5.5.1 YACC Metalanguage

The YACC metalanguage has a form similar to that for LEX:

Rules <-- BNF plus associated actions

User-Written Procedures <-- In C

The Definitions section can contain the typical things found at the top of a C program: constant definitions, structure declarations, include statements, and user variable definitions. Tokens may also be defined here.

The Rules section contains the BNF. The left-hand side is separated from the right-hand side by a color, ":". The actions may appear interspersed in the right-hand side; this means they will be executed when all the tokens or nonterminals to the action's left are shifted. If the action occurs after a production, it will be invoked when a reduction ( of the right-hand side to the left-hand side of a production ) is made.

Thus, a YACC input file looks like the following:

	%{
	#include 1
	#include 2
	#define ...
	%}
	%token ...
	...
	%%
	Nonterminal: Right-Hand side {semantic action 1}
		   | Right-hand side {semantic action 2}
	...
	%%
	C functions

Example 6 shows a YACC input file for the language consisting of sequences of assignment statements.

EXAMPLE 6 Sequences of assignment statements in YACC

	%{
	#include <stdio.h>
	%}
	%start program
	%token Id
	%token Lit
	%%

Program:	Statements	        {printf("Program \n");};
Statements:	Statement Statements	{printf("Statements \n)";};
		| Statement	        {printf("Statements \n");};
Statement:	AsstStmt	        {printf("Statement \n");};
AsstStmt:	Id ":=" Expression	{printf("AsstStmt \n");};
Expression:	Expression "=" Term	{printf("Expression \n");};
		| Expression "-" Term 	{printf("Expression \n");};
		| Term			{printf("Expression \n");};
Term:		Term "*" Factor		{printf("Term \n");};
		| Term "/" Factor	{printf("Term \n");};
		| Factor		{printf("Term \n");};
Factor:		"(" Expression ")"	{printf("Factor \n");};
		| Id			{printf("Factor \n");};
		| Lit			{printf("Factor \n");};
%%
#include lex.yy.c
 
main()
{
  yyparse ();
}

In Example 6, the lexical analyzer is the one output by LEX, or users may write their own ( and call it "lex.yy.c"). Alternatively, a function called "yylex()", consisting of code to find tokens, could be written in either the definition section or the User-Written Procedures section.

YACC contain facilities for specifying precedence and associativity of operators and for specifying errors. An error production in YACC is of the form:

B error { Action }

where is an erroneous construct.

We will look at this example again in Chapter 6 when we discuss semantic analysis.

Send questions and comments to: Karen Lemone