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From: debray@sbcs.UUCP (Saumya Debray)
Newsgroups: net.lang
Subject: Re: Self-modifying code (Prolog)
Message-ID: <566@sbcs.UUCP>
Date: Tue, 10-Jan-84 10:46:00 EST
Article-I.D.: sbcs.566
Posted: Tue Jan 10 10:46:00 1984
Date-Received: Wed, 11-Jan-84 08:17:51 EST
References: <1096@mit-eddie.UUCP>
Organization: SUNY at Stony Brook
Lines: 26

:
Prolog permits users to write self-modifying code pretty easily : in this
language, program and data appear identical, so an assertion into the
program database, or a retraction from it, effectively results in a change
to the program. In fact, this feature is one of the major problems with the
semantics of the language ("pure" Prolog has a very elegant semantics in
terms of Horn Clause logic).

In passing: Prolog has no concept of "global" variables.  Therefore,
attempts at simulating global variables, e.g.  for gensym counters, usually
use the self-modifying feature of Prolog (it could be done by passing the
value of the gensym counter around explicitly as a parameter to everyone who
needed it, but that would be a royal pain in the neck!), though the practice
is frowned upon by Prolog purists.
-- 

Saumya Debray
SUNY at Stony Brook

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