3 * Noteworthy changes in release ?.? (????-??-??) [?]
6 * Noteworthy changes in release 3.6.92 (2020-07-19) [beta]
8 Changes in the display of counterexamples.
14 The bistromathic demonstrates %param and how to quote sources in the error
18 1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
22 * Noteworthy changes in release 3.6.91 (2020-07-09) [beta]
29 * Noteworthy changes in release 3.6.90 (2020-07-04) [beta]
31 ** Deprecated features
33 The YYPRINT macro, which works only with yacc.c and only for tokens, was
34 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
35 It is deprecated and its support will be removed eventually.
37 In conformance with the recommendations of the Graphviz team, in the next
38 version Bison the option `--graph` will generate a *.gv file by default,
39 instead of *.dot. A transition started in Bison 3.4.
43 *** Counterexample Generation
45 Contributed by Vincent Imbimbo.
47 When given `--report=counterexamples` or `-Wcounterexamples`, bison will
48 now output counterexamples for conflicts in the grammar. These are
49 strings in the grammar which can be parsed in two ways due to the
50 conflict. For example:
52 Shift/reduce conflict on token "/":
53 Example exp "+" exp • "/" exp
58 Example exp "+" exp • "/" exp
64 When Bison is installed with text styling enabled, the example is actually
65 shown twice, with colors highlighting the ambiguity.
67 This is a shift/reduce conflict caused by none of the operators having
68 precedence, so the example can be parsed in the two ways shown. When
69 bison cannot find an example that can be derived in two ways, it instead
70 generates two examples that are the same up until the dot:
72 First example expr • ID ',' ID $end
79 Second example expr • ID $end
86 In these cases, the parser usually doesn't have enough lookahead to
87 differentiate the two given examples.
89 The counterexamples are "focused" in two different ways. First, they do
90 not clutter the output with all the derivations from the start symbol,
91 rather they start on the "conflicted nonterminal". They go straight to the
92 point. Second, they don't "expand" nonterminal symbols uselessly.
94 *** File prefix mapping
96 Contributed by Joshua Watt.
98 Bison learned a new argument, `--file-prefix-map OLD=NEW`. Any file path
99 in the output (specifically `#line` directives and `#ifdef` header guards)
100 that begins with the prefix OLD will have it replaced with the prefix NEW,
101 similar to the `-ffile-prefix-map` in GCC. This option can be used to
102 make bison output reproducible.
106 *** Relocatable installation
108 When installed to be relocatable (via `configure --enable-relocatable`),
109 bison will now also look for a relocated m4.
113 The `filename_type` %define variable was renamed `api.filename.type`.
116 %define filename_type "symbol"
120 %define api.filename.type {symbol}
122 (Or let `bison --update` do it for you).
124 It now defaults to `const std::string` instead of `std::string`.
126 *** Deprecated %define variable names
128 The following variables have been renamed for consistency. Backward
129 compatibility is ensured, but upgrading is recommended.
131 filename_type -> api.filename.type
132 package -> api.package
134 *** Push parsers no longer clear their state when parsing is finished
136 Previously push-parsers cleared their state when parsing was finished (on
137 success and on failure). This made it impossible to check if there were
138 parse errors, since `yynerrs` was also reset. This can be especially
139 troublesome when used in autocompletion, since a parser with error
140 recovery would suggest (irrelevant) expected tokens even if there were
143 Now the parser state can be examined when parsing is finished. The parser
144 state is reset when starting a new parse.
148 *** Include the generated header (yacc.c)
150 Historically, when --defines was used, bison generated a header and pasted
151 an exact copy of it into the generated parser implementation file. Since
152 Bison 3.4 it is possible to specify that the header should be `#include`d,
153 and how. For instance
155 %define api.header.include {"parse.h"}
159 %define api.header.include {<parser/parse.h>}
161 Now api.header.include defaults to `"header-basename"`, as was intended in
162 Bison 3.4, where `header-basename` is the basename of the generated
163 header. This is disabled when the generated header is `y.tab.h`, to
164 comply with Automake's ylwrap.
166 *** String aliases are faithfully propagated
168 Bison used to interpret user strings (i.e., decoding backslash escapes)
169 when reading them, and to escape them (i.e., issue non-printable
170 characters as backslash escapes, taking the locale into account) when
171 outputting them. As a consequence non-ASCII strings (say in UTF-8) ended
172 up "ciphered" as sequences of backslash escapes. This happened not only
173 in the generated sources (where the compiler will reinterpret them), but
174 also in all the generated reports (text, xml, html, dot, etc.). Reports
175 were therefore not readable when string aliases were not pure ASCII.
176 Worse yet: the output depended on the user's locale.
178 Now Bison faithfully treats the string aliases exactly the way the user
179 spelled them. This fixes all the aforementioned problems. However, now,
180 string aliases semantically equivalent but syntactically different (e.g.,
181 "A", "\x41", "\101") are considered to be different.
183 *** Crash when generating IELR
185 An old, well hidden, bug in the generation of IELR parsers was fixed.
188 * Noteworthy changes in release 3.6.4 (2020-06-15) [stable]
192 In glr.cc some internal macros leaked in the user's code, and could damage
193 access to the token kinds.
196 * Noteworthy changes in release 3.6.3 (2020-06-03) [stable]
200 Incorrect comments in the generated parsers.
202 Warnings in push parsers (yacc.c).
204 Incorrect display of gotos in LAC traces (lalr1.cc).
207 * Noteworthy changes in release 3.6.2 (2020-05-17) [stable]
211 Some tests were fixed.
213 When token aliases contain comment delimiters:
215 %token FOO "/* foo */"
217 bison used to emit "nested" comments, which is invalid C.
220 * Noteworthy changes in release 3.6.1 (2020-05-10) [stable]
224 Restored ANSI-C compliance in yacc.c.
226 GNU readline portability issues.
228 In C++, yy::parser::symbol_name is now a public member, as was intended.
232 In C++, yy::parser::symbol_type now has a public name() member function.
235 * Noteworthy changes in release 3.6 (2020-05-08) [stable]
237 ** Backward incompatible changes
239 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
241 The YYERROR_VERBOSE macro is no longer supported; the parsers that still
242 depend on it will now produce Yacc-like error messages (just "syntax
243 error"). It was superseded by the "%error-verbose" directive in Bison
244 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that support
245 for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
246 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
247 parse.error verbose".
249 ** Deprecated features
251 The YYPRINT macro, which works only with yacc.c and only for tokens, was
252 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
253 It is deprecated and its support will be removed eventually.
257 *** Improved syntax error messages
259 Two new values for the %define parse.error variable offer more control to
260 the user. Available in all the skeletons (C, C++, Java).
262 **** %define parse.error detailed
264 The behavior of "%define parse.error detailed" is closely resembling that
265 of "%define parse.error verbose" with a few exceptions. First, it is safe
266 to use non-ASCII characters in token aliases (with 'verbose', the result
267 depends on the locale with which bison was run). Second, a yysymbol_name
268 function is exposed to the user, instead of the yytnamerr function and the
269 yytname table. Third, token internationalization is supported (see
272 **** %define parse.error custom
274 With this directive, the user forges and emits the syntax error message
275 herself by defining the yyreport_syntax_error function. A new type,
276 yypcontext_t, captures the circumstances of the error, and provides the
277 user with functions to get details, such as yypcontext_expected_tokens to
278 get the list of expected token kinds.
280 A possible implementation of yyreport_syntax_error is:
283 yyreport_syntax_error (const yypcontext_t *ctx)
286 YY_LOCATION_PRINT (stderr, *yypcontext_location (ctx));
287 fprintf (stderr, ": syntax error");
288 // Report the tokens expected at this point.
290 enum { TOKENMAX = 10 };
291 yysymbol_kind_t expected[TOKENMAX];
292 int n = yypcontext_expected_tokens (ctx, expected, TOKENMAX);
294 // Forward errors to yyparse.
297 for (int i = 0; i < n; ++i)
298 fprintf (stderr, "%s %s",
299 i == 0 ? ": expected" : " or", yysymbol_name (expected[i]));
301 // Report the unexpected token.
303 yysymbol_kind_t lookahead = yypcontext_token (ctx);
304 if (lookahead != YYSYMBOL_YYEMPTY)
305 fprintf (stderr, " before %s", yysymbol_name (lookahead));
307 fprintf (stderr, "\n");
311 **** Token aliases internationalization
313 When the %define variable parse.error is set to `custom` or `detailed`,
314 one may specify which token aliases are to be translated using _(). For
326 In that case the user must define _() and N_(), and yysymbol_name returns
327 the translated symbol (i.e., it returns '_("variable")' rather that
328 '"variable"'). In Java, the user must provide an i18n() function.
330 *** List of expected tokens (yacc.c)
332 Push parsers may invoke yypstate_expected_tokens at any point during
333 parsing (including even before submitting the first token) to get the list
334 of possible tokens. This feature can be used to propose autocompletion
335 (see below the "bistromathic" example).
337 It makes little sense to use this feature without enabling LAC (lookahead
340 *** Returning the error token
342 When the scanner returns an invalid token or the undefined token
343 (YYUNDEF), the parser generates an error message and enters error
344 recovery. Because of that error message, most scanners that find lexical
345 errors generate an error message, and then ignore the invalid input
346 without entering the error-recovery.
348 The scanners may now return YYerror, the error token, to enter the
349 error-recovery mode without triggering an additional error message. See
350 the bistromathic for an example.
352 *** Deep overhaul of the symbol and token kinds
354 To avoid the confusion with types in programming languages, we now refer
355 to token and symbol "kinds" instead of token and symbol "types". The
356 documentation and error messages have been revised.
358 All the skeletons have been updated to use dedicated enum types rather
359 than integral types. Special symbols are now regular citizens, instead of
360 being declared in ad hoc ways.
364 The "token kind" is what is returned by the scanner, e.g., PLUS, NUMBER,
365 LPAREN, etc. While backward compatibility is of course ensured, users are
366 nonetheless invited to replace their uses of "enum yytokentype" by
369 This type now also includes tokens that were previously hidden: YYEOF (end
370 of input), YYUNDEF (undefined token), and YYerror (error token). They
371 now have string aliases, internationalized when internationalization is
372 enabled. Therefore, by default, error messages now refer to "end of file"
373 (internationalized) rather than the cryptic "$end", or to "invalid token"
374 rather than "$undefined".
376 Therefore in most cases it is now useless to define the end-of-line token
379 %token T_EOF 0 "end of file"
381 Rather simply use "YYEOF" in your scanner.
385 The "symbol kinds" is what the parser actually uses. (Unless the
386 api.token.raw %define variable is used, the symbol kind of a terminal
387 differs from the corresponding token kind.)
389 They are now exposed as a enum, "yysymbol_kind_t".
391 This allows users to tailor the error messages the way they want, or to
392 process some symbols in a specific way in autocompletion (see the
393 bistromathic example below).
395 *** Modernize display of explanatory statements in diagnostics
397 Since Bison 2.7, output was indented four spaces for explanatory
398 statements. For example:
400 input.y:2.7-13: error: %type redeclaration for exp
401 input.y:1.7-11: previous declaration
403 Since the introduction of caret-diagnostics, it became less clear. This
404 indentation has been removed and submessages are displayed similarly as in
407 input.y:2.7-13: error: %type redeclaration for exp
408 2 | %type <float> exp
410 input.y:1.7-11: note: previous declaration
414 Contributed by Victor Morales Cayuela.
418 The token and symbol kinds are yy::parser::token_kind_type and
419 yy::parser::symbol_kind_type.
421 The symbol_type::kind() member function allows to get the kind of a
422 symbol. This can be used to write unit tests for scanners, e.g.,
424 yy::parser::symbol_type t = make_NUMBER ("123");
425 assert (t.kind () == yy::parser::symbol_kind::S_NUMBER);
426 assert (t.value.as<int> () == 123);
432 In order to avoid ambiguities with "type" as in "typing", we now refer to
433 the "token kind" (e.g., `PLUS`, `NUMBER`, etc.) rather than the "token
434 type". We now also refer to the "symbol type" (e.g., `PLUS`, `expr`,
439 There are now examples/java: a very simple calculator, and a more complete
440 one (push-parser, location tracking, and debug traces).
442 The lexcalc example (a simple example in C based on Flex and Bison) now
443 also demonstrates location tracking.
446 A new C example, bistromathic, is a fully featured interactive calculator
447 using many Bison features: pure interface, push parser, autocompletion
448 based on the current parser state (using yypstate_expected_tokens),
449 location tracking, internationalized custom error messages, lookahead
450 correction, rich debug traces, etc.
452 It shows how to depend on the symbol kinds to tailor autocompletion. For
453 instance it recognizes the symbol kind "VARIABLE" to propose
454 autocompletion on the existing variables, rather than of the word
458 * Noteworthy changes in release 3.5.4 (2020-04-05) [stable]
460 ** WARNING: Future backward-incompatibilities!
462 TL;DR: replace "#define YYERROR_VERBOSE 1" by "%define parse.error verbose".
464 Bison 3.6 will no longer support the YYERROR_VERBOSE macro; the parsers
465 that still depend on it will produce Yacc-like error messages (just
466 "syntax error"). It was superseded by the "%error-verbose" directive in
467 Bison 1.875 (2003-01-01). Bison 2.6 (2012-07-19) clearly announced that
468 support for YYERROR_VERBOSE would be removed. Note that since Bison 3.0
469 (2013-07-25), "%error-verbose" is deprecated in favor of "%define
470 parse.error verbose".
474 Fix portability issues of the package itself on old compilers.
476 Fix api.token.raw support in Java.
479 * Noteworthy changes in release 3.5.3 (2020-03-08) [stable]
483 Error messages could quote lines containing zero-width characters (such as
484 \005) with incorrect styling. Fixes for similar issues with unexpectedly
485 short lines (e.g., the file was changed between parsing and diagnosing).
487 Several unlikely crashes found by fuzzing have been fixed.
490 * Noteworthy changes in release 3.5.2 (2020-02-13) [stable]
494 Portability issues and minor cosmetic issues.
496 The lalr1.cc skeleton properly rejects unsupported values for parse.lac
500 * Noteworthy changes in release 3.5.1 (2020-01-19) [stable]
506 Fix compiler warnings.
509 * Noteworthy changes in release 3.5 (2019-12-11) [stable]
511 ** Backward incompatible changes
513 Lone carriage-return characters (aka \r or ^M) in the grammar files are no
514 longer treated as end-of-lines. This changes the diagnostics, and in
515 particular their locations.
517 In C++, line numbers and columns are now represented as 'int' not
518 'unsigned', so that integer overflow on positions is easily checkable via
519 'gcc -fsanitize=undefined' and the like. This affects the API for
520 positions. The default position and location classes now expose
521 'counter_type' (int), used to define line and column numbers.
523 ** Deprecated features
525 The YYPRINT macro, which works only with yacc.c and only for tokens, was
526 obsoleted long ago by %printer, introduced in Bison 1.50 (November 2002).
527 It is deprecated and its support will be removed eventually.
531 *** Lookahead correction in C++
533 Contributed by Adrian Vogelsgesang.
535 The C++ deterministic skeleton (lalr1.cc) now supports LAC, via the
536 %define variable parse.lac.
538 *** Variable api.token.raw: Optimized token numbers (all skeletons)
540 In the generated parsers, tokens have two numbers: the "external" token
541 number as returned by yylex (which starts at 257), and the "internal"
542 symbol number (which starts at 3). Each time yylex is called, a table
543 lookup maps the external token number to the internal symbol number.
545 When the %define variable api.token.raw is set, tokens are assigned their
546 internal number, which saves one table lookup per token, and also saves
547 the generation of the mapping table.
549 The gain is typically moderate, but in extreme cases (very simple user
550 actions), a 10% improvement can be observed.
552 *** Generated parsers use better types for states
554 Stacks now use the best integral type for state numbers, instead of always
555 using 15 bits. As a result "small" parsers now have a smaller memory
556 footprint (they use 8 bits), and there is support for large automata (16
557 bits), and extra large (using int, i.e., typically 31 bits).
559 *** Generated parsers prefer signed integer types
561 Bison skeletons now prefer signed to unsigned integer types when either
562 will do, as the signed types are less error-prone and allow for better
563 checking with 'gcc -fsanitize=undefined'. Also, the types chosen are now
564 portable to unusual machines where char, short and int are all the same
565 width. On non-GNU platforms this may entail including <limits.h> and (if
566 available) <stdint.h> to define integer types and constants.
568 *** A skeleton for the D programming language
570 For the last few releases, Bison has shipped a stealth experimental
571 skeleton: lalr1.d. It was first contributed by Oliver Mangold, based on
572 Paolo Bonzini's lalr1.java, and was cleaned and improved thanks to
575 However, because nobody has committed to improving, testing, and
576 documenting this skeleton, it is not clear that it will be supported in
579 The lalr1.d skeleton *is functional*, and works well, as demonstrated in
580 examples/d/calc.d. Please try it, enjoy it, and... commit to support it.
582 *** Debug traces in Java
584 The Java backend no longer emits code and data for parser tracing if the
585 %define variable parse.trace is not defined.
589 *** New diagnostic: -Wdangling-alias
591 String literals, which allow for better error messages, are (too)
592 liberally accepted by Bison, which might result in silent errors. For
595 %type <exVal> cond "condition"
597 does not define "condition" as a string alias to 'cond' (nonterminal
598 symbols do not have string aliases). It is rather equivalent to
601 %token <exVal> "condition"
603 i.e., it gives the type 'exVal' to the "condition" token, which was
604 clearly not the intention.
606 Also, because string aliases need not be defined, typos such as "baz"
607 instead of "bar" will be not reported.
609 The option -Wdangling-alias catches these situations. On
612 %type <ival> foo "foo"
616 bison -Wdangling-alias reports
618 warning: string literal not attached to a symbol
619 | %type <ival> foo "foo"
621 warning: string literal not attached to a symbol
625 The -Wall option does not (yet?) include -Wdangling-alias.
627 *** Better POSIX Yacc compatibility diagnostics
629 POSIX Yacc restricts %type to nonterminals. This is now diagnosed by
633 %type <ival> TOKEN1 TOKEN2 't'
640 input.y:2.15-20: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
641 2 | %type <ival> TOKEN1 TOKEN2 't'
643 input.y:2.29-31: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
644 2 | %type <ival> TOKEN1 TOKEN2 't'
646 input.y:2.22-27: warning: POSIX yacc reserves %type to nonterminals [-Wyacc]
647 2 | %type <ival> TOKEN1 TOKEN2 't'
650 *** Diagnostics with insertion
652 The diagnostics now display the suggestion below the underlined source.
653 Replacement for undeclared symbols are now also suggested.
660 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'?
664 foo.y:2.16: warning: empty rule without %empty [-Wempty-rule]
668 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
670 *** Diagnostics about long lines
672 Quoted sources may now be truncated to fit the screen. For instance, on a
673 30-column wide terminal:
680 foo.y:1.34-36: warning: symbol FOO redeclared [-Wother]
683 foo.y:1.8-10: previous declaration
686 foo.y:1.62-64: warning: symbol FOO redeclared [-Wother]
689 foo.y:1.8-10: previous declaration
695 *** Debugging glr.c and glr.cc
697 The glr.c skeleton always had asserts to check its own behavior (not the
698 user's). These assertions are now under the control of the parse.assert
699 %define variable (disabled by default).
703 Several new compiler warnings in the generated output have been avoided.
704 Some unused features are no longer emitted. Cleaner generated code in
709 Portability issues in the test suite.
711 In theory, parsers using %nonassoc could crash when reporting verbose
712 error messages. This unlikely bug has been fixed.
714 In Java, %define api.prefix was ignored. It now behaves as expected.
717 * Noteworthy changes in release 3.4.2 (2019-09-12) [stable]
721 In some cases, when warnings are disabled, bison could emit tons of white
722 spaces as diagnostics.
724 When running out of memory, bison could crash (found by fuzzing).
726 When defining twice the EOF token, bison would crash.
728 New warnings from recent compilers have been addressed in the generated
729 parsers (yacc.c, glr.c, glr.cc).
731 When lone carriage-return characters appeared in the input file,
732 diagnostics could hang forever.
735 * Noteworthy changes in release 3.4.1 (2019-05-22) [stable]
742 * Noteworthy changes in release 3.4 (2019-05-19) [stable]
744 ** Deprecated features
746 The %pure-parser directive is deprecated in favor of '%define api.pure'
747 since Bison 2.3b (2008-05-27), but no warning was issued; there is one
748 now. Note that since Bison 2.7 you are strongly encouraged to use
749 '%define api.pure full' instead of '%define api.pure'.
753 *** Colored diagnostics
755 As an experimental feature, diagnostics are now colored, controlled by the
756 new options --color and --style.
758 To use them, install the libtextstyle library before configuring Bison.
761 https://alpha.gnu.org/gnu/gettext/
765 https://alpha.gnu.org/gnu/gettext/libtextstyle-0.8.tar.gz
767 The option --color supports the following arguments:
768 - always, yes: Enable colors.
769 - never, no: Disable colors.
770 - auto, tty (default): Enable colors if the output device is a tty.
772 To customize the styles, create a CSS file similar to
776 .error { font-weight: 800; text-decoration: underline; }
779 then invoke bison with --style=bison-bw.css, or set the BISON_STYLE
780 environment variable to "bison-bw.css".
784 When given -fsyntax-only, the diagnostics are reported, but no output is
787 The name of this option is somewhat misleading as bison does more than
788 just checking the syntax: every stage is run (including checking for
789 conflicts for instance), except the generation of the output files.
791 *** Include the generated header (yacc.c)
793 Before, when --defines is used, bison generated a header, and pasted an
794 exact copy of it into the generated parser implementation file. If the
795 header name is not "y.tab.h", it is now #included instead of being
798 To use an '#include' even if the header name is "y.tab.h" (which is what
799 happens with --yacc, or when using the Autotools' ylwrap), define
800 api.header.include to the exact argument to pass to #include. For
803 %define api.header.include {"parse.h"}
807 %define api.header.include {<parser/parse.h>}
809 *** api.location.type is now supported in C (yacc.c, glr.c)
811 The %define variable api.location.type defines the name of the type to use
812 for locations. When defined, Bison no longer defines YYLTYPE.
814 This can be used in programs with several parsers to factor their
815 definition of locations: let one of them generate them, and the others
822 In conformance with the recommendations of the Graphviz team, if %require
823 "3.4" (or better) is specified, the option --graph generates a *.gv file
824 by default, instead of *.dot.
826 *** Diagnostics overhaul
828 Column numbers were wrong with multibyte characters, which would also
829 result in skewed diagnostics with carets. Beside, because we were
830 indenting the quoted source with a single space, lines with tab characters
831 were incorrectly underlined.
833 To address these issues, and to be clearer, Bison now issues diagnostics
834 as GCC9 does. For instance it used to display (there's a tab before the
837 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
838 expr: expr '+' "number" { $$ = $1 + $2; }
842 foo.y:3.37-38: error: $2 of ‘expr’ has no declared type
843 3 | expr: expr '+' "number" { $$ = $1 + $2; }
846 Other constructs now also have better locations, resulting in more precise
849 *** Fix-it hints for %empty
851 Running Bison with -Wempty-rules and --update will remove incorrect %empty
852 annotations, and add the missing ones.
854 *** Generated reports
856 The format of the reports (parse.output) was improved for readability.
858 *** Better support for --no-line.
860 When --no-line is used, the generated files are now cleaner: no lines are
861 generated instead of empty lines. Together with using api.header.include,
862 that should help people saving the generated files into version control
863 systems get smaller diffs.
867 A new example in C shows an simple infix calculator with a hand-written
868 scanner (examples/c/calc).
870 A new example in C shows a reentrant parser (capable of recursive calls)
871 built with Flex and Bison (examples/c/reccalc).
873 There is a new section about the history of Yaccs and Bison.
877 A few obscure bugs were fixed, including the second oldest (known) bug in
878 Bison: it was there when Bison was entered in the RCS version control
879 system, in December 1987. See the NEWS of Bison 3.3 for the previous
883 * Noteworthy changes in release 3.3.2 (2019-02-03) [stable]
887 Bison 3.3 failed to generate parsers for grammars with unused nonterminal
891 * Noteworthy changes in release 3.3.1 (2019-01-27) [stable]
895 The option -y/--yacc used to imply -Werror=yacc, which turns uses of Bison
896 extensions into errors. It now makes them simple warnings (-Wyacc).
899 * Noteworthy changes in release 3.3 (2019-01-26) [stable]
901 A new mailing list was created, Bison Announce. It is low traffic, and is
902 only about announcing new releases and important messages (e.g., polls
903 about major decisions to make).
905 https://lists.gnu.org/mailman/listinfo/bison-announce
907 ** Backward incompatible changes
909 Support for DJGPP, which has been unmaintained and untested for years, is
912 ** Deprecated features
914 A new feature, --update (see below) helps adjusting existing grammars to
917 *** Deprecated directives
919 The %error-verbose directive is deprecated in favor of '%define
920 parse.error verbose' since Bison 3.0, but no warning was issued.
922 The '%name-prefix "xx"' directive is deprecated in favor of '%define
923 api.prefix {xx}' since Bison 3.0, but no warning was issued. These
924 directives are slightly different, you might need to adjust your code.
925 %name-prefix renames only symbols with external linkage, while api.prefix
926 also renames types and macros, including YYDEBUG, YYTOKENTYPE,
927 yytokentype, YYSTYPE, YYLTYPE, etc.
929 Users of Flex that move from '%name-prefix "xx"' to '%define api.prefix
930 {xx}' will typically have to update YY_DECL from
932 #define YY_DECL int xxlex (YYSTYPE *yylval, YYLTYPE *yylloc)
936 #define YY_DECL int xxlex (XXSTYPE *yylval, XXLTYPE *yylloc)
938 *** Deprecated %define variable names
940 The following variables, mostly related to parsers in Java, have been
941 renamed for consistency. Backward compatibility is ensured, but upgrading
944 abstract -> api.parser.abstract
945 annotations -> api.parser.annotations
946 extends -> api.parser.extends
947 final -> api.parser.final
948 implements -> api.parser.implements
949 parser_class_name -> api.parser.class
950 public -> api.parser.public
951 strictfp -> api.parser.strictfp
955 *** Generation of fix-its for IDEs/Editors
957 When given the new option -ffixit (aka -fdiagnostics-parseable-fixits),
958 bison now generates machine readable editing instructions to fix some
959 issues. Currently, this is mostly limited to updating deprecated
960 directives and removing duplicates. For instance:
964 %define parser_class_name "Parser"
965 %define api.parser.class "Parser"
969 See the "fix-it:" lines below:
971 $ bison -ffixit foo.y
972 foo.y:1.1-14: warning: deprecated directive, use '%define parse.error verbose' [-Wdeprecated]
975 fix-it:"foo.y":{1:1-1:15}:"%define parse.error verbose"
976 foo.y:2.1-34: warning: deprecated directive, use '%define api.parser.class {Parser}' [-Wdeprecated]
977 %define parser_class_name "Parser"
978 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
979 fix-it:"foo.y":{2:1-2:35}:"%define api.parser.class {Parser}"
980 foo.y:3.1-33: error: %define variable 'api.parser.class' redefined
981 %define api.parser.class "Parser"
982 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
983 foo.y:2.1-34: previous definition
984 %define parser_class_name "Parser"
985 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
986 fix-it:"foo.y":{3:1-3:34}:""
987 foo.y: warning: fix-its can be applied. Rerun with option '--update'. [-Wother]
989 This uses the same output format as GCC and Clang.
991 *** Updating grammar files
993 Fixes can be applied on the fly. The previous example ends with the
994 suggestion to re-run bison with the option -u/--update, which results in a
995 cleaner grammar file.
997 $ bison --update foo.y
999 bison: file 'foo.y' was updated (backup: 'foo.y~')
1002 %define parse.error verbose
1003 %define api.parser.class {Parser}
1007 *** Bison is now relocatable
1009 If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
1011 A relocatable program can be moved or copied to a different location on
1012 the file system. It can also be used through mount points for network
1013 sharing. It is possible to make symbolic links to the installed and moved
1014 programs, and invoke them through the symbolic link.
1016 *** %expect and %expect-rr modifiers on individual rules
1018 One can now document (and check) which rules participate in shift/reduce
1019 and reduce/reduce conflicts. This is particularly important GLR parsers,
1020 where conflicts are a normal occurrence. For example,
1036 | argument_list ',' expression
1041 Looking at the output from -v, one can see that the shift/reduce conflict
1042 here is due to the fact that the parser does not know whether to reduce
1043 arguments to argument_list until it sees the token _after_ the following
1044 ','. By marking the rule with %expect 1 (because there is a conflict in
1045 one state), we document the source of the 1 overall shift/reduce conflict.
1047 In GLR parsers, we can use %expect-rr in a rule for reduce/reduce
1048 conflicts. In this case, we mark each of the conflicting rules. For
1057 target_list '=' expr ';'
1063 | target ',' target_list
1072 | expr ',' expr_list
1080 In a statement such as
1084 the parser must reduce x to a target or an expr, but does not know which
1085 until it sees the '='. So we notate the two possible reductions to
1086 indicate that each conflicts in one rule.
1088 This feature needs user feedback, and might evolve in the future.
1090 *** C++: Actual token constructors
1092 When variants and token constructors are enabled, in addition to the
1093 type-safe named token constructors (make_ID, make_INT, etc.), we now
1094 generate genuine constructors for symbol_type.
1096 For instance with these declarations
1102 you may use these constructors:
1104 symbol_type (int token, const std::string&);
1105 symbol_type (int token, const int&);
1106 symbol_type (int token);
1108 Correct matching between token types and value types is checked via
1109 'assert'; for instance, 'symbol_type (ID, 42)' would abort. Named
1110 constructors are preferable, as they offer better type safety (for
1111 instance 'make_ID (42)' would not even compile), but symbol_type
1112 constructors may help when token types are discovered at run-time, e.g.,
1115 if (auto i = lookup_keyword (yytext))
1116 return yy::parser::symbol_type (i);
1118 return yy::parser::make_ID (yytext);
1121 *** C++: Variadic emplace
1123 If your application requires C++11 and you don't use symbol constructors,
1124 you may now use a variadic emplace for semantic values:
1126 %define api.value.type variant
1127 %token <std::pair<int, int>> PAIR
1131 int yylex (parser::semantic_type *lvalp)
1133 lvalp->emplace <std::pair<int, int>> (1, 2);
1134 return parser::token::PAIR;
1137 *** C++: Syntax error exceptions in GLR
1139 The glr.cc skeleton now supports syntax_error exceptions thrown from user
1140 actions, or from the scanner.
1142 *** More POSIX Yacc compatibility warnings
1144 More Bison specific directives are now reported with -y or -Wyacc. This
1145 change was ready since the release of Bison 3.0 in September 2015. It was
1146 delayed because Autoconf used to define YACC as `bison -y`, which resulted
1147 in numerous warnings for Bison users that use the GNU Build System.
1149 If you still experience that problem, either redefine YACC as `bison -o
1150 y.tab.c`, or pass -Wno-yacc to Bison.
1152 *** The tables yyrhs and yyphrs are back
1154 Because no Bison skeleton uses them, these tables were removed (no longer
1155 passed to the skeletons, not even computed) in 2008. However, some users
1156 have expressed interest in being able to use them in their own skeletons.
1160 *** Incorrect number of reduce/reduce conflicts
1162 On a grammar such as
1164 exp: "num" | "num" | "num"
1166 bison used to report a single RR conflict, instead of two. This is now
1167 fixed. This was the oldest (known) bug in Bison: it was there when Bison
1168 was entered in the RCS version control system, in December 1987.
1170 Some grammar files might have to adjust their %expect-rr.
1172 *** Parser directives that were not careful enough
1174 Passing invalid arguments to %nterm, for instance character literals, used
1175 to result in unclear error messages.
1179 The examples/ directory (installed in .../share/doc/bison/examples) has
1180 been restructured per language for clarity. The examples come with a
1181 README and a Makefile. Not only can they be used to toy with Bison, they
1182 can also be starting points for your own grammars.
1184 There is now a Java example, and a simple example in C based on Flex and
1185 Bison (examples/c/lexcalc/).
1191 They now use noexcept and constexpr. Please, report missing annotations.
1193 *** Symbol Declarations
1195 The syntax of the variation directives to declare symbols was overhauled
1196 for more consistency, and also better POSIX Yacc compliance (which, for
1197 instance, allows "%type" without actually providing a type). The %nterm
1198 directive, supported by Bison since its inception, is now documented and
1199 officially supported.
1201 The syntax is now as follows:
1203 %token TAG? ( ID NUMBER? STRING? )+ ( TAG ( ID NUMBER? STRING? )+ )*
1204 %left TAG? ( ID NUMBER? )+ ( TAG ( ID NUMBER? )+ )*
1205 %type TAG? ( ID | CHAR | STRING )+ ( TAG ( ID | CHAR | STRING )+ )*
1206 %nterm TAG? ID+ ( TAG ID+ )*
1208 where TAG denotes a type tag such as ‘<ival>’, ID denotes an identifier
1209 such as ‘NUM’, NUMBER a decimal or hexadecimal integer such as ‘300’ or
1210 ‘0x12d’, CHAR a character literal such as ‘'+'’, and STRING a string
1211 literal such as ‘"number"’. The post-fix quantifiers are ‘?’ (zero or
1212 one), ‘*’ (zero or more) and ‘+’ (one or more).
1215 * Noteworthy changes in release 3.2.4 (2018-12-24) [stable]
1219 Fix the move constructor of symbol_type.
1221 Always provide a copy constructor for symbol_type, even in modern C++.
1224 * Noteworthy changes in release 3.2.3 (2018-12-18) [stable]
1228 Properly support token constructors in C++ with types that include commas
1229 (e.g., std::pair<int, int>). A regression introduced in Bison 3.2.
1232 * Noteworthy changes in release 3.2.2 (2018-11-21) [stable]
1236 C++ portability issues.
1239 * Noteworthy changes in release 3.2.1 (2018-11-09) [stable]
1243 Several portability issues have been fixed in the build system, in the
1244 test suite, and in the generated parsers in C++.
1247 * Noteworthy changes in release 3.2 (2018-10-29) [stable]
1249 ** Backward incompatible changes
1251 Support for DJGPP, which has been unmaintained and untested for years, is
1252 obsolete. Unless there is activity to revive it, it will be removed.
1256 %printers should use yyo rather than yyoutput to denote the output stream.
1258 Variant-based symbols in C++ should use emplace() rather than build().
1260 In C++ parsers, parser::operator() is now a synonym for the parser::parse.
1264 A new section, "A Simple C++ Example", is a tutorial for parsers in C++.
1266 A comment in the generated code now emphasizes that users should not
1267 depend upon non-documented implementation details, such as macros starting
1272 *** C++: Support for move semantics (lalr1.cc)
1274 The lalr1.cc skeleton now fully supports C++ move semantics, while
1275 maintaining compatibility with C++98. You may now store move-only types
1276 when using Bison's variants. For instance:
1283 %skeleton "lalr1.cc"
1284 %define api.value.type variant
1288 %token <int> INT "int";
1289 %type <std::unique_ptr<int>> int;
1290 %type <std::vector<std::unique_ptr<int>>> list;
1294 | list int { $$ = std::move($1); $$.emplace_back(std::move($2)); }
1296 int: "int" { $$ = std::make_unique<int>($1); }
1298 *** C++: Implicit move of right-hand side values (lalr1.cc)
1300 In modern C++ (C++11 and later), you should always use 'std::move' with
1301 the values of the right-hand side symbols ($1, $2, etc.), as they will be
1302 popped from the stack anyway. Using 'std::move' is mandatory for
1303 move-only types such as unique_ptr, and it provides a significant speedup
1304 for large types such as std::string, or std::vector, etc.
1306 If '%define api.value.automove' is set, every occurrence '$n' is replaced
1307 by 'std::move ($n)'. The second rule in the previous grammar can be
1310 list: list int { $$ = $1; $$.emplace_back($2); }
1312 With automove enabled, the semantic values are no longer lvalues, so do
1313 not use the swap idiom:
1315 list: list int { std::swap($$, $1); $$.emplace_back($2); }
1317 This idiom is anyway obsolete: it is preferable to move than to swap.
1319 A warning is issued when automove is enabled, and a value is used several
1322 input.yy:16.31-32: warning: multiple occurrences of $2 with api.value.automove enabled [-Wother]
1323 exp: "twice" exp { $$ = $2 + $2; }
1326 Enabling api.value.automove does not require support for modern C++. The
1327 generated code is valid C++98/03, but will use copies instead of moves.
1329 The new examples/c++/variant-11.yy shows these features in action.
1331 *** C++: The implicit default semantic action is always run
1333 When variants are enabled, the default action was not run, so
1341 It now behaves like in all the other cases, as
1343 exp: "number" { $$ = $1; }
1345 possibly using std::move if automove is enabled.
1347 We do not expect backward compatibility issues. However, beware of
1348 forward compatibility issues: if you rely on default actions with
1349 variants, be sure to '%require "3.2"' to avoid older versions of Bison to
1350 generate incorrect parsers.
1352 *** C++: Renaming location.hh
1354 When both %defines and %locations are enabled, Bison generates a
1355 location.hh file. If you don't use locations outside of the parser, you
1356 may avoid its creation with:
1358 %define api.location.file none
1360 However this file is useful if, for instance, your parser builds an AST
1361 decorated with locations: you may use Bison's location independently of
1362 Bison's parser. You can now give it another name, for instance:
1364 %define api.location.file "my-location.hh"
1366 This name can have directory components, and even be absolute. The name
1367 under which the location file is included is controlled by
1368 api.location.include.
1370 This way it is possible to have several parsers share the same location
1373 For instance, in src/foo/parser.hh, generate the include/ast/loc.hh file:
1376 %define api.namespace {foo}
1377 %define api.location.file "include/ast/loc.hh"
1378 %define api.location.include {<ast/loc.hh>}
1380 and use it in src/bar/parser.hh:
1383 %define api.namespace {bar}
1384 %code requires {#include <ast/loc.hh>}
1385 %define api.location.type {bar::location}
1387 Absolute file names are supported, so in your Makefile, passing the flag
1388 -Dapi.location.file='"$(top_srcdir)/include/ast/location.hh"' to bison is
1391 *** C++: stack.hh and position.hh are deprecated
1393 When asked to generate a header file (%defines), the lalr1.cc skeleton
1394 generates a stack.hh file. This file had no interest for users; it is now
1395 made useless: its content is included in the parser definition. It is
1396 still generated for backward compatibility.
1398 When in addition to %defines, location support is requested (%locations),
1399 the file position.hh is also generated. It is now also useless: its
1400 content is now included in location.hh.
1402 These files are no longer generated when your grammar file requires at
1403 least Bison 3.2 (%require "3.2").
1407 Portability issues on MinGW and VS2015.
1409 Portability issues in the test suite.
1411 Portability/warning issues with Flex.
1414 * Noteworthy changes in release 3.1 (2018-08-27) [stable]
1416 ** Backward incompatible changes
1418 Compiling Bison now requires a C99 compiler---as announced during the
1419 release of Bison 3.0, five years ago. Generated parsers do not require a
1422 Support for DJGPP, which has been unmaintained and untested for years, is
1423 obsolete. Unless there is activity to revive it, the next release of Bison
1424 will have it removed.
1428 *** Typed midrule actions
1430 Because their type is unknown to Bison, the values of midrule actions are
1431 not treated like the others: they don't have %printer and %destructor
1432 support. It also prevents C++ (Bison) variants to handle them properly.
1434 Typed midrule actions address these issues. Instead of:
1436 exp: { $<ival>$ = 1; } { $<ival>$ = 2; } { $$ = $<ival>1 + $<ival>2; }
1440 exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; } { $$ = $1 + $2; }
1442 *** Reports include the type of the symbols
1444 The sections about terminal and nonterminal symbols of the '*.output' file
1445 now specify their declared type. For instance, for:
1449 the report now shows '<ival>':
1451 Terminals, with rules where they appear
1455 *** Diagnostics about useless rules
1457 In the following grammar, the 'exp' nonterminal is trivially useless. So,
1458 of course, its rules are useless too.
1462 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1464 Previously all the useless rules were reported, including those whose
1465 left-hand side is the 'exp' nonterminal:
1467 warning: 1 nonterminal useless in grammar [-Wother]
1468 warning: 4 rules useless in grammar [-Wother]
1469 2.14-16: warning: nonterminal useless in grammar: exp [-Wother]
1472 2.14-16: warning: rule useless in grammar [-Wother]
1475 3.6-16: warning: rule useless in grammar [-Wother]
1476 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1478 3.20-30: warning: rule useless in grammar [-Wother]
1479 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1481 3.34-44: warning: rule useless in grammar [-Wother]
1482 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1485 Now, rules whose left-hand side symbol is useless are no longer reported
1486 as useless. The locations of the errors have also been adjusted to point
1487 to the first use of the nonterminal as a left-hand side of a rule:
1489 warning: 1 nonterminal useless in grammar [-Wother]
1490 warning: 4 rules useless in grammar [-Wother]
1491 3.1-3: warning: nonterminal useless in grammar: exp [-Wother]
1492 exp: exp '+' exp | exp '-' exp | '(' exp ')'
1494 2.14-16: warning: rule useless in grammar [-Wother]
1498 *** C++: Generated parsers can be compiled with -fno-exceptions (lalr1.cc)
1500 When compiled with exceptions disabled, the generated parsers no longer
1501 uses try/catch clauses.
1503 Currently only GCC and Clang are supported.
1507 *** A demonstration of variants
1509 A new example was added (installed in .../share/doc/bison/examples),
1510 'variant.yy', which shows how to use (Bison) variants in C++.
1512 The other examples were made nicer to read.
1514 *** Some features are no longer 'experimental'
1516 The following features, mature enough, are no longer flagged as
1517 experimental in the documentation: push parsers, default %printer and
1518 %destructor (typed: <*> and untyped: <>), %define api.value.type union and
1519 variant, Java parsers, XML output, LR family (lr, ielr, lalr), and
1520 semantic predicates (%?).
1524 *** GLR: Predicates support broken by #line directives
1526 Predicates (%?) in GLR such as
1529 %? {new_syntax} 'w' id new_args
1530 | %?{!new_syntax} 'w' id old_args
1532 were issued with #lines in the middle of C code.
1534 *** Printer and destructor with broken #line directives
1536 The #line directives were not properly escaped when emitting the code for
1537 %printer/%destructor, which resulted in compiler errors if there are
1538 backslashes or double-quotes in the grammar file name.
1540 *** Portability on ICC
1542 The Intel compiler claims compatibility with GCC, yet rejects its _Pragma.
1543 Generated parsers now work around this.
1547 There were several small fixes in the test suite and in the build system,
1548 many warnings in bison and in the generated parsers were eliminated. The
1549 documentation also received its share of minor improvements.
1551 Useless code was removed from C++ parsers, and some of the generated
1552 constructors are more 'natural'.
1555 * Noteworthy changes in release 3.0.5 (2018-05-27) [stable]
1559 *** C++: Fix support of 'syntax_error'
1561 One incorrect 'inline' resulted in linking errors about the constructor of
1562 the syntax_error exception.
1564 *** C++: Fix warnings
1566 GCC 7.3 (with -O1 or -O2 but not -O0 or -O3) issued null-dereference
1567 warnings about yyformat being possibly null. It also warned about the
1568 deprecated implicit definition of copy constructors when there's a
1569 user-defined (copy) assignment operator.
1571 *** Location of errors
1573 In C++ parsers, out-of-bounds errors can happen when a rule with an empty
1574 ride-hand side raises a syntax error. The behavior of the default parser
1575 (yacc.c) in such a condition was undefined.
1577 Now all the parsers match the behavior of glr.c: @$ is used as the
1578 location of the error. This handles gracefully rules with and without
1581 *** Portability fixes in the test suite
1583 On some platforms, some Java and/or C++ tests were failing.
1586 * Noteworthy changes in release 3.0.4 (2015-01-23) [stable]
1590 *** C++ with Variants (lalr1.cc)
1592 Fix a compiler warning when no %destructor use $$.
1596 Several portability issues in tests were fixed.
1599 * Noteworthy changes in release 3.0.3 (2015-01-15) [stable]
1603 *** C++ with Variants (lalr1.cc)
1605 Problems with %destructor and '%define parse.assert' have been fixed.
1607 *** Named %union support (yacc.c, glr.c)
1609 Bison 3.0 introduced a regression on named %union such as
1611 %union foo { int ival; };
1613 The possibility to use a name was introduced "for Yacc compatibility".
1614 It is however not required by POSIX Yacc, and its usefulness is not clear.
1616 *** %define api.value.type union with %defines (yacc.c, glr.c)
1618 The C parsers were broken when %defines was used together with "%define
1619 api.value.type union".
1621 *** Redeclarations are reported in proper order
1629 bison used to report:
1631 foo.yy:2.10-11: error: %printer redeclaration for FOO
1634 foo.yy:3.10-11: previous declaration
1638 Now, the "previous" declaration is always the first one.
1643 Bison now installs various files in its docdir (which defaults to
1644 '/usr/local/share/doc/bison'), including the three fully blown examples
1645 extracted from the documentation:
1648 Reverse Polish Calculator, a simple introductory example.
1650 Multi-function Calc, a calculator with memory and functions and located
1653 a calculator in C++ using variant support and token constructors.
1656 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
1660 *** Generated source files when errors are reported
1662 When warnings are issued and -Werror is set, bison would still generate
1663 the source files (*.c, *.h...). As a consequence, some runs of "make"
1664 could fail the first time, but not the second (as the files were generated
1667 This is fixed: bison no longer generates this source files, but, of
1668 course, still produces the various reports (*.output, *.xml, etc.).
1670 *** %empty is used in reports
1672 Empty right-hand sides are denoted by '%empty' in all the reports (text,
1673 dot, XML and formats derived from it).
1675 *** YYERROR and variants
1677 When C++ variant support is enabled, an error triggered via YYERROR, but
1678 not caught via error recovery, resulted in a double deletion.
1681 * Noteworthy changes in release 3.0.1 (2013-11-12) [stable]
1685 *** Errors in caret diagnostics
1687 On some platforms, some errors could result in endless diagnostics.
1689 *** Fixes of the -Werror option
1691 Options such as "-Werror -Wno-error=foo" were still turning "foo"
1692 diagnostics into errors instead of warnings. This is fixed.
1694 Actually, for consistency with GCC, "-Wno-error=foo -Werror" now also
1695 leaves "foo" diagnostics as warnings. Similarly, with "-Werror=foo
1696 -Wno-error", "foo" diagnostics are now errors.
1700 As demonstrated in the documentation, one can now leave spaces between
1705 The yacc.1 man page is no longer installed if --disable-yacc was
1708 *** Fixes in the test suite
1710 Bugs and portability issues.
1713 * Noteworthy changes in release 3.0 (2013-07-25) [stable]
1715 ** WARNING: Future backward-incompatibilities!
1717 Like other GNU packages, Bison will start using some of the C99 features
1718 for its own code, especially the definition of variables after statements.
1719 The generated C parsers still aim at C90.
1721 ** Backward incompatible changes
1723 *** Obsolete features
1725 Support for YYFAIL is removed (deprecated in Bison 2.4.2): use YYERROR.
1727 Support for yystype and yyltype is removed (deprecated in Bison 1.875):
1728 use YYSTYPE and YYLTYPE.
1730 Support for YYLEX_PARAM and YYPARSE_PARAM is removed (deprecated in Bison
1731 1.875): use %lex-param, %parse-param, or %param.
1733 Missing semicolons at the end of actions are no longer added (as announced
1734 in the release 2.5).
1736 *** Use of YACC='bison -y'
1738 TL;DR: With Autoconf <= 2.69, pass -Wno-yacc to (AM_)YFLAGS if you use
1741 Traditional Yacc generates 'y.tab.c' whatever the name of the input file.
1742 Therefore Makefiles written for Yacc expect 'y.tab.c' (and possibly
1743 'y.tab.h' and 'y.output') to be generated from 'foo.y'.
1745 To this end, for ages, AC_PROG_YACC, Autoconf's macro to look for an
1746 implementation of Yacc, was using Bison as 'bison -y'. While it does
1747 ensure compatible output file names, it also enables warnings for
1748 incompatibilities with POSIX Yacc. In other words, 'bison -y' triggers
1749 warnings for Bison extensions.
1751 Autoconf 2.70+ fixes this incompatibility by using YACC='bison -o y.tab.c'
1752 (which also generates 'y.tab.h' and 'y.output' when needed).
1753 Alternatively, disable Yacc warnings by passing '-Wno-yacc' to your Yacc
1754 flags (YFLAGS, or AM_YFLAGS with Automake).
1758 *** The epilogue is no longer affected by internal #defines (glr.c)
1760 The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
1761 generated code. These weren't properly undefined before the inclusion of
1762 the user epilogue, so functions such as the following were butchered by the
1763 preprocessor expansion:
1765 int yylex (YYSTYPE *yylval);
1767 This is fixed: yylval, yynerrs, yychar, and yylloc are now valid
1768 identifiers for user-provided variables.
1770 *** stdio.h is no longer needed when locations are enabled (yacc.c)
1772 Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
1773 locations are enabled. This is fixed.
1775 *** Warnings about useless %pure-parser/%define api.pure are restored
1777 ** Diagnostics reported by Bison
1779 Most of these features were contributed by Théophile Ranquet and Victor
1784 Version 2.7 introduced caret errors, for a prettier output. These are now
1785 activated by default. The old format can still be used by invoking Bison
1786 with -fno-caret (or -fnone).
1788 Some error messages that reproduced excerpts of the grammar are now using
1789 the caret information only. For instance on:
1796 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1797 in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
1801 in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
1802 in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
1806 and "bison -fno-caret" reports:
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]
1811 *** Enhancements of the -Werror option
1813 The -Werror=CATEGORY option is now recognized, and will treat specified
1814 warnings as errors. The warnings need not have been explicitly activated
1815 using the -W option, this is similar to what GCC 4.7 does.
1817 For example, given the following command line, Bison will treat both
1818 warnings related to POSIX Yacc incompatibilities and S/R conflicts as
1819 errors (and only those):
1821 $ bison -Werror=yacc,error=conflicts-sr input.y
1823 If no categories are specified, -Werror will make all active warnings into
1824 errors. For example, the following line does the same the previous example:
1826 $ bison -Werror -Wnone -Wyacc -Wconflicts-sr input.y
1828 (By default -Wconflicts-sr,conflicts-rr,deprecated,other is enabled.)
1830 Note that the categories in this -Werror option may not be prefixed with
1831 "no-". However, -Wno-error[=CATEGORY] is valid.
1833 Note that -y enables -Werror=yacc. Therefore it is now possible to require
1834 Yacc-like behavior (e.g., always generate y.tab.c), but to report
1835 incompatibilities as warnings: "-y -Wno-error=yacc".
1837 *** The display of warnings is now richer
1839 The option that controls a given warning is now displayed:
1841 foo.y:4.6: warning: type clash on default action: <foo> != <bar> [-Wother]
1843 In the case of warnings treated as errors, the prefix is changed from
1844 "warning: " to "error: ", and the suffix is displayed, in a manner similar
1845 to GCC, as [-Werror=CATEGORY].
1847 For instance, where the previous version of Bison would report (and exit
1850 bison: warnings being treated as errors
1851 input.y:1.1: warning: stray ',' treated as white space
1855 input.y:1.1: error: stray ',' treated as white space [-Werror=other]
1857 *** Deprecated constructs
1859 The new 'deprecated' warning category flags obsolete constructs whose
1860 support will be discontinued. It is enabled by default. These warnings
1861 used to be reported as 'other' warnings.
1863 *** Useless semantic types
1865 Bison now warns about useless (uninhabited) semantic types. Since
1866 semantic types are not declared to Bison (they are defined in the opaque
1867 %union structure), it is %printer/%destructor directives about useless
1868 types that trigger the warning:
1872 %printer {} <type1> <type3>
1873 %destructor {} <type2> <type4>
1875 nterm: term { $$ = $1; };
1877 3.28-34: warning: type <type3> is used, but is not associated to any symbol
1878 4.28-34: warning: type <type4> is used, but is not associated to any symbol
1880 *** Undefined but unused symbols
1882 Bison used to raise an error for undefined symbols that are not used in
1883 the grammar. This is now only a warning.
1886 %destructor {} symbol2
1887 %type <type> symbol3
1891 *** Useless destructors or printers
1893 Bison now warns about useless destructors or printers. In the following
1894 example, the printer for <type1>, and the destructor for <type2> are
1895 useless: all symbols of <type1> (token1) already have a printer, and all
1896 symbols of type <type2> (token2) already have a destructor.
1898 %token <type1> token1
1902 %printer {} token1 <type1> <type3>
1903 %destructor {} token2 <type2> <type4>
1907 The warnings and error messages about shift/reduce and reduce/reduce
1908 conflicts have been normalized. For instance on the following foo.y file:
1912 exp: exp '+' exp | '0' | '0';
1914 compare the previous version of bison:
1917 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1918 $ bison -Werror foo.y
1919 bison: warnings being treated as errors
1920 foo.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1922 with the new behavior:
1925 foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
1926 foo.y: warning: 2 reduce/reduce conflicts [-Wconflicts-rr]
1927 $ bison -Werror foo.y
1928 foo.y: error: 1 shift/reduce conflict [-Werror=conflicts-sr]
1929 foo.y: error: 2 reduce/reduce conflicts [-Werror=conflicts-rr]
1931 When %expect or %expect-rr is used, such as with bar.y:
1936 exp: exp '+' exp | '0' | '0';
1941 bar.y: conflicts: 1 shift/reduce, 2 reduce/reduce
1942 bar.y: expected 0 shift/reduce conflicts
1943 bar.y: expected 0 reduce/reduce conflicts
1948 bar.y: error: shift/reduce conflicts: 1 found, 0 expected
1949 bar.y: error: reduce/reduce conflicts: 2 found, 0 expected
1951 ** Incompatibilities with POSIX Yacc
1953 The 'yacc' category is no longer part of '-Wall', enable it explicitly
1956 ** Additional yylex/yyparse arguments
1958 The new directive %param declares additional arguments to both yylex and
1959 yyparse. The %lex-param, %parse-param, and %param directives support one
1960 or more arguments. Instead of
1962 %lex-param {arg1_type *arg1}
1963 %lex-param {arg2_type *arg2}
1964 %parse-param {arg1_type *arg1}
1965 %parse-param {arg2_type *arg2}
1969 %param {arg1_type *arg1} {arg2_type *arg2}
1971 ** Types of values for %define variables
1973 Bison used to make no difference between '%define foo bar' and '%define
1974 foo "bar"'. The former is now called a 'keyword value', and the latter a
1975 'string value'. A third kind was added: 'code values', such as '%define
1978 Keyword variables are used for fixed value sets, e.g.,
1980 %define lr.type lalr
1982 Code variables are used for value in the target language, e.g.,
1984 %define api.value.type {struct semantic_type}
1986 String variables are used remaining cases, e.g. file names.
1988 ** Variable api.token.prefix
1990 The variable api.token.prefix changes the way tokens are identified in
1991 the generated files. This is especially useful to avoid collisions
1992 with identifiers in the target language. For instance
1994 %token FILE for ERROR
1995 %define api.token.prefix {TOK_}
1997 start: FILE for ERROR;
1999 will generate the definition of the symbols TOK_FILE, TOK_for, and
2000 TOK_ERROR in the generated sources. In particular, the scanner must
2001 use these prefixed token names, although the grammar itself still
2002 uses the short names (as in the sample rule given above).
2004 ** Variable api.value.type
2006 This new %define variable supersedes the #define macro YYSTYPE. The use
2007 of YYSTYPE is discouraged. In particular, #defining YYSTYPE *and* either
2008 using %union or %defining api.value.type results in undefined behavior.
2010 Either define api.value.type, or use "%union":
2017 %token <ival> INT "integer"
2018 %token <sval> STRING "string"
2019 %printer { fprintf (yyo, "%d", $$); } <ival>
2020 %destructor { free ($$); } <sval>
2023 yylval.ival = 42; return INT;
2024 yylval.sval = "42"; return STRING;
2026 The %define variable api.value.type supports both keyword and code values.
2028 The keyword value 'union' means that the user provides genuine types, not
2029 union member names such as "ival" and "sval" above (WARNING: will fail if
2030 -y/--yacc/%yacc is enabled).
2032 %define api.value.type union
2033 %token <int> INT "integer"
2034 %token <char *> STRING "string"
2035 %printer { fprintf (yyo, "%d", $$); } <int>
2036 %destructor { free ($$); } <char *>
2039 yylval.INT = 42; return INT;
2040 yylval.STRING = "42"; return STRING;
2042 The keyword value variant is somewhat equivalent, but for C++ special
2043 provision is made to allow classes to be used (more about this below).
2045 %define api.value.type variant
2046 %token <int> INT "integer"
2047 %token <std::string> STRING "string"
2049 Code values (in braces) denote user defined types. This is where YYSTYPE
2067 %define api.value.type {struct my_value}
2068 %token <u.ival> INT "integer"
2069 %token <u.sval> STRING "string"
2070 %printer { fprintf (yyo, "%d", $$); } <u.ival>
2071 %destructor { free ($$); } <u.sval>
2074 yylval.u.ival = 42; return INT;
2075 yylval.u.sval = "42"; return STRING;
2077 ** Variable parse.error
2079 This variable controls the verbosity of error messages. The use of the
2080 %error-verbose directive is deprecated in favor of "%define parse.error
2083 ** Deprecated %define variable names
2085 The following variables have been renamed for consistency. Backward
2086 compatibility is ensured, but upgrading is recommended.
2088 lr.default-reductions -> lr.default-reduction
2089 lr.keep-unreachable-states -> lr.keep-unreachable-state
2090 namespace -> api.namespace
2091 stype -> api.value.type
2093 ** Semantic predicates
2095 Contributed by Paul Hilfinger.
2097 The new, experimental, semantic-predicate feature allows actions of the
2098 form "%?{ BOOLEAN-EXPRESSION }", which cause syntax errors (as for
2099 YYERROR) if the expression evaluates to 0, and are evaluated immediately
2100 in GLR parsers, rather than being deferred. The result is that they allow
2101 the programmer to prune possible parses based on the values of run-time
2104 ** The directive %expect-rr is now an error in non GLR mode
2106 It used to be an error only if used in non GLR mode, _and_ if there are
2107 reduce/reduce conflicts.
2109 ** Tokens are numbered in their order of appearance
2111 Contributed by Valentin Tolmer.
2113 With '%token A B', A had a number less than the one of B. However,
2114 precedence declarations used to generate a reversed order. This is now
2115 fixed, and introducing tokens with any of %token, %left, %right,
2116 %precedence, or %nonassoc yields the same result.
2118 When mixing declarations of tokens with a literal character (e.g., 'a') or
2119 with an identifier (e.g., B) in a precedence declaration, Bison numbered
2120 the literal characters first. For example
2124 would lead to the tokens declared in this order: 'c' 'd' A B. Again, the
2125 input order is now preserved.
2127 These changes were made so that one can remove useless precedence and
2128 associativity declarations (i.e., map %nonassoc, %left or %right to
2129 %precedence, or to %token) and get exactly the same output.
2131 ** Useless precedence and associativity
2133 Contributed by Valentin Tolmer.
2135 When developing and maintaining a grammar, useless associativity and
2136 precedence directives are common. They can be a nuisance: new ambiguities
2137 arising are sometimes masked because their conflicts are resolved due to
2138 the extra precedence or associativity information. Furthermore, it can
2139 hinder the comprehension of a new grammar: one will wonder about the role
2140 of a precedence, where in fact it is useless. The following changes aim
2141 at detecting and reporting these extra directives.
2143 *** Precedence warning category
2145 A new category of warning, -Wprecedence, was introduced. It flags the
2146 useless precedence and associativity directives.
2148 *** Useless associativity
2150 Bison now warns about symbols with a declared associativity that is never
2151 used to resolve conflicts. In that case, using %precedence is sufficient;
2152 the parsing tables will remain unchanged. Solving these warnings may raise
2153 useless precedence warnings, as the symbols no longer have associativity.
2167 warning: useless associativity for '+', use %precedence [-Wprecedence]
2171 *** Useless precedence
2173 Bison now warns about symbols with a declared precedence and no declared
2174 associativity (i.e., declared with %precedence), and whose precedence is
2175 never used. In that case, the symbol can be safely declared with %token
2176 instead, without modifying the parsing tables. For example:
2180 exp: "var" '=' "number";
2184 warning: useless precedence for '=' [-Wprecedence]
2188 *** Useless precedence and associativity
2190 In case of both useless precedence and associativity, the issue is flagged
2195 exp: "var" '=' "number";
2199 warning: useless precedence and associativity for '=' [-Wprecedence]
2205 With help from Joel E. Denny and Gabriel Rassoul.
2207 Empty rules (i.e., with an empty right-hand side) can now be explicitly
2208 marked by the new %empty directive. Using %empty on a non-empty rule is
2209 an error. The new -Wempty-rule warning reports empty rules without
2210 %empty. On the following grammar:
2220 3.4-5: warning: empty rule without %empty [-Wempty-rule]
2223 5.8-13: error: %empty on non-empty rule
2227 ** Java skeleton improvements
2229 The constants for token names were moved to the Lexer interface. Also, it
2230 is possible to add code to the parser's constructors using "%code init"
2231 and "%define init_throws".
2232 Contributed by Paolo Bonzini.
2234 The Java skeleton now supports push parsing.
2235 Contributed by Dennis Heimbigner.
2237 ** C++ skeletons improvements
2239 *** The parser header is no longer mandatory (lalr1.cc, glr.cc)
2241 Using %defines is now optional. Without it, the needed support classes
2242 are defined in the generated parser, instead of additional files (such as
2243 location.hh, position.hh and stack.hh).
2245 *** Locations are no longer mandatory (lalr1.cc, glr.cc)
2247 Both lalr1.cc and glr.cc no longer require %location.
2249 *** syntax_error exception (lalr1.cc)
2251 The C++ parser features a syntax_error exception, which can be
2252 thrown from the scanner or from user rules to raise syntax errors.
2253 This facilitates reporting errors caught in sub-functions (e.g.,
2254 rejecting too large integral literals from a conversion function
2255 used by the scanner, or rejecting invalid combinations from a
2256 factory invoked by the user actions).
2258 *** %define api.value.type variant
2260 This is based on a submission from Michiel De Wilde. With help
2261 from Théophile Ranquet.
2263 In this mode, complex C++ objects can be used as semantic values. For
2266 %token <::std::string> TEXT;
2267 %token <int> NUMBER;
2268 %token SEMICOLON ";"
2269 %type <::std::string> item;
2270 %type <::std::list<std::string>> list;
2273 list { std::cout << $1 << std::endl; }
2277 %empty { /* Generates an empty string list. */ }
2278 | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
2282 TEXT { std::swap ($$, $1); }
2283 | NUMBER { $$ = string_cast ($1); }
2286 *** %define api.token.constructor
2288 When variants are enabled, Bison can generate functions to build the
2289 tokens. This guarantees that the token type (e.g., NUMBER) is consistent
2290 with the semantic value (e.g., int):
2292 parser::symbol_type yylex ()
2294 parser::location_type loc = ...;
2296 return parser::make_TEXT ("Hello, world!", loc);
2298 return parser::make_NUMBER (42, loc);
2300 return parser::make_SEMICOLON (loc);
2306 There are operator- and operator-= for 'location'. Negative line/column
2307 increments can no longer underflow the resulting value.
2310 * Noteworthy changes in release 2.7.1 (2013-04-15) [stable]
2314 *** Fix compiler attribute portability (yacc.c)
2316 With locations enabled, __attribute__ was used unprotected.
2318 *** Fix some compiler warnings (lalr1.cc)
2321 * Noteworthy changes in release 2.7 (2012-12-12) [stable]
2325 Warnings about uninitialized yylloc in yyparse have been fixed.
2327 Restored C90 compliance (yet no report was ever made).
2329 ** Diagnostics are improved
2331 Contributed by Théophile Ranquet.
2333 *** Changes in the format of error messages
2335 This used to be the format of many error reports:
2337 input.y:2.7-12: %type redeclaration for exp
2338 input.y:1.7-12: previous declaration
2342 input.y:2.7-12: error: %type redeclaration for exp
2343 input.y:1.7-12: previous declaration
2345 *** New format for error reports: carets
2347 Caret errors have been added to Bison:
2349 input.y:2.7-12: error: %type redeclaration for exp
2352 input.y:1.7-12: previous declaration
2358 input.y:3.20-23: error: ambiguous reference: '$exp'
2359 exp: exp '+' exp { $exp = $1 + $3; };
2361 input.y:3.1-3: refers to: $exp at $$
2362 exp: exp '+' exp { $exp = $1 + $3; };
2364 input.y:3.6-8: refers to: $exp at $1
2365 exp: exp '+' exp { $exp = $1 + $3; };
2367 input.y:3.14-16: refers to: $exp at $3
2368 exp: exp '+' exp { $exp = $1 + $3; };
2371 The default behavior for now is still not to display these unless
2372 explicitly asked with -fcaret (or -fall). However, in a later release, it
2373 will be made the default behavior (but may still be deactivated with
2376 ** New value for %define variable: api.pure full
2378 The %define variable api.pure requests a pure (reentrant) parser. However,
2379 for historical reasons, using it in a location-tracking Yacc parser
2380 resulted in a yyerror function that did not take a location as a
2381 parameter. With this new value, the user may request a better pure parser,
2382 where yyerror does take a location as a parameter (in location-tracking
2385 The use of "%define api.pure true" is deprecated in favor of this new
2386 "%define api.pure full".
2388 ** New %define variable: api.location.type (glr.cc, lalr1.cc, lalr1.java)
2390 The %define variable api.location.type defines the name of the type to use
2391 for locations. When defined, Bison no longer generates the position.hh
2392 and location.hh files, nor does the parser will include them: the user is
2393 then responsible to define her type.
2395 This can be used in programs with several parsers to factor their location
2396 and position files: let one of them generate them, and the others just use
2399 This feature was actually introduced, but not documented, in Bison 2.5,
2400 under the name "location_type" (which is maintained for backward
2403 For consistency, lalr1.java's %define variables location_type and
2404 position_type are deprecated in favor of api.location.type and
2407 ** Exception safety (lalr1.cc)
2409 The parse function now catches exceptions, uses the %destructors to
2410 release memory (the lookahead symbol and the symbols pushed on the stack)
2411 before re-throwing the exception.
2413 This feature is somewhat experimental. User feedback would be
2416 ** Graph improvements in DOT and XSLT
2418 Contributed by Théophile Ranquet.
2420 The graphical presentation of the states is more readable: their shape is
2421 now rectangular, the state number is clearly displayed, and the items are
2422 numbered and left-justified.
2424 The reductions are now explicitly represented as transitions to other
2425 diamond shaped nodes.
2427 These changes are present in both --graph output and xml2dot.xsl XSLT
2428 processing, with minor (documented) differences.
2430 ** %language is no longer an experimental feature.
2432 The introduction of this feature, in 2.4, was four years ago. The
2433 --language option and the %language directive are no longer experimental.
2437 The sections about shift/reduce and reduce/reduce conflicts resolution
2438 have been fixed and extended.
2440 Although introduced more than four years ago, XML and Graphviz reports
2441 were not properly documented.
2443 The translation of midrule actions is now described.
2446 * Noteworthy changes in release 2.6.5 (2012-11-07) [stable]
2448 We consider compiler warnings about Bison generated parsers to be bugs.
2449 Rather than working around them in your own project, please consider
2450 reporting them to us.
2454 Warnings about uninitialized yylval and/or yylloc for push parsers with a
2455 pure interface have been fixed for GCC 4.0 up to 4.8, and Clang 2.9 to
2458 Other issues in the test suite have been addressed.
2460 Null characters are correctly displayed in error messages.
2462 When possible, yylloc is correctly initialized before calling yylex. It
2463 is no longer necessary to initialize it in the %initial-action.
2466 * Noteworthy changes in release 2.6.4 (2012-10-23) [stable]
2468 Bison 2.6.3's --version was incorrect. This release fixes this issue.
2471 * Noteworthy changes in release 2.6.3 (2012-10-22) [stable]
2475 Bugs and portability issues in the test suite have been fixed.
2477 Some errors in translations have been addressed, and --help now directs
2478 users to the appropriate place to report them.
2480 Stray Info files shipped by accident are removed.
2482 Incorrect definitions of YY_, issued by yacc.c when no parser header is
2483 generated, are removed.
2485 All the generated headers are self-contained.
2487 ** Header guards (yacc.c, glr.c, glr.cc)
2489 In order to avoid collisions, the header guards are now
2490 YY_<PREFIX>_<FILE>_INCLUDED, instead of merely <PREFIX>_<FILE>.
2491 For instance the header generated from
2493 %define api.prefix "calc"
2494 %defines "lib/parse.h"
2496 will use YY_CALC_LIB_PARSE_H_INCLUDED as guard.
2498 ** Fix compiler warnings in the generated parser (yacc.c, glr.c)
2500 The compilation of pure parsers (%define api.pure) can trigger GCC
2503 input.c: In function 'yyparse':
2504 input.c:1503:12: warning: 'yylval' may be used uninitialized in this
2505 function [-Wmaybe-uninitialized]
2509 This is now fixed; pragmas to avoid these warnings are no longer needed.
2511 Warnings from clang ("equality comparison with extraneous parentheses" and
2512 "function declared 'noreturn' should not return") have also been
2516 * Noteworthy changes in release 2.6.2 (2012-08-03) [stable]
2520 Buffer overruns, complaints from Flex, and portability issues in the test
2521 suite have been fixed.
2523 ** Spaces in %lex- and %parse-param (lalr1.cc, glr.cc)
2525 Trailing end-of-lines in %parse-param or %lex-param would result in
2526 invalid C++. This is fixed.
2528 ** Spurious spaces and end-of-lines
2530 The generated files no longer end (nor start) with empty lines.
2533 * Noteworthy changes in release 2.6.1 (2012-07-30) [stable]
2535 Bison no longer executes user-specified M4 code when processing a grammar.
2539 In addition to the removal of the features announced in Bison 2.6, the
2540 next major release will remove the "Temporary hack for adding a semicolon
2541 to the user action", as announced in the release 2.5. Instead of:
2543 exp: exp "+" exp { $$ = $1 + $3 };
2547 exp: exp "+" exp { $$ = $1 + $3; };
2551 *** Type names are now properly escaped.
2553 *** glr.cc: set_debug_level and debug_level work as expected.
2555 *** Stray @ or $ in actions
2557 While Bison used to warn about stray $ or @ in action rules, it did not
2558 for other actions such as printers, destructors, or initial actions. It
2561 ** Type names in actions
2563 For consistency with rule actions, it is now possible to qualify $$ by a
2564 type-name in destructors, printers, and initial actions. For instance:
2566 %printer { fprintf (yyo, "(%d, %f)", $<ival>$, $<fval>$); } <*> <>;
2568 will display two values for each typed and untyped symbol (provided
2569 that YYSTYPE has both "ival" and "fval" fields).
2572 * Noteworthy changes in release 2.6 (2012-07-19) [stable]
2576 The next major release of Bison will drop support for the following
2577 deprecated features. Please report disagreements to bug-bison@gnu.org.
2581 Support for generating parsers in K&R C will be removed. Parsers
2582 generated for C support ISO C90, and are tested with ISO C99 and ISO C11
2585 *** Features deprecated since Bison 1.875
2587 The definitions of yystype and yyltype will be removed; use YYSTYPE and
2590 YYPARSE_PARAM and YYLEX_PARAM, deprecated in favor of %parse-param and
2591 %lex-param, will no longer be supported.
2593 Support for the preprocessor symbol YYERROR_VERBOSE will be removed, use
2596 *** The generated header will be included (yacc.c)
2598 Instead of duplicating the content of the generated header (definition of
2599 YYSTYPE, yyparse declaration etc.), the generated parser will include it,
2600 as is already the case for GLR or C++ parsers. This change is deferred
2601 because existing versions of ylwrap (e.g., Automake 1.12.1) do not support
2604 ** Generated Parser Headers
2606 *** Guards (yacc.c, glr.c, glr.cc)
2608 The generated headers are now guarded, as is already the case for C++
2609 parsers (lalr1.cc). For instance, with --defines=foo.h:
2614 #endif /* !YY_FOO_H */
2616 *** New declarations (yacc.c, glr.c)
2618 The generated header now declares yydebug and yyparse. Both honor
2619 --name-prefix=bar_, and yield
2621 int bar_parse (void);
2625 #define yyparse bar_parse
2628 in order to facilitate the inclusion of several parser headers inside a
2629 single compilation unit.
2631 *** Exported symbols in C++
2633 The symbols YYTOKEN_TABLE and YYERROR_VERBOSE, which were defined in the
2634 header, are removed, as they prevent the possibility of including several
2635 generated headers from a single compilation unit.
2639 For the same reasons, the undocumented and unused macro YYLSP_NEEDED is no
2642 ** New %define variable: api.prefix
2644 Now that the generated headers are more complete and properly protected
2645 against multiple inclusions, constant names, such as YYSTYPE are a
2646 problem. While yyparse and others are properly renamed by %name-prefix,
2647 YYSTYPE, YYDEBUG and others have never been affected by it. Because it
2648 would introduce backward compatibility issues in projects not expecting
2649 YYSTYPE to be renamed, instead of changing the behavior of %name-prefix,
2650 it is deprecated in favor of a new %define variable: api.prefix.
2652 The following examples compares both:
2654 %name-prefix "bar_" | %define api.prefix "bar_"
2655 %token <ival> FOO %token <ival> FOO
2656 %union { int ival; } %union { int ival; }
2662 #ifndef BAR_FOO_H #ifndef BAR_FOO_H
2663 # define BAR_FOO_H # define BAR_FOO_H
2665 /* Enabling traces. */ /* Enabling traces. */
2666 # ifndef YYDEBUG | # ifndef BAR_DEBUG
2667 > # if defined YYDEBUG
2669 > # define BAR_DEBUG 1
2671 > # define BAR_DEBUG 0
2674 # define YYDEBUG 0 | # define BAR_DEBUG 0
2678 # if YYDEBUG | # if BAR_DEBUG
2679 extern int bar_debug; extern int bar_debug;
2682 /* Tokens. */ /* Tokens. */
2683 # ifndef YYTOKENTYPE | # ifndef BAR_TOKENTYPE
2684 # define YYTOKENTYPE | # define BAR_TOKENTYPE
2685 enum yytokentype { | enum bar_tokentype {
2690 #if ! defined YYSTYPE \ | #if ! defined BAR_STYPE \
2691 && ! defined YYSTYPE_IS_DECLARED | && ! defined BAR_STYPE_IS_DECLARED
2692 typedef union YYSTYPE | typedef union BAR_STYPE
2695 } YYSTYPE; | } BAR_STYPE;
2696 # define YYSTYPE_IS_DECLARED 1 | # define BAR_STYPE_IS_DECLARED 1
2699 extern YYSTYPE bar_lval; | extern BAR_STYPE bar_lval;
2701 int bar_parse (void); int bar_parse (void);
2703 #endif /* !BAR_FOO_H */ #endif /* !BAR_FOO_H */
2706 * Noteworthy changes in release 2.5.1 (2012-06-05) [stable]
2710 The next major release will drop support for generating parsers in K&R C.
2712 ** yacc.c: YYBACKUP works as expected.
2714 ** glr.c improvements:
2716 *** Location support is eliminated when not requested:
2718 GLR parsers used to include location-related code even when locations were
2719 not requested, and therefore not even usable.
2721 *** __attribute__ is preserved:
2723 __attribute__ is no longer disabled when __STRICT_ANSI__ is defined (i.e.,
2724 when -std is passed to GCC).
2726 ** lalr1.java: several fixes:
2728 The Java parser no longer throws ArrayIndexOutOfBoundsException if the
2729 first token leads to a syntax error. Some minor clean ups.
2733 *** C++11 compatibility:
2735 C and C++ parsers use "nullptr" instead of "0" when __cplusplus is 201103L
2740 The header files such as "parser.hh", "location.hh", etc. used a constant
2741 name for preprocessor guards, for instance:
2743 #ifndef BISON_LOCATION_HH
2744 # define BISON_LOCATION_HH
2746 #endif // !BISON_LOCATION_HH
2748 The inclusion guard is now computed from "PREFIX/FILE-NAME", where lower
2749 case characters are converted to upper case, and series of
2750 non-alphanumerical characters are converted to an underscore.
2752 With "bison -o lang++/parser.cc", "location.hh" would now include:
2754 #ifndef YY_LANG_LOCATION_HH
2755 # define YY_LANG_LOCATION_HH
2757 #endif // !YY_LANG_LOCATION_HH
2761 The position and location constructors (and their initialize methods)
2762 accept new arguments for line and column. Several issues in the
2763 documentation were fixed.
2765 ** liby is no longer asking for "rpl_fprintf" on some platforms.
2767 ** Changes in the manual:
2769 *** %printer is documented
2771 The "%printer" directive, supported since at least Bison 1.50, is finally
2772 documented. The "mfcalc" example is extended to demonstrate it.
2774 For consistency with the C skeletons, the C++ parsers now also support
2775 "yyoutput" (as an alias to "debug_stream ()").
2777 *** Several improvements have been made:
2779 The layout for grammar excerpts was changed to a more compact scheme.
2780 Named references are motivated. The description of the automaton
2781 description file (*.output) is updated to the current format. Incorrect
2782 index entries were fixed. Some other errors were fixed.
2786 *** Conflicting prototypes with recent/modified Flex.
2788 Fixed build problems with the current, unreleased, version of Flex, and
2789 some modified versions of 2.5.35, which have modified function prototypes.
2791 *** Warnings during the build procedure have been eliminated.
2793 *** Several portability problems in the test suite have been fixed:
2795 This includes warnings with some compilers, unexpected behavior of tools
2796 such as diff, warning messages from the test suite itself, etc.
2798 *** The install-pdf target works properly:
2800 Running "make install-pdf" (or -dvi, -html, -info, and -ps) no longer
2801 halts in the middle of its course.
2804 * Noteworthy changes in release 2.5 (2011-05-14)
2806 ** Grammar symbol names can now contain non-initial dashes:
2808 Consistently with directives (such as %error-verbose) and with
2809 %define variables (e.g. push-pull), grammar symbol names may contain
2810 dashes in any position except the beginning. This is a GNU
2811 extension over POSIX Yacc. Thus, use of this extension is reported
2812 by -Wyacc and rejected in Yacc mode (--yacc).
2814 ** Named references:
2816 Historically, Yacc and Bison have supported positional references
2817 ($n, $$) to allow access to symbol values from inside of semantic
2820 Starting from this version, Bison can also accept named references.
2821 When no ambiguity is possible, original symbol names may be used
2822 as named references:
2824 if_stmt : "if" cond_expr "then" then_stmt ';'
2825 { $if_stmt = mk_if_stmt($cond_expr, $then_stmt); }
2827 In the more common case, explicit names may be declared:
2829 stmt[res] : "if" expr[cond] "then" stmt[then] "else" stmt[else] ';'
2830 { $res = mk_if_stmt($cond, $then, $else); }
2832 Location information is also accessible using @name syntax. When
2833 accessing symbol names containing dots or dashes, explicit bracketing
2834 ($[sym.1]) must be used.
2836 These features are experimental in this version. More user feedback
2837 will help to stabilize them.
2838 Contributed by Alex Rozenman.
2840 ** IELR(1) and canonical LR(1):
2842 IELR(1) is a minimal LR(1) parser table generation algorithm. That
2843 is, given any context-free grammar, IELR(1) generates parser tables
2844 with the full language-recognition power of canonical LR(1) but with
2845 nearly the same number of parser states as LALR(1). This reduction
2846 in parser states is often an order of magnitude. More importantly,
2847 because canonical LR(1)'s extra parser states may contain duplicate
2848 conflicts in the case of non-LR(1) grammars, the number of conflicts
2849 for IELR(1) is often an order of magnitude less as well. This can
2850 significantly reduce the complexity of developing of a grammar.
2852 Bison can now generate IELR(1) and canonical LR(1) parser tables in
2853 place of its traditional LALR(1) parser tables, which remain the
2854 default. You can specify the type of parser tables in the grammar
2855 file with these directives:
2857 %define lr.type lalr
2858 %define lr.type ielr
2859 %define lr.type canonical-lr
2861 The default-reduction optimization in the parser tables can also be
2862 adjusted using "%define lr.default-reductions". For details on both
2863 of these features, see the new section "Tuning LR" in the Bison
2866 These features are experimental. More user feedback will help to
2869 ** LAC (Lookahead Correction) for syntax error handling
2871 Contributed by Joel E. Denny.
2873 Canonical LR, IELR, and LALR can suffer from a couple of problems
2874 upon encountering a syntax error. First, the parser might perform
2875 additional parser stack reductions before discovering the syntax
2876 error. Such reductions can perform user semantic actions that are
2877 unexpected because they are based on an invalid token, and they
2878 cause error recovery to begin in a different syntactic context than
2879 the one in which the invalid token was encountered. Second, when
2880 verbose error messages are enabled (with %error-verbose or the
2881 obsolete "#define YYERROR_VERBOSE"), the expected token list in the
2882 syntax error message can both contain invalid tokens and omit valid
2885 The culprits for the above problems are %nonassoc, default
2886 reductions in inconsistent states, and parser state merging. Thus,
2887 IELR and LALR suffer the most. Canonical LR can suffer only if
2888 %nonassoc is used or if default reductions are enabled for
2889 inconsistent states.
2891 LAC is a new mechanism within the parsing algorithm that solves
2892 these problems for canonical LR, IELR, and LALR without sacrificing
2893 %nonassoc, default reductions, or state merging. When LAC is in
2894 use, canonical LR and IELR behave almost exactly the same for both
2895 syntactically acceptable and syntactically unacceptable input.
2896 While LALR still does not support the full language-recognition
2897 power of canonical LR and IELR, LAC at least enables LALR's syntax
2898 error handling to correctly reflect LALR's language-recognition
2901 Currently, LAC is only supported for deterministic parsers in C.
2902 You can enable LAC with the following directive:
2904 %define parse.lac full
2906 See the new section "LAC" in the Bison manual for additional
2907 details including a few caveats.
2909 LAC is an experimental feature. More user feedback will help to
2912 ** %define improvements:
2914 *** Can now be invoked via the command line:
2916 Each of these command-line options
2919 --define=NAME[=VALUE]
2922 --force-define=NAME[=VALUE]
2924 is equivalent to this grammar file declaration
2926 %define NAME ["VALUE"]
2928 except that the manner in which Bison processes multiple definitions
2929 for the same NAME differs. Most importantly, -F and --force-define
2930 quietly override %define, but -D and --define do not. For further
2931 details, see the section "Bison Options" in the Bison manual.
2933 *** Variables renamed:
2935 The following %define variables
2938 lr.keep_unreachable_states
2940 have been renamed to
2943 lr.keep-unreachable-states
2945 The old names are now deprecated but will be maintained indefinitely
2946 for backward compatibility.
2948 *** Values no longer need to be quoted in the grammar file:
2950 If a %define value is an identifier, it no longer needs to be placed
2951 within quotations marks. For example,
2953 %define api.push-pull "push"
2957 %define api.push-pull push
2959 *** Unrecognized variables are now errors not warnings.
2961 *** Multiple invocations for any variable is now an error not a warning.
2963 ** Unrecognized %code qualifiers are now errors not warnings.
2965 ** Character literals not of length one:
2967 Previously, Bison quietly converted all character literals to length
2968 one. For example, without warning, Bison interpreted the operators in
2969 the following grammar to be the same token:
2975 Bison now warns when a character literal is not of length one. In
2976 some future release, Bison will start reporting an error instead.
2978 ** Destructor calls fixed for lookaheads altered in semantic actions:
2980 Previously for deterministic parsers in C, if a user semantic action
2981 altered yychar, the parser in some cases used the old yychar value to
2982 determine which destructor to call for the lookahead upon a syntax
2983 error or upon parser return. This bug has been fixed.
2985 ** C++ parsers use YYRHSLOC:
2987 Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
2988 macro and use it in the default YYLLOC_DEFAULT. You are encouraged
2989 to use it. If, for instance, your location structure has "first"
2990 and "last" members, instead of
2992 # define YYLLOC_DEFAULT(Current, Rhs, N) \
2996 (Current).first = (Rhs)[1].location.first; \
2997 (Current).last = (Rhs)[N].location.last; \
3001 (Current).first = (Current).last = (Rhs)[0].location.last; \
3007 # define YYLLOC_DEFAULT(Current, Rhs, N) \
3011 (Current).first = YYRHSLOC (Rhs, 1).first; \
3012 (Current).last = YYRHSLOC (Rhs, N).last; \
3016 (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last; \
3020 ** YYLLOC_DEFAULT in C++:
3022 The default implementation of YYLLOC_DEFAULT used to be issued in
3023 the header file. It is now output in the implementation file, after
3024 the user %code sections so that its #ifndef guard does not try to
3025 override the user's YYLLOC_DEFAULT if provided.
3027 ** YYFAIL now produces warnings and Java parsers no longer implement it:
3029 YYFAIL has existed for many years as an undocumented feature of
3030 deterministic parsers in C generated by Bison. More recently, it was
3031 a documented feature of Bison's experimental Java parsers. As
3032 promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
3033 semantic action now produces a deprecation warning, and Java parsers
3034 no longer implement YYFAIL at all. For further details, including a
3035 discussion of how to suppress C preprocessor warnings about YYFAIL
3036 being unused, see the Bison 2.4.2 NEWS entry.
3038 ** Temporary hack for adding a semicolon to the user action:
3040 Previously, Bison appended a semicolon to every user action for
3041 reductions when the output language defaulted to C (specifically, when
3042 neither %yacc, %language, %skeleton, or equivalent command-line
3043 options were specified). This allowed actions such as
3045 exp: exp "+" exp { $$ = $1 + $3 };
3049 exp: exp "+" exp { $$ = $1 + $3; };
3051 As a first step in removing this misfeature, Bison now issues a
3052 warning when it appends a semicolon. Moreover, in cases where Bison
3053 cannot easily determine whether a semicolon is needed (for example, an
3054 action ending with a cpp directive or a braced compound initializer),
3055 it no longer appends one. Thus, the C compiler might now complain
3056 about a missing semicolon where it did not before. Future releases of
3057 Bison will cease to append semicolons entirely.
3059 ** Verbose syntax error message fixes:
3061 When %error-verbose or the obsolete "#define YYERROR_VERBOSE" is
3062 specified, syntax error messages produced by the generated parser
3063 include the unexpected token as well as a list of expected tokens.
3064 The effect of %nonassoc on these verbose messages has been corrected
3065 in two ways, but a more complete fix requires LAC, described above:
3067 *** When %nonassoc is used, there can exist parser states that accept no
3068 tokens, and so the parser does not always require a lookahead token
3069 in order to detect a syntax error. Because no unexpected token or
3070 expected tokens can then be reported, the verbose syntax error
3071 message described above is suppressed, and the parser instead
3072 reports the simpler message, "syntax error". Previously, this
3073 suppression was sometimes erroneously triggered by %nonassoc when a
3074 lookahead was actually required. Now verbose messages are
3075 suppressed only when all previous lookaheads have already been
3076 shifted or discarded.
3078 *** Previously, the list of expected tokens erroneously included tokens
3079 that would actually induce a syntax error because conflicts for them
3080 were resolved with %nonassoc in the current parser state. Such
3081 tokens are now properly omitted from the list.
3083 *** Expected token lists are still often wrong due to state merging
3084 (from LALR or IELR) and default reductions, which can both add
3085 invalid tokens and subtract valid tokens. Canonical LR almost
3086 completely fixes this problem by eliminating state merging and
3087 default reductions. However, there is one minor problem left even
3088 when using canonical LR and even after the fixes above. That is,
3089 if the resolution of a conflict with %nonassoc appears in a later
3090 parser state than the one at which some syntax error is
3091 discovered, the conflicted token is still erroneously included in
3092 the expected token list. Bison's new LAC implementation,
3093 described above, eliminates this problem and the need for
3094 canonical LR. However, LAC is still experimental and is disabled
3097 ** Java skeleton fixes:
3099 *** A location handling bug has been fixed.
3101 *** The top element of each of the value stack and location stack is now
3102 cleared when popped so that it can be garbage collected.
3104 *** Parser traces now print the top element of the stack.
3106 ** -W/--warnings fixes:
3108 *** Bison now properly recognizes the "no-" versions of categories:
3110 For example, given the following command line, Bison now enables all
3111 warnings except warnings for incompatibilities with POSIX Yacc:
3113 bison -Wall,no-yacc gram.y
3115 *** Bison now treats S/R and R/R conflicts like other warnings:
3117 Previously, conflict reports were independent of Bison's normal
3118 warning system. Now, Bison recognizes the warning categories
3119 "conflicts-sr" and "conflicts-rr". This change has important
3120 consequences for the -W and --warnings command-line options. For
3123 bison -Wno-conflicts-sr gram.y # S/R conflicts not reported
3124 bison -Wno-conflicts-rr gram.y # R/R conflicts not reported
3125 bison -Wnone gram.y # no conflicts are reported
3126 bison -Werror gram.y # any conflict is an error
3128 However, as before, if the %expect or %expect-rr directive is
3129 specified, an unexpected number of conflicts is an error, and an
3130 expected number of conflicts is not reported, so -W and --warning
3131 then have no effect on the conflict report.
3133 *** The "none" category no longer disables a preceding "error":
3135 For example, for the following command line, Bison now reports
3136 errors instead of warnings for incompatibilities with POSIX Yacc:
3138 bison -Werror,none,yacc gram.y
3140 *** The "none" category now disables all Bison warnings:
3142 Previously, the "none" category disabled only Bison warnings for
3143 which there existed a specific -W/--warning category. However,
3144 given the following command line, Bison is now guaranteed to
3145 suppress all warnings:
3149 ** Precedence directives can now assign token number 0:
3151 Since Bison 2.3b, which restored the ability of precedence
3152 directives to assign token numbers, doing so for token number 0 has
3153 produced an assertion failure. For example:
3157 This bug has been fixed.
3160 * Noteworthy changes in release 2.4.3 (2010-08-05)
3162 ** Bison now obeys -Werror and --warnings=error for warnings about
3163 grammar rules that are useless in the parser due to conflicts.
3165 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
3168 ** Failures in the test suite for GCC 4.5 have been fixed.
3170 ** Failures in the test suite for some versions of Sun Studio C++ have
3173 ** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
3174 warnings about undefined %prec identifiers will not be converted to
3175 errors in Bison 2.5. They will remain warnings, which should be
3176 sufficient for POSIX while avoiding backward compatibility issues.
3178 ** Minor documentation fixes.
3181 * Noteworthy changes in release 2.4.2 (2010-03-20)
3183 ** Some portability problems that resulted in failures and livelocks
3184 in the test suite on some versions of at least Solaris, AIX, HP-UX,
3185 RHEL4, and Tru64 have been addressed. As a result, fatal Bison
3186 errors should no longer cause M4 to report a broken pipe on the
3189 ** "%prec IDENTIFIER" requires IDENTIFIER to be defined separately.
3191 POSIX specifies that an error be reported for any identifier that does
3192 not appear on the LHS of a grammar rule and that is not defined by
3193 %token, %left, %right, or %nonassoc. Bison 2.3b and later lost this
3194 error report for the case when an identifier appears only after a
3195 %prec directive. It is now restored. However, for backward
3196 compatibility with recent Bison releases, it is only a warning for
3197 now. In Bison 2.5 and later, it will return to being an error.
3198 [Between the 2.4.2 and 2.4.3 releases, it was decided that this
3199 warning will not be converted to an error in Bison 2.5.]
3201 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
3203 ** Warnings from gcc's -Wundef option about undefined YYENABLE_NLS,
3204 YYLTYPE_IS_TRIVIAL, and __STRICT_ANSI__ in C/C++ parsers are now
3207 ** %code is now a permanent feature.
3209 A traditional Yacc prologue directive is written in the form:
3213 To provide a more flexible alternative, Bison 2.3b introduced the
3214 %code directive with the following forms for C/C++:
3217 %code requires {CODE}
3218 %code provides {CODE}
3221 These forms are now considered permanent features of Bison. See the
3222 %code entries in the section "Bison Declaration Summary" in the Bison
3223 manual for a summary of their functionality. See the section
3224 "Prologue Alternatives" for a detailed discussion including the
3225 advantages of %code over the traditional Yacc prologue directive.
3227 Bison's Java feature as a whole including its current usage of %code
3228 is still considered experimental.
3230 ** YYFAIL is deprecated and will eventually be removed.
3232 YYFAIL has existed for many years as an undocumented feature of
3233 deterministic parsers in C generated by Bison. Previously, it was
3234 documented for Bison's experimental Java parsers. YYFAIL is no longer
3235 documented for Java parsers and is formally deprecated in both cases.
3236 Users are strongly encouraged to migrate to YYERROR, which is
3239 Like YYERROR, you can invoke YYFAIL from a semantic action in order to
3240 induce a syntax error. The most obvious difference from YYERROR is
3241 that YYFAIL will automatically invoke yyerror to report the syntax
3242 error so that you don't have to. However, there are several other
3243 subtle differences between YYERROR and YYFAIL, and YYFAIL suffers from
3244 inherent flaws when %error-verbose or "#define YYERROR_VERBOSE" is
3245 used. For a more detailed discussion, see:
3247 http://lists.gnu.org/archive/html/bison-patches/2009-12/msg00024.html
3249 The upcoming Bison 2.5 will remove YYFAIL from Java parsers, but
3250 deterministic parsers in C will continue to implement it. However,
3251 because YYFAIL is already flawed, it seems futile to try to make new
3252 Bison features compatible with it. Thus, during parser generation,
3253 Bison 2.5 will produce a warning whenever it discovers YYFAIL in a
3254 rule action. In a later release, YYFAIL will be disabled for
3255 %error-verbose and "#define YYERROR_VERBOSE". Eventually, YYFAIL will
3256 be removed altogether.
3258 There exists at least one case where Bison 2.5's YYFAIL warning will
3259 be a false positive. Some projects add phony uses of YYFAIL and other
3260 Bison-defined macros for the sole purpose of suppressing C
3261 preprocessor warnings (from GCC cpp's -Wunused-macros, for example).
3262 To avoid Bison's future warning, such YYFAIL uses can be moved to the
3263 epilogue (that is, after the second "%%") in the Bison input file. In
3264 this release (2.4.2), Bison already generates its own code to suppress
3265 C preprocessor warnings for YYFAIL, so projects can remove their own
3266 phony uses of YYFAIL if compatibility with Bison releases prior to
3267 2.4.2 is not necessary.
3269 ** Internationalization.
3271 Fix a regression introduced in Bison 2.4: Under some circumstances,
3272 message translations were not installed although supported by the
3276 * Noteworthy changes in release 2.4.1 (2008-12-11)
3278 ** In the GLR defines file, unexpanded M4 macros in the yylval and yylloc
3279 declarations have been fixed.
3281 ** Temporary hack for adding a semicolon to the user action.
3283 Bison used to prepend a trailing semicolon at the end of the user
3284 action for reductions. This allowed actions such as
3286 exp: exp "+" exp { $$ = $1 + $3 };
3290 exp: exp "+" exp { $$ = $1 + $3; };
3292 Some grammars still depend on this "feature". Bison 2.4.1 restores
3293 the previous behavior in the case of C output (specifically, when
3294 neither %language or %skeleton or equivalent command-line options
3295 are used) to leave more time for grammars depending on the old
3296 behavior to be adjusted. Future releases of Bison will disable this
3299 ** A few minor improvements to the Bison manual.
3302 * Noteworthy changes in release 2.4 (2008-11-02)
3304 ** %language is an experimental feature.
3306 We first introduced this feature in test release 2.3b as a cleaner
3307 alternative to %skeleton. Since then, we have discussed the possibility of
3308 modifying its effect on Bison's output file names. Thus, in this release,
3309 we consider %language to be an experimental feature that will likely evolve
3312 ** Forward compatibility with GNU M4 has been improved.
3314 ** Several bugs in the C++ skeleton and the experimental Java skeleton have been
3318 * Noteworthy changes in release 2.3b (2008-05-27)
3320 ** The quotes around NAME that used to be required in the following directive
3323 %define NAME "VALUE"
3325 ** The directive "%pure-parser" is now deprecated in favor of:
3329 which has the same effect except that Bison is more careful to warn about
3330 unreasonable usage in the latter case.
3334 Bison can now generate an LALR(1) parser in C with a push interface. That
3335 is, instead of invoking "yyparse", which pulls tokens from "yylex", you can
3336 push one token at a time to the parser using "yypush_parse", which will
3337 return to the caller after processing each token. By default, the push
3338 interface is disabled. Either of the following directives will enable it:
3340 %define api.push_pull "push" // Just push; does not require yylex.
3341 %define api.push_pull "both" // Push and pull; requires yylex.
3343 See the new section "A Push Parser" in the Bison manual for details.
3345 The current push parsing interface is experimental and may evolve. More user
3346 feedback will help to stabilize it.
3348 ** The -g and --graph options now output graphs in Graphviz DOT format,
3349 not VCG format. Like --graph, -g now also takes an optional FILE argument
3350 and thus cannot be bundled with other short options.
3354 Bison can now generate an LALR(1) parser in Java. The skeleton is
3355 "data/lalr1.java". Consider using the new %language directive instead of
3356 %skeleton to select it.
3358 See the new section "Java Parsers" in the Bison manual for details.
3360 The current Java interface is experimental and may evolve. More user
3361 feedback will help to stabilize it.
3362 Contributed by Paolo Bonzini.
3366 This new directive specifies the programming language of the generated
3367 parser, which can be C (the default), C++, or Java. Besides the skeleton
3368 that Bison uses, the directive affects the names of the generated files if
3369 the grammar file's name ends in ".y".
3371 ** XML Automaton Report
3373 Bison can now generate an XML report of the LALR(1) automaton using the new
3374 "--xml" option. The current XML schema is experimental and may evolve. More
3375 user feedback will help to stabilize it.
3376 Contributed by Wojciech Polak.
3378 ** The grammar file may now specify the name of the parser header file using
3379 %defines. For example:
3383 ** When reporting useless rules, useless nonterminals, and unused terminals,
3384 Bison now employs the terms "useless in grammar" instead of "useless",
3385 "useless in parser" instead of "never reduced", and "unused in grammar"
3386 instead of "unused".
3388 ** Unreachable State Removal
3390 Previously, Bison sometimes generated parser tables containing unreachable
3391 states. A state can become unreachable during conflict resolution if Bison
3392 disables a shift action leading to it from a predecessor state. Bison now:
3394 1. Removes unreachable states.
3396 2. Does not report any conflicts that appeared in unreachable states.
3397 WARNING: As a result, you may need to update %expect and %expect-rr
3398 directives in existing grammar files.
3400 3. For any rule used only in such states, Bison now reports the rule as
3401 "useless in parser due to conflicts".
3403 This feature can be disabled with the following directive:
3405 %define lr.keep_unreachable_states
3407 See the %define entry in the "Bison Declaration Summary" in the Bison manual
3408 for further discussion.
3410 ** Lookahead Set Correction in the ".output" Report
3412 When instructed to generate a ".output" file including lookahead sets
3413 (using "--report=lookahead", for example), Bison now prints each reduction's
3414 lookahead set only next to the associated state's one item that (1) is
3415 associated with the same rule as the reduction and (2) has its dot at the end
3416 of its RHS. Previously, Bison also erroneously printed the lookahead set
3417 next to all of the state's other items associated with the same rule. This
3418 bug affected only the ".output" file and not the generated parser source
3421 ** --report-file=FILE is a new option to override the default ".output" file
3424 ** The "=" that used to be required in the following directives is now
3427 %file-prefix "parser"
3431 ** An Alternative to "%{...%}" -- "%code QUALIFIER {CODE}"
3433 Bison 2.3a provided a new set of directives as a more flexible alternative to
3434 the traditional Yacc prologue blocks. Those have now been consolidated into
3435 a single %code directive with an optional qualifier field, which identifies
3436 the purpose of the code and thus the location(s) where Bison should generate
3439 1. "%code {CODE}" replaces "%after-header {CODE}"
3440 2. "%code requires {CODE}" replaces "%start-header {CODE}"
3441 3. "%code provides {CODE}" replaces "%end-header {CODE}"
3442 4. "%code top {CODE}" replaces "%before-header {CODE}"
3444 See the %code entries in section "Bison Declaration Summary" in the Bison
3445 manual for a summary of the new functionality. See the new section "Prologue
3446 Alternatives" for a detailed discussion including the advantages of %code
3447 over the traditional Yacc prologues.
3449 The prologue alternatives are experimental. More user feedback will help to
3450 determine whether they should become permanent features.
3452 ** Revised warning: unset or unused midrule values
3454 Since Bison 2.2, Bison has warned about midrule values that are set but not
3455 used within any of the actions of the parent rule. For example, Bison warns
3458 exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
3460 Now, Bison also warns about midrule values that are used but not set. For
3461 example, Bison warns about unset $$ in the midrule action in:
3463 exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
3465 However, Bison now disables both of these warnings by default since they
3466 sometimes prove to be false alarms in existing grammars employing the Yacc
3467 constructs $0 or $-N (where N is some positive integer).
3469 To enable these warnings, specify the option "--warnings=midrule-values" or
3470 "-W", which is a synonym for "--warnings=all".
3472 ** Default %destructor or %printer with "<*>" or "<>"
3474 Bison now recognizes two separate kinds of default %destructor's and
3477 1. Place "<*>" in a %destructor/%printer symbol list to define a default
3478 %destructor/%printer for all grammar symbols for which you have formally
3479 declared semantic type tags.
3481 2. Place "<>" in a %destructor/%printer symbol list to define a default
3482 %destructor/%printer for all grammar symbols without declared semantic
3485 Bison no longer supports the "%symbol-default" notation from Bison 2.3a.
3486 "<*>" and "<>" combined achieve the same effect with one exception: Bison no
3487 longer applies any %destructor to a midrule value if that midrule value is
3488 not actually ever referenced using either $$ or $n in a semantic action.
3490 The default %destructor's and %printer's are experimental. More user
3491 feedback will help to determine whether they should become permanent
3494 See the section "Freeing Discarded Symbols" in the Bison manual for further
3497 ** %left, %right, and %nonassoc can now declare token numbers. This is required
3498 by POSIX. However, see the end of section "Operator Precedence" in the Bison
3499 manual for a caveat concerning the treatment of literal strings.
3501 ** The nonfunctional --no-parser, -n, and %no-parser options have been
3502 completely removed from Bison.
3505 * Noteworthy changes in release 2.3a (2006-09-13)
3507 ** Instead of %union, you can define and use your own union type
3508 YYSTYPE if your grammar contains at least one <type> tag.
3509 Your YYSTYPE need not be a macro; it can be a typedef.
3510 This change is for compatibility with other Yacc implementations,
3511 and is required by POSIX.
3513 ** Locations columns and lines start at 1.
3514 In accordance with the GNU Coding Standards and Emacs.
3516 ** You may now declare per-type and default %destructor's and %printer's:
3520 %union { char *string; }
3521 %token <string> STRING1
3522 %token <string> STRING2
3523 %type <string> string1
3524 %type <string> string2
3525 %union { char character; }
3526 %token <character> CHR
3527 %type <character> chr
3528 %destructor { free ($$); } %symbol-default
3529 %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
3530 %destructor { } <character>
3532 guarantees that, when the parser discards any user-defined symbol that has a
3533 semantic type tag other than "<character>", it passes its semantic value to
3534 "free". However, when the parser discards a "STRING1" or a "string1", it
3535 also prints its line number to "stdout". It performs only the second
3536 "%destructor" in this case, so it invokes "free" only once.
3538 [Although we failed to mention this here in the 2.3a release, the default
3539 %destructor's and %printer's were experimental, and they were rewritten in
3542 ** Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with "-y",
3543 "--yacc", or "%yacc"), Bison no longer generates #define statements for
3544 associating token numbers with token names. Removing the #define statements
3545 helps to sanitize the global namespace during preprocessing, but POSIX Yacc
3546 requires them. Bison still generates an enum for token names in all cases.
3548 ** Handling of traditional Yacc prologue blocks is now more consistent but
3549 potentially incompatible with previous releases of Bison.
3551 As before, you declare prologue blocks in your grammar file with the
3552 "%{ ... %}" syntax. To generate the pre-prologue, Bison concatenates all
3553 prologue blocks that you've declared before the first %union. To generate
3554 the post-prologue, Bison concatenates all prologue blocks that you've
3555 declared after the first %union.
3557 Previous releases of Bison inserted the pre-prologue into both the header
3558 file and the code file in all cases except for LALR(1) parsers in C. In the
3559 latter case, Bison inserted it only into the code file. For parsers in C++,
3560 the point of insertion was before any token definitions (which associate
3561 token numbers with names). For parsers in C, the point of insertion was
3562 after the token definitions.
3564 Now, Bison never inserts the pre-prologue into the header file. In the code
3565 file, it always inserts it before the token definitions.
3567 ** Bison now provides a more flexible alternative to the traditional Yacc
3568 prologue blocks: %before-header, %start-header, %end-header, and
3571 For example, the following declaration order in the grammar file reflects the
3572 order in which Bison will output these code blocks. However, you are free to
3573 declare these code blocks in your grammar file in whatever order is most
3577 /* Bison treats this block like a pre-prologue block: it inserts it into
3578 * the code file before the contents of the header file. It does *not*
3579 * insert it into the header file. This is a good place to put
3580 * #include's that you want at the top of your code file. A common
3581 * example is '#include "system.h"'. */
3584 /* Bison inserts this block into both the header file and the code file.
3585 * In both files, the point of insertion is before any Bison-generated
3586 * token, semantic type, location type, and class definitions. This is a
3587 * good place to define %union dependencies, for example. */
3590 /* Unlike the traditional Yacc prologue blocks, the output order for the
3591 * new %*-header blocks is not affected by their declaration position
3592 * relative to any %union in the grammar file. */
3595 /* Bison inserts this block into both the header file and the code file.
3596 * In both files, the point of insertion is after the Bison-generated
3597 * definitions. This is a good place to declare or define public
3598 * functions or data structures that depend on the Bison-generated
3602 /* Bison treats this block like a post-prologue block: it inserts it into
3603 * the code file after the contents of the header file. It does *not*
3604 * insert it into the header file. This is a good place to declare or
3605 * define internal functions or data structures that depend on the
3606 * Bison-generated definitions. */
3609 If you have multiple occurrences of any one of the above declarations, Bison
3610 will concatenate the contents in declaration order.
3612 [Although we failed to mention this here in the 2.3a release, the prologue
3613 alternatives were experimental, and they were rewritten in future versions.]
3615 ** The option "--report=look-ahead" has been changed to "--report=lookahead".
3616 The old spelling still works, but is not documented and may be removed
3617 in a future release.
3620 * Noteworthy changes in release 2.3 (2006-06-05)
3622 ** GLR grammars should now use "YYRECOVERING ()" instead of "YYRECOVERING",
3623 for compatibility with LALR(1) grammars.
3625 ** It is now documented that any definition of YYSTYPE or YYLTYPE should
3626 be to a type name that does not contain parentheses or brackets.
3629 * Noteworthy changes in release 2.2 (2006-05-19)
3631 ** The distribution terms for all Bison-generated parsers now permit
3632 using the parsers in nonfree programs. Previously, this permission
3633 was granted only for Bison-generated LALR(1) parsers in C.
3635 ** %name-prefix changes the namespace name in C++ outputs.
3637 ** The C++ parsers export their token_type.
3639 ** Bison now allows multiple %union declarations, and concatenates
3640 their contents together.
3642 ** New warning: unused values
3643 Right-hand side symbols whose values are not used are reported,
3644 if the symbols have destructors. For instance:
3646 exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
3650 will trigger a warning about $$ and $5 in the first rule, and $3 in
3651 the second ($1 is copied to $$ by the default rule). This example
3652 most likely contains three errors, and could be rewritten as:
3654 exp: exp "?" exp ":" exp
3655 { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
3657 { $$ = $1 ? $1 : $3; if ($1) free ($3); }
3660 However, if the original actions were really intended, memory leaks
3661 and all, the warnings can be suppressed by letting Bison believe the
3662 values are used, e.g.:
3664 exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
3665 | exp "+" exp { $$ = $1; (void) $3; }
3668 If there are midrule actions, the warning is issued if no action
3669 uses it. The following triggers no warning: $1 and $3 are used.
3671 exp: exp { push ($1); } '+' exp { push ($3); sum (); };
3673 The warning is intended to help catching lost values and memory leaks.
3674 If a value is ignored, its associated memory typically is not reclaimed.
3676 ** %destructor vs. YYABORT, YYACCEPT, and YYERROR.
3677 Destructors are now called when user code invokes YYABORT, YYACCEPT,
3678 and YYERROR, for all objects on the stack, other than objects
3679 corresponding to the right-hand side of the current rule.
3681 ** %expect, %expect-rr
3682 Incorrect numbers of expected conflicts are now actual errors,
3683 instead of warnings.
3685 ** GLR, YACC parsers.
3686 The %parse-params are available in the destructors (and the
3687 experimental printers) as per the documentation.
3689 ** Bison now warns if it finds a stray "$" or "@" in an action.
3691 ** %require "VERSION"
3692 This specifies that the grammar file depends on features implemented
3693 in Bison version VERSION or higher.
3695 ** lalr1.cc: The token and value types are now class members.
3696 The tokens were defined as free form enums and cpp macros. YYSTYPE
3697 was defined as a free form union. They are now class members:
3698 tokens are enumerations of the "yy::parser::token" struct, and the
3699 semantic values have the "yy::parser::semantic_type" type.
3701 If you do not want or can update to this scheme, the directive
3702 '%define "global_tokens_and_yystype" "1"' triggers the global
3703 definition of tokens and YYSTYPE. This change is suitable both
3704 for previous releases of Bison, and this one.
3706 If you wish to update, then make sure older version of Bison will
3707 fail using '%require "2.2"'.
3709 ** DJGPP support added.
3712 * Noteworthy changes in release 2.1 (2005-09-16)
3714 ** The C++ lalr1.cc skeleton supports %lex-param.
3716 ** Bison-generated parsers now support the translation of diagnostics like
3717 "syntax error" into languages other than English. The default
3718 language is still English. For details, please see the new
3719 Internationalization section of the Bison manual. Software
3720 distributors should also see the new PACKAGING file. Thanks to
3721 Bruno Haible for this new feature.
3723 ** Wording in the Bison-generated parsers has been changed slightly to
3724 simplify translation. In particular, the message "memory exhausted"
3725 has replaced "parser stack overflow", as the old message was not
3726 always accurate for modern Bison-generated parsers.
3728 ** Destructors are now called when the parser aborts, for all symbols left
3729 behind on the stack. Also, the start symbol is now destroyed after a
3730 successful parse. In both cases, the behavior was formerly inconsistent.
3732 ** When generating verbose diagnostics, Bison-generated parsers no longer
3733 quote the literal strings associated with tokens. For example, for
3734 a syntax error associated with '%token NUM "number"' they might
3735 print 'syntax error, unexpected number' instead of 'syntax error,
3736 unexpected "number"'.
3739 * Noteworthy changes in release 2.0 (2004-12-25)
3741 ** Possibly-incompatible changes
3743 - Bison-generated parsers no longer default to using the alloca function
3744 (when available) to extend the parser stack, due to widespread
3745 problems in unchecked stack-overflow detection. You can "#define
3746 YYSTACK_USE_ALLOCA 1" to require the use of alloca, but please read
3747 the manual to determine safe values for YYMAXDEPTH in that case.
3749 - Error token location.
3750 During error recovery, the location of the syntax error is updated
3751 to cover the whole sequence covered by the error token: it includes
3752 the shifted symbols thrown away during the first part of the error
3753 recovery, and the lookahead rejected during the second part.
3755 - Semicolon changes:
3756 . Stray semicolons are no longer allowed at the start of a grammar.
3757 . Semicolons are now required after in-grammar declarations.
3759 - Unescaped newlines are no longer allowed in character constants or
3760 string literals. They were never portable, and GCC 3.4.0 has
3761 dropped support for them. Better diagnostics are now generated if
3762 forget a closing quote.
3764 - NUL bytes are no longer allowed in Bison string literals, unfortunately.
3768 - GLR grammars now support locations.
3770 - New directive: %initial-action.
3771 This directive allows the user to run arbitrary code (including
3772 initializing @$) from yyparse before parsing starts.
3774 - A new directive "%expect-rr N" specifies the expected number of
3775 reduce/reduce conflicts in GLR parsers.
3777 - %token numbers can now be hexadecimal integers, e.g., "%token FOO 0x12d".
3778 This is a GNU extension.
3780 - The option "--report=lookahead" was changed to "--report=look-ahead".
3781 [However, this was changed back after 2.3.]
3783 - Experimental %destructor support has been added to lalr1.cc.
3785 - New configure option --disable-yacc, to disable installation of the
3786 yacc command and -ly library introduced in 1.875 for POSIX conformance.
3790 - For now, %expect-count violations are now just warnings, not errors.
3791 This is for compatibility with Bison 1.75 and earlier (when there are
3792 reduce/reduce conflicts) and with Bison 1.30 and earlier (when there
3793 are too many or too few shift/reduce conflicts). However, in future
3794 versions of Bison we plan to improve the %expect machinery so that
3795 these violations will become errors again.
3797 - Within Bison itself, numbers (e.g., goto numbers) are no longer
3798 arbitrarily limited to 16-bit counts.
3800 - Semicolons are now allowed before "|" in grammar rules, as POSIX requires.
3803 * Noteworthy changes in release 1.875 (2003-01-01)
3805 ** The documentation license has been upgraded to version 1.2
3806 of the GNU Free Documentation License.
3808 ** syntax error processing
3810 - In Yacc-style parsers YYLLOC_DEFAULT is now used to compute error
3811 locations too. This fixes bugs in error-location computation.
3814 It is now possible to reclaim the memory associated to symbols
3815 discarded during error recovery. This feature is still experimental.
3818 This new directive is preferred over YYERROR_VERBOSE.
3820 - #defining yyerror to steal internal variables is discouraged.
3821 It is not guaranteed to work forever.
3823 ** POSIX conformance
3825 - Semicolons are once again optional at the end of grammar rules.
3826 This reverts to the behavior of Bison 1.33 and earlier, and improves
3827 compatibility with Yacc.
3829 - "parse error" -> "syntax error"
3830 Bison now uniformly uses the term "syntax error"; formerly, the code
3831 and manual sometimes used the term "parse error" instead. POSIX
3832 requires "syntax error" in diagnostics, and it was thought better to
3835 - The documentation now emphasizes that yylex and yyerror must be
3836 declared before use. C99 requires this.
3838 - Bison now parses C99 lexical constructs like UCNs and
3839 backslash-newline within C escape sequences, as POSIX 1003.1-2001 requires.
3841 - File names are properly escaped in C output. E.g., foo\bar.y is
3842 output as "foo\\bar.y".
3844 - Yacc command and library now available
3845 The Bison distribution now installs a "yacc" command, as POSIX requires.
3846 Also, Bison now installs a small library liby.a containing
3847 implementations of Yacc-compatible yyerror and main functions.
3848 This library is normally not useful, but POSIX requires it.
3850 - Type clashes now generate warnings, not errors.
3852 - If the user does not define YYSTYPE as a macro, Bison now declares it
3853 using typedef instead of defining it as a macro.
3854 For consistency, YYLTYPE is also declared instead of defined.
3856 ** Other compatibility issues
3858 - %union directives can now have a tag before the "{", e.g., the
3859 directive "%union foo {...}" now generates the C code
3860 "typedef union foo { ... } YYSTYPE;"; this is for Yacc compatibility.
3861 The default union tag is "YYSTYPE", for compatibility with Solaris 9 Yacc.
3862 For consistency, YYLTYPE's struct tag is now "YYLTYPE" not "yyltype".
3863 This is for compatibility with both Yacc and Bison 1.35.
3865 - ";" is output before the terminating "}" of an action, for
3866 compatibility with Bison 1.35.
3868 - Bison now uses a Yacc-style format for conflict reports, e.g.,
3869 "conflicts: 2 shift/reduce, 1 reduce/reduce".
3871 - "yystype" and "yyltype" are now obsolescent macros instead of being
3872 typedefs or tags; they are no longer documented and are planned to be
3873 withdrawn in a future release.
3878 Users of Bison have to decide how they handle the portability of the
3881 - "parsing stack overflow..." -> "parser stack overflow"
3882 GLR parsers now report "parser stack overflow" as per the Bison manual.
3884 ** %parse-param and %lex-param
3885 The macros YYPARSE_PARAM and YYLEX_PARAM provide a means to pass
3886 additional context to yyparse and yylex. They suffer from several
3889 - a single argument only can be added,
3890 - their types are weak (void *),
3891 - this context is not passed to ancillary functions such as yyerror,
3892 - only yacc.c parsers support them.
3894 The new %parse-param/%lex-param directives provide a more precise control.
3897 %parse-param {int *nastiness}
3898 %lex-param {int *nastiness}
3899 %parse-param {int *randomness}
3901 results in the following signatures:
3903 int yylex (int *nastiness);
3904 int yyparse (int *nastiness, int *randomness);
3906 or, if both %pure-parser and %locations are used:
3908 int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
3909 int yyparse (int *nastiness, int *randomness);
3911 ** Bison now warns if it detects conflicting outputs to the same file,
3912 e.g., it generates a warning for "bison -d -o foo.h foo.y" since
3913 that command outputs both code and header to foo.h.
3915 ** #line in output files
3916 - --no-line works properly.
3918 ** Bison can no longer be built by a K&R C compiler; it requires C89 or
3919 later to be built. This change originally took place a few versions
3920 ago, but nobody noticed until we recently asked someone to try
3921 building Bison with a K&R C compiler.
3924 * Noteworthy changes in release 1.75 (2002-10-14)
3926 ** Bison should now work on 64-bit hosts.
3928 ** Indonesian translation thanks to Tedi Heriyanto.
3931 Fix spurious parse errors.
3934 Some people redefine yyerror to steal yyparse' private variables.
3935 Reenable this trick until an official feature replaces it.
3938 In agreement with POSIX and with other Yaccs, leaving a default
3939 action is valid when $$ is untyped, and $1 typed:
3943 but the converse remains an error:
3947 ** Values of midrule actions
3950 foo: { ... } { $$ = $1; } ...
3952 was incorrectly rejected: $1 is defined in the second midrule
3953 action, and is equal to the $$ of the first midrule action.
3956 * Noteworthy changes in release 1.50 (2002-10-04)
3961 causes Bison to produce a Generalized LR (GLR) parser, capable of handling
3962 almost any context-free grammar, ambiguous or not. The new declarations
3963 %dprec and %merge on grammar rules allow parse-time resolution of
3964 ambiguities. Contributed by Paul Hilfinger.
3966 Unfortunately Bison 1.50 does not work properly on 64-bit hosts
3967 like the Alpha, so please stick to 32-bit hosts for now.
3970 When not in Yacc compatibility mode, when the output file was not
3971 specified, running "bison foo/bar.y" created "foo/bar.c". It
3972 now creates "bar.c".
3975 The undefined token was systematically mapped to 2 which prevented
3976 the use of 2 by the user. This is no longer the case.
3978 ** Unknown token numbers
3979 If yylex returned an out of range value, yyparse could die. This is
3983 According to POSIX, the error token must be 256.
3984 Bison extends this requirement by making it a preference: *if* the
3985 user specified that one of her tokens is numbered 256, then error
3986 will be mapped onto another number.
3988 ** Verbose error messages
3989 They no longer report "..., expecting error or..." for states where
3990 error recovery is possible.
3993 Defaults to "$end" instead of "$".
3995 ** Error recovery now conforms to documentation and to POSIX
3996 When a Bison-generated parser encounters a syntax error, it now pops
3997 the stack until it finds a state that allows shifting the error
3998 token. Formerly, it popped the stack until it found a state that
3999 allowed some non-error action other than a default reduction on the
4000 error token. The new behavior has long been the documented behavior,
4001 and has long been required by POSIX. For more details, please see
4002 Paul Eggert, "Reductions during Bison error handling" (2002-05-20)
4003 <http://lists.gnu.org/archive/html/bug-bison/2002-05/msg00038.html>.
4006 Popped tokens and nonterminals are now reported.
4009 Larger grammars are now supported (larger token numbers, larger grammar
4010 size (= sum of the LHS and RHS lengths), larger LALR tables).
4011 Formerly, many of these numbers ran afoul of 16-bit limits;
4012 now these limits are 32 bits on most hosts.
4014 ** Explicit initial rule
4015 Bison used to play hacks with the initial rule, which the user does
4016 not write. It is now explicit, and visible in the reports and
4020 Before, Bison reported the useless rules, but, although not used,
4021 included them in the parsers. They are now actually removed.
4023 ** Useless rules, useless nonterminals
4024 They are now reported, as a warning, with their locations.
4026 ** Rules never reduced
4027 Rules that can never be reduced because of conflicts are now
4030 ** Incorrect "Token not used"
4031 On a grammar such as
4033 %token useless useful
4035 exp: '0' %prec useful;
4037 where a token was used to set the precedence of the last rule,
4038 bison reported both "useful" and "useless" as useless tokens.
4040 ** Revert the C++ namespace changes introduced in 1.31
4041 as they caused too many portability hassles.
4043 ** Default locations
4044 By an accident of design, the default computation of @$ was
4045 performed after another default computation was performed: @$ = @1.
4046 The latter is now removed: YYLLOC_DEFAULT is fully responsible of
4047 the computation of @$.
4049 ** Token end-of-file
4050 The token end of file may be specified by the user, in which case,
4051 the user symbol is used in the reports, the graphs, and the verbose
4052 error messages instead of "$end", which remains being the default.
4056 %token MYEOF 0 "end of file"
4059 This old option, which has been broken for ages, is removed.
4062 Brazilian Portuguese, thanks to Alexandre Folle de Menezes.
4063 Croatian, thanks to Denis Lackovic.
4065 ** Incorrect token definitions
4068 bison used to output
4071 ** Token definitions as enums
4072 Tokens are output both as the traditional #define's, and, provided
4073 the compiler supports ANSI C or is a C++ compiler, as enums.
4074 This lets debuggers display names instead of integers.
4077 In addition to --verbose, bison supports --report=THINGS, which
4078 produces additional information:
4080 complete the core item sets with their closure
4081 - lookahead [changed to "look-ahead" in 1.875e through 2.3, but changed back]
4082 explicitly associate lookahead tokens to items
4084 describe shift/reduce conflicts solving.
4085 Bison used to systematically output this information on top of
4086 the report. Solved conflicts are now attached to their states.
4089 Previous versions don't complain when there is a type clash on
4090 the default action if the rule has a midrule action, such as in:
4098 ** GNU M4 is now required when using Bison.
4101 * Noteworthy changes in release 1.35 (2002-03-25)
4104 Some projects use Bison's C parser with C++ compilers, and define
4105 YYSTYPE as a class. The recent adjustment of C parsers for data
4106 alignment and 64 bit architectures made this impossible.
4108 Because for the time being no real solution for C++ parser
4109 generation exists, kludges were implemented in the parser to
4110 maintain this use. In the future, when Bison has C++ parsers, this
4111 kludge will be disabled.
4113 This kludge also addresses some C++ problems when the stack was
4117 * Noteworthy changes in release 1.34 (2002-03-12)
4119 ** File name clashes are detected
4120 $ bison foo.y -d -o foo.x
4121 fatal error: header and parser would both be named "foo.x"
4123 ** A missing ";" at the end of a rule triggers a warning
4124 In accordance with POSIX, and in agreement with other
4125 Yacc implementations, Bison will mandate this semicolon in the near
4126 future. This eases the implementation of a Bison parser of Bison
4127 grammars by making this grammar LALR(1) instead of LR(2). To
4128 facilitate the transition, this release introduces a warning.
4130 ** Revert the C++ namespace changes introduced in 1.31, as they caused too
4131 many portability hassles.
4133 ** DJGPP support added.
4135 ** Fix test suite portability problems.
4138 * Noteworthy changes in release 1.33 (2002-02-07)
4141 Groff could not be compiled for the definition of size_t was lacking
4142 under some conditions.
4148 * Noteworthy changes in release 1.32 (2002-01-23)
4150 ** Fix Yacc output file names
4152 ** Portability fixes
4154 ** Italian, Dutch translations
4157 * Noteworthy changes in release 1.31 (2002-01-14)
4161 ** GNU Gettext and %expect
4162 GNU Gettext asserts 10 s/r conflicts, but there are 7. Now that
4163 Bison dies on incorrect %expectations, we fear there will be
4164 too many bug reports for Gettext, so _for the time being_, %expect
4165 does not trigger an error when the input file is named "plural.y".
4167 ** Use of alloca in parsers
4168 If YYSTACK_USE_ALLOCA is defined to 0, then the parsers will use
4169 malloc exclusively. Since 1.29, but was not NEWS'ed.
4171 alloca is used only when compiled with GCC, to avoid portability
4174 ** yyparse now returns 2 if memory is exhausted; formerly it dumped core.
4176 ** When the generated parser lacks debugging code, YYDEBUG is now 0
4177 (as POSIX requires) instead of being undefined.
4180 Bison has always permitted actions such as { $$ = $1 }: it adds the
4181 ending semicolon. Now if in Yacc compatibility mode, the semicolon
4182 is no longer output: one has to write { $$ = $1; }.
4184 ** Better C++ compliance
4185 The output parsers try to respect C++ namespaces.
4186 [This turned out to be a failed experiment, and it was reverted later.]
4189 Fixed bugs when reporting useless nonterminals.
4192 The parsers work properly on 64 bit hosts.
4195 Some calls to strerror resulted in scrambled or missing error messages.
4198 When the number of shift/reduce conflicts is correct, don't issue
4201 ** The verbose report includes the rule line numbers.
4203 ** Rule line numbers are fixed in traces.
4205 ** Swedish translation
4208 Verbose parse error messages from the parsers are better looking.
4209 Before: parse error: unexpected `'/'', expecting `"number"' or `'-'' or `'(''
4210 Now: parse error: unexpected '/', expecting "number" or '-' or '('
4212 ** Fixed parser memory leaks.
4213 When the generated parser was using malloc to extend its stacks, the
4214 previous allocations were not freed.
4216 ** Fixed verbose output file.
4217 Some newlines were missing.
4218 Some conflicts in state descriptions were missing.
4220 ** Fixed conflict report.
4221 Option -v was needed to get the result.
4225 Mismatches are errors, not warnings.
4227 ** Fixed incorrect processing of some invalid input.
4229 ** Fixed CPP guards: 9foo.h uses BISON_9FOO_H instead of 9FOO_H.
4231 ** Fixed some typos in the documentation.
4233 ** %token MY_EOF 0 is supported.
4234 Before, MY_EOF was silently renumbered as 257.
4236 ** doc/refcard.tex is updated.
4238 ** %output, %file-prefix, %name-prefix.
4242 New, aliasing "--output-file".
4245 * Noteworthy changes in release 1.30 (2001-10-26)
4247 ** "--defines" and "--graph" have now an optional argument which is the
4248 output file name. "-d" and "-g" do not change; they do not take any
4251 ** "%source_extension" and "%header_extension" are removed, failed
4254 ** Portability fixes.
4257 * Noteworthy changes in release 1.29 (2001-09-07)
4259 ** The output file does not define const, as this caused problems when used
4260 with common autoconfiguration schemes. If you still use ancient compilers
4261 that lack const, compile with the equivalent of the C compiler option
4262 "-Dconst=". Autoconf's AC_C_CONST macro provides one way to do this.
4264 ** Added "-g" and "--graph".
4266 ** The Bison manual is now distributed under the terms of the GNU FDL.
4268 ** The input and the output files has automatically a similar extension.
4270 ** Russian translation added.
4272 ** NLS support updated; should hopefully be less troublesome.
4274 ** Added the old Bison reference card.
4276 ** Added "--locations" and "%locations".
4278 ** Added "-S" and "--skeleton".
4280 ** "%raw", "-r", "--raw" is disabled.
4282 ** Special characters are escaped when output. This solves the problems
4283 of the #line lines with path names including backslashes.
4286 "%yacc", "%fixed_output_files", "%defines", "%no_parser", "%verbose",
4287 "%debug", "%source_extension" and "%header_extension".
4290 Automatic location tracking.
4293 * Noteworthy changes in release 1.28 (1999-07-06)
4295 ** Should compile better now with K&R compilers.
4299 ** Fixed a problem with escaping the double quote character.
4301 ** There is now a FAQ.
4304 * Noteworthy changes in release 1.27
4306 ** The make rule which prevented bison.simple from being created on
4307 some systems has been fixed.
4310 * Noteworthy changes in release 1.26
4312 ** Bison now uses Automake.
4314 ** New mailing lists: <bug-bison@gnu.org> and <help-bison@gnu.org>.
4316 ** Token numbers now start at 257 as previously documented, not 258.
4318 ** Bison honors the TMPDIR environment variable.
4320 ** A couple of buffer overruns have been fixed.
4322 ** Problems when closing files should now be reported.
4324 ** Generated parsers should now work even on operating systems which do
4325 not provide alloca().
4328 * Noteworthy changes in release 1.25 (1995-10-16)
4330 ** Errors in the input grammar are not fatal; Bison keeps reading
4331 the grammar file, and reports all the errors found in it.
4333 ** Tokens can now be specified as multiple-character strings: for
4334 example, you could use "<=" for a token which looks like <=, instead
4335 of choosing a name like LESSEQ.
4337 ** The %token_table declaration says to write a table of tokens (names
4338 and numbers) into the parser file. The yylex function can use this
4339 table to recognize multiple-character string tokens, or for other
4342 ** The %no_lines declaration says not to generate any #line preprocessor
4343 directives in the parser file.
4345 ** The %raw declaration says to use internal Bison token numbers, not
4346 Yacc-compatible token numbers, when token names are defined as macros.
4348 ** The --no-parser option produces the parser tables without including
4349 the parser engine; a project can now use its own parser engine.
4350 The actions go into a separate file called NAME.act, in the form of
4351 a switch statement body.
4354 * Noteworthy changes in release 1.23
4356 The user can define YYPARSE_PARAM as the name of an argument to be
4357 passed into yyparse. The argument should have type void *. It should
4358 actually point to an object. Grammar actions can access the variable
4359 by casting it to the proper pointer type.
4361 Line numbers in output file corrected.
4364 * Noteworthy changes in release 1.22
4366 --help option added.
4369 * Noteworthy changes in release 1.20
4371 Output file does not redefine const for C++.
4375 LocalWords: yacc YYBACKUP glr GCC lalr ArrayIndexOutOfBoundsException nullptr
4376 LocalWords: cplusplus liby rpl fprintf mfcalc Wyacc stmt cond expr mk sym lr
4377 LocalWords: IELR ielr Lookahead YYERROR nonassoc LALR's api lookaheads yychar
4378 LocalWords: destructor lookahead YYRHSLOC YYLLOC Rhs ifndef YYFAIL cpp sr rr
4379 LocalWords: preprocessor initializer Wno Wnone Werror FreeBSD prec livelocks
4380 LocalWords: Solaris AIX UX RHEL Tru LHS gcc's Wundef YYENABLE NLS YYLTYPE VCG
4381 LocalWords: yyerror cpp's Wunused yylval yylloc prepend yyparse yylex yypush
4382 LocalWords: Graphviz xml nonterminals midrule destructor's YYSTYPE typedef ly
4383 LocalWords: CHR chr printf stdout namespace preprocessing enum pre include's
4384 LocalWords: YYRECOVERING nonfree destructors YYABORT YYACCEPT params enums de
4385 LocalWords: struct yystype DJGPP lex param Haible NUM alloca YYSTACK NUL goto
4386 LocalWords: YYMAXDEPTH Unescaped UCNs YYLTYPE's yyltype typedefs inline Yaccs
4387 LocalWords: Heriyanto Reenable dprec Hilfinger Eggert MYEOF Folle Menezes EOF
4388 LocalWords: Lackovic define's itemset Groff Gettext malloc NEWS'ed YYDEBUG YY
4389 LocalWords: namespaces strerror const autoconfiguration Dconst Autoconf's FDL
4390 LocalWords: Automake TMPDIR LESSEQ ylwrap endif yydebug YYTOKEN YYLSP ival hh
4391 LocalWords: extern YYTOKENTYPE TOKENTYPE yytokentype tokentype STYPE lval pdf
4392 LocalWords: lang yyoutput dvi html ps POSIX lvalp llocp Wother nterm arg init
4393 LocalWords: TOK calc yyo fval Wconflicts parsers yystackp yyval yynerrs
4394 LocalWords: Théophile Ranquet Santet fno fnone stype associativity Tolmer
4395 LocalWords: Wprecedence Rassoul Wempty Paolo Bonzini parser's Michiel loc
4396 LocalWords: redeclaration sval fcaret reentrant XSLT xsl Wmaybe yyvsp Tedi
4397 LocalWords: pragmas noreturn untyped Rozenman unexpanded Wojciech Polak
4398 LocalWords: Alexandre MERCHANTABILITY yytype emplace ptr automove lvalues
4399 LocalWords: nonterminal yy args Pragma dereference yyformat rhs docdir bw
4400 LocalWords: Redeclarations rpcalc Autoconf YFLAGS Makefiles PROG DECL num
4401 LocalWords: Heimbigner AST src ast Makefile srcdir MinGW xxlex XXSTYPE
4402 LocalWords: XXLTYPE strictfp IDEs ffixit fdiagnostics parseable fixits
4403 LocalWords: Wdeprecated yytext Variadic variadic yyrhs yyphrs RCS README
4404 LocalWords: noexcept constexpr ispell american deprecations backend Teoh
4405 LocalWords: YYPRINT Mangold Bonzini's Wdangling exVal baz checkable gcc
4406 LocalWords: fsanitize Vogelsgesang lis redeclared stdint automata yytname
4407 LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
4408 LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
4409 LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
4410 LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
4411 LocalWords: gotos readline Imbimbo Wcounterexamples
4414 ispell-dictionary: "american"
4419 Copyright (C) 1995-2015, 2018-2020 Free Software Foundation, Inc.
4421 This file is part of Bison, the GNU Parser Generator.
4423 Permission is granted to copy, distribute and/or modify this document
4424 under the terms of the GNU Free Documentation License, Version 1.3 or
4425 any later version published by the Free Software Foundation; with no
4426 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
4427 Texts. A copy of the license is included in the "GNU Free
4428 Documentation License" file as part of this distribution.