3 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
7 *** Counterexample Generation
9 In some cases counterexample generation could crash. This is fixed.
11 *** Fix Table Generation
13 In some very rare conditions, when there are many useless tokens, it was
14 possible to generate incorrect parsers.
16 *** GLR parsers now support %merge together with api.value.type=union.
18 *** C++ parsers use noexcept in more places.
20 *** Generated parsers avoid some warnings about signedness issues.
22 *** C-language parsers now avoid warnings from pedantic clang.
24 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
30 Prefer value_type to semantic_type to denote the semantic value type,
31 specified by the `api.value.type` %define variable.
35 There were not debug traces for deferred calls to user actions. They are
40 *** Option -H, --header and directive %header
42 The option `-H`/`--header` supersedes the option `--defines`, and the
43 directive %header supersedes %defines. Both `--defines` and `%defines`
44 are, of course, maintained for backward compatibility.
48 Since version 2.4 Bison can be used to generate HTML reports. However it
49 was a two-step process: first bison must be invoked with option `--xml`,
50 and then xsltproc must be run to the convert the XML reports into HTML.
52 The new option `--html` combines these steps. The xsltproc program must
55 *** A C++ native GLR parser
57 A new version of the generated C++ GLR parser was added as "glr2.cc". It
58 is forked from the existing glr.c/cc parser, with the objective of making
59 it a more modern, truly C++ parser (instead of a C++ wrapper around a C
60 parser). Down the line, the goal is to support `%define api.value.type
61 variant` and maybe share code with lalr1.cc.
63 The current parser should be identical in terms of interface, functionality
64 and performance to "glr.cc". To try it out, simply use
70 Counterexamples now show the rule numbers, and always show ε for rules
71 with an empty right-hand side. For instance
83 *** Lookahead correction in Java
85 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
88 *** Abort parsing for memory exhaustion (C)
90 The user actions may now use YYNOMEM to abort the current parse with
94 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
98 *** Bug fixes in yacc.c
100 In Yacc mode, all the tokens are defined twice: once as an enum, and then
101 as a macro. YYEMPTY was missing its macro.
103 *** Bug fixes in lalr1.cc
105 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
106 even when the `parse.assert` %define variable is not enabled. It no
109 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
112 When there is a very large number of tokens, some assertions could be long
113 enough to hit arbitrary limits in Visual C++. They have been rewritten to
114 work around this limitation.
118 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
119 skeleton) used to be defined to 1. It is now defined to the version of
120 Bison as an integer (e.g., 30704 for version 3.7.4).
123 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
127 Fix concurrent build issues.
129 The bison executable is no longer linked uselessly against libreadline.
131 Fix incorrect use of yytname in glr.cc.
134 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
136 This release of Bison fixes all known bugs reported for Bison in MITRE's
137 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
138 are only about bison-the-program itself, not the generated code.
140 Although these bugs are typically irrelevant to how Bison is used, they
141 are worth fixing if only to give users peace of mind.
143 There is no known vulnerability in the generated parsers.
147 Fix concurrent build issues (introduced in Bison 3.5).
149 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
151 Fix portability issues of the test suite, and of bison itself.
153 Some unlikely crashes found by fuzzing have been fixed. This is only
154 about bison itself, not the generated parsers.
157 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
161 Crash when a token alias contains a NUL byte.
163 Portability issues with libtextstyle.
165 Portability issues of Bison itself with MSVC.
169 Improvements and fixes in the documentation.
171 More precise location about symbol type redefinitions.
174 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
176 ** Deprecated features
178 The YYPRINT macro, which works only with yacc.c and only for tokens, was
179 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
180 It is deprecated and its support will be removed eventually.
182 In conformance with the recommendations of the Graphviz team, in the next
183 version Bison the option `--graph` will generate a *.gv file by default,
184 instead of *.dot. A transition started in Bison 3.4.
188 *** Counterexample Generation
190 Contributed by Vincent Imbimbo.
192 When given `-Wcounterexamples`/`-Wcex`, bison will now output
193 counterexamples for conflicts.
195 **** Unifying Counterexamples
197 Unifying counterexamples are strings which can be parsed in two ways due
198 to the conflict. For example on a grammar that contains the usual
199 "dangling else" ambiguity:
202 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
203 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
206 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
207 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
208 Example: "if" exp "then" "if" exp "then" exp • "else" exp
211 ↳ "if" exp "then" exp
212 ↳ "if" exp "then" exp • "else" exp
213 Example: "if" exp "then" "if" exp "then" exp • "else" exp
216 ↳ "if" exp "then" exp "else" exp
217 ↳ "if" exp "then" exp •
219 When text styling is enabled, colors are used in the examples and the
220 derivations to highlight the structure of both analyses. In this case,
222 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
226 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
229 The counterexamples are "focused", in two different ways. First, they do
230 not clutter the output with all the derivations from the start symbol,
231 rather they start on the "conflicted nonterminal". They go straight to the
232 point. Second, they don't "expand" nonterminal symbols uselessly.
234 **** Nonunifying Counterexamples
236 In the case of the dangling else, Bison found an example that can be
237 parsed in two ways (therefore proving that the grammar is ambiguous).
238 When it cannot find such an example, it instead generates two examples
239 that are the same up until the dot:
242 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
243 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
244 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
249 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
250 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
251 First example: expr • ID ',' ID $end
258 Second example: expr • ID $end
264 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
268 In these cases, the parser usually doesn't have enough lookahead to
269 differentiate the two given examples.
273 Counterexamples are also included in the report when given
274 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
279 1 exp: "if" exp "then" exp • [$end, "then", "else"]
280 2 | "if" exp "then" exp • "else" exp
282 "else" shift, and go to state 8
284 "else" [reduce using rule 1 (exp)]
285 $default reduce using rule 1 (exp)
287 shift/reduce conflict on token "else":
288 1 exp: "if" exp "then" exp •
289 2 exp: "if" exp "then" exp • "else" exp
290 Example: "if" exp "then" "if" exp "then" exp • "else" exp
293 ↳ "if" exp "then" exp
294 ↳ "if" exp "then" exp • "else" exp
295 Example: "if" exp "then" "if" exp "then" exp • "else" exp
298 ↳ "if" exp "then" exp "else" exp
299 ↳ "if" exp "then" exp •
301 *** File prefix mapping
303 Contributed by Joshua Watt.
305 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
306 in the output (specifically `#line` directives and `#ifdef` header guards)
307 that begins with the prefix OLD will have it replaced with the prefix NEW,
308 similar to the `-ffile-prefix-map` in GCC. This option can be used to
309 make bison output reproducible.
315 When text styling is enabled and the terminal supports it, the warnings
316 now include hyperlinks to the documentation.
318 *** Relocatable installation
320 When installed to be relocatable (via `configure --enable-relocatable`),
321 bison will now also look for a relocated m4.
325 The `filename_type` %define variable was renamed `api.filename.type`.
328 %define filename_type "symbol"
332 %define api.filename.type {symbol}
334 (Or let `bison --update` do it for you).
336 It now defaults to `const std::string` instead of `std::string`.
338 *** Deprecated %define variable names
340 The following variables have been renamed for consistency. Backward
341 compatibility is ensured, but upgrading is recommended.
343 filename_type -> api.filename.type
344 package -> api.package
346 *** Push parsers no longer clear their state when parsing is finished
348 Previously push-parsers cleared their state when parsing was finished (on
349 success and on failure). This made it impossible to check if there were
350 parse errors, since `yynerrs` was also reset. This can be especially
351 troublesome when used in autocompletion, since a parser with error
352 recovery would suggest (irrelevant) expected tokens even if there were
355 Now the parser state can be examined when parsing is finished. The parser
356 state is reset when starting a new parse.
362 The bistromathic demonstrates %param and how to quote sources in the error
366 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
372 *** Include the generated header (yacc.c)
374 Historically, when --defines was used, bison generated a header and pasted
375 an exact copy of it into the generated parser implementation file. Since
376 Bison 3.4 it is possible to specify that the header should be `#include`d,
377 and how. For instance
379 %define api.header.include {"parse.h"}
383 %define api.header.include {<parser/parse.h>}
385 Now api.header.include defaults to `"header-basename"`, as was intended in
386 Bison 3.4, where `header-basename` is the basename of the generated
387 header. This is disabled when the generated header is `y.tab.h`, to
388 comply with Automake's ylwrap.
390 *** String aliases are faithfully propagated
392 Bison used to interpret user strings (i.e., decoding backslash escapes)
393 when reading them, and to escape them (i.e., issue non-printable
394 characters as backslash escapes, taking the locale into account) when
395 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
396 up "ciphered" as sequences of backslash escapes. This happened not only
397 in the generated sources (where the compiler will reinterpret them), but
398 also in all the generated reports (text, xml, html, dot, etc.). Reports
399 were therefore not readable when string aliases were not pure ASCII.
400 Worse yet: the output depended on the user's locale.
402 Now Bison faithfully treats the string aliases exactly the way the user
403 spelled them. This fixes all the aforementioned problems. However, now,
404 string aliases semantically equivalent but syntactically different (e.g.,
405 "A", "\x41", "\101") are considered to be different.
407 *** Crash when generating IELR
409 An old, well hidden, bug in the generation of IELR parsers was fixed.
412 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
416 In glr.cc some internal macros leaked in the user's code, and could damage
417 access to the token kinds.
420 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
424 Incorrect comments in the generated parsers.
426 Warnings in push parsers (yacc.c).
428 Incorrect display of gotos in LAC traces (lalr1.cc).
431 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
435 Some tests were fixed.
437 When token aliases contain comment delimiters:
439 %token FOO "/* foo */"
441 bison used to emit "nested" comments, which is invalid C.
444 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
448 Restored ANSI-C compliance in yacc.c.
450 GNU readline portability issues.
452 In C++, yy::parser::symbol_name is now a public member, as was intended.
456 In C++, yy::parser::symbol_type now has a public name() member function.
459 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
461 ** Backward incompatible changes
463 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
465 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
466 depend on it will now produce Yacc-like error messages (just "syntax
467 error"). It was superseded by the "%error-verbose" directive in Bison
468 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
469 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
470 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
471 parse.error verbose".
473 ** Deprecated features
475 The YYPRINT macro, which works only with yacc.c and only for tokens, was
476 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
477 It is deprecated and its support will be removed eventually.
481 *** Improved syntax error messages
483 Two new values for the %define parse.error variable offer more control to
484 the user. Available in all the skeletons (C, C++, Java).
486 **** %define parse.error detailed
488 The behavior of "%define parse.error detailed" is closely resembling that
489 of "%define parse.error verbose" with a few exceptions. First, it is safe
490 to use non-ASCII characters in token aliases (with 'verbose', the result
491 depends on the locale with which bison was run). Second, a yysymbol_name
492 function is exposed to the user, instead of the yytnamerr function and the
493 yytname table. Third, token internationalization is supported (see
496 **** %define parse.error custom
498 With this directive, the user forges and emits the syntax error message
499 herself by defining the yyreport_syntax_error function. A new type,
500 yypcontext_t, captures the circumstances of the error, and provides the
501 user with functions to get details, such as yypcontext_expected_tokens to
502 get the list of expected token kinds.
504 A possible implementation of yyreport_syntax_error is:
507 yyreport_syntax_error (const yypcontext_t *ctx)
510 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
511 fprintf (stderr, ": syntax error");
512 // Report the tokens expected at this point.
514 enum { TOKENMAX = 10 };
515 yysymbol_kind_t expected[TOKENMAX];
516 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
518 // Forward errors to yyparse.
521 for (int i = 0; i < n; ++i)
522 fprintf (stderr, "%s %s",
523 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
525 // Report the unexpected token.
527 yysymbol_kind_t lookahead = yypcontext_token (ctx);
528 if (lookahead != YYSYMBOL_YYEMPTY)
529 fprintf (stderr, " before %s", yysymbol_name (lookahead));
531 fprintf (stderr, "\n");
535 **** Token aliases internationalization
537 When the %define variable parse.error is set to `custom` or `detailed`,
538 one may specify which token aliases are to be translated using _(). For
550 In that case the user must define _() and N_(), and yysymbol_name returns
551 the translated symbol (i.e., it returns '_("variable")' rather that
552 '"variable"'). In Java, the user must provide an i18n() function.
554 *** List of expected tokens (yacc.c)
556 Push parsers may invoke yypstate_expected_tokens at any point during
557 parsing (including even before submitting the first token) to get the list
558 of possible tokens. This feature can be used to propose autocompletion
559 (see below the "bistromathic" example).
561 It makes little sense to use this feature without enabling LAC (lookahead
564 *** Returning the error token
566 When the scanner returns an invalid token or the undefined token
567 (YYUNDEF), the parser generates an error message and enters error
568 recovery. Because of that error message, most scanners that find lexical
569 errors generate an error message, and then ignore the invalid input
570 without entering the error-recovery.
572 The scanners may now return YYerror, the error token, to enter the
573 error-recovery mode without triggering an additional error message. See
574 the bistromathic for an example.
576 *** Deep overhaul of the symbol and token kinds
578 To avoid the confusion with types in programming languages, we now refer
579 to token and symbol "kinds" instead of token and symbol "types". The
580 documentation and error messages have been revised.
582 All the skeletons have been updated to use dedicated enum types rather
583 than integral types. Special symbols are now regular citizens, instead of
584 being declared in ad hoc ways.
588 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
589 LPAREN, etc. While backward compatibility is of course ensured, users are
590 nonetheless invited to replace their uses of "enum yytokentype" by
593 This type now also includes tokens that were previously hidden: YYEOF (end
594 of input), YYUNDEF (undefined token), and YYerror (error token). They
595 now have string aliases, internationalized when internationalization is
596 enabled. Therefore, by default, error messages now refer to "end of file"
597 (internationalized) rather than the cryptic "$end", or to "invalid token"
598 rather than "$undefined".
600 Therefore in most cases it is now useless to define the end-of-line token
603 %token T_EOF 0 "end of file"
605 Rather simply use "YYEOF" in your scanner.
609 The "symbol kinds" is what the parser actually uses. (Unless the
610 api.token.raw %define variable is used, the symbol kind of a terminal
611 differs from the corresponding token kind.)
613 They are now exposed as a enum, "yysymbol_kind_t".
615 This allows users to tailor the error messages the way they want, or to
616 process some symbols in a specific way in autocompletion (see the
617 bistromathic example below).
619 *** Modernize display of explanatory statements in diagnostics
621 Since Bison 2.7, output was indented four spaces for explanatory
622 statements. For example:
624 input.y:2.7-13: error: %type redeclaration for exp
625 input.y:1.7-11: previous declaration
627 Since the introduction of caret-diagnostics, it became less clear. This
628 indentation has been removed and submessages are displayed similarly as in
631 input.y:2.7-13: error: %type redeclaration for exp
632 2 | %type <float> exp
634 input.y:1.7-11: note: previous declaration
638 Contributed by Victor Morales Cayuela.
642 The token and symbol kinds are yy::parser::token_kind_type and
643 yy::parser::symbol_kind_type.
645 The symbol_type::kind() member function allows to get the kind of a
646 symbol. This can be used to write unit tests for scanners, e.g.,
648 yy::parser::symbol_type t = make_NUMBER ("123");
649 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
650 assert (t.value.as<int> () == 123);
656 In order to avoid ambiguities with "type" as in "typing", we now refer to
657 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
658 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
663 There are now examples/java: a very simple calculator, and a more complete
664 one (push-parser, location tracking, and debug traces).
666 The lexcalc example (a simple example in C based on Flex and Bison) now
667 also demonstrates location tracking.
670 A new C example, bistromathic, is a fully featured interactive calculator
671 using many Bison features: pure interface, push parser, autocompletion
672 based on the current parser state (using yypstate_expected_tokens),
673 location tracking, internationalized custom error messages, lookahead
674 correction, rich debug traces, etc.
676 It shows how to depend on the symbol kinds to tailor autocompletion. For
677 instance it recognizes the symbol kind "VARIABLE" to propose
678 autocompletion on the existing variables, rather than of the word
682 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
684 ** WARNING: Future backward-incompatibilities!
686 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
688 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
689 that still depend on it will produce Yacc-like error messages (just
690 "syntax error"). It was superseded by the "%error-verbose" directive in
691 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
692 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
693 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
694 parse.error verbose".
698 Fix portability issues of the package itself on old compilers.
700 Fix api.token.raw support in Java.
703 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
707 Error messages could quote lines containing zero-width characters (such as
708 \005) with incorrect styling. Fixes for similar issues with unexpectedly
709 short lines (e.g., the file was changed between parsing and diagnosing).
711 Some unlikely crashes found by fuzzing have been fixed. This is only
712 about bison itself, not the generated parsers.
715 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
719 Portability issues and minor cosmetic issues.
721 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
725 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
731 Fix compiler warnings.
734 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
736 ** Backward incompatible changes
738 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
739 longer treated as end-of-lines. This changes the diagnostics, and in
740 particular their locations.
742 In C++, line numbers and columns are now represented as 'int' not
743 'unsigned', so that integer overflow on positions is easily checkable via
744 'gcc -fsanitize=undefined' and the like. This affects the API for
745 positions. The default position and location classes now expose
746 'counter_type' (int), used to define line and column numbers.
748 ** Deprecated features
750 The YYPRINT macro, which works only with yacc.c and only for tokens, was
751 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
752 It is deprecated and its support will be removed eventually.
756 *** Lookahead correction in C++
758 Contributed by Adrian Vogelsgesang.
760 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
761 %define variable parse.lac.
763 *** Variable api.token.raw: Optimized token numbers (all skeletons)
765 In the generated parsers, tokens have two numbers: the "external" token
766 number as returned by yylex (which starts at 257), and the "internal"
767 symbol number (which starts at 3). Each time yylex is called, a table
768 lookup maps the external token number to the internal symbol number.
770 When the %define variable api.token.raw is set, tokens are assigned their
771 internal number, which saves one table lookup per token, and also saves
772 the generation of the mapping table.
774 The gain is typically moderate, but in extreme cases (very simple user
775 actions), a 10% improvement can be observed.
777 *** Generated parsers use better types for states
779 Stacks now use the best integral type for state numbers, instead of always
780 using 15 bits. As a result "small" parsers now have a smaller memory
781 footprint (they use 8 bits), and there is support for large automata (16
782 bits), and extra large (using int, i.e., typically 31 bits).
784 *** Generated parsers prefer signed integer types
786 Bison skeletons now prefer signed to unsigned integer types when either
787 will do, as the signed types are less error-prone and allow for better
788 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
789 portable to unusual machines where char, short and int are all the same
790 width. On non-GNU platforms this may entail including <limits.h> and (if
791 available) <stdint.h> to define integer types and constants.
793 *** A skeleton for the D programming language
795 For the last few releases, Bison has shipped a stealth experimental
796 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
797 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
800 However, because nobody has committed to improving, testing, and
801 documenting this skeleton, it is not clear that it will be supported in
804 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
805 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
807 *** Debug traces in Java
809 The Java backend no longer emits code and data for parser tracing if the
810 %define variable parse.trace is not defined.
814 *** New diagnostic: -Wdangling-alias
816 String literals, which allow for better error messages, are (too)
817 liberally accepted by Bison, which might result in silent errors. For
820 %type <exVal> cond "condition"
822 does not define "condition" as a string alias to 'cond' (nonterminal
823 symbols do not have string aliases). It is rather equivalent to
826 %token <exVal> "condition"
828 i.e., it gives the type 'exVal' to the "condition" token, which was
829 clearly not the intention.
831 Also, because string aliases need not be defined, typos such as "baz"
832 instead of "bar" will be not reported.
834 The option `-Wdangling-alias` catches these situations. On
837 %type <ival> foo "foo"
841 bison -Wdangling-alias reports
843 warning: string literal not attached to a symbol
844 | %type <ival> foo "foo"
846 warning: string literal not attached to a symbol
850 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
852 *** Better POSIX Yacc compatibility diagnostics
854 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
858 %type <ival> TOKEN1 TOKEN2 't'
865 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
866 2 | %type <ival> TOKEN1 TOKEN2 't'
868 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
869 2 | %type <ival> TOKEN1 TOKEN2 't'
871 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
872 2 | %type <ival> TOKEN1 TOKEN2 't'
875 *** Diagnostics with insertion
877 The diagnostics now display the suggestion below the underlined source.
878 Replacement for undeclared symbols are now also suggested.
885 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'?
889 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
893 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
895 *** Diagnostics about long lines
897 Quoted sources may now be truncated to fit the screen. For instance, on a
898 30-column wide terminal:
905 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
908 foo.y:1.8-10: previous declaration
911 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
914 foo.y:1.8-10: previous declaration
920 *** Debugging glr.c and glr.cc
922 The glr.c skeleton always had asserts to check its own behavior (not the
923 user's). These assertions are now under the control of the parse.assert
924 %define variable (disabled by default).
928 Several new compiler warnings in the generated output have been avoided.
929 Some unused features are no longer emitted. Cleaner generated code in
934 Portability issues in the test suite.
936 In theory, parsers using %nonassoc could crash when reporting verbose
937 error messages. This unlikely bug has been fixed.
939 In Java, %define api.prefix was ignored. It now behaves as expected.
942 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
946 In some cases, when warnings are disabled, bison could emit tons of white
947 spaces as diagnostics.
949 When running out of memory, bison could crash (found by fuzzing).
951 When defining twice the EOF token, bison would crash.
953 New warnings from recent compilers have been addressed in the generated
954 parsers (yacc.c, glr.c, glr.cc).
956 When lone carriage-return characters appeared in the input file,
957 diagnostics could hang forever.
960 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
967 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
969 ** Deprecated features
971 The %pure-parser directive is deprecated in favor of '%define api.pure'
972 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
973 now. Note that since Bison 2.7 you are strongly encouraged to use
974 '%define api.pure full' instead of '%define api.pure'.
978 *** Colored diagnostics
980 As an experimental feature, diagnostics are now colored, controlled by the
981 new options --color and --style.
983 To use them, install the libtextstyle library before configuring Bison.
986 https://alpha.gnu.org/gnu/gettext/
990 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
992 The option --color supports the following arguments:
993 - always, yes: Enable colors.
994 - never, no: Disable colors.
995 - auto, tty (default): Enable colors if the output device is a tty.
997 To customize the styles, create a CSS file similar to
1001 .error { font-weight: 800; text-decoration: underline; }
1004 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1005 environment variable to "bison-bw.css".
1007 *** Disabling output
1009 When given -fsyntax-only, the diagnostics are reported, but no output is
1012 The name of this option is somewhat misleading as bison does more than
1013 just checking the syntax: every stage is run (including checking for
1014 conflicts for instance), except the generation of the output files.
1016 *** Include the generated header (yacc.c)
1018 Before, when --defines is used, bison generated a header, and pasted an
1019 exact copy of it into the generated parser implementation file. If the
1020 header name is not "y.tab.h", it is now #included instead of being
1023 To use an '#include' even if the header name is "y.tab.h" (which is what
1024 happens with --yacc, or when using the Autotools' ylwrap), define
1025 api.header.include to the exact argument to pass to #include. For
1028 %define api.header.include {"parse.h"}
1032 %define api.header.include {<parser/parse.h>}
1034 *** api.location.type is now supported in C (yacc.c, glr.c)
1036 The %define variable api.location.type defines the name of the type to use
1037 for locations. When defined, Bison no longer defines YYLTYPE.
1039 This can be used in programs with several parsers to factor their
1040 definition of locations: let one of them generate them, and the others
1047 In conformance with the recommendations of the Graphviz team, if %require
1048 "3.4" (or better) is specified, the option --graph generates a *.gv file
1049 by default, instead of *.dot.
1051 *** Diagnostics overhaul
1053 Column numbers were wrong with multibyte characters, which would also
1054 result in skewed diagnostics with carets. Beside, because we were
1055 indenting the quoted source with a single space, lines with tab characters
1056 were incorrectly underlined.
1058 To address these issues, and to be clearer, Bison now issues diagnostics
1059 as GCC9 does. For instance it used to display (there's a tab before the
1062 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1063 expr: expr '+' "number" { $$ = $1 + $2; }
1067 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1068 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1071 Other constructs now also have better locations, resulting in more precise
1074 *** Fix-it hints for %empty
1076 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1077 annotations, and add the missing ones.
1079 *** Generated reports
1081 The format of the reports (parse.output) was improved for readability.
1083 *** Better support for --no-line.
1085 When --no-line is used, the generated files are now cleaner: no lines are
1086 generated instead of empty lines. Together with using api.header.include,
1087 that should help people saving the generated files into version control
1088 systems get smaller diffs.
1092 A new example in C shows an simple infix calculator with a hand-written
1093 scanner (examples/c/calc).
1095 A new example in C shows a reentrant parser (capable of recursive calls)
1096 built with Flex and Bison (examples/c/reccalc).
1098 There is a new section about the history of Yaccs and Bison.
1102 A few obscure bugs were fixed, including the second oldest (known) bug in
1103 Bison: it was there when Bison was entered in the RCS version control
1104 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1108 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1112 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1116 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1120 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1121 extensions into errors. It now makes them simple warnings (-Wyacc).
1124 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1126 A new mailing list was created, Bison Announce. It is low traffic, and is
1127 only about announcing new releases and important messages (e.g., polls
1128 about major decisions to make).
1130 https://lists.gnu.org/mailman/listinfo/bison-announce
1132 ** Backward incompatible changes
1134 Support for DJGPP, which has been unmaintained and untested for years, is
1137 ** Deprecated features
1139 A new feature, --update (see below) helps adjusting existing grammars to
1142 *** Deprecated directives
1144 The %error-verbose directive is deprecated in favor of '%define
1145 parse.error verbose' since Bison 3.0, but no warning was issued.
1147 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1148 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1149 directives are slightly different, you might need to adjust your code.
1150 %name-prefix renames only symbols with external linkage, while api.prefix
1151 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1152 yytokentype, YYSTYPE, YYLTYPE, etc.
1154 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1155 {xx}' will typically have to update YY_DECL from
1157 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1161 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1163 *** Deprecated %define variable names
1165 The following variables, mostly related to parsers in Java, have been
1166 renamed for consistency. Backward compatibility is ensured, but upgrading
1169 abstract -> api.parser.abstract
1170 annotations -> api.parser.annotations
1171 extends -> api.parser.extends
1172 final -> api.parser.final
1173 implements -> api.parser.implements
1174 parser_class_name -> api.parser.class
1175 public -> api.parser.public
1176 strictfp -> api.parser.strictfp
1180 *** Generation of fix-its for IDEs/Editors
1182 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1183 bison now generates machine readable editing instructions to fix some
1184 issues. Currently, this is mostly limited to updating deprecated
1185 directives and removing duplicates. For instance:
1189 %define parser_class_name "Parser"
1190 %define api.parser.class "Parser"
1194 See the "fix-it:" lines below:
1196 $ bison -ffixit foo.y
1197 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1200 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1201 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1202 %define parser_class_name "Parser"
1203 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1204 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1205 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1206 %define api.parser.class "Parser"
1207 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1208 foo.y:2.1-34: previous definition
1209 %define parser_class_name "Parser"
1210 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1211 fix-it:"foo.y":{3:1-3:34}:""
1212 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1214 This uses the same output format as GCC and Clang.
1216 *** Updating grammar files
1218 Fixes can be applied on the fly. The previous example ends with the
1219 suggestion to re-run bison with the option -u/--update, which results in a
1220 cleaner grammar file.
1222 $ bison --update foo.y
1224 bison: file 'foo.y' was updated (backup: 'foo.y~')
1227 %define parse.error verbose
1228 %define api.parser.class {Parser}
1232 *** Bison is now relocatable
1234 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1236 A relocatable program can be moved or copied to a different location on
1237 the file system. It can also be used through mount points for network
1238 sharing. It is possible to make symbolic links to the installed and moved
1239 programs, and invoke them through the symbolic link.
1241 *** %expect and %expect-rr modifiers on individual rules
1243 One can now document (and check) which rules participate in shift/reduce
1244 and reduce/reduce conflicts. This is particularly important GLR parsers,
1245 where conflicts are a normal occurrence. For example,
1261 | argument_list ',' expression
1266 Looking at the output from -v, one can see that the shift/reduce conflict
1267 here is due to the fact that the parser does not know whether to reduce
1268 arguments to argument_list until it sees the token _after_ the following
1269 ','. By marking the rule with %expect 1 (because there is a conflict in
1270 one state), we document the source of the 1 overall shift/reduce conflict.
1272 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1273 conflicts. In this case, we mark each of the conflicting rules. For
1282 target_list '=' expr ';'
1288 | target ',' target_list
1297 | expr ',' expr_list
1305 In a statement such as
1309 the parser must reduce x to a target or an expr, but does not know which
1310 until it sees the '='. So we notate the two possible reductions to
1311 indicate that each conflicts in one rule.
1313 This feature needs user feedback, and might evolve in the future.
1315 *** C++: Actual token constructors
1317 When variants and token constructors are enabled, in addition to the
1318 type-safe named token constructors (make_ID, make_INT, etc.), we now
1319 generate genuine constructors for symbol_type.
1321 For instance with these declarations
1327 you may use these constructors:
1329 symbol_type (int token, const std::string&);
1330 symbol_type (int token, const int&);
1331 symbol_type (int token);
1333 Correct matching between token types and value types is checked via
1334 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1335 constructors are preferable, as they offer better type safety (for
1336 instance 'make_ID (42)' would not even compile), but symbol_type
1337 constructors may help when token types are discovered at run-time, e.g.,
1340 if (auto i = lookup_keyword (yytext))
1341 return yy::parser::symbol_type (i);
1343 return yy::parser::make_ID (yytext);
1346 *** C++: Variadic emplace
1348 If your application requires C++11 and you don't use symbol constructors,
1349 you may now use a variadic emplace for semantic values:
1351 %define api.value.type variant
1352 %token <std::pair<int, int>> PAIR
1356 int yylex (parser::semantic_type *lvalp)
1358 lvalp->emplace <std::pair<int, int>> (1, 2);
1359 return parser::token::PAIR;
1362 *** C++: Syntax error exceptions in GLR
1364 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1365 actions, or from the scanner.
1367 *** More POSIX Yacc compatibility warnings
1369 More Bison specific directives are now reported with -y or -Wyacc. This
1370 change was ready since the release of Bison 3.0 in September 2015. It was
1371 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1372 in numerous warnings for Bison users that use the GNU Build System.
1374 If you still experience that problem, either redefine YACC as `bison -o
1375 y.tab.c`, or pass -Wno-yacc to Bison.
1377 *** The tables yyrhs and yyphrs are back
1379 Because no Bison skeleton uses them, these tables were removed (no longer
1380 passed to the skeletons, not even computed) in 2008. However, some users
1381 have expressed interest in being able to use them in their own skeletons.
1385 *** Incorrect number of reduce/reduce conflicts
1387 On a grammar such as
1389 exp: "num" | "num" | "num"
1391 bison used to report a single RR conflict, instead of two. This is now
1392 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1393 was entered in the RCS version control system, in December 1987.
1395 Some grammar files might have to adjust their %expect-rr.
1397 *** Parser directives that were not careful enough
1399 Passing invalid arguments to %nterm, for instance character literals, used
1400 to result in unclear error messages.
1404 The examples/ directory (installed in .../share/doc/bison/examples) has
1405 been restructured per language for clarity. The examples come with a
1406 README and a Makefile. Not only can they be used to toy with Bison, they
1407 can also be starting points for your own grammars.
1409 There is now a Java example, and a simple example in C based on Flex and
1410 Bison (examples/c/lexcalc/).
1416 They now use noexcept and constexpr. Please, report missing annotations.
1418 *** Symbol Declarations
1420 The syntax of the variation directives to declare symbols was overhauled
1421 for more consistency, and also better POSIX Yacc compliance (which, for
1422 instance, allows "%type" without actually providing a type). The %nterm
1423 directive, supported by Bison since its inception, is now documented and
1424 officially supported.
1426 The syntax is now as follows:
1428 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1429 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1430 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1431 %nterm TAG? ID+ ( TAG ID+ )*
1433 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1434 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1435 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1436 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1437 one), ‘*’ (zero or more) and ‘+’ (one or more).
1440 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1444 Fix the move constructor of symbol_type.
1446 Always provide a copy constructor for symbol_type, even in modern C++.
1449 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1453 Properly support token constructors in C++ with types that include commas
1454 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1457 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1461 C++ portability issues.
1464 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1468 Several portability issues have been fixed in the build system, in the
1469 test suite, and in the generated parsers in C++.
1472 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1474 ** Backward incompatible changes
1476 Support for DJGPP, which has been unmaintained and untested for years, is
1477 obsolete. Unless there is activity to revive it, it will be removed.
1481 %printers should use yyo rather than yyoutput to denote the output stream.
1483 Variant-based symbols in C++ should use emplace() rather than build().
1485 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1489 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1491 A comment in the generated code now emphasizes that users should not
1492 depend upon non-documented implementation details, such as macros starting
1497 *** C++: Support for move semantics (lalr1.cc)
1499 The lalr1.cc skeleton now fully supports C++ move semantics, while
1500 maintaining compatibility with C++98. You may now store move-only types
1501 when using Bison's variants. For instance:
1508 %skeleton "lalr1.cc"
1509 %define api.value.type variant
1513 %token <int> INT "int";
1514 %type <std::unique_ptr<int>> int;
1515 %type <std::vector<std::unique_ptr<int>>> list;
1519 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1521 int: "int" { $$ = std::make_unique<int>($1); }
1523 *** C++: Implicit move of right-hand side values (lalr1.cc)
1525 In modern C++ (C++11 and later), you should always use 'std::move' with
1526 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1527 popped from the stack anyway. Using 'std::move' is mandatory for
1528 move-only types such as unique_ptr, and it provides a significant speedup
1529 for large types such as std::string, or std::vector, etc.
1531 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1532 by 'std::move ($n)'. The second rule in the previous grammar can be
1535 list: list int { $$ = $1; $$.emplace_back($2); }
1537 With automove enabled, the semantic values are no longer lvalues, so do
1538 not use the swap idiom:
1540 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1542 This idiom is anyway obsolete: it is preferable to move than to swap.
1544 A warning is issued when automove is enabled, and a value is used several
1547 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1548 exp: "twice" exp { $$ = $2 + $2; }
1551 Enabling api.value.automove does not require support for modern C++. The
1552 generated code is valid C++98/03, but will use copies instead of moves.
1554 The new examples/c++/variant-11.yy shows these features in action.
1556 *** C++: The implicit default semantic action is always run
1558 When variants are enabled, the default action was not run, so
1566 It now behaves like in all the other cases, as
1568 exp: "number" { $$ = $1; }
1570 possibly using std::move if automove is enabled.
1572 We do not expect backward compatibility issues. However, beware of
1573 forward compatibility issues: if you rely on default actions with
1574 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1575 generate incorrect parsers.
1577 *** C++: Renaming location.hh
1579 When both %defines and %locations are enabled, Bison generates a
1580 location.hh file. If you don't use locations outside of the parser, you
1581 may avoid its creation with:
1583 %define api.location.file none
1585 However this file is useful if, for instance, your parser builds an AST
1586 decorated with locations: you may use Bison's location independently of
1587 Bison's parser. You can now give it another name, for instance:
1589 %define api.location.file "my-location.hh"
1591 This name can have directory components, and even be absolute. The name
1592 under which the location file is included is controlled by
1593 api.location.include.
1595 This way it is possible to have several parsers share the same location
1598 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1601 %define api.namespace {foo}
1602 %define api.location.file "include/ast/loc.hh"
1603 %define api.location.include {<ast/loc.hh>}
1605 and use it in src/bar/parser.hh:
1608 %define api.namespace {bar}
1609 %code requires {#include <ast/loc.hh>}
1610 %define api.location.type {bar::location}
1612 Absolute file names are supported, so in your Makefile, passing the flag
1613 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1616 *** C++: stack.hh and position.hh are deprecated
1618 When asked to generate a header file (%defines), the lalr1.cc skeleton
1619 generates a stack.hh file. This file had no interest for users; it is now
1620 made useless: its content is included in the parser definition. It is
1621 still generated for backward compatibility.
1623 When in addition to %defines, location support is requested (%locations),
1624 the file position.hh is also generated. It is now also useless: its
1625 content is now included in location.hh.
1627 These files are no longer generated when your grammar file requires at
1628 least Bison 3.2 (%require "3.2").
1632 Portability issues on MinGW and VS2015.
1634 Portability issues in the test suite.
1636 Portability/warning issues with Flex.
1639 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1641 ** Backward incompatible changes
1643 Compiling Bison now requires a C99 compiler---as announced during the
1644 release of Bison 3.0, five years ago. Generated parsers do not require a
1647 Support for DJGPP, which has been unmaintained and untested for years, is
1648 obsolete. Unless there is activity to revive it, the next release of Bison
1649 will have it removed.
1653 *** Typed midrule actions
1655 Because their type is unknown to Bison, the values of midrule actions are
1656 not treated like the others: they don't have %printer and %destructor
1657 support. It also prevents C++ (Bison) variants to handle them properly.
1659 Typed midrule actions address these issues. Instead of:
1661 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1665 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1667 *** Reports include the type of the symbols
1669 The sections about terminal and nonterminal symbols of the '*.output' file
1670 now specify their declared type. For instance, for:
1674 the report now shows '<ival>':
1676 Terminals, with rules where they appear
1680 *** Diagnostics about useless rules
1682 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1683 of course, its rules are useless too.
1687 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1689 Previously all the useless rules were reported, including those whose
1690 left-hand side is the 'exp' nonterminal:
1692 warning: 1 nonterminal useless in grammar [-Wother]
1693 warning: 4 rules useless in grammar [-Wother]
1694 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1697 2.14-16: warning: rule useless in grammar [-Wother]
1700 3.6-16: warning: rule useless in grammar [-Wother]
1701 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1703 3.20-30: warning: rule useless in grammar [-Wother]
1704 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1706 3.34-44: warning: rule useless in grammar [-Wother]
1707 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1710 Now, rules whose left-hand side symbol is useless are no longer reported
1711 as useless. The locations of the errors have also been adjusted to point
1712 to the first use of the nonterminal as a left-hand side of a rule:
1714 warning: 1 nonterminal useless in grammar [-Wother]
1715 warning: 4 rules useless in grammar [-Wother]
1716 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1717 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1719 2.14-16: warning: rule useless in grammar [-Wother]
1723 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1725 When compiled with exceptions disabled, the generated parsers no longer
1726 uses try/catch clauses.
1728 Currently only GCC and Clang are supported.
1732 *** A demonstration of variants
1734 A new example was added (installed in .../share/doc/bison/examples),
1735 'variant.yy', which shows how to use (Bison) variants in C++.
1737 The other examples were made nicer to read.
1739 *** Some features are no longer 'experimental'
1741 The following features, mature enough, are no longer flagged as
1742 experimental in the documentation: push parsers, default %printer and
1743 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1744 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1745 semantic predicates (%?).
1749 *** GLR: Predicates support broken by #line directives
1751 Predicates (%?) in GLR such as
1754 %? {new_syntax} 'w' id new_args
1755 | %?{!new_syntax} 'w' id old_args
1757 were issued with #lines in the middle of C code.
1759 *** Printer and destructor with broken #line directives
1761 The #line directives were not properly escaped when emitting the code for
1762 %printer/%destructor, which resulted in compiler errors if there are
1763 backslashes or double-quotes in the grammar file name.
1765 *** Portability on ICC
1767 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1768 Generated parsers now work around this.
1772 There were several small fixes in the test suite and in the build system,
1773 many warnings in bison and in the generated parsers were eliminated. The
1774 documentation also received its share of minor improvements.
1776 Useless code was removed from C++ parsers, and some of the generated
1777 constructors are more 'natural'.
1780 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1784 *** C++: Fix support of 'syntax_error'
1786 One incorrect 'inline' resulted in linking errors about the constructor of
1787 the syntax_error exception.
1789 *** C++: Fix warnings
1791 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1792 warnings about yyformat being possibly null. It also warned about the
1793 deprecated implicit definition of copy constructors when there's a
1794 user-defined (copy) assignment operator.
1796 *** Location of errors
1798 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1799 ride-hand side raises a syntax error. The behavior of the default parser
1800 (yacc.c) in such a condition was undefined.
1802 Now all the parsers match the behavior of glr.c: @$ is used as the
1803 location of the error. This handles gracefully rules with and without
1806 *** Portability fixes in the test suite
1808 On some platforms, some Java and/or C++ tests were failing.
1811 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1815 *** C++ with Variants (lalr1.cc)
1817 Fix a compiler warning when no %destructor use $$.
1821 Several portability issues in tests were fixed.
1824 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1828 *** C++ with Variants (lalr1.cc)
1830 Problems with %destructor and '%define parse.assert' have been fixed.
1832 *** Named %union support (yacc.c, glr.c)
1834 Bison 3.0 introduced a regression on named %union such as
1836 %union foo { int ival; };
1838 The possibility to use a name was introduced "for Yacc compatibility".
1839 It is however not required by POSIX Yacc, and its usefulness is not clear.
1841 *** %define api.value.type union with %defines (yacc.c, glr.c)
1843 The C parsers were broken when %defines was used together with "%define
1844 api.value.type union".
1846 *** Redeclarations are reported in proper order
1854 bison used to report:
1856 foo.yy:2.10-11: error: %printer redeclaration for FOO
1859 foo.yy:3.10-11: previous declaration
1863 Now, the "previous" declaration is always the first one.
1868 Bison now installs various files in its docdir (which defaults to
1869 '/usr/local/share/doc/bison'), including the three fully blown examples
1870 extracted from the documentation:
1873 Reverse Polish Calculator, a simple introductory example.
1875 Multi-function Calc, a calculator with memory and functions and located
1878 a calculator in C++ using variant support and token constructors.
1881 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1885 *** Generated source files when errors are reported
1887 When warnings are issued and -Werror is set, bison would still generate
1888 the source files (*.c, *.h...). As a consequence, some runs of "make"
1889 could fail the first time, but not the second (as the files were generated
1892 This is fixed: bison no longer generates this source files, but, of
1893 course, still produces the various reports (*.output, *.xml, etc.).
1895 *** %empty is used in reports
1897 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1898 dot, XML and formats derived from it).
1900 *** YYERROR and variants
1902 When C++ variant support is enabled, an error triggered via YYERROR, but
1903 not caught via error recovery, resulted in a double deletion.
1906 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1910 *** Errors in caret diagnostics
1912 On some platforms, some errors could result in endless diagnostics.
1914 *** Fixes of the -Werror option
1916 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1917 diagnostics into errors instead of warnings. This is fixed.
1919 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1920 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1921 -Wno-error", "foo" diagnostics are now errors.
1925 As demonstrated in the documentation, one can now leave spaces between
1930 The yacc.1 man page is no longer installed if --disable-yacc was
1933 *** Fixes in the test suite
1935 Bugs and portability issues.
1938 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1940 ** WARNING: Future backward-incompatibilities!
1942 Like other GNU packages, Bison will start using some of the C99 features
1943 for its own code, especially the definition of variables after statements.
1944 The generated C parsers still aim at C90.
1946 ** Backward incompatible changes
1948 *** Obsolete features
1950 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1952 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1953 use YYSTYPE and YYLTYPE.
1955 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1956 1.875): use %lex-param, %parse-param, or %param.
1958 Missing semicolons at the end of actions are no longer added (as announced
1959 in the release 2.5).
1961 *** Use of YACC='bison -y'
1963 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1966 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1967 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1968 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1970 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1971 implementation of Yacc, was using Bison as 'bison -y'. While it does
1972 ensure compatible output file names, it also enables warnings for
1973 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1974 warnings for Bison extensions.
1976 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1977 (which also generates 'y.tab.h' and 'y.output' when needed).
1978 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1979 flags (YFLAGS, or AM_YFLAGS with Automake).
1983 *** The epilogue is no longer affected by internal #defines (glr.c)
1985 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1986 generated code. These weren't properly undefined before the inclusion of
1987 the user epilogue, so functions such as the following were butchered by the
1988 preprocessor expansion:
1990 int yylex (YYSTYPE *yylval);
1992 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
1993 identifiers for user-provided variables.
1995 *** stdio.h is no longer needed when locations are enabled (yacc.c)
1997 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
1998 locations are enabled. This is fixed.
2000 *** Warnings about useless %pure-parser/%define api.pure are restored
2002 ** Diagnostics reported by Bison
2004 Most of these features were contributed by Théophile Ranquet and Victor
2009 Version 2.7 introduced caret errors, for a prettier output. These are now
2010 activated by default. The old format can still be used by invoking Bison
2011 with -fno-caret (or -fnone).
2013 Some error messages that reproduced excerpts of the grammar are now using
2014 the caret information only. For instance on:
2021 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2022 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2026 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2027 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2031 and "bison -fno-caret" reports:
2033 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2034 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2036 *** Enhancements of the -Werror option
2038 The -Werror=CATEGORY option is now recognized, and will treat specified
2039 warnings as errors. The warnings need not have been explicitly activated
2040 using the -W option, this is similar to what GCC 4.7 does.
2042 For example, given the following command line, Bison will treat both
2043 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2044 errors (and only those):
2046 $ bison -Werror=yacc,error=conflicts-sr input.y
2048 If no categories are specified, -Werror will make all active warnings into
2049 errors. For example, the following line does the same the previous example:
2051 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2053 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2055 Note that the categories in this -Werror option may not be prefixed with
2056 "no-". However, -Wno-error[=CATEGORY] is valid.
2058 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2059 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2060 incompatibilities as warnings: "-y -Wno-error=yacc".
2062 *** The display of warnings is now richer
2064 The option that controls a given warning is now displayed:
2066 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2068 In the case of warnings treated as errors, the prefix is changed from
2069 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2070 to GCC, as [-Werror=CATEGORY].
2072 For instance, where the previous version of Bison would report (and exit
2075 bison: warnings being treated as errors
2076 input.y:1.1: warning: stray ',' treated as white space
2080 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2082 *** Deprecated constructs
2084 The new 'deprecated' warning category flags obsolete constructs whose
2085 support will be discontinued. It is enabled by default. These warnings
2086 used to be reported as 'other' warnings.
2088 *** Useless semantic types
2090 Bison now warns about useless (uninhabited) semantic types. Since
2091 semantic types are not declared to Bison (they are defined in the opaque
2092 %union structure), it is %printer/%destructor directives about useless
2093 types that trigger the warning:
2097 %printer {} <type1> <type3>
2098 %destructor {} <type2> <type4>
2100 nterm: term { $$ = $1; };
2102 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2103 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2105 *** Undefined but unused symbols
2107 Bison used to raise an error for undefined symbols that are not used in
2108 the grammar. This is now only a warning.
2111 %destructor {} symbol2
2112 %type <type> symbol3
2116 *** Useless destructors or printers
2118 Bison now warns about useless destructors or printers. In the following
2119 example, the printer for <type1>, and the destructor for <type2> are
2120 useless: all symbols of <type1> (token1) already have a printer, and all
2121 symbols of type <type2> (token2) already have a destructor.
2123 %token <type1> token1
2127 %printer {} token1 <type1> <type3>
2128 %destructor {} token2 <type2> <type4>
2132 The warnings and error messages about shift/reduce and reduce/reduce
2133 conflicts have been normalized. For instance on the following foo.y file:
2137 exp: exp '+' exp | '0' | '0';
2139 compare the previous version of bison:
2142 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2143 $ bison -Werror foo.y
2144 bison: warnings being treated as errors
2145 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2147 with the new behavior:
2150 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2151 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2152 $ bison -Werror foo.y
2153 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2154 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2156 When %expect or %expect-rr is used, such as with bar.y:
2161 exp: exp '+' exp | '0' | '0';
2166 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2167 bar.y: expected 0 shift/reduce conflicts
2168 bar.y: expected 0 reduce/reduce conflicts
2173 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2174 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2176 ** Incompatibilities with POSIX Yacc
2178 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2181 ** Additional yylex/yyparse arguments
2183 The new directive %param declares additional arguments to both yylex and
2184 yyparse. The %lex-param, %parse-param, and %param directives support one
2185 or more arguments. Instead of
2187 %lex-param {arg1_type *arg1}
2188 %lex-param {arg2_type *arg2}
2189 %parse-param {arg1_type *arg1}
2190 %parse-param {arg2_type *arg2}
2194 %param {arg1_type *arg1} {arg2_type *arg2}
2196 ** Types of values for %define variables
2198 Bison used to make no difference between '%define foo bar' and '%define
2199 foo "bar"'. The former is now called a 'keyword value', and the latter a
2200 'string value'. A third kind was added: 'code values', such as '%define
2203 Keyword variables are used for fixed value sets, e.g.,
2205 %define lr.type lalr
2207 Code variables are used for value in the target language, e.g.,
2209 %define api.value.type {struct semantic_type}
2211 String variables are used remaining cases, e.g. file names.
2213 ** Variable api.token.prefix
2215 The variable api.token.prefix changes the way tokens are identified in
2216 the generated files. This is especially useful to avoid collisions
2217 with identifiers in the target language. For instance
2219 %token FILE for ERROR
2220 %define api.token.prefix {TOK_}
2222 start: FILE for ERROR;
2224 will generate the definition of the symbols TOK_FILE, TOK_for, and
2225 TOK_ERROR in the generated sources. In particular, the scanner must
2226 use these prefixed token names, although the grammar itself still
2227 uses the short names (as in the sample rule given above).
2229 ** Variable api.value.type
2231 This new %define variable supersedes the #define macro YYSTYPE. The use
2232 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2233 using %union or %defining api.value.type results in undefined behavior.
2235 Either define api.value.type, or use "%union":
2242 %token <ival> INT "integer"
2243 %token <sval> STRING "string"
2244 %printer { fprintf (yyo, "%d", $$); } <ival>
2245 %destructor { free ($$); } <sval>
2248 yylval.ival = 42; return INT;
2249 yylval.sval = "42"; return STRING;
2251 The %define variable api.value.type supports both keyword and code values.
2253 The keyword value 'union' means that the user provides genuine types, not
2254 union member names such as "ival" and "sval" above (WARNING: will fail if
2255 -y/--yacc/%yacc is enabled).
2257 %define api.value.type union
2258 %token <int> INT "integer"
2259 %token <char *> STRING "string"
2260 %printer { fprintf (yyo, "%d", $$); } <int>
2261 %destructor { free ($$); } <char *>
2264 yylval.INT = 42; return INT;
2265 yylval.STRING = "42"; return STRING;
2267 The keyword value variant is somewhat equivalent, but for C++ special
2268 provision is made to allow classes to be used (more about this below).
2270 %define api.value.type variant
2271 %token <int> INT "integer"
2272 %token <std::string> STRING "string"
2274 Code values (in braces) denote user defined types. This is where YYSTYPE
2292 %define api.value.type {struct my_value}
2293 %token <u.ival> INT "integer"
2294 %token <u.sval> STRING "string"
2295 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2296 %destructor { free ($$); } <u.sval>
2299 yylval.u.ival = 42; return INT;
2300 yylval.u.sval = "42"; return STRING;
2302 ** Variable parse.error
2304 This variable controls the verbosity of error messages. The use of the
2305 %error-verbose directive is deprecated in favor of "%define parse.error
2308 ** Deprecated %define variable names
2310 The following variables have been renamed for consistency. Backward
2311 compatibility is ensured, but upgrading is recommended.
2313 lr.default-reductions -> lr.default-reduction
2314 lr.keep-unreachable-states -> lr.keep-unreachable-state
2315 namespace -> api.namespace
2316 stype -> api.value.type
2318 ** Semantic predicates
2320 Contributed by Paul Hilfinger.
2322 The new, experimental, semantic-predicate feature allows actions of the
2323 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2324 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2325 in GLR parsers, rather than being deferred. The result is that they allow
2326 the programmer to prune possible parses based on the values of run-time
2329 ** The directive %expect-rr is now an error in non GLR mode
2331 It used to be an error only if used in non GLR mode, _and_ if there are
2332 reduce/reduce conflicts.
2334 ** Tokens are numbered in their order of appearance
2336 Contributed by Valentin Tolmer.
2338 With '%token A B', A had a number less than the one of B. However,
2339 precedence declarations used to generate a reversed order. This is now
2340 fixed, and introducing tokens with any of %token, %left, %right,
2341 %precedence, or %nonassoc yields the same result.
2343 When mixing declarations of tokens with a literal character (e.g., 'a') or
2344 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2345 the literal characters first. For example
2349 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2350 input order is now preserved.
2352 These changes were made so that one can remove useless precedence and
2353 associativity declarations (i.e., map %nonassoc, %left or %right to
2354 %precedence, or to %token) and get exactly the same output.
2356 ** Useless precedence and associativity
2358 Contributed by Valentin Tolmer.
2360 When developing and maintaining a grammar, useless associativity and
2361 precedence directives are common. They can be a nuisance: new ambiguities
2362 arising are sometimes masked because their conflicts are resolved due to
2363 the extra precedence or associativity information. Furthermore, it can
2364 hinder the comprehension of a new grammar: one will wonder about the role
2365 of a precedence, where in fact it is useless. The following changes aim
2366 at detecting and reporting these extra directives.
2368 *** Precedence warning category
2370 A new category of warning, -Wprecedence, was introduced. It flags the
2371 useless precedence and associativity directives.
2373 *** Useless associativity
2375 Bison now warns about symbols with a declared associativity that is never
2376 used to resolve conflicts. In that case, using %precedence is sufficient;
2377 the parsing tables will remain unchanged. Solving these warnings may raise
2378 useless precedence warnings, as the symbols no longer have associativity.
2392 warning: useless associativity for '+', use %precedence [-Wprecedence]
2396 *** Useless precedence
2398 Bison now warns about symbols with a declared precedence and no declared
2399 associativity (i.e., declared with %precedence), and whose precedence is
2400 never used. In that case, the symbol can be safely declared with %token
2401 instead, without modifying the parsing tables. For example:
2405 exp: "var" '=' "number";
2409 warning: useless precedence for '=' [-Wprecedence]
2413 *** Useless precedence and associativity
2415 In case of both useless precedence and associativity, the issue is flagged
2420 exp: "var" '=' "number";
2424 warning: useless precedence and associativity for '=' [-Wprecedence]
2430 With help from Joel E. Denny and Gabriel Rassoul.
2432 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2433 marked by the new %empty directive. Using %empty on a non-empty rule is
2434 an error. The new -Wempty-rule warning reports empty rules without
2435 %empty. On the following grammar:
2445 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2448 5.8-13: error: %empty on non-empty rule
2452 ** Java skeleton improvements
2454 The constants for token names were moved to the Lexer interface. Also, it
2455 is possible to add code to the parser's constructors using "%code init"
2456 and "%define init_throws".
2457 Contributed by Paolo Bonzini.
2459 The Java skeleton now supports push parsing.
2460 Contributed by Dennis Heimbigner.
2462 ** C++ skeletons improvements
2464 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2466 Using %defines is now optional. Without it, the needed support classes
2467 are defined in the generated parser, instead of additional files (such as
2468 location.hh, position.hh and stack.hh).
2470 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2472 Both lalr1.cc and glr.cc no longer require %location.
2474 *** syntax_error exception (lalr1.cc)
2476 The C++ parser features a syntax_error exception, which can be
2477 thrown from the scanner or from user rules to raise syntax errors.
2478 This facilitates reporting errors caught in sub-functions (e.g.,
2479 rejecting too large integral literals from a conversion function
2480 used by the scanner, or rejecting invalid combinations from a
2481 factory invoked by the user actions).
2483 *** %define api.value.type variant
2485 This is based on a submission from Michiel De Wilde. With help
2486 from Théophile Ranquet.
2488 In this mode, complex C++ objects can be used as semantic values. For
2491 %token <::std::string> TEXT;
2492 %token <int> NUMBER;
2493 %token SEMICOLON ";"
2494 %type <::std::string> item;
2495 %type <::std::list<std::string>> list;
2498 list { std::cout << $1 << std::endl; }
2502 %empty { /* Generates an empty string list. */ }
2503 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2507 TEXT { std::swap ($$, $1); }
2508 | NUMBER { $$ = string_cast ($1); }
2511 *** %define api.token.constructor
2513 When variants are enabled, Bison can generate functions to build the
2514 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2515 with the semantic value (e.g., int):
2517 parser::symbol_type yylex ()
2519 parser::location_type loc = ...;
2521 return parser::make_TEXT ("Hello, world!", loc);
2523 return parser::make_NUMBER (42, loc);
2525 return parser::make_SEMICOLON (loc);
2531 There are operator- and operator-= for 'location'. Negative line/column
2532 increments can no longer underflow the resulting value.
2535 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2539 *** Fix compiler attribute portability (yacc.c)
2541 With locations enabled, __attribute__ was used unprotected.
2543 *** Fix some compiler warnings (lalr1.cc)
2546 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2550 Warnings about uninitialized yylloc in yyparse have been fixed.
2552 Restored C90 compliance (yet no report was ever made).
2554 ** Diagnostics are improved
2556 Contributed by Théophile Ranquet.
2558 *** Changes in the format of error messages
2560 This used to be the format of many error reports:
2562 input.y:2.7-12: %type redeclaration for exp
2563 input.y:1.7-12: previous declaration
2567 input.y:2.7-12: error: %type redeclaration for exp
2568 input.y:1.7-12: previous declaration
2570 *** New format for error reports: carets
2572 Caret errors have been added to Bison:
2574 input.y:2.7-12: error: %type redeclaration for exp
2577 input.y:1.7-12: previous declaration
2583 input.y:3.20-23: error: ambiguous reference: '$exp'
2584 exp: exp '+' exp { $exp = $1 + $3; };
2586 input.y:3.1-3: refers to: $exp at $$
2587 exp: exp '+' exp { $exp = $1 + $3; };
2589 input.y:3.6-8: refers to: $exp at $1
2590 exp: exp '+' exp { $exp = $1 + $3; };
2592 input.y:3.14-16: refers to: $exp at $3
2593 exp: exp '+' exp { $exp = $1 + $3; };
2596 The default behavior for now is still not to display these unless
2597 explicitly asked with -fcaret (or -fall). However, in a later release, it
2598 will be made the default behavior (but may still be deactivated with
2601 ** New value for %define variable: api.pure full
2603 The %define variable api.pure requests a pure (reentrant) parser. However,
2604 for historical reasons, using it in a location-tracking Yacc parser
2605 resulted in a yyerror function that did not take a location as a
2606 parameter. With this new value, the user may request a better pure parser,
2607 where yyerror does take a location as a parameter (in location-tracking
2610 The use of "%define api.pure true" is deprecated in favor of this new
2611 "%define api.pure full".
2613 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2615 The %define variable api.location.type defines the name of the type to use
2616 for locations. When defined, Bison no longer generates the position.hh
2617 and location.hh files, nor does the parser will include them: the user is
2618 then responsible to define her type.
2620 This can be used in programs with several parsers to factor their location
2621 and position files: let one of them generate them, and the others just use
2624 This feature was actually introduced, but not documented, in Bison 2.5,
2625 under the name "location_type" (which is maintained for backward
2628 For consistency, lalr1.java's %define variables location_type and
2629 position_type are deprecated in favor of api.location.type and
2632 ** Exception safety (lalr1.cc)
2634 The parse function now catches exceptions, uses the %destructors to
2635 release memory (the lookahead symbol and the symbols pushed on the stack)
2636 before re-throwing the exception.
2638 This feature is somewhat experimental. User feedback would be
2641 ** Graph improvements in DOT and XSLT
2643 Contributed by Théophile Ranquet.
2645 The graphical presentation of the states is more readable: their shape is
2646 now rectangular, the state number is clearly displayed, and the items are
2647 numbered and left-justified.
2649 The reductions are now explicitly represented as transitions to other
2650 diamond shaped nodes.
2652 These changes are present in both --graph output and xml2dot.xsl XSLT
2653 processing, with minor (documented) differences.
2655 ** %language is no longer an experimental feature.
2657 The introduction of this feature, in 2.4, was four years ago. The
2658 --language option and the %language directive are no longer experimental.
2662 The sections about shift/reduce and reduce/reduce conflicts resolution
2663 have been fixed and extended.
2665 Although introduced more than four years ago, XML and Graphviz reports
2666 were not properly documented.
2668 The translation of midrule actions is now described.
2671 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2673 We consider compiler warnings about Bison generated parsers to be bugs.
2674 Rather than working around them in your own project, please consider
2675 reporting them to us.
2679 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2680 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2683 Other issues in the test suite have been addressed.
2685 Null characters are correctly displayed in error messages.
2687 When possible, yylloc is correctly initialized before calling yylex. It
2688 is no longer necessary to initialize it in the %initial-action.
2691 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2693 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2696 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2700 Bugs and portability issues in the test suite have been fixed.
2702 Some errors in translations have been addressed, and --help now directs
2703 users to the appropriate place to report them.
2705 Stray Info files shipped by accident are removed.
2707 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2708 generated, are removed.
2710 All the generated headers are self-contained.
2712 ** Header guards (yacc.c, glr.c, glr.cc)
2714 In order to avoid collisions, the header guards are now
2715 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2716 For instance the header generated from
2718 %define api.prefix "calc"
2719 %defines "lib/parse.h"
2721 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2723 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2725 The compilation of pure parsers (%define api.pure) can trigger GCC
2728 input.c: In function 'yyparse':
2729 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2730 function [-Wmaybe-uninitialized]
2734 This is now fixed; pragmas to avoid these warnings are no longer needed.
2736 Warnings from clang ("equality comparison with extraneous parentheses" and
2737 "function declared 'noreturn' should not return") have also been
2741 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2745 Buffer overruns, complaints from Flex, and portability issues in the test
2746 suite have been fixed.
2748 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2750 Trailing end-of-lines in %parse-param or %lex-param would result in
2751 invalid C++. This is fixed.
2753 ** Spurious spaces and end-of-lines
2755 The generated files no longer end (nor start) with empty lines.
2758 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2760 Bison no longer executes user-specified M4 code when processing a grammar.
2764 In addition to the removal of the features announced in Bison 2.6, the
2765 next major release will remove the "Temporary hack for adding a semicolon
2766 to the user action", as announced in the release 2.5. Instead of:
2768 exp: exp "+" exp { $$ = $1 + $3 };
2772 exp: exp "+" exp { $$ = $1 + $3; };
2776 *** Type names are now properly escaped.
2778 *** glr.cc: set_debug_level and debug_level work as expected.
2780 *** Stray @ or $ in actions
2782 While Bison used to warn about stray $ or @ in action rules, it did not
2783 for other actions such as printers, destructors, or initial actions. It
2786 ** Type names in actions
2788 For consistency with rule actions, it is now possible to qualify $$ by a
2789 type-name in destructors, printers, and initial actions. For instance:
2791 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2793 will display two values for each typed and untyped symbol (provided
2794 that YYSTYPE has both "ival" and "fval" fields).
2797 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2801 The next major release of Bison will drop support for the following
2802 deprecated features. Please report disagreements to bug-bison@gnu.org.
2806 Support for generating parsers in K&R C will be removed. Parsers
2807 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2810 *** Features deprecated since Bison 1.875
2812 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2815 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2816 %lex-param, will no longer be supported.
2818 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2821 *** The generated header will be included (yacc.c)
2823 Instead of duplicating the content of the generated header (definition of
2824 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2825 as is already the case for GLR or C++ parsers. This change is deferred
2826 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2829 ** Generated Parser Headers
2831 *** Guards (yacc.c, glr.c, glr.cc)
2833 The generated headers are now guarded, as is already the case for C++
2834 parsers (lalr1.cc). For instance, with --defines=foo.h:
2839 #endif /* !YY_FOO_H */
2841 *** New declarations (yacc.c, glr.c)
2843 The generated header now declares yydebug and yyparse. Both honor
2844 --name-prefix=bar_, and yield
2846 int bar_parse (void);
2850 #define yyparse bar_parse
2853 in order to facilitate the inclusion of several parser headers inside a
2854 single compilation unit.
2856 *** Exported symbols in C++
2858 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2859 header, are removed, as they prevent the possibility of including several
2860 generated headers from a single compilation unit.
2864 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2867 ** New %define variable: api.prefix
2869 Now that the generated headers are more complete and properly protected
2870 against multiple inclusions, constant names, such as YYSTYPE are a
2871 problem. While yyparse and others are properly renamed by %name-prefix,
2872 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2873 would introduce backward compatibility issues in projects not expecting
2874 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2875 it is deprecated in favor of a new %define variable: api.prefix.
2877 The following examples compares both:
2879 %name-prefix "bar_" | %define api.prefix "bar_"
2880 %token <ival> FOO %token <ival> FOO
2881 %union { int ival; } %union { int ival; }
2887 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2888 # define BAR_FOO_H # define BAR_FOO_H
2890 /* Enabling traces. */ /* Enabling traces. */
2891 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2892 > # if defined YYDEBUG
2894 > # define BAR_DEBUG 1
2896 > # define BAR_DEBUG 0
2899 # define YYDEBUG 0 | # define BAR_DEBUG 0
2903 # if YYDEBUG | # if BAR_DEBUG
2904 extern int bar_debug; extern int bar_debug;
2907 /* Tokens. */ /* Tokens. */
2908 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2909 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2910 enum yytokentype { | enum bar_tokentype {
2915 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2916 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2917 typedef union YYSTYPE | typedef union BAR_STYPE
2920 } YYSTYPE; | } BAR_STYPE;
2921 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2924 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2926 int bar_parse (void); int bar_parse (void);
2928 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2931 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2935 The next major release will drop support for generating parsers in K&R C.
2937 ** yacc.c: YYBACKUP works as expected.
2939 ** glr.c improvements:
2941 *** Location support is eliminated when not requested:
2943 GLR parsers used to include location-related code even when locations were
2944 not requested, and therefore not even usable.
2946 *** __attribute__ is preserved:
2948 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2949 when -std is passed to GCC).
2951 ** lalr1.java: several fixes:
2953 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2954 first token leads to a syntax error. Some minor clean ups.
2958 *** C++11 compatibility:
2960 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2965 The header files such as "parser.hh", "location.hh", etc. used a constant
2966 name for preprocessor guards, for instance:
2968 #ifndef BISON_LOCATION_HH
2969 # define BISON_LOCATION_HH
2971 #endif // !BISON_LOCATION_HH
2973 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2974 case characters are converted to upper case, and series of
2975 non-alphanumerical characters are converted to an underscore.
2977 With "bison -o lang++/parser.cc", "location.hh" would now include:
2979 #ifndef YY_LANG_LOCATION_HH
2980 # define YY_LANG_LOCATION_HH
2982 #endif // !YY_LANG_LOCATION_HH
2986 The position and location constructors (and their initialize methods)
2987 accept new arguments for line and column. Several issues in the
2988 documentation were fixed.
2990 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2992 ** Changes in the manual:
2994 *** %printer is documented
2996 The "%printer" directive, supported since at least Bison 1.50, is finally
2997 documented. The "mfcalc" example is extended to demonstrate it.
2999 For consistency with the C skeletons, the C++ parsers now also support
3000 "yyoutput" (as an alias to "debug_stream ()").
3002 *** Several improvements have been made:
3004 The layout for grammar excerpts was changed to a more compact scheme.
3005 Named references are motivated. The description of the automaton
3006 description file (*.output) is updated to the current format. Incorrect
3007 index entries were fixed. Some other errors were fixed.
3011 *** Conflicting prototypes with recent/modified Flex.
3013 Fixed build problems with the current, unreleased, version of Flex, and
3014 some modified versions of 2.5.35, which have modified function prototypes.
3016 *** Warnings during the build procedure have been eliminated.
3018 *** Several portability problems in the test suite have been fixed:
3020 This includes warnings with some compilers, unexpected behavior of tools
3021 such as diff, warning messages from the test suite itself, etc.
3023 *** The install-pdf target works properly:
3025 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3026 halts in the middle of its course.
3029 * Noteworthy changes in release 2.5 (2011-05-14)
3031 ** Grammar symbol names can now contain non-initial dashes:
3033 Consistently with directives (such as %error-verbose) and with
3034 %define variables (e.g. push-pull), grammar symbol names may contain
3035 dashes in any position except the beginning. This is a GNU
3036 extension over POSIX Yacc. Thus, use of this extension is reported
3037 by -Wyacc and rejected in Yacc mode (--yacc).
3039 ** Named references:
3041 Historically, Yacc and Bison have supported positional references
3042 ($n, $$) to allow access to symbol values from inside of semantic
3045 Starting from this version, Bison can also accept named references.
3046 When no ambiguity is possible, original symbol names may be used
3047 as named references:
3049 if_stmt : "if" cond_expr "then" then_stmt ';'
3050 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3052 In the more common case, explicit names may be declared:
3054 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3055 { $res = mk_if_stmt($cond, $then, $else); }
3057 Location information is also accessible using @name syntax. When
3058 accessing symbol names containing dots or dashes, explicit bracketing
3059 ($[sym.1]) must be used.
3061 These features are experimental in this version. More user feedback
3062 will help to stabilize them.
3063 Contributed by Alex Rozenman.
3065 ** IELR(1) and canonical LR(1):
3067 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3068 is, given any context-free grammar, IELR(1) generates parser tables
3069 with the full language-recognition power of canonical LR(1) but with
3070 nearly the same number of parser states as LALR(1). This reduction
3071 in parser states is often an order of magnitude. More importantly,
3072 because canonical LR(1)'s extra parser states may contain duplicate
3073 conflicts in the case of non-LR(1) grammars, the number of conflicts
3074 for IELR(1) is often an order of magnitude less as well. This can
3075 significantly reduce the complexity of developing of a grammar.
3077 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3078 place of its traditional LALR(1) parser tables, which remain the
3079 default. You can specify the type of parser tables in the grammar
3080 file with these directives:
3082 %define lr.type lalr
3083 %define lr.type ielr
3084 %define lr.type canonical-lr
3086 The default-reduction optimization in the parser tables can also be
3087 adjusted using "%define lr.default-reductions". For details on both
3088 of these features, see the new section "Tuning LR" in the Bison
3091 These features are experimental. More user feedback will help to
3094 ** LAC (Lookahead Correction) for syntax error handling
3096 Contributed by Joel E. Denny.
3098 Canonical LR, IELR, and LALR can suffer from a couple of problems
3099 upon encountering a syntax error. First, the parser might perform
3100 additional parser stack reductions before discovering the syntax
3101 error. Such reductions can perform user semantic actions that are
3102 unexpected because they are based on an invalid token, and they
3103 cause error recovery to begin in a different syntactic context than
3104 the one in which the invalid token was encountered. Second, when
3105 verbose error messages are enabled (with %error-verbose or the
3106 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3107 syntax error message can both contain invalid tokens and omit valid
3110 The culprits for the above problems are %nonassoc, default
3111 reductions in inconsistent states, and parser state merging. Thus,
3112 IELR and LALR suffer the most. Canonical LR can suffer only if
3113 %nonassoc is used or if default reductions are enabled for
3114 inconsistent states.
3116 LAC is a new mechanism within the parsing algorithm that solves
3117 these problems for canonical LR, IELR, and LALR without sacrificing
3118 %nonassoc, default reductions, or state merging. When LAC is in
3119 use, canonical LR and IELR behave almost exactly the same for both
3120 syntactically acceptable and syntactically unacceptable input.
3121 While LALR still does not support the full language-recognition
3122 power of canonical LR and IELR, LAC at least enables LALR's syntax
3123 error handling to correctly reflect LALR's language-recognition
3126 Currently, LAC is only supported for deterministic parsers in C.
3127 You can enable LAC with the following directive:
3129 %define parse.lac full
3131 See the new section "LAC" in the Bison manual for additional
3132 details including a few caveats.
3134 LAC is an experimental feature. More user feedback will help to
3137 ** %define improvements:
3139 *** Can now be invoked via the command line:
3141 Each of these command-line options
3144 --define=NAME[=VALUE]
3147 --force-define=NAME[=VALUE]
3149 is equivalent to this grammar file declaration
3151 %define NAME ["VALUE"]
3153 except that the manner in which Bison processes multiple definitions
3154 for the same NAME differs. Most importantly, -F and --force-define
3155 quietly override %define, but -D and --define do not. For further
3156 details, see the section "Bison Options" in the Bison manual.
3158 *** Variables renamed:
3160 The following %define variables
3163 lr.keep_unreachable_states
3165 have been renamed to
3168 lr.keep-unreachable-states
3170 The old names are now deprecated but will be maintained indefinitely
3171 for backward compatibility.
3173 *** Values no longer need to be quoted in the grammar file:
3175 If a %define value is an identifier, it no longer needs to be placed
3176 within quotations marks. For example,
3178 %define api.push-pull "push"
3182 %define api.push-pull push
3184 *** Unrecognized variables are now errors not warnings.
3186 *** Multiple invocations for any variable is now an error not a warning.
3188 ** Unrecognized %code qualifiers are now errors not warnings.
3190 ** Character literals not of length one:
3192 Previously, Bison quietly converted all character literals to length
3193 one. For example, without warning, Bison interpreted the operators in
3194 the following grammar to be the same token:
3200 Bison now warns when a character literal is not of length one. In
3201 some future release, Bison will start reporting an error instead.
3203 ** Destructor calls fixed for lookaheads altered in semantic actions:
3205 Previously for deterministic parsers in C, if a user semantic action
3206 altered yychar, the parser in some cases used the old yychar value to
3207 determine which destructor to call for the lookahead upon a syntax
3208 error or upon parser return. This bug has been fixed.
3210 ** C++ parsers use YYRHSLOC:
3212 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3213 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3214 to use it. If, for instance, your location structure has "first"
3215 and "last" members, instead of
3217 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3221 (Current).first = (Rhs)[1].location.first; \
3222 (Current).last = (Rhs)[N].location.last; \
3226 (Current).first = (Current).last = (Rhs)[0].location.last; \
3232 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3236 (Current).first = YYRHSLOC (Rhs, 1).first; \
3237 (Current).last = YYRHSLOC (Rhs, N).last; \
3241 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3245 ** YYLLOC_DEFAULT in C++:
3247 The default implementation of YYLLOC_DEFAULT used to be issued in
3248 the header file. It is now output in the implementation file, after
3249 the user %code sections so that its #ifndef guard does not try to
3250 override the user's YYLLOC_DEFAULT if provided.
3252 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3254 YYFAIL has existed for many years as an undocumented feature of
3255 deterministic parsers in C generated by Bison. More recently, it was
3256 a documented feature of Bison's experimental Java parsers. As
3257 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3258 semantic action now produces a deprecation warning, and Java parsers
3259 no longer implement YYFAIL at all. For further details, including a
3260 discussion of how to suppress C preprocessor warnings about YYFAIL
3261 being unused, see the Bison 2.4.2 NEWS entry.
3263 ** Temporary hack for adding a semicolon to the user action:
3265 Previously, Bison appended a semicolon to every user action for
3266 reductions when the output language defaulted to C (specifically, when
3267 neither %yacc, %language, %skeleton, or equivalent command-line
3268 options were specified). This allowed actions such as
3270 exp: exp "+" exp { $$ = $1 + $3 };
3274 exp: exp "+" exp { $$ = $1 + $3; };
3276 As a first step in removing this misfeature, Bison now issues a
3277 warning when it appends a semicolon. Moreover, in cases where Bison
3278 cannot easily determine whether a semicolon is needed (for example, an
3279 action ending with a cpp directive or a braced compound initializer),
3280 it no longer appends one. Thus, the C compiler might now complain
3281 about a missing semicolon where it did not before. Future releases of
3282 Bison will cease to append semicolons entirely.
3284 ** Verbose syntax error message fixes:
3286 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3287 specified, syntax error messages produced by the generated parser
3288 include the unexpected token as well as a list of expected tokens.
3289 The effect of %nonassoc on these verbose messages has been corrected
3290 in two ways, but a more complete fix requires LAC, described above:
3292 *** When %nonassoc is used, there can exist parser states that accept no
3293 tokens, and so the parser does not always require a lookahead token
3294 in order to detect a syntax error. Because no unexpected token or
3295 expected tokens can then be reported, the verbose syntax error
3296 message described above is suppressed, and the parser instead
3297 reports the simpler message, "syntax error". Previously, this
3298 suppression was sometimes erroneously triggered by %nonassoc when a
3299 lookahead was actually required. Now verbose messages are
3300 suppressed only when all previous lookaheads have already been
3301 shifted or discarded.
3303 *** Previously, the list of expected tokens erroneously included tokens
3304 that would actually induce a syntax error because conflicts for them
3305 were resolved with %nonassoc in the current parser state. Such
3306 tokens are now properly omitted from the list.
3308 *** Expected token lists are still often wrong due to state merging
3309 (from LALR or IELR) and default reductions, which can both add
3310 invalid tokens and subtract valid tokens. Canonical LR almost
3311 completely fixes this problem by eliminating state merging and
3312 default reductions. However, there is one minor problem left even
3313 when using canonical LR and even after the fixes above. That is,
3314 if the resolution of a conflict with %nonassoc appears in a later
3315 parser state than the one at which some syntax error is
3316 discovered, the conflicted token is still erroneously included in
3317 the expected token list. Bison's new LAC implementation,
3318 described above, eliminates this problem and the need for
3319 canonical LR. However, LAC is still experimental and is disabled
3322 ** Java skeleton fixes:
3324 *** A location handling bug has been fixed.
3326 *** The top element of each of the value stack and location stack is now
3327 cleared when popped so that it can be garbage collected.
3329 *** Parser traces now print the top element of the stack.
3331 ** -W/--warnings fixes:
3333 *** Bison now properly recognizes the "no-" versions of categories:
3335 For example, given the following command line, Bison now enables all
3336 warnings except warnings for incompatibilities with POSIX Yacc:
3338 bison -Wall,no-yacc gram.y
3340 *** Bison now treats S/R and R/R conflicts like other warnings:
3342 Previously, conflict reports were independent of Bison's normal
3343 warning system. Now, Bison recognizes the warning categories
3344 "conflicts-sr" and "conflicts-rr". This change has important
3345 consequences for the -W and --warnings command-line options. For
3348 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3349 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3350 bison -Wnone gram.y # no conflicts are reported
3351 bison -Werror gram.y # any conflict is an error
3353 However, as before, if the %expect or %expect-rr directive is
3354 specified, an unexpected number of conflicts is an error, and an
3355 expected number of conflicts is not reported, so -W and --warning
3356 then have no effect on the conflict report.
3358 *** The "none" category no longer disables a preceding "error":
3360 For example, for the following command line, Bison now reports
3361 errors instead of warnings for incompatibilities with POSIX Yacc:
3363 bison -Werror,none,yacc gram.y
3365 *** The "none" category now disables all Bison warnings:
3367 Previously, the "none" category disabled only Bison warnings for
3368 which there existed a specific -W/--warning category. However,
3369 given the following command line, Bison is now guaranteed to
3370 suppress all warnings:
3374 ** Precedence directives can now assign token number 0:
3376 Since Bison 2.3b, which restored the ability of precedence
3377 directives to assign token numbers, doing so for token number 0 has
3378 produced an assertion failure. For example:
3382 This bug has been fixed.
3385 * Noteworthy changes in release 2.4.3 (2010-08-05)
3387 ** Bison now obeys -Werror and --warnings=error for warnings about
3388 grammar rules that are useless in the parser due to conflicts.
3390 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3393 ** Failures in the test suite for GCC 4.5 have been fixed.
3395 ** Failures in the test suite for some versions of Sun Studio C++ have
3398 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3399 warnings about undefined %prec identifiers will not be converted to
3400 errors in Bison 2.5. They will remain warnings, which should be
3401 sufficient for POSIX while avoiding backward compatibility issues.
3403 ** Minor documentation fixes.
3406 * Noteworthy changes in release 2.4.2 (2010-03-20)
3408 ** Some portability problems that resulted in failures and livelocks
3409 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3410 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3411 errors should no longer cause M4 to report a broken pipe on the
3414 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3416 POSIX specifies that an error be reported for any identifier that does
3417 not appear on the LHS of a grammar rule and that is not defined by
3418 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3419 error report for the case when an identifier appears only after a
3420 %prec directive. It is now restored. However, for backward
3421 compatibility with recent Bison releases, it is only a warning for
3422 now. In Bison 2.5 and later, it will return to being an error.
3423 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3424 warning will not be converted to an error in Bison 2.5.]
3426 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3428 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3429 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3432 ** %code is now a permanent feature.
3434 A traditional Yacc prologue directive is written in the form:
3438 To provide a more flexible alternative, Bison 2.3b introduced the
3439 %code directive with the following forms for C/C++:
3442 %code requires {CODE}
3443 %code provides {CODE}
3446 These forms are now considered permanent features of Bison. See the
3447 %code entries in the section "Bison Declaration Summary" in the Bison
3448 manual for a summary of their functionality. See the section
3449 "Prologue Alternatives" for a detailed discussion including the
3450 advantages of %code over the traditional Yacc prologue directive.
3452 Bison's Java feature as a whole including its current usage of %code
3453 is still considered experimental.
3455 ** YYFAIL is deprecated and will eventually be removed.
3457 YYFAIL has existed for many years as an undocumented feature of
3458 deterministic parsers in C generated by Bison. Previously, it was
3459 documented for Bison's experimental Java parsers. YYFAIL is no longer
3460 documented for Java parsers and is formally deprecated in both cases.
3461 Users are strongly encouraged to migrate to YYERROR, which is
3464 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3465 induce a syntax error. The most obvious difference from YYERROR is
3466 that YYFAIL will automatically invoke yyerror to report the syntax
3467 error so that you don't have to. However, there are several other
3468 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3469 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3470 used. For a more detailed discussion, see:
3472 http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html
3474 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3475 deterministic parsers in C will continue to implement it. However,
3476 because YYFAIL is already flawed, it seems futile to try to make new
3477 Bison features compatible with it. Thus, during parser generation,
3478 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3479 rule action. In a later release, YYFAIL will be disabled for
3480 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3481 be removed altogether.
3483 There exists at least one case where Bison 2.5's YYFAIL warning will
3484 be a false positive. Some projects add phony uses of YYFAIL and other
3485 Bison-defined macros for the sole purpose of suppressing C
3486 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3487 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3488 epilogue (that is, after the second "%%") in the Bison input file. In
3489 this release (2.4.2), Bison already generates its own code to suppress
3490 C preprocessor warnings for YYFAIL, so projects can remove their own
3491 phony uses of YYFAIL if compatibility with Bison releases prior to
3492 2.4.2 is not necessary.
3494 ** Internationalization.
3496 Fix a regression introduced in Bison 2.4: Under some circumstances,
3497 message translations were not installed although supported by the
3501 * Noteworthy changes in release 2.4.1 (2008-12-11)
3503 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3504 declarations have been fixed.
3506 ** Temporary hack for adding a semicolon to the user action.
3508 Bison used to prepend a trailing semicolon at the end of the user
3509 action for reductions. This allowed actions such as
3511 exp: exp "+" exp { $$ = $1 + $3 };
3515 exp: exp "+" exp { $$ = $1 + $3; };
3517 Some grammars still depend on this "feature". Bison 2.4.1 restores
3518 the previous behavior in the case of C output (specifically, when
3519 neither %language or %skeleton or equivalent command-line options
3520 are used) to leave more time for grammars depending on the old
3521 behavior to be adjusted. Future releases of Bison will disable this
3524 ** A few minor improvements to the Bison manual.
3527 * Noteworthy changes in release 2.4 (2008-11-02)
3529 ** %language is an experimental feature.
3531 We first introduced this feature in test release 2.3b as a cleaner
3532 alternative to %skeleton. Since then, we have discussed the possibility of
3533 modifying its effect on Bison's output file names. Thus, in this release,
3534 we consider %language to be an experimental feature that will likely evolve
3537 ** Forward compatibility with GNU M4 has been improved.
3539 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3543 * Noteworthy changes in release 2.3b (2008-05-27)
3545 ** The quotes around NAME that used to be required in the following directive
3548 %define NAME "VALUE"
3550 ** The directive "%pure-parser" is now deprecated in favor of:
3554 which has the same effect except that Bison is more careful to warn about
3555 unreasonable usage in the latter case.
3559 Bison can now generate an LALR(1) parser in C with a push interface. That
3560 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3561 push one token at a time to the parser using "yypush_parse", which will
3562 return to the caller after processing each token. By default, the push
3563 interface is disabled. Either of the following directives will enable it:
3565 %define api.push_pull "push" // Just push; does not require yylex.
3566 %define api.push_pull "both" // Push and pull; requires yylex.
3568 See the new section "A Push Parser" in the Bison manual for details.
3570 The current push parsing interface is experimental and may evolve. More user
3571 feedback will help to stabilize it.
3573 ** The -g and --graph options now output graphs in Graphviz DOT format,
3574 not VCG format. Like --graph, -g now also takes an optional FILE argument
3575 and thus cannot be bundled with other short options.
3579 Bison can now generate an LALR(1) parser in Java. The skeleton is
3580 "data/lalr1.java". Consider using the new %language directive instead of
3581 %skeleton to select it.
3583 See the new section "Java Parsers" in the Bison manual for details.
3585 The current Java interface is experimental and may evolve. More user
3586 feedback will help to stabilize it.
3587 Contributed by Paolo Bonzini.
3591 This new directive specifies the programming language of the generated
3592 parser, which can be C (the default), C++, or Java. Besides the skeleton
3593 that Bison uses, the directive affects the names of the generated files if
3594 the grammar file's name ends in ".y".
3596 ** XML Automaton Report
3598 Bison can now generate an XML report of the LALR(1) automaton using the new
3599 "--xml" option. The current XML schema is experimental and may evolve. More
3600 user feedback will help to stabilize it.
3601 Contributed by Wojciech Polak.
3603 ** The grammar file may now specify the name of the parser header file using
3604 %defines. For example:
3608 ** When reporting useless rules, useless nonterminals, and unused terminals,
3609 Bison now employs the terms "useless in grammar" instead of "useless",
3610 "useless in parser" instead of "never reduced", and "unused in grammar"
3611 instead of "unused".
3613 ** Unreachable State Removal
3615 Previously, Bison sometimes generated parser tables containing unreachable
3616 states. A state can become unreachable during conflict resolution if Bison
3617 disables a shift action leading to it from a predecessor state. Bison now:
3619 1. Removes unreachable states.
3621 2. Does not report any conflicts that appeared in unreachable states.
3622 WARNING: As a result, you may need to update %expect and %expect-rr
3623 directives in existing grammar files.
3625 3. For any rule used only in such states, Bison now reports the rule as
3626 "useless in parser due to conflicts".
3628 This feature can be disabled with the following directive:
3630 %define lr.keep_unreachable_states
3632 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3633 for further discussion.
3635 ** Lookahead Set Correction in the ".output" Report
3637 When instructed to generate a ".output" file including lookahead sets
3638 (using "--report=lookahead", for example), Bison now prints each reduction's
3639 lookahead set only next to the associated state's one item that (1) is
3640 associated with the same rule as the reduction and (2) has its dot at the end
3641 of its RHS. Previously, Bison also erroneously printed the lookahead set
3642 next to all of the state's other items associated with the same rule. This
3643 bug affected only the ".output" file and not the generated parser source
3646 ** --report-file=FILE is a new option to override the default ".output" file
3649 ** The "=" that used to be required in the following directives is now
3652 %file-prefix "parser"
3656 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3658 Bison 2.3a provided a new set of directives as a more flexible alternative to
3659 the traditional Yacc prologue blocks. Those have now been consolidated into
3660 a single %code directive with an optional qualifier field, which identifies
3661 the purpose of the code and thus the location(s) where Bison should generate
3664 1. "%code {CODE}" replaces "%after-header {CODE}"
3665 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3666 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3667 4. "%code top {CODE}" replaces "%before-header {CODE}"
3669 See the %code entries in section "Bison Declaration Summary" in the Bison
3670 manual for a summary of the new functionality. See the new section "Prologue
3671 Alternatives" for a detailed discussion including the advantages of %code
3672 over the traditional Yacc prologues.
3674 The prologue alternatives are experimental. More user feedback will help to
3675 determine whether they should become permanent features.
3677 ** Revised warning: unset or unused midrule values
3679 Since Bison 2.2, Bison has warned about midrule values that are set but not
3680 used within any of the actions of the parent rule. For example, Bison warns
3683 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3685 Now, Bison also warns about midrule values that are used but not set. For
3686 example, Bison warns about unset $$ in the midrule action in:
3688 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3690 However, Bison now disables both of these warnings by default since they
3691 sometimes prove to be false alarms in existing grammars employing the Yacc
3692 constructs $0 or $-N (where N is some positive integer).
3694 To enable these warnings, specify the option "--warnings=midrule-values" or
3695 "-W", which is a synonym for "--warnings=all".
3697 ** Default %destructor or %printer with "<*>" or "<>"
3699 Bison now recognizes two separate kinds of default %destructor's and
3702 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3703 %destructor/%printer for all grammar symbols for which you have formally
3704 declared semantic type tags.
3706 2. Place "<>" in a %destructor/%printer symbol list to define a default
3707 %destructor/%printer for all grammar symbols without declared semantic
3710 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3711 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3712 longer applies any %destructor to a midrule value if that midrule value is
3713 not actually ever referenced using either $$ or $n in a semantic action.
3715 The default %destructor's and %printer's are experimental. More user
3716 feedback will help to determine whether they should become permanent
3719 See the section "Freeing Discarded Symbols" in the Bison manual for further
3722 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3723 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3724 manual for a caveat concerning the treatment of literal strings.
3726 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3727 completely removed from Bison.
3730 * Noteworthy changes in release 2.3a (2006-09-13)
3732 ** Instead of %union, you can define and use your own union type
3733 YYSTYPE if your grammar contains at least one <type> tag.
3734 Your YYSTYPE need not be a macro; it can be a typedef.
3735 This change is for compatibility with other Yacc implementations,
3736 and is required by POSIX.
3738 ** Locations columns and lines start at 1.
3739 In accordance with the GNU Coding Standards and Emacs.
3741 ** You may now declare per-type and default %destructor's and %printer's:
3745 %union { char *string; }
3746 %token <string> STRING1
3747 %token <string> STRING2
3748 %type <string> string1
3749 %type <string> string2
3750 %union { char character; }
3751 %token <character> CHR
3752 %type <character> chr
3753 %destructor { free ($$); } %symbol-default
3754 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3755 %destructor { } <character>
3757 guarantees that, when the parser discards any user-defined symbol that has a
3758 semantic type tag other than "<character>", it passes its semantic value to
3759 "free". However, when the parser discards a "STRING1" or a "string1", it
3760 also prints its line number to "stdout". It performs only the second
3761 "%destructor" in this case, so it invokes "free" only once.
3763 [Although we failed to mention this here in the 2.3a release, the default
3764 %destructor's and %printer's were experimental, and they were rewritten in
3767 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3768 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3769 associating token numbers with token names. Removing the #define statements
3770 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3771 requires them. Bison still generates an enum for token names in all cases.
3773 ** Handling of traditional Yacc prologue blocks is now more consistent but
3774 potentially incompatible with previous releases of Bison.
3776 As before, you declare prologue blocks in your grammar file with the
3777 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3778 prologue blocks that you've declared before the first %union. To generate
3779 the post-prologue, Bison concatenates all prologue blocks that you've
3780 declared after the first %union.
3782 Previous releases of Bison inserted the pre-prologue into both the header
3783 file and the code file in all cases except for LALR(1) parsers in C. In the
3784 latter case, Bison inserted it only into the code file. For parsers in C++,
3785 the point of insertion was before any token definitions (which associate
3786 token numbers with names). For parsers in C, the point of insertion was
3787 after the token definitions.
3789 Now, Bison never inserts the pre-prologue into the header file. In the code
3790 file, it always inserts it before the token definitions.
3792 ** Bison now provides a more flexible alternative to the traditional Yacc
3793 prologue blocks: %before-header, %start-header, %end-header, and
3796 For example, the following declaration order in the grammar file reflects the
3797 order in which Bison will output these code blocks. However, you are free to
3798 declare these code blocks in your grammar file in whatever order is most
3802 /* Bison treats this block like a pre-prologue block: it inserts it into
3803 * the code file before the contents of the header file. It does *not*
3804 * insert it into the header file. This is a good place to put
3805 * #include's that you want at the top of your code file. A common
3806 * example is '#include "system.h"'. */
3809 /* Bison inserts this block into both the header file and the code file.
3810 * In both files, the point of insertion is before any Bison-generated
3811 * token, semantic type, location type, and class definitions. This is a
3812 * good place to define %union dependencies, for example. */
3815 /* Unlike the traditional Yacc prologue blocks, the output order for the
3816 * new %*-header blocks is not affected by their declaration position
3817 * relative to any %union in the grammar file. */
3820 /* Bison inserts this block into both the header file and the code file.
3821 * In both files, the point of insertion is after the Bison-generated
3822 * definitions. This is a good place to declare or define public
3823 * functions or data structures that depend on the Bison-generated
3827 /* Bison treats this block like a post-prologue block: it inserts it into
3828 * the code file after the contents of the header file. It does *not*
3829 * insert it into the header file. This is a good place to declare or
3830 * define internal functions or data structures that depend on the
3831 * Bison-generated definitions. */
3834 If you have multiple occurrences of any one of the above declarations, Bison
3835 will concatenate the contents in declaration order.
3837 [Although we failed to mention this here in the 2.3a release, the prologue
3838 alternatives were experimental, and they were rewritten in future versions.]
3840 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3841 The old spelling still works, but is not documented and may be removed
3842 in a future release.
3845 * Noteworthy changes in release 2.3 (2006-06-05)
3847 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3848 for compatibility with LALR(1) grammars.
3850 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3851 be to a type name that does not contain parentheses or brackets.
3854 * Noteworthy changes in release 2.2 (2006-05-19)
3856 ** The distribution terms for all Bison-generated parsers now permit
3857 using the parsers in nonfree programs. Previously, this permission
3858 was granted only for Bison-generated LALR(1) parsers in C.
3860 ** %name-prefix changes the namespace name in C++ outputs.
3862 ** The C++ parsers export their token_type.
3864 ** Bison now allows multiple %union declarations, and concatenates
3865 their contents together.
3867 ** New warning: unused values
3868 Right-hand side symbols whose values are not used are reported,
3869 if the symbols have destructors. For instance:
3871 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3875 will trigger a warning about $$ and $5 in the first rule, and $3 in
3876 the second ($1 is copied to $$ by the default rule). This example
3877 most likely contains three errors, and could be rewritten as:
3879 exp: exp "?" exp ":" exp
3880 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3882 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3885 However, if the original actions were really intended, memory leaks
3886 and all, the warnings can be suppressed by letting Bison believe the
3887 values are used, e.g.:
3889 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3890 | exp "+" exp { $$ = $1; (void) $3; }
3893 If there are midrule actions, the warning is issued if no action
3894 uses it. The following triggers no warning: $1 and $3 are used.
3896 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3898 The warning is intended to help catching lost values and memory leaks.
3899 If a value is ignored, its associated memory typically is not reclaimed.
3901 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3902 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3903 and YYERROR, for all objects on the stack, other than objects
3904 corresponding to the right-hand side of the current rule.
3906 ** %expect, %expect-rr
3907 Incorrect numbers of expected conflicts are now actual errors,
3908 instead of warnings.
3910 ** GLR, YACC parsers.
3911 The %parse-params are available in the destructors (and the
3912 experimental printers) as per the documentation.
3914 ** Bison now warns if it finds a stray "$" or "@" in an action.
3916 ** %require "VERSION"
3917 This specifies that the grammar file depends on features implemented
3918 in Bison version VERSION or higher.
3920 ** lalr1.cc: The token and value types are now class members.
3921 The tokens were defined as free form enums and cpp macros. YYSTYPE
3922 was defined as a free form union. They are now class members:
3923 tokens are enumerations of the "yy::parser::token" struct, and the
3924 semantic values have the "yy::parser::semantic_type" type.
3926 If you do not want or can update to this scheme, the directive
3927 '%define "global_tokens_and_yystype" "1"' triggers the global
3928 definition of tokens and YYSTYPE. This change is suitable both
3929 for previous releases of Bison, and this one.
3931 If you wish to update, then make sure older version of Bison will
3932 fail using '%require "2.2"'.
3934 ** DJGPP support added.
3937 * Noteworthy changes in release 2.1 (2005-09-16)
3939 ** The C++ lalr1.cc skeleton supports %lex-param.
3941 ** Bison-generated parsers now support the translation of diagnostics like
3942 "syntax error" into languages other than English. The default
3943 language is still English. For details, please see the new
3944 Internationalization section of the Bison manual. Software
3945 distributors should also see the new PACKAGING file. Thanks to
3946 Bruno Haible for this new feature.
3948 ** Wording in the Bison-generated parsers has been changed slightly to
3949 simplify translation. In particular, the message "memory exhausted"
3950 has replaced "parser stack overflow", as the old message was not
3951 always accurate for modern Bison-generated parsers.
3953 ** Destructors are now called when the parser aborts, for all symbols left
3954 behind on the stack. Also, the start symbol is now destroyed after a
3955 successful parse. In both cases, the behavior was formerly inconsistent.
3957 ** When generating verbose diagnostics, Bison-generated parsers no longer
3958 quote the literal strings associated with tokens. For example, for
3959 a syntax error associated with '%token NUM "number"' they might
3960 print 'syntax error, unexpected number' instead of 'syntax error,
3961 unexpected "number"'.
3964 * Noteworthy changes in release 2.0 (2004-12-25)
3966 ** Possibly-incompatible changes
3968 - Bison-generated parsers no longer default to using the alloca function
3969 (when available) to extend the parser stack, due to widespread
3970 problems in unchecked stack-overflow detection. You can "#define
3971 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3972 the manual to determine safe values for YYMAXDEPTH in that case.
3974 - Error token location.
3975 During error recovery, the location of the syntax error is updated
3976 to cover the whole sequence covered by the error token: it includes
3977 the shifted symbols thrown away during the first part of the error
3978 recovery, and the lookahead rejected during the second part.
3980 - Semicolon changes:
3981 . Stray semicolons are no longer allowed at the start of a grammar.
3982 . Semicolons are now required after in-grammar declarations.
3984 - Unescaped newlines are no longer allowed in character constants or
3985 string literals. They were never portable, and GCC 3.4.0 has
3986 dropped support for them. Better diagnostics are now generated if
3987 forget a closing quote.
3989 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
3993 - GLR grammars now support locations.
3995 - New directive: %initial-action.
3996 This directive allows the user to run arbitrary code (including
3997 initializing @$) from yyparse before parsing starts.
3999 - A new directive "%expect-rr N" specifies the expected number of
4000 reduce/reduce conflicts in GLR parsers.
4002 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4003 This is a GNU extension.
4005 - The option "--report=lookahead" was changed to "--report=look-ahead".
4006 [However, this was changed back after 2.3.]
4008 - Experimental %destructor support has been added to lalr1.cc.
4010 - New configure option --disable-yacc, to disable installation of the
4011 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4015 - For now, %expect-count violations are now just warnings, not errors.
4016 This is for compatibility with Bison 1.75 and earlier (when there are
4017 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4018 are too many or too few shift/reduce conflicts). However, in future
4019 versions of Bison we plan to improve the %expect machinery so that
4020 these violations will become errors again.
4022 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4023 arbitrarily limited to 16-bit counts.
4025 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4028 * Noteworthy changes in release 1.875 (2003-01-01)
4030 ** The documentation license has been upgraded to version 1.2
4031 of the GNU Free Documentation License.
4033 ** syntax error processing
4035 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4036 locations too. This fixes bugs in error-location computation.
4039 It is now possible to reclaim the memory associated to symbols
4040 discarded during error recovery. This feature is still experimental.
4043 This new directive is preferred over YYERROR_VERBOSE.
4045 - #defining yyerror to steal internal variables is discouraged.
4046 It is not guaranteed to work forever.
4048 ** POSIX conformance
4050 - Semicolons are once again optional at the end of grammar rules.
4051 This reverts to the behavior of Bison 1.33 and earlier, and improves
4052 compatibility with Yacc.
4054 - "parse error" -> "syntax error"
4055 Bison now uniformly uses the term "syntax error"; formerly, the code
4056 and manual sometimes used the term "parse error" instead. POSIX
4057 requires "syntax error" in diagnostics, and it was thought better to
4060 - The documentation now emphasizes that yylex and yyerror must be
4061 declared before use. C99 requires this.
4063 - Bison now parses C99 lexical constructs like UCNs and
4064 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4066 - File names are properly escaped in C output. E.g., foo\bar.y is
4067 output as "foo\\bar.y".
4069 - Yacc command and library now available
4070 The Bison distribution now installs a "yacc" command, as POSIX requires.
4071 Also, Bison now installs a small library liby.a containing
4072 implementations of Yacc-compatible yyerror and main functions.
4073 This library is normally not useful, but POSIX requires it.
4075 - Type clashes now generate warnings, not errors.
4077 - If the user does not define YYSTYPE as a macro, Bison now declares it
4078 using typedef instead of defining it as a macro.
4079 For consistency, YYLTYPE is also declared instead of defined.
4081 ** Other compatibility issues
4083 - %union directives can now have a tag before the "{", e.g., the
4084 directive "%union foo {...}" now generates the C code
4085 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4086 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4087 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4088 This is for compatibility with both Yacc and Bison 1.35.
4090 - ";" is output before the terminating "}" of an action, for
4091 compatibility with Bison 1.35.
4093 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4094 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4096 - "yystype" and "yyltype" are now obsolescent macros instead of being
4097 typedefs or tags; they are no longer documented and are planned to be
4098 withdrawn in a future release.
4103 Users of Bison have to decide how they handle the portability of the
4106 - "parsing stack overflow..." -> "parser stack overflow"
4107 GLR parsers now report "parser stack overflow" as per the Bison manual.
4109 ** %parse-param and %lex-param
4110 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4111 additional context to yyparse and yylex. They suffer from several
4114 - a single argument only can be added,
4115 - their types are weak (void *),
4116 - this context is not passed to ancillary functions such as yyerror,
4117 - only yacc.c parsers support them.
4119 The new %parse-param/%lex-param directives provide a more precise control.
4122 %parse-param {int *nastiness}
4123 %lex-param {int *nastiness}
4124 %parse-param {int *randomness}
4126 results in the following signatures:
4128 int yylex (int *nastiness);
4129 int yyparse (int *nastiness, int *randomness);
4131 or, if both %pure-parser and %locations are used:
4133 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4134 int yyparse (int *nastiness, int *randomness);
4136 ** Bison now warns if it detects conflicting outputs to the same file,
4137 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4138 that command outputs both code and header to foo.h.
4140 ** #line in output files
4141 - --no-line works properly.
4143 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4144 later to be built. This change originally took place a few versions
4145 ago, but nobody noticed until we recently asked someone to try
4146 building Bison with a K&R C compiler.
4149 * Noteworthy changes in release 1.75 (2002-10-14)
4151 ** Bison should now work on 64-bit hosts.
4153 ** Indonesian translation thanks to Tedi Heriyanto.
4156 Fix spurious parse errors.
4159 Some people redefine yyerror to steal yyparse' private variables.
4160 Reenable this trick until an official feature replaces it.
4163 In agreement with POSIX and with other Yaccs, leaving a default
4164 action is valid when $$ is untyped, and $1 typed:
4168 but the converse remains an error:
4172 ** Values of midrule actions
4175 foo: { ... } { $$ = $1; } ...
4177 was incorrectly rejected: $1 is defined in the second midrule
4178 action, and is equal to the $$ of the first midrule action.
4181 * Noteworthy changes in release 1.50 (2002-10-04)
4186 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4187 almost any context-free grammar, ambiguous or not. The new declarations
4188 %dprec and %merge on grammar rules allow parse-time resolution of
4189 ambiguities. Contributed by Paul Hilfinger.
4191 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4192 like the Alpha, so please stick to 32-bit hosts for now.
4195 When not in Yacc compatibility mode, when the output file was not
4196 specified, running "bison foo/bar.y" created "foo/bar.c". It
4197 now creates "bar.c".
4200 The undefined token was systematically mapped to 2 which prevented
4201 the use of 2 by the user. This is no longer the case.
4203 ** Unknown token numbers
4204 If yylex returned an out of range value, yyparse could die. This is
4208 According to POSIX, the error token must be 256.
4209 Bison extends this requirement by making it a preference: *if* the
4210 user specified that one of her tokens is numbered 256, then error
4211 will be mapped onto another number.
4213 ** Verbose error messages
4214 They no longer report "..., expecting error or..." for states where
4215 error recovery is possible.
4218 Defaults to "$end" instead of "$".
4220 ** Error recovery now conforms to documentation and to POSIX
4221 When a Bison-generated parser encounters a syntax error, it now pops
4222 the stack until it finds a state that allows shifting the error
4223 token. Formerly, it popped the stack until it found a state that
4224 allowed some non-error action other than a default reduction on the
4225 error token. The new behavior has long been the documented behavior,
4226 and has long been required by POSIX. For more details, please see
4227 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4228 <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.
4231 Popped tokens and nonterminals are now reported.
4234 Larger grammars are now supported (larger token numbers, larger grammar
4235 size (= sum of the LHS and RHS lengths), larger LALR tables).
4236 Formerly, many of these numbers ran afoul of 16-bit limits;
4237 now these limits are 32 bits on most hosts.
4239 ** Explicit initial rule
4240 Bison used to play hacks with the initial rule, which the user does
4241 not write. It is now explicit, and visible in the reports and
4245 Before, Bison reported the useless rules, but, although not used,
4246 included them in the parsers. They are now actually removed.
4248 ** Useless rules, useless nonterminals
4249 They are now reported, as a warning, with their locations.
4251 ** Rules never reduced
4252 Rules that can never be reduced because of conflicts are now
4255 ** Incorrect "Token not used"
4256 On a grammar such as
4258 %token useless useful
4260 exp: '0' %prec useful;
4262 where a token was used to set the precedence of the last rule,
4263 bison reported both "useful" and "useless" as useless tokens.
4265 ** Revert the C++ namespace changes introduced in 1.31
4266 as they caused too many portability hassles.
4268 ** Default locations
4269 By an accident of design, the default computation of @$ was
4270 performed after another default computation was performed: @$ = @1.
4271 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4272 the computation of @$.
4274 ** Token end-of-file
4275 The token end of file may be specified by the user, in which case,
4276 the user symbol is used in the reports, the graphs, and the verbose
4277 error messages instead of "$end", which remains being the default.
4281 %token MYEOF 0 "end of file"
4284 This old option, which has been broken for ages, is removed.
4287 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4288 Croatian, thanks to Denis Lackovic.
4290 ** Incorrect token definitions
4293 bison used to output
4296 ** Token definitions as enums
4297 Tokens are output both as the traditional #define's, and, provided
4298 the compiler supports ANSI C or is a C++ compiler, as enums.
4299 This lets debuggers display names instead of integers.
4302 In addition to --verbose, bison supports --report=THINGS, which
4303 produces additional information:
4305 complete the core item sets with their closure
4306 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4307 explicitly associate lookahead tokens to items
4309 describe shift/reduce conflicts solving.
4310 Bison used to systematically output this information on top of
4311 the report. Solved conflicts are now attached to their states.
4314 Previous versions don't complain when there is a type clash on
4315 the default action if the rule has a midrule action, such as in:
4323 ** GNU M4 is now required when using Bison.
4326 * Noteworthy changes in release 1.35 (2002-03-25)
4329 Some projects use Bison's C parser with C++ compilers, and define
4330 YYSTYPE as a class. The recent adjustment of C parsers for data
4331 alignment and 64 bit architectures made this impossible.
4333 Because for the time being no real solution for C++ parser
4334 generation exists, kludges were implemented in the parser to
4335 maintain this use. In the future, when Bison has C++ parsers, this
4336 kludge will be disabled.
4338 This kludge also addresses some C++ problems when the stack was
4342 * Noteworthy changes in release 1.34 (2002-03-12)
4344 ** File name clashes are detected
4345 $ bison foo.y -d -o foo.x
4346 fatal error: header and parser would both be named "foo.x"
4348 ** A missing ";" at the end of a rule triggers a warning
4349 In accordance with POSIX, and in agreement with other
4350 Yacc implementations, Bison will mandate this semicolon in the near
4351 future. This eases the implementation of a Bison parser of Bison
4352 grammars by making this grammar LALR(1) instead of LR(2). To
4353 facilitate the transition, this release introduces a warning.
4355 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4356 many portability hassles.
4358 ** DJGPP support added.
4360 ** Fix test suite portability problems.
4363 * Noteworthy changes in release 1.33 (2002-02-07)
4366 Groff could not be compiled for the definition of size_t was lacking
4367 under some conditions.
4373 * Noteworthy changes in release 1.32 (2002-01-23)
4375 ** Fix Yacc output file names
4377 ** Portability fixes
4379 ** Italian, Dutch translations
4382 * Noteworthy changes in release 1.31 (2002-01-14)
4386 ** GNU Gettext and %expect
4387 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4388 Bison dies on incorrect %expectations, we fear there will be
4389 too many bug reports for Gettext, so _for the time being_, %expect
4390 does not trigger an error when the input file is named "plural.y".
4392 ** Use of alloca in parsers
4393 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4394 malloc exclusively. Since 1.29, but was not NEWS'ed.
4396 alloca is used only when compiled with GCC, to avoid portability
4399 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4401 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4402 (as POSIX requires) instead of being undefined.
4405 Bison has always permitted actions such as { $$ = $1 }: it adds the
4406 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4407 is no longer output: one has to write { $$ = $1; }.
4409 ** Better C++ compliance
4410 The output parsers try to respect C++ namespaces.
4411 [This turned out to be a failed experiment, and it was reverted later.]
4414 Fixed bugs when reporting useless nonterminals.
4417 The parsers work properly on 64 bit hosts.
4420 Some calls to strerror resulted in scrambled or missing error messages.
4423 When the number of shift/reduce conflicts is correct, don't issue
4426 ** The verbose report includes the rule line numbers.
4428 ** Rule line numbers are fixed in traces.
4430 ** Swedish translation
4433 Verbose parse error messages from the parsers are better looking.
4434 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4435 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4437 ** Fixed parser memory leaks.
4438 When the generated parser was using malloc to extend its stacks, the
4439 previous allocations were not freed.
4441 ** Fixed verbose output file.
4442 Some newlines were missing.
4443 Some conflicts in state descriptions were missing.
4445 ** Fixed conflict report.
4446 Option -v was needed to get the result.
4450 Mismatches are errors, not warnings.
4452 ** Fixed incorrect processing of some invalid input.
4454 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4456 ** Fixed some typos in the documentation.
4458 ** %token MY_EOF 0 is supported.
4459 Before, MY_EOF was silently renumbered as 257.
4461 ** doc/refcard.tex is updated.
4463 ** %output, %file-prefix, %name-prefix.
4467 New, aliasing "--output-file".
4470 * Noteworthy changes in release 1.30 (2001-10-26)
4472 ** "--defines" and "--graph" have now an optional argument which is the
4473 output file name. "-d" and "-g" do not change; they do not take any
4476 ** "%source_extension" and "%header_extension" are removed, failed
4479 ** Portability fixes.
4482 * Noteworthy changes in release 1.29 (2001-09-07)
4484 ** The output file does not define const, as this caused problems when used
4485 with common autoconfiguration schemes. If you still use ancient compilers
4486 that lack const, compile with the equivalent of the C compiler option
4487 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4489 ** Added "-g" and "--graph".
4491 ** The Bison manual is now distributed under the terms of the GNU FDL.
4493 ** The input and the output files has automatically a similar extension.
4495 ** Russian translation added.
4497 ** NLS support updated; should hopefully be less troublesome.
4499 ** Added the old Bison reference card.
4501 ** Added "--locations" and "%locations".
4503 ** Added "-S" and "--skeleton".
4505 ** "%raw", "-r", "--raw" is disabled.
4507 ** Special characters are escaped when output. This solves the problems
4508 of the #line lines with path names including backslashes.
4511 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4512 "%debug", "%source_extension" and "%header_extension".
4515 Automatic location tracking.
4518 * Noteworthy changes in release 1.28 (1999-07-06)
4520 ** Should compile better now with K&R compilers.
4524 ** Fixed a problem with escaping the double quote character.
4526 ** There is now a FAQ.
4529 * Noteworthy changes in release 1.27
4531 ** The make rule which prevented bison.simple from being created on
4532 some systems has been fixed.
4535 * Noteworthy changes in release 1.26
4537 ** Bison now uses Automake.
4539 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4541 ** Token numbers now start at 257 as previously documented, not 258.
4543 ** Bison honors the TMPDIR environment variable.
4545 ** A couple of buffer overruns have been fixed.
4547 ** Problems when closing files should now be reported.
4549 ** Generated parsers should now work even on operating systems which do
4550 not provide alloca().
4553 * Noteworthy changes in release 1.25 (1995-10-16)
4555 ** Errors in the input grammar are not fatal; Bison keeps reading
4556 the grammar file, and reports all the errors found in it.
4558 ** Tokens can now be specified as multiple-character strings: for
4559 example, you could use "<=" for a token which looks like <=, instead
4560 of choosing a name like LESSEQ.
4562 ** The %token_table declaration says to write a table of tokens (names
4563 and numbers) into the parser file. The yylex function can use this
4564 table to recognize multiple-character string tokens, or for other
4567 ** The %no_lines declaration says not to generate any #line preprocessor
4568 directives in the parser file.
4570 ** The %raw declaration says to use internal Bison token numbers, not
4571 Yacc-compatible token numbers, when token names are defined as macros.
4573 ** The --no-parser option produces the parser tables without including
4574 the parser engine; a project can now use its own parser engine.
4575 The actions go into a separate file called NAME.act, in the form of
4576 a switch statement body.
4579 * Noteworthy changes in release 1.23
4581 The user can define YYPARSE_PARAM as the name of an argument to be
4582 passed into yyparse. The argument should have type void *. It should
4583 actually point to an object. Grammar actions can access the variable
4584 by casting it to the proper pointer type.
4586 Line numbers in output file corrected.
4589 * Noteworthy changes in release 1.22
4591 --help option added.
4594 * Noteworthy changes in release 1.20
4596 Output file does not redefine const for C++.
4600 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4601 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4602 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4603 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4604 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4605 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4606 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4607 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4608 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4609 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4610 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4611 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4612 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4613 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4614 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4615 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4616 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4617 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4618 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4619 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4620 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4621 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4622 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4623 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4624 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4625 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4626 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4627 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4628 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4629 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4630 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4631 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4632 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4633 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4634 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4635 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4636 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4639 ispell-dictionary: "american"
4644 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4646 This file is part of Bison, the GNU Parser Generator.
4648 Permission is granted to copy, distribute and/or modify this document
4649 under the terms of the GNU Free Documentation License, Version 1.3 or
4650 any later version published by the Free Software Foundation; with no
4651 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4652 Texts. A copy of the license is included in the "GNU Free
4653 Documentation License" file as part of this distribution.