parse-time resolution of
4261 ambiguities. Contributed by Paul Hilfinger.
4263 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4264 like the Alpha, so please stick to 32-bit hosts for now.
4267 When not in Yacc compatibility mode, when the output file was not
4268 specified, running "bison foo/bar.y" created "foo/bar.c". It
4269 now creates "bar.c".
4272 The undefined token was systematically mapped to 2 which prevented
4273 the use of 2 by the user. This is no longer the case.
4275 ** Unknown token numbers
4276 If yylex returned an out of range value, yyparse could die. This is
4280 According to POSIX, the error token must be 256.
4281 Bison extends this requirement by making it a preference: *if* the
4282 user specified that one of her tokens is numbered 256, then error
4283 will be mapped onto another number.
4285 ** Verbose error messages
4286 They no longer report "..., expecting error or..." for states where
4287 error recovery is possible.
4290 Defaults to "$end" instead of "$".
4292 ** Error recovery now conforms to documentation and to POSIX
4293 When a Bison-generated parser encounters a syntax error, it now pops
4294 the stack until it finds a state that allows shifting the error
4295 token. Formerly, it popped the stack until it found a state that
4296 allowed some non-error action other than a default reduction on the
4297 error token. The new behavior has long been the documented behavior,
4298 and has long been required by POSIX. For more details, please see
4299 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4300 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4303 Popped tokens and nonterminals are now reported.
4306 Larger grammars are now supported (larger token numbers, larger grammar
4307 size (= sum of the LHS and RHS lengths), larger LALR tables).
4308 Formerly, many of these numbers ran afoul of 16-bit limits;
4309 now these limits are 32 bits on most hosts.
4311 ** Explicit initial rule
4312 Bison used to play hacks with the initial rule, which the user does
4313 not write. It is now explicit, and visible in the reports and
4317 Before, Bison reported the useless rules, but, although not used,
4318 included them in the parsers. They are now actually removed.
4320 ** Useless rules, useless nonterminals
4321 They are now reported, as a warning, with their locations.
4323 ** Rules never reduced
4324 Rules that can never be reduced because of conflicts are now
4327 ** Incorrect "Token not used"
4328 On a grammar such as
4330 %token useless useful
4332 exp: '0' %prec useful;
4334 where a token was used to set the precedence of the last rule,
4335 bison reported both "useful" and "useless" as useless tokens.
4337 ** Revert the C++ namespace changes introduced in 1.31
4338 as they caused too many portability hassles.
4340 ** Default locations
4341 By an accident of design, the default computation of @$ was
4342 performed after another default computation was performed: @$ = @1.
4343 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4344 the computation of @$.
4346 ** Token end-of-file
4347 The token end of file may be specified by the user, in which case,
4348 the user symbol is used in the reports, the graphs, and the verbose
4349 error messages instead of "$end", which remains being the default.
4353 %token MYEOF 0 "end of file"
4356 This old option, which has been broken for ages, is removed.
4359 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4360 Croatian, thanks to Denis Lackovic.
4362 ** Incorrect token definitions
4365 bison used to output
4368 ** Token definitions as enums
4369 Tokens are output both as the traditional #define's, and, provided
4370 the compiler supports ANSI C or is a C++ compiler, as enums.
4371 This lets debuggers display names instead of integers.
4374 In addition to --verbose, bison supports --report=THINGS, which
4375 produces additional information:
4377 complete the core item sets with their closure
4378 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4379 explicitly associate lookahead tokens to items
4381 describe shift/reduce conflicts solving.
4382 Bison used to systematically output this information on top of
4383 the report. Solved conflicts are now attached to their states.
4386 Previous versions don't complain when there is a type clash on
4387 the default action if the rule has a midrule action, such as in:
4395 ** GNU M4 is now required when using Bison.
4398 * Noteworthy changes in release 1.35 (2002-03-25)
4401 Some projects use Bison's C parser with C++ compilers, and define
4402 YYSTYPE as a class. The recent adjustment of C parsers for data
4403 alignment and 64 bit architectures made this impossible.
4405 Because for the time being no real solution for C++ parser
4406 generation exists, kludges were implemented in the parser to
4407 maintain this use. In the future, when Bison has C++ parsers, this
4408 kludge will be disabled.
4410 This kludge also addresses some C++ problems when the stack was
4414 * Noteworthy changes in release 1.34 (2002-03-12)
4416 ** File name clashes are detected
4417 $ bison foo.y -d -o foo.x
4418 fatal error: header and parser would both be named "foo.x"
4420 ** A missing ";" at the end of a rule triggers a warning
4421 In accordance with POSIX, and in agreement with other
4422 Yacc implementations, Bison will mandate this semicolon in the near
4423 future. This eases the implementation of a Bison parser of Bison
4424 grammars by making this grammar LALR(1) instead of LR(2). To
4425 facilitate the transition, this release introduces a warning.
4427 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4428 many portability hassles.
4430 ** DJGPP support added.
4432 ** Fix test suite portability problems.
4435 * Noteworthy changes in release 1.33 (2002-02-07)
4438 Groff could not be compiled for the definition of size_t was lacking
4439 under some conditions.
4445 * Noteworthy changes in release 1.32 (2002-01-23)
4447 ** Fix Yacc output file names
4449 ** Portability fixes
4451 ** Italian, Dutch translations
4454 * Noteworthy changes in release 1.31 (2002-01-14)
4458 ** GNU Gettext and %expect
4459 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4460 Bison dies on incorrect %expectations, we fear there will be
4461 too many bug reports for Gettext, so _for the time being_, %expect
4462 does not trigger an error when the input file is named "plural.y".
4464 ** Use of alloca in parsers
4465 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4466 malloc exclusively. Since 1.29, but was not NEWS'ed.
4468 alloca is used only when compiled with GCC, to avoid portability
4471 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4473 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4474 (as POSIX requires) instead of being undefined.
4477 Bison has always permitted actions such as { $$ = $1 }: it adds the
4478 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4479 is no longer output: one has to write { $$ = $1; }.
4481 ** Better C++ compliance
4482 The output parsers try to respect C++ namespaces.
4483 [This turned out to be a failed experiment, and it was reverted later.]
4486 Fixed bugs when reporting useless nonterminals.
4489 The parsers work properly on 64 bit hosts.
4492 Some calls to strerror resulted in scrambled or missing error messages.
4495 When the number of shift/reduce conflicts is correct, don't issue
4498 ** The verbose report includes the rule line numbers.
4500 ** Rule line numbers are fixed in traces.
4502 ** Swedish translation
4505 Verbose parse error messages from the parsers are better looking.
4506 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4507 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4509 ** Fixed parser memory leaks.
4510 When the generated parser was using malloc to extend its stacks, the
4511 previous allocations were not freed.
4513 ** Fixed verbose output file.
4514 Some newlines were missing.
4515 Some conflicts in state descriptions were missing.
4517 ** Fixed conflict report.
4518 Option -v was needed to get the result.
4522 Mismatches are errors, not warnings.
4524 ** Fixed incorrect processing of some invalid input.
4526 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4528 ** Fixed some typos in the documentation.
4530 ** %token MY_EOF 0 is supported.
4531 Before, MY_EOF was silently renumbered as 257.
4533 ** doc/refcard.tex is updated.
4535 ** %output, %file-prefix, %name-prefix.
4539 New, aliasing "--output-file".
4542 * Noteworthy changes in release 1.30 (2001-10-26)
4544 ** "--defines" and "--graph" have now an optional argument which is the
4545 output file name. "-d" and "-g" do not change; they do not take any
4548 ** "%source_extension" and "%header_extension" are removed, failed
4551 ** Portability fixes.
4554 * Noteworthy changes in release 1.29 (2001-09-07)
4556 ** The output file does not define const, as this caused problems when used
4557 with common autoconfiguration schemes. If you still use ancient compilers
4558 that lack const, compile with the equivalent of the C compiler option
4559 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4561 ** Added "-g" and "--graph".
4563 ** The Bison manual is now distributed under the terms of the GNU FDL.
4565 ** The input and the output files has automatically a similar extension.
4567 ** Russian translation added.
4569 ** NLS support updated; should hopefully be less troublesome.
4571 ** Added the old Bison reference card.
4573 ** Added "--locations" and "%locations".
4575 ** Added "-S" and "--skeleton".
4577 ** "%raw", "-r", "--raw" is disabled.
4579 ** Special characters are escaped when output. This solves the problems
4580 of the #line lines with path names including backslashes.
4583 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4584 "%debug", "%source_extension" and "%header_extension".
4587 Automatic location tracking.
4590 * Noteworthy changes in release 1.28 (1999-07-06)
4592 ** Should compile better now with K&R compilers.
4596 ** Fixed a problem with escaping the double quote character.
4598 ** There is now a FAQ.
4601 * Noteworthy changes in release 1.27
4603 ** The make rule which prevented bison.simple from being created on
4604 some systems has been fixed.
4607 * Noteworthy changes in release 1.26
4609 ** Bison now uses Automake.
4611 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4613 ** Token numbers now start at 257 as previously documented, not 258.
4615 ** Bison honors the TMPDIR environment variable.
4617 ** A couple of buffer overruns have been fixed.
4619 ** Problems when closing files should now be reported.
4621 ** Generated parsers should now work even on operating systems which do
4622 not provide alloca().
4625 * Noteworthy changes in release 1.25 (1995-10-16)
4627 ** Errors in the input grammar are not fatal; Bison keeps reading
4628 the grammar file, and reports all the errors found in it.
4630 ** Tokens can now be specified as multiple-character strings: for
4631 example, you could use "<=" for a token which looks like <=, instead
4632 of choosing a name like LESSEQ.
4634 ** The %token_table declaration says to write a table of tokens (names
4635 and numbers) into the parser file. The yylex function can use this
4636 table to recognize multiple-character string tokens, or for other
4639 ** The %no_lines declaration says not to generate any #line preprocessor
4640 directives in the parser file.
4642 ** The %raw declaration says to use internal Bison token numbers, not
4643 Yacc-compatible token numbers, when token names are defined as macros.
4645 ** The --no-parser option produces the parser tables without including
4646 the parser engine; a project can now use its own parser engine.
4647 The actions go into a separate file called NAME.act, in the form of
4648 a switch statement body.
4651 * Noteworthy changes in release 1.23
4653 The user can define YYPARSE_PARAM as the name of an argument to be
4654 passed into yyparse. The argument should have type void *. It should
4655 actually point to an object. Grammar actions can access the variable
4656 by casting it to the proper pointer type.
4658 Line numbers in output file corrected.
4661 * Noteworthy changes in release 1.22
4663 --help option added.
4666 * Noteworthy changes in release 1.20
4668 Output file does not redefine const for C++.
4672 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4673 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4674 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4675 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4676 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4677 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4678 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4679 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4680 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4681 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4682 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4683 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4684 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4685 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4686 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4687 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4688 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4689 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4690 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4691 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4692 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4693 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4694 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4695 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4696 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4697 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4698 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE CVE
4699 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4700 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4701 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4702 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4703 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4704 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4705 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4706 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4707 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4708 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4709 LocalWords: Vais xsltproc YYNOMEM YYLOCATION signedness YYBISON MITRE's
4710 LocalWords: libreadline YYMALLOC YYFREE MSVC redefinitions
4713 ispell-dictionary: "american"
4718 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4720 This file is part of Bison, the GNU Parser Generator.
4722 Permission is granted to copy, distribute and/or modify this document
4723 under the terms of the GNU Free Documentation License, Version 1.3 or
4724 any later version published by the Free Software Foundation; with no
4725 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4726 Texts. A copy of the license is included in the "GNU Free
4727 Documentation License" file as part of this distribution.