Document named references.

* doc/bison.texinfo (Actions): Add new example and xref to
	Using Named References node.
	(Using Named References): New node.

ChangeLog

@@ -1,3 +1,10 @@
+2009-10-03  Alex Rozenman  <rozenman@gmail.com>
+
+	Document named references.
+	* doc/bison.texinfo (Actions): Add new example and xref to
+	Using Named References node.
+	(Using Named References): New node.
+
 2009-10-16  Joel E. Denny  <jdenny@clemson.edu>
 
 	cleanup.

doc/bison.texinfo

@@ -206,6 +206,7 @@ Defining Language Semantics
 * 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
 
@@ -3365,6 +3366,7 @@ the numbers associated with @var{x} and @var{y}.
 * 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
@@ -3426,6 +3428,8 @@ Decl, ,Nonterminal Symbols}).
 @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
@@ -3442,9 +3446,12 @@ Actions, ,Actions in Mid-Rule}).
 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:
@@ -3457,16 +3464,31 @@ exp:    @dots{}
 @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
@@ -3761,6 +3783,93 @@ compound: subroutine
 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
@@ -3816,6 +3925,8 @@ Action Decl, , Performing Actions before Parsing}.
 @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.
@@ -3827,6 +3938,11 @@ The location of the @var{n}th component of the right hand side is
 @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
@@ -9889,6 +10005,16 @@ In an action, the location of the @var{n}-th symbol of the right-hand
 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}.
@@ -9899,6 +10025,16 @@ In an action, the semantic value of the @var{n}-th symbol of the
 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.