This manual (@value{UPDATED}) is for @acronym{GNU} Bison (version
@value{VERSION}), the @acronym{GNU} parser generator.
-Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free
-Software Foundation, Inc.
+Copyright @copyright{} 1988-1993, 1995, 1998-2011 Free Software
+Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the @acronym{GNU} Free Documentation License,
-Version 1.2 or any later version published by the Free Software
+Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, with the Front-Cover texts
being ``A @acronym{GNU} Manual,'' and with the Back-Cover Texts as in
(a) below. A copy of the license is included in the section entitled
* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
-* GLR Semantic Actions:: Deferred semantic actions have special concerns.
+* GLR Semantic Actions:: Considerations for semantic values and deferred actions.
+* Semantic Predicates:: Controlling a parse with arbitrary computations.
* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
Examples
* Mid-Rule Actions:: Most actions go at the end of a rule.
This says when, why and how to use the exceptional
action in the middle of a rule.
+* Named References:: Using named references in actions.
Tracking Locations
@cindex introduction
@dfn{Bison} is a general-purpose parser generator that converts an
-annotated context-free grammar into a deterministic or @acronym{GLR}
-parser employing @acronym{LALR}(1), @acronym{IELR}(1), or canonical
-@acronym{LR}(1) parser tables.
+annotated context-free grammar into a deterministic @acronym{LR} or
+generalized @acronym{LR} (@acronym{GLR}) parser employing
+@acronym{LALR}(1), @acronym{IELR}(1), or canonical @acronym{LR}(1)
+parser tables.
Once you are proficient with Bison, you can use it to develop a wide
range of language parsers, from those used in simple desk calculators to
complex programming languages.
restrictions of @acronym{LALR}(1), which is hard to explain simply.
@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
more information on this.
-To escape these additional restrictions, you can request
-@acronym{IELR}(1) or canonical @acronym{LR}(1) parser tables.
+As an experimental feature, you can escape these additional restrictions by
+requesting @acronym{IELR}(1) or canonical @acronym{LR}(1) parser tables.
@xref{Decl Summary,,lr.type}, to learn how.
@cindex @acronym{GLR} parsing
@menu
* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
-* GLR Semantic Actions:: Deferred semantic actions have special concerns.
+* GLR Semantic Actions:: Considerations for semantic values and deferred actions.
+* Semantic Predicates:: Controlling a parse with arbitrary computations.
* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
@end menu
@node GLR Semantic Actions
@subsection GLR Semantic Actions
+The nature of @acronym{GLR} parsing and the structure of the generated
+parsers give rise to certain restrictions on semantic values and actions.
+
+@subsubsection Deferred semantic actions
@cindex deferred semantic actions
By definition, a deferred semantic action is not performed at the same time as
the associated reduction.
to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
memory referenced by @code{yylval}.
+@subsubsection YYERROR
@findex YYERROR
@cindex @acronym{GLR} parsers and @code{YYERROR}
Another Bison feature requiring special consideration is @code{YYERROR}
initiate error recovery.
During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is
the same as its effect in a deterministic parser.
-In a deferred semantic action, its effect is undefined.
-@c The effect is probably a syntax error at the split point.
+The effect in a deferred action is similar, but the precise point of the
+error is undefined; instead, the parser reverts to deterministic operation,
+selecting an unspecified stack on which to continue with a syntax error.
+In a semantic predicate (see @ref{Semantic Predicates}) during nondeterministic
+parsing, @code{YYERROR} silently prunes
+the parse that invoked the test.
+
+@subsubsection Restrictions on semantic values and locations
+@acronym{GLR} parsers require that you use POD (Plain Old Data) types for
+semantic values and location types when using the generated parsers as
+C++ code.
+
+@node Semantic Predicates
+@subsection Controlling a Parse with Arbitrary Predicates
+@findex %?
+@cindex Semantic predicates in @acronym{GLR} parsers
+
+In addition to the @code{%dprec} and @code{%merge} directives,
+@acronym{GLR} parsers
+allow you to reject parses on the basis of arbitrary computations executed
+in user code, without having Bison treat this rejection as an error
+if there are alternative parses. (This feature is experimental and may
+evolve. We welcome user feedback.) For example,
+
+@smallexample
+widget :
+ %?@{ new_syntax @} "widget" id new_args @{ $$ = f($3, $4); @}
+ | %?@{ !new_syntax @} "widget" id old_args @{ $$ = f($3, $4); @}
+ ;
+@end smallexample
+
+@noindent
+is one way to allow the same parser to handle two different syntaxes for
+widgets. The clause preceded by @code{%?} is treated like an ordinary
+action, except that its text is treated as an expression and is always
+evaluated immediately (even when in nondeterministic mode). If the
+expression yields 0 (false), the clause is treated as a syntax error,
+which, in a nondeterministic parser, causes the stack in which it is reduced
+to die. In a deterministic parser, it acts like YYERROR.
+
+As the example shows, predicates otherwise look like semantic actions, and
+therefore you must be take them into account when determining the numbers
+to use for denoting the semantic values of right-hand side symbols.
+Predicate actions, however, have no defined value, and may not be given
+labels.
+
+There is a subtle difference between semantic predicates and ordinary
+actions in nondeterministic mode, since the latter are deferred.
+For example, we could try to rewrite the previous example as
+
+@smallexample
+widget :
+ @{ if (!new_syntax) YYERROR; @} "widget" id new_args @{ $$ = f($3, $4); @}
+ | @{ if (new_syntax) YYERROR; @} "widget" id old_args @{ $$ = f($3, $4); @}
+ ;
+@end smallexample
-Also, see @ref{Location Default Action, ,Default Action for Locations}, which
-describes a special usage of @code{YYLLOC_DEFAULT} in @acronym{GLR} parsers.
+@noindent
+(reversing the sense of the predicate tests to cause an error when they are
+false). However, this
+does @emph{not} have the same effect if @code{new_args} and @code{old_args}
+have overlapping syntax.
+Since the mid-rule actions testing @code{new_syntax} are deferred,
+a @acronym{GLR} parser first encounters the unresolved ambiguous reduction
+for cases where @code{new_args} and @code{old_args} recognize the same string
+@emph{before} performing the tests of @code{new_syntax}. It therefore
+reports an error.
+
+Finally, be careful in writing predicates: deferred actions have not been
+evaluated, so that using them in a predicate will have undefined effects.
@node Compiler Requirements
@subsection Considerations when Compiling @acronym{GLR} Parsers
* Mid-Rule Actions:: Most actions go at the end of a rule.
This says when, why and how to use the exceptional
action in the middle of a rule.
+* Named References:: Using named references in actions.
@end menu
@node Value Type
@cindex action
@vindex $$
@vindex $@var{n}
+@vindex $@var{name}
+@vindex $[@var{name}]
An action accompanies a syntactic rule and contains C code to be executed
each time an instance of that rule is recognized. The task of most actions
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. Bison translates both of these
+being constructed is @code{$$}. In addition, the semantic values of
+symbols can be accessed with the named references construct
+@code{$@var{name}} or @code{$[@var{name}]}. Bison translates both of these
constructs into expressions of the appropriate type when it copies the
-actions into the parser file. @code{$$} is translated to a modifiable
+actions into the parser file. @code{$$} (or @code{$@var{name}}, when it
+stands for the current grouping) is translated to a modifiable
lvalue, so it can be assigned to.
Here is a typical example:
@end group
@end example
+Or, in terms of named references:
+
+@example
+@group
+exp[result]: @dots{}
+ | exp[left] '+' exp[right]
+ @{ $result = $left + $right; @}
+@end group
+@end example
+
@noindent
This rule constructs an @code{exp} from two smaller @code{exp} groupings
connected by a plus-sign token. In the action, @code{$1} and @code{$3}
+(@code{$left} and @code{$right})
refer to the semantic values of the two component @code{exp} groupings,
which are the first and third symbols on the right hand side of the rule.
-The sum is stored into @code{$$} so that it becomes the semantic value of
+The sum is stored into @code{$$} (@code{$result}) so that it becomes the
+semantic value of
the addition-expression just recognized by the rule. If there were a
useful semantic value associated with the @samp{+} token, it could be
referred to as @code{$2}.
+@xref{Named References,,Using Named References}, for more information
+about using the named references construct.
+
Note that the vertical-bar character @samp{|} is really a rule
separator, and actions are attached to a single rule. This is a
difference with tools like Flex, for which @samp{|} stands for either
Now Bison can execute the action in the rule for @code{subroutine} without
deciding which rule for @code{compound} it will eventually use.
+@node Named References
+@subsection Using Named References
+@cindex named references
+
+While every semantic value can be accessed with positional references
+@code{$@var{n}} and @code{$$}, it's often much more convenient to refer to
+them by name. First of all, original symbol names may be used as named
+references. For example:
+
+@example
+@group
+invocation: op '(' args ')'
+ @{ $invocation = new_invocation ($op, $args, @@invocation); @}
+@end group
+@end example
+
+@noindent
+The positional @code{$$}, @code{@@$}, @code{$n}, and @code{@@n} can be
+mixed with @code{$name} and @code{@@name} arbitrarily. For example:
+
+@example
+@group
+invocation: op '(' args ')'
+ @{ $$ = new_invocation ($op, $args, @@$); @}
+@end group
+@end example
+
+@noindent
+However, sometimes regular symbol names are not sufficient due to
+ambiguities:
+
+@example
+@group
+exp: exp '/' exp
+ @{ $exp = $exp / $exp; @} // $exp is ambiguous.
+
+exp: exp '/' exp
+ @{ $$ = $1 / $exp; @} // One usage is ambiguous.
+
+exp: exp '/' exp
+ @{ $$ = $1 / $3; @} // No error.
+@end group
+@end example
+
+@noindent
+When ambiguity occurs, explicitly declared names may be used for values and
+locations. Explicit names are declared as a bracketed name after a symbol
+appearance in rule definitions. For example:
+@example
+@group
+exp[result]: exp[left] '/' exp[right]
+ @{ $result = $left / $right; @}
+@end group
+@end example
+
+@noindent
+Explicit names may be declared for RHS and for LHS symbols as well. In order
+to access a semantic value generated by a mid-rule action, an explicit name
+may also be declared by putting a bracketed name after the closing brace of
+the mid-rule action code:
+@example
+@group
+exp[res]: exp[x] '+' @{$left = $x;@}[left] exp[right]
+ @{ $res = $left + $right; @}
+@end group
+@end example
+
+@noindent
+
+In references, in order to specify names containing dots and dashes, an explicit
+bracketed syntax @code{$[name]} and @code{@@[name]} must be used:
+@example
+@group
+if-stmt: IF '(' expr ')' THEN then.stmt ';'
+ @{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); @}
+@end group
+@end example
+
+It often happens that named references are followed by a dot, dash or other
+C punctuation marks and operators. By default, Bison will read
+@code{$name.suffix} as a reference to symbol value @code{$name} followed by
+@samp{.suffix}, i.e., an access to the @samp{suffix} field of the semantic
+value. In order to force Bison to recognize @code{name.suffix} in its entirety
+as the name of a semantic value, bracketed syntax @code{$[name.suffix]}
+must be used.
+
+
@node Locations
@section Tracking Locations
@cindex location
@cindex actions, location
@vindex @@$
@vindex @@@var{n}
+@vindex @@@var{name}
+@vindex @@[@var{name}]
Actions are not only useful for defining language semantics, but also for
describing the behavior of the output parser with locations.
@code{@@@var{n}}, while the location of the left hand side grouping is
@code{@@$}.
+In addition, the named references construct @code{@@@var{name}} and
+@code{@@[@var{name}]} may also be used to address the symbol locations.
+@xref{Named References,,Using Named References}, for more information
+about using the named references construct.
+
Here is a basic example using the default data type for locations:
@example
@code{%expect-rr} declaration as well.
@end itemize
-Now Bison will warn you if you introduce an unexpected conflict, but
-will keep silent otherwise.
+Now Bison will report an error if you introduce an unexpected conflict,
+but will keep silent otherwise.
@node Start Decl
@subsection The Start-Symbol
parser (@pxref{Decl Summary,,%define api.push-pull}):
@example
-%define api.push-pull "push"
+%define api.push-pull push
@end example
In almost all cases, you want to ensure that your push parser is also
@example
%define api.pure
-%define api.push-pull "push"
+%define api.push-pull push
@end example
There is a major notable functional difference between the pure push parser
Bison also supports both the push parser interface along with the pull parser
interface in the same generated parser. In order to get this functionality,
-you should replace the @samp{%define api.push-pull "push"} declaration with the
-@samp{%define api.push-pull "both"} declaration. Doing this will create all of
+you should replace the @samp{%define api.push-pull push} declaration with the
+@samp{%define api.push-pull both} declaration. Doing this will create all of
the symbols mentioned earlier along with the two extra symbols, @code{yyparse}
and @code{yypull_parse}. @code{yyparse} can be used exactly as it normally
would be used. However, the user should note that it is implemented in the
generated parser by calling @code{yypull_parse}.
This makes the @code{yyparse} function that is generated with the
-@samp{%define api.push-pull "both"} declaration slower than the normal
+@samp{%define api.push-pull both} declaration slower than the normal
@code{yyparse} function. If the user
calls the @code{yypull_parse} function it will parse the rest of the input
stream. It is possible to @code{yypush_parse} tokens to select a subgrammar
@end example
Adding the @samp{%define api.pure} declaration does exactly the same thing to
-the generated parser with @samp{%define api.push-pull "both"} as it did for
-@samp{%define api.push-pull "push"}.
+the generated parser with @samp{%define api.push-pull both} as it did for
+@samp{%define api.push-pull push}.
@node Decl Summary
@subsection Bison Declaration Summary
@var{qualifier} identifies the purpose of @var{code} and thus the location(s)
where Bison should generate it.
-Not all values of @var{qualifier} are available for all target languages:
+Not all @var{qualifier}s are accepted for all target languages.
+Unaccepted @var{qualifier}s produce an error.
+Some of the accepted @var{qualifier}s are:
@itemize @bullet
@item requires
@end deffn
@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} @var{value}
@deffnx {Directive} %define @var{variable} "@var{value}"
Define a variable to adjust Bison's behavior.
-The possible choices for @var{variable}, as well as their meanings, depend on
-the selected target language and/or the parser skeleton (@pxref{Decl
-Summary,,%language}, @pxref{Decl Summary,,%skeleton}).
It is an error if a @var{variable} is defined by @code{%define} multiple
times, but see @ref{Bison Options,,-D @var{name}[=@var{value}]}.
-Omitting @code{"@var{value}"} is always equivalent to specifying it as
+@var{value} must be placed in quotation marks if it contains any
+character other than a letter, underscore, period, dash, or non-initial
+digit.
+
+Omitting @code{"@var{value}"} entirely is always equivalent to specifying
@code{""}.
-Some @var{variable}s may be used as Booleans.
+Some @var{variable}s take Boolean values.
In this case, Bison will complain if the variable definition does not meet one
of the following four conditions:
@enumerate
-@item @code{"@var{value}"} is @code{"true"}
+@item @code{@var{value}} is @code{true}
-@item @code{"@var{value}"} is omitted (or is @code{""}).
-This is equivalent to @code{"true"}.
+@item @code{@var{value}} is omitted (or @code{""} is specified).
+This is equivalent to @code{true}.
-@item @code{"@var{value}"} is @code{"false"}.
+@item @code{@var{value}} is @code{false}.
@item @var{variable} is never defined.
-In this case, Bison selects a default value, which may depend on the selected
-target language and/or parser skeleton.
+In this case, Bison selects a default value.
@end enumerate
+What @var{variable}s are accepted, as well as their meanings and default
+values, depend on the selected target language and/or the parser
+skeleton (@pxref{Decl Summary,,%language}, @pxref{Decl
+Summary,,%skeleton}).
+Unaccepted @var{variable}s produce an error.
Some of the accepted @var{variable}s are:
@table @code
-@c ================================================== namespace
+@c ================================================== api.namespace
@item api.namespace
@findex %define api.namespace
@itemize
@item Languages(s): C++
-@item Purpose: Specifies the namespace for the parser class.
+@item Purpose: Specify the namespace for the parser class.
For example, if you specify:
@smallexample
@item Accepted Values: Boolean
-@item Default Value: @code{"false"}
+@item Default Value: @code{false}
@end itemize
@c api.pure
@itemize @bullet
@item Language(s): C (deterministic parsers only)
-@item Purpose: Requests a pull parser, a push parser, or both.
+@item Purpose: Request a pull parser, a push parser, or both.
@xref{Push Decl, ,A Push Parser}.
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
-@item Accepted Values: @code{"pull"}, @code{"push"}, @code{"both"}
+@item Accepted Values: @code{pull}, @code{push}, @code{both}
-@item Default Value: @code{"pull"}
+@item Default Value: @code{pull}
@end itemize
@c api.push-pull
+
+
+@c ================================================== api.tokens.prefix
@item api.tokens.prefix
@findex %define api.tokens.prefix
@c api.tokens.prefix
+@c ================================================== lex_symbol
+@item variant
+@findex %define lex_symbol
+
+@itemize @bullet
+@item Language(s):
+C++
+
+@item Purpose:
+When variant-based semantic values are enabled (@pxref{C++ Variants}),
+request that symbols be handled as a whole (type, value, and possibly
+location) in the scanner. @xref{Complete Symbols}, for details.
+
+@item Accepted Values:
+Boolean.
+
+@item Default Value:
+@code{false}
+@end itemize
+@c lex_symbol
+
+
+@c ================================================== lr.default-reductions
+
@item lr.default-reductions
@cindex default reductions
@findex %define lr.default-reductions
@cindex delayed syntax errors
@cindex syntax errors delayed
+@cindex @acronym{LAC}
+@findex %nonassoc
@itemize @bullet
@item Language(s): all
-@item Purpose: Specifies the kind of states that are permitted to
+@item Purpose: Specify the kind of states that are permitted to
contain default reductions.
-That is, in such a state, Bison declares the reduction with the largest
-lookahead set to be the default reduction and then removes that
+That is, in such a state, Bison selects the reduction with the largest
+lookahead set to be the default parser action and then removes that
lookahead set.
-The advantages of default reductions are discussed below.
-The disadvantage is that, when the generated parser encounters a
-syntactically unacceptable token, the parser might then perform
-unnecessary default reductions before it can detect the syntax error.
-
-(This feature is experimental.
+(The ability to specify where default reductions should be used is
+experimental.
More user feedback will help to stabilize it.)
@item Accepted Values:
@itemize
-@item @code{"all"}.
-For @acronym{LALR} and @acronym{IELR} parsers (@pxref{Decl
-Summary,,lr.type}) by default, all states are permitted to contain
-default reductions.
-The advantage is that parser table sizes can be significantly reduced.
-The reason Bison does not by default attempt to address the disadvantage
-of delayed syntax error detection is that this disadvantage is already
-inherent in @acronym{LALR} and @acronym{IELR} parser tables.
-That is, unlike in a canonical @acronym{LR} state, the lookahead sets of
-reductions in an @acronym{LALR} or @acronym{IELR} state can contain
-tokens that are syntactically incorrect for some left contexts.
-
-@item @code{"consistent"}.
+@item @code{all}.
+This is the traditional Bison behavior.
+The main advantage is a significant decrease in the size of the parser
+tables.
+The disadvantage is that, when the generated parser encounters a
+syntactically unacceptable token, the parser might then perform
+unnecessary default reductions before it can detect the syntax error.
+Such delayed syntax error detection is usually inherent in
+@acronym{LALR} and @acronym{IELR} parser tables anyway due to
+@acronym{LR} state merging (@pxref{Decl Summary,,lr.type}).
+Furthermore, the use of @code{%nonassoc} can contribute to delayed
+syntax error detection even in the case of canonical @acronym{LR}.
+As an experimental feature, delayed syntax error detection can be
+overcome in all cases by enabling @acronym{LAC} (@pxref{Decl
+Summary,,parse.lac}, for details, including a discussion of the effects
+of delayed syntax error detection).
+
+@item @code{consistent}.
@cindex consistent states
A consistent state is a state that has only one possible action.
If that action is a reduction, then the parser does not need to request
a lookahead token from the scanner before performing that action.
-However, the parser only recognizes the ability to ignore the lookahead
-token when such a reduction is encoded as a default reduction.
-Thus, if default reductions are permitted in and only in consistent
-states, then a canonical @acronym{LR} parser reports a syntax error as
-soon as it @emph{needs} the syntactically unacceptable token from the
-scanner.
-
-@item @code{"accepting"}.
+However, the parser recognizes the ability to ignore the lookahead token
+in this way only when such a reduction is encoded as a default
+reduction.
+Thus, if default reductions are permitted only in consistent states,
+then a canonical @acronym{LR} parser that does not employ
+@code{%nonassoc} detects a syntax error as soon as it @emph{needs} the
+syntactically unacceptable token from the scanner.
+
+@item @code{accepting}.
@cindex accepting state
-By default, the only default reduction permitted in a canonical
-@acronym{LR} parser is the accept action in the accepting state, which
-the parser reaches only after reading all tokens from the input.
-Thus, the default canonical @acronym{LR} parser reports a syntax error
-as soon as it @emph{reaches} the syntactically unacceptable token
-without performing any extra reductions.
+In the accepting state, the default reduction is actually the accept
+action.
+In this case, a canonical @acronym{LR} parser that does not employ
+@code{%nonassoc} detects a syntax error as soon as it @emph{reaches} the
+syntactically unacceptable token in the input.
+That is, it does not perform any extra reductions.
@end itemize
@item Default Value:
@itemize
-@item @code{"accepting"} if @code{lr.type} is @code{"canonical LR"}.
-@item @code{"all"} otherwise.
+@item @code{accepting} if @code{lr.type} is @code{canonical-lr}.
+@item @code{all} otherwise.
@end itemize
@end itemize
+@c ============================================ lr.keep-unreachable-states
+
@item lr.keep-unreachable-states
@findex %define lr.keep-unreachable-states
@itemize @bullet
@item Language(s): all
-@item Purpose: Requests that Bison allow unreachable parser states to remain in
-the parser tables.
+@item Purpose: Request that Bison allow unreachable parser states to
+remain in the parser tables.
Bison considers a state to be unreachable if there exists no sequence of
transitions from the start state to that state.
A state can become unreachable during conflict resolution if Bison disables a
@item Accepted Values: Boolean
-@item Default Value: @code{"false"}
+@item Default Value: @code{false}
@item Caveats:
@end itemize
@c lr.keep-unreachable-states
+@c ================================================== lr.type
+
@item lr.type
@findex %define lr.type
@cindex @acronym{LALR}
@itemize @bullet
@item Language(s): all
-@item Purpose: Specifies the type of parser tables within the
+@item Purpose: Specify the type of parser tables within the
@acronym{LR}(1) family.
(This feature is experimental.
More user feedback will help to stabilize it.)
@item Accepted Values:
@itemize
-@item @code{"LALR"}.
+@item @code{lalr}.
While Bison generates @acronym{LALR} parser tables by default for
historical reasons, @acronym{IELR} or canonical @acronym{LR} is almost
always preferable for deterministic parsers.
to alter the language accepted by the parser.
@acronym{LALR} parser tables are the smallest parser tables Bison can
currently generate, so they may be preferable.
+Nevertheless, once you begin to resolve conflicts statically,
+@acronym{GLR} begins to behave more like a deterministic parser, and so
+@acronym{IELR} and canonical @acronym{LR} can be helpful to avoid
+@acronym{LALR}'s mysterious behavior.
@item Occasionally during development, an especially malformed grammar
with a major recurring flaw may severely impede the @acronym{IELR} or
from @acronym{IELR} and canonical @acronym{LR}.
@end itemize
-@item @code{"IELR"}.
+@item @code{ielr}.
@acronym{IELR} is a minimal @acronym{LR} algorithm.
That is, given any grammar (@acronym{LR} or non-@acronym{LR}),
@acronym{IELR} and canonical @acronym{LR} always accept exactly the same
of magnitude less as well.
This can significantly reduce the complexity of developing of a grammar.
-@item @code{"canonical LR"}.
+@item @code{canonical-lr}.
@cindex delayed syntax errors
@cindex syntax errors delayed
-The only advantage of canonical @acronym{LR} over @acronym{IELR} is
-that, for every left context of every canonical @acronym{LR} state, the
-set of tokens accepted by that state is the exact set of tokens that is
-syntactically acceptable in that left context.
-Thus, the only difference in parsing behavior is that the canonical
-@acronym{LR} parser can report a syntax error as soon as possible
-without performing any unnecessary reductions.
-@xref{Decl Summary,,lr.default-reductions}, for further details.
-Even when canonical @acronym{LR} behavior is ultimately desired,
-@acronym{IELR}'s elimination of duplicate conflicts should still
-facilitate the development of a grammar.
+@cindex @acronym{LAC}
+@findex %nonassoc
+While inefficient, canonical @acronym{LR} parser tables can be an
+interesting means to explore a grammar because they have a property that
+@acronym{IELR} and @acronym{LALR} tables do not.
+That is, if @code{%nonassoc} is not used and default reductions are left
+disabled (@pxref{Decl Summary,,lr.default-reductions}), then, for every
+left context of every canonical @acronym{LR} state, the set of tokens
+accepted by that state is guaranteed to be the exact set of tokens that
+is syntactically acceptable in that left context.
+It might then seem that an advantage of canonical @acronym{LR} parsers
+in production is that, under the above constraints, they are guaranteed
+to detect a syntax error as soon as possible without performing any
+unnecessary reductions.
+However, @acronym{IELR} parsers using @acronym{LAC} (@pxref{Decl
+Summary,,parse.lac}) are also able to achieve this behavior without
+sacrificing @code{%nonassoc} or default reductions.
@end itemize
-@item Default Value: @code{"LALR"}
+@item Default Value: @code{lalr}
@end itemize
@item Languages(s): C++
@item Purpose: Issue runtime assertions to catch invalid uses.
-In C++, when variants are used, symbols must be constructed and
+In C++, when variants are used (@pxref{C++ Variants}), symbols must be
+constructed and
destroyed properly. This option checks these constraints.
@item Accepted Values: Boolean
@findex %define parse.error
@itemize
@item Languages(s):
-all.
+all
@item Purpose:
Control the kind of error messages passed to the error reporting
function. @xref{Error Reporting, ,The Error Reporting Function
@code{yyerror}}.
@item Accepted Values:
@itemize
-@item @code{"simple"}
+@item @code{simple}
Error messages passed to @code{yyerror} are simply @w{@code{"syntax
error"}}.
-@item @code{"verbose"}
+@item @code{verbose}
Error messages report the unexpected token, and possibly the expected
ones.
@end itemize
@c parse.error
+@c ================================================== parse.lac
+@item parse.lac
+@findex %define parse.lac
+@cindex @acronym{LAC}
+@cindex lookahead correction
+
+@itemize
+@item Languages(s): C
+
+@item Purpose: Enable @acronym{LAC} (lookahead correction) to improve
+syntax error handling.
+
+Canonical @acronym{LR}, @acronym{IELR}, and @acronym{LALR} can suffer
+from a couple of problems upon encountering a syntax error. First, the
+parser might perform additional parser stack reductions before
+discovering the syntax error. Such reductions perform user semantic
+actions that are unexpected because they are based on an invalid token,
+and they cause error recovery to begin in a different syntactic context
+than the one in which the invalid token was encountered. Second, when
+verbose error messages are enabled (with @code{%error-verbose} or
+@code{#define YYERROR_VERBOSE}), the expected token list in the syntax
+error message can both contain invalid tokens and omit valid tokens.
+
+The culprits for the above problems are @code{%nonassoc}, default
+reductions in inconsistent states, and parser state merging. Thus,
+@acronym{IELR} and @acronym{LALR} suffer the most. Canonical
+@acronym{LR} can suffer only if @code{%nonassoc} is used or if default
+reductions are enabled for inconsistent states.
+
+@acronym{LAC} is a new mechanism within the parsing algorithm that
+completely solves these problems for canonical @acronym{LR},
+@acronym{IELR}, and @acronym{LALR} without sacrificing @code{%nonassoc},
+default reductions, or state mering. Conceptually, the mechanism is
+straight-forward. Whenever the parser fetches a new token from the
+scanner so that it can determine the next parser action, it immediately
+suspends normal parsing and performs an exploratory parse using a
+temporary copy of the normal parser state stack. During this
+exploratory parse, the parser does not perform user semantic actions.
+If the exploratory parse reaches a shift action, normal parsing then
+resumes on the normal parser stacks. If the exploratory parse reaches
+an error instead, the parser reports a syntax error. If verbose syntax
+error messages are enabled, the parser must then discover the list of
+expected tokens, so it performs a separate exploratory parse for each
+token in the grammar.
+
+There is one subtlety about the use of @acronym{LAC}. That is, when in
+a consistent parser state with a default reduction, the parser will not
+attempt to fetch a token from the scanner because no lookahead is needed
+to determine the next parser action. Thus, whether default reductions
+are enabled in consistent states (@pxref{Decl
+Summary,,lr.default-reductions}) affects how soon the parser detects a
+syntax error: when it @emph{reaches} an erroneous token or when it
+eventually @emph{needs} that token as a lookahead. The latter behavior
+is probably more intuitive, so Bison currently provides no way to
+achieve the former behavior while default reductions are fully enabled.
+
+Thus, when @acronym{LAC} is in use, for some fixed decision of whether
+to enable default reductions in consistent states, canonical
+@acronym{LR} and @acronym{IELR} behave exactly the same for both
+syntactically acceptable and syntactically unacceptable input. While
+@acronym{LALR} still does not support the full language-recognition
+power of canonical @acronym{LR} and @acronym{IELR}, @acronym{LAC} at
+least enables @acronym{LALR}'s syntax error handling to correctly
+reflect @acronym{LALR}'s language-recognition power.
+
+Because @acronym{LAC} requires many parse actions to be performed twice,
+it can have a performance penalty. However, not all parse actions must
+be performed twice. Specifically, during a series of default reductions
+in consistent states and shift actions, the parser never has to initiate
+an exploratory parse. Moreover, the most time-consuming tasks in a
+parse are often the file I/O, the lexical analysis performed by the
+scanner, and the user's semantic actions, but none of these are
+performed during the exploratory parse. Finally, the base of the
+temporary stack used during an exploratory parse is a pointer into the
+normal parser state stack so that the stack is never physically copied.
+In our experience, the performance penalty of @acronym{LAC} has proven
+insignificant for practical grammars.
+
+@item Accepted Values: @code{none}, @code{full}
+
+@item Default Value: @code{none}
+@end itemize
+@c parse.lac
+
@c ================================================== parse.trace
@item parse.trace
@findex %define parse.trace
@end itemize
@c parse.trace
+@c ================================================== variant
+@item variant
+@findex %define variant
+
+@itemize @bullet
+@item Language(s):
+C++
+
+@item Purpose:
+Request variant-based semantic values.
+@xref{C++ Variants}.
+
+@item Accepted Values:
+Boolean.
+
+@item Default Value:
+@code{false}
+@end itemize
+@c variant
+
+
@end table
@end deffn
@c ---------------------------------------------------------- %define
parameter information to it in a reentrant way. To do so, use the
declaration @code{%parse-param}:
-@deffn {Directive} %parse-param @{@var{argument-declaration}@}
+@deffn {Directive} %parse-param @{@var{argument-declaration}@} @dots{}
@findex %parse-param
-Declare that an argument declared by the braced-code
-@var{argument-declaration} is an additional @code{yyparse} argument.
+Declare that one or more
+@var{argument-declaration} are additional @code{yyparse} arguments.
The @var{argument-declaration} is used when declaring
functions or prototypes. The last identifier in
@var{argument-declaration} must be the argument name.
Here's an example. Write this in the parser:
@example
-%parse-param @{int *nastiness@}
-%parse-param @{int *randomness@}
+%parse-param @{int *nastiness@} @{int *randomness@}
@end example
@noindent
More user feedback will help to stabilize it.)
You call the function @code{yypush_parse} to parse a single token. This
-function is available if either the @samp{%define api.push-pull "push"} or
-@samp{%define api.push-pull "both"} declaration is used.
+function is available if either the @samp{%define api.push-pull push} or
+@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl, ,A Push Parser}.
@deftypefun int yypush_parse (yypstate *yyps)
More user feedback will help to stabilize it.)
You call the function @code{yypull_parse} to parse the rest of the input
-stream. This function is available if the @samp{%define api.push-pull "both"}
+stream. This function is available if the @samp{%define api.push-pull both}
declaration is used.
@xref{Push Decl, ,A Push Parser}.
More user feedback will help to stabilize it.)
You call the function @code{yypstate_new} to create a new parser instance.
-This function is available if either the @samp{%define api.push-pull "push"} or
-@samp{%define api.push-pull "both"} declaration is used.
+This function is available if either the @samp{%define api.push-pull push} or
+@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl, ,A Push Parser}.
@deftypefun yypstate *yypstate_new (void)
-The fuction will return a valid parser instance if there was memory available
+The function will return a valid parser instance if there was memory available
or 0 if no memory was available.
In impure mode, it will also return 0 if a parser instance is currently
allocated.
More user feedback will help to stabilize it.)
You call the function @code{yypstate_delete} to delete a parser instance.
-function is available if either the @samp{%define api.push-pull "push"} or
-@samp{%define api.push-pull "both"} declaration is used.
+function is available if either the @samp{%define api.push-pull push} or
+@samp{%define api.push-pull both} declaration is used.
@xref{Push Decl, ,A Push Parser}.
@deftypefun void yypstate_delete (yypstate *yyps)
this case, omit the second argument; @code{yylex} will be called with
only one argument.
-
-If you wish to pass the additional parameter data to @code{yylex}, use
+If you wish to pass additional arguments to @code{yylex}, use
@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
-Function}).
+Function}). To pass additional arguments to both @code{yylex} and
+@code{yyparse}, use @code{%param}.
-@deffn {Directive} lex-param @{@var{argument-declaration}@}
+@deffn {Directive} %lex-param @{@var{argument-declaration}@} @dots{}
@findex %lex-param
-Declare that the braced-code @var{argument-declaration} is an
-additional @code{yylex} argument declaration.
+Specify that @var{argument-declaration} are additional @code{yylex} argument
+declarations. You may pass one or more such declarations, which is
+equivalent to repeating @code{%lex-param}.
+@end deffn
+
+@deffn {Directive} %param @{@var{argument-declaration}@} @dots{}
+@findex %param
+Specify that @var{argument-declaration} are additional
+@code{yylex}/@code{yyparse} argument declaration. This is equivalent to
+@samp{%lex-param @{@var{argument-declaration}@} @dots{} %parse-param
+@{@var{argument-declaration}@} @dots{}}. You may pass one or more
+declarations, which is equivalent to repeating @code{%param}.
@end deffn
For instance:
@example
-%parse-param @{int *nastiness@}
-%lex-param @{int *nastiness@}
-%parse-param @{int *randomness@}
+%lex-param @{scanner_mode *mode@}
+%parse-param @{parser_mode *mode@}
+%param @{environment_type *env@}
@end example
@noindent
results in the following signature:
@example
-int yylex (int *nastiness);
-int yyparse (int *nastiness, int *randomness);
+int yylex (scanner_mode *mode, environment_type *env);
+int yyparse (parser_mode *mode, environment_type *env);
@end example
If @samp{%define api.pure} is added:
@example
-int yylex (YYSTYPE *lvalp, int *nastiness);
-int yyparse (int *nastiness, int *randomness);
+int yylex (YYSTYPE *lvalp, scanner_mode *mode, environment_type *env);
+int yyparse (parser_mode *mode, environment_type *env);
@end example
@noindent
and finally, if both @samp{%define api.pure} and @code{%locations} are used:
@example
-int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
-int yyparse (int *nastiness, int *randomness);
+int yylex (YYSTYPE *lvalp, YYLTYPE *llocp,
+ scanner_mode *mode, environment_type *env);
+int yyparse (parser_mode *mode, environment_type *env);
@end example
@node Error Reporting
@w{@code{"syntax error"}}.
@findex %define parse.error
-If you invoke @samp{%define parse.error "verbose"} in the Bison
+If you invoke @samp{%define parse.error verbose} in the Bison
declarations section (@pxref{Bison Declarations, ,The Bison Declarations
Section}), then Bison provides a more verbose and specific error message
string instead of just plain @w{@code{"syntax error"}}.
Algol 60 and is called the ``dangling @code{else}'' ambiguity.
To avoid warnings from Bison about predictable, legitimate shift/reduce
-conflicts, use the @code{%expect @var{n}} declaration. There will be no
-warning as long as the number of shift/reduce conflicts is exactly @var{n}.
+conflicts, use the @code{%expect @var{n}} declaration.
+There will be no warning as long as the number of shift/reduce conflicts
+is exactly @var{n}, and Bison will report an error if there is a
+different number.
@xref{Expect Decl, ,Suppressing Conflict Warnings}.
The definition of @code{if_stmt} above is solely to blame for the
related) conflicts that would remain hidden.
The dangling @code{else} ambiguity (@pxref{Shift/Reduce, , Shift/Reduce
-Conflicts}) can be solved explictly. This shift/reduce conflicts occurs
+Conflicts}) can be solved explicitly. This shift/reduce conflicts occurs
in the following situation, where the period denotes the current parsing
state:
The unary-minus is another typical example where associativity is
usually over-specified, see @ref{Infix Calc, , Infix Notation
-Calculator: @code{calc}}. The @code{%left} directive is traditionaly
+Calculator: @code{calc}}. The @code{%left} directive is traditionally
used to declare the precedence of @code{NEG}, which is more than needed
since it also defines its associativity. While this is harmless in the
traditional example, who knows how @code{NEG} might be used in future
Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.
-@c FIXME: C++ output.
-Because of semantical differences between C and C++, the deterministic
-parsers in C produced by Bison cannot grow when compiled
-by C++ compilers. In this precise case (compiling a C parser as C++) you are
-suggested to grow @code{YYINITDEPTH}. The Bison maintainers hope to fix
-this deficiency in a future release.
+You can generate a deterministic parser containing C++ user code from
+the default (C) skeleton, as well as from the C++ skeleton
+(@pxref{C++ Parsers}). However, if you do use the default skeleton
+and want to allow the parsing stack to grow,
+be careful not to use semantic types or location types that require
+non-trivial copy constructors.
+The C skeleton bypasses these constructors when copying data to
+new, larger stacks.
@node Error Recovery
@chapter Error Recovery
@end table
A category can be turned off by prefixing its name with @samp{no-}. For
-instance, @option{-Wno-syntax} will hide the warnings about unused
-variables.
+instance, @option{-Wno-yacc} will hide the warnings about
+@acronym{POSIX} Yacc incompatibilities.
@end table
@noindent
@c - initial action
The C++ deterministic parser is selected using the skeleton directive,
-@samp{%skeleton "lalr1.c"}, or the synonymous command-line option
-@option{--skeleton=lalr1.c}.
+@samp{%skeleton "lalr1.cc"}, or the synonymous command-line option
+@option{--skeleton=lalr1.cc}.
@xref{Decl Summary}.
When run, @command{bison} will create several entities in the @samp{yy}
@item position.hh
@itemx location.hh
The definition of the classes @code{position} and @code{location},
-used for location tracking. @xref{C++ Location Values}.
+used for location tracking when enabled. @xref{C++ Location Values}.
@item stack.hh
An auxiliary class @code{stack} used by the parser.
@c - YYSTYPE
@c - Printer and destructor
+Bison supports two different means to handle semantic values in C++. One is
+alike the C interface, and relies on unions (@pxref{C++ Unions}). As C++
+practitioners know, unions are inconvenient in C++, therefore another
+approach is provided, based on variants (@pxref{C++ Variants}).
+
+@menu
+* C++ Unions:: Semantic values cannot be objects
+* C++ Variants:: Using objects as semantic values
+@end menu
+
+@node C++ Unions
+@subsubsection C++ Unions
+
The @code{%union} directive works as for C, see @ref{Union Decl, ,The
Collection of Value Types}. In particular it produces a genuine
-@code{union}@footnote{In the future techniques to allow complex types
-within pseudo-unions (similar to Boost variants) might be implemented to
-alleviate these issues.}, which have a few specific features in C++.
+@code{union}, which have a few specific features in C++.
@itemize @minus
@item
The type @code{YYSTYPE} is defined but its use is discouraged: rather
only means to avoid leaks. @xref{Destructor Decl, , Freeing Discarded
Symbols}.
+@node C++ Variants
+@subsubsection C++ Variants
+
+Starting with version 2.6, Bison provides a @emph{variant} based
+implementation of semantic values for C++. This alleviates all the
+limitations reported in the previous section, and in particular, object
+types can be used without pointers.
+
+To enable variant-based semantic values, set @code{%define} variable
+@code{variant} (@pxref{Decl Summary, , variant}). Once this defined,
+@code{%union} is ignored, and instead of using the name of the fields of the
+@code{%union} to ``type'' the symbols, use genuine types.
+
+For instance, instead of
+
+@example
+%union
+@{
+ int ival;
+ std::string* sval;
+@}
+%token <ival> NUMBER;
+%token <sval> STRING;
+@end example
+
+@noindent
+write
+
+@example
+%token <int> NUMBER;
+%token <std::string> STRING;
+@end example
+
+@code{STRING} is no longer a pointer, which should fairly simplify the user
+actions in the grammar and in the scanner (in particular the memory
+management).
+
+Since C++ features destructors, and since it is customary to specialize
+@code{operator<<} to support uniform printing of values, variants also
+typically simplify Bison printers and destructors.
+
+Variants are stricter than unions. When based on unions, you may play any
+dirty game with @code{yylval}, say storing an @code{int}, reading a
+@code{char*}, and then storing a @code{double} in it. This is no longer
+possible with variants: they must be initialized, then assigned to, and
+eventually, destroyed.
+
+@deftypemethod {semantic_type} {T&} build<T> ()
+Initialize, but leave empty. Returns the address where the actual value may
+be stored. Requires that the variant was not initialized yet.
+@end deftypemethod
+
+@deftypemethod {semantic_type} {T&} build<T> (const T& @var{t})
+Initialize, and copy-construct from @var{t}.
+@end deftypemethod
+
+
+@strong{Warning}: We do not use Boost.Variant, for two reasons. First, it
+appeared unacceptable to require Boost on the user's machine (i.e., the
+machine on which the generated parser will be compiled, not the machine on
+which @command{bison} was run). Second, for each possible semantic value,
+Boost.Variant not only stores the value, but also a tag specifying its
+type. But the parser already ``knows'' the type of the semantic value, so
+that would be duplicating the information.
+
+Therefore we developed light-weight variants whose type tag is external (so
+they are really like @code{unions} for C++ actually). But our code is much
+less mature that Boost.Variant. So there is a number of limitations in
+(the current implementation of) variants:
+@itemize
+@item
+Alignment must be enforced: values should be aligned in memory according to
+the most demanding type. Computing the smallest alignment possible requires
+meta-programming techniques that are not currently implemented in Bison, and
+therefore, since, as far as we know, @code{double} is the most demanding
+type on all platforms, alignments are enforced for @code{double} whatever
+types are actually used. This may waste space in some cases.
+
+@item
+Our implementation is not conforming with strict aliasing rules. Alias
+analysis is a technique used in optimizing compilers to detect when two
+pointers are disjoint (they cannot ``meet''). Our implementation breaks
+some of the rules that G++ 4.4 uses in its alias analysis, so @emph{strict
+alias analysis must be disabled}. Use the option
+@option{-fno-strict-aliasing} to compile the generated parser.
+
+@item
+There might be portability issues we are not aware of.
+@end itemize
+
+As far as we know, these limitations @emph{can} be alleviated. All it takes
+is some time and/or some talented C++ hacker willing to contribute to Bison.
@node C++ Location Values
@subsection C++ Location Values
it describes an additional member of the parser class, and an
additional argument for its constructor.
-@defcv {Type} {parser} {semantic_value_type}
-@defcvx {Type} {parser} {location_value_type}
-The types for semantics value and locations.
+@defcv {Type} {parser} {semantic_type}
+@defcvx {Type} {parser} {location_type}
+The types for semantic values and locations (if enabled).
+@end defcv
+
+@defcv {Type} {parser} {token}
+A structure that contains (only) the definition of the tokens as the
+@code{yytokentype} enumeration. To refer to the token @code{FOO}, the
+scanner should use @code{yy::parser::token::FOO}. The scanner can use
+@samp{typedef yy::parser::token token;} to ``import'' the token enumeration
+(@pxref{Calc++ Scanner}).
+@end defcv
+
+@defcv {Type} {parser} {syntax_error}
+This class derives from @code{std::runtime_error}. Throw instances of it
+from user actions to raise parse errors. This is equivalent with first
+invoking @code{error} to report the location and message of the syntax
+error, and then to invoke @code{YYERROR} to enter the error-recovery mode.
+But contrary to @code{YYERROR} which can only be invoked from user actions
+(i.e., written in the action itself), the exception can be thrown from
+function invoked from the user action.
@end defcv
@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...)
@samp{%parse-param @{@var{type1} @var{arg1}@}} was used.
@end deftypemethod
+@deftypemethod {syntax_error} {} syntax_error (const location_type& @var{l}, const std::string& @var{m})
+@deftypemethodx {syntax_error} {} syntax_error (const std::string& @var{m})
+Instantiate a syntax-error exception.
+@end deftypemethod
+
@deftypemethod {parser} {int} parse ()
Run the syntactic analysis, and return 0 on success, 1 otherwise.
@end deftypemethod
@end deftypemethod
@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m})
+@deftypemethodx {parser} {void} error (const std::string& @var{m})
The definition for this member function must be supplied by the user:
the parser uses it to report a parser error occurring at @var{l},
-described by @var{m}.
+described by @var{m}. If location tracking is not enabled, the second
+signature is used.
@end deftypemethod
The parser invokes the scanner by calling @code{yylex}. Contrary to C
parsers, C++ parsers are always pure: there is no point in using the
-@samp{%define api.pure} directive. Therefore the interface is as follows.
+@samp{%define api.pure} directive. The actual interface with @code{yylex}
+depends whether you use unions, or variants.
-@deftypemethod {parser} {int} yylex (semantic_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...)
-Return the next token. Its type is the return value, its semantic
-value and location being @var{yylval} and @var{yylloc}. Invocations of
+@menu
+* Split Symbols:: Passing symbols as two/three components
+* Complete Symbols:: Making symbols a whole
+@end menu
+
+@node Split Symbols
+@subsubsection Split Symbols
+
+Therefore the interface is as follows.
+
+@deftypemethod {parser} {int} yylex (semantic_type* @var{yylval}, location_type* @var{yylloc}, @var{type1} @var{arg1}, ...)
+@deftypemethodx {parser} {int} yylex (semantic_type* @var{yylval}, @var{type1} @var{arg1}, ...)
+Return the next token. Its type is the return value, its semantic value and
+location (if enabled) being @var{yylval} and @var{yylloc}. Invocations of
@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
@end deftypemethod
+Note that when using variants, the interface for @code{yylex} is the same,
+but @code{yylval} is handled differently.
+
+Regular union-based code in Lex scanner typically look like:
+
+@example
+[0-9]+ @{
+ yylval.ival = text_to_int (yytext);
+ return yy::parser::INTEGER;
+ @}
+[a-z]+ @{
+ yylval.sval = new std::string (yytext);
+ return yy::parser::IDENTIFIER;
+ @}
+@end example
+
+Using variants, @code{yylval} is already constructed, but it is not
+initialized. So the code would look like:
+
+@example
+[0-9]+ @{
+ yylval.build<int>() = text_to_int (yytext);
+ return yy::parser::INTEGER;
+ @}
+[a-z]+ @{
+ yylval.build<std::string> = yytext;
+ return yy::parser::IDENTIFIER;
+ @}
+@end example
+
+@noindent
+or
+
+@example
+[0-9]+ @{
+ yylval.build(text_to_int (yytext));
+ return yy::parser::INTEGER;
+ @}
+[a-z]+ @{
+ yylval.build(yytext);
+ return yy::parser::IDENTIFIER;
+ @}
+@end example
+
+
+@node Complete Symbols
+@subsubsection Complete Symbols
+
+If you specified both @code{%define variant} and @code{%define lex_symbol},
+the @code{parser} class also defines the class @code{parser::symbol_type}
+which defines a @emph{complete} symbol, aggregating its type (i.e., the
+traditional value returned by @code{yylex}), its semantic value (i.e., the
+value passed in @code{yylval}, and possibly its location (@code{yylloc}).
+
+@deftypemethod {symbol_type} {} symbol_type (token_type @var{type}, const semantic_type& @var{value}, const location_type& @var{location})
+Build a complete terminal symbol which token type is @var{type}, and which
+semantic value is @var{value}. If location tracking is enabled, also pass
+the @var{location}.
+@end deftypemethod
+
+This interface is low-level and should not be used for two reasons. First,
+it is inconvenient, as you still have to build the semantic value, which is
+a variant, and second, because consistency is not enforced: as with unions,
+it is still possible to give an integer as semantic value for a string.
+
+So for each token type, Bison generates named constructors as follows.
+
+@deftypemethod {symbol_type} {} make_@var{token} (const @var{value_type}& @var{value}, const location_type& @var{location})
+@deftypemethodx {symbol_type} {} make_@var{token} (const location_type& @var{location})
+Build a complete terminal symbol for the token type @var{token} (not
+including the @code{api.tokens.prefix}) whose possible semantic value is
+@var{value} of adequate @var{value_type}. If location tracking is enabled,
+also pass the @var{location}.
+@end deftypemethod
+
+For instance, given the following declarations:
+
+@example
+%define api.tokens.prefix "TOK_"
+%token <std::string> IDENTIFIER;
+%token <int> INTEGER;
+%token COLON;
+@end example
+
+@noindent
+Bison generates the following functions:
+
+@example
+symbol_type make_IDENTIFIER(const std::string& v,
+ const location_type& l);
+symbol_type make_INTEGER(const int& v,
+ const location_type& loc);
+symbol_type make_COLON(const location_type& loc);
+@end example
+
+@noindent
+which should be used in a Lex-scanner as follows.
+
+@example
+[0-9]+ return yy::parser::make_INTEGER(text_to_int (yytext), loc);
+[a-z]+ return yy::parser::make_IDENTIFIER(yytext, loc);
+":" return yy::parser::make_COLON(loc);
+@end example
+
+Tokens that do not have an identifier are not accessible: you cannot simply
+use characters such as @code{':'}, they must be declared with @code{%token}.
@node A Complete C++ Example
@subsection A Complete C++ Example
This section demonstrates the use of a C++ parser with a simple but
complete example. This example should be available on your system,
-ready to compile, in the directory @dfn{../bison/examples/calc++}. It
+ready to compile, in the directory @dfn{.../bison/examples/calc++}. It
focuses on the use of Bison, therefore the design of the various C++
classes is very naive: no accessors, no encapsulation of members etc.
We will use a Lex scanner, and more precisely, a Flex scanner, to
-demonstrate the various interaction. A hand written scanner is
+demonstrate the various interactions. A hand-written scanner is
actually easier to interface with.
@menu
@comment file: calc++-driver.hh
@example
// Tell Flex the lexer's prototype ...
-# define YY_DECL \
- yy::calcxx_parser::token_type \
- yylex (yy::calcxx_parser::semantic_type* yylval, \
- yy::calcxx_parser::location_type* yylloc, \
- calcxx_driver& driver)
+# define YY_DECL \
+ yy::calcxx_parser::symbol_type yylex (calcxx_driver& driver)
// ... and declare it for the parser's sake.
YY_DECL;
@end example
@end example
@noindent
-To encapsulate the coordination with the Flex scanner, it is useful to
-have two members function to open and close the scanning phase.
+To encapsulate the coordination with the Flex scanner, it is useful to have
+member functions to open and close the scanning phase.
@comment file: calc++-driver.hh
@example
@comment file: calc++-driver.hh
@example
- // Run the parser. Return 0 on success.
+ // Run the parser on file F.
+ // Return 0 on success.
int parse (const std::string& f);
+ // The name of the file being parsed.
+ // Used later to pass the file name to the location tracker.
std::string file;
+ // Whether parser traces should be generated.
bool trace_parsing;
@end example
%define parser_class_name "calcxx_parser"
@end example
+@noindent
+@findex %define variant
+@findex %define lex_symbol
+This example will use genuine C++ objects as semantic values, therefore, we
+require the variant-based interface. To make sure we properly use it, we
+enable assertions. To fully benefit from type-safety and more natural
+definition of ``symbol'', we enable @code{lex_symbol}.
+
+@comment file: calc++-parser.yy
+@example
+%define variant
+%define parse.assert
+%define lex_symbol
+@end example
+
@noindent
@findex %code requires
-Then come the declarations/inclusions needed to define the
-@code{%union}. Because the parser uses the parsing driver and
-reciprocally, both cannot include the header of the other. Because the
+Then come the declarations/inclusions needed by the semantic values.
+Because the parser uses the parsing driver and reciprocally, both would like
+to include the header of the other, which is, of course, insane. This
+mutual dependency will be broken using forward declarations. Because the
driver's header needs detailed knowledge about the parser class (in
-particular its inner types), it is the parser's header which will simply
-use a forward declaration of the driver.
-@xref{Decl Summary, ,%code}.
+particular its inner types), it is the parser's header which will use a
+forward declaration of the driver. @xref{Decl Summary, ,%code}.
@comment file: calc++-parser.yy
@example
-%code requires @{
+%code requires
+@{
# include <string>
class calcxx_driver;
@}
@comment file: calc++-parser.yy
@example
// The parsing context.
-%parse-param @{ calcxx_driver& driver @}
-%lex-param @{ calcxx_driver& driver @}
+%param @{ calcxx_driver& driver @}
@end example
@noindent
-Then we request the location tracking feature, and initialize the
-first location's file name. Afterwards new locations are computed
+Then we request location tracking, and initialize the
+first location's file name. Afterward new locations are computed
relatively to the previous locations: the file name will be
-automatically propagated.
+propagated.
@comment file: calc++-parser.yy
@example
@end example
@noindent
-Use the two following directives to enable parser tracing and verbose
+Use the following two directives to enable parser tracing and verbose
error messages.
@comment file: calc++-parser.yy
@example
%define parse.trace
-%define parse.error "verbose"
-@end example
-
-@noindent
-Semantic values cannot use ``real'' objects, but only pointers to
-them.
-
-@comment file: calc++-parser.yy
-@example
-// Symbols.
-%union
-@{
- int ival;
- std::string *sval;
-@};
+%define parse.error verbose
@end example
@noindent
@comment file: calc++-parser.yy
@example
-%code @{
+%code
+@{
# include "calc++-driver.hh"
@}
@end example
@comment file: calc++-parser.yy
@example
%define api.tokens.prefix "TOK_"
-%token END 0 "end of file"
-%token ASSIGN ":="
-%token <sval> IDENTIFIER "identifier"
-%token <ival> NUMBER "number"
-%type <ival> exp
+%token
+ END 0 "end of file"
+ ASSIGN ":="
+ MINUS "-"
+ PLUS "+"
+ STAR "*"
+ SLASH "/"
+ LPAREN "("
+ RPAREN ")"
+;
@end example
@noindent
-To enable memory deallocation during error recovery, use
-@code{%destructor}.
+Since we use variant-based semantic values, @code{%union} is not used, and
+both @code{%type} and @code{%token} expect genuine types, as opposed to type
+tags.
-@c FIXME: Document %printer, and mention that it takes a braced-code operand.
@comment file: calc++-parser.yy
@example
-%printer @{ debug_stream () << *$$; @} "identifier"
-%destructor @{ delete $$; @} "identifier"
+%token <std::string> IDENTIFIER "identifier"
+%token <int> NUMBER "number"
+%type <int> exp
+@end example
+
+@noindent
+No @code{%destructor} is needed to enable memory deallocation during error
+recovery; the memory, for strings for instance, will be reclaimed by the
+regular destructors. All the values are printed using their
+@code{operator<<}.
-%printer @{ debug_stream () << $$; @} <ival>
+@c FIXME: Document %printer, and mention that it takes a braced-code operand.
+@comment file: calc++-parser.yy
+@example
+%printer @{ debug_stream () << $$; @} <*>;
@end example
@noindent
-The grammar itself is straightforward.
+The grammar itself is straightforward (@pxref{Location Tracking Calc, ,
+Location Tracking Calculator: @code{ltcalc}}).
@comment file: calc++-parser.yy
@example
| /* Nothing. */ @{@};
assignment:
- "identifier" ":=" exp
- @{ driver.variables[*$1] = $3; delete $1; @};
+ "identifier" ":=" exp @{ driver.variables[$1] = $3; @};
-%left '+' '-';
-%left '*' '/';
+%left "+" "-";
+%left "*" "/";
exp:
- exp '+' exp @{ $$ = $1 + $3; @}
-| exp '-' exp @{ $$ = $1 - $3; @}
-| exp '*' exp @{ $$ = $1 * $3; @}
-| exp '/' exp @{ $$ = $1 / $3; @}
-| '(' exp ')' @{ $$ = $2; @}
-| "identifier" @{ $$ = driver.variables[*$1]; delete $1; @}
-| "number" @{ $$ = $1; @};
+ exp "+" exp @{ $$ = $1 + $3; @}
+| exp "-" exp @{ $$ = $1 - $3; @}
+| exp "*" exp @{ $$ = $1 * $3; @}
+| exp "/" exp @{ $$ = $1 / $3; @}
+| "(" exp ")" @{ std::swap ($$, $2); @}
+| "identifier" @{ $$ = driver.variables[$1]; @}
+| "number" @{ std::swap ($$, $1); @};
%%
@end example
@comment file: calc++-parser.yy
@example
void
-yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l,
+yy::calcxx_parser::error (const location_type& l,
const std::string& m)
@{
driver.error (l, m);
@comment file: calc++-scanner.ll
@example
%@{ /* -*- C++ -*- */
-# include <cstdlib>
# include <cerrno>
# include <climits>
+# include <cstdlib>
# include <string>
# include "calc++-driver.hh"
# include "calc++-parser.hh"
-/* Work around an incompatibility in flex (at least versions
- 2.5.31 through 2.5.33): it generates code that does
- not conform to C89. See Debian bug 333231
- <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>. */
+// Work around an incompatibility in flex (at least versions
+// 2.5.31 through 2.5.33): it generates code that does
+// not conform to C89. See Debian bug 333231
+// <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>.
# undef yywrap
# define yywrap() 1
-/* By default yylex returns an int; we use token_type.
- The default yyterminate implementation returns 0, which is
- not of token_type. */
-#define yyterminate() return TOKEN(END)
+// The location of the current token.
+static yy::location loc;
%@}
@end example
Because there is no @code{#include}-like feature we don't need
@code{yywrap}, we don't need @code{unput} either, and we parse an
actual file, this is not an interactive session with the user.
-Finally we enable the scanner tracing features.
+Finally, we enable scanner tracing.
@comment file: calc++-scanner.ll
@example
@noindent
The following paragraph suffices to track locations accurately. Each
time @code{yylex} is invoked, the begin position is moved onto the end
-position. Then when a pattern is matched, the end position is
-advanced of its width. In case it matched ends of lines, the end
+position. Then when a pattern is matched, its width is added to the end
+column. When matching ends of lines, the end
cursor is adjusted, and each time blanks are matched, the begin cursor
is moved onto the end cursor to effectively ignore the blanks
preceding tokens. Comments would be treated equally.
@comment file: calc++-scanner.ll
@example
%@{
-# define YY_USER_ACTION yylloc->columns (yyleng);
+ // Code run each time a pattern is matched.
+ # define YY_USER_ACTION loc.columns (yyleng);
%@}
%%
%@{
- yylloc->step ();
+ // Code run each time yylex is called.
+ loc.step ();
%@}
-@{blank@}+ yylloc->step ();
-[\n]+ yylloc->lines (yyleng); yylloc->step ();
+@{blank@}+ loc.step ();
+[\n]+ loc.lines (yyleng); loc.step ();
@end example
@noindent
-The rules are simple. The driver is used to report errors. It is
-convenient to use a macro to shorten
-@code{yy::calcxx_parser::token::TOK_@var{Name}} into
-@code{TOKEN(@var{Name})}; note the token prefix, @code{TOK_}.
+The rules are simple. The driver is used to report errors.
@comment file: calc++-scanner.ll
@example
-%@{
-# define TOKEN(Name) \
- yy::calcxx_parser::token::TOK_ ## Name
-%@}
- /* Convert ints to the actual type of tokens. */
-[-+*/()] return yy::calcxx_parser::token_type (yytext[0]);
-":=" return TOKEN(ASSIGN);
+"-" return yy::calcxx_parser::make_MINUS(loc);
+"+" return yy::calcxx_parser::make_PLUS(loc);
+"*" return yy::calcxx_parser::make_STAR(loc);
+"/" return yy::calcxx_parser::make_SLASH(loc);
+"(" return yy::calcxx_parser::make_LPAREN(loc);
+")" return yy::calcxx_parser::make_RPAREN(loc);
+":=" return yy::calcxx_parser::make_ASSIGN(loc);
+
@{int@} @{
errno = 0;
long n = strtol (yytext, NULL, 10);
if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
- driver.error (*yylloc, "integer is out of range");
- yylval->ival = n;
- return TOKEN(NUMBER);
+ driver.error (loc, "integer is out of range");
+ return yy::calcxx_parser::make_NUMBER(n, loc);
@}
-@{id@} @{
- yylval->sval = new std::string (yytext);
- return TOKEN(IDENTIFIER);
-@}
-. driver.error (*yylloc, "invalid character");
+@{id@} return yy::calcxx_parser::make_IDENTIFIER(yytext, loc);
+. driver.error (loc, "invalid character");
+<<EOF>> return yy::calcxx_parser::make_END(loc);
%%
@end example
@noindent
-Finally, because the scanner related driver's member function depend
+Finally, because the scanner-related driver's member-functions depend
on the scanner's data, it is simpler to implement them in this file.
@comment file: calc++-scanner.ll
yyin = stdin;
else if (!(yyin = fopen (file.c_str (), "r")))
@{
- error (std::string ("cannot open ") + file);
+ error (std::string ("cannot open ") + file + ": " + strerror(errno));
exit (1);
@}
@}
access the token names and codes.
Getting a ``code too large'' error from the Java compiler means the code
-hit the 64KB bytecode per method limination of the Java class file.
+hit the 64KB bytecode per method limitation of the Java class file.
Try reducing the amount of code in actions and static initializers;
otherwise, report a bug so that the parser skeleton will be improved.
a range composed of a pair of positions (possibly spanning several
files). The location class is an inner class of the parser; the name
is @code{Location} by default, and may also be renamed using
-@samp{%define location_type "@var{class-name}}.
+@samp{%define location_type "@var{class-name}"}.
The location class treats the position as a completely opaque value.
By default, the class name is @code{Position}, but this can be changed
@deftypeop {Constructor} {YYParser} {} YYParser (@var{lex_param}, @dots{}, @var{parse_param}, @dots{})
Build a new parser object with embedded @code{%code lexer}. There are
-no parameters, unless @code{%parse-param}s and/or @code{%lex-param}s are
-used.
+no parameters, unless @code{%param}s and/or @code{%parse-param}s and/or
+@code{%lex-param}s are used.
Use @code{%code init} for code added to the start of the constructor
body. This is especially useful to initialize superclasses. Use
-@samp{%define init_throws} to specify any uncatch exceptions.
+@samp{%define init_throws} to specify any uncaught exceptions.
@end deftypeop
@deftypeop {Constructor} {YYParser} {} YYParser (Lexer @var{lexer}, @var{parse_param}, @dots{})
Build a new parser object using the specified scanner. There are no
-additional parameters unless @code{%parse-param}s are used.
+additional parameters unless @code{%param}s and/or @code{%parse-param}s are
+used.
If the scanner is defined by @code{%code lexer}, this constructor is
declared @code{protected} and is called automatically with a scanner
-created with the correct @code{%lex-param}s.
+created with the correct @code{%param}s and/or @code{%lex-param}s.
Use @code{%code init} for code added to the start of the constructor
body. This is especially useful to initialize superclasses. Use
@deftypemethod {YYParser} {boolean} getErrorVerbose ()
@deftypemethodx {YYParser} {void} setErrorVerbose (boolean @var{verbose})
Get or set the option to produce verbose error messages. These are only
-available with @samp{%define parse.error "verbose"}, which also turns on
+available with @samp{%define parse.error verbose}, which also turns on
verbose error messages.
@end deftypemethod
@deftypemethod {Lexer} {int} yylex ()
Return the next token. Its type is the return value, its semantic
-value and location are saved and returned by the ther methods in the
+value and location are saved and returned by the their methods in the
interface.
Use @samp{%define lex_throws} to specify any uncaught exceptions.
@end deftypemethod
@deftypemethod {Lexer} {Object} getLVal ()
-Return the semantical value of the last token that yylex returned.
+Return the semantic value of the last token that yylex returned.
The return type can be changed using @samp{%define stype
"@var{class-name}".}
@xref{Error Recovery}.
@end deffn
-@deffn {Statement} {return YYFAIL;}
-Print an error message and start error recovery.
-@xref{Error Recovery}.
-@end deffn
-
@deftypefn {Function} {boolean} recovering ()
Return whether error recovery is being done. In this state, the parser
reads token until it reaches a known state, and then restarts normal
@item
Java lacks unions, so @code{%union} has no effect. Instead, semantic
values have a common base type: @code{Object} or as specified by
-@samp{%define stype}. Angle backets on @code{%token}, @code{type},
+@samp{%define stype}. Angle brackets on @code{%token}, @code{type},
@code{$@var{n}} and @code{$$} specify subtypes rather than fields of
an union. The type of @code{$$}, even with angle brackets, is the base
type since Java casts are not allow on the left-hand side of assignments.
@pxref{Java Action Features}.
@item
-The prolog declarations have a different meaning than in C/C++ code.
+The prologue declarations have a different meaning than in C/C++ code.
@table @asis
@item @code{%code imports}
blocks are placed at the beginning of the Java source code. They may
side of the rule. @xref{Locations, , Locations Overview}.
@end deffn
+@deffn {Variable} @@@var{name}
+In an action, the location of a symbol addressed by name.
+@xref{Locations, , Locations Overview}.
+@end deffn
+
+@deffn {Variable} @@[@var{name}]
+In an action, the location of a symbol addressed by name.
+@xref{Locations, , Locations Overview}.
+@end deffn
+
@deffn {Variable} $$
In an action, the semantic value of the left-hand side of the rule.
@xref{Actions}.
right-hand side of the rule. @xref{Actions}.
@end deffn
+@deffn {Variable} $@var{name}
+In an action, the semantic value of a symbol addressed by name.
+@xref{Actions}.
+@end deffn
+
+@deffn {Variable} $[@var{name}]
+In an action, the semantic value of a symbol addressed by name.
+@xref{Actions}.
+@end deffn
+
@deffn {Delimiter} %%
Delimiter used to separate the grammar rule section from the
Bison declarations section or the epilogue.
Grammar}.
@end deffn
+@deffn {Directive} %?@{@var{expression}@}
+Predicate actions. This is a type of action clause that may appear in
+rules. The expression is evaluated, and if false, causes a syntax error. In
+@acronym{GLR} parsers during nondeterministic operation,
+this silently causes an alternative parse to die. During deterministic
+operation, it is the same as the effect of YYERROR.
+@xref{Semantic Predicates}.
+
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
+@end deffn
+
@deffn {Construct} /*@dots{}*/
Comment delimiters, as in C.
@end deffn
@deffn {Directive} %define @var{define-variable}
@deffnx {Directive} %define @var{define-variable} @var{value}
+@deffnx {Directive} %define @var{define-variable} "@var{value}"
Define a variable to adjust Bison's behavior.
@xref{Decl Summary,,%define}.
@end deffn
@end deffn
@deffn {Directive} %error-verbose
-An obsolete directive standing for @samp{%define parse.error "verbose"}.
+An obsolete directive standing for @samp{%define parse.error verbose}.
@end deffn
@deffn {Directive} %file-prefix "@var{prefix}"
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn
-@deffn {Directive} %lex-param @{@var{argument-declaration}@}
-Bison declaration to specifying an additional parameter that
+@deffn {Directive} %lex-param @{@var{argument-declaration}@} @dots{}
+Bison declaration to specifying additional arguments that
@code{yylex} should accept. @xref{Pure Calling,, Calling Conventions
for Pure Parsers}.
@end deffn
Summary}.
@end deffn
-@deffn {Directive} %parse-param @{@var{argument-declaration}@}
-Bison declaration to specifying an additional parameter that
-@code{yyparse} should accept. @xref{Parser Function,, The Parser
-Function @code{yyparse}}.
+@deffn {Directive} %param @{@var{argument-declaration}@} @dots{}
+Bison declaration to specify additional arguments that both
+@code{yylex} and @code{yyparse} should accept. @xref{Parser Function,, The
+Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@} @dots{}
+Bison declaration to specify additional arguments that @code{yyparse}
+should accept. @xref{Parser Function,, The Parser Function @code{yyparse}}.
@end deffn
@deffn {Directive} %prec
with @code{#define} in the prologue to request verbose, specific error
message strings when @code{yyerror} is called. It doesn't matter what
definition you use for @code{YYERROR_VERBOSE}, just whether you define
-it. Using @samp{%define parse.error "verbose"} is preferred
+it. Using @samp{%define parse.error verbose} is preferred
(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
@end deffn
@item Input stream
A continuous flow of data between devices or programs.
+@item @acronym{LAC} (Lookahead Correction)
+A parsing mechanism that fixes the problem of delayed syntax error
+detection, which is caused by LR state merging, default reductions, and
+the use of @code{%nonassoc}. Delayed syntax error detection results in
+unexpected semantic actions, initiation of error recovery in the wrong
+syntactic context, and an incorrect list of expected tokens in a verbose
+syntax error message. @xref{Decl Summary,,parse.lac}.
+
@item Language construct
One of the typical usage schemas of the language. For example, one of
the constructs of the C language is the @code{if} statement.
@bye
+@c LocalWords: texinfo setfilename settitle setchapternewpage finalout texi FSF
+@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex FSF's
+@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry Naur
+@c LocalWords: ifset vskip pt filll insertcopying sp ISBN Etienne Suvasa Multi
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projects / bison.git / blobdiff