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.TL
YACC \- Yet Another Compiler-Compiler
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Stephen C. Johnson
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Computer program input generally has some structure;
in fact, every computer program which does input can be thought of as defining
an ``input language'' which it accepts.
The input languages may be as complex as a programming language, or as simple as
a sequence of numbers.
Unfortunately, standard input facilities
are restricted, difficult to use and change,
and do not completely check their inputs for validity.
.PP
Yacc provides a general tool for controlling
the input to a computer program.
The Yacc user describes the structures
of his input, together with code which is to be invoked when
each such structure is recognized.
Yacc turns such a specification into a subroutine which may
be invoked to handle 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.
.PP
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, if he wishes,
in terms of higher level constructs such as names and numbers.
The user supplied routine may also handle idiomatic features such as
comment and continuation conventions, which typically defy easy specification.
.PP
Yacc is written in C[7], and runs under UNIX.
The subroutine which is output may be in C or in Ratfor[4], at the user\'s choice;
Ratfor permits translation of the output subroutine into portable Fortran[5].
The class of specifications accepted is a very general one, called LALR(1)
grammars with disambiguating rules.
The theory behind Yacc has been described elsewhere[1,2,3].
.PP
Yacc was originally designed to help produce the ``front end'' of
compilers; in addition to this use, it has been successfully used in many application
programs, including a phototypesetter language, a document retrieval system,
a Fortran debugging system, and the Ratfor compiler.
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