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From: web@edsdrd.eds.com (William E. Bejcek)
Newsgroups: comp.object
Subject: Re: Infinite Loop of Message Passing
Message-ID: <321@intrepid.edsdrd.eds.com>
Date: 28 Jan 91 14:09:35 GMT
References: <26386@uflorida.cis.ufl.EDU> <4172@osc.COM>
Reply-To: web@intrepid.eds.com (William E. Bejcek)
Distribution: usa
Organization: EDS Research & Development
Lines: 32

In article <4172@osc.COM> tma@osc.UUCP (Tim Atkins) writes:
>The buck has to stop somewhere.  It is the very function of the
>Evaluator to take a method as argument to its eval function and
>actually execute it.

Exactly.  We did an environment here, internally named OWL (not
to be confused with Trellis/Owl from DEC), where "everything"
is an object" including the "evaluator".  We incorporated
the concept of an external method (one coded in the native
language) but having an object in the environment that represents
it.  Encapsulating the native code with a consistent protocol
can introduce some really nice features.  For example, in OWL,
you can trap any object with additional instance level behavior.
There are traps for tracing, breaking, & debugging that can be
put on other behavior to aid with debugging.  These work equally
well on external methods as they do with standard intrepreted
methods.  The diffence is that you can trap the individual
"statements" of a standard method (they are just objects that
get eval'ed) giving finer granularity.

Using the "all object" approach for behavior (a term that we
use as a generalization of the standard method term) along
with a consistent protocol provides a very powerful extensible
environment.  We've used this for defining inferencing behavior
(logic, forward & backward chaining), properties, slot (instance
variable) accesses, programable slots (i.v. x method), expainable
slots (how did the value get there), etc.

-- 
 / William E. Bejcek         / They choose the path where no one goes         /
/ Electronic Data Systems   / They hold no quarter, they ask no quarter - LZ /
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