Article View: net.lang.prolog
Article #612Re: Help - how do I avoid circularity here?
From: Chris Moss
Date: Thu, 15 May 1986 14:56
Date: Thu, 15 May 1986 14:56
26 lines
1618 bytes
1618 bytes
In article <1211@lsuc.UUCP> dave@lsuc.UUCP (David Sherman) writes: >I'm designing a large Prolog program for corporate tax planning >under Canadian income tax law. I want to include the following >definitions: > >tptype(Taxpayer, corporation) :- tptype(Taxpayer, ccpc). > >Let's call this a "type 1" definition. ... A type >2 rule might be: > >tptype(Taxpayer, ccpc) :- > tptype(Taxpayer, corporation), >Now comes the problem. When I have both type 1 and type 2 >rules in place, I get circularity. Well known problem. Structural solution as follows: split your predicate into two parts, one of rules, the other facts which do not have any recursive calls. Add a reference from first to second. By this time you will rarely if ever need directly recursive calls. e.g. tptype(Taxpayer, corporation) :- tptypefact(Taxpayer, ccpc). tptype(Taxpayer, ccpc) :- tptypefact(Taxpayer, corporation). tptype(Taxpayer, Type) :- tptypefact(Taxpayer, Type). Chris Moss, Imperial College London. cdsm@ic.doc.ac.uk or cdsm@icdoc.uucp
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Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: arc1-ca.pugleaf.net!utzoo!linus!philabs!cmcl2!seismo!mcvax!ukc!icdoc!cdsm From: cdsm@doc.ic.ac.uk (Chris Moss) Newsgroups: net.lang.prolog Subject: Re: Help - how do I avoid circularity here? Message-ID: <332@ivax.doc.ic.ac.uk> Date: Thu, 15-May-86 10:56:37 EDT Article-I.D.: ivax.332 Posted: Thu May 15 10:56:37 1986 Date-Received: Sun, 18-May-86 15:06:01 EDT References: <1211@lsuc.UUCP> Reply-To: cdsm@doc.ic.ac.uk (Chris Moss) Organization: Dept. of Computing, Imperial College, London, UK. Lines: 26