.RP.ND "July 31, 1978".TLYacc:Yet Another Compiler-Compiler.AU "MH 2C-559" 3968Stephen C. Johnson.AI.MH.AB.PPComputer program input generally has some structure;in fact, every computer program that does input can be thought of as definingan ``input language'' which it accepts.An input language may be as complex as a programming language, or as simple asa sequence of numbers.Unfortunately, usual input facilitiesare limited, difficult to use,and often are lax about checking their inputs for validity..PPYacc provides a general tool for describingthe input to a computer program.The Yacc user specifies the structuresof his input, together with code to be invoked aseach such structure is recognized.Yacc turns such a specification into a subroutine thathandles the input process;frequently, it is convenient and appropriate to have mostof the flow of control in the user's applicationhandled by this subroutine..PPThe input subroutine produced by Yacc calls a user-supplied routine toreturn the next basic input item.Thus, the user can specify his input in terms of individual input characters, orin terms of higher level constructs such as names and numbers.The user-supplied routine may also handle idiomatic features such ascomment and continuation conventions, which typically defy easy grammatical specification..PPYacc is written in portable C.The class of specifications accepted is a very general one: LALR(1)grammars with disambiguating rules..PPIn addition to compilers for C, APL, Pascal, RATFOR, etc., Yacchas also been used for less conventional languages,including a phototypesetter language, several desk calculator languages, a document retrieval system,and a Fortran debugging system..AE.OKComputer LanguagesCompilersFormal Language Theory.CS 23 11 34 0 0 8