3 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
7 It is always recommended to prefer `%define api.value.type foo` to
8 `#define YYSTYPE foo`. The latter is supported in C for compatibility
9 with Yacc, but not in C++. Warnings are now issued if `#define YYSTYPE`
10 is used in C++, and eventually support will be removed.
14 *** Counterexample Generation
16 In some cases counterexample generation could crash. This is fixed.
18 *** Fix Table Generation
20 In some very rare conditions, when there are many useless tokens, it was
21 possible to generate incorrect parsers.
23 *** GLR parsers now support %merge together with api.value.type=union.
25 *** C++ parsers use noexcept in more places.
27 *** Generated parsers avoid some warnings about signedness issues.
29 *** C-language parsers now avoid warnings from pedantic clang.
31 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
37 Prefer value_type to semantic_type to denote the semantic value type,
38 specified by the `api.value.type` %define variable.
42 There were not debug traces for deferred calls to user actions. They are
47 *** Option -H, --header and directive %header
49 The option `-H`/`--header` supersedes the option `--defines`, and the
50 directive %header supersedes %defines. Both `--defines` and `%defines`
51 are, of course, maintained for backward compatibility.
55 Since version 2.4 Bison can be used to generate HTML reports. However it
56 was a two-step process: first bison must be invoked with option `--xml`,
57 and then xsltproc must be run to the convert the XML reports into HTML.
59 The new option `--html` combines these steps. The xsltproc program must
62 *** A C++ native GLR parser
64 A new version of the generated C++ GLR parser was added as "glr2.cc". It
65 is forked from the existing glr.c/cc parser, with the objective of making
66 it a more modern, truly C++ parser (instead of a C++ wrapper around a C
67 parser). Down the line, the goal is to support `%define api.value.type
68 variant` and maybe share code with lalr1.cc.
70 The current parser should be identical in terms of interface, functionality
71 and performance to "glr.cc". To try it out, simply use
77 Counterexamples now show the rule numbers, and always show ε for rules
78 with an empty right-hand side. For instance
90 *** Lookahead correction in Java
92 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
95 *** Abort parsing for memory exhaustion (C)
97 The user actions may now use YYNOMEM to abort the current parse with
101 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
105 *** Bug fixes in yacc.c
107 In Yacc mode, all the tokens are defined twice: once as an enum, and then
108 as a macro. YYEMPTY was missing its macro.
110 *** Bug fixes in lalr1.cc
112 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
113 even when the `parse.assert` %define variable is not enabled. It no
116 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
119 When there is a very large number of tokens, some assertions could be long
120 enough to hit arbitrary limits in Visual C++. They have been rewritten to
121 work around this limitation.
125 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
126 skeleton) used to be defined to 1. It is now defined to the version of
127 Bison as an integer (e.g., 30704 for version 3.7.4).
130 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
134 Fix concurrent build issues.
136 The bison executable is no longer linked uselessly against libreadline.
138 Fix incorrect use of yytname in glr.cc.
141 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
143 This release of Bison fixes all known bugs reported for Bison in MITRE's
144 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
145 are only about bison-the-program itself, not the generated code.
147 Although these bugs are typically irrelevant to how Bison is used, they
148 are worth fixing if only to give users peace of mind.
150 There is no known vulnerability in the generated parsers.
154 Fix concurrent build issues (introduced in Bison 3.5).
156 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
158 Fix portability issues of the test suite, and of bison itself.
160 Some unlikely crashes found by fuzzing have been fixed. This is only
161 about bison itself, not the generated parsers.
164 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
168 Crash when a token alias contains a NUL byte.
170 Portability issues with libtextstyle.
172 Portability issues of Bison itself with MSVC.
176 Improvements and fixes in the documentation.
178 More precise location about symbol type redefinitions.
181 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
183 ** Deprecated features
185 The YYPRINT macro, which works only with yacc.c and only for tokens, was
186 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
187 It is deprecated and its support will be removed eventually.
189 In conformance with the recommendations of the Graphviz team, in the next
190 version Bison the option `--graph` will generate a *.gv file by default,
191 instead of *.dot. A transition started in Bison 3.4.
195 *** Counterexample Generation
197 Contributed by Vincent Imbimbo.
199 When given `-Wcounterexamples`/`-Wcex`, bison will now output
200 counterexamples for conflicts.
202 **** Unifying Counterexamples
204 Unifying counterexamples are strings which can be parsed in two ways due
205 to the conflict. For example on a grammar that contains the usual
206 "dangling else" ambiguity:
209 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
210 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
213 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
214 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
215 Example: "if" exp "then" "if" exp "then" exp • "else" exp
218 ↳ "if" exp "then" exp
219 ↳ "if" exp "then" exp • "else" exp
220 Example: "if" exp "then" "if" exp "then" exp • "else" exp
223 ↳ "if" exp "then" exp "else" exp
224 ↳ "if" exp "then" exp •
226 When text styling is enabled, colors are used in the examples and the
227 derivations to highlight the structure of both analyses. In this case,
229 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
233 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
236 The counterexamples are "focused", in two different ways. First, they do
237 not clutter the output with all the derivations from the start symbol,
238 rather they start on the "conflicted nonterminal". They go straight to the
239 point. Second, they don't "expand" nonterminal symbols uselessly.
241 **** Nonunifying Counterexamples
243 In the case of the dangling else, Bison found an example that can be
244 parsed in two ways (therefore proving that the grammar is ambiguous).
245 When it cannot find such an example, it instead generates two examples
246 that are the same up until the dot:
249 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
250 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
251 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
256 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
257 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
258 First example: expr • ID ',' ID $end
265 Second example: expr • ID $end
271 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
275 In these cases, the parser usually doesn't have enough lookahead to
276 differentiate the two given examples.
280 Counterexamples are also included in the report when given
281 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
286 1 exp: "if" exp "then" exp • [$end, "then", "else"]
287 2 | "if" exp "then" exp • "else" exp
289 "else" shift, and go to state 8
291 "else" [reduce using rule 1 (exp)]
292 $default reduce using rule 1 (exp)
294 shift/reduce conflict on token "else":
295 1 exp: "if" exp "then" exp •
296 2 exp: "if" exp "then" exp • "else" exp
297 Example: "if" exp "then" "if" exp "then" exp • "else" exp
300 ↳ "if" exp "then" exp
301 ↳ "if" exp "then" exp • "else" exp
302 Example: "if" exp "then" "if" exp "then" exp • "else" exp
305 ↳ "if" exp "then" exp "else" exp
306 ↳ "if" exp "then" exp •
308 *** File prefix mapping
310 Contributed by Joshua Watt.
312 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
313 in the output (specifically `#line` directives and `#ifdef` header guards)
314 that begins with the prefix OLD will have it replaced with the prefix NEW,
315 similar to the `-ffile-prefix-map` in GCC. This option can be used to
316 make bison output reproducible.
322 When text styling is enabled and the terminal supports it, the warnings
323 now include hyperlinks to the documentation.
325 *** Relocatable installation
327 When installed to be relocatable (via `configure --enable-relocatable`),
328 bison will now also look for a relocated m4.
332 The `filename_type` %define variable was renamed `api.filename.type`.
335 %define filename_type "symbol"
339 %define api.filename.type {symbol}
341 (Or let `bison --update` do it for you).
343 It now defaults to `const std::string` instead of `std::string`.
345 *** Deprecated %define variable names
347 The following variables have been renamed for consistency. Backward
348 compatibility is ensured, but upgrading is recommended.
350 filename_type -> api.filename.type
351 package -> api.package
353 *** Push parsers no longer clear their state when parsing is finished
355 Previously push-parsers cleared their state when parsing was finished (on
356 success and on failure). This made it impossible to check if there were
357 parse errors, since `yynerrs` was also reset. This can be especially
358 troublesome when used in autocompletion, since a parser with error
359 recovery would suggest (irrelevant) expected tokens even if there were
362 Now the parser state can be examined when parsing is finished. The parser
363 state is reset when starting a new parse.
369 The bistromathic demonstrates %param and how to quote sources in the error
373 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
379 *** Include the generated header (yacc.c)
381 Historically, when --defines was used, bison generated a header and pasted
382 an exact copy of it into the generated parser implementation file. Since
383 Bison 3.4 it is possible to specify that the header should be `#include`d,
384 and how. For instance
386 %define api.header.include {"parse.h"}
390 %define api.header.include {<parser/parse.h>}
392 Now api.header.include defaults to `"header-basename"`, as was intended in
393 Bison 3.4, where `header-basename` is the basename of the generated
394 header. This is disabled when the generated header is `y.tab.h`, to
395 comply with Automake's ylwrap.
397 *** String aliases are faithfully propagated
399 Bison used to interpret user strings (i.e., decoding backslash escapes)
400 when reading them, and to escape them (i.e., issue non-printable
401 characters as backslash escapes, taking the locale into account) when
402 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
403 up "ciphered" as sequences of backslash escapes. This happened not only
404 in the generated sources (where the compiler will reinterpret them), but
405 also in all the generated reports (text, xml, html, dot, etc.). Reports
406 were therefore not readable when string aliases were not pure ASCII.
407 Worse yet: the output depended on the user's locale.
409 Now Bison faithfully treats the string aliases exactly the way the user
410 spelled them. This fixes all the aforementioned problems. However, now,
411 string aliases semantically equivalent but syntactically different (e.g.,
412 "A", "\x41", "\101") are considered to be different.
414 *** Crash when generating IELR
416 An old, well hidden, bug in the generation of IELR parsers was fixed.
419 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
423 In glr.cc some internal macros leaked in the user's code, and could damage
424 access to the token kinds.
427 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
431 Incorrect comments in the generated parsers.
433 Warnings in push parsers (yacc.c).
435 Incorrect display of gotos in LAC traces (lalr1.cc).
438 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
442 Some tests were fixed.
444 When token aliases contain comment delimiters:
446 %token FOO "/* foo */"
448 bison used to emit "nested" comments, which is invalid C.
451 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
455 Restored ANSI-C compliance in yacc.c.
457 GNU readline portability issues.
459 In C++, yy::parser::symbol_name is now a public member, as was intended.
463 In C++, yy::parser::symbol_type now has a public name() member function.
466 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
468 ** Backward incompatible changes
470 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
472 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
473 depend on it will now produce Yacc-like error messages (just "syntax
474 error"). It was superseded by the "%error-verbose" directive in Bison
475 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
476 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
477 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
478 parse.error verbose".
480 ** Deprecated features
482 The YYPRINT macro, which works only with yacc.c and only for tokens, was
483 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
484 It is deprecated and its support will be removed eventually.
488 *** Improved syntax error messages
490 Two new values for the %define parse.error variable offer more control to
491 the user. Available in all the skeletons (C, C++, Java).
493 **** %define parse.error detailed
495 The behavior of "%define parse.error detailed" is closely resembling that
496 of "%define parse.error verbose" with a few exceptions. First, it is safe
497 to use non-ASCII characters in token aliases (with 'verbose', the result
498 depends on the locale with which bison was run). Second, a yysymbol_name
499 function is exposed to the user, instead of the yytnamerr function and the
500 yytname table. Third, token internationalization is supported (see
503 **** %define parse.error custom
505 With this directive, the user forges and emits the syntax error message
506 herself by defining the yyreport_syntax_error function. A new type,
507 yypcontext_t, captures the circumstances of the error, and provides the
508 user with functions to get details, such as yypcontext_expected_tokens to
509 get the list of expected token kinds.
511 A possible implementation of yyreport_syntax_error is:
514 yyreport_syntax_error (const yypcontext_t *ctx)
517 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
518 fprintf (stderr, ": syntax error");
519 // Report the tokens expected at this point.
521 enum { TOKENMAX = 10 };
522 yysymbol_kind_t expected[TOKENMAX];
523 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
525 // Forward errors to yyparse.
528 for (int i = 0; i < n; ++i)
529 fprintf (stderr, "%s %s",
530 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
532 // Report the unexpected token.
534 yysymbol_kind_t lookahead = yypcontext_token (ctx);
535 if (lookahead != YYSYMBOL_YYEMPTY)
536 fprintf (stderr, " before %s", yysymbol_name (lookahead));
538 fprintf (stderr, "\n");
542 **** Token aliases internationalization
544 When the %define variable parse.error is set to `custom` or `detailed`,
545 one may specify which token aliases are to be translated using _(). For
557 In that case the user must define _() and N_(), and yysymbol_name returns
558 the translated symbol (i.e., it returns '_("variable")' rather that
559 '"variable"'). In Java, the user must provide an i18n() function.
561 *** List of expected tokens (yacc.c)
563 Push parsers may invoke yypstate_expected_tokens at any point during
564 parsing (including even before submitting the first token) to get the list
565 of possible tokens. This feature can be used to propose autocompletion
566 (see below the "bistromathic" example).
568 It makes little sense to use this feature without enabling LAC (lookahead
571 *** Returning the error token
573 When the scanner returns an invalid token or the undefined token
574 (YYUNDEF), the parser generates an error message and enters error
575 recovery. Because of that error message, most scanners that find lexical
576 errors generate an error message, and then ignore the invalid input
577 without entering the error-recovery.
579 The scanners may now return YYerror, the error token, to enter the
580 error-recovery mode without triggering an additional error message. See
581 the bistromathic for an example.
583 *** Deep overhaul of the symbol and token kinds
585 To avoid the confusion with types in programming languages, we now refer
586 to token and symbol "kinds" instead of token and symbol "types". The
587 documentation and error messages have been revised.
589 All the skeletons have been updated to use dedicated enum types rather
590 than integral types. Special symbols are now regular citizens, instead of
591 being declared in ad hoc ways.
595 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
596 LPAREN, etc. While backward compatibility is of course ensured, users are
597 nonetheless invited to replace their uses of "enum yytokentype" by
600 This type now also includes tokens that were previously hidden: YYEOF (end
601 of input), YYUNDEF (undefined token), and YYerror (error token). They
602 now have string aliases, internationalized when internationalization is
603 enabled. Therefore, by default, error messages now refer to "end of file"
604 (internationalized) rather than the cryptic "$end", or to "invalid token"
605 rather than "$undefined".
607 Therefore in most cases it is now useless to define the end-of-line token
610 %token T_EOF 0 "end of file"
612 Rather simply use "YYEOF" in your scanner.
616 The "symbol kinds" is what the parser actually uses. (Unless the
617 api.token.raw %define variable is used, the symbol kind of a terminal
618 differs from the corresponding token kind.)
620 They are now exposed as a enum, "yysymbol_kind_t".
622 This allows users to tailor the error messages the way they want, or to
623 process some symbols in a specific way in autocompletion (see the
624 bistromathic example below).
626 *** Modernize display of explanatory statements in diagnostics
628 Since Bison 2.7, output was indented four spaces for explanatory
629 statements. For example:
631 input.y:2.7-13: error: %type redeclaration for exp
632 input.y:1.7-11: previous declaration
634 Since the introduction of caret-diagnostics, it became less clear. This
635 indentation has been removed and submessages are displayed similarly as in
638 input.y:2.7-13: error: %type redeclaration for exp
639 2 | %type <float> exp
641 input.y:1.7-11: note: previous declaration
645 Contributed by Victor Morales Cayuela.
649 The token and symbol kinds are yy::parser::token_kind_type and
650 yy::parser::symbol_kind_type.
652 The symbol_type::kind() member function allows to get the kind of a
653 symbol. This can be used to write unit tests for scanners, e.g.,
655 yy::parser::symbol_type t = make_NUMBER ("123");
656 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
657 assert (t.value.as<int> () == 123);
663 In order to avoid ambiguities with "type" as in "typing", we now refer to
664 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
665 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
670 There are now examples/java: a very simple calculator, and a more complete
671 one (push-parser, location tracking, and debug traces).
673 The lexcalc example (a simple example in C based on Flex and Bison) now
674 also demonstrates location tracking.
677 A new C example, bistromathic, is a fully featured interactive calculator
678 using many Bison features: pure interface, push parser, autocompletion
679 based on the current parser state (using yypstate_expected_tokens),
680 location tracking, internationalized custom error messages, lookahead
681 correction, rich debug traces, etc.
683 It shows how to depend on the symbol kinds to tailor autocompletion. For
684 instance it recognizes the symbol kind "VARIABLE" to propose
685 autocompletion on the existing variables, rather than of the word
689 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
691 ** WARNING: Future backward-incompatibilities!
693 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
695 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
696 that still depend on it will produce Yacc-like error messages (just
697 "syntax error"). It was superseded by the "%error-verbose" directive in
698 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
699 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
700 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
701 parse.error verbose".
705 Fix portability issues of the package itself on old compilers.
707 Fix api.token.raw support in Java.
710 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
714 Error messages could quote lines containing zero-width characters (such as
715 \005) with incorrect styling. Fixes for similar issues with unexpectedly
716 short lines (e.g., the file was changed between parsing and diagnosing).
718 Some unlikely crashes found by fuzzing have been fixed. This is only
719 about bison itself, not the generated parsers.
722 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
726 Portability issues and minor cosmetic issues.
728 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
732 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
738 Fix compiler warnings.
741 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
743 ** Backward incompatible changes
745 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
746 longer treated as end-of-lines. This changes the diagnostics, and in
747 particular their locations.
749 In C++, line numbers and columns are now represented as 'int' not
750 'unsigned', so that integer overflow on positions is easily checkable via
751 'gcc -fsanitize=undefined' and the like. This affects the API for
752 positions. The default position and location classes now expose
753 'counter_type' (int), used to define line and column numbers.
755 ** Deprecated features
757 The YYPRINT macro, which works only with yacc.c and only for tokens, was
758 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
759 It is deprecated and its support will be removed eventually.
763 *** Lookahead correction in C++
765 Contributed by Adrian Vogelsgesang.
767 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
768 %define variable parse.lac.
770 *** Variable api.token.raw: Optimized token numbers (all skeletons)
772 In the generated parsers, tokens have two numbers: the "external" token
773 number as returned by yylex (which starts at 257), and the "internal"
774 symbol number (which starts at 3). Each time yylex is called, a table
775 lookup maps the external token number to the internal symbol number.
777 When the %define variable api.token.raw is set, tokens are assigned their
778 internal number, which saves one table lookup per token, and also saves
779 the generation of the mapping table.
781 The gain is typically moderate, but in extreme cases (very simple user
782 actions), a 10% improvement can be observed.
784 *** Generated parsers use better types for states
786 Stacks now use the best integral type for state numbers, instead of always
787 using 15 bits. As a result "small" parsers now have a smaller memory
788 footprint (they use 8 bits), and there is support for large automata (16
789 bits), and extra large (using int, i.e., typically 31 bits).
791 *** Generated parsers prefer signed integer types
793 Bison skeletons now prefer signed to unsigned integer types when either
794 will do, as the signed types are less error-prone and allow for better
795 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
796 portable to unusual machines where char, short and int are all the same
797 width. On non-GNU platforms this may entail including <limits.h> and (if
798 available) <stdint.h> to define integer types and constants.
800 *** A skeleton for the D programming language
802 For the last few releases, Bison has shipped a stealth experimental
803 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
804 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
807 However, because nobody has committed to improving, testing, and
808 documenting this skeleton, it is not clear that it will be supported in
811 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
812 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
814 *** Debug traces in Java
816 The Java backend no longer emits code and data for parser tracing if the
817 %define variable parse.trace is not defined.
821 *** New diagnostic: -Wdangling-alias
823 String literals, which allow for better error messages, are (too)
824 liberally accepted by Bison, which might result in silent errors. For
827 %type <exVal> cond "condition"
829 does not define "condition" as a string alias to 'cond' (nonterminal
830 symbols do not have string aliases). It is rather equivalent to
833 %token <exVal> "condition"
835 i.e., it gives the type 'exVal' to the "condition" token, which was
836 clearly not the intention.
838 Also, because string aliases need not be defined, typos such as "baz"
839 instead of "bar" will be not reported.
841 The option `-Wdangling-alias` catches these situations. On
844 %type <ival> foo "foo"
848 bison -Wdangling-alias reports
850 warning: string literal not attached to a symbol
851 | %type <ival> foo "foo"
853 warning: string literal not attached to a symbol
857 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
859 *** Better POSIX Yacc compatibility diagnostics
861 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
865 %type <ival> TOKEN1 TOKEN2 't'
872 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
873 2 | %type <ival> TOKEN1 TOKEN2 't'
875 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
876 2 | %type <ival> TOKEN1 TOKEN2 't'
878 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
879 2 | %type <ival> TOKEN1 TOKEN2 't'
882 *** Diagnostics with insertion
884 The diagnostics now display the suggestion below the underlined source.
885 Replacement for undeclared symbols are now also suggested.
892 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'?
896 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
900 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
902 *** Diagnostics about long lines
904 Quoted sources may now be truncated to fit the screen. For instance, on a
905 30-column wide terminal:
912 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
915 foo.y:1.8-10: previous declaration
918 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
921 foo.y:1.8-10: previous declaration
927 *** Debugging glr.c and glr.cc
929 The glr.c skeleton always had asserts to check its own behavior (not the
930 user's). These assertions are now under the control of the parse.assert
931 %define variable (disabled by default).
935 Several new compiler warnings in the generated output have been avoided.
936 Some unused features are no longer emitted. Cleaner generated code in
941 Portability issues in the test suite.
943 In theory, parsers using %nonassoc could crash when reporting verbose
944 error messages. This unlikely bug has been fixed.
946 In Java, %define api.prefix was ignored. It now behaves as expected.
949 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
953 In some cases, when warnings are disabled, bison could emit tons of white
954 spaces as diagnostics.
956 When running out of memory, bison could crash (found by fuzzing).
958 When defining twice the EOF token, bison would crash.
960 New warnings from recent compilers have been addressed in the generated
961 parsers (yacc.c, glr.c, glr.cc).
963 When lone carriage-return characters appeared in the input file,
964 diagnostics could hang forever.
967 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
974 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
976 ** Deprecated features
978 The %pure-parser directive is deprecated in favor of '%define api.pure'
979 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
980 now. Note that since Bison 2.7 you are strongly encouraged to use
981 '%define api.pure full' instead of '%define api.pure'.
985 *** Colored diagnostics
987 As an experimental feature, diagnostics are now colored, controlled by the
988 new options --color and --style.
990 To use them, install the libtextstyle library before configuring Bison.
993 https://alpha.gnu.org/gnu/gettext/
997 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
999 The option --color supports the following arguments:
1000 - always, yes: Enable colors.
1001 - never, no: Disable colors.
1002 - auto, tty (default): Enable colors if the output device is a tty.
1004 To customize the styles, create a CSS file similar to
1008 .error { font-weight: 800; text-decoration: underline; }
1011 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
1012 environment variable to "bison-bw.css".
1014 *** Disabling output
1016 When given -fsyntax-only, the diagnostics are reported, but no output is
1019 The name of this option is somewhat misleading as bison does more than
1020 just checking the syntax: every stage is run (including checking for
1021 conflicts for instance), except the generation of the output files.
1023 *** Include the generated header (yacc.c)
1025 Before, when --defines is used, bison generated a header, and pasted an
1026 exact copy of it into the generated parser implementation file. If the
1027 header name is not "y.tab.h", it is now #included instead of being
1030 To use an '#include' even if the header name is "y.tab.h" (which is what
1031 happens with --yacc, or when using the Autotools' ylwrap), define
1032 api.header.include to the exact argument to pass to #include. For
1035 %define api.header.include {"parse.h"}
1039 %define api.header.include {<parser/parse.h>}
1041 *** api.location.type is now supported in C (yacc.c, glr.c)
1043 The %define variable api.location.type defines the name of the type to use
1044 for locations. When defined, Bison no longer defines YYLTYPE.
1046 This can be used in programs with several parsers to factor their
1047 definition of locations: let one of them generate them, and the others
1054 In conformance with the recommendations of the Graphviz team, if %require
1055 "3.4" (or better) is specified, the option --graph generates a *.gv file
1056 by default, instead of *.dot.
1058 *** Diagnostics overhaul
1060 Column numbers were wrong with multibyte characters, which would also
1061 result in skewed diagnostics with carets. Beside, because we were
1062 indenting the quoted source with a single space, lines with tab characters
1063 were incorrectly underlined.
1065 To address these issues, and to be clearer, Bison now issues diagnostics
1066 as GCC9 does. For instance it used to display (there's a tab before the
1069 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1070 expr: expr '+' "number" { $$ = $1 + $2; }
1074 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1075 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1078 Other constructs now also have better locations, resulting in more precise
1081 *** Fix-it hints for %empty
1083 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1084 annotations, and add the missing ones.
1086 *** Generated reports
1088 The format of the reports (parse.output) was improved for readability.
1090 *** Better support for --no-line.
1092 When --no-line is used, the generated files are now cleaner: no lines are
1093 generated instead of empty lines. Together with using api.header.include,
1094 that should help people saving the generated files into version control
1095 systems get smaller diffs.
1099 A new example in C shows an simple infix calculator with a hand-written
1100 scanner (examples/c/calc).
1102 A new example in C shows a reentrant parser (capable of recursive calls)
1103 built with Flex and Bison (examples/c/reccalc).
1105 There is a new section about the history of Yaccs and Bison.
1109 A few obscure bugs were fixed, including the second oldest (known) bug in
1110 Bison: it was there when Bison was entered in the RCS version control
1111 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1115 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1119 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1123 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1127 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1128 extensions into errors. It now makes them simple warnings (-Wyacc).
1131 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1133 A new mailing list was created, Bison Announce. It is low traffic, and is
1134 only about announcing new releases and important messages (e.g., polls
1135 about major decisions to make).
1137 https://lists.gnu.org/mailman/listinfo/bison-announce
1139 ** Backward incompatible changes
1141 Support for DJGPP, which has been unmaintained and untested for years, is
1144 ** Deprecated features
1146 A new feature, --update (see below) helps adjusting existing grammars to
1149 *** Deprecated directives
1151 The %error-verbose directive is deprecated in favor of '%define
1152 parse.error verbose' since Bison 3.0, but no warning was issued.
1154 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1155 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1156 directives are slightly different, you might need to adjust your code.
1157 %name-prefix renames only symbols with external linkage, while api.prefix
1158 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1159 yytokentype, YYSTYPE, YYLTYPE, etc.
1161 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1162 {xx}' will typically have to update YY_DECL from
1164 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1168 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1170 *** Deprecated %define variable names
1172 The following variables, mostly related to parsers in Java, have been
1173 renamed for consistency. Backward compatibility is ensured, but upgrading
1176 abstract -> api.parser.abstract
1177 annotations -> api.parser.annotations
1178 extends -> api.parser.extends
1179 final -> api.parser.final
1180 implements -> api.parser.implements
1181 parser_class_name -> api.parser.class
1182 public -> api.parser.public
1183 strictfp -> api.parser.strictfp
1187 *** Generation of fix-its for IDEs/Editors
1189 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1190 bison now generates machine readable editing instructions to fix some
1191 issues. Currently, this is mostly limited to updating deprecated
1192 directives and removing duplicates. For instance:
1196 %define parser_class_name "Parser"
1197 %define api.parser.class "Parser"
1201 See the "fix-it:" lines below:
1203 $ bison -ffixit foo.y
1204 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1207 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1208 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1209 %define parser_class_name "Parser"
1210 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1211 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1212 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1213 %define api.parser.class "Parser"
1214 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1215 foo.y:2.1-34: previous definition
1216 %define parser_class_name "Parser"
1217 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1218 fix-it:"foo.y":{3:1-3:34}:""
1219 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1221 This uses the same output format as GCC and Clang.
1223 *** Updating grammar files
1225 Fixes can be applied on the fly. The previous example ends with the
1226 suggestion to re-run bison with the option -u/--update, which results in a
1227 cleaner grammar file.
1229 $ bison --update foo.y
1231 bison: file 'foo.y' was updated (backup: 'foo.y~')
1234 %define parse.error verbose
1235 %define api.parser.class {Parser}
1239 *** Bison is now relocatable
1241 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1243 A relocatable program can be moved or copied to a different location on
1244 the file system. It can also be used through mount points for network
1245 sharing. It is possible to make symbolic links to the installed and moved
1246 programs, and invoke them through the symbolic link.
1248 *** %expect and %expect-rr modifiers on individual rules
1250 One can now document (and check) which rules participate in shift/reduce
1251 and reduce/reduce conflicts. This is particularly important GLR parsers,
1252 where conflicts are a normal occurrence. For example,
1268 | argument_list ',' expression
1273 Looking at the output from -v, one can see that the shift/reduce conflict
1274 here is due to the fact that the parser does not know whether to reduce
1275 arguments to argument_list until it sees the token _after_ the following
1276 ','. By marking the rule with %expect 1 (because there is a conflict in
1277 one state), we document the source of the 1 overall shift/reduce conflict.
1279 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1280 conflicts. In this case, we mark each of the conflicting rules. For
1289 target_list '=' expr ';'
1295 | target ',' target_list
1304 | expr ',' expr_list
1312 In a statement such as
1316 the parser must reduce x to a target or an expr, but does not know which
1317 until it sees the '='. So we notate the two possible reductions to
1318 indicate that each conflicts in one rule.
1320 This feature needs user feedback, and might evolve in the future.
1322 *** C++: Actual token constructors
1324 When variants and token constructors are enabled, in addition to the
1325 type-safe named token constructors (make_ID, make_INT, etc.), we now
1326 generate genuine constructors for symbol_type.
1328 For instance with these declarations
1334 you may use these constructors:
1336 symbol_type (int token, const std::string&);
1337 symbol_type (int token, const int&);
1338 symbol_type (int token);
1340 Correct matching between token types and value types is checked via
1341 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1342 constructors are preferable, as they offer better type safety (for
1343 instance 'make_ID (42)' would not even compile), but symbol_type
1344 constructors may help when token types are discovered at run-time, e.g.,
1347 if (auto i = lookup_keyword (yytext))
1348 return yy::parser::symbol_type (i);
1350 return yy::parser::make_ID (yytext);
1353 *** C++: Variadic emplace
1355 If your application requires C++11 and you don't use symbol constructors,
1356 you may now use a variadic emplace for semantic values:
1358 %define api.value.type variant
1359 %token <std::pair<int, int>> PAIR
1363 int yylex (parser::semantic_type *lvalp)
1365 lvalp->emplace <std::pair<int, int>> (1, 2);
1366 return parser::token::PAIR;
1369 *** C++: Syntax error exceptions in GLR
1371 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1372 actions, or from the scanner.
1374 *** More POSIX Yacc compatibility warnings
1376 More Bison specific directives are now reported with -y or -Wyacc. This
1377 change was ready since the release of Bison 3.0 in September 2015. It was
1378 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1379 in numerous warnings for Bison users that use the GNU Build System.
1381 If you still experience that problem, either redefine YACC as `bison -o
1382 y.tab.c`, or pass -Wno-yacc to Bison.
1384 *** The tables yyrhs and yyphrs are back
1386 Because no Bison skeleton uses them, these tables were removed (no longer
1387 passed to the skeletons, not even computed) in 2008. However, some users
1388 have expressed interest in being able to use them in their own skeletons.
1392 *** Incorrect number of reduce/reduce conflicts
1394 On a grammar such as
1396 exp: "num" | "num" | "num"
1398 bison used to report a single RR conflict, instead of two. This is now
1399 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1400 was entered in the RCS version control system, in December 1987.
1402 Some grammar files might have to adjust their %expect-rr.
1404 *** Parser directives that were not careful enough
1406 Passing invalid arguments to %nterm, for instance character literals, used
1407 to result in unclear error messages.
1411 The examples/ directory (installed in .../share/doc/bison/examples) has
1412 been restructured per language for clarity. The examples come with a
1413 README and a Makefile. Not only can they be used to toy with Bison, they
1414 can also be starting points for your own grammars.
1416 There is now a Java example, and a simple example in C based on Flex and
1417 Bison (examples/c/lexcalc/).
1423 They now use noexcept and constexpr. Please, report missing annotations.
1425 *** Symbol Declarations
1427 The syntax of the variation directives to declare symbols was overhauled
1428 for more consistency, and also better POSIX Yacc compliance (which, for
1429 instance, allows "%type" without actually providing a type). The %nterm
1430 directive, supported by Bison since its inception, is now documented and
1431 officially supported.
1433 The syntax is now as follows:
1435 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1436 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1437 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1438 %nterm TAG? ID+ ( TAG ID+ )*
1440 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1441 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1442 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1443 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1444 one), ‘*’ (zero or more) and ‘+’ (one or more).
1447 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1451 Fix the move constructor of symbol_type.
1453 Always provide a copy constructor for symbol_type, even in modern C++.
1456 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1460 Properly support token constructors in C++ with types that include commas
1461 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1464 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1468 C++ portability issues.
1471 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1475 Several portability issues have been fixed in the build system, in the
1476 test suite, and in the generated parsers in C++.
1479 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1481 ** Backward incompatible changes
1483 Support for DJGPP, which has been unmaintained and untested for years, is
1484 obsolete. Unless there is activity to revive it, it will be removed.
1488 %printers should use yyo rather than yyoutput to denote the output stream.
1490 Variant-based symbols in C++ should use emplace() rather than build().
1492 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1496 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1498 A comment in the generated code now emphasizes that users should not
1499 depend upon non-documented implementation details, such as macros starting
1504 *** C++: Support for move semantics (lalr1.cc)
1506 The lalr1.cc skeleton now fully supports C++ move semantics, while
1507 maintaining compatibility with C++98. You may now store move-only types
1508 when using Bison's variants. For instance:
1515 %skeleton "lalr1.cc"
1516 %define api.value.type variant
1520 %token <int> INT "int";
1521 %type <std::unique_ptr<int>> int;
1522 %type <std::vector<std::unique_ptr<int>>> list;
1526 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1528 int: "int" { $$ = std::make_unique<int>($1); }
1530 *** C++: Implicit move of right-hand side values (lalr1.cc)
1532 In modern C++ (C++11 and later), you should always use 'std::move' with
1533 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1534 popped from the stack anyway. Using 'std::move' is mandatory for
1535 move-only types such as unique_ptr, and it provides a significant speedup
1536 for large types such as std::string, or std::vector, etc.
1538 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1539 by 'std::move ($n)'. The second rule in the previous grammar can be
1542 list: list int { $$ = $1; $$.emplace_back($2); }
1544 With automove enabled, the semantic values are no longer lvalues, so do
1545 not use the swap idiom:
1547 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1549 This idiom is anyway obsolete: it is preferable to move than to swap.
1551 A warning is issued when automove is enabled, and a value is used several
1554 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1555 exp: "twice" exp { $$ = $2 + $2; }
1558 Enabling api.value.automove does not require support for modern C++. The
1559 generated code is valid C++98/03, but will use copies instead of moves.
1561 The new examples/c++/variant-11.yy shows these features in action.
1563 *** C++: The implicit default semantic action is always run
1565 When variants are enabled, the default action was not run, so
1573 It now behaves like in all the other cases, as
1575 exp: "number" { $$ = $1; }
1577 possibly using std::move if automove is enabled.
1579 We do not expect backward compatibility issues. However, beware of
1580 forward compatibility issues: if you rely on default actions with
1581 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1582 generate incorrect parsers.
1584 *** C++: Renaming location.hh
1586 When both %defines and %locations are enabled, Bison generates a
1587 location.hh file. If you don't use locations outside of the parser, you
1588 may avoid its creation with:
1590 %define api.location.file none
1592 However this file is useful if, for instance, your parser builds an AST
1593 decorated with locations: you may use Bison's location independently of
1594 Bison's parser. You can now give it another name, for instance:
1596 %define api.location.file "my-location.hh"
1598 This name can have directory components, and even be absolute. The name
1599 under which the location file is included is controlled by
1600 api.location.include.
1602 This way it is possible to have several parsers share the same location
1605 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1608 %define api.namespace {foo}
1609 %define api.location.file "include/ast/loc.hh"
1610 %define api.location.include {<ast/loc.hh>}
1612 and use it in src/bar/parser.hh:
1615 %define api.namespace {bar}
1616 %code requires {#include <ast/loc.hh>}
1617 %define api.location.type {bar::location}
1619 Absolute file names are supported, so in your Makefile, passing the flag
1620 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1623 *** C++: stack.hh and position.hh are deprecated
1625 When asked to generate a header file (%defines), the lalr1.cc skeleton
1626 generates a stack.hh file. This file had no interest for users; it is now
1627 made useless: its content is included in the parser definition. It is
1628 still generated for backward compatibility.
1630 When in addition to %defines, location support is requested (%locations),
1631 the file position.hh is also generated. It is now also useless: its
1632 content is now included in location.hh.
1634 These files are no longer generated when your grammar file requires at
1635 least Bison 3.2 (%require "3.2").
1639 Portability issues on MinGW and VS2015.
1641 Portability issues in the test suite.
1643 Portability/warning issues with Flex.
1646 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1648 ** Backward incompatible changes
1650 Compiling Bison now requires a C99 compiler---as announced during the
1651 release of Bison 3.0, five years ago. Generated parsers do not require a
1654 Support for DJGPP, which has been unmaintained and untested for years, is
1655 obsolete. Unless there is activity to revive it, the next release of Bison
1656 will have it removed.
1660 *** Typed midrule actions
1662 Because their type is unknown to Bison, the values of midrule actions are
1663 not treated like the others: they don't have %printer and %destructor
1664 support. It also prevents C++ (Bison) variants to handle them properly.
1666 Typed midrule actions address these issues. Instead of:
1668 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1672 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1674 *** Reports include the type of the symbols
1676 The sections about terminal and nonterminal symbols of the '*.output' file
1677 now specify their declared type. For instance, for:
1681 the report now shows '<ival>':
1683 Terminals, with rules where they appear
1687 *** Diagnostics about useless rules
1689 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1690 of course, its rules are useless too.
1694 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1696 Previously all the useless rules were reported, including those whose
1697 left-hand side is the 'exp' nonterminal:
1699 warning: 1 nonterminal useless in grammar [-Wother]
1700 warning: 4 rules useless in grammar [-Wother]
1701 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1704 2.14-16: warning: rule useless in grammar [-Wother]
1707 3.6-16: warning: rule useless in grammar [-Wother]
1708 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1710 3.20-30: warning: rule useless in grammar [-Wother]
1711 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1713 3.34-44: warning: rule useless in grammar [-Wother]
1714 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1717 Now, rules whose left-hand side symbol is useless are no longer reported
1718 as useless. The locations of the errors have also been adjusted to point
1719 to the first use of the nonterminal as a left-hand side of a rule:
1721 warning: 1 nonterminal useless in grammar [-Wother]
1722 warning: 4 rules useless in grammar [-Wother]
1723 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1724 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1726 2.14-16: warning: rule useless in grammar [-Wother]
1730 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1732 When compiled with exceptions disabled, the generated parsers no longer
1733 uses try/catch clauses.
1735 Currently only GCC and Clang are supported.
1739 *** A demonstration of variants
1741 A new example was added (installed in .../share/doc/bison/examples),
1742 'variant.yy', which shows how to use (Bison) variants in C++.
1744 The other examples were made nicer to read.
1746 *** Some features are no longer 'experimental'
1748 The following features, mature enough, are no longer flagged as
1749 experimental in the documentation: push parsers, default %printer and
1750 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1751 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1752 semantic predicates (%?).
1756 *** GLR: Predicates support broken by #line directives
1758 Predicates (%?) in GLR such as
1761 %? {new_syntax} 'w' id new_args
1762 | %?{!new_syntax} 'w' id old_args
1764 were issued with #lines in the middle of C code.
1766 *** Printer and destructor with broken #line directives
1768 The #line directives were not properly escaped when emitting the code for
1769 %printer/%destructor, which resulted in compiler errors if there are
1770 backslashes or double-quotes in the grammar file name.
1772 *** Portability on ICC
1774 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1775 Generated parsers now work around this.
1779 There were several small fixes in the test suite and in the build system,
1780 many warnings in bison and in the generated parsers were eliminated. The
1781 documentation also received its share of minor improvements.
1783 Useless code was removed from C++ parsers, and some of the generated
1784 constructors are more 'natural'.
1787 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1791 *** C++: Fix support of 'syntax_error'
1793 One incorrect 'inline' resulted in linking errors about the constructor of
1794 the syntax_error exception.
1796 *** C++: Fix warnings
1798 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1799 warnings about yyformat being possibly null. It also warned about the
1800 deprecated implicit definition of copy constructors when there's a
1801 user-defined (copy) assignment operator.
1803 *** Location of errors
1805 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1806 ride-hand side raises a syntax error. The behavior of the default parser
1807 (yacc.c) in such a condition was undefined.
1809 Now all the parsers match the behavior of glr.c: @$ is used as the
1810 location of the error. This handles gracefully rules with and without
1813 *** Portability fixes in the test suite
1815 On some platforms, some Java and/or C++ tests were failing.
1818 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1822 *** C++ with Variants (lalr1.cc)
1824 Fix a compiler warning when no %destructor use $$.
1828 Several portability issues in tests were fixed.
1831 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1835 *** C++ with Variants (lalr1.cc)
1837 Problems with %destructor and '%define parse.assert' have been fixed.
1839 *** Named %union support (yacc.c, glr.c)
1841 Bison 3.0 introduced a regression on named %union such as
1843 %union foo { int ival; };
1845 The possibility to use a name was introduced "for Yacc compatibility".
1846 It is however not required by POSIX Yacc, and its usefulness is not clear.
1848 *** %define api.value.type union with %defines (yacc.c, glr.c)
1850 The C parsers were broken when %defines was used together with "%define
1851 api.value.type union".
1853 *** Redeclarations are reported in proper order
1861 bison used to report:
1863 foo.yy:2.10-11: error: %printer redeclaration for FOO
1866 foo.yy:3.10-11: previous declaration
1870 Now, the "previous" declaration is always the first one.
1875 Bison now installs various files in its docdir (which defaults to
1876 '/usr/local/share/doc/bison'), including the three fully blown examples
1877 extracted from the documentation:
1880 Reverse Polish Calculator, a simple introductory example.
1882 Multi-function Calc, a calculator with memory and functions and located
1885 a calculator in C++ using variant support and token constructors.
1888 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1892 *** Generated source files when errors are reported
1894 When warnings are issued and -Werror is set, bison would still generate
1895 the source files (*.c, *.h...). As a consequence, some runs of "make"
1896 could fail the first time, but not the second (as the files were generated
1899 This is fixed: bison no longer generates this source files, but, of
1900 course, still produces the various reports (*.output, *.xml, etc.).
1902 *** %empty is used in reports
1904 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1905 dot, XML and formats derived from it).
1907 *** YYERROR and variants
1909 When C++ variant support is enabled, an error triggered via YYERROR, but
1910 not caught via error recovery, resulted in a double deletion.
1913 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1917 *** Errors in caret diagnostics
1919 On some platforms, some errors could result in endless diagnostics.
1921 *** Fixes of the -Werror option
1923 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1924 diagnostics into errors instead of warnings. This is fixed.
1926 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1927 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1928 -Wno-error", "foo" diagnostics are now errors.
1932 As demonstrated in the documentation, one can now leave spaces between
1937 The yacc.1 man page is no longer installed if --disable-yacc was
1940 *** Fixes in the test suite
1942 Bugs and portability issues.
1945 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1947 ** WARNING: Future backward-incompatibilities!
1949 Like other GNU packages, Bison will start using some of the C99 features
1950 for its own code, especially the definition of variables after statements.
1951 The generated C parsers still aim at C90.
1953 ** Backward incompatible changes
1955 *** Obsolete features
1957 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1959 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1960 use YYSTYPE and YYLTYPE.
1962 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1963 1.875): use %lex-param, %parse-param, or %param.
1965 Missing semicolons at the end of actions are no longer added (as announced
1966 in the release 2.5).
1968 *** Use of YACC='bison -y'
1970 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1973 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1974 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1975 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1977 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1978 implementation of Yacc, was using Bison as 'bison -y'. While it does
1979 ensure compatible output file names, it also enables warnings for
1980 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1981 warnings for Bison extensions.
1983 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1984 (which also generates 'y.tab.h' and 'y.output' when needed).
1985 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1986 flags (YFLAGS, or AM_YFLAGS with Automake).
1990 *** The epilogue is no longer affected by internal #defines (glr.c)
1992 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1993 generated code. These weren't properly undefined before the inclusion of
1994 the user epilogue, so functions such as the following were butchered by the
1995 preprocessor expansion:
1997 int yylex (YYSTYPE *yylval);
1999 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
2000 identifiers for user-provided variables.
2002 *** stdio.h is no longer needed when locations are enabled (yacc.c)
2004 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
2005 locations are enabled. This is fixed.
2007 *** Warnings about useless %pure-parser/%define api.pure are restored
2009 ** Diagnostics reported by Bison
2011 Most of these features were contributed by Théophile Ranquet and Victor
2016 Version 2.7 introduced caret errors, for a prettier output. These are now
2017 activated by default. The old format can still be used by invoking Bison
2018 with -fno-caret (or -fnone).
2020 Some error messages that reproduced excerpts of the grammar are now using
2021 the caret information only. For instance on:
2028 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2029 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2033 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2034 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2038 and "bison -fno-caret" reports:
2040 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2041 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2043 *** Enhancements of the -Werror option
2045 The -Werror=CATEGORY option is now recognized, and will treat specified
2046 warnings as errors. The warnings need not have been explicitly activated
2047 using the -W option, this is similar to what GCC 4.7 does.
2049 For example, given the following command line, Bison will treat both
2050 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2051 errors (and only those):
2053 $ bison -Werror=yacc,error=conflicts-sr input.y
2055 If no categories are specified, -Werror will make all active warnings into
2056 errors. For example, the following line does the same the previous example:
2058 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2060 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2062 Note that the categories in this -Werror option may not be prefixed with
2063 "no-". However, -Wno-error[=CATEGORY] is valid.
2065 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2066 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2067 incompatibilities as warnings: "-y -Wno-error=yacc".
2069 *** The display of warnings is now richer
2071 The option that controls a given warning is now displayed:
2073 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2075 In the case of warnings treated as errors, the prefix is changed from
2076 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2077 to GCC, as [-Werror=CATEGORY].
2079 For instance, where the previous version of Bison would report (and exit
2082 bison: warnings being treated as errors
2083 input.y:1.1: warning: stray ',' treated as white space
2087 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2089 *** Deprecated constructs
2091 The new 'deprecated' warning category flags obsolete constructs whose
2092 support will be discontinued. It is enabled by default. These warnings
2093 used to be reported as 'other' warnings.
2095 *** Useless semantic types
2097 Bison now warns about useless (uninhabited) semantic types. Since
2098 semantic types are not declared to Bison (they are defined in the opaque
2099 %union structure), it is %printer/%destructor directives about useless
2100 types that trigger the warning:
2104 %printer {} <type1> <type3>
2105 %destructor {} <type2> <type4>
2107 nterm: term { $$ = $1; };
2109 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2110 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2112 *** Undefined but unused symbols
2114 Bison used to raise an error for undefined symbols that are not used in
2115 the grammar. This is now only a warning.
2118 %destructor {} symbol2
2119 %type <type> symbol3
2123 *** Useless destructors or printers
2125 Bison now warns about useless destructors or printers. In the following
2126 example, the printer for <type1>, and the destructor for <type2> are
2127 useless: all symbols of <type1> (token1) already have a printer, and all
2128 symbols of type <type2> (token2) already have a destructor.
2130 %token <type1> token1
2134 %printer {} token1 <type1> <type3>
2135 %destructor {} token2 <type2> <type4>
2139 The warnings and error messages about shift/reduce and reduce/reduce
2140 conflicts have been normalized. For instance on the following foo.y file:
2144 exp: exp '+' exp | '0' | '0';
2146 compare the previous version of bison:
2149 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2150 $ bison -Werror foo.y
2151 bison: warnings being treated as errors
2152 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2154 with the new behavior:
2157 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2158 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2159 $ bison -Werror foo.y
2160 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2161 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2163 When %expect or %expect-rr is used, such as with bar.y:
2168 exp: exp '+' exp | '0' | '0';
2173 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2174 bar.y: expected 0 shift/reduce conflicts
2175 bar.y: expected 0 reduce/reduce conflicts
2180 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2181 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2183 ** Incompatibilities with POSIX Yacc
2185 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2188 ** Additional yylex/yyparse arguments
2190 The new directive %param declares additional arguments to both yylex and
2191 yyparse. The %lex-param, %parse-param, and %param directives support one
2192 or more arguments. Instead of
2194 %lex-param {arg1_type *arg1}
2195 %lex-param {arg2_type *arg2}
2196 %parse-param {arg1_type *arg1}
2197 %parse-param {arg2_type *arg2}
2201 %param {arg1_type *arg1} {arg2_type *arg2}
2203 ** Types of values for %define variables
2205 Bison used to make no difference between '%define foo bar' and '%define
2206 foo "bar"'. The former is now called a 'keyword value', and the latter a
2207 'string value'. A third kind was added: 'code values', such as '%define
2210 Keyword variables are used for fixed value sets, e.g.,
2212 %define lr.type lalr
2214 Code variables are used for value in the target language, e.g.,
2216 %define api.value.type {struct semantic_type}
2218 String variables are used remaining cases, e.g. file names.
2220 ** Variable api.token.prefix
2222 The variable api.token.prefix changes the way tokens are identified in
2223 the generated files. This is especially useful to avoid collisions
2224 with identifiers in the target language. For instance
2226 %token FILE for ERROR
2227 %define api.token.prefix {TOK_}
2229 start: FILE for ERROR;
2231 will generate the definition of the symbols TOK_FILE, TOK_for, and
2232 TOK_ERROR in the generated sources. In particular, the scanner must
2233 use these prefixed token names, although the grammar itself still
2234 uses the short names (as in the sample rule given above).
2236 ** Variable api.value.type
2238 This new %define variable supersedes the #define macro YYSTYPE. The use
2239 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2240 using %union or %defining api.value.type results in undefined behavior.
2242 Either define api.value.type, or use "%union":
2249 %token <ival> INT "integer"
2250 %token <sval> STRING "string"
2251 %printer { fprintf (yyo, "%d", $$); } <ival>
2252 %destructor { free ($$); } <sval>
2255 yylval.ival = 42; return INT;
2256 yylval.sval = "42"; return STRING;
2258 The %define variable api.value.type supports both keyword and code values.
2260 The keyword value 'union' means that the user provides genuine types, not
2261 union member names such as "ival" and "sval" above (WARNING: will fail if
2262 -y/--yacc/%yacc is enabled).
2264 %define api.value.type union
2265 %token <int> INT "integer"
2266 %token <char *> STRING "string"
2267 %printer { fprintf (yyo, "%d", $$); } <int>
2268 %destructor { free ($$); } <char *>
2271 yylval.INT = 42; return INT;
2272 yylval.STRING = "42"; return STRING;
2274 The keyword value variant is somewhat equivalent, but for C++ special
2275 provision is made to allow classes to be used (more about this below).
2277 %define api.value.type variant
2278 %token <int> INT "integer"
2279 %token <std::string> STRING "string"
2281 Code values (in braces) denote user defined types. This is where YYSTYPE
2299 %define api.value.type {struct my_value}
2300 %token <u.ival> INT "integer"
2301 %token <u.sval> STRING "string"
2302 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2303 %destructor { free ($$); } <u.sval>
2306 yylval.u.ival = 42; return INT;
2307 yylval.u.sval = "42"; return STRING;
2309 ** Variable parse.error
2311 This variable controls the verbosity of error messages. The use of the
2312 %error-verbose directive is deprecated in favor of "%define parse.error
2315 ** Deprecated %define variable names
2317 The following variables have been renamed for consistency. Backward
2318 compatibility is ensured, but upgrading is recommended.
2320 lr.default-reductions -> lr.default-reduction
2321 lr.keep-unreachable-states -> lr.keep-unreachable-state
2322 namespace -> api.namespace
2323 stype -> api.value.type
2325 ** Semantic predicates
2327 Contributed by Paul Hilfinger.
2329 The new, experimental, semantic-predicate feature allows actions of the
2330 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2331 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2332 in GLR parsers, rather than being deferred. The result is that they allow
2333 the programmer to prune possible parses based on the values of run-time
2336 ** The directive %expect-rr is now an error in non GLR mode
2338 It used to be an error only if used in non GLR mode, _and_ if there are
2339 reduce/reduce conflicts.
2341 ** Tokens are numbered in their order of appearance
2343 Contributed by Valentin Tolmer.
2345 With '%token A B', A had a number less than the one of B. However,
2346 precedence declarations used to generate a reversed order. This is now
2347 fixed, and introducing tokens with any of %token, %left, %right,
2348 %precedence, or %nonassoc yields the same result.
2350 When mixing declarations of tokens with a literal character (e.g., 'a') or
2351 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2352 the literal characters first. For example
2356 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2357 input order is now preserved.
2359 These changes were made so that one can remove useless precedence and
2360 associativity declarations (i.e., map %nonassoc, %left or %right to
2361 %precedence, or to %token) and get exactly the same output.
2363 ** Useless precedence and associativity
2365 Contributed by Valentin Tolmer.
2367 When developing and maintaining a grammar, useless associativity and
2368 precedence directives are common. They can be a nuisance: new ambiguities
2369 arising are sometimes masked because their conflicts are resolved due to
2370 the extra precedence or associativity information. Furthermore, it can
2371 hinder the comprehension of a new grammar: one will wonder about the role
2372 of a precedence, where in fact it is useless. The following changes aim
2373 at detecting and reporting these extra directives.
2375 *** Precedence warning category
2377 A new category of warning, -Wprecedence, was introduced. It flags the
2378 useless precedence and associativity directives.
2380 *** Useless associativity
2382 Bison now warns about symbols with a declared associativity that is never
2383 used to resolve conflicts. In that case, using %precedence is sufficient;
2384 the parsing tables will remain unchanged. Solving these warnings may raise
2385 useless precedence warnings, as the symbols no longer have associativity.
2399 warning: useless associativity for '+', use %precedence [-Wprecedence]
2403 *** Useless precedence
2405 Bison now warns about symbols with a declared precedence and no declared
2406 associativity (i.e., declared with %precedence), and whose precedence is
2407 never used. In that case, the symbol can be safely declared with %token
2408 instead, without modifying the parsing tables. For example:
2412 exp: "var" '=' "number";
2416 warning: useless precedence for '=' [-Wprecedence]
2420 *** Useless precedence and associativity
2422 In case of both useless precedence and associativity, the issue is flagged
2427 exp: "var" '=' "number";
2431 warning: useless precedence and associativity for '=' [-Wprecedence]
2437 With help from Joel E. Denny and Gabriel Rassoul.
2439 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2440 marked by the new %empty directive. Using %empty on a non-empty rule is
2441 an error. The new -Wempty-rule warning reports empty rules without
2442 %empty. On the following grammar:
2452 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2455 5.8-13: error: %empty on non-empty rule
2459 ** Java skeleton improvements
2461 The constants for token names were moved to the Lexer interface. Also, it
2462 is possible to add code to the parser's constructors using "%code init"
2463 and "%define init_throws".
2464 Contributed by Paolo Bonzini.
2466 The Java skeleton now supports push parsing.
2467 Contributed by Dennis Heimbigner.
2469 ** C++ skeletons improvements
2471 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2473 Using %defines is now optional. Without it, the needed support classes
2474 are defined in the generated parser, instead of additional files (such as
2475 location.hh, position.hh and stack.hh).
2477 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2479 Both lalr1.cc and glr.cc no longer require %location.
2481 *** syntax_error exception (lalr1.cc)
2483 The C++ parser features a syntax_error exception, which can be
2484 thrown from the scanner or from user rules to raise syntax errors.
2485 This facilitates reporting errors caught in sub-functions (e.g.,
2486 rejecting too large integral literals from a conversion function
2487 used by the scanner, or rejecting invalid combinations from a
2488 factory invoked by the user actions).
2490 *** %define api.value.type variant
2492 This is based on a submission from Michiel De Wilde. With help
2493 from Théophile Ranquet.
2495 In this mode, complex C++ objects can be used as semantic values. For
2498 %token <::std::string> TEXT;
2499 %token <int> NUMBER;
2500 %token SEMICOLON ";"
2501 %type <::std::string> item;
2502 %type <::std::list<std::string>> list;
2505 list { std::cout << $1 << std::endl; }
2509 %empty { /* Generates an empty string list. */ }
2510 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2514 TEXT { std::swap ($$, $1); }
2515 | NUMBER { $$ = string_cast ($1); }
2518 *** %define api.token.constructor
2520 When variants are enabled, Bison can generate functions to build the
2521 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2522 with the semantic value (e.g., int):
2524 parser::symbol_type yylex ()
2526 parser::location_type loc = ...;
2528 return parser::make_TEXT ("Hello, world!", loc);
2530 return parser::make_NUMBER (42, loc);
2532 return parser::make_SEMICOLON (loc);
2538 There are operator- and operator-= for 'location'. Negative line/column
2539 increments can no longer underflow the resulting value.
2542 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2546 *** Fix compiler attribute portability (yacc.c)
2548 With locations enabled, __attribute__ was used unprotected.
2550 *** Fix some compiler warnings (lalr1.cc)
2553 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2557 Warnings about uninitialized yylloc in yyparse have been fixed.
2559 Restored C90 compliance (yet no report was ever made).
2561 ** Diagnostics are improved
2563 Contributed by Théophile Ranquet.
2565 *** Changes in the format of error messages
2567 This used to be the format of many error reports:
2569 input.y:2.7-12: %type redeclaration for exp
2570 input.y:1.7-12: previous declaration
2574 input.y:2.7-12: error: %type redeclaration for exp
2575 input.y:1.7-12: previous declaration
2577 *** New format for error reports: carets
2579 Caret errors have been added to Bison:
2581 input.y:2.7-12: error: %type redeclaration for exp
2584 input.y:1.7-12: previous declaration
2590 input.y:3.20-23: error: ambiguous reference: '$exp'
2591 exp: exp '+' exp { $exp = $1 + $3; };
2593 input.y:3.1-3: refers to: $exp at $$
2594 exp: exp '+' exp { $exp = $1 + $3; };
2596 input.y:3.6-8: refers to: $exp at $1
2597 exp: exp '+' exp { $exp = $1 + $3; };
2599 input.y:3.14-16: refers to: $exp at $3
2600 exp: exp '+' exp { $exp = $1 + $3; };
2603 The default behavior for now is still not to display these unless
2604 explicitly asked with -fcaret (or -fall). However, in a later release, it
2605 will be made the default behavior (but may still be deactivated with
2608 ** New value for %define variable: api.pure full
2610 The %define variable api.pure requests a pure (reentrant) parser. However,
2611 for historical reasons, using it in a location-tracking Yacc parser
2612 resulted in a yyerror function that did not take a location as a
2613 parameter. With this new value, the user may request a better pure parser,
2614 where yyerror does take a location as a parameter (in location-tracking
2617 The use of "%define api.pure true" is deprecated in favor of this new
2618 "%define api.pure full".
2620 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2622 The %define variable api.location.type defines the name of the type to use
2623 for locations. When defined, Bison no longer generates the position.hh
2624 and location.hh files, nor does the parser will include them: the user is
2625 then responsible to define her type.
2627 This can be used in programs with several parsers to factor their location
2628 and position files: let one of them generate them, and the others just use
2631 This feature was actually introduced, but not documented, in Bison 2.5,
2632 under the name "location_type" (which is maintained for backward
2635 For consistency, lalr1.java's %define variables location_type and
2636 position_type are deprecated in favor of api.location.type and
2639 ** Exception safety (lalr1.cc)
2641 The parse function now catches exceptions, uses the %destructors to
2642 release memory (the lookahead symbol and the symbols pushed on the stack)
2643 before re-throwing the exception.
2645 This feature is somewhat experimental. User feedback would be
2648 ** Graph improvements in DOT and XSLT
2650 Contributed by Théophile Ranquet.
2652 The graphical presentation of the states is more readable: their shape is
2653 now rectangular, the state number is clearly displayed, and the items are
2654 numbered and left-justified.
2656 The reductions are now explicitly represented as transitions to other
2657 diamond shaped nodes.
2659 These changes are present in both --graph output and xml2dot.xsl XSLT
2660 processing, with minor (documented) differences.
2662 ** %language is no longer an experimental feature.
2664 The introduction of this feature, in 2.4, was four years ago. The
2665 --language option and the %language directive are no longer experimental.
2669 The sections about shift/reduce and reduce/reduce conflicts resolution
2670 have been fixed and extended.
2672 Although introduced more than four years ago, XML and Graphviz reports
2673 were not properly documented.
2675 The translation of midrule actions is now described.
2678 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2680 We consider compiler warnings about Bison generated parsers to be bugs.
2681 Rather than working around them in your own project, please consider
2682 reporting them to us.
2686 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2687 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2690 Other issues in the test suite have been addressed.
2692 Null characters are correctly displayed in error messages.
2694 When possible, yylloc is correctly initialized before calling yylex. It
2695 is no longer necessary to initialize it in the %initial-action.
2698 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2700 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2703 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2707 Bugs and portability issues in the test suite have been fixed.
2709 Some errors in translations have been addressed, and --help now directs
2710 users to the appropriate place to report them.
2712 Stray Info files shipped by accident are removed.
2714 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2715 generated, are removed.
2717 All the generated headers are self-contained.
2719 ** Header guards (yacc.c, glr.c, glr.cc)
2721 In order to avoid collisions, the header guards are now
2722 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2723 For instance the header generated from
2725 %define api.prefix "calc"
2726 %defines "lib/parse.h"
2728 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2730 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2732 The compilation of pure parsers (%define api.pure) can trigger GCC
2735 input.c: In function 'yyparse':
2736 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2737 function [-Wmaybe-uninitialized]
2741 This is now fixed; pragmas to avoid these warnings are no longer needed.
2743 Warnings from clang ("equality comparison with extraneous parentheses" and
2744 "function declared 'noreturn' should not return") have also been
2748 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2752 Buffer overruns, complaints from Flex, and portability issues in the test
2753 suite have been fixed.
2755 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2757 Trailing end-of-lines in %parse-param or %lex-param would result in
2758 invalid C++. This is fixed.
2760 ** Spurious spaces and end-of-lines
2762 The generated files no longer end (nor start) with empty lines.
2765 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2767 Bison no longer executes user-specified M4 code when processing a grammar.
2771 In addition to the removal of the features announced in Bison 2.6, the
2772 next major release will remove the "Temporary hack for adding a semicolon
2773 to the user action", as announced in the release 2.5. Instead of:
2775 exp: exp "+" exp { $$ = $1 + $3 };
2779 exp: exp "+" exp { $$ = $1 + $3; };
2783 *** Type names are now properly escaped.
2785 *** glr.cc: set_debug_level and debug_level work as expected.
2787 *** Stray @ or $ in actions
2789 While Bison used to warn about stray $ or @ in action rules, it did not
2790 for other actions such as printers, destructors, or initial actions. It
2793 ** Type names in actions
2795 For consistency with rule actions, it is now possible to qualify $$ by a
2796 type-name in destructors, printers, and initial actions. For instance:
2798 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2800 will display two values for each typed and untyped symbol (provided
2801 that YYSTYPE has both "ival" and "fval" fields).
2804 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2808 The next major release of Bison will drop support for the following
2809 deprecated features. Please report disagreements to bug-bison@gnu.org.
2813 Support for generating parsers in K&R C will be removed. Parsers
2814 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2817 *** Features deprecated since Bison 1.875
2819 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2822 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2823 %lex-param, will no longer be supported.
2825 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2828 *** The generated header will be included (yacc.c)
2830 Instead of duplicating the content of the generated header (definition of
2831 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2832 as is already the case for GLR or C++ parsers. This change is deferred
2833 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2836 ** Generated Parser Headers
2838 *** Guards (yacc.c, glr.c, glr.cc)
2840 The generated headers are now guarded, as is already the case for C++
2841 parsers (lalr1.cc). For instance, with --defines=foo.h:
2846 #endif /* !YY_FOO_H */
2848 *** New declarations (yacc.c, glr.c)
2850 The generated header now declares yydebug and yyparse. Both honor
2851 --name-prefix=bar_, and yield
2853 int bar_parse (void);
2857 #define yyparse bar_parse
2860 in order to facilitate the inclusion of several parser headers inside a
2861 single compilation unit.
2863 *** Exported symbols in C++
2865 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2866 header, are removed, as they prevent the possibility of including several
2867 generated headers from a single compilation unit.
2871 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2874 ** New %define variable: api.prefix
2876 Now that the generated headers are more complete and properly protected
2877 against multiple inclusions, constant names, such as YYSTYPE are a
2878 problem. While yyparse and others are properly renamed by %name-prefix,
2879 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2880 would introduce backward compatibility issues in projects not expecting
2881 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2882 it is deprecated in favor of a new %define variable: api.prefix.
2884 The following examples compares both:
2886 %name-prefix "bar_" | %define api.prefix "bar_"
2887 %token <ival> FOO %token <ival> FOO
2888 %union { int ival; } %union { int ival; }
2894 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2895 # define BAR_FOO_H # define BAR_FOO_H
2897 /* Enabling traces. */ /* Enabling traces. */
2898 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2899 > # if defined YYDEBUG
2901 > # define BAR_DEBUG 1
2903 > # define BAR_DEBUG 0
2906 # define YYDEBUG 0 | # define BAR_DEBUG 0
2910 # if YYDEBUG | # if BAR_DEBUG
2911 extern int bar_debug; extern int bar_debug;
2914 /* Tokens. */ /* Tokens. */
2915 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2916 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2917 enum yytokentype { | enum bar_tokentype {
2922 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2923 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2924 typedef union YYSTYPE | typedef union BAR_STYPE
2927 } YYSTYPE; | } BAR_STYPE;
2928 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2931 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2933 int bar_parse (void); int bar_parse (void);
2935 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2938 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2942 The next major release will drop support for generating parsers in K&R C.
2944 ** yacc.c: YYBACKUP works as expected.
2946 ** glr.c improvements:
2948 *** Location support is eliminated when not requested:
2950 GLR parsers used to include location-related code even when locations were
2951 not requested, and therefore not even usable.
2953 *** __attribute__ is preserved:
2955 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2956 when -std is passed to GCC).
2958 ** lalr1.java: several fixes:
2960 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2961 first token leads to a syntax error. Some minor clean ups.
2965 *** C++11 compatibility:
2967 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2972 The header files such as "parser.hh", "location.hh", etc. used a constant
2973 name for preprocessor guards, for instance:
2975 #ifndef BISON_LOCATION_HH
2976 # define BISON_LOCATION_HH
2978 #endif // !BISON_LOCATION_HH
2980 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2981 case characters are converted to upper case, and series of
2982 non-alphanumerical characters are converted to an underscore.
2984 With "bison -o lang++/parser.cc", "location.hh" would now include:
2986 #ifndef YY_LANG_LOCATION_HH
2987 # define YY_LANG_LOCATION_HH
2989 #endif // !YY_LANG_LOCATION_HH
2993 The position and location constructors (and their initialize methods)
2994 accept new arguments for line and column. Several issues in the
2995 documentation were fixed.
2997 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2999 ** Changes in the manual:
3001 *** %printer is documented
3003 The "%printer" directive, supported since at least Bison 1.50, is finally
3004 documented. The "mfcalc" example is extended to demonstrate it.
3006 For consistency with the C skeletons, the C++ parsers now also support
3007 "yyoutput" (as an alias to "debug_stream ()").
3009 *** Several improvements have been made:
3011 The layout for grammar excerpts was changed to a more compact scheme.
3012 Named references are motivated. The description of the automaton
3013 description file (*.output) is updated to the current format. Incorrect
3014 index entries were fixed. Some other errors were fixed.
3018 *** Conflicting prototypes with recent/modified Flex.
3020 Fixed build problems with the current, unreleased, version of Flex, and
3021 some modified versions of 2.5.35, which have modified function prototypes.
3023 *** Warnings during the build procedure have been eliminated.
3025 *** Several portability problems in the test suite have been fixed:
3027 This includes warnings with some compilers, unexpected behavior of tools
3028 such as diff, warning messages from the test suite itself, etc.
3030 *** The install-pdf target works properly:
3032 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3033 halts in the middle of its course.
3036 * Noteworthy changes in release 2.5 (2011-05-14)
3038 ** Grammar symbol names can now contain non-initial dashes:
3040 Consistently with directives (such as %error-verbose) and with
3041 %define variables (e.g. push-pull), grammar symbol names may contain
3042 dashes in any position except the beginning. This is a GNU
3043 extension over POSIX Yacc. Thus, use of this extension is reported
3044 by -Wyacc and rejected in Yacc mode (--yacc).
3046 ** Named references:
3048 Historically, Yacc and Bison have supported positional references
3049 ($n, $$) to allow access to symbol values from inside of semantic
3052 Starting from this version, Bison can also accept named references.
3053 When no ambiguity is possible, original symbol names may be used
3054 as named references:
3056 if_stmt : "if" cond_expr "then" then_stmt ';'
3057 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3059 In the more common case, explicit names may be declared:
3061 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3062 { $res = mk_if_stmt($cond, $then, $else); }
3064 Location information is also accessible using @name syntax. When
3065 accessing symbol names containing dots or dashes, explicit bracketing
3066 ($[sym.1]) must be used.
3068 These features are experimental in this version. More user feedback
3069 will help to stabilize them.
3070 Contributed by Alex Rozenman.
3072 ** IELR(1) and canonical LR(1):
3074 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3075 is, given any context-free grammar, IELR(1) generates parser tables
3076 with the full language-recognition power of canonical LR(1) but with
3077 nearly the same number of parser states as LALR(1). This reduction
3078 in parser states is often an order of magnitude. More importantly,
3079 because canonical LR(1)'s extra parser states may contain duplicate
3080 conflicts in the case of non-LR(1) grammars, the number of conflicts
3081 for IELR(1) is often an order of magnitude less as well. This can
3082 significantly reduce the complexity of developing of a grammar.
3084 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3085 place of its traditional LALR(1) parser tables, which remain the
3086 default. You can specify the type of parser tables in the grammar
3087 file with these directives:
3089 %define lr.type lalr
3090 %define lr.type ielr
3091 %define lr.type canonical-lr
3093 The default-reduction optimization in the parser tables can also be
3094 adjusted using "%define lr.default-reductions". For details on both
3095 of these features, see the new section "Tuning LR" in the Bison
3098 These features are experimental. More user feedback will help to
3101 ** LAC (Lookahead Correction) for syntax error handling
3103 Contributed by Joel E. Denny.
3105 Canonical LR, IELR, and LALR can suffer from a couple of problems
3106 upon encountering a syntax error. First, the parser might perform
3107 additional parser stack reductions before discovering the syntax
3108 error. Such reductions can perform user semantic actions that are
3109 unexpected because they are based on an invalid token, and they
3110 cause error recovery to begin in a different syntactic context than
3111 the one in which the invalid token was encountered. Second, when
3112 verbose error messages are enabled (with %error-verbose or the
3113 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3114 syntax error message can both contain invalid tokens and omit valid
3117 The culprits for the above problems are %nonassoc, default
3118 reductions in inconsistent states, and parser state merging. Thus,
3119 IELR and LALR suffer the most. Canonical LR can suffer only if
3120 %nonassoc is used or if default reductions are enabled for
3121 inconsistent states.
3123 LAC is a new mechanism within the parsing algorithm that solves
3124 these problems for canonical LR, IELR, and LALR without sacrificing
3125 %nonassoc, default reductions, or state merging. When LAC is in
3126 use, canonical LR and IELR behave almost exactly the same for both
3127 syntactically acceptable and syntactically unacceptable input.
3128 While LALR still does not support the full language-recognition
3129 power of canonical LR and IELR, LAC at least enables LALR's syntax
3130 error handling to correctly reflect LALR's language-recognition
3133 Currently, LAC is only supported for deterministic parsers in C.
3134 You can enable LAC with the following directive:
3136 %define parse.lac full
3138 See the new section "LAC" in the Bison manual for additional
3139 details including a few caveats.
3141 LAC is an experimental feature. More user feedback will help to
3144 ** %define improvements:
3146 *** Can now be invoked via the command line:
3148 Each of these command-line options
3151 --define=NAME[=VALUE]
3154 --force-define=NAME[=VALUE]
3156 is equivalent to this grammar file declaration
3158 %define NAME ["VALUE"]
3160 except that the manner in which Bison processes multiple definitions
3161 for the same NAME differs. Most importantly, -F and --force-define
3162 quietly override %define, but -D and --define do not. For further
3163 details, see the section "Bison Options" in the Bison manual.
3165 *** Variables renamed:
3167 The following %define variables
3170 lr.keep_unreachable_states
3172 have been renamed to
3175 lr.keep-unreachable-states
3177 The old names are now deprecated but will be maintained indefinitely
3178 for backward compatibility.
3180 *** Values no longer need to be quoted in the grammar file:
3182 If a %define value is an identifier, it no longer needs to be placed
3183 within quotations marks. For example,
3185 %define api.push-pull "push"
3189 %define api.push-pull push
3191 *** Unrecognized variables are now errors not warnings.
3193 *** Multiple invocations for any variable is now an error not a warning.
3195 ** Unrecognized %code qualifiers are now errors not warnings.
3197 ** Character literals not of length one:
3199 Previously, Bison quietly converted all character literals to length
3200 one. For example, without warning, Bison interpreted the operators in
3201 the following grammar to be the same token:
3207 Bison now warns when a character literal is not of length one. In
3208 some future release, Bison will start reporting an error instead.
3210 ** Destructor calls fixed for lookaheads altered in semantic actions:
3212 Previously for deterministic parsers in C, if a user semantic action
3213 altered yychar, the parser in some cases used the old yychar value to
3214 determine which destructor to call for the lookahead upon a syntax
3215 error or upon parser return. This bug has been fixed.
3217 ** C++ parsers use YYRHSLOC:
3219 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3220 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3221 to use it. If, for instance, your location structure has "first"
3222 and "last" members, instead of
3224 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3228 (Current).first = (Rhs)[1].location.first; \
3229 (Current).last = (Rhs)[N].location.last; \
3233 (Current).first = (Current).last = (Rhs)[0].location.last; \
3239 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3243 (Current).first = YYRHSLOC (Rhs, 1).first; \
3244 (Current).last = YYRHSLOC (Rhs, N).last; \
3248 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3252 ** YYLLOC_DEFAULT in C++:
3254 The default implementation of YYLLOC_DEFAULT used to be issued in
3255 the header file. It is now output in the implementation file, after
3256 the user %code sections so that its #ifndef guard does not try to
3257 override the user's YYLLOC_DEFAULT if provided.
3259 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3261 YYFAIL has existed for many years as an undocumented feature of
3262 deterministic parsers in C generated by Bison. More recently, it was
3263 a documented feature of Bison's experimental Java parsers. As
3264 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3265 semantic action now produces a deprecation warning, and Java parsers
3266 no longer implement YYFAIL at all. For further details, including a
3267 discussion of how to suppress C preprocessor warnings about YYFAIL
3268 being unused, see the Bison 2.4.2 NEWS entry.
3270 ** Temporary hack for adding a semicolon to the user action:
3272 Previously, Bison appended a semicolon to every user action for
3273 reductions when the output language defaulted to C (specifically, when
3274 neither %yacc, %language, %skeleton, or equivalent command-line
3275 options were specified). This allowed actions such as
3277 exp: exp "+" exp { $$ = $1 + $3 };
3281 exp: exp "+" exp { $$ = $1 + $3; };
3283 As a first step in removing this misfeature, Bison now issues a
3284 warning when it appends a semicolon. Moreover, in cases where Bison
3285 cannot easily determine whether a semicolon is needed (for example, an
3286 action ending with a cpp directive or a braced compound initializer),
3287 it no longer appends one. Thus, the C compiler might now complain
3288 about a missing semicolon where it did not before. Future releases of
3289 Bison will cease to append semicolons entirely.
3291 ** Verbose syntax error message fixes:
3293 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3294 specified, syntax error messages produced by the generated parser
3295 include the unexpected token as well as a list of expected tokens.
3296 The effect of %nonassoc on these verbose messages has been corrected
3297 in two ways, but a more complete fix requires LAC, described above:
3299 *** When %nonassoc is used, there can exist parser states that accept no
3300 tokens, and so the parser does not always require a lookahead token
3301 in order to detect a syntax error. Because no unexpected token or
3302 expected tokens can then be reported, the verbose syntax error
3303 message described above is suppressed, and the parser instead
3304 reports the simpler message, "syntax error". Previously, this
3305 suppression was sometimes erroneously triggered by %nonassoc when a
3306 lookahead was actually required. Now verbose messages are
3307 suppressed only when all previous lookaheads have already been
3308 shifted or discarded.
3310 *** Previously, the list of expected tokens erroneously included tokens
3311 that would actually induce a syntax error because conflicts for them
3312 were resolved with %nonassoc in the current parser state. Such
3313 tokens are now properly omitted from the list.
3315 *** Expected token lists are still often wrong due to state merging
3316 (from LALR or IELR) and default reductions, which can both add
3317 invalid tokens and subtract valid tokens. Canonical LR almost
3318 completely fixes this problem by eliminating state merging and
3319 default reductions. However, there is one minor problem left even
3320 when using canonical LR and even after the fixes above. That is,
3321 if the resolution of a conflict with %nonassoc appears in a later
3322 parser state than the one at which some syntax error is
3323 discovered, the conflicted token is still erroneously included in
3324 the expected token list. Bison's new LAC implementation,
3325 described above, eliminates this problem and the need for
3326 canonical LR. However, LAC is still experimental and is disabled
3329 ** Java skeleton fixes:
3331 *** A location handling bug has been fixed.
3333 *** The top element of each of the value stack and location stack is now
3334 cleared when popped so that it can be garbage collected.
3336 *** Parser traces now print the top element of the stack.
3338 ** -W/--warnings fixes:
3340 *** Bison now properly recognizes the "no-" versions of categories:
3342 For example, given the following command line, Bison now enables all
3343 warnings except warnings for incompatibilities with POSIX Yacc:
3345 bison -Wall,no-yacc gram.y
3347 *** Bison now treats S/R and R/R conflicts like other warnings:
3349 Previously, conflict reports were independent of Bison's normal
3350 warning system. Now, Bison recognizes the warning categories
3351 "conflicts-sr" and "conflicts-rr". This change has important
3352 consequences for the -W and --warnings command-line options. For
3355 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3356 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3357 bison -Wnone gram.y # no conflicts are reported
3358 bison -Werror gram.y # any conflict is an error
3360 However, as before, if the %expect or %expect-rr directive is
3361 specified, an unexpected number of conflicts is an error, and an
3362 expected number of conflicts is not reported, so -W and --warning
3363 then have no effect on the conflict report.
3365 *** The "none" category no longer disables a preceding "error":
3367 For example, for the following command line, Bison now reports
3368 errors instead of warnings for incompatibilities with POSIX Yacc:
3370 bison -Werror,none,yacc gram.y
3372 *** The "none" category now disables all Bison warnings:
3374 Previously, the "none" category disabled only Bison warnings for
3375 which there existed a specific -W/--warning category. However,
3376 given the following command line, Bison is now guaranteed to
3377 suppress all warnings:
3381 ** Precedence directives can now assign token number 0:
3383 Since Bison 2.3b, which restored the ability of precedence
3384 directives to assign token numbers, doing so for token number 0 has
3385 produced an assertion failure. For example:
3389 This bug has been fixed.
3392 * Noteworthy changes in release 2.4.3 (2010-08-05)
3394 ** Bison now obeys -Werror and --warnings=error for warnings about
3395 grammar rules that are useless in the parser due to conflicts.
3397 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3400 ** Failures in the test suite for GCC 4.5 have been fixed.
3402 ** Failures in the test suite for some versions of Sun Studio C++ have
3405 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3406 warnings about undefined %prec identifiers will not be converted to
3407 errors in Bison 2.5. They will remain warnings, which should be
3408 sufficient for POSIX while avoiding backward compatibility issues.
3410 ** Minor documentation fixes.
3413 * Noteworthy changes in release 2.4.2 (2010-03-20)
3415 ** Some portability problems that resulted in failures and livelocks
3416 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3417 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3418 errors should no longer cause M4 to report a broken pipe on the
3421 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3423 POSIX specifies that an error be reported for any identifier that does
3424 not appear on the LHS of a grammar rule and that is not defined by
3425 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3426 error report for the case when an identifier appears only after a
3427 %prec directive. It is now restored. However, for backward
3428 compatibility with recent Bison releases, it is only a warning for
3429 now. In Bison 2.5 and later, it will return to being an error.
3430 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3431 warning will not be converted to an error in Bison 2.5.]
3433 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3435 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3436 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3439 ** %code is now a permanent feature.
3441 A traditional Yacc prologue directive is written in the form:
3445 To provide a more flexible alternative, Bison 2.3b introduced the
3446 %code directive with the following forms for C/C++:
3449 %code requires {CODE}
3450 %code provides {CODE}
3453 These forms are now considered permanent features of Bison. See the
3454 %code entries in the section "Bison Declaration Summary" in the Bison
3455 manual for a summary of their functionality. See the section
3456 "Prologue Alternatives" for a detailed discussion including the
3457 advantages of %code over the traditional Yacc prologue directive.
3459 Bison's Java feature as a whole including its current usage of %code
3460 is still considered experimental.
3462 ** YYFAIL is deprecated and will eventually be removed.
3464 YYFAIL has existed for many years as an undocumented feature of
3465 deterministic parsers in C generated by Bison. Previously, it was
3466 documented for Bison's experimental Java parsers. YYFAIL is no longer
3467 documented for Java parsers and is formally deprecated in both cases.
3468 Users are strongly encouraged to migrate to YYERROR, which is
3471 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3472 induce a syntax error. The most obvious difference from YYERROR is
3473 that YYFAIL will automatically invoke yyerror to report the syntax
3474 error so that you don't have to. However, there are several other
3475 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3476 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3477 used. For a more detailed discussion, see:
3479 https://lists.gnu.org/r/bison-patches/2009-12/msg00024.html
3481 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3482 deterministic parsers in C will continue to implement it. However,
3483 because YYFAIL is already flawed, it seems futile to try to make new
3484 Bison features compatible with it. Thus, during parser generation,
3485 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3486 rule action. In a later release, YYFAIL will be disabled for
3487 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3488 be removed altogether.
3490 There exists at least one case where Bison 2.5's YYFAIL warning will
3491 be a false positive. Some projects add phony uses of YYFAIL and other
3492 Bison-defined macros for the sole purpose of suppressing C
3493 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3494 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3495 epilogue (that is, after the second "%%") in the Bison input file. In
3496 this release (2.4.2), Bison already generates its own code to suppress
3497 C preprocessor warnings for YYFAIL, so projects can remove their own
3498 phony uses of YYFAIL if compatibility with Bison releases prior to
3499 2.4.2 is not necessary.
3501 ** Internationalization.
3503 Fix a regression introduced in Bison 2.4: Under some circumstances,
3504 message translations were not installed although supported by the
3508 * Noteworthy changes in release 2.4.1 (2008-12-11)
3510 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3511 declarations have been fixed.
3513 ** Temporary hack for adding a semicolon to the user action.
3515 Bison used to prepend a trailing semicolon at the end of the user
3516 action for reductions. This allowed actions such as
3518 exp: exp "+" exp { $$ = $1 + $3 };
3522 exp: exp "+" exp { $$ = $1 + $3; };
3524 Some grammars still depend on this "feature". Bison 2.4.1 restores
3525 the previous behavior in the case of C output (specifically, when
3526 neither %language or %skeleton or equivalent command-line options
3527 are used) to leave more time for grammars depending on the old
3528 behavior to be adjusted. Future releases of Bison will disable this
3531 ** A few minor improvements to the Bison manual.
3534 * Noteworthy changes in release 2.4 (2008-11-02)
3536 ** %language is an experimental feature.
3538 We first introduced this feature in test release 2.3b as a cleaner
3539 alternative to %skeleton. Since then, we have discussed the possibility of
3540 modifying its effect on Bison's output file names. Thus, in this release,
3541 we consider %language to be an experimental feature that will likely evolve
3544 ** Forward compatibility with GNU M4 has been improved.
3546 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3550 * Noteworthy changes in release 2.3b (2008-05-27)
3552 ** The quotes around NAME that used to be required in the following directive
3555 %define NAME "VALUE"
3557 ** The directive "%pure-parser" is now deprecated in favor of:
3561 which has the same effect except that Bison is more careful to warn about
3562 unreasonable usage in the latter case.
3566 Bison can now generate an LALR(1) parser in C with a push interface. That
3567 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3568 push one token at a time to the parser using "yypush_parse", which will
3569 return to the caller after processing each token. By default, the push
3570 interface is disabled. Either of the following directives will enable it:
3572 %define api.push_pull "push" // Just push; does not require yylex.
3573 %define api.push_pull "both" // Push and pull; requires yylex.
3575 See the new section "A Push Parser" in the Bison manual for details.
3577 The current push parsing interface is experimental and may evolve. More user
3578 feedback will help to stabilize it.
3580 ** The -g and --graph options now output graphs in Graphviz DOT format,
3581 not VCG format. Like --graph, -g now also takes an optional FILE argument
3582 and thus cannot be bundled with other short options.
3586 Bison can now generate an LALR(1) parser in Java. The skeleton is
3587 "data/lalr1.java". Consider using the new %language directive instead of
3588 %skeleton to select it.
3590 See the new section "Java Parsers" in the Bison manual for details.
3592 The current Java interface is experimental and may evolve. More user
3593 feedback will help to stabilize it.
3594 Contributed by Paolo Bonzini.
3598 This new directive specifies the programming language of the generated
3599 parser, which can be C (the default), C++, or Java. Besides the skeleton
3600 that Bison uses, the directive affects the names of the generated files if
3601 the grammar file's name ends in ".y".
3603 ** XML Automaton Report
3605 Bison can now generate an XML report of the LALR(1) automaton using the new
3606 "--xml" option. The current XML schema is experimental and may evolve. More
3607 user feedback will help to stabilize it.
3608 Contributed by Wojciech Polak.
3610 ** The grammar file may now specify the name of the parser header file using
3611 %defines. For example:
3615 ** When reporting useless rules, useless nonterminals, and unused terminals,
3616 Bison now employs the terms "useless in grammar" instead of "useless",
3617 "useless in parser" instead of "never reduced", and "unused in grammar"
3618 instead of "unused".
3620 ** Unreachable State Removal
3622 Previously, Bison sometimes generated parser tables containing unreachable
3623 states. A state can become unreachable during conflict resolution if Bison
3624 disables a shift action leading to it from a predecessor state. Bison now:
3626 1. Removes unreachable states.
3628 2. Does not report any conflicts that appeared in unreachable states.
3629 WARNING: As a result, you may need to update %expect and %expect-rr
3630 directives in existing grammar files.
3632 3. For any rule used only in such states, Bison now reports the rule as
3633 "useless in parser due to conflicts".
3635 This feature can be disabled with the following directive:
3637 %define lr.keep_unreachable_states
3639 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3640 for further discussion.
3642 ** Lookahead Set Correction in the ".output" Report
3644 When instructed to generate a ".output" file including lookahead sets
3645 (using "--report=lookahead", for example), Bison now prints each reduction's
3646 lookahead set only next to the associated state's one item that (1) is
3647 associated with the same rule as the reduction and (2) has its dot at the end
3648 of its RHS. Previously, Bison also erroneously printed the lookahead set
3649 next to all of the state's other items associated with the same rule. This
3650 bug affected only the ".output" file and not the generated parser source
3653 ** --report-file=FILE is a new option to override the default ".output" file
3656 ** The "=" that used to be required in the following directives is now
3659 %file-prefix "parser"
3663 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3665 Bison 2.3a provided a new set of directives as a more flexible alternative to
3666 the traditional Yacc prologue blocks. Those have now been consolidated into
3667 a single %code directive with an optional qualifier field, which identifies
3668 the purpose of the code and thus the location(s) where Bison should generate
3671 1. "%code {CODE}" replaces "%after-header {CODE}"
3672 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3673 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3674 4. "%code top {CODE}" replaces "%before-header {CODE}"
3676 See the %code entries in section "Bison Declaration Summary" in the Bison
3677 manual for a summary of the new functionality. See the new section "Prologue
3678 Alternatives" for a detailed discussion including the advantages of %code
3679 over the traditional Yacc prologues.
3681 The prologue alternatives are experimental. More user feedback will help to
3682 determine whether they should become permanent features.
3684 ** Revised warning: unset or unused midrule values
3686 Since Bison 2.2, Bison has warned about midrule values that are set but not
3687 used within any of the actions of the parent rule. For example, Bison warns
3690 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3692 Now, Bison also warns about midrule values that are used but not set. For
3693 example, Bison warns about unset $$ in the midrule action in:
3695 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3697 However, Bison now disables both of these warnings by default since they
3698 sometimes prove to be false alarms in existing grammars employing the Yacc
3699 constructs $0 or $-N (where N is some positive integer).
3701 To enable these warnings, specify the option "--warnings=midrule-values" or
3702 "-W", which is a synonym for "--warnings=all".
3704 ** Default %destructor or %printer with "<*>" or "<>"
3706 Bison now recognizes two separate kinds of default %destructor's and
3709 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3710 %destructor/%printer for all grammar symbols for which you have formally
3711 declared semantic type tags.
3713 2. Place "<>" in a %destructor/%printer symbol list to define a default
3714 %destructor/%printer for all grammar symbols without declared semantic
3717 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3718 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3719 longer applies any %destructor to a midrule value if that midrule value is
3720 not actually ever referenced using either $$ or $n in a semantic action.
3722 The default %destructor's and %printer's are experimental. More user
3723 feedback will help to determine whether they should become permanent
3726 See the section "Freeing Discarded Symbols" in the Bison manual for further
3729 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3730 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3731 manual for a caveat concerning the treatment of literal strings.
3733 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3734 completely removed from Bison.
3737 * Noteworthy changes in release 2.3a (2006-09-13)
3739 ** Instead of %union, you can define and use your own union type
3740 YYSTYPE if your grammar contains at least one <type> tag.
3741 Your YYSTYPE need not be a macro; it can be a typedef.
3742 This change is for compatibility with other Yacc implementations,
3743 and is required by POSIX.
3745 ** Locations columns and lines start at 1.
3746 In accordance with the GNU Coding Standards and Emacs.
3748 ** You may now declare per-type and default %destructor's and %printer's:
3752 %union { char *string; }
3753 %token <string> STRING1
3754 %token <string> STRING2
3755 %type <string> string1
3756 %type <string> string2
3757 %union { char character; }
3758 %token <character> CHR
3759 %type <character> chr
3760 %destructor { free ($$); } %symbol-default
3761 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3762 %destructor { } <character>
3764 guarantees that, when the parser discards any user-defined symbol that has a
3765 semantic type tag other than "<character>", it passes its semantic value to
3766 "free". However, when the parser discards a "STRING1" or a "string1", it
3767 also prints its line number to "stdout". It performs only the second
3768 "%destructor" in this case, so it invokes "free" only once.
3770 [Although we failed to mention this here in the 2.3a release, the default
3771 %destructor's and %printer's were experimental, and they were rewritten in
3774 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3775 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3776 associating token numbers with token names. Removing the #define statements
3777 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3778 requires them. Bison still generates an enum for token names in all cases.
3780 ** Handling of traditional Yacc prologue blocks is now more consistent but
3781 potentially incompatible with previous releases of Bison.
3783 As before, you declare prologue blocks in your grammar file with the
3784 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3785 prologue blocks that you've declared before the first %union. To generate
3786 the post-prologue, Bison concatenates all prologue blocks that you've
3787 declared after the first %union.
3789 Previous releases of Bison inserted the pre-prologue into both the header
3790 file and the code file in all cases except for LALR(1) parsers in C. In the
3791 latter case, Bison inserted it only into the code file. For parsers in C++,
3792 the point of insertion was before any token definitions (which associate
3793 token numbers with names). For parsers in C, the point of insertion was
3794 after the token definitions.
3796 Now, Bison never inserts the pre-prologue into the header file. In the code
3797 file, it always inserts it before the token definitions.
3799 ** Bison now provides a more flexible alternative to the traditional Yacc
3800 prologue blocks: %before-header, %start-header, %end-header, and
3803 For example, the following declaration order in the grammar file reflects the
3804 order in which Bison will output these code blocks. However, you are free to
3805 declare these code blocks in your grammar file in whatever order is most
3809 /* Bison treats this block like a pre-prologue block: it inserts it into
3810 * the code file before the contents of the header file. It does *not*
3811 * insert it into the header file. This is a good place to put
3812 * #include's that you want at the top of your code file. A common
3813 * example is '#include "system.h"'. */
3816 /* Bison inserts this block into both the header file and the code file.
3817 * In both files, the point of insertion is before any Bison-generated
3818 * token, semantic type, location type, and class definitions. This is a
3819 * good place to define %union dependencies, for example. */
3822 /* Unlike the traditional Yacc prologue blocks, the output order for the
3823 * new %*-header blocks is not affected by their declaration position
3824 * relative to any %union in the grammar file. */
3827 /* Bison inserts this block into both the header file and the code file.
3828 * In both files, the point of insertion is after the Bison-generated
3829 * definitions. This is a good place to declare or define public
3830 * functions or data structures that depend on the Bison-generated
3834 /* Bison treats this block like a post-prologue block: it inserts it into
3835 * the code file after the contents of the header file. It does *not*
3836 * insert it into the header file. This is a good place to declare or
3837 * define internal functions or data structures that depend on the
3838 * Bison-generated definitions. */
3841 If you have multiple occurrences of any one of the above declarations, Bison
3842 will concatenate the contents in declaration order.
3844 [Although we failed to mention this here in the 2.3a release, the prologue
3845 alternatives were experimental, and they were rewritten in future versions.]
3847 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3848 The old spelling still works, but is not documented and may be removed
3849 in a future release.
3852 * Noteworthy changes in release 2.3 (2006-06-05)
3854 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3855 for compatibility with LALR(1) grammars.
3857 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3858 be to a type name that does not contain parentheses or brackets.
3861 * Noteworthy changes in release 2.2 (2006-05-19)
3863 ** The distribution terms for all Bison-generated parsers now permit
3864 using the parsers in nonfree programs. Previously, this permission
3865 was granted only for Bison-generated LALR(1) parsers in C.
3867 ** %name-prefix changes the namespace name in C++ outputs.
3869 ** The C++ parsers export their token_type.
3871 ** Bison now allows multiple %union declarations, and concatenates
3872 their contents together.
3874 ** New warning: unused values
3875 Right-hand side symbols whose values are not used are reported,
3876 if the symbols have destructors. For instance:
3878 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3882 will trigger a warning about $$ and $5 in the first rule, and $3 in
3883 the second ($1 is copied to $$ by the default rule). This example
3884 most likely contains three errors, and could be rewritten as:
3886 exp: exp "?" exp ":" exp
3887 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3889 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3892 However, if the original actions were really intended, memory leaks
3893 and all, the warnings can be suppressed by letting Bison believe the
3894 values are used, e.g.:
3896 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3897 | exp "+" exp { $$ = $1; (void) $3; }
3900 If there are midrule actions, the warning is issued if no action
3901 uses it. The following triggers no warning: $1 and $3 are used.
3903 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3905 The warning is intended to help catching lost values and memory leaks.
3906 If a value is ignored, its associated memory typically is not reclaimed.
3908 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3909 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3910 and YYERROR, for all objects on the stack, other than objects
3911 corresponding to the right-hand side of the current rule.
3913 ** %expect, %expect-rr
3914 Incorrect numbers of expected conflicts are now actual errors,
3915 instead of warnings.
3917 ** GLR, YACC parsers.
3918 The %parse-params are available in the destructors (and the
3919 experimental printers) as per the documentation.
3921 ** Bison now warns if it finds a stray "$" or "@" in an action.
3923 ** %require "VERSION"
3924 This specifies that the grammar file depends on features implemented
3925 in Bison version VERSION or higher.
3927 ** lalr1.cc: The token and value types are now class members.
3928 The tokens were defined as free form enums and cpp macros. YYSTYPE
3929 was defined as a free form union. They are now class members:
3930 tokens are enumerations of the "yy::parser::token" struct, and the
3931 semantic values have the "yy::parser::semantic_type" type.
3933 If you do not want or can update to this scheme, the directive
3934 '%define "global_tokens_and_yystype" "1"' triggers the global
3935 definition of tokens and YYSTYPE. This change is suitable both
3936 for previous releases of Bison, and this one.
3938 If you wish to update, then make sure older version of Bison will
3939 fail using '%require "2.2"'.
3941 ** DJGPP support added.
3944 * Noteworthy changes in release 2.1 (2005-09-16)
3946 ** The C++ lalr1.cc skeleton supports %lex-param.
3948 ** Bison-generated parsers now support the translation of diagnostics like
3949 "syntax error" into languages other than English. The default
3950 language is still English. For details, please see the new
3951 Internationalization section of the Bison manual. Software
3952 distributors should also see the new PACKAGING file. Thanks to
3953 Bruno Haible for this new feature.
3955 ** Wording in the Bison-generated parsers has been changed slightly to
3956 simplify translation. In particular, the message "memory exhausted"
3957 has replaced "parser stack overflow", as the old message was not
3958 always accurate for modern Bison-generated parsers.
3960 ** Destructors are now called when the parser aborts, for all symbols left
3961 behind on the stack. Also, the start symbol is now destroyed after a
3962 successful parse. In both cases, the behavior was formerly inconsistent.
3964 ** When generating verbose diagnostics, Bison-generated parsers no longer
3965 quote the literal strings associated with tokens. For example, for
3966 a syntax error associated with '%token NUM "number"' they might
3967 print 'syntax error, unexpected number' instead of 'syntax error,
3968 unexpected "number"'.
3971 * Noteworthy changes in release 2.0 (2004-12-25)
3973 ** Possibly-incompatible changes
3975 - Bison-generated parsers no longer default to using the alloca function
3976 (when available) to extend the parser stack, due to widespread
3977 problems in unchecked stack-overflow detection. You can "#define
3978 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3979 the manual to determine safe values for YYMAXDEPTH in that case.
3981 - Error token location.
3982 During error recovery, the location of the syntax error is updated
3983 to cover the whole sequence covered by the error token: it includes
3984 the shifted symbols thrown away during the first part of the error
3985 recovery, and the lookahead rejected during the second part.
3987 - Semicolon changes:
3988 . Stray semicolons are no longer allowed at the start of a grammar.
3989 . Semicolons are now required after in-grammar declarations.
3991 - Unescaped newlines are no longer allowed in character constants or
3992 string literals. They were never portable, and GCC 3.4.0 has
3993 dropped support for them. Better diagnostics are now generated if
3994 forget a closing quote.
3996 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
4000 - GLR grammars now support locations.
4002 - New directive: %initial-action.
4003 This directive allows the user to run arbitrary code (including
4004 initializing @$) from yyparse before parsing starts.
4006 - A new directive "%expect-rr N" specifies the expected number of
4007 reduce/reduce conflicts in GLR parsers.
4009 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
4010 This is a GNU extension.
4012 - The option "--report=lookahead" was changed to "--report=look-ahead".
4013 [However, this was changed back after 2.3.]
4015 - Experimental %destructor support has been added to lalr1.cc.
4017 - New configure option --disable-yacc, to disable installation of the
4018 yacc command and -ly library introduced in 1.875 for POSIX conformance.
4022 - For now, %expect-count violations are now just warnings, not errors.
4023 This is for compatibility with Bison 1.75 and earlier (when there are
4024 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
4025 are too many or too few shift/reduce conflicts). However, in future
4026 versions of Bison we plan to improve the %expect machinery so that
4027 these violations will become errors again.
4029 - Within Bison itself, numbers (e.g., goto numbers) are no longer
4030 arbitrarily limited to 16-bit counts.
4032 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4035 * Noteworthy changes in release 1.875 (2003-01-01)
4037 ** The documentation license has been upgraded to version 1.2
4038 of the GNU Free Documentation License.
4040 ** syntax error processing
4042 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4043 locations too. This fixes bugs in error-location computation.
4046 It is now possible to reclaim the memory associated to symbols
4047 discarded during error recovery. This feature is still experimental.
4050 This new directive is preferred over YYERROR_VERBOSE.
4052 - #defining yyerror to steal internal variables is discouraged.
4053 It is not guaranteed to work forever.
4055 ** POSIX conformance
4057 - Semicolons are once again optional at the end of grammar rules.
4058 This reverts to the behavior of Bison 1.33 and earlier, and improves
4059 compatibility with Yacc.
4061 - "parse error" -> "syntax error"
4062 Bison now uniformly uses the term "syntax error"; formerly, the code
4063 and manual sometimes used the term "parse error" instead. POSIX
4064 requires "syntax error" in diagnostics, and it was thought better to
4067 - The documentation now emphasizes that yylex and yyerror must be
4068 declared before use. C99 requires this.
4070 - Bison now parses C99 lexical constructs like UCNs and
4071 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4073 - File names are properly escaped in C output. E.g., foo\bar.y is
4074 output as "foo\\bar.y".
4076 - Yacc command and library now available
4077 The Bison distribution now installs a "yacc" command, as POSIX requires.
4078 Also, Bison now installs a small library liby.a containing
4079 implementations of Yacc-compatible yyerror and main functions.
4080 This library is normally not useful, but POSIX requires it.
4082 - Type clashes now generate warnings, not errors.
4084 - If the user does not define YYSTYPE as a macro, Bison now declares it
4085 using typedef instead of defining it as a macro.
4086 For consistency, YYLTYPE is also declared instead of defined.
4088 ** Other compatibility issues
4090 - %union directives can now have a tag before the "{", e.g., the
4091 directive "%union foo {...}" now generates the C code
4092 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4093 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4094 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4095 This is for compatibility with both Yacc and Bison 1.35.
4097 - ";" is output before the terminating "}" of an action, for
4098 compatibility with Bison 1.35.
4100 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4101 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4103 - "yystype" and "yyltype" are now obsolescent macros instead of being
4104 typedefs or tags; they are no longer documented and are planned to be
4105 withdrawn in a future release.
4110 Users of Bison have to decide how they handle the portability of the
4113 - "parsing stack overflow..." -> "parser stack overflow"
4114 GLR parsers now report "parser stack overflow" as per the Bison manual.
4116 ** %parse-param and %lex-param
4117 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4118 additional context to yyparse and yylex. They suffer from several
4121 - a single argument only can be added,
4122 - their types are weak (void *),
4123 - this context is not passed to ancillary functions such as yyerror,
4124 - only yacc.c parsers support them.
4126 The new %parse-param/%lex-param directives provide a more precise control.
4129 %parse-param {int *nastiness}
4130 %lex-param {int *nastiness}
4131 %parse-param {int *randomness}
4133 results in the following signatures:
4135 int yylex (int *nastiness);
4136 int yyparse (int *nastiness, int *randomness);
4138 or, if both %pure-parser and %locations are used:
4140 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4141 int yyparse (int *nastiness, int *randomness);
4143 ** Bison now warns if it detects conflicting outputs to the same file,
4144 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4145 that command outputs both code and header to foo.h.
4147 ** #line in output files
4148 - --no-line works properly.
4150 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4151 later to be built. This change originally took place a few versions
4152 ago, but nobody noticed until we recently asked someone to try
4153 building Bison with a K&R C compiler.
4156 * Noteworthy changes in release 1.75 (2002-10-14)
4158 ** Bison should now work on 64-bit hosts.
4160 ** Indonesian translation thanks to Tedi Heriyanto.
4163 Fix spurious parse errors.
4166 Some people redefine yyerror to steal yyparse' private variables.
4167 Reenable this trick until an official feature replaces it.
4170 In agreement with POSIX and with other Yaccs, leaving a default
4171 action is valid when $$ is untyped, and $1 typed:
4175 but the converse remains an error:
4179 ** Values of midrule actions
4182 foo: { ... } { $$ = $1; } ...
4184 was incorrectly rejected: $1 is defined in the second midrule
4185 action, and is equal to the $$ of the first midrule action.
4188 * Noteworthy changes in release 1.50 (2002-10-04)
4193 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4194 almost any context-free grammar, ambiguous or not. The new declarations
4195 %dprec and %merge on grammar rules allow parse-time resolution of
4196 ambiguities. Contributed by Paul Hilfinger.
4198 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4199 like the Alpha, so please stick to 32-bit hosts for now.
4202 When not in Yacc compatibility mode, when the output file was not
4203 specified, running "bison foo/bar.y" created "foo/bar.c". It
4204 now creates "bar.c".
4207 The undefined token was systematically mapped to 2 which prevented
4208 the use of 2 by the user. This is no longer the case.
4210 ** Unknown token numbers
4211 If yylex returned an out of range value, yyparse could die. This is
4215 According to POSIX, the error token must be 256.
4216 Bison extends this requirement by making it a preference: *if* the
4217 user specified that one of her tokens is numbered 256, then error
4218 will be mapped onto another number.
4220 ** Verbose error messages
4221 They no longer report "..., expecting error or..." for states where
4222 error recovery is possible.
4225 Defaults to "$end" instead of "$".
4227 ** Error recovery now conforms to documentation and to POSIX
4228 When a Bison-generated parser encounters a syntax error, it now pops
4229 the stack until it finds a state that allows shifting the error
4230 token. Formerly, it popped the stack until it found a state that
4231 allowed some non-error action other than a default reduction on the
4232 error token. The new behavior has long been the documented behavior,
4233 and has long been required by POSIX. For more details, please see
4234 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4235 <https://lists.gnu.org/r/bug-bison/2002-05/msg00038.html>.
4238 Popped tokens and nonterminals are now reported.
4241 Larger grammars are now supported (larger token numbers, larger grammar
4242 size (= sum of the LHS and RHS lengths), larger LALR tables).
4243 Formerly, many of these numbers ran afoul of 16-bit limits;
4244 now these limits are 32 bits on most hosts.
4246 ** Explicit initial rule
4247 Bison used to play hacks with the initial rule, which the user does
4248 not write. It is now explicit, and visible in the reports and
4252 Before, Bison reported the useless rules, but, although not used,
4253 included them in the parsers. They are now actually removed.
4255 ** Useless rules, useless nonterminals
4256 They are now reported, as a warning, with their locations.
4258 ** Rules never reduced
4259 Rules that can never be reduced because of conflicts are now
4262 ** Incorrect "Token not used"
4263 On a grammar such as
4265 %token useless useful
4267 exp: '0' %prec useful;
4269 where a token was used to set the precedence of the last rule,
4270 bison reported both "useful" and "useless" as useless tokens.
4272 ** Revert the C++ namespace changes introduced in 1.31
4273 as they caused too many portability hassles.
4275 ** Default locations
4276 By an accident of design, the default computation of @$ was
4277 performed after another default computation was performed: @$ = @1.
4278 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4279 the computation of @$.
4281 ** Token end-of-file
4282 The token end of file may be specified by the user, in which case,
4283 the user symbol is used in the reports, the graphs, and the verbose
4284 error messages instead of "$end", which remains being the default.
4288 %token MYEOF 0 "end of file"
4291 This old option, which has been broken for ages, is removed.
4294 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4295 Croatian, thanks to Denis Lackovic.
4297 ** Incorrect token definitions
4300 bison used to output
4303 ** Token definitions as enums
4304 Tokens are output both as the traditional #define's, and, provided
4305 the compiler supports ANSI C or is a C++ compiler, as enums.
4306 This lets debuggers display names instead of integers.
4309 In addition to --verbose, bison supports --report=THINGS, which
4310 produces additional information:
4312 complete the core item sets with their closure
4313 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4314 explicitly associate lookahead tokens to items
4316 describe shift/reduce conflicts solving.
4317 Bison used to systematically output this information on top of
4318 the report. Solved conflicts are now attached to their states.
4321 Previous versions don't complain when there is a type clash on
4322 the default action if the rule has a midrule action, such as in:
4330 ** GNU M4 is now required when using Bison.
4333 * Noteworthy changes in release 1.35 (2002-03-25)
4336 Some projects use Bison's C parser with C++ compilers, and define
4337 YYSTYPE as a class. The recent adjustment of C parsers for data
4338 alignment and 64 bit architectures made this impossible.
4340 Because for the time being no real solution for C++ parser
4341 generation exists, kludges were implemented in the parser to
4342 maintain this use. In the future, when Bison has C++ parsers, this
4343 kludge will be disabled.
4345 This kludge also addresses some C++ problems when the stack was
4349 * Noteworthy changes in release 1.34 (2002-03-12)
4351 ** File name clashes are detected
4352 $ bison foo.y -d -o foo.x
4353 fatal error: header and parser would both be named "foo.x"
4355 ** A missing ";" at the end of a rule triggers a warning
4356 In accordance with POSIX, and in agreement with other
4357 Yacc implementations, Bison will mandate this semicolon in the near
4358 future. This eases the implementation of a Bison parser of Bison
4359 grammars by making this grammar LALR(1) instead of LR(2). To
4360 facilitate the transition, this release introduces a warning.
4362 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4363 many portability hassles.
4365 ** DJGPP support added.
4367 ** Fix test suite portability problems.
4370 * Noteworthy changes in release 1.33 (2002-02-07)
4373 Groff could not be compiled for the definition of size_t was lacking
4374 under some conditions.
4380 * Noteworthy changes in release 1.32 (2002-01-23)
4382 ** Fix Yacc output file names
4384 ** Portability fixes
4386 ** Italian, Dutch translations
4389 * Noteworthy changes in release 1.31 (2002-01-14)
4393 ** GNU Gettext and %expect
4394 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4395 Bison dies on incorrect %expectations, we fear there will be
4396 too many bug reports for Gettext, so _for the time being_, %expect
4397 does not trigger an error when the input file is named "plural.y".
4399 ** Use of alloca in parsers
4400 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4401 malloc exclusively. Since 1.29, but was not NEWS'ed.
4403 alloca is used only when compiled with GCC, to avoid portability
4406 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4408 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4409 (as POSIX requires) instead of being undefined.
4412 Bison has always permitted actions such as { $$ = $1 }: it adds the
4413 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4414 is no longer output: one has to write { $$ = $1; }.
4416 ** Better C++ compliance
4417 The output parsers try to respect C++ namespaces.
4418 [This turned out to be a failed experiment, and it was reverted later.]
4421 Fixed bugs when reporting useless nonterminals.
4424 The parsers work properly on 64 bit hosts.
4427 Some calls to strerror resulted in scrambled or missing error messages.
4430 When the number of shift/reduce conflicts is correct, don't issue
4433 ** The verbose report includes the rule line numbers.
4435 ** Rule line numbers are fixed in traces.
4437 ** Swedish translation
4440 Verbose parse error messages from the parsers are better looking.
4441 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4442 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4444 ** Fixed parser memory leaks.
4445 When the generated parser was using malloc to extend its stacks, the
4446 previous allocations were not freed.
4448 ** Fixed verbose output file.
4449 Some newlines were missing.
4450 Some conflicts in state descriptions were missing.
4452 ** Fixed conflict report.
4453 Option -v was needed to get the result.
4457 Mismatches are errors, not warnings.
4459 ** Fixed incorrect processing of some invalid input.
4461 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4463 ** Fixed some typos in the documentation.
4465 ** %token MY_EOF 0 is supported.
4466 Before, MY_EOF was silently renumbered as 257.
4468 ** doc/refcard.tex is updated.
4470 ** %output, %file-prefix, %name-prefix.
4474 New, aliasing "--output-file".
4477 * Noteworthy changes in release 1.30 (2001-10-26)
4479 ** "--defines" and "--graph" have now an optional argument which is the
4480 output file name. "-d" and "-g" do not change; they do not take any
4483 ** "%source_extension" and "%header_extension" are removed, failed
4486 ** Portability fixes.
4489 * Noteworthy changes in release 1.29 (2001-09-07)
4491 ** The output file does not define const, as this caused problems when used
4492 with common autoconfiguration schemes. If you still use ancient compilers
4493 that lack const, compile with the equivalent of the C compiler option
4494 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4496 ** Added "-g" and "--graph".
4498 ** The Bison manual is now distributed under the terms of the GNU FDL.
4500 ** The input and the output files has automatically a similar extension.
4502 ** Russian translation added.
4504 ** NLS support updated; should hopefully be less troublesome.
4506 ** Added the old Bison reference card.
4508 ** Added "--locations" and "%locations".
4510 ** Added "-S" and "--skeleton".
4512 ** "%raw", "-r", "--raw" is disabled.
4514 ** Special characters are escaped when output. This solves the problems
4515 of the #line lines with path names including backslashes.
4518 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4519 "%debug", "%source_extension" and "%header_extension".
4522 Automatic location tracking.
4525 * Noteworthy changes in release 1.28 (1999-07-06)
4527 ** Should compile better now with K&R compilers.
4531 ** Fixed a problem with escaping the double quote character.
4533 ** There is now a FAQ.
4536 * Noteworthy changes in release 1.27
4538 ** The make rule which prevented bison.simple from being created on
4539 some systems has been fixed.
4542 * Noteworthy changes in release 1.26
4544 ** Bison now uses Automake.
4546 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4548 ** Token numbers now start at 257 as previously documented, not 258.
4550 ** Bison honors the TMPDIR environment variable.
4552 ** A couple of buffer overruns have been fixed.
4554 ** Problems when closing files should now be reported.
4556 ** Generated parsers should now work even on operating systems which do
4557 not provide alloca().
4560 * Noteworthy changes in release 1.25 (1995-10-16)
4562 ** Errors in the input grammar are not fatal; Bison keeps reading
4563 the grammar file, and reports all the errors found in it.
4565 ** Tokens can now be specified as multiple-character strings: for
4566 example, you could use "<=" for a token which looks like <=, instead
4567 of choosing a name like LESSEQ.
4569 ** The %token_table declaration says to write a table of tokens (names
4570 and numbers) into the parser file. The yylex function can use this
4571 table to recognize multiple-character string tokens, or for other
4574 ** The %no_lines declaration says not to generate any #line preprocessor
4575 directives in the parser file.
4577 ** The %raw declaration says to use internal Bison token numbers, not
4578 Yacc-compatible token numbers, when token names are defined as macros.
4580 ** The --no-parser option produces the parser tables without including
4581 the parser engine; a project can now use its own parser engine.
4582 The actions go into a separate file called NAME.act, in the form of
4583 a switch statement body.
4586 * Noteworthy changes in release 1.23
4588 The user can define YYPARSE_PARAM as the name of an argument to be
4589 passed into yyparse. The argument should have type void *. It should
4590 actually point to an object. Grammar actions can access the variable
4591 by casting it to the proper pointer type.
4593 Line numbers in output file corrected.
4596 * Noteworthy changes in release 1.22
4598 --help option added.
4601 * Noteworthy changes in release 1.20
4603 Output file does not redefine const for C++.
4607 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4608 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4609 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4610 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4611 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4612 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4613 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4614 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4615 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4616 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4617 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4618 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4619 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4620 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4621 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4622 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4623 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4624 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4625 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4626 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4627 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4628 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4629 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4630 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4631 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4632 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4633 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4634 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4635 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4636 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4637 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4638 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4639 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4640 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4641 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4642 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4643 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4646 ispell-dictionary: "american"
4651 Copyright (C) 1995-2015, 2018-2021 Free Software Foundation, Inc.
4653 This file is part of Bison, the GNU Parser Generator.
4655 Permission is granted to copy, distribute and/or modify this document
4656 under the terms of the GNU Free Documentation License, Version 1.3 or
4657 any later version published by the Free Software Foundation; with no
4658 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4659 Texts. A copy of the license is included in the "GNU Free
4660 Documentation License" file as part of this distribution.