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

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Mon, 17 Sep 2001 16:08:56 -0400
To: sandro@w3.org
Cc: www-rdf-rules@w3.org
Message-Id: <20010917160856N.pfps@research.bell-labs.com>
From: Sandro Hawke <sandro@w3.org>
Subject: Re: Making Rules Out of RDF-Graphs (Re: What is an RDF Query?) 
Date: Mon, 17 Sep 2001 15:28:16 -0400

> >                                                              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. 
> 
> Sounds like a good practice.  Anyone out there psyched to axiomatize
> RDFS?  I'd naively imagine it would be relatively easy given the DAML
> axiomatization.

Sure, you could just take the RDF(S) parts of that axiomatization.
However, that axiomatization is for official RDF(S), i.e., no
existentials.  Adding existentials may or may not be difficult.

> Meanwhile, rules (for many other applications) seem more important to
> me than RDFS....    (maybe that's an overstatement.   they are
> differently-imporant, for different applications.)
> 
> > 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.
> 
> I don't see the rules I'm talking about as "an RDFS rule extension".
> I see them as a basic tool one can use, along side RDF data, to do a
> variety of things.     One of those things might be data validation.
> One of the approaches to data validation might use RDFS.

But, again, why constrain RDFS to fit within this rule formalism?

> > 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.
> 
> Maybe I'm not really understanding "holds".  The implementation I'm
> thinking of is XSB's HiLog, which rewrites all uses of the predicates
> you name as "higher-order" into a form like "apply(predicate,
> arguments)".  I was under the impression "holds" was being used in a
> similar essentially-syntactic manner, to combine HOL-syntax and
> FOL-semantics.
> 
> When I'm actually writing code (in prolog or FOL), I use something like
>    rdf(subject, predicate, object)
> so the RDF "predicate" element has no special meaning in the language,
> which sees only the 3-ary predicate "rdf".

But then consider what your Horn-syntax is translated into.  Is it anything
like Horn rules?  Is it as powerful as/more powerful than Horn rules?  You
have to be extremely cautious when combining things.  Assumptions in one
area (e.g., holds is just a trick and doesn't change anything) can have
strange consequences in other areas (e.g., restricting the form of formulae
to get some operational benefits).  I'm not saying that it wouldn't work,
I'm just saying that it is up to you to show that it would work.

> > > > > 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 ...
> >
> > peter
> 
> Maybe I'm just a guy holding a hammer called "Horn" but it seems
> useful to me.  There are a variety of ways one should evaluate a
> proposed technical standard, and this seems like a useful angle,
> somewhere between a pure-theory analysis and running code.
> 
>      -- sandro

Why?  What is the benefit to viewing RDF or RDFS or DAML+OIL through the
``Horn'' lens?  I think that it is up to you to provide a convincing
argument that there is some significant benefit!  If you can't, then why
should anyone worry about it.

peter
Received on Monday, 17 September 2001 16:09:49 GMT

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