Re: Non-declarative Constructors?

macgregor@ISI.EDU
Message-id: <199210022124.AA19641@quark.isi.edu>
Date: Fri, 2 Oct 1992 14:24:23 -0800
To: phayes@herodotus.cs.uiuc.edu, Tom Gruber <Gruber@Sumex-AIM.Stanford.edu>,
        interlingua@ISI.EDU
From: macgregor@ISI.EDU
Subject: Re: Non-declarative Constructors?

Pat, thanks for cleaning up my example.  Substituting "(the-location 30 40)"
for an object identifier is fine.  From Gruber's response, here
is a KIF definition of "the-location":

(DEFFUNCTION THE-LOCATION
 (<=> (THE-LOCATION ?LAT ?LON ?LOC)
      (AND (LOCATION ?LOC)
           (LEGAL-LATITUDE ?LAT)
           (LEGAL-LONGITUDE ?LON)
           (LATITUDE ?LOC ?LAT)
           (LONGITUDE ?LOC ?LON)))
 (FUNCTION THE-LOCATION)
 (ARITY THE-LOCATION 3))

This definition does NOT include any assertion about the existence
of location terms (I still think of them as objects, but that's 
irrelevant).  Thus, I claim that starting from a knowledge base
containing only the above declaration, that the statement

   (retrieve ?loc (the-location 30 40 ?loc))

should return nil.  However, my impression was that the whole purpose of the
"the-foo" constructor was to introduce an equivalent of a call to
"make-instance" in logic.  From one of Tom's earlier messages:

> I've been using the "the" prefix for constructors, and I think it
> works pretty well.  In KIF, you don't want to say MAKE-FOO because that
> sounds imperative.  But you can define a function that given some
> arguments returns an instance of a class.  So, the function the-foo
> (arg1 arg2) is defined so that the thing it denotes is determined from
> its arguments.

If the above semantics is the approved one, then I don't
see how that effect is obtained.  Possibly you are saying, Tom, that
in order to obtain the desired effect, I should define the function
"the-location" as being a total function?  Let us pretend that I've
done that: (assert (total the-location)).  Then I come back to my 
original question.  Here is a
variation of it that Ramesh came up with:

   (ask (the-location 40 40 (the-location 30 40)))
       => false
   (retrieve ?x (location ?x)) 
       => ??

One possibility is that nothing comes back.  Another is that I
get back the term (the-location 30 40), since I called the function
once.  Another is that the outer call to the predicate "the-function"
executed a call to the function "the-function", having the side-effect
of creating (the-location 40 40), so I get back two terms.  Another 
possibility is that the system refuses to 
return an answer.  I have doubts about the utility of a system
that returns either the first or last response.

> It looks like you are merging the use of terms in first-order reasoning and
> their use in LISP-type functional reasoning, but this may be a mistake. 

Perhaps so.  What I wish to do is, from Lisp, retrieve terms from an
embedded logic system.  If I can't retrieve terms (other
than built-ins like numbers), then the logic system can't be integrated with
any kind of object-oriented application.  I was hoping that constructors like
"the-location" would make the system more useful.  Depending on how the
system interacts with queries like "retrieve", this may or may not 
be the case.  

BTW, I've seen a PACT ontology that includes constructors
such as "the-quantity", but haven't seen anything indicating how they
are used in practice.  If there is a use for these constructors that
is very different from the thrust of my retrieve examples, I'd be 
interested seeing some examples.

Cheers, Bob

>PS, in any case, shouldnt the last answer here be loc-0?
Yes.

Robert M. MacGregor                                     macgregor@isi.edu
USC/ISI, 4676 Admiralty Way, Marina del Rey, CA 90292      (310) 822-1511