Path: utzoo!attcan!uunet!decwrl!ucbvax!pisces!vanroy
From: vanroy@pisces (Peter Van Roy)
Newsgroups: comp.lang.prolog
Subject: Re: Simple Prolog Question
Message-ID: <38122@ucbvax.BERKELEY.EDU>
Date: 13 Aug 90 19:51:58 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: vanroy@pisces (Peter Van Roy)
Organization: University of California at Berkeley
Lines: 26

In article <3553@goanna.cs.rmit.oz.au> ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
>
>I'd prefer that too.  Indeed, Debray's SB-Prolog will let you write that
>last clause as
>       remove_leading_blanks([H|T], L) :-
>               (   H =:= " ",
>                   remove_leading_blanks(T, L)
>               ;   H =\= " ",
>                   L = [H|T]
>               ).
>and the compiler is smart enough to notice that the two tests conflict,
>and it will plant the cut for you.  That's the _really_ nice way to write it.
>(Does the new Berkeley compiler do this too?)

Yes, the Berkeley compiler generates deterministic code for this example, but
it does not "plant a cut".  A single conditional branch is generated to
distinguish between the two arms of the disjunction.  Even if the two tests
do not conflict entirely, the compiler is smart enough to generate a choice
point only when they overlap.  For example, consider:

        max(A, B, C) :- A>=B, C=A.
        max(A, B, C) :- A=<B, C=B.

A choice point is generated only if A=:=B.
Sincerely,
        Peter Van Roy