Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!usc!wuarchive!emory!athena.cs.uga.edu!mcovingt
From: mcovingt@athena.cs.uga.edu (Michael A. Covington)
Newsgroups: comp.lang.prolog
Subject: Re: '->' operator
Message-ID: <1991Jan4.030918.24996@athena.cs.uga.edu>
Date: 4 Jan 91 03:09:18 GMT
References: <291@valverde.cs.utexas.edu>
Organization: University of Georgia, Athens
Lines: 32

In article <291@valverde.cs.utexas.edu> bradley@cs.utexas.edu (Bradley L. Richards) writes:
>I have a (probably silly) question regarding the '->' operator in Quintus.
>Suppose I say
>
>   Condition -> Predicate
>
>If the condition is true, the predicate will execute.  Fine.
>
>If the condition is true and the predicate fails, the whole conditional fails.
>  This makes sense since logically (true -> false) is false.
>
>If the condition is true and the predicate is true, the whole thing succeeds.
>  This makes sense since (true -> true) is true.
>
>BUT if the conditional is false, the ugly little beastie fails--this doesn't
>  make sense since logically (false -> anything) is *true*.
>
>Why is this implemented the way it is?
>
>Thanks,
>Bradley
>bradley@cs.utexas.edu

Generally, "false -> anything" is not true in Prolog. Prolog is not quite
the same as classical logic. The main differences are (1) limitations on the
form of formulas, and (2) absence of explicit negation (in Prolog, "not P"
means "unable to prove P").
  
  Further, the -> operator is not intended to be an encoding of anything
  in classical logic. It is a Pascal-like, *procedural* if-then-else
  construct, designed to make it easier to express algorithms.
  The equivalent of classical logic "implies" is :- .