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From: ok@edai.UUCP (Richard O'Keefe)
Newsgroups: net.lang.prolog
Subject: Re: Problem With "Not"
Message-ID: <3863@edai.UUCP>
Date: Sat, 27-Aug-83 07:42:44 EDT
Article-I.D.: edai.3863
Posted: Sat Aug 27 07:42:44 1983
Date-Received: Sun, 21-Aug-83 05:42:46 EDT
References: sri-arpa.4230
Lines: 30

A full reply to the "Problem with not" message has been sent to the ARPAnet
Prolog digest.  The points I make in it are
1) Please say which utility (it was Not.Hlp, which is not in fact a
   utility) and QUOTE ACCURATELY.
2) The reason Not.Hlp tells you to use '\+'/1 rather than 'not'/1 is that
   the latter doesn't exist in Dec-10 Prolog.  If you are running the bare
   Dec-10 Prolog system, not(X) fails for ANY X, including not(fail).  The
   \+ symbol is as close as you can get to |-/- without hacking the tokeniser.
3) Anybody who says that the order shouldn't make any difference isn't
   talking about Dec-10 Prolog, where the fact that the order makes a lot of
   difference to virtually EVERYTHING is both clearly stated in the manual and
   in the Clocksin & Mellish book, and is vital to getting the efficiency
   which separates Dec-10 Prolog from all other Prologs.  (E.g. Dec-10
   Prolog on a KL-10 : LM-Prolog on an LM2 = 10:1, both compiled.)
4) Dec-10 Prolog is step 3 on the way to getting an efficient logic
   programming language.  It manages the efficient bit ok, but the logic
   didn't fare quite so well.  MU Prolog, available from the University of
   Melbourne, IS order-independent, and the not problem simply doesn't
   exist in it.  Negated goals in MU Prolog are suspended until they are
   ground.  MU Prolog is step 4 on the way to getting an efficient logic
   programming language.  Now if only they could figure out how to make
   it run fast...  (Though it's better than a lot of the junk being sold
   commercially.)
5) When there are unbound variables in a negated goal, there are two
   possible interpretations, and NO theory which justifies either.  I
   claim that what Dec-10 Prolog does is correct (because negation flips
   quantifier type), and have never understood why some people expect a
   negation to bind variables.  If you want a logic programming language
   which has explicit quantifiers so that both interpretations can be
   expressed, have a look at Seif Haridi's LPL.