3 * Noteworthy changes in release ?.? (????-??-??) [?]
5 ** Backward incompatible changes
7 In conformance with the recommendations of the Graphviz team
8 (https://marc.info/?l=graphviz-devel&m=129418103126092), `-g`/`--graph`
9 now generates a *.gv file by default, instead of *.dot. A transition
12 To comply with the latest POSIX standard, in Yacc compatibility mode
13 (options `-y`/`--yacc`) Bison now generates prototypes for yyerror and
14 yylex. In some situations, this is breaking compatibility: if the user
15 has already declared these functions but with some differences (e.g., to
16 declare them as static, or to use specific attributes), the generated
17 parser will fail to compile. To disable these prototypes, #define yyerror
18 (to `yyerror`), and likewise for yylex.
20 ** Deprecated features
22 Support for the YYPRINT macro is removed. It worked only with yacc.c and
23 only for tokens. It was obsoleted by %printer, introduced in Bison 1.50
26 It has always been recommended to prefer `%define api.value.type foo` to
27 `#define YYSTYPE foo`. The latter is supported in C for compatibility
28 with Yacc, but not in C++. Warnings are now issued if `#define YYSTYPE`
29 is used in C++, and eventually support will be removed.
31 In C++ code, prefer value_type to semantic_type to denote the semantic
32 value type, which is specified by the `api.value.type` %define variable.
36 *** A skeleton for the D programming language
38 The "lalr1.d" skeleton is now officially part of Bison.
40 It was originally contributed by Oliver Mangold, based on Paolo Bonzini's
41 lalr1.java, and was improved by H. S. Teoh. Adela Vais then took over
42 maintenance and invested a lot of efforts to complete, test and document
45 It now supports all the bells and whistles of the other deterministic
46 parsers, which include: pull/push interfaces, verbose and custom error
47 messages, lookahead correction, token constructors, internationalization,
48 locations, printers, token and symbol prefixes, etc.
50 Two examples demonstrate the D parsers: a basic one (examples/d/simple),
51 and an advanced one (examples/d/calc).
53 *** Option -H, --header and directive %header
55 The option `-H`/`--header` supersedes the option `--defines`, and the
56 directive %header supersedes %defines. Both `--defines` and `%defines`
57 are, of course, maintained for backward compatibility.
61 Since version 2.4 Bison can be used to generate HTML reports. However it
62 was a two-step process: first bison must be invoked with option `--xml`,
63 and then xsltproc must be run to the convert the XML reports into HTML.
65 The new option `--html` combines these steps. The xsltproc program must
68 *** A C++ native GLR parser
70 A new version of the generated C++ GLR parser was added as "glr2.cc". It
71 is forked from the existing glr.c/cc parser, with the objective of making
72 it a more modern, truly C++ parser (instead of a C++ wrapper around a C
73 parser). Down the line, the goal is to support `%define api.value.type
74 variant` and maybe share code with lalr1.cc.
76 The current parser should be identical in terms of interface, functionality
77 and performance to "glr.cc". To try it out, simply use
81 It will eventually replace "glr.cc". However we need user feedback on
82 this skeleton. _Please_ report your results and comments about it.
86 Counterexamples now show the rule numbers, and always show ε for rules
87 with an empty right-hand side. For instance
99 *** Lookahead correction in Java
101 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
104 *** Abort parsing for memory exhaustion (C)
106 User actions may now use `YYNOMEM` (similar to `YYACCEPT` and `YYABORT`)
107 to abort the current parse with memory exhaustion.
109 *** Printing locations in debug traces (C)
111 The `YYLOCATION_PRINT(File, Loc)` macro prints a location. It is defined
112 when (i) locations are enabled, (ii) the default type for locations is
113 used, (iii) debug traces are enabled, and (iv) `YYLOCATION_PRINT` is not
116 Users may define `YYLOCATION_PRINT` to cover other cases.
120 There were no debug traces for deferred calls to user actions. They are
124 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
128 *** Reused Push Parsers
130 When a push-parser state structure is used for multiple parses, it was
131 possible for some state to leak from one run into the following one.
133 *** Fix Table Generation
135 In some very rare conditions, when there are many useless tokens, it was
136 possible to generate incorrect parsers.
139 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
143 *** Counterexample Generation
145 In some cases counterexample generation could crash. This is fixed.
147 *** Fix Table Generation
149 In some very rare conditions, when there are many useless tokens, it was
150 possible to generate incorrect parsers.
152 *** GLR parsers now support %merge together with api.value.type=union.
154 *** C++ parsers use noexcept in more places.
156 *** Generated parsers avoid some warnings about signedness issues.
158 *** C-language parsers now avoid warnings from pedantic clang.
160 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
163 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
167 *** Bug fixes in yacc.c
169 In Yacc mode, all the tokens are defined twice: once as an enum, and then
170 as a macro. YYEMPTY was missing its macro.
172 *** Bug fixes in lalr1.cc
174 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
175 even when the `parse.assert` %define variable is not enabled. It no
178 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
181 When there is a very large number of tokens, some assertions could be long
182 enough to hit arbitrary limits in Visual C++. They have been rewritten to
183 work around this limitation.
187 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
188 skeleton) used to be defined to 1. It is now defined to the version of
189 Bison as an integer (e.g., 30704 for version 3.7.4).
192 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
196 Fix concurrent build issues.
198 The bison executable is no longer linked uselessly against libreadline.
200 Fix incorrect use of yytname in glr.cc.
203 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
205 This release of Bison fixes all known bugs reported for Bison in MITRE's
206 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
207 are only about bison-the-program itself, not the generated code.
209 Although these bugs are typically irrelevant to how Bison is used, they
210 are worth fixing if only to give users peace of mind.
212 There is no known vulnerability in the generated parsers.
216 Fix concurrent build issues (introduced in Bison 3.5).
218 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
220 Fix portability issues of the test suite, and of bison itself.
222 Some unlikely crashes found by fuzzing have been fixed. This is only
223 about bison itself, not the generated parsers.
226 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
230 Crash when a token alias contains a NUL byte.
232 Portability issues with libtextstyle.
234 Portability issues of Bison itself with MSVC.
238 Improvements and fixes in the documentation.
240 More precise location about symbol type redefinitions.
243 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
245 ** Deprecated features
247 The YYPRINT macro, which works only with yacc.c and only for tokens, was
248 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
249 It is deprecated and its support will be removed eventually.
251 In conformance with the recommendations of the Graphviz team, in the next
252 version Bison the option `--graph` will generate a *.gv file by default,
253 instead of *.dot. A transition started in Bison 3.4.
257 *** Counterexample Generation
259 Contributed by Vincent Imbimbo.
261 When given `-Wcounterexamples`/`-Wcex`, bison will now output
262 counterexamples for conflicts.
264 **** Unifying Counterexamples
266 Unifying counterexamples are strings which can be parsed in two ways due
267 to the conflict. For example on a grammar that contains the usual
268 "dangling else" ambiguity:
271 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
272 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
275 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
276 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
277 Example: "if" exp "then" "if" exp "then" exp • "else" exp
280 ↳ "if" exp "then" exp
281 ↳ "if" exp "then" exp • "else" exp
282 Example: "if" exp "then" "if" exp "then" exp • "else" exp
285 ↳ "if" exp "then" exp "else" exp
286 ↳ "if" exp "then" exp •
288 When text styling is enabled, colors are used in the examples and the
289 derivations to highlight the structure of both analyses. In this case,
291 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
295 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
298 The counterexamples are "focused", in two different ways. First, they do
299 not clutter the output with all the derivations from the start symbol,
300 rather they start on the "conflicted nonterminal". They go straight to the
301 point. Second, they don't "expand" nonterminal symbols uselessly.
303 **** Nonunifying Counterexamples
305 In the case of the dangling else, Bison found an example that can be
306 parsed in two ways (therefore proving that the grammar is ambiguous).
307 When it cannot find such an example, it instead generates two examples
308 that are the same up until the dot:
311 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
312 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
313 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
318 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
319 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
320 First example: expr • ID ',' ID $end
327 Second example: expr • ID $end
333 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
337 In these cases, the parser usually doesn't have enough lookahead to
338 differentiate the two given examples.
342 Counterexamples are also included in the report when given
343 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
348 1 exp: "if" exp "then" exp • [$end, "then", "else"]
349 2 | "if" exp "then" exp • "else" exp
351 "else" shift, and go to state 8
353 "else" [reduce using rule 1 (exp)]
354 $default reduce using rule 1 (exp)
356 shift/reduce conflict on token "else":
357 1 exp: "if" exp "then" exp •
358 2 exp: "if" exp "then" exp • "else" exp
359 Example: "if" exp "then" "if" exp "then" exp • "else" exp
362 ↳ "if" exp "then" exp
363 ↳ "if" exp "then" exp • "else" exp
364 Example: "if" exp "then" "if" exp "then" exp • "else" exp
367 ↳ "if" exp "then" exp "else" exp
368 ↳ "if" exp "then" exp •
370 *** File prefix mapping
372 Contributed by Joshua Watt.
374 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
375 in the output (specifically `#line` directives and `#ifdef` header guards)
376 that begins with the prefix OLD will have it replaced with the prefix NEW,
377 similar to the `-ffile-prefix-map` in GCC. This option can be used to
378 make bison output reproducible.
384 When text styling is enabled and the terminal supports it, the warnings
385 now include hyperlinks to the documentation.
387 *** Relocatable installation
389 When installed to be relocatable (via `configure --enable-relocatable`),
390 bison will now also look for a relocated m4.
394 The `filename_type` %define variable was renamed `api.filename.type`.
397 %define filename_type "symbol"
401 %define api.filename.type {symbol}
403 (Or let `bison --update` do it for you).
405 It now defaults to `const std::string` instead of `std::string`.
407 *** Deprecated %define variable names
409 The following variables have been renamed for consistency. Backward
410 compatibility is ensured, but upgrading is recommended.
412 filename_type -> api.filename.type
413 package -> api.package
415 *** Push parsers no longer clear their state when parsing is finished
417 Previously push-parsers cleared their state when parsing was finished (on
418 success and on failure). This made it impossible to check if there were
419 parse errors, since `yynerrs` was also reset. This can be especially
420 troublesome when used in autocompletion, since a parser with error
421 recovery would suggest (irrelevant) expected tokens even if there were
424 Now the parser state can be examined when parsing is finished. The parser
425 state is reset when starting a new parse.
431 The bistromathic demonstrates %param and how to quote sources in the error
435 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
441 *** Include the generated header (yacc.c)
443 Historically, when --defines was used, bison generated a header and pasted
444 an exact copy of it into the generated parser implementation file. Since
445 Bison 3.4 it is possible to specify that the header should be `#include`d,
446 and how. For instance
448 %define api.header.include {"parse.h"}
452 %define api.header.include {<parser/parse.h>}
454 Now api.header.include defaults to `"header-basename"`, as was intended in
455 Bison 3.4, where `header-basename` is the basename of the generated
456 header. This is disabled when the generated header is `y.tab.h`, to
457 comply with Automake's ylwrap.
459 *** String aliases are faithfully propagated
461 Bison used to interpret user strings (i.e., decoding backslash escapes)
462 when reading them, and to escape them (i.e., issue non-printable
463 characters as backslash escapes, taking the locale into account) when
464 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
465 up "ciphered" as sequences of backslash escapes. This happened not only
466 in the generated sources (where the compiler will reinterpret them), but
467 also in all the generated reports (text, xml, html, dot, etc.). Reports
468 were therefore not readable when string aliases were not pure ASCII.
469 Worse yet: the output depended on the user's locale.
471 Now Bison faithfully treats the string aliases exactly the way the user
472 spelled them. This fixes all the aforementioned problems. However, now,
473 string aliases semantically equivalent but syntactically different (e.g.,
474 "A", "\x41", "\101") are considered to be different.
476 *** Crash when generating IELR
478 An old, well hidden, bug in the generation of IELR parsers was fixed.
481 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
485 In glr.cc some internal macros leaked in the user's code, and could damage
486 access to the token kinds.
489 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
493 Incorrect comments in the generated parsers.
495 Warnings in push parsers (yacc.c).
497 Incorrect display of gotos in LAC traces (lalr1.cc).
500 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
504 Some tests were fixed.
506 When token aliases contain comment delimiters:
508 %token FOO "/* foo */"
510 bison used to emit "nested" comments, which is invalid C.
513 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
517 Restored ANSI-C compliance in yacc.c.
519 GNU readline portability issues.
521 In C++, yy::parser::symbol_name is now a public member, as was intended.
525 In C++, yy::parser::symbol_type now has a public name() member function.
528 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
530 ** Backward incompatible changes
532 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
534 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
535 depend on it will now produce Yacc-like error messages (just "syntax
536 error"). It was superseded by the "%error-verbose" directive in Bison
537 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
538 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
539 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
540 parse.error verbose".
542 ** Deprecated features
544 The YYPRINT macro, which works only with yacc.c and only for tokens, was
545 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
546 It is deprecated and its support will be removed eventually.
550 *** Improved syntax error messages
552 Two new values for the %define parse.error variable offer more control to
553 the user. Available in all the skeletons (C, C++, Java).
555 **** %define parse.error detailed
557 The behavior of "%define parse.error detailed" is closely resembling that
558 of "%define parse.error verbose" with a few exceptions. First, it is safe
559 to use non-ASCII characters in token aliases (with 'verbose', the result
560 depends on the locale with which bison was run). Second, a yysymbol_name
561 function is exposed to the user, instead of the yytnamerr function and the
562 yytname table. Third, token internationalization is supported (see
565 **** %define parse.error custom
567 With this directive, the user forges and emits the syntax error message
568 herself by defining the yyreport_syntax_error function. A new type,
569 yypcontext_t, captures the circumstances of the error, and provides the
570 user with functions to get details, such as yypcontext_expected_tokens to
571 get the list of expected token kinds.
573 A possible implementation of yyreport_syntax_error is:
576 yyreport_syntax_error (const yypcontext_t *ctx)
579 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
580 fprintf (stderr, ": syntax error");
581 // Report the tokens expected at this point.
583 enum { TOKENMAX = 10 };
584 yysymbol_kind_t expected[TOKENMAX];
585 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
587 // Forward errors to yyparse.
590 for (int i = 0; i < n; ++i)
591 fprintf (stderr, "%s %s",
592 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
594 // Report the unexpected token.
596 yysymbol_kind_t lookahead = yypcontext_token (ctx);
597 if (lookahead != YYSYMBOL_YYEMPTY)
598 fprintf (stderr, " before %s", yysymbol_name (lookahead));
600 fprintf (stderr, "\n");
604 **** Token aliases internationalization
606 When the %define variable parse.error is set to `custom` or `detailed`,
607 one may specify which token aliases are to be translated using _(). For
619 In that case the user must define _() and N_(), and yysymbol_name returns
620 the translated symbol (i.e., it returns '_("variable")' rather that
621 '"variable"'). In Java, the user must provide an i18n() function.
623 *** List of expected tokens (yacc.c)
625 Push parsers may invoke yypstate_expected_tokens at any point during
626 parsing (including even before submitting the first token) to get the list
627 of possible tokens. This feature can be used to propose autocompletion
628 (see below the "bistromathic" example).
630 It makes little sense to use this feature without enabling LAC (lookahead
633 *** Returning the error token
635 When the scanner returns an invalid token or the undefined token
636 (YYUNDEF), the parser generates an error message and enters error
637 recovery. Because of that error message, most scanners that find lexical
638 errors generate an error message, and then ignore the invalid input
639 without entering the error-recovery.
641 The scanners may now return YYerror, the error token, to enter the
642 error-recovery mode without triggering an additional error message. See
643 the bistromathic for an example.
645 *** Deep overhaul of the symbol and token kinds
647 To avoid the confusion with types in programming languages, we now refer
648 to token and symbol "kinds" instead of token and symbol "types". The
649 documentation and error messages have been revised.
651 All the skeletons have been updated to use dedicated enum types rather
652 than integral types. Special symbols are now regular citizens, instead of
653 being declared in ad hoc ways.
657 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
658 LPAREN, etc. While backward compatibility is of course ensured, users are
659 nonetheless invited to replace their uses of "enum yytokentype" by
662 This type now also includes tokens that were previously hidden: YYEOF (end
663 of input), YYUNDEF (undefined token), and YYerror (error token). They
664 now have string aliases, internationalized when internationalization is
665 enabled. Therefore, by default, error messages now refer to "end of file"
666 (internationalized) rather than the cryptic "$end", or to "invalid token"
667 rather than "$undefined".
669 Therefore in most cases it is now useless to define the end-of-line token
672 %token T_EOF 0 "end of file"
674 Rather simply use "YYEOF" in your scanner.
678 The "symbol kinds" is what the parser actually uses. (Unless the
679 api.token.raw %define variable is used, the symbol kind of a terminal
680 differs from the corresponding token kind.)
682 They are now exposed as a enum, "yysymbol_kind_t".
684 This allows users to tailor the error messages the way they want, or to
685 process some symbols in a specific way in autocompletion (see the
686 bistromathic example below).
688 *** Modernize display of explanatory statements in diagnostics
690 Since Bison 2.7, output was indented four spaces for explanatory
691 statements. For example:
693 input.y:2.7-13: error: %type redeclaration for exp
694 input.y:1.7-11: previous declaration
696 Since the introduction of caret-diagnostics, it became less clear. This
697 indentation has been removed and submessages are displayed similarly as in
700 input.y:2.7-13: error: %type redeclaration for exp
701 2 | %type <float> exp
703 input.y:1.7-11: note: previous declaration
707 Contributed by Victor Morales Cayuela.
711 The token and symbol kinds are yy::parser::token_kind_type and
712 yy::parser::symbol_kind_type.
714 The symbol_type::kind() member function allows to get the kind of a
715 symbol. This can be used to write unit tests for scanners, e.g.,
717 yy::parser::symbol_type t = make_NUMBER ("123");
718 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
719 assert (t.value.as<int> () == 123);
725 In order to avoid ambiguities with "type" as in "typing", we now refer to
726 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
727 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
732 There are now examples/java: a very simple calculator, and a more complete
733 one (push-parser, location tracking, and debug traces).
735 The lexcalc example (a simple example in C based on Flex and Bison) now
736 also demonstrates location tracking.
739 A new C example, bistromathic, is a fully featured interactive calculator
740 using many Bison features: pure interface, push parser, autocompletion
741 based on the current parser state (using yypstate_expected_tokens),
742 location tracking, internationalized custom error messages, lookahead
743 correction, rich debug traces, etc.
745 It shows how to depend on the symbol kinds to tailor autocompletion. For
746 instance it recognizes the symbol kind "VARIABLE" to propose
747 autocompletion on the existing variables, rather than of the word
751 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
753 ** WARNING: Future backward-incompatibilities!
755 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
757 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
758 that still depend on it will produce Yacc-like error messages (just
759 "syntax error"). It was superseded by the "%error-verbose" directive in
760 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
761 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
762 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
763 parse.error verbose".
767 Fix portability issues of the package itself on old compilers.
769 Fix api.token.raw support in Java.
772 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
776 Error messages could quote lines containing zero-width characters (such as
777 \005) with incorrect styling. Fixes for similar issues with unexpectedly
778 short lines (e.g., the file was changed between parsing and diagnosing).
780 Some unlikely crashes found by fuzzing have been fixed. This is only
781 about bison itself, not the generated parsers.
784 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
788 Portability issues and minor cosmetic issues.
790 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
794 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
800 Fix compiler warnings.
803 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
805 ** Backward incompatible changes
807 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
808 longer treated as end-of-lines. This changes the diagnostics, and in
809 particular their locations.
811 In C++, line numbers and columns are now represented as 'int' not
812 'unsigned', so that integer overflow on positions is easily checkable via
813 'gcc -fsanitize=undefined' and the like. This affects the API for
814 positions. The default position and location classes now expose
815 'counter_type' (int), used to define line and column numbers.
817 ** Deprecated features
819 The YYPRINT macro, which works only with yacc.c and only for tokens, was
820 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
821 It is deprecated and its support will be removed eventually.
825 *** Lookahead correction in C++
827 Contributed by Adrian Vogelsgesang.
829 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
830 %define variable parse.lac.
832 *** Variable api.token.raw: Optimized token numbers (all skeletons)
834 In the generated parsers, tokens have two numbers: the "external" token
835 number as returned by yylex (which starts at 257), and the "internal"
836 symbol number (which starts at 3). Each time yylex is called, a table
837 lookup maps the external token number to the internal symbol number.
839 When the %define variable api.token.raw is set, tokens are assigned their
840 internal number, which saves one table lookup per token, and also saves
841 the generation of the mapping table.
843 The gain is typically moderate, but in extreme cases (very simple user
844 actions), a 10% improvement can be observed.
846 *** Generated parsers use better types for states
848 Stacks now use the best integral type for state numbers, instead of always
849 using 15 bits. As a result "small" parsers now have a smaller memory
850 footprint (they use 8 bits), and there is support for large automata (16
851 bits), and extra large (using int, i.e., typically 31 bits).
853 *** Generated parsers prefer signed integer types
855 Bison skeletons now prefer signed to unsigned integer types when either
856 will do, as the signed types are less error-prone and allow for better
857 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
858 portable to unusual machines where char, short and int are all the same
859 width. On non-GNU platforms this may entail including <limits.h> and (if
860 available) <stdint.h> to define integer types and constants.
862 *** A skeleton for the D programming language
864 For the last few releases, Bison has shipped a stealth experimental
865 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
866 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
869 However, because nobody has committed to improving, testing, and
870 documenting this skeleton, it is not clear that it will be supported in
873 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
874 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
876 *** Debug traces in Java
878 The Java backend no longer emits code and data for parser tracing if the
879 %define variable parse.trace is not defined.
883 *** New diagnostic: -Wdangling-alias
885 String literals, which allow for better error messages, are (too)
886 liberally accepted by Bison, which might result in silent errors. For
889 %type <exVal> cond "condition"
891 does not define "condition" as a string alias to 'cond' (nonterminal
892 symbols do not have string aliases). It is rather equivalent to
895 %token <exVal> "condition"
897 i.e., it gives the type 'exVal' to the "condition" token, which was
898 clearly not the intention.
900 Also, because string aliases need not be defined, typos such as "baz"
901 instead of "bar" will be not reported.
903 The option `-Wdangling-alias` catches these situations. On
906 %type <ival> foo "foo"
910 bison -Wdangling-alias reports
912 warning: string literal not attached to a symbol
913 | %type <ival> foo "foo"
915 warning: string literal not attached to a symbol
919 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
921 *** Better POSIX Yacc compatibility diagnostics
923 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
927 %type <ival> TOKEN1 TOKEN2 't'
934 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
935 2 | %type <ival> TOKEN1 TOKEN2 't'
937 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
938 2 | %type <ival> TOKEN1 TOKEN2 't'
940 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
941 2 | %type <ival> TOKEN1 TOKEN2 't'
944 *** Diagnostics with insertion
946 The diagnostics now display the suggestion below the underlined source.
947 Replacement for undeclared symbols are now also suggested.
954 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'?
958 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
962 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
964 *** Diagnostics about long lines
966 Quoted sources may now be truncated to fit the screen. For instance, on a
967 30-column wide terminal:
974 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
977 foo.y:1.8-10: previous declaration
980 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
983 foo.y:1.8-10: previous declaration
989 *** Debugging glr.c and glr.cc
991 The glr.c skeleton always had asserts to check its own behavior (not the
992 user's). These assertions are now under the control of the parse.assert
993 %define variable (disabled by default).
997 Several new compiler warnings in the generated output have been avoided.
998 Some unused features are no longer emitted. Cleaner generated code in
1003 Portability issues in the test suite.
1005 In theory, parsers using %nonassoc could crash when reporting verbose
1006 error messages. This unlikely bug has been fixed.
1008 In Java, %define api.prefix was ignored. It now behaves as expected.
1011 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
1015 In some cases, when warnings are disabled, bison could emit tons of white
1016 spaces as diagnostics.
1018 When running out of memory, bison could crash (found by fuzzing).
1020 When defining twice the EOF token, bison would crash.
1022 New warnings from recent compilers have been addressed in the generated
1023 parsers (yacc.c, glr.c, glr.cc).
1025 When lone carriage-return characters appeared in the input file,
1026 diagnostics could hang forever.
1029 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
1036 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
1038 ** Deprecated features
1040 The %pure-parser directive is deprecated in favor of '%define api.pure'
1041 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
1042 now. Note that since Bison 2.7 you are strongly encouraged to use
1043 '%define api.pure full' instead of '%define api.pure'.
1047 *** Colored diagnostics
1049 As an experimental feature, diagnostics are now colored, controlled by the
1050 new options --color and --style.
1052 To use them, install the libtextstyle library before configuring Bison.
1053 It is available from
1055 https://alpha.gnu.org/gnu/gettext/
1059 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
1061 The option --color supports the following arguments:
1062 - always, yes: Enable colors.
1063 - never, no: Disable colors.
1064 - auto, tty (default): Enable colors if the output device is a tty.
1066 To customize the styles, create a CSS file similar to
1070 .error { font-weight: 800; text-decoration: underline; }
1073 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1074 environment variable to "bison-bw.css".
1076 *** Disabling output
1078 When given -fsyntax-only, the diagnostics are reported, but no output is
1081 The name of this option is somewhat misleading as bison does more than
1082 just checking the syntax: every stage is run (including checking for
1083 conflicts for instance), except the generation of the output files.
1085 *** Include the generated header (yacc.c)
1087 Before, when --defines is used, bison generated a header, and pasted an
1088 exact copy of it into the generated parser implementation file. If the
1089 header name is not "y.tab.h", it is now #included instead of being
1092 To use an '#include' even if the header name is "y.tab.h" (which is what
1093 happens with --yacc, or when using the Autotools' ylwrap), define
1094 api.header.include to the exact argument to pass to #include. For
1097 %define api.header.include {"parse.h"}
1101 %define api.header.include {<parser/parse.h>}
1103 *** api.location.type is now supported in C (yacc.c, glr.c)
1105 The %define variable api.location.type defines the name of the type to use
1106 for locations. When defined, Bison no longer defines YYLTYPE.
1108 This can be used in programs with several parsers to factor their
1109 definition of locations: let one of them generate them, and the others
1116 In conformance with the recommendations of the Graphviz team, if %require
1117 "3.4" (or better) is specified, the option --graph generates a *.gv file
1118 by default, instead of *.dot.
1120 *** Diagnostics overhaul
1122 Column numbers were wrong with multibyte characters, which would also
1123 result in skewed diagnostics with carets. Beside, because we were
1124 indenting the quoted source with a single space, lines with tab characters
1125 were incorrectly underlined.
1127 To address these issues, and to be clearer, Bison now issues diagnostics
1128 as GCC9 does. For instance it used to display (there's a tab before the
1131 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1132 expr: expr '+' "number" { $$ = $1 + $2; }
1136 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1137 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1140 Other constructs now also have better locations, resulting in more precise
1143 *** Fix-it hints for %empty
1145 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1146 annotations, and add the missing ones.
1148 *** Generated reports
1150 The format of the reports (parse.output) was improved for readability.
1152 *** Better support for --no-line.
1154 When --no-line is used, the generated files are now cleaner: no lines are
1155 generated instead of empty lines. Together with using api.header.include,
1156 that should help people saving the generated files into version control
1157 systems get smaller diffs.
1161 A new example in C shows an simple infix calculator with a hand-written
1162 scanner (examples/c/calc).
1164 A new example in C shows a reentrant parser (capable of recursive calls)
1165 built with Flex and Bison (examples/c/reccalc).
1167 There is a new section about the history of Yaccs and Bison.
1171 A few obscure bugs were fixed, including the second oldest (known) bug in
1172 Bison: it was there when Bison was entered in the RCS version control
1173 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1177 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1181 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1185 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1189 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1190 extensions into errors. It now makes them simple warnings (-Wyacc).
1193 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1195 A new mailing list was created, Bison Announce. It is low traffic, and is
1196 only about announcing new releases and important messages (e.g., polls
1197 about major decisions to make).
1199 https://lists.gnu.org/mailman/listinfo/bison-announce
1201 ** Backward incompatible changes
1203 Support for DJGPP, which has been unmaintained and untested for years, is
1206 ** Deprecated features
1208 A new feature, --update (see below) helps adjusting existing grammars to
1211 *** Deprecated directives
1213 The %error-verbose directive is deprecated in favor of '%define
1214 parse.error verbose' since Bison 3.0, but no warning was issued.
1216 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1217 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1218 directives are slightly different, you might need to adjust your code.
1219 %name-prefix renames only symbols with external linkage, while api.prefix
1220 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1221 yytokentype, YYSTYPE, YYLTYPE, etc.
1223 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1224 {xx}' will typically have to update YY_DECL from
1226 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1230 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1232 *** Deprecated %define variable names
1234 The following variables, mostly related to parsers in Java, have been
1235 renamed for consistency. Backward compatibility is ensured, but upgrading
1238 abstract -> api.parser.abstract
1239 annotations -> api.parser.annotations
1240 extends -> api.parser.extends
1241 final -> api.parser.final
1242 implements -> api.parser.implements
1243 parser_class_name -> api.parser.class
1244 public -> api.parser.public
1245 strictfp -> api.parser.strictfp
1249 *** Generation of fix-its for IDEs/Editors
1251 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1252 bison now generates machine readable editing instructions to fix some
1253 issues. Currently, this is mostly limited to updating deprecated
1254 directives and removing duplicates. For instance:
1258 %define parser_class_name "Parser"
1259 %define api.parser.class "Parser"
1263 See the "fix-it:" lines below:
1265 $ bison -ffixit foo.y
1266 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1269 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1270 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1271 %define parser_class_name "Parser"
1272 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1273 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1274 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1275 %define api.parser.class "Parser"
1276 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1277 foo.y:2.1-34: previous definition
1278 %define parser_class_name "Parser"
1279 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1280 fix-it:"foo.y":{3:1-3:34}:""
1281 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1283 This uses the same output format as GCC and Clang.
1285 *** Updating grammar files
1287 Fixes can be applied on the fly. The previous example ends with the
1288 suggestion to re-run bison with the option -u/--update, which results in a
1289 cleaner grammar file.
1291 $ bison --update foo.y
1293 bison: file 'foo.y' was updated (backup: 'foo.y~')
1296 %define parse.error verbose
1297 %define api.parser.class {Parser}
1301 *** Bison is now relocatable
1303 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1305 A relocatable program can be moved or copied to a different location on
1306 the file system. It can also be used through mount points for network
1307 sharing. It is possible to make symbolic links to the installed and moved
1308 programs, and invoke them through the symbolic link.
1310 *** %expect and %expect-rr modifiers on individual rules
1312 One can now document (and check) which rules participate in shift/reduce
1313 and reduce/reduce conflicts. This is particularly important GLR parsers,
1314 where conflicts are a normal occurrence. For example,
1330 | argument_list ',' expression
1335 Looking at the output from -v, one can see that the shift/reduce conflict
1336 here is due to the fact that the parser does not know whether to reduce
1337 arguments to argument_list until it sees the token _after_ the following
1338 ','. By marking the rule with %expect 1 (because there is a conflict in
1339 one state), we document the source of the 1 overall shift/reduce conflict.
1341 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1342 conflicts. In this case, we mark each of the conflicting rules. For
1351 target_list '=' expr ';'
1357 | target ',' target_list
1366 | expr ',' expr_list
1374 In a statement such as
1378 the parser must reduce x to a target or an expr, but does not know which
1379 until it sees the '='. So we notate the two possible reductions to
1380 indicate that each conflicts in one rule.
1382 This feature needs user feedback, and might evolve in the future.
1384 *** C++: Actual token constructors
1386 When variants and token constructors are enabled, in addition to the
1387 type-safe named token constructors (make_ID, make_INT, etc.), we now
1388 generate genuine constructors for symbol_type.
1390 For instance with these declarations
1396 you may use these constructors:
1398 symbol_type (int token, const std::string&);
1399 symbol_type (int token, const int&);
1400 symbol_type (int token);
1402 Correct matching between token types and value types is checked via
1403 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1404 constructors are preferable, as they offer better type safety (for
1405 instance 'make_ID (42)' would not even compile), but symbol_type
1406 constructors may help when token types are discovered at run-time, e.g.,
1409 if (auto i = lookup_keyword (yytext))
1410 return yy::parser::symbol_type (i);
1412 return yy::parser::make_ID (yytext);
1415 *** C++: Variadic emplace
1417 If your application requires C++11 and you don't use symbol constructors,
1418 you may now use a variadic emplace for semantic values:
1420 %define api.value.type variant
1421 %token <std::pair<int, int>> PAIR
1425 int yylex (parser::semantic_type *lvalp)
1427 lvalp->emplace <std::pair<int, int>> (1, 2);
1428 return parser::token::PAIR;
1431 *** C++: Syntax error exceptions in GLR
1433 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1434 actions, or from the scanner.
1436 *** More POSIX Yacc compatibility warnings
1438 More Bison specific directives are now reported with -y or -Wyacc. This
1439 change was ready since the release of Bison 3.0 in September 2015. It was
1440 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1441 in numerous warnings for Bison users that use the GNU Build System.
1443 If you still experience that problem, either redefine YACC as `bison -o
1444 y.tab.c`, or pass -Wno-yacc to Bison.
1446 *** The tables yyrhs and yyphrs are back
1448 Because no Bison skeleton uses them, these tables were removed (no longer
1449 passed to the skeletons, not even computed) in 2008. However, some users
1450 have expressed interest in being able to use them in their own skeletons.
1454 *** Incorrect number of reduce/reduce conflicts
1456 On a grammar such as
1458 exp: "num" | "num" | "num"
1460 bison used to report a single RR conflict, instead of two. This is now
1461 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1462 was entered in the RCS version control system, in December 1987.
1464 Some grammar files might have to adjust their %expect-rr.
1466 *** Parser directives that were not careful enough
1468 Passing invalid arguments to %nterm, for instance character literals, used
1469 to result in unclear error messages.
1473 The examples/ directory (installed in .../share/doc/bison/examples) has
1474 been restructured per language for clarity. The examples come with a
1475 README and a Makefile. Not only can they be used to toy with Bison, they
1476 can also be starting points for your own grammars.
1478 There is now a Java example, and a simple example in C based on Flex and
1479 Bison (examples/c/lexcalc/).
1485 They now use noexcept and constexpr. Please, report missing annotations.
1487 *** Symbol Declarations
1489 The syntax of the variation directives to declare symbols was overhauled
1490 for more consistency, and also better POSIX Yacc compliance (which, for
1491 instance, allows "%type" without actually providing a type). The %nterm
1492 directive, supported by Bison since its inception, is now documented and
1493 officially supported.
1495 The syntax is now as follows:
1497 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1498 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1499 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1500 %nterm TAG? ID+ ( TAG ID+ )*
1502 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1503 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1504 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1505 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1506 one), ‘*’ (zero or more) and ‘+’ (one or more).
1509 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1513 Fix the move constructor of symbol_type.
1515 Always provide a copy constructor for symbol_type, even in modern C++.
1518 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1522 Properly support token constructors in C++ with types that include commas
1523 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1526 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1530 C++ portability issues.
1533 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1537 Several portability issues have been fixed in the build system, in the
1538 test suite, and in the generated parsers in C++.
1541 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1543 ** Backward incompatible changes
1545 Support for DJGPP, which has been unmaintained and untested for years, is
1546 obsolete. Unless there is activity to revive it, it will be removed.
1550 %printers should use yyo rather than yyoutput to denote the output stream.
1552 Variant-based symbols in C++ should use emplace() rather than build().
1554 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1558 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1560 A comment in the generated code now emphasizes that users should not
1561 depend upon non-documented implementation details, such as macros starting
1566 *** C++: Support for move semantics (lalr1.cc)
1568 The lalr1.cc skeleton now fully supports C++ move semantics, while
1569 maintaining compatibility with C++98. You may now store move-only types
1570 when using Bison's variants. For instance:
1577 %skeleton "lalr1.cc"
1578 %define api.value.type variant
1582 %token <int> INT "int";
1583 %type <std::unique_ptr<int>> int;
1584 %type <std::vector<std::unique_ptr<int>>> list;
1588 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1590 int: "int" { $$ = std::make_unique<int>($1); }
1592 *** C++: Implicit move of right-hand side values (lalr1.cc)
1594 In modern C++ (C++11 and later), you should always use 'std::move' with
1595 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1596 popped from the stack anyway. Using 'std::move' is mandatory for
1597 move-only types such as unique_ptr, and it provides a significant speedup
1598 for large types such as std::string, or std::vector, etc.
1600 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1601 by 'std::move ($n)'. The second rule in the previous grammar can be
1604 list: list int { $$ = $1; $$.emplace_back($2); }
1606 With automove enabled, the semantic values are no longer lvalues, so do
1607 not use the swap idiom:
1609 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1611 This idiom is anyway obsolete: it is preferable to move than to swap.
1613 A warning is issued when automove is enabled, and a value is used several
1616 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1617 exp: "twice" exp { $$ = $2 + $2; }
1620 Enabling api.value.automove does not require support for modern C++. The
1621 generated code is valid C++98/03, but will use copies instead of moves.
1623 The new examples/c++/variant-11.yy shows these features in action.
1625 *** C++: The implicit default semantic action is always run
1627 When variants are enabled, the default action was not run, so
1635 It now behaves like in all the other cases, as
1637 exp: "number" { $$ = $1; }
1639 possibly using std::move if automove is enabled.
1641 We do not expect backward compatibility issues. However, beware of
1642 forward compatibility issues: if you rely on default actions with
1643 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1644 generate incorrect parsers.
1646 *** C++: Renaming location.hh
1648 When both %defines and %locations are enabled, Bison generates a
1649 location.hh file. If you don't use locations outside of the parser, you
1650 may avoid its creation with:
1652 %define api.location.file none
1654 However this file is useful if, for instance, your parser builds an AST
1655 decorated with locations: you may use Bison's location independently of
1656 Bison's parser. You can now give it another name, for instance:
1658 %define api.location.file "my-location.hh"
1660 This name can have directory components, and even be absolute. The name
1661 under which the location file is included is controlled by
1662 api.location.include.
1664 This way it is possible to have several parsers share the same location
1667 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1670 %define api.namespace {foo}
1671 %define api.location.file "include/ast/loc.hh"
1672 %define api.location.include {<ast/loc.hh>}
1674 and use it in src/bar/parser.hh:
1677 %define api.namespace {bar}
1678 %code requires {#include <ast/loc.hh>}
1679 %define api.location.type {bar::location}
1681 Absolute file names are supported, so in your Makefile, passing the flag
1682 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1685 *** C++: stack.hh and position.hh are deprecated
1687 When asked to generate a header file (%defines), the lalr1.cc skeleton
1688 generates a stack.hh file. This file had no interest for users; it is now
1689 made useless: its content is included in the parser definition. It is
1690 still generated for backward compatibility.
1692 When in addition to %defines, location support is requested (%locations),
1693 the file position.hh is also generated. It is now also useless: its
1694 content is now included in location.hh.
1696 These files are no longer generated when your grammar file requires at
1697 least Bison 3.2 (%require "3.2").
1701 Portability issues on MinGW and VS2015.
1703 Portability issues in the test suite.
1705 Portability/warning issues with Flex.
1708 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1710 ** Backward incompatible changes
1712 Compiling Bison now requires a C99 compiler---as announced during the
1713 release of Bison 3.0, five years ago. Generated parsers do not require a
1716 Support for DJGPP, which has been unmaintained and untested for years, is
1717 obsolete. Unless there is activity to revive it, the next release of Bison
1718 will have it removed.
1722 *** Typed midrule actions
1724 Because their type is unknown to Bison, the values of midrule actions are
1725 not treated like the others: they don't have %printer and %destructor
1726 support. It also prevents C++ (Bison) variants to handle them properly.
1728 Typed midrule actions address these issues. Instead of:
1730 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1734 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1736 *** Reports include the type of the symbols
1738 The sections about terminal and nonterminal symbols of the '*.output' file
1739 now specify their declared type. For instance, for:
1743 the report now shows '<ival>':
1745 Terminals, with rules where they appear
1749 *** Diagnostics about useless rules
1751 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1752 of course, its rules are useless too.
1756 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1758 Previously all the useless rules were reported, including those whose
1759 left-hand side is the 'exp' nonterminal:
1761 warning: 1 nonterminal useless in grammar [-Wother]
1762 warning: 4 rules useless in grammar [-Wother]
1763 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1766 2.14-16: warning: rule useless in grammar [-Wother]
1769 3.6-16: warning: rule useless in grammar [-Wother]
1770 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1772 3.20-30: warning: rule useless in grammar [-Wother]
1773 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1775 3.34-44: warning: rule useless in grammar [-Wother]
1776 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1779 Now, rules whose left-hand side symbol is useless are no longer reported
1780 as useless. The locations of the errors have also been adjusted to point
1781 to the first use of the nonterminal as a left-hand side of a rule:
1783 warning: 1 nonterminal useless in grammar [-Wother]
1784 warning: 4 rules useless in grammar [-Wother]
1785 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1786 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1788 2.14-16: warning: rule useless in grammar [-Wother]
1792 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1794 When compiled with exceptions disabled, the generated parsers no longer
1795 uses try/catch clauses.
1797 Currently only GCC and Clang are supported.
1801 *** A demonstration of variants
1803 A new example was added (installed in .../share/doc/bison/examples),
1804 'variant.yy', which shows how to use (Bison) variants in C++.
1806 The other examples were made nicer to read.
1808 *** Some features are no longer 'experimental'
1810 The following features, mature enough, are no longer flagged as
1811 experimental in the documentation: push parsers, default %printer and
1812 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1813 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1814 semantic predicates (%?).
1818 *** GLR: Predicates support broken by #line directives
1820 Predicates (%?) in GLR such as
1823 %? {new_syntax} 'w' id new_args
1824 | %?{!new_syntax} 'w' id old_args
1826 were issued with #lines in the middle of C code.
1828 *** Printer and destructor with broken #line directives
1830 The #line directives were not properly escaped when emitting the code for
1831 %printer/%destructor, which resulted in compiler errors if there are
1832 backslashes or double-quotes in the grammar file name.
1834 *** Portability on ICC
1836 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1837 Generated parsers now work around this.
1841 There were several small fixes in the test suite and in the build system,
1842 many warnings in bison and in the generated parsers were eliminated. The
1843 documentation also received its share of minor improvements.
1845 Useless code was removed from C++ parsers, and some of the generated
1846 constructors are more 'natural'.
1849 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1853 *** C++: Fix support of 'syntax_error'
1855 One incorrect 'inline' resulted in linking errors about the constructor of
1856 the syntax_error exception.
1858 *** C++: Fix warnings
1860 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1861 warnings about yyformat being possibly null. It also warned about the
1862 deprecated implicit definition of copy constructors when there's a
1863 user-defined (copy) assignment operator.
1865 *** Location of errors
1867 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1868 ride-hand side raises a syntax error. The behavior of the default parser
1869 (yacc.c) in such a condition was undefined.
1871 Now all the parsers match the behavior of glr.c: @$ is used as the
1872 location of the error. This handles gracefully rules with and without
1875 *** Portability fixes in the test suite
1877 On some platforms, some Java and/or C++ tests were failing.
1880 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1884 *** C++ with Variants (lalr1.cc)
1886 Fix a compiler warning when no %destructor use $$.
1890 Several portability issues in tests were fixed.
1893 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1897 *** C++ with Variants (lalr1.cc)
1899 Problems with %destructor and '%define parse.assert' have been fixed.
1901 *** Named %union support (yacc.c, glr.c)
1903 Bison 3.0 introduced a regression on named %union such as
1905 %union foo { int ival; };
1907 The possibility to use a name was introduced "for Yacc compatibility".
1908 It is however not required by POSIX Yacc, and its usefulness is not clear.
1910 *** %define api.value.type union with %defines (yacc.c, glr.c)
1912 The C parsers were broken when %defines was used together with "%define
1913 api.value.type union".
1915 *** Redeclarations are reported in proper order
1923 bison used to report:
1925 foo.yy:2.10-11: error: %printer redeclaration for FOO
1928 foo.yy:3.10-11: previous declaration
1932 Now, the "previous" declaration is always the first one.
1937 Bison now installs various files in its docdir (which defaults to
1938 '/usr/local/share/doc/bison'), including the three fully blown examples
1939 extracted from the documentation:
1942 Reverse Polish Calculator, a simple introductory example.
1944 Multi-function Calc, a calculator with memory and functions and located
1947 a calculator in C++ using variant support and token constructors.
1950 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1954 *** Generated source files when errors are reported
1956 When warnings are issued and -Werror is set, bison would still generate
1957 the source files (*.c, *.h...). As a consequence, some runs of "make"
1958 could fail the first time, but not the second (as the files were generated
1961 This is fixed: bison no longer generates this source files, but, of
1962 course, still produces the various reports (*.output, *.xml, etc.).
1964 *** %empty is used in reports
1966 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1967 dot, XML and formats derived from it).
1969 *** YYERROR and variants
1971 When C++ variant support is enabled, an error triggered via YYERROR, but
1972 not caught via error recovery, resulted in a double deletion.
1975 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1979 *** Errors in caret diagnostics
1981 On some platforms, some errors could result in endless diagnostics.
1983 *** Fixes of the -Werror option
1985 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1986 diagnostics into errors instead of warnings. This is fixed.
1988 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1989 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1990 -Wno-error", "foo" diagnostics are now errors.
1994 As demonstrated in the documentation, one can now leave spaces between
1999 The yacc.1 man page is no longer installed if --disable-yacc was
2002 *** Fixes in the test suite
2004 Bugs and portability issues.
2007 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
2009 ** WARNING: Future backward-incompatibilities!
2011 Like other GNU packages, Bison will start using some of the C99 features
2012 for its own code, especially the definition of variables after statements.
2013 The generated C parsers still aim at C90.
2015 ** Backward incompatible changes
2017 *** Obsolete features
2019 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
2021 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
2022 use YYSTYPE and YYLTYPE.
2024 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
2025 1.875): use %lex-param, %parse-param, or %param.
2027 Missing semicolons at the end of actions are no longer added (as announced
2028 in the release 2.5).
2030 *** Use of YACC='bison -y'
2032 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
2035 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
2036 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
2037 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
2039 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
2040 implementation of Yacc, was using Bison as 'bison -y'. While it does
2041 ensure compatible output file names, it also enables warnings for
2042 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
2043 warnings for Bison extensions.
2045 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
2046 (which also generates 'y.tab.h' and 'y.output' when needed).
2047 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
2048 flags (YFLAGS, or AM_YFLAGS with Automake).
2052 *** The epilogue is no longer affected by internal #defines (glr.c)
2054 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
2055 generated code. These weren't properly undefined before the inclusion of
2056 the user epilogue, so functions such as the following were butchered by the
2057 preprocessor expansion:
2059 int yylex (YYSTYPE *yylval);
2061 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2062 identifiers for user-provided variables.
2064 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2066 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2067 locations are enabled. This is fixed.
2069 *** Warnings about useless %pure-parser/%define api.pure are restored
2071 ** Diagnostics reported by Bison
2073 Most of these features were contributed by Théophile Ranquet and Victor
2078 Version 2.7 introduced caret errors, for a prettier output. These are now
2079 activated by default. The old format can still be used by invoking Bison
2080 with -fno-caret (or -fnone).
2082 Some error messages that reproduced excerpts of the grammar are now using
2083 the caret information only. For instance on:
2090 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2091 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2095 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2096 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2100 and "bison -fno-caret" reports:
2102 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2103 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2105 *** Enhancements of the -Werror option
2107 The -Werror=CATEGORY option is now recognized, and will treat specified
2108 warnings as errors. The warnings need not have been explicitly activated
2109 using the -W option, this is similar to what GCC 4.7 does.
2111 For example, given the following command line, Bison will treat both
2112 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2113 errors (and only those):
2115 $ bison -Werror=yacc,error=conflicts-sr input.y
2117 If no categories are specified, -Werror will make all active warnings into
2118 errors. For example, the following line does the same the previous example:
2120 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2122 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2124 Note that the categories in this -Werror option may not be prefixed with
2125 "no-". However, -Wno-error[=CATEGORY] is valid.
2127 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2128 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2129 incompatibilities as warnings: "-y -Wno-error=yacc".
2131 *** The display of warnings is now richer
2133 The option that controls a given warning is now displayed:
2135 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2137 In the case of warnings treated as errors, the prefix is changed from
2138 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2139 to GCC, as [-Werror=CATEGORY].
2141 For instance, where the previous version of Bison would report (and exit
2144 bison: warnings being treated as errors
2145 input.y:1.1: warning: stray ',' treated as white space
2149 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2151 *** Deprecated constructs
2153 The new 'deprecated' warning category flags obsolete constructs whose
2154 support will be discontinued. It is enabled by default. These warnings
2155 used to be reported as 'other' warnings.
2157 *** Useless semantic types
2159 Bison now warns about useless (uninhabited) semantic types. Since
2160 semantic types are not declared to Bison (they are defined in the opaque
2161 %union structure), it is %printer/%destructor directives about useless
2162 types that trigger the warning:
2166 %printer {} <type1> <type3>
2167 %destructor {} <type2> <type4>
2169 nterm: term { $$ = $1; };
2171 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2172 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2174 *** Undefined but unused symbols
2176 Bison used to raise an error for undefined symbols that are not used in
2177 the grammar. This is now only a warning.
2180 %destructor {} symbol2
2181 %type <type> symbol3
2185 *** Useless destructors or printers
2187 Bison now warns about useless destructors or printers. In the following
2188 example, the printer for <type1>, and the destructor for <type2> are
2189 useless: all symbols of <type1> (token1) already have a printer, and all
2190 symbols of type <type2> (token2) already have a destructor.
2192 %token <type1> token1
2196 %printer {} token1 <type1> <type3>
2197 %destructor {} token2 <type2> <type4>
2201 The warnings and error messages about shift/reduce and reduce/reduce
2202 conflicts have been normalized. For instance on the following foo.y file:
2206 exp: exp '+' exp | '0' | '0';
2208 compare the previous version of bison:
2211 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2212 $ bison -Werror foo.y
2213 bison: warnings being treated as errors
2214 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2216 with the new behavior:
2219 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2220 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2221 $ bison -Werror foo.y
2222 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2223 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2225 When %expect or %expect-rr is used, such as with bar.y:
2230 exp: exp '+' exp | '0' | '0';
2235 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2236 bar.y: expected 0 shift/reduce conflicts
2237 bar.y: expected 0 reduce/reduce conflicts
2242 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2243 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2245 ** Incompatibilities with POSIX Yacc
2247 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2250 ** Additional yylex/yyparse arguments
2252 The new directive %param declares additional arguments to both yylex and
2253 yyparse. The %lex-param, %parse-param, and %param directives support one
2254 or more arguments. Instead of
2256 %lex-param {arg1_type *arg1}
2257 %lex-param {arg2_type *arg2}
2258 %parse-param {arg1_type *arg1}
2259 %parse-param {arg2_type *arg2}
2263 %param {arg1_type *arg1} {arg2_type *arg2}
2265 ** Types of values for %define variables
2267 Bison used to make no difference between '%define foo bar' and '%define
2268 foo "bar"'. The former is now called a 'keyword value', and the latter a
2269 'string value'. A third kind was added: 'code values', such as '%define
2272 Keyword variables are used for fixed value sets, e.g.,
2274 %define lr.type lalr
2276 Code variables are used for value in the target language, e.g.,
2278 %define api.value.type {struct semantic_type}
2280 String variables are used remaining cases, e.g. file names.
2282 ** Variable api.token.prefix
2284 The variable api.token.prefix changes the way tokens are identified in
2285 the generated files. This is especially useful to avoid collisions
2286 with identifiers in the target language. For instance
2288 %token FILE for ERROR
2289 %define api.token.prefix {TOK_}
2291 start: FILE for ERROR;
2293 will generate the definition of the symbols TOK_FILE, TOK_for, and
2294 TOK_ERROR in the generated sources. In particular, the scanner must
2295 use these prefixed token names, although the grammar itself still
2296 uses the short names (as in the sample rule given above).
2298 ** Variable api.value.type
2300 This new %define variable supersedes the #define macro YYSTYPE. The use
2301 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2302 using %union or %defining api.value.type results in undefined behavior.
2304 Either define api.value.type, or use "%union":
2311 %token <ival> INT "integer"
2312 %token <sval> STRING "string"
2313 %printer { fprintf (yyo, "%d", $$); } <ival>
2314 %destructor { free ($$); } <sval>
2317 yylval.ival = 42; return INT;
2318 yylval.sval = "42"; return STRING;
2320 The %define variable api.value.type supports both keyword and code values.
2322 The keyword value 'union' means that the user provides genuine types, not
2323 union member names such as "ival" and "sval" above (WARNING: will fail if
2324 -y/--yacc/%yacc is enabled).
2326 %define api.value.type union
2327 %token <int> INT "integer"
2328 %token <char *> STRING "string"
2329 %printer { fprintf (yyo, "%d", $$); } <int>
2330 %destructor { free ($$); } <char *>
2333 yylval.INT = 42; return INT;
2334 yylval.STRING = "42"; return STRING;
2336 The keyword value variant is somewhat equivalent, but for C++ special
2337 provision is made to allow classes to be used (more about this below).
2339 %define api.value.type variant
2340 %token <int> INT "integer"
2341 %token <std::string> STRING "string"
2343 Code values (in braces) denote user defined types. This is where YYSTYPE
2361 %define api.value.type {struct my_value}
2362 %token <u.ival> INT "integer"
2363 %token <u.sval> STRING "string"
2364 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2365 %destructor { free ($$); } <u.sval>
2368 yylval.u.ival = 42; return INT;
2369 yylval.u.sval = "42"; return STRING;
2371 ** Variable parse.error
2373 This variable controls the verbosity of error messages. The use of the
2374 %error-verbose directive is deprecated in favor of "%define parse.error
2377 ** Deprecated %define variable names
2379 The following variables have been renamed for consistency. Backward
2380 compatibility is ensured, but upgrading is recommended.
2382 lr.default-reductions -> lr.default-reduction
2383 lr.keep-unreachable-states -> lr.keep-unreachable-state
2384 namespace -> api.namespace
2385 stype -> api.value.type
2387 ** Semantic predicates
2389 Contributed by Paul Hilfinger.
2391 The new, experimental, semantic-predicate feature allows actions of the
2392 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2393 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2394 in GLR parsers, rather than being deferred. The result is that they allow
2395 the programmer to prune possible parses based on the values of run-time
2398 ** The directive %expect-rr is now an error in non GLR mode
2400 It used to be an error only if used in non GLR mode, _and_ if there are
2401 reduce/reduce conflicts.
2403 ** Tokens are numbered in their order of appearance
2405 Contributed by Valentin Tolmer.
2407 With '%token A B', A had a number less than the one of B. However,
2408 precedence declarations used to generate a reversed order. This is now
2409 fixed, and introducing tokens with any of %token, %left, %right,
2410 %precedence, or %nonassoc yields the same result.
2412 When mixing declarations of tokens with a literal character (e.g., 'a') or
2413 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2414 the literal characters first. For example
2418 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2419 input order is now preserved.
2421 These changes were made so that one can remove useless precedence and
2422 associativity declarations (i.e., map %nonassoc, %left or %right to
2423 %precedence, or to %token) and get exactly the same output.
2425 ** Useless precedence and associativity
2427 Contributed by Valentin Tolmer.
2429 When developing and maintaining a grammar, useless associativity and
2430 precedence directives are common. They can be a nuisance: new ambiguities
2431 arising are sometimes masked because their conflicts are resolved due to
2432 the extra precedence or associativity information. Furthermore, it can
2433 hinder the comprehension of a new grammar: one will wonder about the role
2434 of a precedence, where in fact it is useless. The following changes aim
2435 at detecting and reporting these extra directives.
2437 *** Precedence warning category
2439 A new category of warning, -Wprecedence, was introduced. It flags the
2440 useless precedence and associativity directives.
2442 *** Useless associativity
2444 Bison now warns about symbols with a declared associativity that is never
2445 used to resolve conflicts. In that case, using %precedence is sufficient;
2446 the parsing tables will remain unchanged. Solving these warnings may raise
2447 useless precedence warnings, as the symbols no longer have associativity.
2461 warning: useless associativity for '+', use %precedence [-Wprecedence]
2465 *** Useless precedence
2467 Bison now warns about symbols with a declared precedence and no declared
2468 associativity (i.e., declared with %precedence), and whose precedence is
2469 never used. In that case, the symbol can be safely declared with %token
2470 instead, without modifying the parsing tables. For example:
2474 exp: "var" '=' "number";
2478 warning: useless precedence for '=' [-Wprecedence]
2482 *** Useless precedence and associativity
2484 In case of both useless precedence and associativity, the issue is flagged
2489 exp: "var" '=' "number";
2493 warning: useless precedence and associativity for '=' [-Wprecedence]
2499 With help from Joel E. Denny and Gabriel Rassoul.
2501 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2502 marked by the new %empty directive. Using %empty on a non-empty rule is
2503 an error. The new -Wempty-rule warning reports empty rules without
2504 %empty. On the following grammar:
2514 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2517 5.8-13: error: %empty on non-empty rule
2521 ** Java skeleton improvements
2523 The constants for token names were moved to the Lexer interface. Also, it
2524 is possible to add code to the parser's constructors using "%code init"
2525 and "%define init_throws".
2526 Contributed by Paolo Bonzini.
2528 The Java skeleton now supports push parsing.
2529 Contributed by Dennis Heimbigner.
2531 ** C++ skeletons improvements
2533 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2535 Using %defines is now optional. Without it, the needed support classes
2536 are defined in the generated parser, instead of additional files (such as
2537 location.hh, position.hh and stack.hh).
2539 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2541 Both lalr1.cc and glr.cc no longer require %location.
2543 *** syntax_error exception (lalr1.cc)
2545 The C++ parser features a syntax_error exception, which can be
2546 thrown from the scanner or from user rules to raise syntax errors.
2547 This facilitates reporting errors caught in sub-functions (e.g.,
2548 rejecting too large integral literals from a conversion function
2549 used by the scanner, or rejecting invalid combinations from a
2550 factory invoked by the user actions).
2552 *** %define api.value.type variant
2554 This is based on a submission from Michiel De Wilde. With help
2555 from Théophile Ranquet.
2557 In this mode, complex C++ objects can be used as semantic values. For
2560 %token <::std::string> TEXT;
2561 %token <int> NUMBER;
2562 %token SEMICOLON ";"
2563 %type <::std::string> item;
2564 %type <::std::list<std::string>> list;
2567 list { std::cout << $1 << std::endl; }
2571 %empty { /* Generates an empty string list. */ }
2572 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2576 TEXT { std::swap ($$, $1); }
2577 | NUMBER { $$ = string_cast ($1); }
2580 *** %define api.token.constructor
2582 When variants are enabled, Bison can generate functions to build the
2583 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2584 with the semantic value (e.g., int):
2586 parser::symbol_type yylex ()
2588 parser::location_type loc = ...;
2590 return parser::make_TEXT ("Hello, world!", loc);
2592 return parser::make_NUMBER (42, loc);
2594 return parser::make_SEMICOLON (loc);
2600 There are operator- and operator-= for 'location'. Negative line/column
2601 increments can no longer underflow the resulting value.
2604 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2608 *** Fix compiler attribute portability (yacc.c)
2610 With locations enabled, __attribute__ was used unprotected.
2612 *** Fix some compiler warnings (lalr1.cc)
2615 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2619 Warnings about uninitialized yylloc in yyparse have been fixed.
2621 Restored C90 compliance (yet no report was ever made).
2623 ** Diagnostics are improved
2625 Contributed by Théophile Ranquet.
2627 *** Changes in the format of error messages
2629 This used to be the format of many error reports:
2631 input.y:2.7-12: %type redeclaration for exp
2632 input.y:1.7-12: previous declaration
2636 input.y:2.7-12: error: %type redeclaration for exp
2637 input.y:1.7-12: previous declaration
2639 *** New format for error reports: carets
2641 Caret errors have been added to Bison:
2643 input.y:2.7-12: error: %type redeclaration for exp
2646 input.y:1.7-12: previous declaration
2652 input.y:3.20-23: error: ambiguous reference: '$exp'
2653 exp: exp '+' exp { $exp = $1 + $3; };
2655 input.y:3.1-3: refers to: $exp at $$
2656 exp: exp '+' exp { $exp = $1 + $3; };
2658 input.y:3.6-8: refers to: $exp at $1
2659 exp: exp '+' exp { $exp = $1 + $3; };
2661 input.y:3.14-16: refers to: $exp at $3
2662 exp: exp '+' exp { $exp = $1 + $3; };
2665 The default behavior for now is still not to display these unless
2666 explicitly asked with -fcaret (or -fall). However, in a later release, it
2667 will be made the default behavior (but may still be deactivated with
2670 ** New value for %define variable: api.pure full
2672 The %define variable api.pure requests a pure (reentrant) parser. However,
2673 for historical reasons, using it in a location-tracking Yacc parser
2674 resulted in a yyerror function that did not take a location as a
2675 parameter. With this new value, the user may request a better pure parser,
2676 where yyerror does take a location as a parameter (in location-tracking
2679 The use of "%define api.pure true" is deprecated in favor of this new
2680 "%define api.pure full".
2682 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2684 The %define variable api.location.type defines the name of the type to use
2685 for locations. When defined, Bison no longer generates the position.hh
2686 and location.hh files, nor does the parser will include them: the user is
2687 then responsible to define her type.
2689 This can be used in programs with several parsers to factor their location
2690 and position files: let one of them generate them, and the others just use
2693 This feature was actually introduced, but not documented, in Bison 2.5,
2694 under the name "location_type" (which is maintained for backward
2697 For consistency, lalr1.java's %define variables location_type and
2698 position_type are deprecated in favor of api.location.type and
2701 ** Exception safety (lalr1.cc)
2703 The parse function now catches exceptions, uses the %destructors to
2704 release memory (the lookahead symbol and the symbols pushed on the stack)
2705 before re-throwing the exception.
2707 This feature is somewhat experimental. User feedback would be
2710 ** Graph improvements in DOT and XSLT
2712 Contributed by Théophile Ranquet.
2714 The graphical presentation of the states is more readable: their shape is
2715 now rectangular, the state number is clearly displayed, and the items are
2716 numbered and left-justified.
2718 The reductions are now explicitly represented as transitions to other
2719 diamond shaped nodes.
2721 These changes are present in both --graph output and xml2dot.xsl XSLT
2722 processing, with minor (documented) differences.
2724 ** %language is no longer an experimental feature.
2726 The introduction of this feature, in 2.4, was four years ago. The
2727 --language option and the %language directive are no longer experimental.
2731 The sections about shift/reduce and reduce/reduce conflicts resolution
2732 have been fixed and extended.
2734 Although introduced more than four years ago, XML and Graphviz reports
2735 were not properly documented.
2737 The translation of midrule actions is now described.
2740 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2742 We consider compiler warnings about Bison generated parsers to be bugs.
2743 Rather than working around them in your own project, please consider
2744 reporting them to us.
2748 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2749 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2752 Other issues in the test suite have been addressed.
2754 Null characters are correctly displayed in error messages.
2756 When possible, yylloc is correctly initialized before calling yylex. It
2757 is no longer necessary to initialize it in the %initial-action.
2760 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2762 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2765 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2769 Bugs and portability issues in the test suite have been fixed.
2771 Some errors in translations have been addressed, and --help now directs
2772 users to the appropriate place to report them.
2774 Stray Info files shipped by accident are removed.
2776 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2777 generated, are removed.
2779 All the generated headers are self-contained.
2781 ** Header guards (yacc.c, glr.c, glr.cc)
2783 In order to avoid collisions, the header guards are now
2784 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2785 For instance the header generated from
2787 %define api.prefix "calc"
2788 %defines "lib/parse.h"
2790 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2792 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2794 The compilation of pure parsers (%define api.pure) can trigger GCC
2797 input.c: In function 'yyparse':
2798 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2799 function [-Wmaybe-uninitialized]
2803 This is now fixed; pragmas to avoid these warnings are no longer needed.
2805 Warnings from clang ("equality comparison with extraneous parentheses" and
2806 "function declared 'noreturn' should not return") have also been
2810 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2814 Buffer overruns, complaints from Flex, and portability issues in the test
2815 suite have been fixed.
2817 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2819 Trailing end-of-lines in %parse-param or %lex-param would result in
2820 invalid C++. This is fixed.
2822 ** Spurious spaces and end-of-lines
2824 The generated files no longer end (nor start) with empty lines.
2827 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2829 Bison no longer executes user-specified M4 code when processing a grammar.
2833 In addition to the removal of the features announced in Bison 2.6, the
2834 next major release will remove the "Temporary hack for adding a semicolon
2835 to the user action", as announced in the release 2.5. Instead of:
2837 exp: exp "+" exp { $$ = $1 + $3 };
2841 exp: exp "+" exp { $$ = $1 + $3; };
2845 *** Type names are now properly escaped.
2847 *** glr.cc: set_debug_level and debug_level work as expected.
2849 *** Stray @ or $ in actions
2851 While Bison used to warn about stray $ or @ in action rules, it did not
2852 for other actions such as printers, destructors, or initial actions. It
2855 ** Type names in actions
2857 For consistency with rule actions, it is now possible to qualify $$ by a
2858 type-name in destructors, printers, and initial actions. For instance:
2860 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2862 will display two values for each typed and untyped symbol (provided
2863 that YYSTYPE has both "ival" and "fval" fields).
2866 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2870 The next major release of Bison will drop support for the following
2871 deprecated features. Please report disagreements to bug-bison@gnu.org.
2875 Support for generating parsers in K&R C will be removed. Parsers
2876 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2879 *** Features deprecated since Bison 1.875
2881 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2884 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2885 %lex-param, will no longer be supported.
2887 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2890 *** The generated header will be included (yacc.c)
2892 Instead of duplicating the content of the generated header (definition of
2893 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2894 as is already the case for GLR or C++ parsers. This change is deferred
2895 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2898 ** Generated Parser Headers
2900 *** Guards (yacc.c, glr.c, glr.cc)
2902 The generated headers are now guarded, as is already the case for C++
2903 parsers (lalr1.cc). For instance, with --defines=foo.h:
2908 #endif /* !YY_FOO_H */
2910 *** New declarations (yacc.c, glr.c)
2912 The generated header now declares yydebug and yyparse. Both honor
2913 --name-prefix=bar_, and yield
2915 int bar_parse (void);
2919 #define yyparse bar_parse
2922 in order to facilitate the inclusion of several parser headers inside a
2923 single compilation unit.
2925 *** Exported symbols in C++
2927 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2928 header, are removed, as they prevent the possibility of including several
2929 generated headers from a single compilation unit.
2933 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2936 ** New %define variable: api.prefix
2938 Now that the generated headers are more complete and properly protected
2939 against multiple inclusions, constant names, such as YYSTYPE are a
2940 problem. While yyparse and others are properly renamed by %name-prefix,
2941 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2942 would introduce backward compatibility issues in projects not expecting
2943 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2944 it is deprecated in favor of a new %define variable: api.prefix.
2946 The following examples compares both:
2948 %name-prefix "bar_" | %define api.prefix "bar_"
2949 %token <ival> FOO %token <ival> FOO
2950 %union { int ival; } %union { int ival; }
2956 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2957 # define BAR_FOO_H # define BAR_FOO_H
2959 /* Enabling traces. */ /* Enabling traces. */
2960 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2961 > # if defined YYDEBUG
2963 > # define BAR_DEBUG 1
2965 > # define BAR_DEBUG 0
2968 # define YYDEBUG 0 | # define BAR_DEBUG 0
2972 # if YYDEBUG | # if BAR_DEBUG
2973 extern int bar_debug; extern int bar_debug;
2976 /* Tokens. */ /* Tokens. */
2977 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2978 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2979 enum yytokentype { | enum bar_tokentype {
2984 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2985 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2986 typedef union YYSTYPE | typedef union BAR_STYPE
2989 } YYSTYPE; | } BAR_STYPE;
2990 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2993 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2995 int bar_parse (void); int bar_parse (void);
2997 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
3000 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
3004 The next major release will drop support for generating parsers in K&R C.
3006 ** yacc.c: YYBACKUP works as expected.
3008 ** glr.c improvements:
3010 *** Location support is eliminated when not requested:
3012 GLR parsers used to include location-related code even when locations were
3013 not requested, and therefore not even usable.
3015 *** __attribute__ is preserved:
3017 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
3018 when -std is passed to GCC).
3020 ** lalr1.java: several fixes:
3022 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
3023 first token leads to a syntax error. Some minor clean ups.
3027 *** C++11 compatibility:
3029 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
3034 The header files such as "parser.hh", "location.hh", etc. used a constant
3035 name for preprocessor guards, for instance:
3037 #ifndef BISON_LOCATION_HH
3038 # define BISON_LOCATION_HH
3040 #endif // !BISON_LOCATION_HH
3042 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
3043 case characters are converted to upper case, and series of
3044 non-alphanumerical characters are converted to an underscore.
3046 With "bison -o lang++/parser.cc", "location.hh" would now include:
3048 #ifndef YY_LANG_LOCATION_HH
3049 # define YY_LANG_LOCATION_HH
3051 #endif // !YY_LANG_LOCATION_HH
3055 The position and location constructors (and their initialize methods)
3056 accept new arguments for line and column. Several issues in the
3057 documentation were fixed.
3059 ** liby is no longer asking for "rpl_fprintf" on some platforms.
3061 ** Changes in the manual:
3063 *** %printer is documented
3065 The "%printer" directive, supported since at least Bison 1.50, is finally
3066 documented. The "mfcalc" example is extended to demonstrate it.
3068 For consistency with the C skeletons, the C++ parsers now also support
3069 "yyoutput" (as an alias to "debug_stream ()").
3071 *** Several improvements have been made:
3073 The layout for grammar excerpts was changed to a more compact scheme.
3074 Named references are motivated. The description of the automaton
3075 description file (*.output) is updated to the current format. Incorrect
3076 index entries were fixed. Some other errors were fixed.
3080 *** Conflicting prototypes with recent/modified Flex.
3082 Fixed build problems with the current, unreleased, version of Flex, and
3083 some modified versions of 2.5.35, which have modified function prototypes.
3085 *** Warnings during the build procedure have been eliminated.
3087 *** Several portability problems in the test suite have been fixed:
3089 This includes warnings with some compilers, unexpected behavior of tools
3090 such as diff, warning messages from the test suite itself, etc.
3092 *** The install-pdf target works properly:
3094 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3095 halts in the middle of its course.
3098 * Noteworthy changes in release 2.5 (2011-05-14)
3100 ** Grammar symbol names can now contain non-initial dashes:
3102 Consistently with directives (such as %error-verbose) and with
3103 %define variables (e.g. push-pull), grammar symbol names may contain
3104 dashes in any position except the beginning. This is a GNU
3105 extension over POSIX Yacc. Thus, use of this extension is reported
3106 by -Wyacc and rejected in Yacc mode (--yacc).
3108 ** Named references:
3110 Historically, Yacc and Bison have supported positional references
3111 ($n, $$) to allow access to symbol values from inside of semantic
3114 Starting from this version, Bison can also accept named references.
3115 When no ambiguity is possible, original symbol names may be used
3116 as named references:
3118 if_stmt : "if" cond_expr "then" then_stmt ';'
3119 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3121 In the more common case, explicit names may be declared:
3123 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3124 { $res = mk_if_stmt($cond, $then, $else); }
3126 Location information is also accessible using @name syntax. When
3127 accessing symbol names containing dots or dashes, explicit bracketing
3128 ($[sym.1]) must be used.
3130 These features are experimental in this version. More user feedback
3131 will help to stabilize them.
3132 Contributed by Alex Rozenman.
3134 ** IELR(1) and canonical LR(1):
3136 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3137 is, given any context-free grammar, IELR(1) generates parser tables
3138 with the full language-recognition power of canonical LR(1) but with
3139 nearly the same number of parser states as LALR(1). This reduction
3140 in parser states is often an order of magnitude. More importantly,
3141 because canonical LR(1)'s extra parser states may contain duplicate
3142 conflicts in the case of non-LR(1) grammars, the number of conflicts
3143 for IELR(1) is often an order of magnitude less as well. This can
3144 significantly reduce the complexity of developing of a grammar.
3146 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3147 place of its traditional LALR(1) parser tables, which remain the
3148 default. You can specify the type of parser tables in the grammar
3149 file with these directives:
3151 %define lr.type lalr
3152 %define lr.type ielr
3153 %define lr.type canonical-lr
3155 The default-reduction optimization in the parser tables can also be
3156 adjusted using "%define lr.default-reductions". For details on both
3157 of these features, see the new section "Tuning LR" in the Bison
3160 These features are experimental. More user feedback will help to
3163 ** LAC (Lookahead Correction) for syntax error handling
3165 Contributed by Joel E. Denny.
3167 Canonical LR, IELR, and LALR can suffer from a couple of problems
3168 upon encountering a syntax error. First, the parser might perform
3169 additional parser stack reductions before discovering the syntax
3170 error. Such reductions can perform user semantic actions that are
3171 unexpected because they are based on an invalid token, and they
3172 cause error recovery to begin in a different syntactic context than
3173 the one in which the invalid token was encountered. Second, when
3174 verbose error messages are enabled (with %error-verbose or the
3175 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3176 syntax error message can both contain invalid tokens and omit valid
3179 The culprits for the above problems are %nonassoc, default
3180 reductions in inconsistent states, and parser state merging. Thus,
3181 IELR and LALR suffer the most. Canonical LR can suffer only if
3182 %nonassoc is used or if default reductions are enabled for
3183 inconsistent states.
3185 LAC is a new mechanism within the parsing algorithm that solves
3186 these problems for canonical LR, IELR, and LALR without sacrificing
3187 %nonassoc, default reductions, or state merging. When LAC is in
3188 use, canonical LR and IELR behave almost exactly the same for both
3189 syntactically acceptable and syntactically unacceptable input.
3190 While LALR still does not support the full language-recognition
3191 power of canonical LR and IELR, LAC at least enables LALR's syntax
3192 error handling to correctly reflect LALR's language-recognition
3195 Currently, LAC is only supported for deterministic parsers in C.
3196 You can enable LAC with the following directive:
3198 %define parse.lac full
3200 See the new section "LAC" in the Bison manual for additional
3201 details including a few caveats.
3203 LAC is an experimental feature. More user feedback will help to
3206 ** %define improvements:
3208 *** Can now be invoked via the command line:
3210 Each of these command-line options
3213 --define=NAME[=VALUE]
3216 --force-define=NAME[=VALUE]
3218 is equivalent to this grammar file declaration
3220 %define NAME ["VALUE"]
3222 except that the manner in which Bison processes multiple definitions
3223 for the same NAME differs. Most importantly, -F and --force-define
3224 quietly override %define, but -D and --define do not. For further
3225 details, see the section "Bison Options" in the Bison manual.
3227 *** Variables renamed:
3229 The following %define variables
3232 lr.keep_unreachable_states
3234 have been renamed to
3237 lr.keep-unreachable-states
3239 The old names are now deprecated but will be maintained indefinitely
3240 for backward compatibility.
3242 *** Values no longer need to be quoted in the grammar file:
3244 If a %define value is an identifier, it no longer needs to be placed
3245 within quotations marks. For example,
3247 %define api.push-pull "push"
3251 %define api.push-pull push
3253 *** Unrecognized variables are now errors not warnings.
3255 *** Multiple invocations for any variable is now an error not a warning.
3257 ** Unrecognized %code qualifiers are now errors not warnings.
3259 ** Character literals not of length one:
3261 Previously, Bison quietly converted all character literals to length
3262 one. For example, without warning, Bison interpreted the operators in
3263 the following grammar to be the same token:
3269 Bison now warns when a character literal is not of length one. In
3270 some future release, Bison will start reporting an error instead.
3272 ** Destructor calls fixed for lookaheads altered in semantic actions:
3274 Previously for deterministic parsers in C, if a user semantic action
3275 altered yychar, the parser in some cases used the old yychar value to
3276 determine which destructor to call for the lookahead upon a syntax
3277 error or upon parser return. This bug has been fixed.
3279 ** C++ parsers use YYRHSLOC:
3281 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3282 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3283 to use it. If, for instance, your location structure has "first"
3284 and "last" members, instead of
3286 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3290 (Current).first = (Rhs)[1].location.first; \
3291 (Current).last = (Rhs)[N].location.last; \
3295 (Current).first = (Current).last = (Rhs)[0].location.last; \
3301 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3305 (Current).first = YYRHSLOC (Rhs, 1).first; \
3306 (Current).last = YYRHSLOC (Rhs, N).last; \
3310 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3314 ** YYLLOC_DEFAULT in C++:
3316 The default implementation of YYLLOC_DEFAULT used to be issued in
3317 the header file. It is now output in the implementation file, after
3318 the user %code sections so that its #ifndef guard does not try to
3319 override the user's YYLLOC_DEFAULT if provided.
3321 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3323 YYFAIL has existed for many years as an undocumented feature of
3324 deterministic parsers in C generated by Bison. More recently, it was
3325 a documented feature of Bison's experimental Java parsers. As
3326 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3327 semantic action now produces a deprecation warning, and Java parsers
3328 no longer implement YYFAIL at all. For further details, including a
3329 discussion of how to suppress C preprocessor warnings about YYFAIL
3330 being unused, see the Bison 2.4.2 NEWS entry.
3332 ** Temporary hack for adding a semicolon to the user action:
3334 Previously, Bison appended a semicolon to every user action for
3335 reductions when the output language defaulted to C (specifically, when
3336 neither %yacc, %language, %skeleton, or equivalent command-line
3337 options were specified). This allowed actions such as
3339 exp: exp "+" exp { $$ = $1 + $3 };
3343 exp: exp "+" exp { $$ = $1 + $3; };
3345 As a first step in removing this misfeature, Bison now issues a
3346 warning when it appends a semicolon. Moreover, in cases where Bison
3347 cannot easily determine whether a semicolon is needed (for example, an
3348 action ending with a cpp directive or a braced compound initializer),
3349 it no longer appends one. Thus, the C compiler might now complain
3350 about a missing semicolon where it did not before. Future releases of
3351 Bison will cease to append semicolons entirely.
3353 ** Verbose syntax error message fixes:
3355 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3356 specified, syntax error messages produced by the generated parser
3357 include the unexpected token as well as a list of expected tokens.
3358 The effect of %nonassoc on these verbose messages has been corrected
3359 in two ways, but a more complete fix requires LAC, described above:
3361 *** When %nonassoc is used, there can exist parser states that accept no
3362 tokens, and so the parser does not always require a lookahead token
3363 in order to detect a syntax error. Because no unexpected token or
3364 expected tokens can then be reported, the verbose syntax error
3365 message described above is suppressed, and the parser instead
3366 reports the simpler message, "syntax error". Previously, this
3367 suppression was sometimes erroneously triggered by %nonassoc when a
3368 lookahead was actually required. Now verbose messages are
3369 suppressed only when all previous lookaheads have already been
3370 shifted or discarded.
3372 *** Previously, the list of expected tokens erroneously included tokens
3373 that would actually induce a syntax error because conflicts for them
3374 were resolved with %nonassoc in the current parser state. Such
3375 tokens are now properly omitted from the list.
3377 *** Expected token lists are still often wrong due to state merging
3378 (from LALR or IELR) and default reductions, which can both add
3379 invalid tokens and subtract valid tokens. Canonical LR almost
3380 completely fixes this problem by eliminating state merging and
3381 default reductions. However, there is one minor problem left even
3382 when using canonical LR and even after the fixes above. That is,
3383 if the resolution of a conflict with %nonassoc appears in a later
3384 parser state than the one at which some syntax error is
3385 discovered, the conflicted token is still erroneously included in
3386 the expected token list. Bison's new LAC implementation,
3387 described above, eliminates this problem and the need for
3388 canonical LR. However, LAC is still experimental and is disabled
3391 ** Java skeleton fixes:
3393 *** A location handling bug has been fixed.
3395 *** The top element of each of the value stack and location stack is now
3396 cleared when popped so that it can be garbage collected.
3398 *** Parser traces now print the top element of the stack.
3400 ** -W/--warnings fixes:
3402 *** Bison now properly recognizes the "no-" versions of categories:
3404 For example, given the following command line, Bison now enables all
3405 warnings except warnings for incompatibilities with POSIX Yacc:
3407 bison -Wall,no-yacc gram.y
3409 *** Bison now treats S/R and R/R conflicts like other warnings:
3411 Previously, conflict reports were independent of Bison's normal
3412 warning system. Now, Bison recognizes the warning categories
3413 "conflicts-sr" and "conflicts-rr". This change has important
3414 consequences for the -W and --warnings command-line options. For
3417 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3418 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3419 bison -Wnone gram.y # no conflicts are reported
3420 bison -Werror gram.y # any conflict is an error
3422 However, as before, if the %expect or %expect-rr directive is
3423 specified, an unexpected number of conflicts is an error, and an
3424 expected number of conflicts is not reported, so -W and --warning
3425 then have no effect on the conflict report.
3427 *** The "none" category no longer disables a preceding "error":
3429 For example, for the following command line, Bison now reports
3430 errors instead of warnings for incompatibilities with POSIX Yacc:
3432 bison -Werror,none,yacc gram.y
3434 *** The "none" category now disables all Bison warnings:
3436 Previously, the "none" category disabled only Bison warnings for
3437 which there existed a specific -W/--warning category. However,
3438 given the following command line, Bison is now guaranteed to
3439 suppress all warnings:
3443 ** Precedence directives can now assign token number 0:
3445 Since Bison 2.3b, which restored the ability of precedence
3446 directives to assign token numbers, doing so for token number 0 has
3447 produced an assertion failure. For example:
3451 This bug has been fixed.
3454 * Noteworthy changes in release 2.4.3 (2010-08-05)
3456 ** Bison now obeys -Werror and --warnings=error for warnings about
3457 grammar rules that are useless in the parser due to conflicts.
3459 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3462 ** Failures in the test suite for GCC 4.5 have been fixed.
3464 ** Failures in the test suite for some versions of Sun Studio C++ have
3467 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3468 warnings about undefined %prec identifiers will not be converted to
3469 errors in Bison 2.5. They will remain warnings, which should be
3470 sufficient for POSIX while avoiding backward compatibility issues.
3472 ** Minor documentation fixes.
3475 * Noteworthy changes in release 2.4.2 (2010-03-20)
3477 ** Some portability problems that resulted in failures and livelocks
3478 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3479 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3480 errors should no longer cause M4 to report a broken pipe on the
3483 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3485 POSIX specifies that an error be reported for any identifier that does
3486 not appear on the LHS of a grammar rule and that is not defined by
3487 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3488 error report for the case when an identifier appears only after a
3489 %prec directive. It is now restored. However, for backward
3490 compatibility with recent Bison releases, it is only a warning for
3491 now. In Bison 2.5 and later, it will return to being an error.
3492 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3493 warning will not be converted to an error in Bison 2.5.]
3495 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3497 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3498 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3501 ** %code is now a permanent feature.
3503 A traditional Yacc prologue directive is written in the form:
3507 To provide a more flexible alternative, Bison 2.3b introduced the
3508 %code directive with the following forms for C/C++:
3511 %code requires {CODE}
3512 %code provides {CODE}
3515 These forms are now considered permanent features of Bison. See the
3516 %code entries in the section "Bison Declaration Summary" in the Bison
3517 manual for a summary of their functionality. See the section
3518 "Prologue Alternatives" for a detailed discussion including the
3519 advantages of %code over the traditional Yacc prologue directive.
3521 Bison's Java feature as a whole including its current usage of %code
3522 is still considered experimental.
3524 ** YYFAIL is deprecated and will eventually be removed.
3526 YYFAIL has existed for many years as an undocumented feature of
3527 deterministic parsers in C generated by Bison. Previously, it was
3528 documented for Bison's experimental Java parsers. YYFAIL is no longer
3529 documented for Java parsers and is formally deprecated in both cases.
3530 Users are strongly encouraged to migrate to YYERROR, which is
3533 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3534 induce a syntax error. The most obvious difference from YYERROR is
3535 that YYFAIL will automatically invoke yyerror to report the syntax
3536 error so that you don't have to. However, there are several other
3537 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3538 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3539 used. For a more detailed discussion, see:
3541 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3543 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3544 deterministic parsers in C will continue to implement it. However,
3545 because YYFAIL is already flawed, it seems futile to try to make new
3546 Bison features compatible with it. Thus, during parser generation,
3547 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3548 rule action. In a later release, YYFAIL will be disabled for
3549 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3550 be removed altogether.
3552 There exists at least one case where Bison 2.5's YYFAIL warning will
3553 be a false positive. Some projects add phony uses of YYFAIL and other
3554 Bison-defined macros for the sole purpose of suppressing C
3555 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3556 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3557 epilogue (that is, after the second "%%") in the Bison input file. In
3558 this release (2.4.2), Bison already generates its own code to suppress
3559 C preprocessor warnings for YYFAIL, so projects can remove their own
3560 phony uses of YYFAIL if compatibility with Bison releases prior to
3561 2.4.2 is not necessary.
3563 ** Internationalization.
3565 Fix a regression introduced in Bison 2.4: Under some circumstances,
3566 message translations were not installed although supported by the
3570 * Noteworthy changes in release 2.4.1 (2008-12-11)
3572 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3573 declarations have been fixed.
3575 ** Temporary hack for adding a semicolon to the user action.
3577 Bison used to prepend a trailing semicolon at the end of the user
3578 action for reductions. This allowed actions such as
3580 exp: exp "+" exp { $$ = $1 + $3 };
3584 exp: exp "+" exp { $$ = $1 + $3; };
3586 Some grammars still depend on this "feature". Bison 2.4.1 restores
3587 the previous behavior in the case of C output (specifically, when
3588 neither %language or %skeleton or equivalent command-line options
3589 are used) to leave more time for grammars depending on the old
3590 behavior to be adjusted. Future releases of Bison will disable this
3593 ** A few minor improvements to the Bison manual.
3596 * Noteworthy changes in release 2.4 (2008-11-02)
3598 ** %language is an experimental feature.
3600 We first introduced this feature in test release 2.3b as a cleaner
3601 alternative to %skeleton. Since then, we have discussed the possibility of
3602 modifying its effect on Bison's output file names. Thus, in this release,
3603 we consider %language to be an experimental feature that will likely evolve
3606 ** Forward compatibility with GNU M4 has been improved.
3608 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3612 * Noteworthy changes in release 2.3b (2008-05-27)
3614 ** The quotes around NAME that used to be required in the following directive
3617 %define NAME "VALUE"
3619 ** The directive "%pure-parser" is now deprecated in favor of:
3623 which has the same effect except that Bison is more careful to warn about
3624 unreasonable usage in the latter case.
3628 Bison can now generate an LALR(1) parser in C with a push interface. That
3629 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3630 push one token at a time to the parser using "yypush_parse", which will
3631 return to the caller after processing each token. By default, the push
3632 interface is disabled. Either of the following directives will enable it:
3634 %define api.push_pull "push" // Just push; does not require yylex.
3635 %define api.push_pull "both" // Push and pull; requires yylex.
3637 See the new section "A Push Parser" in the Bison manual for details.
3639 The current push parsing interface is experimental and may evolve. More user
3640 feedback will help to stabilize it.
3642 ** The -g and --graph options now output graphs in Graphviz DOT format,
3643 not VCG format. Like --graph, -g now also takes an optional FILE argument
3644 and thus cannot be bundled with other short options.
3648 Bison can now generate an LALR(1) parser in Java. The skeleton is
3649 "data/lalr1.java". Consider using the new %language directive instead of
3650 %skeleton to select it.
3652 See the new section "Java Parsers" in the Bison manual for details.
3654 The current Java interface is experimental and may evolve. More user
3655 feedback will help to stabilize it.
3656 Contributed by Paolo Bonzini.
3660 This new directive specifies the programming language of the generated
3661 parser, which can be C (the default), C++, or Java. Besides the skeleton
3662 that Bison uses, the directive affects the names of the generated files if
3663 the grammar file's name ends in ".y".
3665 ** XML Automaton Report
3667 Bison can now generate an XML report of the LALR(1) automaton using the new
3668 "--xml" option. The current XML schema is experimental and may evolve. More
3669 user feedback will help to stabilize it.
3670 Contributed by Wojciech Polak.
3672 ** The grammar file may now specify the name of the parser header file using
3673 %defines. For example:
3677 ** When reporting useless rules, useless nonterminals, and unused terminals,
3678 Bison now employs the terms "useless in grammar" instead of "useless",
3679 "useless in parser" instead of "never reduced", and "unused in grammar"
3680 instead of "unused".
3682 ** Unreachable State Removal
3684 Previously, Bison sometimes generated parser tables containing unreachable
3685 states. A state can become unreachable during conflict resolution if Bison
3686 disables a shift action leading to it from a predecessor state. Bison now:
3688 1. Removes unreachable states.
3690 2. Does not report any conflicts that appeared in unreachable states.
3691 WARNING: As a result, you may need to update %expect and %expect-rr
3692 directives in existing grammar files.
3694 3. For any rule used only in such states, Bison now reports the rule as
3695 "useless in parser due to conflicts".
3697 This feature can be disabled with the following directive:
3699 %define lr.keep_unreachable_states
3701 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3702 for further discussion.
3704 ** Lookahead Set Correction in the ".output" Report
3706 When instructed to generate a ".output" file including lookahead sets
3707 (using "--report=lookahead", for example), Bison now prints each reduction's
3708 lookahead set only next to the associated state's one item that (1) is
3709 associated with the same rule as the reduction and (2) has its dot at the end
3710 of its RHS. Previously, Bison also erroneously printed the lookahead set
3711 next to all of the state's other items associated with the same rule. This
3712 bug affected only the ".output" file and not the generated parser source
3715 ** --report-file=FILE is a new option to override the default ".output" file
3718 ** The "=" that used to be required in the following directives is now
3721 %file-prefix "parser"
3725 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3727 Bison 2.3a provided a new set of directives as a more flexible alternative to
3728 the traditional Yacc prologue blocks. Those have now been consolidated into
3729 a single %code directive with an optional qualifier field, which identifies
3730 the purpose of the code and thus the location(s) where Bison should generate
3733 1. "%code {CODE}" replaces "%after-header {CODE}"
3734 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3735 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3736 4. "%code top {CODE}" replaces "%before-header {CODE}"
3738 See the %code entries in section "Bison Declaration Summary" in the Bison
3739 manual for a summary of the new functionality. See the new section "Prologue
3740 Alternatives" for a detailed discussion including the advantages of %code
3741 over the traditional Yacc prologues.
3743 The prologue alternatives are experimental. More user feedback will help to
3744 determine whether they should become permanent features.
3746 ** Revised warning: unset or unused midrule values
3748 Since Bison 2.2, Bison has warned about midrule values that are set but not
3749 used within any of the actions of the parent rule. For example, Bison warns
3752 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3754 Now, Bison also warns about midrule values that are used but not set. For
3755 example, Bison warns about unset $$ in the midrule action in:
3757 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3759 However, Bison now disables both of these warnings by default since they
3760 sometimes prove to be false alarms in existing grammars employing the Yacc
3761 constructs $0 or $-N (where N is some positive integer).
3763 To enable these warnings, specify the option "--warnings=midrule-values" or
3764 "-W", which is a synonym for "--warnings=all".
3766 ** Default %destructor or %printer with "<*>" or "<>"
3768 Bison now recognizes two separate kinds of default %destructor's and
3771 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3772 %destructor/%printer for all grammar symbols for which you have formally
3773 declared semantic type tags.
3775 2. Place "<>" in a %destructor/%printer symbol list to define a default
3776 %destructor/%printer for all grammar symbols without declared semantic
3779 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3780 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3781 longer applies any %destructor to a midrule value if that midrule value is
3782 not actually ever referenced using either $$ or $n in a semantic action.
3784 The default %destructor's and %printer's are experimental. More user
3785 feedback will help to determine whether they should become permanent
3788 See the section "Freeing Discarded Symbols" in the Bison manual for further
3791 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3792 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3793 manual for a caveat concerning the treatment of literal strings.
3795 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3796 completely removed from Bison.
3799 * Noteworthy changes in release 2.3a (2006-09-13)
3801 ** Instead of %union, you can define and use your own union type
3802 YYSTYPE if your grammar contains at least one <type> tag.
3803 Your YYSTYPE need not be a macro; it can be a typedef.
3804 This change is for compatibility with other Yacc implementations,
3805 and is required by POSIX.
3807 ** Locations columns and lines start at 1.
3808 In accordance with the GNU Coding Standards and Emacs.
3810 ** You may now declare per-type and default %destructor's and %printer's:
3814 %union { char *string; }
3815 %token <string> STRING1
3816 %token <string> STRING2
3817 %type <string> string1
3818 %type <string> string2
3819 %union { char character; }
3820 %token <character> CHR
3821 %type <character> chr
3822 %destructor { free ($$); } %symbol-default
3823 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3824 %destructor { } <character>
3826 guarantees that, when the parser discards any user-defined symbol that has a
3827 semantic type tag other than "<character>", it passes its semantic value to
3828 "free". However, when the parser discards a "STRING1" or a "string1", it
3829 also prints its line number to "stdout". It performs only the second
3830 "%destructor" in this case, so it invokes "free" only once.
3832 [Although we failed to mention this here in the 2.3a release, the default
3833 %destructor's and %printer's were experimental, and they were rewritten in
3836 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3837 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3838 associating token numbers with token names. Removing the #define statements
3839 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3840 requires them. Bison still generates an enum for token names in all cases.
3842 ** Handling of traditional Yacc prologue blocks is now more consistent but
3843 potentially incompatible with previous releases of Bison.
3845 As before, you declare prologue blocks in your grammar file with the
3846 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3847 prologue blocks that you've declared before the first %union. To generate
3848 the post-prologue, Bison concatenates all prologue blocks that you've
3849 declared after the first %union.
3851 Previous releases of Bison inserted the pre-prologue into both the header
3852 file and the code file in all cases except for LALR(1) parsers in C. In the
3853 latter case, Bison inserted it only into the code file. For parsers in C++,
3854 the point of insertion was before any token definitions (which associate
3855 token numbers with names). For parsers in C, the point of insertion was
3856 after the token definitions.
3858 Now, Bison never inserts the pre-prologue into the header file. In the code
3859 file, it always inserts it before the token definitions.
3861 ** Bison now provides a more flexible alternative to the traditional Yacc
3862 prologue blocks: %before-header, %start-header, %end-header, and
3865 For example, the following declaration order in the grammar file reflects the
3866 order in which Bison will output these code blocks. However, you are free to
3867 declare these code blocks in your grammar file in whatever order is most
3871 /* Bison treats this block like a pre-prologue block: it inserts it into
3872 * the code file before the contents of the header file. It does *not*
3873 * insert it into the header file. This is a good place to put
3874 * #include's that you want at the top of your code file. A common
3875 * example is '#include "system.h"'. */
3878 /* Bison inserts this block into both the header file and the code file.
3879 * In both files, the point of insertion is before any Bison-generated
3880 * token, semantic type, location type, and class definitions. This is a
3881 * good place to define %union dependencies, for example. */
3884 /* Unlike the traditional Yacc prologue blocks, the output order for the
3885 * new %*-header blocks is not affected by their declaration position
3886 * relative to any %union in the grammar file. */
3889 /* Bison inserts this block into both the header file and the code file.
3890 * In both files, the point of insertion is after the Bison-generated
3891 * definitions. This is a good place to declare or define public
3892 * functions or data structures that depend on the Bison-generated
3896 /* Bison treats this block like a post-prologue block: it inserts it into
3897 * the code file after the contents of the header file. It does *not*
3898 * insert it into the header file. This is a good place to declare or
3899 * define internal functions or data structures that depend on the
3900 * Bison-generated definitions. */
3903 If you have multiple occurrences of any one of the above declarations, Bison
3904 will concatenate the contents in declaration order.
3906 [Although we failed to mention this here in the 2.3a release, the prologue
3907 alternatives were experimental, and they were rewritten in future versions.]
3909 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3910 The old spelling still works, but is not documented and may be removed
3911 in a future release.
3914 * Noteworthy changes in release 2.3 (2006-06-05)
3916 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3917 for compatibility with LALR(1) grammars.
3919 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3920 be to a type name that does not contain parentheses or brackets.
3923 * Noteworthy changes in release 2.2 (2006-05-19)
3925 ** The distribution terms for all Bison-generated parsers now permit
3926 using the parsers in nonfree programs. Previously, this permission
3927 was granted only for Bison-generated LALR(1) parsers in C.
3929 ** %name-prefix changes the namespace name in C++ outputs.
3931 ** The C++ parsers export their token_type.
3933 ** Bison now allows multiple %union declarations, and concatenates
3934 their contents together.
3936 ** New warning: unused values
3937 Right-hand side symbols whose values are not used are reported,
3938 if the symbols have destructors. For instance:
3940 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3944 will trigger a warning about $$ and $5 in the first rule, and $3 in
3945 the second ($1 is copied to $$ by the default rule). This example
3946 most likely contains three errors, and could be rewritten as:
3948 exp: exp "?" exp ":" exp
3949 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3951 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3954 However, if the original actions were really intended, memory leaks
3955 and all, the warnings can be suppressed by letting Bison believe the
3956 values are used, e.g.:
3958 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3959 | exp "+" exp { $$ = $1; (void) $3; }
3962 If there are midrule actions, the warning is issued if no action
3963 uses it. The following triggers no warning: $1 and $3 are used.
3965 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3967 The warning is intended to help catching lost values and memory leaks.
3968 If a value is ignored, its associated memory typically is not reclaimed.
3970 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3971 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3972 and YYERROR, for all objects on the stack, other than objects
3973 corresponding to the right-hand side of the current rule.
3975 ** %expect, %expect-rr
3976 Incorrect numbers of expected conflicts are now actual errors,
3977 instead of warnings.
3979 ** GLR, YACC parsers.
3980 The %parse-params are available in the destructors (and the
3981 experimental printers) as per the documentation.
3983 ** Bison now warns if it finds a stray "$" or "@" in an action.
3985 ** %require "VERSION"
3986 This specifies that the grammar file depends on features implemented
3987 in Bison version VERSION or higher.
3989 ** lalr1.cc: The token and value types are now class members.
3990 The tokens were defined as free form enums and cpp macros. YYSTYPE
3991 was defined as a free form union. They are now class members:
3992 tokens are enumerations of the "yy::parser::token" struct, and the
3993 semantic values have the "yy::parser::semantic_type" type.
3995 If you do not want or can update to this scheme, the directive
3996 '%define "global_tokens_and_yystype" "1"' triggers the global
3997 definition of tokens and YYSTYPE. This change is suitable both
3998 for previous releases of Bison, and this one.
4000 If you wish to update, then make sure older version of Bison will
4001 fail using '%require "2.2"'.
4003 ** DJGPP support added.
4006 * Noteworthy changes in release 2.1 (2005-09-16)
4008 ** The C++ lalr1.cc skeleton supports %lex-param.
4010 ** Bison-generated parsers now support the translation of diagnostics like
4011 "syntax error" into languages other than English. The default
4012 language is still English. For details, please see the new
4013 Internationalization section of the Bison manual. Software
4014 distributors should also see the new PACKAGING file. Thanks to
4015 Bruno Haible for this new feature.
4017 ** Wording in the Bison-generated parsers has been changed slightly to
4018 simplify translation. In particular, the message "memory exhausted"
4019 has replaced "parser stack overflow", as the old message was not
4020 always accurate for modern Bison-generated parsers.
4022 ** Destructors are now called when the parser aborts, for all symbols left
4023 behind on the stack. Also, the start symbol is now destroyed after a
4024 successful parse. In both cases, the behavior was formerly inconsistent.
4026 ** When generating verbose diagnostics, Bison-generated parsers no longer
4027 quote the literal strings associated with tokens. For example, for
4028 a syntax error associated with '%token NUM "number"' they might
4029 print 'syntax error, unexpected number' instead of 'syntax error,
4030 unexpected "number"'.
4033 * Noteworthy changes in release 2.0 (2004-12-25)
4035 ** Possibly-incompatible changes
4037 - Bison-generated parsers no longer default to using the alloca function
4038 (when available) to extend the parser stack, due to widespread
4039 problems in unchecked stack-overflow detection. You can "#define
4040 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
4041 the manual to determine safe values for YYMAXDEPTH in that case.
4043 - Error token location.
4044 During error recovery, the location of the syntax error is updated
4045 to cover the whole sequence covered by the error token: it includes
4046 the shifted symbols thrown away during the first part of the error
4047 recovery, and the lookahead rejected during the second part.
4049 - Semicolon changes:
4050 . Stray semicolons are no longer allowed at the start of a grammar.
4051 . Semicolons are now required after in-grammar declarations.
4053 - Unescaped newlines are no longer allowed in character constants or
4054 string literals. They were never portable, and GCC 3.4.0 has
4055 dropped support for them. Better diagnostics are now generated if
4056 forget a closing quote.
4058 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4062 - GLR grammars now support locations.
4064 - New directive: %initial-action.
4065 This directive allows the user to run arbitrary code (including
4066 initializing @$) from yyparse before parsing starts.
4068 - A new directive "%expect-rr N" specifies the expected number of
4069 reduce/reduce conflicts in GLR parsers.
4071 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4072 This is a GNU extension.
4074 - The option "--report=lookahead" was changed to "--report=look-ahead".
4075 [However, this was changed back after 2.3.]
4077 - Experimental %destructor support has been added to lalr1.cc.
4079 - New configure option --disable-yacc, to disable installation of the
4080 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4084 - For now, %expect-count violations are now just warnings, not errors.
4085 This is for compatibility with Bison 1.75 and earlier (when there are
4086 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4087 are too many or too few shift/reduce conflicts). However, in future
4088 versions of Bison we plan to improve the %expect machinery so that
4089 these violations will become errors again.
4091 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4092 arbitrarily limited to 16-bit counts.
4094 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4097 * Noteworthy changes in release 1.875 (2003-01-01)
4099 ** The documentation license has been upgraded to version 1.2
4100 of the GNU Free Documentation License.
4102 ** syntax error processing
4104 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4105 locations too. This fixes bugs in error-location computation.
4108 It is now possible to reclaim the memory associated to symbols
4109 discarded during error recovery. This feature is still experimental.
4112 This new directive is preferred over YYERROR_VERBOSE.
4114 - #defining yyerror to steal internal variables is discouraged.
4115 It is not guaranteed to work forever.
4117 ** POSIX conformance
4119 - Semicolons are once again optional at the end of grammar rules.
4120 This reverts to the behavior of Bison 1.33 and earlier, and improves
4121 compatibility with Yacc.
4123 - "parse error" -> "syntax error"
4124 Bison now uniformly uses the term "syntax error"; formerly, the code
4125 and manual sometimes used the term "parse error" instead. POSIX
4126 requires "syntax error" in diagnostics, and it was thought better to
4129 - The documentation now emphasizes that yylex and yyerror must be
4130 declared before use. C99 requires this.
4132 - Bison now parses C99 lexical constructs like UCNs and
4133 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4135 - File names are properly escaped in C output. E.g., foo\bar.y is
4136 output as "foo\\bar.y".
4138 - Yacc command and library now available
4139 The Bison distribution now installs a "yacc" command, as POSIX requires.
4140 Also, Bison now installs a small library liby.a containing
4141 implementations of Yacc-compatible yyerror and main functions.
4142 This library is normally not useful, but POSIX requires it.
4144 - Type clashes now generate warnings, not errors.
4146 - If the user does not define YYSTYPE as a macro, Bison now declares it
4147 using typedef instead of defining it as a macro.
4148 For consistency, YYLTYPE is also declared instead of defined.
4150 ** Other compatibility issues
4152 - %union directives can now have a tag before the "{", e.g., the
4153 directive "%union foo {...}" now generates the C code
4154 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4155 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4156 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4157 This is for compatibility with both Yacc and Bison 1.35.
4159 - ";" is output before the terminating "}" of an action, for
4160 compatibility with Bison 1.35.
4162 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4163 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4165 - "yystype" and "yyltype" are now obsolescent macros instead of being
4166 typedefs or tags; they are no longer documented and are planned to be
4167 withdrawn in a future release.
4172 Users of Bison have to decide how they handle the portability of the
4175 - "parsing stack overflow..." -> "parser stack overflow"
4176 GLR parsers now report "parser stack overflow" as per the Bison manual.
4178 ** %parse-param and %lex-param
4179 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4180 additional context to yyparse and yylex. They suffer from several
4183 - a single argument only can be added,
4184 - their types are weak (void *),
4185 - this context is not passed to ancillary functions such as yyerror,
4186 - only yacc.c parsers support them.
4188 The new %parse-param/%lex-param directives provide a more precise control.
4191 %parse-param {int *nastiness}
4192 %lex-param {int *nastiness}
4193 %parse-param {int *randomness}
4195 results in the following signatures:
4197 int yylex (int *nastiness);
4198 int yyparse (int *nastiness, int *randomness);
4200 or, if both %pure-parser and %locations are used:
4202 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4203 int yyparse (int *nastiness, int *randomness);
4205 ** Bison now warns if it detects conflicting outputs to the same file,
4206 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4207 that command outputs both code and header to foo.h.
4209 ** #line in output files
4210 - --no-line works properly.
4212 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4213 later to be built. This change originally took place a few versions
4214 ago, but nobody noticed until we recently asked someone to try
4215 building Bison with a K&R C compiler.
4218 * Noteworthy changes in release 1.75 (2002-10-14)
4220 ** Bison should now work on 64-bit hosts.
4222 ** Indonesian translation thanks to Tedi Heriyanto.
4225 Fix spurious parse errors.
4228 Some people redefine yyerror to steal yyparse' private variables.
4229 Reenable this trick until an official feature replaces it.
4232 In agreement with POSIX and with other Yaccs, leaving a default
4233 action is valid when $$ is untyped, and $1 typed:
4237 but the converse remains an error:
4241 ** Values of midrule actions
4244 foo: { ... } { $$ = $1; } ...
4246 was incorrectly rejected: $1 is defined in the second midrule
4247 action, and is equal to the $$ of the first midrule action.
4250 * Noteworthy changes in release 1.50 (2002-10-04)
4255 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4256 almost any context-free grammar, ambiguous or not. The new declarations
4257 %dprec and %merge on grammar rules allow parse-time resolution of
4258 ambiguities. Contributed by Paul Hilfinger.
4260 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4261 like the Alpha, so please stick to 32-bit hosts for now.
4264 When not in Yacc compatibility mode, when the output file was not
4265 specified, running "bison foo/bar.y" created "foo/bar.c". It
4266 now creates "bar.c".
4269 The undefined token was systematically mapped to 2 which prevented
4270 the use of 2 by the user. This is no longer the case.
4272 ** Unknown token numbers
4273 If yylex returned an out of range value, yyparse could die. This is
4277 According to POSIX, the error token must be 256.
4278 Bison extends this requirement by making it a preference: *if* the
4279 user specified that one of her tokens is numbered 256, then error
4280 will be mapped onto another number.
4282 ** Verbose error messages
4283 They no longer report "..., expecting error or..." for states where
4284 error recovery is possible.
4287 Defaults to "$end" instead of "$".
4289 ** Error recovery now conforms to documentation and to POSIX
4290 When a Bison-generated parser encounters a syntax error, it now pops
4291 the stack until it finds a state that allows shifting the error
4292 token. Formerly, it popped the stack until it found a state that
4293 allowed some non-error action other than a default reduction on the
4294 error token. The new behavior has long been the documented behavior,
4295 and has long been required by POSIX. For more details, please see
4296 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4297 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4300 Popped tokens and nonterminals are now reported.
4303 Larger grammars are now supported (larger token numbers, larger grammar
4304 size (= sum of the LHS and RHS lengths), larger LALR tables).
4305 Formerly, many of these numbers ran afoul of 16-bit limits;
4306 now these limits are 32 bits on most hosts.
4308 ** Explicit initial rule
4309 Bison used to play hacks with the initial rule, which the user does
4310 not write. It is now explicit, and visible in the reports and
4314 Before, Bison reported the useless rules, but, although not used,
4315 included them in the parsers. They are now actually removed.
4317 ** Useless rules, useless nonterminals
4318 They are now reported, as a warning, with their locations.
4320 ** Rules never reduced
4321 Rules that can never be reduced because of conflicts are now
4324 ** Incorrect "Token not used"
4325 On a grammar such as
4327 %token useless useful
4329 exp: '0' %prec useful;
4331 where a token was used to set the precedence of the last rule,
4332 bison reported both "useful" and "useless" as useless tokens.
4334 ** Revert the C++ namespace changes introduced in 1.31
4335 as they caused too many portability hassles.
4337 ** Default locations
4338 By an accident of design, the default computation of @$ was
4339 performed after another default computation was performed: @$ = @1.
4340 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4341 the computation of @$.
4343 ** Token end-of-file
4344 The token end of file may be specified by the user, in which case,
4345 the user symbol is used in the reports, the graphs, and the verbose
4346 error messages instead of "$end", which remains being the default.
4350 %token MYEOF 0 "end of file"
4353 This old option, which has been broken for ages, is removed.
4356 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4357 Croatian, thanks to Denis Lackovic.
4359 ** Incorrect token definitions
4362 bison used to output
4365 ** Token definitions as enums
4366 Tokens are output both as the traditional #define's, and, provided
4367 the compiler supports ANSI C or is a C++ compiler, as enums.
4368 This lets debuggers display names instead of integers.
4371 In addition to --verbose, bison supports --report=THINGS, which
4372 produces additional information:
4374 complete the core item sets with their closure
4375 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4376 explicitly associate lookahead tokens to items
4378 describe shift/reduce conflicts solving.
4379 Bison used to systematically output this information on top of
4380 the report. Solved conflicts are now attached to their states.
4383 Previous versions don't complain when there is a type clash on
4384 the default action if the rule has a midrule action, such as in:
4392 ** GNU M4 is now required when using Bison.
4395 * Noteworthy changes in release 1.35 (2002-03-25)
4398 Some projects use Bison's C parser with C++ compilers, and define
4399 YYSTYPE as a class. The recent adjustment of C parsers for data
4400 alignment and 64 bit architectures made this impossible.
4402 Because for the time being no real solution for C++ parser
4403 generation exists, kludges were implemented in the parser to
4404 maintain this use. In the future, when Bison has C++ parsers, this
4405 kludge will be disabled.
4407 This kludge also addresses some C++ problems when the stack was
4411 * Noteworthy changes in release 1.34 (2002-03-12)
4413 ** File name clashes are detected
4414 $ bison foo.y -d -o foo.x
4415 fatal error: header and parser would both be named "foo.x"
4417 ** A missing ";" at the end of a rule triggers a warning
4418 In accordance with POSIX, and in agreement with other
4419 Yacc implementations, Bison will mandate this semicolon in the near
4420 future. This eases the implementation of a Bison parser of Bison
4421 grammars by making this grammar LALR(1) instead of LR(2). To
4422 facilitate the transition, this release introduces a warning.
4424 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4425 many portability hassles.
4427 ** DJGPP support added.
4429 ** Fix test suite portability problems.
4432 * Noteworthy changes in release 1.33 (2002-02-07)
4435 Groff could not be compiled for the definition of size_t was lacking
4436 under some conditions.
4442 * Noteworthy changes in release 1.32 (2002-01-23)
4444 ** Fix Yacc output file names
4446 ** Portability fixes
4448 ** Italian, Dutch translations
4451 * Noteworthy changes in release 1.31 (2002-01-14)
4455 ** GNU Gettext and %expect
4456 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4457 Bison dies on incorrect %expectations, we fear there will be
4458 too many bug reports for Gettext, so _for the time being_, %expect
4459 does not trigger an error when the input file is named "plural.y".
4461 ** Use of alloca in parsers
4462 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4463 malloc exclusively. Since 1.29, but was not NEWS'ed.
4465 alloca is used only when compiled with GCC, to avoid portability
4468 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4470 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4471 (as POSIX requires) instead of being undefined.
4474 Bison has always permitted actions such as { $$ = $1 }: it adds the
4475 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4476 is no longer output: one has to write { $$ = $1; }.
4478 ** Better C++ compliance
4479 The output parsers try to respect C++ namespaces.
4480 [This turned out to be a failed experiment, and it was reverted later.]
4483 Fixed bugs when reporting useless nonterminals.
4486 The parsers work properly on 64 bit hosts.
4489 Some calls to strerror resulted in scrambled or missing error messages.
4492 When the number of shift/reduce conflicts is correct, don't issue
4495 ** The verbose report includes the rule line numbers.
4497 ** Rule line numbers are fixed in traces.
4499 ** Swedish translation
4502 Verbose parse error messages from the parsers are better looking.
4503 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4504 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4506 ** Fixed parser memory leaks.
4507 When the generated parser was using malloc to extend its stacks, the
4508 previous allocations were not freed.
4510 ** Fixed verbose output file.
4511 Some newlines were missing.
4512 Some conflicts in state descriptions were missing.
4514 ** Fixed conflict report.
4515 Option -v was needed to get the result.
4519 Mismatches are errors, not warnings.
4521 ** Fixed incorrect processing of some invalid input.
4523 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4525 ** Fixed some typos in the documentation.
4527 ** %token MY_EOF 0 is supported.
4528 Before, MY_EOF was silently renumbered as 257.
4530 ** doc/refcard.tex is updated.
4532 ** %output, %file-prefix, %name-prefix.
4536 New, aliasing "--output-file".
4539 * Noteworthy changes in release 1.30 (2001-10-26)
4541 ** "--defines" and "--graph" have now an optional argument which is the
4542 output file name. "-d" and "-g" do not change; they do not take any
4545 ** "%source_extension" and "%header_extension" are removed, failed
4548 ** Portability fixes.
4551 * Noteworthy changes in release 1.29 (2001-09-07)
4553 ** The output file does not define const, as this caused problems when used
4554 with common autoconfiguration schemes. If you still use ancient compilers
4555 that lack const, compile with the equivalent of the C compiler option
4556 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4558 ** Added "-g" and "--graph".
4560 ** The Bison manual is now distributed under the terms of the GNU FDL.
4562 ** The input and the output files has automatically a similar extension.
4564 ** Russian translation added.
4566 ** NLS support updated; should hopefully be less troublesome.
4568 ** Added the old Bison reference card.
4570 ** Added "--locations" and "%locations".
4572 ** Added "-S" and "--skeleton".
4574 ** "%raw", "-r", "--raw" is disabled.
4576 ** Special characters are escaped when output. This solves the problems
4577 of the #line lines with path names including backslashes.
4580 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4581 "%debug", "%source_extension" and "%header_extension".
4584 Automatic location tracking.
4587 * Noteworthy changes in release 1.28 (1999-07-06)
4589 ** Should compile better now with K&R compilers.
4593 ** Fixed a problem with escaping the double quote character.
4595 ** There is now a FAQ.
4598 * Noteworthy changes in release 1.27
4600 ** The make rule which prevented bison.simple from being created on
4601 some systems has been fixed.
4604 * Noteworthy changes in release 1.26
4606 ** Bison now uses Automake.
4608 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4610 ** Token numbers now start at 257 as previously documented, not 258.
4612 ** Bison honors the TMPDIR environment variable.
4614 ** A couple of buffer overruns have been fixed.
4616 ** Problems when closing files should now be reported.
4618 ** Generated parsers should now work even on operating systems which do
4619 not provide alloca().
4622 * Noteworthy changes in release 1.25 (1995-10-16)
4624 ** Errors in the input grammar are not fatal; Bison keeps reading
4625 the grammar file, and reports all the errors found in it.
4627 ** Tokens can now be specified as multiple-character strings: for
4628 example, you could use "<=" for a token which looks like <=, instead
4629 of choosing a name like LESSEQ.
4631 ** The %token_table declaration says to write a table of tokens (names
4632 and numbers) into the parser file. The yylex function can use this
4633 table to recognize multiple-character string tokens, or for other
4636 ** The %no_lines declaration says not to generate any #line preprocessor
4637 directives in the parser file.
4639 ** The %raw declaration says to use internal Bison token numbers, not
4640 Yacc-compatible token numbers, when token names are defined as macros.
4642 ** The --no-parser option produces the parser tables without including
4643 the parser engine; a project can now use its own parser engine.
4644 The actions go into a separate file called NAME.act, in the form of
4645 a switch statement body.
4648 * Noteworthy changes in release 1.23
4650 The user can define YYPARSE_PARAM as the name of an argument to be
4651 passed into yyparse. The argument should have type void *. It should
4652 actually point to an object. Grammar actions can access the variable
4653 by casting it to the proper pointer type.
4655 Line numbers in output file corrected.
4658 * Noteworthy changes in release 1.22
4660 --help option added.
4663 * Noteworthy changes in release 1.20
4665 Output file does not redefine const for C++.
4669 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4670 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4671 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4672 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4673 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4674 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4675 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4676 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4677 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4678 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4679 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4680 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4681 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4682 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4683 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4684 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4685 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4686 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4687 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4688 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4689 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4690 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4691 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4692 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4693 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4694 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4695 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE CVE
4696 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4697 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4698 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4699 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4700 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4701 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4702 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4703 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4704 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4705 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4706 LocalWords: Vais xsltproc YYNOMEM YYLOCATION signedness YYBISON MITRE's
4707 LocalWords: libreadline YYMALLOC YYFREE MSVC redefinitions
4710 ispell-dictionary: "american"
4715 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4717 This file is part of Bison, the GNU Parser Generator.
4719 Permission is granted to copy, distribute and/or modify this document
4720 under the terms of the GNU Free Documentation License, Version 1.3 or
4721 any later version published by the Free Software Foundation; with no
4722 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4723 Texts. A copy of the license is included in the "GNU Free
4724 Documentation License" file as part of this distribution.