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From: roland@sics.se (Roland Karlsson)
Newsgroups: comp.lang.prolog
Subject: Re: immediate update
Message-ID: <1990Nov27.095143.28162@sics.se>
Date: 27 Nov 90 09:51:43 GMT
References: <1386@n_kulcscs.kuleuven.ac.be>
Sender: news@sics.se
Organization: Swedish Institute of Computer Science, Kista
Lines: 32
In-Reply-To: bimbart@kulcs.cs.kuleuven.ac.be's message of 26 Nov 90 21:45:46 GMT


> a/1 is dynamic (and immediate) and there is one clause for a/1:
> a(1) .

> question 1
> what is the output of the query
> ?- a(X) , write(X) , retract(a(1)) , asserta(a(2)) , fail .

> question 2
> what is the output of the query
> ?- a(X) , write(X) , retract(a(1)) , assertz(a(2)) , fail .

In Aurora and Muse parallel Prologs there are similar implementations
of immediate update of the data base.  Both implementations keep a
choice point until ready with the immediate predicate.  So the above
queries should give the output '1' and '12' respectively.  But
unfortunately both implementations are WRONG.  If you remove the
retract(a(1)) literal from both queries above then they will correctly
give the output '1" and '12222...' respectively.  As the one who have
programmed this code for Muse I feel very ashamed (:-).  I thought it
was RIGHT.  Looking at the code I see that retract will incorrectly
remove the clause a(1) from the list of possible clauses.  When adding
a(2) then it will be added non reachable from the removed clause a(1).
Muse had a correct, but more inefficient, implementation before.  Then
all clauses was kept, until reaching prolog top level, and only marked
as dead.  After a while this implementation resultet in VERY long
search time in long lists of dead clauses.  Hmmmm...  Might there be
an efficient AND correct solution?
--
Roland Karlsson
SICS, PO Box 1263, S-164 28 KISTA, SWEDEN	Internet: roland@sics.se
Tel: +46 8 752 15 40	Ttx: 812 61 54 SICS S	Fax: +46 8 751 72 30