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[Jeff Morrill: MOP hacks]
- To: kanderso@DINO.BBN.COM, mop@arisia.Xerox.COM
- Subject: [Jeff Morrill: MOP hacks]
- From: Scott Cyphers <Cyphers@JASPER.SCRC.Symbolics.COM>
- Date: Fri, 2 Nov 1990 13:11:00 PST
- Cc: jmorrill@DINO.BBN.COM, reinhard@DINO.BBN.COM
- In-reply-to: <90Nov2.firstname.lastname@example.org>
Date: Fri, 2 Nov 1990 15:20 EST
I asked some of our slot hackers to describe their experience to
support this discussion. This is a response from Jeff Morrill. Tom
Reinhardt also has some interesting experience with this, and
described some of it in detail at a CLOS workshop at OPSLA 1989.
------- Forwarded Message
Date: Fri, 2 Nov 90 13:33 EST
From: Jeff Morrill <jmorrill@BBN.COM>
Subject: MOP hacks
cc: jmorrill@BBN.COM, delatizky@BBN.COM, treinhardt@BBN.COM
You asked for a description of what I have done with the mop. Here are the highlights.
1. Definition of SLOT-FILLER-MIXIN, a metaclass.
In my expert system, a "slot-value" is really a structure containing (1) the
"real" value, and (2) a pointer to the RULE responsible for inferring that
value (used to construct explanations). Once uncertainty is supported, I
should like to include a DISTRIBUTION OF BELIEF as the "value" of the slot.
I have hacs to slot-value-using-class (:around methods) that get the structure
and pull out the "real" value. I have added a method called slot-explanation-using-class
that gets the structure and pulls out the rule pointer.
I was very upset that Symbolics chose to hardcode the optimization of SLOT-VALUE,
thus I have no recourse as I did in PCL to un-optimize calls for certain classes.
Thus my code doesn't work unless I call slot-value-using-class directly. So I have
my own version of slot-value in a different package that never gets optimized.
You can't tell when not to do the optimization, since the method doesn't
know the exact class. However, mapping tables have escape mechanisms in
them for just this sort of thing, so this can be made to work, as long
as only one value was to be returned. It would be impossible to make an
optimized SLOT-VALUE return multiple values or no values (I also think
that this would be an invalid thing to do in ANSI CL).