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

From: Sandro Hawke <sandro@w3.org>
Subject: Making Rules Out of RDF-Graphs (Re: What is an RDF Query?)
Date: Mon, 17 Sep 2001 14:18:42 -0400

> > > 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.

You are asking for a very large bag of nightmares if you expect to have an
informal semantics for a representation system.  Just look at all the heat
generated about RDFS so far.

Now if you want to have an axiomatization for RDFS, I have no problem with
that.  I just don't see why there should be any semantic need to have the
axiomatization fit into some particular implemented system.  Basically I am
saying that you should first axiomatize RDFS and only later consider rule
systems that might be expressive enough to capture this axiomatization.  To
require that an RDFS rule extension be powerful enough to capture the
axiomatization of RDFS is, to me, putting some cart before some horse that
is supposed to pull it.

> > 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?

Aside from any other problems, if you use ``holds'', then you can't appeal
to any existing version of Horn rules, at least any version that I know of,
as none of them use ``holds''.  Your syntax will have to be given
independant meaning, and may work very differently from anyone's intuitions
of Horn rules.

> > > 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

Why?  DAML+OIL doesn't concern itself with rules.  Therefore it shouldn't
need to say whether its can be implemented in Horn rules, just as RDFS
should neither have to say whether it can be implemented in Horn rules nor
should it have to conform to Horn rules.

peter

Received on Monday, 17 September 2001 14:55:48 UTC