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.SH
Section 2: Actions
.PP
To each grammar rule, the user may associate an action to be performed each time
the rule is recognized in the input process.
This action may return a value, and may obtain the values returned by previous
actions in the grammar rule.
In addition, the lexical analyzer can return values
for tokens, if desired.
.PP
When invoking Yacc, the user specifies a programming language; currently, Ratfor and C are supported.
An action is an arbitrary statement in this language, and as such can do
input and output, call subprograms, and alter
external vectors and variables (recall that a ``statement'' in both C and Ratfor can be compound
and do many distinct tasks).
An action is specified by an equal sign ``=''
at the end of a grammar rule, followed by
one or more statements, enclosed in curly braces ``{'' and ``}''.
For example,
.DS
A: \'(\' B \')\' = { hello( 1, "abc" );  }
.DE
and
.DS
XXX: YYY ZZZ =
	{
		printf("a message\en");
		flag = 25;
	}
.DE
are grammar rules with actions in C.
A grammar rule with an action need not end with a semicolon; in fact, it is an error to have a semicolon
before the equal sign.
.PP
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.
.PP
To return a value, the action normally sets the
pseudo-variable ``$$'' to some integer value.
For example, an action which does nothing but return the value 1 is
.DS
= { $$ = 1; }
.DE
.PP
To obtain the values returned by previous actions and the lexical analyzer, the
action may use the (integer) 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
.DS
A: B C D ;
.DE
for example, then $2 has the value returned by C, and $3 the value returned by D.
.PP
As a more concrete example, we might have the rule
.DS
expression: \'(\' expression \')\' ;
.DE
We wish the value returned by this rule to be the value of the expression in parentheses.
Then we write
.DS
expression: \'(\' expression \')\'    = { $$ = $2 ; }
.DE
.PP
As a default, the value of a rule is the value of the first element in it ($1).
This is true even if there is no explicit action given for the rule.
Thus, grammar rules of the form
.DS
A: B ;
.DE
frequently need not have an explict action.
.PP
Notice that, although the values of actions are integers, these integers may in fact
contain pointers (in C) or indices into an array (in Ratfor); in this way,
actions can return and reference more complex data structures.
.PP
Sometimes, we wish to get control before a rule is fully parsed, as well as at the
end of the rule.
There is no explicit mechanism in Yacc to allow this; the same effect can be obtained, however,
by introducing a new symbol which matches the empty string, and inserting an action for this symbol.
For example, we might have a rule describing an ``if'' statement:
.DS
statement: IF \'(\' expr \')\' THEN statement
.DE
Suppose that we wish to get control after seeing the right parenthesis
in order to output some code.
We might accomplish this by the rules:
.DS
statement:  IF \'(\' expr \')\' actn THEN statement 
	= { call action1 }

actn:   /* matches the empty string */
	= { call action2 }
.DE
.PP
Thus, the new nonterminal symbol actn matches no input, but serves only to call action2 after the
right parenthesis is seen.
.PP
Frequently, it is more natural in such cases to break the rule into
parts where the action is needed.
Thus, the above example might also have been written
.DS
statement:  ifpart THEN statement
	= { call action1 }

ifpart:      IF \'(\' expr \')\'
	= { call action2 }
.DE
.PP
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 which build and maintain the tree
structure desired.
For example, suppose we have a C function
``node'', written so that the call
.DS
node( L, n1, n2 )
.DE
creates a node with label L, and descendants n1 and n2, and returns a pointer
to the newly created node.
Then we can cause a parse tree to be built by supplying actions such as:
.DS
expr: expr \'+\' expr 
	= { $$ = node( \'+\', $1, $3 ); }
.DE
in our specification.
.PP
The user may define other variables to be used by the actions.
Declarations and definitions can appear in two places in the
Yacc specification: in the declarations section, and at the head of the rules sections, before the
first grammar rule.
In each case, the declarations and definitions are enclosed in the marks ``%{'' and ``%}''.
Declarations and definitions placed in the declarations section have global scope, 
and are thus known to the action statements and the lexical analyzer.
Declarations and definitions placed at the head of the rules section have scope local to
the action statements.
Thus, in the above example, we might have included
.DS
%{ int variable 0; %}
.DE
in the declarations section, or, perhaps,
.DS
%{ static int variable; %}
.DE
at the head of the rules section.
If we were writing Ratfor actions, we might want to include some
COMMON statements at the beginning of the rules section, to allow for
easy communication between the actions and other routines.
For both C and Ratfor, Yacc has used only external names beginning in ``yy'';
the user should avoid such names.