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 ** Deprecated features
14 Support for the YYPRINT macro is removed. It worked only with yacc.c and
15 only for tokens. It was obsoleted by %printer, introduced in Bison 1.50
18 It has always been recommended to prefer `%define api.value.type foo` to
19 `#define YYSTYPE foo`. The latter is supported in C for compatibility
20 with Yacc, but not in C++. Warnings are now issued if `#define YYSTYPE`
21 is used in C++, and eventually support will be removed.
23 In C++ code, prefer value_type to semantic_type to denote the semantic
24 value type, which is specified by the `api.value.type` %define variable.
28 *** A skeleton for the D programming language
30 The "lalr1.d" skeleton is now officially part of Bison.
32 It was originally contributed by Oliver Mangold, based on Paolo Bonzini's
33 lalr1.java, and was improved by H. S. Teoh. Adela Vais then took over
34 maintenance and invested a lot of efforts to complete, test and document
37 It now supports all the bells and whistles of the other deterministic
38 parsers, which include: pull/push interfaces, verbose and custom error
39 messages, lookahead correction, token constructors, internationalization,
40 locations, printers, token and symbol prefixes, etc.
42 Two examples demonstrate the D parsers: a basic one (examples/d/simple),
43 and an advanced one (examples/d/calc).
45 *** Option -H, --header and directive %header
47 The option `-H`/`--header` supersedes the option `--defines`, and the
48 directive %header supersedes %defines. Both `--defines` and `%defines`
49 are, of course, maintained for backward compatibility.
53 Since version 2.4 Bison can be used to generate HTML reports. However it
54 was a two-step process: first bison must be invoked with option `--xml`,
55 and then xsltproc must be run to the convert the XML reports into HTML.
57 The new option `--html` combines these steps. The xsltproc program must
60 *** A C++ native GLR parser
62 A new version of the generated C++ GLR parser was added as "glr2.cc". It
63 is forked from the existing glr.c/cc parser, with the objective of making
64 it a more modern, truly C++ parser (instead of a C++ wrapper around a C
65 parser). Down the line, the goal is to support `%define api.value.type
66 variant` and maybe share code with lalr1.cc.
68 The current parser should be identical in terms of interface, functionality
69 and performance to "glr.cc". To try it out, simply use
73 It will eventually replace "glr.cc". However we need user feedback on
74 this skeleton. _Please_ report your results and comments about it.
78 Counterexamples now show the rule numbers, and always show ε for rules
79 with an empty right-hand side. For instance
91 *** Lookahead correction in Java
93 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
96 *** Abort parsing for memory exhaustion (C)
98 User actions may now use `YYNOMEM` (similar to `YYACCEPT` and `YYABORT`)
99 to abort the current parse with memory exhaustion.
101 *** Printing locations in debug traces (C)
103 The `YYLOCATION_PRINT(File, Loc)` macro prints a location. It is defined
104 when (i) locations are enabled, (ii) the default type for locations is
105 used, (iii) debug traces are enabled, and (iv) `YYLOCATION_PRINT` is not
108 Users may define `YYLOCATION_PRINT` to cover other cases.
112 There were no debug traces for deferred calls to user actions. They are
116 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
120 *** Reused Push Parsers
122 When a push-parser state structure is used for multiple parses, it was
123 possible for some state to leak from one run into the following one.
125 *** Fix Table Generation
127 In some very rare conditions, when there are many useless tokens, it was
128 possible to generate incorrect parsers.
131 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
135 *** Counterexample Generation
137 In some cases counterexample generation could crash. This is fixed.
139 *** Fix Table Generation
141 In some very rare conditions, when there are many useless tokens, it was
142 possible to generate incorrect parsers.
144 *** GLR parsers now support %merge together with api.value.type=union.
146 *** C++ parsers use noexcept in more places.
148 *** Generated parsers avoid some warnings about signedness issues.
150 *** C-language parsers now avoid warnings from pedantic clang.
152 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
155 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
159 *** Bug fixes in yacc.c
161 In Yacc mode, all the tokens are defined twice: once as an enum, and then
162 as a macro. YYEMPTY was missing its macro.
164 *** Bug fixes in lalr1.cc
166 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
167 even when the `parse.assert` %define variable is not enabled. It no
170 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
173 When there is a very large number of tokens, some assertions could be long
174 enough to hit arbitrary limits in Visual C++. They have been rewritten to
175 work around this limitation.
179 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
180 skeleton) used to be defined to 1. It is now defined to the version of
181 Bison as an integer (e.g., 30704 for version 3.7.4).
184 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
188 Fix concurrent build issues.
190 The bison executable is no longer linked uselessly against libreadline.
192 Fix incorrect use of yytname in glr.cc.
195 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
197 This release of Bison fixes all known bugs reported for Bison in MITRE's
198 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
199 are only about bison-the-program itself, not the generated code.
201 Although these bugs are typically irrelevant to how Bison is used, they
202 are worth fixing if only to give users peace of mind.
204 There is no known vulnerability in the generated parsers.
208 Fix concurrent build issues (introduced in Bison 3.5).
210 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
212 Fix portability issues of the test suite, and of bison itself.
214 Some unlikely crashes found by fuzzing have been fixed. This is only
215 about bison itself, not the generated parsers.
218 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
222 Crash when a token alias contains a NUL byte.
224 Portability issues with libtextstyle.
226 Portability issues of Bison itself with MSVC.
230 Improvements and fixes in the documentation.
232 More precise location about symbol type redefinitions.
235 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
237 ** Deprecated features
239 The YYPRINT macro, which works only with yacc.c and only for tokens, was
240 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
241 It is deprecated and its support will be removed eventually.
243 In conformance with the recommendations of the Graphviz team, in the next
244 version Bison the option `--graph` will generate a *.gv file by default,
245 instead of *.dot. A transition started in Bison 3.4.
249 *** Counterexample Generation
251 Contributed by Vincent Imbimbo.
253 When given `-Wcounterexamples`/`-Wcex`, bison will now output
254 counterexamples for conflicts.
256 **** Unifying Counterexamples
258 Unifying counterexamples are strings which can be parsed in two ways due
259 to the conflict. For example on a grammar that contains the usual
260 "dangling else" ambiguity:
263 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
264 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
267 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
268 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
269 Example: "if" exp "then" "if" exp "then" exp • "else" exp
272 ↳ "if" exp "then" exp
273 ↳ "if" exp "then" exp • "else" exp
274 Example: "if" exp "then" "if" exp "then" exp • "else" exp
277 ↳ "if" exp "then" exp "else" exp
278 ↳ "if" exp "then" exp •
280 When text styling is enabled, colors are used in the examples and the
281 derivations to highlight the structure of both analyses. In this case,
283 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
287 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
290 The counterexamples are "focused", in two different ways. First, they do
291 not clutter the output with all the derivations from the start symbol,
292 rather they start on the "conflicted nonterminal". They go straight to the
293 point. Second, they don't "expand" nonterminal symbols uselessly.
295 **** Nonunifying Counterexamples
297 In the case of the dangling else, Bison found an example that can be
298 parsed in two ways (therefore proving that the grammar is ambiguous).
299 When it cannot find such an example, it instead generates two examples
300 that are the same up until the dot:
303 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
304 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
305 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
310 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
311 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
312 First example: expr • ID ',' ID $end
319 Second example: expr • ID $end
325 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
329 In these cases, the parser usually doesn't have enough lookahead to
330 differentiate the two given examples.
334 Counterexamples are also included in the report when given
335 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
340 1 exp: "if" exp "then" exp • [$end, "then", "else"]
341 2 | "if" exp "then" exp • "else" exp
343 "else" shift, and go to state 8
345 "else" [reduce using rule 1 (exp)]
346 $default reduce using rule 1 (exp)
348 shift/reduce conflict on token "else":
349 1 exp: "if" exp "then" exp •
350 2 exp: "if" exp "then" exp • "else" exp
351 Example: "if" exp "then" "if" exp "then" exp • "else" exp
354 ↳ "if" exp "then" exp
355 ↳ "if" exp "then" exp • "else" exp
356 Example: "if" exp "then" "if" exp "then" exp • "else" exp
359 ↳ "if" exp "then" exp "else" exp
360 ↳ "if" exp "then" exp •
362 *** File prefix mapping
364 Contributed by Joshua Watt.
366 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
367 in the output (specifically `#line` directives and `#ifdef` header guards)
368 that begins with the prefix OLD will have it replaced with the prefix NEW,
369 similar to the `-ffile-prefix-map` in GCC. This option can be used to
370 make bison output reproducible.
376 When text styling is enabled and the terminal supports it, the warnings
377 now include hyperlinks to the documentation.
379 *** Relocatable installation
381 When installed to be relocatable (via `configure --enable-relocatable`),
382 bison will now also look for a relocated m4.
386 The `filename_type` %define variable was renamed `api.filename.type`.
389 %define filename_type "symbol"
393 %define api.filename.type {symbol}
395 (Or let `bison --update` do it for you).
397 It now defaults to `const std::string` instead of `std::string`.
399 *** Deprecated %define variable names
401 The following variables have been renamed for consistency. Backward
402 compatibility is ensured, but upgrading is recommended.
404 filename_type -> api.filename.type
405 package -> api.package
407 *** Push parsers no longer clear their state when parsing is finished
409 Previously push-parsers cleared their state when parsing was finished (on
410 success and on failure). This made it impossible to check if there were
411 parse errors, since `yynerrs` was also reset. This can be especially
412 troublesome when used in autocompletion, since a parser with error
413 recovery would suggest (irrelevant) expected tokens even if there were
416 Now the parser state can be examined when parsing is finished. The parser
417 state is reset when starting a new parse.
423 The bistromathic demonstrates %param and how to quote sources in the error
427 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
433 *** Include the generated header (yacc.c)
435 Historically, when --defines was used, bison generated a header and pasted
436 an exact copy of it into the generated parser implementation file. Since
437 Bison 3.4 it is possible to specify that the header should be `#include`d,
438 and how. For instance
440 %define api.header.include {"parse.h"}
444 %define api.header.include {<parser/parse.h>}
446 Now api.header.include defaults to `"header-basename"`, as was intended in
447 Bison 3.4, where `header-basename` is the basename of the generated
448 header. This is disabled when the generated header is `y.tab.h`, to
449 comply with Automake's ylwrap.
451 *** String aliases are faithfully propagated
453 Bison used to interpret user strings (i.e., decoding backslash escapes)
454 when reading them, and to escape them (i.e., issue non-printable
455 characters as backslash escapes, taking the locale into account) when
456 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
457 up "ciphered" as sequences of backslash escapes. This happened not only
458 in the generated sources (where the compiler will reinterpret them), but
459 also in all the generated reports (text, xml, html, dot, etc.). Reports
460 were therefore not readable when string aliases were not pure ASCII.
461 Worse yet: the output depended on the user's locale.
463 Now Bison faithfully treats the string aliases exactly the way the user
464 spelled them. This fixes all the aforementioned problems. However, now,
465 string aliases semantically equivalent but syntactically different (e.g.,
466 "A", "\x41", "\101") are considered to be different.
468 *** Crash when generating IELR
470 An old, well hidden, bug in the generation of IELR parsers was fixed.
473 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
477 In glr.cc some internal macros leaked in the user's code, and could damage
478 access to the token kinds.
481 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
485 Incorrect comments in the generated parsers.
487 Warnings in push parsers (yacc.c).
489 Incorrect display of gotos in LAC traces (lalr1.cc).
492 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
496 Some tests were fixed.
498 When token aliases contain comment delimiters:
500 %token FOO "/* foo */"
502 bison used to emit "nested" comments, which is invalid C.
505 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
509 Restored ANSI-C compliance in yacc.c.
511 GNU readline portability issues.
513 In C++, yy::parser::symbol_name is now a public member, as was intended.
517 In C++, yy::parser::symbol_type now has a public name() member function.
520 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
522 ** Backward incompatible changes
524 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
526 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
527 depend on it will now produce Yacc-like error messages (just "syntax
528 error"). It was superseded by the "%error-verbose" directive in Bison
529 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
530 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
531 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
532 parse.error verbose".
534 ** Deprecated features
536 The YYPRINT macro, which works only with yacc.c and only for tokens, was
537 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
538 It is deprecated and its support will be removed eventually.
542 *** Improved syntax error messages
544 Two new values for the %define parse.error variable offer more control to
545 the user. Available in all the skeletons (C, C++, Java).
547 **** %define parse.error detailed
549 The behavior of "%define parse.error detailed" is closely resembling that
550 of "%define parse.error verbose" with a few exceptions. First, it is safe
551 to use non-ASCII characters in token aliases (with 'verbose', the result
552 depends on the locale with which bison was run). Second, a yysymbol_name
553 function is exposed to the user, instead of the yytnamerr function and the
554 yytname table. Third, token internationalization is supported (see
557 **** %define parse.error custom
559 With this directive, the user forges and emits the syntax error message
560 herself by defining the yyreport_syntax_error function. A new type,
561 yypcontext_t, captures the circumstances of the error, and provides the
562 user with functions to get details, such as yypcontext_expected_tokens to
563 get the list of expected token kinds.
565 A possible implementation of yyreport_syntax_error is:
568 yyreport_syntax_error (const yypcontext_t *ctx)
571 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
572 fprintf (stderr, ": syntax error");
573 // Report the tokens expected at this point.
575 enum { TOKENMAX = 10 };
576 yysymbol_kind_t expected[TOKENMAX];
577 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
579 // Forward errors to yyparse.
582 for (int i = 0; i < n; ++i)
583 fprintf (stderr, "%s %s",
584 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
586 // Report the unexpected token.
588 yysymbol_kind_t lookahead = yypcontext_token (ctx);
589 if (lookahead != YYSYMBOL_YYEMPTY)
590 fprintf (stderr, " before %s", yysymbol_name (lookahead));
592 fprintf (stderr, "\n");
596 **** Token aliases internationalization
598 When the %define variable parse.error is set to `custom` or `detailed`,
599 one may specify which token aliases are to be translated using _(). For
611 In that case the user must define _() and N_(), and yysymbol_name returns
612 the translated symbol (i.e., it returns '_("variable")' rather that
613 '"variable"'). In Java, the user must provide an i18n() function.
615 *** List of expected tokens (yacc.c)
617 Push parsers may invoke yypstate_expected_tokens at any point during
618 parsing (including even before submitting the first token) to get the list
619 of possible tokens. This feature can be used to propose autocompletion
620 (see below the "bistromathic" example).
622 It makes little sense to use this feature without enabling LAC (lookahead
625 *** Returning the error token
627 When the scanner returns an invalid token or the undefined token
628 (YYUNDEF), the parser generates an error message and enters error
629 recovery. Because of that error message, most scanners that find lexical
630 errors generate an error message, and then ignore the invalid input
631 without entering the error-recovery.
633 The scanners may now return YYerror, the error token, to enter the
634 error-recovery mode without triggering an additional error message. See
635 the bistromathic for an example.
637 *** Deep overhaul of the symbol and token kinds
639 To avoid the confusion with types in programming languages, we now refer
640 to token and symbol "kinds" instead of token and symbol "types". The
641 documentation and error messages have been revised.
643 All the skeletons have been updated to use dedicated enum types rather
644 than integral types. Special symbols are now regular citizens, instead of
645 being declared in ad hoc ways.
649 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
650 LPAREN, etc. While backward compatibility is of course ensured, users are
651 nonetheless invited to replace their uses of "enum yytokentype" by
654 This type now also includes tokens that were previously hidden: YYEOF (end
655 of input), YYUNDEF (undefined token), and YYerror (error token). They
656 now have string aliases, internationalized when internationalization is
657 enabled. Therefore, by default, error messages now refer to "end of file"
658 (internationalized) rather than the cryptic "$end", or to "invalid token"
659 rather than "$undefined".
661 Therefore in most cases it is now useless to define the end-of-line token
664 %token T_EOF 0 "end of file"
666 Rather simply use "YYEOF" in your scanner.
670 The "symbol kinds" is what the parser actually uses. (Unless the
671 api.token.raw %define variable is used, the symbol kind of a terminal
672 differs from the corresponding token kind.)
674 They are now exposed as a enum, "yysymbol_kind_t".
676 This allows users to tailor the error messages the way they want, or to
677 process some symbols in a specific way in autocompletion (see the
678 bistromathic example below).
680 *** Modernize display of explanatory statements in diagnostics
682 Since Bison 2.7, output was indented four spaces for explanatory
683 statements. For example:
685 input.y:2.7-13: error: %type redeclaration for exp
686 input.y:1.7-11: previous declaration
688 Since the introduction of caret-diagnostics, it became less clear. This
689 indentation has been removed and submessages are displayed similarly as in
692 input.y:2.7-13: error: %type redeclaration for exp
693 2 | %type <float> exp
695 input.y:1.7-11: note: previous declaration
699 Contributed by Victor Morales Cayuela.
703 The token and symbol kinds are yy::parser::token_kind_type and
704 yy::parser::symbol_kind_type.
706 The symbol_type::kind() member function allows to get the kind of a
707 symbol. This can be used to write unit tests for scanners, e.g.,
709 yy::parser::symbol_type t = make_NUMBER ("123");
710 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
711 assert (t.value.as<int> () == 123);
717 In order to avoid ambiguities with "type" as in "typing", we now refer to
718 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
719 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
724 There are now examples/java: a very simple calculator, and a more complete
725 one (push-parser, location tracking, and debug traces).
727 The lexcalc example (a simple example in C based on Flex and Bison) now
728 also demonstrates location tracking.
731 A new C example, bistromathic, is a fully featured interactive calculator
732 using many Bison features: pure interface, push parser, autocompletion
733 based on the current parser state (using yypstate_expected_tokens),
734 location tracking, internationalized custom error messages, lookahead
735 correction, rich debug traces, etc.
737 It shows how to depend on the symbol kinds to tailor autocompletion. For
738 instance it recognizes the symbol kind "VARIABLE" to propose
739 autocompletion on the existing variables, rather than of the word
743 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
745 ** WARNING: Future backward-incompatibilities!
747 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
749 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
750 that still depend on it will produce Yacc-like error messages (just
751 "syntax error"). It was superseded by the "%error-verbose" directive in
752 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
753 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
754 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
755 parse.error verbose".
759 Fix portability issues of the package itself on old compilers.
761 Fix api.token.raw support in Java.
764 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
768 Error messages could quote lines containing zero-width characters (such as
769 \005) with incorrect styling. Fixes for similar issues with unexpectedly
770 short lines (e.g., the file was changed between parsing and diagnosing).
772 Some unlikely crashes found by fuzzing have been fixed. This is only
773 about bison itself, not the generated parsers.
776 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
780 Portability issues and minor cosmetic issues.
782 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
786 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
792 Fix compiler warnings.
795 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
797 ** Backward incompatible changes
799 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
800 longer treated as end-of-lines. This changes the diagnostics, and in
801 particular their locations.
803 In C++, line numbers and columns are now represented as 'int' not
804 'unsigned', so that integer overflow on positions is easily checkable via
805 'gcc -fsanitize=undefined' and the like. This affects the API for
806 positions. The default position and location classes now expose
807 'counter_type' (int), used to define line and column numbers.
809 ** Deprecated features
811 The YYPRINT macro, which works only with yacc.c and only for tokens, was
812 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
813 It is deprecated and its support will be removed eventually.
817 *** Lookahead correction in C++
819 Contributed by Adrian Vogelsgesang.
821 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
822 %define variable parse.lac.
824 *** Variable api.token.raw: Optimized token numbers (all skeletons)
826 In the generated parsers, tokens have two numbers: the "external" token
827 number as returned by yylex (which starts at 257), and the "internal"
828 symbol number (which starts at 3). Each time yylex is called, a table
829 lookup maps the external token number to the internal symbol number.
831 When the %define variable api.token.raw is set, tokens are assigned their
832 internal number, which saves one table lookup per token, and also saves
833 the generation of the mapping table.
835 The gain is typically moderate, but in extreme cases (very simple user
836 actions), a 10% improvement can be observed.
838 *** Generated parsers use better types for states
840 Stacks now use the best integral type for state numbers, instead of always
841 using 15 bits. As a result "small" parsers now have a smaller memory
842 footprint (they use 8 bits), and there is support for large automata (16
843 bits), and extra large (using int, i.e., typically 31 bits).
845 *** Generated parsers prefer signed integer types
847 Bison skeletons now prefer signed to unsigned integer types when either
848 will do, as the signed types are less error-prone and allow for better
849 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
850 portable to unusual machines where char, short and int are all the same
851 width. On non-GNU platforms this may entail including <limits.h> and (if
852 available) <stdint.h> to define integer types and constants.
854 *** A skeleton for the D programming language
856 For the last few releases, Bison has shipped a stealth experimental
857 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
858 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
861 However, because nobody has committed to improving, testing, and
862 documenting this skeleton, it is not clear that it will be supported in
865 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
866 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
868 *** Debug traces in Java
870 The Java backend no longer emits code and data for parser tracing if the
871 %define variable parse.trace is not defined.
875 *** New diagnostic: -Wdangling-alias
877 String literals, which allow for better error messages, are (too)
878 liberally accepted by Bison, which might result in silent errors. For
881 %type <exVal> cond "condition"
883 does not define "condition" as a string alias to 'cond' (nonterminal
884 symbols do not have string aliases). It is rather equivalent to
887 %token <exVal> "condition"
889 i.e., it gives the type 'exVal' to the "condition" token, which was
890 clearly not the intention.
892 Also, because string aliases need not be defined, typos such as "baz"
893 instead of "bar" will be not reported.
895 The option `-Wdangling-alias` catches these situations. On
898 %type <ival> foo "foo"
902 bison -Wdangling-alias reports
904 warning: string literal not attached to a symbol
905 | %type <ival> foo "foo"
907 warning: string literal not attached to a symbol
911 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
913 *** Better POSIX Yacc compatibility diagnostics
915 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
919 %type <ival> TOKEN1 TOKEN2 't'
926 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
927 2 | %type <ival> TOKEN1 TOKEN2 't'
929 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
930 2 | %type <ival> TOKEN1 TOKEN2 't'
932 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
933 2 | %type <ival> TOKEN1 TOKEN2 't'
936 *** Diagnostics with insertion
938 The diagnostics now display the suggestion below the underlined source.
939 Replacement for undeclared symbols are now also suggested.
946 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'?
950 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
954 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
956 *** Diagnostics about long lines
958 Quoted sources may now be truncated to fit the screen. For instance, on a
959 30-column wide terminal:
966 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
969 foo.y:1.8-10: previous declaration
972 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
975 foo.y:1.8-10: previous declaration
981 *** Debugging glr.c and glr.cc
983 The glr.c skeleton always had asserts to check its own behavior (not the
984 user's). These assertions are now under the control of the parse.assert
985 %define variable (disabled by default).
989 Several new compiler warnings in the generated output have been avoided.
990 Some unused features are no longer emitted. Cleaner generated code in
995 Portability issues in the test suite.
997 In theory, parsers using %nonassoc could crash when reporting verbose
998 error messages. This unlikely bug has been fixed.
1000 In Java, %define api.prefix was ignored. It now behaves as expected.
1003 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
1007 In some cases, when warnings are disabled, bison could emit tons of white
1008 spaces as diagnostics.
1010 When running out of memory, bison could crash (found by fuzzing).
1012 When defining twice the EOF token, bison would crash.
1014 New warnings from recent compilers have been addressed in the generated
1015 parsers (yacc.c, glr.c, glr.cc).
1017 When lone carriage-return characters appeared in the input file,
1018 diagnostics could hang forever.
1021 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
1028 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
1030 ** Deprecated features
1032 The %pure-parser directive is deprecated in favor of '%define api.pure'
1033 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
1034 now. Note that since Bison 2.7 you are strongly encouraged to use
1035 '%define api.pure full' instead of '%define api.pure'.
1039 *** Colored diagnostics
1041 As an experimental feature, diagnostics are now colored, controlled by the
1042 new options --color and --style.
1044 To use them, install the libtextstyle library before configuring Bison.
1045 It is available from
1047 https://alpha.gnu.org/gnu/gettext/
1051 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
1053 The option --color supports the following arguments:
1054 - always, yes: Enable colors.
1055 - never, no: Disable colors.
1056 - auto, tty (default): Enable colors if the output device is a tty.
1058 To customize the styles, create a CSS file similar to
1062 .error { font-weight: 800; text-decoration: underline; }
1065 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1066 environment variable to "bison-bw.css".
1068 *** Disabling output
1070 When given -fsyntax-only, the diagnostics are reported, but no output is
1073 The name of this option is somewhat misleading as bison does more than
1074 just checking the syntax: every stage is run (including checking for
1075 conflicts for instance), except the generation of the output files.
1077 *** Include the generated header (yacc.c)
1079 Before, when --defines is used, bison generated a header, and pasted an
1080 exact copy of it into the generated parser implementation file. If the
1081 header name is not "y.tab.h", it is now #included instead of being
1084 To use an '#include' even if the header name is "y.tab.h" (which is what
1085 happens with --yacc, or when using the Autotools' ylwrap), define
1086 api.header.include to the exact argument to pass to #include. For
1089 %define api.header.include {"parse.h"}
1093 %define api.header.include {<parser/parse.h>}
1095 *** api.location.type is now supported in C (yacc.c, glr.c)
1097 The %define variable api.location.type defines the name of the type to use
1098 for locations. When defined, Bison no longer defines YYLTYPE.
1100 This can be used in programs with several parsers to factor their
1101 definition of locations: let one of them generate them, and the others
1108 In conformance with the recommendations of the Graphviz team, if %require
1109 "3.4" (or better) is specified, the option --graph generates a *.gv file
1110 by default, instead of *.dot.
1112 *** Diagnostics overhaul
1114 Column numbers were wrong with multibyte characters, which would also
1115 result in skewed diagnostics with carets. Beside, because we were
1116 indenting the quoted source with a single space, lines with tab characters
1117 were incorrectly underlined.
1119 To address these issues, and to be clearer, Bison now issues diagnostics
1120 as GCC9 does. For instance it used to display (there's a tab before the
1123 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1124 expr: expr '+' "number" { $$ = $1 + $2; }
1128 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1129 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1132 Other constructs now also have better locations, resulting in more precise
1135 *** Fix-it hints for %empty
1137 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1138 annotations, and add the missing ones.
1140 *** Generated reports
1142 The format of the reports (parse.output) was improved for readability.
1144 *** Better support for --no-line.
1146 When --no-line is used, the generated files are now cleaner: no lines are
1147 generated instead of empty lines. Together with using api.header.include,
1148 that should help people saving the generated files into version control
1149 systems get smaller diffs.
1153 A new example in C shows an simple infix calculator with a hand-written
1154 scanner (examples/c/calc).
1156 A new example in C shows a reentrant parser (capable of recursive calls)
1157 built with Flex and Bison (examples/c/reccalc).
1159 There is a new section about the history of Yaccs and Bison.
1163 A few obscure bugs were fixed, including the second oldest (known) bug in
1164 Bison: it was there when Bison was entered in the RCS version control
1165 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1169 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1173 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1177 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1181 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1182 extensions into errors. It now makes them simple warnings (-Wyacc).
1185 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1187 A new mailing list was created, Bison Announce. It is low traffic, and is
1188 only about announcing new releases and important messages (e.g., polls
1189 about major decisions to make).
1191 https://lists.gnu.org/mailman/listinfo/bison-announce
1193 ** Backward incompatible changes
1195 Support for DJGPP, which has been unmaintained and untested for years, is
1198 ** Deprecated features
1200 A new feature, --update (see below) helps adjusting existing grammars to
1203 *** Deprecated directives
1205 The %error-verbose directive is deprecated in favor of '%define
1206 parse.error verbose' since Bison 3.0, but no warning was issued.
1208 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1209 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1210 directives are slightly different, you might need to adjust your code.
1211 %name-prefix renames only symbols with external linkage, while api.prefix
1212 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1213 yytokentype, YYSTYPE, YYLTYPE, etc.
1215 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1216 {xx}' will typically have to update YY_DECL from
1218 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1222 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1224 *** Deprecated %define variable names
1226 The following variables, mostly related to parsers in Java, have been
1227 renamed for consistency. Backward compatibility is ensured, but upgrading
1230 abstract -> api.parser.abstract
1231 annotations -> api.parser.annotations
1232 extends -> api.parser.extends
1233 final -> api.parser.final
1234 implements -> api.parser.implements
1235 parser_class_name -> api.parser.class
1236 public -> api.parser.public
1237 strictfp -> api.parser.strictfp
1241 *** Generation of fix-its for IDEs/Editors
1243 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1244 bison now generates machine readable editing instructions to fix some
1245 issues. Currently, this is mostly limited to updating deprecated
1246 directives and removing duplicates. For instance:
1250 %define parser_class_name "Parser"
1251 %define api.parser.class "Parser"
1255 See the "fix-it:" lines below:
1257 $ bison -ffixit foo.y
1258 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1261 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1262 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1263 %define parser_class_name "Parser"
1264 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1265 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1266 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1267 %define api.parser.class "Parser"
1268 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1269 foo.y:2.1-34: previous definition
1270 %define parser_class_name "Parser"
1271 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1272 fix-it:"foo.y":{3:1-3:34}:""
1273 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1275 This uses the same output format as GCC and Clang.
1277 *** Updating grammar files
1279 Fixes can be applied on the fly. The previous example ends with the
1280 suggestion to re-run bison with the option -u/--update, which results in a
1281 cleaner grammar file.
1283 $ bison --update foo.y
1285 bison: file 'foo.y' was updated (backup: 'foo.y~')
1288 %define parse.error verbose
1289 %define api.parser.class {Parser}
1293 *** Bison is now relocatable
1295 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1297 A relocatable program can be moved or copied to a different location on
1298 the file system. It can also be used through mount points for network
1299 sharing. It is possible to make symbolic links to the installed and moved
1300 programs, and invoke them through the symbolic link.
1302 *** %expect and %expect-rr modifiers on individual rules
1304 One can now document (and check) which rules participate in shift/reduce
1305 and reduce/reduce conflicts. This is particularly important GLR parsers,
1306 where conflicts are a normal occurrence. For example,
1322 | argument_list ',' expression
1327 Looking at the output from -v, one can see that the shift/reduce conflict
1328 here is due to the fact that the parser does not know whether to reduce
1329 arguments to argument_list until it sees the token _after_ the following
1330 ','. By marking the rule with %expect 1 (because there is a conflict in
1331 one state), we document the source of the 1 overall shift/reduce conflict.
1333 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1334 conflicts. In this case, we mark each of the conflicting rules. For
1343 target_list '=' expr ';'
1349 | target ',' target_list
1358 | expr ',' expr_list
1366 In a statement such as
1370 the parser must reduce x to a target or an expr, but does not know which
1371 until it sees the '='. So we notate the two possible reductions to
1372 indicate that each conflicts in one rule.
1374 This feature needs user feedback, and might evolve in the future.
1376 *** C++: Actual token constructors
1378 When variants and token constructors are enabled, in addition to the
1379 type-safe named token constructors (make_ID, make_INT, etc.), we now
1380 generate genuine constructors for symbol_type.
1382 For instance with these declarations
1388 you may use these constructors:
1390 symbol_type (int token, const std::string&);
1391 symbol_type (int token, const int&);
1392 symbol_type (int token);
1394 Correct matching between token types and value types is checked via
1395 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1396 constructors are preferable, as they offer better type safety (for
1397 instance 'make_ID (42)' would not even compile), but symbol_type
1398 constructors may help when token types are discovered at run-time, e.g.,
1401 if (auto i = lookup_keyword (yytext))
1402 return yy::parser::symbol_type (i);
1404 return yy::parser::make_ID (yytext);
1407 *** C++: Variadic emplace
1409 If your application requires C++11 and you don't use symbol constructors,
1410 you may now use a variadic emplace for semantic values:
1412 %define api.value.type variant
1413 %token <std::pair<int, int>> PAIR
1417 int yylex (parser::semantic_type *lvalp)
1419 lvalp->emplace <std::pair<int, int>> (1, 2);
1420 return parser::token::PAIR;
1423 *** C++: Syntax error exceptions in GLR
1425 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1426 actions, or from the scanner.
1428 *** More POSIX Yacc compatibility warnings
1430 More Bison specific directives are now reported with -y or -Wyacc. This
1431 change was ready since the release of Bison 3.0 in September 2015. It was
1432 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1433 in numerous warnings for Bison users that use the GNU Build System.
1435 If you still experience that problem, either redefine YACC as `bison -o
1436 y.tab.c`, or pass -Wno-yacc to Bison.
1438 *** The tables yyrhs and yyphrs are back
1440 Because no Bison skeleton uses them, these tables were removed (no longer
1441 passed to the skeletons, not even computed) in 2008. However, some users
1442 have expressed interest in being able to use them in their own skeletons.
1446 *** Incorrect number of reduce/reduce conflicts
1448 On a grammar such as
1450 exp: "num" | "num" | "num"
1452 bison used to report a single RR conflict, instead of two. This is now
1453 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1454 was entered in the RCS version control system, in December 1987.
1456 Some grammar files might have to adjust their %expect-rr.
1458 *** Parser directives that were not careful enough
1460 Passing invalid arguments to %nterm, for instance character literals, used
1461 to result in unclear error messages.
1465 The examples/ directory (installed in .../share/doc/bison/examples) has
1466 been restructured per language for clarity. The examples come with a
1467 README and a Makefile. Not only can they be used to toy with Bison, they
1468 can also be starting points for your own grammars.
1470 There is now a Java example, and a simple example in C based on Flex and
1471 Bison (examples/c/lexcalc/).
1477 They now use noexcept and constexpr. Please, report missing annotations.
1479 *** Symbol Declarations
1481 The syntax of the variation directives to declare symbols was overhauled
1482 for more consistency, and also better POSIX Yacc compliance (which, for
1483 instance, allows "%type" without actually providing a type). The %nterm
1484 directive, supported by Bison since its inception, is now documented and
1485 officially supported.
1487 The syntax is now as follows:
1489 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1490 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1491 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1492 %nterm TAG? ID+ ( TAG ID+ )*
1494 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1495 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1496 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1497 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1498 one), ‘*’ (zero or more) and ‘+’ (one or more).
1501 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1505 Fix the move constructor of symbol_type.
1507 Always provide a copy constructor for symbol_type, even in modern C++.
1510 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1514 Properly support token constructors in C++ with types that include commas
1515 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1518 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1522 C++ portability issues.
1525 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1529 Several portability issues have been fixed in the build system, in the
1530 test suite, and in the generated parsers in C++.
1533 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1535 ** Backward incompatible changes
1537 Support for DJGPP, which has been unmaintained and untested for years, is
1538 obsolete. Unless there is activity to revive it, it will be removed.
1542 %printers should use yyo rather than yyoutput to denote the output stream.
1544 Variant-based symbols in C++ should use emplace() rather than build().
1546 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1550 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1552 A comment in the generated code now emphasizes that users should not
1553 depend upon non-documented implementation details, such as macros starting
1558 *** C++: Support for move semantics (lalr1.cc)
1560 The lalr1.cc skeleton now fully supports C++ move semantics, while
1561 maintaining compatibility with C++98. You may now store move-only types
1562 when using Bison's variants. For instance:
1569 %skeleton "lalr1.cc"
1570 %define api.value.type variant
1574 %token <int> INT "int";
1575 %type <std::unique_ptr<int>> int;
1576 %type <std::vector<std::unique_ptr<int>>> list;
1580 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1582 int: "int" { $$ = std::make_unique<int>($1); }
1584 *** C++: Implicit move of right-hand side values (lalr1.cc)
1586 In modern C++ (C++11 and later), you should always use 'std::move' with
1587 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1588 popped from the stack anyway. Using 'std::move' is mandatory for
1589 move-only types such as unique_ptr, and it provides a significant speedup
1590 for large types such as std::string, or std::vector, etc.
1592 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1593 by 'std::move ($n)'. The second rule in the previous grammar can be
1596 list: list int { $$ = $1; $$.emplace_back($2); }
1598 With automove enabled, the semantic values are no longer lvalues, so do
1599 not use the swap idiom:
1601 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1603 This idiom is anyway obsolete: it is preferable to move than to swap.
1605 A warning is issued when automove is enabled, and a value is used several
1608 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1609 exp: "twice" exp { $$ = $2 + $2; }
1612 Enabling api.value.automove does not require support for modern C++. The
1613 generated code is valid C++98/03, but will use copies instead of moves.
1615 The new examples/c++/variant-11.yy shows these features in action.
1617 *** C++: The implicit default semantic action is always run
1619 When variants are enabled, the default action was not run, so
1627 It now behaves like in all the other cases, as
1629 exp: "number" { $$ = $1; }
1631 possibly using std::move if automove is enabled.
1633 We do not expect backward compatibility issues. However, beware of
1634 forward compatibility issues: if you rely on default actions with
1635 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1636 generate incorrect parsers.
1638 *** C++: Renaming location.hh
1640 When both %defines and %locations are enabled, Bison generates a
1641 location.hh file. If you don't use locations outside of the parser, you
1642 may avoid its creation with:
1644 %define api.location.file none
1646 However this file is useful if, for instance, your parser builds an AST
1647 decorated with locations: you may use Bison's location independently of
1648 Bison's parser. You can now give it another name, for instance:
1650 %define api.location.file "my-location.hh"
1652 This name can have directory components, and even be absolute. The name
1653 under which the location file is included is controlled by
1654 api.location.include.
1656 This way it is possible to have several parsers share the same location
1659 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1662 %define api.namespace {foo}
1663 %define api.location.file "include/ast/loc.hh"
1664 %define api.location.include {<ast/loc.hh>}
1666 and use it in src/bar/parser.hh:
1669 %define api.namespace {bar}
1670 %code requires {#include <ast/loc.hh>}
1671 %define api.location.type {bar::location}
1673 Absolute file names are supported, so in your Makefile, passing the flag
1674 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1677 *** C++: stack.hh and position.hh are deprecated
1679 When asked to generate a header file (%defines), the lalr1.cc skeleton
1680 generates a stack.hh file. This file had no interest for users; it is now
1681 made useless: its content is included in the parser definition. It is
1682 still generated for backward compatibility.
1684 When in addition to %defines, location support is requested (%locations),
1685 the file position.hh is also generated. It is now also useless: its
1686 content is now included in location.hh.
1688 These files are no longer generated when your grammar file requires at
1689 least Bison 3.2 (%require "3.2").
1693 Portability issues on MinGW and VS2015.
1695 Portability issues in the test suite.
1697 Portability/warning issues with Flex.
1700 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1702 ** Backward incompatible changes
1704 Compiling Bison now requires a C99 compiler---as announced during the
1705 release of Bison 3.0, five years ago. Generated parsers do not require a
1708 Support for DJGPP, which has been unmaintained and untested for years, is
1709 obsolete. Unless there is activity to revive it, the next release of Bison
1710 will have it removed.
1714 *** Typed midrule actions
1716 Because their type is unknown to Bison, the values of midrule actions are
1717 not treated like the others: they don't have %printer and %destructor
1718 support. It also prevents C++ (Bison) variants to handle them properly.
1720 Typed midrule actions address these issues. Instead of:
1722 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1726 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1728 *** Reports include the type of the symbols
1730 The sections about terminal and nonterminal symbols of the '*.output' file
1731 now specify their declared type. For instance, for:
1735 the report now shows '<ival>':
1737 Terminals, with rules where they appear
1741 *** Diagnostics about useless rules
1743 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1744 of course, its rules are useless too.
1748 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1750 Previously all the useless rules were reported, including those whose
1751 left-hand side is the 'exp' nonterminal:
1753 warning: 1 nonterminal useless in grammar [-Wother]
1754 warning: 4 rules useless in grammar [-Wother]
1755 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1758 2.14-16: warning: rule useless in grammar [-Wother]
1761 3.6-16: warning: rule useless in grammar [-Wother]
1762 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1764 3.20-30: warning: rule useless in grammar [-Wother]
1765 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1767 3.34-44: warning: rule useless in grammar [-Wother]
1768 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1771 Now, rules whose left-hand side symbol is useless are no longer reported
1772 as useless. The locations of the errors have also been adjusted to point
1773 to the first use of the nonterminal as a left-hand side of a rule:
1775 warning: 1 nonterminal useless in grammar [-Wother]
1776 warning: 4 rules useless in grammar [-Wother]
1777 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1778 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1780 2.14-16: warning: rule useless in grammar [-Wother]
1784 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1786 When compiled with exceptions disabled, the generated parsers no longer
1787 uses try/catch clauses.
1789 Currently only GCC and Clang are supported.
1793 *** A demonstration of variants
1795 A new example was added (installed in .../share/doc/bison/examples),
1796 'variant.yy', which shows how to use (Bison) variants in C++.
1798 The other examples were made nicer to read.
1800 *** Some features are no longer 'experimental'
1802 The following features, mature enough, are no longer flagged as
1803 experimental in the documentation: push parsers, default %printer and
1804 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1805 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1806 semantic predicates (%?).
1810 *** GLR: Predicates support broken by #line directives
1812 Predicates (%?) in GLR such as
1815 %? {new_syntax} 'w' id new_args
1816 | %?{!new_syntax} 'w' id old_args
1818 were issued with #lines in the middle of C code.
1820 *** Printer and destructor with broken #line directives
1822 The #line directives were not properly escaped when emitting the code for
1823 %printer/%destructor, which resulted in compiler errors if there are
1824 backslashes or double-quotes in the grammar file name.
1826 *** Portability on ICC
1828 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1829 Generated parsers now work around this.
1833 There were several small fixes in the test suite and in the build system,
1834 many warnings in bison and in the generated parsers were eliminated. The
1835 documentation also received its share of minor improvements.
1837 Useless code was removed from C++ parsers, and some of the generated
1838 constructors are more 'natural'.
1841 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1845 *** C++: Fix support of 'syntax_error'
1847 One incorrect 'inline' resulted in linking errors about the constructor of
1848 the syntax_error exception.
1850 *** C++: Fix warnings
1852 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1853 warnings about yyformat being possibly null. It also warned about the
1854 deprecated implicit definition of copy constructors when there's a
1855 user-defined (copy) assignment operator.
1857 *** Location of errors
1859 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1860 ride-hand side raises a syntax error. The behavior of the default parser
1861 (yacc.c) in such a condition was undefined.
1863 Now all the parsers match the behavior of glr.c: @$ is used as the
1864 location of the error. This handles gracefully rules with and without
1867 *** Portability fixes in the test suite
1869 On some platforms, some Java and/or C++ tests were failing.
1872 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1876 *** C++ with Variants (lalr1.cc)
1878 Fix a compiler warning when no %destructor use $$.
1882 Several portability issues in tests were fixed.
1885 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1889 *** C++ with Variants (lalr1.cc)
1891 Problems with %destructor and '%define parse.assert' have been fixed.
1893 *** Named %union support (yacc.c, glr.c)
1895 Bison 3.0 introduced a regression on named %union such as
1897 %union foo { int ival; };
1899 The possibility to use a name was introduced "for Yacc compatibility".
1900 It is however not required by POSIX Yacc, and its usefulness is not clear.
1902 *** %define api.value.type union with %defines (yacc.c, glr.c)
1904 The C parsers were broken when %defines was used together with "%define
1905 api.value.type union".
1907 *** Redeclarations are reported in proper order
1915 bison used to report:
1917 foo.yy:2.10-11: error: %printer redeclaration for FOO
1920 foo.yy:3.10-11: previous declaration
1924 Now, the "previous" declaration is always the first one.
1929 Bison now installs various files in its docdir (which defaults to
1930 '/usr/local/share/doc/bison'), including the three fully blown examples
1931 extracted from the documentation:
1934 Reverse Polish Calculator, a simple introductory example.
1936 Multi-function Calc, a calculator with memory and functions and located
1939 a calculator in C++ using variant support and token constructors.
1942 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1946 *** Generated source files when errors are reported
1948 When warnings are issued and -Werror is set, bison would still generate
1949 the source files (*.c, *.h...). As a consequence, some runs of "make"
1950 could fail the first time, but not the second (as the files were generated
1953 This is fixed: bison no longer generates this source files, but, of
1954 course, still produces the various reports (*.output, *.xml, etc.).
1956 *** %empty is used in reports
1958 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1959 dot, XML and formats derived from it).
1961 *** YYERROR and variants
1963 When C++ variant support is enabled, an error triggered via YYERROR, but
1964 not caught via error recovery, resulted in a double deletion.
1967 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1971 *** Errors in caret diagnostics
1973 On some platforms, some errors could result in endless diagnostics.
1975 *** Fixes of the -Werror option
1977 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1978 diagnostics into errors instead of warnings. This is fixed.
1980 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1981 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1982 -Wno-error", "foo" diagnostics are now errors.
1986 As demonstrated in the documentation, one can now leave spaces between
1991 The yacc.1 man page is no longer installed if --disable-yacc was
1994 *** Fixes in the test suite
1996 Bugs and portability issues.
1999 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
2001 ** WARNING: Future backward-incompatibilities!
2003 Like other GNU packages, Bison will start using some of the C99 features
2004 for its own code, especially the definition of variables after statements.
2005 The generated C parsers still aim at C90.
2007 ** Backward incompatible changes
2009 *** Obsolete features
2011 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
2013 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
2014 use YYSTYPE and YYLTYPE.
2016 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
2017 1.875): use %lex-param, %parse-param, or %param.
2019 Missing semicolons at the end of actions are no longer added (as announced
2020 in the release 2.5).
2022 *** Use of YACC='bison -y'
2024 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
2027 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
2028 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
2029 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
2031 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
2032 implementation of Yacc, was using Bison as 'bison -y'. While it does
2033 ensure compatible output file names, it also enables warnings for
2034 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
2035 warnings for Bison extensions.
2037 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
2038 (which also generates 'y.tab.h' and 'y.output' when needed).
2039 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
2040 flags (YFLAGS, or AM_YFLAGS with Automake).
2044 *** The epilogue is no longer affected by internal #defines (glr.c)
2046 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
2047 generated code. These weren't properly undefined before the inclusion of
2048 the user epilogue, so functions such as the following were butchered by the
2049 preprocessor expansion:
2051 int yylex (YYSTYPE *yylval);
2053 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2054 identifiers for user-provided variables.
2056 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2058 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2059 locations are enabled. This is fixed.
2061 *** Warnings about useless %pure-parser/%define api.pure are restored
2063 ** Diagnostics reported by Bison
2065 Most of these features were contributed by Théophile Ranquet and Victor
2070 Version 2.7 introduced caret errors, for a prettier output. These are now
2071 activated by default. The old format can still be used by invoking Bison
2072 with -fno-caret (or -fnone).
2074 Some error messages that reproduced excerpts of the grammar are now using
2075 the caret information only. For instance on:
2082 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2083 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2087 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2088 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2092 and "bison -fno-caret" reports:
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]
2097 *** Enhancements of the -Werror option
2099 The -Werror=CATEGORY option is now recognized, and will treat specified
2100 warnings as errors. The warnings need not have been explicitly activated
2101 using the -W option, this is similar to what GCC 4.7 does.
2103 For example, given the following command line, Bison will treat both
2104 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2105 errors (and only those):
2107 $ bison -Werror=yacc,error=conflicts-sr input.y
2109 If no categories are specified, -Werror will make all active warnings into
2110 errors. For example, the following line does the same the previous example:
2112 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2114 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2116 Note that the categories in this -Werror option may not be prefixed with
2117 "no-". However, -Wno-error[=CATEGORY] is valid.
2119 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2120 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2121 incompatibilities as warnings: "-y -Wno-error=yacc".
2123 *** The display of warnings is now richer
2125 The option that controls a given warning is now displayed:
2127 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2129 In the case of warnings treated as errors, the prefix is changed from
2130 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2131 to GCC, as [-Werror=CATEGORY].
2133 For instance, where the previous version of Bison would report (and exit
2136 bison: warnings being treated as errors
2137 input.y:1.1: warning: stray ',' treated as white space
2141 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2143 *** Deprecated constructs
2145 The new 'deprecated' warning category flags obsolete constructs whose
2146 support will be discontinued. It is enabled by default. These warnings
2147 used to be reported as 'other' warnings.
2149 *** Useless semantic types
2151 Bison now warns about useless (uninhabited) semantic types. Since
2152 semantic types are not declared to Bison (they are defined in the opaque
2153 %union structure), it is %printer/%destructor directives about useless
2154 types that trigger the warning:
2158 %printer {} <type1> <type3>
2159 %destructor {} <type2> <type4>
2161 nterm: term { $$ = $1; };
2163 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2164 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2166 *** Undefined but unused symbols
2168 Bison used to raise an error for undefined symbols that are not used in
2169 the grammar. This is now only a warning.
2172 %destructor {} symbol2
2173 %type <type> symbol3
2177 *** Useless destructors or printers
2179 Bison now warns about useless destructors or printers. In the following
2180 example, the printer for <type1>, and the destructor for <type2> are
2181 useless: all symbols of <type1> (token1) already have a printer, and all
2182 symbols of type <type2> (token2) already have a destructor.
2184 %token <type1> token1
2188 %printer {} token1 <type1> <type3>
2189 %destructor {} token2 <type2> <type4>
2193 The warnings and error messages about shift/reduce and reduce/reduce
2194 conflicts have been normalized. For instance on the following foo.y file:
2198 exp: exp '+' exp | '0' | '0';
2200 compare the previous version of bison:
2203 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2204 $ bison -Werror foo.y
2205 bison: warnings being treated as errors
2206 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2208 with the new behavior:
2211 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2212 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2213 $ bison -Werror foo.y
2214 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2215 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2217 When %expect or %expect-rr is used, such as with bar.y:
2222 exp: exp '+' exp | '0' | '0';
2227 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2228 bar.y: expected 0 shift/reduce conflicts
2229 bar.y: expected 0 reduce/reduce conflicts
2234 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2235 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2237 ** Incompatibilities with POSIX Yacc
2239 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2242 ** Additional yylex/yyparse arguments
2244 The new directive %param declares additional arguments to both yylex and
2245 yyparse. The %lex-param, %parse-param, and %param directives support one
2246 or more arguments. Instead of
2248 %lex-param {arg1_type *arg1}
2249 %lex-param {arg2_type *arg2}
2250 %parse-param {arg1_type *arg1}
2251 %parse-param {arg2_type *arg2}
2255 %param {arg1_type *arg1} {arg2_type *arg2}
2257 ** Types of values for %define variables
2259 Bison used to make no difference between '%define foo bar' and '%define
2260 foo "bar"'. The former is now called a 'keyword value', and the latter a
2261 'string value'. A third kind was added: 'code values', such as '%define
2264 Keyword variables are used for fixed value sets, e.g.,
2266 %define lr.type lalr
2268 Code variables are used for value in the target language, e.g.,
2270 %define api.value.type {struct semantic_type}
2272 String variables are used remaining cases, e.g. file names.
2274 ** Variable api.token.prefix
2276 The variable api.token.prefix changes the way tokens are identified in
2277 the generated files. This is especially useful to avoid collisions
2278 with identifiers in the target language. For instance
2280 %token FILE for ERROR
2281 %define api.token.prefix {TOK_}
2283 start: FILE for ERROR;
2285 will generate the definition of the symbols TOK_FILE, TOK_for, and
2286 TOK_ERROR in the generated sources. In particular, the scanner must
2287 use these prefixed token names, although the grammar itself still
2288 uses the short names (as in the sample rule given above).
2290 ** Variable api.value.type
2292 This new %define variable supersedes the #define macro YYSTYPE. The use
2293 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2294 using %union or %defining api.value.type results in undefined behavior.
2296 Either define api.value.type, or use "%union":
2303 %token <ival> INT "integer"
2304 %token <sval> STRING "string"
2305 %printer { fprintf (yyo, "%d", $$); } <ival>
2306 %destructor { free ($$); } <sval>
2309 yylval.ival = 42; return INT;
2310 yylval.sval = "42"; return STRING;
2312 The %define variable api.value.type supports both keyword and code values.
2314 The keyword value 'union' means that the user provides genuine types, not
2315 union member names such as "ival" and "sval" above (WARNING: will fail if
2316 -y/--yacc/%yacc is enabled).
2318 %define api.value.type union
2319 %token <int> INT "integer"
2320 %token <char *> STRING "string"
2321 %printer { fprintf (yyo, "%d", $$); } <int>
2322 %destructor { free ($$); } <char *>
2325 yylval.INT = 42; return INT;
2326 yylval.STRING = "42"; return STRING;
2328 The keyword value variant is somewhat equivalent, but for C++ special
2329 provision is made to allow classes to be used (more about this below).
2331 %define api.value.type variant
2332 %token <int> INT "integer"
2333 %token <std::string> STRING "string"
2335 Code values (in braces) denote user defined types. This is where YYSTYPE
2353 %define api.value.type {struct my_value}
2354 %token <u.ival> INT "integer"
2355 %token <u.sval> STRING "string"
2356 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2357 %destructor { free ($$); } <u.sval>
2360 yylval.u.ival = 42; return INT;
2361 yylval.u.sval = "42"; return STRING;
2363 ** Variable parse.error
2365 This variable controls the verbosity of error messages. The use of the
2366 %error-verbose directive is deprecated in favor of "%define parse.error
2369 ** Deprecated %define variable names
2371 The following variables have been renamed for consistency. Backward
2372 compatibility is ensured, but upgrading is recommended.
2374 lr.default-reductions -> lr.default-reduction
2375 lr.keep-unreachable-states -> lr.keep-unreachable-state
2376 namespace -> api.namespace
2377 stype -> api.value.type
2379 ** Semantic predicates
2381 Contributed by Paul Hilfinger.
2383 The new, experimental, semantic-predicate feature allows actions of the
2384 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2385 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2386 in GLR parsers, rather than being deferred. The result is that they allow
2387 the programmer to prune possible parses based on the values of run-time
2390 ** The directive %expect-rr is now an error in non GLR mode
2392 It used to be an error only if used in non GLR mode, _and_ if there are
2393 reduce/reduce conflicts.
2395 ** Tokens are numbered in their order of appearance
2397 Contributed by Valentin Tolmer.
2399 With '%token A B', A had a number less than the one of B. However,
2400 precedence declarations used to generate a reversed order. This is now
2401 fixed, and introducing tokens with any of %token, %left, %right,
2402 %precedence, or %nonassoc yields the same result.
2404 When mixing declarations of tokens with a literal character (e.g., 'a') or
2405 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2406 the literal characters first. For example
2410 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2411 input order is now preserved.
2413 These changes were made so that one can remove useless precedence and
2414 associativity declarations (i.e., map %nonassoc, %left or %right to
2415 %precedence, or to %token) and get exactly the same output.
2417 ** Useless precedence and associativity
2419 Contributed by Valentin Tolmer.
2421 When developing and maintaining a grammar, useless associativity and
2422 precedence directives are common. They can be a nuisance: new ambiguities
2423 arising are sometimes masked because their conflicts are resolved due to
2424 the extra precedence or associativity information. Furthermore, it can
2425 hinder the comprehension of a new grammar: one will wonder about the role
2426 of a precedence, where in fact it is useless. The following changes aim
2427 at detecting and reporting these extra directives.
2429 *** Precedence warning category
2431 A new category of warning, -Wprecedence, was introduced. It flags the
2432 useless precedence and associativity directives.
2434 *** Useless associativity
2436 Bison now warns about symbols with a declared associativity that is never
2437 used to resolve conflicts. In that case, using %precedence is sufficient;
2438 the parsing tables will remain unchanged. Solving these warnings may raise
2439 useless precedence warnings, as the symbols no longer have associativity.
2453 warning: useless associativity for '+', use %precedence [-Wprecedence]
2457 *** Useless precedence
2459 Bison now warns about symbols with a declared precedence and no declared
2460 associativity (i.e., declared with %precedence), and whose precedence is
2461 never used. In that case, the symbol can be safely declared with %token
2462 instead, without modifying the parsing tables. For example:
2466 exp: "var" '=' "number";
2470 warning: useless precedence for '=' [-Wprecedence]
2474 *** Useless precedence and associativity
2476 In case of both useless precedence and associativity, the issue is flagged
2481 exp: "var" '=' "number";
2485 warning: useless precedence and associativity for '=' [-Wprecedence]
2491 With help from Joel E. Denny and Gabriel Rassoul.
2493 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2494 marked by the new %empty directive. Using %empty on a non-empty rule is
2495 an error. The new -Wempty-rule warning reports empty rules without
2496 %empty. On the following grammar:
2506 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2509 5.8-13: error: %empty on non-empty rule
2513 ** Java skeleton improvements
2515 The constants for token names were moved to the Lexer interface. Also, it
2516 is possible to add code to the parser's constructors using "%code init"
2517 and "%define init_throws".
2518 Contributed by Paolo Bonzini.
2520 The Java skeleton now supports push parsing.
2521 Contributed by Dennis Heimbigner.
2523 ** C++ skeletons improvements
2525 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2527 Using %defines is now optional. Without it, the needed support classes
2528 are defined in the generated parser, instead of additional files (such as
2529 location.hh, position.hh and stack.hh).
2531 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2533 Both lalr1.cc and glr.cc no longer require %location.
2535 *** syntax_error exception (lalr1.cc)
2537 The C++ parser features a syntax_error exception, which can be
2538 thrown from the scanner or from user rules to raise syntax errors.
2539 This facilitates reporting errors caught in sub-functions (e.g.,
2540 rejecting too large integral literals from a conversion function
2541 used by the scanner, or rejecting invalid combinations from a
2542 factory invoked by the user actions).
2544 *** %define api.value.type variant
2546 This is based on a submission from Michiel De Wilde. With help
2547 from Théophile Ranquet.
2549 In this mode, complex C++ objects can be used as semantic values. For
2552 %token <::std::string> TEXT;
2553 %token <int> NUMBER;
2554 %token SEMICOLON ";"
2555 %type <::std::string> item;
2556 %type <::std::list<std::string>> list;
2559 list { std::cout << $1 << std::endl; }
2563 %empty { /* Generates an empty string list. */ }
2564 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2568 TEXT { std::swap ($$, $1); }
2569 | NUMBER { $$ = string_cast ($1); }
2572 *** %define api.token.constructor
2574 When variants are enabled, Bison can generate functions to build the
2575 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2576 with the semantic value (e.g., int):
2578 parser::symbol_type yylex ()
2580 parser::location_type loc = ...;
2582 return parser::make_TEXT ("Hello, world!", loc);
2584 return parser::make_NUMBER (42, loc);
2586 return parser::make_SEMICOLON (loc);
2592 There are operator- and operator-= for 'location'. Negative line/column
2593 increments can no longer underflow the resulting value.
2596 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2600 *** Fix compiler attribute portability (yacc.c)
2602 With locations enabled, __attribute__ was used unprotected.
2604 *** Fix some compiler warnings (lalr1.cc)
2607 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2611 Warnings about uninitialized yylloc in yyparse have been fixed.
2613 Restored C90 compliance (yet no report was ever made).
2615 ** Diagnostics are improved
2617 Contributed by Théophile Ranquet.
2619 *** Changes in the format of error messages
2621 This used to be the format of many error reports:
2623 input.y:2.7-12: %type redeclaration for exp
2624 input.y:1.7-12: previous declaration
2628 input.y:2.7-12: error: %type redeclaration for exp
2629 input.y:1.7-12: previous declaration
2631 *** New format for error reports: carets
2633 Caret errors have been added to Bison:
2635 input.y:2.7-12: error: %type redeclaration for exp
2638 input.y:1.7-12: previous declaration
2644 input.y:3.20-23: error: ambiguous reference: '$exp'
2645 exp: exp '+' exp { $exp = $1 + $3; };
2647 input.y:3.1-3: refers to: $exp at $$
2648 exp: exp '+' exp { $exp = $1 + $3; };
2650 input.y:3.6-8: refers to: $exp at $1
2651 exp: exp '+' exp { $exp = $1 + $3; };
2653 input.y:3.14-16: refers to: $exp at $3
2654 exp: exp '+' exp { $exp = $1 + $3; };
2657 The default behavior for now is still not to display these unless
2658 explicitly asked with -fcaret (or -fall). However, in a later release, it
2659 will be made the default behavior (but may still be deactivated with
2662 ** New value for %define variable: api.pure full
2664 The %define variable api.pure requests a pure (reentrant) parser. However,
2665 for historical reasons, using it in a location-tracking Yacc parser
2666 resulted in a yyerror function that did not take a location as a
2667 parameter. With this new value, the user may request a better pure parser,
2668 where yyerror does take a location as a parameter (in location-tracking
2671 The use of "%define api.pure true" is deprecated in favor of this new
2672 "%define api.pure full".
2674 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2676 The %define variable api.location.type defines the name of the type to use
2677 for locations. When defined, Bison no longer generates the position.hh
2678 and location.hh files, nor does the parser will include them: the user is
2679 then responsible to define her type.
2681 This can be used in programs with several parsers to factor their location
2682 and position files: let one of them generate them, and the others just use
2685 This feature was actually introduced, but not documented, in Bison 2.5,
2686 under the name "location_type" (which is maintained for backward
2689 For consistency, lalr1.java's %define variables location_type and
2690 position_type are deprecated in favor of api.location.type and
2693 ** Exception safety (lalr1.cc)
2695 The parse function now catches exceptions, uses the %destructors to
2696 release memory (the lookahead symbol and the symbols pushed on the stack)
2697 before re-throwing the exception.
2699 This feature is somewhat experimental. User feedback would be
2702 ** Graph improvements in DOT and XSLT
2704 Contributed by Théophile Ranquet.
2706 The graphical presentation of the states is more readable: their shape is
2707 now rectangular, the state number is clearly displayed, and the items are
2708 numbered and left-justified.
2710 The reductions are now explicitly represented as transitions to other
2711 diamond shaped nodes.
2713 These changes are present in both --graph output and xml2dot.xsl XSLT
2714 processing, with minor (documented) differences.
2716 ** %language is no longer an experimental feature.
2718 The introduction of this feature, in 2.4, was four years ago. The
2719 --language option and the %language directive are no longer experimental.
2723 The sections about shift/reduce and reduce/reduce conflicts resolution
2724 have been fixed and extended.
2726 Although introduced more than four years ago, XML and Graphviz reports
2727 were not properly documented.
2729 The translation of midrule actions is now described.
2732 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2734 We consider compiler warnings about Bison generated parsers to be bugs.
2735 Rather than working around them in your own project, please consider
2736 reporting them to us.
2740 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2741 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2744 Other issues in the test suite have been addressed.
2746 Null characters are correctly displayed in error messages.
2748 When possible, yylloc is correctly initialized before calling yylex. It
2749 is no longer necessary to initialize it in the %initial-action.
2752 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2754 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2757 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2761 Bugs and portability issues in the test suite have been fixed.
2763 Some errors in translations have been addressed, and --help now directs
2764 users to the appropriate place to report them.
2766 Stray Info files shipped by accident are removed.
2768 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2769 generated, are removed.
2771 All the generated headers are self-contained.
2773 ** Header guards (yacc.c, glr.c, glr.cc)
2775 In order to avoid collisions, the header guards are now
2776 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2777 For instance the header generated from
2779 %define api.prefix "calc"
2780 %defines "lib/parse.h"
2782 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2784 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2786 The compilation of pure parsers (%define api.pure) can trigger GCC
2789 input.c: In function 'yyparse':
2790 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2791 function [-Wmaybe-uninitialized]
2795 This is now fixed; pragmas to avoid these warnings are no longer needed.
2797 Warnings from clang ("equality comparison with extraneous parentheses" and
2798 "function declared 'noreturn' should not return") have also been
2802 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2806 Buffer overruns, complaints from Flex, and portability issues in the test
2807 suite have been fixed.
2809 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2811 Trailing end-of-lines in %parse-param or %lex-param would result in
2812 invalid C++. This is fixed.
2814 ** Spurious spaces and end-of-lines
2816 The generated files no longer end (nor start) with empty lines.
2819 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2821 Bison no longer executes user-specified M4 code when processing a grammar.
2825 In addition to the removal of the features announced in Bison 2.6, the
2826 next major release will remove the "Temporary hack for adding a semicolon
2827 to the user action", as announced in the release 2.5. Instead of:
2829 exp: exp "+" exp { $$ = $1 + $3 };
2833 exp: exp "+" exp { $$ = $1 + $3; };
2837 *** Type names are now properly escaped.
2839 *** glr.cc: set_debug_level and debug_level work as expected.
2841 *** Stray @ or $ in actions
2843 While Bison used to warn about stray $ or @ in action rules, it did not
2844 for other actions such as printers, destructors, or initial actions. It
2847 ** Type names in actions
2849 For consistency with rule actions, it is now possible to qualify $$ by a
2850 type-name in destructors, printers, and initial actions. For instance:
2852 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2854 will display two values for each typed and untyped symbol (provided
2855 that YYSTYPE has both "ival" and "fval" fields).
2858 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2862 The next major release of Bison will drop support for the following
2863 deprecated features. Please report disagreements to bug-bison@gnu.org.
2867 Support for generating parsers in K&R C will be removed. Parsers
2868 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2871 *** Features deprecated since Bison 1.875
2873 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2876 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2877 %lex-param, will no longer be supported.
2879 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2882 *** The generated header will be included (yacc.c)
2884 Instead of duplicating the content of the generated header (definition of
2885 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2886 as is already the case for GLR or C++ parsers. This change is deferred
2887 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2890 ** Generated Parser Headers
2892 *** Guards (yacc.c, glr.c, glr.cc)
2894 The generated headers are now guarded, as is already the case for C++
2895 parsers (lalr1.cc). For instance, with --defines=foo.h:
2900 #endif /* !YY_FOO_H */
2902 *** New declarations (yacc.c, glr.c)
2904 The generated header now declares yydebug and yyparse. Both honor
2905 --name-prefix=bar_, and yield
2907 int bar_parse (void);
2911 #define yyparse bar_parse
2914 in order to facilitate the inclusion of several parser headers inside a
2915 single compilation unit.
2917 *** Exported symbols in C++
2919 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2920 header, are removed, as they prevent the possibility of including several
2921 generated headers from a single compilation unit.
2925 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2928 ** New %define variable: api.prefix
2930 Now that the generated headers are more complete and properly protected
2931 against multiple inclusions, constant names, such as YYSTYPE are a
2932 problem. While yyparse and others are properly renamed by %name-prefix,
2933 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2934 would introduce backward compatibility issues in projects not expecting
2935 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2936 it is deprecated in favor of a new %define variable: api.prefix.
2938 The following examples compares both:
2940 %name-prefix "bar_" | %define api.prefix "bar_"
2941 %token <ival> FOO %token <ival> FOO
2942 %union { int ival; } %union { int ival; }
2948 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2949 # define BAR_FOO_H # define BAR_FOO_H
2951 /* Enabling traces. */ /* Enabling traces. */
2952 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2953 > # if defined YYDEBUG
2955 > # define BAR_DEBUG 1
2957 > # define BAR_DEBUG 0
2960 # define YYDEBUG 0 | # define BAR_DEBUG 0
2964 # if YYDEBUG | # if BAR_DEBUG
2965 extern int bar_debug; extern int bar_debug;
2968 /* Tokens. */ /* Tokens. */
2969 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2970 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2971 enum yytokentype { | enum bar_tokentype {
2976 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2977 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2978 typedef union YYSTYPE | typedef union BAR_STYPE
2981 } YYSTYPE; | } BAR_STYPE;
2982 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2985 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2987 int bar_parse (void); int bar_parse (void);
2989 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2992 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2996 The next major release will drop support for generating parsers in K&R C.
2998 ** yacc.c: YYBACKUP works as expected.
3000 ** glr.c improvements:
3002 *** Location support is eliminated when not requested:
3004 GLR parsers used to include location-related code even when locations were
3005 not requested, and therefore not even usable.
3007 *** __attribute__ is preserved:
3009 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
3010 when -std is passed to GCC).
3012 ** lalr1.java: several fixes:
3014 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
3015 first token leads to a syntax error. Some minor clean ups.
3019 *** C++11 compatibility:
3021 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
3026 The header files such as "parser.hh", "location.hh", etc. used a constant
3027 name for preprocessor guards, for instance:
3029 #ifndef BISON_LOCATION_HH
3030 # define BISON_LOCATION_HH
3032 #endif // !BISON_LOCATION_HH
3034 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
3035 case characters are converted to upper case, and series of
3036 non-alphanumerical characters are converted to an underscore.
3038 With "bison -o lang++/parser.cc", "location.hh" would now include:
3040 #ifndef YY_LANG_LOCATION_HH
3041 # define YY_LANG_LOCATION_HH
3043 #endif // !YY_LANG_LOCATION_HH
3047 The position and location constructors (and their initialize methods)
3048 accept new arguments for line and column. Several issues in the
3049 documentation were fixed.
3051 ** liby is no longer asking for "rpl_fprintf" on some platforms.
3053 ** Changes in the manual:
3055 *** %printer is documented
3057 The "%printer" directive, supported since at least Bison 1.50, is finally
3058 documented. The "mfcalc" example is extended to demonstrate it.
3060 For consistency with the C skeletons, the C++ parsers now also support
3061 "yyoutput" (as an alias to "debug_stream ()").
3063 *** Several improvements have been made:
3065 The layout for grammar excerpts was changed to a more compact scheme.
3066 Named references are motivated. The description of the automaton
3067 description file (*.output) is updated to the current format. Incorrect
3068 index entries were fixed. Some other errors were fixed.
3072 *** Conflicting prototypes with recent/modified Flex.
3074 Fixed build problems with the current, unreleased, version of Flex, and
3075 some modified versions of 2.5.35, which have modified function prototypes.
3077 *** Warnings during the build procedure have been eliminated.
3079 *** Several portability problems in the test suite have been fixed:
3081 This includes warnings with some compilers, unexpected behavior of tools
3082 such as diff, warning messages from the test suite itself, etc.
3084 *** The install-pdf target works properly:
3086 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3087 halts in the middle of its course.
3090 * Noteworthy changes in release 2.5 (2011-05-14)
3092 ** Grammar symbol names can now contain non-initial dashes:
3094 Consistently with directives (such as %error-verbose) and with
3095 %define variables (e.g. push-pull), grammar symbol names may contain
3096 dashes in any position except the beginning. This is a GNU
3097 extension over POSIX Yacc. Thus, use of this extension is reported
3098 by -Wyacc and rejected in Yacc mode (--yacc).
3100 ** Named references:
3102 Historically, Yacc and Bison have supported positional references
3103 ($n, $$) to allow access to symbol values from inside of semantic
3106 Starting from this version, Bison can also accept named references.
3107 When no ambiguity is possible, original symbol names may be used
3108 as named references:
3110 if_stmt : "if" cond_expr "then" then_stmt ';'
3111 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3113 In the more common case, explicit names may be declared:
3115 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3116 { $res = mk_if_stmt($cond, $then, $else); }
3118 Location information is also accessible using @name syntax. When
3119 accessing symbol names containing dots or dashes, explicit bracketing
3120 ($[sym.1]) must be used.
3122 These features are experimental in this version. More user feedback
3123 will help to stabilize them.
3124 Contributed by Alex Rozenman.
3126 ** IELR(1) and canonical LR(1):
3128 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3129 is, given any context-free grammar, IELR(1) generates parser tables
3130 with the full language-recognition power of canonical LR(1) but with
3131 nearly the same number of parser states as LALR(1). This reduction
3132 in parser states is often an order of magnitude. More importantly,
3133 because canonical LR(1)'s extra parser states may contain duplicate
3134 conflicts in the case of non-LR(1) grammars, the number of conflicts
3135 for IELR(1) is often an order of magnitude less as well. This can
3136 significantly reduce the complexity of developing of a grammar.
3138 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3139 place of its traditional LALR(1) parser tables, which remain the
3140 default. You can specify the type of parser tables in the grammar
3141 file with these directives:
3143 %define lr.type lalr
3144 %define lr.type ielr
3145 %define lr.type canonical-lr
3147 The default-reduction optimization in the parser tables can also be
3148 adjusted using "%define lr.default-reductions". For details on both
3149 of these features, see the new section "Tuning LR" in the Bison
3152 These features are experimental. More user feedback will help to
3155 ** LAC (Lookahead Correction) for syntax error handling
3157 Contributed by Joel E. Denny.
3159 Canonical LR, IELR, and LALR can suffer from a couple of problems
3160 upon encountering a syntax error. First, the parser might perform
3161 additional parser stack reductions before discovering the syntax
3162 error. Such reductions can perform user semantic actions that are
3163 unexpected because they are based on an invalid token, and they
3164 cause error recovery to begin in a different syntactic context than
3165 the one in which the invalid token was encountered. Second, when
3166 verbose error messages are enabled (with %error-verbose or the
3167 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3168 syntax error message can both contain invalid tokens and omit valid
3171 The culprits for the above problems are %nonassoc, default
3172 reductions in inconsistent states, and parser state merging. Thus,
3173 IELR and LALR suffer the most. Canonical LR can suffer only if
3174 %nonassoc is used or if default reductions are enabled for
3175 inconsistent states.
3177 LAC is a new mechanism within the parsing algorithm that solves
3178 these problems for canonical LR, IELR, and LALR without sacrificing
3179 %nonassoc, default reductions, or state merging. When LAC is in
3180 use, canonical LR and IELR behave almost exactly the same for both
3181 syntactically acceptable and syntactically unacceptable input.
3182 While LALR still does not support the full language-recognition
3183 power of canonical LR and IELR, LAC at least enables LALR's syntax
3184 error handling to correctly reflect LALR's language-recognition
3187 Currently, LAC is only supported for deterministic parsers in C.
3188 You can enable LAC with the following directive:
3190 %define parse.lac full
3192 See the new section "LAC" in the Bison manual for additional
3193 details including a few caveats.
3195 LAC is an experimental feature. More user feedback will help to
3198 ** %define improvements:
3200 *** Can now be invoked via the command line:
3202 Each of these command-line options
3205 --define=NAME[=VALUE]
3208 --force-define=NAME[=VALUE]
3210 is equivalent to this grammar file declaration
3212 %define NAME ["VALUE"]
3214 except that the manner in which Bison processes multiple definitions
3215 for the same NAME differs. Most importantly, -F and --force-define
3216 quietly override %define, but -D and --define do not. For further
3217 details, see the section "Bison Options" in the Bison manual.
3219 *** Variables renamed:
3221 The following %define variables
3224 lr.keep_unreachable_states
3226 have been renamed to
3229 lr.keep-unreachable-states
3231 The old names are now deprecated but will be maintained indefinitely
3232 for backward compatibility.
3234 *** Values no longer need to be quoted in the grammar file:
3236 If a %define value is an identifier, it no longer needs to be placed
3237 within quotations marks. For example,
3239 %define api.push-pull "push"
3243 %define api.push-pull push
3245 *** Unrecognized variables are now errors not warnings.
3247 *** Multiple invocations for any variable is now an error not a warning.
3249 ** Unrecognized %code qualifiers are now errors not warnings.
3251 ** Character literals not of length one:
3253 Previously, Bison quietly converted all character literals to length
3254 one. For example, without warning, Bison interpreted the operators in
3255 the following grammar to be the same token:
3261 Bison now warns when a character literal is not of length one. In
3262 some future release, Bison will start reporting an error instead.
3264 ** Destructor calls fixed for lookaheads altered in semantic actions:
3266 Previously for deterministic parsers in C, if a user semantic action
3267 altered yychar, the parser in some cases used the old yychar value to
3268 determine which destructor to call for the lookahead upon a syntax
3269 error or upon parser return. This bug has been fixed.
3271 ** C++ parsers use YYRHSLOC:
3273 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3274 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3275 to use it. If, for instance, your location structure has "first"
3276 and "last" members, instead of
3278 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3282 (Current).first = (Rhs)[1].location.first; \
3283 (Current).last = (Rhs)[N].location.last; \
3287 (Current).first = (Current).last = (Rhs)[0].location.last; \
3293 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3297 (Current).first = YYRHSLOC (Rhs, 1).first; \
3298 (Current).last = YYRHSLOC (Rhs, N).last; \
3302 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3306 ** YYLLOC_DEFAULT in C++:
3308 The default implementation of YYLLOC_DEFAULT used to be issued in
3309 the header file. It is now output in the implementation file, after
3310 the user %code sections so that its #ifndef guard does not try to
3311 override the user's YYLLOC_DEFAULT if provided.
3313 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3315 YYFAIL has existed for many years as an undocumented feature of
3316 deterministic parsers in C generated by Bison. More recently, it was
3317 a documented feature of Bison's experimental Java parsers. As
3318 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3319 semantic action now produces a deprecation warning, and Java parsers
3320 no longer implement YYFAIL at all. For further details, including a
3321 discussion of how to suppress C preprocessor warnings about YYFAIL
3322 being unused, see the Bison 2.4.2 NEWS entry.
3324 ** Temporary hack for adding a semicolon to the user action:
3326 Previously, Bison appended a semicolon to every user action for
3327 reductions when the output language defaulted to C (specifically, when
3328 neither %yacc, %language, %skeleton, or equivalent command-line
3329 options were specified). This allowed actions such as
3331 exp: exp "+" exp { $$ = $1 + $3 };
3335 exp: exp "+" exp { $$ = $1 + $3; };
3337 As a first step in removing this misfeature, Bison now issues a
3338 warning when it appends a semicolon. Moreover, in cases where Bison
3339 cannot easily determine whether a semicolon is needed (for example, an
3340 action ending with a cpp directive or a braced compound initializer),
3341 it no longer appends one. Thus, the C compiler might now complain
3342 about a missing semicolon where it did not before. Future releases of
3343 Bison will cease to append semicolons entirely.
3345 ** Verbose syntax error message fixes:
3347 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3348 specified, syntax error messages produced by the generated parser
3349 include the unexpected token as well as a list of expected tokens.
3350 The effect of %nonassoc on these verbose messages has been corrected
3351 in two ways, but a more complete fix requires LAC, described above:
3353 *** When %nonassoc is used, there can exist parser states that accept no
3354 tokens, and so the parser does not always require a lookahead token
3355 in order to detect a syntax error. Because no unexpected token or
3356 expected tokens can then be reported, the verbose syntax error
3357 message described above is suppressed, and the parser instead
3358 reports the simpler message, "syntax error". Previously, this
3359 suppression was sometimes erroneously triggered by %nonassoc when a
3360 lookahead was actually required. Now verbose messages are
3361 suppressed only when all previous lookaheads have already been
3362 shifted or discarded.
3364 *** Previously, the list of expected tokens erroneously included tokens
3365 that would actually induce a syntax error because conflicts for them
3366 were resolved with %nonassoc in the current parser state. Such
3367 tokens are now properly omitted from the list.
3369 *** Expected token lists are still often wrong due to state merging
3370 (from LALR or IELR) and default reductions, which can both add
3371 invalid tokens and subtract valid tokens. Canonical LR almost
3372 completely fixes this problem by eliminating state merging and
3373 default reductions. However, there is one minor problem left even
3374 when using canonical LR and even after the fixes above. That is,
3375 if the resolution of a conflict with %nonassoc appears in a later
3376 parser state than the one at which some syntax error is
3377 discovered, the conflicted token is still erroneously included in
3378 the expected token list. Bison's new LAC implementation,
3379 described above, eliminates this problem and the need for
3380 canonical LR. However, LAC is still experimental and is disabled
3383 ** Java skeleton fixes:
3385 *** A location handling bug has been fixed.
3387 *** The top element of each of the value stack and location stack is now
3388 cleared when popped so that it can be garbage collected.
3390 *** Parser traces now print the top element of the stack.
3392 ** -W/--warnings fixes:
3394 *** Bison now properly recognizes the "no-" versions of categories:
3396 For example, given the following command line, Bison now enables all
3397 warnings except warnings for incompatibilities with POSIX Yacc:
3399 bison -Wall,no-yacc gram.y
3401 *** Bison now treats S/R and R/R conflicts like other warnings:
3403 Previously, conflict reports were independent of Bison's normal
3404 warning system. Now, Bison recognizes the warning categories
3405 "conflicts-sr" and "conflicts-rr". This change has important
3406 consequences for the -W and --warnings command-line options. For
3409 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3410 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3411 bison -Wnone gram.y # no conflicts are reported
3412 bison -Werror gram.y # any conflict is an error
3414 However, as before, if the %expect or %expect-rr directive is
3415 specified, an unexpected number of conflicts is an error, and an
3416 expected number of conflicts is not reported, so -W and --warning
3417 then have no effect on the conflict report.
3419 *** The "none" category no longer disables a preceding "error":
3421 For example, for the following command line, Bison now reports
3422 errors instead of warnings for incompatibilities with POSIX Yacc:
3424 bison -Werror,none,yacc gram.y
3426 *** The "none" category now disables all Bison warnings:
3428 Previously, the "none" category disabled only Bison warnings for
3429 which there existed a specific -W/--warning category. However,
3430 given the following command line, Bison is now guaranteed to
3431 suppress all warnings:
3435 ** Precedence directives can now assign token number 0:
3437 Since Bison 2.3b, which restored the ability of precedence
3438 directives to assign token numbers, doing so for token number 0 has
3439 produced an assertion failure. For example:
3443 This bug has been fixed.
3446 * Noteworthy changes in release 2.4.3 (2010-08-05)
3448 ** Bison now obeys -Werror and --warnings=error for warnings about
3449 grammar rules that are useless in the parser due to conflicts.
3451 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3454 ** Failures in the test suite for GCC 4.5 have been fixed.
3456 ** Failures in the test suite for some versions of Sun Studio C++ have
3459 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3460 warnings about undefined %prec identifiers will not be converted to
3461 errors in Bison 2.5. They will remain warnings, which should be
3462 sufficient for POSIX while avoiding backward compatibility issues.
3464 ** Minor documentation fixes.
3467 * Noteworthy changes in release 2.4.2 (2010-03-20)
3469 ** Some portability problems that resulted in failures and livelocks
3470 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3471 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3472 errors should no longer cause M4 to report a broken pipe on the
3475 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3477 POSIX specifies that an error be reported for any identifier that does
3478 not appear on the LHS of a grammar rule and that is not defined by
3479 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3480 error report for the case when an identifier appears only after a
3481 %prec directive. It is now restored. However, for backward
3482 compatibility with recent Bison releases, it is only a warning for
3483 now. In Bison 2.5 and later, it will return to being an error.
3484 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3485 warning will not be converted to an error in Bison 2.5.]
3487 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3489 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3490 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3493 ** %code is now a permanent feature.
3495 A traditional Yacc prologue directive is written in the form:
3499 To provide a more flexible alternative, Bison 2.3b introduced the
3500 %code directive with the following forms for C/C++:
3503 %code requires {CODE}
3504 %code provides {CODE}
3507 These forms are now considered permanent features of Bison. See the
3508 %code entries in the section "Bison Declaration Summary" in the Bison
3509 manual for a summary of their functionality. See the section
3510 "Prologue Alternatives" for a detailed discussion including the
3511 advantages of %code over the traditional Yacc prologue directive.
3513 Bison's Java feature as a whole including its current usage of %code
3514 is still considered experimental.
3516 ** YYFAIL is deprecated and will eventually be removed.
3518 YYFAIL has existed for many years as an undocumented feature of
3519 deterministic parsers in C generated by Bison. Previously, it was
3520 documented for Bison's experimental Java parsers. YYFAIL is no longer
3521 documented for Java parsers and is formally deprecated in both cases.
3522 Users are strongly encouraged to migrate to YYERROR, which is
3525 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3526 induce a syntax error. The most obvious difference from YYERROR is
3527 that YYFAIL will automatically invoke yyerror to report the syntax
3528 error so that you don't have to. However, there are several other
3529 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3530 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3531 used. For a more detailed discussion, see:
3533 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3535 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3536 deterministic parsers in C will continue to implement it. However,
3537 because YYFAIL is already flawed, it seems futile to try to make new
3538 Bison features compatible with it. Thus, during parser generation,
3539 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3540 rule action. In a later release, YYFAIL will be disabled for
3541 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3542 be removed altogether.
3544 There exists at least one case where Bison 2.5's YYFAIL warning will
3545 be a false positive. Some projects add phony uses of YYFAIL and other
3546 Bison-defined macros for the sole purpose of suppressing C
3547 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3548 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3549 epilogue (that is, after the second "%%") in the Bison input file. In
3550 this release (2.4.2), Bison already generates its own code to suppress
3551 C preprocessor warnings for YYFAIL, so projects can remove their own
3552 phony uses of YYFAIL if compatibility with Bison releases prior to
3553 2.4.2 is not necessary.
3555 ** Internationalization.
3557 Fix a regression introduced in Bison 2.4: Under some circumstances,
3558 message translations were not installed although supported by the
3562 * Noteworthy changes in release 2.4.1 (2008-12-11)
3564 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3565 declarations have been fixed.
3567 ** Temporary hack for adding a semicolon to the user action.
3569 Bison used to prepend a trailing semicolon at the end of the user
3570 action for reductions. This allowed actions such as
3572 exp: exp "+" exp { $$ = $1 + $3 };
3576 exp: exp "+" exp { $$ = $1 + $3; };
3578 Some grammars still depend on this "feature". Bison 2.4.1 restores
3579 the previous behavior in the case of C output (specifically, when
3580 neither %language or %skeleton or equivalent command-line options
3581 are used) to leave more time for grammars depending on the old
3582 behavior to be adjusted. Future releases of Bison will disable this
3585 ** A few minor improvements to the Bison manual.
3588 * Noteworthy changes in release 2.4 (2008-11-02)
3590 ** %language is an experimental feature.
3592 We first introduced this feature in test release 2.3b as a cleaner
3593 alternative to %skeleton. Since then, we have discussed the possibility of
3594 modifying its effect on Bison's output file names. Thus, in this release,
3595 we consider %language to be an experimental feature that will likely evolve
3598 ** Forward compatibility with GNU M4 has been improved.
3600 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3604 * Noteworthy changes in release 2.3b (2008-05-27)
3606 ** The quotes around NAME that used to be required in the following directive
3609 %define NAME "VALUE"
3611 ** The directive "%pure-parser" is now deprecated in favor of:
3615 which has the same effect except that Bison is more careful to warn about
3616 unreasonable usage in the latter case.
3620 Bison can now generate an LALR(1) parser in C with a push interface. That
3621 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3622 push one token at a time to the parser using "yypush_parse", which will
3623 return to the caller after processing each token. By default, the push
3624 interface is disabled. Either of the following directives will enable it:
3626 %define api.push_pull "push" // Just push; does not require yylex.
3627 %define api.push_pull "both" // Push and pull; requires yylex.
3629 See the new section "A Push Parser" in the Bison manual for details.
3631 The current push parsing interface is experimental and may evolve. More user
3632 feedback will help to stabilize it.
3634 ** The -g and --graph options now output graphs in Graphviz DOT format,
3635 not VCG format. Like --graph, -g now also takes an optional FILE argument
3636 and thus cannot be bundled with other short options.
3640 Bison can now generate an LALR(1) parser in Java. The skeleton is
3641 "data/lalr1.java". Consider using the new %language directive instead of
3642 %skeleton to select it.
3644 See the new section "Java Parsers" in the Bison manual for details.
3646 The current Java interface is experimental and may evolve. More user
3647 feedback will help to stabilize it.
3648 Contributed by Paolo Bonzini.
3652 This new directive specifies the programming language of the generated
3653 parser, which can be C (the default), C++, or Java. Besides the skeleton
3654 that Bison uses, the directive affects the names of the generated files if
3655 the grammar file's name ends in ".y".
3657 ** XML Automaton Report
3659 Bison can now generate an XML report of the LALR(1) automaton using the new
3660 "--xml" option. The current XML schema is experimental and may evolve. More
3661 user feedback will help to stabilize it.
3662 Contributed by Wojciech Polak.
3664 ** The grammar file may now specify the name of the parser header file using
3665 %defines. For example:
3669 ** When reporting useless rules, useless nonterminals, and unused terminals,
3670 Bison now employs the terms "useless in grammar" instead of "useless",
3671 "useless in parser" instead of "never reduced", and "unused in grammar"
3672 instead of "unused".
3674 ** Unreachable State Removal
3676 Previously, Bison sometimes generated parser tables containing unreachable
3677 states. A state can become unreachable during conflict resolution if Bison
3678 disables a shift action leading to it from a predecessor state. Bison now:
3680 1. Removes unreachable states.
3682 2. Does not report any conflicts that appeared in unreachable states.
3683 WARNING: As a result, you may need to update %expect and %expect-rr
3684 directives in existing grammar files.
3686 3. For any rule used only in such states, Bison now reports the rule as
3687 "useless in parser due to conflicts".
3689 This feature can be disabled with the following directive:
3691 %define lr.keep_unreachable_states
3693 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3694 for further discussion.
3696 ** Lookahead Set Correction in the ".output" Report
3698 When instructed to generate a ".output" file including lookahead sets
3699 (using "--report=lookahead", for example), Bison now prints each reduction's
3700 lookahead set only next to the associated state's one item that (1) is
3701 associated with the same rule as the reduction and (2) has its dot at the end
3702 of its RHS. Previously, Bison also erroneously printed the lookahead set
3703 next to all of the state's other items associated with the same rule. This
3704 bug affected only the ".output" file and not the generated parser source
3707 ** --report-file=FILE is a new option to override the default ".output" file
3710 ** The "=" that used to be required in the following directives is now
3713 %file-prefix "parser"
3717 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3719 Bison 2.3a provided a new set of directives as a more flexible alternative to
3720 the traditional Yacc prologue blocks. Those have now been consolidated into
3721 a single %code directive with an optional qualifier field, which identifies
3722 the purpose of the code and thus the location(s) where Bison should generate
3725 1. "%code {CODE}" replaces "%after-header {CODE}"
3726 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3727 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3728 4. "%code top {CODE}" replaces "%before-header {CODE}"
3730 See the %code entries in section "Bison Declaration Summary" in the Bison
3731 manual for a summary of the new functionality. See the new section "Prologue
3732 Alternatives" for a detailed discussion including the advantages of %code
3733 over the traditional Yacc prologues.
3735 The prologue alternatives are experimental. More user feedback will help to
3736 determine whether they should become permanent features.
3738 ** Revised warning: unset or unused midrule values
3740 Since Bison 2.2, Bison has warned about midrule values that are set but not
3741 used within any of the actions of the parent rule. For example, Bison warns
3744 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3746 Now, Bison also warns about midrule values that are used but not set. For
3747 example, Bison warns about unset $$ in the midrule action in:
3749 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3751 However, Bison now disables both of these warnings by default since they
3752 sometimes prove to be false alarms in existing grammars employing the Yacc
3753 constructs $0 or $-N (where N is some positive integer).
3755 To enable these warnings, specify the option "--warnings=midrule-values" or
3756 "-W", which is a synonym for "--warnings=all".
3758 ** Default %destructor or %printer with "<*>" or "<>"
3760 Bison now recognizes two separate kinds of default %destructor's and
3763 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3764 %destructor/%printer for all grammar symbols for which you have formally
3765 declared semantic type tags.
3767 2. Place "<>" in a %destructor/%printer symbol list to define a default
3768 %destructor/%printer for all grammar symbols without declared semantic
3771 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3772 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3773 longer applies any %destructor to a midrule value if that midrule value is
3774 not actually ever referenced using either $$ or $n in a semantic action.
3776 The default %destructor's and %printer's are experimental. More user
3777 feedback will help to determine whether they should become permanent
3780 See the section "Freeing Discarded Symbols" in the Bison manual for further
3783 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3784 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3785 manual for a caveat concerning the treatment of literal strings.
3787 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3788 completely removed from Bison.
3791 * Noteworthy changes in release 2.3a (2006-09-13)
3793 ** Instead of %union, you can define and use your own union type
3794 YYSTYPE if your grammar contains at least one <type> tag.
3795 Your YYSTYPE need not be a macro; it can be a typedef.
3796 This change is for compatibility with other Yacc implementations,
3797 and is required by POSIX.
3799 ** Locations columns and lines start at 1.
3800 In accordance with the GNU Coding Standards and Emacs.
3802 ** You may now declare per-type and default %destructor's and %printer's:
3806 %union { char *string; }
3807 %token <string> STRING1
3808 %token <string> STRING2
3809 %type <string> string1
3810 %type <string> string2
3811 %union { char character; }
3812 %token <character> CHR
3813 %type <character> chr
3814 %destructor { free ($$); } %symbol-default
3815 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3816 %destructor { } <character>
3818 guarantees that, when the parser discards any user-defined symbol that has a
3819 semantic type tag other than "<character>", it passes its semantic value to
3820 "free". However, when the parser discards a "STRING1" or a "string1", it
3821 also prints its line number to "stdout". It performs only the second
3822 "%destructor" in this case, so it invokes "free" only once.
3824 [Although we failed to mention this here in the 2.3a release, the default
3825 %destructor's and %printer's were experimental, and they were rewritten in
3828 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3829 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3830 associating token numbers with token names. Removing the #define statements
3831 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3832 requires them. Bison still generates an enum for token names in all cases.
3834 ** Handling of traditional Yacc prologue blocks is now more consistent but
3835 potentially incompatible with previous releases of Bison.
3837 As before, you declare prologue blocks in your grammar file with the
3838 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3839 prologue blocks that you've declared before the first %union. To generate
3840 the post-prologue, Bison concatenates all prologue blocks that you've
3841 declared after the first %union.
3843 Previous releases of Bison inserted the pre-prologue into both the header
3844 file and the code file in all cases except for LALR(1) parsers in C. In the
3845 latter case, Bison inserted it only into the code file. For parsers in C++,
3846 the point of insertion was before any token definitions (which associate
3847 token numbers with names). For parsers in C, the point of insertion was
3848 after the token definitions.
3850 Now, Bison never inserts the pre-prologue into the header file. In the code
3851 file, it always inserts it before the token definitions.
3853 ** Bison now provides a more flexible alternative to the traditional Yacc
3854 prologue blocks: %before-header, %start-header, %end-header, and
3857 For example, the following declaration order in the grammar file reflects the
3858 order in which Bison will output these code blocks. However, you are free to
3859 declare these code blocks in your grammar file in whatever order is most
3863 /* Bison treats this block like a pre-prologue block: it inserts it into
3864 * the code file before the contents of the header file. It does *not*
3865 * insert it into the header file. This is a good place to put
3866 * #include's that you want at the top of your code file. A common
3867 * example is '#include "system.h"'. */
3870 /* Bison inserts this block into both the header file and the code file.
3871 * In both files, the point of insertion is before any Bison-generated
3872 * token, semantic type, location type, and class definitions. This is a
3873 * good place to define %union dependencies, for example. */
3876 /* Unlike the traditional Yacc prologue blocks, the output order for the
3877 * new %*-header blocks is not affected by their declaration position
3878 * relative to any %union in the grammar file. */
3881 /* Bison inserts this block into both the header file and the code file.
3882 * In both files, the point of insertion is after the Bison-generated
3883 * definitions. This is a good place to declare or define public
3884 * functions or data structures that depend on the Bison-generated
3888 /* Bison treats this block like a post-prologue block: it inserts it into
3889 * the code file after the contents of the header file. It does *not*
3890 * insert it into the header file. This is a good place to declare or
3891 * define internal functions or data structures that depend on the
3892 * Bison-generated definitions. */
3895 If you have multiple occurrences of any one of the above declarations, Bison
3896 will concatenate the contents in declaration order.
3898 [Although we failed to mention this here in the 2.3a release, the prologue
3899 alternatives were experimental, and they were rewritten in future versions.]
3901 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3902 The old spelling still works, but is not documented and may be removed
3903 in a future release.
3906 * Noteworthy changes in release 2.3 (2006-06-05)
3908 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3909 for compatibility with LALR(1) grammars.
3911 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3912 be to a type name that does not contain parentheses or brackets.
3915 * Noteworthy changes in release 2.2 (2006-05-19)
3917 ** The distribution terms for all Bison-generated parsers now permit
3918 using the parsers in nonfree programs. Previously, this permission
3919 was granted only for Bison-generated LALR(1) parsers in C.
3921 ** %name-prefix changes the namespace name in C++ outputs.
3923 ** The C++ parsers export their token_type.
3925 ** Bison now allows multiple %union declarations, and concatenates
3926 their contents together.
3928 ** New warning: unused values
3929 Right-hand side symbols whose values are not used are reported,
3930 if the symbols have destructors. For instance:
3932 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3936 will trigger a warning about $$ and $5 in the first rule, and $3 in
3937 the second ($1 is copied to $$ by the default rule). This example
3938 most likely contains three errors, and could be rewritten as:
3940 exp: exp "?" exp ":" exp
3941 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3943 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3946 However, if the original actions were really intended, memory leaks
3947 and all, the warnings can be suppressed by letting Bison believe the
3948 values are used, e.g.:
3950 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3951 | exp "+" exp { $$ = $1; (void) $3; }
3954 If there are midrule actions, the warning is issued if no action
3955 uses it. The following triggers no warning: $1 and $3 are used.
3957 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3959 The warning is intended to help catching lost values and memory leaks.
3960 If a value is ignored, its associated memory typically is not reclaimed.
3962 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3963 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3964 and YYERROR, for all objects on the stack, other than objects
3965 corresponding to the right-hand side of the current rule.
3967 ** %expect, %expect-rr
3968 Incorrect numbers of expected conflicts are now actual errors,
3969 instead of warnings.
3971 ** GLR, YACC parsers.
3972 The %parse-params are available in the destructors (and the
3973 experimental printers) as per the documentation.
3975 ** Bison now warns if it finds a stray "$" or "@" in an action.
3977 ** %require "VERSION"
3978 This specifies that the grammar file depends on features implemented
3979 in Bison version VERSION or higher.
3981 ** lalr1.cc: The token and value types are now class members.
3982 The tokens were defined as free form enums and cpp macros. YYSTYPE
3983 was defined as a free form union. They are now class members:
3984 tokens are enumerations of the "yy::parser::token" struct, and the
3985 semantic values have the "yy::parser::semantic_type" type.
3987 If you do not want or can update to this scheme, the directive
3988 '%define "global_tokens_and_yystype" "1"' triggers the global
3989 definition of tokens and YYSTYPE. This change is suitable both
3990 for previous releases of Bison, and this one.
3992 If you wish to update, then make sure older version of Bison will
3993 fail using '%require "2.2"'.
3995 ** DJGPP support added.
3998 * Noteworthy changes in release 2.1 (2005-09-16)
4000 ** The C++ lalr1.cc skeleton supports %lex-param.
4002 ** Bison-generated parsers now support the translation of diagnostics like
4003 "syntax error" into languages other than English. The default
4004 language is still English. For details, please see the new
4005 Internationalization section of the Bison manual. Software
4006 distributors should also see the new PACKAGING file. Thanks to
4007 Bruno Haible for this new feature.
4009 ** Wording in the Bison-generated parsers has been changed slightly to
4010 simplify translation. In particular, the message "memory exhausted"
4011 has replaced "parser stack overflow", as the old message was not
4012 always accurate for modern Bison-generated parsers.
4014 ** Destructors are now called when the parser aborts, for all symbols left
4015 behind on the stack. Also, the start symbol is now destroyed after a
4016 successful parse. In both cases, the behavior was formerly inconsistent.
4018 ** When generating verbose diagnostics, Bison-generated parsers no longer
4019 quote the literal strings associated with tokens. For example, for
4020 a syntax error associated with '%token NUM "number"' they might
4021 print 'syntax error, unexpected number' instead of 'syntax error,
4022 unexpected "number"'.
4025 * Noteworthy changes in release 2.0 (2004-12-25)
4027 ** Possibly-incompatible changes
4029 - Bison-generated parsers no longer default to using the alloca function
4030 (when available) to extend the parser stack, due to widespread
4031 problems in unchecked stack-overflow detection. You can "#define
4032 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
4033 the manual to determine safe values for YYMAXDEPTH in that case.
4035 - Error token location.
4036 During error recovery, the location of the syntax error is updated
4037 to cover the whole sequence covered by the error token: it includes
4038 the shifted symbols thrown away during the first part of the error
4039 recovery, and the lookahead rejected during the second part.
4041 - Semicolon changes:
4042 . Stray semicolons are no longer allowed at the start of a grammar.
4043 . Semicolons are now required after in-grammar declarations.
4045 - Unescaped newlines are no longer allowed in character constants or
4046 string literals. They were never portable, and GCC 3.4.0 has
4047 dropped support for them. Better diagnostics are now generated if
4048 forget a closing quote.
4050 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4054 - GLR grammars now support locations.
4056 - New directive: %initial-action.
4057 This directive allows the user to run arbitrary code (including
4058 initializing @$) from yyparse before parsing starts.
4060 - A new directive "%expect-rr N" specifies the expected number of
4061 reduce/reduce conflicts in GLR parsers.
4063 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4064 This is a GNU extension.
4066 - The option "--report=lookahead" was changed to "--report=look-ahead".
4067 [However, this was changed back after 2.3.]
4069 - Experimental %destructor support has been added to lalr1.cc.
4071 - New configure option --disable-yacc, to disable installation of the
4072 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4076 - For now, %expect-count violations are now just warnings, not errors.
4077 This is for compatibility with Bison 1.75 and earlier (when there are
4078 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4079 are too many or too few shift/reduce conflicts). However, in future
4080 versions of Bison we plan to improve the %expect machinery so that
4081 these violations will become errors again.
4083 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4084 arbitrarily limited to 16-bit counts.
4086 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4089 * Noteworthy changes in release 1.875 (2003-01-01)
4091 ** The documentation license has been upgraded to version 1.2
4092 of the GNU Free Documentation License.
4094 ** syntax error processing
4096 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4097 locations too. This fixes bugs in error-location computation.
4100 It is now possible to reclaim the memory associated to symbols
4101 discarded during error recovery. This feature is still experimental.
4104 This new directive is preferred over YYERROR_VERBOSE.
4106 - #defining yyerror to steal internal variables is discouraged.
4107 It is not guaranteed to work forever.
4109 ** POSIX conformance
4111 - Semicolons are once again optional at the end of grammar rules.
4112 This reverts to the behavior of Bison 1.33 and earlier, and improves
4113 compatibility with Yacc.
4115 - "parse error" -> "syntax error"
4116 Bison now uniformly uses the term "syntax error"; formerly, the code
4117 and manual sometimes used the term "parse error" instead. POSIX
4118 requires "syntax error" in diagnostics, and it was thought better to
4121 - The documentation now emphasizes that yylex and yyerror must be
4122 declared before use. C99 requires this.
4124 - Bison now parses C99 lexical constructs like UCNs and
4125 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4127 - File names are properly escaped in C output. E.g., foo\bar.y is
4128 output as "foo\\bar.y".
4130 - Yacc command and library now available
4131 The Bison distribution now installs a "yacc" command, as POSIX requires.
4132 Also, Bison now installs a small library liby.a containing
4133 implementations of Yacc-compatible yyerror and main functions.
4134 This library is normally not useful, but POSIX requires it.
4136 - Type clashes now generate warnings, not errors.
4138 - If the user does not define YYSTYPE as a macro, Bison now declares it
4139 using typedef instead of defining it as a macro.
4140 For consistency, YYLTYPE is also declared instead of defined.
4142 ** Other compatibility issues
4144 - %union directives can now have a tag before the "{", e.g., the
4145 directive "%union foo {...}" now generates the C code
4146 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4147 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4148 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4149 This is for compatibility with both Yacc and Bison 1.35.
4151 - ";" is output before the terminating "}" of an action, for
4152 compatibility with Bison 1.35.
4154 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4155 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4157 - "yystype" and "yyltype" are now obsolescent macros instead of being
4158 typedefs or tags; they are no longer documented and are planned to be
4159 withdrawn in a future release.
4164 Users of Bison have to decide how they handle the portability of the
4167 - "parsing stack overflow..." -> "parser stack overflow"
4168 GLR parsers now report "parser stack overflow" as per the Bison manual.
4170 ** %parse-param and %lex-param
4171 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4172 additional context to yyparse and yylex. They suffer from several
4175 - a single argument only can be added,
4176 - their types are weak (void *),
4177 - this context is not passed to ancillary functions such as yyerror,
4178 - only yacc.c parsers support them.
4180 The new %parse-param/%lex-param directives provide a more precise control.
4183 %parse-param {int *nastiness}
4184 %lex-param {int *nastiness}
4185 %parse-param {int *randomness}
4187 results in the following signatures:
4189 int yylex (int *nastiness);
4190 int yyparse (int *nastiness, int *randomness);
4192 or, if both %pure-parser and %locations are used:
4194 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4195 int yyparse (int *nastiness, int *randomness);
4197 ** Bison now warns if it detects conflicting outputs to the same file,
4198 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4199 that command outputs both code and header to foo.h.
4201 ** #line in output files
4202 - --no-line works properly.
4204 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4205 later to be built. This change originally took place a few versions
4206 ago, but nobody noticed until we recently asked someone to try
4207 building Bison with a K&R C compiler.
4210 * Noteworthy changes in release 1.75 (2002-10-14)
4212 ** Bison should now work on 64-bit hosts.
4214 ** Indonesian translation thanks to Tedi Heriyanto.
4217 Fix spurious parse errors.
4220 Some people redefine yyerror to steal yyparse' private variables.
4221 Reenable this trick until an official feature replaces it.
4224 In agreement with POSIX and with other Yaccs, leaving a default
4225 action is valid when $$ is untyped, and $1 typed:
4229 but the converse remains an error:
4233 ** Values of midrule actions
4236 foo: { ... } { $$ = $1; } ...
4238 was incorrectly rejected: $1 is defined in the second midrule
4239 action, and is equal to the $$ of the first midrule action.
4242 * Noteworthy changes in release 1.50 (2002-10-04)
4247 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4248 almost any context-free grammar, ambiguous or not. The new declarations
4249 %dprec and %merge on grammar rules allow parse-time resolution of
4250 ambiguities. Contributed by Paul Hilfinger.
4252 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4253 like the Alpha, so please stick to 32-bit hosts for now.
4256 When not in Yacc compatibility mode, when the output file was not
4257 specified, running "bison foo/bar.y" created "foo/bar.c". It
4258 now creates "bar.c".
4261 The undefined token was systematically mapped to 2 which prevented
4262 the use of 2 by the user. This is no longer the case.
4264 ** Unknown token numbers
4265 If yylex returned an out of range value, yyparse could die. This is
4269 According to POSIX, the error token must be 256.
4270 Bison extends this requirement by making it a preference: *if* the
4271 user specified that one of her tokens is numbered 256, then error
4272 will be mapped onto another number.
4274 ** Verbose error messages
4275 They no longer report "..., expecting error or..." for states where
4276 error recovery is possible.
4279 Defaults to "$end" instead of "$".
4281 ** Error recovery now conforms to documentation and to POSIX
4282 When a Bison-generated parser encounters a syntax error, it now pops
4283 the stack until it finds a state that allows shifting the error
4284 token. Formerly, it popped the stack until it found a state that
4285 allowed some non-error action other than a default reduction on the
4286 error token. The new behavior has long been the documented behavior,
4287 and has long been required by POSIX. For more details, please see
4288 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4289 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4292 Popped tokens and nonterminals are now reported.
4295 Larger grammars are now supported (larger token numbers, larger grammar
4296 size (= sum of the LHS and RHS lengths), larger LALR tables).
4297 Formerly, many of these numbers ran afoul of 16-bit limits;
4298 now these limits are 32 bits on most hosts.
4300 ** Explicit initial rule
4301 Bison used to play hacks with the initial rule, which the user does
4302 not write. It is now explicit, and visible in the reports and
4306 Before, Bison reported the useless rules, but, although not used,
4307 included them in the parsers. They are now actually removed.
4309 ** Useless rules, useless nonterminals
4310 They are now reported, as a warning, with their locations.
4312 ** Rules never reduced
4313 Rules that can never be reduced because of conflicts are now
4316 ** Incorrect "Token not used"
4317 On a grammar such as
4319 %token useless useful
4321 exp: '0' %prec useful;
4323 where a token was used to set the precedence of the last rule,
4324 bison reported both "useful" and "useless" as useless tokens.
4326 ** Revert the C++ namespace changes introduced in 1.31
4327 as they caused too many portability hassles.
4329 ** Default locations
4330 By an accident of design, the default computation of @$ was
4331 performed after another default computation was performed: @$ = @1.
4332 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4333 the computation of @$.
4335 ** Token end-of-file
4336 The token end of file may be specified by the user, in which case,
4337 the user symbol is used in the reports, the graphs, and the verbose
4338 error messages instead of "$end", which remains being the default.
4342 %token MYEOF 0 "end of file"
4345 This old option, which has been broken for ages, is removed.
4348 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4349 Croatian, thanks to Denis Lackovic.
4351 ** Incorrect token definitions
4354 bison used to output
4357 ** Token definitions as enums
4358 Tokens are output both as the traditional #define's, and, provided
4359 the compiler supports ANSI C or is a C++ compiler, as enums.
4360 This lets debuggers display names instead of integers.
4363 In addition to --verbose, bison supports --report=THINGS, which
4364 produces additional information:
4366 complete the core item sets with their closure
4367 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4368 explicitly associate lookahead tokens to items
4370 describe shift/reduce conflicts solving.
4371 Bison used to systematically output this information on top of
4372 the report. Solved conflicts are now attached to their states.
4375 Previous versions don't complain when there is a type clash on
4376 the default action if the rule has a midrule action, such as in:
4384 ** GNU M4 is now required when using Bison.
4387 * Noteworthy changes in release 1.35 (2002-03-25)
4390 Some projects use Bison's C parser with C++ compilers, and define
4391 YYSTYPE as a class. The recent adjustment of C parsers for data
4392 alignment and 64 bit architectures made this impossible.
4394 Because for the time being no real solution for C++ parser
4395 generation exists, kludges were implemented in the parser to
4396 maintain this use. In the future, when Bison has C++ parsers, this
4397 kludge will be disabled.
4399 This kludge also addresses some C++ problems when the stack was
4403 * Noteworthy changes in release 1.34 (2002-03-12)
4405 ** File name clashes are detected
4406 $ bison foo.y -d -o foo.x
4407 fatal error: header and parser would both be named "foo.x"
4409 ** A missing ";" at the end of a rule triggers a warning
4410 In accordance with POSIX, and in agreement with other
4411 Yacc implementations, Bison will mandate this semicolon in the near
4412 future. This eases the implementation of a Bison parser of Bison
4413 grammars by making this grammar LALR(1) instead of LR(2). To
4414 facilitate the transition, this release introduces a warning.
4416 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4417 many portability hassles.
4419 ** DJGPP support added.
4421 ** Fix test suite portability problems.
4424 * Noteworthy changes in release 1.33 (2002-02-07)
4427 Groff could not be compiled for the definition of size_t was lacking
4428 under some conditions.
4434 * Noteworthy changes in release 1.32 (2002-01-23)
4436 ** Fix Yacc output file names
4438 ** Portability fixes
4440 ** Italian, Dutch translations
4443 * Noteworthy changes in release 1.31 (2002-01-14)
4447 ** GNU Gettext and %expect
4448 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4449 Bison dies on incorrect %expectations, we fear there will be
4450 too many bug reports for Gettext, so _for the time being_, %expect
4451 does not trigger an error when the input file is named "plural.y".
4453 ** Use of alloca in parsers
4454 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4455 malloc exclusively. Since 1.29, but was not NEWS'ed.
4457 alloca is used only when compiled with GCC, to avoid portability
4460 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4462 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4463 (as POSIX requires) instead of being undefined.
4466 Bison has always permitted actions such as { $$ = $1 }: it adds the
4467 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4468 is no longer output: one has to write { $$ = $1; }.
4470 ** Better C++ compliance
4471 The output parsers try to respect C++ namespaces.
4472 [This turned out to be a failed experiment, and it was reverted later.]
4475 Fixed bugs when reporting useless nonterminals.
4478 The parsers work properly on 64 bit hosts.
4481 Some calls to strerror resulted in scrambled or missing error messages.
4484 When the number of shift/reduce conflicts is correct, don't issue
4487 ** The verbose report includes the rule line numbers.
4489 ** Rule line numbers are fixed in traces.
4491 ** Swedish translation
4494 Verbose parse error messages from the parsers are better looking.
4495 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4496 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4498 ** Fixed parser memory leaks.
4499 When the generated parser was using malloc to extend its stacks, the
4500 previous allocations were not freed.
4502 ** Fixed verbose output file.
4503 Some newlines were missing.
4504 Some conflicts in state descriptions were missing.
4506 ** Fixed conflict report.
4507 Option -v was needed to get the result.
4511 Mismatches are errors, not warnings.
4513 ** Fixed incorrect processing of some invalid input.
4515 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4517 ** Fixed some typos in the documentation.
4519 ** %token MY_EOF 0 is supported.
4520 Before, MY_EOF was silently renumbered as 257.
4522 ** doc/refcard.tex is updated.
4524 ** %output, %file-prefix, %name-prefix.
4528 New, aliasing "--output-file".
4531 * Noteworthy changes in release 1.30 (2001-10-26)
4533 ** "--defines" and "--graph" have now an optional argument which is the
4534 output file name. "-d" and "-g" do not change; they do not take any
4537 ** "%source_extension" and "%header_extension" are removed, failed
4540 ** Portability fixes.
4543 * Noteworthy changes in release 1.29 (2001-09-07)
4545 ** The output file does not define const, as this caused problems when used
4546 with common autoconfiguration schemes. If you still use ancient compilers
4547 that lack const, compile with the equivalent of the C compiler option
4548 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4550 ** Added "-g" and "--graph".
4552 ** The Bison manual is now distributed under the terms of the GNU FDL.
4554 ** The input and the output files has automatically a similar extension.
4556 ** Russian translation added.
4558 ** NLS support updated; should hopefully be less troublesome.
4560 ** Added the old Bison reference card.
4562 ** Added "--locations" and "%locations".
4564 ** Added "-S" and "--skeleton".
4566 ** "%raw", "-r", "--raw" is disabled.
4568 ** Special characters are escaped when output. This solves the problems
4569 of the #line lines with path names including backslashes.
4572 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4573 "%debug", "%source_extension" and "%header_extension".
4576 Automatic location tracking.
4579 * Noteworthy changes in release 1.28 (1999-07-06)
4581 ** Should compile better now with K&R compilers.
4585 ** Fixed a problem with escaping the double quote character.
4587 ** There is now a FAQ.
4590 * Noteworthy changes in release 1.27
4592 ** The make rule which prevented bison.simple from being created on
4593 some systems has been fixed.
4596 * Noteworthy changes in release 1.26
4598 ** Bison now uses Automake.
4600 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4602 ** Token numbers now start at 257 as previously documented, not 258.
4604 ** Bison honors the TMPDIR environment variable.
4606 ** A couple of buffer overruns have been fixed.
4608 ** Problems when closing files should now be reported.
4610 ** Generated parsers should now work even on operating systems which do
4611 not provide alloca().
4614 * Noteworthy changes in release 1.25 (1995-10-16)
4616 ** Errors in the input grammar are not fatal; Bison keeps reading
4617 the grammar file, and reports all the errors found in it.
4619 ** Tokens can now be specified as multiple-character strings: for
4620 example, you could use "<=" for a token which looks like <=, instead
4621 of choosing a name like LESSEQ.
4623 ** The %token_table declaration says to write a table of tokens (names
4624 and numbers) into the parser file. The yylex function can use this
4625 table to recognize multiple-character string tokens, or for other
4628 ** The %no_lines declaration says not to generate any #line preprocessor
4629 directives in the parser file.
4631 ** The %raw declaration says to use internal Bison token numbers, not
4632 Yacc-compatible token numbers, when token names are defined as macros.
4634 ** The --no-parser option produces the parser tables without including
4635 the parser engine; a project can now use its own parser engine.
4636 The actions go into a separate file called NAME.act, in the form of
4637 a switch statement body.
4640 * Noteworthy changes in release 1.23
4642 The user can define YYPARSE_PARAM as the name of an argument to be
4643 passed into yyparse. The argument should have type void *. It should
4644 actually point to an object. Grammar actions can access the variable
4645 by casting it to the proper pointer type.
4647 Line numbers in output file corrected.
4650 * Noteworthy changes in release 1.22
4652 --help option added.
4655 * Noteworthy changes in release 1.20
4657 Output file does not redefine const for C++.
4661 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4662 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4663 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4664 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4665 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4666 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4667 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4668 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4669 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4670 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4671 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4672 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4673 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4674 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4675 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4676 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4677 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4678 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4679 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4680 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4681 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4682 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4683 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4684 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4685 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4686 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4687 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE CVE
4688 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4689 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4690 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4691 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4692 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4693 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4694 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4695 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4696 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4697 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4698 LocalWords: Vais xsltproc YYNOMEM YYLOCATION signedness YYBISON MITRE's
4699 LocalWords: libreadline YYMALLOC YYFREE MSVC redefinitions
4702 ispell-dictionary: "american"
4707 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4709 This file is part of Bison, the GNU Parser Generator.
4711 Permission is granted to copy, distribute and/or modify this document
4712 under the terms of the GNU Free Documentation License, Version 1.3 or
4713 any later version published by the Free Software Foundation; with no
4714 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4715 Texts. A copy of the license is included in the "GNU Free
4716 Documentation License" file as part of this distribution.