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From: markh@csd4.csd.uwm.edu (Mark William Hopkins)
Newsgroups: comp.lang.prolog
Subject: Re:  general data structures are [not] impossible
Message-ID: <9732@uwm.edu>
Date: 24 Feb 91 02:28:36 GMT
References: <6406@skye.cs.ed.ac.uk>
Sender: news@uwm.edu
Organization: University of Wisconsin - Milwaukee
Lines: 39

In article <6406@skye.cs.ed.ac.uk> jha@lfcs.ed.ac.uk (Jamie Andrews) writes:
>o Manipulations of such data structures are greatly facilitated
>  by languages with explicit types, named record fields, and so
>  on.  Pascal has these; most Prologs don't.

Predicates ARE records only in disguise...

PASCAL:
type Complex = record RE, IM: real end;

PROLOG:
complex(RE, IM) :- real(RE), real(IM).

if floating point came with your Prolog complete with its own real() predicate.

Or better yet as an example:
PASCAL:
type NodeTag = (RE, INT);
     Node = record case Tag:NodeTag of
	RE: (RVal: real);
	INT: (IVal: integer)
     end

PROLOG:
node(RE, X) :- !, real(X).
node(INT, X) :- !, integer(X).

Prolog types are far more powerful than corresponding Pascal types,
furthermore, since types in "procedure call"-s can be matched by Unification
in Prolog.  The closest thing you have in Pascal is the limited Polymorphism
that the Standard attempted with array types.

A similar argument also proves that Prolog is a very powerful object-oriented
language in disguise.

Also, if you consult the lambda calculus and functional programming literature,
you'll find out about results concerning the correspondence (isomorphism)
between types and propositions which underlies this observation
(records <-> AND, unions/variants <-> OR, functions <-> IMPLICATION, etc.)