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Path: utzoo!mnetor!uunet!munnari!mulga!lee
From: lee@mulga.oz (Lee Naish)
Newsgroups: comp.lang.prolog
Subject: Standard of Prolog code
Message-ID: <2265@mulga.oz>
Date: Sun, 27-Sep-87 00:28:45 EDT
Article-I.D.: mulga.2265
Posted: Sun Sep 27 00:28:45 1987
Date-Received: Mon, 28-Sep-87 01:43:37 EDT
Reply-To: lee@mulga.UUCP (Lee Naish)
Distribution: world
Organization: Comp Sci, Melbourne Uni, Australia
Lines: 25

Virtually every time someone posts Prolog code to the network I groan.
I see code like this:

>merge([],[],[]).
>merge([],List2,List2):- !, fail.
>merge(List1,[],List1):- !, fail.
>merge([Head1|Tail1],[Head2|Tail2],[Head1,Head2|R]):- merge(Tail1,Tail2,R), !.

I wince, I tear my hair out, I fall to my knees and bang my head
against the nearest rock crying; is this the Prolog code people write?
What happened to logic programming?  Am I wasting my time?

Then I compose myself.  I grit my teeth, take hold of my electronic
butchers knife and attack the code.  Out with the filth!  Out with the
cuts, the dirty negation, the asserts.  Wash it clean.  Just leave the
essence; pure beautiful logic:

merge([],[],[]).
merge(Head1.Tail1, Head2.Tail2, Head1.Head2.R) :-
	merge(Tail1,Tail2,R).

Then I look on it anew.  I see it can be run as Prolog.  No choice point
will be created!  It will be fast!  It can be run backwards!  It can be
run any way need dictates.  I breath a sigh of relief and my confidence
in logic programming and Prolog are restored.