Path: utzoo!attcan!uunet!husc6!think!ames!pasteur!agate!labrea!sri-unix!quintus!ok
From: ok@quintus.uucp (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: representation of infinite lists
Keywords: infinite data structures
Message-ID: <190@quintus.UUCP>
Date: 23 Jul 88 04:33:22 GMT
References: <7037@burdvax.PRC.Unisys.COM>
Sender: news@quintus.UUCP
Reply-To: ok@quintus.UUCP (Richard A. O'Keefe)
Organization: Quintus Computer Systems, Inc.
Lines: 26

In article <7037@burdvax.PRC.Unisys.COM> lang@othello.PRC.Unisys.COM (Francois-Michel Lang) writes:
>[In Quintus Prolog release 2.2]
>If I call the goal 
>| ?- L = [x|L].
>[he gets the output he expected]
>However, if I call the goal
>| ?- L = [x|L], M = [x|M].

(0) Why not send technical questions about Quintus Prolog to Quintus?
    We're more likely to know the answers than the net.

(1) Undefined is undefined: if you create cyclic terms you're on your own.
    Quintus Prolog makes no pretence of doing anything clever with them.

(2) You've been hit by a feature:  the Quintus Prolog top level, before
    printing out the bindings of the variables, checks to see whether any
    of them are identical.  For example,
	| ?- L = a, M = a.
    prints
	L = M = a
    If QP ever got around to noticing that L and M *are* identical, it
    would print them out as
	L = M = [x,x,x,x,x,x,x,x,....
    The difference between the two examples was that the first had only
    one variable, so there was nothing to check, while the second had two
    variables with isomorphic cyclic values.