Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site decwrl.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!think!harvard!seismo!umcp-cs!gymble!lll-crg!dual!decwrl!dec-rhea!dec-logic!vantreeck
From: vantreeck@logic.DEC
Newsgroups: net.lang.prolog
Subject: Re: Event handling in Prolog
Message-ID: <2434@decwrl.UUCP>
Date: Mon, 3-Jun-85 12:39:10 EDT
Article-I.D.: decwrl.2434
Posted: Mon Jun  3 12:39:10 1985
Date-Received: Thu, 6-Jun-85 02:29:38 EDT
Sender: daemon@decwrl.UUCP
Organization: DEC Engineering Network
Lines: 22


      I'm not a theorist. From a programming point of view, the ease of
establishing event handlers depends on the hardware/OS support more than some
limitation of Prolog. Adding event handlers to Prolog implementations on
VAX/VMS should be a trivial task. Adding an Prolog predicate to generate an
exception, should also be easy. 

      Besides control-C, probably the best other example common of a Prolog
event handler, would be a garbage collection which occurs on the event of a
stack running into a protected page. Establishing a handler for integer or
floating point underflow over flow is easy - provided that the calculations are
done with routines that generate an exception. I don't know if implementations
like QUINTAS Prolog, which uses their own routines for their short floats,
generate exceptions on underflow/overflow. 

      If I were to write a concurrent Prolog, I think I would probably use an
event handler to manage the concurrency. I hope that the newer Prologs that
will be hitting the market in the next year or two of years will have predicates
for establishing and handling events. 

						George Van Treeck
						Digital Equip. Corp.