3 * Noteworthy changes in release ?.? (????-??-??) [?]
7 It is always recommended to prefer `%define api.value.type foo` to
8 `#define YYSTYPE foo`. The latter is supported in C for compatibility
9 with Yacc, but not in C++. Warnings are now issued if `#define YYSTYPE`
10 is used in C++, and eventually support will be removed.
12 Support for the YYPRINT macro is removed. It worked only with yacc.c and
13 only for tokens. It was obsoleted by %printer, introduced in Bison 1.50
20 In conformance with the recommendations of the Graphviz team
21 (https://marc.info/?l=graphviz-devel&m=129418103126092), `-g`/`--graph`
22 now generates a *.gv file by default, instead of *.dot. A transition
27 Prefer value_type to semantic_type to denote the semantic value type,
28 specified by the `api.value.type` %define variable.
32 There were no debug traces for deferred calls to user actions. They are
37 *** Option -H, --header and directive %header
39 The option `-H`/`--header` supersedes the option `--defines`, and the
40 directive %header supersedes %defines. Both `--defines` and `%defines`
41 are, of course, maintained for backward compatibility.
45 Since version 2.4 Bison can be used to generate HTML reports. However it
46 was a two-step process: first bison must be invoked with option `--xml`,
47 and then xsltproc must be run to the convert the XML reports into HTML.
49 The new option `--html` combines these steps. The xsltproc program must
52 *** A C++ native GLR parser
54 A new version of the generated C++ GLR parser was added as "glr2.cc". It
55 is forked from the existing glr.c/cc parser, with the objective of making
56 it a more modern, truly C++ parser (instead of a C++ wrapper around a C
57 parser). Down the line, the goal is to support `%define api.value.type
58 variant` and maybe share code with lalr1.cc.
60 The current parser should be identical in terms of interface, functionality
61 and performance to "glr.cc". To try it out, simply use
67 Counterexamples now show the rule numbers, and always show ε for rules
68 with an empty right-hand side. For instance
80 *** Lookahead correction in Java
82 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
85 *** Abort parsing for memory exhaustion (C)
87 User actions may now use `YYNOMEM` (similar to `YYACCEPT` and `YYABORT`)
88 to abort the current parse with memory exhaustion.
90 *** Printing locations in debug traces (C)
92 The `YYLOCATION_PRINT(File, Loc)` macro prints a location. It is defined
93 when (i) locations are enabled, (ii) the default type for locations is
94 used, (iii) debug traces are enabled, and (iv) `YYLOCATION_PRINT` is not
97 Users may define `YYLOCATION_PRINT` to cover other cases.
100 * Noteworthy changes in release 3.7.6 (2021-03-08) [stable]
104 *** Reused Push Parsers
106 When a push-parser state structure is used for multiple parses, it was
107 possible for some state to leak from one run into the following one.
109 *** Fix Table Generation
111 In some very rare conditions, when there are many useless tokens, it was
112 possible to generate incorrect parsers.
115 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
119 *** Counterexample Generation
121 In some cases counterexample generation could crash. This is fixed.
123 *** Fix Table Generation
125 In some very rare conditions, when there are many useless tokens, it was
126 possible to generate incorrect parsers.
128 *** GLR parsers now support %merge together with api.value.type=union.
130 *** C++ parsers use noexcept in more places.
132 *** Generated parsers avoid some warnings about signedness issues.
134 *** C-language parsers now avoid warnings from pedantic clang.
136 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
139 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
143 *** Bug fixes in yacc.c
145 In Yacc mode, all the tokens are defined twice: once as an enum, and then
146 as a macro. YYEMPTY was missing its macro.
148 *** Bug fixes in lalr1.cc
150 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
151 even when the `parse.assert` %define variable is not enabled. It no
154 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
157 When there is a very large number of tokens, some assertions could be long
158 enough to hit arbitrary limits in Visual C++. They have been rewritten to
159 work around this limitation.
163 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
164 skeleton) used to be defined to 1. It is now defined to the version of
165 Bison as an integer (e.g., 30704 for version 3.7.4).
168 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
172 Fix concurrent build issues.
174 The bison executable is no longer linked uselessly against libreadline.
176 Fix incorrect use of yytname in glr.cc.
179 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
181 This release of Bison fixes all known bugs reported for Bison in MITRE's
182 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
183 are only about bison-the-program itself, not the generated code.
185 Although these bugs are typically irrelevant to how Bison is used, they
186 are worth fixing if only to give users peace of mind.
188 There is no known vulnerability in the generated parsers.
192 Fix concurrent build issues (introduced in Bison 3.5).
194 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
196 Fix portability issues of the test suite, and of bison itself.
198 Some unlikely crashes found by fuzzing have been fixed. This is only
199 about bison itself, not the generated parsers.
202 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
206 Crash when a token alias contains a NUL byte.
208 Portability issues with libtextstyle.
210 Portability issues of Bison itself with MSVC.
214 Improvements and fixes in the documentation.
216 More precise location about symbol type redefinitions.
219 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
221 ** Deprecated features
223 The YYPRINT macro, which works only with yacc.c and only for tokens, was
224 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
225 It is deprecated and its support will be removed eventually.
227 In conformance with the recommendations of the Graphviz team, in the next
228 version Bison the option `--graph` will generate a *.gv file by default,
229 instead of *.dot. A transition started in Bison 3.4.
233 *** Counterexample Generation
235 Contributed by Vincent Imbimbo.
237 When given `-Wcounterexamples`/`-Wcex`, bison will now output
238 counterexamples for conflicts.
240 **** Unifying Counterexamples
242 Unifying counterexamples are strings which can be parsed in two ways due
243 to the conflict. For example on a grammar that contains the usual
244 "dangling else" ambiguity:
247 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
248 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
251 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
252 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
253 Example: "if" exp "then" "if" exp "then" exp • "else" exp
256 ↳ "if" exp "then" exp
257 ↳ "if" exp "then" exp • "else" exp
258 Example: "if" exp "then" "if" exp "then" exp • "else" exp
261 ↳ "if" exp "then" exp "else" exp
262 ↳ "if" exp "then" exp •
264 When text styling is enabled, colors are used in the examples and the
265 derivations to highlight the structure of both analyses. In this case,
267 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
271 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
274 The counterexamples are "focused", in two different ways. First, they do
275 not clutter the output with all the derivations from the start symbol,
276 rather they start on the "conflicted nonterminal". They go straight to the
277 point. Second, they don't "expand" nonterminal symbols uselessly.
279 **** Nonunifying Counterexamples
281 In the case of the dangling else, Bison found an example that can be
282 parsed in two ways (therefore proving that the grammar is ambiguous).
283 When it cannot find such an example, it instead generates two examples
284 that are the same up until the dot:
287 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
288 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
289 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
294 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
295 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
296 First example: expr • ID ',' ID $end
303 Second example: expr • ID $end
309 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
313 In these cases, the parser usually doesn't have enough lookahead to
314 differentiate the two given examples.
318 Counterexamples are also included in the report when given
319 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
324 1 exp: "if" exp "then" exp • [$end, "then", "else"]
325 2 | "if" exp "then" exp • "else" exp
327 "else" shift, and go to state 8
329 "else" [reduce using rule 1 (exp)]
330 $default reduce using rule 1 (exp)
332 shift/reduce conflict on token "else":
333 1 exp: "if" exp "then" exp •
334 2 exp: "if" exp "then" exp • "else" exp
335 Example: "if" exp "then" "if" exp "then" exp • "else" exp
338 ↳ "if" exp "then" exp
339 ↳ "if" exp "then" exp • "else" exp
340 Example: "if" exp "then" "if" exp "then" exp • "else" exp
343 ↳ "if" exp "then" exp "else" exp
344 ↳ "if" exp "then" exp •
346 *** File prefix mapping
348 Contributed by Joshua Watt.
350 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
351 in the output (specifically `#line` directives and `#ifdef` header guards)
352 that begins with the prefix OLD will have it replaced with the prefix NEW,
353 similar to the `-ffile-prefix-map` in GCC. This option can be used to
354 make bison output reproducible.
360 When text styling is enabled and the terminal supports it, the warnings
361 now include hyperlinks to the documentation.
363 *** Relocatable installation
365 When installed to be relocatable (via `configure --enable-relocatable`),
366 bison will now also look for a relocated m4.
370 The `filename_type` %define variable was renamed `api.filename.type`.
373 %define filename_type "symbol"
377 %define api.filename.type {symbol}
379 (Or let `bison --update` do it for you).
381 It now defaults to `const std::string` instead of `std::string`.
383 *** Deprecated %define variable names
385 The following variables have been renamed for consistency. Backward
386 compatibility is ensured, but upgrading is recommended.
388 filename_type -> api.filename.type
389 package -> api.package
391 *** Push parsers no longer clear their state when parsing is finished
393 Previously push-parsers cleared their state when parsing was finished (on
394 success and on failure). This made it impossible to check if there were
395 parse errors, since `yynerrs` was also reset. This can be especially
396 troublesome when used in autocompletion, since a parser with error
397 recovery would suggest (irrelevant) expected tokens even if there were
400 Now the parser state can be examined when parsing is finished. The parser
401 state is reset when starting a new parse.
407 The bistromathic demonstrates %param and how to quote sources in the error
411 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
417 *** Include the generated header (yacc.c)
419 Historically, when --defines was used, bison generated a header and pasted
420 an exact copy of it into the generated parser implementation file. Since
421 Bison 3.4 it is possible to specify that the header should be `#include`d,
422 and how. For instance
424 %define api.header.include {"parse.h"}
428 %define api.header.include {<parser/parse.h>}
430 Now api.header.include defaults to `"header-basename"`, as was intended in
431 Bison 3.4, where `header-basename` is the basename of the generated
432 header. This is disabled when the generated header is `y.tab.h`, to
433 comply with Automake's ylwrap.
435 *** String aliases are faithfully propagated
437 Bison used to interpret user strings (i.e., decoding backslash escapes)
438 when reading them, and to escape them (i.e., issue non-printable
439 characters as backslash escapes, taking the locale into account) when
440 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
441 up "ciphered" as sequences of backslash escapes. This happened not only
442 in the generated sources (where the compiler will reinterpret them), but
443 also in all the generated reports (text, xml, html, dot, etc.). Reports
444 were therefore not readable when string aliases were not pure ASCII.
445 Worse yet: the output depended on the user's locale.
447 Now Bison faithfully treats the string aliases exactly the way the user
448 spelled them. This fixes all the aforementioned problems. However, now,
449 string aliases semantically equivalent but syntactically different (e.g.,
450 "A", "\x41", "\101") are considered to be different.
452 *** Crash when generating IELR
454 An old, well hidden, bug in the generation of IELR parsers was fixed.
457 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
461 In glr.cc some internal macros leaked in the user's code, and could damage
462 access to the token kinds.
465 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
469 Incorrect comments in the generated parsers.
471 Warnings in push parsers (yacc.c).
473 Incorrect display of gotos in LAC traces (lalr1.cc).
476 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
480 Some tests were fixed.
482 When token aliases contain comment delimiters:
484 %token FOO "/* foo */"
486 bison used to emit "nested" comments, which is invalid C.
489 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
493 Restored ANSI-C compliance in yacc.c.
495 GNU readline portability issues.
497 In C++, yy::parser::symbol_name is now a public member, as was intended.
501 In C++, yy::parser::symbol_type now has a public name() member function.
504 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
506 ** Backward incompatible changes
508 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
510 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
511 depend on it will now produce Yacc-like error messages (just "syntax
512 error"). It was superseded by the "%error-verbose" directive in Bison
513 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
514 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
515 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
516 parse.error verbose".
518 ** Deprecated features
520 The YYPRINT macro, which works only with yacc.c and only for tokens, was
521 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
522 It is deprecated and its support will be removed eventually.
526 *** Improved syntax error messages
528 Two new values for the %define parse.error variable offer more control to
529 the user. Available in all the skeletons (C, C++, Java).
531 **** %define parse.error detailed
533 The behavior of "%define parse.error detailed" is closely resembling that
534 of "%define parse.error verbose" with a few exceptions. First, it is safe
535 to use non-ASCII characters in token aliases (with 'verbose', the result
536 depends on the locale with which bison was run). Second, a yysymbol_name
537 function is exposed to the user, instead of the yytnamerr function and the
538 yytname table. Third, token internationalization is supported (see
541 **** %define parse.error custom
543 With this directive, the user forges and emits the syntax error message
544 herself by defining the yyreport_syntax_error function. A new type,
545 yypcontext_t, captures the circumstances of the error, and provides the
546 user with functions to get details, such as yypcontext_expected_tokens to
547 get the list of expected token kinds.
549 A possible implementation of yyreport_syntax_error is:
552 yyreport_syntax_error (const yypcontext_t *ctx)
555 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
556 fprintf (stderr, ": syntax error");
557 // Report the tokens expected at this point.
559 enum { TOKENMAX = 10 };
560 yysymbol_kind_t expected[TOKENMAX];
561 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
563 // Forward errors to yyparse.
566 for (int i = 0; i < n; ++i)
567 fprintf (stderr, "%s %s",
568 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
570 // Report the unexpected token.
572 yysymbol_kind_t lookahead = yypcontext_token (ctx);
573 if (lookahead != YYSYMBOL_YYEMPTY)
574 fprintf (stderr, " before %s", yysymbol_name (lookahead));
576 fprintf (stderr, "\n");
580 **** Token aliases internationalization
582 When the %define variable parse.error is set to `custom` or `detailed`,
583 one may specify which token aliases are to be translated using _(). For
595 In that case the user must define _() and N_(), and yysymbol_name returns
596 the translated symbol (i.e., it returns '_("variable")' rather that
597 '"variable"'). In Java, the user must provide an i18n() function.
599 *** List of expected tokens (yacc.c)
601 Push parsers may invoke yypstate_expected_tokens at any point during
602 parsing (including even before submitting the first token) to get the list
603 of possible tokens. This feature can be used to propose autocompletion
604 (see below the "bistromathic" example).
606 It makes little sense to use this feature without enabling LAC (lookahead
609 *** Returning the error token
611 When the scanner returns an invalid token or the undefined token
612 (YYUNDEF), the parser generates an error message and enters error
613 recovery. Because of that error message, most scanners that find lexical
614 errors generate an error message, and then ignore the invalid input
615 without entering the error-recovery.
617 The scanners may now return YYerror, the error token, to enter the
618 error-recovery mode without triggering an additional error message. See
619 the bistromathic for an example.
621 *** Deep overhaul of the symbol and token kinds
623 To avoid the confusion with types in programming languages, we now refer
624 to token and symbol "kinds" instead of token and symbol "types". The
625 documentation and error messages have been revised.
627 All the skeletons have been updated to use dedicated enum types rather
628 than integral types. Special symbols are now regular citizens, instead of
629 being declared in ad hoc ways.
633 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
634 LPAREN, etc. While backward compatibility is of course ensured, users are
635 nonetheless invited to replace their uses of "enum yytokentype" by
638 This type now also includes tokens that were previously hidden: YYEOF (end
639 of input), YYUNDEF (undefined token), and YYerror (error token). They
640 now have string aliases, internationalized when internationalization is
641 enabled. Therefore, by default, error messages now refer to "end of file"
642 (internationalized) rather than the cryptic "$end", or to "invalid token"
643 rather than "$undefined".
645 Therefore in most cases it is now useless to define the end-of-line token
648 %token T_EOF 0 "end of file"
650 Rather simply use "YYEOF" in your scanner.
654 The "symbol kinds" is what the parser actually uses. (Unless the
655 api.token.raw %define variable is used, the symbol kind of a terminal
656 differs from the corresponding token kind.)
658 They are now exposed as a enum, "yysymbol_kind_t".
660 This allows users to tailor the error messages the way they want, or to
661 process some symbols in a specific way in autocompletion (see the
662 bistromathic example below).
664 *** Modernize display of explanatory statements in diagnostics
666 Since Bison 2.7, output was indented four spaces for explanatory
667 statements. For example:
669 input.y:2.7-13: error: %type redeclaration for exp
670 input.y:1.7-11: previous declaration
672 Since the introduction of caret-diagnostics, it became less clear. This
673 indentation has been removed and submessages are displayed similarly as in
676 input.y:2.7-13: error: %type redeclaration for exp
677 2 | %type <float> exp
679 input.y:1.7-11: note: previous declaration
683 Contributed by Victor Morales Cayuela.
687 The token and symbol kinds are yy::parser::token_kind_type and
688 yy::parser::symbol_kind_type.
690 The symbol_type::kind() member function allows to get the kind of a
691 symbol. This can be used to write unit tests for scanners, e.g.,
693 yy::parser::symbol_type t = make_NUMBER ("123");
694 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
695 assert (t.value.as<int> () == 123);
701 In order to avoid ambiguities with "type" as in "typing", we now refer to
702 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
703 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
708 There are now examples/java: a very simple calculator, and a more complete
709 one (push-parser, location tracking, and debug traces).
711 The lexcalc example (a simple example in C based on Flex and Bison) now
712 also demonstrates location tracking.
715 A new C example, bistromathic, is a fully featured interactive calculator
716 using many Bison features: pure interface, push parser, autocompletion
717 based on the current parser state (using yypstate_expected_tokens),
718 location tracking, internationalized custom error messages, lookahead
719 correction, rich debug traces, etc.
721 It shows how to depend on the symbol kinds to tailor autocompletion. For
722 instance it recognizes the symbol kind "VARIABLE" to propose
723 autocompletion on the existing variables, rather than of the word
727 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
729 ** WARNING: Future backward-incompatibilities!
731 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
733 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
734 that still depend on it will produce Yacc-like error messages (just
735 "syntax error"). It was superseded by the "%error-verbose" directive in
736 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
737 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
738 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
739 parse.error verbose".
743 Fix portability issues of the package itself on old compilers.
745 Fix api.token.raw support in Java.
748 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
752 Error messages could quote lines containing zero-width characters (such as
753 \005) with incorrect styling. Fixes for similar issues with unexpectedly
754 short lines (e.g., the file was changed between parsing and diagnosing).
756 Some unlikely crashes found by fuzzing have been fixed. This is only
757 about bison itself, not the generated parsers.
760 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
764 Portability issues and minor cosmetic issues.
766 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
770 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
776 Fix compiler warnings.
779 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
781 ** Backward incompatible changes
783 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
784 longer treated as end-of-lines. This changes the diagnostics, and in
785 particular their locations.
787 In C++, line numbers and columns are now represented as 'int' not
788 'unsigned', so that integer overflow on positions is easily checkable via
789 'gcc -fsanitize=undefined' and the like. This affects the API for
790 positions. The default position and location classes now expose
791 'counter_type' (int), used to define line and column numbers.
793 ** Deprecated features
795 The YYPRINT macro, which works only with yacc.c and only for tokens, was
796 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
797 It is deprecated and its support will be removed eventually.
801 *** Lookahead correction in C++
803 Contributed by Adrian Vogelsgesang.
805 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
806 %define variable parse.lac.
808 *** Variable api.token.raw: Optimized token numbers (all skeletons)
810 In the generated parsers, tokens have two numbers: the "external" token
811 number as returned by yylex (which starts at 257), and the "internal"
812 symbol number (which starts at 3). Each time yylex is called, a table
813 lookup maps the external token number to the internal symbol number.
815 When the %define variable api.token.raw is set, tokens are assigned their
816 internal number, which saves one table lookup per token, and also saves
817 the generation of the mapping table.
819 The gain is typically moderate, but in extreme cases (very simple user
820 actions), a 10% improvement can be observed.
822 *** Generated parsers use better types for states
824 Stacks now use the best integral type for state numbers, instead of always
825 using 15 bits. As a result "small" parsers now have a smaller memory
826 footprint (they use 8 bits), and there is support for large automata (16
827 bits), and extra large (using int, i.e., typically 31 bits).
829 *** Generated parsers prefer signed integer types
831 Bison skeletons now prefer signed to unsigned integer types when either
832 will do, as the signed types are less error-prone and allow for better
833 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
834 portable to unusual machines where char, short and int are all the same
835 width. On non-GNU platforms this may entail including <limits.h> and (if
836 available) <stdint.h> to define integer types and constants.
838 *** A skeleton for the D programming language
840 For the last few releases, Bison has shipped a stealth experimental
841 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
842 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
845 However, because nobody has committed to improving, testing, and
846 documenting this skeleton, it is not clear that it will be supported in
849 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
850 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
852 *** Debug traces in Java
854 The Java backend no longer emits code and data for parser tracing if the
855 %define variable parse.trace is not defined.
859 *** New diagnostic: -Wdangling-alias
861 String literals, which allow for better error messages, are (too)
862 liberally accepted by Bison, which might result in silent errors. For
865 %type <exVal> cond "condition"
867 does not define "condition" as a string alias to 'cond' (nonterminal
868 symbols do not have string aliases). It is rather equivalent to
871 %token <exVal> "condition"
873 i.e., it gives the type 'exVal' to the "condition" token, which was
874 clearly not the intention.
876 Also, because string aliases need not be defined, typos such as "baz"
877 instead of "bar" will be not reported.
879 The option `-Wdangling-alias` catches these situations. On
882 %type <ival> foo "foo"
886 bison -Wdangling-alias reports
888 warning: string literal not attached to a symbol
889 | %type <ival> foo "foo"
891 warning: string literal not attached to a symbol
895 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
897 *** Better POSIX Yacc compatibility diagnostics
899 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
903 %type <ival> TOKEN1 TOKEN2 't'
910 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
911 2 | %type <ival> TOKEN1 TOKEN2 't'
913 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
914 2 | %type <ival> TOKEN1 TOKEN2 't'
916 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
917 2 | %type <ival> TOKEN1 TOKEN2 't'
920 *** Diagnostics with insertion
922 The diagnostics now display the suggestion below the underlined source.
923 Replacement for undeclared symbols are now also suggested.
930 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'?
934 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
938 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
940 *** Diagnostics about long lines
942 Quoted sources may now be truncated to fit the screen. For instance, on a
943 30-column wide terminal:
950 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
953 foo.y:1.8-10: previous declaration
956 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
959 foo.y:1.8-10: previous declaration
965 *** Debugging glr.c and glr.cc
967 The glr.c skeleton always had asserts to check its own behavior (not the
968 user's). These assertions are now under the control of the parse.assert
969 %define variable (disabled by default).
973 Several new compiler warnings in the generated output have been avoided.
974 Some unused features are no longer emitted. Cleaner generated code in
979 Portability issues in the test suite.
981 In theory, parsers using %nonassoc could crash when reporting verbose
982 error messages. This unlikely bug has been fixed.
984 In Java, %define api.prefix was ignored. It now behaves as expected.
987 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
991 In some cases, when warnings are disabled, bison could emit tons of white
992 spaces as diagnostics.
994 When running out of memory, bison could crash (found by fuzzing).
996 When defining twice the EOF token, bison would crash.
998 New warnings from recent compilers have been addressed in the generated
999 parsers (yacc.c, glr.c, glr.cc).
1001 When lone carriage-return characters appeared in the input file,
1002 diagnostics could hang forever.
1005 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
1012 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
1014 ** Deprecated features
1016 The %pure-parser directive is deprecated in favor of '%define api.pure'
1017 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
1018 now. Note that since Bison 2.7 you are strongly encouraged to use
1019 '%define api.pure full' instead of '%define api.pure'.
1023 *** Colored diagnostics
1025 As an experimental feature, diagnostics are now colored, controlled by the
1026 new options --color and --style.
1028 To use them, install the libtextstyle library before configuring Bison.
1029 It is available from
1031 https://alpha.gnu.org/gnu/gettext/
1035 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
1037 The option --color supports the following arguments:
1038 - always, yes: Enable colors.
1039 - never, no: Disable colors.
1040 - auto, tty (default): Enable colors if the output device is a tty.
1042 To customize the styles, create a CSS file similar to
1046 .error { font-weight: 800; text-decoration: underline; }
1049 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1050 environment variable to "bison-bw.css".
1052 *** Disabling output
1054 When given -fsyntax-only, the diagnostics are reported, but no output is
1057 The name of this option is somewhat misleading as bison does more than
1058 just checking the syntax: every stage is run (including checking for
1059 conflicts for instance), except the generation of the output files.
1061 *** Include the generated header (yacc.c)
1063 Before, when --defines is used, bison generated a header, and pasted an
1064 exact copy of it into the generated parser implementation file. If the
1065 header name is not "y.tab.h", it is now #included instead of being
1068 To use an '#include' even if the header name is "y.tab.h" (which is what
1069 happens with --yacc, or when using the Autotools' ylwrap), define
1070 api.header.include to the exact argument to pass to #include. For
1073 %define api.header.include {"parse.h"}
1077 %define api.header.include {<parser/parse.h>}
1079 *** api.location.type is now supported in C (yacc.c, glr.c)
1081 The %define variable api.location.type defines the name of the type to use
1082 for locations. When defined, Bison no longer defines YYLTYPE.
1084 This can be used in programs with several parsers to factor their
1085 definition of locations: let one of them generate them, and the others
1092 In conformance with the recommendations of the Graphviz team, if %require
1093 "3.4" (or better) is specified, the option --graph generates a *.gv file
1094 by default, instead of *.dot.
1096 *** Diagnostics overhaul
1098 Column numbers were wrong with multibyte characters, which would also
1099 result in skewed diagnostics with carets. Beside, because we were
1100 indenting the quoted source with a single space, lines with tab characters
1101 were incorrectly underlined.
1103 To address these issues, and to be clearer, Bison now issues diagnostics
1104 as GCC9 does. For instance it used to display (there's a tab before the
1107 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1108 expr: expr '+' "number" { $$ = $1 + $2; }
1112 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1113 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1116 Other constructs now also have better locations, resulting in more precise
1119 *** Fix-it hints for %empty
1121 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1122 annotations, and add the missing ones.
1124 *** Generated reports
1126 The format of the reports (parse.output) was improved for readability.
1128 *** Better support for --no-line.
1130 When --no-line is used, the generated files are now cleaner: no lines are
1131 generated instead of empty lines. Together with using api.header.include,
1132 that should help people saving the generated files into version control
1133 systems get smaller diffs.
1137 A new example in C shows an simple infix calculator with a hand-written
1138 scanner (examples/c/calc).
1140 A new example in C shows a reentrant parser (capable of recursive calls)
1141 built with Flex and Bison (examples/c/reccalc).
1143 There is a new section about the history of Yaccs and Bison.
1147 A few obscure bugs were fixed, including the second oldest (known) bug in
1148 Bison: it was there when Bison was entered in the RCS version control
1149 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1153 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1157 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1161 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1165 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1166 extensions into errors. It now makes them simple warnings (-Wyacc).
1169 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1171 A new mailing list was created, Bison Announce. It is low traffic, and is
1172 only about announcing new releases and important messages (e.g., polls
1173 about major decisions to make).
1175 https://lists.gnu.org/mailman/listinfo/bison-announce
1177 ** Backward incompatible changes
1179 Support for DJGPP, which has been unmaintained and untested for years, is
1182 ** Deprecated features
1184 A new feature, --update (see below) helps adjusting existing grammars to
1187 *** Deprecated directives
1189 The %error-verbose directive is deprecated in favor of '%define
1190 parse.error verbose' since Bison 3.0, but no warning was issued.
1192 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1193 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1194 directives are slightly different, you might need to adjust your code.
1195 %name-prefix renames only symbols with external linkage, while api.prefix
1196 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1197 yytokentype, YYSTYPE, YYLTYPE, etc.
1199 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1200 {xx}' will typically have to update YY_DECL from
1202 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1206 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1208 *** Deprecated %define variable names
1210 The following variables, mostly related to parsers in Java, have been
1211 renamed for consistency. Backward compatibility is ensured, but upgrading
1214 abstract -> api.parser.abstract
1215 annotations -> api.parser.annotations
1216 extends -> api.parser.extends
1217 final -> api.parser.final
1218 implements -> api.parser.implements
1219 parser_class_name -> api.parser.class
1220 public -> api.parser.public
1221 strictfp -> api.parser.strictfp
1225 *** Generation of fix-its for IDEs/Editors
1227 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1228 bison now generates machine readable editing instructions to fix some
1229 issues. Currently, this is mostly limited to updating deprecated
1230 directives and removing duplicates. For instance:
1234 %define parser_class_name "Parser"
1235 %define api.parser.class "Parser"
1239 See the "fix-it:" lines below:
1241 $ bison -ffixit foo.y
1242 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1245 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1246 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1247 %define parser_class_name "Parser"
1248 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1249 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1250 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1251 %define api.parser.class "Parser"
1252 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1253 foo.y:2.1-34: previous definition
1254 %define parser_class_name "Parser"
1255 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1256 fix-it:"foo.y":{3:1-3:34}:""
1257 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1259 This uses the same output format as GCC and Clang.
1261 *** Updating grammar files
1263 Fixes can be applied on the fly. The previous example ends with the
1264 suggestion to re-run bison with the option -u/--update, which results in a
1265 cleaner grammar file.
1267 $ bison --update foo.y
1269 bison: file 'foo.y' was updated (backup: 'foo.y~')
1272 %define parse.error verbose
1273 %define api.parser.class {Parser}
1277 *** Bison is now relocatable
1279 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1281 A relocatable program can be moved or copied to a different location on
1282 the file system. It can also be used through mount points for network
1283 sharing. It is possible to make symbolic links to the installed and moved
1284 programs, and invoke them through the symbolic link.
1286 *** %expect and %expect-rr modifiers on individual rules
1288 One can now document (and check) which rules participate in shift/reduce
1289 and reduce/reduce conflicts. This is particularly important GLR parsers,
1290 where conflicts are a normal occurrence. For example,
1306 | argument_list ',' expression
1311 Looking at the output from -v, one can see that the shift/reduce conflict
1312 here is due to the fact that the parser does not know whether to reduce
1313 arguments to argument_list until it sees the token _after_ the following
1314 ','. By marking the rule with %expect 1 (because there is a conflict in
1315 one state), we document the source of the 1 overall shift/reduce conflict.
1317 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1318 conflicts. In this case, we mark each of the conflicting rules. For
1327 target_list '=' expr ';'
1333 | target ',' target_list
1342 | expr ',' expr_list
1350 In a statement such as
1354 the parser must reduce x to a target or an expr, but does not know which
1355 until it sees the '='. So we notate the two possible reductions to
1356 indicate that each conflicts in one rule.
1358 This feature needs user feedback, and might evolve in the future.
1360 *** C++: Actual token constructors
1362 When variants and token constructors are enabled, in addition to the
1363 type-safe named token constructors (make_ID, make_INT, etc.), we now
1364 generate genuine constructors for symbol_type.
1366 For instance with these declarations
1372 you may use these constructors:
1374 symbol_type (int token, const std::string&);
1375 symbol_type (int token, const int&);
1376 symbol_type (int token);
1378 Correct matching between token types and value types is checked via
1379 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1380 constructors are preferable, as they offer better type safety (for
1381 instance 'make_ID (42)' would not even compile), but symbol_type
1382 constructors may help when token types are discovered at run-time, e.g.,
1385 if (auto i = lookup_keyword (yytext))
1386 return yy::parser::symbol_type (i);
1388 return yy::parser::make_ID (yytext);
1391 *** C++: Variadic emplace
1393 If your application requires C++11 and you don't use symbol constructors,
1394 you may now use a variadic emplace for semantic values:
1396 %define api.value.type variant
1397 %token <std::pair<int, int>> PAIR
1401 int yylex (parser::semantic_type *lvalp)
1403 lvalp->emplace <std::pair<int, int>> (1, 2);
1404 return parser::token::PAIR;
1407 *** C++: Syntax error exceptions in GLR
1409 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1410 actions, or from the scanner.
1412 *** More POSIX Yacc compatibility warnings
1414 More Bison specific directives are now reported with -y or -Wyacc. This
1415 change was ready since the release of Bison 3.0 in September 2015. It was
1416 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1417 in numerous warnings for Bison users that use the GNU Build System.
1419 If you still experience that problem, either redefine YACC as `bison -o
1420 y.tab.c`, or pass -Wno-yacc to Bison.
1422 *** The tables yyrhs and yyphrs are back
1424 Because no Bison skeleton uses them, these tables were removed (no longer
1425 passed to the skeletons, not even computed) in 2008. However, some users
1426 have expressed interest in being able to use them in their own skeletons.
1430 *** Incorrect number of reduce/reduce conflicts
1432 On a grammar such as
1434 exp: "num" | "num" | "num"
1436 bison used to report a single RR conflict, instead of two. This is now
1437 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1438 was entered in the RCS version control system, in December 1987.
1440 Some grammar files might have to adjust their %expect-rr.
1442 *** Parser directives that were not careful enough
1444 Passing invalid arguments to %nterm, for instance character literals, used
1445 to result in unclear error messages.
1449 The examples/ directory (installed in .../share/doc/bison/examples) has
1450 been restructured per language for clarity. The examples come with a
1451 README and a Makefile. Not only can they be used to toy with Bison, they
1452 can also be starting points for your own grammars.
1454 There is now a Java example, and a simple example in C based on Flex and
1455 Bison (examples/c/lexcalc/).
1461 They now use noexcept and constexpr. Please, report missing annotations.
1463 *** Symbol Declarations
1465 The syntax of the variation directives to declare symbols was overhauled
1466 for more consistency, and also better POSIX Yacc compliance (which, for
1467 instance, allows "%type" without actually providing a type). The %nterm
1468 directive, supported by Bison since its inception, is now documented and
1469 officially supported.
1471 The syntax is now as follows:
1473 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1474 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1475 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1476 %nterm TAG? ID+ ( TAG ID+ )*
1478 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1479 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1480 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1481 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1482 one), ‘*’ (zero or more) and ‘+’ (one or more).
1485 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1489 Fix the move constructor of symbol_type.
1491 Always provide a copy constructor for symbol_type, even in modern C++.
1494 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1498 Properly support token constructors in C++ with types that include commas
1499 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1502 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1506 C++ portability issues.
1509 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1513 Several portability issues have been fixed in the build system, in the
1514 test suite, and in the generated parsers in C++.
1517 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1519 ** Backward incompatible changes
1521 Support for DJGPP, which has been unmaintained and untested for years, is
1522 obsolete. Unless there is activity to revive it, it will be removed.
1526 %printers should use yyo rather than yyoutput to denote the output stream.
1528 Variant-based symbols in C++ should use emplace() rather than build().
1530 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1534 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1536 A comment in the generated code now emphasizes that users should not
1537 depend upon non-documented implementation details, such as macros starting
1542 *** C++: Support for move semantics (lalr1.cc)
1544 The lalr1.cc skeleton now fully supports C++ move semantics, while
1545 maintaining compatibility with C++98. You may now store move-only types
1546 when using Bison's variants. For instance:
1553 %skeleton "lalr1.cc"
1554 %define api.value.type variant
1558 %token <int> INT "int";
1559 %type <std::unique_ptr<int>> int;
1560 %type <std::vector<std::unique_ptr<int>>> list;
1564 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1566 int: "int" { $$ = std::make_unique<int>($1); }
1568 *** C++: Implicit move of right-hand side values (lalr1.cc)
1570 In modern C++ (C++11 and later), you should always use 'std::move' with
1571 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1572 popped from the stack anyway. Using 'std::move' is mandatory for
1573 move-only types such as unique_ptr, and it provides a significant speedup
1574 for large types such as std::string, or std::vector, etc.
1576 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1577 by 'std::move ($n)'. The second rule in the previous grammar can be
1580 list: list int { $$ = $1; $$.emplace_back($2); }
1582 With automove enabled, the semantic values are no longer lvalues, so do
1583 not use the swap idiom:
1585 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1587 This idiom is anyway obsolete: it is preferable to move than to swap.
1589 A warning is issued when automove is enabled, and a value is used several
1592 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1593 exp: "twice" exp { $$ = $2 + $2; }
1596 Enabling api.value.automove does not require support for modern C++. The
1597 generated code is valid C++98/03, but will use copies instead of moves.
1599 The new examples/c++/variant-11.yy shows these features in action.
1601 *** C++: The implicit default semantic action is always run
1603 When variants are enabled, the default action was not run, so
1611 It now behaves like in all the other cases, as
1613 exp: "number" { $$ = $1; }
1615 possibly using std::move if automove is enabled.
1617 We do not expect backward compatibility issues. However, beware of
1618 forward compatibility issues: if you rely on default actions with
1619 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1620 generate incorrect parsers.
1622 *** C++: Renaming location.hh
1624 When both %defines and %locations are enabled, Bison generates a
1625 location.hh file. If you don't use locations outside of the parser, you
1626 may avoid its creation with:
1628 %define api.location.file none
1630 However this file is useful if, for instance, your parser builds an AST
1631 decorated with locations: you may use Bison's location independently of
1632 Bison's parser. You can now give it another name, for instance:
1634 %define api.location.file "my-location.hh"
1636 This name can have directory components, and even be absolute. The name
1637 under which the location file is included is controlled by
1638 api.location.include.
1640 This way it is possible to have several parsers share the same location
1643 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1646 %define api.namespace {foo}
1647 %define api.location.file "include/ast/loc.hh"
1648 %define api.location.include {<ast/loc.hh>}
1650 and use it in src/bar/parser.hh:
1653 %define api.namespace {bar}
1654 %code requires {#include <ast/loc.hh>}
1655 %define api.location.type {bar::location}
1657 Absolute file names are supported, so in your Makefile, passing the flag
1658 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1661 *** C++: stack.hh and position.hh are deprecated
1663 When asked to generate a header file (%defines), the lalr1.cc skeleton
1664 generates a stack.hh file. This file had no interest for users; it is now
1665 made useless: its content is included in the parser definition. It is
1666 still generated for backward compatibility.
1668 When in addition to %defines, location support is requested (%locations),
1669 the file position.hh is also generated. It is now also useless: its
1670 content is now included in location.hh.
1672 These files are no longer generated when your grammar file requires at
1673 least Bison 3.2 (%require "3.2").
1677 Portability issues on MinGW and VS2015.
1679 Portability issues in the test suite.
1681 Portability/warning issues with Flex.
1684 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1686 ** Backward incompatible changes
1688 Compiling Bison now requires a C99 compiler---as announced during the
1689 release of Bison 3.0, five years ago. Generated parsers do not require a
1692 Support for DJGPP, which has been unmaintained and untested for years, is
1693 obsolete. Unless there is activity to revive it, the next release of Bison
1694 will have it removed.
1698 *** Typed midrule actions
1700 Because their type is unknown to Bison, the values of midrule actions are
1701 not treated like the others: they don't have %printer and %destructor
1702 support. It also prevents C++ (Bison) variants to handle them properly.
1704 Typed midrule actions address these issues. Instead of:
1706 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1710 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1712 *** Reports include the type of the symbols
1714 The sections about terminal and nonterminal symbols of the '*.output' file
1715 now specify their declared type. For instance, for:
1719 the report now shows '<ival>':
1721 Terminals, with rules where they appear
1725 *** Diagnostics about useless rules
1727 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1728 of course, its rules are useless too.
1732 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1734 Previously all the useless rules were reported, including those whose
1735 left-hand side is the 'exp' nonterminal:
1737 warning: 1 nonterminal useless in grammar [-Wother]
1738 warning: 4 rules useless in grammar [-Wother]
1739 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1742 2.14-16: warning: rule useless in grammar [-Wother]
1745 3.6-16: warning: rule useless in grammar [-Wother]
1746 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1748 3.20-30: warning: rule useless in grammar [-Wother]
1749 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1751 3.34-44: warning: rule useless in grammar [-Wother]
1752 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1755 Now, rules whose left-hand side symbol is useless are no longer reported
1756 as useless. The locations of the errors have also been adjusted to point
1757 to the first use of the nonterminal as a left-hand side of a rule:
1759 warning: 1 nonterminal useless in grammar [-Wother]
1760 warning: 4 rules useless in grammar [-Wother]
1761 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1762 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1764 2.14-16: warning: rule useless in grammar [-Wother]
1768 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1770 When compiled with exceptions disabled, the generated parsers no longer
1771 uses try/catch clauses.
1773 Currently only GCC and Clang are supported.
1777 *** A demonstration of variants
1779 A new example was added (installed in .../share/doc/bison/examples),
1780 'variant.yy', which shows how to use (Bison) variants in C++.
1782 The other examples were made nicer to read.
1784 *** Some features are no longer 'experimental'
1786 The following features, mature enough, are no longer flagged as
1787 experimental in the documentation: push parsers, default %printer and
1788 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1789 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1790 semantic predicates (%?).
1794 *** GLR: Predicates support broken by #line directives
1796 Predicates (%?) in GLR such as
1799 %? {new_syntax} 'w' id new_args
1800 | %?{!new_syntax} 'w' id old_args
1802 were issued with #lines in the middle of C code.
1804 *** Printer and destructor with broken #line directives
1806 The #line directives were not properly escaped when emitting the code for
1807 %printer/%destructor, which resulted in compiler errors if there are
1808 backslashes or double-quotes in the grammar file name.
1810 *** Portability on ICC
1812 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1813 Generated parsers now work around this.
1817 There were several small fixes in the test suite and in the build system,
1818 many warnings in bison and in the generated parsers were eliminated. The
1819 documentation also received its share of minor improvements.
1821 Useless code was removed from C++ parsers, and some of the generated
1822 constructors are more 'natural'.
1825 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1829 *** C++: Fix support of 'syntax_error'
1831 One incorrect 'inline' resulted in linking errors about the constructor of
1832 the syntax_error exception.
1834 *** C++: Fix warnings
1836 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1837 warnings about yyformat being possibly null. It also warned about the
1838 deprecated implicit definition of copy constructors when there's a
1839 user-defined (copy) assignment operator.
1841 *** Location of errors
1843 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1844 ride-hand side raises a syntax error. The behavior of the default parser
1845 (yacc.c) in such a condition was undefined.
1847 Now all the parsers match the behavior of glr.c: @$ is used as the
1848 location of the error. This handles gracefully rules with and without
1851 *** Portability fixes in the test suite
1853 On some platforms, some Java and/or C++ tests were failing.
1856 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1860 *** C++ with Variants (lalr1.cc)
1862 Fix a compiler warning when no %destructor use $$.
1866 Several portability issues in tests were fixed.
1869 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1873 *** C++ with Variants (lalr1.cc)
1875 Problems with %destructor and '%define parse.assert' have been fixed.
1877 *** Named %union support (yacc.c, glr.c)
1879 Bison 3.0 introduced a regression on named %union such as
1881 %union foo { int ival; };
1883 The possibility to use a name was introduced "for Yacc compatibility".
1884 It is however not required by POSIX Yacc, and its usefulness is not clear.
1886 *** %define api.value.type union with %defines (yacc.c, glr.c)
1888 The C parsers were broken when %defines was used together with "%define
1889 api.value.type union".
1891 *** Redeclarations are reported in proper order
1899 bison used to report:
1901 foo.yy:2.10-11: error: %printer redeclaration for FOO
1904 foo.yy:3.10-11: previous declaration
1908 Now, the "previous" declaration is always the first one.
1913 Bison now installs various files in its docdir (which defaults to
1914 '/usr/local/share/doc/bison'), including the three fully blown examples
1915 extracted from the documentation:
1918 Reverse Polish Calculator, a simple introductory example.
1920 Multi-function Calc, a calculator with memory and functions and located
1923 a calculator in C++ using variant support and token constructors.
1926 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1930 *** Generated source files when errors are reported
1932 When warnings are issued and -Werror is set, bison would still generate
1933 the source files (*.c, *.h...). As a consequence, some runs of "make"
1934 could fail the first time, but not the second (as the files were generated
1937 This is fixed: bison no longer generates this source files, but, of
1938 course, still produces the various reports (*.output, *.xml, etc.).
1940 *** %empty is used in reports
1942 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1943 dot, XML and formats derived from it).
1945 *** YYERROR and variants
1947 When C++ variant support is enabled, an error triggered via YYERROR, but
1948 not caught via error recovery, resulted in a double deletion.
1951 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1955 *** Errors in caret diagnostics
1957 On some platforms, some errors could result in endless diagnostics.
1959 *** Fixes of the -Werror option
1961 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1962 diagnostics into errors instead of warnings. This is fixed.
1964 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1965 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1966 -Wno-error", "foo" diagnostics are now errors.
1970 As demonstrated in the documentation, one can now leave spaces between
1975 The yacc.1 man page is no longer installed if --disable-yacc was
1978 *** Fixes in the test suite
1980 Bugs and portability issues.
1983 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1985 ** WARNING: Future backward-incompatibilities!
1987 Like other GNU packages, Bison will start using some of the C99 features
1988 for its own code, especially the definition of variables after statements.
1989 The generated C parsers still aim at C90.
1991 ** Backward incompatible changes
1993 *** Obsolete features
1995 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1997 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1998 use YYSTYPE and YYLTYPE.
2000 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
2001 1.875): use %lex-param, %parse-param, or %param.
2003 Missing semicolons at the end of actions are no longer added (as announced
2004 in the release 2.5).
2006 *** Use of YACC='bison -y'
2008 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
2011 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
2012 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
2013 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
2015 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
2016 implementation of Yacc, was using Bison as 'bison -y'. While it does
2017 ensure compatible output file names, it also enables warnings for
2018 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
2019 warnings for Bison extensions.
2021 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
2022 (which also generates 'y.tab.h' and 'y.output' when needed).
2023 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
2024 flags (YFLAGS, or AM_YFLAGS with Automake).
2028 *** The epilogue is no longer affected by internal #defines (glr.c)
2030 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
2031 generated code. These weren't properly undefined before the inclusion of
2032 the user epilogue, so functions such as the following were butchered by the
2033 preprocessor expansion:
2035 int yylex (YYSTYPE *yylval);
2037 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2038 identifiers for user-provided variables.
2040 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2042 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2043 locations are enabled. This is fixed.
2045 *** Warnings about useless %pure-parser/%define api.pure are restored
2047 ** Diagnostics reported by Bison
2049 Most of these features were contributed by Théophile Ranquet and Victor
2054 Version 2.7 introduced caret errors, for a prettier output. These are now
2055 activated by default. The old format can still be used by invoking Bison
2056 with -fno-caret (or -fnone).
2058 Some error messages that reproduced excerpts of the grammar are now using
2059 the caret information only. For instance on:
2066 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2067 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2071 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2072 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2076 and "bison -fno-caret" reports:
2078 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2079 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2081 *** Enhancements of the -Werror option
2083 The -Werror=CATEGORY option is now recognized, and will treat specified
2084 warnings as errors. The warnings need not have been explicitly activated
2085 using the -W option, this is similar to what GCC 4.7 does.
2087 For example, given the following command line, Bison will treat both
2088 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2089 errors (and only those):
2091 $ bison -Werror=yacc,error=conflicts-sr input.y
2093 If no categories are specified, -Werror will make all active warnings into
2094 errors. For example, the following line does the same the previous example:
2096 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2098 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2100 Note that the categories in this -Werror option may not be prefixed with
2101 "no-". However, -Wno-error[=CATEGORY] is valid.
2103 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2104 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2105 incompatibilities as warnings: "-y -Wno-error=yacc".
2107 *** The display of warnings is now richer
2109 The option that controls a given warning is now displayed:
2111 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2113 In the case of warnings treated as errors, the prefix is changed from
2114 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2115 to GCC, as [-Werror=CATEGORY].
2117 For instance, where the previous version of Bison would report (and exit
2120 bison: warnings being treated as errors
2121 input.y:1.1: warning: stray ',' treated as white space
2125 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2127 *** Deprecated constructs
2129 The new 'deprecated' warning category flags obsolete constructs whose
2130 support will be discontinued. It is enabled by default. These warnings
2131 used to be reported as 'other' warnings.
2133 *** Useless semantic types
2135 Bison now warns about useless (uninhabited) semantic types. Since
2136 semantic types are not declared to Bison (they are defined in the opaque
2137 %union structure), it is %printer/%destructor directives about useless
2138 types that trigger the warning:
2142 %printer {} <type1> <type3>
2143 %destructor {} <type2> <type4>
2145 nterm: term { $$ = $1; };
2147 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2148 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2150 *** Undefined but unused symbols
2152 Bison used to raise an error for undefined symbols that are not used in
2153 the grammar. This is now only a warning.
2156 %destructor {} symbol2
2157 %type <type> symbol3
2161 *** Useless destructors or printers
2163 Bison now warns about useless destructors or printers. In the following
2164 example, the printer for <type1>, and the destructor for <type2> are
2165 useless: all symbols of <type1> (token1) already have a printer, and all
2166 symbols of type <type2> (token2) already have a destructor.
2168 %token <type1> token1
2172 %printer {} token1 <type1> <type3>
2173 %destructor {} token2 <type2> <type4>
2177 The warnings and error messages about shift/reduce and reduce/reduce
2178 conflicts have been normalized. For instance on the following foo.y file:
2182 exp: exp '+' exp | '0' | '0';
2184 compare the previous version of bison:
2187 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2188 $ bison -Werror foo.y
2189 bison: warnings being treated as errors
2190 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2192 with the new behavior:
2195 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2196 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2197 $ bison -Werror foo.y
2198 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2199 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2201 When %expect or %expect-rr is used, such as with bar.y:
2206 exp: exp '+' exp | '0' | '0';
2211 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2212 bar.y: expected 0 shift/reduce conflicts
2213 bar.y: expected 0 reduce/reduce conflicts
2218 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2219 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2221 ** Incompatibilities with POSIX Yacc
2223 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2226 ** Additional yylex/yyparse arguments
2228 The new directive %param declares additional arguments to both yylex and
2229 yyparse. The %lex-param, %parse-param, and %param directives support one
2230 or more arguments. Instead of
2232 %lex-param {arg1_type *arg1}
2233 %lex-param {arg2_type *arg2}
2234 %parse-param {arg1_type *arg1}
2235 %parse-param {arg2_type *arg2}
2239 %param {arg1_type *arg1} {arg2_type *arg2}
2241 ** Types of values for %define variables
2243 Bison used to make no difference between '%define foo bar' and '%define
2244 foo "bar"'. The former is now called a 'keyword value', and the latter a
2245 'string value'. A third kind was added: 'code values', such as '%define
2248 Keyword variables are used for fixed value sets, e.g.,
2250 %define lr.type lalr
2252 Code variables are used for value in the target language, e.g.,
2254 %define api.value.type {struct semantic_type}
2256 String variables are used remaining cases, e.g. file names.
2258 ** Variable api.token.prefix
2260 The variable api.token.prefix changes the way tokens are identified in
2261 the generated files. This is especially useful to avoid collisions
2262 with identifiers in the target language. For instance
2264 %token FILE for ERROR
2265 %define api.token.prefix {TOK_}
2267 start: FILE for ERROR;
2269 will generate the definition of the symbols TOK_FILE, TOK_for, and
2270 TOK_ERROR in the generated sources. In particular, the scanner must
2271 use these prefixed token names, although the grammar itself still
2272 uses the short names (as in the sample rule given above).
2274 ** Variable api.value.type
2276 This new %define variable supersedes the #define macro YYSTYPE. The use
2277 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2278 using %union or %defining api.value.type results in undefined behavior.
2280 Either define api.value.type, or use "%union":
2287 %token <ival> INT "integer"
2288 %token <sval> STRING "string"
2289 %printer { fprintf (yyo, "%d", $$); } <ival>
2290 %destructor { free ($$); } <sval>
2293 yylval.ival = 42; return INT;
2294 yylval.sval = "42"; return STRING;
2296 The %define variable api.value.type supports both keyword and code values.
2298 The keyword value 'union' means that the user provides genuine types, not
2299 union member names such as "ival" and "sval" above (WARNING: will fail if
2300 -y/--yacc/%yacc is enabled).
2302 %define api.value.type union
2303 %token <int> INT "integer"
2304 %token <char *> STRING "string"
2305 %printer { fprintf (yyo, "%d", $$); } <int>
2306 %destructor { free ($$); } <char *>
2309 yylval.INT = 42; return INT;
2310 yylval.STRING = "42"; return STRING;
2312 The keyword value variant is somewhat equivalent, but for C++ special
2313 provision is made to allow classes to be used (more about this below).
2315 %define api.value.type variant
2316 %token <int> INT "integer"
2317 %token <std::string> STRING "string"
2319 Code values (in braces) denote user defined types. This is where YYSTYPE
2337 %define api.value.type {struct my_value}
2338 %token <u.ival> INT "integer"
2339 %token <u.sval> STRING "string"
2340 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2341 %destructor { free ($$); } <u.sval>
2344 yylval.u.ival = 42; return INT;
2345 yylval.u.sval = "42"; return STRING;
2347 ** Variable parse.error
2349 This variable controls the verbosity of error messages. The use of the
2350 %error-verbose directive is deprecated in favor of "%define parse.error
2353 ** Deprecated %define variable names
2355 The following variables have been renamed for consistency. Backward
2356 compatibility is ensured, but upgrading is recommended.
2358 lr.default-reductions -> lr.default-reduction
2359 lr.keep-unreachable-states -> lr.keep-unreachable-state
2360 namespace -> api.namespace
2361 stype -> api.value.type
2363 ** Semantic predicates
2365 Contributed by Paul Hilfinger.
2367 The new, experimental, semantic-predicate feature allows actions of the
2368 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2369 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2370 in GLR parsers, rather than being deferred. The result is that they allow
2371 the programmer to prune possible parses based on the values of run-time
2374 ** The directive %expect-rr is now an error in non GLR mode
2376 It used to be an error only if used in non GLR mode, _and_ if there are
2377 reduce/reduce conflicts.
2379 ** Tokens are numbered in their order of appearance
2381 Contributed by Valentin Tolmer.
2383 With '%token A B', A had a number less than the one of B. However,
2384 precedence declarations used to generate a reversed order. This is now
2385 fixed, and introducing tokens with any of %token, %left, %right,
2386 %precedence, or %nonassoc yields the same result.
2388 When mixing declarations of tokens with a literal character (e.g., 'a') or
2389 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2390 the literal characters first. For example
2394 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2395 input order is now preserved.
2397 These changes were made so that one can remove useless precedence and
2398 associativity declarations (i.e., map %nonassoc, %left or %right to
2399 %precedence, or to %token) and get exactly the same output.
2401 ** Useless precedence and associativity
2403 Contributed by Valentin Tolmer.
2405 When developing and maintaining a grammar, useless associativity and
2406 precedence directives are common. They can be a nuisance: new ambiguities
2407 arising are sometimes masked because their conflicts are resolved due to
2408 the extra precedence or associativity information. Furthermore, it can
2409 hinder the comprehension of a new grammar: one will wonder about the role
2410 of a precedence, where in fact it is useless. The following changes aim
2411 at detecting and reporting these extra directives.
2413 *** Precedence warning category
2415 A new category of warning, -Wprecedence, was introduced. It flags the
2416 useless precedence and associativity directives.
2418 *** Useless associativity
2420 Bison now warns about symbols with a declared associativity that is never
2421 used to resolve conflicts. In that case, using %precedence is sufficient;
2422 the parsing tables will remain unchanged. Solving these warnings may raise
2423 useless precedence warnings, as the symbols no longer have associativity.
2437 warning: useless associativity for '+', use %precedence [-Wprecedence]
2441 *** Useless precedence
2443 Bison now warns about symbols with a declared precedence and no declared
2444 associativity (i.e., declared with %precedence), and whose precedence is
2445 never used. In that case, the symbol can be safely declared with %token
2446 instead, without modifying the parsing tables. For example:
2450 exp: "var" '=' "number";
2454 warning: useless precedence for '=' [-Wprecedence]
2458 *** Useless precedence and associativity
2460 In case of both useless precedence and associativity, the issue is flagged
2465 exp: "var" '=' "number";
2469 warning: useless precedence and associativity for '=' [-Wprecedence]
2475 With help from Joel E. Denny and Gabriel Rassoul.
2477 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2478 marked by the new %empty directive. Using %empty on a non-empty rule is
2479 an error. The new -Wempty-rule warning reports empty rules without
2480 %empty. On the following grammar:
2490 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2493 5.8-13: error: %empty on non-empty rule
2497 ** Java skeleton improvements
2499 The constants for token names were moved to the Lexer interface. Also, it
2500 is possible to add code to the parser's constructors using "%code init"
2501 and "%define init_throws".
2502 Contributed by Paolo Bonzini.
2504 The Java skeleton now supports push parsing.
2505 Contributed by Dennis Heimbigner.
2507 ** C++ skeletons improvements
2509 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2511 Using %defines is now optional. Without it, the needed support classes
2512 are defined in the generated parser, instead of additional files (such as
2513 location.hh, position.hh and stack.hh).
2515 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2517 Both lalr1.cc and glr.cc no longer require %location.
2519 *** syntax_error exception (lalr1.cc)
2521 The C++ parser features a syntax_error exception, which can be
2522 thrown from the scanner or from user rules to raise syntax errors.
2523 This facilitates reporting errors caught in sub-functions (e.g.,
2524 rejecting too large integral literals from a conversion function
2525 used by the scanner, or rejecting invalid combinations from a
2526 factory invoked by the user actions).
2528 *** %define api.value.type variant
2530 This is based on a submission from Michiel De Wilde. With help
2531 from Théophile Ranquet.
2533 In this mode, complex C++ objects can be used as semantic values. For
2536 %token <::std::string> TEXT;
2537 %token <int> NUMBER;
2538 %token SEMICOLON ";"
2539 %type <::std::string> item;
2540 %type <::std::list<std::string>> list;
2543 list { std::cout << $1 << std::endl; }
2547 %empty { /* Generates an empty string list. */ }
2548 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2552 TEXT { std::swap ($$, $1); }
2553 | NUMBER { $$ = string_cast ($1); }
2556 *** %define api.token.constructor
2558 When variants are enabled, Bison can generate functions to build the
2559 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2560 with the semantic value (e.g., int):
2562 parser::symbol_type yylex ()
2564 parser::location_type loc = ...;
2566 return parser::make_TEXT ("Hello, world!", loc);
2568 return parser::make_NUMBER (42, loc);
2570 return parser::make_SEMICOLON (loc);
2576 There are operator- and operator-= for 'location'. Negative line/column
2577 increments can no longer underflow the resulting value.
2580 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2584 *** Fix compiler attribute portability (yacc.c)
2586 With locations enabled, __attribute__ was used unprotected.
2588 *** Fix some compiler warnings (lalr1.cc)
2591 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2595 Warnings about uninitialized yylloc in yyparse have been fixed.
2597 Restored C90 compliance (yet no report was ever made).
2599 ** Diagnostics are improved
2601 Contributed by Théophile Ranquet.
2603 *** Changes in the format of error messages
2605 This used to be the format of many error reports:
2607 input.y:2.7-12: %type redeclaration for exp
2608 input.y:1.7-12: previous declaration
2612 input.y:2.7-12: error: %type redeclaration for exp
2613 input.y:1.7-12: previous declaration
2615 *** New format for error reports: carets
2617 Caret errors have been added to Bison:
2619 input.y:2.7-12: error: %type redeclaration for exp
2622 input.y:1.7-12: previous declaration
2628 input.y:3.20-23: error: ambiguous reference: '$exp'
2629 exp: exp '+' exp { $exp = $1 + $3; };
2631 input.y:3.1-3: refers to: $exp at $$
2632 exp: exp '+' exp { $exp = $1 + $3; };
2634 input.y:3.6-8: refers to: $exp at $1
2635 exp: exp '+' exp { $exp = $1 + $3; };
2637 input.y:3.14-16: refers to: $exp at $3
2638 exp: exp '+' exp { $exp = $1 + $3; };
2641 The default behavior for now is still not to display these unless
2642 explicitly asked with -fcaret (or -fall). However, in a later release, it
2643 will be made the default behavior (but may still be deactivated with
2646 ** New value for %define variable: api.pure full
2648 The %define variable api.pure requests a pure (reentrant) parser. However,
2649 for historical reasons, using it in a location-tracking Yacc parser
2650 resulted in a yyerror function that did not take a location as a
2651 parameter. With this new value, the user may request a better pure parser,
2652 where yyerror does take a location as a parameter (in location-tracking
2655 The use of "%define api.pure true" is deprecated in favor of this new
2656 "%define api.pure full".
2658 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2660 The %define variable api.location.type defines the name of the type to use
2661 for locations. When defined, Bison no longer generates the position.hh
2662 and location.hh files, nor does the parser will include them: the user is
2663 then responsible to define her type.
2665 This can be used in programs with several parsers to factor their location
2666 and position files: let one of them generate them, and the others just use
2669 This feature was actually introduced, but not documented, in Bison 2.5,
2670 under the name "location_type" (which is maintained for backward
2673 For consistency, lalr1.java's %define variables location_type and
2674 position_type are deprecated in favor of api.location.type and
2677 ** Exception safety (lalr1.cc)
2679 The parse function now catches exceptions, uses the %destructors to
2680 release memory (the lookahead symbol and the symbols pushed on the stack)
2681 before re-throwing the exception.
2683 This feature is somewhat experimental. User feedback would be
2686 ** Graph improvements in DOT and XSLT
2688 Contributed by Théophile Ranquet.
2690 The graphical presentation of the states is more readable: their shape is
2691 now rectangular, the state number is clearly displayed, and the items are
2692 numbered and left-justified.
2694 The reductions are now explicitly represented as transitions to other
2695 diamond shaped nodes.
2697 These changes are present in both --graph output and xml2dot.xsl XSLT
2698 processing, with minor (documented) differences.
2700 ** %language is no longer an experimental feature.
2702 The introduction of this feature, in 2.4, was four years ago. The
2703 --language option and the %language directive are no longer experimental.
2707 The sections about shift/reduce and reduce/reduce conflicts resolution
2708 have been fixed and extended.
2710 Although introduced more than four years ago, XML and Graphviz reports
2711 were not properly documented.
2713 The translation of midrule actions is now described.
2716 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2718 We consider compiler warnings about Bison generated parsers to be bugs.
2719 Rather than working around them in your own project, please consider
2720 reporting them to us.
2724 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2725 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2728 Other issues in the test suite have been addressed.
2730 Null characters are correctly displayed in error messages.
2732 When possible, yylloc is correctly initialized before calling yylex. It
2733 is no longer necessary to initialize it in the %initial-action.
2736 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2738 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2741 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2745 Bugs and portability issues in the test suite have been fixed.
2747 Some errors in translations have been addressed, and --help now directs
2748 users to the appropriate place to report them.
2750 Stray Info files shipped by accident are removed.
2752 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2753 generated, are removed.
2755 All the generated headers are self-contained.
2757 ** Header guards (yacc.c, glr.c, glr.cc)
2759 In order to avoid collisions, the header guards are now
2760 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2761 For instance the header generated from
2763 %define api.prefix "calc"
2764 %defines "lib/parse.h"
2766 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2768 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2770 The compilation of pure parsers (%define api.pure) can trigger GCC
2773 input.c: In function 'yyparse':
2774 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2775 function [-Wmaybe-uninitialized]
2779 This is now fixed; pragmas to avoid these warnings are no longer needed.
2781 Warnings from clang ("equality comparison with extraneous parentheses" and
2782 "function declared 'noreturn' should not return") have also been
2786 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2790 Buffer overruns, complaints from Flex, and portability issues in the test
2791 suite have been fixed.
2793 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2795 Trailing end-of-lines in %parse-param or %lex-param would result in
2796 invalid C++. This is fixed.
2798 ** Spurious spaces and end-of-lines
2800 The generated files no longer end (nor start) with empty lines.
2803 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2805 Bison no longer executes user-specified M4 code when processing a grammar.
2809 In addition to the removal of the features announced in Bison 2.6, the
2810 next major release will remove the "Temporary hack for adding a semicolon
2811 to the user action", as announced in the release 2.5. Instead of:
2813 exp: exp "+" exp { $$ = $1 + $3 };
2817 exp: exp "+" exp { $$ = $1 + $3; };
2821 *** Type names are now properly escaped.
2823 *** glr.cc: set_debug_level and debug_level work as expected.
2825 *** Stray @ or $ in actions
2827 While Bison used to warn about stray $ or @ in action rules, it did not
2828 for other actions such as printers, destructors, or initial actions. It
2831 ** Type names in actions
2833 For consistency with rule actions, it is now possible to qualify $$ by a
2834 type-name in destructors, printers, and initial actions. For instance:
2836 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2838 will display two values for each typed and untyped symbol (provided
2839 that YYSTYPE has both "ival" and "fval" fields).
2842 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2846 The next major release of Bison will drop support for the following
2847 deprecated features. Please report disagreements to bug-bison@gnu.org.
2851 Support for generating parsers in K&R C will be removed. Parsers
2852 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2855 *** Features deprecated since Bison 1.875
2857 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2860 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2861 %lex-param, will no longer be supported.
2863 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2866 *** The generated header will be included (yacc.c)
2868 Instead of duplicating the content of the generated header (definition of
2869 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2870 as is already the case for GLR or C++ parsers. This change is deferred
2871 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2874 ** Generated Parser Headers
2876 *** Guards (yacc.c, glr.c, glr.cc)
2878 The generated headers are now guarded, as is already the case for C++
2879 parsers (lalr1.cc). For instance, with --defines=foo.h:
2884 #endif /* !YY_FOO_H */
2886 *** New declarations (yacc.c, glr.c)
2888 The generated header now declares yydebug and yyparse. Both honor
2889 --name-prefix=bar_, and yield
2891 int bar_parse (void);
2895 #define yyparse bar_parse
2898 in order to facilitate the inclusion of several parser headers inside a
2899 single compilation unit.
2901 *** Exported symbols in C++
2903 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2904 header, are removed, as they prevent the possibility of including several
2905 generated headers from a single compilation unit.
2909 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2912 ** New %define variable: api.prefix
2914 Now that the generated headers are more complete and properly protected
2915 against multiple inclusions, constant names, such as YYSTYPE are a
2916 problem. While yyparse and others are properly renamed by %name-prefix,
2917 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2918 would introduce backward compatibility issues in projects not expecting
2919 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2920 it is deprecated in favor of a new %define variable: api.prefix.
2922 The following examples compares both:
2924 %name-prefix "bar_" | %define api.prefix "bar_"
2925 %token <ival> FOO %token <ival> FOO
2926 %union { int ival; } %union { int ival; }
2932 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2933 # define BAR_FOO_H # define BAR_FOO_H
2935 /* Enabling traces. */ /* Enabling traces. */
2936 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2937 > # if defined YYDEBUG
2939 > # define BAR_DEBUG 1
2941 > # define BAR_DEBUG 0
2944 # define YYDEBUG 0 | # define BAR_DEBUG 0
2948 # if YYDEBUG | # if BAR_DEBUG
2949 extern int bar_debug; extern int bar_debug;
2952 /* Tokens. */ /* Tokens. */
2953 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2954 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2955 enum yytokentype { | enum bar_tokentype {
2960 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2961 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2962 typedef union YYSTYPE | typedef union BAR_STYPE
2965 } YYSTYPE; | } BAR_STYPE;
2966 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2969 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2971 int bar_parse (void); int bar_parse (void);
2973 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2976 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2980 The next major release will drop support for generating parsers in K&R C.
2982 ** yacc.c: YYBACKUP works as expected.
2984 ** glr.c improvements:
2986 *** Location support is eliminated when not requested:
2988 GLR parsers used to include location-related code even when locations were
2989 not requested, and therefore not even usable.
2991 *** __attribute__ is preserved:
2993 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2994 when -std is passed to GCC).
2996 ** lalr1.java: several fixes:
2998 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2999 first token leads to a syntax error. Some minor clean ups.
3003 *** C++11 compatibility:
3005 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
3010 The header files such as "parser.hh", "location.hh", etc. used a constant
3011 name for preprocessor guards, for instance:
3013 #ifndef BISON_LOCATION_HH
3014 # define BISON_LOCATION_HH
3016 #endif // !BISON_LOCATION_HH
3018 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
3019 case characters are converted to upper case, and series of
3020 non-alphanumerical characters are converted to an underscore.
3022 With "bison -o lang++/parser.cc", "location.hh" would now include:
3024 #ifndef YY_LANG_LOCATION_HH
3025 # define YY_LANG_LOCATION_HH
3027 #endif // !YY_LANG_LOCATION_HH
3031 The position and location constructors (and their initialize methods)
3032 accept new arguments for line and column. Several issues in the
3033 documentation were fixed.
3035 ** liby is no longer asking for "rpl_fprintf" on some platforms.
3037 ** Changes in the manual:
3039 *** %printer is documented
3041 The "%printer" directive, supported since at least Bison 1.50, is finally
3042 documented. The "mfcalc" example is extended to demonstrate it.
3044 For consistency with the C skeletons, the C++ parsers now also support
3045 "yyoutput" (as an alias to "debug_stream ()").
3047 *** Several improvements have been made:
3049 The layout for grammar excerpts was changed to a more compact scheme.
3050 Named references are motivated. The description of the automaton
3051 description file (*.output) is updated to the current format. Incorrect
3052 index entries were fixed. Some other errors were fixed.
3056 *** Conflicting prototypes with recent/modified Flex.
3058 Fixed build problems with the current, unreleased, version of Flex, and
3059 some modified versions of 2.5.35, which have modified function prototypes.
3061 *** Warnings during the build procedure have been eliminated.
3063 *** Several portability problems in the test suite have been fixed:
3065 This includes warnings with some compilers, unexpected behavior of tools
3066 such as diff, warning messages from the test suite itself, etc.
3068 *** The install-pdf target works properly:
3070 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3071 halts in the middle of its course.
3074 * Noteworthy changes in release 2.5 (2011-05-14)
3076 ** Grammar symbol names can now contain non-initial dashes:
3078 Consistently with directives (such as %error-verbose) and with
3079 %define variables (e.g. push-pull), grammar symbol names may contain
3080 dashes in any position except the beginning. This is a GNU
3081 extension over POSIX Yacc. Thus, use of this extension is reported
3082 by -Wyacc and rejected in Yacc mode (--yacc).
3084 ** Named references:
3086 Historically, Yacc and Bison have supported positional references
3087 ($n, $$) to allow access to symbol values from inside of semantic
3090 Starting from this version, Bison can also accept named references.
3091 When no ambiguity is possible, original symbol names may be used
3092 as named references:
3094 if_stmt : "if" cond_expr "then" then_stmt ';'
3095 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3097 In the more common case, explicit names may be declared:
3099 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3100 { $res = mk_if_stmt($cond, $then, $else); }
3102 Location information is also accessible using @name syntax. When
3103 accessing symbol names containing dots or dashes, explicit bracketing
3104 ($[sym.1]) must be used.
3106 These features are experimental in this version. More user feedback
3107 will help to stabilize them.
3108 Contributed by Alex Rozenman.
3110 ** IELR(1) and canonical LR(1):
3112 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3113 is, given any context-free grammar, IELR(1) generates parser tables
3114 with the full language-recognition power of canonical LR(1) but with
3115 nearly the same number of parser states as LALR(1). This reduction
3116 in parser states is often an order of magnitude. More importantly,
3117 because canonical LR(1)'s extra parser states may contain duplicate
3118 conflicts in the case of non-LR(1) grammars, the number of conflicts
3119 for IELR(1) is often an order of magnitude less as well. This can
3120 significantly reduce the complexity of developing of a grammar.
3122 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3123 place of its traditional LALR(1) parser tables, which remain the
3124 default. You can specify the type of parser tables in the grammar
3125 file with these directives:
3127 %define lr.type lalr
3128 %define lr.type ielr
3129 %define lr.type canonical-lr
3131 The default-reduction optimization in the parser tables can also be
3132 adjusted using "%define lr.default-reductions". For details on both
3133 of these features, see the new section "Tuning LR" in the Bison
3136 These features are experimental. More user feedback will help to
3139 ** LAC (Lookahead Correction) for syntax error handling
3141 Contributed by Joel E. Denny.
3143 Canonical LR, IELR, and LALR can suffer from a couple of problems
3144 upon encountering a syntax error. First, the parser might perform
3145 additional parser stack reductions before discovering the syntax
3146 error. Such reductions can perform user semantic actions that are
3147 unexpected because they are based on an invalid token, and they
3148 cause error recovery to begin in a different syntactic context than
3149 the one in which the invalid token was encountered. Second, when
3150 verbose error messages are enabled (with %error-verbose or the
3151 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3152 syntax error message can both contain invalid tokens and omit valid
3155 The culprits for the above problems are %nonassoc, default
3156 reductions in inconsistent states, and parser state merging. Thus,
3157 IELR and LALR suffer the most. Canonical LR can suffer only if
3158 %nonassoc is used or if default reductions are enabled for
3159 inconsistent states.
3161 LAC is a new mechanism within the parsing algorithm that solves
3162 these problems for canonical LR, IELR, and LALR without sacrificing
3163 %nonassoc, default reductions, or state merging. When LAC is in
3164 use, canonical LR and IELR behave almost exactly the same for both
3165 syntactically acceptable and syntactically unacceptable input.
3166 While LALR still does not support the full language-recognition
3167 power of canonical LR and IELR, LAC at least enables LALR's syntax
3168 error handling to correctly reflect LALR's language-recognition
3171 Currently, LAC is only supported for deterministic parsers in C.
3172 You can enable LAC with the following directive:
3174 %define parse.lac full
3176 See the new section "LAC" in the Bison manual for additional
3177 details including a few caveats.
3179 LAC is an experimental feature. More user feedback will help to
3182 ** %define improvements:
3184 *** Can now be invoked via the command line:
3186 Each of these command-line options
3189 --define=NAME[=VALUE]
3192 --force-define=NAME[=VALUE]
3194 is equivalent to this grammar file declaration
3196 %define NAME ["VALUE"]
3198 except that the manner in which Bison processes multiple definitions
3199 for the same NAME differs. Most importantly, -F and --force-define
3200 quietly override %define, but -D and --define do not. For further
3201 details, see the section "Bison Options" in the Bison manual.
3203 *** Variables renamed:
3205 The following %define variables
3208 lr.keep_unreachable_states
3210 have been renamed to
3213 lr.keep-unreachable-states
3215 The old names are now deprecated but will be maintained indefinitely
3216 for backward compatibility.
3218 *** Values no longer need to be quoted in the grammar file:
3220 If a %define value is an identifier, it no longer needs to be placed
3221 within quotations marks. For example,
3223 %define api.push-pull "push"
3227 %define api.push-pull push
3229 *** Unrecognized variables are now errors not warnings.
3231 *** Multiple invocations for any variable is now an error not a warning.
3233 ** Unrecognized %code qualifiers are now errors not warnings.
3235 ** Character literals not of length one:
3237 Previously, Bison quietly converted all character literals to length
3238 one. For example, without warning, Bison interpreted the operators in
3239 the following grammar to be the same token:
3245 Bison now warns when a character literal is not of length one. In
3246 some future release, Bison will start reporting an error instead.
3248 ** Destructor calls fixed for lookaheads altered in semantic actions:
3250 Previously for deterministic parsers in C, if a user semantic action
3251 altered yychar, the parser in some cases used the old yychar value to
3252 determine which destructor to call for the lookahead upon a syntax
3253 error or upon parser return. This bug has been fixed.
3255 ** C++ parsers use YYRHSLOC:
3257 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3258 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3259 to use it. If, for instance, your location structure has "first"
3260 and "last" members, instead of
3262 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3266 (Current).first = (Rhs)[1].location.first; \
3267 (Current).last = (Rhs)[N].location.last; \
3271 (Current).first = (Current).last = (Rhs)[0].location.last; \
3277 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3281 (Current).first = YYRHSLOC (Rhs, 1).first; \
3282 (Current).last = YYRHSLOC (Rhs, N).last; \
3286 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3290 ** YYLLOC_DEFAULT in C++:
3292 The default implementation of YYLLOC_DEFAULT used to be issued in
3293 the header file. It is now output in the implementation file, after
3294 the user %code sections so that its #ifndef guard does not try to
3295 override the user's YYLLOC_DEFAULT if provided.
3297 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3299 YYFAIL has existed for many years as an undocumented feature of
3300 deterministic parsers in C generated by Bison. More recently, it was
3301 a documented feature of Bison's experimental Java parsers. As
3302 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3303 semantic action now produces a deprecation warning, and Java parsers
3304 no longer implement YYFAIL at all. For further details, including a
3305 discussion of how to suppress C preprocessor warnings about YYFAIL
3306 being unused, see the Bison 2.4.2 NEWS entry.
3308 ** Temporary hack for adding a semicolon to the user action:
3310 Previously, Bison appended a semicolon to every user action for
3311 reductions when the output language defaulted to C (specifically, when
3312 neither %yacc, %language, %skeleton, or equivalent command-line
3313 options were specified). This allowed actions such as
3315 exp: exp "+" exp { $$ = $1 + $3 };
3319 exp: exp "+" exp { $$ = $1 + $3; };
3321 As a first step in removing this misfeature, Bison now issues a
3322 warning when it appends a semicolon. Moreover, in cases where Bison
3323 cannot easily determine whether a semicolon is needed (for example, an
3324 action ending with a cpp directive or a braced compound initializer),
3325 it no longer appends one. Thus, the C compiler might now complain
3326 about a missing semicolon where it did not before. Future releases of
3327 Bison will cease to append semicolons entirely.
3329 ** Verbose syntax error message fixes:
3331 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3332 specified, syntax error messages produced by the generated parser
3333 include the unexpected token as well as a list of expected tokens.
3334 The effect of %nonassoc on these verbose messages has been corrected
3335 in two ways, but a more complete fix requires LAC, described above:
3337 *** When %nonassoc is used, there can exist parser states that accept no
3338 tokens, and so the parser does not always require a lookahead token
3339 in order to detect a syntax error. Because no unexpected token or
3340 expected tokens can then be reported, the verbose syntax error
3341 message described above is suppressed, and the parser instead
3342 reports the simpler message, "syntax error". Previously, this
3343 suppression was sometimes erroneously triggered by %nonassoc when a
3344 lookahead was actually required. Now verbose messages are
3345 suppressed only when all previous lookaheads have already been
3346 shifted or discarded.
3348 *** Previously, the list of expected tokens erroneously included tokens
3349 that would actually induce a syntax error because conflicts for them
3350 were resolved with %nonassoc in the current parser state. Such
3351 tokens are now properly omitted from the list.
3353 *** Expected token lists are still often wrong due to state merging
3354 (from LALR or IELR) and default reductions, which can both add
3355 invalid tokens and subtract valid tokens. Canonical LR almost
3356 completely fixes this problem by eliminating state merging and
3357 default reductions. However, there is one minor problem left even
3358 when using canonical LR and even after the fixes above. That is,
3359 if the resolution of a conflict with %nonassoc appears in a later
3360 parser state than the one at which some syntax error is
3361 discovered, the conflicted token is still erroneously included in
3362 the expected token list. Bison's new LAC implementation,
3363 described above, eliminates this problem and the need for
3364 canonical LR. However, LAC is still experimental and is disabled
3367 ** Java skeleton fixes:
3369 *** A location handling bug has been fixed.
3371 *** The top element of each of the value stack and location stack is now
3372 cleared when popped so that it can be garbage collected.
3374 *** Parser traces now print the top element of the stack.
3376 ** -W/--warnings fixes:
3378 *** Bison now properly recognizes the "no-" versions of categories:
3380 For example, given the following command line, Bison now enables all
3381 warnings except warnings for incompatibilities with POSIX Yacc:
3383 bison -Wall,no-yacc gram.y
3385 *** Bison now treats S/R and R/R conflicts like other warnings:
3387 Previously, conflict reports were independent of Bison's normal
3388 warning system. Now, Bison recognizes the warning categories
3389 "conflicts-sr" and "conflicts-rr". This change has important
3390 consequences for the -W and --warnings command-line options. For
3393 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3394 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3395 bison -Wnone gram.y # no conflicts are reported
3396 bison -Werror gram.y # any conflict is an error
3398 However, as before, if the %expect or %expect-rr directive is
3399 specified, an unexpected number of conflicts is an error, and an
3400 expected number of conflicts is not reported, so -W and --warning
3401 then have no effect on the conflict report.
3403 *** The "none" category no longer disables a preceding "error":
3405 For example, for the following command line, Bison now reports
3406 errors instead of warnings for incompatibilities with POSIX Yacc:
3408 bison -Werror,none,yacc gram.y
3410 *** The "none" category now disables all Bison warnings:
3412 Previously, the "none" category disabled only Bison warnings for
3413 which there existed a specific -W/--warning category. However,
3414 given the following command line, Bison is now guaranteed to
3415 suppress all warnings:
3419 ** Precedence directives can now assign token number 0:
3421 Since Bison 2.3b, which restored the ability of precedence
3422 directives to assign token numbers, doing so for token number 0 has
3423 produced an assertion failure. For example:
3427 This bug has been fixed.
3430 * Noteworthy changes in release 2.4.3 (2010-08-05)
3432 ** Bison now obeys -Werror and --warnings=error for warnings about
3433 grammar rules that are useless in the parser due to conflicts.
3435 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3438 ** Failures in the test suite for GCC 4.5 have been fixed.
3440 ** Failures in the test suite for some versions of Sun Studio C++ have
3443 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3444 warnings about undefined %prec identifiers will not be converted to
3445 errors in Bison 2.5. They will remain warnings, which should be
3446 sufficient for POSIX while avoiding backward compatibility issues.
3448 ** Minor documentation fixes.
3451 * Noteworthy changes in release 2.4.2 (2010-03-20)
3453 ** Some portability problems that resulted in failures and livelocks
3454 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3455 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3456 errors should no longer cause M4 to report a broken pipe on the
3459 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3461 POSIX specifies that an error be reported for any identifier that does
3462 not appear on the LHS of a grammar rule and that is not defined by
3463 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3464 error report for the case when an identifier appears only after a
3465 %prec directive. It is now restored. However, for backward
3466 compatibility with recent Bison releases, it is only a warning for
3467 now. In Bison 2.5 and later, it will return to being an error.
3468 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3469 warning will not be converted to an error in Bison 2.5.]
3471 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3473 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3474 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3477 ** %code is now a permanent feature.
3479 A traditional Yacc prologue directive is written in the form:
3483 To provide a more flexible alternative, Bison 2.3b introduced the
3484 %code directive with the following forms for C/C++:
3487 %code requires {CODE}
3488 %code provides {CODE}
3491 These forms are now considered permanent features of Bison. See the
3492 %code entries in the section "Bison Declaration Summary" in the Bison
3493 manual for a summary of their functionality. See the section
3494 "Prologue Alternatives" for a detailed discussion including the
3495 advantages of %code over the traditional Yacc prologue directive.
3497 Bison's Java feature as a whole including its current usage of %code
3498 is still considered experimental.
3500 ** YYFAIL is deprecated and will eventually be removed.
3502 YYFAIL has existed for many years as an undocumented feature of
3503 deterministic parsers in C generated by Bison. Previously, it was
3504 documented for Bison's experimental Java parsers. YYFAIL is no longer
3505 documented for Java parsers and is formally deprecated in both cases.
3506 Users are strongly encouraged to migrate to YYERROR, which is
3509 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3510 induce a syntax error. The most obvious difference from YYERROR is
3511 that YYFAIL will automatically invoke yyerror to report the syntax
3512 error so that you don't have to. However, there are several other
3513 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3514 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3515 used. For a more detailed discussion, see:
3517 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3519 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3520 deterministic parsers in C will continue to implement it. However,
3521 because YYFAIL is already flawed, it seems futile to try to make new
3522 Bison features compatible with it. Thus, during parser generation,
3523 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3524 rule action. In a later release, YYFAIL will be disabled for
3525 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3526 be removed altogether.
3528 There exists at least one case where Bison 2.5's YYFAIL warning will
3529 be a false positive. Some projects add phony uses of YYFAIL and other
3530 Bison-defined macros for the sole purpose of suppressing C
3531 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3532 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3533 epilogue (that is, after the second "%%") in the Bison input file. In
3534 this release (2.4.2), Bison already generates its own code to suppress
3535 C preprocessor warnings for YYFAIL, so projects can remove their own
3536 phony uses of YYFAIL if compatibility with Bison releases prior to
3537 2.4.2 is not necessary.
3539 ** Internationalization.
3541 Fix a regression introduced in Bison 2.4: Under some circumstances,
3542 message translations were not installed although supported by the
3546 * Noteworthy changes in release 2.4.1 (2008-12-11)
3548 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3549 declarations have been fixed.
3551 ** Temporary hack for adding a semicolon to the user action.
3553 Bison used to prepend a trailing semicolon at the end of the user
3554 action for reductions. This allowed actions such as
3556 exp: exp "+" exp { $$ = $1 + $3 };
3560 exp: exp "+" exp { $$ = $1 + $3; };
3562 Some grammars still depend on this "feature". Bison 2.4.1 restores
3563 the previous behavior in the case of C output (specifically, when
3564 neither %language or %skeleton or equivalent command-line options
3565 are used) to leave more time for grammars depending on the old
3566 behavior to be adjusted. Future releases of Bison will disable this
3569 ** A few minor improvements to the Bison manual.
3572 * Noteworthy changes in release 2.4 (2008-11-02)
3574 ** %language is an experimental feature.
3576 We first introduced this feature in test release 2.3b as a cleaner
3577 alternative to %skeleton. Since then, we have discussed the possibility of
3578 modifying its effect on Bison's output file names. Thus, in this release,
3579 we consider %language to be an experimental feature that will likely evolve
3582 ** Forward compatibility with GNU M4 has been improved.
3584 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3588 * Noteworthy changes in release 2.3b (2008-05-27)
3590 ** The quotes around NAME that used to be required in the following directive
3593 %define NAME "VALUE"
3595 ** The directive "%pure-parser" is now deprecated in favor of:
3599 which has the same effect except that Bison is more careful to warn about
3600 unreasonable usage in the latter case.
3604 Bison can now generate an LALR(1) parser in C with a push interface. That
3605 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3606 push one token at a time to the parser using "yypush_parse", which will
3607 return to the caller after processing each token. By default, the push
3608 interface is disabled. Either of the following directives will enable it:
3610 %define api.push_pull "push" // Just push; does not require yylex.
3611 %define api.push_pull "both" // Push and pull; requires yylex.
3613 See the new section "A Push Parser" in the Bison manual for details.
3615 The current push parsing interface is experimental and may evolve. More user
3616 feedback will help to stabilize it.
3618 ** The -g and --graph options now output graphs in Graphviz DOT format,
3619 not VCG format. Like --graph, -g now also takes an optional FILE argument
3620 and thus cannot be bundled with other short options.
3624 Bison can now generate an LALR(1) parser in Java. The skeleton is
3625 "data/lalr1.java". Consider using the new %language directive instead of
3626 %skeleton to select it.
3628 See the new section "Java Parsers" in the Bison manual for details.
3630 The current Java interface is experimental and may evolve. More user
3631 feedback will help to stabilize it.
3632 Contributed by Paolo Bonzini.
3636 This new directive specifies the programming language of the generated
3637 parser, which can be C (the default), C++, or Java. Besides the skeleton
3638 that Bison uses, the directive affects the names of the generated files if
3639 the grammar file's name ends in ".y".
3641 ** XML Automaton Report
3643 Bison can now generate an XML report of the LALR(1) automaton using the new
3644 "--xml" option. The current XML schema is experimental and may evolve. More
3645 user feedback will help to stabilize it.
3646 Contributed by Wojciech Polak.
3648 ** The grammar file may now specify the name of the parser header file using
3649 %defines. For example:
3653 ** When reporting useless rules, useless nonterminals, and unused terminals,
3654 Bison now employs the terms "useless in grammar" instead of "useless",
3655 "useless in parser" instead of "never reduced", and "unused in grammar"
3656 instead of "unused".
3658 ** Unreachable State Removal
3660 Previously, Bison sometimes generated parser tables containing unreachable
3661 states. A state can become unreachable during conflict resolution if Bison
3662 disables a shift action leading to it from a predecessor state. Bison now:
3664 1. Removes unreachable states.
3666 2. Does not report any conflicts that appeared in unreachable states.
3667 WARNING: As a result, you may need to update %expect and %expect-rr
3668 directives in existing grammar files.
3670 3. For any rule used only in such states, Bison now reports the rule as
3671 "useless in parser due to conflicts".
3673 This feature can be disabled with the following directive:
3675 %define lr.keep_unreachable_states
3677 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3678 for further discussion.
3680 ** Lookahead Set Correction in the ".output" Report
3682 When instructed to generate a ".output" file including lookahead sets
3683 (using "--report=lookahead", for example), Bison now prints each reduction's
3684 lookahead set only next to the associated state's one item that (1) is
3685 associated with the same rule as the reduction and (2) has its dot at the end
3686 of its RHS. Previously, Bison also erroneously printed the lookahead set
3687 next to all of the state's other items associated with the same rule. This
3688 bug affected only the ".output" file and not the generated parser source
3691 ** --report-file=FILE is a new option to override the default ".output" file
3694 ** The "=" that used to be required in the following directives is now
3697 %file-prefix "parser"
3701 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3703 Bison 2.3a provided a new set of directives as a more flexible alternative to
3704 the traditional Yacc prologue blocks. Those have now been consolidated into
3705 a single %code directive with an optional qualifier field, which identifies
3706 the purpose of the code and thus the location(s) where Bison should generate
3709 1. "%code {CODE}" replaces "%after-header {CODE}"
3710 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3711 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3712 4. "%code top {CODE}" replaces "%before-header {CODE}"
3714 See the %code entries in section "Bison Declaration Summary" in the Bison
3715 manual for a summary of the new functionality. See the new section "Prologue
3716 Alternatives" for a detailed discussion including the advantages of %code
3717 over the traditional Yacc prologues.
3719 The prologue alternatives are experimental. More user feedback will help to
3720 determine whether they should become permanent features.
3722 ** Revised warning: unset or unused midrule values
3724 Since Bison 2.2, Bison has warned about midrule values that are set but not
3725 used within any of the actions of the parent rule. For example, Bison warns
3728 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3730 Now, Bison also warns about midrule values that are used but not set. For
3731 example, Bison warns about unset $$ in the midrule action in:
3733 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3735 However, Bison now disables both of these warnings by default since they
3736 sometimes prove to be false alarms in existing grammars employing the Yacc
3737 constructs $0 or $-N (where N is some positive integer).
3739 To enable these warnings, specify the option "--warnings=midrule-values" or
3740 "-W", which is a synonym for "--warnings=all".
3742 ** Default %destructor or %printer with "<*>" or "<>"
3744 Bison now recognizes two separate kinds of default %destructor's and
3747 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3748 %destructor/%printer for all grammar symbols for which you have formally
3749 declared semantic type tags.
3751 2. Place "<>" in a %destructor/%printer symbol list to define a default
3752 %destructor/%printer for all grammar symbols without declared semantic
3755 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3756 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3757 longer applies any %destructor to a midrule value if that midrule value is
3758 not actually ever referenced using either $$ or $n in a semantic action.
3760 The default %destructor's and %printer's are experimental. More user
3761 feedback will help to determine whether they should become permanent
3764 See the section "Freeing Discarded Symbols" in the Bison manual for further
3767 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3768 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3769 manual for a caveat concerning the treatment of literal strings.
3771 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3772 completely removed from Bison.
3775 * Noteworthy changes in release 2.3a (2006-09-13)
3777 ** Instead of %union, you can define and use your own union type
3778 YYSTYPE if your grammar contains at least one <type> tag.
3779 Your YYSTYPE need not be a macro; it can be a typedef.
3780 This change is for compatibility with other Yacc implementations,
3781 and is required by POSIX.
3783 ** Locations columns and lines start at 1.
3784 In accordance with the GNU Coding Standards and Emacs.
3786 ** You may now declare per-type and default %destructor's and %printer's:
3790 %union { char *string; }
3791 %token <string> STRING1
3792 %token <string> STRING2
3793 %type <string> string1
3794 %type <string> string2
3795 %union { char character; }
3796 %token <character> CHR
3797 %type <character> chr
3798 %destructor { free ($$); } %symbol-default
3799 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3800 %destructor { } <character>
3802 guarantees that, when the parser discards any user-defined symbol that has a
3803 semantic type tag other than "<character>", it passes its semantic value to
3804 "free". However, when the parser discards a "STRING1" or a "string1", it
3805 also prints its line number to "stdout". It performs only the second
3806 "%destructor" in this case, so it invokes "free" only once.
3808 [Although we failed to mention this here in the 2.3a release, the default
3809 %destructor's and %printer's were experimental, and they were rewritten in
3812 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3813 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3814 associating token numbers with token names. Removing the #define statements
3815 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3816 requires them. Bison still generates an enum for token names in all cases.
3818 ** Handling of traditional Yacc prologue blocks is now more consistent but
3819 potentially incompatible with previous releases of Bison.
3821 As before, you declare prologue blocks in your grammar file with the
3822 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3823 prologue blocks that you've declared before the first %union. To generate
3824 the post-prologue, Bison concatenates all prologue blocks that you've
3825 declared after the first %union.
3827 Previous releases of Bison inserted the pre-prologue into both the header
3828 file and the code file in all cases except for LALR(1) parsers in C. In the
3829 latter case, Bison inserted it only into the code file. For parsers in C++,
3830 the point of insertion was before any token definitions (which associate
3831 token numbers with names). For parsers in C, the point of insertion was
3832 after the token definitions.
3834 Now, Bison never inserts the pre-prologue into the header file. In the code
3835 file, it always inserts it before the token definitions.
3837 ** Bison now provides a more flexible alternative to the traditional Yacc
3838 prologue blocks: %before-header, %start-header, %end-header, and
3841 For example, the following declaration order in the grammar file reflects the
3842 order in which Bison will output these code blocks. However, you are free to
3843 declare these code blocks in your grammar file in whatever order is most
3847 /* Bison treats this block like a pre-prologue block: it inserts it into
3848 * the code file before the contents of the header file. It does *not*
3849 * insert it into the header file. This is a good place to put
3850 * #include's that you want at the top of your code file. A common
3851 * example is '#include "system.h"'. */
3854 /* Bison inserts this block into both the header file and the code file.
3855 * In both files, the point of insertion is before any Bison-generated
3856 * token, semantic type, location type, and class definitions. This is a
3857 * good place to define %union dependencies, for example. */
3860 /* Unlike the traditional Yacc prologue blocks, the output order for the
3861 * new %*-header blocks is not affected by their declaration position
3862 * relative to any %union in the grammar file. */
3865 /* Bison inserts this block into both the header file and the code file.
3866 * In both files, the point of insertion is after the Bison-generated
3867 * definitions. This is a good place to declare or define public
3868 * functions or data structures that depend on the Bison-generated
3872 /* Bison treats this block like a post-prologue block: it inserts it into
3873 * the code file after the contents of the header file. It does *not*
3874 * insert it into the header file. This is a good place to declare or
3875 * define internal functions or data structures that depend on the
3876 * Bison-generated definitions. */
3879 If you have multiple occurrences of any one of the above declarations, Bison
3880 will concatenate the contents in declaration order.
3882 [Although we failed to mention this here in the 2.3a release, the prologue
3883 alternatives were experimental, and they were rewritten in future versions.]
3885 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3886 The old spelling still works, but is not documented and may be removed
3887 in a future release.
3890 * Noteworthy changes in release 2.3 (2006-06-05)
3892 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3893 for compatibility with LALR(1) grammars.
3895 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3896 be to a type name that does not contain parentheses or brackets.
3899 * Noteworthy changes in release 2.2 (2006-05-19)
3901 ** The distribution terms for all Bison-generated parsers now permit
3902 using the parsers in nonfree programs. Previously, this permission
3903 was granted only for Bison-generated LALR(1) parsers in C.
3905 ** %name-prefix changes the namespace name in C++ outputs.
3907 ** The C++ parsers export their token_type.
3909 ** Bison now allows multiple %union declarations, and concatenates
3910 their contents together.
3912 ** New warning: unused values
3913 Right-hand side symbols whose values are not used are reported,
3914 if the symbols have destructors. For instance:
3916 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3920 will trigger a warning about $$ and $5 in the first rule, and $3 in
3921 the second ($1 is copied to $$ by the default rule). This example
3922 most likely contains three errors, and could be rewritten as:
3924 exp: exp "?" exp ":" exp
3925 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3927 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3930 However, if the original actions were really intended, memory leaks
3931 and all, the warnings can be suppressed by letting Bison believe the
3932 values are used, e.g.:
3934 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3935 | exp "+" exp { $$ = $1; (void) $3; }
3938 If there are midrule actions, the warning is issued if no action
3939 uses it. The following triggers no warning: $1 and $3 are used.
3941 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3943 The warning is intended to help catching lost values and memory leaks.
3944 If a value is ignored, its associated memory typically is not reclaimed.
3946 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3947 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3948 and YYERROR, for all objects on the stack, other than objects
3949 corresponding to the right-hand side of the current rule.
3951 ** %expect, %expect-rr
3952 Incorrect numbers of expected conflicts are now actual errors,
3953 instead of warnings.
3955 ** GLR, YACC parsers.
3956 The %parse-params are available in the destructors (and the
3957 experimental printers) as per the documentation.
3959 ** Bison now warns if it finds a stray "$" or "@" in an action.
3961 ** %require "VERSION"
3962 This specifies that the grammar file depends on features implemented
3963 in Bison version VERSION or higher.
3965 ** lalr1.cc: The token and value types are now class members.
3966 The tokens were defined as free form enums and cpp macros. YYSTYPE
3967 was defined as a free form union. They are now class members:
3968 tokens are enumerations of the "yy::parser::token" struct, and the
3969 semantic values have the "yy::parser::semantic_type" type.
3971 If you do not want or can update to this scheme, the directive
3972 '%define "global_tokens_and_yystype" "1"' triggers the global
3973 definition of tokens and YYSTYPE. This change is suitable both
3974 for previous releases of Bison, and this one.
3976 If you wish to update, then make sure older version of Bison will
3977 fail using '%require "2.2"'.
3979 ** DJGPP support added.
3982 * Noteworthy changes in release 2.1 (2005-09-16)
3984 ** The C++ lalr1.cc skeleton supports %lex-param.
3986 ** Bison-generated parsers now support the translation of diagnostics like
3987 "syntax error" into languages other than English. The default
3988 language is still English. For details, please see the new
3989 Internationalization section of the Bison manual. Software
3990 distributors should also see the new PACKAGING file. Thanks to
3991 Bruno Haible for this new feature.
3993 ** Wording in the Bison-generated parsers has been changed slightly to
3994 simplify translation. In particular, the message "memory exhausted"
3995 has replaced "parser stack overflow", as the old message was not
3996 always accurate for modern Bison-generated parsers.
3998 ** Destructors are now called when the parser aborts, for all symbols left
3999 behind on the stack. Also, the start symbol is now destroyed after a
4000 successful parse. In both cases, the behavior was formerly inconsistent.
4002 ** When generating verbose diagnostics, Bison-generated parsers no longer
4003 quote the literal strings associated with tokens. For example, for
4004 a syntax error associated with '%token NUM "number"' they might
4005 print 'syntax error, unexpected number' instead of 'syntax error,
4006 unexpected "number"'.
4009 * Noteworthy changes in release 2.0 (2004-12-25)
4011 ** Possibly-incompatible changes
4013 - Bison-generated parsers no longer default to using the alloca function
4014 (when available) to extend the parser stack, due to widespread
4015 problems in unchecked stack-overflow detection. You can "#define
4016 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
4017 the manual to determine safe values for YYMAXDEPTH in that case.
4019 - Error token location.
4020 During error recovery, the location of the syntax error is updated
4021 to cover the whole sequence covered by the error token: it includes
4022 the shifted symbols thrown away during the first part of the error
4023 recovery, and the lookahead rejected during the second part.
4025 - Semicolon changes:
4026 . Stray semicolons are no longer allowed at the start of a grammar.
4027 . Semicolons are now required after in-grammar declarations.
4029 - Unescaped newlines are no longer allowed in character constants or
4030 string literals. They were never portable, and GCC 3.4.0 has
4031 dropped support for them. Better diagnostics are now generated if
4032 forget a closing quote.
4034 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4038 - GLR grammars now support locations.
4040 - New directive: %initial-action.
4041 This directive allows the user to run arbitrary code (including
4042 initializing @$) from yyparse before parsing starts.
4044 - A new directive "%expect-rr N" specifies the expected number of
4045 reduce/reduce conflicts in GLR parsers.
4047 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4048 This is a GNU extension.
4050 - The option "--report=lookahead" was changed to "--report=look-ahead".
4051 [However, this was changed back after 2.3.]
4053 - Experimental %destructor support has been added to lalr1.cc.
4055 - New configure option --disable-yacc, to disable installation of the
4056 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4060 - For now, %expect-count violations are now just warnings, not errors.
4061 This is for compatibility with Bison 1.75 and earlier (when there are
4062 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4063 are too many or too few shift/reduce conflicts). However, in future
4064 versions of Bison we plan to improve the %expect machinery so that
4065 these violations will become errors again.
4067 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4068 arbitrarily limited to 16-bit counts.
4070 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4073 * Noteworthy changes in release 1.875 (2003-01-01)
4075 ** The documentation license has been upgraded to version 1.2
4076 of the GNU Free Documentation License.
4078 ** syntax error processing
4080 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4081 locations too. This fixes bugs in error-location computation.
4084 It is now possible to reclaim the memory associated to symbols
4085 discarded during error recovery. This feature is still experimental.
4088 This new directive is preferred over YYERROR_VERBOSE.
4090 - #defining yyerror to steal internal variables is discouraged.
4091 It is not guaranteed to work forever.
4093 ** POSIX conformance
4095 - Semicolons are once again optional at the end of grammar rules.
4096 This reverts to the behavior of Bison 1.33 and earlier, and improves
4097 compatibility with Yacc.
4099 - "parse error" -> "syntax error"
4100 Bison now uniformly uses the term "syntax error"; formerly, the code
4101 and manual sometimes used the term "parse error" instead. POSIX
4102 requires "syntax error" in diagnostics, and it was thought better to
4105 - The documentation now emphasizes that yylex and yyerror must be
4106 declared before use. C99 requires this.
4108 - Bison now parses C99 lexical constructs like UCNs and
4109 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4111 - File names are properly escaped in C output. E.g., foo\bar.y is
4112 output as "foo\\bar.y".
4114 - Yacc command and library now available
4115 The Bison distribution now installs a "yacc" command, as POSIX requires.
4116 Also, Bison now installs a small library liby.a containing
4117 implementations of Yacc-compatible yyerror and main functions.
4118 This library is normally not useful, but POSIX requires it.
4120 - Type clashes now generate warnings, not errors.
4122 - If the user does not define YYSTYPE as a macro, Bison now declares it
4123 using typedef instead of defining it as a macro.
4124 For consistency, YYLTYPE is also declared instead of defined.
4126 ** Other compatibility issues
4128 - %union directives can now have a tag before the "{", e.g., the
4129 directive "%union foo {...}" now generates the C code
4130 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4131 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4132 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4133 This is for compatibility with both Yacc and Bison 1.35.
4135 - ";" is output before the terminating "}" of an action, for
4136 compatibility with Bison 1.35.
4138 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4139 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4141 - "yystype" and "yyltype" are now obsolescent macros instead of being
4142 typedefs or tags; they are no longer documented and are planned to be
4143 withdrawn in a future release.
4148 Users of Bison have to decide how they handle the portability of the
4151 - "parsing stack overflow..." -> "parser stack overflow"
4152 GLR parsers now report "parser stack overflow" as per the Bison manual.
4154 ** %parse-param and %lex-param
4155 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4156 additional context to yyparse and yylex. They suffer from several
4159 - a single argument only can be added,
4160 - their types are weak (void *),
4161 - this context is not passed to ancillary functions such as yyerror,
4162 - only yacc.c parsers support them.
4164 The new %parse-param/%lex-param directives provide a more precise control.
4167 %parse-param {int *nastiness}
4168 %lex-param {int *nastiness}
4169 %parse-param {int *randomness}
4171 results in the following signatures:
4173 int yylex (int *nastiness);
4174 int yyparse (int *nastiness, int *randomness);
4176 or, if both %pure-parser and %locations are used:
4178 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4179 int yyparse (int *nastiness, int *randomness);
4181 ** Bison now warns if it detects conflicting outputs to the same file,
4182 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4183 that command outputs both code and header to foo.h.
4185 ** #line in output files
4186 - --no-line works properly.
4188 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4189 later to be built. This change originally took place a few versions
4190 ago, but nobody noticed until we recently asked someone to try
4191 building Bison with a K&R C compiler.
4194 * Noteworthy changes in release 1.75 (2002-10-14)
4196 ** Bison should now work on 64-bit hosts.
4198 ** Indonesian translation thanks to Tedi Heriyanto.
4201 Fix spurious parse errors.
4204 Some people redefine yyerror to steal yyparse' private variables.
4205 Reenable this trick until an official feature replaces it.
4208 In agreement with POSIX and with other Yaccs, leaving a default
4209 action is valid when $$ is untyped, and $1 typed:
4213 but the converse remains an error:
4217 ** Values of midrule actions
4220 foo: { ... } { $$ = $1; } ...
4222 was incorrectly rejected: $1 is defined in the second midrule
4223 action, and is equal to the $$ of the first midrule action.
4226 * Noteworthy changes in release 1.50 (2002-10-04)
4231 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4232 almost any context-free grammar, ambiguous or not. The new declarations
4233 %dprec and %merge on grammar rules allow parse-time resolution of
4234 ambiguities. Contributed by Paul Hilfinger.
4236 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4237 like the Alpha, so please stick to 32-bit hosts for now.
4240 When not in Yacc compatibility mode, when the output file was not
4241 specified, running "bison foo/bar.y" created "foo/bar.c". It
4242 now creates "bar.c".
4245 The undefined token was systematically mapped to 2 which prevented
4246 the use of 2 by the user. This is no longer the case.
4248 ** Unknown token numbers
4249 If yylex returned an out of range value, yyparse could die. This is
4253 According to POSIX, the error token must be 256.
4254 Bison extends this requirement by making it a preference: *if* the
4255 user specified that one of her tokens is numbered 256, then error
4256 will be mapped onto another number.
4258 ** Verbose error messages
4259 They no longer report "..., expecting error or..." for states where
4260 error recovery is possible.
4263 Defaults to "$end" instead of "$".
4265 ** Error recovery now conforms to documentation and to POSIX
4266 When a Bison-generated parser encounters a syntax error, it now pops
4267 the stack until it finds a state that allows shifting the error
4268 token. Formerly, it popped the stack until it found a state that
4269 allowed some non-error action other than a default reduction on the
4270 error token. The new behavior has long been the documented behavior,
4271 and has long been required by POSIX. For more details, please see
4272 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4273 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4276 Popped tokens and nonterminals are now reported.
4279 Larger grammars are now supported (larger token numbers, larger grammar
4280 size (= sum of the LHS and RHS lengths), larger LALR tables).
4281 Formerly, many of these numbers ran afoul of 16-bit limits;
4282 now these limits are 32 bits on most hosts.
4284 ** Explicit initial rule
4285 Bison used to play hacks with the initial rule, which the user does
4286 not write. It is now explicit, and visible in the reports and
4290 Before, Bison reported the useless rules, but, although not used,
4291 included them in the parsers. They are now actually removed.
4293 ** Useless rules, useless nonterminals
4294 They are now reported, as a warning, with their locations.
4296 ** Rules never reduced
4297 Rules that can never be reduced because of conflicts are now
4300 ** Incorrect "Token not used"
4301 On a grammar such as
4303 %token useless useful
4305 exp: '0' %prec useful;
4307 where a token was used to set the precedence of the last rule,
4308 bison reported both "useful" and "useless" as useless tokens.
4310 ** Revert the C++ namespace changes introduced in 1.31
4311 as they caused too many portability hassles.
4313 ** Default locations
4314 By an accident of design, the default computation of @$ was
4315 performed after another default computation was performed: @$ = @1.
4316 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4317 the computation of @$.
4319 ** Token end-of-file
4320 The token end of file may be specified by the user, in which case,
4321 the user symbol is used in the reports, the graphs, and the verbose
4322 error messages instead of "$end", which remains being the default.
4326 %token MYEOF 0 "end of file"
4329 This old option, which has been broken for ages, is removed.
4332 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4333 Croatian, thanks to Denis Lackovic.
4335 ** Incorrect token definitions
4338 bison used to output
4341 ** Token definitions as enums
4342 Tokens are output both as the traditional #define's, and, provided
4343 the compiler supports ANSI C or is a C++ compiler, as enums.
4344 This lets debuggers display names instead of integers.
4347 In addition to --verbose, bison supports --report=THINGS, which
4348 produces additional information:
4350 complete the core item sets with their closure
4351 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4352 explicitly associate lookahead tokens to items
4354 describe shift/reduce conflicts solving.
4355 Bison used to systematically output this information on top of
4356 the report. Solved conflicts are now attached to their states.
4359 Previous versions don't complain when there is a type clash on
4360 the default action if the rule has a midrule action, such as in:
4368 ** GNU M4 is now required when using Bison.
4371 * Noteworthy changes in release 1.35 (2002-03-25)
4374 Some projects use Bison's C parser with C++ compilers, and define
4375 YYSTYPE as a class. The recent adjustment of C parsers for data
4376 alignment and 64 bit architectures made this impossible.
4378 Because for the time being no real solution for C++ parser
4379 generation exists, kludges were implemented in the parser to
4380 maintain this use. In the future, when Bison has C++ parsers, this
4381 kludge will be disabled.
4383 This kludge also addresses some C++ problems when the stack was
4387 * Noteworthy changes in release 1.34 (2002-03-12)
4389 ** File name clashes are detected
4390 $ bison foo.y -d -o foo.x
4391 fatal error: header and parser would both be named "foo.x"
4393 ** A missing ";" at the end of a rule triggers a warning
4394 In accordance with POSIX, and in agreement with other
4395 Yacc implementations, Bison will mandate this semicolon in the near
4396 future. This eases the implementation of a Bison parser of Bison
4397 grammars by making this grammar LALR(1) instead of LR(2). To
4398 facilitate the transition, this release introduces a warning.
4400 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4401 many portability hassles.
4403 ** DJGPP support added.
4405 ** Fix test suite portability problems.
4408 * Noteworthy changes in release 1.33 (2002-02-07)
4411 Groff could not be compiled for the definition of size_t was lacking
4412 under some conditions.
4418 * Noteworthy changes in release 1.32 (2002-01-23)
4420 ** Fix Yacc output file names
4422 ** Portability fixes
4424 ** Italian, Dutch translations
4427 * Noteworthy changes in release 1.31 (2002-01-14)
4431 ** GNU Gettext and %expect
4432 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4433 Bison dies on incorrect %expectations, we fear there will be
4434 too many bug reports for Gettext, so _for the time being_, %expect
4435 does not trigger an error when the input file is named "plural.y".
4437 ** Use of alloca in parsers
4438 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4439 malloc exclusively. Since 1.29, but was not NEWS'ed.
4441 alloca is used only when compiled with GCC, to avoid portability
4444 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4446 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4447 (as POSIX requires) instead of being undefined.
4450 Bison has always permitted actions such as { $$ = $1 }: it adds the
4451 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4452 is no longer output: one has to write { $$ = $1; }.
4454 ** Better C++ compliance
4455 The output parsers try to respect C++ namespaces.
4456 [This turned out to be a failed experiment, and it was reverted later.]
4459 Fixed bugs when reporting useless nonterminals.
4462 The parsers work properly on 64 bit hosts.
4465 Some calls to strerror resulted in scrambled or missing error messages.
4468 When the number of shift/reduce conflicts is correct, don't issue
4471 ** The verbose report includes the rule line numbers.
4473 ** Rule line numbers are fixed in traces.
4475 ** Swedish translation
4478 Verbose parse error messages from the parsers are better looking.
4479 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4480 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4482 ** Fixed parser memory leaks.
4483 When the generated parser was using malloc to extend its stacks, the
4484 previous allocations were not freed.
4486 ** Fixed verbose output file.
4487 Some newlines were missing.
4488 Some conflicts in state descriptions were missing.
4490 ** Fixed conflict report.
4491 Option -v was needed to get the result.
4495 Mismatches are errors, not warnings.
4497 ** Fixed incorrect processing of some invalid input.
4499 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4501 ** Fixed some typos in the documentation.
4503 ** %token MY_EOF 0 is supported.
4504 Before, MY_EOF was silently renumbered as 257.
4506 ** doc/refcard.tex is updated.
4508 ** %output, %file-prefix, %name-prefix.
4512 New, aliasing "--output-file".
4515 * Noteworthy changes in release 1.30 (2001-10-26)
4517 ** "--defines" and "--graph" have now an optional argument which is the
4518 output file name. "-d" and "-g" do not change; they do not take any
4521 ** "%source_extension" and "%header_extension" are removed, failed
4524 ** Portability fixes.
4527 * Noteworthy changes in release 1.29 (2001-09-07)
4529 ** The output file does not define const, as this caused problems when used
4530 with common autoconfiguration schemes. If you still use ancient compilers
4531 that lack const, compile with the equivalent of the C compiler option
4532 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4534 ** Added "-g" and "--graph".
4536 ** The Bison manual is now distributed under the terms of the GNU FDL.
4538 ** The input and the output files has automatically a similar extension.
4540 ** Russian translation added.
4542 ** NLS support updated; should hopefully be less troublesome.
4544 ** Added the old Bison reference card.
4546 ** Added "--locations" and "%locations".
4548 ** Added "-S" and "--skeleton".
4550 ** "%raw", "-r", "--raw" is disabled.
4552 ** Special characters are escaped when output. This solves the problems
4553 of the #line lines with path names including backslashes.
4556 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4557 "%debug", "%source_extension" and "%header_extension".
4560 Automatic location tracking.
4563 * Noteworthy changes in release 1.28 (1999-07-06)
4565 ** Should compile better now with K&R compilers.
4569 ** Fixed a problem with escaping the double quote character.
4571 ** There is now a FAQ.
4574 * Noteworthy changes in release 1.27
4576 ** The make rule which prevented bison.simple from being created on
4577 some systems has been fixed.
4580 * Noteworthy changes in release 1.26
4582 ** Bison now uses Automake.
4584 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4586 ** Token numbers now start at 257 as previously documented, not 258.
4588 ** Bison honors the TMPDIR environment variable.
4590 ** A couple of buffer overruns have been fixed.
4592 ** Problems when closing files should now be reported.
4594 ** Generated parsers should now work even on operating systems which do
4595 not provide alloca().
4598 * Noteworthy changes in release 1.25 (1995-10-16)
4600 ** Errors in the input grammar are not fatal; Bison keeps reading
4601 the grammar file, and reports all the errors found in it.
4603 ** Tokens can now be specified as multiple-character strings: for
4604 example, you could use "<=" for a token which looks like <=, instead
4605 of choosing a name like LESSEQ.
4607 ** The %token_table declaration says to write a table of tokens (names
4608 and numbers) into the parser file. The yylex function can use this
4609 table to recognize multiple-character string tokens, or for other
4612 ** The %no_lines declaration says not to generate any #line preprocessor
4613 directives in the parser file.
4615 ** The %raw declaration says to use internal Bison token numbers, not
4616 Yacc-compatible token numbers, when token names are defined as macros.
4618 ** The --no-parser option produces the parser tables without including
4619 the parser engine; a project can now use its own parser engine.
4620 The actions go into a separate file called NAME.act, in the form of
4621 a switch statement body.
4624 * Noteworthy changes in release 1.23
4626 The user can define YYPARSE_PARAM as the name of an argument to be
4627 passed into yyparse. The argument should have type void *. It should
4628 actually point to an object. Grammar actions can access the variable
4629 by casting it to the proper pointer type.
4631 Line numbers in output file corrected.
4634 * Noteworthy changes in release 1.22
4636 --help option added.
4639 * Noteworthy changes in release 1.20
4641 Output file does not redefine const for C++.
4645 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4646 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4647 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4648 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4649 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4650 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4651 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4652 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4653 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4654 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4655 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4656 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4657 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4658 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4659 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4660 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4661 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4662 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4663 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4664 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4665 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4666 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4667 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4668 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4669 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4670 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4671 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4672 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4673 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4674 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4675 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4676 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4677 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4678 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4679 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4680 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4681 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4684 ispell-dictionary: "american"
4689 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4691 This file is part of Bison, the GNU Parser Generator.
4693 Permission is granted to copy, distribute and/or modify this document
4694 under the terms of the GNU Free Documentation License, Version 1.3 or
4695 any later version published by the Free Software Foundation; with no
4696 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4697 Texts. A copy of the license is included in the "GNU Free
4698 Documentation License" file as part of this distribution.