Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!icdoc!sappho!cdsm
From: cdsm@sappho.doc.ic.ac.uk (Chris Moss)
Newsgroups: comp.lang.prolog
Subject: Re: Legitimate uses of "assert"?
Message-ID: <1649@gould.doc.ic.ac.uk>
Date: 28 Feb 90 19:41:13 GMT
References: <8205@pt.cs.cmu.edu>
Sender: news@doc.ic.ac.uk
Reply-To: cdsm@doc.ic.ac.uk (Chris Moss)
Organization: Logic Group, Dept. of Computing, Imperial College, London, UK.
Lines: 19

In article <8205@pt.cs.cmu.edu> fp@PROOF.ERGO.CS.CMU.EDU (Frank Pfenning) writes:
>I am looking for papers/reports/code/information on "legitimate" uses
>of "assert" and "retract" in logic programming, that is, uses which are
>considered to be in good taste by the connoisseurs of the field.

Many of the best uses are for assert WITHOUT retract. They generally come into
the area of "lemmas": asserting things you've found to be true so you don't
have to prove them all over again. This is mentioned in Sterling and Shapiro
(The Art of Prolog) p.181 - section entitled "Memo Functions" with an example
of Towers of Hanoi.

(retract is used simply to clean up at the end)

Another example is the (bottom-up) chart parser in Rob Simmons' "Computations
from the English" p.202.  Here again the information is additive, because every
part of the well-formed substring table remains valid.


Chris Moss.