3 * Noteworthy changes in release ?.? (????-??-??) [?]
6 * Noteworthy changes in release 3.7 (2020-07-23) [stable]
10 The YYPRINT macro, which works only with yacc.c and only for tokens, was
11 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
12 It is deprecated and its support will be removed eventually.
14 In conformance with the recommendations of the Graphviz team, in the next
15 version Bison the option `--graph` will generate a *.gv file by default,
16 instead of *.dot. A transition started in Bison 3.4.
20 *** Counterexample Generation
22 Contributed by Vincent Imbimbo.
24 When given `-Wcounterexamples`/`-Wcex`, bison will now output
25 counterexamples for conflicts.
27 **** Unifying Counterexamples
29 Unifying counterexamples are strings which can be parsed in two ways due
30 to the conflict. For example on a grammar that contains the usual
31 "dangling else" ambiguity:
34 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
35 else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
38 else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
39 else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
40 Example: "if" exp "then" "if" exp "then" exp • "else" exp
44 ↳ "if" exp "then" exp • "else" exp
45 Example: "if" exp "then" "if" exp "then" exp • "else" exp
48 ↳ "if" exp "then" exp "else" exp
49 ↳ "if" exp "then" exp •
51 When text styling is enabled, colors are used in the examples and the
52 derivations to highlight the structure of both analyses. In this case,
54 "if" exp "then" [ "if" exp "then" exp • ] "else" exp
58 "if" exp "then" [ "if" exp "then" exp • "else" exp ]
61 The counterexamples are "focused", in two different ways. First, they do
62 not clutter the output with all the derivations from the start symbol,
63 rather they start on the "conflicted nonterminal". They go straight to the
64 point. Second, they don't "expand" nonterminal symbols uselessly.
66 **** Nonunifying Counterexamples
68 In the case of the dangling else, Bison found an example that can be
69 parsed in two ways (therefore proving that the grammar is ambiguous).
70 When it cannot find such an example, it instead generates two examples
71 that are the same up until the dot:
74 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
75 foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
76 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
81 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
82 foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
83 First example: expr • ID ',' ID $end
90 Second example: expr • ID $end
96 foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
100 In these cases, the parser usually doesn't have enough lookahead to
101 differentiate the two given examples.
105 Counterexamples are also included in the report when given
106 `--report=counterexamples`/`-rcex` (or `--report=all`), with more
111 1 exp: "if" exp "then" exp • [$end, "then", "else"]
112 2 | "if" exp "then" exp • "else" exp
114 "else" shift, and go to state 8
116 "else" [reduce using rule 1 (exp)]
117 $default reduce using rule 1 (exp)
119 shift/reduce conflict on token "else":
120 1 exp: "if" exp "then" exp •
121 2 exp: "if" exp "then" exp • "else" exp
122 Example: "if" exp "then" "if" exp "then" exp • "else" exp
125 ↳ "if" exp "then" exp
126 ↳ "if" exp "then" exp • "else" exp
127 Example: "if" exp "then" "if" exp "then" exp • "else" exp
130 ↳ "if" exp "then" exp "else" exp
131 ↳ "if" exp "then" exp •
133 *** File prefix mapping
135 Contributed by Joshua Watt.
137 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
138 in the output (specifically `#line` directives and `#ifdef` header guards)
139 that begins with the prefix OLD will have it replaced with the prefix NEW,
140 similar to the `-ffile-prefix-map` in GCC. This option can be used to
141 make bison output reproducible.
147 When text styling is enabled and the terminal supports it, the warnings
148 now include hyperlinks to the documentation.
150 *** Relocatable installation
152 When installed to be relocatable (via `configure --enable-relocatable`),
153 bison will now also look for a relocated m4.
157 The `filename_type` %define variable was renamed `api.filename.type`.
160 %define filename_type "symbol"
164 %define api.filename.type {symbol}
166 (Or let `bison --update` do it for you).
168 It now defaults to `const std::string` instead of `std::string`.
170 *** Deprecated %define variable names
172 The following variables have been renamed for consistency. Backward
173 compatibility is ensured, but upgrading is recommended.
175 filename_type -> api.filename.type
176 package -> api.package
178 *** Push parsers no longer clear their state when parsing is finished
180 Previously push-parsers cleared their state when parsing was finished (on
181 success and on failure). This made it impossible to check if there were
182 parse errors, since `yynerrs` was also reset. This can be especially
183 troublesome when used in autocompletion, since a parser with error
184 recovery would suggest (irrelevant) expected tokens even if there were
187 Now the parser state can be examined when parsing is finished. The parser
188 state is reset when starting a new parse.
194 The bistromathic demonstrates %param and how to quote sources in the error
198 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
204 *** Include the generated header (yacc.c)
206 Historically, when --defines was used, bison generated a header and pasted
207 an exact copy of it into the generated parser implementation file. Since
208 Bison 3.4 it is possible to specify that the header should be `#include`d,
209 and how. For instance
211 %define api.header.include {"parse.h"}
215 %define api.header.include {<parser/parse.h>}
217 Now api.header.include defaults to `"header-basename"`, as was intended in
218 Bison 3.4, where `header-basename` is the basename of the generated
219 header. This is disabled when the generated header is `y.tab.h`, to
220 comply with Automake's ylwrap.
222 *** String aliases are faithfully propagated
224 Bison used to interpret user strings (i.e., decoding backslash escapes)
225 when reading them, and to escape them (i.e., issue non-printable
226 characters as backslash escapes, taking the locale into account) when
227 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
228 up "ciphered" as sequences of backslash escapes. This happened not only
229 in the generated sources (where the compiler will reinterpret them), but
230 also in all the generated reports (text, xml, html, dot, etc.). Reports
231 were therefore not readable when string aliases were not pure ASCII.
232 Worse yet: the output depended on the user's locale.
234 Now Bison faithfully treats the string aliases exactly the way the user
235 spelled them. This fixes all the aforementioned problems. However, now,
236 string aliases semantically equivalent but syntactically different (e.g.,
237 "A", "\x41", "\101") are considered to be different.
239 *** Crash when generating IELR
241 An old, well hidden, bug in the generation of IELR parsers was fixed.
244 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
248 In glr.cc some internal macros leaked in the user's code, and could damage
249 access to the token kinds.
252 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
256 Incorrect comments in the generated parsers.
258 Warnings in push parsers (yacc.c).
260 Incorrect display of gotos in LAC traces (lalr1.cc).
263 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
267 Some tests were fixed.
269 When token aliases contain comment delimiters:
271 %token FOO "/* foo */"
273 bison used to emit "nested" comments, which is invalid C.
276 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
280 Restored ANSI-C compliance in yacc.c.
282 GNU readline portability issues.
284 In C++, yy::parser::symbol_name is now a public member, as was intended.
288 In C++, yy::parser::symbol_type now has a public name() member function.
291 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
293 ** Backward incompatible changes
295 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
297 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
298 depend on it will now produce Yacc-like error messages (just "syntax
299 error"). It was superseded by the "%error-verbose" directive in Bison
300 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
301 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
302 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
303 parse.error verbose".
305 ** Deprecated features
307 The YYPRINT macro, which works only with yacc.c and only for tokens, was
308 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
309 It is deprecated and its support will be removed eventually.
313 *** Improved syntax error messages
315 Two new values for the %define parse.error variable offer more control to
316 the user. Available in all the skeletons (C, C++, Java).
318 **** %define parse.error detailed
320 The behavior of "%define parse.error detailed" is closely resembling that
321 of "%define parse.error verbose" with a few exceptions. First, it is safe
322 to use non-ASCII characters in token aliases (with 'verbose', the result
323 depends on the locale with which bison was run). Second, a yysymbol_name
324 function is exposed to the user, instead of the yytnamerr function and the
325 yytname table. Third, token internationalization is supported (see
328 **** %define parse.error custom
330 With this directive, the user forges and emits the syntax error message
331 herself by defining the yyreport_syntax_error function. A new type,
332 yypcontext_t, captures the circumstances of the error, and provides the
333 user with functions to get details, such as yypcontext_expected_tokens to
334 get the list of expected token kinds.
336 A possible implementation of yyreport_syntax_error is:
339 yyreport_syntax_error (const yypcontext_t *ctx)
342 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
343 fprintf (stderr, ": syntax error");
344 // Report the tokens expected at this point.
346 enum { TOKENMAX = 10 };
347 yysymbol_kind_t expected[TOKENMAX];
348 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
350 // Forward errors to yyparse.
353 for (int i = 0; i < n; ++i)
354 fprintf (stderr, "%s %s",
355 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
357 // Report the unexpected token.
359 yysymbol_kind_t lookahead = yypcontext_token (ctx);
360 if (lookahead != YYSYMBOL_YYEMPTY)
361 fprintf (stderr, " before %s", yysymbol_name (lookahead));
363 fprintf (stderr, "\n");
367 **** Token aliases internationalization
369 When the %define variable parse.error is set to `custom` or `detailed`,
370 one may specify which token aliases are to be translated using _(). For
382 In that case the user must define _() and N_(), and yysymbol_name returns
383 the translated symbol (i.e., it returns '_("variable")' rather that
384 '"variable"'). In Java, the user must provide an i18n() function.
386 *** List of expected tokens (yacc.c)
388 Push parsers may invoke yypstate_expected_tokens at any point during
389 parsing (including even before submitting the first token) to get the list
390 of possible tokens. This feature can be used to propose autocompletion
391 (see below the "bistromathic" example).
393 It makes little sense to use this feature without enabling LAC (lookahead
396 *** Returning the error token
398 When the scanner returns an invalid token or the undefined token
399 (YYUNDEF), the parser generates an error message and enters error
400 recovery. Because of that error message, most scanners that find lexical
401 errors generate an error message, and then ignore the invalid input
402 without entering the error-recovery.
404 The scanners may now return YYerror, the error token, to enter the
405 error-recovery mode without triggering an additional error message. See
406 the bistromathic for an example.
408 *** Deep overhaul of the symbol and token kinds
410 To avoid the confusion with types in programming languages, we now refer
411 to token and symbol "kinds" instead of token and symbol "types". The
412 documentation and error messages have been revised.
414 All the skeletons have been updated to use dedicated enum types rather
415 than integral types. Special symbols are now regular citizens, instead of
416 being declared in ad hoc ways.
420 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
421 LPAREN, etc. While backward compatibility is of course ensured, users are
422 nonetheless invited to replace their uses of "enum yytokentype" by
425 This type now also includes tokens that were previously hidden: YYEOF (end
426 of input), YYUNDEF (undefined token), and YYerror (error token). They
427 now have string aliases, internationalized when internationalization is
428 enabled. Therefore, by default, error messages now refer to "end of file"
429 (internationalized) rather than the cryptic "$end", or to "invalid token"
430 rather than "$undefined".
432 Therefore in most cases it is now useless to define the end-of-line token
435 %token T_EOF 0 "end of file"
437 Rather simply use "YYEOF" in your scanner.
441 The "symbol kinds" is what the parser actually uses. (Unless the
442 api.token.raw %define variable is used, the symbol kind of a terminal
443 differs from the corresponding token kind.)
445 They are now exposed as a enum, "yysymbol_kind_t".
447 This allows users to tailor the error messages the way they want, or to
448 process some symbols in a specific way in autocompletion (see the
449 bistromathic example below).
451 *** Modernize display of explanatory statements in diagnostics
453 Since Bison 2.7, output was indented four spaces for explanatory
454 statements. For example:
456 input.y:2.7-13: error: %type redeclaration for exp
457 input.y:1.7-11: previous declaration
459 Since the introduction of caret-diagnostics, it became less clear. This
460 indentation has been removed and submessages are displayed similarly as in
463 input.y:2.7-13: error: %type redeclaration for exp
464 2 | %type <float> exp
466 input.y:1.7-11: note: previous declaration
470 Contributed by Victor Morales Cayuela.
474 The token and symbol kinds are yy::parser::token_kind_type and
475 yy::parser::symbol_kind_type.
477 The symbol_type::kind() member function allows to get the kind of a
478 symbol. This can be used to write unit tests for scanners, e.g.,
480 yy::parser::symbol_type t = make_NUMBER ("123");
481 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
482 assert (t.value.as<int> () == 123);
488 In order to avoid ambiguities with "type" as in "typing", we now refer to
489 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
490 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
495 There are now examples/java: a very simple calculator, and a more complete
496 one (push-parser, location tracking, and debug traces).
498 The lexcalc example (a simple example in C based on Flex and Bison) now
499 also demonstrates location tracking.
502 A new C example, bistromathic, is a fully featured interactive calculator
503 using many Bison features: pure interface, push parser, autocompletion
504 based on the current parser state (using yypstate_expected_tokens),
505 location tracking, internationalized custom error messages, lookahead
506 correction, rich debug traces, etc.
508 It shows how to depend on the symbol kinds to tailor autocompletion. For
509 instance it recognizes the symbol kind "VARIABLE" to propose
510 autocompletion on the existing variables, rather than of the word
514 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
516 ** WARNING: Future backward-incompatibilities!
518 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
520 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
521 that still depend on it will produce Yacc-like error messages (just
522 "syntax error"). It was superseded by the "%error-verbose" directive in
523 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
524 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
525 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
526 parse.error verbose".
530 Fix portability issues of the package itself on old compilers.
532 Fix api.token.raw support in Java.
535 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
539 Error messages could quote lines containing zero-width characters (such as
540 \005) with incorrect styling. Fixes for similar issues with unexpectedly
541 short lines (e.g., the file was changed between parsing and diagnosing).
543 Several unlikely crashes found by fuzzing have been fixed.
546 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
550 Portability issues and minor cosmetic issues.
552 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
556 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
562 Fix compiler warnings.
565 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
567 ** Backward incompatible changes
569 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
570 longer treated as end-of-lines. This changes the diagnostics, and in
571 particular their locations.
573 In C++, line numbers and columns are now represented as 'int' not
574 'unsigned', so that integer overflow on positions is easily checkable via
575 'gcc -fsanitize=undefined' and the like. This affects the API for
576 positions. The default position and location classes now expose
577 'counter_type' (int), used to define line and column numbers.
579 ** Deprecated features
581 The YYPRINT macro, which works only with yacc.c and only for tokens, was
582 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
583 It is deprecated and its support will be removed eventually.
587 *** Lookahead correction in C++
589 Contributed by Adrian Vogelsgesang.
591 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
592 %define variable parse.lac.
594 *** Variable api.token.raw: Optimized token numbers (all skeletons)
596 In the generated parsers, tokens have two numbers: the "external" token
597 number as returned by yylex (which starts at 257), and the "internal"
598 symbol number (which starts at 3). Each time yylex is called, a table
599 lookup maps the external token number to the internal symbol number.
601 When the %define variable api.token.raw is set, tokens are assigned their
602 internal number, which saves one table lookup per token, and also saves
603 the generation of the mapping table.
605 The gain is typically moderate, but in extreme cases (very simple user
606 actions), a 10% improvement can be observed.
608 *** Generated parsers use better types for states
610 Stacks now use the best integral type for state numbers, instead of always
611 using 15 bits. As a result "small" parsers now have a smaller memory
612 footprint (they use 8 bits), and there is support for large automata (16
613 bits), and extra large (using int, i.e., typically 31 bits).
615 *** Generated parsers prefer signed integer types
617 Bison skeletons now prefer signed to unsigned integer types when either
618 will do, as the signed types are less error-prone and allow for better
619 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
620 portable to unusual machines where char, short and int are all the same
621 width. On non-GNU platforms this may entail including <limits.h> and (if
622 available) <stdint.h> to define integer types and constants.
624 *** A skeleton for the D programming language
626 For the last few releases, Bison has shipped a stealth experimental
627 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
628 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
631 However, because nobody has committed to improving, testing, and
632 documenting this skeleton, it is not clear that it will be supported in
635 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
636 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
638 *** Debug traces in Java
640 The Java backend no longer emits code and data for parser tracing if the
641 %define variable parse.trace is not defined.
645 *** New diagnostic: -Wdangling-alias
647 String literals, which allow for better error messages, are (too)
648 liberally accepted by Bison, which might result in silent errors. For
651 %type <exVal> cond "condition"
653 does not define "condition" as a string alias to 'cond' (nonterminal
654 symbols do not have string aliases). It is rather equivalent to
657 %token <exVal> "condition"
659 i.e., it gives the type 'exVal' to the "condition" token, which was
660 clearly not the intention.
662 Also, because string aliases need not be defined, typos such as "baz"
663 instead of "bar" will be not reported.
665 The option -Wdangling-alias catches these situations. On
668 %type <ival> foo "foo"
672 bison -Wdangling-alias reports
674 warning: string literal not attached to a symbol
675 | %type <ival> foo "foo"
677 warning: string literal not attached to a symbol
681 The -Wall option does not (yet?) include -Wdangling-alias.
683 *** Better POSIX Yacc compatibility diagnostics
685 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
689 %type <ival> TOKEN1 TOKEN2 't'
696 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
697 2 | %type <ival> TOKEN1 TOKEN2 't'
699 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
700 2 | %type <ival> TOKEN1 TOKEN2 't'
702 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
703 2 | %type <ival> TOKEN1 TOKEN2 't'
706 *** Diagnostics with insertion
708 The diagnostics now display the suggestion below the underlined source.
709 Replacement for undeclared symbols are now also suggested.
716 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'?
720 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
724 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
726 *** Diagnostics about long lines
728 Quoted sources may now be truncated to fit the screen. For instance, on a
729 30-column wide terminal:
736 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
739 foo.y:1.8-10: previous declaration
742 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
745 foo.y:1.8-10: previous declaration
751 *** Debugging glr.c and glr.cc
753 The glr.c skeleton always had asserts to check its own behavior (not the
754 user's). These assertions are now under the control of the parse.assert
755 %define variable (disabled by default).
759 Several new compiler warnings in the generated output have been avoided.
760 Some unused features are no longer emitted. Cleaner generated code in
765 Portability issues in the test suite.
767 In theory, parsers using %nonassoc could crash when reporting verbose
768 error messages. This unlikely bug has been fixed.
770 In Java, %define api.prefix was ignored. It now behaves as expected.
773 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
777 In some cases, when warnings are disabled, bison could emit tons of white
778 spaces as diagnostics.
780 When running out of memory, bison could crash (found by fuzzing).
782 When defining twice the EOF token, bison would crash.
784 New warnings from recent compilers have been addressed in the generated
785 parsers (yacc.c, glr.c, glr.cc).
787 When lone carriage-return characters appeared in the input file,
788 diagnostics could hang forever.
791 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
798 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
800 ** Deprecated features
802 The %pure-parser directive is deprecated in favor of '%define api.pure'
803 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
804 now. Note that since Bison 2.7 you are strongly encouraged to use
805 '%define api.pure full' instead of '%define api.pure'.
809 *** Colored diagnostics
811 As an experimental feature, diagnostics are now colored, controlled by the
812 new options --color and --style.
814 To use them, install the libtextstyle library before configuring Bison.
817 https://alpha.gnu.org/gnu/gettext/
821 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
823 The option --color supports the following arguments:
824 - always, yes: Enable colors.
825 - never, no: Disable colors.
826 - auto, tty (default): Enable colors if the output device is a tty.
828 To customize the styles, create a CSS file similar to
832 .error { font-weight: 800; text-decoration: underline; }
835 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
836 environment variable to "bison-bw.css".
840 When given -fsyntax-only, the diagnostics are reported, but no output is
843 The name of this option is somewhat misleading as bison does more than
844 just checking the syntax: every stage is run (including checking for
845 conflicts for instance), except the generation of the output files.
847 *** Include the generated header (yacc.c)
849 Before, when --defines is used, bison generated a header, and pasted an
850 exact copy of it into the generated parser implementation file. If the
851 header name is not "y.tab.h", it is now #included instead of being
854 To use an '#include' even if the header name is "y.tab.h" (which is what
855 happens with --yacc, or when using the Autotools' ylwrap), define
856 api.header.include to the exact argument to pass to #include. For
859 %define api.header.include {"parse.h"}
863 %define api.header.include {<parser/parse.h>}
865 *** api.location.type is now supported in C (yacc.c, glr.c)
867 The %define variable api.location.type defines the name of the type to use
868 for locations. When defined, Bison no longer defines YYLTYPE.
870 This can be used in programs with several parsers to factor their
871 definition of locations: let one of them generate them, and the others
878 In conformance with the recommendations of the Graphviz team, if %require
879 "3.4" (or better) is specified, the option --graph generates a *.gv file
880 by default, instead of *.dot.
882 *** Diagnostics overhaul
884 Column numbers were wrong with multibyte characters, which would also
885 result in skewed diagnostics with carets. Beside, because we were
886 indenting the quoted source with a single space, lines with tab characters
887 were incorrectly underlined.
889 To address these issues, and to be clearer, Bison now issues diagnostics
890 as GCC9 does. For instance it used to display (there's a tab before the
893 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
894 expr: expr '+' "number" { $$ = $1 + $2; }
898 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
899 3 | expr: expr '+' "number" { $$ = $1 + $2; }
902 Other constructs now also have better locations, resulting in more precise
905 *** Fix-it hints for %empty
907 Running Bison with -Wempty-rules and --update will remove incorrect %empty
908 annotations, and add the missing ones.
910 *** Generated reports
912 The format of the reports (parse.output) was improved for readability.
914 *** Better support for --no-line.
916 When --no-line is used, the generated files are now cleaner: no lines are
917 generated instead of empty lines. Together with using api.header.include,
918 that should help people saving the generated files into version control
919 systems get smaller diffs.
923 A new example in C shows an simple infix calculator with a hand-written
924 scanner (examples/c/calc).
926 A new example in C shows a reentrant parser (capable of recursive calls)
927 built with Flex and Bison (examples/c/reccalc).
929 There is a new section about the history of Yaccs and Bison.
933 A few obscure bugs were fixed, including the second oldest (known) bug in
934 Bison: it was there when Bison was entered in the RCS version control
935 system, in December 1987. See the NEWS of Bison 3.3 for the previous
939 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
943 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
947 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
951 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
952 extensions into errors. It now makes them simple warnings (-Wyacc).
955 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
957 A new mailing list was created, Bison Announce. It is low traffic, and is
958 only about announcing new releases and important messages (e.g., polls
959 about major decisions to make).
961 https://lists.gnu.org/mailman/listinfo/bison-announce
963 ** Backward incompatible changes
965 Support for DJGPP, which has been unmaintained and untested for years, is
968 ** Deprecated features
970 A new feature, --update (see below) helps adjusting existing grammars to
973 *** Deprecated directives
975 The %error-verbose directive is deprecated in favor of '%define
976 parse.error verbose' since Bison 3.0, but no warning was issued.
978 The '%name-prefix "xx"' directive is deprecated in favor of '%define
979 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
980 directives are slightly different, you might need to adjust your code.
981 %name-prefix renames only symbols with external linkage, while api.prefix
982 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
983 yytokentype, YYSTYPE, YYLTYPE, etc.
985 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
986 {xx}' will typically have to update YY_DECL from
988 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
992 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
994 *** Deprecated %define variable names
996 The following variables, mostly related to parsers in Java, have been
997 renamed for consistency. Backward compatibility is ensured, but upgrading
1000 abstract -> api.parser.abstract
1001 annotations -> api.parser.annotations
1002 extends -> api.parser.extends
1003 final -> api.parser.final
1004 implements -> api.parser.implements
1005 parser_class_name -> api.parser.class
1006 public -> api.parser.public
1007 strictfp -> api.parser.strictfp
1011 *** Generation of fix-its for IDEs/Editors
1013 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
1014 bison now generates machine readable editing instructions to fix some
1015 issues. Currently, this is mostly limited to updating deprecated
1016 directives and removing duplicates. For instance:
1020 %define parser_class_name "Parser"
1021 %define api.parser.class "Parser"
1025 See the "fix-it:" lines below:
1027 $ bison -ffixit foo.y
1028 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
1031 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
1032 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
1033 %define parser_class_name "Parser"
1034 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1035 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
1036 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
1037 %define api.parser.class "Parser"
1038 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1039 foo.y:2.1-34: previous definition
1040 %define parser_class_name "Parser"
1041 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1042 fix-it:"foo.y":{3:1-3:34}:""
1043 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
1045 This uses the same output format as GCC and Clang.
1047 *** Updating grammar files
1049 Fixes can be applied on the fly. The previous example ends with the
1050 suggestion to re-run bison with the option -u/--update, which results in a
1051 cleaner grammar file.
1053 $ bison --update foo.y
1055 bison: file 'foo.y' was updated (backup: 'foo.y~')
1058 %define parse.error verbose
1059 %define api.parser.class {Parser}
1063 *** Bison is now relocatable
1065 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1067 A relocatable program can be moved or copied to a different location on
1068 the file system. It can also be used through mount points for network
1069 sharing. It is possible to make symbolic links to the installed and moved
1070 programs, and invoke them through the symbolic link.
1072 *** %expect and %expect-rr modifiers on individual rules
1074 One can now document (and check) which rules participate in shift/reduce
1075 and reduce/reduce conflicts. This is particularly important GLR parsers,
1076 where conflicts are a normal occurrence. For example,
1092 | argument_list ',' expression
1097 Looking at the output from -v, one can see that the shift/reduce conflict
1098 here is due to the fact that the parser does not know whether to reduce
1099 arguments to argument_list until it sees the token _after_ the following
1100 ','. By marking the rule with %expect 1 (because there is a conflict in
1101 one state), we document the source of the 1 overall shift/reduce conflict.
1103 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1104 conflicts. In this case, we mark each of the conflicting rules. For
1113 target_list '=' expr ';'
1119 | target ',' target_list
1128 | expr ',' expr_list
1136 In a statement such as
1140 the parser must reduce x to a target or an expr, but does not know which
1141 until it sees the '='. So we notate the two possible reductions to
1142 indicate that each conflicts in one rule.
1144 This feature needs user feedback, and might evolve in the future.
1146 *** C++: Actual token constructors
1148 When variants and token constructors are enabled, in addition to the
1149 type-safe named token constructors (make_ID, make_INT, etc.), we now
1150 generate genuine constructors for symbol_type.
1152 For instance with these declarations
1158 you may use these constructors:
1160 symbol_type (int token, const std::string&);
1161 symbol_type (int token, const int&);
1162 symbol_type (int token);
1164 Correct matching between token types and value types is checked via
1165 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1166 constructors are preferable, as they offer better type safety (for
1167 instance 'make_ID (42)' would not even compile), but symbol_type
1168 constructors may help when token types are discovered at run-time, e.g.,
1171 if (auto i = lookup_keyword (yytext))
1172 return yy::parser::symbol_type (i);
1174 return yy::parser::make_ID (yytext);
1177 *** C++: Variadic emplace
1179 If your application requires C++11 and you don't use symbol constructors,
1180 you may now use a variadic emplace for semantic values:
1182 %define api.value.type variant
1183 %token <std::pair<int, int>> PAIR
1187 int yylex (parser::semantic_type *lvalp)
1189 lvalp->emplace <std::pair<int, int>> (1, 2);
1190 return parser::token::PAIR;
1193 *** C++: Syntax error exceptions in GLR
1195 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1196 actions, or from the scanner.
1198 *** More POSIX Yacc compatibility warnings
1200 More Bison specific directives are now reported with -y or -Wyacc. This
1201 change was ready since the release of Bison 3.0 in September 2015. It was
1202 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1203 in numerous warnings for Bison users that use the GNU Build System.
1205 If you still experience that problem, either redefine YACC as `bison -o
1206 y.tab.c`, or pass -Wno-yacc to Bison.
1208 *** The tables yyrhs and yyphrs are back
1210 Because no Bison skeleton uses them, these tables were removed (no longer
1211 passed to the skeletons, not even computed) in 2008. However, some users
1212 have expressed interest in being able to use them in their own skeletons.
1216 *** Incorrect number of reduce/reduce conflicts
1218 On a grammar such as
1220 exp: "num" | "num" | "num"
1222 bison used to report a single RR conflict, instead of two. This is now
1223 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1224 was entered in the RCS version control system, in December 1987.
1226 Some grammar files might have to adjust their %expect-rr.
1228 *** Parser directives that were not careful enough
1230 Passing invalid arguments to %nterm, for instance character literals, used
1231 to result in unclear error messages.
1235 The examples/ directory (installed in .../share/doc/bison/examples) has
1236 been restructured per language for clarity. The examples come with a
1237 README and a Makefile. Not only can they be used to toy with Bison, they
1238 can also be starting points for your own grammars.
1240 There is now a Java example, and a simple example in C based on Flex and
1241 Bison (examples/c/lexcalc/).
1247 They now use noexcept and constexpr. Please, report missing annotations.
1249 *** Symbol Declarations
1251 The syntax of the variation directives to declare symbols was overhauled
1252 for more consistency, and also better POSIX Yacc compliance (which, for
1253 instance, allows "%type" without actually providing a type). The %nterm
1254 directive, supported by Bison since its inception, is now documented and
1255 officially supported.
1257 The syntax is now as follows:
1259 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1260 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1261 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1262 %nterm TAG? ID+ ( TAG ID+ )*
1264 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1265 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1266 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1267 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1268 one), ‘*’ (zero or more) and ‘+’ (one or more).
1271 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1275 Fix the move constructor of symbol_type.
1277 Always provide a copy constructor for symbol_type, even in modern C++.
1280 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1284 Properly support token constructors in C++ with types that include commas
1285 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1288 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1292 C++ portability issues.
1295 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1299 Several portability issues have been fixed in the build system, in the
1300 test suite, and in the generated parsers in C++.
1303 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1305 ** Backward incompatible changes
1307 Support for DJGPP, which has been unmaintained and untested for years, is
1308 obsolete. Unless there is activity to revive it, it will be removed.
1312 %printers should use yyo rather than yyoutput to denote the output stream.
1314 Variant-based symbols in C++ should use emplace() rather than build().
1316 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1320 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1322 A comment in the generated code now emphasizes that users should not
1323 depend upon non-documented implementation details, such as macros starting
1328 *** C++: Support for move semantics (lalr1.cc)
1330 The lalr1.cc skeleton now fully supports C++ move semantics, while
1331 maintaining compatibility with C++98. You may now store move-only types
1332 when using Bison's variants. For instance:
1339 %skeleton "lalr1.cc"
1340 %define api.value.type variant
1344 %token <int> INT "int";
1345 %type <std::unique_ptr<int>> int;
1346 %type <std::vector<std::unique_ptr<int>>> list;
1350 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1352 int: "int" { $$ = std::make_unique<int>($1); }
1354 *** C++: Implicit move of right-hand side values (lalr1.cc)
1356 In modern C++ (C++11 and later), you should always use 'std::move' with
1357 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1358 popped from the stack anyway. Using 'std::move' is mandatory for
1359 move-only types such as unique_ptr, and it provides a significant speedup
1360 for large types such as std::string, or std::vector, etc.
1362 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1363 by 'std::move ($n)'. The second rule in the previous grammar can be
1366 list: list int { $$ = $1; $$.emplace_back($2); }
1368 With automove enabled, the semantic values are no longer lvalues, so do
1369 not use the swap idiom:
1371 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1373 This idiom is anyway obsolete: it is preferable to move than to swap.
1375 A warning is issued when automove is enabled, and a value is used several
1378 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1379 exp: "twice" exp { $$ = $2 + $2; }
1382 Enabling api.value.automove does not require support for modern C++. The
1383 generated code is valid C++98/03, but will use copies instead of moves.
1385 The new examples/c++/variant-11.yy shows these features in action.
1387 *** C++: The implicit default semantic action is always run
1389 When variants are enabled, the default action was not run, so
1397 It now behaves like in all the other cases, as
1399 exp: "number" { $$ = $1; }
1401 possibly using std::move if automove is enabled.
1403 We do not expect backward compatibility issues. However, beware of
1404 forward compatibility issues: if you rely on default actions with
1405 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1406 generate incorrect parsers.
1408 *** C++: Renaming location.hh
1410 When both %defines and %locations are enabled, Bison generates a
1411 location.hh file. If you don't use locations outside of the parser, you
1412 may avoid its creation with:
1414 %define api.location.file none
1416 However this file is useful if, for instance, your parser builds an AST
1417 decorated with locations: you may use Bison's location independently of
1418 Bison's parser. You can now give it another name, for instance:
1420 %define api.location.file "my-location.hh"
1422 This name can have directory components, and even be absolute. The name
1423 under which the location file is included is controlled by
1424 api.location.include.
1426 This way it is possible to have several parsers share the same location
1429 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1432 %define api.namespace {foo}
1433 %define api.location.file "include/ast/loc.hh"
1434 %define api.location.include {<ast/loc.hh>}
1436 and use it in src/bar/parser.hh:
1439 %define api.namespace {bar}
1440 %code requires {#include <ast/loc.hh>}
1441 %define api.location.type {bar::location}
1443 Absolute file names are supported, so in your Makefile, passing the flag
1444 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1447 *** C++: stack.hh and position.hh are deprecated
1449 When asked to generate a header file (%defines), the lalr1.cc skeleton
1450 generates a stack.hh file. This file had no interest for users; it is now
1451 made useless: its content is included in the parser definition. It is
1452 still generated for backward compatibility.
1454 When in addition to %defines, location support is requested (%locations),
1455 the file position.hh is also generated. It is now also useless: its
1456 content is now included in location.hh.
1458 These files are no longer generated when your grammar file requires at
1459 least Bison 3.2 (%require "3.2").
1463 Portability issues on MinGW and VS2015.
1465 Portability issues in the test suite.
1467 Portability/warning issues with Flex.
1470 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1472 ** Backward incompatible changes
1474 Compiling Bison now requires a C99 compiler---as announced during the
1475 release of Bison 3.0, five years ago. Generated parsers do not require a
1478 Support for DJGPP, which has been unmaintained and untested for years, is
1479 obsolete. Unless there is activity to revive it, the next release of Bison
1480 will have it removed.
1484 *** Typed midrule actions
1486 Because their type is unknown to Bison, the values of midrule actions are
1487 not treated like the others: they don't have %printer and %destructor
1488 support. It also prevents C++ (Bison) variants to handle them properly.
1490 Typed midrule actions address these issues. Instead of:
1492 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1496 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1498 *** Reports include the type of the symbols
1500 The sections about terminal and nonterminal symbols of the '*.output' file
1501 now specify their declared type. For instance, for:
1505 the report now shows '<ival>':
1507 Terminals, with rules where they appear
1511 *** Diagnostics about useless rules
1513 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1514 of course, its rules are useless too.
1518 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1520 Previously all the useless rules were reported, including those whose
1521 left-hand side is the 'exp' nonterminal:
1523 warning: 1 nonterminal useless in grammar [-Wother]
1524 warning: 4 rules useless in grammar [-Wother]
1525 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1528 2.14-16: warning: rule useless in grammar [-Wother]
1531 3.6-16: warning: rule useless in grammar [-Wother]
1532 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1534 3.20-30: warning: rule useless in grammar [-Wother]
1535 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1537 3.34-44: warning: rule useless in grammar [-Wother]
1538 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1541 Now, rules whose left-hand side symbol is useless are no longer reported
1542 as useless. The locations of the errors have also been adjusted to point
1543 to the first use of the nonterminal as a left-hand side of a rule:
1545 warning: 1 nonterminal useless in grammar [-Wother]
1546 warning: 4 rules useless in grammar [-Wother]
1547 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1548 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1550 2.14-16: warning: rule useless in grammar [-Wother]
1554 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1556 When compiled with exceptions disabled, the generated parsers no longer
1557 uses try/catch clauses.
1559 Currently only GCC and Clang are supported.
1563 *** A demonstration of variants
1565 A new example was added (installed in .../share/doc/bison/examples),
1566 'variant.yy', which shows how to use (Bison) variants in C++.
1568 The other examples were made nicer to read.
1570 *** Some features are no longer 'experimental'
1572 The following features, mature enough, are no longer flagged as
1573 experimental in the documentation: push parsers, default %printer and
1574 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1575 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1576 semantic predicates (%?).
1580 *** GLR: Predicates support broken by #line directives
1582 Predicates (%?) in GLR such as
1585 %? {new_syntax} 'w' id new_args
1586 | %?{!new_syntax} 'w' id old_args
1588 were issued with #lines in the middle of C code.
1590 *** Printer and destructor with broken #line directives
1592 The #line directives were not properly escaped when emitting the code for
1593 %printer/%destructor, which resulted in compiler errors if there are
1594 backslashes or double-quotes in the grammar file name.
1596 *** Portability on ICC
1598 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1599 Generated parsers now work around this.
1603 There were several small fixes in the test suite and in the build system,
1604 many warnings in bison and in the generated parsers were eliminated. The
1605 documentation also received its share of minor improvements.
1607 Useless code was removed from C++ parsers, and some of the generated
1608 constructors are more 'natural'.
1611 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1615 *** C++: Fix support of 'syntax_error'
1617 One incorrect 'inline' resulted in linking errors about the constructor of
1618 the syntax_error exception.
1620 *** C++: Fix warnings
1622 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1623 warnings about yyformat being possibly null. It also warned about the
1624 deprecated implicit definition of copy constructors when there's a
1625 user-defined (copy) assignment operator.
1627 *** Location of errors
1629 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1630 ride-hand side raises a syntax error. The behavior of the default parser
1631 (yacc.c) in such a condition was undefined.
1633 Now all the parsers match the behavior of glr.c: @$ is used as the
1634 location of the error. This handles gracefully rules with and without
1637 *** Portability fixes in the test suite
1639 On some platforms, some Java and/or C++ tests were failing.
1642 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1646 *** C++ with Variants (lalr1.cc)
1648 Fix a compiler warning when no %destructor use $$.
1652 Several portability issues in tests were fixed.
1655 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1659 *** C++ with Variants (lalr1.cc)
1661 Problems with %destructor and '%define parse.assert' have been fixed.
1663 *** Named %union support (yacc.c, glr.c)
1665 Bison 3.0 introduced a regression on named %union such as
1667 %union foo { int ival; };
1669 The possibility to use a name was introduced "for Yacc compatibility".
1670 It is however not required by POSIX Yacc, and its usefulness is not clear.
1672 *** %define api.value.type union with %defines (yacc.c, glr.c)
1674 The C parsers were broken when %defines was used together with "%define
1675 api.value.type union".
1677 *** Redeclarations are reported in proper order
1685 bison used to report:
1687 foo.yy:2.10-11: error: %printer redeclaration for FOO
1690 foo.yy:3.10-11: previous declaration
1694 Now, the "previous" declaration is always the first one.
1699 Bison now installs various files in its docdir (which defaults to
1700 '/usr/local/share/doc/bison'), including the three fully blown examples
1701 extracted from the documentation:
1704 Reverse Polish Calculator, a simple introductory example.
1706 Multi-function Calc, a calculator with memory and functions and located
1709 a calculator in C++ using variant support and token constructors.
1712 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1716 *** Generated source files when errors are reported
1718 When warnings are issued and -Werror is set, bison would still generate
1719 the source files (*.c, *.h...). As a consequence, some runs of "make"
1720 could fail the first time, but not the second (as the files were generated
1723 This is fixed: bison no longer generates this source files, but, of
1724 course, still produces the various reports (*.output, *.xml, etc.).
1726 *** %empty is used in reports
1728 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1729 dot, XML and formats derived from it).
1731 *** YYERROR and variants
1733 When C++ variant support is enabled, an error triggered via YYERROR, but
1734 not caught via error recovery, resulted in a double deletion.
1737 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1741 *** Errors in caret diagnostics
1743 On some platforms, some errors could result in endless diagnostics.
1745 *** Fixes of the -Werror option
1747 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1748 diagnostics into errors instead of warnings. This is fixed.
1750 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1751 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1752 -Wno-error", "foo" diagnostics are now errors.
1756 As demonstrated in the documentation, one can now leave spaces between
1761 The yacc.1 man page is no longer installed if --disable-yacc was
1764 *** Fixes in the test suite
1766 Bugs and portability issues.
1769 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1771 ** WARNING: Future backward-incompatibilities!
1773 Like other GNU packages, Bison will start using some of the C99 features
1774 for its own code, especially the definition of variables after statements.
1775 The generated C parsers still aim at C90.
1777 ** Backward incompatible changes
1779 *** Obsolete features
1781 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1783 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1784 use YYSTYPE and YYLTYPE.
1786 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1787 1.875): use %lex-param, %parse-param, or %param.
1789 Missing semicolons at the end of actions are no longer added (as announced
1790 in the release 2.5).
1792 *** Use of YACC='bison -y'
1794 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1797 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1798 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1799 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1801 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1802 implementation of Yacc, was using Bison as 'bison -y'. While it does
1803 ensure compatible output file names, it also enables warnings for
1804 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1805 warnings for Bison extensions.
1807 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1808 (which also generates 'y.tab.h' and 'y.output' when needed).
1809 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1810 flags (YFLAGS, or AM_YFLAGS with Automake).
1814 *** The epilogue is no longer affected by internal #defines (glr.c)
1816 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1817 generated code. These weren't properly undefined before the inclusion of
1818 the user epilogue, so functions such as the following were butchered by the
1819 preprocessor expansion:
1821 int yylex (YYSTYPE *yylval);
1823 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
1824 identifiers for user-provided variables.
1826 *** stdio.h is no longer needed when locations are enabled (yacc.c)
1828 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
1829 locations are enabled. This is fixed.
1831 *** Warnings about useless %pure-parser/%define api.pure are restored
1833 ** Diagnostics reported by Bison
1835 Most of these features were contributed by Théophile Ranquet and Victor
1840 Version 2.7 introduced caret errors, for a prettier output. These are now
1841 activated by default. The old format can still be used by invoking Bison
1842 with -fno-caret (or -fnone).
1844 Some error messages that reproduced excerpts of the grammar are now using
1845 the caret information only. For instance on:
1852 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1853 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
1857 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1858 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
1862 and "bison -fno-caret" reports:
1864 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1865 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
1867 *** Enhancements of the -Werror option
1869 The -Werror=CATEGORY option is now recognized, and will treat specified
1870 warnings as errors. The warnings need not have been explicitly activated
1871 using the -W option, this is similar to what GCC 4.7 does.
1873 For example, given the following command line, Bison will treat both
1874 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
1875 errors (and only those):
1877 $ bison -Werror=yacc,error=conflicts-sr input.y
1879 If no categories are specified, -Werror will make all active warnings into
1880 errors. For example, the following line does the same the previous example:
1882 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
1884 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
1886 Note that the categories in this -Werror option may not be prefixed with
1887 "no-". However, -Wno-error[=CATEGORY] is valid.
1889 Note that -y enables -Werror=yacc. Therefore it is now possible to require
1890 Yacc-like behavior (e.g., always generate y.tab.c), but to report
1891 incompatibilities as warnings: "-y -Wno-error=yacc".
1893 *** The display of warnings is now richer
1895 The option that controls a given warning is now displayed:
1897 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
1899 In the case of warnings treated as errors, the prefix is changed from
1900 "warning: " to "error: ", and the suffix is displayed, in a manner similar
1901 to GCC, as [-Werror=CATEGORY].
1903 For instance, where the previous version of Bison would report (and exit
1906 bison: warnings being treated as errors
1907 input.y:1.1: warning: stray ',' treated as white space
1911 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
1913 *** Deprecated constructs
1915 The new 'deprecated' warning category flags obsolete constructs whose
1916 support will be discontinued. It is enabled by default. These warnings
1917 used to be reported as 'other' warnings.
1919 *** Useless semantic types
1921 Bison now warns about useless (uninhabited) semantic types. Since
1922 semantic types are not declared to Bison (they are defined in the opaque
1923 %union structure), it is %printer/%destructor directives about useless
1924 types that trigger the warning:
1928 %printer {} <type1> <type3>
1929 %destructor {} <type2> <type4>
1931 nterm: term { $$ = $1; };
1933 3.28-34: warning: type <type3> is used, but is not associated to any symbol
1934 4.28-34: warning: type <type4> is used, but is not associated to any symbol
1936 *** Undefined but unused symbols
1938 Bison used to raise an error for undefined symbols that are not used in
1939 the grammar. This is now only a warning.
1942 %destructor {} symbol2
1943 %type <type> symbol3
1947 *** Useless destructors or printers
1949 Bison now warns about useless destructors or printers. In the following
1950 example, the printer for <type1>, and the destructor for <type2> are
1951 useless: all symbols of <type1> (token1) already have a printer, and all
1952 symbols of type <type2> (token2) already have a destructor.
1954 %token <type1> token1
1958 %printer {} token1 <type1> <type3>
1959 %destructor {} token2 <type2> <type4>
1963 The warnings and error messages about shift/reduce and reduce/reduce
1964 conflicts have been normalized. For instance on the following foo.y file:
1968 exp: exp '+' exp | '0' | '0';
1970 compare the previous version of bison:
1973 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1974 $ bison -Werror foo.y
1975 bison: warnings being treated as errors
1976 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1978 with the new behavior:
1981 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
1982 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
1983 $ bison -Werror foo.y
1984 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
1985 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
1987 When %expect or %expect-rr is used, such as with bar.y:
1992 exp: exp '+' exp | '0' | '0';
1997 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1998 bar.y: expected 0 shift/reduce conflicts
1999 bar.y: expected 0 reduce/reduce conflicts
2004 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
2005 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
2007 ** Incompatibilities with POSIX Yacc
2009 The 'yacc' category is no longer part of '-Wall', enable it explicitly
2012 ** Additional yylex/yyparse arguments
2014 The new directive %param declares additional arguments to both yylex and
2015 yyparse. The %lex-param, %parse-param, and %param directives support one
2016 or more arguments. Instead of
2018 %lex-param {arg1_type *arg1}
2019 %lex-param {arg2_type *arg2}
2020 %parse-param {arg1_type *arg1}
2021 %parse-param {arg2_type *arg2}
2025 %param {arg1_type *arg1} {arg2_type *arg2}
2027 ** Types of values for %define variables
2029 Bison used to make no difference between '%define foo bar' and '%define
2030 foo "bar"'. The former is now called a 'keyword value', and the latter a
2031 'string value'. A third kind was added: 'code values', such as '%define
2034 Keyword variables are used for fixed value sets, e.g.,
2036 %define lr.type lalr
2038 Code variables are used for value in the target language, e.g.,
2040 %define api.value.type {struct semantic_type}
2042 String variables are used remaining cases, e.g. file names.
2044 ** Variable api.token.prefix
2046 The variable api.token.prefix changes the way tokens are identified in
2047 the generated files. This is especially useful to avoid collisions
2048 with identifiers in the target language. For instance
2050 %token FILE for ERROR
2051 %define api.token.prefix {TOK_}
2053 start: FILE for ERROR;
2055 will generate the definition of the symbols TOK_FILE, TOK_for, and
2056 TOK_ERROR in the generated sources. In particular, the scanner must
2057 use these prefixed token names, although the grammar itself still
2058 uses the short names (as in the sample rule given above).
2060 ** Variable api.value.type
2062 This new %define variable supersedes the #define macro YYSTYPE. The use
2063 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2064 using %union or %defining api.value.type results in undefined behavior.
2066 Either define api.value.type, or use "%union":
2073 %token <ival> INT "integer"
2074 %token <sval> STRING "string"
2075 %printer { fprintf (yyo, "%d", $$); } <ival>
2076 %destructor { free ($$); } <sval>
2079 yylval.ival = 42; return INT;
2080 yylval.sval = "42"; return STRING;
2082 The %define variable api.value.type supports both keyword and code values.
2084 The keyword value 'union' means that the user provides genuine types, not
2085 union member names such as "ival" and "sval" above (WARNING: will fail if
2086 -y/--yacc/%yacc is enabled).
2088 %define api.value.type union
2089 %token <int> INT "integer"
2090 %token <char *> STRING "string"
2091 %printer { fprintf (yyo, "%d", $$); } <int>
2092 %destructor { free ($$); } <char *>
2095 yylval.INT = 42; return INT;
2096 yylval.STRING = "42"; return STRING;
2098 The keyword value variant is somewhat equivalent, but for C++ special
2099 provision is made to allow classes to be used (more about this below).
2101 %define api.value.type variant
2102 %token <int> INT "integer"
2103 %token <std::string> STRING "string"
2105 Code values (in braces) denote user defined types. This is where YYSTYPE
2123 %define api.value.type {struct my_value}
2124 %token <u.ival> INT "integer"
2125 %token <u.sval> STRING "string"
2126 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2127 %destructor { free ($$); } <u.sval>
2130 yylval.u.ival = 42; return INT;
2131 yylval.u.sval = "42"; return STRING;
2133 ** Variable parse.error
2135 This variable controls the verbosity of error messages. The use of the
2136 %error-verbose directive is deprecated in favor of "%define parse.error
2139 ** Deprecated %define variable names
2141 The following variables have been renamed for consistency. Backward
2142 compatibility is ensured, but upgrading is recommended.
2144 lr.default-reductions -> lr.default-reduction
2145 lr.keep-unreachable-states -> lr.keep-unreachable-state
2146 namespace -> api.namespace
2147 stype -> api.value.type
2149 ** Semantic predicates
2151 Contributed by Paul Hilfinger.
2153 The new, experimental, semantic-predicate feature allows actions of the
2154 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2155 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2156 in GLR parsers, rather than being deferred. The result is that they allow
2157 the programmer to prune possible parses based on the values of run-time
2160 ** The directive %expect-rr is now an error in non GLR mode
2162 It used to be an error only if used in non GLR mode, _and_ if there are
2163 reduce/reduce conflicts.
2165 ** Tokens are numbered in their order of appearance
2167 Contributed by Valentin Tolmer.
2169 With '%token A B', A had a number less than the one of B. However,
2170 precedence declarations used to generate a reversed order. This is now
2171 fixed, and introducing tokens with any of %token, %left, %right,
2172 %precedence, or %nonassoc yields the same result.
2174 When mixing declarations of tokens with a literal character (e.g., 'a') or
2175 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2176 the literal characters first. For example
2180 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2181 input order is now preserved.
2183 These changes were made so that one can remove useless precedence and
2184 associativity declarations (i.e., map %nonassoc, %left or %right to
2185 %precedence, or to %token) and get exactly the same output.
2187 ** Useless precedence and associativity
2189 Contributed by Valentin Tolmer.
2191 When developing and maintaining a grammar, useless associativity and
2192 precedence directives are common. They can be a nuisance: new ambiguities
2193 arising are sometimes masked because their conflicts are resolved due to
2194 the extra precedence or associativity information. Furthermore, it can
2195 hinder the comprehension of a new grammar: one will wonder about the role
2196 of a precedence, where in fact it is useless. The following changes aim
2197 at detecting and reporting these extra directives.
2199 *** Precedence warning category
2201 A new category of warning, -Wprecedence, was introduced. It flags the
2202 useless precedence and associativity directives.
2204 *** Useless associativity
2206 Bison now warns about symbols with a declared associativity that is never
2207 used to resolve conflicts. In that case, using %precedence is sufficient;
2208 the parsing tables will remain unchanged. Solving these warnings may raise
2209 useless precedence warnings, as the symbols no longer have associativity.
2223 warning: useless associativity for '+', use %precedence [-Wprecedence]
2227 *** Useless precedence
2229 Bison now warns about symbols with a declared precedence and no declared
2230 associativity (i.e., declared with %precedence), and whose precedence is
2231 never used. In that case, the symbol can be safely declared with %token
2232 instead, without modifying the parsing tables. For example:
2236 exp: "var" '=' "number";
2240 warning: useless precedence for '=' [-Wprecedence]
2244 *** Useless precedence and associativity
2246 In case of both useless precedence and associativity, the issue is flagged
2251 exp: "var" '=' "number";
2255 warning: useless precedence and associativity for '=' [-Wprecedence]
2261 With help from Joel E. Denny and Gabriel Rassoul.
2263 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2264 marked by the new %empty directive. Using %empty on a non-empty rule is
2265 an error. The new -Wempty-rule warning reports empty rules without
2266 %empty. On the following grammar:
2276 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2279 5.8-13: error: %empty on non-empty rule
2283 ** Java skeleton improvements
2285 The constants for token names were moved to the Lexer interface. Also, it
2286 is possible to add code to the parser's constructors using "%code init"
2287 and "%define init_throws".
2288 Contributed by Paolo Bonzini.
2290 The Java skeleton now supports push parsing.
2291 Contributed by Dennis Heimbigner.
2293 ** C++ skeletons improvements
2295 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2297 Using %defines is now optional. Without it, the needed support classes
2298 are defined in the generated parser, instead of additional files (such as
2299 location.hh, position.hh and stack.hh).
2301 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2303 Both lalr1.cc and glr.cc no longer require %location.
2305 *** syntax_error exception (lalr1.cc)
2307 The C++ parser features a syntax_error exception, which can be
2308 thrown from the scanner or from user rules to raise syntax errors.
2309 This facilitates reporting errors caught in sub-functions (e.g.,
2310 rejecting too large integral literals from a conversion function
2311 used by the scanner, or rejecting invalid combinations from a
2312 factory invoked by the user actions).
2314 *** %define api.value.type variant
2316 This is based on a submission from Michiel De Wilde. With help
2317 from Théophile Ranquet.
2319 In this mode, complex C++ objects can be used as semantic values. For
2322 %token <::std::string> TEXT;
2323 %token <int> NUMBER;
2324 %token SEMICOLON ";"
2325 %type <::std::string> item;
2326 %type <::std::list<std::string>> list;
2329 list { std::cout << $1 << std::endl; }
2333 %empty { /* Generates an empty string list. */ }
2334 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2338 TEXT { std::swap ($$, $1); }
2339 | NUMBER { $$ = string_cast ($1); }
2342 *** %define api.token.constructor
2344 When variants are enabled, Bison can generate functions to build the
2345 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2346 with the semantic value (e.g., int):
2348 parser::symbol_type yylex ()
2350 parser::location_type loc = ...;
2352 return parser::make_TEXT ("Hello, world!", loc);
2354 return parser::make_NUMBER (42, loc);
2356 return parser::make_SEMICOLON (loc);
2362 There are operator- and operator-= for 'location'. Negative line/column
2363 increments can no longer underflow the resulting value.
2366 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2370 *** Fix compiler attribute portability (yacc.c)
2372 With locations enabled, __attribute__ was used unprotected.
2374 *** Fix some compiler warnings (lalr1.cc)
2377 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2381 Warnings about uninitialized yylloc in yyparse have been fixed.
2383 Restored C90 compliance (yet no report was ever made).
2385 ** Diagnostics are improved
2387 Contributed by Théophile Ranquet.
2389 *** Changes in the format of error messages
2391 This used to be the format of many error reports:
2393 input.y:2.7-12: %type redeclaration for exp
2394 input.y:1.7-12: previous declaration
2398 input.y:2.7-12: error: %type redeclaration for exp
2399 input.y:1.7-12: previous declaration
2401 *** New format for error reports: carets
2403 Caret errors have been added to Bison:
2405 input.y:2.7-12: error: %type redeclaration for exp
2408 input.y:1.7-12: previous declaration
2414 input.y:3.20-23: error: ambiguous reference: '$exp'
2415 exp: exp '+' exp { $exp = $1 + $3; };
2417 input.y:3.1-3: refers to: $exp at $$
2418 exp: exp '+' exp { $exp = $1 + $3; };
2420 input.y:3.6-8: refers to: $exp at $1
2421 exp: exp '+' exp { $exp = $1 + $3; };
2423 input.y:3.14-16: refers to: $exp at $3
2424 exp: exp '+' exp { $exp = $1 + $3; };
2427 The default behavior for now is still not to display these unless
2428 explicitly asked with -fcaret (or -fall). However, in a later release, it
2429 will be made the default behavior (but may still be deactivated with
2432 ** New value for %define variable: api.pure full
2434 The %define variable api.pure requests a pure (reentrant) parser. However,
2435 for historical reasons, using it in a location-tracking Yacc parser
2436 resulted in a yyerror function that did not take a location as a
2437 parameter. With this new value, the user may request a better pure parser,
2438 where yyerror does take a location as a parameter (in location-tracking
2441 The use of "%define api.pure true" is deprecated in favor of this new
2442 "%define api.pure full".
2444 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2446 The %define variable api.location.type defines the name of the type to use
2447 for locations. When defined, Bison no longer generates the position.hh
2448 and location.hh files, nor does the parser will include them: the user is
2449 then responsible to define her type.
2451 This can be used in programs with several parsers to factor their location
2452 and position files: let one of them generate them, and the others just use
2455 This feature was actually introduced, but not documented, in Bison 2.5,
2456 under the name "location_type" (which is maintained for backward
2459 For consistency, lalr1.java's %define variables location_type and
2460 position_type are deprecated in favor of api.location.type and
2463 ** Exception safety (lalr1.cc)
2465 The parse function now catches exceptions, uses the %destructors to
2466 release memory (the lookahead symbol and the symbols pushed on the stack)
2467 before re-throwing the exception.
2469 This feature is somewhat experimental. User feedback would be
2472 ** Graph improvements in DOT and XSLT
2474 Contributed by Théophile Ranquet.
2476 The graphical presentation of the states is more readable: their shape is
2477 now rectangular, the state number is clearly displayed, and the items are
2478 numbered and left-justified.
2480 The reductions are now explicitly represented as transitions to other
2481 diamond shaped nodes.
2483 These changes are present in both --graph output and xml2dot.xsl XSLT
2484 processing, with minor (documented) differences.
2486 ** %language is no longer an experimental feature.
2488 The introduction of this feature, in 2.4, was four years ago. The
2489 --language option and the %language directive are no longer experimental.
2493 The sections about shift/reduce and reduce/reduce conflicts resolution
2494 have been fixed and extended.
2496 Although introduced more than four years ago, XML and Graphviz reports
2497 were not properly documented.
2499 The translation of midrule actions is now described.
2502 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2504 We consider compiler warnings about Bison generated parsers to be bugs.
2505 Rather than working around them in your own project, please consider
2506 reporting them to us.
2510 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2511 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2514 Other issues in the test suite have been addressed.
2516 Null characters are correctly displayed in error messages.
2518 When possible, yylloc is correctly initialized before calling yylex. It
2519 is no longer necessary to initialize it in the %initial-action.
2522 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2524 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2527 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2531 Bugs and portability issues in the test suite have been fixed.
2533 Some errors in translations have been addressed, and --help now directs
2534 users to the appropriate place to report them.
2536 Stray Info files shipped by accident are removed.
2538 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2539 generated, are removed.
2541 All the generated headers are self-contained.
2543 ** Header guards (yacc.c, glr.c, glr.cc)
2545 In order to avoid collisions, the header guards are now
2546 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2547 For instance the header generated from
2549 %define api.prefix "calc"
2550 %defines "lib/parse.h"
2552 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2554 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2556 The compilation of pure parsers (%define api.pure) can trigger GCC
2559 input.c: In function 'yyparse':
2560 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2561 function [-Wmaybe-uninitialized]
2565 This is now fixed; pragmas to avoid these warnings are no longer needed.
2567 Warnings from clang ("equality comparison with extraneous parentheses" and
2568 "function declared 'noreturn' should not return") have also been
2572 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2576 Buffer overruns, complaints from Flex, and portability issues in the test
2577 suite have been fixed.
2579 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2581 Trailing end-of-lines in %parse-param or %lex-param would result in
2582 invalid C++. This is fixed.
2584 ** Spurious spaces and end-of-lines
2586 The generated files no longer end (nor start) with empty lines.
2589 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2591 Bison no longer executes user-specified M4 code when processing a grammar.
2595 In addition to the removal of the features announced in Bison 2.6, the
2596 next major release will remove the "Temporary hack for adding a semicolon
2597 to the user action", as announced in the release 2.5. Instead of:
2599 exp: exp "+" exp { $$ = $1 + $3 };
2603 exp: exp "+" exp { $$ = $1 + $3; };
2607 *** Type names are now properly escaped.
2609 *** glr.cc: set_debug_level and debug_level work as expected.
2611 *** Stray @ or $ in actions
2613 While Bison used to warn about stray $ or @ in action rules, it did not
2614 for other actions such as printers, destructors, or initial actions. It
2617 ** Type names in actions
2619 For consistency with rule actions, it is now possible to qualify $$ by a
2620 type-name in destructors, printers, and initial actions. For instance:
2622 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2624 will display two values for each typed and untyped symbol (provided
2625 that YYSTYPE has both "ival" and "fval" fields).
2628 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2632 The next major release of Bison will drop support for the following
2633 deprecated features. Please report disagreements to bug-bison@gnu.org.
2637 Support for generating parsers in K&R C will be removed. Parsers
2638 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2641 *** Features deprecated since Bison 1.875
2643 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2646 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2647 %lex-param, will no longer be supported.
2649 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2652 *** The generated header will be included (yacc.c)
2654 Instead of duplicating the content of the generated header (definition of
2655 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2656 as is already the case for GLR or C++ parsers. This change is deferred
2657 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2660 ** Generated Parser Headers
2662 *** Guards (yacc.c, glr.c, glr.cc)
2664 The generated headers are now guarded, as is already the case for C++
2665 parsers (lalr1.cc). For instance, with --defines=foo.h:
2670 #endif /* !YY_FOO_H */
2672 *** New declarations (yacc.c, glr.c)
2674 The generated header now declares yydebug and yyparse. Both honor
2675 --name-prefix=bar_, and yield
2677 int bar_parse (void);
2681 #define yyparse bar_parse
2684 in order to facilitate the inclusion of several parser headers inside a
2685 single compilation unit.
2687 *** Exported symbols in C++
2689 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2690 header, are removed, as they prevent the possibility of including several
2691 generated headers from a single compilation unit.
2695 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2698 ** New %define variable: api.prefix
2700 Now that the generated headers are more complete and properly protected
2701 against multiple inclusions, constant names, such as YYSTYPE are a
2702 problem. While yyparse and others are properly renamed by %name-prefix,
2703 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2704 would introduce backward compatibility issues in projects not expecting
2705 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2706 it is deprecated in favor of a new %define variable: api.prefix.
2708 The following examples compares both:
2710 %name-prefix "bar_" | %define api.prefix "bar_"
2711 %token <ival> FOO %token <ival> FOO
2712 %union { int ival; } %union { int ival; }
2718 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2719 # define BAR_FOO_H # define BAR_FOO_H
2721 /* Enabling traces. */ /* Enabling traces. */
2722 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2723 > # if defined YYDEBUG
2725 > # define BAR_DEBUG 1
2727 > # define BAR_DEBUG 0
2730 # define YYDEBUG 0 | # define BAR_DEBUG 0
2734 # if YYDEBUG | # if BAR_DEBUG
2735 extern int bar_debug; extern int bar_debug;
2738 /* Tokens. */ /* Tokens. */
2739 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2740 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2741 enum yytokentype { | enum bar_tokentype {
2746 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2747 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2748 typedef union YYSTYPE | typedef union BAR_STYPE
2751 } YYSTYPE; | } BAR_STYPE;
2752 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2755 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2757 int bar_parse (void); int bar_parse (void);
2759 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2762 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2766 The next major release will drop support for generating parsers in K&R C.
2768 ** yacc.c: YYBACKUP works as expected.
2770 ** glr.c improvements:
2772 *** Location support is eliminated when not requested:
2774 GLR parsers used to include location-related code even when locations were
2775 not requested, and therefore not even usable.
2777 *** __attribute__ is preserved:
2779 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2780 when -std is passed to GCC).
2782 ** lalr1.java: several fixes:
2784 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2785 first token leads to a syntax error. Some minor clean ups.
2789 *** C++11 compatibility:
2791 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2796 The header files such as "parser.hh", "location.hh", etc. used a constant
2797 name for preprocessor guards, for instance:
2799 #ifndef BISON_LOCATION_HH
2800 # define BISON_LOCATION_HH
2802 #endif // !BISON_LOCATION_HH
2804 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2805 case characters are converted to upper case, and series of
2806 non-alphanumerical characters are converted to an underscore.
2808 With "bison -o lang++/parser.cc", "location.hh" would now include:
2810 #ifndef YY_LANG_LOCATION_HH
2811 # define YY_LANG_LOCATION_HH
2813 #endif // !YY_LANG_LOCATION_HH
2817 The position and location constructors (and their initialize methods)
2818 accept new arguments for line and column. Several issues in the
2819 documentation were fixed.
2821 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2823 ** Changes in the manual:
2825 *** %printer is documented
2827 The "%printer" directive, supported since at least Bison 1.50, is finally
2828 documented. The "mfcalc" example is extended to demonstrate it.
2830 For consistency with the C skeletons, the C++ parsers now also support
2831 "yyoutput" (as an alias to "debug_stream ()").
2833 *** Several improvements have been made:
2835 The layout for grammar excerpts was changed to a more compact scheme.
2836 Named references are motivated. The description of the automaton
2837 description file (*.output) is updated to the current format. Incorrect
2838 index entries were fixed. Some other errors were fixed.
2842 *** Conflicting prototypes with recent/modified Flex.
2844 Fixed build problems with the current, unreleased, version of Flex, and
2845 some modified versions of 2.5.35, which have modified function prototypes.
2847 *** Warnings during the build procedure have been eliminated.
2849 *** Several portability problems in the test suite have been fixed:
2851 This includes warnings with some compilers, unexpected behavior of tools
2852 such as diff, warning messages from the test suite itself, etc.
2854 *** The install-pdf target works properly:
2856 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
2857 halts in the middle of its course.
2860 * Noteworthy changes in release 2.5 (2011-05-14)
2862 ** Grammar symbol names can now contain non-initial dashes:
2864 Consistently with directives (such as %error-verbose) and with
2865 %define variables (e.g. push-pull), grammar symbol names may contain
2866 dashes in any position except the beginning. This is a GNU
2867 extension over POSIX Yacc. Thus, use of this extension is reported
2868 by -Wyacc and rejected in Yacc mode (--yacc).
2870 ** Named references:
2872 Historically, Yacc and Bison have supported positional references
2873 ($n, $$) to allow access to symbol values from inside of semantic
2876 Starting from this version, Bison can also accept named references.
2877 When no ambiguity is possible, original symbol names may be used
2878 as named references:
2880 if_stmt : "if" cond_expr "then" then_stmt ';'
2881 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
2883 In the more common case, explicit names may be declared:
2885 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
2886 { $res = mk_if_stmt($cond, $then, $else); }
2888 Location information is also accessible using @name syntax. When
2889 accessing symbol names containing dots or dashes, explicit bracketing
2890 ($[sym.1]) must be used.
2892 These features are experimental in this version. More user feedback
2893 will help to stabilize them.
2894 Contributed by Alex Rozenman.
2896 ** IELR(1) and canonical LR(1):
2898 IELR(1) is a minimal LR(1) parser table generation algorithm. That
2899 is, given any context-free grammar, IELR(1) generates parser tables
2900 with the full language-recognition power of canonical LR(1) but with
2901 nearly the same number of parser states as LALR(1). This reduction
2902 in parser states is often an order of magnitude. More importantly,
2903 because canonical LR(1)'s extra parser states may contain duplicate
2904 conflicts in the case of non-LR(1) grammars, the number of conflicts
2905 for IELR(1) is often an order of magnitude less as well. This can
2906 significantly reduce the complexity of developing of a grammar.
2908 Bison can now generate IELR(1) and canonical LR(1) parser tables in
2909 place of its traditional LALR(1) parser tables, which remain the
2910 default. You can specify the type of parser tables in the grammar
2911 file with these directives:
2913 %define lr.type lalr
2914 %define lr.type ielr
2915 %define lr.type canonical-lr
2917 The default-reduction optimization in the parser tables can also be
2918 adjusted using "%define lr.default-reductions". For details on both
2919 of these features, see the new section "Tuning LR" in the Bison
2922 These features are experimental. More user feedback will help to
2925 ** LAC (Lookahead Correction) for syntax error handling
2927 Contributed by Joel E. Denny.
2929 Canonical LR, IELR, and LALR can suffer from a couple of problems
2930 upon encountering a syntax error. First, the parser might perform
2931 additional parser stack reductions before discovering the syntax
2932 error. Such reductions can perform user semantic actions that are
2933 unexpected because they are based on an invalid token, and they
2934 cause error recovery to begin in a different syntactic context than
2935 the one in which the invalid token was encountered. Second, when
2936 verbose error messages are enabled (with %error-verbose or the
2937 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
2938 syntax error message can both contain invalid tokens and omit valid
2941 The culprits for the above problems are %nonassoc, default
2942 reductions in inconsistent states, and parser state merging. Thus,
2943 IELR and LALR suffer the most. Canonical LR can suffer only if
2944 %nonassoc is used or if default reductions are enabled for
2945 inconsistent states.
2947 LAC is a new mechanism within the parsing algorithm that solves
2948 these problems for canonical LR, IELR, and LALR without sacrificing
2949 %nonassoc, default reductions, or state merging. When LAC is in
2950 use, canonical LR and IELR behave almost exactly the same for both
2951 syntactically acceptable and syntactically unacceptable input.
2952 While LALR still does not support the full language-recognition
2953 power of canonical LR and IELR, LAC at least enables LALR's syntax
2954 error handling to correctly reflect LALR's language-recognition
2957 Currently, LAC is only supported for deterministic parsers in C.
2958 You can enable LAC with the following directive:
2960 %define parse.lac full
2962 See the new section "LAC" in the Bison manual for additional
2963 details including a few caveats.
2965 LAC is an experimental feature. More user feedback will help to
2968 ** %define improvements:
2970 *** Can now be invoked via the command line:
2972 Each of these command-line options
2975 --define=NAME[=VALUE]
2978 --force-define=NAME[=VALUE]
2980 is equivalent to this grammar file declaration
2982 %define NAME ["VALUE"]
2984 except that the manner in which Bison processes multiple definitions
2985 for the same NAME differs. Most importantly, -F and --force-define
2986 quietly override %define, but -D and --define do not. For further
2987 details, see the section "Bison Options" in the Bison manual.
2989 *** Variables renamed:
2991 The following %define variables
2994 lr.keep_unreachable_states
2996 have been renamed to
2999 lr.keep-unreachable-states
3001 The old names are now deprecated but will be maintained indefinitely
3002 for backward compatibility.
3004 *** Values no longer need to be quoted in the grammar file:
3006 If a %define value is an identifier, it no longer needs to be placed
3007 within quotations marks. For example,
3009 %define api.push-pull "push"
3013 %define api.push-pull push
3015 *** Unrecognized variables are now errors not warnings.
3017 *** Multiple invocations for any variable is now an error not a warning.
3019 ** Unrecognized %code qualifiers are now errors not warnings.
3021 ** Character literals not of length one:
3023 Previously, Bison quietly converted all character literals to length
3024 one. For example, without warning, Bison interpreted the operators in
3025 the following grammar to be the same token:
3031 Bison now warns when a character literal is not of length one. In
3032 some future release, Bison will start reporting an error instead.
3034 ** Destructor calls fixed for lookaheads altered in semantic actions:
3036 Previously for deterministic parsers in C, if a user semantic action
3037 altered yychar, the parser in some cases used the old yychar value to
3038 determine which destructor to call for the lookahead upon a syntax
3039 error or upon parser return. This bug has been fixed.
3041 ** C++ parsers use YYRHSLOC:
3043 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
3044 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
3045 to use it. If, for instance, your location structure has "first"
3046 and "last" members, instead of
3048 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3052 (Current).first = (Rhs)[1].location.first; \
3053 (Current).last = (Rhs)[N].location.last; \
3057 (Current).first = (Current).last = (Rhs)[0].location.last; \
3063 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3067 (Current).first = YYRHSLOC (Rhs, 1).first; \
3068 (Current).last = YYRHSLOC (Rhs, N).last; \
3072 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3076 ** YYLLOC_DEFAULT in C++:
3078 The default implementation of YYLLOC_DEFAULT used to be issued in
3079 the header file. It is now output in the implementation file, after
3080 the user %code sections so that its #ifndef guard does not try to
3081 override the user's YYLLOC_DEFAULT if provided.
3083 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3085 YYFAIL has existed for many years as an undocumented feature of
3086 deterministic parsers in C generated by Bison. More recently, it was
3087 a documented feature of Bison's experimental Java parsers. As
3088 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3089 semantic action now produces a deprecation warning, and Java parsers
3090 no longer implement YYFAIL at all. For further details, including a
3091 discussion of how to suppress C preprocessor warnings about YYFAIL
3092 being unused, see the Bison 2.4.2 NEWS entry.
3094 ** Temporary hack for adding a semicolon to the user action:
3096 Previously, Bison appended a semicolon to every user action for
3097 reductions when the output language defaulted to C (specifically, when
3098 neither %yacc, %language, %skeleton, or equivalent command-line
3099 options were specified). This allowed actions such as
3101 exp: exp "+" exp { $$ = $1 + $3 };
3105 exp: exp "+" exp { $$ = $1 + $3; };
3107 As a first step in removing this misfeature, Bison now issues a
3108 warning when it appends a semicolon. Moreover, in cases where Bison
3109 cannot easily determine whether a semicolon is needed (for example, an
3110 action ending with a cpp directive or a braced compound initializer),
3111 it no longer appends one. Thus, the C compiler might now complain
3112 about a missing semicolon where it did not before. Future releases of
3113 Bison will cease to append semicolons entirely.
3115 ** Verbose syntax error message fixes:
3117 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3118 specified, syntax error messages produced by the generated parser
3119 include the unexpected token as well as a list of expected tokens.
3120 The effect of %nonassoc on these verbose messages has been corrected
3121 in two ways, but a more complete fix requires LAC, described above:
3123 *** When %nonassoc is used, there can exist parser states that accept no
3124 tokens, and so the parser does not always require a lookahead token
3125 in order to detect a syntax error. Because no unexpected token or
3126 expected tokens can then be reported, the verbose syntax error
3127 message described above is suppressed, and the parser instead
3128 reports the simpler message, "syntax error". Previously, this
3129 suppression was sometimes erroneously triggered by %nonassoc when a
3130 lookahead was actually required. Now verbose messages are
3131 suppressed only when all previous lookaheads have already been
3132 shifted or discarded.
3134 *** Previously, the list of expected tokens erroneously included tokens
3135 that would actually induce a syntax error because conflicts for them
3136 were resolved with %nonassoc in the current parser state. Such
3137 tokens are now properly omitted from the list.
3139 *** Expected token lists are still often wrong due to state merging
3140 (from LALR or IELR) and default reductions, which can both add
3141 invalid tokens and subtract valid tokens. Canonical LR almost
3142 completely fixes this problem by eliminating state merging and
3143 default reductions. However, there is one minor problem left even
3144 when using canonical LR and even after the fixes above. That is,
3145 if the resolution of a conflict with %nonassoc appears in a later
3146 parser state than the one at which some syntax error is
3147 discovered, the conflicted token is still erroneously included in
3148 the expected token list. Bison's new LAC implementation,
3149 described above, eliminates this problem and the need for
3150 canonical LR. However, LAC is still experimental and is disabled
3153 ** Java skeleton fixes:
3155 *** A location handling bug has been fixed.
3157 *** The top element of each of the value stack and location stack is now
3158 cleared when popped so that it can be garbage collected.
3160 *** Parser traces now print the top element of the stack.
3162 ** -W/--warnings fixes:
3164 *** Bison now properly recognizes the "no-" versions of categories:
3166 For example, given the following command line, Bison now enables all
3167 warnings except warnings for incompatibilities with POSIX Yacc:
3169 bison -Wall,no-yacc gram.y
3171 *** Bison now treats S/R and R/R conflicts like other warnings:
3173 Previously, conflict reports were independent of Bison's normal
3174 warning system. Now, Bison recognizes the warning categories
3175 "conflicts-sr" and "conflicts-rr". This change has important
3176 consequences for the -W and --warnings command-line options. For
3179 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3180 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3181 bison -Wnone gram.y # no conflicts are reported
3182 bison -Werror gram.y # any conflict is an error
3184 However, as before, if the %expect or %expect-rr directive is
3185 specified, an unexpected number of conflicts is an error, and an
3186 expected number of conflicts is not reported, so -W and --warning
3187 then have no effect on the conflict report.
3189 *** The "none" category no longer disables a preceding "error":
3191 For example, for the following command line, Bison now reports
3192 errors instead of warnings for incompatibilities with POSIX Yacc:
3194 bison -Werror,none,yacc gram.y
3196 *** The "none" category now disables all Bison warnings:
3198 Previously, the "none" category disabled only Bison warnings for
3199 which there existed a specific -W/--warning category. However,
3200 given the following command line, Bison is now guaranteed to
3201 suppress all warnings:
3205 ** Precedence directives can now assign token number 0:
3207 Since Bison 2.3b, which restored the ability of precedence
3208 directives to assign token numbers, doing so for token number 0 has
3209 produced an assertion failure. For example:
3213 This bug has been fixed.
3216 * Noteworthy changes in release 2.4.3 (2010-08-05)
3218 ** Bison now obeys -Werror and --warnings=error for warnings about
3219 grammar rules that are useless in the parser due to conflicts.
3221 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3224 ** Failures in the test suite for GCC 4.5 have been fixed.
3226 ** Failures in the test suite for some versions of Sun Studio C++ have
3229 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3230 warnings about undefined %prec identifiers will not be converted to
3231 errors in Bison 2.5. They will remain warnings, which should be
3232 sufficient for POSIX while avoiding backward compatibility issues.
3234 ** Minor documentation fixes.
3237 * Noteworthy changes in release 2.4.2 (2010-03-20)
3239 ** Some portability problems that resulted in failures and livelocks
3240 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3241 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3242 errors should no longer cause M4 to report a broken pipe on the
3245 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3247 POSIX specifies that an error be reported for any identifier that does
3248 not appear on the LHS of a grammar rule and that is not defined by
3249 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3250 error report for the case when an identifier appears only after a
3251 %prec directive. It is now restored. However, for backward
3252 compatibility with recent Bison releases, it is only a warning for
3253 now. In Bison 2.5 and later, it will return to being an error.
3254 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3255 warning will not be converted to an error in Bison 2.5.]
3257 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3259 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3260 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3263 ** %code is now a permanent feature.
3265 A traditional Yacc prologue directive is written in the form:
3269 To provide a more flexible alternative, Bison 2.3b introduced the
3270 %code directive with the following forms for C/C++:
3273 %code requires {CODE}
3274 %code provides {CODE}
3277 These forms are now considered permanent features of Bison. See the
3278 %code entries in the section "Bison Declaration Summary" in the Bison
3279 manual for a summary of their functionality. See the section
3280 "Prologue Alternatives" for a detailed discussion including the
3281 advantages of %code over the traditional Yacc prologue directive.
3283 Bison's Java feature as a whole including its current usage of %code
3284 is still considered experimental.
3286 ** YYFAIL is deprecated and will eventually be removed.
3288 YYFAIL has existed for many years as an undocumented feature of
3289 deterministic parsers in C generated by Bison. Previously, it was
3290 documented for Bison's experimental Java parsers. YYFAIL is no longer
3291 documented for Java parsers and is formally deprecated in both cases.
3292 Users are strongly encouraged to migrate to YYERROR, which is
3295 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3296 induce a syntax error. The most obvious difference from YYERROR is
3297 that YYFAIL will automatically invoke yyerror to report the syntax
3298 error so that you don't have to. However, there are several other
3299 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3300 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3301 used. For a more detailed discussion, see:
3303 http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html
3305 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3306 deterministic parsers in C will continue to implement it. However,
3307 because YYFAIL is already flawed, it seems futile to try to make new
3308 Bison features compatible with it. Thus, during parser generation,
3309 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3310 rule action. In a later release, YYFAIL will be disabled for
3311 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3312 be removed altogether.
3314 There exists at least one case where Bison 2.5's YYFAIL warning will
3315 be a false positive. Some projects add phony uses of YYFAIL and other
3316 Bison-defined macros for the sole purpose of suppressing C
3317 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3318 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3319 epilogue (that is, after the second "%%") in the Bison input file. In
3320 this release (2.4.2), Bison already generates its own code to suppress
3321 C preprocessor warnings for YYFAIL, so projects can remove their own
3322 phony uses of YYFAIL if compatibility with Bison releases prior to
3323 2.4.2 is not necessary.
3325 ** Internationalization.
3327 Fix a regression introduced in Bison 2.4: Under some circumstances,
3328 message translations were not installed although supported by the
3332 * Noteworthy changes in release 2.4.1 (2008-12-11)
3334 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3335 declarations have been fixed.
3337 ** Temporary hack for adding a semicolon to the user action.
3339 Bison used to prepend a trailing semicolon at the end of the user
3340 action for reductions. This allowed actions such as
3342 exp: exp "+" exp { $$ = $1 + $3 };
3346 exp: exp "+" exp { $$ = $1 + $3; };
3348 Some grammars still depend on this "feature". Bison 2.4.1 restores
3349 the previous behavior in the case of C output (specifically, when
3350 neither %language or %skeleton or equivalent command-line options
3351 are used) to leave more time for grammars depending on the old
3352 behavior to be adjusted. Future releases of Bison will disable this
3355 ** A few minor improvements to the Bison manual.
3358 * Noteworthy changes in release 2.4 (2008-11-02)
3360 ** %language is an experimental feature.
3362 We first introduced this feature in test release 2.3b as a cleaner
3363 alternative to %skeleton. Since then, we have discussed the possibility of
3364 modifying its effect on Bison's output file names. Thus, in this release,
3365 we consider %language to be an experimental feature that will likely evolve
3368 ** Forward compatibility with GNU M4 has been improved.
3370 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3374 * Noteworthy changes in release 2.3b (2008-05-27)
3376 ** The quotes around NAME that used to be required in the following directive
3379 %define NAME "VALUE"
3381 ** The directive "%pure-parser" is now deprecated in favor of:
3385 which has the same effect except that Bison is more careful to warn about
3386 unreasonable usage in the latter case.
3390 Bison can now generate an LALR(1) parser in C with a push interface. That
3391 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3392 push one token at a time to the parser using "yypush_parse", which will
3393 return to the caller after processing each token. By default, the push
3394 interface is disabled. Either of the following directives will enable it:
3396 %define api.push_pull "push" // Just push; does not require yylex.
3397 %define api.push_pull "both" // Push and pull; requires yylex.
3399 See the new section "A Push Parser" in the Bison manual for details.
3401 The current push parsing interface is experimental and may evolve. More user
3402 feedback will help to stabilize it.
3404 ** The -g and --graph options now output graphs in Graphviz DOT format,
3405 not VCG format. Like --graph, -g now also takes an optional FILE argument
3406 and thus cannot be bundled with other short options.
3410 Bison can now generate an LALR(1) parser in Java. The skeleton is
3411 "data/lalr1.java". Consider using the new %language directive instead of
3412 %skeleton to select it.
3414 See the new section "Java Parsers" in the Bison manual for details.
3416 The current Java interface is experimental and may evolve. More user
3417 feedback will help to stabilize it.
3418 Contributed by Paolo Bonzini.
3422 This new directive specifies the programming language of the generated
3423 parser, which can be C (the default), C++, or Java. Besides the skeleton
3424 that Bison uses, the directive affects the names of the generated files if
3425 the grammar file's name ends in ".y".
3427 ** XML Automaton Report
3429 Bison can now generate an XML report of the LALR(1) automaton using the new
3430 "--xml" option. The current XML schema is experimental and may evolve. More
3431 user feedback will help to stabilize it.
3432 Contributed by Wojciech Polak.
3434 ** The grammar file may now specify the name of the parser header file using
3435 %defines. For example:
3439 ** When reporting useless rules, useless nonterminals, and unused terminals,
3440 Bison now employs the terms "useless in grammar" instead of "useless",
3441 "useless in parser" instead of "never reduced", and "unused in grammar"
3442 instead of "unused".
3444 ** Unreachable State Removal
3446 Previously, Bison sometimes generated parser tables containing unreachable
3447 states. A state can become unreachable during conflict resolution if Bison
3448 disables a shift action leading to it from a predecessor state. Bison now:
3450 1. Removes unreachable states.
3452 2. Does not report any conflicts that appeared in unreachable states.
3453 WARNING: As a result, you may need to update %expect and %expect-rr
3454 directives in existing grammar files.
3456 3. For any rule used only in such states, Bison now reports the rule as
3457 "useless in parser due to conflicts".
3459 This feature can be disabled with the following directive:
3461 %define lr.keep_unreachable_states
3463 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3464 for further discussion.
3466 ** Lookahead Set Correction in the ".output" Report
3468 When instructed to generate a ".output" file including lookahead sets
3469 (using "--report=lookahead", for example), Bison now prints each reduction's
3470 lookahead set only next to the associated state's one item that (1) is
3471 associated with the same rule as the reduction and (2) has its dot at the end
3472 of its RHS. Previously, Bison also erroneously printed the lookahead set
3473 next to all of the state's other items associated with the same rule. This
3474 bug affected only the ".output" file and not the generated parser source
3477 ** --report-file=FILE is a new option to override the default ".output" file
3480 ** The "=" that used to be required in the following directives is now
3483 %file-prefix "parser"
3487 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3489 Bison 2.3a provided a new set of directives as a more flexible alternative to
3490 the traditional Yacc prologue blocks. Those have now been consolidated into
3491 a single %code directive with an optional qualifier field, which identifies
3492 the purpose of the code and thus the location(s) where Bison should generate
3495 1. "%code {CODE}" replaces "%after-header {CODE}"
3496 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3497 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3498 4. "%code top {CODE}" replaces "%before-header {CODE}"
3500 See the %code entries in section "Bison Declaration Summary" in the Bison
3501 manual for a summary of the new functionality. See the new section "Prologue
3502 Alternatives" for a detailed discussion including the advantages of %code
3503 over the traditional Yacc prologues.
3505 The prologue alternatives are experimental. More user feedback will help to
3506 determine whether they should become permanent features.
3508 ** Revised warning: unset or unused midrule values
3510 Since Bison 2.2, Bison has warned about midrule values that are set but not
3511 used within any of the actions of the parent rule. For example, Bison warns
3514 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3516 Now, Bison also warns about midrule values that are used but not set. For
3517 example, Bison warns about unset $$ in the midrule action in:
3519 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3521 However, Bison now disables both of these warnings by default since they
3522 sometimes prove to be false alarms in existing grammars employing the Yacc
3523 constructs $0 or $-N (where N is some positive integer).
3525 To enable these warnings, specify the option "--warnings=midrule-values" or
3526 "-W", which is a synonym for "--warnings=all".
3528 ** Default %destructor or %printer with "<*>" or "<>"
3530 Bison now recognizes two separate kinds of default %destructor's and
3533 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3534 %destructor/%printer for all grammar symbols for which you have formally
3535 declared semantic type tags.
3537 2. Place "<>" in a %destructor/%printer symbol list to define a default
3538 %destructor/%printer for all grammar symbols without declared semantic
3541 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3542 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3543 longer applies any %destructor to a midrule value if that midrule value is
3544 not actually ever referenced using either $$ or $n in a semantic action.
3546 The default %destructor's and %printer's are experimental. More user
3547 feedback will help to determine whether they should become permanent
3550 See the section "Freeing Discarded Symbols" in the Bison manual for further
3553 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3554 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3555 manual for a caveat concerning the treatment of literal strings.
3557 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3558 completely removed from Bison.
3561 * Noteworthy changes in release 2.3a (2006-09-13)
3563 ** Instead of %union, you can define and use your own union type
3564 YYSTYPE if your grammar contains at least one <type> tag.
3565 Your YYSTYPE need not be a macro; it can be a typedef.
3566 This change is for compatibility with other Yacc implementations,
3567 and is required by POSIX.
3569 ** Locations columns and lines start at 1.
3570 In accordance with the GNU Coding Standards and Emacs.
3572 ** You may now declare per-type and default %destructor's and %printer's:
3576 %union { char *string; }
3577 %token <string> STRING1
3578 %token <string> STRING2
3579 %type <string> string1
3580 %type <string> string2
3581 %union { char character; }
3582 %token <character> CHR
3583 %type <character> chr
3584 %destructor { free ($$); } %symbol-default
3585 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3586 %destructor { } <character>
3588 guarantees that, when the parser discards any user-defined symbol that has a
3589 semantic type tag other than "<character>", it passes its semantic value to
3590 "free". However, when the parser discards a "STRING1" or a "string1", it
3591 also prints its line number to "stdout". It performs only the second
3592 "%destructor" in this case, so it invokes "free" only once.
3594 [Although we failed to mention this here in the 2.3a release, the default
3595 %destructor's and %printer's were experimental, and they were rewritten in
3598 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3599 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3600 associating token numbers with token names. Removing the #define statements
3601 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3602 requires them. Bison still generates an enum for token names in all cases.
3604 ** Handling of traditional Yacc prologue blocks is now more consistent but
3605 potentially incompatible with previous releases of Bison.
3607 As before, you declare prologue blocks in your grammar file with the
3608 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3609 prologue blocks that you've declared before the first %union. To generate
3610 the post-prologue, Bison concatenates all prologue blocks that you've
3611 declared after the first %union.
3613 Previous releases of Bison inserted the pre-prologue into both the header
3614 file and the code file in all cases except for LALR(1) parsers in C. In the
3615 latter case, Bison inserted it only into the code file. For parsers in C++,
3616 the point of insertion was before any token definitions (which associate
3617 token numbers with names). For parsers in C, the point of insertion was
3618 after the token definitions.
3620 Now, Bison never inserts the pre-prologue into the header file. In the code
3621 file, it always inserts it before the token definitions.
3623 ** Bison now provides a more flexible alternative to the traditional Yacc
3624 prologue blocks: %before-header, %start-header, %end-header, and
3627 For example, the following declaration order in the grammar file reflects the
3628 order in which Bison will output these code blocks. However, you are free to
3629 declare these code blocks in your grammar file in whatever order is most
3633 /* Bison treats this block like a pre-prologue block: it inserts it into
3634 * the code file before the contents of the header file. It does *not*
3635 * insert it into the header file. This is a good place to put
3636 * #include's that you want at the top of your code file. A common
3637 * example is '#include "system.h"'. */
3640 /* Bison inserts this block into both the header file and the code file.
3641 * In both files, the point of insertion is before any Bison-generated
3642 * token, semantic type, location type, and class definitions. This is a
3643 * good place to define %union dependencies, for example. */
3646 /* Unlike the traditional Yacc prologue blocks, the output order for the
3647 * new %*-header blocks is not affected by their declaration position
3648 * relative to any %union in the grammar file. */
3651 /* Bison inserts this block into both the header file and the code file.
3652 * In both files, the point of insertion is after the Bison-generated
3653 * definitions. This is a good place to declare or define public
3654 * functions or data structures that depend on the Bison-generated
3658 /* Bison treats this block like a post-prologue block: it inserts it into
3659 * the code file after the contents of the header file. It does *not*
3660 * insert it into the header file. This is a good place to declare or
3661 * define internal functions or data structures that depend on the
3662 * Bison-generated definitions. */
3665 If you have multiple occurrences of any one of the above declarations, Bison
3666 will concatenate the contents in declaration order.
3668 [Although we failed to mention this here in the 2.3a release, the prologue
3669 alternatives were experimental, and they were rewritten in future versions.]
3671 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3672 The old spelling still works, but is not documented and may be removed
3673 in a future release.
3676 * Noteworthy changes in release 2.3 (2006-06-05)
3678 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3679 for compatibility with LALR(1) grammars.
3681 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3682 be to a type name that does not contain parentheses or brackets.
3685 * Noteworthy changes in release 2.2 (2006-05-19)
3687 ** The distribution terms for all Bison-generated parsers now permit
3688 using the parsers in nonfree programs. Previously, this permission
3689 was granted only for Bison-generated LALR(1) parsers in C.
3691 ** %name-prefix changes the namespace name in C++ outputs.
3693 ** The C++ parsers export their token_type.
3695 ** Bison now allows multiple %union declarations, and concatenates
3696 their contents together.
3698 ** New warning: unused values
3699 Right-hand side symbols whose values are not used are reported,
3700 if the symbols have destructors. For instance:
3702 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3706 will trigger a warning about $$ and $5 in the first rule, and $3 in
3707 the second ($1 is copied to $$ by the default rule). This example
3708 most likely contains three errors, and could be rewritten as:
3710 exp: exp "?" exp ":" exp
3711 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3713 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3716 However, if the original actions were really intended, memory leaks
3717 and all, the warnings can be suppressed by letting Bison believe the
3718 values are used, e.g.:
3720 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3721 | exp "+" exp { $$ = $1; (void) $3; }
3724 If there are midrule actions, the warning is issued if no action
3725 uses it. The following triggers no warning: $1 and $3 are used.
3727 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3729 The warning is intended to help catching lost values and memory leaks.
3730 If a value is ignored, its associated memory typically is not reclaimed.
3732 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3733 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3734 and YYERROR, for all objects on the stack, other than objects
3735 corresponding to the right-hand side of the current rule.
3737 ** %expect, %expect-rr
3738 Incorrect numbers of expected conflicts are now actual errors,
3739 instead of warnings.
3741 ** GLR, YACC parsers.
3742 The %parse-params are available in the destructors (and the
3743 experimental printers) as per the documentation.
3745 ** Bison now warns if it finds a stray "$" or "@" in an action.
3747 ** %require "VERSION"
3748 This specifies that the grammar file depends on features implemented
3749 in Bison version VERSION or higher.
3751 ** lalr1.cc: The token and value types are now class members.
3752 The tokens were defined as free form enums and cpp macros. YYSTYPE
3753 was defined as a free form union. They are now class members:
3754 tokens are enumerations of the "yy::parser::token" struct, and the
3755 semantic values have the "yy::parser::semantic_type" type.
3757 If you do not want or can update to this scheme, the directive
3758 '%define "global_tokens_and_yystype" "1"' triggers the global
3759 definition of tokens and YYSTYPE. This change is suitable both
3760 for previous releases of Bison, and this one.
3762 If you wish to update, then make sure older version of Bison will
3763 fail using '%require "2.2"'.
3765 ** DJGPP support added.
3768 * Noteworthy changes in release 2.1 (2005-09-16)
3770 ** The C++ lalr1.cc skeleton supports %lex-param.
3772 ** Bison-generated parsers now support the translation of diagnostics like
3773 "syntax error" into languages other than English. The default
3774 language is still English. For details, please see the new
3775 Internationalization section of the Bison manual. Software
3776 distributors should also see the new PACKAGING file. Thanks to
3777 Bruno Haible for this new feature.
3779 ** Wording in the Bison-generated parsers has been changed slightly to
3780 simplify translation. In particular, the message "memory exhausted"
3781 has replaced "parser stack overflow", as the old message was not
3782 always accurate for modern Bison-generated parsers.
3784 ** Destructors are now called when the parser aborts, for all symbols left
3785 behind on the stack. Also, the start symbol is now destroyed after a
3786 successful parse. In both cases, the behavior was formerly inconsistent.
3788 ** When generating verbose diagnostics, Bison-generated parsers no longer
3789 quote the literal strings associated with tokens. For example, for
3790 a syntax error associated with '%token NUM "number"' they might
3791 print 'syntax error, unexpected number' instead of 'syntax error,
3792 unexpected "number"'.
3795 * Noteworthy changes in release 2.0 (2004-12-25)
3797 ** Possibly-incompatible changes
3799 - Bison-generated parsers no longer default to using the alloca function
3800 (when available) to extend the parser stack, due to widespread
3801 problems in unchecked stack-overflow detection. You can "#define
3802 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3803 the manual to determine safe values for YYMAXDEPTH in that case.
3805 - Error token location.
3806 During error recovery, the location of the syntax error is updated
3807 to cover the whole sequence covered by the error token: it includes
3808 the shifted symbols thrown away during the first part of the error
3809 recovery, and the lookahead rejected during the second part.
3811 - Semicolon changes:
3812 . Stray semicolons are no longer allowed at the start of a grammar.
3813 . Semicolons are now required after in-grammar declarations.
3815 - Unescaped newlines are no longer allowed in character constants or
3816 string literals. They were never portable, and GCC 3.4.0 has
3817 dropped support for them. Better diagnostics are now generated if
3818 forget a closing quote.
3820 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
3824 - GLR grammars now support locations.
3826 - New directive: %initial-action.
3827 This directive allows the user to run arbitrary code (including
3828 initializing @$) from yyparse before parsing starts.
3830 - A new directive "%expect-rr N" specifies the expected number of
3831 reduce/reduce conflicts in GLR parsers.
3833 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
3834 This is a GNU extension.
3836 - The option "--report=lookahead" was changed to "--report=look-ahead".
3837 [However, this was changed back after 2.3.]
3839 - Experimental %destructor support has been added to lalr1.cc.
3841 - New configure option --disable-yacc, to disable installation of the
3842 yacc command and -ly library introduced in 1.875 for POSIX conformance.
3846 - For now, %expect-count violations are now just warnings, not errors.
3847 This is for compatibility with Bison 1.75 and earlier (when there are
3848 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
3849 are too many or too few shift/reduce conflicts). However, in future
3850 versions of Bison we plan to improve the %expect machinery so that
3851 these violations will become errors again.
3853 - Within Bison itself, numbers (e.g., goto numbers) are no longer
3854 arbitrarily limited to 16-bit counts.
3856 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
3859 * Noteworthy changes in release 1.875 (2003-01-01)
3861 ** The documentation license has been upgraded to version 1.2
3862 of the GNU Free Documentation License.
3864 ** syntax error processing
3866 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
3867 locations too. This fixes bugs in error-location computation.
3870 It is now possible to reclaim the memory associated to symbols
3871 discarded during error recovery. This feature is still experimental.
3874 This new directive is preferred over YYERROR_VERBOSE.
3876 - #defining yyerror to steal internal variables is discouraged.
3877 It is not guaranteed to work forever.
3879 ** POSIX conformance
3881 - Semicolons are once again optional at the end of grammar rules.
3882 This reverts to the behavior of Bison 1.33 and earlier, and improves
3883 compatibility with Yacc.
3885 - "parse error" -> "syntax error"
3886 Bison now uniformly uses the term "syntax error"; formerly, the code
3887 and manual sometimes used the term "parse error" instead. POSIX
3888 requires "syntax error" in diagnostics, and it was thought better to
3891 - The documentation now emphasizes that yylex and yyerror must be
3892 declared before use. C99 requires this.
3894 - Bison now parses C99 lexical constructs like UCNs and
3895 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
3897 - File names are properly escaped in C output. E.g., foo\bar.y is
3898 output as "foo\\bar.y".
3900 - Yacc command and library now available
3901 The Bison distribution now installs a "yacc" command, as POSIX requires.
3902 Also, Bison now installs a small library liby.a containing
3903 implementations of Yacc-compatible yyerror and main functions.
3904 This library is normally not useful, but POSIX requires it.
3906 - Type clashes now generate warnings, not errors.
3908 - If the user does not define YYSTYPE as a macro, Bison now declares it
3909 using typedef instead of defining it as a macro.
3910 For consistency, YYLTYPE is also declared instead of defined.
3912 ** Other compatibility issues
3914 - %union directives can now have a tag before the "{", e.g., the
3915 directive "%union foo {...}" now generates the C code
3916 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
3917 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
3918 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
3919 This is for compatibility with both Yacc and Bison 1.35.
3921 - ";" is output before the terminating "}" of an action, for
3922 compatibility with Bison 1.35.
3924 - Bison now uses a Yacc-style format for conflict reports, e.g.,
3925 "conflicts: 2 shift/reduce, 1 reduce/reduce".
3927 - "yystype" and "yyltype" are now obsolescent macros instead of being
3928 typedefs or tags; they are no longer documented and are planned to be
3929 withdrawn in a future release.
3934 Users of Bison have to decide how they handle the portability of the
3937 - "parsing stack overflow..." -> "parser stack overflow"
3938 GLR parsers now report "parser stack overflow" as per the Bison manual.
3940 ** %parse-param and %lex-param
3941 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
3942 additional context to yyparse and yylex. They suffer from several
3945 - a single argument only can be added,
3946 - their types are weak (void *),
3947 - this context is not passed to ancillary functions such as yyerror,
3948 - only yacc.c parsers support them.
3950 The new %parse-param/%lex-param directives provide a more precise control.
3953 %parse-param {int *nastiness}
3954 %lex-param {int *nastiness}
3955 %parse-param {int *randomness}
3957 results in the following signatures:
3959 int yylex (int *nastiness);
3960 int yyparse (int *nastiness, int *randomness);
3962 or, if both %pure-parser and %locations are used:
3964 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
3965 int yyparse (int *nastiness, int *randomness);
3967 ** Bison now warns if it detects conflicting outputs to the same file,
3968 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
3969 that command outputs both code and header to foo.h.
3971 ** #line in output files
3972 - --no-line works properly.
3974 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
3975 later to be built. This change originally took place a few versions
3976 ago, but nobody noticed until we recently asked someone to try
3977 building Bison with a K&R C compiler.
3980 * Noteworthy changes in release 1.75 (2002-10-14)
3982 ** Bison should now work on 64-bit hosts.
3984 ** Indonesian translation thanks to Tedi Heriyanto.
3987 Fix spurious parse errors.
3990 Some people redefine yyerror to steal yyparse' private variables.
3991 Reenable this trick until an official feature replaces it.
3994 In agreement with POSIX and with other Yaccs, leaving a default
3995 action is valid when $$ is untyped, and $1 typed:
3999 but the converse remains an error:
4003 ** Values of midrule actions
4006 foo: { ... } { $$ = $1; } ...
4008 was incorrectly rejected: $1 is defined in the second midrule
4009 action, and is equal to the $$ of the first midrule action.
4012 * Noteworthy changes in release 1.50 (2002-10-04)
4017 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
4018 almost any context-free grammar, ambiguous or not. The new declarations
4019 %dprec and %merge on grammar rules allow parse-time resolution of
4020 ambiguities. Contributed by Paul Hilfinger.
4022 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
4023 like the Alpha, so please stick to 32-bit hosts for now.
4026 When not in Yacc compatibility mode, when the output file was not
4027 specified, running "bison foo/bar.y" created "foo/bar.c". It
4028 now creates "bar.c".
4031 The undefined token was systematically mapped to 2 which prevented
4032 the use of 2 by the user. This is no longer the case.
4034 ** Unknown token numbers
4035 If yylex returned an out of range value, yyparse could die. This is
4039 According to POSIX, the error token must be 256.
4040 Bison extends this requirement by making it a preference: *if* the
4041 user specified that one of her tokens is numbered 256, then error
4042 will be mapped onto another number.
4044 ** Verbose error messages
4045 They no longer report "..., expecting error or..." for states where
4046 error recovery is possible.
4049 Defaults to "$end" instead of "$".
4051 ** Error recovery now conforms to documentation and to POSIX
4052 When a Bison-generated parser encounters a syntax error, it now pops
4053 the stack until it finds a state that allows shifting the error
4054 token. Formerly, it popped the stack until it found a state that
4055 allowed some non-error action other than a default reduction on the
4056 error token. The new behavior has long been the documented behavior,
4057 and has long been required by POSIX. For more details, please see
4058 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4059 <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.
4062 Popped tokens and nonterminals are now reported.
4065 Larger grammars are now supported (larger token numbers, larger grammar
4066 size (= sum of the LHS and RHS lengths), larger LALR tables).
4067 Formerly, many of these numbers ran afoul of 16-bit limits;
4068 now these limits are 32 bits on most hosts.
4070 ** Explicit initial rule
4071 Bison used to play hacks with the initial rule, which the user does
4072 not write. It is now explicit, and visible in the reports and
4076 Before, Bison reported the useless rules, but, although not used,
4077 included them in the parsers. They are now actually removed.
4079 ** Useless rules, useless nonterminals
4080 They are now reported, as a warning, with their locations.
4082 ** Rules never reduced
4083 Rules that can never be reduced because of conflicts are now
4086 ** Incorrect "Token not used"
4087 On a grammar such as
4089 %token useless useful
4091 exp: '0' %prec useful;
4093 where a token was used to set the precedence of the last rule,
4094 bison reported both "useful" and "useless" as useless tokens.
4096 ** Revert the C++ namespace changes introduced in 1.31
4097 as they caused too many portability hassles.
4099 ** Default locations
4100 By an accident of design, the default computation of @$ was
4101 performed after another default computation was performed: @$ = @1.
4102 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4103 the computation of @$.
4105 ** Token end-of-file
4106 The token end of file may be specified by the user, in which case,
4107 the user symbol is used in the reports, the graphs, and the verbose
4108 error messages instead of "$end", which remains being the default.
4112 %token MYEOF 0 "end of file"
4115 This old option, which has been broken for ages, is removed.
4118 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4119 Croatian, thanks to Denis Lackovic.
4121 ** Incorrect token definitions
4124 bison used to output
4127 ** Token definitions as enums
4128 Tokens are output both as the traditional #define's, and, provided
4129 the compiler supports ANSI C or is a C++ compiler, as enums.
4130 This lets debuggers display names instead of integers.
4133 In addition to --verbose, bison supports --report=THINGS, which
4134 produces additional information:
4136 complete the core item sets with their closure
4137 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4138 explicitly associate lookahead tokens to items
4140 describe shift/reduce conflicts solving.
4141 Bison used to systematically output this information on top of
4142 the report. Solved conflicts are now attached to their states.
4145 Previous versions don't complain when there is a type clash on
4146 the default action if the rule has a midrule action, such as in:
4154 ** GNU M4 is now required when using Bison.
4157 * Noteworthy changes in release 1.35 (2002-03-25)
4160 Some projects use Bison's C parser with C++ compilers, and define
4161 YYSTYPE as a class. The recent adjustment of C parsers for data
4162 alignment and 64 bit architectures made this impossible.
4164 Because for the time being no real solution for C++ parser
4165 generation exists, kludges were implemented in the parser to
4166 maintain this use. In the future, when Bison has C++ parsers, this
4167 kludge will be disabled.
4169 This kludge also addresses some C++ problems when the stack was
4173 * Noteworthy changes in release 1.34 (2002-03-12)
4175 ** File name clashes are detected
4176 $ bison foo.y -d -o foo.x
4177 fatal error: header and parser would both be named "foo.x"
4179 ** A missing ";" at the end of a rule triggers a warning
4180 In accordance with POSIX, and in agreement with other
4181 Yacc implementations, Bison will mandate this semicolon in the near
4182 future. This eases the implementation of a Bison parser of Bison
4183 grammars by making this grammar LALR(1) instead of LR(2). To
4184 facilitate the transition, this release introduces a warning.
4186 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4187 many portability hassles.
4189 ** DJGPP support added.
4191 ** Fix test suite portability problems.
4194 * Noteworthy changes in release 1.33 (2002-02-07)
4197 Groff could not be compiled for the definition of size_t was lacking
4198 under some conditions.
4204 * Noteworthy changes in release 1.32 (2002-01-23)
4206 ** Fix Yacc output file names
4208 ** Portability fixes
4210 ** Italian, Dutch translations
4213 * Noteworthy changes in release 1.31 (2002-01-14)
4217 ** GNU Gettext and %expect
4218 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4219 Bison dies on incorrect %expectations, we fear there will be
4220 too many bug reports for Gettext, so _for the time being_, %expect
4221 does not trigger an error when the input file is named "plural.y".
4223 ** Use of alloca in parsers
4224 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4225 malloc exclusively. Since 1.29, but was not NEWS'ed.
4227 alloca is used only when compiled with GCC, to avoid portability
4230 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4232 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4233 (as POSIX requires) instead of being undefined.
4236 Bison has always permitted actions such as { $$ = $1 }: it adds the
4237 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4238 is no longer output: one has to write { $$ = $1; }.
4240 ** Better C++ compliance
4241 The output parsers try to respect C++ namespaces.
4242 [This turned out to be a failed experiment, and it was reverted later.]
4245 Fixed bugs when reporting useless nonterminals.
4248 The parsers work properly on 64 bit hosts.
4251 Some calls to strerror resulted in scrambled or missing error messages.
4254 When the number of shift/reduce conflicts is correct, don't issue
4257 ** The verbose report includes the rule line numbers.
4259 ** Rule line numbers are fixed in traces.
4261 ** Swedish translation
4264 Verbose parse error messages from the parsers are better looking.
4265 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4266 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4268 ** Fixed parser memory leaks.
4269 When the generated parser was using malloc to extend its stacks, the
4270 previous allocations were not freed.
4272 ** Fixed verbose output file.
4273 Some newlines were missing.
4274 Some conflicts in state descriptions were missing.
4276 ** Fixed conflict report.
4277 Option -v was needed to get the result.
4281 Mismatches are errors, not warnings.
4283 ** Fixed incorrect processing of some invalid input.
4285 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4287 ** Fixed some typos in the documentation.
4289 ** %token MY_EOF 0 is supported.
4290 Before, MY_EOF was silently renumbered as 257.
4292 ** doc/refcard.tex is updated.
4294 ** %output, %file-prefix, %name-prefix.
4298 New, aliasing "--output-file".
4301 * Noteworthy changes in release 1.30 (2001-10-26)
4303 ** "--defines" and "--graph" have now an optional argument which is the
4304 output file name. "-d" and "-g" do not change; they do not take any
4307 ** "%source_extension" and "%header_extension" are removed, failed
4310 ** Portability fixes.
4313 * Noteworthy changes in release 1.29 (2001-09-07)
4315 ** The output file does not define const, as this caused problems when used
4316 with common autoconfiguration schemes. If you still use ancient compilers
4317 that lack const, compile with the equivalent of the C compiler option
4318 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4320 ** Added "-g" and "--graph".
4322 ** The Bison manual is now distributed under the terms of the GNU FDL.
4324 ** The input and the output files has automatically a similar extension.
4326 ** Russian translation added.
4328 ** NLS support updated; should hopefully be less troublesome.
4330 ** Added the old Bison reference card.
4332 ** Added "--locations" and "%locations".
4334 ** Added "-S" and "--skeleton".
4336 ** "%raw", "-r", "--raw" is disabled.
4338 ** Special characters are escaped when output. This solves the problems
4339 of the #line lines with path names including backslashes.
4342 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4343 "%debug", "%source_extension" and "%header_extension".
4346 Automatic location tracking.
4349 * Noteworthy changes in release 1.28 (1999-07-06)
4351 ** Should compile better now with K&R compilers.
4355 ** Fixed a problem with escaping the double quote character.
4357 ** There is now a FAQ.
4360 * Noteworthy changes in release 1.27
4362 ** The make rule which prevented bison.simple from being created on
4363 some systems has been fixed.
4366 * Noteworthy changes in release 1.26
4368 ** Bison now uses Automake.
4370 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4372 ** Token numbers now start at 257 as previously documented, not 258.
4374 ** Bison honors the TMPDIR environment variable.
4376 ** A couple of buffer overruns have been fixed.
4378 ** Problems when closing files should now be reported.
4380 ** Generated parsers should now work even on operating systems which do
4381 not provide alloca().
4384 * Noteworthy changes in release 1.25 (1995-10-16)
4386 ** Errors in the input grammar are not fatal; Bison keeps reading
4387 the grammar file, and reports all the errors found in it.
4389 ** Tokens can now be specified as multiple-character strings: for
4390 example, you could use "<=" for a token which looks like <=, instead
4391 of choosing a name like LESSEQ.
4393 ** The %token_table declaration says to write a table of tokens (names
4394 and numbers) into the parser file. The yylex function can use this
4395 table to recognize multiple-character string tokens, or for other
4398 ** The %no_lines declaration says not to generate any #line preprocessor
4399 directives in the parser file.
4401 ** The %raw declaration says to use internal Bison token numbers, not
4402 Yacc-compatible token numbers, when token names are defined as macros.
4404 ** The --no-parser option produces the parser tables without including
4405 the parser engine; a project can now use its own parser engine.
4406 The actions go into a separate file called NAME.act, in the form of
4407 a switch statement body.
4410 * Noteworthy changes in release 1.23
4412 The user can define YYPARSE_PARAM as the name of an argument to be
4413 passed into yyparse. The argument should have type void *. It should
4414 actually point to an object. Grammar actions can access the variable
4415 by casting it to the proper pointer type.
4417 Line numbers in output file corrected.
4420 * Noteworthy changes in release 1.22
4422 --help option added.
4425 * Noteworthy changes in release 1.20
4427 Output file does not redefine const for C++.
4431 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4432 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4433 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4434 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4435 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4436 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4437 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4438 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4439 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4440 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4441 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4442 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4443 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4444 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4445 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4446 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4447 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4448 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4449 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4450 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4451 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4452 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4453 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4454 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4455 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4456 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4457 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4458 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4459 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4460 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4461 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4462 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4463 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4464 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4465 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4466 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4467 LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
4470 ispell-dictionary: "american"
4475 Copyright (C) 1995-2015, 2018-2020 Free Software Foundation, Inc.
4477 This file is part of Bison, the GNU Parser Generator.
4479 Permission is granted to copy, distribute and/or modify this document
4480 under the terms of the GNU Free Documentation License, Version 1.3 or
4481 any later version published by the Free Software Foundation; with no
4482 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4483 Texts. A copy of the license is included in the "GNU Free
4484 Documentation License" file as part of this distribution.