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Making Rules Out of RDF-Graphs (Re: What is an RDF Query?)

From: Sandro Hawke <sandro@w3.org>
Date: Mon, 17 Sep 2001 14:18:42 -0400
Message-Id: <200109171818.f8HIIg231452@wadimousa.hawke.org>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
cc: www-rdf-rules@w3.org

> > That is, I'm assuming a basic system
> > will know nothing of rdfs (or reification or containers or even
> > rdf:types) until a set of rules providing the axiomatic semantics for
> > such things are loaded (in the same manner, whatever it may be, as
> > any other rules are loaded).
> 
> Oh.  You are redefining RDF(S).  This could work, sort of, but would
> require a redefinition of RDF(S) into a core formalism and the full
> formalism.  It also would require work on the rule system to be completed
> before full RDF(S) can be defined.  

RDFS could be defined informally like most practical specifications
(and like it has been so far, I think) and then have the axiomatic
semantics come along later.  Not the order one might wish, but workable.

> Further, some aspects of RDFS cannot be captured in Horn rules, at least not
> without some ``extensions''.  
> 
> For example, how are you going to capture the meaning of domain and range
> restrictions?   You can't do something like
> 
> 	domain(?r,?c), ?r(?x,?y) -> ?c(?x)
> 
> because neither ?r nor ?c can be in the predicate position.  
> Neither can you capture inheritance, for similar reasons.
> 
> You could, if you really wanted to, use some sort of holds predicate, but
> then you have lost the intuitive connection between RDF and Horn rules,
> which you probably don't want.  (See the KIF axiomatization of DAML+OIL for
> an example of this sort of game.)

Yes, I should have said something about that in my first mention of
mapping RDF to FOL, which is that I do use "holds" when I'm thinking
about these HOL-like axioms.  Some people are happy with just using
HOL, of course.    

> > Does my approach here seem reasonable?  
> 
> My view is that this approach is not reasonable, for (at least) the above
> reasons.

None of those reasons seems serious to me.   So probably we're a long
way from convincing each other here.   Has anyone else been following
this?   Care to weigh in on (1) whether this approach has been well
explained, and (2) whether it's reasonable to explore?

> > My big concern with it is that
> > I hear DAML+OIL cannot be expressed axiomatically in Horn logic.
> > Obviously there may be performance issues to this approach, but I
> > think that can be addressed behind the scenes, without changing the
> > general query/rule model.
> 
> Certainly there are many portions of DAML+OIL that cannot be captured in
> Horn rules, including number restrictions.

I think these ought to be clearly spelled out in some DAML docs, when
someone gets a chance.

    -- sandro
Received on Monday, 17 September 2001 14:22:41 GMT

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