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From: kirshenb@hplabsz.HPL.HP.COM (Evan Kirshenbaum)
Newsgroups: comp.lang.prolog
Subject: Lists (was Arrays) in Prolog
Message-ID: <6009@hplabsz.HPL.HP.COM>
Date: 4 Oct 90 00:00:12 GMT
References: <1238@ecrc.de> <1990Sep19.075314.14372@irisa.fr> <1403@ecrc.de> <34487@cup.portal.com>
Reply-To: kirshenb@hplabsz.UUCP (Evan Kirshenbaum)
Organization: Hewlett-Packard Laboratories
Lines: 49

In article <34487@cup.portal.com> pgl@cup.portal.com (Peter G Ludemann) writes:
>> I would like to point out that there is no such thing as
>Can we change the subject a bit?  How about list structures
>which can be as easily accessed from the end as from the front?
>That, to me, is the big advantage of arrays; not the performance
>improvement.  Does anyone have a nice notation for getting the
>last element of a list as opposed to the first element?
>

Once upon a time, I was playing with the design of a language based on
prolog, and one of the extensions was exactly this.  The notation I
used was:
	[A,B,C]		a sequence containing exactly A, B, and C
	[A,...]		a sequence starting with A
	[A,..As]	a sequence with head A and tail As
	[...,L]		a sequence ending in L
	[..Ls,L]	a sequence ending in L with everything up to it Ls
	[...,X,...]	a sequence containing X
	[..Pre,..Mid,..Post]	a sequence partitioned into Pre, Mid,
                        and Post
	[...,X,...,Y,...] a sequence in which X occurs before Y


I called ..X a splice (... was equivalent to .._), and it was an
actual object in the sequence (although you couldn't get one outside
of a sequence).  Unifying a sequence that contained splices was
possibly nondeterministic, but was reasonably easy to implement.
Multiple solutions were obtained on backtracking.  This allowed you to
define
	append(Xs,Ys,[..Xs,..Ys]).
	member(X,[...,X,...]).
	last(L,[...,L]).

If you unified [..ButLast,Last] with [..Pre,X,..Post] you got
	ButLast = Pre = Post = [], Last = X
	and
	ButLast = [..Pre,X,..As], Post = [..As,L]
	
My system also contained a notion of anonymous variables, so sequences
with anonymous splices were relatively cheap as well (no structures
needed to be consed).

Evan Kirshenbaum
    HP Laboratories
    3500 Deer Creek Road, Building 26U
    Palo Alto, CA  94304

    kirshenbaum@hplabs.hp.com
    (415)857-7572