* Noteworthy changes in release ?.? (????-??-??) [?]
-** Future changes
+** WARNING: Future backward-incompatibilities!
- Bison will stop adding a semicolon at the end of the actions:
+ Bison will stop adding a semicolon at the end of the actions (as announced
+ in the release 2.5):
foo.y:2.22: warning: a ';' might be needed at the end of action code
exp: "num" { $$ = $1 }
for its own code, especially the definition of variables after statements.
The generated C parsers still aim at C90.
-** Incompatible changes
+** Backward incompatible changes
*** Obsolete features
** Bug fixes
-*** The epilogue is no longer affected by internal #defines
+*** The epilogue is no longer affected by internal #defines (glr.c)
The glr.c skeleton uses defines such as #define yylval (yystackp->yyval) in
generated code. These weren't properly undefined before the inclusion of
This is has been fixed: yylval, yynerrs, yychar, and yylloc are now valid
identifiers for user-provided variables.
+*** stdio.h is no longer needed when locations are enabled (yacc.c)
+
+ Changes in Bison 2.7 introduced a dependency on FILE and fprintf when
+ locations are enabled. This is fixed.
+
** Diagnostics reported by Bison
Most of these features were contributed by Théophile Ranquet and Victor
activated by default. The old format can still be used by invoking Bison
with -fno-caret (or -fnone).
+ Some error messages that reproduced excerpts of the grammar are now using
+ the caret information only. For instance on:
+
+ %%
+ exp: 'a' | 'a';
+
+ Bison 2.7 reports:
+
+ in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
+ in.y:2.12-14: warning: rule useless in parser due to conflicts: exp: 'a' [-Wother]
+
+ Now bison reports:
+
+ in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
+ in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
+ exp: 'a' | 'a';
+ ^^^
+
+ and "bison -fno-caret" reports:
+
+ in.y: warning: 1 reduce/reduce conflict [-Wconflicts-rr]
+ in.y:2.12-14: warning: rule useless in parser due to conflicts [-Wother]
+
*** Enhancements of the -Werror option
The -Werror=CATEGORY option is now recognized, and will treat specified
%param {arg1_type *arg1} {arg2_type *arg2}
-** Java skeleton improvements
-
- Contributed by Paolo Bonzini.
-
- The constants for token names were moved to the Lexer interface. Also, it
- is possible to add code to the parser's constructors using "%code init"
- and "%define init_throws".
-
-** C++ skeletons improvements
-
-*** The parser header is no longer mandatory (lalr1.cc, glr.cc)
-
- Using %defines is now optional. Without it, the needed support classes
- are defined in the generated parser, instead of additional files (such as
- location.hh, position.hh and stack.hh).
-
-*** Locations are no longer mandatory (lalr1.cc, glr.cc)
-
- Both lalr1.cc and glr.cc no longer require %location.
-
-*** syntax_error exception (lalr1.cc)
-
- The C++ parser features a syntax_error exception, which can be
- thrown from the scanner or from user rules to raise syntax errors.
- This facilitates reporting errors caught in sub-functions (e.g.,
- rejecting too large integral literals from a conversion function
- used by the scanner, or rejecting invalid combinations from a
- factory invoked by the user actions).
-
-** Renamed %define variables
-
- The following variables have been renamed for consistency. Backward
- compatibility is ensured, but upgrading is recommended.
-
- lr.default-reductions -> lr.default-reduction
- lr.keep-unreachable-states -> lr.keep-unreachable-state
- namespace -> api.namespace
- stype -> api.value.type
-
** Variable api.token.prefix
The variable api.token.prefix changes the way tokens are identified in
%error-verbose directive is deprecated in favor of "%define parse.error
verbose".
+** Renamed %define variables
+
+ The following variables have been renamed for consistency. Backward
+ compatibility is ensured, but upgrading is recommended.
+
+ lr.default-reductions -> lr.default-reduction
+ lr.keep-unreachable-states -> lr.keep-unreachable-state
+ namespace -> api.namespace
+ stype -> api.value.type
+
** Semantic predicates
Contributed by Paul Hilfinger.
It used to be an error only if used in non GLR mode, _and_ if there are
reduce/reduce conflicts.
+** Token numbering has changed to preserve the user-defined order
+
+ When declaring %token A B, the numbering for A is inferior to B. Up to now,
+ when declaring associativity at the same time, with %left (or %right,
+ %precedence, %nonassoc), B was inferior to A.
+
+** Useless precedence and associativity
+
+ Contributed by Valentin Tolmer.
+
+ When developping and maintaining a grammar, useless associativity and
+ precedence directives are common. They can be a nuisance: new ambiguities
+ arising are sometimes masked because their conflicts are resolved due to
+ the extra precedence or associativity information. Furthermore, it can
+ hinder the comprehension of a new grammar: one will wonder about the role
+ of a precedence, where in fact it is useless. The following changes aim
+ at detecting and reporting these extra directives.
+
+*** Precedence warning category
+
+ A new category of warning, -Wprecedence, was introduced. It flags the
+ useless precedence and associativity directives.
+
+*** Useless associativity
+
+ Bison now warns about symbols with a declared associativity that is never
+ used to resolve conflicts. In that case, using %precedence is sufficient;
+ the parsing tables will remain unchanged. Solving these warnings may raise
+ useless precedence warnings, as the symbols no longer have associativity.
+ For example:
+
+ %left '+'
+ %left '*'
+ %%
+ exp:
+ "num"
+ | exp '+' "num"
+ | exp '*' exp
+ ;
+
+ will produce a
+
+ warning: useless associativity for '+', use %precedence [-Wprecedence]
+ %left '+'
+ ^^^
+
+*** Useless precedence
+
+ Bison now warns about symbols with a declared precedence and no declared
+ associativity (i.e., declared with %precedence), and whose precedence is
+ never used. In that case, the symbol can be safely declared with %token
+ instead, without modifying the parsing tables. For example:
+
+ %precedence '='
+ %%
+ exp: "var" '=' "num";
+
+ will produce a
+
+ warning: useless precedence for '=' [-Wprecedence]
+ %precedence '='
+ ^^^
+
+*** Useless precedence and associativity
+
+ In case of both useless precedence and associativity, the issue is flagged
+ as follows:
+
+ %nonassoc '='
+ %%
+ exp: "var" '=' "num";
+
+ The warning is:
+
+ warning: useless precedence and associativity for '=' [-Wprecedence]
+ %nonassoc '='
+ ^^^
+
+** Empty rules
+
+ Empty rules (i.e., with an empty right-hand side) can now be explicitly
+ marked by the new %empty directive. Using %empty on a non-empty rule is
+ an error. The new -Wempty-rule warning reports empty rules without
+ %empty. On the following grammar:
+
+ %%
+ s: a b c;
+ a: ;
+ b: %empty;
+ c: 'a' %empty;
+
+ bison reports:
+
+ 3.4-5: warning: empty rule without %empty [-Wempty-rule]
+ a: {}
+ ^^
+ 5.8-13: error: %empty on non-empty rule
+ c: 'a' %empty {};
+ ^^^^^^
+
+** Java skeleton improvements
+
+ Contributed by Paolo Bonzini.
+
+ The constants for token names were moved to the Lexer interface. Also, it
+ is possible to add code to the parser's constructors using "%code init"
+ and "%define init_throws".
+
+** C++ skeletons improvements
+
+*** The parser header is no longer mandatory (lalr1.cc, glr.cc)
+
+ Using %defines is now optional. Without it, the needed support classes
+ are defined in the generated parser, instead of additional files (such as
+ location.hh, position.hh and stack.hh).
+
+*** Locations are no longer mandatory (lalr1.cc, glr.cc)
+
+ Both lalr1.cc and glr.cc no longer require %location.
+
+*** syntax_error exception (lalr1.cc)
+
+ The C++ parser features a syntax_error exception, which can be
+ thrown from the scanner or from user rules to raise syntax errors.
+ This facilitates reporting errors caught in sub-functions (e.g.,
+ rejecting too large integral literals from a conversion function
+ used by the scanner, or rejecting invalid combinations from a
+ factory invoked by the user actions).
+
+*** %define api.value.type variant
+
+ This is based on a submission from Michiel De Wilde. With help
+ from Théophile Ranquet.
+
+ In this mode, complex C++ objects can be used as semantic values. For
+ instance:
+
+ %token <::std::string> TEXT;
+ %token <int> NUMBER;
+ %token SEMICOLON ";"
+ %type <::std::string> item;
+ %type <::std::list<std::string>> list;
+ %%
+ result:
+ list { std::cout << $1 << std::endl; }
+ ;
+
+ list:
+ /* nothing */ { /* Generates an empty string list */ }
+ | list item ";" { std::swap ($$, $1); $$.push_back ($2); }
+ ;
+
+ item:
+ TEXT { std::swap ($$, $1); }
+ | NUMBER { $$ = string_cast ($1); }
+ ;
+
+*** %define api.token.constructor
+
+ When variants are enabled, Bison can generate functions to build the
+ tokens. This guarantees that the token type (e.g., NUMBER) is consistent
+ with the semantic value (e.g., int):
+
+ parser::symbol_type yylex ()
+ {
+ parser::location_type loc = ...;
+ ...
+ return parser::make_TEXT ("Hello, world!", loc);
+ ...
+ return parser::make_NUMBER (42, loc);
+ ...
+ return parser::make_SEMICOLON (loc);
+ ...
+ }
+
* Noteworthy changes in release 2.7 (2012-12-12) [stable]
** Bug fixes
** Diagnostics are improved
+ Contributed by Théophile Ranquet.
+
*** Changes in the format of error messages
This used to be the format of many error reports:
** Graph improvements in DOT and XSLT
+ Contributed by Théophile Ranquet.
+
The graphical presentation of the states is more readable: their shape is
now rectangular, the state number is clearly displayed, and the items are
numbered and left-justified.
These features are experimental. More user feedback will help to
stabilize them.
-** LAC (Lookahead Correction) for syntax error handling:
+** LAC (Lookahead Correction) for syntax error handling
+
+ Contributed by Joel E. Denny.
Canonical LR, IELR, and LALR can suffer from a couple of problems
upon encountering a syntax error. First, the parser might perform
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