3 * Noteworthy changes in release ?.? (????-??-??) [?]
9 Prefer value_type to semantic_type to denote the semantic value type,
10 specified by the `api.value.type` %define variable.
14 There were not debug traces for deferred calls to user actions. They are
19 *** Option -H, --header and directive %header
21 The option `-H`/`--header` supersedes the option `--defines`, and the
22 directive %header supersedes %defines. Both `--defines` and `%defines`
23 are, of course, maintained for backward compatibility.
27 Since version 2.4 Bison can be used to generate HTML reports. However it
28 was a two-step process: first bison must be invoked with option `--xml`,
29 and then xsltproc must be run to the convert the XML reports into HTML.
31 The new option `--html` combines these steps. The xsltproc program must
34 *** A C++ native GLR parser
36 A new version of the generated C++ GLR parser was added as "glr2.cc". It
37 is forked from the existing glr.c/cc parser, with the objective of making
38 it a more modern, truly C++ parser (instead of a C++ wrapper around a C
39 parser). Down the line, the goal is to support `%define api.value.type
40 variant` and maybe share code with lalr1.cc.
42 The current parser should be identical in terms of interface, functionality
43 and performance to "glr.cc". To try it out, simply use
49 Counterexamples now show the rule numbers, and always show ε for rules
50 with an empty right-hand side. For instance
62 *** Lookahead correction in Java
64 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
67 *** Abort parsing for memory exhaustion (C)
69 The user actions may now use YYNOMEM to abort the current parse with
73 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
77 *** Bug fixes in yacc.c
79 In Yacc mode, all the tokens are defined twice: once as an enum, and then
80 as a macro. YYEMPTY was missing its macro.
82 *** Bug fixes in lalr1.cc
84 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
85 even when the `parse.assert` %define variable is not enabled. It no
88 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
91 When there is a very large number of tokens, some assertions could be long
92 enough to hit arbitrary limits in Visual C++. They have been rewritten to
93 work around this limitation.
97 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
98 skeleton) used to be defined to 1. It is now defined to the version of
99 Bison as an integer (e.g., 30704 for version 3.7.4).
102 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
106 Fix concurrent build issues.
108 The bison executable is no longer linked uselessly against libreadline.
110 Fix incorrect use of yytname in glr.cc.
113 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
115 This release of Bison fixes all known bugs reported for Bison in MITRE's
116 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
117 are only about bison-the-program itself, not the generated code.
119 Although these bugs are typically irrelevant to how Bison is used, they
120 are worth fixing if only to give users peace of mind.
122 There is no known vulnerability in the generated parsers.
126 Fix concurrent build issues (introduced in Bison 3.5).
128 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
130 Fix portability issues of the test suite, and of bison itself.
132 Some unlikely crashes found by fuzzing have been fixed. This is only
133 about bison itself, not the generated parsers.
136 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
140 Crash when a token alias contains a NUL byte.
142 Portability issues with libtextstyle.
144 Portability issues of Bison itself with MSVC.
148 Improvements and fixes in the documentation.
150 More precise location about symbol type redefinitions.
153 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
155 ** Deprecated features
157 The YYPRINT macro, which works only with yacc.c and only for tokens, was
158 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
159 It is deprecated and its support will be removed eventually.
161 In conformance with the recommendations of the Graphviz team, in the next
162 version Bison the option `--graph` will generate a *.gv file by default,
163 instead of *.dot. A transition started in Bison 3.4.
167 *** Counterexample Generation
169 Contributed by Vincent Imbimbo.
171 When given `-Wcounterexamples`/`-Wcex`, bison will now output
172 counterexamples for conflicts.
174 **** Unifying Counterexamples
176 Unifying counterexamples are strings which can be parsed in two ways due
177 to the conflict. For example on a grammar that contains the usual
178 "dangling else" ambiguity:
181 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
182 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
185 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
186 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
187 Example: "if" exp "then" "if" exp "then" exp • "else" exp
190 ↳ "if" exp "then" exp
191 ↳ "if" exp "then" exp • "else" exp
192 Example: "if" exp "then" "if" exp "then" exp • "else" exp
195 ↳ "if" exp "then" exp "else" exp
196 ↳ "if" exp "then" exp •
198 When text styling is enabled, colors are used in the examples and the
199 derivations to highlight the structure of both analyses. In this case,
201 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
205 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
208 The counterexamples are "focused", in two different ways. First, they do
209 not clutter the output with all the derivations from the start symbol,
210 rather they start on the "conflicted nonterminal". They go straight to the
211 point. Second, they don't "expand" nonterminal symbols uselessly.
213 **** Nonunifying Counterexamples
215 In the case of the dangling else, Bison found an example that can be
216 parsed in two ways (therefore proving that the grammar is ambiguous).
217 When it cannot find such an example, it instead generates two examples
218 that are the same up until the dot:
221 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
222 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
223 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
228 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
229 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
230 First example: expr • ID ',' ID $end
237 Second example: expr • ID $end
243 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
247 In these cases, the parser usually doesn't have enough lookahead to
248 differentiate the two given examples.
252 Counterexamples are also included in the report when given
253 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
258 1 exp: "if" exp "then" exp • [$end, "then", "else"]
259 2 | "if" exp "then" exp • "else" exp
261 "else" shift, and go to state 8
263 "else" [reduce using rule 1 (exp)]
264 $default reduce using rule 1 (exp)
266 shift/reduce conflict on token "else":
267 1 exp: "if" exp "then" exp •
268 2 exp: "if" exp "then" exp • "else" exp
269 Example: "if" exp "then" "if" exp "then" exp • "else" exp
272 ↳ "if" exp "then" exp
273 ↳ "if" exp "then" exp • "else" exp
274 Example: "if" exp "then" "if" exp "then" exp • "else" exp
277 ↳ "if" exp "then" exp "else" exp
278 ↳ "if" exp "then" exp •
280 *** File prefix mapping
282 Contributed by Joshua Watt.
284 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
285 in the output (specifically `#line` directives and `#ifdef` header guards)
286 that begins with the prefix OLD will have it replaced with the prefix NEW,
287 similar to the `-ffile-prefix-map` in GCC. This option can be used to
288 make bison output reproducible.
294 When text styling is enabled and the terminal supports it, the warnings
295 now include hyperlinks to the documentation.
297 *** Relocatable installation
299 When installed to be relocatable (via `configure --enable-relocatable`),
300 bison will now also look for a relocated m4.
304 The `filename_type` %define variable was renamed `api.filename.type`.
307 %define filename_type "symbol"
311 %define api.filename.type {symbol}
313 (Or let `bison --update` do it for you).
315 It now defaults to `const std::string` instead of `std::string`.
317 *** Deprecated %define variable names
319 The following variables have been renamed for consistency. Backward
320 compatibility is ensured, but upgrading is recommended.
322 filename_type -> api.filename.type
323 package -> api.package
325 *** Push parsers no longer clear their state when parsing is finished
327 Previously push-parsers cleared their state when parsing was finished (on
328 success and on failure). This made it impossible to check if there were
329 parse errors, since `yynerrs` was also reset. This can be especially
330 troublesome when used in autocompletion, since a parser with error
331 recovery would suggest (irrelevant) expected tokens even if there were
334 Now the parser state can be examined when parsing is finished. The parser
335 state is reset when starting a new parse.
341 The bistromathic demonstrates %param and how to quote sources in the error
345 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
351 *** Include the generated header (yacc.c)
353 Historically, when --defines was used, bison generated a header and pasted
354 an exact copy of it into the generated parser implementation file. Since
355 Bison 3.4 it is possible to specify that the header should be `#include`d,
356 and how. For instance
358 %define api.header.include {"parse.h"}
362 %define api.header.include {<parser/parse.h>}
364 Now api.header.include defaults to `"header-basename"`, as was intended in
365 Bison 3.4, where `header-basename` is the basename of the generated
366 header. This is disabled when the generated header is `y.tab.h`, to
367 comply with Automake's ylwrap.
369 *** String aliases are faithfully propagated
371 Bison used to interpret user strings (i.e., decoding backslash escapes)
372 when reading them, and to escape them (i.e., issue non-printable
373 characters as backslash escapes, taking the locale into account) when
374 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
375 up "ciphered" as sequences of backslash escapes. This happened not only
376 in the generated sources (where the compiler will reinterpret them), but
377 also in all the generated reports (text, xml, html, dot, etc.). Reports
378 were therefore not readable when string aliases were not pure ASCII.
379 Worse yet: the output depended on the user's locale.
381 Now Bison faithfully treats the string aliases exactly the way the user
382 spelled them. This fixes all the aforementioned problems. However, now,
383 string aliases semantically equivalent but syntactically different (e.g.,
384 "A", "\x41", "\101") are considered to be different.
386 *** Crash when generating IELR
388 An old, well hidden, bug in the generation of IELR parsers was fixed.
391 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
395 In glr.cc some internal macros leaked in the user's code, and could damage
396 access to the token kinds.
399 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
403 Incorrect comments in the generated parsers.
405 Warnings in push parsers (yacc.c).
407 Incorrect display of gotos in LAC traces (lalr1.cc).
410 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
414 Some tests were fixed.
416 When token aliases contain comment delimiters:
418 %token FOO "/* foo */"
420 bison used to emit "nested" comments, which is invalid C.
423 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
427 Restored ANSI-C compliance in yacc.c.
429 GNU readline portability issues.
431 In C++, yy::parser::symbol_name is now a public member, as was intended.
435 In C++, yy::parser::symbol_type now has a public name() member function.
438 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
440 ** Backward incompatible changes
442 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
444 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
445 depend on it will now produce Yacc-like error messages (just "syntax
446 error"). It was superseded by the "%error-verbose" directive in Bison
447 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
448 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
449 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
450 parse.error verbose".
452 ** Deprecated features
454 The YYPRINT macro, which works only with yacc.c and only for tokens, was
455 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
456 It is deprecated and its support will be removed eventually.
460 *** Improved syntax error messages
462 Two new values for the %define parse.error variable offer more control to
463 the user. Available in all the skeletons (C, C++, Java).
465 **** %define parse.error detailed
467 The behavior of "%define parse.error detailed" is closely resembling that
468 of "%define parse.error verbose" with a few exceptions. First, it is safe
469 to use non-ASCII characters in token aliases (with 'verbose', the result
470 depends on the locale with which bison was run). Second, a yysymbol_name
471 function is exposed to the user, instead of the yytnamerr function and the
472 yytname table. Third, token internationalization is supported (see
475 **** %define parse.error custom
477 With this directive, the user forges and emits the syntax error message
478 herself by defining the yyreport_syntax_error function. A new type,
479 yypcontext_t, captures the circumstances of the error, and provides the
480 user with functions to get details, such as yypcontext_expected_tokens to
481 get the list of expected token kinds.
483 A possible implementation of yyreport_syntax_error is:
486 yyreport_syntax_error (const yypcontext_t *ctx)
489 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
490 fprintf (stderr, ": syntax error");
491 // Report the tokens expected at this point.
493 enum { TOKENMAX = 10 };
494 yysymbol_kind_t expected[TOKENMAX];
495 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
497 // Forward errors to yyparse.
500 for (int i = 0; i < n; ++i)
501 fprintf (stderr, "%s %s",
502 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
504 // Report the unexpected token.
506 yysymbol_kind_t lookahead = yypcontext_token (ctx);
507 if (lookahead != YYSYMBOL_YYEMPTY)
508 fprintf (stderr, " before %s", yysymbol_name (lookahead));
510 fprintf (stderr, "\n");
514 **** Token aliases internationalization
516 When the %define variable parse.error is set to `custom` or `detailed`,
517 one may specify which token aliases are to be translated using _(). For
529 In that case the user must define _() and N_(), and yysymbol_name returns
530 the translated symbol (i.e., it returns '_("variable")' rather that
531 '"variable"'). In Java, the user must provide an i18n() function.
533 *** List of expected tokens (yacc.c)
535 Push parsers may invoke yypstate_expected_tokens at any point during
536 parsing (including even before submitting the first token) to get the list
537 of possible tokens. This feature can be used to propose autocompletion
538 (see below the "bistromathic" example).
540 It makes little sense to use this feature without enabling LAC (lookahead
543 *** Returning the error token
545 When the scanner returns an invalid token or the undefined token
546 (YYUNDEF), the parser generates an error message and enters error
547 recovery. Because of that error message, most scanners that find lexical
548 errors generate an error message, and then ignore the invalid input
549 without entering the error-recovery.
551 The scanners may now return YYerror, the error token, to enter the
552 error-recovery mode without triggering an additional error message. See
553 the bistromathic for an example.
555 *** Deep overhaul of the symbol and token kinds
557 To avoid the confusion with types in programming languages, we now refer
558 to token and symbol "kinds" instead of token and symbol "types". The
559 documentation and error messages have been revised.
561 All the skeletons have been updated to use dedicated enum types rather
562 than integral types. Special symbols are now regular citizens, instead of
563 being declared in ad hoc ways.
567 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
568 LPAREN, etc. While backward compatibility is of course ensured, users are
569 nonetheless invited to replace their uses of "enum yytokentype" by
572 This type now also includes tokens that were previously hidden: YYEOF (end
573 of input), YYUNDEF (undefined token), and YYerror (error token). They
574 now have string aliases, internationalized when internationalization is
575 enabled. Therefore, by default, error messages now refer to "end of file"
576 (internationalized) rather than the cryptic "$end", or to "invalid token"
577 rather than "$undefined".
579 Therefore in most cases it is now useless to define the end-of-line token
582 %token T_EOF 0 "end of file"
584 Rather simply use "YYEOF" in your scanner.
588 The "symbol kinds" is what the parser actually uses. (Unless the
589 api.token.raw %define variable is used, the symbol kind of a terminal
590 differs from the corresponding token kind.)
592 They are now exposed as a enum, "yysymbol_kind_t".
594 This allows users to tailor the error messages the way they want, or to
595 process some symbols in a specific way in autocompletion (see the
596 bistromathic example below).
598 *** Modernize display of explanatory statements in diagnostics
600 Since Bison 2.7, output was indented four spaces for explanatory
601 statements. For example:
603 input.y:2.7-13: error: %type redeclaration for exp
604 input.y:1.7-11: previous declaration
606 Since the introduction of caret-diagnostics, it became less clear. This
607 indentation has been removed and submessages are displayed similarly as in
610 input.y:2.7-13: error: %type redeclaration for exp
611 2 | %type <float> exp
613 input.y:1.7-11: note: previous declaration
617 Contributed by Victor Morales Cayuela.
621 The token and symbol kinds are yy::parser::token_kind_type and
622 yy::parser::symbol_kind_type.
624 The symbol_type::kind() member function allows to get the kind of a
625 symbol. This can be used to write unit tests for scanners, e.g.,
627 yy::parser::symbol_type t = make_NUMBER ("123");
628 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
629 assert (t.value.as<int> () == 123);
635 In order to avoid ambiguities with "type" as in "typing", we now refer to
636 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
637 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
642 There are now examples/java: a very simple calculator, and a more complete
643 one (push-parser, location tracking, and debug traces).
645 The lexcalc example (a simple example in C based on Flex and Bison) now
646 also demonstrates location tracking.
649 A new C example, bistromathic, is a fully featured interactive calculator
650 using many Bison features: pure interface, push parser, autocompletion
651 based on the current parser state (using yypstate_expected_tokens),
652 location tracking, internationalized custom error messages, lookahead
653 correction, rich debug traces, etc.
655 It shows how to depend on the symbol kinds to tailor autocompletion. For
656 instance it recognizes the symbol kind "VARIABLE" to propose
657 autocompletion on the existing variables, rather than of the word
661 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
663 ** WARNING: Future backward-incompatibilities!
665 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
667 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
668 that still depend on it will produce Yacc-like error messages (just
669 "syntax error"). It was superseded by the "%error-verbose" directive in
670 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
671 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
672 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
673 parse.error verbose".
677 Fix portability issues of the package itself on old compilers.
679 Fix api.token.raw support in Java.
682 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
686 Error messages could quote lines containing zero-width characters (such as
687 \005) with incorrect styling. Fixes for similar issues with unexpectedly
688 short lines (e.g., the file was changed between parsing and diagnosing).
690 Some unlikely crashes found by fuzzing have been fixed. This is only
691 about bison itself, not the generated parsers.
694 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
698 Portability issues and minor cosmetic issues.
700 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
704 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
710 Fix compiler warnings.
713 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
715 ** Backward incompatible changes
717 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
718 longer treated as end-of-lines. This changes the diagnostics, and in
719 particular their locations.
721 In C++, line numbers and columns are now represented as 'int' not
722 'unsigned', so that integer overflow on positions is easily checkable via
723 'gcc -fsanitize=undefined' and the like. This affects the API for
724 positions. The default position and location classes now expose
725 'counter_type' (int), used to define line and column numbers.
727 ** Deprecated features
729 The YYPRINT macro, which works only with yacc.c and only for tokens, was
730 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
731 It is deprecated and its support will be removed eventually.
735 *** Lookahead correction in C++
737 Contributed by Adrian Vogelsgesang.
739 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
740 %define variable parse.lac.
742 *** Variable api.token.raw: Optimized token numbers (all skeletons)
744 In the generated parsers, tokens have two numbers: the "external" token
745 number as returned by yylex (which starts at 257), and the "internal"
746 symbol number (which starts at 3). Each time yylex is called, a table
747 lookup maps the external token number to the internal symbol number.
749 When the %define variable api.token.raw is set, tokens are assigned their
750 internal number, which saves one table lookup per token, and also saves
751 the generation of the mapping table.
753 The gain is typically moderate, but in extreme cases (very simple user
754 actions), a 10% improvement can be observed.
756 *** Generated parsers use better types for states
758 Stacks now use the best integral type for state numbers, instead of always
759 using 15 bits. As a result "small" parsers now have a smaller memory
760 footprint (they use 8 bits), and there is support for large automata (16
761 bits), and extra large (using int, i.e., typically 31 bits).
763 *** Generated parsers prefer signed integer types
765 Bison skeletons now prefer signed to unsigned integer types when either
766 will do, as the signed types are less error-prone and allow for better
767 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
768 portable to unusual machines where char, short and int are all the same
769 width. On non-GNU platforms this may entail including <limits.h> and (if
770 available) <stdint.h> to define integer types and constants.
772 *** A skeleton for the D programming language
774 For the last few releases, Bison has shipped a stealth experimental
775 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
776 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
779 However, because nobody has committed to improving, testing, and
780 documenting this skeleton, it is not clear that it will be supported in
783 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
784 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
786 *** Debug traces in Java
788 The Java backend no longer emits code and data for parser tracing if the
789 %define variable parse.trace is not defined.
793 *** New diagnostic: -Wdangling-alias
795 String literals, which allow for better error messages, are (too)
796 liberally accepted by Bison, which might result in silent errors. For
799 %type <exVal> cond "condition"
801 does not define "condition" as a string alias to 'cond' (nonterminal
802 symbols do not have string aliases). It is rather equivalent to
805 %token <exVal> "condition"
807 i.e., it gives the type 'exVal' to the "condition" token, which was
808 clearly not the intention.
810 Also, because string aliases need not be defined, typos such as "baz"
811 instead of "bar" will be not reported.
813 The option `-Wdangling-alias` catches these situations. On
816 %type <ival> foo "foo"
820 bison -Wdangling-alias reports
822 warning: string literal not attached to a symbol
823 | %type <ival> foo "foo"
825 warning: string literal not attached to a symbol
829 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
831 *** Better POSIX Yacc compatibility diagnostics
833 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
837 %type <ival> TOKEN1 TOKEN2 't'
844 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
845 2 | %type <ival> TOKEN1 TOKEN2 't'
847 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
848 2 | %type <ival> TOKEN1 TOKEN2 't'
850 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
851 2 | %type <ival> TOKEN1 TOKEN2 't'
854 *** Diagnostics with insertion
856 The diagnostics now display the suggestion below the underlined source.
857 Replacement for undeclared symbols are now also suggested.
864 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'?
868 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
872 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
874 *** Diagnostics about long lines
876 Quoted sources may now be truncated to fit the screen. For instance, on a
877 30-column wide terminal:
884 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
887 foo.y:1.8-10: previous declaration
890 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
893 foo.y:1.8-10: previous declaration
899 *** Debugging glr.c and glr.cc
901 The glr.c skeleton always had asserts to check its own behavior (not the
902 user's). These assertions are now under the control of the parse.assert
903 %define variable (disabled by default).
907 Several new compiler warnings in the generated output have been avoided.
908 Some unused features are no longer emitted. Cleaner generated code in
913 Portability issues in the test suite.
915 In theory, parsers using %nonassoc could crash when reporting verbose
916 error messages. This unlikely bug has been fixed.
918 In Java, %define api.prefix was ignored. It now behaves as expected.
921 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
925 In some cases, when warnings are disabled, bison could emit tons of white
926 spaces as diagnostics.
928 When running out of memory, bison could crash (found by fuzzing).
930 When defining twice the EOF token, bison would crash.
932 New warnings from recent compilers have been addressed in the generated
933 parsers (yacc.c, glr.c, glr.cc).
935 When lone carriage-return characters appeared in the input file,
936 diagnostics could hang forever.
939 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
946 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
948 ** Deprecated features
950 The %pure-parser directive is deprecated in favor of '%define api.pure'
951 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
952 now. Note that since Bison 2.7 you are strongly encouraged to use
953 '%define api.pure full' instead of '%define api.pure'.
957 *** Colored diagnostics
959 As an experimental feature, diagnostics are now colored, controlled by the
960 new options --color and --style.
962 To use them, install the libtextstyle library before configuring Bison.
965 https://alpha.gnu.org/gnu/gettext/
969 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
971 The option --color supports the following arguments:
972 - always, yes: Enable colors.
973 - never, no: Disable colors.
974 - auto, tty (default): Enable colors if the output device is a tty.
976 To customize the styles, create a CSS file similar to
980 .error { font-weight: 800; text-decoration: underline; }
983 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
984 environment variable to "bison-bw.css".
988 When given -fsyntax-only, the diagnostics are reported, but no output is
991 The name of this option is somewhat misleading as bison does more than
992 just checking the syntax: every stage is run (including checking for
993 conflicts for instance), except the generation of the output files.
995 *** Include the generated header (yacc.c)
997 Before, when --defines is used, bison generated a header, and pasted an
998 exact copy of it into the generated parser implementation file. If the
999 header name is not "y.tab.h", it is now #included instead of being
1002 To use an '#include' even if the header name is "y.tab.h" (which is what
1003 happens with --yacc, or when using the Autotools' ylwrap), define
1004 api.header.include to the exact argument to pass to #include. For
1007 %define api.header.include {"parse.h"}
1011 %define api.header.include {<parser/parse.h>}
1013 *** api.location.type is now supported in C (yacc.c, glr.c)
1015 The %define variable api.location.type defines the name of the type to use
1016 for locations. When defined, Bison no longer defines YYLTYPE.
1018 This can be used in programs with several parsers to factor their
1019 definition of locations: let one of them generate them, and the others
1026 In conformance with the recommendations of the Graphviz team, if %require
1027 "3.4" (or better) is specified, the option --graph generates a *.gv file
1028 by default, instead of *.dot.
1030 *** Diagnostics overhaul
1032 Column numbers were wrong with multibyte characters, which would also
1033 result in skewed diagnostics with carets. Beside, because we were
1034 indenting the quoted source with a single space, lines with tab characters
1035 were incorrectly underlined.
1037 To address these issues, and to be clearer, Bison now issues diagnostics
1038 as GCC9 does. For instance it used to display (there's a tab before the
1041 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1042 expr: expr '+' "number" { $$ = $1 + $2; }
1046 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1047 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1050 Other constructs now also have better locations, resulting in more precise
1053 *** Fix-it hints for %empty
1055 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1056 annotations, and add the missing ones.
1058 *** Generated reports
1060 The format of the reports (parse.output) was improved for readability.
1062 *** Better support for --no-line.
1064 When --no-line is used, the generated files are now cleaner: no lines are
1065 generated instead of empty lines. Together with using api.header.include,
1066 that should help people saving the generated files into version control
1067 systems get smaller diffs.
1071 A new example in C shows an simple infix calculator with a hand-written
1072 scanner (examples/c/calc).
1074 A new example in C shows a reentrant parser (capable of recursive calls)
1075 built with Flex and Bison (examples/c/reccalc).
1077 There is a new section about the history of Yaccs and Bison.
1081 A few obscure bugs were fixed, including the second oldest (known) bug in
1082 Bison: it was there when Bison was entered in the RCS version control
1083 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1087 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1091 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1095 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1099 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1100 extensions into errors. It now makes them simple warnings (-Wyacc).
1103 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1105 A new mailing list was created, Bison Announce. It is low traffic, and is
1106 only about announcing new releases and important messages (e.g., polls
1107 about major decisions to make).
1109 https://lists.gnu.org/mailman/listinfo/bison-announce
1111 ** Backward incompatible changes
1113 Support for DJGPP, which has been unmaintained and untested for years, is
1116 ** Deprecated features
1118 A new feature, --update (see below) helps adjusting existing grammars to
1121 *** Deprecated directives
1123 The %error-verbose directive is deprecated in favor of '%define
1124 parse.error verbose' since Bison 3.0, but no warning was issued.
1126 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1127 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1128 directives are slightly different, you might need to adjust your code.
1129 %name-prefix renames only symbols with external linkage, while api.prefix
1130 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1131 yytokentype, YYSTYPE, YYLTYPE, etc.
1133 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1134 {xx}' will typically have to update YY_DECL from
1136 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1140 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1142 *** Deprecated %define variable names
1144 The following variables, mostly related to parsers in Java, have been
1145 renamed for consistency. Backward compatibility is ensured, but upgrading
1148 abstract -> api.parser.abstract
1149 annotations -> api.parser.annotations
1150 extends -> api.parser.extends
1151 final -> api.parser.final
1152 implements -> api.parser.implements
1153 parser_class_name -> api.parser.class
1154 public -> api.parser.public
1155 strictfp -> api.parser.strictfp
1159 *** Generation of fix-its for IDEs/Editors
1161 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1162 bison now generates machine readable editing instructions to fix some
1163 issues. Currently, this is mostly limited to updating deprecated
1164 directives and removing duplicates. For instance:
1168 %define parser_class_name "Parser"
1169 %define api.parser.class "Parser"
1173 See the "fix-it:" lines below:
1175 $ bison -ffixit foo.y
1176 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1179 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1180 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1181 %define parser_class_name "Parser"
1182 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1183 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1184 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1185 %define api.parser.class "Parser"
1186 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1187 foo.y:2.1-34: previous definition
1188 %define parser_class_name "Parser"
1189 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1190 fix-it:"foo.y":{3:1-3:34}:""
1191 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1193 This uses the same output format as GCC and Clang.
1195 *** Updating grammar files
1197 Fixes can be applied on the fly. The previous example ends with the
1198 suggestion to re-run bison with the option -u/--update, which results in a
1199 cleaner grammar file.
1201 $ bison --update foo.y
1203 bison: file 'foo.y' was updated (backup: 'foo.y~')
1206 %define parse.error verbose
1207 %define api.parser.class {Parser}
1211 *** Bison is now relocatable
1213 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1215 A relocatable program can be moved or copied to a different location on
1216 the file system. It can also be used through mount points for network
1217 sharing. It is possible to make symbolic links to the installed and moved
1218 programs, and invoke them through the symbolic link.
1220 *** %expect and %expect-rr modifiers on individual rules
1222 One can now document (and check) which rules participate in shift/reduce
1223 and reduce/reduce conflicts. This is particularly important GLR parsers,
1224 where conflicts are a normal occurrence. For example,
1240 | argument_list ',' expression
1245 Looking at the output from -v, one can see that the shift/reduce conflict
1246 here is due to the fact that the parser does not know whether to reduce
1247 arguments to argument_list until it sees the token _after_ the following
1248 ','. By marking the rule with %expect 1 (because there is a conflict in
1249 one state), we document the source of the 1 overall shift/reduce conflict.
1251 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1252 conflicts. In this case, we mark each of the conflicting rules. For
1261 target_list '=' expr ';'
1267 | target ',' target_list
1276 | expr ',' expr_list
1284 In a statement such as
1288 the parser must reduce x to a target or an expr, but does not know which
1289 until it sees the '='. So we notate the two possible reductions to
1290 indicate that each conflicts in one rule.
1292 This feature needs user feedback, and might evolve in the future.
1294 *** C++: Actual token constructors
1296 When variants and token constructors are enabled, in addition to the
1297 type-safe named token constructors (make_ID, make_INT, etc.), we now
1298 generate genuine constructors for symbol_type.
1300 For instance with these declarations
1306 you may use these constructors:
1308 symbol_type (int token, const std::string&);
1309 symbol_type (int token, const int&);
1310 symbol_type (int token);
1312 Correct matching between token types and value types is checked via
1313 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1314 constructors are preferable, as they offer better type safety (for
1315 instance 'make_ID (42)' would not even compile), but symbol_type
1316 constructors may help when token types are discovered at run-time, e.g.,
1319 if (auto i = lookup_keyword (yytext))
1320 return yy::parser::symbol_type (i);
1322 return yy::parser::make_ID (yytext);
1325 *** C++: Variadic emplace
1327 If your application requires C++11 and you don't use symbol constructors,
1328 you may now use a variadic emplace for semantic values:
1330 %define api.value.type variant
1331 %token <std::pair<int, int>> PAIR
1335 int yylex (parser::semantic_type *lvalp)
1337 lvalp->emplace <std::pair<int, int>> (1, 2);
1338 return parser::token::PAIR;
1341 *** C++: Syntax error exceptions in GLR
1343 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1344 actions, or from the scanner.
1346 *** More POSIX Yacc compatibility warnings
1348 More Bison specific directives are now reported with -y or -Wyacc. This
1349 change was ready since the release of Bison 3.0 in September 2015. It was
1350 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1351 in numerous warnings for Bison users that use the GNU Build System.
1353 If you still experience that problem, either redefine YACC as `bison -o
1354 y.tab.c`, or pass -Wno-yacc to Bison.
1356 *** The tables yyrhs and yyphrs are back
1358 Because no Bison skeleton uses them, these tables were removed (no longer
1359 passed to the skeletons, not even computed) in 2008. However, some users
1360 have expressed interest in being able to use them in their own skeletons.
1364 *** Incorrect number of reduce/reduce conflicts
1366 On a grammar such as
1368 exp: "num" | "num" | "num"
1370 bison used to report a single RR conflict, instead of two. This is now
1371 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1372 was entered in the RCS version control system, in December 1987.
1374 Some grammar files might have to adjust their %expect-rr.
1376 *** Parser directives that were not careful enough
1378 Passing invalid arguments to %nterm, for instance character literals, used
1379 to result in unclear error messages.
1383 The examples/ directory (installed in .../share/doc/bison/examples) has
1384 been restructured per language for clarity. The examples come with a
1385 README and a Makefile. Not only can they be used to toy with Bison, they
1386 can also be starting points for your own grammars.
1388 There is now a Java example, and a simple example in C based on Flex and
1389 Bison (examples/c/lexcalc/).
1395 They now use noexcept and constexpr. Please, report missing annotations.
1397 *** Symbol Declarations
1399 The syntax of the variation directives to declare symbols was overhauled
1400 for more consistency, and also better POSIX Yacc compliance (which, for
1401 instance, allows "%type" without actually providing a type). The %nterm
1402 directive, supported by Bison since its inception, is now documented and
1403 officially supported.
1405 The syntax is now as follows:
1407 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1408 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1409 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1410 %nterm TAG? ID+ ( TAG ID+ )*
1412 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1413 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1414 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1415 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1416 one), ‘*’ (zero or more) and ‘+’ (one or more).
1419 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1423 Fix the move constructor of symbol_type.
1425 Always provide a copy constructor for symbol_type, even in modern C++.
1428 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1432 Properly support token constructors in C++ with types that include commas
1433 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1436 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1440 C++ portability issues.
1443 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1447 Several portability issues have been fixed in the build system, in the
1448 test suite, and in the generated parsers in C++.
1451 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1453 ** Backward incompatible changes
1455 Support for DJGPP, which has been unmaintained and untested for years, is
1456 obsolete. Unless there is activity to revive it, it will be removed.
1460 %printers should use yyo rather than yyoutput to denote the output stream.
1462 Variant-based symbols in C++ should use emplace() rather than build().
1464 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1468 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1470 A comment in the generated code now emphasizes that users should not
1471 depend upon non-documented implementation details, such as macros starting
1476 *** C++: Support for move semantics (lalr1.cc)
1478 The lalr1.cc skeleton now fully supports C++ move semantics, while
1479 maintaining compatibility with C++98. You may now store move-only types
1480 when using Bison's variants. For instance:
1487 %skeleton "lalr1.cc"
1488 %define api.value.type variant
1492 %token <int> INT "int";
1493 %type <std::unique_ptr<int>> int;
1494 %type <std::vector<std::unique_ptr<int>>> list;
1498 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1500 int: "int" { $$ = std::make_unique<int>($1); }
1502 *** C++: Implicit move of right-hand side values (lalr1.cc)
1504 In modern C++ (C++11 and later), you should always use 'std::move' with
1505 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1506 popped from the stack anyway. Using 'std::move' is mandatory for
1507 move-only types such as unique_ptr, and it provides a significant speedup
1508 for large types such as std::string, or std::vector, etc.
1510 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1511 by 'std::move ($n)'. The second rule in the previous grammar can be
1514 list: list int { $$ = $1; $$.emplace_back($2); }
1516 With automove enabled, the semantic values are no longer lvalues, so do
1517 not use the swap idiom:
1519 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1521 This idiom is anyway obsolete: it is preferable to move than to swap.
1523 A warning is issued when automove is enabled, and a value is used several
1526 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1527 exp: "twice" exp { $$ = $2 + $2; }
1530 Enabling api.value.automove does not require support for modern C++. The
1531 generated code is valid C++98/03, but will use copies instead of moves.
1533 The new examples/c++/variant-11.yy shows these features in action.
1535 *** C++: The implicit default semantic action is always run
1537 When variants are enabled, the default action was not run, so
1545 It now behaves like in all the other cases, as
1547 exp: "number" { $$ = $1; }
1549 possibly using std::move if automove is enabled.
1551 We do not expect backward compatibility issues. However, beware of
1552 forward compatibility issues: if you rely on default actions with
1553 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1554 generate incorrect parsers.
1556 *** C++: Renaming location.hh
1558 When both %defines and %locations are enabled, Bison generates a
1559 location.hh file. If you don't use locations outside of the parser, you
1560 may avoid its creation with:
1562 %define api.location.file none
1564 However this file is useful if, for instance, your parser builds an AST
1565 decorated with locations: you may use Bison's location independently of
1566 Bison's parser. You can now give it another name, for instance:
1568 %define api.location.file "my-location.hh"
1570 This name can have directory components, and even be absolute. The name
1571 under which the location file is included is controlled by
1572 api.location.include.
1574 This way it is possible to have several parsers share the same location
1577 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1580 %define api.namespace {foo}
1581 %define api.location.file "include/ast/loc.hh"
1582 %define api.location.include {<ast/loc.hh>}
1584 and use it in src/bar/parser.hh:
1587 %define api.namespace {bar}
1588 %code requires {#include <ast/loc.hh>}
1589 %define api.location.type {bar::location}
1591 Absolute file names are supported, so in your Makefile, passing the flag
1592 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1595 *** C++: stack.hh and position.hh are deprecated
1597 When asked to generate a header file (%defines), the lalr1.cc skeleton
1598 generates a stack.hh file. This file had no interest for users; it is now
1599 made useless: its content is included in the parser definition. It is
1600 still generated for backward compatibility.
1602 When in addition to %defines, location support is requested (%locations),
1603 the file position.hh is also generated. It is now also useless: its
1604 content is now included in location.hh.
1606 These files are no longer generated when your grammar file requires at
1607 least Bison 3.2 (%require "3.2").
1611 Portability issues on MinGW and VS2015.
1613 Portability issues in the test suite.
1615 Portability/warning issues with Flex.
1618 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1620 ** Backward incompatible changes
1622 Compiling Bison now requires a C99 compiler---as announced during the
1623 release of Bison 3.0, five years ago. Generated parsers do not require a
1626 Support for DJGPP, which has been unmaintained and untested for years, is
1627 obsolete. Unless there is activity to revive it, the next release of Bison
1628 will have it removed.
1632 *** Typed midrule actions
1634 Because their type is unknown to Bison, the values of midrule actions are
1635 not treated like the others: they don't have %printer and %destructor
1636 support. It also prevents C++ (Bison) variants to handle them properly.
1638 Typed midrule actions address these issues. Instead of:
1640 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1644 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1646 *** Reports include the type of the symbols
1648 The sections about terminal and nonterminal symbols of the '*.output' file
1649 now specify their declared type. For instance, for:
1653 the report now shows '<ival>':
1655 Terminals, with rules where they appear
1659 *** Diagnostics about useless rules
1661 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1662 of course, its rules are useless too.
1666 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1668 Previously all the useless rules were reported, including those whose
1669 left-hand side is the 'exp' nonterminal:
1671 warning: 1 nonterminal useless in grammar [-Wother]
1672 warning: 4 rules useless in grammar [-Wother]
1673 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1676 2.14-16: warning: rule useless in grammar [-Wother]
1679 3.6-16: warning: rule useless in grammar [-Wother]
1680 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1682 3.20-30: warning: rule useless in grammar [-Wother]
1683 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1685 3.34-44: warning: rule useless in grammar [-Wother]
1686 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1689 Now, rules whose left-hand side symbol is useless are no longer reported
1690 as useless. The locations of the errors have also been adjusted to point
1691 to the first use of the nonterminal as a left-hand side of a rule:
1693 warning: 1 nonterminal useless in grammar [-Wother]
1694 warning: 4 rules useless in grammar [-Wother]
1695 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1696 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1698 2.14-16: warning: rule useless in grammar [-Wother]
1702 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1704 When compiled with exceptions disabled, the generated parsers no longer
1705 uses try/catch clauses.
1707 Currently only GCC and Clang are supported.
1711 *** A demonstration of variants
1713 A new example was added (installed in .../share/doc/bison/examples),
1714 'variant.yy', which shows how to use (Bison) variants in C++.
1716 The other examples were made nicer to read.
1718 *** Some features are no longer 'experimental'
1720 The following features, mature enough, are no longer flagged as
1721 experimental in the documentation: push parsers, default %printer and
1722 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1723 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1724 semantic predicates (%?).
1728 *** GLR: Predicates support broken by #line directives
1730 Predicates (%?) in GLR such as
1733 %? {new_syntax} 'w' id new_args
1734 | %?{!new_syntax} 'w' id old_args
1736 were issued with #lines in the middle of C code.
1738 *** Printer and destructor with broken #line directives
1740 The #line directives were not properly escaped when emitting the code for
1741 %printer/%destructor, which resulted in compiler errors if there are
1742 backslashes or double-quotes in the grammar file name.
1744 *** Portability on ICC
1746 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1747 Generated parsers now work around this.
1751 There were several small fixes in the test suite and in the build system,
1752 many warnings in bison and in the generated parsers were eliminated. The
1753 documentation also received its share of minor improvements.
1755 Useless code was removed from C++ parsers, and some of the generated
1756 constructors are more 'natural'.
1759 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1763 *** C++: Fix support of 'syntax_error'
1765 One incorrect 'inline' resulted in linking errors about the constructor of
1766 the syntax_error exception.
1768 *** C++: Fix warnings
1770 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1771 warnings about yyformat being possibly null. It also warned about the
1772 deprecated implicit definition of copy constructors when there's a
1773 user-defined (copy) assignment operator.
1775 *** Location of errors
1777 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1778 ride-hand side raises a syntax error. The behavior of the default parser
1779 (yacc.c) in such a condition was undefined.
1781 Now all the parsers match the behavior of glr.c: @$ is used as the
1782 location of the error. This handles gracefully rules with and without
1785 *** Portability fixes in the test suite
1787 On some platforms, some Java and/or C++ tests were failing.
1790 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1794 *** C++ with Variants (lalr1.cc)
1796 Fix a compiler warning when no %destructor use $$.
1800 Several portability issues in tests were fixed.
1803 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1807 *** C++ with Variants (lalr1.cc)
1809 Problems with %destructor and '%define parse.assert' have been fixed.
1811 *** Named %union support (yacc.c, glr.c)
1813 Bison 3.0 introduced a regression on named %union such as
1815 %union foo { int ival; };
1817 The possibility to use a name was introduced "for Yacc compatibility".
1818 It is however not required by POSIX Yacc, and its usefulness is not clear.
1820 *** %define api.value.type union with %defines (yacc.c, glr.c)
1822 The C parsers were broken when %defines was used together with "%define
1823 api.value.type union".
1825 *** Redeclarations are reported in proper order
1833 bison used to report:
1835 foo.yy:2.10-11: error: %printer redeclaration for FOO
1838 foo.yy:3.10-11: previous declaration
1842 Now, the "previous" declaration is always the first one.
1847 Bison now installs various files in its docdir (which defaults to
1848 '/usr/local/share/doc/bison'), including the three fully blown examples
1849 extracted from the documentation:
1852 Reverse Polish Calculator, a simple introductory example.
1854 Multi-function Calc, a calculator with memory and functions and located
1857 a calculator in C++ using variant support and token constructors.
1860 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1864 *** Generated source files when errors are reported
1866 When warnings are issued and -Werror is set, bison would still generate
1867 the source files (*.c, *.h...). As a consequence, some runs of "make"
1868 could fail the first time, but not the second (as the files were generated
1871 This is fixed: bison no longer generates this source files, but, of
1872 course, still produces the various reports (*.output, *.xml, etc.).
1874 *** %empty is used in reports
1876 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1877 dot, XML and formats derived from it).
1879 *** YYERROR and variants
1881 When C++ variant support is enabled, an error triggered via YYERROR, but
1882 not caught via error recovery, resulted in a double deletion.
1885 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1889 *** Errors in caret diagnostics
1891 On some platforms, some errors could result in endless diagnostics.
1893 *** Fixes of the -Werror option
1895 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1896 diagnostics into errors instead of warnings. This is fixed.
1898 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1899 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1900 -Wno-error", "foo" diagnostics are now errors.
1904 As demonstrated in the documentation, one can now leave spaces between
1909 The yacc.1 man page is no longer installed if --disable-yacc was
1912 *** Fixes in the test suite
1914 Bugs and portability issues.
1917 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1919 ** WARNING: Future backward-incompatibilities!
1921 Like other GNU packages, Bison will start using some of the C99 features
1922 for its own code, especially the definition of variables after statements.
1923 The generated C parsers still aim at C90.
1925 ** Backward incompatible changes
1927 *** Obsolete features
1929 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1931 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1932 use YYSTYPE and YYLTYPE.
1934 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1935 1.875): use %lex-param, %parse-param, or %param.
1937 Missing semicolons at the end of actions are no longer added (as announced
1938 in the release 2.5).
1940 *** Use of YACC='bison -y'
1942 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1945 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1946 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1947 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1949 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1950 implementation of Yacc, was using Bison as 'bison -y'. While it does
1951 ensure compatible output file names, it also enables warnings for
1952 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1953 warnings for Bison extensions.
1955 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1956 (which also generates 'y.tab.h' and 'y.output' when needed).
1957 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1958 flags (YFLAGS, or AM_YFLAGS with Automake).
1962 *** The epilogue is no longer affected by internal #defines (glr.c)
1964 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1965 generated code. These weren't properly undefined before the inclusion of
1966 the user epilogue, so functions such as the following were butchered by the
1967 preprocessor expansion:
1969 int yylex (YYSTYPE *yylval);
1971 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
1972 identifiers for user-provided variables.
1974 *** stdio.h is no longer needed when locations are enabled (yacc.c)
1976 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
1977 locations are enabled. This is fixed.
1979 *** Warnings about useless %pure-parser/%define api.pure are restored
1981 ** Diagnostics reported by Bison
1983 Most of these features were contributed by Théophile Ranquet and Victor
1988 Version 2.7 introduced caret errors, for a prettier output. These are now
1989 activated by default. The old format can still be used by invoking Bison
1990 with -fno-caret (or -fnone).
1992 Some error messages that reproduced excerpts of the grammar are now using
1993 the caret information only. For instance on:
2000 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2001 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2005 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2006 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2010 and "bison -fno-caret" reports:
2012 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2013 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2015 *** Enhancements of the -Werror option
2017 The -Werror=CATEGORY option is now recognized, and will treat specified
2018 warnings as errors. The warnings need not have been explicitly activated
2019 using the -W option, this is similar to what GCC 4.7 does.
2021 For example, given the following command line, Bison will treat both
2022 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2023 errors (and only those):
2025 $ bison -Werror=yacc,error=conflicts-sr input.y
2027 If no categories are specified, -Werror will make all active warnings into
2028 errors. For example, the following line does the same the previous example:
2030 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2032 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2034 Note that the categories in this -Werror option may not be prefixed with
2035 "no-". However, -Wno-error[=CATEGORY] is valid.
2037 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2038 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2039 incompatibilities as warnings: "-y -Wno-error=yacc".
2041 *** The display of warnings is now richer
2043 The option that controls a given warning is now displayed:
2045 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2047 In the case of warnings treated as errors, the prefix is changed from
2048 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2049 to GCC, as [-Werror=CATEGORY].
2051 For instance, where the previous version of Bison would report (and exit
2054 bison: warnings being treated as errors
2055 input.y:1.1: warning: stray ',' treated as white space
2059 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2061 *** Deprecated constructs
2063 The new 'deprecated' warning category flags obsolete constructs whose
2064 support will be discontinued. It is enabled by default. These warnings
2065 used to be reported as 'other' warnings.
2067 *** Useless semantic types
2069 Bison now warns about useless (uninhabited) semantic types. Since
2070 semantic types are not declared to Bison (they are defined in the opaque
2071 %union structure), it is %printer/%destructor directives about useless
2072 types that trigger the warning:
2076 %printer {} <type1> <type3>
2077 %destructor {} <type2> <type4>
2079 nterm: term { $$ = $1; };
2081 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2082 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2084 *** Undefined but unused symbols
2086 Bison used to raise an error for undefined symbols that are not used in
2087 the grammar. This is now only a warning.
2090 %destructor {} symbol2
2091 %type <type> symbol3
2095 *** Useless destructors or printers
2097 Bison now warns about useless destructors or printers. In the following
2098 example, the printer for <type1>, and the destructor for <type2> are
2099 useless: all symbols of <type1> (token1) already have a printer, and all
2100 symbols of type <type2> (token2) already have a destructor.
2102 %token <type1> token1
2106 %printer {} token1 <type1> <type3>
2107 %destructor {} token2 <type2> <type4>
2111 The warnings and error messages about shift/reduce and reduce/reduce
2112 conflicts have been normalized. For instance on the following foo.y file:
2116 exp: exp '+' exp | '0' | '0';
2118 compare the previous version of bison:
2121 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2122 $ bison -Werror foo.y
2123 bison: warnings being treated as errors
2124 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2126 with the new behavior:
2129 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2130 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2131 $ bison -Werror foo.y
2132 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2133 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2135 When %expect or %expect-rr is used, such as with bar.y:
2140 exp: exp '+' exp | '0' | '0';
2145 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2146 bar.y: expected 0 shift/reduce conflicts
2147 bar.y: expected 0 reduce/reduce conflicts
2152 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2153 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2155 ** Incompatibilities with POSIX Yacc
2157 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2160 ** Additional yylex/yyparse arguments
2162 The new directive %param declares additional arguments to both yylex and
2163 yyparse. The %lex-param, %parse-param, and %param directives support one
2164 or more arguments. Instead of
2166 %lex-param {arg1_type *arg1}
2167 %lex-param {arg2_type *arg2}
2168 %parse-param {arg1_type *arg1}
2169 %parse-param {arg2_type *arg2}
2173 %param {arg1_type *arg1} {arg2_type *arg2}
2175 ** Types of values for %define variables
2177 Bison used to make no difference between '%define foo bar' and '%define
2178 foo "bar"'. The former is now called a 'keyword value', and the latter a
2179 'string value'. A third kind was added: 'code values', such as '%define
2182 Keyword variables are used for fixed value sets, e.g.,
2184 %define lr.type lalr
2186 Code variables are used for value in the target language, e.g.,
2188 %define api.value.type {struct semantic_type}
2190 String variables are used remaining cases, e.g. file names.
2192 ** Variable api.token.prefix
2194 The variable api.token.prefix changes the way tokens are identified in
2195 the generated files. This is especially useful to avoid collisions
2196 with identifiers in the target language. For instance
2198 %token FILE for ERROR
2199 %define api.token.prefix {TOK_}
2201 start: FILE for ERROR;
2203 will generate the definition of the symbols TOK_FILE, TOK_for, and
2204 TOK_ERROR in the generated sources. In particular, the scanner must
2205 use these prefixed token names, although the grammar itself still
2206 uses the short names (as in the sample rule given above).
2208 ** Variable api.value.type
2210 This new %define variable supersedes the #define macro YYSTYPE. The use
2211 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2212 using %union or %defining api.value.type results in undefined behavior.
2214 Either define api.value.type, or use "%union":
2221 %token <ival> INT "integer"
2222 %token <sval> STRING "string"
2223 %printer { fprintf (yyo, "%d", $$); } <ival>
2224 %destructor { free ($$); } <sval>
2227 yylval.ival = 42; return INT;
2228 yylval.sval = "42"; return STRING;
2230 The %define variable api.value.type supports both keyword and code values.
2232 The keyword value 'union' means that the user provides genuine types, not
2233 union member names such as "ival" and "sval" above (WARNING: will fail if
2234 -y/--yacc/%yacc is enabled).
2236 %define api.value.type union
2237 %token <int> INT "integer"
2238 %token <char *> STRING "string"
2239 %printer { fprintf (yyo, "%d", $$); } <int>
2240 %destructor { free ($$); } <char *>
2243 yylval.INT = 42; return INT;
2244 yylval.STRING = "42"; return STRING;
2246 The keyword value variant is somewhat equivalent, but for C++ special
2247 provision is made to allow classes to be used (more about this below).
2249 %define api.value.type variant
2250 %token <int> INT "integer"
2251 %token <std::string> STRING "string"
2253 Code values (in braces) denote user defined types. This is where YYSTYPE
2271 %define api.value.type {struct my_value}
2272 %token <u.ival> INT "integer"
2273 %token <u.sval> STRING "string"
2274 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2275 %destructor { free ($$); } <u.sval>
2278 yylval.u.ival = 42; return INT;
2279 yylval.u.sval = "42"; return STRING;
2281 ** Variable parse.error
2283 This variable controls the verbosity of error messages. The use of the
2284 %error-verbose directive is deprecated in favor of "%define parse.error
2287 ** Deprecated %define variable names
2289 The following variables have been renamed for consistency. Backward
2290 compatibility is ensured, but upgrading is recommended.
2292 lr.default-reductions -> lr.default-reduction
2293 lr.keep-unreachable-states -> lr.keep-unreachable-state
2294 namespace -> api.namespace
2295 stype -> api.value.type
2297 ** Semantic predicates
2299 Contributed by Paul Hilfinger.
2301 The new, experimental, semantic-predicate feature allows actions of the
2302 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2303 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2304 in GLR parsers, rather than being deferred. The result is that they allow
2305 the programmer to prune possible parses based on the values of run-time
2308 ** The directive %expect-rr is now an error in non GLR mode
2310 It used to be an error only if used in non GLR mode, _and_ if there are
2311 reduce/reduce conflicts.
2313 ** Tokens are numbered in their order of appearance
2315 Contributed by Valentin Tolmer.
2317 With '%token A B', A had a number less than the one of B. However,
2318 precedence declarations used to generate a reversed order. This is now
2319 fixed, and introducing tokens with any of %token, %left, %right,
2320 %precedence, or %nonassoc yields the same result.
2322 When mixing declarations of tokens with a literal character (e.g., 'a') or
2323 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2324 the literal characters first. For example
2328 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2329 input order is now preserved.
2331 These changes were made so that one can remove useless precedence and
2332 associativity declarations (i.e., map %nonassoc, %left or %right to
2333 %precedence, or to %token) and get exactly the same output.
2335 ** Useless precedence and associativity
2337 Contributed by Valentin Tolmer.
2339 When developing and maintaining a grammar, useless associativity and
2340 precedence directives are common. They can be a nuisance: new ambiguities
2341 arising are sometimes masked because their conflicts are resolved due to
2342 the extra precedence or associativity information. Furthermore, it can
2343 hinder the comprehension of a new grammar: one will wonder about the role
2344 of a precedence, where in fact it is useless. The following changes aim
2345 at detecting and reporting these extra directives.
2347 *** Precedence warning category
2349 A new category of warning, -Wprecedence, was introduced. It flags the
2350 useless precedence and associativity directives.
2352 *** Useless associativity
2354 Bison now warns about symbols with a declared associativity that is never
2355 used to resolve conflicts. In that case, using %precedence is sufficient;
2356 the parsing tables will remain unchanged. Solving these warnings may raise
2357 useless precedence warnings, as the symbols no longer have associativity.
2371 warning: useless associativity for '+', use %precedence [-Wprecedence]
2375 *** Useless precedence
2377 Bison now warns about symbols with a declared precedence and no declared
2378 associativity (i.e., declared with %precedence), and whose precedence is
2379 never used. In that case, the symbol can be safely declared with %token
2380 instead, without modifying the parsing tables. For example:
2384 exp: "var" '=' "number";
2388 warning: useless precedence for '=' [-Wprecedence]
2392 *** Useless precedence and associativity
2394 In case of both useless precedence and associativity, the issue is flagged
2399 exp: "var" '=' "number";
2403 warning: useless precedence and associativity for '=' [-Wprecedence]
2409 With help from Joel E. Denny and Gabriel Rassoul.
2411 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2412 marked by the new %empty directive. Using %empty on a non-empty rule is
2413 an error. The new -Wempty-rule warning reports empty rules without
2414 %empty. On the following grammar:
2424 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2427 5.8-13: error: %empty on non-empty rule
2431 ** Java skeleton improvements
2433 The constants for token names were moved to the Lexer interface. Also, it
2434 is possible to add code to the parser's constructors using "%code init"
2435 and "%define init_throws".
2436 Contributed by Paolo Bonzini.
2438 The Java skeleton now supports push parsing.
2439 Contributed by Dennis Heimbigner.
2441 ** C++ skeletons improvements
2443 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2445 Using %defines is now optional. Without it, the needed support classes
2446 are defined in the generated parser, instead of additional files (such as
2447 location.hh, position.hh and stack.hh).
2449 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2451 Both lalr1.cc and glr.cc no longer require %location.
2453 *** syntax_error exception (lalr1.cc)
2455 The C++ parser features a syntax_error exception, which can be
2456 thrown from the scanner or from user rules to raise syntax errors.
2457 This facilitates reporting errors caught in sub-functions (e.g.,
2458 rejecting too large integral literals from a conversion function
2459 used by the scanner, or rejecting invalid combinations from a
2460 factory invoked by the user actions).
2462 *** %define api.value.type variant
2464 This is based on a submission from Michiel De Wilde. With help
2465 from Théophile Ranquet.
2467 In this mode, complex C++ objects can be used as semantic values. For
2470 %token <::std::string> TEXT;
2471 %token <int> NUMBER;
2472 %token SEMICOLON ";"
2473 %type <::std::string> item;
2474 %type <::std::list<std::string>> list;
2477 list { std::cout << $1 << std::endl; }
2481 %empty { /* Generates an empty string list. */ }
2482 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2486 TEXT { std::swap ($$, $1); }
2487 | NUMBER { $$ = string_cast ($1); }
2490 *** %define api.token.constructor
2492 When variants are enabled, Bison can generate functions to build the
2493 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2494 with the semantic value (e.g., int):
2496 parser::symbol_type yylex ()
2498 parser::location_type loc = ...;
2500 return parser::make_TEXT ("Hello, world!", loc);
2502 return parser::make_NUMBER (42, loc);
2504 return parser::make_SEMICOLON (loc);
2510 There are operator- and operator-= for 'location'. Negative line/column
2511 increments can no longer underflow the resulting value.
2514 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2518 *** Fix compiler attribute portability (yacc.c)
2520 With locations enabled, __attribute__ was used unprotected.
2522 *** Fix some compiler warnings (lalr1.cc)
2525 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2529 Warnings about uninitialized yylloc in yyparse have been fixed.
2531 Restored C90 compliance (yet no report was ever made).
2533 ** Diagnostics are improved
2535 Contributed by Théophile Ranquet.
2537 *** Changes in the format of error messages
2539 This used to be the format of many error reports:
2541 input.y:2.7-12: %type redeclaration for exp
2542 input.y:1.7-12: previous declaration
2546 input.y:2.7-12: error: %type redeclaration for exp
2547 input.y:1.7-12: previous declaration
2549 *** New format for error reports: carets
2551 Caret errors have been added to Bison:
2553 input.y:2.7-12: error: %type redeclaration for exp
2556 input.y:1.7-12: previous declaration
2562 input.y:3.20-23: error: ambiguous reference: '$exp'
2563 exp: exp '+' exp { $exp = $1 + $3; };
2565 input.y:3.1-3: refers to: $exp at $$
2566 exp: exp '+' exp { $exp = $1 + $3; };
2568 input.y:3.6-8: refers to: $exp at $1
2569 exp: exp '+' exp { $exp = $1 + $3; };
2571 input.y:3.14-16: refers to: $exp at $3
2572 exp: exp '+' exp { $exp = $1 + $3; };
2575 The default behavior for now is still not to display these unless
2576 explicitly asked with -fcaret (or -fall). However, in a later release, it
2577 will be made the default behavior (but may still be deactivated with
2580 ** New value for %define variable: api.pure full
2582 The %define variable api.pure requests a pure (reentrant) parser. However,
2583 for historical reasons, using it in a location-tracking Yacc parser
2584 resulted in a yyerror function that did not take a location as a
2585 parameter. With this new value, the user may request a better pure parser,
2586 where yyerror does take a location as a parameter (in location-tracking
2589 The use of "%define api.pure true" is deprecated in favor of this new
2590 "%define api.pure full".
2592 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2594 The %define variable api.location.type defines the name of the type to use
2595 for locations. When defined, Bison no longer generates the position.hh
2596 and location.hh files, nor does the parser will include them: the user is
2597 then responsible to define her type.
2599 This can be used in programs with several parsers to factor their location
2600 and position files: let one of them generate them, and the others just use
2603 This feature was actually introduced, but not documented, in Bison 2.5,
2604 under the name "location_type" (which is maintained for backward
2607 For consistency, lalr1.java's %define variables location_type and
2608 position_type are deprecated in favor of api.location.type and
2611 ** Exception safety (lalr1.cc)
2613 The parse function now catches exceptions, uses the %destructors to
2614 release memory (the lookahead symbol and the symbols pushed on the stack)
2615 before re-throwing the exception.
2617 This feature is somewhat experimental. User feedback would be
2620 ** Graph improvements in DOT and XSLT
2622 Contributed by Théophile Ranquet.
2624 The graphical presentation of the states is more readable: their shape is
2625 now rectangular, the state number is clearly displayed, and the items are
2626 numbered and left-justified.
2628 The reductions are now explicitly represented as transitions to other
2629 diamond shaped nodes.
2631 These changes are present in both --graph output and xml2dot.xsl XSLT
2632 processing, with minor (documented) differences.
2634 ** %language is no longer an experimental feature.
2636 The introduction of this feature, in 2.4, was four years ago. The
2637 --language option and the %language directive are no longer experimental.
2641 The sections about shift/reduce and reduce/reduce conflicts resolution
2642 have been fixed and extended.
2644 Although introduced more than four years ago, XML and Graphviz reports
2645 were not properly documented.
2647 The translation of midrule actions is now described.
2650 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2652 We consider compiler warnings about Bison generated parsers to be bugs.
2653 Rather than working around them in your own project, please consider
2654 reporting them to us.
2658 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2659 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2662 Other issues in the test suite have been addressed.
2664 Null characters are correctly displayed in error messages.
2666 When possible, yylloc is correctly initialized before calling yylex. It
2667 is no longer necessary to initialize it in the %initial-action.
2670 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2672 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2675 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2679 Bugs and portability issues in the test suite have been fixed.
2681 Some errors in translations have been addressed, and --help now directs
2682 users to the appropriate place to report them.
2684 Stray Info files shipped by accident are removed.
2686 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2687 generated, are removed.
2689 All the generated headers are self-contained.
2691 ** Header guards (yacc.c, glr.c, glr.cc)
2693 In order to avoid collisions, the header guards are now
2694 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2695 For instance the header generated from
2697 %define api.prefix "calc"
2698 %defines "lib/parse.h"
2700 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2702 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2704 The compilation of pure parsers (%define api.pure) can trigger GCC
2707 input.c: In function 'yyparse':
2708 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2709 function [-Wmaybe-uninitialized]
2713 This is now fixed; pragmas to avoid these warnings are no longer needed.
2715 Warnings from clang ("equality comparison with extraneous parentheses" and
2716 "function declared 'noreturn' should not return") have also been
2720 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2724 Buffer overruns, complaints from Flex, and portability issues in the test
2725 suite have been fixed.
2727 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2729 Trailing end-of-lines in %parse-param or %lex-param would result in
2730 invalid C++. This is fixed.
2732 ** Spurious spaces and end-of-lines
2734 The generated files no longer end (nor start) with empty lines.
2737 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2739 Bison no longer executes user-specified M4 code when processing a grammar.
2743 In addition to the removal of the features announced in Bison 2.6, the
2744 next major release will remove the "Temporary hack for adding a semicolon
2745 to the user action", as announced in the release 2.5. Instead of:
2747 exp: exp "+" exp { $$ = $1 + $3 };
2751 exp: exp "+" exp { $$ = $1 + $3; };
2755 *** Type names are now properly escaped.
2757 *** glr.cc: set_debug_level and debug_level work as expected.
2759 *** Stray @ or $ in actions
2761 While Bison used to warn about stray $ or @ in action rules, it did not
2762 for other actions such as printers, destructors, or initial actions. It
2765 ** Type names in actions
2767 For consistency with rule actions, it is now possible to qualify $$ by a
2768 type-name in destructors, printers, and initial actions. For instance:
2770 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2772 will display two values for each typed and untyped symbol (provided
2773 that YYSTYPE has both "ival" and "fval" fields).
2776 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2780 The next major release of Bison will drop support for the following
2781 deprecated features. Please report disagreements to bug-bison@gnu.org.
2785 Support for generating parsers in K&R C will be removed. Parsers
2786 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2789 *** Features deprecated since Bison 1.875
2791 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2794 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2795 %lex-param, will no longer be supported.
2797 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2800 *** The generated header will be included (yacc.c)
2802 Instead of duplicating the content of the generated header (definition of
2803 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2804 as is already the case for GLR or C++ parsers. This change is deferred
2805 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2808 ** Generated Parser Headers
2810 *** Guards (yacc.c, glr.c, glr.cc)
2812 The generated headers are now guarded, as is already the case for C++
2813 parsers (lalr1.cc). For instance, with --defines=foo.h:
2818 #endif /* !YY_FOO_H */
2820 *** New declarations (yacc.c, glr.c)
2822 The generated header now declares yydebug and yyparse. Both honor
2823 --name-prefix=bar_, and yield
2825 int bar_parse (void);
2829 #define yyparse bar_parse
2832 in order to facilitate the inclusion of several parser headers inside a
2833 single compilation unit.
2835 *** Exported symbols in C++
2837 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2838 header, are removed, as they prevent the possibility of including several
2839 generated headers from a single compilation unit.
2843 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2846 ** New %define variable: api.prefix
2848 Now that the generated headers are more complete and properly protected
2849 against multiple inclusions, constant names, such as YYSTYPE are a
2850 problem. While yyparse and others are properly renamed by %name-prefix,
2851 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2852 would introduce backward compatibility issues in projects not expecting
2853 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2854 it is deprecated in favor of a new %define variable: api.prefix.
2856 The following examples compares both:
2858 %name-prefix "bar_" | %define api.prefix "bar_"
2859 %token <ival> FOO %token <ival> FOO
2860 %union { int ival; } %union { int ival; }
2866 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2867 # define BAR_FOO_H # define BAR_FOO_H
2869 /* Enabling traces. */ /* Enabling traces. */
2870 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2871 > # if defined YYDEBUG
2873 > # define BAR_DEBUG 1
2875 > # define BAR_DEBUG 0
2878 # define YYDEBUG 0 | # define BAR_DEBUG 0
2882 # if YYDEBUG | # if BAR_DEBUG
2883 extern int bar_debug; extern int bar_debug;
2886 /* Tokens. */ /* Tokens. */
2887 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2888 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2889 enum yytokentype { | enum bar_tokentype {
2894 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2895 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2896 typedef union YYSTYPE | typedef union BAR_STYPE
2899 } YYSTYPE; | } BAR_STYPE;
2900 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2903 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2905 int bar_parse (void); int bar_parse (void);
2907 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2910 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2914 The next major release will drop support for generating parsers in K&R C.
2916 ** yacc.c: YYBACKUP works as expected.
2918 ** glr.c improvements:
2920 *** Location support is eliminated when not requested:
2922 GLR parsers used to include location-related code even when locations were
2923 not requested, and therefore not even usable.
2925 *** __attribute__ is preserved:
2927 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2928 when -std is passed to GCC).
2930 ** lalr1.java: several fixes:
2932 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2933 first token leads to a syntax error. Some minor clean ups.
2937 *** C++11 compatibility:
2939 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2944 The header files such as "parser.hh", "location.hh", etc. used a constant
2945 name for preprocessor guards, for instance:
2947 #ifndef BISON_LOCATION_HH
2948 # define BISON_LOCATION_HH
2950 #endif // !BISON_LOCATION_HH
2952 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2953 case characters are converted to upper case, and series of
2954 non-alphanumerical characters are converted to an underscore.
2956 With "bison -o lang++/parser.cc", "location.hh" would now include:
2958 #ifndef YY_LANG_LOCATION_HH
2959 # define YY_LANG_LOCATION_HH
2961 #endif // !YY_LANG_LOCATION_HH
2965 The position and location constructors (and their initialize methods)
2966 accept new arguments for line and column. Several issues in the
2967 documentation were fixed.
2969 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2971 ** Changes in the manual:
2973 *** %printer is documented
2975 The "%printer" directive, supported since at least Bison 1.50, is finally
2976 documented. The "mfcalc" example is extended to demonstrate it.
2978 For consistency with the C skeletons, the C++ parsers now also support
2979 "yyoutput" (as an alias to "debug_stream ()").
2981 *** Several improvements have been made:
2983 The layout for grammar excerpts was changed to a more compact scheme.
2984 Named references are motivated. The description of the automaton
2985 description file (*.output) is updated to the current format. Incorrect
2986 index entries were fixed. Some other errors were fixed.
2990 *** Conflicting prototypes with recent/modified Flex.
2992 Fixed build problems with the current, unreleased, version of Flex, and
2993 some modified versions of 2.5.35, which have modified function prototypes.
2995 *** Warnings during the build procedure have been eliminated.
2997 *** Several portability problems in the test suite have been fixed:
2999 This includes warnings with some compilers, unexpected behavior of tools
3000 such as diff, warning messages from the test suite itself, etc.
3002 *** The install-pdf target works properly:
3004 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3005 halts in the middle of its course.
3008 * Noteworthy changes in release 2.5 (2011-05-14)
3010 ** Grammar symbol names can now contain non-initial dashes:
3012 Consistently with directives (such as %error-verbose) and with
3013 %define variables (e.g. push-pull), grammar symbol names may contain
3014 dashes in any position except the beginning. This is a GNU
3015 extension over POSIX Yacc. Thus, use of this extension is reported
3016 by -Wyacc and rejected in Yacc mode (--yacc).
3018 ** Named references:
3020 Historically, Yacc and Bison have supported positional references
3021 ($n, $$) to allow access to symbol values from inside of semantic
3024 Starting from this version, Bison can also accept named references.
3025 When no ambiguity is possible, original symbol names may be used
3026 as named references:
3028 if_stmt : "if" cond_expr "then" then_stmt ';'
3029 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3031 In the more common case, explicit names may be declared:
3033 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3034 { $res = mk_if_stmt($cond, $then, $else); }
3036 Location information is also accessible using @name syntax. When
3037 accessing symbol names containing dots or dashes, explicit bracketing
3038 ($[sym.1]) must be used.
3040 These features are experimental in this version. More user feedback
3041 will help to stabilize them.
3042 Contributed by Alex Rozenman.
3044 ** IELR(1) and canonical LR(1):
3046 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3047 is, given any context-free grammar, IELR(1) generates parser tables
3048 with the full language-recognition power of canonical LR(1) but with
3049 nearly the same number of parser states as LALR(1). This reduction
3050 in parser states is often an order of magnitude. More importantly,
3051 because canonical LR(1)'s extra parser states may contain duplicate
3052 conflicts in the case of non-LR(1) grammars, the number of conflicts
3053 for IELR(1) is often an order of magnitude less as well. This can
3054 significantly reduce the complexity of developing of a grammar.
3056 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3057 place of its traditional LALR(1) parser tables, which remain the
3058 default. You can specify the type of parser tables in the grammar
3059 file with these directives:
3061 %define lr.type lalr
3062 %define lr.type ielr
3063 %define lr.type canonical-lr
3065 The default-reduction optimization in the parser tables can also be
3066 adjusted using "%define lr.default-reductions". For details on both
3067 of these features, see the new section "Tuning LR" in the Bison
3070 These features are experimental. More user feedback will help to
3073 ** LAC (Lookahead Correction) for syntax error handling
3075 Contributed by Joel E. Denny.
3077 Canonical LR, IELR, and LALR can suffer from a couple of problems
3078 upon encountering a syntax error. First, the parser might perform
3079 additional parser stack reductions before discovering the syntax
3080 error. Such reductions can perform user semantic actions that are
3081 unexpected because they are based on an invalid token, and they
3082 cause error recovery to begin in a different syntactic context than
3083 the one in which the invalid token was encountered. Second, when
3084 verbose error messages are enabled (with %error-verbose or the
3085 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3086 syntax error message can both contain invalid tokens and omit valid
3089 The culprits for the above problems are %nonassoc, default
3090 reductions in inconsistent states, and parser state merging. Thus,
3091 IELR and LALR suffer the most. Canonical LR can suffer only if
3092 %nonassoc is used or if default reductions are enabled for
3093 inconsistent states.
3095 LAC is a new mechanism within the parsing algorithm that solves
3096 these problems for canonical LR, IELR, and LALR without sacrificing
3097 %nonassoc, default reductions, or state merging. When LAC is in
3098 use, canonical LR and IELR behave almost exactly the same for both
3099 syntactically acceptable and syntactically unacceptable input.
3100 While LALR still does not support the full language-recognition
3101 power of canonical LR and IELR, LAC at least enables LALR's syntax
3102 error handling to correctly reflect LALR's language-recognition
3105 Currently, LAC is only supported for deterministic parsers in C.
3106 You can enable LAC with the following directive:
3108 %define parse.lac full
3110 See the new section "LAC" in the Bison manual for additional
3111 details including a few caveats.
3113 LAC is an experimental feature. More user feedback will help to
3116 ** %define improvements:
3118 *** Can now be invoked via the command line:
3120 Each of these command-line options
3123 --define=NAME[=VALUE]
3126 --force-define=NAME[=VALUE]
3128 is equivalent to this grammar file declaration
3130 %define NAME ["VALUE"]
3132 except that the manner in which Bison processes multiple definitions
3133 for the same NAME differs. Most importantly, -F and --force-define
3134 quietly override %define, but -D and --define do not. For further
3135 details, see the section "Bison Options" in the Bison manual.
3137 *** Variables renamed:
3139 The following %define variables
3142 lr.keep_unreachable_states
3144 have been renamed to
3147 lr.keep-unreachable-states
3149 The old names are now deprecated but will be maintained indefinitely
3150 for backward compatibility.
3152 *** Values no longer need to be quoted in the grammar file:
3154 If a %define value is an identifier, it no longer needs to be placed
3155 within quotations marks. For example,
3157 %define api.push-pull "push"
3161 %define api.push-pull push
3163 *** Unrecognized variables are now errors not warnings.
3165 *** Multiple invocations for any variable is now an error not a warning.
3167 ** Unrecognized %code qualifiers are now errors not warnings.
3169 ** Character literals not of length one:
3171 Previously, Bison quietly converted all character literals to length
3172 one. For example, without warning, Bison interpreted the operators in
3173 the following grammar to be the same token:
3179 Bison now warns when a character literal is not of length one. In
3180 some future release, Bison will start reporting an error instead.
3182 ** Destructor calls fixed for lookaheads altered in semantic actions:
3184 Previously for deterministic parsers in C, if a user semantic action
3185 altered yychar, the parser in some cases used the old yychar value to
3186 determine which destructor to call for the lookahead upon a syntax
3187 error or upon parser return. This bug has been fixed.
3189 ** C++ parsers use YYRHSLOC:
3191 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3192 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3193 to use it. If, for instance, your location structure has "first"
3194 and "last" members, instead of
3196 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3200 (Current).first = (Rhs)[1].location.first; \
3201 (Current).last = (Rhs)[N].location.last; \
3205 (Current).first = (Current).last = (Rhs)[0].location.last; \
3211 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3215 (Current).first = YYRHSLOC (Rhs, 1).first; \
3216 (Current).last = YYRHSLOC (Rhs, N).last; \
3220 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3224 ** YYLLOC_DEFAULT in C++:
3226 The default implementation of YYLLOC_DEFAULT used to be issued in
3227 the header file. It is now output in the implementation file, after
3228 the user %code sections so that its #ifndef guard does not try to
3229 override the user's YYLLOC_DEFAULT if provided.
3231 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3233 YYFAIL has existed for many years as an undocumented feature of
3234 deterministic parsers in C generated by Bison. More recently, it was
3235 a documented feature of Bison's experimental Java parsers. As
3236 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3237 semantic action now produces a deprecation warning, and Java parsers
3238 no longer implement YYFAIL at all. For further details, including a
3239 discussion of how to suppress C preprocessor warnings about YYFAIL
3240 being unused, see the Bison 2.4.2 NEWS entry.
3242 ** Temporary hack for adding a semicolon to the user action:
3244 Previously, Bison appended a semicolon to every user action for
3245 reductions when the output language defaulted to C (specifically, when
3246 neither %yacc, %language, %skeleton, or equivalent command-line
3247 options were specified). This allowed actions such as
3249 exp: exp "+" exp { $$ = $1 + $3 };
3253 exp: exp "+" exp { $$ = $1 + $3; };
3255 As a first step in removing this misfeature, Bison now issues a
3256 warning when it appends a semicolon. Moreover, in cases where Bison
3257 cannot easily determine whether a semicolon is needed (for example, an
3258 action ending with a cpp directive or a braced compound initializer),
3259 it no longer appends one. Thus, the C compiler might now complain
3260 about a missing semicolon where it did not before. Future releases of
3261 Bison will cease to append semicolons entirely.
3263 ** Verbose syntax error message fixes:
3265 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3266 specified, syntax error messages produced by the generated parser
3267 include the unexpected token as well as a list of expected tokens.
3268 The effect of %nonassoc on these verbose messages has been corrected
3269 in two ways, but a more complete fix requires LAC, described above:
3271 *** When %nonassoc is used, there can exist parser states that accept no
3272 tokens, and so the parser does not always require a lookahead token
3273 in order to detect a syntax error. Because no unexpected token or
3274 expected tokens can then be reported, the verbose syntax error
3275 message described above is suppressed, and the parser instead
3276 reports the simpler message, "syntax error". Previously, this
3277 suppression was sometimes erroneously triggered by %nonassoc when a
3278 lookahead was actually required. Now verbose messages are
3279 suppressed only when all previous lookaheads have already been
3280 shifted or discarded.
3282 *** Previously, the list of expected tokens erroneously included tokens
3283 that would actually induce a syntax error because conflicts for them
3284 were resolved with %nonassoc in the current parser state. Such
3285 tokens are now properly omitted from the list.
3287 *** Expected token lists are still often wrong due to state merging
3288 (from LALR or IELR) and default reductions, which can both add
3289 invalid tokens and subtract valid tokens. Canonical LR almost
3290 completely fixes this problem by eliminating state merging and
3291 default reductions. However, there is one minor problem left even
3292 when using canonical LR and even after the fixes above. That is,
3293 if the resolution of a conflict with %nonassoc appears in a later
3294 parser state than the one at which some syntax error is
3295 discovered, the conflicted token is still erroneously included in
3296 the expected token list. Bison's new LAC implementation,
3297 described above, eliminates this problem and the need for
3298 canonical LR. However, LAC is still experimental and is disabled
3301 ** Java skeleton fixes:
3303 *** A location handling bug has been fixed.
3305 *** The top element of each of the value stack and location stack is now
3306 cleared when popped so that it can be garbage collected.
3308 *** Parser traces now print the top element of the stack.
3310 ** -W/--warnings fixes:
3312 *** Bison now properly recognizes the "no-" versions of categories:
3314 For example, given the following command line, Bison now enables all
3315 warnings except warnings for incompatibilities with POSIX Yacc:
3317 bison -Wall,no-yacc gram.y
3319 *** Bison now treats S/R and R/R conflicts like other warnings:
3321 Previously, conflict reports were independent of Bison's normal
3322 warning system. Now, Bison recognizes the warning categories
3323 "conflicts-sr" and "conflicts-rr". This change has important
3324 consequences for the -W and --warnings command-line options. For
3327 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3328 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3329 bison -Wnone gram.y # no conflicts are reported
3330 bison -Werror gram.y # any conflict is an error
3332 However, as before, if the %expect or %expect-rr directive is
3333 specified, an unexpected number of conflicts is an error, and an
3334 expected number of conflicts is not reported, so -W and --warning
3335 then have no effect on the conflict report.
3337 *** The "none" category no longer disables a preceding "error":
3339 For example, for the following command line, Bison now reports
3340 errors instead of warnings for incompatibilities with POSIX Yacc:
3342 bison -Werror,none,yacc gram.y
3344 *** The "none" category now disables all Bison warnings:
3346 Previously, the "none" category disabled only Bison warnings for
3347 which there existed a specific -W/--warning category. However,
3348 given the following command line, Bison is now guaranteed to
3349 suppress all warnings:
3353 ** Precedence directives can now assign token number 0:
3355 Since Bison 2.3b, which restored the ability of precedence
3356 directives to assign token numbers, doing so for token number 0 has
3357 produced an assertion failure. For example:
3361 This bug has been fixed.
3364 * Noteworthy changes in release 2.4.3 (2010-08-05)
3366 ** Bison now obeys -Werror and --warnings=error for warnings about
3367 grammar rules that are useless in the parser due to conflicts.
3369 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3372 ** Failures in the test suite for GCC 4.5 have been fixed.
3374 ** Failures in the test suite for some versions of Sun Studio C++ have
3377 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3378 warnings about undefined %prec identifiers will not be converted to
3379 errors in Bison 2.5. They will remain warnings, which should be
3380 sufficient for POSIX while avoiding backward compatibility issues.
3382 ** Minor documentation fixes.
3385 * Noteworthy changes in release 2.4.2 (2010-03-20)
3387 ** Some portability problems that resulted in failures and livelocks
3388 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3389 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3390 errors should no longer cause M4 to report a broken pipe on the
3393 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3395 POSIX specifies that an error be reported for any identifier that does
3396 not appear on the LHS of a grammar rule and that is not defined by
3397 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3398 error report for the case when an identifier appears only after a
3399 %prec directive. It is now restored. However, for backward
3400 compatibility with recent Bison releases, it is only a warning for
3401 now. In Bison 2.5 and later, it will return to being an error.
3402 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3403 warning will not be converted to an error in Bison 2.5.]
3405 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3407 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3408 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3411 ** %code is now a permanent feature.
3413 A traditional Yacc prologue directive is written in the form:
3417 To provide a more flexible alternative, Bison 2.3b introduced the
3418 %code directive with the following forms for C/C++:
3421 %code requires {CODE}
3422 %code provides {CODE}
3425 These forms are now considered permanent features of Bison. See the
3426 %code entries in the section "Bison Declaration Summary" in the Bison
3427 manual for a summary of their functionality. See the section
3428 "Prologue Alternatives" for a detailed discussion including the
3429 advantages of %code over the traditional Yacc prologue directive.
3431 Bison's Java feature as a whole including its current usage of %code
3432 is still considered experimental.
3434 ** YYFAIL is deprecated and will eventually be removed.
3436 YYFAIL has existed for many years as an undocumented feature of
3437 deterministic parsers in C generated by Bison. Previously, it was
3438 documented for Bison's experimental Java parsers. YYFAIL is no longer
3439 documented for Java parsers and is formally deprecated in both cases.
3440 Users are strongly encouraged to migrate to YYERROR, which is
3443 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3444 induce a syntax error. The most obvious difference from YYERROR is
3445 that YYFAIL will automatically invoke yyerror to report the syntax
3446 error so that you don't have to. However, there are several other
3447 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3448 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3449 used. For a more detailed discussion, see:
3451 http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html
3453 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3454 deterministic parsers in C will continue to implement it. However,
3455 because YYFAIL is already flawed, it seems futile to try to make new
3456 Bison features compatible with it. Thus, during parser generation,
3457 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3458 rule action. In a later release, YYFAIL will be disabled for
3459 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3460 be removed altogether.
3462 There exists at least one case where Bison 2.5's YYFAIL warning will
3463 be a false positive. Some projects add phony uses of YYFAIL and other
3464 Bison-defined macros for the sole purpose of suppressing C
3465 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3466 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3467 epilogue (that is, after the second "%%") in the Bison input file. In
3468 this release (2.4.2), Bison already generates its own code to suppress
3469 C preprocessor warnings for YYFAIL, so projects can remove their own
3470 phony uses of YYFAIL if compatibility with Bison releases prior to
3471 2.4.2 is not necessary.
3473 ** Internationalization.
3475 Fix a regression introduced in Bison 2.4: Under some circumstances,
3476 message translations were not installed although supported by the
3480 * Noteworthy changes in release 2.4.1 (2008-12-11)
3482 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3483 declarations have been fixed.
3485 ** Temporary hack for adding a semicolon to the user action.
3487 Bison used to prepend a trailing semicolon at the end of the user
3488 action for reductions. This allowed actions such as
3490 exp: exp "+" exp { $$ = $1 + $3 };
3494 exp: exp "+" exp { $$ = $1 + $3; };
3496 Some grammars still depend on this "feature". Bison 2.4.1 restores
3497 the previous behavior in the case of C output (specifically, when
3498 neither %language or %skeleton or equivalent command-line options
3499 are used) to leave more time for grammars depending on the old
3500 behavior to be adjusted. Future releases of Bison will disable this
3503 ** A few minor improvements to the Bison manual.
3506 * Noteworthy changes in release 2.4 (2008-11-02)
3508 ** %language is an experimental feature.
3510 We first introduced this feature in test release 2.3b as a cleaner
3511 alternative to %skeleton. Since then, we have discussed the possibility of
3512 modifying its effect on Bison's output file names. Thus, in this release,
3513 we consider %language to be an experimental feature that will likely evolve
3516 ** Forward compatibility with GNU M4 has been improved.
3518 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3522 * Noteworthy changes in release 2.3b (2008-05-27)
3524 ** The quotes around NAME that used to be required in the following directive
3527 %define NAME "VALUE"
3529 ** The directive "%pure-parser" is now deprecated in favor of:
3533 which has the same effect except that Bison is more careful to warn about
3534 unreasonable usage in the latter case.
3538 Bison can now generate an LALR(1) parser in C with a push interface. That
3539 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3540 push one token at a time to the parser using "yypush_parse", which will
3541 return to the caller after processing each token. By default, the push
3542 interface is disabled. Either of the following directives will enable it:
3544 %define api.push_pull "push" // Just push; does not require yylex.
3545 %define api.push_pull "both" // Push and pull; requires yylex.
3547 See the new section "A Push Parser" in the Bison manual for details.
3549 The current push parsing interface is experimental and may evolve. More user
3550 feedback will help to stabilize it.
3552 ** The -g and --graph options now output graphs in Graphviz DOT format,
3553 not VCG format. Like --graph, -g now also takes an optional FILE argument
3554 and thus cannot be bundled with other short options.
3558 Bison can now generate an LALR(1) parser in Java. The skeleton is
3559 "data/lalr1.java". Consider using the new %language directive instead of
3560 %skeleton to select it.
3562 See the new section "Java Parsers" in the Bison manual for details.
3564 The current Java interface is experimental and may evolve. More user
3565 feedback will help to stabilize it.
3566 Contributed by Paolo Bonzini.
3570 This new directive specifies the programming language of the generated
3571 parser, which can be C (the default), C++, or Java. Besides the skeleton
3572 that Bison uses, the directive affects the names of the generated files if
3573 the grammar file's name ends in ".y".
3575 ** XML Automaton Report
3577 Bison can now generate an XML report of the LALR(1) automaton using the new
3578 "--xml" option. The current XML schema is experimental and may evolve. More
3579 user feedback will help to stabilize it.
3580 Contributed by Wojciech Polak.
3582 ** The grammar file may now specify the name of the parser header file using
3583 %defines. For example:
3587 ** When reporting useless rules, useless nonterminals, and unused terminals,
3588 Bison now employs the terms "useless in grammar" instead of "useless",
3589 "useless in parser" instead of "never reduced", and "unused in grammar"
3590 instead of "unused".
3592 ** Unreachable State Removal
3594 Previously, Bison sometimes generated parser tables containing unreachable
3595 states. A state can become unreachable during conflict resolution if Bison
3596 disables a shift action leading to it from a predecessor state. Bison now:
3598 1. Removes unreachable states.
3600 2. Does not report any conflicts that appeared in unreachable states.
3601 WARNING: As a result, you may need to update %expect and %expect-rr
3602 directives in existing grammar files.
3604 3. For any rule used only in such states, Bison now reports the rule as
3605 "useless in parser due to conflicts".
3607 This feature can be disabled with the following directive:
3609 %define lr.keep_unreachable_states
3611 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3612 for further discussion.
3614 ** Lookahead Set Correction in the ".output" Report
3616 When instructed to generate a ".output" file including lookahead sets
3617 (using "--report=lookahead", for example), Bison now prints each reduction's
3618 lookahead set only next to the associated state's one item that (1) is
3619 associated with the same rule as the reduction and (2) has its dot at the end
3620 of its RHS. Previously, Bison also erroneously printed the lookahead set
3621 next to all of the state's other items associated with the same rule. This
3622 bug affected only the ".output" file and not the generated parser source
3625 ** --report-file=FILE is a new option to override the default ".output" file
3628 ** The "=" that used to be required in the following directives is now
3631 %file-prefix "parser"
3635 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3637 Bison 2.3a provided a new set of directives as a more flexible alternative to
3638 the traditional Yacc prologue blocks. Those have now been consolidated into
3639 a single %code directive with an optional qualifier field, which identifies
3640 the purpose of the code and thus the location(s) where Bison should generate
3643 1. "%code {CODE}" replaces "%after-header {CODE}"
3644 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3645 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3646 4. "%code top {CODE}" replaces "%before-header {CODE}"
3648 See the %code entries in section "Bison Declaration Summary" in the Bison
3649 manual for a summary of the new functionality. See the new section "Prologue
3650 Alternatives" for a detailed discussion including the advantages of %code
3651 over the traditional Yacc prologues.
3653 The prologue alternatives are experimental. More user feedback will help to
3654 determine whether they should become permanent features.
3656 ** Revised warning: unset or unused midrule values
3658 Since Bison 2.2, Bison has warned about midrule values that are set but not
3659 used within any of the actions of the parent rule. For example, Bison warns
3662 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3664 Now, Bison also warns about midrule values that are used but not set. For
3665 example, Bison warns about unset $$ in the midrule action in:
3667 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3669 However, Bison now disables both of these warnings by default since they
3670 sometimes prove to be false alarms in existing grammars employing the Yacc
3671 constructs $0 or $-N (where N is some positive integer).
3673 To enable these warnings, specify the option "--warnings=midrule-values" or
3674 "-W", which is a synonym for "--warnings=all".
3676 ** Default %destructor or %printer with "<*>" or "<>"
3678 Bison now recognizes two separate kinds of default %destructor's and
3681 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3682 %destructor/%printer for all grammar symbols for which you have formally
3683 declared semantic type tags.
3685 2. Place "<>" in a %destructor/%printer symbol list to define a default
3686 %destructor/%printer for all grammar symbols without declared semantic
3689 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3690 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3691 longer applies any %destructor to a midrule value if that midrule value is
3692 not actually ever referenced using either $$ or $n in a semantic action.
3694 The default %destructor's and %printer's are experimental. More user
3695 feedback will help to determine whether they should become permanent
3698 See the section "Freeing Discarded Symbols" in the Bison manual for further
3701 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3702 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3703 manual for a caveat concerning the treatment of literal strings.
3705 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3706 completely removed from Bison.
3709 * Noteworthy changes in release 2.3a (2006-09-13)
3711 ** Instead of %union, you can define and use your own union type
3712 YYSTYPE if your grammar contains at least one <type> tag.
3713 Your YYSTYPE need not be a macro; it can be a typedef.
3714 This change is for compatibility with other Yacc implementations,
3715 and is required by POSIX.
3717 ** Locations columns and lines start at 1.
3718 In accordance with the GNU Coding Standards and Emacs.
3720 ** You may now declare per-type and default %destructor's and %printer's:
3724 %union { char *string; }
3725 %token <string> STRING1
3726 %token <string> STRING2
3727 %type <string> string1
3728 %type <string> string2
3729 %union { char character; }
3730 %token <character> CHR
3731 %type <character> chr
3732 %destructor { free ($$); } %symbol-default
3733 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3734 %destructor { } <character>
3736 guarantees that, when the parser discards any user-defined symbol that has a
3737 semantic type tag other than "<character>", it passes its semantic value to
3738 "free". However, when the parser discards a "STRING1" or a "string1", it
3739 also prints its line number to "stdout". It performs only the second
3740 "%destructor" in this case, so it invokes "free" only once.
3742 [Although we failed to mention this here in the 2.3a release, the default
3743 %destructor's and %printer's were experimental, and they were rewritten in
3746 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3747 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3748 associating token numbers with token names. Removing the #define statements
3749 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3750 requires them. Bison still generates an enum for token names in all cases.
3752 ** Handling of traditional Yacc prologue blocks is now more consistent but
3753 potentially incompatible with previous releases of Bison.
3755 As before, you declare prologue blocks in your grammar file with the
3756 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3757 prologue blocks that you've declared before the first %union. To generate
3758 the post-prologue, Bison concatenates all prologue blocks that you've
3759 declared after the first %union.
3761 Previous releases of Bison inserted the pre-prologue into both the header
3762 file and the code file in all cases except for LALR(1) parsers in C. In the
3763 latter case, Bison inserted it only into the code file. For parsers in C++,
3764 the point of insertion was before any token definitions (which associate
3765 token numbers with names). For parsers in C, the point of insertion was
3766 after the token definitions.
3768 Now, Bison never inserts the pre-prologue into the header file. In the code
3769 file, it always inserts it before the token definitions.
3771 ** Bison now provides a more flexible alternative to the traditional Yacc
3772 prologue blocks: %before-header, %start-header, %end-header, and
3775 For example, the following declaration order in the grammar file reflects the
3776 order in which Bison will output these code blocks. However, you are free to
3777 declare these code blocks in your grammar file in whatever order is most
3781 /* Bison treats this block like a pre-prologue block: it inserts it into
3782 * the code file before the contents of the header file. It does *not*
3783 * insert it into the header file. This is a good place to put
3784 * #include's that you want at the top of your code file. A common
3785 * example is '#include "system.h"'. */
3788 /* Bison inserts this block into both the header file and the code file.
3789 * In both files, the point of insertion is before any Bison-generated
3790 * token, semantic type, location type, and class definitions. This is a
3791 * good place to define %union dependencies, for example. */
3794 /* Unlike the traditional Yacc prologue blocks, the output order for the
3795 * new %*-header blocks is not affected by their declaration position
3796 * relative to any %union in the grammar file. */
3799 /* Bison inserts this block into both the header file and the code file.
3800 * In both files, the point of insertion is after the Bison-generated
3801 * definitions. This is a good place to declare or define public
3802 * functions or data structures that depend on the Bison-generated
3806 /* Bison treats this block like a post-prologue block: it inserts it into
3807 * the code file after the contents of the header file. It does *not*
3808 * insert it into the header file. This is a good place to declare or
3809 * define internal functions or data structures that depend on the
3810 * Bison-generated definitions. */
3813 If you have multiple occurrences of any one of the above declarations, Bison
3814 will concatenate the contents in declaration order.
3816 [Although we failed to mention this here in the 2.3a release, the prologue
3817 alternatives were experimental, and they were rewritten in future versions.]
3819 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3820 The old spelling still works, but is not documented and may be removed
3821 in a future release.
3824 * Noteworthy changes in release 2.3 (2006-06-05)
3826 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3827 for compatibility with LALR(1) grammars.
3829 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3830 be to a type name that does not contain parentheses or brackets.
3833 * Noteworthy changes in release 2.2 (2006-05-19)
3835 ** The distribution terms for all Bison-generated parsers now permit
3836 using the parsers in nonfree programs. Previously, this permission
3837 was granted only for Bison-generated LALR(1) parsers in C.
3839 ** %name-prefix changes the namespace name in C++ outputs.
3841 ** The C++ parsers export their token_type.
3843 ** Bison now allows multiple %union declarations, and concatenates
3844 their contents together.
3846 ** New warning: unused values
3847 Right-hand side symbols whose values are not used are reported,
3848 if the symbols have destructors. For instance:
3850 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3854 will trigger a warning about $$ and $5 in the first rule, and $3 in
3855 the second ($1 is copied to $$ by the default rule). This example
3856 most likely contains three errors, and could be rewritten as:
3858 exp: exp "?" exp ":" exp
3859 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3861 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3864 However, if the original actions were really intended, memory leaks
3865 and all, the warnings can be suppressed by letting Bison believe the
3866 values are used, e.g.:
3868 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3869 | exp "+" exp { $$ = $1; (void) $3; }
3872 If there are midrule actions, the warning is issued if no action
3873 uses it. The following triggers no warning: $1 and $3 are used.
3875 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3877 The warning is intended to help catching lost values and memory leaks.
3878 If a value is ignored, its associated memory typically is not reclaimed.
3880 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3881 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3882 and YYERROR, for all objects on the stack, other than objects
3883 corresponding to the right-hand side of the current rule.
3885 ** %expect, %expect-rr
3886 Incorrect numbers of expected conflicts are now actual errors,
3887 instead of warnings.
3889 ** GLR, YACC parsers.
3890 The %parse-params are available in the destructors (and the
3891 experimental printers) as per the documentation.
3893 ** Bison now warns if it finds a stray "$" or "@" in an action.
3895 ** %require "VERSION"
3896 This specifies that the grammar file depends on features implemented
3897 in Bison version VERSION or higher.
3899 ** lalr1.cc: The token and value types are now class members.
3900 The tokens were defined as free form enums and cpp macros. YYSTYPE
3901 was defined as a free form union. They are now class members:
3902 tokens are enumerations of the "yy::parser::token" struct, and the
3903 semantic values have the "yy::parser::semantic_type" type.
3905 If you do not want or can update to this scheme, the directive
3906 '%define "global_tokens_and_yystype" "1"' triggers the global
3907 definition of tokens and YYSTYPE. This change is suitable both
3908 for previous releases of Bison, and this one.
3910 If you wish to update, then make sure older version of Bison will
3911 fail using '%require "2.2"'.
3913 ** DJGPP support added.
3916 * Noteworthy changes in release 2.1 (2005-09-16)
3918 ** The C++ lalr1.cc skeleton supports %lex-param.
3920 ** Bison-generated parsers now support the translation of diagnostics like
3921 "syntax error" into languages other than English. The default
3922 language is still English. For details, please see the new
3923 Internationalization section of the Bison manual. Software
3924 distributors should also see the new PACKAGING file. Thanks to
3925 Bruno Haible for this new feature.
3927 ** Wording in the Bison-generated parsers has been changed slightly to
3928 simplify translation. In particular, the message "memory exhausted"
3929 has replaced "parser stack overflow", as the old message was not
3930 always accurate for modern Bison-generated parsers.
3932 ** Destructors are now called when the parser aborts, for all symbols left
3933 behind on the stack. Also, the start symbol is now destroyed after a
3934 successful parse. In both cases, the behavior was formerly inconsistent.
3936 ** When generating verbose diagnostics, Bison-generated parsers no longer
3937 quote the literal strings associated with tokens. For example, for
3938 a syntax error associated with '%token NUM "number"' they might
3939 print 'syntax error, unexpected number' instead of 'syntax error,
3940 unexpected "number"'.
3943 * Noteworthy changes in release 2.0 (2004-12-25)
3945 ** Possibly-incompatible changes
3947 - Bison-generated parsers no longer default to using the alloca function
3948 (when available) to extend the parser stack, due to widespread
3949 problems in unchecked stack-overflow detection. You can "#define
3950 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3951 the manual to determine safe values for YYMAXDEPTH in that case.
3953 - Error token location.
3954 During error recovery, the location of the syntax error is updated
3955 to cover the whole sequence covered by the error token: it includes
3956 the shifted symbols thrown away during the first part of the error
3957 recovery, and the lookahead rejected during the second part.
3959 - Semicolon changes:
3960 . Stray semicolons are no longer allowed at the start of a grammar.
3961 . Semicolons are now required after in-grammar declarations.
3963 - Unescaped newlines are no longer allowed in character constants or
3964 string literals. They were never portable, and GCC 3.4.0 has
3965 dropped support for them. Better diagnostics are now generated if
3966 forget a closing quote.
3968 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
3972 - GLR grammars now support locations.
3974 - New directive: %initial-action.
3975 This directive allows the user to run arbitrary code (including
3976 initializing @$) from yyparse before parsing starts.
3978 - A new directive "%expect-rr N" specifies the expected number of
3979 reduce/reduce conflicts in GLR parsers.
3981 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
3982 This is a GNU extension.
3984 - The option "--report=lookahead" was changed to "--report=look-ahead".
3985 [However, this was changed back after 2.3.]
3987 - Experimental %destructor support has been added to lalr1.cc.
3989 - New configure option --disable-yacc, to disable installation of the
3990 yacc command and -ly library introduced in 1.875 for POSIX conformance.
3994 - For now, %expect-count violations are now just warnings, not errors.
3995 This is for compatibility with Bison 1.75 and earlier (when there are
3996 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
3997 are too many or too few shift/reduce conflicts). However, in future
3998 versions of Bison we plan to improve the %expect machinery so that
3999 these violations will become errors again.
4001 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4002 arbitrarily limited to 16-bit counts.
4004 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4007 * Noteworthy changes in release 1.875 (2003-01-01)
4009 ** The documentation license has been upgraded to version 1.2
4010 of the GNU Free Documentation License.
4012 ** syntax error processing
4014 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4015 locations too. This fixes bugs in error-location computation.
4018 It is now possible to reclaim the memory associated to symbols
4019 discarded during error recovery. This feature is still experimental.
4022 This new directive is preferred over YYERROR_VERBOSE.
4024 - #defining yyerror to steal internal variables is discouraged.
4025 It is not guaranteed to work forever.
4027 ** POSIX conformance
4029 - Semicolons are once again optional at the end of grammar rules.
4030 This reverts to the behavior of Bison 1.33 and earlier, and improves
4031 compatibility with Yacc.
4033 - "parse error" -> "syntax error"
4034 Bison now uniformly uses the term "syntax error"; formerly, the code
4035 and manual sometimes used the term "parse error" instead. POSIX
4036 requires "syntax error" in diagnostics, and it was thought better to
4039 - The documentation now emphasizes that yylex and yyerror must be
4040 declared before use. C99 requires this.
4042 - Bison now parses C99 lexical constructs like UCNs and
4043 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4045 - File names are properly escaped in C output. E.g., foo\bar.y is
4046 output as "foo\\bar.y".
4048 - Yacc command and library now available
4049 The Bison distribution now installs a "yacc" command, as POSIX requires.
4050 Also, Bison now installs a small library liby.a containing
4051 implementations of Yacc-compatible yyerror and main functions.
4052 This library is normally not useful, but POSIX requires it.
4054 - Type clashes now generate warnings, not errors.
4056 - If the user does not define YYSTYPE as a macro, Bison now declares it
4057 using typedef instead of defining it as a macro.
4058 For consistency, YYLTYPE is also declared instead of defined.
4060 ** Other compatibility issues
4062 - %union directives can now have a tag before the "{", e.g., the
4063 directive "%union foo {...}" now generates the C code
4064 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4065 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4066 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4067 This is for compatibility with both Yacc and Bison 1.35.
4069 - ";" is output before the terminating "}" of an action, for
4070 compatibility with Bison 1.35.
4072 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4073 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4075 - "yystype" and "yyltype" are now obsolescent macros instead of being
4076 typedefs or tags; they are no longer documented and are planned to be
4077 withdrawn in a future release.
4082 Users of Bison have to decide how they handle the portability of the
4085 - "parsing stack overflow..." -> "parser stack overflow"
4086 GLR parsers now report "parser stack overflow" as per the Bison manual.
4088 ** %parse-param and %lex-param
4089 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4090 additional context to yyparse and yylex. They suffer from several
4093 - a single argument only can be added,
4094 - their types are weak (void *),
4095 - this context is not passed to ancillary functions such as yyerror,
4096 - only yacc.c parsers support them.
4098 The new %parse-param/%lex-param directives provide a more precise control.
4101 %parse-param {int *nastiness}
4102 %lex-param {int *nastiness}
4103 %parse-param {int *randomness}
4105 results in the following signatures:
4107 int yylex (int *nastiness);
4108 int yyparse (int *nastiness, int *randomness);
4110 or, if both %pure-parser and %locations are used:
4112 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4113 int yyparse (int *nastiness, int *randomness);
4115 ** Bison now warns if it detects conflicting outputs to the same file,
4116 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4117 that command outputs both code and header to foo.h.
4119 ** #line in output files
4120 - --no-line works properly.
4122 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4123 later to be built. This change originally took place a few versions
4124 ago, but nobody noticed until we recently asked someone to try
4125 building Bison with a K&R C compiler.
4128 * Noteworthy changes in release 1.75 (2002-10-14)
4130 ** Bison should now work on 64-bit hosts.
4132 ** Indonesian translation thanks to Tedi Heriyanto.
4135 Fix spurious parse errors.
4138 Some people redefine yyerror to steal yyparse' private variables.
4139 Reenable this trick until an official feature replaces it.
4142 In agreement with POSIX and with other Yaccs, leaving a default
4143 action is valid when $$ is untyped, and $1 typed:
4147 but the converse remains an error:
4151 ** Values of midrule actions
4154 foo: { ... } { $$ = $1; } ...
4156 was incorrectly rejected: $1 is defined in the second midrule
4157 action, and is equal to the $$ of the first midrule action.
4160 * Noteworthy changes in release 1.50 (2002-10-04)
4165 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4166 almost any context-free grammar, ambiguous or not. The new declarations
4167 %dprec and %merge on grammar rules allow parse-time resolution of
4168 ambiguities. Contributed by Paul Hilfinger.
4170 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4171 like the Alpha, so please stick to 32-bit hosts for now.
4174 When not in Yacc compatibility mode, when the output file was not
4175 specified, running "bison foo/bar.y" created "foo/bar.c". It
4176 now creates "bar.c".
4179 The undefined token was systematically mapped to 2 which prevented
4180 the use of 2 by the user. This is no longer the case.
4182 ** Unknown token numbers
4183 If yylex returned an out of range value, yyparse could die. This is
4187 According to POSIX, the error token must be 256.
4188 Bison extends this requirement by making it a preference: *if* the
4189 user specified that one of her tokens is numbered 256, then error
4190 will be mapped onto another number.
4192 ** Verbose error messages
4193 They no longer report "..., expecting error or..." for states where
4194 error recovery is possible.
4197 Defaults to "$end" instead of "$".
4199 ** Error recovery now conforms to documentation and to POSIX
4200 When a Bison-generated parser encounters a syntax error, it now pops
4201 the stack until it finds a state that allows shifting the error
4202 token. Formerly, it popped the stack until it found a state that
4203 allowed some non-error action other than a default reduction on the
4204 error token. The new behavior has long been the documented behavior,
4205 and has long been required by POSIX. For more details, please see
4206 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4207 <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.
4210 Popped tokens and nonterminals are now reported.
4213 Larger grammars are now supported (larger token numbers, larger grammar
4214 size (= sum of the LHS and RHS lengths), larger LALR tables).
4215 Formerly, many of these numbers ran afoul of 16-bit limits;
4216 now these limits are 32 bits on most hosts.
4218 ** Explicit initial rule
4219 Bison used to play hacks with the initial rule, which the user does
4220 not write. It is now explicit, and visible in the reports and
4224 Before, Bison reported the useless rules, but, although not used,
4225 included them in the parsers. They are now actually removed.
4227 ** Useless rules, useless nonterminals
4228 They are now reported, as a warning, with their locations.
4230 ** Rules never reduced
4231 Rules that can never be reduced because of conflicts are now
4234 ** Incorrect "Token not used"
4235 On a grammar such as
4237 %token useless useful
4239 exp: '0' %prec useful;
4241 where a token was used to set the precedence of the last rule,
4242 bison reported both "useful" and "useless" as useless tokens.
4244 ** Revert the C++ namespace changes introduced in 1.31
4245 as they caused too many portability hassles.
4247 ** Default locations
4248 By an accident of design, the default computation of @$ was
4249 performed after another default computation was performed: @$ = @1.
4250 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4251 the computation of @$.
4253 ** Token end-of-file
4254 The token end of file may be specified by the user, in which case,
4255 the user symbol is used in the reports, the graphs, and the verbose
4256 error messages instead of "$end", which remains being the default.
4260 %token MYEOF 0 "end of file"
4263 This old option, which has been broken for ages, is removed.
4266 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4267 Croatian, thanks to Denis Lackovic.
4269 ** Incorrect token definitions
4272 bison used to output
4275 ** Token definitions as enums
4276 Tokens are output both as the traditional #define's, and, provided
4277 the compiler supports ANSI C or is a C++ compiler, as enums.
4278 This lets debuggers display names instead of integers.
4281 In addition to --verbose, bison supports --report=THINGS, which
4282 produces additional information:
4284 complete the core item sets with their closure
4285 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4286 explicitly associate lookahead tokens to items
4288 describe shift/reduce conflicts solving.
4289 Bison used to systematically output this information on top of
4290 the report. Solved conflicts are now attached to their states.
4293 Previous versions don't complain when there is a type clash on
4294 the default action if the rule has a midrule action, such as in:
4302 ** GNU M4 is now required when using Bison.
4305 * Noteworthy changes in release 1.35 (2002-03-25)
4308 Some projects use Bison's C parser with C++ compilers, and define
4309 YYSTYPE as a class. The recent adjustment of C parsers for data
4310 alignment and 64 bit architectures made this impossible.
4312 Because for the time being no real solution for C++ parser
4313 generation exists, kludges were implemented in the parser to
4314 maintain this use. In the future, when Bison has C++ parsers, this
4315 kludge will be disabled.
4317 This kludge also addresses some C++ problems when the stack was
4321 * Noteworthy changes in release 1.34 (2002-03-12)
4323 ** File name clashes are detected
4324 $ bison foo.y -d -o foo.x
4325 fatal error: header and parser would both be named "foo.x"
4327 ** A missing ";" at the end of a rule triggers a warning
4328 In accordance with POSIX, and in agreement with other
4329 Yacc implementations, Bison will mandate this semicolon in the near
4330 future. This eases the implementation of a Bison parser of Bison
4331 grammars by making this grammar LALR(1) instead of LR(2). To
4332 facilitate the transition, this release introduces a warning.
4334 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4335 many portability hassles.
4337 ** DJGPP support added.
4339 ** Fix test suite portability problems.
4342 * Noteworthy changes in release 1.33 (2002-02-07)
4345 Groff could not be compiled for the definition of size_t was lacking
4346 under some conditions.
4352 * Noteworthy changes in release 1.32 (2002-01-23)
4354 ** Fix Yacc output file names
4356 ** Portability fixes
4358 ** Italian, Dutch translations
4361 * Noteworthy changes in release 1.31 (2002-01-14)
4365 ** GNU Gettext and %expect
4366 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4367 Bison dies on incorrect %expectations, we fear there will be
4368 too many bug reports for Gettext, so _for the time being_, %expect
4369 does not trigger an error when the input file is named "plural.y".
4371 ** Use of alloca in parsers
4372 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4373 malloc exclusively. Since 1.29, but was not NEWS'ed.
4375 alloca is used only when compiled with GCC, to avoid portability
4378 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4380 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4381 (as POSIX requires) instead of being undefined.
4384 Bison has always permitted actions such as { $$ = $1 }: it adds the
4385 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4386 is no longer output: one has to write { $$ = $1; }.
4388 ** Better C++ compliance
4389 The output parsers try to respect C++ namespaces.
4390 [This turned out to be a failed experiment, and it was reverted later.]
4393 Fixed bugs when reporting useless nonterminals.
4396 The parsers work properly on 64 bit hosts.
4399 Some calls to strerror resulted in scrambled or missing error messages.
4402 When the number of shift/reduce conflicts is correct, don't issue
4405 ** The verbose report includes the rule line numbers.
4407 ** Rule line numbers are fixed in traces.
4409 ** Swedish translation
4412 Verbose parse error messages from the parsers are better looking.
4413 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4414 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4416 ** Fixed parser memory leaks.
4417 When the generated parser was using malloc to extend its stacks, the
4418 previous allocations were not freed.
4420 ** Fixed verbose output file.
4421 Some newlines were missing.
4422 Some conflicts in state descriptions were missing.
4424 ** Fixed conflict report.
4425 Option -v was needed to get the result.
4429 Mismatches are errors, not warnings.
4431 ** Fixed incorrect processing of some invalid input.
4433 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4435 ** Fixed some typos in the documentation.
4437 ** %token MY_EOF 0 is supported.
4438 Before, MY_EOF was silently renumbered as 257.
4440 ** doc/refcard.tex is updated.
4442 ** %output, %file-prefix, %name-prefix.
4446 New, aliasing "--output-file".
4449 * Noteworthy changes in release 1.30 (2001-10-26)
4451 ** "--defines" and "--graph" have now an optional argument which is the
4452 output file name. "-d" and "-g" do not change; they do not take any
4455 ** "%source_extension" and "%header_extension" are removed, failed
4458 ** Portability fixes.
4461 * Noteworthy changes in release 1.29 (2001-09-07)
4463 ** The output file does not define const, as this caused problems when used
4464 with common autoconfiguration schemes. If you still use ancient compilers
4465 that lack const, compile with the equivalent of the C compiler option
4466 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4468 ** Added "-g" and "--graph".
4470 ** The Bison manual is now distributed under the terms of the GNU FDL.
4472 ** The input and the output files has automatically a similar extension.
4474 ** Russian translation added.
4476 ** NLS support updated; should hopefully be less troublesome.
4478 ** Added the old Bison reference card.
4480 ** Added "--locations" and "%locations".
4482 ** Added "-S" and "--skeleton".
4484 ** "%raw", "-r", "--raw" is disabled.
4486 ** Special characters are escaped when output. This solves the problems
4487 of the #line lines with path names including backslashes.
4490 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4491 "%debug", "%source_extension" and "%header_extension".
4494 Automatic location tracking.
4497 * Noteworthy changes in release 1.28 (1999-07-06)
4499 ** Should compile better now with K&R compilers.
4503 ** Fixed a problem with escaping the double quote character.
4505 ** There is now a FAQ.
4508 * Noteworthy changes in release 1.27
4510 ** The make rule which prevented bison.simple from being created on
4511 some systems has been fixed.
4514 * Noteworthy changes in release 1.26
4516 ** Bison now uses Automake.
4518 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4520 ** Token numbers now start at 257 as previously documented, not 258.
4522 ** Bison honors the TMPDIR environment variable.
4524 ** A couple of buffer overruns have been fixed.
4526 ** Problems when closing files should now be reported.
4528 ** Generated parsers should now work even on operating systems which do
4529 not provide alloca().
4532 * Noteworthy changes in release 1.25 (1995-10-16)
4534 ** Errors in the input grammar are not fatal; Bison keeps reading
4535 the grammar file, and reports all the errors found in it.
4537 ** Tokens can now be specified as multiple-character strings: for
4538 example, you could use "<=" for a token which looks like <=, instead
4539 of choosing a name like LESSEQ.
4541 ** The %token_table declaration says to write a table of tokens (names
4542 and numbers) into the parser file. The yylex function can use this
4543 table to recognize multiple-character string tokens, or for other
4546 ** The %no_lines declaration says not to generate any #line preprocessor
4547 directives in the parser file.
4549 ** The %raw declaration says to use internal Bison token numbers, not
4550 Yacc-compatible token numbers, when token names are defined as macros.
4552 ** The --no-parser option produces the parser tables without including
4553 the parser engine; a project can now use its own parser engine.
4554 The actions go into a separate file called NAME.act, in the form of
4555 a switch statement body.
4558 * Noteworthy changes in release 1.23
4560 The user can define YYPARSE_PARAM as the name of an argument to be
4561 passed into yyparse. The argument should have type void *. It should
4562 actually point to an object. Grammar actions can access the variable
4563 by casting it to the proper pointer type.
4565 Line numbers in output file corrected.
4568 * Noteworthy changes in release 1.22
4570 --help option added.
4573 * Noteworthy changes in release 1.20
4575 Output file does not redefine const for C++.
4579 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4580 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4581 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4582 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4583 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4584 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4585 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4586 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4587 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4588 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4589 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4590 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4591 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4592 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4593 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4594 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4595 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4596 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4597 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4598 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4599 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4600 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4601 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4602 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4603 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4604 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4605 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4606 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4607 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4608 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4609 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4610 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4611 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4612 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4613 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4614 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4615 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4618 ispell-dictionary: "american"
4623 Copyright (C) 1995-2015, 2018-2020 Free Software Foundation, Inc.
4625 This file is part of Bison, the GNU Parser Generator.
4627 Permission is granted to copy, distribute and/or modify this document
4628 under the terms of the GNU Free Documentation License, Version 1.3 or
4629 any later version published by the Free Software Foundation; with no
4630 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4631 Texts. A copy of the license is included in the "GNU Free
4632 Documentation License" file as part of this distribution.