Re: Help on problem needed!

Article View: comp.lang.prolog

Article #723

#723

From: Michel BILLAUD
Date: Tue, 25 Aug 1992 07:46

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In article <meskes.713709771@ulysses> meskes@ulysses.informatik.rwth-aachen.de (Michael Meskes) writes:

>Can you program a IF...THEN...ELSE in Prolog without using the cut?

No. a proof by contradiction:

Suppose you can program IF..THEN ...ELSE, then you could also
program NOT ... (negation as failure) because
	NOT(X) <==> IF X THEN FAIL ELSE TRUE

Obviously NOT(TRUE) <=> FAIL     and NOT(FAIL) <=> TRUE

But every program P built from AND/OR/TRUE/FAIL only has the following property
  P(TRUE) "produces more solutions than" P(FAIL)
					(not very hard to prove)
Which is not the case for NOT.
----
Also, it is not possible to replace CUT by IF...THEN..ELSE
for a similar reason

Michel
-- 
Michel BILLAUD                 :  billaud@geocub.greco-prog.fr
Departement d'Informatique     :  phone W: 56.84.57.92  // 56.84.69.22
IUT "A", Universite Bordeaux I :  "Je veux jouer de l'helicon
33405 Talence  (FRANCE)        :        pon pon pon pon !    "

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Message-ID: <1992Aug25.074623.25450@greco-prog.fr>

Path: arc1-ca.pugleaf.net!sparky!uunet!mcsun!corton!geocub!billaud

References: <meskes.713709771@ulysses>

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Path: arc1-ca.pugleaf.net!sparky!uunet!mcsun!corton!geocub!billaud
From: billaud@greco-prog.fr (Michel BILLAUD)
Newsgroups: comp.lang.prolog
Subject: Re: Help on problem needed!
Keywords: help
Message-ID: <1992Aug25.074623.25450@greco-prog.fr>
Date: 25 Aug 92 07:46:23 GMT
References: <meskes.713709771@ulysses>
Reply-To: billaud@geocub.UUCP (Michel BILLAUD)
Organization: LaBRI - Universite Bordeaux I - France
Lines: 26

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