Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!gem.mps.ohio-state.edu!apple!usc!henry.jpl.nasa.gov!elroy.jpl.nasa.gov!peregrine!ccicpg!taurus!capone!ndjc
From: ndjc@capone.UUCP (Nick Crossley)
Newsgroups: comp.unix.questions
Subject: Re: Any YACC / BISON gurus out there?
Summary: You might be able to use YYBACKUP
Keywords: yacc bison
Message-ID: <278@capone.UUCP>
Date: 27 Oct 89 00:37:37 GMT
References: <1341@ssf>
Reply-To: ndjc@ccicpg.UUCP (Nick Crossley)
Organization: ICL NA, Irvine CA
Lines: 24

In article <1341@ssf> richard@ssf.uucp (Richard Thombs) writes:
>I need to be able to get YACC (or BISON if need be) to fail a parse when an
>action tells it to.

Some versions of yacc have a YYBACKUP macro in the parser skeleton.
This allows limited backup, possibly not as much as you want.  I have
found it most useful to reduce the number of 'reserved words' in a
grammar:

rule	:	identifier	{ YYBACKUP(special($1)); }
	|	token1		{ ... }
	|	token2		{ ... }
	...
	;

The lexer never returns 'token1', etc.  The function 'special'
examines the identifier and returns token1, etc., as appropriate.
So token1, etc., are only recognised as keywords in the correct context;
elsewhere, they are just identifiers.
-- 

<<< standard disclaimers >>>
Nick Crossley, ICL NA, 9801 Muirlands, Irvine, CA 92718-2521, USA 714-458-7282
uunet!ccicpg!ndjc  /  ndjc@ccicpg.UUCP