The user-supplied routine may also handle idiomatic features such as comment and continuation conventions, which typically defy easy grammatical specification. The class of specifications accepted is a very general one: LALR(1) grammars with disambiguating rules.
In addition to compilers for C, APL, Pascal, RATFOR, etc., Yacc has also been used for less conventional languages, including a phototypesetter language, several desk calculator languages, a document retrieval system, and a Fortran debugging system.
The Yacc user specifies the structures of his input, together with code to be invoked as each such structure is recognized.
Yacc turns such a specification into a subroutine that handles the input process; frequently, it is convenient and appropriate to have most of the flow of control in the user's application handled by this subroutine.
Yacc then generates a function to control the input process.
This function, called a parser, calls the user-supplied low-level input routine (the lexical analyzer) to pick up the basic items (called tokens) from the input stream.
Unfortunately, usual input facilities are limited, difficult to use, and often are lax about checking their inputs for validity.
Yacc provides a general tool for describing the input to a computer program.
Yacc provides a general tool for imposing structure on the input to a computer program.
The Yacc user prepares a specification of the input process; this includes rules describing the input structure, code to be invoked when these rules are recognized, and a low-level routine to do the basic input.
Johnson AT&T Bell Laboratories Murray Hill, New Jersey 07974 Computer program input generally has some structure; in fact, every computer program that does input can be thought of as defining an ``input language'' which it accepts.
An input language may be as complex as a programming language, or as simple as a sequence of numbers.
The input subroutine produced by Yacc calls a user-supplied routine to return the next basic input item.
Thus, the user can specify his input in terms of individual input characters, or in terms of higher level constructs such as names and numbers.