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From: dep@allegra.UUCP (Dewayne E. Perry)
Newsgroups: net.lang.prolog
Subject: Re: A Cautionary tale.
Message-ID: <2599@allegra.UUCP>
Date: Fri, 13-Jul-84 10:38:35 EDT
Article-I.D.: allegra.2599
Posted: Fri Jul 13 10:38:35 1984
Date-Received: Sat, 14-Jul-84 01:10:52 EDT
References: <1809@sri-arpa.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 22

offering :- all_the _previous_lines, leave_this_line_if_appeased.

The rules for the operation reverse were reported to be faster because
they were measured to be faster using C Prolog (measured by doing
each version 200 times on a list with 36 elements).

You may recall that I warned about optimizations that appeal to your
intuition as improving the average case, etc, but that do not have
that effect in a prolog implementation.  This phenomena comes from
the complex underlying structure that does the data base management
and the inferences.  There seem to be two levels of understanding 
needed to exploit this structure:  the logical structure of prolog -
ie, the simplifications made to first order predicate calculus and the
underlying theory in order tomake prolog a practical working system -
and the implementation of this logical structure - ie, the particular
quirks of a particular version of prolog.  Unfortunately, these latter
considerations vary from version to version.  Besides, you shouldn't
depend on those kinds of "features" anyway.

?- author(X)

X = Dewayne E. Perry