3 * Noteworthy changes in release ?.? (????-??-??) [?]
9 Prefer value_type to semantic_type to denote the semantic value type,
10 specified by the `api.value.type` %define variable.
14 There were not debug traces for deferred calls to user actions. They are
19 *** Option -H, --header and directive %header
21 The option `-H`/`--header` supersedes the option `--defines`, and the
22 directive %header supersedes %defines. Both `--defines` and `%defines`
23 are, of course, maintained for backward compatibility.
27 Since version 2.4 Bison can be used to generate HTML reports. However it
28 was a two-step process: first bison must be invoked with option `--xml`,
29 and then xsltproc must be run to the convert the XML reports into HTML.
31 The new option `--html` combines these steps. The xsltproc program must
34 *** A C++ native GLR parser
36 A new version of the generated C++ GLR parser was added as "glr2.cc". It
37 is forked from the existing glr.c/cc parser, with the objective of making
38 it a more modern, truly C++ parser (instead of a C++ wrapper around a C
39 parser). Down the line, the goal is to support `%define api.value.type
40 variant` and maybe share code with lalr1.cc.
42 The current parser should be identical in terms of interface, functionality
43 and performance to "glr.cc". To try it out, simply use
49 Counterexamples now show the rule numbers, and always show ε for rules
50 with an empty right-hand side. For instance
62 *** Lookahead correction in Java
64 The Java skeleton (lalr1.java) now supports LAC, via the `parse.lac`
68 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
72 *** Bug fixes in yacc.c
74 In Yacc mode, all the tokens are defined twice: once as an enum, and then
75 as a macro. YYEMPTY was missing its macro.
77 *** Bug fixes in lalr1.cc
79 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
80 even when the `parse.assert` %define variable is not enabled. It no
83 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
86 When there is a very large number of tokens, some assertions could be long
87 enough to hit arbitrary limits in Visual C++. They have been rewritten to
88 work around this limitation.
92 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
93 skeleton) used to be defined to 1. It is now defined to the version of
94 Bison as an integer (e.g., 30704 for version 3.7.4).
97 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
101 Fix concurrent build issues.
103 The bison executable is no longer linked uselessly against libreadline.
105 Fix incorrect use of yytname in glr.cc.
108 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
110 This release of Bison fixes all known bugs reported for Bison in MITRE's
111 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
112 are only about bison-the-program itself, not the generated code.
114 Although these bugs are typically irrelevant to how Bison is used, they
115 are worth fixing if only to give users peace of mind.
117 There is no known vulnerability in the generated parsers.
121 Fix concurrent build issues (introduced in Bison 3.5).
123 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
125 Fix portability issues of the test suite, and of bison itself.
127 Some unlikely crashes found by fuzzing have been fixed. This is only
128 about bison itself, not the generated parsers.
131 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
135 Crash when a token alias contains a NUL byte.
137 Portability issues with libtextstyle.
139 Portability issues of Bison itself with MSVC.
143 Improvements and fixes in the documentation.
145 More precise location about symbol type redefinitions.
148 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
150 ** Deprecated features
152 The YYPRINT macro, which works only with yacc.c and only for tokens, was
153 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
154 It is deprecated and its support will be removed eventually.
156 In conformance with the recommendations of the Graphviz team, in the next
157 version Bison the option `--graph` will generate a *.gv file by default,
158 instead of *.dot. A transition started in Bison 3.4.
162 *** Counterexample Generation
164 Contributed by Vincent Imbimbo.
166 When given `-Wcounterexamples`/`-Wcex`, bison will now output
167 counterexamples for conflicts.
169 **** Unifying Counterexamples
171 Unifying counterexamples are strings which can be parsed in two ways due
172 to the conflict. For example on a grammar that contains the usual
173 "dangling else" ambiguity:
176 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
177 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
180 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
181 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
182 Example: "if" exp "then" "if" exp "then" exp • "else" exp
185 ↳ "if" exp "then" exp
186 ↳ "if" exp "then" exp • "else" exp
187 Example: "if" exp "then" "if" exp "then" exp • "else" exp
190 ↳ "if" exp "then" exp "else" exp
191 ↳ "if" exp "then" exp •
193 When text styling is enabled, colors are used in the examples and the
194 derivations to highlight the structure of both analyses. In this case,
196 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
200 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
203 The counterexamples are "focused", in two different ways. First, they do
204 not clutter the output with all the derivations from the start symbol,
205 rather they start on the "conflicted nonterminal". They go straight to the
206 point. Second, they don't "expand" nonterminal symbols uselessly.
208 **** Nonunifying Counterexamples
210 In the case of the dangling else, Bison found an example that can be
211 parsed in two ways (therefore proving that the grammar is ambiguous).
212 When it cannot find such an example, it instead generates two examples
213 that are the same up until the dot:
216 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
217 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
218 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
223 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
224 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
225 First example: expr • ID ',' ID $end
232 Second example: expr • ID $end
238 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
242 In these cases, the parser usually doesn't have enough lookahead to
243 differentiate the two given examples.
247 Counterexamples are also included in the report when given
248 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
253 1 exp: "if" exp "then" exp • [$end, "then", "else"]
254 2 | "if" exp "then" exp • "else" exp
256 "else" shift, and go to state 8
258 "else" [reduce using rule 1 (exp)]
259 $default reduce using rule 1 (exp)
261 shift/reduce conflict on token "else":
262 1 exp: "if" exp "then" exp •
263 2 exp: "if" exp "then" exp • "else" exp
264 Example: "if" exp "then" "if" exp "then" exp • "else" exp
267 ↳ "if" exp "then" exp
268 ↳ "if" exp "then" exp • "else" exp
269 Example: "if" exp "then" "if" exp "then" exp • "else" exp
272 ↳ "if" exp "then" exp "else" exp
273 ↳ "if" exp "then" exp •
275 *** File prefix mapping
277 Contributed by Joshua Watt.
279 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
280 in the output (specifically `#line` directives and `#ifdef` header guards)
281 that begins with the prefix OLD will have it replaced with the prefix NEW,
282 similar to the `-ffile-prefix-map` in GCC. This option can be used to
283 make bison output reproducible.
289 When text styling is enabled and the terminal supports it, the warnings
290 now include hyperlinks to the documentation.
292 *** Relocatable installation
294 When installed to be relocatable (via `configure --enable-relocatable`),
295 bison will now also look for a relocated m4.
299 The `filename_type` %define variable was renamed `api.filename.type`.
302 %define filename_type "symbol"
306 %define api.filename.type {symbol}
308 (Or let `bison --update` do it for you).
310 It now defaults to `const std::string` instead of `std::string`.
312 *** Deprecated %define variable names
314 The following variables have been renamed for consistency. Backward
315 compatibility is ensured, but upgrading is recommended.
317 filename_type -> api.filename.type
318 package -> api.package
320 *** Push parsers no longer clear their state when parsing is finished
322 Previously push-parsers cleared their state when parsing was finished (on
323 success and on failure). This made it impossible to check if there were
324 parse errors, since `yynerrs` was also reset. This can be especially
325 troublesome when used in autocompletion, since a parser with error
326 recovery would suggest (irrelevant) expected tokens even if there were
329 Now the parser state can be examined when parsing is finished. The parser
330 state is reset when starting a new parse.
336 The bistromathic demonstrates %param and how to quote sources in the error
340 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
346 *** Include the generated header (yacc.c)
348 Historically, when --defines was used, bison generated a header and pasted
349 an exact copy of it into the generated parser implementation file. Since
350 Bison 3.4 it is possible to specify that the header should be `#include`d,
351 and how. For instance
353 %define api.header.include {"parse.h"}
357 %define api.header.include {<parser/parse.h>}
359 Now api.header.include defaults to `"header-basename"`, as was intended in
360 Bison 3.4, where `header-basename` is the basename of the generated
361 header. This is disabled when the generated header is `y.tab.h`, to
362 comply with Automake's ylwrap.
364 *** String aliases are faithfully propagated
366 Bison used to interpret user strings (i.e., decoding backslash escapes)
367 when reading them, and to escape them (i.e., issue non-printable
368 characters as backslash escapes, taking the locale into account) when
369 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
370 up "ciphered" as sequences of backslash escapes. This happened not only
371 in the generated sources (where the compiler will reinterpret them), but
372 also in all the generated reports (text, xml, html, dot, etc.). Reports
373 were therefore not readable when string aliases were not pure ASCII.
374 Worse yet: the output depended on the user's locale.
376 Now Bison faithfully treats the string aliases exactly the way the user
377 spelled them. This fixes all the aforementioned problems. However, now,
378 string aliases semantically equivalent but syntactically different (e.g.,
379 "A", "\x41", "\101") are considered to be different.
381 *** Crash when generating IELR
383 An old, well hidden, bug in the generation of IELR parsers was fixed.
386 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
390 In glr.cc some internal macros leaked in the user's code, and could damage
391 access to the token kinds.
394 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
398 Incorrect comments in the generated parsers.
400 Warnings in push parsers (yacc.c).
402 Incorrect display of gotos in LAC traces (lalr1.cc).
405 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
409 Some tests were fixed.
411 When token aliases contain comment delimiters:
413 %token FOO "/* foo */"
415 bison used to emit "nested" comments, which is invalid C.
418 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
422 Restored ANSI-C compliance in yacc.c.
424 GNU readline portability issues.
426 In C++, yy::parser::symbol_name is now a public member, as was intended.
430 In C++, yy::parser::symbol_type now has a public name() member function.
433 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
435 ** Backward incompatible changes
437 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
439 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
440 depend on it will now produce Yacc-like error messages (just "syntax
441 error"). It was superseded by the "%error-verbose" directive in Bison
442 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
443 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
444 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
445 parse.error verbose".
447 ** Deprecated features
449 The YYPRINT macro, which works only with yacc.c and only for tokens, was
450 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
451 It is deprecated and its support will be removed eventually.
455 *** Improved syntax error messages
457 Two new values for the %define parse.error variable offer more control to
458 the user. Available in all the skeletons (C, C++, Java).
460 **** %define parse.error detailed
462 The behavior of "%define parse.error detailed" is closely resembling that
463 of "%define parse.error verbose" with a few exceptions. First, it is safe
464 to use non-ASCII characters in token aliases (with 'verbose', the result
465 depends on the locale with which bison was run). Second, a yysymbol_name
466 function is exposed to the user, instead of the yytnamerr function and the
467 yytname table. Third, token internationalization is supported (see
470 **** %define parse.error custom
472 With this directive, the user forges and emits the syntax error message
473 herself by defining the yyreport_syntax_error function. A new type,
474 yypcontext_t, captures the circumstances of the error, and provides the
475 user with functions to get details, such as yypcontext_expected_tokens to
476 get the list of expected token kinds.
478 A possible implementation of yyreport_syntax_error is:
481 yyreport_syntax_error (const yypcontext_t *ctx)
484 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
485 fprintf (stderr, ": syntax error");
486 // Report the tokens expected at this point.
488 enum { TOKENMAX = 10 };
489 yysymbol_kind_t expected[TOKENMAX];
490 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
492 // Forward errors to yyparse.
495 for (int i = 0; i < n; ++i)
496 fprintf (stderr, "%s %s",
497 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
499 // Report the unexpected token.
501 yysymbol_kind_t lookahead = yypcontext_token (ctx);
502 if (lookahead != YYSYMBOL_YYEMPTY)
503 fprintf (stderr, " before %s", yysymbol_name (lookahead));
505 fprintf (stderr, "\n");
509 **** Token aliases internationalization
511 When the %define variable parse.error is set to `custom` or `detailed`,
512 one may specify which token aliases are to be translated using _(). For
524 In that case the user must define _() and N_(), and yysymbol_name returns
525 the translated symbol (i.e., it returns '_("variable")' rather that
526 '"variable"'). In Java, the user must provide an i18n() function.
528 *** List of expected tokens (yacc.c)
530 Push parsers may invoke yypstate_expected_tokens at any point during
531 parsing (including even before submitting the first token) to get the list
532 of possible tokens. This feature can be used to propose autocompletion
533 (see below the "bistromathic" example).
535 It makes little sense to use this feature without enabling LAC (lookahead
538 *** Returning the error token
540 When the scanner returns an invalid token or the undefined token
541 (YYUNDEF), the parser generates an error message and enters error
542 recovery. Because of that error message, most scanners that find lexical
543 errors generate an error message, and then ignore the invalid input
544 without entering the error-recovery.
546 The scanners may now return YYerror, the error token, to enter the
547 error-recovery mode without triggering an additional error message. See
548 the bistromathic for an example.
550 *** Deep overhaul of the symbol and token kinds
552 To avoid the confusion with types in programming languages, we now refer
553 to token and symbol "kinds" instead of token and symbol "types". The
554 documentation and error messages have been revised.
556 All the skeletons have been updated to use dedicated enum types rather
557 than integral types. Special symbols are now regular citizens, instead of
558 being declared in ad hoc ways.
562 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
563 LPAREN, etc. While backward compatibility is of course ensured, users are
564 nonetheless invited to replace their uses of "enum yytokentype" by
567 This type now also includes tokens that were previously hidden: YYEOF (end
568 of input), YYUNDEF (undefined token), and YYerror (error token). They
569 now have string aliases, internationalized when internationalization is
570 enabled. Therefore, by default, error messages now refer to "end of file"
571 (internationalized) rather than the cryptic "$end", or to "invalid token"
572 rather than "$undefined".
574 Therefore in most cases it is now useless to define the end-of-line token
577 %token T_EOF 0 "end of file"
579 Rather simply use "YYEOF" in your scanner.
583 The "symbol kinds" is what the parser actually uses. (Unless the
584 api.token.raw %define variable is used, the symbol kind of a terminal
585 differs from the corresponding token kind.)
587 They are now exposed as a enum, "yysymbol_kind_t".
589 This allows users to tailor the error messages the way they want, or to
590 process some symbols in a specific way in autocompletion (see the
591 bistromathic example below).
593 *** Modernize display of explanatory statements in diagnostics
595 Since Bison 2.7, output was indented four spaces for explanatory
596 statements. For example:
598 input.y:2.7-13: error: %type redeclaration for exp
599 input.y:1.7-11: previous declaration
601 Since the introduction of caret-diagnostics, it became less clear. This
602 indentation has been removed and submessages are displayed similarly as in
605 input.y:2.7-13: error: %type redeclaration for exp
606 2 | %type <float> exp
608 input.y:1.7-11: note: previous declaration
612 Contributed by Victor Morales Cayuela.
616 The token and symbol kinds are yy::parser::token_kind_type and
617 yy::parser::symbol_kind_type.
619 The symbol_type::kind() member function allows to get the kind of a
620 symbol. This can be used to write unit tests for scanners, e.g.,
622 yy::parser::symbol_type t = make_NUMBER ("123");
623 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
624 assert (t.value.as<int> () == 123);
630 In order to avoid ambiguities with "type" as in "typing", we now refer to
631 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
632 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
637 There are now examples/java: a very simple calculator, and a more complete
638 one (push-parser, location tracking, and debug traces).
640 The lexcalc example (a simple example in C based on Flex and Bison) now
641 also demonstrates location tracking.
644 A new C example, bistromathic, is a fully featured interactive calculator
645 using many Bison features: pure interface, push parser, autocompletion
646 based on the current parser state (using yypstate_expected_tokens),
647 location tracking, internationalized custom error messages, lookahead
648 correction, rich debug traces, etc.
650 It shows how to depend on the symbol kinds to tailor autocompletion. For
651 instance it recognizes the symbol kind "VARIABLE" to propose
652 autocompletion on the existing variables, rather than of the word
656 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
658 ** WARNING: Future backward-incompatibilities!
660 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
662 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
663 that still depend on it will produce Yacc-like error messages (just
664 "syntax error"). It was superseded by the "%error-verbose" directive in
665 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
666 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
667 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
668 parse.error verbose".
672 Fix portability issues of the package itself on old compilers.
674 Fix api.token.raw support in Java.
677 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
681 Error messages could quote lines containing zero-width characters (such as
682 \005) with incorrect styling. Fixes for similar issues with unexpectedly
683 short lines (e.g., the file was changed between parsing and diagnosing).
685 Some unlikely crashes found by fuzzing have been fixed. This is only
686 about bison itself, not the generated parsers.
689 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
693 Portability issues and minor cosmetic issues.
695 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
699 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
705 Fix compiler warnings.
708 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
710 ** Backward incompatible changes
712 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
713 longer treated as end-of-lines. This changes the diagnostics, and in
714 particular their locations.
716 In C++, line numbers and columns are now represented as 'int' not
717 'unsigned', so that integer overflow on positions is easily checkable via
718 'gcc -fsanitize=undefined' and the like. This affects the API for
719 positions. The default position and location classes now expose
720 'counter_type' (int), used to define line and column numbers.
722 ** Deprecated features
724 The YYPRINT macro, which works only with yacc.c and only for tokens, was
725 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
726 It is deprecated and its support will be removed eventually.
730 *** Lookahead correction in C++
732 Contributed by Adrian Vogelsgesang.
734 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
735 %define variable parse.lac.
737 *** Variable api.token.raw: Optimized token numbers (all skeletons)
739 In the generated parsers, tokens have two numbers: the "external" token
740 number as returned by yylex (which starts at 257), and the "internal"
741 symbol number (which starts at 3). Each time yylex is called, a table
742 lookup maps the external token number to the internal symbol number.
744 When the %define variable api.token.raw is set, tokens are assigned their
745 internal number, which saves one table lookup per token, and also saves
746 the generation of the mapping table.
748 The gain is typically moderate, but in extreme cases (very simple user
749 actions), a 10% improvement can be observed.
751 *** Generated parsers use better types for states
753 Stacks now use the best integral type for state numbers, instead of always
754 using 15 bits. As a result "small" parsers now have a smaller memory
755 footprint (they use 8 bits), and there is support for large automata (16
756 bits), and extra large (using int, i.e., typically 31 bits).
758 *** Generated parsers prefer signed integer types
760 Bison skeletons now prefer signed to unsigned integer types when either
761 will do, as the signed types are less error-prone and allow for better
762 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
763 portable to unusual machines where char, short and int are all the same
764 width. On non-GNU platforms this may entail including <limits.h> and (if
765 available) <stdint.h> to define integer types and constants.
767 *** A skeleton for the D programming language
769 For the last few releases, Bison has shipped a stealth experimental
770 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
771 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
774 However, because nobody has committed to improving, testing, and
775 documenting this skeleton, it is not clear that it will be supported in
778 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
779 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
781 *** Debug traces in Java
783 The Java backend no longer emits code and data for parser tracing if the
784 %define variable parse.trace is not defined.
788 *** New diagnostic: -Wdangling-alias
790 String literals, which allow for better error messages, are (too)
791 liberally accepted by Bison, which might result in silent errors. For
794 %type <exVal> cond "condition"
796 does not define "condition" as a string alias to 'cond' (nonterminal
797 symbols do not have string aliases). It is rather equivalent to
800 %token <exVal> "condition"
802 i.e., it gives the type 'exVal' to the "condition" token, which was
803 clearly not the intention.
805 Also, because string aliases need not be defined, typos such as "baz"
806 instead of "bar" will be not reported.
808 The option `-Wdangling-alias` catches these situations. On
811 %type <ival> foo "foo"
815 bison -Wdangling-alias reports
817 warning: string literal not attached to a symbol
818 | %type <ival> foo "foo"
820 warning: string literal not attached to a symbol
824 The `-Wall` option does not (yet?) include `-Wdangling-alias`.
826 *** Better POSIX Yacc compatibility diagnostics
828 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
832 %type <ival> TOKEN1 TOKEN2 't'
839 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
840 2 | %type <ival> TOKEN1 TOKEN2 't'
842 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
843 2 | %type <ival> TOKEN1 TOKEN2 't'
845 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
846 2 | %type <ival> TOKEN1 TOKEN2 't'
849 *** Diagnostics with insertion
851 The diagnostics now display the suggestion below the underlined source.
852 Replacement for undeclared symbols are now also suggested.
859 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'?
863 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
867 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
869 *** Diagnostics about long lines
871 Quoted sources may now be truncated to fit the screen. For instance, on a
872 30-column wide terminal:
879 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
882 foo.y:1.8-10: previous declaration
885 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
888 foo.y:1.8-10: previous declaration
894 *** Debugging glr.c and glr.cc
896 The glr.c skeleton always had asserts to check its own behavior (not the
897 user's). These assertions are now under the control of the parse.assert
898 %define variable (disabled by default).
902 Several new compiler warnings in the generated output have been avoided.
903 Some unused features are no longer emitted. Cleaner generated code in
908 Portability issues in the test suite.
910 In theory, parsers using %nonassoc could crash when reporting verbose
911 error messages. This unlikely bug has been fixed.
913 In Java, %define api.prefix was ignored. It now behaves as expected.
916 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
920 In some cases, when warnings are disabled, bison could emit tons of white
921 spaces as diagnostics.
923 When running out of memory, bison could crash (found by fuzzing).
925 When defining twice the EOF token, bison would crash.
927 New warnings from recent compilers have been addressed in the generated
928 parsers (yacc.c, glr.c, glr.cc).
930 When lone carriage-return characters appeared in the input file,
931 diagnostics could hang forever.
934 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
941 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
943 ** Deprecated features
945 The %pure-parser directive is deprecated in favor of '%define api.pure'
946 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
947 now. Note that since Bison 2.7 you are strongly encouraged to use
948 '%define api.pure full' instead of '%define api.pure'.
952 *** Colored diagnostics
954 As an experimental feature, diagnostics are now colored, controlled by the
955 new options --color and --style.
957 To use them, install the libtextstyle library before configuring Bison.
960 https://alpha.gnu.org/gnu/gettext/
964 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
966 The option --color supports the following arguments:
967 - always, yes: Enable colors.
968 - never, no: Disable colors.
969 - auto, tty (default): Enable colors if the output device is a tty.
971 To customize the styles, create a CSS file similar to
975 .error { font-weight: 800; text-decoration: underline; }
978 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
979 environment variable to "bison-bw.css".
983 When given -fsyntax-only, the diagnostics are reported, but no output is
986 The name of this option is somewhat misleading as bison does more than
987 just checking the syntax: every stage is run (including checking for
988 conflicts for instance), except the generation of the output files.
990 *** Include the generated header (yacc.c)
992 Before, when --defines is used, bison generated a header, and pasted an
993 exact copy of it into the generated parser implementation file. If the
994 header name is not "y.tab.h", it is now #included instead of being
997 To use an '#include' even if the header name is "y.tab.h" (which is what
998 happens with --yacc, or when using the Autotools' ylwrap), define
999 api.header.include to the exact argument to pass to #include. For
1002 %define api.header.include {"parse.h"}
1006 %define api.header.include {<parser/parse.h>}
1008 *** api.location.type is now supported in C (yacc.c, glr.c)
1010 The %define variable api.location.type defines the name of the type to use
1011 for locations. When defined, Bison no longer defines YYLTYPE.
1013 This can be used in programs with several parsers to factor their
1014 definition of locations: let one of them generate them, and the others
1021 In conformance with the recommendations of the Graphviz team, if %require
1022 "3.4" (or better) is specified, the option --graph generates a *.gv file
1023 by default, instead of *.dot.
1025 *** Diagnostics overhaul
1027 Column numbers were wrong with multibyte characters, which would also
1028 result in skewed diagnostics with carets. Beside, because we were
1029 indenting the quoted source with a single space, lines with tab characters
1030 were incorrectly underlined.
1032 To address these issues, and to be clearer, Bison now issues diagnostics
1033 as GCC9 does. For instance it used to display (there's a tab before the
1036 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1037 expr: expr '+' "number" { $$ = $1 + $2; }
1041 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
1042 3 | expr: expr '+' "number" { $$ = $1 + $2; }
1045 Other constructs now also have better locations, resulting in more precise
1048 *** Fix-it hints for %empty
1050 Running Bison with -Wempty-rules and --update will remove incorrect %empty
1051 annotations, and add the missing ones.
1053 *** Generated reports
1055 The format of the reports (parse.output) was improved for readability.
1057 *** Better support for --no-line.
1059 When --no-line is used, the generated files are now cleaner: no lines are
1060 generated instead of empty lines. Together with using api.header.include,
1061 that should help people saving the generated files into version control
1062 systems get smaller diffs.
1066 A new example in C shows an simple infix calculator with a hand-written
1067 scanner (examples/c/calc).
1069 A new example in C shows a reentrant parser (capable of recursive calls)
1070 built with Flex and Bison (examples/c/reccalc).
1072 There is a new section about the history of Yaccs and Bison.
1076 A few obscure bugs were fixed, including the second oldest (known) bug in
1077 Bison: it was there when Bison was entered in the RCS version control
1078 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1082 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1086 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1090 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1094 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1095 extensions into errors. It now makes them simple warnings (-Wyacc).
1098 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1100 A new mailing list was created, Bison Announce. It is low traffic, and is
1101 only about announcing new releases and important messages (e.g., polls
1102 about major decisions to make).
1104 https://lists.gnu.org/mailman/listinfo/bison-announce
1106 ** Backward incompatible changes
1108 Support for DJGPP, which has been unmaintained and untested for years, is
1111 ** Deprecated features
1113 A new feature, --update (see below) helps adjusting existing grammars to
1116 *** Deprecated directives
1118 The %error-verbose directive is deprecated in favor of '%define
1119 parse.error verbose' since Bison 3.0, but no warning was issued.
1121 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1122 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1123 directives are slightly different, you might need to adjust your code.
1124 %name-prefix renames only symbols with external linkage, while api.prefix
1125 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1126 yytokentype, YYSTYPE, YYLTYPE, etc.
1128 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1129 {xx}' will typically have to update YY_DECL from
1131 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1135 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1137 *** Deprecated %define variable names
1139 The following variables, mostly related to parsers in Java, have been
1140 renamed for consistency. Backward compatibility is ensured, but upgrading
1143 abstract -> api.parser.abstract
1144 annotations -> api.parser.annotations
1145 extends -> api.parser.extends
1146 final -> api.parser.final
1147 implements -> api.parser.implements
1148 parser_class_name -> api.parser.class
1149 public -> api.parser.public
1150 strictfp -> api.parser.strictfp
1154 *** Generation of fix-its for IDEs/Editors
1156 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1157 bison now generates machine readable editing instructions to fix some
1158 issues. Currently, this is mostly limited to updating deprecated
1159 directives and removing duplicates. For instance:
1163 %define parser_class_name "Parser"
1164 %define api.parser.class "Parser"
1168 See the "fix-it:" lines below:
1170 $ bison -ffixit foo.y
1171 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1174 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1175 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1176 %define parser_class_name "Parser"
1177 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1178 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1179 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1180 %define api.parser.class "Parser"
1181 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1182 foo.y:2.1-34: previous definition
1183 %define parser_class_name "Parser"
1184 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1185 fix-it:"foo.y":{3:1-3:34}:""
1186 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1188 This uses the same output format as GCC and Clang.
1190 *** Updating grammar files
1192 Fixes can be applied on the fly. The previous example ends with the
1193 suggestion to re-run bison with the option -u/--update, which results in a
1194 cleaner grammar file.
1196 $ bison --update foo.y
1198 bison: file 'foo.y' was updated (backup: 'foo.y~')
1201 %define parse.error verbose
1202 %define api.parser.class {Parser}
1206 *** Bison is now relocatable
1208 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1210 A relocatable program can be moved or copied to a different location on
1211 the file system. It can also be used through mount points for network
1212 sharing. It is possible to make symbolic links to the installed and moved
1213 programs, and invoke them through the symbolic link.
1215 *** %expect and %expect-rr modifiers on individual rules
1217 One can now document (and check) which rules participate in shift/reduce
1218 and reduce/reduce conflicts. This is particularly important GLR parsers,
1219 where conflicts are a normal occurrence. For example,
1235 | argument_list ',' expression
1240 Looking at the output from -v, one can see that the shift/reduce conflict
1241 here is due to the fact that the parser does not know whether to reduce
1242 arguments to argument_list until it sees the token _after_ the following
1243 ','. By marking the rule with %expect 1 (because there is a conflict in
1244 one state), we document the source of the 1 overall shift/reduce conflict.
1246 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1247 conflicts. In this case, we mark each of the conflicting rules. For
1256 target_list '=' expr ';'
1262 | target ',' target_list
1271 | expr ',' expr_list
1279 In a statement such as
1283 the parser must reduce x to a target or an expr, but does not know which
1284 until it sees the '='. So we notate the two possible reductions to
1285 indicate that each conflicts in one rule.
1287 This feature needs user feedback, and might evolve in the future.
1289 *** C++: Actual token constructors
1291 When variants and token constructors are enabled, in addition to the
1292 type-safe named token constructors (make_ID, make_INT, etc.), we now
1293 generate genuine constructors for symbol_type.
1295 For instance with these declarations
1301 you may use these constructors:
1303 symbol_type (int token, const std::string&);
1304 symbol_type (int token, const int&);
1305 symbol_type (int token);
1307 Correct matching between token types and value types is checked via
1308 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1309 constructors are preferable, as they offer better type safety (for
1310 instance 'make_ID (42)' would not even compile), but symbol_type
1311 constructors may help when token types are discovered at run-time, e.g.,
1314 if (auto i = lookup_keyword (yytext))
1315 return yy::parser::symbol_type (i);
1317 return yy::parser::make_ID (yytext);
1320 *** C++: Variadic emplace
1322 If your application requires C++11 and you don't use symbol constructors,
1323 you may now use a variadic emplace for semantic values:
1325 %define api.value.type variant
1326 %token <std::pair<int, int>> PAIR
1330 int yylex (parser::semantic_type *lvalp)
1332 lvalp->emplace <std::pair<int, int>> (1, 2);
1333 return parser::token::PAIR;
1336 *** C++: Syntax error exceptions in GLR
1338 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1339 actions, or from the scanner.
1341 *** More POSIX Yacc compatibility warnings
1343 More Bison specific directives are now reported with -y or -Wyacc. This
1344 change was ready since the release of Bison 3.0 in September 2015. It was
1345 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1346 in numerous warnings for Bison users that use the GNU Build System.
1348 If you still experience that problem, either redefine YACC as `bison -o
1349 y.tab.c`, or pass -Wno-yacc to Bison.
1351 *** The tables yyrhs and yyphrs are back
1353 Because no Bison skeleton uses them, these tables were removed (no longer
1354 passed to the skeletons, not even computed) in 2008. However, some users
1355 have expressed interest in being able to use them in their own skeletons.
1359 *** Incorrect number of reduce/reduce conflicts
1361 On a grammar such as
1363 exp: "num" | "num" | "num"
1365 bison used to report a single RR conflict, instead of two. This is now
1366 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1367 was entered in the RCS version control system, in December 1987.
1369 Some grammar files might have to adjust their %expect-rr.
1371 *** Parser directives that were not careful enough
1373 Passing invalid arguments to %nterm, for instance character literals, used
1374 to result in unclear error messages.
1378 The examples/ directory (installed in .../share/doc/bison/examples) has
1379 been restructured per language for clarity. The examples come with a
1380 README and a Makefile. Not only can they be used to toy with Bison, they
1381 can also be starting points for your own grammars.
1383 There is now a Java example, and a simple example in C based on Flex and
1384 Bison (examples/c/lexcalc/).
1390 They now use noexcept and constexpr. Please, report missing annotations.
1392 *** Symbol Declarations
1394 The syntax of the variation directives to declare symbols was overhauled
1395 for more consistency, and also better POSIX Yacc compliance (which, for
1396 instance, allows "%type" without actually providing a type). The %nterm
1397 directive, supported by Bison since its inception, is now documented and
1398 officially supported.
1400 The syntax is now as follows:
1402 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1403 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1404 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1405 %nterm TAG? ID+ ( TAG ID+ )*
1407 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1408 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1409 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1410 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1411 one), ‘*’ (zero or more) and ‘+’ (one or more).
1414 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1418 Fix the move constructor of symbol_type.
1420 Always provide a copy constructor for symbol_type, even in modern C++.
1423 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1427 Properly support token constructors in C++ with types that include commas
1428 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1431 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1435 C++ portability issues.
1438 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1442 Several portability issues have been fixed in the build system, in the
1443 test suite, and in the generated parsers in C++.
1446 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1448 ** Backward incompatible changes
1450 Support for DJGPP, which has been unmaintained and untested for years, is
1451 obsolete. Unless there is activity to revive it, it will be removed.
1455 %printers should use yyo rather than yyoutput to denote the output stream.
1457 Variant-based symbols in C++ should use emplace() rather than build().
1459 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1463 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1465 A comment in the generated code now emphasizes that users should not
1466 depend upon non-documented implementation details, such as macros starting
1471 *** C++: Support for move semantics (lalr1.cc)
1473 The lalr1.cc skeleton now fully supports C++ move semantics, while
1474 maintaining compatibility with C++98. You may now store move-only types
1475 when using Bison's variants. For instance:
1482 %skeleton "lalr1.cc"
1483 %define api.value.type variant
1487 %token <int> INT "int";
1488 %type <std::unique_ptr<int>> int;
1489 %type <std::vector<std::unique_ptr<int>>> list;
1493 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1495 int: "int" { $$ = std::make_unique<int>($1); }
1497 *** C++: Implicit move of right-hand side values (lalr1.cc)
1499 In modern C++ (C++11 and later), you should always use 'std::move' with
1500 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1501 popped from the stack anyway. Using 'std::move' is mandatory for
1502 move-only types such as unique_ptr, and it provides a significant speedup
1503 for large types such as std::string, or std::vector, etc.
1505 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1506 by 'std::move ($n)'. The second rule in the previous grammar can be
1509 list: list int { $$ = $1; $$.emplace_back($2); }
1511 With automove enabled, the semantic values are no longer lvalues, so do
1512 not use the swap idiom:
1514 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1516 This idiom is anyway obsolete: it is preferable to move than to swap.
1518 A warning is issued when automove is enabled, and a value is used several
1521 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1522 exp: "twice" exp { $$ = $2 + $2; }
1525 Enabling api.value.automove does not require support for modern C++. The
1526 generated code is valid C++98/03, but will use copies instead of moves.
1528 The new examples/c++/variant-11.yy shows these features in action.
1530 *** C++: The implicit default semantic action is always run
1532 When variants are enabled, the default action was not run, so
1540 It now behaves like in all the other cases, as
1542 exp: "number" { $$ = $1; }
1544 possibly using std::move if automove is enabled.
1546 We do not expect backward compatibility issues. However, beware of
1547 forward compatibility issues: if you rely on default actions with
1548 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1549 generate incorrect parsers.
1551 *** C++: Renaming location.hh
1553 When both %defines and %locations are enabled, Bison generates a
1554 location.hh file. If you don't use locations outside of the parser, you
1555 may avoid its creation with:
1557 %define api.location.file none
1559 However this file is useful if, for instance, your parser builds an AST
1560 decorated with locations: you may use Bison's location independently of
1561 Bison's parser. You can now give it another name, for instance:
1563 %define api.location.file "my-location.hh"
1565 This name can have directory components, and even be absolute. The name
1566 under which the location file is included is controlled by
1567 api.location.include.
1569 This way it is possible to have several parsers share the same location
1572 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1575 %define api.namespace {foo}
1576 %define api.location.file "include/ast/loc.hh"
1577 %define api.location.include {<ast/loc.hh>}
1579 and use it in src/bar/parser.hh:
1582 %define api.namespace {bar}
1583 %code requires {#include <ast/loc.hh>}
1584 %define api.location.type {bar::location}
1586 Absolute file names are supported, so in your Makefile, passing the flag
1587 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1590 *** C++: stack.hh and position.hh are deprecated
1592 When asked to generate a header file (%defines), the lalr1.cc skeleton
1593 generates a stack.hh file. This file had no interest for users; it is now
1594 made useless: its content is included in the parser definition. It is
1595 still generated for backward compatibility.
1597 When in addition to %defines, location support is requested (%locations),
1598 the file position.hh is also generated. It is now also useless: its
1599 content is now included in location.hh.
1601 These files are no longer generated when your grammar file requires at
1602 least Bison 3.2 (%require "3.2").
1606 Portability issues on MinGW and VS2015.
1608 Portability issues in the test suite.
1610 Portability/warning issues with Flex.
1613 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1615 ** Backward incompatible changes
1617 Compiling Bison now requires a C99 compiler---as announced during the
1618 release of Bison 3.0, five years ago. Generated parsers do not require a
1621 Support for DJGPP, which has been unmaintained and untested for years, is
1622 obsolete. Unless there is activity to revive it, the next release of Bison
1623 will have it removed.
1627 *** Typed midrule actions
1629 Because their type is unknown to Bison, the values of midrule actions are
1630 not treated like the others: they don't have %printer and %destructor
1631 support. It also prevents C++ (Bison) variants to handle them properly.
1633 Typed midrule actions address these issues. Instead of:
1635 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1639 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1641 *** Reports include the type of the symbols
1643 The sections about terminal and nonterminal symbols of the '*.output' file
1644 now specify their declared type. For instance, for:
1648 the report now shows '<ival>':
1650 Terminals, with rules where they appear
1654 *** Diagnostics about useless rules
1656 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1657 of course, its rules are useless too.
1661 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1663 Previously all the useless rules were reported, including those whose
1664 left-hand side is the 'exp' nonterminal:
1666 warning: 1 nonterminal useless in grammar [-Wother]
1667 warning: 4 rules useless in grammar [-Wother]
1668 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1671 2.14-16: warning: rule useless in grammar [-Wother]
1674 3.6-16: warning: rule useless in grammar [-Wother]
1675 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1677 3.20-30: warning: rule useless in grammar [-Wother]
1678 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1680 3.34-44: warning: rule useless in grammar [-Wother]
1681 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1684 Now, rules whose left-hand side symbol is useless are no longer reported
1685 as useless. The locations of the errors have also been adjusted to point
1686 to the first use of the nonterminal as a left-hand side of a rule:
1688 warning: 1 nonterminal useless in grammar [-Wother]
1689 warning: 4 rules useless in grammar [-Wother]
1690 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1691 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1693 2.14-16: warning: rule useless in grammar [-Wother]
1697 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1699 When compiled with exceptions disabled, the generated parsers no longer
1700 uses try/catch clauses.
1702 Currently only GCC and Clang are supported.
1706 *** A demonstration of variants
1708 A new example was added (installed in .../share/doc/bison/examples),
1709 'variant.yy', which shows how to use (Bison) variants in C++.
1711 The other examples were made nicer to read.
1713 *** Some features are no longer 'experimental'
1715 The following features, mature enough, are no longer flagged as
1716 experimental in the documentation: push parsers, default %printer and
1717 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1718 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1719 semantic predicates (%?).
1723 *** GLR: Predicates support broken by #line directives
1725 Predicates (%?) in GLR such as
1728 %? {new_syntax} 'w' id new_args
1729 | %?{!new_syntax} 'w' id old_args
1731 were issued with #lines in the middle of C code.
1733 *** Printer and destructor with broken #line directives
1735 The #line directives were not properly escaped when emitting the code for
1736 %printer/%destructor, which resulted in compiler errors if there are
1737 backslashes or double-quotes in the grammar file name.
1739 *** Portability on ICC
1741 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1742 Generated parsers now work around this.
1746 There were several small fixes in the test suite and in the build system,
1747 many warnings in bison and in the generated parsers were eliminated. The
1748 documentation also received its share of minor improvements.
1750 Useless code was removed from C++ parsers, and some of the generated
1751 constructors are more 'natural'.
1754 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1758 *** C++: Fix support of 'syntax_error'
1760 One incorrect 'inline' resulted in linking errors about the constructor of
1761 the syntax_error exception.
1763 *** C++: Fix warnings
1765 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1766 warnings about yyformat being possibly null. It also warned about the
1767 deprecated implicit definition of copy constructors when there's a
1768 user-defined (copy) assignment operator.
1770 *** Location of errors
1772 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1773 ride-hand side raises a syntax error. The behavior of the default parser
1774 (yacc.c) in such a condition was undefined.
1776 Now all the parsers match the behavior of glr.c: @$ is used as the
1777 location of the error. This handles gracefully rules with and without
1780 *** Portability fixes in the test suite
1782 On some platforms, some Java and/or C++ tests were failing.
1785 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1789 *** C++ with Variants (lalr1.cc)
1791 Fix a compiler warning when no %destructor use $$.
1795 Several portability issues in tests were fixed.
1798 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1802 *** C++ with Variants (lalr1.cc)
1804 Problems with %destructor and '%define parse.assert' have been fixed.
1806 *** Named %union support (yacc.c, glr.c)
1808 Bison 3.0 introduced a regression on named %union such as
1810 %union foo { int ival; };
1812 The possibility to use a name was introduced "for Yacc compatibility".
1813 It is however not required by POSIX Yacc, and its usefulness is not clear.
1815 *** %define api.value.type union with %defines (yacc.c, glr.c)
1817 The C parsers were broken when %defines was used together with "%define
1818 api.value.type union".
1820 *** Redeclarations are reported in proper order
1828 bison used to report:
1830 foo.yy:2.10-11: error: %printer redeclaration for FOO
1833 foo.yy:3.10-11: previous declaration
1837 Now, the "previous" declaration is always the first one.
1842 Bison now installs various files in its docdir (which defaults to
1843 '/usr/local/share/doc/bison'), including the three fully blown examples
1844 extracted from the documentation:
1847 Reverse Polish Calculator, a simple introductory example.
1849 Multi-function Calc, a calculator with memory and functions and located
1852 a calculator in C++ using variant support and token constructors.
1855 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1859 *** Generated source files when errors are reported
1861 When warnings are issued and -Werror is set, bison would still generate
1862 the source files (*.c, *.h...). As a consequence, some runs of "make"
1863 could fail the first time, but not the second (as the files were generated
1866 This is fixed: bison no longer generates this source files, but, of
1867 course, still produces the various reports (*.output, *.xml, etc.).
1869 *** %empty is used in reports
1871 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1872 dot, XML and formats derived from it).
1874 *** YYERROR and variants
1876 When C++ variant support is enabled, an error triggered via YYERROR, but
1877 not caught via error recovery, resulted in a double deletion.
1880 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1884 *** Errors in caret diagnostics
1886 On some platforms, some errors could result in endless diagnostics.
1888 *** Fixes of the -Werror option
1890 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1891 diagnostics into errors instead of warnings. This is fixed.
1893 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1894 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1895 -Wno-error", "foo" diagnostics are now errors.
1899 As demonstrated in the documentation, one can now leave spaces between
1904 The yacc.1 man page is no longer installed if --disable-yacc was
1907 *** Fixes in the test suite
1909 Bugs and portability issues.
1912 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1914 ** WARNING: Future backward-incompatibilities!
1916 Like other GNU packages, Bison will start using some of the C99 features
1917 for its own code, especially the definition of variables after statements.
1918 The generated C parsers still aim at C90.
1920 ** Backward incompatible changes
1922 *** Obsolete features
1924 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1926 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1927 use YYSTYPE and YYLTYPE.
1929 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1930 1.875): use %lex-param, %parse-param, or %param.
1932 Missing semicolons at the end of actions are no longer added (as announced
1933 in the release 2.5).
1935 *** Use of YACC='bison -y'
1937 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1940 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1941 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1942 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1944 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1945 implementation of Yacc, was using Bison as 'bison -y'. While it does
1946 ensure compatible output file names, it also enables warnings for
1947 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1948 warnings for Bison extensions.
1950 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1951 (which also generates 'y.tab.h' and 'y.output' when needed).
1952 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1953 flags (YFLAGS, or AM_YFLAGS with Automake).
1957 *** The epilogue is no longer affected by internal #defines (glr.c)
1959 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1960 generated code. These weren't properly undefined before the inclusion of
1961 the user epilogue, so functions such as the following were butchered by the
1962 preprocessor expansion:
1964 int yylex (YYSTYPE *yylval);
1966 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
1967 identifiers for user-provided variables.
1969 *** stdio.h is no longer needed when locations are enabled (yacc.c)
1971 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
1972 locations are enabled. This is fixed.
1974 *** Warnings about useless %pure-parser/%define api.pure are restored
1976 ** Diagnostics reported by Bison
1978 Most of these features were contributed by Théophile Ranquet and Victor
1983 Version 2.7 introduced caret errors, for a prettier output. These are now
1984 activated by default. The old format can still be used by invoking Bison
1985 with -fno-caret (or -fnone).
1987 Some error messages that reproduced excerpts of the grammar are now using
1988 the caret information only. For instance on:
1995 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1996 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
2000 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2001 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2005 and "bison -fno-caret" reports:
2007 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
2008 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
2010 *** Enhancements of the -Werror option
2012 The -Werror=CATEGORY option is now recognized, and will treat specified
2013 warnings as errors. The warnings need not have been explicitly activated
2014 using the -W option, this is similar to what GCC 4.7 does.
2016 For example, given the following command line, Bison will treat both
2017 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
2018 errors (and only those):
2020 $ bison -Werror=yacc,error=conflicts-sr input.y
2022 If no categories are specified, -Werror will make all active warnings into
2023 errors. For example, the following line does the same the previous example:
2025 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
2027 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
2029 Note that the categories in this -Werror option may not be prefixed with
2030 "no-". However, -Wno-error[=CATEGORY] is valid.
2032 Note that -y enables -Werror=yacc. Therefore it is now possible to require
2033 Yacc-like behavior (e.g., always generate y.tab.c), but to report
2034 incompatibilities as warnings: "-y -Wno-error=yacc".
2036 *** The display of warnings is now richer
2038 The option that controls a given warning is now displayed:
2040 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
2042 In the case of warnings treated as errors, the prefix is changed from
2043 "warning: " to "error: ", and the suffix is displayed, in a manner similar
2044 to GCC, as [-Werror=CATEGORY].
2046 For instance, where the previous version of Bison would report (and exit
2049 bison: warnings being treated as errors
2050 input.y:1.1: warning: stray ',' treated as white space
2054 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
2056 *** Deprecated constructs
2058 The new 'deprecated' warning category flags obsolete constructs whose
2059 support will be discontinued. It is enabled by default. These warnings
2060 used to be reported as 'other' warnings.
2062 *** Useless semantic types
2064 Bison now warns about useless (uninhabited) semantic types. Since
2065 semantic types are not declared to Bison (they are defined in the opaque
2066 %union structure), it is %printer/%destructor directives about useless
2067 types that trigger the warning:
2071 %printer {} <type1> <type3>
2072 %destructor {} <type2> <type4>
2074 nterm: term { $$ = $1; };
2076 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2077 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2079 *** Undefined but unused symbols
2081 Bison used to raise an error for undefined symbols that are not used in
2082 the grammar. This is now only a warning.
2085 %destructor {} symbol2
2086 %type <type> symbol3
2090 *** Useless destructors or printers
2092 Bison now warns about useless destructors or printers. In the following
2093 example, the printer for <type1>, and the destructor for <type2> are
2094 useless: all symbols of <type1> (token1) already have a printer, and all
2095 symbols of type <type2> (token2) already have a destructor.
2097 %token <type1> token1
2101 %printer {} token1 <type1> <type3>
2102 %destructor {} token2 <type2> <type4>
2106 The warnings and error messages about shift/reduce and reduce/reduce
2107 conflicts have been normalized. For instance on the following foo.y file:
2111 exp: exp '+' exp | '0' | '0';
2113 compare the previous version of bison:
2116 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2117 $ bison -Werror foo.y
2118 bison: warnings being treated as errors
2119 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2121 with the new behavior:
2124 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2125 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2126 $ bison -Werror foo.y
2127 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2128 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2130 When %expect or %expect-rr is used, such as with bar.y:
2135 exp: exp '+' exp | '0' | '0';
2140 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2141 bar.y: expected 0 shift/reduce conflicts
2142 bar.y: expected 0 reduce/reduce conflicts
2147 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2148 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2150 ** Incompatibilities with POSIX Yacc
2152 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2155 ** Additional yylex/yyparse arguments
2157 The new directive %param declares additional arguments to both yylex and
2158 yyparse. The %lex-param, %parse-param, and %param directives support one
2159 or more arguments. Instead of
2161 %lex-param {arg1_type *arg1}
2162 %lex-param {arg2_type *arg2}
2163 %parse-param {arg1_type *arg1}
2164 %parse-param {arg2_type *arg2}
2168 %param {arg1_type *arg1} {arg2_type *arg2}
2170 ** Types of values for %define variables
2172 Bison used to make no difference between '%define foo bar' and '%define
2173 foo "bar"'. The former is now called a 'keyword value', and the latter a
2174 'string value'. A third kind was added: 'code values', such as '%define
2177 Keyword variables are used for fixed value sets, e.g.,
2179 %define lr.type lalr
2181 Code variables are used for value in the target language, e.g.,
2183 %define api.value.type {struct semantic_type}
2185 String variables are used remaining cases, e.g. file names.
2187 ** Variable api.token.prefix
2189 The variable api.token.prefix changes the way tokens are identified in
2190 the generated files. This is especially useful to avoid collisions
2191 with identifiers in the target language. For instance
2193 %token FILE for ERROR
2194 %define api.token.prefix {TOK_}
2196 start: FILE for ERROR;
2198 will generate the definition of the symbols TOK_FILE, TOK_for, and
2199 TOK_ERROR in the generated sources. In particular, the scanner must
2200 use these prefixed token names, although the grammar itself still
2201 uses the short names (as in the sample rule given above).
2203 ** Variable api.value.type
2205 This new %define variable supersedes the #define macro YYSTYPE. The use
2206 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2207 using %union or %defining api.value.type results in undefined behavior.
2209 Either define api.value.type, or use "%union":
2216 %token <ival> INT "integer"
2217 %token <sval> STRING "string"
2218 %printer { fprintf (yyo, "%d", $$); } <ival>
2219 %destructor { free ($$); } <sval>
2222 yylval.ival = 42; return INT;
2223 yylval.sval = "42"; return STRING;
2225 The %define variable api.value.type supports both keyword and code values.
2227 The keyword value 'union' means that the user provides genuine types, not
2228 union member names such as "ival" and "sval" above (WARNING: will fail if
2229 -y/--yacc/%yacc is enabled).
2231 %define api.value.type union
2232 %token <int> INT "integer"
2233 %token <char *> STRING "string"
2234 %printer { fprintf (yyo, "%d", $$); } <int>
2235 %destructor { free ($$); } <char *>
2238 yylval.INT = 42; return INT;
2239 yylval.STRING = "42"; return STRING;
2241 The keyword value variant is somewhat equivalent, but for C++ special
2242 provision is made to allow classes to be used (more about this below).
2244 %define api.value.type variant
2245 %token <int> INT "integer"
2246 %token <std::string> STRING "string"
2248 Code values (in braces) denote user defined types. This is where YYSTYPE
2266 %define api.value.type {struct my_value}
2267 %token <u.ival> INT "integer"
2268 %token <u.sval> STRING "string"
2269 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2270 %destructor { free ($$); } <u.sval>
2273 yylval.u.ival = 42; return INT;
2274 yylval.u.sval = "42"; return STRING;
2276 ** Variable parse.error
2278 This variable controls the verbosity of error messages. The use of the
2279 %error-verbose directive is deprecated in favor of "%define parse.error
2282 ** Deprecated %define variable names
2284 The following variables have been renamed for consistency. Backward
2285 compatibility is ensured, but upgrading is recommended.
2287 lr.default-reductions -> lr.default-reduction
2288 lr.keep-unreachable-states -> lr.keep-unreachable-state
2289 namespace -> api.namespace
2290 stype -> api.value.type
2292 ** Semantic predicates
2294 Contributed by Paul Hilfinger.
2296 The new, experimental, semantic-predicate feature allows actions of the
2297 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2298 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2299 in GLR parsers, rather than being deferred. The result is that they allow
2300 the programmer to prune possible parses based on the values of run-time
2303 ** The directive %expect-rr is now an error in non GLR mode
2305 It used to be an error only if used in non GLR mode, _and_ if there are
2306 reduce/reduce conflicts.
2308 ** Tokens are numbered in their order of appearance
2310 Contributed by Valentin Tolmer.
2312 With '%token A B', A had a number less than the one of B. However,
2313 precedence declarations used to generate a reversed order. This is now
2314 fixed, and introducing tokens with any of %token, %left, %right,
2315 %precedence, or %nonassoc yields the same result.
2317 When mixing declarations of tokens with a literal character (e.g., 'a') or
2318 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2319 the literal characters first. For example
2323 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2324 input order is now preserved.
2326 These changes were made so that one can remove useless precedence and
2327 associativity declarations (i.e., map %nonassoc, %left or %right to
2328 %precedence, or to %token) and get exactly the same output.
2330 ** Useless precedence and associativity
2332 Contributed by Valentin Tolmer.
2334 When developing and maintaining a grammar, useless associativity and
2335 precedence directives are common. They can be a nuisance: new ambiguities
2336 arising are sometimes masked because their conflicts are resolved due to
2337 the extra precedence or associativity information. Furthermore, it can
2338 hinder the comprehension of a new grammar: one will wonder about the role
2339 of a precedence, where in fact it is useless. The following changes aim
2340 at detecting and reporting these extra directives.
2342 *** Precedence warning category
2344 A new category of warning, -Wprecedence, was introduced. It flags the
2345 useless precedence and associativity directives.
2347 *** Useless associativity
2349 Bison now warns about symbols with a declared associativity that is never
2350 used to resolve conflicts. In that case, using %precedence is sufficient;
2351 the parsing tables will remain unchanged. Solving these warnings may raise
2352 useless precedence warnings, as the symbols no longer have associativity.
2366 warning: useless associativity for '+', use %precedence [-Wprecedence]
2370 *** Useless precedence
2372 Bison now warns about symbols with a declared precedence and no declared
2373 associativity (i.e., declared with %precedence), and whose precedence is
2374 never used. In that case, the symbol can be safely declared with %token
2375 instead, without modifying the parsing tables. For example:
2379 exp: "var" '=' "number";
2383 warning: useless precedence for '=' [-Wprecedence]
2387 *** Useless precedence and associativity
2389 In case of both useless precedence and associativity, the issue is flagged
2394 exp: "var" '=' "number";
2398 warning: useless precedence and associativity for '=' [-Wprecedence]
2404 With help from Joel E. Denny and Gabriel Rassoul.
2406 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2407 marked by the new %empty directive. Using %empty on a non-empty rule is
2408 an error. The new -Wempty-rule warning reports empty rules without
2409 %empty. On the following grammar:
2419 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2422 5.8-13: error: %empty on non-empty rule
2426 ** Java skeleton improvements
2428 The constants for token names were moved to the Lexer interface. Also, it
2429 is possible to add code to the parser's constructors using "%code init"
2430 and "%define init_throws".
2431 Contributed by Paolo Bonzini.
2433 The Java skeleton now supports push parsing.
2434 Contributed by Dennis Heimbigner.
2436 ** C++ skeletons improvements
2438 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2440 Using %defines is now optional. Without it, the needed support classes
2441 are defined in the generated parser, instead of additional files (such as
2442 location.hh, position.hh and stack.hh).
2444 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2446 Both lalr1.cc and glr.cc no longer require %location.
2448 *** syntax_error exception (lalr1.cc)
2450 The C++ parser features a syntax_error exception, which can be
2451 thrown from the scanner or from user rules to raise syntax errors.
2452 This facilitates reporting errors caught in sub-functions (e.g.,
2453 rejecting too large integral literals from a conversion function
2454 used by the scanner, or rejecting invalid combinations from a
2455 factory invoked by the user actions).
2457 *** %define api.value.type variant
2459 This is based on a submission from Michiel De Wilde. With help
2460 from Théophile Ranquet.
2462 In this mode, complex C++ objects can be used as semantic values. For
2465 %token <::std::string> TEXT;
2466 %token <int> NUMBER;
2467 %token SEMICOLON ";"
2468 %type <::std::string> item;
2469 %type <::std::list<std::string>> list;
2472 list { std::cout << $1 << std::endl; }
2476 %empty { /* Generates an empty string list. */ }
2477 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2481 TEXT { std::swap ($$, $1); }
2482 | NUMBER { $$ = string_cast ($1); }
2485 *** %define api.token.constructor
2487 When variants are enabled, Bison can generate functions to build the
2488 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2489 with the semantic value (e.g., int):
2491 parser::symbol_type yylex ()
2493 parser::location_type loc = ...;
2495 return parser::make_TEXT ("Hello, world!", loc);
2497 return parser::make_NUMBER (42, loc);
2499 return parser::make_SEMICOLON (loc);
2505 There are operator- and operator-= for 'location'. Negative line/column
2506 increments can no longer underflow the resulting value.
2509 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2513 *** Fix compiler attribute portability (yacc.c)
2515 With locations enabled, __attribute__ was used unprotected.
2517 *** Fix some compiler warnings (lalr1.cc)
2520 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2524 Warnings about uninitialized yylloc in yyparse have been fixed.
2526 Restored C90 compliance (yet no report was ever made).
2528 ** Diagnostics are improved
2530 Contributed by Théophile Ranquet.
2532 *** Changes in the format of error messages
2534 This used to be the format of many error reports:
2536 input.y:2.7-12: %type redeclaration for exp
2537 input.y:1.7-12: previous declaration
2541 input.y:2.7-12: error: %type redeclaration for exp
2542 input.y:1.7-12: previous declaration
2544 *** New format for error reports: carets
2546 Caret errors have been added to Bison:
2548 input.y:2.7-12: error: %type redeclaration for exp
2551 input.y:1.7-12: previous declaration
2557 input.y:3.20-23: error: ambiguous reference: '$exp'
2558 exp: exp '+' exp { $exp = $1 + $3; };
2560 input.y:3.1-3: refers to: $exp at $$
2561 exp: exp '+' exp { $exp = $1 + $3; };
2563 input.y:3.6-8: refers to: $exp at $1
2564 exp: exp '+' exp { $exp = $1 + $3; };
2566 input.y:3.14-16: refers to: $exp at $3
2567 exp: exp '+' exp { $exp = $1 + $3; };
2570 The default behavior for now is still not to display these unless
2571 explicitly asked with -fcaret (or -fall). However, in a later release, it
2572 will be made the default behavior (but may still be deactivated with
2575 ** New value for %define variable: api.pure full
2577 The %define variable api.pure requests a pure (reentrant) parser. However,
2578 for historical reasons, using it in a location-tracking Yacc parser
2579 resulted in a yyerror function that did not take a location as a
2580 parameter. With this new value, the user may request a better pure parser,
2581 where yyerror does take a location as a parameter (in location-tracking
2584 The use of "%define api.pure true" is deprecated in favor of this new
2585 "%define api.pure full".
2587 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2589 The %define variable api.location.type defines the name of the type to use
2590 for locations. When defined, Bison no longer generates the position.hh
2591 and location.hh files, nor does the parser will include them: the user is
2592 then responsible to define her type.
2594 This can be used in programs with several parsers to factor their location
2595 and position files: let one of them generate them, and the others just use
2598 This feature was actually introduced, but not documented, in Bison 2.5,
2599 under the name "location_type" (which is maintained for backward
2602 For consistency, lalr1.java's %define variables location_type and
2603 position_type are deprecated in favor of api.location.type and
2606 ** Exception safety (lalr1.cc)
2608 The parse function now catches exceptions, uses the %destructors to
2609 release memory (the lookahead symbol and the symbols pushed on the stack)
2610 before re-throwing the exception.
2612 This feature is somewhat experimental. User feedback would be
2615 ** Graph improvements in DOT and XSLT
2617 Contributed by Théophile Ranquet.
2619 The graphical presentation of the states is more readable: their shape is
2620 now rectangular, the state number is clearly displayed, and the items are
2621 numbered and left-justified.
2623 The reductions are now explicitly represented as transitions to other
2624 diamond shaped nodes.
2626 These changes are present in both --graph output and xml2dot.xsl XSLT
2627 processing, with minor (documented) differences.
2629 ** %language is no longer an experimental feature.
2631 The introduction of this feature, in 2.4, was four years ago. The
2632 --language option and the %language directive are no longer experimental.
2636 The sections about shift/reduce and reduce/reduce conflicts resolution
2637 have been fixed and extended.
2639 Although introduced more than four years ago, XML and Graphviz reports
2640 were not properly documented.
2642 The translation of midrule actions is now described.
2645 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2647 We consider compiler warnings about Bison generated parsers to be bugs.
2648 Rather than working around them in your own project, please consider
2649 reporting them to us.
2653 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2654 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2657 Other issues in the test suite have been addressed.
2659 Null characters are correctly displayed in error messages.
2661 When possible, yylloc is correctly initialized before calling yylex. It
2662 is no longer necessary to initialize it in the %initial-action.
2665 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2667 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2670 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2674 Bugs and portability issues in the test suite have been fixed.
2676 Some errors in translations have been addressed, and --help now directs
2677 users to the appropriate place to report them.
2679 Stray Info files shipped by accident are removed.
2681 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2682 generated, are removed.
2684 All the generated headers are self-contained.
2686 ** Header guards (yacc.c, glr.c, glr.cc)
2688 In order to avoid collisions, the header guards are now
2689 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2690 For instance the header generated from
2692 %define api.prefix "calc"
2693 %defines "lib/parse.h"
2695 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2697 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2699 The compilation of pure parsers (%define api.pure) can trigger GCC
2702 input.c: In function 'yyparse':
2703 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2704 function [-Wmaybe-uninitialized]
2708 This is now fixed; pragmas to avoid these warnings are no longer needed.
2710 Warnings from clang ("equality comparison with extraneous parentheses" and
2711 "function declared 'noreturn' should not return") have also been
2715 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2719 Buffer overruns, complaints from Flex, and portability issues in the test
2720 suite have been fixed.
2722 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2724 Trailing end-of-lines in %parse-param or %lex-param would result in
2725 invalid C++. This is fixed.
2727 ** Spurious spaces and end-of-lines
2729 The generated files no longer end (nor start) with empty lines.
2732 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2734 Bison no longer executes user-specified M4 code when processing a grammar.
2738 In addition to the removal of the features announced in Bison 2.6, the
2739 next major release will remove the "Temporary hack for adding a semicolon
2740 to the user action", as announced in the release 2.5. Instead of:
2742 exp: exp "+" exp { $$ = $1 + $3 };
2746 exp: exp "+" exp { $$ = $1 + $3; };
2750 *** Type names are now properly escaped.
2752 *** glr.cc: set_debug_level and debug_level work as expected.
2754 *** Stray @ or $ in actions
2756 While Bison used to warn about stray $ or @ in action rules, it did not
2757 for other actions such as printers, destructors, or initial actions. It
2760 ** Type names in actions
2762 For consistency with rule actions, it is now possible to qualify $$ by a
2763 type-name in destructors, printers, and initial actions. For instance:
2765 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2767 will display two values for each typed and untyped symbol (provided
2768 that YYSTYPE has both "ival" and "fval" fields).
2771 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2775 The next major release of Bison will drop support for the following
2776 deprecated features. Please report disagreements to bug-bison@gnu.org.
2780 Support for generating parsers in K&R C will be removed. Parsers
2781 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2784 *** Features deprecated since Bison 1.875
2786 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2789 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2790 %lex-param, will no longer be supported.
2792 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2795 *** The generated header will be included (yacc.c)
2797 Instead of duplicating the content of the generated header (definition of
2798 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2799 as is already the case for GLR or C++ parsers. This change is deferred
2800 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2803 ** Generated Parser Headers
2805 *** Guards (yacc.c, glr.c, glr.cc)
2807 The generated headers are now guarded, as is already the case for C++
2808 parsers (lalr1.cc). For instance, with --defines=foo.h:
2813 #endif /* !YY_FOO_H */
2815 *** New declarations (yacc.c, glr.c)
2817 The generated header now declares yydebug and yyparse. Both honor
2818 --name-prefix=bar_, and yield
2820 int bar_parse (void);
2824 #define yyparse bar_parse
2827 in order to facilitate the inclusion of several parser headers inside a
2828 single compilation unit.
2830 *** Exported symbols in C++
2832 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2833 header, are removed, as they prevent the possibility of including several
2834 generated headers from a single compilation unit.
2838 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2841 ** New %define variable: api.prefix
2843 Now that the generated headers are more complete and properly protected
2844 against multiple inclusions, constant names, such as YYSTYPE are a
2845 problem. While yyparse and others are properly renamed by %name-prefix,
2846 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2847 would introduce backward compatibility issues in projects not expecting
2848 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2849 it is deprecated in favor of a new %define variable: api.prefix.
2851 The following examples compares both:
2853 %name-prefix "bar_" | %define api.prefix "bar_"
2854 %token <ival> FOO %token <ival> FOO
2855 %union { int ival; } %union { int ival; }
2861 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2862 # define BAR_FOO_H # define BAR_FOO_H
2864 /* Enabling traces. */ /* Enabling traces. */
2865 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2866 > # if defined YYDEBUG
2868 > # define BAR_DEBUG 1
2870 > # define BAR_DEBUG 0
2873 # define YYDEBUG 0 | # define BAR_DEBUG 0
2877 # if YYDEBUG | # if BAR_DEBUG
2878 extern int bar_debug; extern int bar_debug;
2881 /* Tokens. */ /* Tokens. */
2882 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2883 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2884 enum yytokentype { | enum bar_tokentype {
2889 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2890 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2891 typedef union YYSTYPE | typedef union BAR_STYPE
2894 } YYSTYPE; | } BAR_STYPE;
2895 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2898 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2900 int bar_parse (void); int bar_parse (void);
2902 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2905 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2909 The next major release will drop support for generating parsers in K&R C.
2911 ** yacc.c: YYBACKUP works as expected.
2913 ** glr.c improvements:
2915 *** Location support is eliminated when not requested:
2917 GLR parsers used to include location-related code even when locations were
2918 not requested, and therefore not even usable.
2920 *** __attribute__ is preserved:
2922 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2923 when -std is passed to GCC).
2925 ** lalr1.java: several fixes:
2927 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2928 first token leads to a syntax error. Some minor clean ups.
2932 *** C++11 compatibility:
2934 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2939 The header files such as "parser.hh", "location.hh", etc. used a constant
2940 name for preprocessor guards, for instance:
2942 #ifndef BISON_LOCATION_HH
2943 # define BISON_LOCATION_HH
2945 #endif // !BISON_LOCATION_HH
2947 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2948 case characters are converted to upper case, and series of
2949 non-alphanumerical characters are converted to an underscore.
2951 With "bison -o lang++/parser.cc", "location.hh" would now include:
2953 #ifndef YY_LANG_LOCATION_HH
2954 # define YY_LANG_LOCATION_HH
2956 #endif // !YY_LANG_LOCATION_HH
2960 The position and location constructors (and their initialize methods)
2961 accept new arguments for line and column. Several issues in the
2962 documentation were fixed.
2964 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2966 ** Changes in the manual:
2968 *** %printer is documented
2970 The "%printer" directive, supported since at least Bison 1.50, is finally
2971 documented. The "mfcalc" example is extended to demonstrate it.
2973 For consistency with the C skeletons, the C++ parsers now also support
2974 "yyoutput" (as an alias to "debug_stream ()").
2976 *** Several improvements have been made:
2978 The layout for grammar excerpts was changed to a more compact scheme.
2979 Named references are motivated. The description of the automaton
2980 description file (*.output) is updated to the current format. Incorrect
2981 index entries were fixed. Some other errors were fixed.
2985 *** Conflicting prototypes with recent/modified Flex.
2987 Fixed build problems with the current, unreleased, version of Flex, and
2988 some modified versions of 2.5.35, which have modified function prototypes.
2990 *** Warnings during the build procedure have been eliminated.
2992 *** Several portability problems in the test suite have been fixed:
2994 This includes warnings with some compilers, unexpected behavior of tools
2995 such as diff, warning messages from the test suite itself, etc.
2997 *** The install-pdf target works properly:
2999 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
3000 halts in the middle of its course.
3003 * Noteworthy changes in release 2.5 (2011-05-14)
3005 ** Grammar symbol names can now contain non-initial dashes:
3007 Consistently with directives (such as %error-verbose) and with
3008 %define variables (e.g. push-pull), grammar symbol names may contain
3009 dashes in any position except the beginning. This is a GNU
3010 extension over POSIX Yacc. Thus, use of this extension is reported
3011 by -Wyacc and rejected in Yacc mode (--yacc).
3013 ** Named references:
3015 Historically, Yacc and Bison have supported positional references
3016 ($n, $$) to allow access to symbol values from inside of semantic
3019 Starting from this version, Bison can also accept named references.
3020 When no ambiguity is possible, original symbol names may be used
3021 as named references:
3023 if_stmt : "if" cond_expr "then" then_stmt ';'
3024 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
3026 In the more common case, explicit names may be declared:
3028 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
3029 { $res = mk_if_stmt($cond, $then, $else); }
3031 Location information is also accessible using @name syntax. When
3032 accessing symbol names containing dots or dashes, explicit bracketing
3033 ($[sym.1]) must be used.
3035 These features are experimental in this version. More user feedback
3036 will help to stabilize them.
3037 Contributed by Alex Rozenman.
3039 ** IELR(1) and canonical LR(1):
3041 IELR(1) is a minimal LR(1) parser table generation algorithm. That
3042 is, given any context-free grammar, IELR(1) generates parser tables
3043 with the full language-recognition power of canonical LR(1) but with
3044 nearly the same number of parser states as LALR(1). This reduction
3045 in parser states is often an order of magnitude. More importantly,
3046 because canonical LR(1)'s extra parser states may contain duplicate
3047 conflicts in the case of non-LR(1) grammars, the number of conflicts
3048 for IELR(1) is often an order of magnitude less as well. This can
3049 significantly reduce the complexity of developing of a grammar.
3051 Bison can now generate IELR(1) and canonical LR(1) parser tables in
3052 place of its traditional LALR(1) parser tables, which remain the
3053 default. You can specify the type of parser tables in the grammar
3054 file with these directives:
3056 %define lr.type lalr
3057 %define lr.type ielr
3058 %define lr.type canonical-lr
3060 The default-reduction optimization in the parser tables can also be
3061 adjusted using "%define lr.default-reductions". For details on both
3062 of these features, see the new section "Tuning LR" in the Bison
3065 These features are experimental. More user feedback will help to
3068 ** LAC (Lookahead Correction) for syntax error handling
3070 Contributed by Joel E. Denny.
3072 Canonical LR, IELR, and LALR can suffer from a couple of problems
3073 upon encountering a syntax error. First, the parser might perform
3074 additional parser stack reductions before discovering the syntax
3075 error. Such reductions can perform user semantic actions that are
3076 unexpected because they are based on an invalid token, and they
3077 cause error recovery to begin in a different syntactic context than
3078 the one in which the invalid token was encountered. Second, when
3079 verbose error messages are enabled (with %error-verbose or the
3080 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3081 syntax error message can both contain invalid tokens and omit valid
3084 The culprits for the above problems are %nonassoc, default
3085 reductions in inconsistent states, and parser state merging. Thus,
3086 IELR and LALR suffer the most. Canonical LR can suffer only if
3087 %nonassoc is used or if default reductions are enabled for
3088 inconsistent states.
3090 LAC is a new mechanism within the parsing algorithm that solves
3091 these problems for canonical LR, IELR, and LALR without sacrificing
3092 %nonassoc, default reductions, or state merging. When LAC is in
3093 use, canonical LR and IELR behave almost exactly the same for both
3094 syntactically acceptable and syntactically unacceptable input.
3095 While LALR still does not support the full language-recognition
3096 power of canonical LR and IELR, LAC at least enables LALR's syntax
3097 error handling to correctly reflect LALR's language-recognition
3100 Currently, LAC is only supported for deterministic parsers in C.
3101 You can enable LAC with the following directive:
3103 %define parse.lac full
3105 See the new section "LAC" in the Bison manual for additional
3106 details including a few caveats.
3108 LAC is an experimental feature. More user feedback will help to
3111 ** %define improvements:
3113 *** Can now be invoked via the command line:
3115 Each of these command-line options
3118 --define=NAME[=VALUE]
3121 --force-define=NAME[=VALUE]
3123 is equivalent to this grammar file declaration
3125 %define NAME ["VALUE"]
3127 except that the manner in which Bison processes multiple definitions
3128 for the same NAME differs. Most importantly, -F and --force-define
3129 quietly override %define, but -D and --define do not. For further
3130 details, see the section "Bison Options" in the Bison manual.
3132 *** Variables renamed:
3134 The following %define variables
3137 lr.keep_unreachable_states
3139 have been renamed to
3142 lr.keep-unreachable-states
3144 The old names are now deprecated but will be maintained indefinitely
3145 for backward compatibility.
3147 *** Values no longer need to be quoted in the grammar file:
3149 If a %define value is an identifier, it no longer needs to be placed
3150 within quotations marks. For example,
3152 %define api.push-pull "push"
3156 %define api.push-pull push
3158 *** Unrecognized variables are now errors not warnings.
3160 *** Multiple invocations for any variable is now an error not a warning.
3162 ** Unrecognized %code qualifiers are now errors not warnings.
3164 ** Character literals not of length one:
3166 Previously, Bison quietly converted all character literals to length
3167 one. For example, without warning, Bison interpreted the operators in
3168 the following grammar to be the same token:
3174 Bison now warns when a character literal is not of length one. In
3175 some future release, Bison will start reporting an error instead.
3177 ** Destructor calls fixed for lookaheads altered in semantic actions:
3179 Previously for deterministic parsers in C, if a user semantic action
3180 altered yychar, the parser in some cases used the old yychar value to
3181 determine which destructor to call for the lookahead upon a syntax
3182 error or upon parser return. This bug has been fixed.
3184 ** C++ parsers use YYRHSLOC:
3186 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3187 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3188 to use it. If, for instance, your location structure has "first"
3189 and "last" members, instead of
3191 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3195 (Current).first = (Rhs)[1].location.first; \
3196 (Current).last = (Rhs)[N].location.last; \
3200 (Current).first = (Current).last = (Rhs)[0].location.last; \
3206 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3210 (Current).first = YYRHSLOC (Rhs, 1).first; \
3211 (Current).last = YYRHSLOC (Rhs, N).last; \
3215 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3219 ** YYLLOC_DEFAULT in C++:
3221 The default implementation of YYLLOC_DEFAULT used to be issued in
3222 the header file. It is now output in the implementation file, after
3223 the user %code sections so that its #ifndef guard does not try to
3224 override the user's YYLLOC_DEFAULT if provided.
3226 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3228 YYFAIL has existed for many years as an undocumented feature of
3229 deterministic parsers in C generated by Bison. More recently, it was
3230 a documented feature of Bison's experimental Java parsers. As
3231 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3232 semantic action now produces a deprecation warning, and Java parsers
3233 no longer implement YYFAIL at all. For further details, including a
3234 discussion of how to suppress C preprocessor warnings about YYFAIL
3235 being unused, see the Bison 2.4.2 NEWS entry.
3237 ** Temporary hack for adding a semicolon to the user action:
3239 Previously, Bison appended a semicolon to every user action for
3240 reductions when the output language defaulted to C (specifically, when
3241 neither %yacc, %language, %skeleton, or equivalent command-line
3242 options were specified). This allowed actions such as
3244 exp: exp "+" exp { $$ = $1 + $3 };
3248 exp: exp "+" exp { $$ = $1 + $3; };
3250 As a first step in removing this misfeature, Bison now issues a
3251 warning when it appends a semicolon. Moreover, in cases where Bison
3252 cannot easily determine whether a semicolon is needed (for example, an
3253 action ending with a cpp directive or a braced compound initializer),
3254 it no longer appends one. Thus, the C compiler might now complain
3255 about a missing semicolon where it did not before. Future releases of
3256 Bison will cease to append semicolons entirely.
3258 ** Verbose syntax error message fixes:
3260 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3261 specified, syntax error messages produced by the generated parser
3262 include the unexpected token as well as a list of expected tokens.
3263 The effect of %nonassoc on these verbose messages has been corrected
3264 in two ways, but a more complete fix requires LAC, described above:
3266 *** When %nonassoc is used, there can exist parser states that accept no
3267 tokens, and so the parser does not always require a lookahead token
3268 in order to detect a syntax error. Because no unexpected token or
3269 expected tokens can then be reported, the verbose syntax error
3270 message described above is suppressed, and the parser instead
3271 reports the simpler message, "syntax error". Previously, this
3272 suppression was sometimes erroneously triggered by %nonassoc when a
3273 lookahead was actually required. Now verbose messages are
3274 suppressed only when all previous lookaheads have already been
3275 shifted or discarded.
3277 *** Previously, the list of expected tokens erroneously included tokens
3278 that would actually induce a syntax error because conflicts for them
3279 were resolved with %nonassoc in the current parser state. Such
3280 tokens are now properly omitted from the list.
3282 *** Expected token lists are still often wrong due to state merging
3283 (from LALR or IELR) and default reductions, which can both add
3284 invalid tokens and subtract valid tokens. Canonical LR almost
3285 completely fixes this problem by eliminating state merging and
3286 default reductions. However, there is one minor problem left even
3287 when using canonical LR and even after the fixes above. That is,
3288 if the resolution of a conflict with %nonassoc appears in a later
3289 parser state than the one at which some syntax error is
3290 discovered, the conflicted token is still erroneously included in
3291 the expected token list. Bison's new LAC implementation,
3292 described above, eliminates this problem and the need for
3293 canonical LR. However, LAC is still experimental and is disabled
3296 ** Java skeleton fixes:
3298 *** A location handling bug has been fixed.
3300 *** The top element of each of the value stack and location stack is now
3301 cleared when popped so that it can be garbage collected.
3303 *** Parser traces now print the top element of the stack.
3305 ** -W/--warnings fixes:
3307 *** Bison now properly recognizes the "no-" versions of categories:
3309 For example, given the following command line, Bison now enables all
3310 warnings except warnings for incompatibilities with POSIX Yacc:
3312 bison -Wall,no-yacc gram.y
3314 *** Bison now treats S/R and R/R conflicts like other warnings:
3316 Previously, conflict reports were independent of Bison's normal
3317 warning system. Now, Bison recognizes the warning categories
3318 "conflicts-sr" and "conflicts-rr". This change has important
3319 consequences for the -W and --warnings command-line options. For
3322 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3323 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3324 bison -Wnone gram.y # no conflicts are reported
3325 bison -Werror gram.y # any conflict is an error
3327 However, as before, if the %expect or %expect-rr directive is
3328 specified, an unexpected number of conflicts is an error, and an
3329 expected number of conflicts is not reported, so -W and --warning
3330 then have no effect on the conflict report.
3332 *** The "none" category no longer disables a preceding "error":
3334 For example, for the following command line, Bison now reports
3335 errors instead of warnings for incompatibilities with POSIX Yacc:
3337 bison -Werror,none,yacc gram.y
3339 *** The "none" category now disables all Bison warnings:
3341 Previously, the "none" category disabled only Bison warnings for
3342 which there existed a specific -W/--warning category. However,
3343 given the following command line, Bison is now guaranteed to
3344 suppress all warnings:
3348 ** Precedence directives can now assign token number 0:
3350 Since Bison 2.3b, which restored the ability of precedence
3351 directives to assign token numbers, doing so for token number 0 has
3352 produced an assertion failure. For example:
3356 This bug has been fixed.
3359 * Noteworthy changes in release 2.4.3 (2010-08-05)
3361 ** Bison now obeys -Werror and --warnings=error for warnings about
3362 grammar rules that are useless in the parser due to conflicts.
3364 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3367 ** Failures in the test suite for GCC 4.5 have been fixed.
3369 ** Failures in the test suite for some versions of Sun Studio C++ have
3372 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3373 warnings about undefined %prec identifiers will not be converted to
3374 errors in Bison 2.5. They will remain warnings, which should be
3375 sufficient for POSIX while avoiding backward compatibility issues.
3377 ** Minor documentation fixes.
3380 * Noteworthy changes in release 2.4.2 (2010-03-20)
3382 ** Some portability problems that resulted in failures and livelocks
3383 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3384 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3385 errors should no longer cause M4 to report a broken pipe on the
3388 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3390 POSIX specifies that an error be reported for any identifier that does
3391 not appear on the LHS of a grammar rule and that is not defined by
3392 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3393 error report for the case when an identifier appears only after a
3394 %prec directive. It is now restored. However, for backward
3395 compatibility with recent Bison releases, it is only a warning for
3396 now. In Bison 2.5 and later, it will return to being an error.
3397 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3398 warning will not be converted to an error in Bison 2.5.]
3400 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3402 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3403 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3406 ** %code is now a permanent feature.
3408 A traditional Yacc prologue directive is written in the form:
3412 To provide a more flexible alternative, Bison 2.3b introduced the
3413 %code directive with the following forms for C/C++:
3416 %code requires {CODE}
3417 %code provides {CODE}
3420 These forms are now considered permanent features of Bison. See the
3421 %code entries in the section "Bison Declaration Summary" in the Bison
3422 manual for a summary of their functionality. See the section
3423 "Prologue Alternatives" for a detailed discussion including the
3424 advantages of %code over the traditional Yacc prologue directive.
3426 Bison's Java feature as a whole including its current usage of %code
3427 is still considered experimental.
3429 ** YYFAIL is deprecated and will eventually be removed.
3431 YYFAIL has existed for many years as an undocumented feature of
3432 deterministic parsers in C generated by Bison. Previously, it was
3433 documented for Bison's experimental Java parsers. YYFAIL is no longer
3434 documented for Java parsers and is formally deprecated in both cases.
3435 Users are strongly encouraged to migrate to YYERROR, which is
3438 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3439 induce a syntax error. The most obvious difference from YYERROR is
3440 that YYFAIL will automatically invoke yyerror to report the syntax
3441 error so that you don't have to. However, there are several other
3442 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3443 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3444 used. For a more detailed discussion, see:
3446 http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html
3448 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3449 deterministic parsers in C will continue to implement it. However,
3450 because YYFAIL is already flawed, it seems futile to try to make new
3451 Bison features compatible with it. Thus, during parser generation,
3452 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3453 rule action. In a later release, YYFAIL will be disabled for
3454 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3455 be removed altogether.
3457 There exists at least one case where Bison 2.5's YYFAIL warning will
3458 be a false positive. Some projects add phony uses of YYFAIL and other
3459 Bison-defined macros for the sole purpose of suppressing C
3460 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3461 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3462 epilogue (that is, after the second "%%") in the Bison input file. In
3463 this release (2.4.2), Bison already generates its own code to suppress
3464 C preprocessor warnings for YYFAIL, so projects can remove their own
3465 phony uses of YYFAIL if compatibility with Bison releases prior to
3466 2.4.2 is not necessary.
3468 ** Internationalization.
3470 Fix a regression introduced in Bison 2.4: Under some circumstances,
3471 message translations were not installed although supported by the
3475 * Noteworthy changes in release 2.4.1 (2008-12-11)
3477 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3478 declarations have been fixed.
3480 ** Temporary hack for adding a semicolon to the user action.
3482 Bison used to prepend a trailing semicolon at the end of the user
3483 action for reductions. This allowed actions such as
3485 exp: exp "+" exp { $$ = $1 + $3 };
3489 exp: exp "+" exp { $$ = $1 + $3; };
3491 Some grammars still depend on this "feature". Bison 2.4.1 restores
3492 the previous behavior in the case of C output (specifically, when
3493 neither %language or %skeleton or equivalent command-line options
3494 are used) to leave more time for grammars depending on the old
3495 behavior to be adjusted. Future releases of Bison will disable this
3498 ** A few minor improvements to the Bison manual.
3501 * Noteworthy changes in release 2.4 (2008-11-02)
3503 ** %language is an experimental feature.
3505 We first introduced this feature in test release 2.3b as a cleaner
3506 alternative to %skeleton. Since then, we have discussed the possibility of
3507 modifying its effect on Bison's output file names. Thus, in this release,
3508 we consider %language to be an experimental feature that will likely evolve
3511 ** Forward compatibility with GNU M4 has been improved.
3513 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3517 * Noteworthy changes in release 2.3b (2008-05-27)
3519 ** The quotes around NAME that used to be required in the following directive
3522 %define NAME "VALUE"
3524 ** The directive "%pure-parser" is now deprecated in favor of:
3528 which has the same effect except that Bison is more careful to warn about
3529 unreasonable usage in the latter case.
3533 Bison can now generate an LALR(1) parser in C with a push interface. That
3534 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3535 push one token at a time to the parser using "yypush_parse", which will
3536 return to the caller after processing each token. By default, the push
3537 interface is disabled. Either of the following directives will enable it:
3539 %define api.push_pull "push" // Just push; does not require yylex.
3540 %define api.push_pull "both" // Push and pull; requires yylex.
3542 See the new section "A Push Parser" in the Bison manual for details.
3544 The current push parsing interface is experimental and may evolve. More user
3545 feedback will help to stabilize it.
3547 ** The -g and --graph options now output graphs in Graphviz DOT format,
3548 not VCG format. Like --graph, -g now also takes an optional FILE argument
3549 and thus cannot be bundled with other short options.
3553 Bison can now generate an LALR(1) parser in Java. The skeleton is
3554 "data/lalr1.java". Consider using the new %language directive instead of
3555 %skeleton to select it.
3557 See the new section "Java Parsers" in the Bison manual for details.
3559 The current Java interface is experimental and may evolve. More user
3560 feedback will help to stabilize it.
3561 Contributed by Paolo Bonzini.
3565 This new directive specifies the programming language of the generated
3566 parser, which can be C (the default), C++, or Java. Besides the skeleton
3567 that Bison uses, the directive affects the names of the generated files if
3568 the grammar file's name ends in ".y".
3570 ** XML Automaton Report
3572 Bison can now generate an XML report of the LALR(1) automaton using the new
3573 "--xml" option. The current XML schema is experimental and may evolve. More
3574 user feedback will help to stabilize it.
3575 Contributed by Wojciech Polak.
3577 ** The grammar file may now specify the name of the parser header file using
3578 %defines. For example:
3582 ** When reporting useless rules, useless nonterminals, and unused terminals,
3583 Bison now employs the terms "useless in grammar" instead of "useless",
3584 "useless in parser" instead of "never reduced", and "unused in grammar"
3585 instead of "unused".
3587 ** Unreachable State Removal
3589 Previously, Bison sometimes generated parser tables containing unreachable
3590 states. A state can become unreachable during conflict resolution if Bison
3591 disables a shift action leading to it from a predecessor state. Bison now:
3593 1. Removes unreachable states.
3595 2. Does not report any conflicts that appeared in unreachable states.
3596 WARNING: As a result, you may need to update %expect and %expect-rr
3597 directives in existing grammar files.
3599 3. For any rule used only in such states, Bison now reports the rule as
3600 "useless in parser due to conflicts".
3602 This feature can be disabled with the following directive:
3604 %define lr.keep_unreachable_states
3606 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3607 for further discussion.
3609 ** Lookahead Set Correction in the ".output" Report
3611 When instructed to generate a ".output" file including lookahead sets
3612 (using "--report=lookahead", for example), Bison now prints each reduction's
3613 lookahead set only next to the associated state's one item that (1) is
3614 associated with the same rule as the reduction and (2) has its dot at the end
3615 of its RHS. Previously, Bison also erroneously printed the lookahead set
3616 next to all of the state's other items associated with the same rule. This
3617 bug affected only the ".output" file and not the generated parser source
3620 ** --report-file=FILE is a new option to override the default ".output" file
3623 ** The "=" that used to be required in the following directives is now
3626 %file-prefix "parser"
3630 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3632 Bison 2.3a provided a new set of directives as a more flexible alternative to
3633 the traditional Yacc prologue blocks. Those have now been consolidated into
3634 a single %code directive with an optional qualifier field, which identifies
3635 the purpose of the code and thus the location(s) where Bison should generate
3638 1. "%code {CODE}" replaces "%after-header {CODE}"
3639 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3640 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3641 4. "%code top {CODE}" replaces "%before-header {CODE}"
3643 See the %code entries in section "Bison Declaration Summary" in the Bison
3644 manual for a summary of the new functionality. See the new section "Prologue
3645 Alternatives" for a detailed discussion including the advantages of %code
3646 over the traditional Yacc prologues.
3648 The prologue alternatives are experimental. More user feedback will help to
3649 determine whether they should become permanent features.
3651 ** Revised warning: unset or unused midrule values
3653 Since Bison 2.2, Bison has warned about midrule values that are set but not
3654 used within any of the actions of the parent rule. For example, Bison warns
3657 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3659 Now, Bison also warns about midrule values that are used but not set. For
3660 example, Bison warns about unset $$ in the midrule action in:
3662 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3664 However, Bison now disables both of these warnings by default since they
3665 sometimes prove to be false alarms in existing grammars employing the Yacc
3666 constructs $0 or $-N (where N is some positive integer).
3668 To enable these warnings, specify the option "--warnings=midrule-values" or
3669 "-W", which is a synonym for "--warnings=all".
3671 ** Default %destructor or %printer with "<*>" or "<>"
3673 Bison now recognizes two separate kinds of default %destructor's and
3676 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3677 %destructor/%printer for all grammar symbols for which you have formally
3678 declared semantic type tags.
3680 2. Place "<>" in a %destructor/%printer symbol list to define a default
3681 %destructor/%printer for all grammar symbols without declared semantic
3684 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3685 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3686 longer applies any %destructor to a midrule value if that midrule value is
3687 not actually ever referenced using either $$ or $n in a semantic action.
3689 The default %destructor's and %printer's are experimental. More user
3690 feedback will help to determine whether they should become permanent
3693 See the section "Freeing Discarded Symbols" in the Bison manual for further
3696 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3697 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3698 manual for a caveat concerning the treatment of literal strings.
3700 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3701 completely removed from Bison.
3704 * Noteworthy changes in release 2.3a (2006-09-13)
3706 ** Instead of %union, you can define and use your own union type
3707 YYSTYPE if your grammar contains at least one <type> tag.
3708 Your YYSTYPE need not be a macro; it can be a typedef.
3709 This change is for compatibility with other Yacc implementations,
3710 and is required by POSIX.
3712 ** Locations columns and lines start at 1.
3713 In accordance with the GNU Coding Standards and Emacs.
3715 ** You may now declare per-type and default %destructor's and %printer's:
3719 %union { char *string; }
3720 %token <string> STRING1
3721 %token <string> STRING2
3722 %type <string> string1
3723 %type <string> string2
3724 %union { char character; }
3725 %token <character> CHR
3726 %type <character> chr
3727 %destructor { free ($$); } %symbol-default
3728 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3729 %destructor { } <character>
3731 guarantees that, when the parser discards any user-defined symbol that has a
3732 semantic type tag other than "<character>", it passes its semantic value to
3733 "free". However, when the parser discards a "STRING1" or a "string1", it
3734 also prints its line number to "stdout". It performs only the second
3735 "%destructor" in this case, so it invokes "free" only once.
3737 [Although we failed to mention this here in the 2.3a release, the default
3738 %destructor's and %printer's were experimental, and they were rewritten in
3741 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3742 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3743 associating token numbers with token names. Removing the #define statements
3744 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3745 requires them. Bison still generates an enum for token names in all cases.
3747 ** Handling of traditional Yacc prologue blocks is now more consistent but
3748 potentially incompatible with previous releases of Bison.
3750 As before, you declare prologue blocks in your grammar file with the
3751 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3752 prologue blocks that you've declared before the first %union. To generate
3753 the post-prologue, Bison concatenates all prologue blocks that you've
3754 declared after the first %union.
3756 Previous releases of Bison inserted the pre-prologue into both the header
3757 file and the code file in all cases except for LALR(1) parsers in C. In the
3758 latter case, Bison inserted it only into the code file. For parsers in C++,
3759 the point of insertion was before any token definitions (which associate
3760 token numbers with names). For parsers in C, the point of insertion was
3761 after the token definitions.
3763 Now, Bison never inserts the pre-prologue into the header file. In the code
3764 file, it always inserts it before the token definitions.
3766 ** Bison now provides a more flexible alternative to the traditional Yacc
3767 prologue blocks: %before-header, %start-header, %end-header, and
3770 For example, the following declaration order in the grammar file reflects the
3771 order in which Bison will output these code blocks. However, you are free to
3772 declare these code blocks in your grammar file in whatever order is most
3776 /* Bison treats this block like a pre-prologue block: it inserts it into
3777 * the code file before the contents of the header file. It does *not*
3778 * insert it into the header file. This is a good place to put
3779 * #include's that you want at the top of your code file. A common
3780 * example is '#include "system.h"'. */
3783 /* Bison inserts this block into both the header file and the code file.
3784 * In both files, the point of insertion is before any Bison-generated
3785 * token, semantic type, location type, and class definitions. This is a
3786 * good place to define %union dependencies, for example. */
3789 /* Unlike the traditional Yacc prologue blocks, the output order for the
3790 * new %*-header blocks is not affected by their declaration position
3791 * relative to any %union in the grammar file. */
3794 /* Bison inserts this block into both the header file and the code file.
3795 * In both files, the point of insertion is after the Bison-generated
3796 * definitions. This is a good place to declare or define public
3797 * functions or data structures that depend on the Bison-generated
3801 /* Bison treats this block like a post-prologue block: it inserts it into
3802 * the code file after the contents of the header file. It does *not*
3803 * insert it into the header file. This is a good place to declare or
3804 * define internal functions or data structures that depend on the
3805 * Bison-generated definitions. */
3808 If you have multiple occurrences of any one of the above declarations, Bison
3809 will concatenate the contents in declaration order.
3811 [Although we failed to mention this here in the 2.3a release, the prologue
3812 alternatives were experimental, and they were rewritten in future versions.]
3814 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3815 The old spelling still works, but is not documented and may be removed
3816 in a future release.
3819 * Noteworthy changes in release 2.3 (2006-06-05)
3821 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3822 for compatibility with LALR(1) grammars.
3824 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3825 be to a type name that does not contain parentheses or brackets.
3828 * Noteworthy changes in release 2.2 (2006-05-19)
3830 ** The distribution terms for all Bison-generated parsers now permit
3831 using the parsers in nonfree programs. Previously, this permission
3832 was granted only for Bison-generated LALR(1) parsers in C.
3834 ** %name-prefix changes the namespace name in C++ outputs.
3836 ** The C++ parsers export their token_type.
3838 ** Bison now allows multiple %union declarations, and concatenates
3839 their contents together.
3841 ** New warning: unused values
3842 Right-hand side symbols whose values are not used are reported,
3843 if the symbols have destructors. For instance:
3845 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3849 will trigger a warning about $$ and $5 in the first rule, and $3 in
3850 the second ($1 is copied to $$ by the default rule). This example
3851 most likely contains three errors, and could be rewritten as:
3853 exp: exp "?" exp ":" exp
3854 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3856 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3859 However, if the original actions were really intended, memory leaks
3860 and all, the warnings can be suppressed by letting Bison believe the
3861 values are used, e.g.:
3863 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3864 | exp "+" exp { $$ = $1; (void) $3; }
3867 If there are midrule actions, the warning is issued if no action
3868 uses it. The following triggers no warning: $1 and $3 are used.
3870 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3872 The warning is intended to help catching lost values and memory leaks.
3873 If a value is ignored, its associated memory typically is not reclaimed.
3875 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3876 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3877 and YYERROR, for all objects on the stack, other than objects
3878 corresponding to the right-hand side of the current rule.
3880 ** %expect, %expect-rr
3881 Incorrect numbers of expected conflicts are now actual errors,
3882 instead of warnings.
3884 ** GLR, YACC parsers.
3885 The %parse-params are available in the destructors (and the
3886 experimental printers) as per the documentation.
3888 ** Bison now warns if it finds a stray "$" or "@" in an action.
3890 ** %require "VERSION"
3891 This specifies that the grammar file depends on features implemented
3892 in Bison version VERSION or higher.
3894 ** lalr1.cc: The token and value types are now class members.
3895 The tokens were defined as free form enums and cpp macros. YYSTYPE
3896 was defined as a free form union. They are now class members:
3897 tokens are enumerations of the "yy::parser::token" struct, and the
3898 semantic values have the "yy::parser::semantic_type" type.
3900 If you do not want or can update to this scheme, the directive
3901 '%define "global_tokens_and_yystype" "1"' triggers the global
3902 definition of tokens and YYSTYPE. This change is suitable both
3903 for previous releases of Bison, and this one.
3905 If you wish to update, then make sure older version of Bison will
3906 fail using '%require "2.2"'.
3908 ** DJGPP support added.
3911 * Noteworthy changes in release 2.1 (2005-09-16)
3913 ** The C++ lalr1.cc skeleton supports %lex-param.
3915 ** Bison-generated parsers now support the translation of diagnostics like
3916 "syntax error" into languages other than English. The default
3917 language is still English. For details, please see the new
3918 Internationalization section of the Bison manual. Software
3919 distributors should also see the new PACKAGING file. Thanks to
3920 Bruno Haible for this new feature.
3922 ** Wording in the Bison-generated parsers has been changed slightly to
3923 simplify translation. In particular, the message "memory exhausted"
3924 has replaced "parser stack overflow", as the old message was not
3925 always accurate for modern Bison-generated parsers.
3927 ** Destructors are now called when the parser aborts, for all symbols left
3928 behind on the stack. Also, the start symbol is now destroyed after a
3929 successful parse. In both cases, the behavior was formerly inconsistent.
3931 ** When generating verbose diagnostics, Bison-generated parsers no longer
3932 quote the literal strings associated with tokens. For example, for
3933 a syntax error associated with '%token NUM "number"' they might
3934 print 'syntax error, unexpected number' instead of 'syntax error,
3935 unexpected "number"'.
3938 * Noteworthy changes in release 2.0 (2004-12-25)
3940 ** Possibly-incompatible changes
3942 - Bison-generated parsers no longer default to using the alloca function
3943 (when available) to extend the parser stack, due to widespread
3944 problems in unchecked stack-overflow detection. You can "#define
3945 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3946 the manual to determine safe values for YYMAXDEPTH in that case.
3948 - Error token location.
3949 During error recovery, the location of the syntax error is updated
3950 to cover the whole sequence covered by the error token: it includes
3951 the shifted symbols thrown away during the first part of the error
3952 recovery, and the lookahead rejected during the second part.
3954 - Semicolon changes:
3955 . Stray semicolons are no longer allowed at the start of a grammar.
3956 . Semicolons are now required after in-grammar declarations.
3958 - Unescaped newlines are no longer allowed in character constants or
3959 string literals. They were never portable, and GCC 3.4.0 has
3960 dropped support for them. Better diagnostics are now generated if
3961 forget a closing quote.
3963 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
3967 - GLR grammars now support locations.
3969 - New directive: %initial-action.
3970 This directive allows the user to run arbitrary code (including
3971 initializing @$) from yyparse before parsing starts.
3973 - A new directive "%expect-rr N" specifies the expected number of
3974 reduce/reduce conflicts in GLR parsers.
3976 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
3977 This is a GNU extension.
3979 - The option "--report=lookahead" was changed to "--report=look-ahead".
3980 [However, this was changed back after 2.3.]
3982 - Experimental %destructor support has been added to lalr1.cc.
3984 - New configure option --disable-yacc, to disable installation of the
3985 yacc command and -ly library introduced in 1.875 for POSIX conformance.
3989 - For now, %expect-count violations are now just warnings, not errors.
3990 This is for compatibility with Bison 1.75 and earlier (when there are
3991 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
3992 are too many or too few shift/reduce conflicts). However, in future
3993 versions of Bison we plan to improve the %expect machinery so that
3994 these violations will become errors again.
3996 - Within Bison itself, numbers (e.g., goto numbers) are no longer
3997 arbitrarily limited to 16-bit counts.
3999 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
4002 * Noteworthy changes in release 1.875 (2003-01-01)
4004 ** The documentation license has been upgraded to version 1.2
4005 of the GNU Free Documentation License.
4007 ** syntax error processing
4009 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
4010 locations too. This fixes bugs in error-location computation.
4013 It is now possible to reclaim the memory associated to symbols
4014 discarded during error recovery. This feature is still experimental.
4017 This new directive is preferred over YYERROR_VERBOSE.
4019 - #defining yyerror to steal internal variables is discouraged.
4020 It is not guaranteed to work forever.
4022 ** POSIX conformance
4024 - Semicolons are once again optional at the end of grammar rules.
4025 This reverts to the behavior of Bison 1.33 and earlier, and improves
4026 compatibility with Yacc.
4028 - "parse error" -> "syntax error"
4029 Bison now uniformly uses the term "syntax error"; formerly, the code
4030 and manual sometimes used the term "parse error" instead. POSIX
4031 requires "syntax error" in diagnostics, and it was thought better to
4034 - The documentation now emphasizes that yylex and yyerror must be
4035 declared before use. C99 requires this.
4037 - Bison now parses C99 lexical constructs like UCNs and
4038 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
4040 - File names are properly escaped in C output. E.g., foo\bar.y is
4041 output as "foo\\bar.y".
4043 - Yacc command and library now available
4044 The Bison distribution now installs a "yacc" command, as POSIX requires.
4045 Also, Bison now installs a small library liby.a containing
4046 implementations of Yacc-compatible yyerror and main functions.
4047 This library is normally not useful, but POSIX requires it.
4049 - Type clashes now generate warnings, not errors.
4051 - If the user does not define YYSTYPE as a macro, Bison now declares it
4052 using typedef instead of defining it as a macro.
4053 For consistency, YYLTYPE is also declared instead of defined.
4055 ** Other compatibility issues
4057 - %union directives can now have a tag before the "{", e.g., the
4058 directive "%union foo {...}" now generates the C code
4059 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
4060 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4061 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4062 This is for compatibility with both Yacc and Bison 1.35.
4064 - ";" is output before the terminating "}" of an action, for
4065 compatibility with Bison 1.35.
4067 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4068 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4070 - "yystype" and "yyltype" are now obsolescent macros instead of being
4071 typedefs or tags; they are no longer documented and are planned to be
4072 withdrawn in a future release.
4077 Users of Bison have to decide how they handle the portability of the
4080 - "parsing stack overflow..." -> "parser stack overflow"
4081 GLR parsers now report "parser stack overflow" as per the Bison manual.
4083 ** %parse-param and %lex-param
4084 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4085 additional context to yyparse and yylex. They suffer from several
4088 - a single argument only can be added,
4089 - their types are weak (void *),
4090 - this context is not passed to ancillary functions such as yyerror,
4091 - only yacc.c parsers support them.
4093 The new %parse-param/%lex-param directives provide a more precise control.
4096 %parse-param {int *nastiness}
4097 %lex-param {int *nastiness}
4098 %parse-param {int *randomness}
4100 results in the following signatures:
4102 int yylex (int *nastiness);
4103 int yyparse (int *nastiness, int *randomness);
4105 or, if both %pure-parser and %locations are used:
4107 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4108 int yyparse (int *nastiness, int *randomness);
4110 ** Bison now warns if it detects conflicting outputs to the same file,
4111 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4112 that command outputs both code and header to foo.h.
4114 ** #line in output files
4115 - --no-line works properly.
4117 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4118 later to be built. This change originally took place a few versions
4119 ago, but nobody noticed until we recently asked someone to try
4120 building Bison with a K&R C compiler.
4123 * Noteworthy changes in release 1.75 (2002-10-14)
4125 ** Bison should now work on 64-bit hosts.
4127 ** Indonesian translation thanks to Tedi Heriyanto.
4130 Fix spurious parse errors.
4133 Some people redefine yyerror to steal yyparse' private variables.
4134 Reenable this trick until an official feature replaces it.
4137 In agreement with POSIX and with other Yaccs, leaving a default
4138 action is valid when $$ is untyped, and $1 typed:
4142 but the converse remains an error:
4146 ** Values of midrule actions
4149 foo: { ... } { $$ = $1; } ...
4151 was incorrectly rejected: $1 is defined in the second midrule
4152 action, and is equal to the $$ of the first midrule action.
4155 * Noteworthy changes in release 1.50 (2002-10-04)
4160 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4161 almost any context-free grammar, ambiguous or not. The new declarations
4162 %dprec and %merge on grammar rules allow parse-time resolution of
4163 ambiguities. Contributed by Paul Hilfinger.
4165 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4166 like the Alpha, so please stick to 32-bit hosts for now.
4169 When not in Yacc compatibility mode, when the output file was not
4170 specified, running "bison foo/bar.y" created "foo/bar.c". It
4171 now creates "bar.c".
4174 The undefined token was systematically mapped to 2 which prevented
4175 the use of 2 by the user. This is no longer the case.
4177 ** Unknown token numbers
4178 If yylex returned an out of range value, yyparse could die. This is
4182 According to POSIX, the error token must be 256.
4183 Bison extends this requirement by making it a preference: *if* the
4184 user specified that one of her tokens is numbered 256, then error
4185 will be mapped onto another number.
4187 ** Verbose error messages
4188 They no longer report "..., expecting error or..." for states where
4189 error recovery is possible.
4192 Defaults to "$end" instead of "$".
4194 ** Error recovery now conforms to documentation and to POSIX
4195 When a Bison-generated parser encounters a syntax error, it now pops
4196 the stack until it finds a state that allows shifting the error
4197 token. Formerly, it popped the stack until it found a state that
4198 allowed some non-error action other than a default reduction on the
4199 error token. The new behavior has long been the documented behavior,
4200 and has long been required by POSIX. For more details, please see
4201 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4202 <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.
4205 Popped tokens and nonterminals are now reported.
4208 Larger grammars are now supported (larger token numbers, larger grammar
4209 size (= sum of the LHS and RHS lengths), larger LALR tables).
4210 Formerly, many of these numbers ran afoul of 16-bit limits;
4211 now these limits are 32 bits on most hosts.
4213 ** Explicit initial rule
4214 Bison used to play hacks with the initial rule, which the user does
4215 not write. It is now explicit, and visible in the reports and
4219 Before, Bison reported the useless rules, but, although not used,
4220 included them in the parsers. They are now actually removed.
4222 ** Useless rules, useless nonterminals
4223 They are now reported, as a warning, with their locations.
4225 ** Rules never reduced
4226 Rules that can never be reduced because of conflicts are now
4229 ** Incorrect "Token not used"
4230 On a grammar such as
4232 %token useless useful
4234 exp: '0' %prec useful;
4236 where a token was used to set the precedence of the last rule,
4237 bison reported both "useful" and "useless" as useless tokens.
4239 ** Revert the C++ namespace changes introduced in 1.31
4240 as they caused too many portability hassles.
4242 ** Default locations
4243 By an accident of design, the default computation of @$ was
4244 performed after another default computation was performed: @$ = @1.
4245 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4246 the computation of @$.
4248 ** Token end-of-file
4249 The token end of file may be specified by the user, in which case,
4250 the user symbol is used in the reports, the graphs, and the verbose
4251 error messages instead of "$end", which remains being the default.
4255 %token MYEOF 0 "end of file"
4258 This old option, which has been broken for ages, is removed.
4261 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4262 Croatian, thanks to Denis Lackovic.
4264 ** Incorrect token definitions
4267 bison used to output
4270 ** Token definitions as enums
4271 Tokens are output both as the traditional #define's, and, provided
4272 the compiler supports ANSI C or is a C++ compiler, as enums.
4273 This lets debuggers display names instead of integers.
4276 In addition to --verbose, bison supports --report=THINGS, which
4277 produces additional information:
4279 complete the core item sets with their closure
4280 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4281 explicitly associate lookahead tokens to items
4283 describe shift/reduce conflicts solving.
4284 Bison used to systematically output this information on top of
4285 the report. Solved conflicts are now attached to their states.
4288 Previous versions don't complain when there is a type clash on
4289 the default action if the rule has a midrule action, such as in:
4297 ** GNU M4 is now required when using Bison.
4300 * Noteworthy changes in release 1.35 (2002-03-25)
4303 Some projects use Bison's C parser with C++ compilers, and define
4304 YYSTYPE as a class. The recent adjustment of C parsers for data
4305 alignment and 64 bit architectures made this impossible.
4307 Because for the time being no real solution for C++ parser
4308 generation exists, kludges were implemented in the parser to
4309 maintain this use. In the future, when Bison has C++ parsers, this
4310 kludge will be disabled.
4312 This kludge also addresses some C++ problems when the stack was
4316 * Noteworthy changes in release 1.34 (2002-03-12)
4318 ** File name clashes are detected
4319 $ bison foo.y -d -o foo.x
4320 fatal error: header and parser would both be named "foo.x"
4322 ** A missing ";" at the end of a rule triggers a warning
4323 In accordance with POSIX, and in agreement with other
4324 Yacc implementations, Bison will mandate this semicolon in the near
4325 future. This eases the implementation of a Bison parser of Bison
4326 grammars by making this grammar LALR(1) instead of LR(2). To
4327 facilitate the transition, this release introduces a warning.
4329 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4330 many portability hassles.
4332 ** DJGPP support added.
4334 ** Fix test suite portability problems.
4337 * Noteworthy changes in release 1.33 (2002-02-07)
4340 Groff could not be compiled for the definition of size_t was lacking
4341 under some conditions.
4347 * Noteworthy changes in release 1.32 (2002-01-23)
4349 ** Fix Yacc output file names
4351 ** Portability fixes
4353 ** Italian, Dutch translations
4356 * Noteworthy changes in release 1.31 (2002-01-14)
4360 ** GNU Gettext and %expect
4361 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4362 Bison dies on incorrect %expectations, we fear there will be
4363 too many bug reports for Gettext, so _for the time being_, %expect
4364 does not trigger an error when the input file is named "plural.y".
4366 ** Use of alloca in parsers
4367 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4368 malloc exclusively. Since 1.29, but was not NEWS'ed.
4370 alloca is used only when compiled with GCC, to avoid portability
4373 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4375 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4376 (as POSIX requires) instead of being undefined.
4379 Bison has always permitted actions such as { $$ = $1 }: it adds the
4380 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4381 is no longer output: one has to write { $$ = $1; }.
4383 ** Better C++ compliance
4384 The output parsers try to respect C++ namespaces.
4385 [This turned out to be a failed experiment, and it was reverted later.]
4388 Fixed bugs when reporting useless nonterminals.
4391 The parsers work properly on 64 bit hosts.
4394 Some calls to strerror resulted in scrambled or missing error messages.
4397 When the number of shift/reduce conflicts is correct, don't issue
4400 ** The verbose report includes the rule line numbers.
4402 ** Rule line numbers are fixed in traces.
4404 ** Swedish translation
4407 Verbose parse error messages from the parsers are better looking.
4408 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4409 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4411 ** Fixed parser memory leaks.
4412 When the generated parser was using malloc to extend its stacks, the
4413 previous allocations were not freed.
4415 ** Fixed verbose output file.
4416 Some newlines were missing.
4417 Some conflicts in state descriptions were missing.
4419 ** Fixed conflict report.
4420 Option -v was needed to get the result.
4424 Mismatches are errors, not warnings.
4426 ** Fixed incorrect processing of some invalid input.
4428 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4430 ** Fixed some typos in the documentation.
4432 ** %token MY_EOF 0 is supported.
4433 Before, MY_EOF was silently renumbered as 257.
4435 ** doc/refcard.tex is updated.
4437 ** %output, %file-prefix, %name-prefix.
4441 New, aliasing "--output-file".
4444 * Noteworthy changes in release 1.30 (2001-10-26)
4446 ** "--defines" and "--graph" have now an optional argument which is the
4447 output file name. "-d" and "-g" do not change; they do not take any
4450 ** "%source_extension" and "%header_extension" are removed, failed
4453 ** Portability fixes.
4456 * Noteworthy changes in release 1.29 (2001-09-07)
4458 ** The output file does not define const, as this caused problems when used
4459 with common autoconfiguration schemes. If you still use ancient compilers
4460 that lack const, compile with the equivalent of the C compiler option
4461 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4463 ** Added "-g" and "--graph".
4465 ** The Bison manual is now distributed under the terms of the GNU FDL.
4467 ** The input and the output files has automatically a similar extension.
4469 ** Russian translation added.
4471 ** NLS support updated; should hopefully be less troublesome.
4473 ** Added the old Bison reference card.
4475 ** Added "--locations" and "%locations".
4477 ** Added "-S" and "--skeleton".
4479 ** "%raw", "-r", "--raw" is disabled.
4481 ** Special characters are escaped when output. This solves the problems
4482 of the #line lines with path names including backslashes.
4485 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4486 "%debug", "%source_extension" and "%header_extension".
4489 Automatic location tracking.
4492 * Noteworthy changes in release 1.28 (1999-07-06)
4494 ** Should compile better now with K&R compilers.
4498 ** Fixed a problem with escaping the double quote character.
4500 ** There is now a FAQ.
4503 * Noteworthy changes in release 1.27
4505 ** The make rule which prevented bison.simple from being created on
4506 some systems has been fixed.
4509 * Noteworthy changes in release 1.26
4511 ** Bison now uses Automake.
4513 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4515 ** Token numbers now start at 257 as previously documented, not 258.
4517 ** Bison honors the TMPDIR environment variable.
4519 ** A couple of buffer overruns have been fixed.
4521 ** Problems when closing files should now be reported.
4523 ** Generated parsers should now work even on operating systems which do
4524 not provide alloca().
4527 * Noteworthy changes in release 1.25 (1995-10-16)
4529 ** Errors in the input grammar are not fatal; Bison keeps reading
4530 the grammar file, and reports all the errors found in it.
4532 ** Tokens can now be specified as multiple-character strings: for
4533 example, you could use "<=" for a token which looks like <=, instead
4534 of choosing a name like LESSEQ.
4536 ** The %token_table declaration says to write a table of tokens (names
4537 and numbers) into the parser file. The yylex function can use this
4538 table to recognize multiple-character string tokens, or for other
4541 ** The %no_lines declaration says not to generate any #line preprocessor
4542 directives in the parser file.
4544 ** The %raw declaration says to use internal Bison token numbers, not
4545 Yacc-compatible token numbers, when token names are defined as macros.
4547 ** The --no-parser option produces the parser tables without including
4548 the parser engine; a project can now use its own parser engine.
4549 The actions go into a separate file called NAME.act, in the form of
4550 a switch statement body.
4553 * Noteworthy changes in release 1.23
4555 The user can define YYPARSE_PARAM as the name of an argument to be
4556 passed into yyparse. The argument should have type void *. It should
4557 actually point to an object. Grammar actions can access the variable
4558 by casting it to the proper pointer type.
4560 Line numbers in output file corrected.
4563 * Noteworthy changes in release 1.22
4565 --help option added.
4568 * Noteworthy changes in release 1.20
4570 Output file does not redefine const for C++.
4574 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4575 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4576 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4577 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4578 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4579 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4580 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4581 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4582 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4583 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4584 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4585 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4586 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4587 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4588 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4589 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4590 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4591 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4592 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4593 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4594 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4595 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4596 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4597 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4598 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4599 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4600 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4601 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4602 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4603 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4604 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4605 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4606 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4607 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4608 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4609 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4610 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4613 ispell-dictionary: "american"
4618 Copyright (C) 1995-2015, 2018-2020 Free Software Foundation, Inc.
4620 This file is part of Bison, the GNU Parser Generator.
4622 Permission is granted to copy, distribute and/or modify this document
4623 under the terms of the GNU Free Documentation License, Version 1.3 or
4624 any later version published by the Free Software Foundation; with no
4625 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4626 Texts. A copy of the license is included in the "GNU Free
4627 Documentation License" file as part of this distribution.