Path: utzoo!utgpu!attcan!uunet!seismo!sundc!pitstop!sun!quintus!ok
From: ok@quintus.uucp (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: In defence of difference-lists.
Message-ID: <559@quintus.UUCP>
Date: 21 Oct 88 04:24:45 GMT
References: <3002@mulga.oz> <16400002@clio>
Sender: news@quintus.UUCP
Reply-To: ok@quintus.UUCP (Richard A. O'Keefe)
Organization: Quintus Computer Systems, Inc.
Lines: 24

In article <16400002@clio> gerdeman@clio.las.uiuc.edu writes:
>Did something go wrong in the Eureka step here?
	NO.
>What does the variable X do?
	It is called for by the original specification.
	Namely, the INPUT of double/4 is a difference pair
	as well as the OUTPUT.

>	double(X0, X, Y0, Y) :-
>		double(X0, X, Y0, Y1, Y1, Y).

The "input" is the "virtual" list X0\X, just as
the "output" is the "virtual" list Y0\Y.

>	double(X, X, Y, Y, Z, Z).

Testing whether there is nothing between the first two arguments
is the equivalent of testing whether the "virtual" input is empty.

>Why not write ;
>        double(X0,Y0,Y):-
>                double(X0,Y0,Y1,Y1,Y).

Because it wouldn't meet the original specification, that's why not.