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Re: What is an RDF Query?

From: pat hayes <phayes@ai.uwf.edu>
Date: Mon, 10 Sep 2001 21:07:50 -0500
Message-Id: <v04210117b7c31fde3ef2@[205.160.76.173]>
To: Sandro Hawke <sandro@w3.org>
Cc: www-rdf-rules@w3.org
> > > Yep.   This works, but it ends up more complicated than we need, if we
> > > just handle existential variables properly.
> >
> > I don't understand the need to encode everything in RDF.  Encoding
> > everything in RDF ends up with a very complicated system, and most of the
> > semantic import will be in the encodings, which are not part of RDF.
>
>The "need" is a perhaps more of an opportunity.  When one is
>programming in LISP, one tends to use LISP syntax for almost
>everything because one's mental and software machinery is all geared
>up for it.
>
>Once we've made it easy to represent and work with knowledge in RDF,
>we might as well do it with all our meta-knowledge, control-knowledge,
>etc...

No, we will *not* be able to do that, becuase RDF is intrinsically 
unable to express most of this stuff. It is a *very* weak assertional 
language, and can hardly say anything more than simple facts. It 
definitely is not going to be adequate for the entire semantic web. 
It cannot represent anything but very trivial control knowledge (eg 
it can't represent implications or conditionals), and cannot 
represent true meta-knowledge at all. The analogy with LISP (or any 
programming langauge) is misleading, since it doesnt take much to 
have a Turing-universal programming language. But assertional 
languages are different, and one cannot extend the expressive power 
of an assertional language by implementing something in it.

>
>Except for (1) it is sometimes more confusing and (2) it might have big
>performance penalties.
>
>DAML+OIL has this same issue, right?  The instance knowledge is in
>RDF, which makes perfect sense.  It's a great language for listing the
>property/value pairs for objects.

Its not a GREAT language for anything, but in any case that is *all* it can do.

> But then the ontology (describing
>what kind of properties and values are allowed) is also encoded in
>RDF.

partially describing; but that is a very strong 'partial'. Even the 
simple assumption of subclass inheritance can't be said in RDF; it 
needs RDFS. And the fact that, for example, sequences and bags are 
disjoint classes can't be said in RDFS; it needs something like DAML.

> What's more, the characterists of the ontology language are
>*also* encoded in RDF in http://www.daml.org/2001/03/daml+oil.daml!

No, they are not. DAML+OIL is much more expressive than RDF, and 
cannot be translated into, or expressed in, RDF.

>So why not describe a query of an RDF dataset in an RDF dataset?

How?

Pat

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Received on Monday, 10 September 2001 22:06:23 GMT

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