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 C++ GLR parser was added: "glr2.cc". It generates
71 "true C++11", instead of a C++ wrapper around a C parser as does the
72 existing "glr.cc" parser. As a first significant consequence, it supports
73 `%define api.value.type variant`, contrary to glr.cc.
75 It should be upward compatible in terms of interface, feature and
76 performance to "glr.cc". To try it out, simply use
80 It will eventually replace "glr.cc". However we need user feedback on
81 this skeleton. _Please_ report your results and comments about it.
85 Counterexamples now show the rule numbers, and always show ε for rules
86 with an empty right-hand side. For instance
98 *** Lookahead correction in Java
100 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
103 *** Abort parsing for memory exhaustion (C)
105 User actions may now use `YYNOMEM` (similar to `YYACCEPT` and `YYABORT`)
106 to abort the current parse with memory exhaustion.
108 *** Printing locations in debug traces (C)
110 The `YYLOCATION_PRINT(File, Loc)` macro prints a location. It is defined
111 when (i) locations are enabled, (ii) the default type for locations is
112 used, (iii) debug traces are enabled, and (iv) `YYLOCATION_PRINT` is not
115 Users may define `YYLOCATION_PRINT` to cover other cases.
119 There were no debug traces for deferred calls to user actions. They are
123 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
127 *** Reused Push Parsers
129 When a push-parser state structure is used for multiple parses, it was
130 possible for some state to leak from one run into the following one.
132 *** Fix Table Generation
134 In some very rare conditions, when there are many useless tokens, it was
135 possible to generate incorrect parsers.
138 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
142 *** Counterexample Generation
144 In some cases counterexample generation could crash. This is fixed.
146 *** Fix Table Generation
148 In some very rare conditions, when there are many useless tokens, it was
149 possible to generate incorrect parsers.
151 *** GLR parsers now support %merge together with api.value.type=union.
153 *** C++ parsers use noexcept in more places.
155 *** Generated parsers avoid some warnings about signedness issues.
157 *** C-language parsers now avoid warnings from pedantic clang.
159 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
162 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
166 *** Bug fixes in yacc.c
168 In Yacc mode, all the tokens are defined twice: once as an enum, and then
169 as a macro. YYEMPTY was missing its macro.
171 *** Bug fixes in lalr1.cc
173 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
174 even when the `parse.assert` %define variable is not enabled. It no
177 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
180 When there is a very large number of tokens, some assertions could be long
181 enough to hit arbitrary limits in Visual C++. They have been rewritten to
182 work around this limitation.
186 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
187 skeleton) used to be defined to 1. It is now defined to the version of
188 Bison as an integer (e.g., 30704 for version 3.7.4).
191 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
195 Fix concurrent build issues.
197 The bison executable is no longer linked uselessly against libreadline.
199 Fix incorrect use of yytname in glr.cc.
202 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
204 This release of Bison fixes all known bugs reported for Bison in MITRE's
205 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
206 are only about bison-the-program itself, not the generated code.
208 Although these bugs are typically irrelevant to how Bison is used, they
209 are worth fixing if only to give users peace of mind.
211 There is no known vulnerability in the generated parsers.
215 Fix concurrent build issues (introduced in Bison 3.5).
217 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
219 Fix portability issues of the test suite, and of bison itself.
221 Some unlikely crashes found by fuzzing have been fixed. This is only
222 about bison itself, not the generated parsers.
225 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
229 Crash when a token alias contains a NUL byte.
231 Portability issues with libtextstyle.
233 Portability issues of Bison itself with MSVC.
237 Improvements and fixes in the documentation.
239 More precise location about symbol type redefinitions.
242 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
244 ** Deprecated features
246 The YYPRINT macro, which works only with yacc.c and only for tokens, was
247 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
248 It is deprecated and its support will be removed eventually.
250 In conformance with the recommendations of the Graphviz team, in the next
251 version Bison the option `--graph` will generate a *.gv file by default,
252 instead of *.dot. A transition started in Bison 3.4.
256 *** Counterexample Generation
258 Contributed by Vincent Imbimbo.
260 When given `-Wcounterexamples`/`-Wcex`, bison will now output
261 counterexamples for conflicts.
263 **** Unifying Counterexamples
265 Unifying counterexamples are strings which can be parsed in two ways due
266 to the conflict. For example on a grammar that contains the usual
267 "dangling else" ambiguity:
270 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
271 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
274 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
275 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
276 Example: "if" exp "then" "if" exp "then" exp • "else" exp
279 ↳ "if" exp "then" exp
280 ↳ "if" exp "then" exp • "else" exp
281 Example: "if" exp "then" "if" exp "then" exp • "else" exp
284 ↳ "if" exp "then" exp "else" exp
285 ↳ "if" exp "then" exp •
287 When text styling is enabled, colors are used in the examples and the
288 derivations to highlight the structure of both analyses. In this case,
290 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
294 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
297 The counterexamples are "focused", in two different ways. First, they do
298 not clutter the output with all the derivations from the start symbol,
299 rather they start on the "conflicted nonterminal". They go straight to the
300 point. Second, they don't "expand" nonterminal symbols uselessly.
302 **** Nonunifying Counterexamples
304 In the case of the dangling else, Bison found an example that can be
305 parsed in two ways (therefore proving that the grammar is ambiguous).
306 When it cannot find such an example, it instead generates two examples
307 that are the same up until the dot:
310 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
311 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
312 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
317 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
318 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
319 First example: expr • ID ',' ID $end
326 Second example: expr • ID $end
332 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
336 In these cases, the parser usually doesn't have enough lookahead to
337 differentiate the two given examples.
341 Counterexamples are also included in the report when given
342 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
347 1 exp: "if" exp "then" exp • [$end, "then", "else"]
348 2 | "if" exp "then" exp • "else" exp
350 "else" shift, and go to state 8
352 "else" [reduce using rule 1 (exp)]
353 $default reduce using rule 1 (exp)
355 shift/reduce conflict on token "else":
356 1 exp: "if" exp "then" exp •
357 2 exp: "if" exp "then" exp • "else" exp
358 Example: "if" exp "then" "if" exp "then" exp • "else" exp
361 ↳ "if" exp "then" exp
362 ↳ "if" exp "then" exp • "else" exp
363 Example: "if" exp "then" "if" exp "then" exp • "else" exp
366 ↳ "if" exp "then" exp "else" exp
367 ↳ "if" exp "then" exp •
369 *** File prefix mapping
371 Contributed by Joshua Watt.
373 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
374 in the output (specifically `#line` directives and `#ifdef` header guards)
375 that begins with the prefix OLD will have it replaced with the prefix NEW,
376 similar to the `-ffile-prefix-map` in GCC. This option can be used to
377 make bison output reproducible.
383 When text styling is enabled and the terminal supports it, the warnings
384 now include hyperlinks to the documentation.
386 *** Relocatable installation
388 When installed to be relocatable (via `configure --enable-relocatable`),
389 bison will now also look for a relocated m4.
393 The `filename_type` %define variable was renamed `api.filename.type`.
396 %define filename_type "symbol"
400 %define api.filename.type {symbol}
402 (Or let `bison --update` do it for you).
404 It now defaults to `const std::string` instead of `std::string`.
406 *** Deprecated %define variable names
408 The following variables have been renamed for consistency. Backward
409 compatibility is ensured, but upgrading is recommended.
411 filename_type -> api.filename.type
412 package -> api.package
414 *** Push parsers no longer clear their state when parsing is finished
416 Previously push-parsers cleared their state when parsing was finished (on
417 success and on failure). This made it impossible to check if there were
418 parse errors, since `yynerrs` was also reset. This can be especially
419 troublesome when used in autocompletion, since a parser with error
420 recovery would suggest (irrelevant) expected tokens even if there were
423 Now the parser state can be examined when parsing is finished. The parser
424 state is reset when starting a new parse.
430 The bistromathic demonstrates %param and how to quote sources in the error
434 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
440 *** Include the generated header (yacc.c)
442 Historically, when --defines was used, bison generated a header and pasted
443 an exact copy of it into the generated parser implementation file. Since
444 Bison 3.4 it is possible to specify that the header should be `#include`d,
445 and how. For instance
447 %define api.header.include {"parse.h"}
451 %define api.header.include {<parser/parse.h>}
453 Now api.header.include defaults to `"header-basename"`, as was intended in
454 Bison 3.4, where `header-basename` is the basename of the generated
455 header. This is disabled when the generated header is `y.tab.h`, to
456 comply with Automake's ylwrap.
458 *** String aliases are faithfully propagated
460 Bison used to interpret user strings (i.e., decoding backslash escapes)
461 when reading them, and to escape them (i.e., issue non-printable
462 characters as backslash escapes, taking the locale into account) when
463 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
464 up "ciphered" as sequences of backslash escapes. This happened not only
465 in the generated sources (where the compiler will reinterpret them), but
466 also in all the generated reports (text, xml, html, dot, etc.). Reports
467 were therefore not readable when string aliases were not pure ASCII.
468 Worse yet: the output depended on the user's locale.
470 Now Bison faithfully treats the string aliases exactly the way the user
471 spelled them. This fixes all the aforementioned problems. However, now,
472 string aliases semantically equivalent but syntactically different (e.g.,
473 "A", "\x41", "\101") are considered to be different.
475 *** Crash when generating IELR
477 An old, well hidden, bug in the generation of IELR parsers was fixed.
480 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
484 In glr.cc some internal macros leaked in the user's code, and could damage
485 access to the token kinds.
488 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
492 Incorrect comments in the generated parsers.
494 Warnings in push parsers (yacc.c).
496 Incorrect display of gotos in LAC traces (lalr1.cc).
499 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
503 Some tests were fixed.
505 When token aliases contain comment delimiters:
507 %token FOO "/* foo */"
509 bison used to emit "nested" comments, which is invalid C.
512 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
516 Restored ANSI-C compliance in yacc.c.
518 GNU readline portability issues.
520 In C++, yy::parser::symbol_name is now a public member, as was intended.
524 In C++, yy::parser::symbol_type now has a public name() member function.
527 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
529 ** Backward incompatible changes
531 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
533 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
534 depend on it will now produce Yacc-like error messages (just "syntax
535 error"). It was superseded by the "%error-verbose" directive in Bison
536 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
537 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
538 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
539 parse.error verbose".
541 ** Deprecated features
543 The YYPRINT macro, which works only with yacc.c and only for tokens, was
544 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
545 It is deprecated and its support will be removed eventually.
549 *** Improved syntax error messages
551 Two new values for the %define parse.error variable offer more control to
552 the user. Available in all the skeletons (C, C++, Java).
554 **** %define parse.error detailed
556 The behavior of "%define parse.error detailed" is closely resembling that
557 of "%define parse.error verbose" with a few exceptions. First, it is safe
558 to use non-ASCII characters in token aliases (with 'verbose', the result
559 depends on the locale with which bison was run). Second, a yysymbol_name
560 function is exposed to the user, instead of the yytnamerr function and the
561 yytname table. Third, token internationalization is supported (see
564 **** %define parse.error custom
566 With this directive, the user forges and emits the syntax error message
567 herself by defining the yyreport_syntax_error function. A new type,
568 yypcontext_t, captures the circumstances of the error, and provides the
569 user with functions to get details, such as yypcontext_expected_tokens to
570 get the list of expected token kinds.
572 A possible implementation of yyreport_syntax_error is:
575 yyreport_syntax_error (const yypcontext_t *ctx)
578 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
579 fprintf (stderr, ": syntax error");
580 // Report the tokens expected at this point.
582 enum { TOKENMAX = 10 };
583 yysymbol_kind_t expected[TOKENMAX];
584 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
586 // Forward errors to yyparse.
589 for (int i = 0; i < n; ++i)
590 fprintf (stderr, "%s %s",
591 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
593 // Report the unexpected token.
595 yysymbol_kind_t lookahead = yypcontext_token (ctx);
596 if (lookahead != YYSYMBOL_YYEMPTY)
597 fprintf (stderr, " before %s", yysymbol_name (lookahead));
599 fprintf (stderr, "\n");
603 **** Token aliases internationalization
605 When the %define variable parse.error is set to `custom` or `detailed`,
606 one may specify which token aliases are to be translated using _(). For
618 In that case the user must define _() and N_(), and yysymbol_name returns
619 the translated symbol (i.e., it returns '_("variable")' rather that
620 '"variable"'). In Java, the user must provide an i18n() function.
622 *** List of expected tokens (yacc.c)
624 Push parsers may invoke yypstate_expected_tokens at any point during
625 parsing (including even before submitting the first token) to get the list
626 of possible tokens. This feature can be used to propose autocompletion
627 (see below the "bistromathic" example).
629 It makes little sense to use this feature without enabling LAC (lookahead
632 *** Returning the error token
634 When the scanner returns an invalid token or the undefined token
635 (YYUNDEF), the parser generates an error message and enters error
636 recovery. Because of that error message, most scanners that find lexical
637 errors generate an error message, and then ignore the invalid input
638 without entering the error-recovery.
640 The scanners may now return YYerror, the error token, to enter the
641 error-recovery mode without triggering an additional error message. See
642 the bistromathic for an example.
644 *** Deep overhaul of the symbol and token kinds
646 To avoid the confusion with types in programming languages, we now refer
647 to token and symbol "kinds" instead of token and symbol "types". The
648 documentation and error messages have been revised.
650 All the skeletons have been updated to use dedicated enum types rather
651 than integral types. Special symbols are now regular citizens, instead of
652 being declared in ad hoc ways.
656 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
657 LPAREN, etc. While backward compatibility is of course ensured, users are
658 nonetheless invited to replace their uses of "enum yytokentype" by
661 This type now also includes tokens that were previously hidden: YYEOF (end
662 of input), YYUNDEF (undefined token), and YYerror (error token). They
663 now have string aliases, internationalized when internationalization is
664 enabled. Therefore, by default, error messages now refer to "end of file"
665 (internationalized) rather than the cryptic "$end", or to "invalid token"
666 rather than "$undefined".
668 Therefore in most cases it is now useless to define the end-of-line token
671 %token T_EOF 0 "end of file"
673 Rather simply use "YYEOF" in your scanner.
677 The "symbol kinds" is what the parser actually uses. (Unless the
678 api.token.raw %define variable is used, the symbol kind of a terminal
679 differs from the corresponding token kind.)
681 They are now exposed as a enum, "yysymbol_kind_t".
683 This allows users to tailor the error messages the way they want, or to
684 process some symbols in a specific way in autocompletion (see the
685 bistromathic example below).
687 *** Modernize display of explanatory statements in diagnostics
689 Since Bison 2.7, output was indented four spaces for explanatory
690 statements. For example:
692 input.y:2.7-13: error: %type redeclaration for exp
693 input.y:1.7-11: previous declaration
695 Since the introduction of caret-diagnostics, it became less clear. This
696 indentation has been removed and submessages are displayed similarly as in
699 input.y:2.7-13: error: %type redeclaration for exp
700 2 | %type <float> exp
702 input.y:1.7-11: note: previous declaration
706 Contributed by Victor Morales Cayuela.
710 The token and symbol kinds are yy::parser::token_kind_type and
711 yy::parser::symbol_kind_type.
713 The symbol_type::kind() member function allows to get the kind of a
714 symbol. This can be used to write unit tests for scanners, e.g.,
716 yy::parser::symbol_type t = make_NUMBER ("123");
717 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
718 assert (t.value.as<int> () == 123);
724 In order to avoid ambiguities with "type" as in "typing", we now refer to
725 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
726 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
731 There are now examples/java: a very simple calculator, and a more complete
732 one (push-parser, location tracking, and debug traces).
734 The lexcalc example (a simple example in C based on Flex and Bison) now
735 also demonstrates location tracking.
738 A new C example, bistromathic, is a fully featured interactive calculator
739 using many Bison features: pure interface, push parser, autocompletion
740 based on the current parser state (using yypstate_expected_tokens),
741 location tracking, internationalized custom error messages, lookahead
742 correction, rich debug traces, etc.
744 It shows how to depend on the symbol kinds to tailor autocompletion. For
745 instance it recognizes the symbol kind "VARIABLE" to propose
746 autocompletion on the existing variables, rather than of the word
750 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
752 ** WARNING: Future backward-incompatibilities!
754 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
756 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
757 that still depend on it will produce Yacc-like error messages (just
758 "syntax error"). It was superseded by the "%error-verbose" directive in
759 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
760 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
761 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
762 parse.error verbose".
766 Fix portability issues of the package itself on old compilers.
768 Fix api.token.raw support in Java.
771 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
775 Error messages could quote lines containing zero-width characters (such as
776 \005) with incorrect styling. Fixes for similar issues with unexpectedly
777 short lines (e.g., the file was changed between parsing and diagnosing).
779 Some unlikely crashes found by fuzzing have been fixed. This is only
780 about bison itself, not the generated parsers.
783 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
787 Portability issues and minor cosmetic issues.
789 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
793 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
799 Fix compiler warnings.
802 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
804 ** Backward incompatible changes
806 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
807 longer treated as end-of-lines. This changes the diagnostics, and in
808 particular their locations.
810 In C++, line numbers and columns are now represented as 'int' not
811 'unsigned', so that integer overflow on positions is easily checkable via
812 'gcc -fsanitize=undefined' and the like. This affects the API for
813 positions. The default position and location classes now expose
814 'counter_type' (int), used to define line and column numbers.
816 ** Deprecated features
818 The YYPRINT macro, which works only with yacc.c and only for tokens, was
819 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
820 It is deprecated and its support will be removed eventually.
824 *** Lookahead correction in C++
826 Contributed by Adrian Vogelsgesang.
828 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
829 %define variable parse.lac.
831 *** Variable api.token.raw: Optimized token numbers (all skeletons)
833 In the generated parsers, tokens have two numbers: the "external" token
834 number as returned by yylex (which starts at 257), and the "internal"
835 symbol number (which starts at 3). Each time yylex is called, a table
836 lookup maps the external token number to the internal symbol number.
838 When the %define variable api.token.raw is set, tokens are assigned their
839 internal number, which saves one table lookup per token, and also saves
840 the generation of the mapping table.
842 The gain is typically moderate, but in extreme cases (very simple user
843 actions), a 10% improvement can be observed.
845 *** Generated parsers use better types for states
847 Stacks now use the best integral type for state numbers, instead of always
848 using 15 bits. As a result "small" parsers now have a smaller memory
849 footprint (they use 8 bits), and there is support for large automata (16
850 bits), and extra large (using int, i.e., typically 31 bits).
852 *** Generated parsers prefer signed integer types
854 Bison skeletons now prefer signed to unsigned integer types when either
855 will do, as the signed types are less error-prone and allow for better
856 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
857 portable to unusual machines where char, short and int are all the same
858 width. On non-GNU platforms this may entail including <limits.h> and (if
859 available) <stdint.h> to define integer types and constants.
861 *** A skeleton for the D programming language
863 For the last few releases, Bison has shipped a stealth experimental
864 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
865 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
868 However, because nobody has committed to improving, testing, and
869 documenting this skeleton, it is not clear that it will be supported in
872 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
873 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
875 *** Debug traces in Java
877 The Java backend no longer emits code and data for parser tracing if the
878 %define variable parse.trace is not defined.
882 *** New diagnostic: -Wdangling-alias
884 String literals, which allow for better error messages, are (too)
885 liberally accepted by Bison, which might result in silent errors. For
888 %type <exVal> cond "condition"
890 does not define "condition" as a string alias to 'cond' (nonterminal
891 symbols do not have string aliases). It is rather equivalent to
894 %token <exVal> "condition"
896 i.e., it gives the type 'exVal' to the "condition" token, which was
897 clearly not the intention.
899 Also, because string aliases need not be defined, typos such as "baz"
900 instead of "bar" will be not reported.
902 The option `-Wdangling-alias` catches these situations. On
905 %type <ival> foo "foo"
909 bison -Wdangling-alias reports
911 warning: string literal not attached to a symbol
912 | %type <ival> foo "foo"
914 warning: string literal not attached to a symbol
918 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
920 *** Better POSIX Yacc compatibility diagnostics
922 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
926 %type <ival> TOKEN1 TOKEN2 't'
933 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
934 2 | %type <ival> TOKEN1 TOKEN2 't'
936 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
937 2 | %type <ival> TOKEN1 TOKEN2 't'
939 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
940 2 | %type <ival> TOKEN1 TOKEN2 't'
943 *** Diagnostics with insertion
945 The diagnostics now display the suggestion below the underlined source.
946 Replacement for undeclared symbols are now also suggested.
953 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'?
957 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
961 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
963 *** Diagnostics about long lines
965 Quoted sources may now be truncated to fit the screen. For instance, on a
966 30-column wide terminal:
973 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
976 foo.y:1.8-10: previous declaration
979 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
982 foo.y:1.8-10: previous declaration
988 *** Debugging glr.c and glr.cc
990 The glr.c skeleton always had asserts to check its own behavior (not the
991 user's). These assertions are now under the control of the parse.assert
992 %define variable (disabled by default).
996 Several new compiler warnings in the generated output have been avoided.
997 Some unused features are no longer emitted. Cleaner generated code in
1002 Portability issues in the test suite.
1004 In theory, parsers using %nonassoc could crash when reporting verbose
1005 error messages. This unlikely bug has been fixed.
1007 In Java, %define api.prefix was ignored. It now behaves as expected.
1010 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
1014 In some cases, when warnings are disabled, bison could emit tons of white
1015 spaces as diagnostics.
1017 When running out of memory, bison could crash (found by fuzzing).
1019 When defining twice the EOF token, bison would crash.
1021 New warnings from recent compilers have been addressed in the generated
1022 parsers (yacc.c, glr.c, glr.cc).
1024 When lone carriage-return characters appeared in the input file,
1025 diagnostics could hang forever.
1028 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
1035 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
1037 ** Deprecated features
1039 The %pure-parser directive is deprecated in favor of '%define api.pure'
1040 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
1041 now. Note that since Bison 2.7 you are strongly encouraged to use
1042 '%define api.pure full' instead of '%define api.pure'.
1046 *** Colored diagnostics
1048 As an experimental feature, diagnostics are now colored, controlled by the
1049 new options --color and --style.
1051 To use them, install the libtextstyle library before configuring Bison.
1052 It is available from
1054 https://alpha.gnu.org/gnu/gettext/
1058 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
1060 The option --color supports the following arguments:
1061 - always, yes: Enable colors.
1062 - never, no: Disable colors.
1063 - auto, tty (default): Enable colors if the output device is a tty.
1065 To customize the styles, create a CSS file similar to
1069 .error { font-weight: 800; text-decoration: underline; }
1072 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1073 environment variable to "bison-bw.css".
1075 *** Disabling output
1077 When given -fsyntax-only, the diagnostics are reported, but no output is
1080 The name of this option is somewhat misleading as bison does more than
1081 just checking the syntax: every stage is run (including checking for
1082 conflicts for instance), except the generation of the output files.
1084 *** Include the generated header (yacc.c)
1086 Before, when --defines is used, bison generated a header, and pasted an
1087 exact copy of it into the generated parser implementation file. If the
1088 header name is not "y.tab.h", it is now #included instead of being
1091 To use an '#include' even if the header name is "y.tab.h" (which is what
1092 happens with --yacc, or when using the Autotools' ylwrap), define
1093 api.header.include to the exact argument to pass to #include. For
1096 %define api.header.include {"parse.h"}
1100 %define api.header.include {<parser/parse.h>}
1102 *** api.location.type is now supported in C (yacc.c, glr.c)
1104 The %define variable api.location.type defines the name of the type to use
1105 for locations. When defined, Bison no longer defines YYLTYPE.
1107 This can be used in programs with several parsers to factor their
1108 definition of locations: let one of them generate them, and the others
1115 In conformance with the recommendations of the Graphviz team, if %require
1116 "3.4" (or better) is specified, the option --graph generates a *.gv file
1117 by default, instead of *.dot.
1119 *** Diagnostics overhaul
1121 Column numbers were wrong with multibyte characters, which would also
1122 result in skewed diagnostics with carets. Beside, because we were
1123 indenting the quoted source with a single space, lines with tab characters
1124 were incorrectly underlined.
1126 To address these issues, and to be clearer, Bison now issues diagnostics
1127 as GCC9 does. For instance it used to display (there's a tab before the
1130 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1131 expr: expr '+' "number" { $$ = $1 + $2; }
1135 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1136 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1139 Other constructs now also have better locations, resulting in more precise
1142 *** Fix-it hints for %empty
1144 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1145 annotations, and add the missing ones.
1147 *** Generated reports
1149 The format of the reports (parse.output) was improved for readability.
1151 *** Better support for --no-line.
1153 When --no-line is used, the generated files are now cleaner: no lines are
1154 generated instead of empty lines. Together with using api.header.include,
1155 that should help people saving the generated files into version control
1156 systems get smaller diffs.
1160 A new example in C shows an simple infix calculator with a hand-written
1161 scanner (examples/c/calc).
1163 A new example in C shows a reentrant parser (capable of recursive calls)
1164 built with Flex and Bison (examples/c/reccalc).
1166 There is a new section about the history of Yaccs and Bison.
1170 A few obscure bugs were fixed, including the second oldest (known) bug in
1171 Bison: it was there when Bison was entered in the RCS version control
1172 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1176 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1180 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1184 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1188 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1189 extensions into errors. It now makes them simple warnings (-Wyacc).
1192 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1194 A new mailing list was created, Bison Announce. It is low traffic, and is
1195 only about announcing new releases and important messages (e.g., polls
1196 about major decisions to make).
1198 https://lists.gnu.org/mailman/listinfo/bison-announce
1200 ** Backward incompatible changes
1202 Support for DJGPP, which has been unmaintained and untested for years, is
1205 ** Deprecated features
1207 A new feature, --update (see below) helps adjusting existing grammars to
1210 *** Deprecated directives
1212 The %error-verbose directive is deprecated in favor of '%define
1213 parse.error verbose' since Bison 3.0, but no warning was issued.
1215 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1216 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1217 directives are slightly different, you might need to adjust your code.
1218 %name-prefix renames only symbols with external linkage, while api.prefix
1219 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1220 yytokentype, YYSTYPE, YYLTYPE, etc.
1222 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1223 {xx}' will typically have to update YY_DECL from
1225 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1229 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1231 *** Deprecated %define variable names
1233 The following variables, mostly related to parsers in Java, have been
1234 renamed for consistency. Backward compatibility is ensured, but upgrading
1237 abstract -> api.parser.abstract
1238 annotations -> api.parser.annotations
1239 extends -> api.parser.extends
1240 final -> api.parser.final
1241 implements -> api.parser.implements
1242 parser_class_name -> api.parser.class
1243 public -> api.parser.public
1244 strictfp -> api.parser.strictfp
1248 *** Generation of fix-its for IDEs/Editors
1250 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1251 bison now generates machine readable editing instructions to fix some
1252 issues. Currently, this is mostly limited to updating deprecated
1253 directives and removing duplicates. For instance:
1257 %define parser_class_name "Parser"
1258 %define api.parser.class "Parser"
1262 See the "fix-it:" lines below:
1264 $ bison -ffixit foo.y
1265 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1268 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1269 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1270 %define parser_class_name "Parser"
1271 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1272 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1273 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1274 %define api.parser.class "Parser"
1275 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1276 foo.y:2.1-34: previous definition
1277 %define parser_class_name "Parser"
1278 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1279 fix-it:"foo.y":{3:1-3:34}:""
1280 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1282 This uses the same output format as GCC and Clang.
1284 *** Updating grammar files
1286 Fixes can be applied on the fly. The previous example ends with the
1287 suggestion to re-run bison with the option -u/--update, which results in a
1288 cleaner grammar file.
1290 $ bison --update foo.y
1292 bison: file 'foo.y' was updated (backup: 'foo.y~')
1295 %define parse.error verbose
1296 %define api.parser.class {Parser}
1300 *** Bison is now relocatable
1302 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1304 A relocatable program can be moved or copied to a different location on
1305 the file system. It can also be used through mount points for network
1306 sharing. It is possible to make symbolic links to the installed and moved
1307 programs, and invoke them through the symbolic link.
1309 *** %expect and %expect-rr modifiers on individual rules
1311 One can now document (and check) which rules participate in shift/reduce
1312 and reduce/reduce conflicts. This is particularly important GLR parsers,
1313 where conflicts are a normal occurrence. For example,
1329 | argument_list ',' expression
1334 Looking at the output from -v, one can see that the shift/reduce conflict
1335 here is due to the fact that the parser does not know whether to reduce
1336 arguments to argument_list until it sees the token _after_ the following
1337 ','. By marking the rule with %expect 1 (because there is a conflict in
1338 one state), we document the source of the 1 overall shift/reduce conflict.
1340 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1341 conflicts. In this case, we mark each of the conflicting rules. For
1350 target_list '=' expr ';'
1356 | target ',' target_list
1365 | expr ',' expr_list
1373 In a statement such as
1377 the parser must reduce x to a target or an expr, but does not know which
1378 until it sees the '='. So we notate the two possible reductions to
1379 indicate that each conflicts in one rule.
1381 This feature needs user feedback, and might evolve in the future.
1383 *** C++: Actual token constructors
1385 When variants and token constructors are enabled, in addition to the
1386 type-safe named token constructors (make_ID, make_INT, etc.), we now
1387 generate genuine constructors for symbol_type.
1389 For instance with these declarations
1395 you may use these constructors:
1397 symbol_type (int token, const std::string&);
1398 symbol_type (int token, const int&);
1399 symbol_type (int token);
1401 Correct matching between token types and value types is checked via
1402 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1403 constructors are preferable, as they offer better type safety (for
1404 instance 'make_ID (42)' would not even compile), but symbol_type
1405 constructors may help when token types are discovered at run-time, e.g.,
1408 if (auto i = lookup_keyword (yytext))
1409 return yy::parser::symbol_type (i);
1411 return yy::parser::make_ID (yytext);
1414 *** C++: Variadic emplace
1416 If your application requires C++11 and you don't use symbol constructors,
1417 you may now use a variadic emplace for semantic values:
1419 %define api.value.type variant
1420 %token <std::pair<int, int>> PAIR
1424 int yylex (parser::semantic_type *lvalp)
1426 lvalp->emplace <std::pair<int, int>> (1, 2);
1427 return parser::token::PAIR;
1430 *** C++: Syntax error exceptions in GLR
1432 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1433 actions, or from the scanner.
1435 *** More POSIX Yacc compatibility warnings
1437 More Bison specific directives are now reported with -y or -Wyacc. This
1438 change was ready since the release of Bison 3.0 in September 2015. It was
1439 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1440 in numerous warnings for Bison users that use the GNU Build System.
1442 If you still experience that problem, either redefine YACC as `bison -o
1443 y.tab.c`, or pass -Wno-yacc to Bison.
1445 *** The tables yyrhs and yyphrs are back
1447 Because no Bison skeleton uses them, these tables were removed (no longer
1448 passed to the skeletons, not even computed) in 2008. However, some users
1449 have expressed interest in being able to use them in their own skeletons.
1453 *** Incorrect number of reduce/reduce conflicts
1455 On a grammar such as
1457 exp: "num" | "num" | "num"
1459 bison used to report a single RR conflict, instead of two. This is now
1460 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1461 was entered in the RCS version control system, in December 1987.
1463 Some grammar files might have to adjust their %expect-rr.
1465 *** Parser directives that were not careful enough
1467 Passing invalid arguments to %nterm, for instance character literals, used
1468 to result in unclear error messages.
1472 The examples/ directory (installed in .../share/doc/bison/examples) has
1473 been restructured per language for clarity. The examples come with a
1474 README and a Makefile. Not only can they be used to toy with Bison, they
1475 can also be starting points for your own grammars.
1477 There is now a Java example, and a simple example in C based on Flex and
1478 Bison (examples/c/lexcalc/).
1484 They now use noexcept and constexpr. Please, report missing annotations.
1486 *** Symbol Declarations
1488 The syntax of the variation directives to declare symbols was overhauled
1489 for more consistency, and also better POSIX Yacc compliance (which, for
1490 instance, allows "%type" without actually providing a type). The %nterm
1491 directive, supported by Bison since its inception, is now documented and
1492 officially supported.
1494 The syntax is now as follows:
1496 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1497 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1498 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1499 %nterm TAG? ID+ ( TAG ID+ )*
1501 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1502 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1503 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1504 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1505 one), ‘*’ (zero or more) and ‘+’ (one or more).
1508 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1512 Fix the move constructor of symbol_type.
1514 Always provide a copy constructor for symbol_type, even in modern C++.
1517 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1521 Properly support token constructors in C++ with types that include commas
1522 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1525 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1529 C++ portability issues.
1532 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1536 Several portability issues have been fixed in the build system, in the
1537 test suite, and in the generated parsers in C++.
1540 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1542 ** Backward incompatible changes
1544 Support for DJGPP, which has been unmaintained and untested for years, is
1545 obsolete. Unless there is activity to revive it, it will be removed.
1549 %printers should use yyo rather than yyoutput to denote the output stream.
1551 Variant-based symbols in C++ should use emplace() rather than build().
1553 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1557 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1559 A comment in the generated code now emphasizes that users should not
1560 depend upon non-documented implementation details, such as macros starting
1565 *** C++: Support for move semantics (lalr1.cc)
1567 The lalr1.cc skeleton now fully supports C++ move semantics, while
1568 maintaining compatibility with C++98. You may now store move-only types
1569 when using Bison's variants. For instance:
1576 %skeleton "lalr1.cc"
1577 %define api.value.type variant
1581 %token <int> INT "int";
1582 %type <std::unique_ptr<int>> int;
1583 %type <std::vector<std::unique_ptr<int>>> list;
1587 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1589 int: "int" { $$ = std::make_unique<int>($1); }
1591 *** C++: Implicit move of right-hand side values (lalr1.cc)
1593 In modern C++ (C++11 and later), you should always use 'std::move' with
1594 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1595 popped from the stack anyway. Using 'std::move' is mandatory for
1596 move-only types such as unique_ptr, and it provides a significant speedup
1597 for large types such as std::string, or std::vector, etc.
1599 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1600 by 'std::move ($n)'. The second rule in the previous grammar can be
1603 list: list int { $$ = $1; $$.emplace_back($2); }
1605 With automove enabled, the semantic values are no longer lvalues, so do
1606 not use the swap idiom:
1608 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1610 This idiom is anyway obsolete: it is preferable to move than to swap.
1612 A warning is issued when automove is enabled, and a value is used several
1615 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1616 exp: "twice" exp { $$ = $2 + $2; }
1619 Enabling api.value.automove does not require support for modern C++. The
1620 generated code is valid C++98/03, but will use copies instead of moves.
1622 The new examples/c++/variant-11.yy shows these features in action.
1624 *** C++: The implicit default semantic action is always run
1626 When variants are enabled, the default action was not run, so
1634 It now behaves like in all the other cases, as
1636 exp: "number" { $$ = $1; }
1638 possibly using std::move if automove is enabled.
1640 We do not expect backward compatibility issues. However, beware of
1641 forward compatibility issues: if you rely on default actions with
1642 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1643 generate incorrect parsers.
1645 *** C++: Renaming location.hh
1647 When both %defines and %locations are enabled, Bison generates a
1648 location.hh file. If you don't use locations outside of the parser, you
1649 may avoid its creation with:
1651 %define api.location.file none
1653 However this file is useful if, for instance, your parser builds an AST
1654 decorated with locations: you may use Bison's location independently of
1655 Bison's parser. You can now give it another name, for instance:
1657 %define api.location.file "my-location.hh"
1659 This name can have directory components, and even be absolute. The name
1660 under which the location file is included is controlled by
1661 api.location.include.
1663 This way it is possible to have several parsers share the same location
1666 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1669 %define api.namespace {foo}
1670 %define api.location.file "include/ast/loc.hh"
1671 %define api.location.include {<ast/loc.hh>}
1673 and use it in src/bar/parser.hh:
1676 %define api.namespace {bar}
1677 %code requires {#include <ast/loc.hh>}
1678 %define api.location.type {bar::location}
1680 Absolute file names are supported, so in your Makefile, passing the flag
1681 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1684 *** C++: stack.hh and position.hh are deprecated
1686 When asked to generate a header file (%defines), the lalr1.cc skeleton
1687 generates a stack.hh file. This file had no interest for users; it is now
1688 made useless: its content is included in the parser definition. It is
1689 still generated for backward compatibility.
1691 When in addition to %defines, location support is requested (%locations),
1692 the file position.hh is also generated. It is now also useless: its
1693 content is now included in location.hh.
1695 These files are no longer generated when your grammar file requires at
1696 least Bison 3.2 (%require "3.2").
1700 Portability issues on MinGW and VS2015.
1702 Portability issues in the test suite.
1704 Portability/warning issues with Flex.
1707 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1709 ** Backward incompatible changes
1711 Compiling Bison now requires a C99 compiler---as announced during the
1712 release of Bison 3.0, five years ago. Generated parsers do not require a
1715 Support for DJGPP, which has been unmaintained and untested for years, is
1716 obsolete. Unless there is activity to revive it, the next release of Bison
1717 will have it removed.
1721 *** Typed midrule actions
1723 Because their type is unknown to Bison, the values of midrule actions are
1724 not treated like the others: they don't have %printer and %destructor
1725 support. It also prevents C++ (Bison) variants to handle them properly.
1727 Typed midrule actions address these issues. Instead of:
1729 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1733 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1735 *** Reports include the type of the symbols
1737 The sections about terminal and nonterminal symbols of the '*.output' file
1738 now specify their declared type. For instance, for:
1742 the report now shows '<ival>':
1744 Terminals, with rules where they appear
1748 *** Diagnostics about useless rules
1750 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1751 of course, its rules are useless too.
1755 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1757 Previously all the useless rules were reported, including those whose
1758 left-hand side is the 'exp' nonterminal:
1760 warning: 1 nonterminal useless in grammar [-Wother]
1761 warning: 4 rules useless in grammar [-Wother]
1762 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1765 2.14-16: warning: rule useless in grammar [-Wother]
1768 3.6-16: warning: rule useless in grammar [-Wother]
1769 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1771 3.20-30: warning: rule useless in grammar [-Wother]
1772 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1774 3.34-44: warning: rule useless in grammar [-Wother]
1775 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1778 Now, rules whose left-hand side symbol is useless are no longer reported
1779 as useless. The locations of the errors have also been adjusted to point
1780 to the first use of the nonterminal as a left-hand side of a rule:
1782 warning: 1 nonterminal useless in grammar [-Wother]
1783 warning: 4 rules useless in grammar [-Wother]
1784 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1785 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1787 2.14-16: warning: rule useless in grammar [-Wother]
1791 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1793 When compiled with exceptions disabled, the generated parsers no longer
1794 uses try/catch clauses.
1796 Currently only GCC and Clang are supported.
1800 *** A demonstration of variants
1802 A new example was added (installed in .../share/doc/bison/examples),
1803 'variant.yy', which shows how to use (Bison) variants in C++.
1805 The other examples were made nicer to read.
1807 *** Some features are no longer 'experimental'
1809 The following features, mature enough, are no longer flagged as
1810 experimental in the documentation: push parsers, default %printer and
1811 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1812 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1813 semantic predicates (%?).
1817 *** GLR: Predicates support broken by #line directives
1819 Predicates (%?) in GLR such as
1822 %? {new_syntax} 'w' id new_args
1823 | %?{!new_syntax} 'w' id old_args
1825 were issued with #lines in the middle of C code.
1827 *** Printer and destructor with broken #line directives
1829 The #line directives were not properly escaped when emitting the code for
1830 %printer/%destructor, which resulted in compiler errors if there are
1831 backslashes or double-quotes in the grammar file name.
1833 *** Portability on ICC
1835 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1836 Generated parsers now work around this.
1840 There were several small fixes in the test suite and in the build system,
1841 many warnings in bison and in the generated parsers were eliminated. The
1842 documentation also received its share of minor improvements.
1844 Useless code was removed from C++ parsers, and some of the generated
1845 constructors are more 'natural'.
1848 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1852 *** C++: Fix support of 'syntax_error'
1854 One incorrect 'inline' resulted in linking errors about the constructor of
1855 the syntax_error exception.
1857 *** C++: Fix warnings
1859 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1860 warnings about yyformat being possibly null. It also warned about the
1861 deprecated implicit definition of copy constructors when there's a
1862 user-defined (copy) assignment operator.
1864 *** Location of errors
1866 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1867 ride-hand side raises a syntax error. The behavior of the default parser
1868 (yacc.c) in such a condition was undefined.
1870 Now all the parsers match the behavior of glr.c: @$ is used as the
1871 location of the error. This handles gracefully rules with and without
1874 *** Portability fixes in the test suite
1876 On some platforms, some Java and/or C++ tests were failing.
1879 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1883 *** C++ with Variants (lalr1.cc)
1885 Fix a compiler warning when no %destructor use $$.
1889 Several portability issues in tests were fixed.
1892 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1896 *** C++ with Variants (lalr1.cc)
1898 Problems with %destructor and '%define parse.assert' have been fixed.
1900 *** Named %union support (yacc.c, glr.c)
1902 Bison 3.0 introduced a regression on named %union such as
1904 %union foo { int ival; };
1906 The possibility to use a name was introduced "for Yacc compatibility".
1907 It is however not required by POSIX Yacc, and its usefulness is not clear.
1909 *** %define api.value.type union with %defines (yacc.c, glr.c)
1911 The C parsers were broken when %defines was used together with "%define
1912 api.value.type union".
1914 *** Redeclarations are reported in proper order
1922 bison used to report:
1924 foo.yy:2.10-11: error: %printer redeclaration for FOO
1927 foo.yy:3.10-11: previous declaration
1931 Now, the "previous" declaration is always the first one.
1936 Bison now installs various files in its docdir (which defaults to
1937 '/usr/local/share/doc/bison'), including the three fully blown examples
1938 extracted from the documentation:
1941 Reverse Polish Calculator, a simple introductory example.
1943 Multi-function Calc, a calculator with memory and functions and located
1946 a calculator in C++ using variant support and token constructors.
1949 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1953 *** Generated source files when errors are reported
1955 When warnings are issued and -Werror is set, bison would still generate
1956 the source files (*.c, *.h...). As a consequence, some runs of "make"
1957 could fail the first time, but not the second (as the files were generated
1960 This is fixed: bison no longer generates this source files, but, of
1961 course, still produces the various reports (*.output, *.xml, etc.).
1963 *** %empty is used in reports
1965 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1966 dot, XML and formats derived from it).
1968 *** YYERROR and variants
1970 When C++ variant support is enabled, an error triggered via YYERROR, but
1971 not caught via error recovery, resulted in a double deletion.
1974 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1978 *** Errors in caret diagnostics
1980 On some platforms, some errors could result in endless diagnostics.
1982 *** Fixes of the -Werror option
1984 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1985 diagnostics into errors instead of warnings. This is fixed.
1987 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1988 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1989 -Wno-error", "foo" diagnostics are now errors.
1993 As demonstrated in the documentation, one can now leave spaces between
1998 The yacc.1 man page is no longer installed if --disable-yacc was
2001 *** Fixes in the test suite
2003 Bugs and portability issues.
2006 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
2008 ** WARNING: Future backward-incompatibilities!
2010 Like other GNU packages, Bison will start using some of the C99 features
2011 for its own code, especially the definition of variables after statements.
2012 The generated C parsers still aim at C90.
2014 ** Backward incompatible changes
2016 *** Obsolete features
2018 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
2020 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
2021 use YYSTYPE and YYLTYPE.
2023 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
2024 1.875): use %lex-param, %parse-param, or %param.
2026 Missing semicolons at the end of actions are no longer added (as announced
2027 in the release 2.5).
2029 *** Use of YACC='bison -y'
2031 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
2034 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
2035 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
2036 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
2038 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
2039 implementation of Yacc, was using Bison as 'bison -y'. While it does
2040 ensure compatible output file names, it also enables warnings for
2041 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
2042 warnings for Bison extensions.
2044 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
2045 (which also generates 'y.tab.h' and 'y.output' when needed).
2046 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
2047 flags (YFLAGS, or AM_YFLAGS with Automake).
2051 *** The epilogue is no longer affected by internal #defines (glr.c)
2053 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
2054 generated code. These weren't properly undefined before the inclusion of
2055 the user epilogue, so functions such as the following were butchered by the
2056 preprocessor expansion:
2058 int yylex (YYSTYPE *yylval);
2060 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2061 identifiers for user-provided variables.
2063 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2065 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2066 locations are enabled. This is fixed.
2068 *** Warnings about useless %pure-parser/%define api.pure are restored
2070 ** Diagnostics reported by Bison
2072 Most of these features were contributed by Théophile Ranquet and Victor
2077 Version 2.7 introduced caret errors, for a prettier output. These are now
2078 activated by default. The old format can still be used by invoking Bison
2079 with -fno-caret (or -fnone).
2081 Some error messages that reproduced excerpts of the grammar are now using
2082 the caret information only. For instance on:
2089 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2090 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2094 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2095 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2099 and "bison -fno-caret" reports:
2101 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2102 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2104 *** Enhancements of the -Werror option
2106 The -Werror=CATEGORY option is now recognized, and will treat specified
2107 warnings as errors. The warnings need not have been explicitly activated
2108 using the -W option, this is similar to what GCC 4.7 does.
2110 For example, given the following command line, Bison will treat both
2111 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2112 errors (and only those):
2114 $ bison -Werror=yacc,error=conflicts-sr input.y
2116 If no categories are specified, -Werror will make all active warnings into
2117 errors. For example, the following line does the same the previous example:
2119 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2121 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2123 Note that the categories in this -Werror option may not be prefixed with
2124 "no-". However, -Wno-error[=CATEGORY] is valid.
2126 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2127 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2128 incompatibilities as warnings: "-y -Wno-error=yacc".
2130 *** The display of warnings is now richer
2132 The option that controls a given warning is now displayed:
2134 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2136 In the case of warnings treated as errors, the prefix is changed from
2137 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2138 to GCC, as [-Werror=CATEGORY].
2140 For instance, where the previous version of Bison would report (and exit
2143 bison: warnings being treated as errors
2144 input.y:1.1: warning: stray ',' treated as white space
2148 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2150 *** Deprecated constructs
2152 The new 'deprecated' warning category flags obsolete constructs whose
2153 support will be discontinued. It is enabled by default. These warnings
2154 used to be reported as 'other' warnings.
2156 *** Useless semantic types
2158 Bison now warns about useless (uninhabited) semantic types. Since
2159 semantic types are not declared to Bison (they are defined in the opaque
2160 %union structure), it is %printer/%destructor directives about useless
2161 types that trigger the warning:
2165 %printer {} <type1> <type3>
2166 %destructor {} <type2> <type4>
2168 nterm: term { $$ = $1; };
2170 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2171 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2173 *** Undefined but unused symbols
2175 Bison used to raise an error for undefined symbols that are not used in
2176 the grammar. This is now only a warning.
2179 %destructor {} symbol2
2180 %type <type> symbol3
2184 *** Useless destructors or printers
2186 Bison now warns about useless destructors or printers. In the following
2187 example, the printer for <type1>, and the destructor for <type2> are
2188 useless: all symbols of <type1> (token1) already have a printer, and all
2189 symbols of type <type2> (token2) already have a destructor.
2191 %token <type1> token1
2195 %printer {} token1 <type1> <type3>
2196 %destructor {} token2 <type2> <type4>
2200 The warnings and error messages about shift/reduce and reduce/reduce
2201 conflicts have been normalized. For instance on the following foo.y file:
2205 exp: exp '+' exp | '0' | '0';
2207 compare the previous version of bison:
2210 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2211 $ bison -Werror foo.y
2212 bison: warnings being treated as errors
2213 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2215 with the new behavior:
2218 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2219 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2220 $ bison -Werror foo.y
2221 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2222 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2224 When %expect or %expect-rr is used, such as with bar.y:
2229 exp: exp '+' exp | '0' | '0';
2234 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2235 bar.y: expected 0 shift/reduce conflicts
2236 bar.y: expected 0 reduce/reduce conflicts
2241 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2242 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2244 ** Incompatibilities with POSIX Yacc
2246 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2249 ** Additional yylex/yyparse arguments
2251 The new directive %param declares additional arguments to both yylex and
2252 yyparse. The %lex-param, %parse-param, and %param directives support one
2253 or more arguments. Instead of
2255 %lex-param {arg1_type *arg1}
2256 %lex-param {arg2_type *arg2}
2257 %parse-param {arg1_type *arg1}
2258 %parse-param {arg2_type *arg2}
2262 %param {arg1_type *arg1} {arg2_type *arg2}
2264 ** Types of values for %define variables
2266 Bison used to make no difference between '%define foo bar' and '%define
2267 foo "bar"'. The former is now called a 'keyword value', and the latter a
2268 'string value'. A third kind was added: 'code values', such as '%define
2271 Keyword variables are used for fixed value sets, e.g.,
2273 %define lr.type lalr
2275 Code variables are used for value in the target language, e.g.,
2277 %define api.value.type {struct semantic_type}
2279 String variables are used remaining cases, e.g. file names.
2281 ** Variable api.token.prefix
2283 The variable api.token.prefix changes the way tokens are identified in
2284 the generated files. This is especially useful to avoid collisions
2285 with identifiers in the target language. For instance
2287 %token FILE for ERROR
2288 %define api.token.prefix {TOK_}
2290 start: FILE for ERROR;
2292 will generate the definition of the symbols TOK_FILE, TOK_for, and
2293 TOK_ERROR in the generated sources. In particular, the scanner must
2294 use these prefixed token names, although the grammar itself still
2295 uses the short names (as in the sample rule given above).
2297 ** Variable api.value.type
2299 This new %define variable supersedes the #define macro YYSTYPE. The use
2300 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2301 using %union or %defining api.value.type results in undefined behavior.
2303 Either define api.value.type, or use "%union":
2310 %token <ival> INT "integer"
2311 %token <sval> STRING "string"
2312 %printer { fprintf (yyo, "%d", $$); } <ival>
2313 %destructor { free ($$); } <sval>
2316 yylval.ival = 42; return INT;
2317 yylval.sval = "42"; return STRING;
2319 The %define variable api.value.type supports both keyword and code values.
2321 The keyword value 'union' means that the user provides genuine types, not
2322 union member names such as "ival" and "sval" above (WARNING: will fail if
2323 -y/--yacc/%yacc is enabled).
2325 %define api.value.type union
2326 %token <int> INT "integer"
2327 %token <char *> STRING "string"
2328 %printer { fprintf (yyo, "%d", $$); } <int>
2329 %destructor { free ($$); } <char *>
2332 yylval.INT = 42; return INT;
2333 yylval.STRING = "42"; return STRING;
2335 The keyword value variant is somewhat equivalent, but for C++ special
2336 provision is made to allow classes to be used (more about this below).
2338 %define api.value.type variant
2339 %token <int> INT "integer"
2340 %token <std::string> STRING "string"
2342 Code values (in braces) denote user defined types. This is where YYSTYPE
2360 %define api.value.type {struct my_value}
2361 %token <u.ival> INT "integer"
2362 %token <u.sval> STRING "string"
2363 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2364 %destructor { free ($$); } <u.sval>
2367 yylval.u.ival = 42; return INT;
2368 yylval.u.sval = "42"; return STRING;
2370 ** Variable parse.error
2372 This variable controls the verbosity of error messages. The use of the
2373 %error-verbose directive is deprecated in favor of "%define parse.error
2376 ** Deprecated %define variable names
2378 The following variables have been renamed for consistency. Backward
2379 compatibility is ensured, but upgrading is recommended.
2381 lr.default-reductions -> lr.default-reduction
2382 lr.keep-unreachable-states -> lr.keep-unreachable-state
2383 namespace -> api.namespace
2384 stype -> api.value.type
2386 ** Semantic predicates
2388 Contributed by Paul Hilfinger.
2390 The new, experimental, semantic-predicate feature allows actions of the
2391 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2392 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2393 in GLR parsers, rather than being deferred. The result is that they allow
2394 the programmer to prune possible parses based on the values of run-time
2397 ** The directive %expect-rr is now an error in non GLR mode
2399 It used to be an error only if used in non GLR mode, _and_ if there are
2400 reduce/reduce conflicts.
2402 ** Tokens are numbered in their order of appearance
2404 Contributed by Valentin Tolmer.
2406 With '%token A B', A had a number less than the one of B. However,
2407 precedence declarations used to generate a reversed order. This is now
2408 fixed, and introducing tokens with any of %token, %left, %right,
2409 %precedence, or %nonassoc yields the same result.
2411 When mixing declarations of tokens with a literal character (e.g., 'a') or
2412 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2413 the literal characters first. For example
2417 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2418 input order is now preserved.
2420 These changes were made so that one can remove useless precedence and
2421 associativity declarations (i.e., map %nonassoc, %left or %right to
2422 %precedence, or to %token) and get exactly the same output.
2424 ** Useless precedence and associativity
2426 Contributed by Valentin Tolmer.
2428 When developing and maintaining a grammar, useless associativity and
2429 precedence directives are common. They can be a nuisance: new ambiguities
2430 arising are sometimes masked because their conflicts are resolved due to
2431 the extra precedence or associativity information. Furthermore, it can
2432 hinder the comprehension of a new grammar: one will wonder about the role
2433 of a precedence, where in fact it is useless. The following changes aim
2434 at detecting and reporting these extra directives.
2436 *** Precedence warning category
2438 A new category of warning, -Wprecedence, was introduced. It flags the
2439 useless precedence and associativity directives.
2441 *** Useless associativity
2443 Bison now warns about symbols with a declared associativity that is never
2444 used to resolve conflicts. In that case, using %precedence is sufficient;
2445 the parsing tables will remain unchanged. Solving these warnings may raise
2446 useless precedence warnings, as the symbols no longer have associativity.
2460 warning: useless associativity for '+', use %precedence [-Wprecedence]
2464 *** Useless precedence
2466 Bison now warns about symbols with a declared precedence and no declared
2467 associativity (i.e., declared with %precedence), and whose precedence is
2468 never used. In that case, the symbol can be safely declared with %token
2469 instead, without modifying the parsing tables. For example:
2473 exp: "var" '=' "number";
2477 warning: useless precedence for '=' [-Wprecedence]
2481 *** Useless precedence and associativity
2483 In case of both useless precedence and associativity, the issue is flagged
2488 exp: "var" '=' "number";
2492 warning: useless precedence and associativity for '=' [-Wprecedence]
2498 With help from Joel E. Denny and Gabriel Rassoul.
2500 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2501 marked by the new %empty directive. Using %empty on a non-empty rule is
2502 an error. The new -Wempty-rule warning reports empty rules without
2503 %empty. On the following grammar:
2513 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2516 5.8-13: error: %empty on non-empty rule
2520 ** Java skeleton improvements
2522 The constants for token names were moved to the Lexer interface. Also, it
2523 is possible to add code to the parser's constructors using "%code init"
2524 and "%define init_throws".
2525 Contributed by Paolo Bonzini.
2527 The Java skeleton now supports push parsing.
2528 Contributed by Dennis Heimbigner.
2530 ** C++ skeletons improvements
2532 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2534 Using %defines is now optional. Without it, the needed support classes
2535 are defined in the generated parser, instead of additional files (such as
2536 location.hh, position.hh and stack.hh).
2538 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2540 Both lalr1.cc and glr.cc no longer require %location.
2542 *** syntax_error exception (lalr1.cc)
2544 The C++ parser features a syntax_error exception, which can be
2545 thrown from the scanner or from user rules to raise syntax errors.
2546 This facilitates reporting errors caught in sub-functions (e.g.,
2547 rejecting too large integral literals from a conversion function
2548 used by the scanner, or rejecting invalid combinations from a
2549 factory invoked by the user actions).
2551 *** %define api.value.type variant
2553 This is based on a submission from Michiel De Wilde. With help
2554 from Théophile Ranquet.
2556 In this mode, complex C++ objects can be used as semantic values. For
2559 %token <::std::string> TEXT;
2560 %token <int> NUMBER;
2561 %token SEMICOLON ";"
2562 %type <::std::string> item;
2563 %type <::std::list<std::string>> list;
2566 list { std::cout << $1 << std::endl; }
2570 %empty { /* Generates an empty string list. */ }
2571 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2575 TEXT { std::swap ($$, $1); }
2576 | NUMBER { $$ = string_cast ($1); }
2579 *** %define api.token.constructor
2581 When variants are enabled, Bison can generate functions to build the
2582 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2583 with the semantic value (e.g., int):
2585 parser::symbol_type yylex ()
2587 parser::location_type loc = ...;
2589 return parser::make_TEXT ("Hello, world!", loc);
2591 return parser::make_NUMBER (42, loc);
2593 return parser::make_SEMICOLON (loc);
2599 There are operator- and operator-= for 'location'. Negative line/column
2600 increments can no longer underflow the resulting value.
2603 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2607 *** Fix compiler attribute portability (yacc.c)
2609 With locations enabled, __attribute__ was used unprotected.
2611 *** Fix some compiler warnings (lalr1.cc)
2614 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2618 Warnings about uninitialized yylloc in yyparse have been fixed.
2620 Restored C90 compliance (yet no report was ever made).
2622 ** Diagnostics are improved
2624 Contributed by Théophile Ranquet.
2626 *** Changes in the format of error messages
2628 This used to be the format of many error reports:
2630 input.y:2.7-12: %type redeclaration for exp
2631 input.y:1.7-12: previous declaration
2635 input.y:2.7-12: error: %type redeclaration for exp
2636 input.y:1.7-12: previous declaration
2638 *** New format for error reports: carets
2640 Caret errors have been added to Bison:
2642 input.y:2.7-12: error: %type redeclaration for exp
2645 input.y:1.7-12: previous declaration
2651 input.y:3.20-23: error: ambiguous reference: '$exp'
2652 exp: exp '+' exp { $exp = $1 + $3; };
2654 input.y:3.1-3: refers to: $exp at $$
2655 exp: exp '+' exp { $exp = $1 + $3; };
2657 input.y:3.6-8: refers to: $exp at $1
2658 exp: exp '+' exp { $exp = $1 + $3; };
2660 input.y:3.14-16: refers to: $exp at $3
2661 exp: exp '+' exp { $exp = $1 + $3; };
2664 The default behavior for now is still not to display these unless
2665 explicitly asked with -fcaret (or -fall). However, in a later release, it
2666 will be made the default behavior (but may still be deactivated with
2669 ** New value for %define variable: api.pure full
2671 The %define variable api.pure requests a pure (reentrant) parser. However,
2672 for historical reasons, using it in a location-tracking Yacc parser
2673 resulted in a yyerror function that did not take a location as a
2674 parameter. With this new value, the user may request a better pure parser,
2675 where yyerror does take a location as a parameter (in location-tracking
2678 The use of "%define api.pure true" is deprecated in favor of this new
2679 "%define api.pure full".
2681 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2683 The %define variable api.location.type defines the name of the type to use
2684 for locations. When defined, Bison no longer generates the position.hh
2685 and location.hh files, nor does the parser will include them: the user is
2686 then responsible to define her type.
2688 This can be used in programs with several parsers to factor their location
2689 and position files: let one of them generate them, and the others just use
2692 This feature was actually introduced, but not documented, in Bison 2.5,
2693 under the name "location_type" (which is maintained for backward
2696 For consistency, lalr1.java's %define variables location_type and
2697 position_type are deprecated in favor of api.location.type and
2700 ** Exception safety (lalr1.cc)
2702 The parse function now catches exceptions, uses the %destructors to
2703 release memory (the lookahead symbol and the symbols pushed on the stack)
2704 before re-throwing the exception.
2706 This feature is somewhat experimental. User feedback would be
2709 ** Graph improvements in DOT and XSLT
2711 Contributed by Théophile Ranquet.
2713 The graphical presentation of the states is more readable: their shape is
2714 now rectangular, the state number is clearly displayed, and the items are
2715 numbered and left-justified.
2717 The reductions are now explicitly represented as transitions to other
2718 diamond shaped nodes.
2720 These changes are present in both --graph output and xml2dot.xsl XSLT
2721 processing, with minor (documented) differences.
2723 ** %language is no longer an experimental feature.
2725 The introduction of this feature, in 2.4, was four years ago. The
2726 --language option and the %language directive are no longer experimental.
2730 The sections about shift/reduce and reduce/reduce conflicts resolution
2731 have been fixed and extended.
2733 Although introduced more than four years ago, XML and Graphviz reports
2734 were not properly documented.
2736 The translation of midrule actions is now described.
2739 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2741 We consider compiler warnings about Bison generated parsers to be bugs.
2742 Rather than working around them in your own project, please consider
2743 reporting them to us.
2747 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2748 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2751 Other issues in the test suite have been addressed.
2753 Null characters are correctly displayed in error messages.
2755 When possible, yylloc is correctly initialized before calling yylex. It
2756 is no longer necessary to initialize it in the %initial-action.
2759 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2761 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2764 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2768 Bugs and portability issues in the test suite have been fixed.
2770 Some errors in translations have been addressed, and --help now directs
2771 users to the appropriate place to report them.
2773 Stray Info files shipped by accident are removed.
2775 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2776 generated, are removed.
2778 All the generated headers are self-contained.
2780 ** Header guards (yacc.c, glr.c, glr.cc)
2782 In order to avoid collisions, the header guards are now
2783 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2784 For instance the header generated from
2786 %define api.prefix "calc"
2787 %defines "lib/parse.h"
2789 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2791 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2793 The compilation of pure parsers (%define api.pure) can trigger GCC
2796 input.c: In function 'yyparse':
2797 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2798 function [-Wmaybe-uninitialized]
2802 This is now fixed; pragmas to avoid these warnings are no longer needed.
2804 Warnings from clang ("equality comparison with extraneous parentheses" and
2805 "function declared 'noreturn' should not return") have also been
2809 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2813 Buffer overruns, complaints from Flex, and portability issues in the test
2814 suite have been fixed.
2816 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2818 Trailing end-of-lines in %parse-param or %lex-param would result in
2819 invalid C++. This is fixed.
2821 ** Spurious spaces and end-of-lines
2823 The generated files no longer end (nor start) with empty lines.
2826 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2828 Bison no longer executes user-specified M4 code when processing a grammar.
2832 In addition to the removal of the features announced in Bison 2.6, the
2833 next major release will remove the "Temporary hack for adding a semicolon
2834 to the user action", as announced in the release 2.5. Instead of:
2836 exp: exp "+" exp { $$ = $1 + $3 };
2840 exp: exp "+" exp { $$ = $1 + $3; };
2844 *** Type names are now properly escaped.
2846 *** glr.cc: set_debug_level and debug_level work as expected.
2848 *** Stray @ or $ in actions
2850 While Bison used to warn about stray $ or @ in action rules, it did not
2851 for other actions such as printers, destructors, or initial actions. It
2854 ** Type names in actions
2856 For consistency with rule actions, it is now possible to qualify $$ by a
2857 type-name in destructors, printers, and initial actions. For instance:
2859 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2861 will display two values for each typed and untyped symbol (provided
2862 that YYSTYPE has both "ival" and "fval" fields).
2865 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2869 The next major release of Bison will drop support for the following
2870 deprecated features. Please report disagreements to bug-bison@gnu.org.
2874 Support for generating parsers in K&R C will be removed. Parsers
2875 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2878 *** Features deprecated since Bison 1.875
2880 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2883 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2884 %lex-param, will no longer be supported.
2886 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2889 *** The generated header will be included (yacc.c)
2891 Instead of duplicating the content of the generated header (definition of
2892 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2893 as is already the case for GLR or C++ parsers. This change is deferred
2894 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2897 ** Generated Parser Headers
2899 *** Guards (yacc.c, glr.c, glr.cc)
2901 The generated headers are now guarded, as is already the case for C++
2902 parsers (lalr1.cc). For instance, with --defines=foo.h:
2907 #endif /* !YY_FOO_H */
2909 *** New declarations (yacc.c, glr.c)
2911 The generated header now declares yydebug and yyparse. Both honor
2912 --name-prefix=bar_, and yield
2914 int bar_parse (void);
2918 #define yyparse bar_parse
2921 in order to facilitate the inclusion of several parser headers inside a
2922 single compilation unit.
2924 *** Exported symbols in C++
2926 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2927 header, are removed, as they prevent the possibility of including several
2928 generated headers from a single compilation unit.
2932 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2935 ** New %define variable: api.prefix
2937 Now that the generated headers are more complete and properly protected
2938 against multiple inclusions, constant names, such as YYSTYPE are a
2939 problem. While yyparse and others are properly renamed by %name-prefix,
2940 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2941 would introduce backward compatibility issues in projects not expecting
2942 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2943 it is deprecated in favor of a new %define variable: api.prefix.
2945 The following examples compares both:
2947 %name-prefix "bar_" | %define api.prefix "bar_"
2948 %token <ival> FOO %token <ival> FOO
2949 %union { int ival; } %union { int ival; }
2955 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2956 # define BAR_FOO_H # define BAR_FOO_H
2958 /* Enabling traces. */ /* Enabling traces. */
2959 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2960 > # if defined YYDEBUG
2962 > # define BAR_DEBUG 1
2964 > # define BAR_DEBUG 0
2967 # define YYDEBUG 0 | # define BAR_DEBUG 0
2971 # if YYDEBUG | # if BAR_DEBUG
2972 extern int bar_debug; extern int bar_debug;
2975 /* Tokens. */ /* Tokens. */
2976 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2977 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2978 enum yytokentype { | enum bar_tokentype {
2983 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2984 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2985 typedef union YYSTYPE | typedef union BAR_STYPE
2988 } YYSTYPE; | } BAR_STYPE;
2989 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2992 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2994 int bar_parse (void); int bar_parse (void);
2996 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2999 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
3003 The next major release will drop support for generating parsers in K&R C.
3005 ** yacc.c: YYBACKUP works as expected.
3007 ** glr.c improvements:
3009 *** Location support is eliminated when not requested:
3011 GLR parsers used to include location-related code even when locations were
3012 not requested, and therefore not even usable.
3014 *** __attribute__ is preserved:
3016 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
3017 when -std is passed to GCC).
3019 ** lalr1.java: several fixes:
3021 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
3022 first token leads to a syntax error. Some minor clean ups.
3026 *** C++11 compatibility:
3028 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
3033 The header files such as "parser.hh", "location.hh", etc. used a constant
3034 name for preprocessor guards, for instance:
3036 #ifndef BISON_LOCATION_HH
3037 # define BISON_LOCATION_HH
3039 #endif // !BISON_LOCATION_HH
3041 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
3042 case characters are converted to upper case, and series of
3043 non-alphanumerical characters are converted to an underscore.
3045 With "bison -o lang++/parser.cc", "location.hh" would now include:
3047 #ifndef YY_LANG_LOCATION_HH
3048 # define YY_LANG_LOCATION_HH
3050 #endif // !YY_LANG_LOCATION_HH
3054 The position and location constructors (and their initialize methods)
3055 accept new arguments for line and column. Several issues in the
3056 documentation were fixed.
3058 ** liby is no longer asking for "rpl_fprintf" on some platforms.
3060 ** Changes in the manual:
3062 *** %printer is documented
3064 The "%printer" directive, supported since at least Bison 1.50, is finally
3065 documented. The "mfcalc" example is extended to demonstrate it.
3067 For consistency with the C skeletons, the C++ parsers now also support
3068 "yyoutput" (as an alias to "debug_stream ()").
3070 *** Several improvements have been made:
3072 The layout for grammar excerpts was changed to a more compact scheme.
3073 Named references are motivated. The description of the automaton
3074 description file (*.output) is updated to the current format. Incorrect
3075 index entries were fixed. Some other errors were fixed.
3079 *** Conflicting prototypes with recent/modified Flex.
3081 Fixed build problems with the current, unreleased, version of Flex, and
3082 some modified versions of 2.5.35, which have modified function prototypes.
3084 *** Warnings during the build procedure have been eliminated.
3086 *** Several portability problems in the test suite have been fixed:
3088 This includes warnings with some compilers, unexpected behavior of tools
3089 such as diff, warning messages from the test suite itself, etc.
3091 *** The install-pdf target works properly:
3093 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3094 halts in the middle of its course.
3097 * Noteworthy changes in release 2.5 (2011-05-14)
3099 ** Grammar symbol names can now contain non-initial dashes:
3101 Consistently with directives (such as %error-verbose) and with
3102 %define variables (e.g. push-pull), grammar symbol names may contain
3103 dashes in any position except the beginning. This is a GNU
3104 extension over POSIX Yacc. Thus, use of this extension is reported
3105 by -Wyacc and rejected in Yacc mode (--yacc).
3107 ** Named references:
3109 Historically, Yacc and Bison have supported positional references
3110 ($n, $$) to allow access to symbol values from inside of semantic
3113 Starting from this version, Bison can also accept named references.
3114 When no ambiguity is possible, original symbol names may be used
3115 as named references:
3117 if_stmt : "if" cond_expr "then" then_stmt ';'
3118 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3120 In the more common case, explicit names may be declared:
3122 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3123 { $res = mk_if_stmt($cond, $then, $else); }
3125 Location information is also accessible using @name syntax. When
3126 accessing symbol names containing dots or dashes, explicit bracketing
3127 ($[sym.1]) must be used.
3129 These features are experimental in this version. More user feedback
3130 will help to stabilize them.
3131 Contributed by Alex Rozenman.
3133 ** IELR(1) and canonical LR(1):
3135 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3136 is, given any context-free grammar, IELR(1) generates parser tables
3137 with the full language-recognition power of canonical LR(1) but with
3138 nearly the same number of parser states as LALR(1). This reduction
3139 in parser states is often an order of magnitude. More importantly,
3140 because canonical LR(1)'s extra parser states may contain duplicate
3141 conflicts in the case of non-LR(1) grammars, the number of conflicts
3142 for IELR(1) is often an order of magnitude less as well. This can
3143 significantly reduce the complexity of developing of a grammar.
3145 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3146 place of its traditional LALR(1) parser tables, which remain the
3147 default. You can specify the type of parser tables in the grammar
3148 file with these directives:
3150 %define lr.type lalr
3151 %define lr.type ielr
3152 %define lr.type canonical-lr
3154 The default-reduction optimization in the parser tables can also be
3155 adjusted using "%define lr.default-reductions". For details on both
3156 of these features, see the new section "Tuning LR" in the Bison
3159 These features are experimental. More user feedback will help to
3162 ** LAC (Lookahead Correction) for syntax error handling
3164 Contributed by Joel E. Denny.
3166 Canonical LR, IELR, and LALR can suffer from a couple of problems
3167 upon encountering a syntax error. First, the parser might perform
3168 additional parser stack reductions before discovering the syntax
3169 error. Such reductions can perform user semantic actions that are
3170 unexpected because they are based on an invalid token, and they
3171 cause error recovery to begin in a different syntactic context than
3172 the one in which the invalid token was encountered. Second, when
3173 verbose error messages are enabled (with %error-verbose or the
3174 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3175 syntax error message can both contain invalid tokens and omit valid
3178 The culprits for the above problems are %nonassoc, default
3179 reductions in inconsistent states, and parser state merging. Thus,
3180 IELR and LALR suffer the most. Canonical LR can suffer only if
3181 %nonassoc is used or if default reductions are enabled for
3182 inconsistent states.
3184 LAC is a new mechanism within the parsing algorithm that solves
3185 these problems for canonical LR, IELR, and LALR without sacrificing
3186 %nonassoc, default reductions, or state merging. When LAC is in
3187 use, canonical LR and IELR behave almost exactly the same for both
3188 syntactically acceptable and syntactically unacceptable input.
3189 While LALR still does not support the full language-recognition
3190 power of canonical LR and IELR, LAC at least enables LALR's syntax
3191 error handling to correctly reflect LALR's language-recognition
3194 Currently, LAC is only supported for deterministic parsers in C.
3195 You can enable LAC with the following directive:
3197 %define parse.lac full
3199 See the new section "LAC" in the Bison manual for additional
3200 details including a few caveats.
3202 LAC is an experimental feature. More user feedback will help to
3205 ** %define improvements:
3207 *** Can now be invoked via the command line:
3209 Each of these command-line options
3212 --define=NAME[=VALUE]
3215 --force-define=NAME[=VALUE]
3217 is equivalent to this grammar file declaration
3219 %define NAME ["VALUE"]
3221 except that the manner in which Bison processes multiple definitions
3222 for the same NAME differs. Most importantly, -F and --force-define
3223 quietly override %define, but -D and --define do not. For further
3224 details, see the section "Bison Options" in the Bison manual.
3226 *** Variables renamed:
3228 The following %define variables
3231 lr.keep_unreachable_states
3233 have been renamed to
3236 lr.keep-unreachable-states
3238 The old names are now deprecated but will be maintained indefinitely
3239 for backward compatibility.
3241 *** Values no longer need to be quoted in the grammar file:
3243 If a %define value is an identifier, it no longer needs to be placed
3244 within quotations marks. For example,
3246 %define api.push-pull "push"
3250 %define api.push-pull push
3252 *** Unrecognized variables are now errors not warnings.
3254 *** Multiple invocations for any variable is now an error not a warning.
3256 ** Unrecognized %code qualifiers are now errors not warnings.
3258 ** Character literals not of length one:
3260 Previously, Bison quietly converted all character literals to length
3261 one. For example, without warning, Bison interpreted the operators in
3262 the following grammar to be the same token:
3268 Bison now warns when a character literal is not of length one. In
3269 some future release, Bison will start reporting an error instead.
3271 ** Destructor calls fixed for lookaheads altered in semantic actions:
3273 Previously for deterministic parsers in C, if a user semantic action
3274 altered yychar, the parser in some cases used the old yychar value to
3275 determine which destructor to call for the lookahead upon a syntax
3276 error or upon parser return. This bug has been fixed.
3278 ** C++ parsers use YYRHSLOC:
3280 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3281 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3282 to use it. If, for instance, your location structure has "first"
3283 and "last" members, instead of
3285 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3289 (Current).first = (Rhs)[1].location.first; \
3290 (Current).last = (Rhs)[N].location.last; \
3294 (Current).first = (Current).last = (Rhs)[0].location.last; \
3300 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3304 (Current).first = YYRHSLOC (Rhs, 1).first; \
3305 (Current).last = YYRHSLOC (Rhs, N).last; \
3309 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3313 ** YYLLOC_DEFAULT in C++:
3315 The default implementation of YYLLOC_DEFAULT used to be issued in
3316 the header file. It is now output in the implementation file, after
3317 the user %code sections so that its #ifndef guard does not try to
3318 override the user's YYLLOC_DEFAULT if provided.
3320 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3322 YYFAIL has existed for many years as an undocumented feature of
3323 deterministic parsers in C generated by Bison. More recently, it was
3324 a documented feature of Bison's experimental Java parsers. As
3325 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3326 semantic action now produces a deprecation warning, and Java parsers
3327 no longer implement YYFAIL at all. For further details, including a
3328 discussion of how to suppress C preprocessor warnings about YYFAIL
3329 being unused, see the Bison 2.4.2 NEWS entry.
3331 ** Temporary hack for adding a semicolon to the user action:
3333 Previously, Bison appended a semicolon to every user action for
3334 reductions when the output language defaulted to C (specifically, when
3335 neither %yacc, %language, %skeleton, or equivalent command-line
3336 options were specified). This allowed actions such as
3338 exp: exp "+" exp { $$ = $1 + $3 };
3342 exp: exp "+" exp { $$ = $1 + $3; };
3344 As a first step in removing this misfeature, Bison now issues a
3345 warning when it appends a semicolon. Moreover, in cases where Bison
3346 cannot easily determine whether a semicolon is needed (for example, an
3347 action ending with a cpp directive or a braced compound initializer),
3348 it no longer appends one. Thus, the C compiler might now complain
3349 about a missing semicolon where it did not before. Future releases of
3350 Bison will cease to append semicolons entirely.
3352 ** Verbose syntax error message fixes:
3354 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3355 specified, syntax error messages produced by the generated parser
3356 include the unexpected token as well as a list of expected tokens.
3357 The effect of %nonassoc on these verbose messages has been corrected
3358 in two ways, but a more complete fix requires LAC, described above:
3360 *** When %nonassoc is used, there can exist parser states that accept no
3361 tokens, and so the parser does not always require a lookahead token
3362 in order to detect a syntax error. Because no unexpected token or
3363 expected tokens can then be reported, the verbose syntax error
3364 message described above is suppressed, and the parser instead
3365 reports the simpler message, "syntax error". Previously, this
3366 suppression was sometimes erroneously triggered by %nonassoc when a
3367 lookahead was actually required. Now verbose messages are
3368 suppressed only when all previous lookaheads have already been
3369 shifted or discarded.
3371 *** Previously, the list of expected tokens erroneously included tokens
3372 that would actually induce a syntax error because conflicts for them
3373 were resolved with %nonassoc in the current parser state. Such
3374 tokens are now properly omitted from the list.
3376 *** Expected token lists are still often wrong due to state merging
3377 (from LALR or IELR) and default reductions, which can both add
3378 invalid tokens and subtract valid tokens. Canonical LR almost
3379 completely fixes this problem by eliminating state merging and
3380 default reductions. However, there is one minor problem left even
3381 when using canonical LR and even after the fixes above. That is,
3382 if the resolution of a conflict with %nonassoc appears in a later
3383 parser state than the one at which some syntax error is
3384 discovered, the conflicted token is still erroneously included in
3385 the expected token list. Bison's new LAC implementation,
3386 described above, eliminates this problem and the need for
3387 canonical LR. However, LAC is still experimental and is disabled
3390 ** Java skeleton fixes:
3392 *** A location handling bug has been fixed.
3394 *** The top element of each of the value stack and location stack is now
3395 cleared when popped so that it can be garbage collected.
3397 *** Parser traces now print the top element of the stack.
3399 ** -W/--warnings fixes:
3401 *** Bison now properly recognizes the "no-" versions of categories:
3403 For example, given the following command line, Bison now enables all
3404 warnings except warnings for incompatibilities with POSIX Yacc:
3406 bison -Wall,no-yacc gram.y
3408 *** Bison now treats S/R and R/R conflicts like other warnings:
3410 Previously, conflict reports were independent of Bison's normal
3411 warning system. Now, Bison recognizes the warning categories
3412 "conflicts-sr" and "conflicts-rr". This change has important
3413 consequences for the -W and --warnings command-line options. For
3416 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3417 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3418 bison -Wnone gram.y # no conflicts are reported
3419 bison -Werror gram.y # any conflict is an error
3421 However, as before, if the %expect or %expect-rr directive is
3422 specified, an unexpected number of conflicts is an error, and an
3423 expected number of conflicts is not reported, so -W and --warning
3424 then have no effect on the conflict report.
3426 *** The "none" category no longer disables a preceding "error":
3428 For example, for the following command line, Bison now reports
3429 errors instead of warnings for incompatibilities with POSIX Yacc:
3431 bison -Werror,none,yacc gram.y
3433 *** The "none" category now disables all Bison warnings:
3435 Previously, the "none" category disabled only Bison warnings for
3436 which there existed a specific -W/--warning category. However,
3437 given the following command line, Bison is now guaranteed to
3438 suppress all warnings:
3442 ** Precedence directives can now assign token number 0:
3444 Since Bison 2.3b, which restored the ability of precedence
3445 directives to assign token numbers, doing so for token number 0 has
3446 produced an assertion failure. For example:
3450 This bug has been fixed.
3453 * Noteworthy changes in release 2.4.3 (2010-08-05)
3455 ** Bison now obeys -Werror and --warnings=error for warnings about
3456 grammar rules that are useless in the parser due to conflicts.
3458 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3461 ** Failures in the test suite for GCC 4.5 have been fixed.
3463 ** Failures in the test suite for some versions of Sun Studio C++ have
3466 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3467 warnings about undefined %prec identifiers will not be converted to
3468 errors in Bison 2.5. They will remain warnings, which should be
3469 sufficient for POSIX while avoiding backward compatibility issues.
3471 ** Minor documentation fixes.
3474 * Noteworthy changes in release 2.4.2 (2010-03-20)
3476 ** Some portability problems that resulted in failures and livelocks
3477 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3478 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3479 errors should no longer cause M4 to report a broken pipe on the
3482 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3484 POSIX specifies that an error be reported for any identifier that does
3485 not appear on the LHS of a grammar rule and that is not defined by
3486 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3487 error report for the case when an identifier appears only after a
3488 %prec directive. It is now restored. However, for backward
3489 compatibility with recent Bison releases, it is only a warning for
3490 now. In Bison 2.5 and later, it will return to being an error.
3491 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3492 warning will not be converted to an error in Bison 2.5.]
3494 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3496 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3497 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3500 ** %code is now a permanent feature.
3502 A traditional Yacc prologue directive is written in the form:
3506 To provide a more flexible alternative, Bison 2.3b introduced the
3507 %code directive with the following forms for C/C++:
3510 %code requires {CODE}
3511 %code provides {CODE}
3514 These forms are now considered permanent features of Bison. See the
3515 %code entries in the section "Bison Declaration Summary" in the Bison
3516 manual for a summary of their functionality. See the section
3517 "Prologue Alternatives" for a detailed discussion including the
3518 advantages of %code over the traditional Yacc prologue directive.
3520 Bison's Java feature as a whole including its current usage of %code
3521 is still considered experimental.
3523 ** YYFAIL is deprecated and will eventually be removed.
3525 YYFAIL has existed for many years as an undocumented feature of
3526 deterministic parsers in C generated by Bison. Previously, it was
3527 documented for Bison's experimental Java parsers. YYFAIL is no longer
3528 documented for Java parsers and is formally deprecated in both cases.
3529 Users are strongly encouraged to migrate to YYERROR, which is
3532 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3533 induce a syntax error. The most obvious difference from YYERROR is
3534 that YYFAIL will automatically invoke yyerror to report the syntax
3535 error so that you don't have to. However, there are several other
3536 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3537 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3538 used. For a more detailed discussion, see:
3540 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3542 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3543 deterministic parsers in C will continue to implement it. However,
3544 because YYFAIL is already flawed, it seems futile to try to make new
3545 Bison features compatible with it. Thus, during parser generation,
3546 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3547 rule action. In a later release, YYFAIL will be disabled for
3548 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3549 be removed altogether.
3551 There exists at least one case where Bison 2.5's YYFAIL warning will
3552 be a false positive. Some projects add phony uses of YYFAIL and other
3553 Bison-defined macros for the sole purpose of suppressing C
3554 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3555 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3556 epilogue (that is, after the second "%%") in the Bison input file. In
3557 this release (2.4.2), Bison already generates its own code to suppress
3558 C preprocessor warnings for YYFAIL, so projects can remove their own
3559 phony uses of YYFAIL if compatibility with Bison releases prior to
3560 2.4.2 is not necessary.
3562 ** Internationalization.
3564 Fix a regression introduced in Bison 2.4: Under some circumstances,
3565 message translations were not installed although supported by the
3569 * Noteworthy changes in release 2.4.1 (2008-12-11)
3571 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3572 declarations have been fixed.
3574 ** Temporary hack for adding a semicolon to the user action.
3576 Bison used to prepend a trailing semicolon at the end of the user
3577 action for reductions. This allowed actions such as
3579 exp: exp "+" exp { $$ = $1 + $3 };
3583 exp: exp "+" exp { $$ = $1 + $3; };
3585 Some grammars still depend on this "feature". Bison 2.4.1 restores
3586 the previous behavior in the case of C output (specifically, when
3587 neither %language or %skeleton or equivalent command-line options
3588 are used) to leave more time for grammars depending on the old
3589 behavior to be adjusted. Future releases of Bison will disable this
3592 ** A few minor improvements to the Bison manual.
3595 * Noteworthy changes in release 2.4 (2008-11-02)
3597 ** %language is an experimental feature.
3599 We first introduced this feature in test release 2.3b as a cleaner
3600 alternative to %skeleton. Since then, we have discussed the possibility of
3601 modifying its effect on Bison's output file names. Thus, in this release,
3602 we consider %language to be an experimental feature that will likely evolve
3605 ** Forward compatibility with GNU M4 has been improved.
3607 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3611 * Noteworthy changes in release 2.3b (2008-05-27)
3613 ** The quotes around NAME that used to be required in the following directive
3616 %define NAME "VALUE"
3618 ** The directive "%pure-parser" is now deprecated in favor of:
3622 which has the same effect except that Bison is more careful to warn about
3623 unreasonable usage in the latter case.
3627 Bison can now generate an LALR(1) parser in C with a push interface. That
3628 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3629 push one token at a time to the parser using "yypush_parse", which will
3630 return to the caller after processing each token. By default, the push
3631 interface is disabled. Either of the following directives will enable it:
3633 %define api.push_pull "push" // Just push; does not require yylex.
3634 %define api.push_pull "both" // Push and pull; requires yylex.
3636 See the new section "A Push Parser" in the Bison manual for details.
3638 The current push parsing interface is experimental and may evolve. More user
3639 feedback will help to stabilize it.
3641 ** The -g and --graph options now output graphs in Graphviz DOT format,
3642 not VCG format. Like --graph, -g now also takes an optional FILE argument
3643 and thus cannot be bundled with other short options.
3647 Bison can now generate an LALR(1) parser in Java. The skeleton is
3648 "data/lalr1.java". Consider using the new %language directive instead of
3649 %skeleton to select it.
3651 See the new section "Java Parsers" in the Bison manual for details.
3653 The current Java interface is experimental and may evolve. More user
3654 feedback will help to stabilize it.
3655 Contributed by Paolo Bonzini.
3659 This new directive specifies the programming language of the generated
3660 parser, which can be C (the default), C++, or Java. Besides the skeleton
3661 that Bison uses, the directive affects the names of the generated files if
3662 the grammar file's name ends in ".y".
3664 ** XML Automaton Report
3666 Bison can now generate an XML report of the LALR(1) automaton using the new
3667 "--xml" option. The current XML schema is experimental and may evolve. More
3668 user feedback will help to stabilize it.
3669 Contributed by Wojciech Polak.
3671 ** The grammar file may now specify the name of the parser header file using
3672 %defines. For example:
3676 ** When reporting useless rules, useless nonterminals, and unused terminals,
3677 Bison now employs the terms "useless in grammar" instead of "useless",
3678 "useless in parser" instead of "never reduced", and "unused in grammar"
3679 instead of "unused".
3681 ** Unreachable State Removal
3683 Previously, Bison sometimes generated parser tables containing unreachable
3684 states. A state can become unreachable during conflict resolution if Bison
3685 disables a shift action leading to it from a predecessor state. Bison now:
3687 1. Removes unreachable states.
3689 2. Does not report any conflicts that appeared in unreachable states.
3690 WARNING: As a result, you may need to update %expect and %expect-rr
3691 directives in existing grammar files.
3693 3. For any rule used only in such states, Bison now reports the rule as
3694 "useless in parser due to conflicts".
3696 This feature can be disabled with the following directive:
3698 %define lr.keep_unreachable_states
3700 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3701 for further discussion.
3703 ** Lookahead Set Correction in the ".output" Report
3705 When instructed to generate a ".output" file including lookahead sets
3706 (using "--report=lookahead", for example), Bison now prints each reduction's
3707 lookahead set only next to the associated state's one item that (1) is
3708 associated with the same rule as the reduction and (2) has its dot at the end
3709 of its RHS. Previously, Bison also erroneously printed the lookahead set
3710 next to all of the state's other items associated with the same rule. This
3711 bug affected only the ".output" file and not the generated parser source
3714 ** --report-file=FILE is a new option to override the default ".output" file
3717 ** The "=" that used to be required in the following directives is now
3720 %file-prefix "parser"
3724 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3726 Bison 2.3a provided a new set of directives as a more flexible alternative to
3727 the traditional Yacc prologue blocks. Those have now been consolidated into
3728 a single %code directive with an optional qualifier field, which identifies
3729 the purpose of the code and thus the location(s) where Bison should generate
3732 1. "%code {CODE}" replaces "%after-header {CODE}"
3733 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3734 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3735 4. "%code top {CODE}" replaces "%before-header {CODE}"
3737 See the %code entries in section "Bison Declaration Summary" in the Bison
3738 manual for a summary of the new functionality. See the new section "Prologue
3739 Alternatives" for a detailed discussion including the advantages of %code
3740 over the traditional Yacc prologues.
3742 The prologue alternatives are experimental. More user feedback will help to
3743 determine whether they should become permanent features.
3745 ** Revised warning: unset or unused midrule values
3747 Since Bison 2.2, Bison has warned about midrule values that are set but not
3748 used within any of the actions of the parent rule. For example, Bison warns
3751 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3753 Now, Bison also warns about midrule values that are used but not set. For
3754 example, Bison warns about unset $$ in the midrule action in:
3756 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3758 However, Bison now disables both of these warnings by default since they
3759 sometimes prove to be false alarms in existing grammars employing the Yacc
3760 constructs $0 or $-N (where N is some positive integer).
3762 To enable these warnings, specify the option "--warnings=midrule-values" or
3763 "-W", which is a synonym for "--warnings=all".
3765 ** Default %destructor or %printer with "<*>" or "<>"
3767 Bison now recognizes two separate kinds of default %destructor's and
3770 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3771 %destructor/%printer for all grammar symbols for which you have formally
3772 declared semantic type tags.
3774 2. Place "<>" in a %destructor/%printer symbol list to define a default
3775 %destructor/%printer for all grammar symbols without declared semantic
3778 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3779 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3780 longer applies any %destructor to a midrule value if that midrule value is
3781 not actually ever referenced using either $$ or $n in a semantic action.
3783 The default %destructor's and %printer's are experimental. More user
3784 feedback will help to determine whether they should become permanent
3787 See the section "Freeing Discarded Symbols" in the Bison manual for further
3790 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3791 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3792 manual for a caveat concerning the treatment of literal strings.
3794 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3795 completely removed from Bison.
3798 * Noteworthy changes in release 2.3a (2006-09-13)
3800 ** Instead of %union, you can define and use your own union type
3801 YYSTYPE if your grammar contains at least one <type> tag.
3802 Your YYSTYPE need not be a macro; it can be a typedef.
3803 This change is for compatibility with other Yacc implementations,
3804 and is required by POSIX.
3806 ** Locations columns and lines start at 1.
3807 In accordance with the GNU Coding Standards and Emacs.
3809 ** You may now declare per-type and default %destructor's and %printer's:
3813 %union { char *string; }
3814 %token <string> STRING1
3815 %token <string> STRING2
3816 %type <string> string1
3817 %type <string> string2
3818 %union { char character; }
3819 %token <character> CHR
3820 %type <character> chr
3821 %destructor { free ($$); } %symbol-default
3822 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3823 %destructor { } <character>
3825 guarantees that, when the parser discards any user-defined symbol that has a
3826 semantic type tag other than "<character>", it passes its semantic value to
3827 "free". However, when the parser discards a "STRING1" or a "string1", it
3828 also prints its line number to "stdout". It performs only the second
3829 "%destructor" in this case, so it invokes "free" only once.
3831 [Although we failed to mention this here in the 2.3a release, the default
3832 %destructor's and %printer's were experimental, and they were rewritten in
3835 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3836 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3837 associating token numbers with token names. Removing the #define statements
3838 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3839 requires them. Bison still generates an enum for token names in all cases.
3841 ** Handling of traditional Yacc prologue blocks is now more consistent but
3842 potentially incompatible with previous releases of Bison.
3844 As before, you declare prologue blocks in your grammar file with the
3845 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3846 prologue blocks that you've declared before the first %union. To generate
3847 the post-prologue, Bison concatenates all prologue blocks that you've
3848 declared after the first %union.
3850 Previous releases of Bison inserted the pre-prologue into both the header
3851 file and the code file in all cases except for LALR(1) parsers in C. In the
3852 latter case, Bison inserted it only into the code file. For parsers in C++,
3853 the point of insertion was before any token definitions (which associate
3854 token numbers with names). For parsers in C, the point of insertion was
3855 after the token definitions.
3857 Now, Bison never inserts the pre-prologue into the header file. In the code
3858 file, it always inserts it before the token definitions.
3860 ** Bison now provides a more flexible alternative to the traditional Yacc
3861 prologue blocks: %before-header, %start-header, %end-header, and
3864 For example, the following declaration order in the grammar file reflects the
3865 order in which Bison will output these code blocks. However, you are free to
3866 declare these code blocks in your grammar file in whatever order is most
3870 /* Bison treats this block like a pre-prologue block: it inserts it into
3871 * the code file before the contents of the header file. It does *not*
3872 * insert it into the header file. This is a good place to put
3873 * #include's that you want at the top of your code file. A common
3874 * example is '#include "system.h"'. */
3877 /* Bison inserts this block into both the header file and the code file.
3878 * In both files, the point of insertion is before any Bison-generated
3879 * token, semantic type, location type, and class definitions. This is a
3880 * good place to define %union dependencies, for example. */
3883 /* Unlike the traditional Yacc prologue blocks, the output order for the
3884 * new %*-header blocks is not affected by their declaration position
3885 * relative to any %union in the grammar file. */
3888 /* Bison inserts this block into both the header file and the code file.
3889 * In both files, the point of insertion is after the Bison-generated
3890 * definitions. This is a good place to declare or define public
3891 * functions or data structures that depend on the Bison-generated
3895 /* Bison treats this block like a post-prologue block: it inserts it into
3896 * the code file after the contents of the header file. It does *not*
3897 * insert it into the header file. This is a good place to declare or
3898 * define internal functions or data structures that depend on the
3899 * Bison-generated definitions. */
3902 If you have multiple occurrences of any one of the above declarations, Bison
3903 will concatenate the contents in declaration order.
3905 [Although we failed to mention this here in the 2.3a release, the prologue
3906 alternatives were experimental, and they were rewritten in future versions.]
3908 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3909 The old spelling still works, but is not documented and may be removed
3910 in a future release.
3913 * Noteworthy changes in release 2.3 (2006-06-05)
3915 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3916 for compatibility with LALR(1) grammars.
3918 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3919 be to a type name that does not contain parentheses or brackets.
3922 * Noteworthy changes in release 2.2 (2006-05-19)
3924 ** The distribution terms for all Bison-generated parsers now permit
3925 using the parsers in nonfree programs. Previously, this permission
3926 was granted only for Bison-generated LALR(1) parsers in C.
3928 ** %name-prefix changes the namespace name in C++ outputs.
3930 ** The C++ parsers export their token_type.
3932 ** Bison now allows multiple %union declarations, and concatenates
3933 their contents together.
3935 ** New warning: unused values
3936 Right-hand side symbols whose values are not used are reported,
3937 if the symbols have destructors. For instance:
3939 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3943 will trigger a warning about $$ and $5 in the first rule, and $3 in
3944 the second ($1 is copied to $$ by the default rule). This example
3945 most likely contains three errors, and could be rewritten as:
3947 exp: exp "?" exp ":" exp
3948 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3950 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3953 However, if the original actions were really intended, memory leaks
3954 and all, the warnings can be suppressed by letting Bison believe the
3955 values are used, e.g.:
3957 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3958 | exp "+" exp { $$ = $1; (void) $3; }
3961 If there are midrule actions, the warning is issued if no action
3962 uses it. The following triggers no warning: $1 and $3 are used.
3964 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3966 The warning is intended to help catching lost values and memory leaks.
3967 If a value is ignored, its associated memory typically is not reclaimed.
3969 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3970 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3971 and YYERROR, for all objects on the stack, other than objects
3972 corresponding to the right-hand side of the current rule.
3974 ** %expect, %expect-rr
3975 Incorrect numbers of expected conflicts are now actual errors,
3976 instead of warnings.
3978 ** GLR, YACC parsers.
3979 The %parse-params are available in the destructors (and the
3980 experimental printers) as per the documentation.
3982 ** Bison now warns if it finds a stray "$" or "@" in an action.
3984 ** %require "VERSION"
3985 This specifies that the grammar file depends on features implemented
3986 in Bison version VERSION or higher.
3988 ** lalr1.cc: The token and value types are now class members.
3989 The tokens were defined as free form enums and cpp macros. YYSTYPE
3990 was defined as a free form union. They are now class members:
3991 tokens are enumerations of the "yy::parser::token" struct, and the
3992 semantic values have the "yy::parser::semantic_type" type.
3994 If you do not want or can update to this scheme, the directive
3995 '%define "global_tokens_and_yystype" "1"' triggers the global
3996 definition of tokens and YYSTYPE. This change is suitable both
3997 for previous releases of Bison, and this one.
3999 If you wish to update, then make sure older version of Bison will
4000 fail using '%require "2.2"'.
4002 ** DJGPP support added.
4005 * Noteworthy changes in release 2.1 (2005-09-16)
4007 ** The C++ lalr1.cc skeleton supports %lex-param.
4009 ** Bison-generated parsers now support the translation of diagnostics like
4010 "syntax error" into languages other than English. The default
4011 language is still English. For details, please see the new
4012 Internationalization section of the Bison manual. Software
4013 distributors should also see the new PACKAGING file. Thanks to
4014 Bruno Haible for this new feature.
4016 ** Wording in the Bison-generated parsers has been changed slightly to
4017 simplify translation. In particular, the message "memory exhausted"
4018 has replaced "parser stack overflow", as the old message was not
4019 always accurate for modern Bison-generated parsers.
4021 ** Destructors are now called when the parser aborts, for all symbols left
4022 behind on the stack. Also, the start symbol is now destroyed after a
4023 successful parse. In both cases, the behavior was formerly inconsistent.
4025 ** When generating verbose diagnostics, Bison-generated parsers no longer
4026 quote the literal strings associated with tokens. For example, for
4027 a syntax error associated with '%token NUM "number"' they might
4028 print 'syntax error, unexpected number' instead of 'syntax error,
4029 unexpected "number"'.
4032 * Noteworthy changes in release 2.0 (2004-12-25)
4034 ** Possibly-incompatible changes
4036 - Bison-generated parsers no longer default to using the alloca function
4037 (when available) to extend the parser stack, due to widespread
4038 problems in unchecked stack-overflow detection. You can "#define
4039 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
4040 the manual to determine safe values for YYMAXDEPTH in that case.
4042 - Error token location.
4043 During error recovery, the location of the syntax error is updated
4044 to cover the whole sequence covered by the error token: it includes
4045 the shifted symbols thrown away during the first part of the error
4046 recovery, and the lookahead rejected during the second part.
4048 - Semicolon changes:
4049 . Stray semicolons are no longer allowed at the start of a grammar.
4050 . Semicolons are now required after in-grammar declarations.
4052 - Unescaped newlines are no longer allowed in character constants or
4053 string literals. They were never portable, and GCC 3.4.0 has
4054 dropped support for them. Better diagnostics are now generated if
4055 forget a closing quote.
4057 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4061 - GLR grammars now support locations.
4063 - New directive: %initial-action.
4064 This directive allows the user to run arbitrary code (including
4065 initializing @$) from yyparse before parsing starts.
4067 - A new directive "%expect-rr N" specifies the expected number of
4068 reduce/reduce conflicts in GLR parsers.
4070 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4071 This is a GNU extension.
4073 - The option "--report=lookahead" was changed to "--report=look-ahead".
4074 [However, this was changed back after 2.3.]
4076 - Experimental %destructor support has been added to lalr1.cc.
4078 - New configure option --disable-yacc, to disable installation of the
4079 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4083 - For now, %expect-count violations are now just warnings, not errors.
4084 This is for compatibility with Bison 1.75 and earlier (when there are
4085 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4086 are too many or too few shift/reduce conflicts). However, in future
4087 versions of Bison we plan to improve the %expect machinery so that
4088 these violations will become errors again.
4090 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4091 arbitrarily limited to 16-bit counts.
4093 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4096 * Noteworthy changes in release 1.875 (2003-01-01)
4098 ** The documentation license has been upgraded to version 1.2
4099 of the GNU Free Documentation License.
4101 ** syntax error processing
4103 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4104 locations too. This fixes bugs in error-location computation.
4107 It is now possible to reclaim the memory associated to symbols
4108 discarded during error recovery. This feature is still experimental.
4111 This new directive is preferred over YYERROR_VERBOSE.
4113 - #defining yyerror to steal internal variables is discouraged.
4114 It is not guaranteed to work forever.
4116 ** POSIX conformance
4118 - Semicolons are once again optional at the end of grammar rules.
4119 This reverts to the behavior of Bison 1.33 and earlier, and improves
4120 compatibility with Yacc.
4122 - "parse error" -> "syntax error"
4123 Bison now uniformly uses the term "syntax error"; formerly, the code
4124 and manual sometimes used the term "parse error" instead. POSIX
4125 requires "syntax error" in diagnostics, and it was thought better to
4128 - The documentation now emphasizes that yylex and yyerror must be
4129 declared before use. C99 requires this.
4131 - Bison now parses C99 lexical constructs like UCNs and
4132 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4134 - File names are properly escaped in C output. E.g., foo\bar.y is
4135 output as "foo\\bar.y".
4137 - Yacc command and library now available
4138 The Bison distribution now installs a "yacc" command, as POSIX requires.
4139 Also, Bison now installs a small library liby.a containing
4140 implementations of Yacc-compatible yyerror and main functions.
4141 This library is normally not useful, but POSIX requires it.
4143 - Type clashes now generate warnings, not errors.
4145 - If the user does not define YYSTYPE as a macro, Bison now declares it
4146 using typedef instead of defining it as a macro.
4147 For consistency, YYLTYPE is also declared instead of defined.
4149 ** Other compatibility issues
4151 - %union directives can now have a tag before the "{", e.g., the
4152 directive "%union foo {...}" now generates the C code
4153 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4154 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4155 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4156 This is for compatibility with both Yacc and Bison 1.35.
4158 - ";" is output before the terminating "}" of an action, for
4159 compatibility with Bison 1.35.
4161 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4162 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4164 - "yystype" and "yyltype" are now obsolescent macros instead of being
4165 typedefs or tags; they are no longer documented and are planned to be
4166 withdrawn in a future release.
4171 Users of Bison have to decide how they handle the portability of the
4174 - "parsing stack overflow..." -> "parser stack overflow"
4175 GLR parsers now report "parser stack overflow" as per the Bison manual.
4177 ** %parse-param and %lex-param
4178 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4179 additional context to yyparse and yylex. They suffer from several
4182 - a single argument only can be added,
4183 - their types are weak (void *),
4184 - this context is not passed to ancillary functions such as yyerror,
4185 - only yacc.c parsers support them.
4187 The new %parse-param/%lex-param directives provide a more precise control.
4190 %parse-param {int *nastiness}
4191 %lex-param {int *nastiness}
4192 %parse-param {int *randomness}
4194 results in the following signatures:
4196 int yylex (int *nastiness);
4197 int yyparse (int *nastiness, int *randomness);
4199 or, if both %pure-parser and %locations are used:
4201 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4202 int yyparse (int *nastiness, int *randomness);
4204 ** Bison now warns if it detects conflicting outputs to the same file,
4205 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4206 that command outputs both code and header to foo.h.
4208 ** #line in output files
4209 - --no-line works properly.
4211 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4212 later to be built. This change originally took place a few versions
4213 ago, but nobody noticed until we recently asked someone to try
4214 building Bison with a K&R C compiler.
4217 * Noteworthy changes in release 1.75 (2002-10-14)
4219 ** Bison should now work on 64-bit hosts.
4221 ** Indonesian translation thanks to Tedi Heriyanto.
4224 Fix spurious parse errors.
4227 Some people redefine yyerror to steal yyparse' private variables.
4228 Reenable this trick until an official feature replaces it.
4231 In agreement with POSIX and with other Yaccs, leaving a default
4232 action is valid when $$ is untyped, and $1 typed:
4236 but the converse remains an error:
4240 ** Values of midrule actions
4243 foo: { ... } { $$ = $1; } ...
4245 was incorrectly rejected: $1 is defined in the second midrule
4246 action, and is equal to the $$ of the first midrule action.
4249 * Noteworthy changes in release 1.50 (2002-10-04)
4254 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4255 almost any context-free grammar, ambiguous or not. The new declarations
4256 %dprec and %merge on grammar rules allow parse-time resolution of
4257 ambiguities. Contributed by Paul Hilfinger.
4259 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4260 like the Alpha, so please stick to 32-bit hosts for now.
4263 When not in Yacc compatibility mode, when the output file was not
4264 specified, running "bison foo/bar.y" created "foo/bar.c". It
4265 now creates "bar.c".
4268 The undefined token was systematically mapped to 2 which prevented
4269 the use of 2 by the user. This is no longer the case.
4271 ** Unknown token numbers
4272 If yylex returned an out of range value, yyparse could die. This is
4276 According to POSIX, the error token must be 256.
4277 Bison extends this requirement by making it a preference: *if* the
4278 user specified that one of her tokens is numbered 256, then error
4279 will be mapped onto another number.
4281 ** Verbose error messages
4282 They no longer report "..., expecting error or..." for states where
4283 error recovery is possible.
4286 Defaults to "$end" instead of "$".
4288 ** Error recovery now conforms to documentation and to POSIX
4289 When a Bison-generated parser encounters a syntax error, it now pops
4290 the stack until it finds a state that allows shifting the error
4291 token. Formerly, it popped the stack until it found a state that
4292 allowed some non-error action other than a default reduction on the
4293 error token. The new behavior has long been the documented behavior,
4294 and has long been required by POSIX. For more details, please see
4295 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4296 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4299 Popped tokens and nonterminals are now reported.
4302 Larger grammars are now supported (larger token numbers, larger grammar
4303 size (= sum of the LHS and RHS lengths), larger LALR tables).
4304 Formerly, many of these numbers ran afoul of 16-bit limits;
4305 now these limits are 32 bits on most hosts.
4307 ** Explicit initial rule
4308 Bison used to play hacks with the initial rule, which the user does
4309 not write. It is now explicit, and visible in the reports and
4313 Before, Bison reported the useless rules, but, although not used,
4314 included them in the parsers. They are now actually removed.
4316 ** Useless rules, useless nonterminals
4317 They are now reported, as a warning, with their locations.
4319 ** Rules never reduced
4320 Rules that can never be reduced because of conflicts are now
4323 ** Incorrect "Token not used"
4324 On a grammar such as
4326 %token useless useful
4328 exp: '0' %prec useful;
4330 where a token was used to set the precedence of the last rule,
4331 bison reported both "useful" and "useless" as useless tokens.
4333 ** Revert the C++ namespace changes introduced in 1.31
4334 as they caused too many portability hassles.
4336 ** Default locations
4337 By an accident of design, the default computation of @$ was
4338 performed after another default computation was performed: @$ = @1.
4339 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4340 the computation of @$.
4342 ** Token end-of-file
4343 The token end of file may be specified by the user, in which case,
4344 the user symbol is used in the reports, the graphs, and the verbose
4345 error messages instead of "$end", which remains being the default.
4349 %token MYEOF 0 "end of file"
4352 This old option, which has been broken for ages, is removed.
4355 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4356 Croatian, thanks to Denis Lackovic.
4358 ** Incorrect token definitions
4361 bison used to output
4364 ** Token definitions as enums
4365 Tokens are output both as the traditional #define's, and, provided
4366 the compiler supports ANSI C or is a C++ compiler, as enums.
4367 This lets debuggers display names instead of integers.
4370 In addition to --verbose, bison supports --report=THINGS, which
4371 produces additional information:
4373 complete the core item sets with their closure
4374 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4375 explicitly associate lookahead tokens to items
4377 describe shift/reduce conflicts solving.
4378 Bison used to systematically output this information on top of
4379 the report. Solved conflicts are now attached to their states.
4382 Previous versions don't complain when there is a type clash on
4383 the default action if the rule has a midrule action, such as in:
4391 ** GNU M4 is now required when using Bison.
4394 * Noteworthy changes in release 1.35 (2002-03-25)
4397 Some projects use Bison's C parser with C++ compilers, and define
4398 YYSTYPE as a class. The recent adjustment of C parsers for data
4399 alignment and 64 bit architectures made this impossible.
4401 Because for the time being no real solution for C++ parser
4402 generation exists, kludges were implemented in the parser to
4403 maintain this use. In the future, when Bison has C++ parsers, this
4404 kludge will be disabled.
4406 This kludge also addresses some C++ problems when the stack was
4410 * Noteworthy changes in release 1.34 (2002-03-12)
4412 ** File name clashes are detected
4413 $ bison foo.y -d -o foo.x
4414 fatal error: header and parser would both be named "foo.x"
4416 ** A missing ";" at the end of a rule triggers a warning
4417 In accordance with POSIX, and in agreement with other
4418 Yacc implementations, Bison will mandate this semicolon in the near
4419 future. This eases the implementation of a Bison parser of Bison
4420 grammars by making this grammar LALR(1) instead of LR(2). To
4421 facilitate the transition, this release introduces a warning.
4423 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4424 many portability hassles.
4426 ** DJGPP support added.
4428 ** Fix test suite portability problems.
4431 * Noteworthy changes in release 1.33 (2002-02-07)
4434 Groff could not be compiled for the definition of size_t was lacking
4435 under some conditions.
4441 * Noteworthy changes in release 1.32 (2002-01-23)
4443 ** Fix Yacc output file names
4445 ** Portability fixes
4447 ** Italian, Dutch translations
4450 * Noteworthy changes in release 1.31 (2002-01-14)
4454 ** GNU Gettext and %expect
4455 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4456 Bison dies on incorrect %expectations, we fear there will be
4457 too many bug reports for Gettext, so _for the time being_, %expect
4458 does not trigger an error when the input file is named "plural.y".
4460 ** Use of alloca in parsers
4461 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4462 malloc exclusively. Since 1.29, but was not NEWS'ed.
4464 alloca is used only when compiled with GCC, to avoid portability
4467 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4469 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4470 (as POSIX requires) instead of being undefined.
4473 Bison has always permitted actions such as { $$ = $1 }: it adds the
4474 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4475 is no longer output: one has to write { $$ = $1; }.
4477 ** Better C++ compliance
4478 The output parsers try to respect C++ namespaces.
4479 [This turned out to be a failed experiment, and it was reverted later.]
4482 Fixed bugs when reporting useless nonterminals.
4485 The parsers work properly on 64 bit hosts.
4488 Some calls to strerror resulted in scrambled or missing error messages.
4491 When the number of shift/reduce conflicts is correct, don't issue
4494 ** The verbose report includes the rule line numbers.
4496 ** Rule line numbers are fixed in traces.
4498 ** Swedish translation
4501 Verbose parse error messages from the parsers are better looking.
4502 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4503 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4505 ** Fixed parser memory leaks.
4506 When the generated parser was using malloc to extend its stacks, the
4507 previous allocations were not freed.
4509 ** Fixed verbose output file.
4510 Some newlines were missing.
4511 Some conflicts in state descriptions were missing.
4513 ** Fixed conflict report.
4514 Option -v was needed to get the result.
4518 Mismatches are errors, not warnings.
4520 ** Fixed incorrect processing of some invalid input.
4522 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4524 ** Fixed some typos in the documentation.
4526 ** %token MY_EOF 0 is supported.
4527 Before, MY_EOF was silently renumbered as 257.
4529 ** doc/refcard.tex is updated.
4531 ** %output, %file-prefix, %name-prefix.
4535 New, aliasing "--output-file".
4538 * Noteworthy changes in release 1.30 (2001-10-26)
4540 ** "--defines" and "--graph" have now an optional argument which is the
4541 output file name. "-d" and "-g" do not change; they do not take any
4544 ** "%source_extension" and "%header_extension" are removed, failed
4547 ** Portability fixes.
4550 * Noteworthy changes in release 1.29 (2001-09-07)
4552 ** The output file does not define const, as this caused problems when used
4553 with common autoconfiguration schemes. If you still use ancient compilers
4554 that lack const, compile with the equivalent of the C compiler option
4555 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4557 ** Added "-g" and "--graph".
4559 ** The Bison manual is now distributed under the terms of the GNU FDL.
4561 ** The input and the output files has automatically a similar extension.
4563 ** Russian translation added.
4565 ** NLS support updated; should hopefully be less troublesome.
4567 ** Added the old Bison reference card.
4569 ** Added "--locations" and "%locations".
4571 ** Added "-S" and "--skeleton".
4573 ** "%raw", "-r", "--raw" is disabled.
4575 ** Special characters are escaped when output. This solves the problems
4576 of the #line lines with path names including backslashes.
4579 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4580 "%debug", "%source_extension" and "%header_extension".
4583 Automatic location tracking.
4586 * Noteworthy changes in release 1.28 (1999-07-06)
4588 ** Should compile better now with K&R compilers.
4592 ** Fixed a problem with escaping the double quote character.
4594 ** There is now a FAQ.
4597 * Noteworthy changes in release 1.27
4599 ** The make rule which prevented bison.simple from being created on
4600 some systems has been fixed.
4603 * Noteworthy changes in release 1.26
4605 ** Bison now uses Automake.
4607 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4609 ** Token numbers now start at 257 as previously documented, not 258.
4611 ** Bison honors the TMPDIR environment variable.
4613 ** A couple of buffer overruns have been fixed.
4615 ** Problems when closing files should now be reported.
4617 ** Generated parsers should now work even on operating systems which do
4618 not provide alloca().
4621 * Noteworthy changes in release 1.25 (1995-10-16)
4623 ** Errors in the input grammar are not fatal; Bison keeps reading
4624 the grammar file, and reports all the errors found in it.
4626 ** Tokens can now be specified as multiple-character strings: for
4627 example, you could use "<=" for a token which looks like <=, instead
4628 of choosing a name like LESSEQ.
4630 ** The %token_table declaration says to write a table of tokens (names
4631 and numbers) into the parser file. The yylex function can use this
4632 table to recognize multiple-character string tokens, or for other
4635 ** The %no_lines declaration says not to generate any #line preprocessor
4636 directives in the parser file.
4638 ** The %raw declaration says to use internal Bison token numbers, not
4639 Yacc-compatible token numbers, when token names are defined as macros.
4641 ** The --no-parser option produces the parser tables without including
4642 the parser engine; a project can now use its own parser engine.
4643 The actions go into a separate file called NAME.act, in the form of
4644 a switch statement body.
4647 * Noteworthy changes in release 1.23
4649 The user can define YYPARSE_PARAM as the name of an argument to be
4650 passed into yyparse. The argument should have type void *. It should
4651 actually point to an object. Grammar actions can access the variable
4652 by casting it to the proper pointer type.
4654 Line numbers in output file corrected.
4657 * Noteworthy changes in release 1.22
4659 --help option added.
4662 * Noteworthy changes in release 1.20
4664 Output file does not redefine const for C++.
4668 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4669 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4670 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4671 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4672 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4673 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4674 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4675 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4676 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4677 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4678 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4679 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4680 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4681 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4682 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4683 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4684 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4685 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4686 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4687 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4688 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4689 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4690 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4691 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4692 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4693 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4694 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE CVE
4695 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4696 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4697 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4698 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4699 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4700 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4701 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4702 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4703 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4704 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4705 LocalWords: Vais xsltproc YYNOMEM YYLOCATION signedness YYBISON MITRE's
4706 LocalWords: libreadline YYMALLOC YYFREE MSVC redefinitions
4709 ispell-dictionary: "american"
4714 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4716 This file is part of Bison, the GNU Parser Generator.
4718 Permission is granted to copy, distribute and/or modify this document
4719 under the terms of the GNU Free Documentation License, Version 1.3 or
4720 any later version published by the Free Software Foundation; with no
4721 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4722 Texts. A copy of the license is included in the "GNU Free
4723 Documentation License" file as part of this distribution.