(bison.info) Actions

 
 Actions
 -------
 
    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 is to compute a semantic value for the grouping built by
 the rule from the semantic values associated with tokens or smaller
 groupings.
 
    An action consists of C statements surrounded by braces, much like a
 compound statement in C.  It can be placed at any position in the rule;
 it is executed at that position.  Most rules have just one action at
 the end of the rule, following all the components.  Actions in the
 middle of a rule are tricky and used only for special purposes (
 Actions in Mid-Rule Mid-Rule Actions.).
 
    The C code in an action can refer to the semantic values of the
 components matched by the rule with the construct `$N', which stands for
 the value of the Nth component.  The semantic value for the grouping
 being constructed is `$$'.  (Bison translates both of these constructs
 into array element references when it copies the actions into the parser
 file.)
 
    Here is a typical example:
 
      exp:    ...
              | exp '+' exp
                  { $$ = $1 + $3; }
 
 This rule constructs an `exp' from two smaller `exp' groupings
 connected by a plus-sign token.  In the action, `$1' and `$3' refer to
 the semantic values of the two component `exp' groupings, which are the
 first and third symbols on the right hand side of the rule.  The sum is
 stored into `$$' 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 `+' token, it could be
 referred to as `$2'.
 
    If you don't specify an action for a rule, Bison supplies a default:
 `$$ = $1'.  Thus, the value of the first symbol in the rule becomes the
 value of the whole rule.  Of course, the default rule is valid only if
 the two data types match.  There is no meaningful default action for an
 empty rule; every empty rule must have an explicit action unless the
 rule's value does not matter.
 
    `$N' with N zero or negative is allowed for reference to tokens and
 groupings on the stack _before_ those that match the current rule.
 This is a very risky practice, and to use it reliably you must be
 certain of the context in which the rule is applied.  Here is a case in
 which you can use this reliably:
 
      foo:      expr bar '+' expr  { ... }
              | expr bar '-' expr  { ... }
              ;
      
      bar:      /* empty */
              { previous_expr = $0; }
              ;
 
    As long as `bar' is used only in the fashion shown here, `$0' always
 refers to the `expr' which precedes `bar' in the definition of `foo'.
Info Catalog
(bison.info) Multiple Types
(bison.info) Semantics
(bison.info) Action Types
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