2: Actions

With each grammar rule, the user may associate actions to be performed each time the rule is recognized in the input process. These actions may return values, and may obtain the values returned by previous actions. Moreover, the lexical analyzer can return values for tokens, if desired.

An action is an arbitrary C statement, and as such can do input and output, call subprograms, and alter external vectors and variables. An action is specified by one or more statements, enclosed in curly braces ``{'' and ``}''. For example,

A	:	'('  B  ')'
			{	hello( 1, "abc" );  }
			{	printf("a message\n");
				flag = 25;   }
are grammar rules with actions.

To facilitate easy communication between the actions and the parser, the action statements are altered slightly. The symbol ``dollar sign'' ``$'' is used as a signal to Yacc in this context.

To return a value, the action normally sets the pseudo-variable ``$$'' to some value. For example, an action that does nothing but return the value 1 is

	{  $$ = 1;  }

To obtain the values returned by previous actions and the lexical analyzer, the action may use the pseudo-variables $1, $2, . . ., which refer to the values returned by the components of the right side of a rule, reading from left to right. Thus, if the rule is

A	:	B  C  D   ;
for example, then $2 has the value returned by C, and $3 the value returned by D.

As a more concrete example, consider the rule

expr	:	'('  expr  ')'   ;
The value returned by this rule is usually the value of the expr in parentheses. This can be indicated by
expr	:	 '('  expr  ')'		{  $$ = $2 ;  }

By default, the value of a rule is the value of the first element in it ($1). Thus, grammar rules of the form

A	:	B    ;
frequently need not have an explicit action.

In the examples above, all the actions came at the end of their rules. Sometimes, it is desirable to get control before a rule is fully parsed. Yacc permits an action to be written in the middle of a rule as well as at the end. This rule is assumed to return a value, accessible through the usual $ mechanism by the actions to the right of it. In turn, it may access the values returned by the symbols to its left. Thus, in the rule

A	:	B
			{  $$ = 1;  }
			{   x = $2;   y = $3;  }
the effect is to set x to 1, and y to the value returned by C.

Actions that do not terminate a rule are actually handled by Yacc by manufacturing a new nonterminal symbol name, and a new rule matching this name to the empty string. The interior action is the action triggered off by recognizing this added rule. Yacc actually treats the above example as if it had been written:

$ACT	:	/* empty */
			{  $$ = 1;  }

A	:	B  $ACT  C
			{   x = $2;   y = $3;  }

In many applications, output is not done directly by the actions; rather, a data structure, such as a parse tree, is constructed in memory, and transformations are applied to it before output is generated. Parse trees are particularly easy to construct, given routines to build and maintain the tree structure desired. For example, suppose there is a C function node , written so that the call

node( L, n1, n2 )
creates a node with label L, and descendants n1 and n2, and returns the index of the newly created node. Then parse tree can be built by supplying actions such as:
expr	:	expr  '+'  expr  
			{  $$ = node( '+', $1, $3 );  }
in the specification.

The user may define other variables to be used by the actions. Declarations and definitions can appear in the declarations section, enclosed in the marks ``%{'' and ``%}''. These declarations and definitions have global scope, so they are known to the action statements and the lexical analyzer. For example,

%{   int variable = 0;   %}
could be placed in the declarations section, making variable accessible to all of the actions. The Yacc parser uses only names beginning in ``yy''; the user should avoid such names.

In these examples, all the values are integers: a discussion of values of other types will be found in Section 10.