[cl-unification-devel] Re: question about CL-UNIFICATION

[Marco Antoniotti]

Hi

thanks for the comments.

On May 27, 2005, at 12:25 PM, Robert P. Goldman wrote:

>
> I was following up on CL-UNIFICATION based on your email to the cells
> list, and read over the web site docs.  I was left with the question
> of how to unify together two structures or objects.  I see how one can
> unify a structure (or object) with a template, and how that might be
> used for ML-style pattern-matching.
>
> But what if you really want to know if two structures themselves
> unify?  Presumably one might want to do something like the following:
>
> (setf x #S(FOO A 42 S NIL D NIL))
> (setf y #S(FOO A 42 S NIL D NIL))
>
> where x and y are EQUALP, but not EQ, and you'd like to do
>
> (unify x y)
>
> but this is impossible.

Ooops.  Not good.



>  Is the following correct:
>
> (unify x #T(FOO foo-a (foo-a y) foo-s (foo-s y) foo-d (foo-d y)))

Yes.  This would work.


>
> and, if so, is there some less cumbersome way to make this happen?
> I.e., to have a unification method definition that would say "whenever
> I unify together a FOO and a FOO, I want to apply this template to the
> second argument?

You make a very good point.

Right now, the only defined methods for UNIFY with STRUCTURE-OBJECTs are

unify structure-object template
unify template structure-object

TRT would be to write a method for

unify structure-object structure-object

This would be a limited method in any case, just testing for equality 
with EQUALP.  E.g. given

(setf x #S(FOO A ?x))
(setf y #S(FOO A 42))

(unify x y)

would return an empty environment.  I.e. the unifier would not recur 
into the slots.
In alternative the unifier could generate two subtests

(unify x #T(FOO foo-a (foo-a y) foo-s (foo-s y) foo-d (foo-d y)))
(unify #T(FOO foo-a (foo-a x) foo-s (foo-s x) foo-d (foo-d x)) y)

This is needed to ensure cross unifications.

I know I could do away with all of this by invoking implementation 
dependent code  getting the structure slots.  But I am kind of against 
it.  As it is the unification code is completely portable.
Yet, given enough pressure, I will concede and add the the extra code.

Cheers

Marco

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--
Marco Antoniotti					http://bioinformatics.nyu.edu
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