3 * Noteworthy changes in release ?.? (????-??-??) [?]
6 * Noteworthy changes in release 3.6.93 (2020-07-20) [beta]
13 * Noteworthy changes in release 3.6.92 (2020-07-19) [beta]
15 Changes in the display of counterexamples.
21 The bistromathic demonstrates %param and how to quote sources in the error
25 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
29 * Noteworthy changes in release 3.6.91 (2020-07-09) [beta]
36 * Noteworthy changes in release 3.6.90 (2020-07-04) [beta]
38 ** Deprecated features
40 The YYPRINT macro, which works only with yacc.c and only for tokens, was
41 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
42 It is deprecated and its support will be removed eventually.
44 In conformance with the recommendations of the Graphviz team, in the next
45 version Bison the option `--graph` will generate a *.gv file by default,
46 instead of *.dot. A transition started in Bison 3.4.
50 *** Counterexample Generation
52 Contributed by Vincent Imbimbo.
54 When given `--report=counterexamples` or `-Wcounterexamples`, bison will
55 now output counterexamples for conflicts in the grammar. These are
56 strings in the grammar which can be parsed in two ways due to the
57 conflict. For example:
59 Shift/reduce conflict on token "/":
60 Example exp "+" exp • "/" exp
65 Example exp "+" exp • "/" exp
71 When Bison is installed with text styling enabled, the example is actually
72 shown twice, with colors highlighting the ambiguity.
74 This is a shift/reduce conflict caused by none of the operators having
75 precedence, so the example can be parsed in the two ways shown. When
76 bison cannot find an example that can be derived in two ways, it instead
77 generates two examples that are the same up until the dot:
79 First example expr • ID ',' ID $end
86 Second example expr • ID $end
93 In these cases, the parser usually doesn't have enough lookahead to
94 differentiate the two given examples.
96 The counterexamples are "focused" in two different ways. First, they do
97 not clutter the output with all the derivations from the start symbol,
98 rather they start on the "conflicted nonterminal". They go straight to the
99 point. Second, they don't "expand" nonterminal symbols uselessly.
101 *** File prefix mapping
103 Contributed by Joshua Watt.
105 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
106 in the output (specifically `#line` directives and `#ifdef` header guards)
107 that begins with the prefix OLD will have it replaced with the prefix NEW,
108 similar to the `-ffile-prefix-map` in GCC. This option can be used to
109 make bison output reproducible.
113 *** Relocatable installation
115 When installed to be relocatable (via `configure --enable-relocatable`),
116 bison will now also look for a relocated m4.
120 The `filename_type` %define variable was renamed `api.filename.type`.
123 %define filename_type "symbol"
127 %define api.filename.type {symbol}
129 (Or let `bison --update` do it for you).
131 It now defaults to `const std::string` instead of `std::string`.
133 *** Deprecated %define variable names
135 The following variables have been renamed for consistency. Backward
136 compatibility is ensured, but upgrading is recommended.
138 filename_type -> api.filename.type
139 package -> api.package
141 *** Push parsers no longer clear their state when parsing is finished
143 Previously push-parsers cleared their state when parsing was finished (on
144 success and on failure). This made it impossible to check if there were
145 parse errors, since `yynerrs` was also reset. This can be especially
146 troublesome when used in autocompletion, since a parser with error
147 recovery would suggest (irrelevant) expected tokens even if there were
150 Now the parser state can be examined when parsing is finished. The parser
151 state is reset when starting a new parse.
155 *** Include the generated header (yacc.c)
157 Historically, when --defines was used, bison generated a header and pasted
158 an exact copy of it into the generated parser implementation file. Since
159 Bison 3.4 it is possible to specify that the header should be `#include`d,
160 and how. For instance
162 %define api.header.include {"parse.h"}
166 %define api.header.include {<parser/parse.h>}
168 Now api.header.include defaults to `"header-basename"`, as was intended in
169 Bison 3.4, where `header-basename` is the basename of the generated
170 header. This is disabled when the generated header is `y.tab.h`, to
171 comply with Automake's ylwrap.
173 *** String aliases are faithfully propagated
175 Bison used to interpret user strings (i.e., decoding backslash escapes)
176 when reading them, and to escape them (i.e., issue non-printable
177 characters as backslash escapes, taking the locale into account) when
178 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
179 up "ciphered" as sequences of backslash escapes. This happened not only
180 in the generated sources (where the compiler will reinterpret them), but
181 also in all the generated reports (text, xml, html, dot, etc.). Reports
182 were therefore not readable when string aliases were not pure ASCII.
183 Worse yet: the output depended on the user's locale.
185 Now Bison faithfully treats the string aliases exactly the way the user
186 spelled them. This fixes all the aforementioned problems. However, now,
187 string aliases semantically equivalent but syntactically different (e.g.,
188 "A", "\x41", "\101") are considered to be different.
190 *** Crash when generating IELR
192 An old, well hidden, bug in the generation of IELR parsers was fixed.
195 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
199 In glr.cc some internal macros leaked in the user's code, and could damage
200 access to the token kinds.
203 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
207 Incorrect comments in the generated parsers.
209 Warnings in push parsers (yacc.c).
211 Incorrect display of gotos in LAC traces (lalr1.cc).
214 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
218 Some tests were fixed.
220 When token aliases contain comment delimiters:
222 %token FOO "/* foo */"
224 bison used to emit "nested" comments, which is invalid C.
227 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
231 Restored ANSI-C compliance in yacc.c.
233 GNU readline portability issues.
235 In C++, yy::parser::symbol_name is now a public member, as was intended.
239 In C++, yy::parser::symbol_type now has a public name() member function.
242 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
244 ** Backward incompatible changes
246 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
248 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
249 depend on it will now produce Yacc-like error messages (just "syntax
250 error"). It was superseded by the "%error-verbose" directive in Bison
251 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
252 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
253 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
254 parse.error verbose".
256 ** Deprecated features
258 The YYPRINT macro, which works only with yacc.c and only for tokens, was
259 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
260 It is deprecated and its support will be removed eventually.
264 *** Improved syntax error messages
266 Two new values for the %define parse.error variable offer more control to
267 the user. Available in all the skeletons (C, C++, Java).
269 **** %define parse.error detailed
271 The behavior of "%define parse.error detailed" is closely resembling that
272 of "%define parse.error verbose" with a few exceptions. First, it is safe
273 to use non-ASCII characters in token aliases (with 'verbose', the result
274 depends on the locale with which bison was run). Second, a yysymbol_name
275 function is exposed to the user, instead of the yytnamerr function and the
276 yytname table. Third, token internationalization is supported (see
279 **** %define parse.error custom
281 With this directive, the user forges and emits the syntax error message
282 herself by defining the yyreport_syntax_error function. A new type,
283 yypcontext_t, captures the circumstances of the error, and provides the
284 user with functions to get details, such as yypcontext_expected_tokens to
285 get the list of expected token kinds.
287 A possible implementation of yyreport_syntax_error is:
290 yyreport_syntax_error (const yypcontext_t *ctx)
293 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
294 fprintf (stderr, ": syntax error");
295 // Report the tokens expected at this point.
297 enum { TOKENMAX = 10 };
298 yysymbol_kind_t expected[TOKENMAX];
299 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
301 // Forward errors to yyparse.
304 for (int i = 0; i < n; ++i)
305 fprintf (stderr, "%s %s",
306 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
308 // Report the unexpected token.
310 yysymbol_kind_t lookahead = yypcontext_token (ctx);
311 if (lookahead != YYSYMBOL_YYEMPTY)
312 fprintf (stderr, " before %s", yysymbol_name (lookahead));
314 fprintf (stderr, "\n");
318 **** Token aliases internationalization
320 When the %define variable parse.error is set to `custom` or `detailed`,
321 one may specify which token aliases are to be translated using _(). For
333 In that case the user must define _() and N_(), and yysymbol_name returns
334 the translated symbol (i.e., it returns '_("variable")' rather that
335 '"variable"'). In Java, the user must provide an i18n() function.
337 *** List of expected tokens (yacc.c)
339 Push parsers may invoke yypstate_expected_tokens at any point during
340 parsing (including even before submitting the first token) to get the list
341 of possible tokens. This feature can be used to propose autocompletion
342 (see below the "bistromathic" example).
344 It makes little sense to use this feature without enabling LAC (lookahead
347 *** Returning the error token
349 When the scanner returns an invalid token or the undefined token
350 (YYUNDEF), the parser generates an error message and enters error
351 recovery. Because of that error message, most scanners that find lexical
352 errors generate an error message, and then ignore the invalid input
353 without entering the error-recovery.
355 The scanners may now return YYerror, the error token, to enter the
356 error-recovery mode without triggering an additional error message. See
357 the bistromathic for an example.
359 *** Deep overhaul of the symbol and token kinds
361 To avoid the confusion with types in programming languages, we now refer
362 to token and symbol "kinds" instead of token and symbol "types". The
363 documentation and error messages have been revised.
365 All the skeletons have been updated to use dedicated enum types rather
366 than integral types. Special symbols are now regular citizens, instead of
367 being declared in ad hoc ways.
371 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
372 LPAREN, etc. While backward compatibility is of course ensured, users are
373 nonetheless invited to replace their uses of "enum yytokentype" by
376 This type now also includes tokens that were previously hidden: YYEOF (end
377 of input), YYUNDEF (undefined token), and YYerror (error token). They
378 now have string aliases, internationalized when internationalization is
379 enabled. Therefore, by default, error messages now refer to "end of file"
380 (internationalized) rather than the cryptic "$end", or to "invalid token"
381 rather than "$undefined".
383 Therefore in most cases it is now useless to define the end-of-line token
386 %token T_EOF 0 "end of file"
388 Rather simply use "YYEOF" in your scanner.
392 The "symbol kinds" is what the parser actually uses. (Unless the
393 api.token.raw %define variable is used, the symbol kind of a terminal
394 differs from the corresponding token kind.)
396 They are now exposed as a enum, "yysymbol_kind_t".
398 This allows users to tailor the error messages the way they want, or to
399 process some symbols in a specific way in autocompletion (see the
400 bistromathic example below).
402 *** Modernize display of explanatory statements in diagnostics
404 Since Bison 2.7, output was indented four spaces for explanatory
405 statements. For example:
407 input.y:2.7-13: error: %type redeclaration for exp
408 input.y:1.7-11: previous declaration
410 Since the introduction of caret-diagnostics, it became less clear. This
411 indentation has been removed and submessages are displayed similarly as in
414 input.y:2.7-13: error: %type redeclaration for exp
415 2 | %type <float> exp
417 input.y:1.7-11: note: previous declaration
421 Contributed by Victor Morales Cayuela.
425 The token and symbol kinds are yy::parser::token_kind_type and
426 yy::parser::symbol_kind_type.
428 The symbol_type::kind() member function allows to get the kind of a
429 symbol. This can be used to write unit tests for scanners, e.g.,
431 yy::parser::symbol_type t = make_NUMBER ("123");
432 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
433 assert (t.value.as<int> () == 123);
439 In order to avoid ambiguities with "type" as in "typing", we now refer to
440 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
441 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
446 There are now examples/java: a very simple calculator, and a more complete
447 one (push-parser, location tracking, and debug traces).
449 The lexcalc example (a simple example in C based on Flex and Bison) now
450 also demonstrates location tracking.
453 A new C example, bistromathic, is a fully featured interactive calculator
454 using many Bison features: pure interface, push parser, autocompletion
455 based on the current parser state (using yypstate_expected_tokens),
456 location tracking, internationalized custom error messages, lookahead
457 correction, rich debug traces, etc.
459 It shows how to depend on the symbol kinds to tailor autocompletion. For
460 instance it recognizes the symbol kind "VARIABLE" to propose
461 autocompletion on the existing variables, rather than of the word
465 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
467 ** WARNING: Future backward-incompatibilities!
469 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
471 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
472 that still depend on it will produce Yacc-like error messages (just
473 "syntax error"). It was superseded by the "%error-verbose" directive in
474 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
475 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
476 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
477 parse.error verbose".
481 Fix portability issues of the package itself on old compilers.
483 Fix api.token.raw support in Java.
486 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
490 Error messages could quote lines containing zero-width characters (such as
491 \005) with incorrect styling. Fixes for similar issues with unexpectedly
492 short lines (e.g., the file was changed between parsing and diagnosing).
494 Several unlikely crashes found by fuzzing have been fixed.
497 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
501 Portability issues and minor cosmetic issues.
503 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
507 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
513 Fix compiler warnings.
516 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
518 ** Backward incompatible changes
520 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
521 longer treated as end-of-lines. This changes the diagnostics, and in
522 particular their locations.
524 In C++, line numbers and columns are now represented as 'int' not
525 'unsigned', so that integer overflow on positions is easily checkable via
526 'gcc -fsanitize=undefined' and the like. This affects the API for
527 positions. The default position and location classes now expose
528 'counter_type' (int), used to define line and column numbers.
530 ** Deprecated features
532 The YYPRINT macro, which works only with yacc.c and only for tokens, was
533 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
534 It is deprecated and its support will be removed eventually.
538 *** Lookahead correction in C++
540 Contributed by Adrian Vogelsgesang.
542 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
543 %define variable parse.lac.
545 *** Variable api.token.raw: Optimized token numbers (all skeletons)
547 In the generated parsers, tokens have two numbers: the "external" token
548 number as returned by yylex (which starts at 257), and the "internal"
549 symbol number (which starts at 3). Each time yylex is called, a table
550 lookup maps the external token number to the internal symbol number.
552 When the %define variable api.token.raw is set, tokens are assigned their
553 internal number, which saves one table lookup per token, and also saves
554 the generation of the mapping table.
556 The gain is typically moderate, but in extreme cases (very simple user
557 actions), a 10% improvement can be observed.
559 *** Generated parsers use better types for states
561 Stacks now use the best integral type for state numbers, instead of always
562 using 15 bits. As a result "small" parsers now have a smaller memory
563 footprint (they use 8 bits), and there is support for large automata (16
564 bits), and extra large (using int, i.e., typically 31 bits).
566 *** Generated parsers prefer signed integer types
568 Bison skeletons now prefer signed to unsigned integer types when either
569 will do, as the signed types are less error-prone and allow for better
570 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
571 portable to unusual machines where char, short and int are all the same
572 width. On non-GNU platforms this may entail including <limits.h> and (if
573 available) <stdint.h> to define integer types and constants.
575 *** A skeleton for the D programming language
577 For the last few releases, Bison has shipped a stealth experimental
578 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
579 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
582 However, because nobody has committed to improving, testing, and
583 documenting this skeleton, it is not clear that it will be supported in
586 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
587 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
589 *** Debug traces in Java
591 The Java backend no longer emits code and data for parser tracing if the
592 %define variable parse.trace is not defined.
596 *** New diagnostic: -Wdangling-alias
598 String literals, which allow for better error messages, are (too)
599 liberally accepted by Bison, which might result in silent errors. For
602 %type <exVal> cond "condition"
604 does not define "condition" as a string alias to 'cond' (nonterminal
605 symbols do not have string aliases). It is rather equivalent to
608 %token <exVal> "condition"
610 i.e., it gives the type 'exVal' to the "condition" token, which was
611 clearly not the intention.
613 Also, because string aliases need not be defined, typos such as "baz"
614 instead of "bar" will be not reported.
616 The option -Wdangling-alias catches these situations. On
619 %type <ival> foo "foo"
623 bison -Wdangling-alias reports
625 warning: string literal not attached to a symbol
626 | %type <ival> foo "foo"
628 warning: string literal not attached to a symbol
632 The -Wall option does not (yet?) include -Wdangling-alias.
634 *** Better POSIX Yacc compatibility diagnostics
636 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
640 %type <ival> TOKEN1 TOKEN2 't'
647 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
648 2 | %type <ival> TOKEN1 TOKEN2 't'
650 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
651 2 | %type <ival> TOKEN1 TOKEN2 't'
653 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
654 2 | %type <ival> TOKEN1 TOKEN2 't'
657 *** Diagnostics with insertion
659 The diagnostics now display the suggestion below the underlined source.
660 Replacement for undeclared symbols are now also suggested.
667 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'?
671 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
675 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
677 *** Diagnostics about long lines
679 Quoted sources may now be truncated to fit the screen. For instance, on a
680 30-column wide terminal:
687 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
690 foo.y:1.8-10: previous declaration
693 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
696 foo.y:1.8-10: previous declaration
702 *** Debugging glr.c and glr.cc
704 The glr.c skeleton always had asserts to check its own behavior (not the
705 user's). These assertions are now under the control of the parse.assert
706 %define variable (disabled by default).
710 Several new compiler warnings in the generated output have been avoided.
711 Some unused features are no longer emitted. Cleaner generated code in
716 Portability issues in the test suite.
718 In theory, parsers using %nonassoc could crash when reporting verbose
719 error messages. This unlikely bug has been fixed.
721 In Java, %define api.prefix was ignored. It now behaves as expected.
724 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
728 In some cases, when warnings are disabled, bison could emit tons of white
729 spaces as diagnostics.
731 When running out of memory, bison could crash (found by fuzzing).
733 When defining twice the EOF token, bison would crash.
735 New warnings from recent compilers have been addressed in the generated
736 parsers (yacc.c, glr.c, glr.cc).
738 When lone carriage-return characters appeared in the input file,
739 diagnostics could hang forever.
742 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
749 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
751 ** Deprecated features
753 The %pure-parser directive is deprecated in favor of '%define api.pure'
754 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
755 now. Note that since Bison 2.7 you are strongly encouraged to use
756 '%define api.pure full' instead of '%define api.pure'.
760 *** Colored diagnostics
762 As an experimental feature, diagnostics are now colored, controlled by the
763 new options --color and --style.
765 To use them, install the libtextstyle library before configuring Bison.
768 https://alpha.gnu.org/gnu/gettext/
772 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
774 The option --color supports the following arguments:
775 - always, yes: Enable colors.
776 - never, no: Disable colors.
777 - auto, tty (default): Enable colors if the output device is a tty.
779 To customize the styles, create a CSS file similar to
783 .error { font-weight: 800; text-decoration: underline; }
786 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
787 environment variable to "bison-bw.css".
791 When given -fsyntax-only, the diagnostics are reported, but no output is
794 The name of this option is somewhat misleading as bison does more than
795 just checking the syntax: every stage is run (including checking for
796 conflicts for instance), except the generation of the output files.
798 *** Include the generated header (yacc.c)
800 Before, when --defines is used, bison generated a header, and pasted an
801 exact copy of it into the generated parser implementation file. If the
802 header name is not "y.tab.h", it is now #included instead of being
805 To use an '#include' even if the header name is "y.tab.h" (which is what
806 happens with --yacc, or when using the Autotools' ylwrap), define
807 api.header.include to the exact argument to pass to #include. For
810 %define api.header.include {"parse.h"}
814 %define api.header.include {<parser/parse.h>}
816 *** api.location.type is now supported in C (yacc.c, glr.c)
818 The %define variable api.location.type defines the name of the type to use
819 for locations. When defined, Bison no longer defines YYLTYPE.
821 This can be used in programs with several parsers to factor their
822 definition of locations: let one of them generate them, and the others
829 In conformance with the recommendations of the Graphviz team, if %require
830 "3.4" (or better) is specified, the option --graph generates a *.gv file
831 by default, instead of *.dot.
833 *** Diagnostics overhaul
835 Column numbers were wrong with multibyte characters, which would also
836 result in skewed diagnostics with carets. Beside, because we were
837 indenting the quoted source with a single space, lines with tab characters
838 were incorrectly underlined.
840 To address these issues, and to be clearer, Bison now issues diagnostics
841 as GCC9 does. For instance it used to display (there's a tab before the
844 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
845 expr: expr '+' "number" { $$ = $1 + $2; }
849 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
850 3 | expr: expr '+' "number" { $$ = $1 + $2; }
853 Other constructs now also have better locations, resulting in more precise
856 *** Fix-it hints for %empty
858 Running Bison with -Wempty-rules and --update will remove incorrect %empty
859 annotations, and add the missing ones.
861 *** Generated reports
863 The format of the reports (parse.output) was improved for readability.
865 *** Better support for --no-line.
867 When --no-line is used, the generated files are now cleaner: no lines are
868 generated instead of empty lines. Together with using api.header.include,
869 that should help people saving the generated files into version control
870 systems get smaller diffs.
874 A new example in C shows an simple infix calculator with a hand-written
875 scanner (examples/c/calc).
877 A new example in C shows a reentrant parser (capable of recursive calls)
878 built with Flex and Bison (examples/c/reccalc).
880 There is a new section about the history of Yaccs and Bison.
884 A few obscure bugs were fixed, including the second oldest (known) bug in
885 Bison: it was there when Bison was entered in the RCS version control
886 system, in December 1987. See the NEWS of Bison 3.3 for the previous
890 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
894 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
898 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
902 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
903 extensions into errors. It now makes them simple warnings (-Wyacc).
906 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
908 A new mailing list was created, Bison Announce. It is low traffic, and is
909 only about announcing new releases and important messages (e.g., polls
910 about major decisions to make).
912 https://lists.gnu.org/mailman/listinfo/bison-announce
914 ** Backward incompatible changes
916 Support for DJGPP, which has been unmaintained and untested for years, is
919 ** Deprecated features
921 A new feature, --update (see below) helps adjusting existing grammars to
924 *** Deprecated directives
926 The %error-verbose directive is deprecated in favor of '%define
927 parse.error verbose' since Bison 3.0, but no warning was issued.
929 The '%name-prefix "xx"' directive is deprecated in favor of '%define
930 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
931 directives are slightly different, you might need to adjust your code.
932 %name-prefix renames only symbols with external linkage, while api.prefix
933 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
934 yytokentype, YYSTYPE, YYLTYPE, etc.
936 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
937 {xx}' will typically have to update YY_DECL from
939 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
943 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
945 *** Deprecated %define variable names
947 The following variables, mostly related to parsers in Java, have been
948 renamed for consistency. Backward compatibility is ensured, but upgrading
951 abstract -> api.parser.abstract
952 annotations -> api.parser.annotations
953 extends -> api.parser.extends
954 final -> api.parser.final
955 implements -> api.parser.implements
956 parser_class_name -> api.parser.class
957 public -> api.parser.public
958 strictfp -> api.parser.strictfp
962 *** Generation of fix-its for IDEs/Editors
964 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
965 bison now generates machine readable editing instructions to fix some
966 issues. Currently, this is mostly limited to updating deprecated
967 directives and removing duplicates. For instance:
971 %define parser_class_name "Parser"
972 %define api.parser.class "Parser"
976 See the "fix-it:" lines below:
978 $ bison -ffixit foo.y
979 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
982 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
983 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
984 %define parser_class_name "Parser"
985 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
986 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
987 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
988 %define api.parser.class "Parser"
989 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
990 foo.y:2.1-34: previous definition
991 %define parser_class_name "Parser"
992 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
993 fix-it:"foo.y":{3:1-3:34}:""
994 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
996 This uses the same output format as GCC and Clang.
998 *** Updating grammar files
1000 Fixes can be applied on the fly. The previous example ends with the
1001 suggestion to re-run bison with the option -u/--update, which results in a
1002 cleaner grammar file.
1004 $ bison --update foo.y
1006 bison: file 'foo.y' was updated (backup: 'foo.y~')
1009 %define parse.error verbose
1010 %define api.parser.class {Parser}
1014 *** Bison is now relocatable
1016 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1018 A relocatable program can be moved or copied to a different location on
1019 the file system. It can also be used through mount points for network
1020 sharing. It is possible to make symbolic links to the installed and moved
1021 programs, and invoke them through the symbolic link.
1023 *** %expect and %expect-rr modifiers on individual rules
1025 One can now document (and check) which rules participate in shift/reduce
1026 and reduce/reduce conflicts. This is particularly important GLR parsers,
1027 where conflicts are a normal occurrence. For example,
1043 | argument_list ',' expression
1048 Looking at the output from -v, one can see that the shift/reduce conflict
1049 here is due to the fact that the parser does not know whether to reduce
1050 arguments to argument_list until it sees the token _after_ the following
1051 ','. By marking the rule with %expect 1 (because there is a conflict in
1052 one state), we document the source of the 1 overall shift/reduce conflict.
1054 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1055 conflicts. In this case, we mark each of the conflicting rules. For
1064 target_list '=' expr ';'
1070 | target ',' target_list
1079 | expr ',' expr_list
1087 In a statement such as
1091 the parser must reduce x to a target or an expr, but does not know which
1092 until it sees the '='. So we notate the two possible reductions to
1093 indicate that each conflicts in one rule.
1095 This feature needs user feedback, and might evolve in the future.
1097 *** C++: Actual token constructors
1099 When variants and token constructors are enabled, in addition to the
1100 type-safe named token constructors (make_ID, make_INT, etc.), we now
1101 generate genuine constructors for symbol_type.
1103 For instance with these declarations
1109 you may use these constructors:
1111 symbol_type (int token, const std::string&);
1112 symbol_type (int token, const int&);
1113 symbol_type (int token);
1115 Correct matching between token types and value types is checked via
1116 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1117 constructors are preferable, as they offer better type safety (for
1118 instance 'make_ID (42)' would not even compile), but symbol_type
1119 constructors may help when token types are discovered at run-time, e.g.,
1122 if (auto i = lookup_keyword (yytext))
1123 return yy::parser::symbol_type (i);
1125 return yy::parser::make_ID (yytext);
1128 *** C++: Variadic emplace
1130 If your application requires C++11 and you don't use symbol constructors,
1131 you may now use a variadic emplace for semantic values:
1133 %define api.value.type variant
1134 %token <std::pair<int, int>> PAIR
1138 int yylex (parser::semantic_type *lvalp)
1140 lvalp->emplace <std::pair<int, int>> (1, 2);
1141 return parser::token::PAIR;
1144 *** C++: Syntax error exceptions in GLR
1146 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1147 actions, or from the scanner.
1149 *** More POSIX Yacc compatibility warnings
1151 More Bison specific directives are now reported with -y or -Wyacc. This
1152 change was ready since the release of Bison 3.0 in September 2015. It was
1153 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1154 in numerous warnings for Bison users that use the GNU Build System.
1156 If you still experience that problem, either redefine YACC as `bison -o
1157 y.tab.c`, or pass -Wno-yacc to Bison.
1159 *** The tables yyrhs and yyphrs are back
1161 Because no Bison skeleton uses them, these tables were removed (no longer
1162 passed to the skeletons, not even computed) in 2008. However, some users
1163 have expressed interest in being able to use them in their own skeletons.
1167 *** Incorrect number of reduce/reduce conflicts
1169 On a grammar such as
1171 exp: "num" | "num" | "num"
1173 bison used to report a single RR conflict, instead of two. This is now
1174 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1175 was entered in the RCS version control system, in December 1987.
1177 Some grammar files might have to adjust their %expect-rr.
1179 *** Parser directives that were not careful enough
1181 Passing invalid arguments to %nterm, for instance character literals, used
1182 to result in unclear error messages.
1186 The examples/ directory (installed in .../share/doc/bison/examples) has
1187 been restructured per language for clarity. The examples come with a
1188 README and a Makefile. Not only can they be used to toy with Bison, they
1189 can also be starting points for your own grammars.
1191 There is now a Java example, and a simple example in C based on Flex and
1192 Bison (examples/c/lexcalc/).
1198 They now use noexcept and constexpr. Please, report missing annotations.
1200 *** Symbol Declarations
1202 The syntax of the variation directives to declare symbols was overhauled
1203 for more consistency, and also better POSIX Yacc compliance (which, for
1204 instance, allows "%type" without actually providing a type). The %nterm
1205 directive, supported by Bison since its inception, is now documented and
1206 officially supported.
1208 The syntax is now as follows:
1210 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1211 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1212 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1213 %nterm TAG? ID+ ( TAG ID+ )*
1215 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1216 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1217 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1218 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1219 one), ‘*’ (zero or more) and ‘+’ (one or more).
1222 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1226 Fix the move constructor of symbol_type.
1228 Always provide a copy constructor for symbol_type, even in modern C++.
1231 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1235 Properly support token constructors in C++ with types that include commas
1236 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1239 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1243 C++ portability issues.
1246 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1250 Several portability issues have been fixed in the build system, in the
1251 test suite, and in the generated parsers in C++.
1254 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1256 ** Backward incompatible changes
1258 Support for DJGPP, which has been unmaintained and untested for years, is
1259 obsolete. Unless there is activity to revive it, it will be removed.
1263 %printers should use yyo rather than yyoutput to denote the output stream.
1265 Variant-based symbols in C++ should use emplace() rather than build().
1267 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1271 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1273 A comment in the generated code now emphasizes that users should not
1274 depend upon non-documented implementation details, such as macros starting
1279 *** C++: Support for move semantics (lalr1.cc)
1281 The lalr1.cc skeleton now fully supports C++ move semantics, while
1282 maintaining compatibility with C++98. You may now store move-only types
1283 when using Bison's variants. For instance:
1290 %skeleton "lalr1.cc"
1291 %define api.value.type variant
1295 %token <int> INT "int";
1296 %type <std::unique_ptr<int>> int;
1297 %type <std::vector<std::unique_ptr<int>>> list;
1301 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1303 int: "int" { $$ = std::make_unique<int>($1); }
1305 *** C++: Implicit move of right-hand side values (lalr1.cc)
1307 In modern C++ (C++11 and later), you should always use 'std::move' with
1308 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1309 popped from the stack anyway. Using 'std::move' is mandatory for
1310 move-only types such as unique_ptr, and it provides a significant speedup
1311 for large types such as std::string, or std::vector, etc.
1313 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1314 by 'std::move ($n)'. The second rule in the previous grammar can be
1317 list: list int { $$ = $1; $$.emplace_back($2); }
1319 With automove enabled, the semantic values are no longer lvalues, so do
1320 not use the swap idiom:
1322 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1324 This idiom is anyway obsolete: it is preferable to move than to swap.
1326 A warning is issued when automove is enabled, and a value is used several
1329 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1330 exp: "twice" exp { $$ = $2 + $2; }
1333 Enabling api.value.automove does not require support for modern C++. The
1334 generated code is valid C++98/03, but will use copies instead of moves.
1336 The new examples/c++/variant-11.yy shows these features in action.
1338 *** C++: The implicit default semantic action is always run
1340 When variants are enabled, the default action was not run, so
1348 It now behaves like in all the other cases, as
1350 exp: "number" { $$ = $1; }
1352 possibly using std::move if automove is enabled.
1354 We do not expect backward compatibility issues. However, beware of
1355 forward compatibility issues: if you rely on default actions with
1356 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1357 generate incorrect parsers.
1359 *** C++: Renaming location.hh
1361 When both %defines and %locations are enabled, Bison generates a
1362 location.hh file. If you don't use locations outside of the parser, you
1363 may avoid its creation with:
1365 %define api.location.file none
1367 However this file is useful if, for instance, your parser builds an AST
1368 decorated with locations: you may use Bison's location independently of
1369 Bison's parser. You can now give it another name, for instance:
1371 %define api.location.file "my-location.hh"
1373 This name can have directory components, and even be absolute. The name
1374 under which the location file is included is controlled by
1375 api.location.include.
1377 This way it is possible to have several parsers share the same location
1380 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1383 %define api.namespace {foo}
1384 %define api.location.file "include/ast/loc.hh"
1385 %define api.location.include {<ast/loc.hh>}
1387 and use it in src/bar/parser.hh:
1390 %define api.namespace {bar}
1391 %code requires {#include <ast/loc.hh>}
1392 %define api.location.type {bar::location}
1394 Absolute file names are supported, so in your Makefile, passing the flag
1395 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1398 *** C++: stack.hh and position.hh are deprecated
1400 When asked to generate a header file (%defines), the lalr1.cc skeleton
1401 generates a stack.hh file. This file had no interest for users; it is now
1402 made useless: its content is included in the parser definition. It is
1403 still generated for backward compatibility.
1405 When in addition to %defines, location support is requested (%locations),
1406 the file position.hh is also generated. It is now also useless: its
1407 content is now included in location.hh.
1409 These files are no longer generated when your grammar file requires at
1410 least Bison 3.2 (%require "3.2").
1414 Portability issues on MinGW and VS2015.
1416 Portability issues in the test suite.
1418 Portability/warning issues with Flex.
1421 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1423 ** Backward incompatible changes
1425 Compiling Bison now requires a C99 compiler---as announced during the
1426 release of Bison 3.0, five years ago. Generated parsers do not require a
1429 Support for DJGPP, which has been unmaintained and untested for years, is
1430 obsolete. Unless there is activity to revive it, the next release of Bison
1431 will have it removed.
1435 *** Typed midrule actions
1437 Because their type is unknown to Bison, the values of midrule actions are
1438 not treated like the others: they don't have %printer and %destructor
1439 support. It also prevents C++ (Bison) variants to handle them properly.
1441 Typed midrule actions address these issues. Instead of:
1443 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1447 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1449 *** Reports include the type of the symbols
1451 The sections about terminal and nonterminal symbols of the '*.output' file
1452 now specify their declared type. For instance, for:
1456 the report now shows '<ival>':
1458 Terminals, with rules where they appear
1462 *** Diagnostics about useless rules
1464 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1465 of course, its rules are useless too.
1469 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1471 Previously all the useless rules were reported, including those whose
1472 left-hand side is the 'exp' nonterminal:
1474 warning: 1 nonterminal useless in grammar [-Wother]
1475 warning: 4 rules useless in grammar [-Wother]
1476 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1479 2.14-16: warning: rule useless in grammar [-Wother]
1482 3.6-16: warning: rule useless in grammar [-Wother]
1483 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1485 3.20-30: warning: rule useless in grammar [-Wother]
1486 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1488 3.34-44: warning: rule useless in grammar [-Wother]
1489 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1492 Now, rules whose left-hand side symbol is useless are no longer reported
1493 as useless. The locations of the errors have also been adjusted to point
1494 to the first use of the nonterminal as a left-hand side of a rule:
1496 warning: 1 nonterminal useless in grammar [-Wother]
1497 warning: 4 rules useless in grammar [-Wother]
1498 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1499 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1501 2.14-16: warning: rule useless in grammar [-Wother]
1505 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1507 When compiled with exceptions disabled, the generated parsers no longer
1508 uses try/catch clauses.
1510 Currently only GCC and Clang are supported.
1514 *** A demonstration of variants
1516 A new example was added (installed in .../share/doc/bison/examples),
1517 'variant.yy', which shows how to use (Bison) variants in C++.
1519 The other examples were made nicer to read.
1521 *** Some features are no longer 'experimental'
1523 The following features, mature enough, are no longer flagged as
1524 experimental in the documentation: push parsers, default %printer and
1525 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1526 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1527 semantic predicates (%?).
1531 *** GLR: Predicates support broken by #line directives
1533 Predicates (%?) in GLR such as
1536 %? {new_syntax} 'w' id new_args
1537 | %?{!new_syntax} 'w' id old_args
1539 were issued with #lines in the middle of C code.
1541 *** Printer and destructor with broken #line directives
1543 The #line directives were not properly escaped when emitting the code for
1544 %printer/%destructor, which resulted in compiler errors if there are
1545 backslashes or double-quotes in the grammar file name.
1547 *** Portability on ICC
1549 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1550 Generated parsers now work around this.
1554 There were several small fixes in the test suite and in the build system,
1555 many warnings in bison and in the generated parsers were eliminated. The
1556 documentation also received its share of minor improvements.
1558 Useless code was removed from C++ parsers, and some of the generated
1559 constructors are more 'natural'.
1562 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1566 *** C++: Fix support of 'syntax_error'
1568 One incorrect 'inline' resulted in linking errors about the constructor of
1569 the syntax_error exception.
1571 *** C++: Fix warnings
1573 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1574 warnings about yyformat being possibly null. It also warned about the
1575 deprecated implicit definition of copy constructors when there's a
1576 user-defined (copy) assignment operator.
1578 *** Location of errors
1580 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1581 ride-hand side raises a syntax error. The behavior of the default parser
1582 (yacc.c) in such a condition was undefined.
1584 Now all the parsers match the behavior of glr.c: @$ is used as the
1585 location of the error. This handles gracefully rules with and without
1588 *** Portability fixes in the test suite
1590 On some platforms, some Java and/or C++ tests were failing.
1593 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1597 *** C++ with Variants (lalr1.cc)
1599 Fix a compiler warning when no %destructor use $$.
1603 Several portability issues in tests were fixed.
1606 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1610 *** C++ with Variants (lalr1.cc)
1612 Problems with %destructor and '%define parse.assert' have been fixed.
1614 *** Named %union support (yacc.c, glr.c)
1616 Bison 3.0 introduced a regression on named %union such as
1618 %union foo { int ival; };
1620 The possibility to use a name was introduced "for Yacc compatibility".
1621 It is however not required by POSIX Yacc, and its usefulness is not clear.
1623 *** %define api.value.type union with %defines (yacc.c, glr.c)
1625 The C parsers were broken when %defines was used together with "%define
1626 api.value.type union".
1628 *** Redeclarations are reported in proper order
1636 bison used to report:
1638 foo.yy:2.10-11: error: %printer redeclaration for FOO
1641 foo.yy:3.10-11: previous declaration
1645 Now, the "previous" declaration is always the first one.
1650 Bison now installs various files in its docdir (which defaults to
1651 '/usr/local/share/doc/bison'), including the three fully blown examples
1652 extracted from the documentation:
1655 Reverse Polish Calculator, a simple introductory example.
1657 Multi-function Calc, a calculator with memory and functions and located
1660 a calculator in C++ using variant support and token constructors.
1663 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1667 *** Generated source files when errors are reported
1669 When warnings are issued and -Werror is set, bison would still generate
1670 the source files (*.c, *.h...). As a consequence, some runs of "make"
1671 could fail the first time, but not the second (as the files were generated
1674 This is fixed: bison no longer generates this source files, but, of
1675 course, still produces the various reports (*.output, *.xml, etc.).
1677 *** %empty is used in reports
1679 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1680 dot, XML and formats derived from it).
1682 *** YYERROR and variants
1684 When C++ variant support is enabled, an error triggered via YYERROR, but
1685 not caught via error recovery, resulted in a double deletion.
1688 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1692 *** Errors in caret diagnostics
1694 On some platforms, some errors could result in endless diagnostics.
1696 *** Fixes of the -Werror option
1698 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1699 diagnostics into errors instead of warnings. This is fixed.
1701 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1702 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1703 -Wno-error", "foo" diagnostics are now errors.
1707 As demonstrated in the documentation, one can now leave spaces between
1712 The yacc.1 man page is no longer installed if --disable-yacc was
1715 *** Fixes in the test suite
1717 Bugs and portability issues.
1720 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1722 ** WARNING: Future backward-incompatibilities!
1724 Like other GNU packages, Bison will start using some of the C99 features
1725 for its own code, especially the definition of variables after statements.
1726 The generated C parsers still aim at C90.
1728 ** Backward incompatible changes
1730 *** Obsolete features
1732 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1734 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1735 use YYSTYPE and YYLTYPE.
1737 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1738 1.875): use %lex-param, %parse-param, or %param.
1740 Missing semicolons at the end of actions are no longer added (as announced
1741 in the release 2.5).
1743 *** Use of YACC='bison -y'
1745 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1748 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1749 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1750 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1752 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1753 implementation of Yacc, was using Bison as 'bison -y'. While it does
1754 ensure compatible output file names, it also enables warnings for
1755 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1756 warnings for Bison extensions.
1758 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1759 (which also generates 'y.tab.h' and 'y.output' when needed).
1760 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1761 flags (YFLAGS, or AM_YFLAGS with Automake).
1765 *** The epilogue is no longer affected by internal #defines (glr.c)
1767 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1768 generated code. These weren't properly undefined before the inclusion of
1769 the user epilogue, so functions such as the following were butchered by the
1770 preprocessor expansion:
1772 int yylex (YYSTYPE *yylval);
1774 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
1775 identifiers for user-provided variables.
1777 *** stdio.h is no longer needed when locations are enabled (yacc.c)
1779 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
1780 locations are enabled. This is fixed.
1782 *** Warnings about useless %pure-parser/%define api.pure are restored
1784 ** Diagnostics reported by Bison
1786 Most of these features were contributed by Théophile Ranquet and Victor
1791 Version 2.7 introduced caret errors, for a prettier output. These are now
1792 activated by default. The old format can still be used by invoking Bison
1793 with -fno-caret (or -fnone).
1795 Some error messages that reproduced excerpts of the grammar are now using
1796 the caret information only. For instance on:
1803 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1804 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
1808 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1809 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
1813 and "bison -fno-caret" reports:
1815 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1816 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
1818 *** Enhancements of the -Werror option
1820 The -Werror=CATEGORY option is now recognized, and will treat specified
1821 warnings as errors. The warnings need not have been explicitly activated
1822 using the -W option, this is similar to what GCC 4.7 does.
1824 For example, given the following command line, Bison will treat both
1825 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
1826 errors (and only those):
1828 $ bison -Werror=yacc,error=conflicts-sr input.y
1830 If no categories are specified, -Werror will make all active warnings into
1831 errors. For example, the following line does the same the previous example:
1833 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
1835 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
1837 Note that the categories in this -Werror option may not be prefixed with
1838 "no-". However, -Wno-error[=CATEGORY] is valid.
1840 Note that -y enables -Werror=yacc. Therefore it is now possible to require
1841 Yacc-like behavior (e.g., always generate y.tab.c), but to report
1842 incompatibilities as warnings: "-y -Wno-error=yacc".
1844 *** The display of warnings is now richer
1846 The option that controls a given warning is now displayed:
1848 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
1850 In the case of warnings treated as errors, the prefix is changed from
1851 "warning: " to "error: ", and the suffix is displayed, in a manner similar
1852 to GCC, as [-Werror=CATEGORY].
1854 For instance, where the previous version of Bison would report (and exit
1857 bison: warnings being treated as errors
1858 input.y:1.1: warning: stray ',' treated as white space
1862 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
1864 *** Deprecated constructs
1866 The new 'deprecated' warning category flags obsolete constructs whose
1867 support will be discontinued. It is enabled by default. These warnings
1868 used to be reported as 'other' warnings.
1870 *** Useless semantic types
1872 Bison now warns about useless (uninhabited) semantic types. Since
1873 semantic types are not declared to Bison (they are defined in the opaque
1874 %union structure), it is %printer/%destructor directives about useless
1875 types that trigger the warning:
1879 %printer {} <type1> <type3>
1880 %destructor {} <type2> <type4>
1882 nterm: term { $$ = $1; };
1884 3.28-34: warning: type <type3> is used, but is not associated to any symbol
1885 4.28-34: warning: type <type4> is used, but is not associated to any symbol
1887 *** Undefined but unused symbols
1889 Bison used to raise an error for undefined symbols that are not used in
1890 the grammar. This is now only a warning.
1893 %destructor {} symbol2
1894 %type <type> symbol3
1898 *** Useless destructors or printers
1900 Bison now warns about useless destructors or printers. In the following
1901 example, the printer for <type1>, and the destructor for <type2> are
1902 useless: all symbols of <type1> (token1) already have a printer, and all
1903 symbols of type <type2> (token2) already have a destructor.
1905 %token <type1> token1
1909 %printer {} token1 <type1> <type3>
1910 %destructor {} token2 <type2> <type4>
1914 The warnings and error messages about shift/reduce and reduce/reduce
1915 conflicts have been normalized. For instance on the following foo.y file:
1919 exp: exp '+' exp | '0' | '0';
1921 compare the previous version of bison:
1924 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1925 $ bison -Werror foo.y
1926 bison: warnings being treated as errors
1927 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1929 with the new behavior:
1932 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
1933 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
1934 $ bison -Werror foo.y
1935 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
1936 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
1938 When %expect or %expect-rr is used, such as with bar.y:
1943 exp: exp '+' exp | '0' | '0';
1948 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1949 bar.y: expected 0 shift/reduce conflicts
1950 bar.y: expected 0 reduce/reduce conflicts
1955 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
1956 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
1958 ** Incompatibilities with POSIX Yacc
1960 The 'yacc' category is no longer part of '-Wall', enable it explicitly
1963 ** Additional yylex/yyparse arguments
1965 The new directive %param declares additional arguments to both yylex and
1966 yyparse. The %lex-param, %parse-param, and %param directives support one
1967 or more arguments. Instead of
1969 %lex-param {arg1_type *arg1}
1970 %lex-param {arg2_type *arg2}
1971 %parse-param {arg1_type *arg1}
1972 %parse-param {arg2_type *arg2}
1976 %param {arg1_type *arg1} {arg2_type *arg2}
1978 ** Types of values for %define variables
1980 Bison used to make no difference between '%define foo bar' and '%define
1981 foo "bar"'. The former is now called a 'keyword value', and the latter a
1982 'string value'. A third kind was added: 'code values', such as '%define
1985 Keyword variables are used for fixed value sets, e.g.,
1987 %define lr.type lalr
1989 Code variables are used for value in the target language, e.g.,
1991 %define api.value.type {struct semantic_type}
1993 String variables are used remaining cases, e.g. file names.
1995 ** Variable api.token.prefix
1997 The variable api.token.prefix changes the way tokens are identified in
1998 the generated files. This is especially useful to avoid collisions
1999 with identifiers in the target language. For instance
2001 %token FILE for ERROR
2002 %define api.token.prefix {TOK_}
2004 start: FILE for ERROR;
2006 will generate the definition of the symbols TOK_FILE, TOK_for, and
2007 TOK_ERROR in the generated sources. In particular, the scanner must
2008 use these prefixed token names, although the grammar itself still
2009 uses the short names (as in the sample rule given above).
2011 ** Variable api.value.type
2013 This new %define variable supersedes the #define macro YYSTYPE. The use
2014 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2015 using %union or %defining api.value.type results in undefined behavior.
2017 Either define api.value.type, or use "%union":
2024 %token <ival> INT "integer"
2025 %token <sval> STRING "string"
2026 %printer { fprintf (yyo, "%d", $$); } <ival>
2027 %destructor { free ($$); } <sval>
2030 yylval.ival = 42; return INT;
2031 yylval.sval = "42"; return STRING;
2033 The %define variable api.value.type supports both keyword and code values.
2035 The keyword value 'union' means that the user provides genuine types, not
2036 union member names such as "ival" and "sval" above (WARNING: will fail if
2037 -y/--yacc/%yacc is enabled).
2039 %define api.value.type union
2040 %token <int> INT "integer"
2041 %token <char *> STRING "string"
2042 %printer { fprintf (yyo, "%d", $$); } <int>
2043 %destructor { free ($$); } <char *>
2046 yylval.INT = 42; return INT;
2047 yylval.STRING = "42"; return STRING;
2049 The keyword value variant is somewhat equivalent, but for C++ special
2050 provision is made to allow classes to be used (more about this below).
2052 %define api.value.type variant
2053 %token <int> INT "integer"
2054 %token <std::string> STRING "string"
2056 Code values (in braces) denote user defined types. This is where YYSTYPE
2074 %define api.value.type {struct my_value}
2075 %token <u.ival> INT "integer"
2076 %token <u.sval> STRING "string"
2077 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2078 %destructor { free ($$); } <u.sval>
2081 yylval.u.ival = 42; return INT;
2082 yylval.u.sval = "42"; return STRING;
2084 ** Variable parse.error
2086 This variable controls the verbosity of error messages. The use of the
2087 %error-verbose directive is deprecated in favor of "%define parse.error
2090 ** Deprecated %define variable names
2092 The following variables have been renamed for consistency. Backward
2093 compatibility is ensured, but upgrading is recommended.
2095 lr.default-reductions -> lr.default-reduction
2096 lr.keep-unreachable-states -> lr.keep-unreachable-state
2097 namespace -> api.namespace
2098 stype -> api.value.type
2100 ** Semantic predicates
2102 Contributed by Paul Hilfinger.
2104 The new, experimental, semantic-predicate feature allows actions of the
2105 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2106 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2107 in GLR parsers, rather than being deferred. The result is that they allow
2108 the programmer to prune possible parses based on the values of run-time
2111 ** The directive %expect-rr is now an error in non GLR mode
2113 It used to be an error only if used in non GLR mode, _and_ if there are
2114 reduce/reduce conflicts.
2116 ** Tokens are numbered in their order of appearance
2118 Contributed by Valentin Tolmer.
2120 With '%token A B', A had a number less than the one of B. However,
2121 precedence declarations used to generate a reversed order. This is now
2122 fixed, and introducing tokens with any of %token, %left, %right,
2123 %precedence, or %nonassoc yields the same result.
2125 When mixing declarations of tokens with a literal character (e.g., 'a') or
2126 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2127 the literal characters first. For example
2131 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2132 input order is now preserved.
2134 These changes were made so that one can remove useless precedence and
2135 associativity declarations (i.e., map %nonassoc, %left or %right to
2136 %precedence, or to %token) and get exactly the same output.
2138 ** Useless precedence and associativity
2140 Contributed by Valentin Tolmer.
2142 When developing and maintaining a grammar, useless associativity and
2143 precedence directives are common. They can be a nuisance: new ambiguities
2144 arising are sometimes masked because their conflicts are resolved due to
2145 the extra precedence or associativity information. Furthermore, it can
2146 hinder the comprehension of a new grammar: one will wonder about the role
2147 of a precedence, where in fact it is useless. The following changes aim
2148 at detecting and reporting these extra directives.
2150 *** Precedence warning category
2152 A new category of warning, -Wprecedence, was introduced. It flags the
2153 useless precedence and associativity directives.
2155 *** Useless associativity
2157 Bison now warns about symbols with a declared associativity that is never
2158 used to resolve conflicts. In that case, using %precedence is sufficient;
2159 the parsing tables will remain unchanged. Solving these warnings may raise
2160 useless precedence warnings, as the symbols no longer have associativity.
2174 warning: useless associativity for '+', use %precedence [-Wprecedence]
2178 *** Useless precedence
2180 Bison now warns about symbols with a declared precedence and no declared
2181 associativity (i.e., declared with %precedence), and whose precedence is
2182 never used. In that case, the symbol can be safely declared with %token
2183 instead, without modifying the parsing tables. For example:
2187 exp: "var" '=' "number";
2191 warning: useless precedence for '=' [-Wprecedence]
2195 *** Useless precedence and associativity
2197 In case of both useless precedence and associativity, the issue is flagged
2202 exp: "var" '=' "number";
2206 warning: useless precedence and associativity for '=' [-Wprecedence]
2212 With help from Joel E. Denny and Gabriel Rassoul.
2214 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2215 marked by the new %empty directive. Using %empty on a non-empty rule is
2216 an error. The new -Wempty-rule warning reports empty rules without
2217 %empty. On the following grammar:
2227 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2230 5.8-13: error: %empty on non-empty rule
2234 ** Java skeleton improvements
2236 The constants for token names were moved to the Lexer interface. Also, it
2237 is possible to add code to the parser's constructors using "%code init"
2238 and "%define init_throws".
2239 Contributed by Paolo Bonzini.
2241 The Java skeleton now supports push parsing.
2242 Contributed by Dennis Heimbigner.
2244 ** C++ skeletons improvements
2246 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2248 Using %defines is now optional. Without it, the needed support classes
2249 are defined in the generated parser, instead of additional files (such as
2250 location.hh, position.hh and stack.hh).
2252 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2254 Both lalr1.cc and glr.cc no longer require %location.
2256 *** syntax_error exception (lalr1.cc)
2258 The C++ parser features a syntax_error exception, which can be
2259 thrown from the scanner or from user rules to raise syntax errors.
2260 This facilitates reporting errors caught in sub-functions (e.g.,
2261 rejecting too large integral literals from a conversion function
2262 used by the scanner, or rejecting invalid combinations from a
2263 factory invoked by the user actions).
2265 *** %define api.value.type variant
2267 This is based on a submission from Michiel De Wilde. With help
2268 from Théophile Ranquet.
2270 In this mode, complex C++ objects can be used as semantic values. For
2273 %token <::std::string> TEXT;
2274 %token <int> NUMBER;
2275 %token SEMICOLON ";"
2276 %type <::std::string> item;
2277 %type <::std::list<std::string>> list;
2280 list { std::cout << $1 << std::endl; }
2284 %empty { /* Generates an empty string list. */ }
2285 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2289 TEXT { std::swap ($$, $1); }
2290 | NUMBER { $$ = string_cast ($1); }
2293 *** %define api.token.constructor
2295 When variants are enabled, Bison can generate functions to build the
2296 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2297 with the semantic value (e.g., int):
2299 parser::symbol_type yylex ()
2301 parser::location_type loc = ...;
2303 return parser::make_TEXT ("Hello, world!", loc);
2305 return parser::make_NUMBER (42, loc);
2307 return parser::make_SEMICOLON (loc);
2313 There are operator- and operator-= for 'location'. Negative line/column
2314 increments can no longer underflow the resulting value.
2317 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2321 *** Fix compiler attribute portability (yacc.c)
2323 With locations enabled, __attribute__ was used unprotected.
2325 *** Fix some compiler warnings (lalr1.cc)
2328 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2332 Warnings about uninitialized yylloc in yyparse have been fixed.
2334 Restored C90 compliance (yet no report was ever made).
2336 ** Diagnostics are improved
2338 Contributed by Théophile Ranquet.
2340 *** Changes in the format of error messages
2342 This used to be the format of many error reports:
2344 input.y:2.7-12: %type redeclaration for exp
2345 input.y:1.7-12: previous declaration
2349 input.y:2.7-12: error: %type redeclaration for exp
2350 input.y:1.7-12: previous declaration
2352 *** New format for error reports: carets
2354 Caret errors have been added to Bison:
2356 input.y:2.7-12: error: %type redeclaration for exp
2359 input.y:1.7-12: previous declaration
2365 input.y:3.20-23: error: ambiguous reference: '$exp'
2366 exp: exp '+' exp { $exp = $1 + $3; };
2368 input.y:3.1-3: refers to: $exp at $$
2369 exp: exp '+' exp { $exp = $1 + $3; };
2371 input.y:3.6-8: refers to: $exp at $1
2372 exp: exp '+' exp { $exp = $1 + $3; };
2374 input.y:3.14-16: refers to: $exp at $3
2375 exp: exp '+' exp { $exp = $1 + $3; };
2378 The default behavior for now is still not to display these unless
2379 explicitly asked with -fcaret (or -fall). However, in a later release, it
2380 will be made the default behavior (but may still be deactivated with
2383 ** New value for %define variable: api.pure full
2385 The %define variable api.pure requests a pure (reentrant) parser. However,
2386 for historical reasons, using it in a location-tracking Yacc parser
2387 resulted in a yyerror function that did not take a location as a
2388 parameter. With this new value, the user may request a better pure parser,
2389 where yyerror does take a location as a parameter (in location-tracking
2392 The use of "%define api.pure true" is deprecated in favor of this new
2393 "%define api.pure full".
2395 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2397 The %define variable api.location.type defines the name of the type to use
2398 for locations. When defined, Bison no longer generates the position.hh
2399 and location.hh files, nor does the parser will include them: the user is
2400 then responsible to define her type.
2402 This can be used in programs with several parsers to factor their location
2403 and position files: let one of them generate them, and the others just use
2406 This feature was actually introduced, but not documented, in Bison 2.5,
2407 under the name "location_type" (which is maintained for backward
2410 For consistency, lalr1.java's %define variables location_type and
2411 position_type are deprecated in favor of api.location.type and
2414 ** Exception safety (lalr1.cc)
2416 The parse function now catches exceptions, uses the %destructors to
2417 release memory (the lookahead symbol and the symbols pushed on the stack)
2418 before re-throwing the exception.
2420 This feature is somewhat experimental. User feedback would be
2423 ** Graph improvements in DOT and XSLT
2425 Contributed by Théophile Ranquet.
2427 The graphical presentation of the states is more readable: their shape is
2428 now rectangular, the state number is clearly displayed, and the items are
2429 numbered and left-justified.
2431 The reductions are now explicitly represented as transitions to other
2432 diamond shaped nodes.
2434 These changes are present in both --graph output and xml2dot.xsl XSLT
2435 processing, with minor (documented) differences.
2437 ** %language is no longer an experimental feature.
2439 The introduction of this feature, in 2.4, was four years ago. The
2440 --language option and the %language directive are no longer experimental.
2444 The sections about shift/reduce and reduce/reduce conflicts resolution
2445 have been fixed and extended.
2447 Although introduced more than four years ago, XML and Graphviz reports
2448 were not properly documented.
2450 The translation of midrule actions is now described.
2453 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2455 We consider compiler warnings about Bison generated parsers to be bugs.
2456 Rather than working around them in your own project, please consider
2457 reporting them to us.
2461 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2462 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2465 Other issues in the test suite have been addressed.
2467 Null characters are correctly displayed in error messages.
2469 When possible, yylloc is correctly initialized before calling yylex. It
2470 is no longer necessary to initialize it in the %initial-action.
2473 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2475 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2478 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2482 Bugs and portability issues in the test suite have been fixed.
2484 Some errors in translations have been addressed, and --help now directs
2485 users to the appropriate place to report them.
2487 Stray Info files shipped by accident are removed.
2489 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2490 generated, are removed.
2492 All the generated headers are self-contained.
2494 ** Header guards (yacc.c, glr.c, glr.cc)
2496 In order to avoid collisions, the header guards are now
2497 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2498 For instance the header generated from
2500 %define api.prefix "calc"
2501 %defines "lib/parse.h"
2503 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2505 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2507 The compilation of pure parsers (%define api.pure) can trigger GCC
2510 input.c: In function 'yyparse':
2511 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2512 function [-Wmaybe-uninitialized]
2516 This is now fixed; pragmas to avoid these warnings are no longer needed.
2518 Warnings from clang ("equality comparison with extraneous parentheses" and
2519 "function declared 'noreturn' should not return") have also been
2523 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2527 Buffer overruns, complaints from Flex, and portability issues in the test
2528 suite have been fixed.
2530 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2532 Trailing end-of-lines in %parse-param or %lex-param would result in
2533 invalid C++. This is fixed.
2535 ** Spurious spaces and end-of-lines
2537 The generated files no longer end (nor start) with empty lines.
2540 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2542 Bison no longer executes user-specified M4 code when processing a grammar.
2546 In addition to the removal of the features announced in Bison 2.6, the
2547 next major release will remove the "Temporary hack for adding a semicolon
2548 to the user action", as announced in the release 2.5. Instead of:
2550 exp: exp "+" exp { $$ = $1 + $3 };
2554 exp: exp "+" exp { $$ = $1 + $3; };
2558 *** Type names are now properly escaped.
2560 *** glr.cc: set_debug_level and debug_level work as expected.
2562 *** Stray @ or $ in actions
2564 While Bison used to warn about stray $ or @ in action rules, it did not
2565 for other actions such as printers, destructors, or initial actions. It
2568 ** Type names in actions
2570 For consistency with rule actions, it is now possible to qualify $$ by a
2571 type-name in destructors, printers, and initial actions. For instance:
2573 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2575 will display two values for each typed and untyped symbol (provided
2576 that YYSTYPE has both "ival" and "fval" fields).
2579 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2583 The next major release of Bison will drop support for the following
2584 deprecated features. Please report disagreements to bug-bison@gnu.org.
2588 Support for generating parsers in K&R C will be removed. Parsers
2589 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2592 *** Features deprecated since Bison 1.875
2594 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2597 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2598 %lex-param, will no longer be supported.
2600 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2603 *** The generated header will be included (yacc.c)
2605 Instead of duplicating the content of the generated header (definition of
2606 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2607 as is already the case for GLR or C++ parsers. This change is deferred
2608 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2611 ** Generated Parser Headers
2613 *** Guards (yacc.c, glr.c, glr.cc)
2615 The generated headers are now guarded, as is already the case for C++
2616 parsers (lalr1.cc). For instance, with --defines=foo.h:
2621 #endif /* !YY_FOO_H */
2623 *** New declarations (yacc.c, glr.c)
2625 The generated header now declares yydebug and yyparse. Both honor
2626 --name-prefix=bar_, and yield
2628 int bar_parse (void);
2632 #define yyparse bar_parse
2635 in order to facilitate the inclusion of several parser headers inside a
2636 single compilation unit.
2638 *** Exported symbols in C++
2640 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2641 header, are removed, as they prevent the possibility of including several
2642 generated headers from a single compilation unit.
2646 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2649 ** New %define variable: api.prefix
2651 Now that the generated headers are more complete and properly protected
2652 against multiple inclusions, constant names, such as YYSTYPE are a
2653 problem. While yyparse and others are properly renamed by %name-prefix,
2654 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2655 would introduce backward compatibility issues in projects not expecting
2656 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2657 it is deprecated in favor of a new %define variable: api.prefix.
2659 The following examples compares both:
2661 %name-prefix "bar_" | %define api.prefix "bar_"
2662 %token <ival> FOO %token <ival> FOO
2663 %union { int ival; } %union { int ival; }
2669 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2670 # define BAR_FOO_H # define BAR_FOO_H
2672 /* Enabling traces. */ /* Enabling traces. */
2673 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2674 > # if defined YYDEBUG
2676 > # define BAR_DEBUG 1
2678 > # define BAR_DEBUG 0
2681 # define YYDEBUG 0 | # define BAR_DEBUG 0
2685 # if YYDEBUG | # if BAR_DEBUG
2686 extern int bar_debug; extern int bar_debug;
2689 /* Tokens. */ /* Tokens. */
2690 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2691 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2692 enum yytokentype { | enum bar_tokentype {
2697 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2698 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2699 typedef union YYSTYPE | typedef union BAR_STYPE
2702 } YYSTYPE; | } BAR_STYPE;
2703 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2706 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2708 int bar_parse (void); int bar_parse (void);
2710 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2713 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2717 The next major release will drop support for generating parsers in K&R C.
2719 ** yacc.c: YYBACKUP works as expected.
2721 ** glr.c improvements:
2723 *** Location support is eliminated when not requested:
2725 GLR parsers used to include location-related code even when locations were
2726 not requested, and therefore not even usable.
2728 *** __attribute__ is preserved:
2730 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2731 when -std is passed to GCC).
2733 ** lalr1.java: several fixes:
2735 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2736 first token leads to a syntax error. Some minor clean ups.
2740 *** C++11 compatibility:
2742 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2747 The header files such as "parser.hh", "location.hh", etc. used a constant
2748 name for preprocessor guards, for instance:
2750 #ifndef BISON_LOCATION_HH
2751 # define BISON_LOCATION_HH
2753 #endif // !BISON_LOCATION_HH
2755 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2756 case characters are converted to upper case, and series of
2757 non-alphanumerical characters are converted to an underscore.
2759 With "bison -o lang++/parser.cc", "location.hh" would now include:
2761 #ifndef YY_LANG_LOCATION_HH
2762 # define YY_LANG_LOCATION_HH
2764 #endif // !YY_LANG_LOCATION_HH
2768 The position and location constructors (and their initialize methods)
2769 accept new arguments for line and column. Several issues in the
2770 documentation were fixed.
2772 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2774 ** Changes in the manual:
2776 *** %printer is documented
2778 The "%printer" directive, supported since at least Bison 1.50, is finally
2779 documented. The "mfcalc" example is extended to demonstrate it.
2781 For consistency with the C skeletons, the C++ parsers now also support
2782 "yyoutput" (as an alias to "debug_stream ()").
2784 *** Several improvements have been made:
2786 The layout for grammar excerpts was changed to a more compact scheme.
2787 Named references are motivated. The description of the automaton
2788 description file (*.output) is updated to the current format. Incorrect
2789 index entries were fixed. Some other errors were fixed.
2793 *** Conflicting prototypes with recent/modified Flex.
2795 Fixed build problems with the current, unreleased, version of Flex, and
2796 some modified versions of 2.5.35, which have modified function prototypes.
2798 *** Warnings during the build procedure have been eliminated.
2800 *** Several portability problems in the test suite have been fixed:
2802 This includes warnings with some compilers, unexpected behavior of tools
2803 such as diff, warning messages from the test suite itself, etc.
2805 *** The install-pdf target works properly:
2807 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
2808 halts in the middle of its course.
2811 * Noteworthy changes in release 2.5 (2011-05-14)
2813 ** Grammar symbol names can now contain non-initial dashes:
2815 Consistently with directives (such as %error-verbose) and with
2816 %define variables (e.g. push-pull), grammar symbol names may contain
2817 dashes in any position except the beginning. This is a GNU
2818 extension over POSIX Yacc. Thus, use of this extension is reported
2819 by -Wyacc and rejected in Yacc mode (--yacc).
2821 ** Named references:
2823 Historically, Yacc and Bison have supported positional references
2824 ($n, $$) to allow access to symbol values from inside of semantic
2827 Starting from this version, Bison can also accept named references.
2828 When no ambiguity is possible, original symbol names may be used
2829 as named references:
2831 if_stmt : "if" cond_expr "then" then_stmt ';'
2832 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
2834 In the more common case, explicit names may be declared:
2836 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
2837 { $res = mk_if_stmt($cond, $then, $else); }
2839 Location information is also accessible using @name syntax. When
2840 accessing symbol names containing dots or dashes, explicit bracketing
2841 ($[sym.1]) must be used.
2843 These features are experimental in this version. More user feedback
2844 will help to stabilize them.
2845 Contributed by Alex Rozenman.
2847 ** IELR(1) and canonical LR(1):
2849 IELR(1) is a minimal LR(1) parser table generation algorithm. That
2850 is, given any context-free grammar, IELR(1) generates parser tables
2851 with the full language-recognition power of canonical LR(1) but with
2852 nearly the same number of parser states as LALR(1). This reduction
2853 in parser states is often an order of magnitude. More importantly,
2854 because canonical LR(1)'s extra parser states may contain duplicate
2855 conflicts in the case of non-LR(1) grammars, the number of conflicts
2856 for IELR(1) is often an order of magnitude less as well. This can
2857 significantly reduce the complexity of developing of a grammar.
2859 Bison can now generate IELR(1) and canonical LR(1) parser tables in
2860 place of its traditional LALR(1) parser tables, which remain the
2861 default. You can specify the type of parser tables in the grammar
2862 file with these directives:
2864 %define lr.type lalr
2865 %define lr.type ielr
2866 %define lr.type canonical-lr
2868 The default-reduction optimization in the parser tables can also be
2869 adjusted using "%define lr.default-reductions". For details on both
2870 of these features, see the new section "Tuning LR" in the Bison
2873 These features are experimental. More user feedback will help to
2876 ** LAC (Lookahead Correction) for syntax error handling
2878 Contributed by Joel E. Denny.
2880 Canonical LR, IELR, and LALR can suffer from a couple of problems
2881 upon encountering a syntax error. First, the parser might perform
2882 additional parser stack reductions before discovering the syntax
2883 error. Such reductions can perform user semantic actions that are
2884 unexpected because they are based on an invalid token, and they
2885 cause error recovery to begin in a different syntactic context than
2886 the one in which the invalid token was encountered. Second, when
2887 verbose error messages are enabled (with %error-verbose or the
2888 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
2889 syntax error message can both contain invalid tokens and omit valid
2892 The culprits for the above problems are %nonassoc, default
2893 reductions in inconsistent states, and parser state merging. Thus,
2894 IELR and LALR suffer the most. Canonical LR can suffer only if
2895 %nonassoc is used or if default reductions are enabled for
2896 inconsistent states.
2898 LAC is a new mechanism within the parsing algorithm that solves
2899 these problems for canonical LR, IELR, and LALR without sacrificing
2900 %nonassoc, default reductions, or state merging. When LAC is in
2901 use, canonical LR and IELR behave almost exactly the same for both
2902 syntactically acceptable and syntactically unacceptable input.
2903 While LALR still does not support the full language-recognition
2904 power of canonical LR and IELR, LAC at least enables LALR's syntax
2905 error handling to correctly reflect LALR's language-recognition
2908 Currently, LAC is only supported for deterministic parsers in C.
2909 You can enable LAC with the following directive:
2911 %define parse.lac full
2913 See the new section "LAC" in the Bison manual for additional
2914 details including a few caveats.
2916 LAC is an experimental feature. More user feedback will help to
2919 ** %define improvements:
2921 *** Can now be invoked via the command line:
2923 Each of these command-line options
2926 --define=NAME[=VALUE]
2929 --force-define=NAME[=VALUE]
2931 is equivalent to this grammar file declaration
2933 %define NAME ["VALUE"]
2935 except that the manner in which Bison processes multiple definitions
2936 for the same NAME differs. Most importantly, -F and --force-define
2937 quietly override %define, but -D and --define do not. For further
2938 details, see the section "Bison Options" in the Bison manual.
2940 *** Variables renamed:
2942 The following %define variables
2945 lr.keep_unreachable_states
2947 have been renamed to
2950 lr.keep-unreachable-states
2952 The old names are now deprecated but will be maintained indefinitely
2953 for backward compatibility.
2955 *** Values no longer need to be quoted in the grammar file:
2957 If a %define value is an identifier, it no longer needs to be placed
2958 within quotations marks. For example,
2960 %define api.push-pull "push"
2964 %define api.push-pull push
2966 *** Unrecognized variables are now errors not warnings.
2968 *** Multiple invocations for any variable is now an error not a warning.
2970 ** Unrecognized %code qualifiers are now errors not warnings.
2972 ** Character literals not of length one:
2974 Previously, Bison quietly converted all character literals to length
2975 one. For example, without warning, Bison interpreted the operators in
2976 the following grammar to be the same token:
2982 Bison now warns when a character literal is not of length one. In
2983 some future release, Bison will start reporting an error instead.
2985 ** Destructor calls fixed for lookaheads altered in semantic actions:
2987 Previously for deterministic parsers in C, if a user semantic action
2988 altered yychar, the parser in some cases used the old yychar value to
2989 determine which destructor to call for the lookahead upon a syntax
2990 error or upon parser return. This bug has been fixed.
2992 ** C++ parsers use YYRHSLOC:
2994 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
2995 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
2996 to use it. If, for instance, your location structure has "first"
2997 and "last" members, instead of
2999 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3003 (Current).first = (Rhs)[1].location.first; \
3004 (Current).last = (Rhs)[N].location.last; \
3008 (Current).first = (Current).last = (Rhs)[0].location.last; \
3014 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3018 (Current).first = YYRHSLOC (Rhs, 1).first; \
3019 (Current).last = YYRHSLOC (Rhs, N).last; \
3023 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3027 ** YYLLOC_DEFAULT in C++:
3029 The default implementation of YYLLOC_DEFAULT used to be issued in
3030 the header file. It is now output in the implementation file, after
3031 the user %code sections so that its #ifndef guard does not try to
3032 override the user's YYLLOC_DEFAULT if provided.
3034 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3036 YYFAIL has existed for many years as an undocumented feature of
3037 deterministic parsers in C generated by Bison. More recently, it was
3038 a documented feature of Bison's experimental Java parsers. As
3039 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3040 semantic action now produces a deprecation warning, and Java parsers
3041 no longer implement YYFAIL at all. For further details, including a
3042 discussion of how to suppress C preprocessor warnings about YYFAIL
3043 being unused, see the Bison 2.4.2 NEWS entry.
3045 ** Temporary hack for adding a semicolon to the user action:
3047 Previously, Bison appended a semicolon to every user action for
3048 reductions when the output language defaulted to C (specifically, when
3049 neither %yacc, %language, %skeleton, or equivalent command-line
3050 options were specified). This allowed actions such as
3052 exp: exp "+" exp { $$ = $1 + $3 };
3056 exp: exp "+" exp { $$ = $1 + $3; };
3058 As a first step in removing this misfeature, Bison now issues a
3059 warning when it appends a semicolon. Moreover, in cases where Bison
3060 cannot easily determine whether a semicolon is needed (for example, an
3061 action ending with a cpp directive or a braced compound initializer),
3062 it no longer appends one. Thus, the C compiler might now complain
3063 about a missing semicolon where it did not before. Future releases of
3064 Bison will cease to append semicolons entirely.
3066 ** Verbose syntax error message fixes:
3068 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3069 specified, syntax error messages produced by the generated parser
3070 include the unexpected token as well as a list of expected tokens.
3071 The effect of %nonassoc on these verbose messages has been corrected
3072 in two ways, but a more complete fix requires LAC, described above:
3074 *** When %nonassoc is used, there can exist parser states that accept no
3075 tokens, and so the parser does not always require a lookahead token
3076 in order to detect a syntax error. Because no unexpected token or
3077 expected tokens can then be reported, the verbose syntax error
3078 message described above is suppressed, and the parser instead
3079 reports the simpler message, "syntax error". Previously, this
3080 suppression was sometimes erroneously triggered by %nonassoc when a
3081 lookahead was actually required. Now verbose messages are
3082 suppressed only when all previous lookaheads have already been
3083 shifted or discarded.
3085 *** Previously, the list of expected tokens erroneously included tokens
3086 that would actually induce a syntax error because conflicts for them
3087 were resolved with %nonassoc in the current parser state. Such
3088 tokens are now properly omitted from the list.
3090 *** Expected token lists are still often wrong due to state merging
3091 (from LALR or IELR) and default reductions, which can both add
3092 invalid tokens and subtract valid tokens. Canonical LR almost
3093 completely fixes this problem by eliminating state merging and
3094 default reductions. However, there is one minor problem left even
3095 when using canonical LR and even after the fixes above. That is,
3096 if the resolution of a conflict with %nonassoc appears in a later
3097 parser state than the one at which some syntax error is
3098 discovered, the conflicted token is still erroneously included in
3099 the expected token list. Bison's new LAC implementation,
3100 described above, eliminates this problem and the need for
3101 canonical LR. However, LAC is still experimental and is disabled
3104 ** Java skeleton fixes:
3106 *** A location handling bug has been fixed.
3108 *** The top element of each of the value stack and location stack is now
3109 cleared when popped so that it can be garbage collected.
3111 *** Parser traces now print the top element of the stack.
3113 ** -W/--warnings fixes:
3115 *** Bison now properly recognizes the "no-" versions of categories:
3117 For example, given the following command line, Bison now enables all
3118 warnings except warnings for incompatibilities with POSIX Yacc:
3120 bison -Wall,no-yacc gram.y
3122 *** Bison now treats S/R and R/R conflicts like other warnings:
3124 Previously, conflict reports were independent of Bison's normal
3125 warning system. Now, Bison recognizes the warning categories
3126 "conflicts-sr" and "conflicts-rr". This change has important
3127 consequences for the -W and --warnings command-line options. For
3130 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3131 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3132 bison -Wnone gram.y # no conflicts are reported
3133 bison -Werror gram.y # any conflict is an error
3135 However, as before, if the %expect or %expect-rr directive is
3136 specified, an unexpected number of conflicts is an error, and an
3137 expected number of conflicts is not reported, so -W and --warning
3138 then have no effect on the conflict report.
3140 *** The "none" category no longer disables a preceding "error":
3142 For example, for the following command line, Bison now reports
3143 errors instead of warnings for incompatibilities with POSIX Yacc:
3145 bison -Werror,none,yacc gram.y
3147 *** The "none" category now disables all Bison warnings:
3149 Previously, the "none" category disabled only Bison warnings for
3150 which there existed a specific -W/--warning category. However,
3151 given the following command line, Bison is now guaranteed to
3152 suppress all warnings:
3156 ** Precedence directives can now assign token number 0:
3158 Since Bison 2.3b, which restored the ability of precedence
3159 directives to assign token numbers, doing so for token number 0 has
3160 produced an assertion failure. For example:
3164 This bug has been fixed.
3167 * Noteworthy changes in release 2.4.3 (2010-08-05)
3169 ** Bison now obeys -Werror and --warnings=error for warnings about
3170 grammar rules that are useless in the parser due to conflicts.
3172 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3175 ** Failures in the test suite for GCC 4.5 have been fixed.
3177 ** Failures in the test suite for some versions of Sun Studio C++ have
3180 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3181 warnings about undefined %prec identifiers will not be converted to
3182 errors in Bison 2.5. They will remain warnings, which should be
3183 sufficient for POSIX while avoiding backward compatibility issues.
3185 ** Minor documentation fixes.
3188 * Noteworthy changes in release 2.4.2 (2010-03-20)
3190 ** Some portability problems that resulted in failures and livelocks
3191 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3192 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3193 errors should no longer cause M4 to report a broken pipe on the
3196 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3198 POSIX specifies that an error be reported for any identifier that does
3199 not appear on the LHS of a grammar rule and that is not defined by
3200 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3201 error report for the case when an identifier appears only after a
3202 %prec directive. It is now restored. However, for backward
3203 compatibility with recent Bison releases, it is only a warning for
3204 now. In Bison 2.5 and later, it will return to being an error.
3205 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3206 warning will not be converted to an error in Bison 2.5.]
3208 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3210 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3211 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3214 ** %code is now a permanent feature.
3216 A traditional Yacc prologue directive is written in the form:
3220 To provide a more flexible alternative, Bison 2.3b introduced the
3221 %code directive with the following forms for C/C++:
3224 %code requires {CODE}
3225 %code provides {CODE}
3228 These forms are now considered permanent features of Bison. See the
3229 %code entries in the section "Bison Declaration Summary" in the Bison
3230 manual for a summary of their functionality. See the section
3231 "Prologue Alternatives" for a detailed discussion including the
3232 advantages of %code over the traditional Yacc prologue directive.
3234 Bison's Java feature as a whole including its current usage of %code
3235 is still considered experimental.
3237 ** YYFAIL is deprecated and will eventually be removed.
3239 YYFAIL has existed for many years as an undocumented feature of
3240 deterministic parsers in C generated by Bison. Previously, it was
3241 documented for Bison's experimental Java parsers. YYFAIL is no longer
3242 documented for Java parsers and is formally deprecated in both cases.
3243 Users are strongly encouraged to migrate to YYERROR, which is
3246 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3247 induce a syntax error. The most obvious difference from YYERROR is
3248 that YYFAIL will automatically invoke yyerror to report the syntax
3249 error so that you don't have to. However, there are several other
3250 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3251 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3252 used. For a more detailed discussion, see:
3254 http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html
3256 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3257 deterministic parsers in C will continue to implement it. However,
3258 because YYFAIL is already flawed, it seems futile to try to make new
3259 Bison features compatible with it. Thus, during parser generation,
3260 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3261 rule action. In a later release, YYFAIL will be disabled for
3262 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3263 be removed altogether.
3265 There exists at least one case where Bison 2.5's YYFAIL warning will
3266 be a false positive. Some projects add phony uses of YYFAIL and other
3267 Bison-defined macros for the sole purpose of suppressing C
3268 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3269 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3270 epilogue (that is, after the second "%%") in the Bison input file. In
3271 this release (2.4.2), Bison already generates its own code to suppress
3272 C preprocessor warnings for YYFAIL, so projects can remove their own
3273 phony uses of YYFAIL if compatibility with Bison releases prior to
3274 2.4.2 is not necessary.
3276 ** Internationalization.
3278 Fix a regression introduced in Bison 2.4: Under some circumstances,
3279 message translations were not installed although supported by the
3283 * Noteworthy changes in release 2.4.1 (2008-12-11)
3285 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3286 declarations have been fixed.
3288 ** Temporary hack for adding a semicolon to the user action.
3290 Bison used to prepend a trailing semicolon at the end of the user
3291 action for reductions. This allowed actions such as
3293 exp: exp "+" exp { $$ = $1 + $3 };
3297 exp: exp "+" exp { $$ = $1 + $3; };
3299 Some grammars still depend on this "feature". Bison 2.4.1 restores
3300 the previous behavior in the case of C output (specifically, when
3301 neither %language or %skeleton or equivalent command-line options
3302 are used) to leave more time for grammars depending on the old
3303 behavior to be adjusted. Future releases of Bison will disable this
3306 ** A few minor improvements to the Bison manual.
3309 * Noteworthy changes in release 2.4 (2008-11-02)
3311 ** %language is an experimental feature.
3313 We first introduced this feature in test release 2.3b as a cleaner
3314 alternative to %skeleton. Since then, we have discussed the possibility of
3315 modifying its effect on Bison's output file names. Thus, in this release,
3316 we consider %language to be an experimental feature that will likely evolve
3319 ** Forward compatibility with GNU M4 has been improved.
3321 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3325 * Noteworthy changes in release 2.3b (2008-05-27)
3327 ** The quotes around NAME that used to be required in the following directive
3330 %define NAME "VALUE"
3332 ** The directive "%pure-parser" is now deprecated in favor of:
3336 which has the same effect except that Bison is more careful to warn about
3337 unreasonable usage in the latter case.
3341 Bison can now generate an LALR(1) parser in C with a push interface. That
3342 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3343 push one token at a time to the parser using "yypush_parse", which will
3344 return to the caller after processing each token. By default, the push
3345 interface is disabled. Either of the following directives will enable it:
3347 %define api.push_pull "push" // Just push; does not require yylex.
3348 %define api.push_pull "both" // Push and pull; requires yylex.
3350 See the new section "A Push Parser" in the Bison manual for details.
3352 The current push parsing interface is experimental and may evolve. More user
3353 feedback will help to stabilize it.
3355 ** The -g and --graph options now output graphs in Graphviz DOT format,
3356 not VCG format. Like --graph, -g now also takes an optional FILE argument
3357 and thus cannot be bundled with other short options.
3361 Bison can now generate an LALR(1) parser in Java. The skeleton is
3362 "data/lalr1.java". Consider using the new %language directive instead of
3363 %skeleton to select it.
3365 See the new section "Java Parsers" in the Bison manual for details.
3367 The current Java interface is experimental and may evolve. More user
3368 feedback will help to stabilize it.
3369 Contributed by Paolo Bonzini.
3373 This new directive specifies the programming language of the generated
3374 parser, which can be C (the default), C++, or Java. Besides the skeleton
3375 that Bison uses, the directive affects the names of the generated files if
3376 the grammar file's name ends in ".y".
3378 ** XML Automaton Report
3380 Bison can now generate an XML report of the LALR(1) automaton using the new
3381 "--xml" option. The current XML schema is experimental and may evolve. More
3382 user feedback will help to stabilize it.
3383 Contributed by Wojciech Polak.
3385 ** The grammar file may now specify the name of the parser header file using
3386 %defines. For example:
3390 ** When reporting useless rules, useless nonterminals, and unused terminals,
3391 Bison now employs the terms "useless in grammar" instead of "useless",
3392 "useless in parser" instead of "never reduced", and "unused in grammar"
3393 instead of "unused".
3395 ** Unreachable State Removal
3397 Previously, Bison sometimes generated parser tables containing unreachable
3398 states. A state can become unreachable during conflict resolution if Bison
3399 disables a shift action leading to it from a predecessor state. Bison now:
3401 1. Removes unreachable states.
3403 2. Does not report any conflicts that appeared in unreachable states.
3404 WARNING: As a result, you may need to update %expect and %expect-rr
3405 directives in existing grammar files.
3407 3. For any rule used only in such states, Bison now reports the rule as
3408 "useless in parser due to conflicts".
3410 This feature can be disabled with the following directive:
3412 %define lr.keep_unreachable_states
3414 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3415 for further discussion.
3417 ** Lookahead Set Correction in the ".output" Report
3419 When instructed to generate a ".output" file including lookahead sets
3420 (using "--report=lookahead", for example), Bison now prints each reduction's
3421 lookahead set only next to the associated state's one item that (1) is
3422 associated with the same rule as the reduction and (2) has its dot at the end
3423 of its RHS. Previously, Bison also erroneously printed the lookahead set
3424 next to all of the state's other items associated with the same rule. This
3425 bug affected only the ".output" file and not the generated parser source
3428 ** --report-file=FILE is a new option to override the default ".output" file
3431 ** The "=" that used to be required in the following directives is now
3434 %file-prefix "parser"
3438 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3440 Bison 2.3a provided a new set of directives as a more flexible alternative to
3441 the traditional Yacc prologue blocks. Those have now been consolidated into
3442 a single %code directive with an optional qualifier field, which identifies
3443 the purpose of the code and thus the location(s) where Bison should generate
3446 1. "%code {CODE}" replaces "%after-header {CODE}"
3447 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3448 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3449 4. "%code top {CODE}" replaces "%before-header {CODE}"
3451 See the %code entries in section "Bison Declaration Summary" in the Bison
3452 manual for a summary of the new functionality. See the new section "Prologue
3453 Alternatives" for a detailed discussion including the advantages of %code
3454 over the traditional Yacc prologues.
3456 The prologue alternatives are experimental. More user feedback will help to
3457 determine whether they should become permanent features.
3459 ** Revised warning: unset or unused midrule values
3461 Since Bison 2.2, Bison has warned about midrule values that are set but not
3462 used within any of the actions of the parent rule. For example, Bison warns
3465 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3467 Now, Bison also warns about midrule values that are used but not set. For
3468 example, Bison warns about unset $$ in the midrule action in:
3470 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3472 However, Bison now disables both of these warnings by default since they
3473 sometimes prove to be false alarms in existing grammars employing the Yacc
3474 constructs $0 or $-N (where N is some positive integer).
3476 To enable these warnings, specify the option "--warnings=midrule-values" or
3477 "-W", which is a synonym for "--warnings=all".
3479 ** Default %destructor or %printer with "<*>" or "<>"
3481 Bison now recognizes two separate kinds of default %destructor's and
3484 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3485 %destructor/%printer for all grammar symbols for which you have formally
3486 declared semantic type tags.
3488 2. Place "<>" in a %destructor/%printer symbol list to define a default
3489 %destructor/%printer for all grammar symbols without declared semantic
3492 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3493 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3494 longer applies any %destructor to a midrule value if that midrule value is
3495 not actually ever referenced using either $$ or $n in a semantic action.
3497 The default %destructor's and %printer's are experimental. More user
3498 feedback will help to determine whether they should become permanent
3501 See the section "Freeing Discarded Symbols" in the Bison manual for further
3504 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3505 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3506 manual for a caveat concerning the treatment of literal strings.
3508 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3509 completely removed from Bison.
3512 * Noteworthy changes in release 2.3a (2006-09-13)
3514 ** Instead of %union, you can define and use your own union type
3515 YYSTYPE if your grammar contains at least one <type> tag.
3516 Your YYSTYPE need not be a macro; it can be a typedef.
3517 This change is for compatibility with other Yacc implementations,
3518 and is required by POSIX.
3520 ** Locations columns and lines start at 1.
3521 In accordance with the GNU Coding Standards and Emacs.
3523 ** You may now declare per-type and default %destructor's and %printer's:
3527 %union { char *string; }
3528 %token <string> STRING1
3529 %token <string> STRING2
3530 %type <string> string1
3531 %type <string> string2
3532 %union { char character; }
3533 %token <character> CHR
3534 %type <character> chr
3535 %destructor { free ($$); } %symbol-default
3536 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3537 %destructor { } <character>
3539 guarantees that, when the parser discards any user-defined symbol that has a
3540 semantic type tag other than "<character>", it passes its semantic value to
3541 "free". However, when the parser discards a "STRING1" or a "string1", it
3542 also prints its line number to "stdout". It performs only the second
3543 "%destructor" in this case, so it invokes "free" only once.
3545 [Although we failed to mention this here in the 2.3a release, the default
3546 %destructor's and %printer's were experimental, and they were rewritten in
3549 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3550 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3551 associating token numbers with token names. Removing the #define statements
3552 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3553 requires them. Bison still generates an enum for token names in all cases.
3555 ** Handling of traditional Yacc prologue blocks is now more consistent but
3556 potentially incompatible with previous releases of Bison.
3558 As before, you declare prologue blocks in your grammar file with the
3559 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3560 prologue blocks that you've declared before the first %union. To generate
3561 the post-prologue, Bison concatenates all prologue blocks that you've
3562 declared after the first %union.
3564 Previous releases of Bison inserted the pre-prologue into both the header
3565 file and the code file in all cases except for LALR(1) parsers in C. In the
3566 latter case, Bison inserted it only into the code file. For parsers in C++,
3567 the point of insertion was before any token definitions (which associate
3568 token numbers with names). For parsers in C, the point of insertion was
3569 after the token definitions.
3571 Now, Bison never inserts the pre-prologue into the header file. In the code
3572 file, it always inserts it before the token definitions.
3574 ** Bison now provides a more flexible alternative to the traditional Yacc
3575 prologue blocks: %before-header, %start-header, %end-header, and
3578 For example, the following declaration order in the grammar file reflects the
3579 order in which Bison will output these code blocks. However, you are free to
3580 declare these code blocks in your grammar file in whatever order is most
3584 /* Bison treats this block like a pre-prologue block: it inserts it into
3585 * the code file before the contents of the header file. It does *not*
3586 * insert it into the header file. This is a good place to put
3587 * #include's that you want at the top of your code file. A common
3588 * example is '#include "system.h"'. */
3591 /* Bison inserts this block into both the header file and the code file.
3592 * In both files, the point of insertion is before any Bison-generated
3593 * token, semantic type, location type, and class definitions. This is a
3594 * good place to define %union dependencies, for example. */
3597 /* Unlike the traditional Yacc prologue blocks, the output order for the
3598 * new %*-header blocks is not affected by their declaration position
3599 * relative to any %union in the grammar file. */
3602 /* Bison inserts this block into both the header file and the code file.
3603 * In both files, the point of insertion is after the Bison-generated
3604 * definitions. This is a good place to declare or define public
3605 * functions or data structures that depend on the Bison-generated
3609 /* Bison treats this block like a post-prologue block: it inserts it into
3610 * the code file after the contents of the header file. It does *not*
3611 * insert it into the header file. This is a good place to declare or
3612 * define internal functions or data structures that depend on the
3613 * Bison-generated definitions. */
3616 If you have multiple occurrences of any one of the above declarations, Bison
3617 will concatenate the contents in declaration order.
3619 [Although we failed to mention this here in the 2.3a release, the prologue
3620 alternatives were experimental, and they were rewritten in future versions.]
3622 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3623 The old spelling still works, but is not documented and may be removed
3624 in a future release.
3627 * Noteworthy changes in release 2.3 (2006-06-05)
3629 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3630 for compatibility with LALR(1) grammars.
3632 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3633 be to a type name that does not contain parentheses or brackets.
3636 * Noteworthy changes in release 2.2 (2006-05-19)
3638 ** The distribution terms for all Bison-generated parsers now permit
3639 using the parsers in nonfree programs. Previously, this permission
3640 was granted only for Bison-generated LALR(1) parsers in C.
3642 ** %name-prefix changes the namespace name in C++ outputs.
3644 ** The C++ parsers export their token_type.
3646 ** Bison now allows multiple %union declarations, and concatenates
3647 their contents together.
3649 ** New warning: unused values
3650 Right-hand side symbols whose values are not used are reported,
3651 if the symbols have destructors. For instance:
3653 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3657 will trigger a warning about $$ and $5 in the first rule, and $3 in
3658 the second ($1 is copied to $$ by the default rule). This example
3659 most likely contains three errors, and could be rewritten as:
3661 exp: exp "?" exp ":" exp
3662 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3664 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3667 However, if the original actions were really intended, memory leaks
3668 and all, the warnings can be suppressed by letting Bison believe the
3669 values are used, e.g.:
3671 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3672 | exp "+" exp { $$ = $1; (void) $3; }
3675 If there are midrule actions, the warning is issued if no action
3676 uses it. The following triggers no warning: $1 and $3 are used.
3678 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3680 The warning is intended to help catching lost values and memory leaks.
3681 If a value is ignored, its associated memory typically is not reclaimed.
3683 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3684 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3685 and YYERROR, for all objects on the stack, other than objects
3686 corresponding to the right-hand side of the current rule.
3688 ** %expect, %expect-rr
3689 Incorrect numbers of expected conflicts are now actual errors,
3690 instead of warnings.
3692 ** GLR, YACC parsers.
3693 The %parse-params are available in the destructors (and the
3694 experimental printers) as per the documentation.
3696 ** Bison now warns if it finds a stray "$" or "@" in an action.
3698 ** %require "VERSION"
3699 This specifies that the grammar file depends on features implemented
3700 in Bison version VERSION or higher.
3702 ** lalr1.cc: The token and value types are now class members.
3703 The tokens were defined as free form enums and cpp macros. YYSTYPE
3704 was defined as a free form union. They are now class members:
3705 tokens are enumerations of the "yy::parser::token" struct, and the
3706 semantic values have the "yy::parser::semantic_type" type.
3708 If you do not want or can update to this scheme, the directive
3709 '%define "global_tokens_and_yystype" "1"' triggers the global
3710 definition of tokens and YYSTYPE. This change is suitable both
3711 for previous releases of Bison, and this one.
3713 If you wish to update, then make sure older version of Bison will
3714 fail using '%require "2.2"'.
3716 ** DJGPP support added.
3719 * Noteworthy changes in release 2.1 (2005-09-16)
3721 ** The C++ lalr1.cc skeleton supports %lex-param.
3723 ** Bison-generated parsers now support the translation of diagnostics like
3724 "syntax error" into languages other than English. The default
3725 language is still English. For details, please see the new
3726 Internationalization section of the Bison manual. Software
3727 distributors should also see the new PACKAGING file. Thanks to
3728 Bruno Haible for this new feature.
3730 ** Wording in the Bison-generated parsers has been changed slightly to
3731 simplify translation. In particular, the message "memory exhausted"
3732 has replaced "parser stack overflow", as the old message was not
3733 always accurate for modern Bison-generated parsers.
3735 ** Destructors are now called when the parser aborts, for all symbols left
3736 behind on the stack. Also, the start symbol is now destroyed after a
3737 successful parse. In both cases, the behavior was formerly inconsistent.
3739 ** When generating verbose diagnostics, Bison-generated parsers no longer
3740 quote the literal strings associated with tokens. For example, for
3741 a syntax error associated with '%token NUM "number"' they might
3742 print 'syntax error, unexpected number' instead of 'syntax error,
3743 unexpected "number"'.
3746 * Noteworthy changes in release 2.0 (2004-12-25)
3748 ** Possibly-incompatible changes
3750 - Bison-generated parsers no longer default to using the alloca function
3751 (when available) to extend the parser stack, due to widespread
3752 problems in unchecked stack-overflow detection. You can "#define
3753 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3754 the manual to determine safe values for YYMAXDEPTH in that case.
3756 - Error token location.
3757 During error recovery, the location of the syntax error is updated
3758 to cover the whole sequence covered by the error token: it includes
3759 the shifted symbols thrown away during the first part of the error
3760 recovery, and the lookahead rejected during the second part.
3762 - Semicolon changes:
3763 . Stray semicolons are no longer allowed at the start of a grammar.
3764 . Semicolons are now required after in-grammar declarations.
3766 - Unescaped newlines are no longer allowed in character constants or
3767 string literals. They were never portable, and GCC 3.4.0 has
3768 dropped support for them. Better diagnostics are now generated if
3769 forget a closing quote.
3771 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
3775 - GLR grammars now support locations.
3777 - New directive: %initial-action.
3778 This directive allows the user to run arbitrary code (including
3779 initializing @$) from yyparse before parsing starts.
3781 - A new directive "%expect-rr N" specifies the expected number of
3782 reduce/reduce conflicts in GLR parsers.
3784 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
3785 This is a GNU extension.
3787 - The option "--report=lookahead" was changed to "--report=look-ahead".
3788 [However, this was changed back after 2.3.]
3790 - Experimental %destructor support has been added to lalr1.cc.
3792 - New configure option --disable-yacc, to disable installation of the
3793 yacc command and -ly library introduced in 1.875 for POSIX conformance.
3797 - For now, %expect-count violations are now just warnings, not errors.
3798 This is for compatibility with Bison 1.75 and earlier (when there are
3799 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
3800 are too many or too few shift/reduce conflicts). However, in future
3801 versions of Bison we plan to improve the %expect machinery so that
3802 these violations will become errors again.
3804 - Within Bison itself, numbers (e.g., goto numbers) are no longer
3805 arbitrarily limited to 16-bit counts.
3807 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
3810 * Noteworthy changes in release 1.875 (2003-01-01)
3812 ** The documentation license has been upgraded to version 1.2
3813 of the GNU Free Documentation License.
3815 ** syntax error processing
3817 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
3818 locations too. This fixes bugs in error-location computation.
3821 It is now possible to reclaim the memory associated to symbols
3822 discarded during error recovery. This feature is still experimental.
3825 This new directive is preferred over YYERROR_VERBOSE.
3827 - #defining yyerror to steal internal variables is discouraged.
3828 It is not guaranteed to work forever.
3830 ** POSIX conformance
3832 - Semicolons are once again optional at the end of grammar rules.
3833 This reverts to the behavior of Bison 1.33 and earlier, and improves
3834 compatibility with Yacc.
3836 - "parse error" -> "syntax error"
3837 Bison now uniformly uses the term "syntax error"; formerly, the code
3838 and manual sometimes used the term "parse error" instead. POSIX
3839 requires "syntax error" in diagnostics, and it was thought better to
3842 - The documentation now emphasizes that yylex and yyerror must be
3843 declared before use. C99 requires this.
3845 - Bison now parses C99 lexical constructs like UCNs and
3846 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
3848 - File names are properly escaped in C output. E.g., foo\bar.y is
3849 output as "foo\\bar.y".
3851 - Yacc command and library now available
3852 The Bison distribution now installs a "yacc" command, as POSIX requires.
3853 Also, Bison now installs a small library liby.a containing
3854 implementations of Yacc-compatible yyerror and main functions.
3855 This library is normally not useful, but POSIX requires it.
3857 - Type clashes now generate warnings, not errors.
3859 - If the user does not define YYSTYPE as a macro, Bison now declares it
3860 using typedef instead of defining it as a macro.
3861 For consistency, YYLTYPE is also declared instead of defined.
3863 ** Other compatibility issues
3865 - %union directives can now have a tag before the "{", e.g., the
3866 directive "%union foo {...}" now generates the C code
3867 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
3868 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
3869 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
3870 This is for compatibility with both Yacc and Bison 1.35.
3872 - ";" is output before the terminating "}" of an action, for
3873 compatibility with Bison 1.35.
3875 - Bison now uses a Yacc-style format for conflict reports, e.g.,
3876 "conflicts: 2 shift/reduce, 1 reduce/reduce".
3878 - "yystype" and "yyltype" are now obsolescent macros instead of being
3879 typedefs or tags; they are no longer documented and are planned to be
3880 withdrawn in a future release.
3885 Users of Bison have to decide how they handle the portability of the
3888 - "parsing stack overflow..." -> "parser stack overflow"
3889 GLR parsers now report "parser stack overflow" as per the Bison manual.
3891 ** %parse-param and %lex-param
3892 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
3893 additional context to yyparse and yylex. They suffer from several
3896 - a single argument only can be added,
3897 - their types are weak (void *),
3898 - this context is not passed to ancillary functions such as yyerror,
3899 - only yacc.c parsers support them.
3901 The new %parse-param/%lex-param directives provide a more precise control.
3904 %parse-param {int *nastiness}
3905 %lex-param {int *nastiness}
3906 %parse-param {int *randomness}
3908 results in the following signatures:
3910 int yylex (int *nastiness);
3911 int yyparse (int *nastiness, int *randomness);
3913 or, if both %pure-parser and %locations are used:
3915 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
3916 int yyparse (int *nastiness, int *randomness);
3918 ** Bison now warns if it detects conflicting outputs to the same file,
3919 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
3920 that command outputs both code and header to foo.h.
3922 ** #line in output files
3923 - --no-line works properly.
3925 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
3926 later to be built. This change originally took place a few versions
3927 ago, but nobody noticed until we recently asked someone to try
3928 building Bison with a K&R C compiler.
3931 * Noteworthy changes in release 1.75 (2002-10-14)
3933 ** Bison should now work on 64-bit hosts.
3935 ** Indonesian translation thanks to Tedi Heriyanto.
3938 Fix spurious parse errors.
3941 Some people redefine yyerror to steal yyparse' private variables.
3942 Reenable this trick until an official feature replaces it.
3945 In agreement with POSIX and with other Yaccs, leaving a default
3946 action is valid when $$ is untyped, and $1 typed:
3950 but the converse remains an error:
3954 ** Values of midrule actions
3957 foo: { ... } { $$ = $1; } ...
3959 was incorrectly rejected: $1 is defined in the second midrule
3960 action, and is equal to the $$ of the first midrule action.
3963 * Noteworthy changes in release 1.50 (2002-10-04)
3968 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
3969 almost any context-free grammar, ambiguous or not. The new declarations
3970 %dprec and %merge on grammar rules allow parse-time resolution of
3971 ambiguities. Contributed by Paul Hilfinger.
3973 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
3974 like the Alpha, so please stick to 32-bit hosts for now.
3977 When not in Yacc compatibility mode, when the output file was not
3978 specified, running "bison foo/bar.y" created "foo/bar.c". It
3979 now creates "bar.c".
3982 The undefined token was systematically mapped to 2 which prevented
3983 the use of 2 by the user. This is no longer the case.
3985 ** Unknown token numbers
3986 If yylex returned an out of range value, yyparse could die. This is
3990 According to POSIX, the error token must be 256.
3991 Bison extends this requirement by making it a preference: *if* the
3992 user specified that one of her tokens is numbered 256, then error
3993 will be mapped onto another number.
3995 ** Verbose error messages
3996 They no longer report "..., expecting error or..." for states where
3997 error recovery is possible.
4000 Defaults to "$end" instead of "$".
4002 ** Error recovery now conforms to documentation and to POSIX
4003 When a Bison-generated parser encounters a syntax error, it now pops
4004 the stack until it finds a state that allows shifting the error
4005 token. Formerly, it popped the stack until it found a state that
4006 allowed some non-error action other than a default reduction on the
4007 error token. The new behavior has long been the documented behavior,
4008 and has long been required by POSIX. For more details, please see
4009 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4010 <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.
4013 Popped tokens and nonterminals are now reported.
4016 Larger grammars are now supported (larger token numbers, larger grammar
4017 size (= sum of the LHS and RHS lengths), larger LALR tables).
4018 Formerly, many of these numbers ran afoul of 16-bit limits;
4019 now these limits are 32 bits on most hosts.
4021 ** Explicit initial rule
4022 Bison used to play hacks with the initial rule, which the user does
4023 not write. It is now explicit, and visible in the reports and
4027 Before, Bison reported the useless rules, but, although not used,
4028 included them in the parsers. They are now actually removed.
4030 ** Useless rules, useless nonterminals
4031 They are now reported, as a warning, with their locations.
4033 ** Rules never reduced
4034 Rules that can never be reduced because of conflicts are now
4037 ** Incorrect "Token not used"
4038 On a grammar such as
4040 %token useless useful
4042 exp: '0' %prec useful;
4044 where a token was used to set the precedence of the last rule,
4045 bison reported both "useful" and "useless" as useless tokens.
4047 ** Revert the C++ namespace changes introduced in 1.31
4048 as they caused too many portability hassles.
4050 ** Default locations
4051 By an accident of design, the default computation of @$ was
4052 performed after another default computation was performed: @$ = @1.
4053 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4054 the computation of @$.
4056 ** Token end-of-file
4057 The token end of file may be specified by the user, in which case,
4058 the user symbol is used in the reports, the graphs, and the verbose
4059 error messages instead of "$end", which remains being the default.
4063 %token MYEOF 0 "end of file"
4066 This old option, which has been broken for ages, is removed.
4069 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4070 Croatian, thanks to Denis Lackovic.
4072 ** Incorrect token definitions
4075 bison used to output
4078 ** Token definitions as enums
4079 Tokens are output both as the traditional #define's, and, provided
4080 the compiler supports ANSI C or is a C++ compiler, as enums.
4081 This lets debuggers display names instead of integers.
4084 In addition to --verbose, bison supports --report=THINGS, which
4085 produces additional information:
4087 complete the core item sets with their closure
4088 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4089 explicitly associate lookahead tokens to items
4091 describe shift/reduce conflicts solving.
4092 Bison used to systematically output this information on top of
4093 the report. Solved conflicts are now attached to their states.
4096 Previous versions don't complain when there is a type clash on
4097 the default action if the rule has a midrule action, such as in:
4105 ** GNU M4 is now required when using Bison.
4108 * Noteworthy changes in release 1.35 (2002-03-25)
4111 Some projects use Bison's C parser with C++ compilers, and define
4112 YYSTYPE as a class. The recent adjustment of C parsers for data
4113 alignment and 64 bit architectures made this impossible.
4115 Because for the time being no real solution for C++ parser
4116 generation exists, kludges were implemented in the parser to
4117 maintain this use. In the future, when Bison has C++ parsers, this
4118 kludge will be disabled.
4120 This kludge also addresses some C++ problems when the stack was
4124 * Noteworthy changes in release 1.34 (2002-03-12)
4126 ** File name clashes are detected
4127 $ bison foo.y -d -o foo.x
4128 fatal error: header and parser would both be named "foo.x"
4130 ** A missing ";" at the end of a rule triggers a warning
4131 In accordance with POSIX, and in agreement with other
4132 Yacc implementations, Bison will mandate this semicolon in the near
4133 future. This eases the implementation of a Bison parser of Bison
4134 grammars by making this grammar LALR(1) instead of LR(2). To
4135 facilitate the transition, this release introduces a warning.
4137 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4138 many portability hassles.
4140 ** DJGPP support added.
4142 ** Fix test suite portability problems.
4145 * Noteworthy changes in release 1.33 (2002-02-07)
4148 Groff could not be compiled for the definition of size_t was lacking
4149 under some conditions.
4155 * Noteworthy changes in release 1.32 (2002-01-23)
4157 ** Fix Yacc output file names
4159 ** Portability fixes
4161 ** Italian, Dutch translations
4164 * Noteworthy changes in release 1.31 (2002-01-14)
4168 ** GNU Gettext and %expect
4169 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4170 Bison dies on incorrect %expectations, we fear there will be
4171 too many bug reports for Gettext, so _for the time being_, %expect
4172 does not trigger an error when the input file is named "plural.y".
4174 ** Use of alloca in parsers
4175 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4176 malloc exclusively. Since 1.29, but was not NEWS'ed.
4178 alloca is used only when compiled with GCC, to avoid portability
4181 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4183 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4184 (as POSIX requires) instead of being undefined.
4187 Bison has always permitted actions such as { $$ = $1 }: it adds the
4188 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4189 is no longer output: one has to write { $$ = $1; }.
4191 ** Better C++ compliance
4192 The output parsers try to respect C++ namespaces.
4193 [This turned out to be a failed experiment, and it was reverted later.]
4196 Fixed bugs when reporting useless nonterminals.
4199 The parsers work properly on 64 bit hosts.
4202 Some calls to strerror resulted in scrambled or missing error messages.
4205 When the number of shift/reduce conflicts is correct, don't issue
4208 ** The verbose report includes the rule line numbers.
4210 ** Rule line numbers are fixed in traces.
4212 ** Swedish translation
4215 Verbose parse error messages from the parsers are better looking.
4216 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4217 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4219 ** Fixed parser memory leaks.
4220 When the generated parser was using malloc to extend its stacks, the
4221 previous allocations were not freed.
4223 ** Fixed verbose output file.
4224 Some newlines were missing.
4225 Some conflicts in state descriptions were missing.
4227 ** Fixed conflict report.
4228 Option -v was needed to get the result.
4232 Mismatches are errors, not warnings.
4234 ** Fixed incorrect processing of some invalid input.
4236 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4238 ** Fixed some typos in the documentation.
4240 ** %token MY_EOF 0 is supported.
4241 Before, MY_EOF was silently renumbered as 257.
4243 ** doc/refcard.tex is updated.
4245 ** %output, %file-prefix, %name-prefix.
4249 New, aliasing "--output-file".
4252 * Noteworthy changes in release 1.30 (2001-10-26)
4254 ** "--defines" and "--graph" have now an optional argument which is the
4255 output file name. "-d" and "-g" do not change; they do not take any
4258 ** "%source_extension" and "%header_extension" are removed, failed
4261 ** Portability fixes.
4264 * Noteworthy changes in release 1.29 (2001-09-07)
4266 ** The output file does not define const, as this caused problems when used
4267 with common autoconfiguration schemes. If you still use ancient compilers
4268 that lack const, compile with the equivalent of the C compiler option
4269 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4271 ** Added "-g" and "--graph".
4273 ** The Bison manual is now distributed under the terms of the GNU FDL.
4275 ** The input and the output files has automatically a similar extension.
4277 ** Russian translation added.
4279 ** NLS support updated; should hopefully be less troublesome.
4281 ** Added the old Bison reference card.
4283 ** Added "--locations" and "%locations".
4285 ** Added "-S" and "--skeleton".
4287 ** "%raw", "-r", "--raw" is disabled.
4289 ** Special characters are escaped when output. This solves the problems
4290 of the #line lines with path names including backslashes.
4293 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4294 "%debug", "%source_extension" and "%header_extension".
4297 Automatic location tracking.
4300 * Noteworthy changes in release 1.28 (1999-07-06)
4302 ** Should compile better now with K&R compilers.
4306 ** Fixed a problem with escaping the double quote character.
4308 ** There is now a FAQ.
4311 * Noteworthy changes in release 1.27
4313 ** The make rule which prevented bison.simple from being created on
4314 some systems has been fixed.
4317 * Noteworthy changes in release 1.26
4319 ** Bison now uses Automake.
4321 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4323 ** Token numbers now start at 257 as previously documented, not 258.
4325 ** Bison honors the TMPDIR environment variable.
4327 ** A couple of buffer overruns have been fixed.
4329 ** Problems when closing files should now be reported.
4331 ** Generated parsers should now work even on operating systems which do
4332 not provide alloca().
4335 * Noteworthy changes in release 1.25 (1995-10-16)
4337 ** Errors in the input grammar are not fatal; Bison keeps reading
4338 the grammar file, and reports all the errors found in it.
4340 ** Tokens can now be specified as multiple-character strings: for
4341 example, you could use "<=" for a token which looks like <=, instead
4342 of choosing a name like LESSEQ.
4344 ** The %token_table declaration says to write a table of tokens (names
4345 and numbers) into the parser file. The yylex function can use this
4346 table to recognize multiple-character string tokens, or for other
4349 ** The %no_lines declaration says not to generate any #line preprocessor
4350 directives in the parser file.
4352 ** The %raw declaration says to use internal Bison token numbers, not
4353 Yacc-compatible token numbers, when token names are defined as macros.
4355 ** The --no-parser option produces the parser tables without including
4356 the parser engine; a project can now use its own parser engine.
4357 The actions go into a separate file called NAME.act, in the form of
4358 a switch statement body.
4361 * Noteworthy changes in release 1.23
4363 The user can define YYPARSE_PARAM as the name of an argument to be
4364 passed into yyparse. The argument should have type void *. It should
4365 actually point to an object. Grammar actions can access the variable
4366 by casting it to the proper pointer type.
4368 Line numbers in output file corrected.
4371 * Noteworthy changes in release 1.22
4373 --help option added.
4376 * Noteworthy changes in release 1.20
4378 Output file does not redefine const for C++.
4382 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4383 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4384 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4385 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4386 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4387 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4388 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4389 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4390 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4391 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4392 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4393 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4394 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4395 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4396 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4397 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4398 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4399 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4400 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4401 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4402 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4403 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4404 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4405 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4406 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4407 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4408 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4409 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4410 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4411 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4412 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4413 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4414 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4415 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4416 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4417 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4418 LocalWords: gotos readline Imbimbo Wcounterexamples
4421 ispell-dictionary: "american"
4426 Copyright (C) 1995-2015, 2018-2020 Free Software Foundation, Inc.
4428 This file is part of Bison, the GNU Parser Generator.
4430 Permission is granted to copy, distribute and/or modify this document
4431 under the terms of the GNU Free Documentation License, Version 1.3 or
4432 any later version published by the Free Software Foundation; with no
4433 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4434 Texts. A copy of the license is included in the "GNU Free
4435 Documentation License" file as part of this distribution.