* Recursion:: Writing recursive rules.
* Semantics:: Semantic values and actions.
* Locations:: Locations and actions.
+* Named References:: Using named references in actions.
* Declarations:: All kinds of Bison declarations are described here.
* Multiple Parsers:: Putting more than one Bison parser in one program.
* 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
* Recursion:: Writing recursive rules.
* Semantics:: Semantic values and actions.
* Locations:: Locations and actions.
+* Named References:: Using named references in actions.
* Declarations:: All kinds of Bison declarations are described here.
* Multiple Parsers:: Putting more than one Bison parser in one program.
@end menu
* 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
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
- @{ $$ = $1 / $3; @} // No error.
-@end group
-@end example
-
-@noindent
-When ambiguity occurs, explicitly declared names may be used for values and
-locations. Explicit names are declared as a bracketed name after a symbol
-appearance in rule definitions. For example:
-@example
-@group
-exp[result]: exp[left] '/' exp[right]
- @{ $result = $left / $right; @}
-@end group
-@end example
-
-@noindent
-Explicit names may be declared for RHS and for LHS symbols as well. In order
-to access a semantic value generated by a mid-rule action, an explicit name
-may also be declared by putting a bracketed name after the closing brace of
-the mid-rule action code:
-@example
-@group
-exp[res]: exp[x] '+' @{$left = $x;@}[left] exp[right]
- @{ $res = $left + $right; @}
-@end group
-@end example
-
-@noindent
-
-In references, in order to specify names containing dots and dashes, an explicit
-bracketed syntax @code{$[name]} and @code{@@[name]} must be used:
-@example
-@group
-if-stmt: IF '(' expr ')' THEN then.stmt ';'
- @{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); @}
-@end group
-@end example
-
-It often happens that named references are followed by a dot, dash or other
-C punctuation marks and operators. By default, Bison will read
-@code{$name.suffix} as a reference to symbol value @code{$name} followed by
-@samp{.suffix}, i.e., an access to the @samp{suffix} field of the semantic
-value. In order to force Bison to recognize @code{name.suffix} in its entirety
-as the name of a semantic value, bracketed syntax @code{$[name.suffix]}
-must be used.
-
-
@node Locations
@section Tracking Locations
@cindex location
statement when it is followed by a semicolon.
@end itemize
+@node Named References
+@section Using Named References
+@cindex named references
+
+As described in the preceding sections, the traditional way to refer to any
+semantic value or location is a @dfn{positional reference}, which takes the
+form @code{$@var{n}}, @code{$$}, @code{@@@var{n}}, and @code{@@$}. However,
+such a reference is not very descriptive. Moreover, if you later decide to
+insert or remove symbols in the right-hand side of a grammar rule, the need
+to renumber such references can be tedious and error-prone.
+
+To avoid these issues, you can also refer to a semantic value or location
+using a @dfn{named reference}. 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
+Positional and named references can be mixed 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 Declarations
@section Bison Declarations
@cindex declarations, Bison
projects / bison.git / blobdiff