3 * Noteworthy changes in release ?.? (????-??-??) [?]
7 *** C-language parsers now work around quirks of HP-UX 11.23 (2003).
9 * Noteworthy changes in release 3.7.4 (2020-11-14) [stable]
13 *** Bug fixes in yacc.c
15 In Yacc mode, all the tokens are defined twice: once as an enum, and then
16 as a macro. YYEMPTY was missing its macro.
18 *** Bug fixes in lalr1.cc
20 The lalr1.cc skeleton used to emit internal assertions (using YY_ASSERT)
21 even when the `parse.assert` %define variable is not enabled. It no
24 The private internal macro YY_ASSERT now obeys the `api.prefix` %define
27 When there is a very large number of tokens, some assertions could be long
28 enough to hit arbitrary limits in Visual C++. They have been rewritten to
29 work around this limitation.
33 The YYBISON macro in generated "regular C parsers" (from the "yacc.c"
34 skeleton) used to be defined to 1. It is now defined to the version of
35 Bison as an integer (e.g., 30704 for version 3.7.4).
37 * Noteworthy changes in release 3.7.3 (2020-10-13) [stable]
41 Fix concurrent build issues.
43 The bison executable is no longer linked uselessly against libreadline.
45 Fix incorrect use of yytname in glr.cc.
47 * Noteworthy changes in release 3.7.2 (2020-09-05) [stable]
49 This release of Bison fixes all known bugs reported for Bison in MITRE's
50 Common Vulnerabilities and Exposures (CVE) system. These vulnerabilities
51 are only about bison-the-program itself, not the generated code.
53 Although these bugs are typically irrelevant to how Bison is used, they
54 are worth fixing if only to give users peace of mind.
56 There is no known vulnerability in the generated parsers.
60 Fix concurrent build issues (introduced in Bison 3.5).
62 Push parsers always use YYMALLOC/YYFREE (no direct calls to malloc/free).
64 Fix portability issues of the test suite, and of bison itself.
66 Some unlikely crashes found by fuzzing have been fixed. This is only
67 about bison itself, not the generated parsers.
70 * Noteworthy changes in release 3.7.1 (2020-08-02) [stable]
74 Crash when a token alias contains a NUL byte.
76 Portability issues with libtextstyle.
78 Portability issues of Bison itself with MSVC.
82 Improvements and fixes in the documentation.
84 More precise location about symbol type redefinitions.
87 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
89 ** Deprecated features
91 The YYPRINT macro, which works only with yacc.c and only for tokens, was
92 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
93 It is deprecated and its support will be removed eventually.
95 In conformance with the recommendations of the Graphviz team, in the next
96 version Bison the option `--graph` will generate a *.gv file by default,
97 instead of *.dot. A transition started in Bison 3.4.
101 *** Counterexample Generation
103 Contributed by Vincent Imbimbo.
105 When given `-Wcounterexamples`/`-Wcex`, bison will now output
106 counterexamples for conflicts.
108 **** Unifying Counterexamples
110 Unifying counterexamples are strings which can be parsed in two ways due
111 to the conflict. For example on a grammar that contains the usual
112 "dangling else" ambiguity:
115 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
116 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
119 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
120 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
121 Example: "if" exp "then" "if" exp "then" exp • "else" exp
124 ↳ "if" exp "then" exp
125 ↳ "if" exp "then" exp • "else" exp
126 Example: "if" exp "then" "if" exp "then" exp • "else" exp
129 ↳ "if" exp "then" exp "else" exp
130 ↳ "if" exp "then" exp •
132 When text styling is enabled, colors are used in the examples and the
133 derivations to highlight the structure of both analyses. In this case,
135 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
139 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
142 The counterexamples are "focused", in two different ways. First, they do
143 not clutter the output with all the derivations from the start symbol,
144 rather they start on the "conflicted nonterminal". They go straight to the
145 point. Second, they don't "expand" nonterminal symbols uselessly.
147 **** Nonunifying Counterexamples
149 In the case of the dangling else, Bison found an example that can be
150 parsed in two ways (therefore proving that the grammar is ambiguous).
151 When it cannot find such an example, it instead generates two examples
152 that are the same up until the dot:
155 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
156 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
157 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
162 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
163 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
164 First example: expr • ID ',' ID $end
171 Second example: expr • ID $end
177 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
181 In these cases, the parser usually doesn't have enough lookahead to
182 differentiate the two given examples.
186 Counterexamples are also included in the report when given
187 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
192 1 exp: "if" exp "then" exp • [$end, "then", "else"]
193 2 | "if" exp "then" exp • "else" exp
195 "else" shift, and go to state 8
197 "else" [reduce using rule 1 (exp)]
198 $default reduce using rule 1 (exp)
200 shift/reduce conflict on token "else":
201 1 exp: "if" exp "then" exp •
202 2 exp: "if" exp "then" exp • "else" exp
203 Example: "if" exp "then" "if" exp "then" exp • "else" exp
206 ↳ "if" exp "then" exp
207 ↳ "if" exp "then" exp • "else" exp
208 Example: "if" exp "then" "if" exp "then" exp • "else" exp
211 ↳ "if" exp "then" exp "else" exp
212 ↳ "if" exp "then" exp •
214 *** File prefix mapping
216 Contributed by Joshua Watt.
218 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
219 in the output (specifically `#line` directives and `#ifdef` header guards)
220 that begins with the prefix OLD will have it replaced with the prefix NEW,
221 similar to the `-ffile-prefix-map` in GCC. This option can be used to
222 make bison output reproducible.
228 When text styling is enabled and the terminal supports it, the warnings
229 now include hyperlinks to the documentation.
231 *** Relocatable installation
233 When installed to be relocatable (via `configure --enable-relocatable`),
234 bison will now also look for a relocated m4.
238 The `filename_type` %define variable was renamed `api.filename.type`.
241 %define filename_type "symbol"
245 %define api.filename.type {symbol}
247 (Or let `bison --update` do it for you).
249 It now defaults to `const std::string` instead of `std::string`.
251 *** Deprecated %define variable names
253 The following variables have been renamed for consistency. Backward
254 compatibility is ensured, but upgrading is recommended.
256 filename_type -> api.filename.type
257 package -> api.package
259 *** Push parsers no longer clear their state when parsing is finished
261 Previously push-parsers cleared their state when parsing was finished (on
262 success and on failure). This made it impossible to check if there were
263 parse errors, since `yynerrs` was also reset. This can be especially
264 troublesome when used in autocompletion, since a parser with error
265 recovery would suggest (irrelevant) expected tokens even if there were
268 Now the parser state can be examined when parsing is finished. The parser
269 state is reset when starting a new parse.
275 The bistromathic demonstrates %param and how to quote sources in the error
279 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
285 *** Include the generated header (yacc.c)
287 Historically, when --defines was used, bison generated a header and pasted
288 an exact copy of it into the generated parser implementation file. Since
289 Bison 3.4 it is possible to specify that the header should be `#include`d,
290 and how. For instance
292 %define api.header.include {"parse.h"}
296 %define api.header.include {<parser/parse.h>}
298 Now api.header.include defaults to `"header-basename"`, as was intended in
299 Bison 3.4, where `header-basename` is the basename of the generated
300 header. This is disabled when the generated header is `y.tab.h`, to
301 comply with Automake's ylwrap.
303 *** String aliases are faithfully propagated
305 Bison used to interpret user strings (i.e., decoding backslash escapes)
306 when reading them, and to escape them (i.e., issue non-printable
307 characters as backslash escapes, taking the locale into account) when
308 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
309 up "ciphered" as sequences of backslash escapes. This happened not only
310 in the generated sources (where the compiler will reinterpret them), but
311 also in all the generated reports (text, xml, html, dot, etc.). Reports
312 were therefore not readable when string aliases were not pure ASCII.
313 Worse yet: the output depended on the user's locale.
315 Now Bison faithfully treats the string aliases exactly the way the user
316 spelled them. This fixes all the aforementioned problems. However, now,
317 string aliases semantically equivalent but syntactically different (e.g.,
318 "A", "\x41", "\101") are considered to be different.
320 *** Crash when generating IELR
322 An old, well hidden, bug in the generation of IELR parsers was fixed.
325 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
329 In glr.cc some internal macros leaked in the user's code, and could damage
330 access to the token kinds.
333 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
337 Incorrect comments in the generated parsers.
339 Warnings in push parsers (yacc.c).
341 Incorrect display of gotos in LAC traces (lalr1.cc).
344 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
348 Some tests were fixed.
350 When token aliases contain comment delimiters:
352 %token FOO "/* foo */"
354 bison used to emit "nested" comments, which is invalid C.
357 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
361 Restored ANSI-C compliance in yacc.c.
363 GNU readline portability issues.
365 In C++, yy::parser::symbol_name is now a public member, as was intended.
369 In C++, yy::parser::symbol_type now has a public name() member function.
372 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
374 ** Backward incompatible changes
376 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
378 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
379 depend on it will now produce Yacc-like error messages (just "syntax
380 error"). It was superseded by the "%error-verbose" directive in Bison
381 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
382 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
383 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
384 parse.error verbose".
386 ** Deprecated features
388 The YYPRINT macro, which works only with yacc.c and only for tokens, was
389 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
390 It is deprecated and its support will be removed eventually.
394 *** Improved syntax error messages
396 Two new values for the %define parse.error variable offer more control to
397 the user. Available in all the skeletons (C, C++, Java).
399 **** %define parse.error detailed
401 The behavior of "%define parse.error detailed" is closely resembling that
402 of "%define parse.error verbose" with a few exceptions. First, it is safe
403 to use non-ASCII characters in token aliases (with 'verbose', the result
404 depends on the locale with which bison was run). Second, a yysymbol_name
405 function is exposed to the user, instead of the yytnamerr function and the
406 yytname table. Third, token internationalization is supported (see
409 **** %define parse.error custom
411 With this directive, the user forges and emits the syntax error message
412 herself by defining the yyreport_syntax_error function. A new type,
413 yypcontext_t, captures the circumstances of the error, and provides the
414 user with functions to get details, such as yypcontext_expected_tokens to
415 get the list of expected token kinds.
417 A possible implementation of yyreport_syntax_error is:
420 yyreport_syntax_error (const yypcontext_t *ctx)
423 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
424 fprintf (stderr, ": syntax error");
425 // Report the tokens expected at this point.
427 enum { TOKENMAX = 10 };
428 yysymbol_kind_t expected[TOKENMAX];
429 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
431 // Forward errors to yyparse.
434 for (int i = 0; i < n; ++i)
435 fprintf (stderr, "%s %s",
436 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
438 // Report the unexpected token.
440 yysymbol_kind_t lookahead = yypcontext_token (ctx);
441 if (lookahead != YYSYMBOL_YYEMPTY)
442 fprintf (stderr, " before %s", yysymbol_name (lookahead));
444 fprintf (stderr, "\n");
448 **** Token aliases internationalization
450 When the %define variable parse.error is set to `custom` or `detailed`,
451 one may specify which token aliases are to be translated using _(). For
463 In that case the user must define _() and N_(), and yysymbol_name returns
464 the translated symbol (i.e., it returns '_("variable")' rather that
465 '"variable"'). In Java, the user must provide an i18n() function.
467 *** List of expected tokens (yacc.c)
469 Push parsers may invoke yypstate_expected_tokens at any point during
470 parsing (including even before submitting the first token) to get the list
471 of possible tokens. This feature can be used to propose autocompletion
472 (see below the "bistromathic" example).
474 It makes little sense to use this feature without enabling LAC (lookahead
477 *** Returning the error token
479 When the scanner returns an invalid token or the undefined token
480 (YYUNDEF), the parser generates an error message and enters error
481 recovery. Because of that error message, most scanners that find lexical
482 errors generate an error message, and then ignore the invalid input
483 without entering the error-recovery.
485 The scanners may now return YYerror, the error token, to enter the
486 error-recovery mode without triggering an additional error message. See
487 the bistromathic for an example.
489 *** Deep overhaul of the symbol and token kinds
491 To avoid the confusion with types in programming languages, we now refer
492 to token and symbol "kinds" instead of token and symbol "types". The
493 documentation and error messages have been revised.
495 All the skeletons have been updated to use dedicated enum types rather
496 than integral types. Special symbols are now regular citizens, instead of
497 being declared in ad hoc ways.
501 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
502 LPAREN, etc. While backward compatibility is of course ensured, users are
503 nonetheless invited to replace their uses of "enum yytokentype" by
506 This type now also includes tokens that were previously hidden: YYEOF (end
507 of input), YYUNDEF (undefined token), and YYerror (error token). They
508 now have string aliases, internationalized when internationalization is
509 enabled. Therefore, by default, error messages now refer to "end of file"
510 (internationalized) rather than the cryptic "$end", or to "invalid token"
511 rather than "$undefined".
513 Therefore in most cases it is now useless to define the end-of-line token
516 %token T_EOF 0 "end of file"
518 Rather simply use "YYEOF" in your scanner.
522 The "symbol kinds" is what the parser actually uses. (Unless the
523 api.token.raw %define variable is used, the symbol kind of a terminal
524 differs from the corresponding token kind.)
526 They are now exposed as a enum, "yysymbol_kind_t".
528 This allows users to tailor the error messages the way they want, or to
529 process some symbols in a specific way in autocompletion (see the
530 bistromathic example below).
532 *** Modernize display of explanatory statements in diagnostics
534 Since Bison 2.7, output was indented four spaces for explanatory
535 statements. For example:
537 input.y:2.7-13: error: %type redeclaration for exp
538 input.y:1.7-11: previous declaration
540 Since the introduction of caret-diagnostics, it became less clear. This
541 indentation has been removed and submessages are displayed similarly as in
544 input.y:2.7-13: error: %type redeclaration for exp
545 2 | %type <float> exp
547 input.y:1.7-11: note: previous declaration
551 Contributed by Victor Morales Cayuela.
555 The token and symbol kinds are yy::parser::token_kind_type and
556 yy::parser::symbol_kind_type.
558 The symbol_type::kind() member function allows to get the kind of a
559 symbol. This can be used to write unit tests for scanners, e.g.,
561 yy::parser::symbol_type t = make_NUMBER ("123");
562 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
563 assert (t.value.as<int> () == 123);
569 In order to avoid ambiguities with "type" as in "typing", we now refer to
570 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
571 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
576 There are now examples/java: a very simple calculator, and a more complete
577 one (push-parser, location tracking, and debug traces).
579 The lexcalc example (a simple example in C based on Flex and Bison) now
580 also demonstrates location tracking.
583 A new C example, bistromathic, is a fully featured interactive calculator
584 using many Bison features: pure interface, push parser, autocompletion
585 based on the current parser state (using yypstate_expected_tokens),
586 location tracking, internationalized custom error messages, lookahead
587 correction, rich debug traces, etc.
589 It shows how to depend on the symbol kinds to tailor autocompletion. For
590 instance it recognizes the symbol kind "VARIABLE" to propose
591 autocompletion on the existing variables, rather than of the word
595 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
597 ** WARNING: Future backward-incompatibilities!
599 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
601 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
602 that still depend on it will produce Yacc-like error messages (just
603 "syntax error"). It was superseded by the "%error-verbose" directive in
604 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
605 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
606 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
607 parse.error verbose".
611 Fix portability issues of the package itself on old compilers.
613 Fix api.token.raw support in Java.
616 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
620 Error messages could quote lines containing zero-width characters (such as
621 \005) with incorrect styling. Fixes for similar issues with unexpectedly
622 short lines (e.g., the file was changed between parsing and diagnosing).
624 Some unlikely crashes found by fuzzing have been fixed. This is only
625 about bison itself, not the generated parsers.
628 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
632 Portability issues and minor cosmetic issues.
634 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
638 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
644 Fix compiler warnings.
647 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
649 ** Backward incompatible changes
651 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
652 longer treated as end-of-lines. This changes the diagnostics, and in
653 particular their locations.
655 In C++, line numbers and columns are now represented as 'int' not
656 'unsigned', so that integer overflow on positions is easily checkable via
657 'gcc -fsanitize=undefined' and the like. This affects the API for
658 positions. The default position and location classes now expose
659 'counter_type' (int), used to define line and column numbers.
661 ** Deprecated features
663 The YYPRINT macro, which works only with yacc.c and only for tokens, was
664 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
665 It is deprecated and its support will be removed eventually.
669 *** Lookahead correction in C++
671 Contributed by Adrian Vogelsgesang.
673 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
674 %define variable parse.lac.
676 *** Variable api.token.raw: Optimized token numbers (all skeletons)
678 In the generated parsers, tokens have two numbers: the "external" token
679 number as returned by yylex (which starts at 257), and the "internal"
680 symbol number (which starts at 3). Each time yylex is called, a table
681 lookup maps the external token number to the internal symbol number.
683 When the %define variable api.token.raw is set, tokens are assigned their
684 internal number, which saves one table lookup per token, and also saves
685 the generation of the mapping table.
687 The gain is typically moderate, but in extreme cases (very simple user
688 actions), a 10% improvement can be observed.
690 *** Generated parsers use better types for states
692 Stacks now use the best integral type for state numbers, instead of always
693 using 15 bits. As a result "small" parsers now have a smaller memory
694 footprint (they use 8 bits), and there is support for large automata (16
695 bits), and extra large (using int, i.e., typically 31 bits).
697 *** Generated parsers prefer signed integer types
699 Bison skeletons now prefer signed to unsigned integer types when either
700 will do, as the signed types are less error-prone and allow for better
701 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
702 portable to unusual machines where char, short and int are all the same
703 width. On non-GNU platforms this may entail including <limits.h> and (if
704 available) <stdint.h> to define integer types and constants.
706 *** A skeleton for the D programming language
708 For the last few releases, Bison has shipped a stealth experimental
709 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
710 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
713 However, because nobody has committed to improving, testing, and
714 documenting this skeleton, it is not clear that it will be supported in
717 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
718 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
720 *** Debug traces in Java
722 The Java backend no longer emits code and data for parser tracing if the
723 %define variable parse.trace is not defined.
727 *** New diagnostic: -Wdangling-alias
729 String literals, which allow for better error messages, are (too)
730 liberally accepted by Bison, which might result in silent errors. For
733 %type <exVal> cond "condition"
735 does not define "condition" as a string alias to 'cond' (nonterminal
736 symbols do not have string aliases). It is rather equivalent to
739 %token <exVal> "condition"
741 i.e., it gives the type 'exVal' to the "condition" token, which was
742 clearly not the intention.
744 Also, because string aliases need not be defined, typos such as "baz"
745 instead of "bar" will be not reported.
747 The option -Wdangling-alias catches these situations. On
750 %type <ival> foo "foo"
754 bison -Wdangling-alias reports
756 warning: string literal not attached to a symbol
757 | %type <ival> foo "foo"
759 warning: string literal not attached to a symbol
763 The -Wall option does not (yet?) include -Wdangling-alias.
765 *** Better POSIX Yacc compatibility diagnostics
767 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
771 %type <ival> TOKEN1 TOKEN2 't'
778 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
779 2 | %type <ival> TOKEN1 TOKEN2 't'
781 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
782 2 | %type <ival> TOKEN1 TOKEN2 't'
784 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
785 2 | %type <ival> TOKEN1 TOKEN2 't'
788 *** Diagnostics with insertion
790 The diagnostics now display the suggestion below the underlined source.
791 Replacement for undeclared symbols are now also suggested.
798 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'?
802 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
806 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
808 *** Diagnostics about long lines
810 Quoted sources may now be truncated to fit the screen. For instance, on a
811 30-column wide terminal:
818 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
821 foo.y:1.8-10: previous declaration
824 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
827 foo.y:1.8-10: previous declaration
833 *** Debugging glr.c and glr.cc
835 The glr.c skeleton always had asserts to check its own behavior (not the
836 user's). These assertions are now under the control of the parse.assert
837 %define variable (disabled by default).
841 Several new compiler warnings in the generated output have been avoided.
842 Some unused features are no longer emitted. Cleaner generated code in
847 Portability issues in the test suite.
849 In theory, parsers using %nonassoc could crash when reporting verbose
850 error messages. This unlikely bug has been fixed.
852 In Java, %define api.prefix was ignored. It now behaves as expected.
855 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
859 In some cases, when warnings are disabled, bison could emit tons of white
860 spaces as diagnostics.
862 When running out of memory, bison could crash (found by fuzzing).
864 When defining twice the EOF token, bison would crash.
866 New warnings from recent compilers have been addressed in the generated
867 parsers (yacc.c, glr.c, glr.cc).
869 When lone carriage-return characters appeared in the input file,
870 diagnostics could hang forever.
873 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
880 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
882 ** Deprecated features
884 The %pure-parser directive is deprecated in favor of '%define api.pure'
885 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
886 now. Note that since Bison 2.7 you are strongly encouraged to use
887 '%define api.pure full' instead of '%define api.pure'.
891 *** Colored diagnostics
893 As an experimental feature, diagnostics are now colored, controlled by the
894 new options --color and --style.
896 To use them, install the libtextstyle library before configuring Bison.
899 https://alpha.gnu.org/gnu/gettext/
903 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
905 The option --color supports the following arguments:
906 - always, yes: Enable colors.
907 - never, no: Disable colors.
908 - auto, tty (default): Enable colors if the output device is a tty.
910 To customize the styles, create a CSS file similar to
914 .error { font-weight: 800; text-decoration: underline; }
917 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
918 environment variable to "bison-bw.css".
922 When given -fsyntax-only, the diagnostics are reported, but no output is
925 The name of this option is somewhat misleading as bison does more than
926 just checking the syntax: every stage is run (including checking for
927 conflicts for instance), except the generation of the output files.
929 *** Include the generated header (yacc.c)
931 Before, when --defines is used, bison generated a header, and pasted an
932 exact copy of it into the generated parser implementation file. If the
933 header name is not "y.tab.h", it is now #included instead of being
936 To use an '#include' even if the header name is "y.tab.h" (which is what
937 happens with --yacc, or when using the Autotools' ylwrap), define
938 api.header.include to the exact argument to pass to #include. For
941 %define api.header.include {"parse.h"}
945 %define api.header.include {<parser/parse.h>}
947 *** api.location.type is now supported in C (yacc.c, glr.c)
949 The %define variable api.location.type defines the name of the type to use
950 for locations. When defined, Bison no longer defines YYLTYPE.
952 This can be used in programs with several parsers to factor their
953 definition of locations: let one of them generate them, and the others
960 In conformance with the recommendations of the Graphviz team, if %require
961 "3.4" (or better) is specified, the option --graph generates a *.gv file
962 by default, instead of *.dot.
964 *** Diagnostics overhaul
966 Column numbers were wrong with multibyte characters, which would also
967 result in skewed diagnostics with carets. Beside, because we were
968 indenting the quoted source with a single space, lines with tab characters
969 were incorrectly underlined.
971 To address these issues, and to be clearer, Bison now issues diagnostics
972 as GCC9 does. For instance it used to display (there's a tab before the
975 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
976 expr: expr '+' "number" { $$ = $1 + $2; }
980 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
981 3 | expr: expr '+' "number" { $$ = $1 + $2; }
984 Other constructs now also have better locations, resulting in more precise
987 *** Fix-it hints for %empty
989 Running Bison with -Wempty-rules and --update will remove incorrect %empty
990 annotations, and add the missing ones.
992 *** Generated reports
994 The format of the reports (parse.output) was improved for readability.
996 *** Better support for --no-line.
998 When --no-line is used, the generated files are now cleaner: no lines are
999 generated instead of empty lines. Together with using api.header.include,
1000 that should help people saving the generated files into version control
1001 systems get smaller diffs.
1005 A new example in C shows an simple infix calculator with a hand-written
1006 scanner (examples/c/calc).
1008 A new example in C shows a reentrant parser (capable of recursive calls)
1009 built with Flex and Bison (examples/c/reccalc).
1011 There is a new section about the history of Yaccs and Bison.
1015 A few obscure bugs were fixed, including the second oldest (known) bug in
1016 Bison: it was there when Bison was entered in the RCS version control
1017 system, in December 1987. See the NEWS of Bison 3.3 for the previous
1021 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
1025 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
1029 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
1033 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
1034 extensions into errors. It now makes them simple warnings (-Wyacc).
1037 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
1039 A new mailing list was created, Bison Announce. It is low traffic, and is
1040 only about announcing new releases and important messages (e.g., polls
1041 about major decisions to make).
1043 https://lists.gnu.org/mailman/listinfo/bison-announce
1045 ** Backward incompatible changes
1047 Support for DJGPP, which has been unmaintained and untested for years, is
1050 ** Deprecated features
1052 A new feature, --update (see below) helps adjusting existing grammars to
1055 *** Deprecated directives
1057 The %error-verbose directive is deprecated in favor of '%define
1058 parse.error verbose' since Bison 3.0, but no warning was issued.
1060 The '%name-prefix "xx"' directive is deprecated in favor of '%define
1061 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
1062 directives are slightly different, you might need to adjust your code.
1063 %name-prefix renames only symbols with external linkage, while api.prefix
1064 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
1065 yytokentype, YYSTYPE, YYLTYPE, etc.
1067 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
1068 {xx}' will typically have to update YY_DECL from
1070 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
1074 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
1076 *** Deprecated %define variable names
1078 The following variables, mostly related to parsers in Java, have been
1079 renamed for consistency. Backward compatibility is ensured, but upgrading
1082 abstract -> api.parser.abstract
1083 annotations -> api.parser.annotations
1084 extends -> api.parser.extends
1085 final -> api.parser.final
1086 implements -> api.parser.implements
1087 parser_class_name -> api.parser.class
1088 public -> api.parser.public
1089 strictfp -> api.parser.strictfp
1093 *** Generation of fix-its for IDEs/Editors
1095 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1096 bison now generates machine readable editing instructions to fix some
1097 issues. Currently, this is mostly limited to updating deprecated
1098 directives and removing duplicates. For instance:
1102 %define parser_class_name "Parser"
1103 %define api.parser.class "Parser"
1107 See the "fix-it:" lines below:
1109 $ bison -ffixit foo.y
1110 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1113 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1114 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1115 %define parser_class_name "Parser"
1116 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1117 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1118 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1119 %define api.parser.class "Parser"
1120 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1121 foo.y:2.1-34: previous definition
1122 %define parser_class_name "Parser"
1123 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1124 fix-it:"foo.y":{3:1-3:34}:""
1125 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1127 This uses the same output format as GCC and Clang.
1129 *** Updating grammar files
1131 Fixes can be applied on the fly. The previous example ends with the
1132 suggestion to re-run bison with the option -u/--update, which results in a
1133 cleaner grammar file.
1135 $ bison --update foo.y
1137 bison: file 'foo.y' was updated (backup: 'foo.y~')
1140 %define parse.error verbose
1141 %define api.parser.class {Parser}
1145 *** Bison is now relocatable
1147 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1149 A relocatable program can be moved or copied to a different location on
1150 the file system. It can also be used through mount points for network
1151 sharing. It is possible to make symbolic links to the installed and moved
1152 programs, and invoke them through the symbolic link.
1154 *** %expect and %expect-rr modifiers on individual rules
1156 One can now document (and check) which rules participate in shift/reduce
1157 and reduce/reduce conflicts. This is particularly important GLR parsers,
1158 where conflicts are a normal occurrence. For example,
1174 | argument_list ',' expression
1179 Looking at the output from -v, one can see that the shift/reduce conflict
1180 here is due to the fact that the parser does not know whether to reduce
1181 arguments to argument_list until it sees the token _after_ the following
1182 ','. By marking the rule with %expect 1 (because there is a conflict in
1183 one state), we document the source of the 1 overall shift/reduce conflict.
1185 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1186 conflicts. In this case, we mark each of the conflicting rules. For
1195 target_list '=' expr ';'
1201 | target ',' target_list
1210 | expr ',' expr_list
1218 In a statement such as
1222 the parser must reduce x to a target or an expr, but does not know which
1223 until it sees the '='. So we notate the two possible reductions to
1224 indicate that each conflicts in one rule.
1226 This feature needs user feedback, and might evolve in the future.
1228 *** C++: Actual token constructors
1230 When variants and token constructors are enabled, in addition to the
1231 type-safe named token constructors (make_ID, make_INT, etc.), we now
1232 generate genuine constructors for symbol_type.
1234 For instance with these declarations
1240 you may use these constructors:
1242 symbol_type (int token, const std::string&);
1243 symbol_type (int token, const int&);
1244 symbol_type (int token);
1246 Correct matching between token types and value types is checked via
1247 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1248 constructors are preferable, as they offer better type safety (for
1249 instance 'make_ID (42)' would not even compile), but symbol_type
1250 constructors may help when token types are discovered at run-time, e.g.,
1253 if (auto i = lookup_keyword (yytext))
1254 return yy::parser::symbol_type (i);
1256 return yy::parser::make_ID (yytext);
1259 *** C++: Variadic emplace
1261 If your application requires C++11 and you don't use symbol constructors,
1262 you may now use a variadic emplace for semantic values:
1264 %define api.value.type variant
1265 %token <std::pair<int, int>> PAIR
1269 int yylex (parser::semantic_type *lvalp)
1271 lvalp->emplace <std::pair<int, int>> (1, 2);
1272 return parser::token::PAIR;
1275 *** C++: Syntax error exceptions in GLR
1277 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1278 actions, or from the scanner.
1280 *** More POSIX Yacc compatibility warnings
1282 More Bison specific directives are now reported with -y or -Wyacc. This
1283 change was ready since the release of Bison 3.0 in September 2015. It was
1284 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1285 in numerous warnings for Bison users that use the GNU Build System.
1287 If you still experience that problem, either redefine YACC as `bison -o
1288 y.tab.c`, or pass -Wno-yacc to Bison.
1290 *** The tables yyrhs and yyphrs are back
1292 Because no Bison skeleton uses them, these tables were removed (no longer
1293 passed to the skeletons, not even computed) in 2008. However, some users
1294 have expressed interest in being able to use them in their own skeletons.
1298 *** Incorrect number of reduce/reduce conflicts
1300 On a grammar such as
1302 exp: "num" | "num" | "num"
1304 bison used to report a single RR conflict, instead of two. This is now
1305 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1306 was entered in the RCS version control system, in December 1987.
1308 Some grammar files might have to adjust their %expect-rr.
1310 *** Parser directives that were not careful enough
1312 Passing invalid arguments to %nterm, for instance character literals, used
1313 to result in unclear error messages.
1317 The examples/ directory (installed in .../share/doc/bison/examples) has
1318 been restructured per language for clarity. The examples come with a
1319 README and a Makefile. Not only can they be used to toy with Bison, they
1320 can also be starting points for your own grammars.
1322 There is now a Java example, and a simple example in C based on Flex and
1323 Bison (examples/c/lexcalc/).
1329 They now use noexcept and constexpr. Please, report missing annotations.
1331 *** Symbol Declarations
1333 The syntax of the variation directives to declare symbols was overhauled
1334 for more consistency, and also better POSIX Yacc compliance (which, for
1335 instance, allows "%type" without actually providing a type). The %nterm
1336 directive, supported by Bison since its inception, is now documented and
1337 officially supported.
1339 The syntax is now as follows:
1341 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1342 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1343 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1344 %nterm TAG? ID+ ( TAG ID+ )*
1346 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1347 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1348 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1349 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1350 one), ‘*’ (zero or more) and ‘+’ (one or more).
1353 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1357 Fix the move constructor of symbol_type.
1359 Always provide a copy constructor for symbol_type, even in modern C++.
1362 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1366 Properly support token constructors in C++ with types that include commas
1367 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1370 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1374 C++ portability issues.
1377 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1381 Several portability issues have been fixed in the build system, in the
1382 test suite, and in the generated parsers in C++.
1385 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1387 ** Backward incompatible changes
1389 Support for DJGPP, which has been unmaintained and untested for years, is
1390 obsolete. Unless there is activity to revive it, it will be removed.
1394 %printers should use yyo rather than yyoutput to denote the output stream.
1396 Variant-based symbols in C++ should use emplace() rather than build().
1398 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1402 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1404 A comment in the generated code now emphasizes that users should not
1405 depend upon non-documented implementation details, such as macros starting
1410 *** C++: Support for move semantics (lalr1.cc)
1412 The lalr1.cc skeleton now fully supports C++ move semantics, while
1413 maintaining compatibility with C++98. You may now store move-only types
1414 when using Bison's variants. For instance:
1421 %skeleton "lalr1.cc"
1422 %define api.value.type variant
1426 %token <int> INT "int";
1427 %type <std::unique_ptr<int>> int;
1428 %type <std::vector<std::unique_ptr<int>>> list;
1432 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1434 int: "int" { $$ = std::make_unique<int>($1); }
1436 *** C++: Implicit move of right-hand side values (lalr1.cc)
1438 In modern C++ (C++11 and later), you should always use 'std::move' with
1439 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1440 popped from the stack anyway. Using 'std::move' is mandatory for
1441 move-only types such as unique_ptr, and it provides a significant speedup
1442 for large types such as std::string, or std::vector, etc.
1444 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1445 by 'std::move ($n)'. The second rule in the previous grammar can be
1448 list: list int { $$ = $1; $$.emplace_back($2); }
1450 With automove enabled, the semantic values are no longer lvalues, so do
1451 not use the swap idiom:
1453 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1455 This idiom is anyway obsolete: it is preferable to move than to swap.
1457 A warning is issued when automove is enabled, and a value is used several
1460 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1461 exp: "twice" exp { $$ = $2 + $2; }
1464 Enabling api.value.automove does not require support for modern C++. The
1465 generated code is valid C++98/03, but will use copies instead of moves.
1467 The new examples/c++/variant-11.yy shows these features in action.
1469 *** C++: The implicit default semantic action is always run
1471 When variants are enabled, the default action was not run, so
1479 It now behaves like in all the other cases, as
1481 exp: "number" { $$ = $1; }
1483 possibly using std::move if automove is enabled.
1485 We do not expect backward compatibility issues. However, beware of
1486 forward compatibility issues: if you rely on default actions with
1487 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1488 generate incorrect parsers.
1490 *** C++: Renaming location.hh
1492 When both %defines and %locations are enabled, Bison generates a
1493 location.hh file. If you don't use locations outside of the parser, you
1494 may avoid its creation with:
1496 %define api.location.file none
1498 However this file is useful if, for instance, your parser builds an AST
1499 decorated with locations: you may use Bison's location independently of
1500 Bison's parser. You can now give it another name, for instance:
1502 %define api.location.file "my-location.hh"
1504 This name can have directory components, and even be absolute. The name
1505 under which the location file is included is controlled by
1506 api.location.include.
1508 This way it is possible to have several parsers share the same location
1511 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1514 %define api.namespace {foo}
1515 %define api.location.file "include/ast/loc.hh"
1516 %define api.location.include {<ast/loc.hh>}
1518 and use it in src/bar/parser.hh:
1521 %define api.namespace {bar}
1522 %code requires {#include <ast/loc.hh>}
1523 %define api.location.type {bar::location}
1525 Absolute file names are supported, so in your Makefile, passing the flag
1526 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1529 *** C++: stack.hh and position.hh are deprecated
1531 When asked to generate a header file (%defines), the lalr1.cc skeleton
1532 generates a stack.hh file. This file had no interest for users; it is now
1533 made useless: its content is included in the parser definition. It is
1534 still generated for backward compatibility.
1536 When in addition to %defines, location support is requested (%locations),
1537 the file position.hh is also generated. It is now also useless: its
1538 content is now included in location.hh.
1540 These files are no longer generated when your grammar file requires at
1541 least Bison 3.2 (%require "3.2").
1545 Portability issues on MinGW and VS2015.
1547 Portability issues in the test suite.
1549 Portability/warning issues with Flex.
1552 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1554 ** Backward incompatible changes
1556 Compiling Bison now requires a C99 compiler---as announced during the
1557 release of Bison 3.0, five years ago. Generated parsers do not require a
1560 Support for DJGPP, which has been unmaintained and untested for years, is
1561 obsolete. Unless there is activity to revive it, the next release of Bison
1562 will have it removed.
1566 *** Typed midrule actions
1568 Because their type is unknown to Bison, the values of midrule actions are
1569 not treated like the others: they don't have %printer and %destructor
1570 support. It also prevents C++ (Bison) variants to handle them properly.
1572 Typed midrule actions address these issues. Instead of:
1574 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1578 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1580 *** Reports include the type of the symbols
1582 The sections about terminal and nonterminal symbols of the '*.output' file
1583 now specify their declared type. For instance, for:
1587 the report now shows '<ival>':
1589 Terminals, with rules where they appear
1593 *** Diagnostics about useless rules
1595 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1596 of course, its rules are useless too.
1600 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1602 Previously all the useless rules were reported, including those whose
1603 left-hand side is the 'exp' nonterminal:
1605 warning: 1 nonterminal useless in grammar [-Wother]
1606 warning: 4 rules useless in grammar [-Wother]
1607 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1610 2.14-16: warning: rule useless in grammar [-Wother]
1613 3.6-16: warning: rule useless in grammar [-Wother]
1614 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1616 3.20-30: warning: rule useless in grammar [-Wother]
1617 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1619 3.34-44: warning: rule useless in grammar [-Wother]
1620 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1623 Now, rules whose left-hand side symbol is useless are no longer reported
1624 as useless. The locations of the errors have also been adjusted to point
1625 to the first use of the nonterminal as a left-hand side of a rule:
1627 warning: 1 nonterminal useless in grammar [-Wother]
1628 warning: 4 rules useless in grammar [-Wother]
1629 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1630 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1632 2.14-16: warning: rule useless in grammar [-Wother]
1636 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1638 When compiled with exceptions disabled, the generated parsers no longer
1639 uses try/catch clauses.
1641 Currently only GCC and Clang are supported.
1645 *** A demonstration of variants
1647 A new example was added (installed in .../share/doc/bison/examples),
1648 'variant.yy', which shows how to use (Bison) variants in C++.
1650 The other examples were made nicer to read.
1652 *** Some features are no longer 'experimental'
1654 The following features, mature enough, are no longer flagged as
1655 experimental in the documentation: push parsers, default %printer and
1656 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1657 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1658 semantic predicates (%?).
1662 *** GLR: Predicates support broken by #line directives
1664 Predicates (%?) in GLR such as
1667 %? {new_syntax} 'w' id new_args
1668 | %?{!new_syntax} 'w' id old_args
1670 were issued with #lines in the middle of C code.
1672 *** Printer and destructor with broken #line directives
1674 The #line directives were not properly escaped when emitting the code for
1675 %printer/%destructor, which resulted in compiler errors if there are
1676 backslashes or double-quotes in the grammar file name.
1678 *** Portability on ICC
1680 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1681 Generated parsers now work around this.
1685 There were several small fixes in the test suite and in the build system,
1686 many warnings in bison and in the generated parsers were eliminated. The
1687 documentation also received its share of minor improvements.
1689 Useless code was removed from C++ parsers, and some of the generated
1690 constructors are more 'natural'.
1693 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1697 *** C++: Fix support of 'syntax_error'
1699 One incorrect 'inline' resulted in linking errors about the constructor of
1700 the syntax_error exception.
1702 *** C++: Fix warnings
1704 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1705 warnings about yyformat being possibly null. It also warned about the
1706 deprecated implicit definition of copy constructors when there's a
1707 user-defined (copy) assignment operator.
1709 *** Location of errors
1711 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1712 ride-hand side raises a syntax error. The behavior of the default parser
1713 (yacc.c) in such a condition was undefined.
1715 Now all the parsers match the behavior of glr.c: @$ is used as the
1716 location of the error. This handles gracefully rules with and without
1719 *** Portability fixes in the test suite
1721 On some platforms, some Java and/or C++ tests were failing.
1724 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1728 *** C++ with Variants (lalr1.cc)
1730 Fix a compiler warning when no %destructor use $$.
1734 Several portability issues in tests were fixed.
1737 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1741 *** C++ with Variants (lalr1.cc)
1743 Problems with %destructor and '%define parse.assert' have been fixed.
1745 *** Named %union support (yacc.c, glr.c)
1747 Bison 3.0 introduced a regression on named %union such as
1749 %union foo { int ival; };
1751 The possibility to use a name was introduced "for Yacc compatibility".
1752 It is however not required by POSIX Yacc, and its usefulness is not clear.
1754 *** %define api.value.type union with %defines (yacc.c, glr.c)
1756 The C parsers were broken when %defines was used together with "%define
1757 api.value.type union".
1759 *** Redeclarations are reported in proper order
1767 bison used to report:
1769 foo.yy:2.10-11: error: %printer redeclaration for FOO
1772 foo.yy:3.10-11: previous declaration
1776 Now, the "previous" declaration is always the first one.
1781 Bison now installs various files in its docdir (which defaults to
1782 '/usr/local/share/doc/bison'), including the three fully blown examples
1783 extracted from the documentation:
1786 Reverse Polish Calculator, a simple introductory example.
1788 Multi-function Calc, a calculator with memory and functions and located
1791 a calculator in C++ using variant support and token constructors.
1794 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1798 *** Generated source files when errors are reported
1800 When warnings are issued and -Werror is set, bison would still generate
1801 the source files (*.c, *.h...). As a consequence, some runs of "make"
1802 could fail the first time, but not the second (as the files were generated
1805 This is fixed: bison no longer generates this source files, but, of
1806 course, still produces the various reports (*.output, *.xml, etc.).
1808 *** %empty is used in reports
1810 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1811 dot, XML and formats derived from it).
1813 *** YYERROR and variants
1815 When C++ variant support is enabled, an error triggered via YYERROR, but
1816 not caught via error recovery, resulted in a double deletion.
1819 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1823 *** Errors in caret diagnostics
1825 On some platforms, some errors could result in endless diagnostics.
1827 *** Fixes of the -Werror option
1829 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1830 diagnostics into errors instead of warnings. This is fixed.
1832 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1833 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1834 -Wno-error", "foo" diagnostics are now errors.
1838 As demonstrated in the documentation, one can now leave spaces between
1843 The yacc.1 man page is no longer installed if --disable-yacc was
1846 *** Fixes in the test suite
1848 Bugs and portability issues.
1851 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1853 ** WARNING: Future backward-incompatibilities!
1855 Like other GNU packages, Bison will start using some of the C99 features
1856 for its own code, especially the definition of variables after statements.
1857 The generated C parsers still aim at C90.
1859 ** Backward incompatible changes
1861 *** Obsolete features
1863 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1865 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1866 use YYSTYPE and YYLTYPE.
1868 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1869 1.875): use %lex-param, %parse-param, or %param.
1871 Missing semicolons at the end of actions are no longer added (as announced
1872 in the release 2.5).
1874 *** Use of YACC='bison -y'
1876 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1879 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1880 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1881 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1883 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1884 implementation of Yacc, was using Bison as 'bison -y'. While it does
1885 ensure compatible output file names, it also enables warnings for
1886 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1887 warnings for Bison extensions.
1889 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1890 (which also generates 'y.tab.h' and 'y.output' when needed).
1891 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1892 flags (YFLAGS, or AM_YFLAGS with Automake).
1896 *** The epilogue is no longer affected by internal #defines (glr.c)
1898 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1899 generated code. These weren't properly undefined before the inclusion of
1900 the user epilogue, so functions such as the following were butchered by the
1901 preprocessor expansion:
1903 int yylex (YYSTYPE *yylval);
1905 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
1906 identifiers for user-provided variables.
1908 *** stdio.h is no longer needed when locations are enabled (yacc.c)
1910 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
1911 locations are enabled. This is fixed.
1913 *** Warnings about useless %pure-parser/%define api.pure are restored
1915 ** Diagnostics reported by Bison
1917 Most of these features were contributed by Théophile Ranquet and Victor
1922 Version 2.7 introduced caret errors, for a prettier output. These are now
1923 activated by default. The old format can still be used by invoking Bison
1924 with -fno-caret (or -fnone).
1926 Some error messages that reproduced excerpts of the grammar are now using
1927 the caret information only. For instance on:
1934 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1935 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
1939 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1940 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
1944 and "bison -fno-caret" reports:
1946 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1947 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
1949 *** Enhancements of the -Werror option
1951 The -Werror=CATEGORY option is now recognized, and will treat specified
1952 warnings as errors. The warnings need not have been explicitly activated
1953 using the -W option, this is similar to what GCC 4.7 does.
1955 For example, given the following command line, Bison will treat both
1956 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
1957 errors (and only those):
1959 $ bison -Werror=yacc,error=conflicts-sr input.y
1961 If no categories are specified, -Werror will make all active warnings into
1962 errors. For example, the following line does the same the previous example:
1964 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
1966 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
1968 Note that the categories in this -Werror option may not be prefixed with
1969 "no-". However, -Wno-error[=CATEGORY] is valid.
1971 Note that -y enables -Werror=yacc. Therefore it is now possible to require
1972 Yacc-like behavior (e.g., always generate y.tab.c), but to report
1973 incompatibilities as warnings: "-y -Wno-error=yacc".
1975 *** The display of warnings is now richer
1977 The option that controls a given warning is now displayed:
1979 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
1981 In the case of warnings treated as errors, the prefix is changed from
1982 "warning: " to "error: ", and the suffix is displayed, in a manner similar
1983 to GCC, as [-Werror=CATEGORY].
1985 For instance, where the previous version of Bison would report (and exit
1988 bison: warnings being treated as errors
1989 input.y:1.1: warning: stray ',' treated as white space
1993 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
1995 *** Deprecated constructs
1997 The new 'deprecated' warning category flags obsolete constructs whose
1998 support will be discontinued. It is enabled by default. These warnings
1999 used to be reported as 'other' warnings.
2001 *** Useless semantic types
2003 Bison now warns about useless (uninhabited) semantic types. Since
2004 semantic types are not declared to Bison (they are defined in the opaque
2005 %union structure), it is %printer/%destructor directives about useless
2006 types that trigger the warning:
2010 %printer {} <type1> <type3>
2011 %destructor {} <type2> <type4>
2013 nterm: term { $$ = $1; };
2015 3.28-34: warning: type <type3> is used, but is not associated to any symbol
2016 4.28-34: warning: type <type4> is used, but is not associated to any symbol
2018 *** Undefined but unused symbols
2020 Bison used to raise an error for undefined symbols that are not used in
2021 the grammar. This is now only a warning.
2024 %destructor {} symbol2
2025 %type <type> symbol3
2029 *** Useless destructors or printers
2031 Bison now warns about useless destructors or printers. In the following
2032 example, the printer for <type1>, and the destructor for <type2> are
2033 useless: all symbols of <type1> (token1) already have a printer, and all
2034 symbols of type <type2> (token2) already have a destructor.
2036 %token <type1> token1
2040 %printer {} token1 <type1> <type3>
2041 %destructor {} token2 <type2> <type4>
2045 The warnings and error messages about shift/reduce and reduce/reduce
2046 conflicts have been normalized. For instance on the following foo.y file:
2050 exp: exp '+' exp | '0' | '0';
2052 compare the previous version of bison:
2055 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2056 $ bison -Werror foo.y
2057 bison: warnings being treated as errors
2058 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2060 with the new behavior:
2063 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
2064 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
2065 $ bison -Werror foo.y
2066 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
2067 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
2069 When %expect or %expect-rr is used, such as with bar.y:
2074 exp: exp '+' exp | '0' | '0';
2079 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
2080 bar.y: expected 0 shift/reduce conflicts
2081 bar.y: expected 0 reduce/reduce conflicts
2086 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2087 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2089 ** Incompatibilities with POSIX Yacc
2091 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2094 ** Additional yylex/yyparse arguments
2096 The new directive %param declares additional arguments to both yylex and
2097 yyparse. The %lex-param, %parse-param, and %param directives support one
2098 or more arguments. Instead of
2100 %lex-param {arg1_type *arg1}
2101 %lex-param {arg2_type *arg2}
2102 %parse-param {arg1_type *arg1}
2103 %parse-param {arg2_type *arg2}
2107 %param {arg1_type *arg1} {arg2_type *arg2}
2109 ** Types of values for %define variables
2111 Bison used to make no difference between '%define foo bar' and '%define
2112 foo "bar"'. The former is now called a 'keyword value', and the latter a
2113 'string value'. A third kind was added: 'code values', such as '%define
2116 Keyword variables are used for fixed value sets, e.g.,
2118 %define lr.type lalr
2120 Code variables are used for value in the target language, e.g.,
2122 %define api.value.type {struct semantic_type}
2124 String variables are used remaining cases, e.g. file names.
2126 ** Variable api.token.prefix
2128 The variable api.token.prefix changes the way tokens are identified in
2129 the generated files. This is especially useful to avoid collisions
2130 with identifiers in the target language. For instance
2132 %token FILE for ERROR
2133 %define api.token.prefix {TOK_}
2135 start: FILE for ERROR;
2137 will generate the definition of the symbols TOK_FILE, TOK_for, and
2138 TOK_ERROR in the generated sources. In particular, the scanner must
2139 use these prefixed token names, although the grammar itself still
2140 uses the short names (as in the sample rule given above).
2142 ** Variable api.value.type
2144 This new %define variable supersedes the #define macro YYSTYPE. The use
2145 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2146 using %union or %defining api.value.type results in undefined behavior.
2148 Either define api.value.type, or use "%union":
2155 %token <ival> INT "integer"
2156 %token <sval> STRING "string"
2157 %printer { fprintf (yyo, "%d", $$); } <ival>
2158 %destructor { free ($$); } <sval>
2161 yylval.ival = 42; return INT;
2162 yylval.sval = "42"; return STRING;
2164 The %define variable api.value.type supports both keyword and code values.
2166 The keyword value 'union' means that the user provides genuine types, not
2167 union member names such as "ival" and "sval" above (WARNING: will fail if
2168 -y/--yacc/%yacc is enabled).
2170 %define api.value.type union
2171 %token <int> INT "integer"
2172 %token <char *> STRING "string"
2173 %printer { fprintf (yyo, "%d", $$); } <int>
2174 %destructor { free ($$); } <char *>
2177 yylval.INT = 42; return INT;
2178 yylval.STRING = "42"; return STRING;
2180 The keyword value variant is somewhat equivalent, but for C++ special
2181 provision is made to allow classes to be used (more about this below).
2183 %define api.value.type variant
2184 %token <int> INT "integer"
2185 %token <std::string> STRING "string"
2187 Code values (in braces) denote user defined types. This is where YYSTYPE
2205 %define api.value.type {struct my_value}
2206 %token <u.ival> INT "integer"
2207 %token <u.sval> STRING "string"
2208 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2209 %destructor { free ($$); } <u.sval>
2212 yylval.u.ival = 42; return INT;
2213 yylval.u.sval = "42"; return STRING;
2215 ** Variable parse.error
2217 This variable controls the verbosity of error messages. The use of the
2218 %error-verbose directive is deprecated in favor of "%define parse.error
2221 ** Deprecated %define variable names
2223 The following variables have been renamed for consistency. Backward
2224 compatibility is ensured, but upgrading is recommended.
2226 lr.default-reductions -> lr.default-reduction
2227 lr.keep-unreachable-states -> lr.keep-unreachable-state
2228 namespace -> api.namespace
2229 stype -> api.value.type
2231 ** Semantic predicates
2233 Contributed by Paul Hilfinger.
2235 The new, experimental, semantic-predicate feature allows actions of the
2236 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2237 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2238 in GLR parsers, rather than being deferred. The result is that they allow
2239 the programmer to prune possible parses based on the values of run-time
2242 ** The directive %expect-rr is now an error in non GLR mode
2244 It used to be an error only if used in non GLR mode, _and_ if there are
2245 reduce/reduce conflicts.
2247 ** Tokens are numbered in their order of appearance
2249 Contributed by Valentin Tolmer.
2251 With '%token A B', A had a number less than the one of B. However,
2252 precedence declarations used to generate a reversed order. This is now
2253 fixed, and introducing tokens with any of %token, %left, %right,
2254 %precedence, or %nonassoc yields the same result.
2256 When mixing declarations of tokens with a literal character (e.g., 'a') or
2257 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2258 the literal characters first. For example
2262 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2263 input order is now preserved.
2265 These changes were made so that one can remove useless precedence and
2266 associativity declarations (i.e., map %nonassoc, %left or %right to
2267 %precedence, or to %token) and get exactly the same output.
2269 ** Useless precedence and associativity
2271 Contributed by Valentin Tolmer.
2273 When developing and maintaining a grammar, useless associativity and
2274 precedence directives are common. They can be a nuisance: new ambiguities
2275 arising are sometimes masked because their conflicts are resolved due to
2276 the extra precedence or associativity information. Furthermore, it can
2277 hinder the comprehension of a new grammar: one will wonder about the role
2278 of a precedence, where in fact it is useless. The following changes aim
2279 at detecting and reporting these extra directives.
2281 *** Precedence warning category
2283 A new category of warning, -Wprecedence, was introduced. It flags the
2284 useless precedence and associativity directives.
2286 *** Useless associativity
2288 Bison now warns about symbols with a declared associativity that is never
2289 used to resolve conflicts. In that case, using %precedence is sufficient;
2290 the parsing tables will remain unchanged. Solving these warnings may raise
2291 useless precedence warnings, as the symbols no longer have associativity.
2305 warning: useless associativity for '+', use %precedence [-Wprecedence]
2309 *** Useless precedence
2311 Bison now warns about symbols with a declared precedence and no declared
2312 associativity (i.e., declared with %precedence), and whose precedence is
2313 never used. In that case, the symbol can be safely declared with %token
2314 instead, without modifying the parsing tables. For example:
2318 exp: "var" '=' "number";
2322 warning: useless precedence for '=' [-Wprecedence]
2326 *** Useless precedence and associativity
2328 In case of both useless precedence and associativity, the issue is flagged
2333 exp: "var" '=' "number";
2337 warning: useless precedence and associativity for '=' [-Wprecedence]
2343 With help from Joel E. Denny and Gabriel Rassoul.
2345 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2346 marked by the new %empty directive. Using %empty on a non-empty rule is
2347 an error. The new -Wempty-rule warning reports empty rules without
2348 %empty. On the following grammar:
2358 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2361 5.8-13: error: %empty on non-empty rule
2365 ** Java skeleton improvements
2367 The constants for token names were moved to the Lexer interface. Also, it
2368 is possible to add code to the parser's constructors using "%code init"
2369 and "%define init_throws".
2370 Contributed by Paolo Bonzini.
2372 The Java skeleton now supports push parsing.
2373 Contributed by Dennis Heimbigner.
2375 ** C++ skeletons improvements
2377 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2379 Using %defines is now optional. Without it, the needed support classes
2380 are defined in the generated parser, instead of additional files (such as
2381 location.hh, position.hh and stack.hh).
2383 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2385 Both lalr1.cc and glr.cc no longer require %location.
2387 *** syntax_error exception (lalr1.cc)
2389 The C++ parser features a syntax_error exception, which can be
2390 thrown from the scanner or from user rules to raise syntax errors.
2391 This facilitates reporting errors caught in sub-functions (e.g.,
2392 rejecting too large integral literals from a conversion function
2393 used by the scanner, or rejecting invalid combinations from a
2394 factory invoked by the user actions).
2396 *** %define api.value.type variant
2398 This is based on a submission from Michiel De Wilde. With help
2399 from Théophile Ranquet.
2401 In this mode, complex C++ objects can be used as semantic values. For
2404 %token <::std::string> TEXT;
2405 %token <int> NUMBER;
2406 %token SEMICOLON ";"
2407 %type <::std::string> item;
2408 %type <::std::list<std::string>> list;
2411 list { std::cout << $1 << std::endl; }
2415 %empty { /* Generates an empty string list. */ }
2416 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2420 TEXT { std::swap ($$, $1); }
2421 | NUMBER { $$ = string_cast ($1); }
2424 *** %define api.token.constructor
2426 When variants are enabled, Bison can generate functions to build the
2427 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2428 with the semantic value (e.g., int):
2430 parser::symbol_type yylex ()
2432 parser::location_type loc = ...;
2434 return parser::make_TEXT ("Hello, world!", loc);
2436 return parser::make_NUMBER (42, loc);
2438 return parser::make_SEMICOLON (loc);
2444 There are operator- and operator-= for 'location'. Negative line/column
2445 increments can no longer underflow the resulting value.
2448 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2452 *** Fix compiler attribute portability (yacc.c)
2454 With locations enabled, __attribute__ was used unprotected.
2456 *** Fix some compiler warnings (lalr1.cc)
2459 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2463 Warnings about uninitialized yylloc in yyparse have been fixed.
2465 Restored C90 compliance (yet no report was ever made).
2467 ** Diagnostics are improved
2469 Contributed by Théophile Ranquet.
2471 *** Changes in the format of error messages
2473 This used to be the format of many error reports:
2475 input.y:2.7-12: %type redeclaration for exp
2476 input.y:1.7-12: previous declaration
2480 input.y:2.7-12: error: %type redeclaration for exp
2481 input.y:1.7-12: previous declaration
2483 *** New format for error reports: carets
2485 Caret errors have been added to Bison:
2487 input.y:2.7-12: error: %type redeclaration for exp
2490 input.y:1.7-12: previous declaration
2496 input.y:3.20-23: error: ambiguous reference: '$exp'
2497 exp: exp '+' exp { $exp = $1 + $3; };
2499 input.y:3.1-3: refers to: $exp at $$
2500 exp: exp '+' exp { $exp = $1 + $3; };
2502 input.y:3.6-8: refers to: $exp at $1
2503 exp: exp '+' exp { $exp = $1 + $3; };
2505 input.y:3.14-16: refers to: $exp at $3
2506 exp: exp '+' exp { $exp = $1 + $3; };
2509 The default behavior for now is still not to display these unless
2510 explicitly asked with -fcaret (or -fall). However, in a later release, it
2511 will be made the default behavior (but may still be deactivated with
2514 ** New value for %define variable: api.pure full
2516 The %define variable api.pure requests a pure (reentrant) parser. However,
2517 for historical reasons, using it in a location-tracking Yacc parser
2518 resulted in a yyerror function that did not take a location as a
2519 parameter. With this new value, the user may request a better pure parser,
2520 where yyerror does take a location as a parameter (in location-tracking
2523 The use of "%define api.pure true" is deprecated in favor of this new
2524 "%define api.pure full".
2526 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2528 The %define variable api.location.type defines the name of the type to use
2529 for locations. When defined, Bison no longer generates the position.hh
2530 and location.hh files, nor does the parser will include them: the user is
2531 then responsible to define her type.
2533 This can be used in programs with several parsers to factor their location
2534 and position files: let one of them generate them, and the others just use
2537 This feature was actually introduced, but not documented, in Bison 2.5,
2538 under the name "location_type" (which is maintained for backward
2541 For consistency, lalr1.java's %define variables location_type and
2542 position_type are deprecated in favor of api.location.type and
2545 ** Exception safety (lalr1.cc)
2547 The parse function now catches exceptions, uses the %destructors to
2548 release memory (the lookahead symbol and the symbols pushed on the stack)
2549 before re-throwing the exception.
2551 This feature is somewhat experimental. User feedback would be
2554 ** Graph improvements in DOT and XSLT
2556 Contributed by Théophile Ranquet.
2558 The graphical presentation of the states is more readable: their shape is
2559 now rectangular, the state number is clearly displayed, and the items are
2560 numbered and left-justified.
2562 The reductions are now explicitly represented as transitions to other
2563 diamond shaped nodes.
2565 These changes are present in both --graph output and xml2dot.xsl XSLT
2566 processing, with minor (documented) differences.
2568 ** %language is no longer an experimental feature.
2570 The introduction of this feature, in 2.4, was four years ago. The
2571 --language option and the %language directive are no longer experimental.
2575 The sections about shift/reduce and reduce/reduce conflicts resolution
2576 have been fixed and extended.
2578 Although introduced more than four years ago, XML and Graphviz reports
2579 were not properly documented.
2581 The translation of midrule actions is now described.
2584 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2586 We consider compiler warnings about Bison generated parsers to be bugs.
2587 Rather than working around them in your own project, please consider
2588 reporting them to us.
2592 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2593 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2596 Other issues in the test suite have been addressed.
2598 Null characters are correctly displayed in error messages.
2600 When possible, yylloc is correctly initialized before calling yylex. It
2601 is no longer necessary to initialize it in the %initial-action.
2604 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2606 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2609 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2613 Bugs and portability issues in the test suite have been fixed.
2615 Some errors in translations have been addressed, and --help now directs
2616 users to the appropriate place to report them.
2618 Stray Info files shipped by accident are removed.
2620 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2621 generated, are removed.
2623 All the generated headers are self-contained.
2625 ** Header guards (yacc.c, glr.c, glr.cc)
2627 In order to avoid collisions, the header guards are now
2628 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2629 For instance the header generated from
2631 %define api.prefix "calc"
2632 %defines "lib/parse.h"
2634 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2636 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2638 The compilation of pure parsers (%define api.pure) can trigger GCC
2641 input.c: In function 'yyparse':
2642 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2643 function [-Wmaybe-uninitialized]
2647 This is now fixed; pragmas to avoid these warnings are no longer needed.
2649 Warnings from clang ("equality comparison with extraneous parentheses" and
2650 "function declared 'noreturn' should not return") have also been
2654 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2658 Buffer overruns, complaints from Flex, and portability issues in the test
2659 suite have been fixed.
2661 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2663 Trailing end-of-lines in %parse-param or %lex-param would result in
2664 invalid C++. This is fixed.
2666 ** Spurious spaces and end-of-lines
2668 The generated files no longer end (nor start) with empty lines.
2671 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2673 Bison no longer executes user-specified M4 code when processing a grammar.
2677 In addition to the removal of the features announced in Bison 2.6, the
2678 next major release will remove the "Temporary hack for adding a semicolon
2679 to the user action", as announced in the release 2.5. Instead of:
2681 exp: exp "+" exp { $$ = $1 + $3 };
2685 exp: exp "+" exp { $$ = $1 + $3; };
2689 *** Type names are now properly escaped.
2691 *** glr.cc: set_debug_level and debug_level work as expected.
2693 *** Stray @ or $ in actions
2695 While Bison used to warn about stray $ or @ in action rules, it did not
2696 for other actions such as printers, destructors, or initial actions. It
2699 ** Type names in actions
2701 For consistency with rule actions, it is now possible to qualify $$ by a
2702 type-name in destructors, printers, and initial actions. For instance:
2704 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2706 will display two values for each typed and untyped symbol (provided
2707 that YYSTYPE has both "ival" and "fval" fields).
2710 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2714 The next major release of Bison will drop support for the following
2715 deprecated features. Please report disagreements to bug-bison@gnu.org.
2719 Support for generating parsers in K&R C will be removed. Parsers
2720 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2723 *** Features deprecated since Bison 1.875
2725 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2728 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2729 %lex-param, will no longer be supported.
2731 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2734 *** The generated header will be included (yacc.c)
2736 Instead of duplicating the content of the generated header (definition of
2737 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2738 as is already the case for GLR or C++ parsers. This change is deferred
2739 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2742 ** Generated Parser Headers
2744 *** Guards (yacc.c, glr.c, glr.cc)
2746 The generated headers are now guarded, as is already the case for C++
2747 parsers (lalr1.cc). For instance, with --defines=foo.h:
2752 #endif /* !YY_FOO_H */
2754 *** New declarations (yacc.c, glr.c)
2756 The generated header now declares yydebug and yyparse. Both honor
2757 --name-prefix=bar_, and yield
2759 int bar_parse (void);
2763 #define yyparse bar_parse
2766 in order to facilitate the inclusion of several parser headers inside a
2767 single compilation unit.
2769 *** Exported symbols in C++
2771 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2772 header, are removed, as they prevent the possibility of including several
2773 generated headers from a single compilation unit.
2777 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2780 ** New %define variable: api.prefix
2782 Now that the generated headers are more complete and properly protected
2783 against multiple inclusions, constant names, such as YYSTYPE are a
2784 problem. While yyparse and others are properly renamed by %name-prefix,
2785 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2786 would introduce backward compatibility issues in projects not expecting
2787 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2788 it is deprecated in favor of a new %define variable: api.prefix.
2790 The following examples compares both:
2792 %name-prefix "bar_" | %define api.prefix "bar_"
2793 %token <ival> FOO %token <ival> FOO
2794 %union { int ival; } %union { int ival; }
2800 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2801 # define BAR_FOO_H # define BAR_FOO_H
2803 /* Enabling traces. */ /* Enabling traces. */
2804 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2805 > # if defined YYDEBUG
2807 > # define BAR_DEBUG 1
2809 > # define BAR_DEBUG 0
2812 # define YYDEBUG 0 | # define BAR_DEBUG 0
2816 # if YYDEBUG | # if BAR_DEBUG
2817 extern int bar_debug; extern int bar_debug;
2820 /* Tokens. */ /* Tokens. */
2821 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2822 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2823 enum yytokentype { | enum bar_tokentype {
2828 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2829 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2830 typedef union YYSTYPE | typedef union BAR_STYPE
2833 } YYSTYPE; | } BAR_STYPE;
2834 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2837 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2839 int bar_parse (void); int bar_parse (void);
2841 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2844 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2848 The next major release will drop support for generating parsers in K&R C.
2850 ** yacc.c: YYBACKUP works as expected.
2852 ** glr.c improvements:
2854 *** Location support is eliminated when not requested:
2856 GLR parsers used to include location-related code even when locations were
2857 not requested, and therefore not even usable.
2859 *** __attribute__ is preserved:
2861 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2862 when -std is passed to GCC).
2864 ** lalr1.java: several fixes:
2866 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2867 first token leads to a syntax error. Some minor clean ups.
2871 *** C++11 compatibility:
2873 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2878 The header files such as "parser.hh", "location.hh", etc. used a constant
2879 name for preprocessor guards, for instance:
2881 #ifndef BISON_LOCATION_HH
2882 # define BISON_LOCATION_HH
2884 #endif // !BISON_LOCATION_HH
2886 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2887 case characters are converted to upper case, and series of
2888 non-alphanumerical characters are converted to an underscore.
2890 With "bison -o lang++/parser.cc", "location.hh" would now include:
2892 #ifndef YY_LANG_LOCATION_HH
2893 # define YY_LANG_LOCATION_HH
2895 #endif // !YY_LANG_LOCATION_HH
2899 The position and location constructors (and their initialize methods)
2900 accept new arguments for line and column. Several issues in the
2901 documentation were fixed.
2903 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2905 ** Changes in the manual:
2907 *** %printer is documented
2909 The "%printer" directive, supported since at least Bison 1.50, is finally
2910 documented. The "mfcalc" example is extended to demonstrate it.
2912 For consistency with the C skeletons, the C++ parsers now also support
2913 "yyoutput" (as an alias to "debug_stream ()").
2915 *** Several improvements have been made:
2917 The layout for grammar excerpts was changed to a more compact scheme.
2918 Named references are motivated. The description of the automaton
2919 description file (*.output) is updated to the current format. Incorrect
2920 index entries were fixed. Some other errors were fixed.
2924 *** Conflicting prototypes with recent/modified Flex.
2926 Fixed build problems with the current, unreleased, version of Flex, and
2927 some modified versions of 2.5.35, which have modified function prototypes.
2929 *** Warnings during the build procedure have been eliminated.
2931 *** Several portability problems in the test suite have been fixed:
2933 This includes warnings with some compilers, unexpected behavior of tools
2934 such as diff, warning messages from the test suite itself, etc.
2936 *** The install-pdf target works properly:
2938 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
2939 halts in the middle of its course.
2942 * Noteworthy changes in release 2.5 (2011-05-14)
2944 ** Grammar symbol names can now contain non-initial dashes:
2946 Consistently with directives (such as %error-verbose) and with
2947 %define variables (e.g. push-pull), grammar symbol names may contain
2948 dashes in any position except the beginning. This is a GNU
2949 extension over POSIX Yacc. Thus, use of this extension is reported
2950 by -Wyacc and rejected in Yacc mode (--yacc).
2952 ** Named references:
2954 Historically, Yacc and Bison have supported positional references
2955 ($n, $$) to allow access to symbol values from inside of semantic
2958 Starting from this version, Bison can also accept named references.
2959 When no ambiguity is possible, original symbol names may be used
2960 as named references:
2962 if_stmt : "if" cond_expr "then" then_stmt ';'
2963 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
2965 In the more common case, explicit names may be declared:
2967 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
2968 { $res = mk_if_stmt($cond, $then, $else); }
2970 Location information is also accessible using @name syntax. When
2971 accessing symbol names containing dots or dashes, explicit bracketing
2972 ($[sym.1]) must be used.
2974 These features are experimental in this version. More user feedback
2975 will help to stabilize them.
2976 Contributed by Alex Rozenman.
2978 ** IELR(1) and canonical LR(1):
2980 IELR(1) is a minimal LR(1) parser table generation algorithm. That
2981 is, given any context-free grammar, IELR(1) generates parser tables
2982 with the full language-recognition power of canonical LR(1) but with
2983 nearly the same number of parser states as LALR(1). This reduction
2984 in parser states is often an order of magnitude. More importantly,
2985 because canonical LR(1)'s extra parser states may contain duplicate
2986 conflicts in the case of non-LR(1) grammars, the number of conflicts
2987 for IELR(1) is often an order of magnitude less as well. This can
2988 significantly reduce the complexity of developing of a grammar.
2990 Bison can now generate IELR(1) and canonical LR(1) parser tables in
2991 place of its traditional LALR(1) parser tables, which remain the
2992 default. You can specify the type of parser tables in the grammar
2993 file with these directives:
2995 %define lr.type lalr
2996 %define lr.type ielr
2997 %define lr.type canonical-lr
2999 The default-reduction optimization in the parser tables can also be
3000 adjusted using "%define lr.default-reductions". For details on both
3001 of these features, see the new section "Tuning LR" in the Bison
3004 These features are experimental. More user feedback will help to
3007 ** LAC (Lookahead Correction) for syntax error handling
3009 Contributed by Joel E. Denny.
3011 Canonical LR, IELR, and LALR can suffer from a couple of problems
3012 upon encountering a syntax error. First, the parser might perform
3013 additional parser stack reductions before discovering the syntax
3014 error. Such reductions can perform user semantic actions that are
3015 unexpected because they are based on an invalid token, and they
3016 cause error recovery to begin in a different syntactic context than
3017 the one in which the invalid token was encountered. Second, when
3018 verbose error messages are enabled (with %error-verbose or the
3019 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
3020 syntax error message can both contain invalid tokens and omit valid
3023 The culprits for the above problems are %nonassoc, default
3024 reductions in inconsistent states, and parser state merging. Thus,
3025 IELR and LALR suffer the most. Canonical LR can suffer only if
3026 %nonassoc is used or if default reductions are enabled for
3027 inconsistent states.
3029 LAC is a new mechanism within the parsing algorithm that solves
3030 these problems for canonical LR, IELR, and LALR without sacrificing
3031 %nonassoc, default reductions, or state merging. When LAC is in
3032 use, canonical LR and IELR behave almost exactly the same for both
3033 syntactically acceptable and syntactically unacceptable input.
3034 While LALR still does not support the full language-recognition
3035 power of canonical LR and IELR, LAC at least enables LALR's syntax
3036 error handling to correctly reflect LALR's language-recognition
3039 Currently, LAC is only supported for deterministic parsers in C.
3040 You can enable LAC with the following directive:
3042 %define parse.lac full
3044 See the new section "LAC" in the Bison manual for additional
3045 details including a few caveats.
3047 LAC is an experimental feature. More user feedback will help to
3050 ** %define improvements:
3052 *** Can now be invoked via the command line:
3054 Each of these command-line options
3057 --define=NAME[=VALUE]
3060 --force-define=NAME[=VALUE]
3062 is equivalent to this grammar file declaration
3064 %define NAME ["VALUE"]
3066 except that the manner in which Bison processes multiple definitions
3067 for the same NAME differs. Most importantly, -F and --force-define
3068 quietly override %define, but -D and --define do not. For further
3069 details, see the section "Bison Options" in the Bison manual.
3071 *** Variables renamed:
3073 The following %define variables
3076 lr.keep_unreachable_states
3078 have been renamed to
3081 lr.keep-unreachable-states
3083 The old names are now deprecated but will be maintained indefinitely
3084 for backward compatibility.
3086 *** Values no longer need to be quoted in the grammar file:
3088 If a %define value is an identifier, it no longer needs to be placed
3089 within quotations marks. For example,
3091 %define api.push-pull "push"
3095 %define api.push-pull push
3097 *** Unrecognized variables are now errors not warnings.
3099 *** Multiple invocations for any variable is now an error not a warning.
3101 ** Unrecognized %code qualifiers are now errors not warnings.
3103 ** Character literals not of length one:
3105 Previously, Bison quietly converted all character literals to length
3106 one. For example, without warning, Bison interpreted the operators in
3107 the following grammar to be the same token:
3113 Bison now warns when a character literal is not of length one. In
3114 some future release, Bison will start reporting an error instead.
3116 ** Destructor calls fixed for lookaheads altered in semantic actions:
3118 Previously for deterministic parsers in C, if a user semantic action
3119 altered yychar, the parser in some cases used the old yychar value to
3120 determine which destructor to call for the lookahead upon a syntax
3121 error or upon parser return. This bug has been fixed.
3123 ** C++ parsers use YYRHSLOC:
3125 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3126 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3127 to use it. If, for instance, your location structure has "first"
3128 and "last" members, instead of
3130 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3134 (Current).first = (Rhs)[1].location.first; \
3135 (Current).last = (Rhs)[N].location.last; \
3139 (Current).first = (Current).last = (Rhs)[0].location.last; \
3145 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3149 (Current).first = YYRHSLOC (Rhs, 1).first; \
3150 (Current).last = YYRHSLOC (Rhs, N).last; \
3154 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3158 ** YYLLOC_DEFAULT in C++:
3160 The default implementation of YYLLOC_DEFAULT used to be issued in
3161 the header file. It is now output in the implementation file, after
3162 the user %code sections so that its #ifndef guard does not try to
3163 override the user's YYLLOC_DEFAULT if provided.
3165 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3167 YYFAIL has existed for many years as an undocumented feature of
3168 deterministic parsers in C generated by Bison. More recently, it was
3169 a documented feature of Bison's experimental Java parsers. As
3170 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3171 semantic action now produces a deprecation warning, and Java parsers
3172 no longer implement YYFAIL at all. For further details, including a
3173 discussion of how to suppress C preprocessor warnings about YYFAIL
3174 being unused, see the Bison 2.4.2 NEWS entry.
3176 ** Temporary hack for adding a semicolon to the user action:
3178 Previously, Bison appended a semicolon to every user action for
3179 reductions when the output language defaulted to C (specifically, when
3180 neither %yacc, %language, %skeleton, or equivalent command-line
3181 options were specified). This allowed actions such as
3183 exp: exp "+" exp { $$ = $1 + $3 };
3187 exp: exp "+" exp { $$ = $1 + $3; };
3189 As a first step in removing this misfeature, Bison now issues a
3190 warning when it appends a semicolon. Moreover, in cases where Bison
3191 cannot easily determine whether a semicolon is needed (for example, an
3192 action ending with a cpp directive or a braced compound initializer),
3193 it no longer appends one. Thus, the C compiler might now complain
3194 about a missing semicolon where it did not before. Future releases of
3195 Bison will cease to append semicolons entirely.
3197 ** Verbose syntax error message fixes:
3199 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3200 specified, syntax error messages produced by the generated parser
3201 include the unexpected token as well as a list of expected tokens.
3202 The effect of %nonassoc on these verbose messages has been corrected
3203 in two ways, but a more complete fix requires LAC, described above:
3205 *** When %nonassoc is used, there can exist parser states that accept no
3206 tokens, and so the parser does not always require a lookahead token
3207 in order to detect a syntax error. Because no unexpected token or
3208 expected tokens can then be reported, the verbose syntax error
3209 message described above is suppressed, and the parser instead
3210 reports the simpler message, "syntax error". Previously, this
3211 suppression was sometimes erroneously triggered by %nonassoc when a
3212 lookahead was actually required. Now verbose messages are
3213 suppressed only when all previous lookaheads have already been
3214 shifted or discarded.
3216 *** Previously, the list of expected tokens erroneously included tokens
3217 that would actually induce a syntax error because conflicts for them
3218 were resolved with %nonassoc in the current parser state. Such
3219 tokens are now properly omitted from the list.
3221 *** Expected token lists are still often wrong due to state merging
3222 (from LALR or IELR) and default reductions, which can both add
3223 invalid tokens and subtract valid tokens. Canonical LR almost
3224 completely fixes this problem by eliminating state merging and
3225 default reductions. However, there is one minor problem left even
3226 when using canonical LR and even after the fixes above. That is,
3227 if the resolution of a conflict with %nonassoc appears in a later
3228 parser state than the one at which some syntax error is
3229 discovered, the conflicted token is still erroneously included in
3230 the expected token list. Bison's new LAC implementation,
3231 described above, eliminates this problem and the need for
3232 canonical LR. However, LAC is still experimental and is disabled
3235 ** Java skeleton fixes:
3237 *** A location handling bug has been fixed.
3239 *** The top element of each of the value stack and location stack is now
3240 cleared when popped so that it can be garbage collected.
3242 *** Parser traces now print the top element of the stack.
3244 ** -W/--warnings fixes:
3246 *** Bison now properly recognizes the "no-" versions of categories:
3248 For example, given the following command line, Bison now enables all
3249 warnings except warnings for incompatibilities with POSIX Yacc:
3251 bison -Wall,no-yacc gram.y
3253 *** Bison now treats S/R and R/R conflicts like other warnings:
3255 Previously, conflict reports were independent of Bison's normal
3256 warning system. Now, Bison recognizes the warning categories
3257 "conflicts-sr" and "conflicts-rr". This change has important
3258 consequences for the -W and --warnings command-line options. For
3261 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3262 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3263 bison -Wnone gram.y # no conflicts are reported
3264 bison -Werror gram.y # any conflict is an error
3266 However, as before, if the %expect or %expect-rr directive is
3267 specified, an unexpected number of conflicts is an error, and an
3268 expected number of conflicts is not reported, so -W and --warning
3269 then have no effect on the conflict report.
3271 *** The "none" category no longer disables a preceding "error":
3273 For example, for the following command line, Bison now reports
3274 errors instead of warnings for incompatibilities with POSIX Yacc:
3276 bison -Werror,none,yacc gram.y
3278 *** The "none" category now disables all Bison warnings:
3280 Previously, the "none" category disabled only Bison warnings for
3281 which there existed a specific -W/--warning category. However,
3282 given the following command line, Bison is now guaranteed to
3283 suppress all warnings:
3287 ** Precedence directives can now assign token number 0:
3289 Since Bison 2.3b, which restored the ability of precedence
3290 directives to assign token numbers, doing so for token number 0 has
3291 produced an assertion failure. For example:
3295 This bug has been fixed.
3298 * Noteworthy changes in release 2.4.3 (2010-08-05)
3300 ** Bison now obeys -Werror and --warnings=error for warnings about
3301 grammar rules that are useless in the parser due to conflicts.
3303 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3306 ** Failures in the test suite for GCC 4.5 have been fixed.
3308 ** Failures in the test suite for some versions of Sun Studio C++ have
3311 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3312 warnings about undefined %prec identifiers will not be converted to
3313 errors in Bison 2.5. They will remain warnings, which should be
3314 sufficient for POSIX while avoiding backward compatibility issues.
3316 ** Minor documentation fixes.
3319 * Noteworthy changes in release 2.4.2 (2010-03-20)
3321 ** Some portability problems that resulted in failures and livelocks
3322 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3323 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3324 errors should no longer cause M4 to report a broken pipe on the
3327 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3329 POSIX specifies that an error be reported for any identifier that does
3330 not appear on the LHS of a grammar rule and that is not defined by
3331 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3332 error report for the case when an identifier appears only after a
3333 %prec directive. It is now restored. However, for backward
3334 compatibility with recent Bison releases, it is only a warning for
3335 now. In Bison 2.5 and later, it will return to being an error.
3336 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3337 warning will not be converted to an error in Bison 2.5.]
3339 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3341 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3342 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3345 ** %code is now a permanent feature.
3347 A traditional Yacc prologue directive is written in the form:
3351 To provide a more flexible alternative, Bison 2.3b introduced the
3352 %code directive with the following forms for C/C++:
3355 %code requires {CODE}
3356 %code provides {CODE}
3359 These forms are now considered permanent features of Bison. See the
3360 %code entries in the section "Bison Declaration Summary" in the Bison
3361 manual for a summary of their functionality. See the section
3362 "Prologue Alternatives" for a detailed discussion including the
3363 advantages of %code over the traditional Yacc prologue directive.
3365 Bison's Java feature as a whole including its current usage of %code
3366 is still considered experimental.
3368 ** YYFAIL is deprecated and will eventually be removed.
3370 YYFAIL has existed for many years as an undocumented feature of
3371 deterministic parsers in C generated by Bison. Previously, it was
3372 documented for Bison's experimental Java parsers. YYFAIL is no longer
3373 documented for Java parsers and is formally deprecated in both cases.
3374 Users are strongly encouraged to migrate to YYERROR, which is
3377 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3378 induce a syntax error. The most obvious difference from YYERROR is
3379 that YYFAIL will automatically invoke yyerror to report the syntax
3380 error so that you don't have to. However, there are several other
3381 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3382 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3383 used. For a more detailed discussion, see:
3385 http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html
3387 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3388 deterministic parsers in C will continue to implement it. However,
3389 because YYFAIL is already flawed, it seems futile to try to make new
3390 Bison features compatible with it. Thus, during parser generation,
3391 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3392 rule action. In a later release, YYFAIL will be disabled for
3393 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3394 be removed altogether.
3396 There exists at least one case where Bison 2.5's YYFAIL warning will
3397 be a false positive. Some projects add phony uses of YYFAIL and other
3398 Bison-defined macros for the sole purpose of suppressing C
3399 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3400 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3401 epilogue (that is, after the second "%%") in the Bison input file. In
3402 this release (2.4.2), Bison already generates its own code to suppress
3403 C preprocessor warnings for YYFAIL, so projects can remove their own
3404 phony uses of YYFAIL if compatibility with Bison releases prior to
3405 2.4.2 is not necessary.
3407 ** Internationalization.
3409 Fix a regression introduced in Bison 2.4: Under some circumstances,
3410 message translations were not installed although supported by the
3414 * Noteworthy changes in release 2.4.1 (2008-12-11)
3416 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3417 declarations have been fixed.
3419 ** Temporary hack for adding a semicolon to the user action.
3421 Bison used to prepend a trailing semicolon at the end of the user
3422 action for reductions. This allowed actions such as
3424 exp: exp "+" exp { $$ = $1 + $3 };
3428 exp: exp "+" exp { $$ = $1 + $3; };
3430 Some grammars still depend on this "feature". Bison 2.4.1 restores
3431 the previous behavior in the case of C output (specifically, when
3432 neither %language or %skeleton or equivalent command-line options
3433 are used) to leave more time for grammars depending on the old
3434 behavior to be adjusted. Future releases of Bison will disable this
3437 ** A few minor improvements to the Bison manual.
3440 * Noteworthy changes in release 2.4 (2008-11-02)
3442 ** %language is an experimental feature.
3444 We first introduced this feature in test release 2.3b as a cleaner
3445 alternative to %skeleton. Since then, we have discussed the possibility of
3446 modifying its effect on Bison's output file names. Thus, in this release,
3447 we consider %language to be an experimental feature that will likely evolve
3450 ** Forward compatibility with GNU M4 has been improved.
3452 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3456 * Noteworthy changes in release 2.3b (2008-05-27)
3458 ** The quotes around NAME that used to be required in the following directive
3461 %define NAME "VALUE"
3463 ** The directive "%pure-parser" is now deprecated in favor of:
3467 which has the same effect except that Bison is more careful to warn about
3468 unreasonable usage in the latter case.
3472 Bison can now generate an LALR(1) parser in C with a push interface. That
3473 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3474 push one token at a time to the parser using "yypush_parse", which will
3475 return to the caller after processing each token. By default, the push
3476 interface is disabled. Either of the following directives will enable it:
3478 %define api.push_pull "push" // Just push; does not require yylex.
3479 %define api.push_pull "both" // Push and pull; requires yylex.
3481 See the new section "A Push Parser" in the Bison manual for details.
3483 The current push parsing interface is experimental and may evolve. More user
3484 feedback will help to stabilize it.
3486 ** The -g and --graph options now output graphs in Graphviz DOT format,
3487 not VCG format. Like --graph, -g now also takes an optional FILE argument
3488 and thus cannot be bundled with other short options.
3492 Bison can now generate an LALR(1) parser in Java. The skeleton is
3493 "data/lalr1.java". Consider using the new %language directive instead of
3494 %skeleton to select it.
3496 See the new section "Java Parsers" in the Bison manual for details.
3498 The current Java interface is experimental and may evolve. More user
3499 feedback will help to stabilize it.
3500 Contributed by Paolo Bonzini.
3504 This new directive specifies the programming language of the generated
3505 parser, which can be C (the default), C++, or Java. Besides the skeleton
3506 that Bison uses, the directive affects the names of the generated files if
3507 the grammar file's name ends in ".y".
3509 ** XML Automaton Report
3511 Bison can now generate an XML report of the LALR(1) automaton using the new
3512 "--xml" option. The current XML schema is experimental and may evolve. More
3513 user feedback will help to stabilize it.
3514 Contributed by Wojciech Polak.
3516 ** The grammar file may now specify the name of the parser header file using
3517 %defines. For example:
3521 ** When reporting useless rules, useless nonterminals, and unused terminals,
3522 Bison now employs the terms "useless in grammar" instead of "useless",
3523 "useless in parser" instead of "never reduced", and "unused in grammar"
3524 instead of "unused".
3526 ** Unreachable State Removal
3528 Previously, Bison sometimes generated parser tables containing unreachable
3529 states. A state can become unreachable during conflict resolution if Bison
3530 disables a shift action leading to it from a predecessor state. Bison now:
3532 1. Removes unreachable states.
3534 2. Does not report any conflicts that appeared in unreachable states.
3535 WARNING: As a result, you may need to update %expect and %expect-rr
3536 directives in existing grammar files.
3538 3. For any rule used only in such states, Bison now reports the rule as
3539 "useless in parser due to conflicts".
3541 This feature can be disabled with the following directive:
3543 %define lr.keep_unreachable_states
3545 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3546 for further discussion.
3548 ** Lookahead Set Correction in the ".output" Report
3550 When instructed to generate a ".output" file including lookahead sets
3551 (using "--report=lookahead", for example), Bison now prints each reduction's
3552 lookahead set only next to the associated state's one item that (1) is
3553 associated with the same rule as the reduction and (2) has its dot at the end
3554 of its RHS. Previously, Bison also erroneously printed the lookahead set
3555 next to all of the state's other items associated with the same rule. This
3556 bug affected only the ".output" file and not the generated parser source
3559 ** --report-file=FILE is a new option to override the default ".output" file
3562 ** The "=" that used to be required in the following directives is now
3565 %file-prefix "parser"
3569 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3571 Bison 2.3a provided a new set of directives as a more flexible alternative to
3572 the traditional Yacc prologue blocks. Those have now been consolidated into
3573 a single %code directive with an optional qualifier field, which identifies
3574 the purpose of the code and thus the location(s) where Bison should generate
3577 1. "%code {CODE}" replaces "%after-header {CODE}"
3578 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3579 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3580 4. "%code top {CODE}" replaces "%before-header {CODE}"
3582 See the %code entries in section "Bison Declaration Summary" in the Bison
3583 manual for a summary of the new functionality. See the new section "Prologue
3584 Alternatives" for a detailed discussion including the advantages of %code
3585 over the traditional Yacc prologues.
3587 The prologue alternatives are experimental. More user feedback will help to
3588 determine whether they should become permanent features.
3590 ** Revised warning: unset or unused midrule values
3592 Since Bison 2.2, Bison has warned about midrule values that are set but not
3593 used within any of the actions of the parent rule. For example, Bison warns
3596 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3598 Now, Bison also warns about midrule values that are used but not set. For
3599 example, Bison warns about unset $$ in the midrule action in:
3601 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3603 However, Bison now disables both of these warnings by default since they
3604 sometimes prove to be false alarms in existing grammars employing the Yacc
3605 constructs $0 or $-N (where N is some positive integer).
3607 To enable these warnings, specify the option "--warnings=midrule-values" or
3608 "-W", which is a synonym for "--warnings=all".
3610 ** Default %destructor or %printer with "<*>" or "<>"
3612 Bison now recognizes two separate kinds of default %destructor's and
3615 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3616 %destructor/%printer for all grammar symbols for which you have formally
3617 declared semantic type tags.
3619 2. Place "<>" in a %destructor/%printer symbol list to define a default
3620 %destructor/%printer for all grammar symbols without declared semantic
3623 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3624 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3625 longer applies any %destructor to a midrule value if that midrule value is
3626 not actually ever referenced using either $$ or $n in a semantic action.
3628 The default %destructor's and %printer's are experimental. More user
3629 feedback will help to determine whether they should become permanent
3632 See the section "Freeing Discarded Symbols" in the Bison manual for further
3635 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3636 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3637 manual for a caveat concerning the treatment of literal strings.
3639 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3640 completely removed from Bison.
3643 * Noteworthy changes in release 2.3a (2006-09-13)
3645 ** Instead of %union, you can define and use your own union type
3646 YYSTYPE if your grammar contains at least one <type> tag.
3647 Your YYSTYPE need not be a macro; it can be a typedef.
3648 This change is for compatibility with other Yacc implementations,
3649 and is required by POSIX.
3651 ** Locations columns and lines start at 1.
3652 In accordance with the GNU Coding Standards and Emacs.
3654 ** You may now declare per-type and default %destructor's and %printer's:
3658 %union { char *string; }
3659 %token <string> STRING1
3660 %token <string> STRING2
3661 %type <string> string1
3662 %type <string> string2
3663 %union { char character; }
3664 %token <character> CHR
3665 %type <character> chr
3666 %destructor { free ($$); } %symbol-default
3667 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3668 %destructor { } <character>
3670 guarantees that, when the parser discards any user-defined symbol that has a
3671 semantic type tag other than "<character>", it passes its semantic value to
3672 "free". However, when the parser discards a "STRING1" or a "string1", it
3673 also prints its line number to "stdout". It performs only the second
3674 "%destructor" in this case, so it invokes "free" only once.
3676 [Although we failed to mention this here in the 2.3a release, the default
3677 %destructor's and %printer's were experimental, and they were rewritten in
3680 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3681 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3682 associating token numbers with token names. Removing the #define statements
3683 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3684 requires them. Bison still generates an enum for token names in all cases.
3686 ** Handling of traditional Yacc prologue blocks is now more consistent but
3687 potentially incompatible with previous releases of Bison.
3689 As before, you declare prologue blocks in your grammar file with the
3690 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3691 prologue blocks that you've declared before the first %union. To generate
3692 the post-prologue, Bison concatenates all prologue blocks that you've
3693 declared after the first %union.
3695 Previous releases of Bison inserted the pre-prologue into both the header
3696 file and the code file in all cases except for LALR(1) parsers in C. In the
3697 latter case, Bison inserted it only into the code file. For parsers in C++,
3698 the point of insertion was before any token definitions (which associate
3699 token numbers with names). For parsers in C, the point of insertion was
3700 after the token definitions.
3702 Now, Bison never inserts the pre-prologue into the header file. In the code
3703 file, it always inserts it before the token definitions.
3705 ** Bison now provides a more flexible alternative to the traditional Yacc
3706 prologue blocks: %before-header, %start-header, %end-header, and
3709 For example, the following declaration order in the grammar file reflects the
3710 order in which Bison will output these code blocks. However, you are free to
3711 declare these code blocks in your grammar file in whatever order is most
3715 /* Bison treats this block like a pre-prologue block: it inserts it into
3716 * the code file before the contents of the header file. It does *not*
3717 * insert it into the header file. This is a good place to put
3718 * #include's that you want at the top of your code file. A common
3719 * example is '#include "system.h"'. */
3722 /* Bison inserts this block into both the header file and the code file.
3723 * In both files, the point of insertion is before any Bison-generated
3724 * token, semantic type, location type, and class definitions. This is a
3725 * good place to define %union dependencies, for example. */
3728 /* Unlike the traditional Yacc prologue blocks, the output order for the
3729 * new %*-header blocks is not affected by their declaration position
3730 * relative to any %union in the grammar file. */
3733 /* Bison inserts this block into both the header file and the code file.
3734 * In both files, the point of insertion is after the Bison-generated
3735 * definitions. This is a good place to declare or define public
3736 * functions or data structures that depend on the Bison-generated
3740 /* Bison treats this block like a post-prologue block: it inserts it into
3741 * the code file after the contents of the header file. It does *not*
3742 * insert it into the header file. This is a good place to declare or
3743 * define internal functions or data structures that depend on the
3744 * Bison-generated definitions. */
3747 If you have multiple occurrences of any one of the above declarations, Bison
3748 will concatenate the contents in declaration order.
3750 [Although we failed to mention this here in the 2.3a release, the prologue
3751 alternatives were experimental, and they were rewritten in future versions.]
3753 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3754 The old spelling still works, but is not documented and may be removed
3755 in a future release.
3758 * Noteworthy changes in release 2.3 (2006-06-05)
3760 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3761 for compatibility with LALR(1) grammars.
3763 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3764 be to a type name that does not contain parentheses or brackets.
3767 * Noteworthy changes in release 2.2 (2006-05-19)
3769 ** The distribution terms for all Bison-generated parsers now permit
3770 using the parsers in nonfree programs. Previously, this permission
3771 was granted only for Bison-generated LALR(1) parsers in C.
3773 ** %name-prefix changes the namespace name in C++ outputs.
3775 ** The C++ parsers export their token_type.
3777 ** Bison now allows multiple %union declarations, and concatenates
3778 their contents together.
3780 ** New warning: unused values
3781 Right-hand side symbols whose values are not used are reported,
3782 if the symbols have destructors. For instance:
3784 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3788 will trigger a warning about $$ and $5 in the first rule, and $3 in
3789 the second ($1 is copied to $$ by the default rule). This example
3790 most likely contains three errors, and could be rewritten as:
3792 exp: exp "?" exp ":" exp
3793 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3795 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3798 However, if the original actions were really intended, memory leaks
3799 and all, the warnings can be suppressed by letting Bison believe the
3800 values are used, e.g.:
3802 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3803 | exp "+" exp { $$ = $1; (void) $3; }
3806 If there are midrule actions, the warning is issued if no action
3807 uses it. The following triggers no warning: $1 and $3 are used.
3809 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3811 The warning is intended to help catching lost values and memory leaks.
3812 If a value is ignored, its associated memory typically is not reclaimed.
3814 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3815 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3816 and YYERROR, for all objects on the stack, other than objects
3817 corresponding to the right-hand side of the current rule.
3819 ** %expect, %expect-rr
3820 Incorrect numbers of expected conflicts are now actual errors,
3821 instead of warnings.
3823 ** GLR, YACC parsers.
3824 The %parse-params are available in the destructors (and the
3825 experimental printers) as per the documentation.
3827 ** Bison now warns if it finds a stray "$" or "@" in an action.
3829 ** %require "VERSION"
3830 This specifies that the grammar file depends on features implemented
3831 in Bison version VERSION or higher.
3833 ** lalr1.cc: The token and value types are now class members.
3834 The tokens were defined as free form enums and cpp macros. YYSTYPE
3835 was defined as a free form union. They are now class members:
3836 tokens are enumerations of the "yy::parser::token" struct, and the
3837 semantic values have the "yy::parser::semantic_type" type.
3839 If you do not want or can update to this scheme, the directive
3840 '%define "global_tokens_and_yystype" "1"' triggers the global
3841 definition of tokens and YYSTYPE. This change is suitable both
3842 for previous releases of Bison, and this one.
3844 If you wish to update, then make sure older version of Bison will
3845 fail using '%require "2.2"'.
3847 ** DJGPP support added.
3850 * Noteworthy changes in release 2.1 (2005-09-16)
3852 ** The C++ lalr1.cc skeleton supports %lex-param.
3854 ** Bison-generated parsers now support the translation of diagnostics like
3855 "syntax error" into languages other than English. The default
3856 language is still English. For details, please see the new
3857 Internationalization section of the Bison manual. Software
3858 distributors should also see the new PACKAGING file. Thanks to
3859 Bruno Haible for this new feature.
3861 ** Wording in the Bison-generated parsers has been changed slightly to
3862 simplify translation. In particular, the message "memory exhausted"
3863 has replaced "parser stack overflow", as the old message was not
3864 always accurate for modern Bison-generated parsers.
3866 ** Destructors are now called when the parser aborts, for all symbols left
3867 behind on the stack. Also, the start symbol is now destroyed after a
3868 successful parse. In both cases, the behavior was formerly inconsistent.
3870 ** When generating verbose diagnostics, Bison-generated parsers no longer
3871 quote the literal strings associated with tokens. For example, for
3872 a syntax error associated with '%token NUM "number"' they might
3873 print 'syntax error, unexpected number' instead of 'syntax error,
3874 unexpected "number"'.
3877 * Noteworthy changes in release 2.0 (2004-12-25)
3879 ** Possibly-incompatible changes
3881 - Bison-generated parsers no longer default to using the alloca function
3882 (when available) to extend the parser stack, due to widespread
3883 problems in unchecked stack-overflow detection. You can "#define
3884 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3885 the manual to determine safe values for YYMAXDEPTH in that case.
3887 - Error token location.
3888 During error recovery, the location of the syntax error is updated
3889 to cover the whole sequence covered by the error token: it includes
3890 the shifted symbols thrown away during the first part of the error
3891 recovery, and the lookahead rejected during the second part.
3893 - Semicolon changes:
3894 . Stray semicolons are no longer allowed at the start of a grammar.
3895 . Semicolons are now required after in-grammar declarations.
3897 - Unescaped newlines are no longer allowed in character constants or
3898 string literals. They were never portable, and GCC 3.4.0 has
3899 dropped support for them. Better diagnostics are now generated if
3900 forget a closing quote.
3902 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
3906 - GLR grammars now support locations.
3908 - New directive: %initial-action.
3909 This directive allows the user to run arbitrary code (including
3910 initializing @$) from yyparse before parsing starts.
3912 - A new directive "%expect-rr N" specifies the expected number of
3913 reduce/reduce conflicts in GLR parsers.
3915 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
3916 This is a GNU extension.
3918 - The option "--report=lookahead" was changed to "--report=look-ahead".
3919 [However, this was changed back after 2.3.]
3921 - Experimental %destructor support has been added to lalr1.cc.
3923 - New configure option --disable-yacc, to disable installation of the
3924 yacc command and -ly library introduced in 1.875 for POSIX conformance.
3928 - For now, %expect-count violations are now just warnings, not errors.
3929 This is for compatibility with Bison 1.75 and earlier (when there are
3930 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
3931 are too many or too few shift/reduce conflicts). However, in future
3932 versions of Bison we plan to improve the %expect machinery so that
3933 these violations will become errors again.
3935 - Within Bison itself, numbers (e.g., goto numbers) are no longer
3936 arbitrarily limited to 16-bit counts.
3938 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
3941 * Noteworthy changes in release 1.875 (2003-01-01)
3943 ** The documentation license has been upgraded to version 1.2
3944 of the GNU Free Documentation License.
3946 ** syntax error processing
3948 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
3949 locations too. This fixes bugs in error-location computation.
3952 It is now possible to reclaim the memory associated to symbols
3953 discarded during error recovery. This feature is still experimental.
3956 This new directive is preferred over YYERROR_VERBOSE.
3958 - #defining yyerror to steal internal variables is discouraged.
3959 It is not guaranteed to work forever.
3961 ** POSIX conformance
3963 - Semicolons are once again optional at the end of grammar rules.
3964 This reverts to the behavior of Bison 1.33 and earlier, and improves
3965 compatibility with Yacc.
3967 - "parse error" -> "syntax error"
3968 Bison now uniformly uses the term "syntax error"; formerly, the code
3969 and manual sometimes used the term "parse error" instead. POSIX
3970 requires "syntax error" in diagnostics, and it was thought better to
3973 - The documentation now emphasizes that yylex and yyerror must be
3974 declared before use. C99 requires this.
3976 - Bison now parses C99 lexical constructs like UCNs and
3977 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
3979 - File names are properly escaped in C output. E.g., foo\bar.y is
3980 output as "foo\\bar.y".
3982 - Yacc command and library now available
3983 The Bison distribution now installs a "yacc" command, as POSIX requires.
3984 Also, Bison now installs a small library liby.a containing
3985 implementations of Yacc-compatible yyerror and main functions.
3986 This library is normally not useful, but POSIX requires it.
3988 - Type clashes now generate warnings, not errors.
3990 - If the user does not define YYSTYPE as a macro, Bison now declares it
3991 using typedef instead of defining it as a macro.
3992 For consistency, YYLTYPE is also declared instead of defined.
3994 ** Other compatibility issues
3996 - %union directives can now have a tag before the "{", e.g., the
3997 directive "%union foo {...}" now generates the C code
3998 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
3999 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
4000 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
4001 This is for compatibility with both Yacc and Bison 1.35.
4003 - ";" is output before the terminating "}" of an action, for
4004 compatibility with Bison 1.35.
4006 - Bison now uses a Yacc-style format for conflict reports, e.g.,
4007 "conflicts: 2 shift/reduce, 1 reduce/reduce".
4009 - "yystype" and "yyltype" are now obsolescent macros instead of being
4010 typedefs or tags; they are no longer documented and are planned to be
4011 withdrawn in a future release.
4016 Users of Bison have to decide how they handle the portability of the
4019 - "parsing stack overflow..." -> "parser stack overflow"
4020 GLR parsers now report "parser stack overflow" as per the Bison manual.
4022 ** %parse-param and %lex-param
4023 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
4024 additional context to yyparse and yylex. They suffer from several
4027 - a single argument only can be added,
4028 - their types are weak (void *),
4029 - this context is not passed to ancillary functions such as yyerror,
4030 - only yacc.c parsers support them.
4032 The new %parse-param/%lex-param directives provide a more precise control.
4035 %parse-param {int *nastiness}
4036 %lex-param {int *nastiness}
4037 %parse-param {int *randomness}
4039 results in the following signatures:
4041 int yylex (int *nastiness);
4042 int yyparse (int *nastiness, int *randomness);
4044 or, if both %pure-parser and %locations are used:
4046 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
4047 int yyparse (int *nastiness, int *randomness);
4049 ** Bison now warns if it detects conflicting outputs to the same file,
4050 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
4051 that command outputs both code and header to foo.h.
4053 ** #line in output files
4054 - --no-line works properly.
4056 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
4057 later to be built. This change originally took place a few versions
4058 ago, but nobody noticed until we recently asked someone to try
4059 building Bison with a K&R C compiler.
4062 * Noteworthy changes in release 1.75 (2002-10-14)
4064 ** Bison should now work on 64-bit hosts.
4066 ** Indonesian translation thanks to Tedi Heriyanto.
4069 Fix spurious parse errors.
4072 Some people redefine yyerror to steal yyparse' private variables.
4073 Reenable this trick until an official feature replaces it.
4076 In agreement with POSIX and with other Yaccs, leaving a default
4077 action is valid when $$ is untyped, and $1 typed:
4081 but the converse remains an error:
4085 ** Values of midrule actions
4088 foo: { ... } { $$ = $1; } ...
4090 was incorrectly rejected: $1 is defined in the second midrule
4091 action, and is equal to the $$ of the first midrule action.
4094 * Noteworthy changes in release 1.50 (2002-10-04)
4099 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4100 almost any context-free grammar, ambiguous or not. The new declarations
4101 %dprec and %merge on grammar rules allow parse-time resolution of
4102 ambiguities. Contributed by Paul Hilfinger.
4104 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4105 like the Alpha, so please stick to 32-bit hosts for now.
4108 When not in Yacc compatibility mode, when the output file was not
4109 specified, running "bison foo/bar.y" created "foo/bar.c". It
4110 now creates "bar.c".
4113 The undefined token was systematically mapped to 2 which prevented
4114 the use of 2 by the user. This is no longer the case.
4116 ** Unknown token numbers
4117 If yylex returned an out of range value, yyparse could die. This is
4121 According to POSIX, the error token must be 256.
4122 Bison extends this requirement by making it a preference: *if* the
4123 user specified that one of her tokens is numbered 256, then error
4124 will be mapped onto another number.
4126 ** Verbose error messages
4127 They no longer report "..., expecting error or..." for states where
4128 error recovery is possible.
4131 Defaults to "$end" instead of "$".
4133 ** Error recovery now conforms to documentation and to POSIX
4134 When a Bison-generated parser encounters a syntax error, it now pops
4135 the stack until it finds a state that allows shifting the error
4136 token. Formerly, it popped the stack until it found a state that
4137 allowed some non-error action other than a default reduction on the
4138 error token. The new behavior has long been the documented behavior,
4139 and has long been required by POSIX. For more details, please see
4140 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4141 <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.
4144 Popped tokens and nonterminals are now reported.
4147 Larger grammars are now supported (larger token numbers, larger grammar
4148 size (= sum of the LHS and RHS lengths), larger LALR tables).
4149 Formerly, many of these numbers ran afoul of 16-bit limits;
4150 now these limits are 32 bits on most hosts.
4152 ** Explicit initial rule
4153 Bison used to play hacks with the initial rule, which the user does
4154 not write. It is now explicit, and visible in the reports and
4158 Before, Bison reported the useless rules, but, although not used,
4159 included them in the parsers. They are now actually removed.
4161 ** Useless rules, useless nonterminals
4162 They are now reported, as a warning, with their locations.
4164 ** Rules never reduced
4165 Rules that can never be reduced because of conflicts are now
4168 ** Incorrect "Token not used"
4169 On a grammar such as
4171 %token useless useful
4173 exp: '0' %prec useful;
4175 where a token was used to set the precedence of the last rule,
4176 bison reported both "useful" and "useless" as useless tokens.
4178 ** Revert the C++ namespace changes introduced in 1.31
4179 as they caused too many portability hassles.
4181 ** Default locations
4182 By an accident of design, the default computation of @$ was
4183 performed after another default computation was performed: @$ = @1.
4184 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4185 the computation of @$.
4187 ** Token end-of-file
4188 The token end of file may be specified by the user, in which case,
4189 the user symbol is used in the reports, the graphs, and the verbose
4190 error messages instead of "$end", which remains being the default.
4194 %token MYEOF 0 "end of file"
4197 This old option, which has been broken for ages, is removed.
4200 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4201 Croatian, thanks to Denis Lackovic.
4203 ** Incorrect token definitions
4206 bison used to output
4209 ** Token definitions as enums
4210 Tokens are output both as the traditional #define's, and, provided
4211 the compiler supports ANSI C or is a C++ compiler, as enums.
4212 This lets debuggers display names instead of integers.
4215 In addition to --verbose, bison supports --report=THINGS, which
4216 produces additional information:
4218 complete the core item sets with their closure
4219 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4220 explicitly associate lookahead tokens to items
4222 describe shift/reduce conflicts solving.
4223 Bison used to systematically output this information on top of
4224 the report. Solved conflicts are now attached to their states.
4227 Previous versions don't complain when there is a type clash on
4228 the default action if the rule has a midrule action, such as in:
4236 ** GNU M4 is now required when using Bison.
4239 * Noteworthy changes in release 1.35 (2002-03-25)
4242 Some projects use Bison's C parser with C++ compilers, and define
4243 YYSTYPE as a class. The recent adjustment of C parsers for data
4244 alignment and 64 bit architectures made this impossible.
4246 Because for the time being no real solution for C++ parser
4247 generation exists, kludges were implemented in the parser to
4248 maintain this use. In the future, when Bison has C++ parsers, this
4249 kludge will be disabled.
4251 This kludge also addresses some C++ problems when the stack was
4255 * Noteworthy changes in release 1.34 (2002-03-12)
4257 ** File name clashes are detected
4258 $ bison foo.y -d -o foo.x
4259 fatal error: header and parser would both be named "foo.x"
4261 ** A missing ";" at the end of a rule triggers a warning
4262 In accordance with POSIX, and in agreement with other
4263 Yacc implementations, Bison will mandate this semicolon in the near
4264 future. This eases the implementation of a Bison parser of Bison
4265 grammars by making this grammar LALR(1) instead of LR(2). To
4266 facilitate the transition, this release introduces a warning.
4268 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4269 many portability hassles.
4271 ** DJGPP support added.
4273 ** Fix test suite portability problems.
4276 * Noteworthy changes in release 1.33 (2002-02-07)
4279 Groff could not be compiled for the definition of size_t was lacking
4280 under some conditions.
4286 * Noteworthy changes in release 1.32 (2002-01-23)
4288 ** Fix Yacc output file names
4290 ** Portability fixes
4292 ** Italian, Dutch translations
4295 * Noteworthy changes in release 1.31 (2002-01-14)
4299 ** GNU Gettext and %expect
4300 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4301 Bison dies on incorrect %expectations, we fear there will be
4302 too many bug reports for Gettext, so _for the time being_, %expect
4303 does not trigger an error when the input file is named "plural.y".
4305 ** Use of alloca in parsers
4306 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4307 malloc exclusively. Since 1.29, but was not NEWS'ed.
4309 alloca is used only when compiled with GCC, to avoid portability
4312 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4314 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4315 (as POSIX requires) instead of being undefined.
4318 Bison has always permitted actions such as { $$ = $1 }: it adds the
4319 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4320 is no longer output: one has to write { $$ = $1; }.
4322 ** Better C++ compliance
4323 The output parsers try to respect C++ namespaces.
4324 [This turned out to be a failed experiment, and it was reverted later.]
4327 Fixed bugs when reporting useless nonterminals.
4330 The parsers work properly on 64 bit hosts.
4333 Some calls to strerror resulted in scrambled or missing error messages.
4336 When the number of shift/reduce conflicts is correct, don't issue
4339 ** The verbose report includes the rule line numbers.
4341 ** Rule line numbers are fixed in traces.
4343 ** Swedish translation
4346 Verbose parse error messages from the parsers are better looking.
4347 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4348 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4350 ** Fixed parser memory leaks.
4351 When the generated parser was using malloc to extend its stacks, the
4352 previous allocations were not freed.
4354 ** Fixed verbose output file.
4355 Some newlines were missing.
4356 Some conflicts in state descriptions were missing.
4358 ** Fixed conflict report.
4359 Option -v was needed to get the result.
4363 Mismatches are errors, not warnings.
4365 ** Fixed incorrect processing of some invalid input.
4367 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4369 ** Fixed some typos in the documentation.
4371 ** %token MY_EOF 0 is supported.
4372 Before, MY_EOF was silently renumbered as 257.
4374 ** doc/refcard.tex is updated.
4376 ** %output, %file-prefix, %name-prefix.
4380 New, aliasing "--output-file".
4383 * Noteworthy changes in release 1.30 (2001-10-26)
4385 ** "--defines" and "--graph" have now an optional argument which is the
4386 output file name. "-d" and "-g" do not change; they do not take any
4389 ** "%source_extension" and "%header_extension" are removed, failed
4392 ** Portability fixes.
4395 * Noteworthy changes in release 1.29 (2001-09-07)
4397 ** The output file does not define const, as this caused problems when used
4398 with common autoconfiguration schemes. If you still use ancient compilers
4399 that lack const, compile with the equivalent of the C compiler option
4400 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4402 ** Added "-g" and "--graph".
4404 ** The Bison manual is now distributed under the terms of the GNU FDL.
4406 ** The input and the output files has automatically a similar extension.
4408 ** Russian translation added.
4410 ** NLS support updated; should hopefully be less troublesome.
4412 ** Added the old Bison reference card.
4414 ** Added "--locations" and "%locations".
4416 ** Added "-S" and "--skeleton".
4418 ** "%raw", "-r", "--raw" is disabled.
4420 ** Special characters are escaped when output. This solves the problems
4421 of the #line lines with path names including backslashes.
4424 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4425 "%debug", "%source_extension" and "%header_extension".
4428 Automatic location tracking.
4431 * Noteworthy changes in release 1.28 (1999-07-06)
4433 ** Should compile better now with K&R compilers.
4437 ** Fixed a problem with escaping the double quote character.
4439 ** There is now a FAQ.
4442 * Noteworthy changes in release 1.27
4444 ** The make rule which prevented bison.simple from being created on
4445 some systems has been fixed.
4448 * Noteworthy changes in release 1.26
4450 ** Bison now uses Automake.
4452 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4454 ** Token numbers now start at 257 as previously documented, not 258.
4456 ** Bison honors the TMPDIR environment variable.
4458 ** A couple of buffer overruns have been fixed.
4460 ** Problems when closing files should now be reported.
4462 ** Generated parsers should now work even on operating systems which do
4463 not provide alloca().
4466 * Noteworthy changes in release 1.25 (1995-10-16)
4468 ** Errors in the input grammar are not fatal; Bison keeps reading
4469 the grammar file, and reports all the errors found in it.
4471 ** Tokens can now be specified as multiple-character strings: for
4472 example, you could use "<=" for a token which looks like <=, instead
4473 of choosing a name like LESSEQ.
4475 ** The %token_table declaration says to write a table of tokens (names
4476 and numbers) into the parser file. The yylex function can use this
4477 table to recognize multiple-character string tokens, or for other
4480 ** The %no_lines declaration says not to generate any #line preprocessor
4481 directives in the parser file.
4483 ** The %raw declaration says to use internal Bison token numbers, not
4484 Yacc-compatible token numbers, when token names are defined as macros.
4486 ** The --no-parser option produces the parser tables without including
4487 the parser engine; a project can now use its own parser engine.
4488 The actions go into a separate file called NAME.act, in the form of
4489 a switch statement body.
4492 * Noteworthy changes in release 1.23
4494 The user can define YYPARSE_PARAM as the name of an argument to be
4495 passed into yyparse. The argument should have type void *. It should
4496 actually point to an object. Grammar actions can access the variable
4497 by casting it to the proper pointer type.
4499 Line numbers in output file corrected.
4502 * Noteworthy changes in release 1.22
4504 --help option added.
4507 * Noteworthy changes in release 1.20
4509 Output file does not redefine const for C++.
4513 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4514 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4515 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4516 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4517 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4518 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4519 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4520 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4521 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4522 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4523 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4524 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4525 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4526 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4527 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4528 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4529 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4530 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4531 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4532 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4533 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4534 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4535 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4536 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4537 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4538 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4539 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4540 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4541 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4542 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4543 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4544 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4545 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4546 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4547 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4548 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4549 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4552 ispell-dictionary: "american"
4557 Copyright (C) 1995-2015, 2018-2020 Free Software Foundation, Inc.
4559 This file is part of Bison, the GNU Parser Generator.
4561 Permission is granted to copy, distribute and/or modify this document
4562 under the terms of the GNU Free Documentation License, Version 1.3 or
4563 any later version published by the Free Software Foundation; with no
4564 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4565 Texts. A copy of the license is included in the "GNU Free
4566 Documentation License" file as part of this distribution.