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From: tim@linc.cis.upenn.edu (Tim Finin)
Newsgroups: comp.lang.prolog
Subject: multiple copies of a clause in the DB
Message-ID: <1696@super.upenn.edu>
Date: Fri, 7-Aug-87 09:54:12 EDT
Article-I.D.: super.1696
Posted: Fri Aug  7 09:54:12 1987
Date-Received: Sun, 9-Aug-87 01:39:48 EDT
Sender: news@super.upenn.edu
Reply-To: tim@linc.cis.upenn.edu.UUCP (Tim Finin)
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Organization: University of Pennsylvania
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Keywords: database

I'm studying various ways to extend Prolog's simple model of the
database (e.g. a flat, global collections of clauses) to a richer
hierarchical one with inheritance.  I am trying to decide whether to
allow multiple instances of a clause in a resulting database view.

Most Prolog implementations, at least those descendant from DEC-10
Prolog, do allow the database to contain two identical clauses.  Most
of the non-Prolog logic programming languages that I am familiar with
do not.  I am interested in discovering what use, if any, people have
made of the ability to assert multiple copies of a clause into the
database.  

I, for one, have never found a use for this in practice.  In fact, it
has only effected my life by being a source of bugs.  It is easy
enough to accidentally get multiple copies of a clause in the database
by consulting a file instead of reconsulting it or by defining the
same predicate in two different files.  This can easily mess up your
program unless you use a rather pure logic programming style which
doen't depend on the order in which the clauses are stored in the
database.

Has anyone out there found a good use for this Prolog "feature"?

Tim