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Re: problems with RDF datatyping

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Tue, 21 Jan 2003 09:48:05 -0500 (EST)
Message-Id: <20030121.094805.05459064.pfps@research.bell-labs.com>
To: phayes@ai.uwf.edu
Cc: bwm@hplb.hpl.hp.com, w3c-rdfcore-wg@w3.org

From: pat hayes <phayes@ai.uwf.edu>
Subject: Re: Fwd: problems with RDF datatyping
Date: Sun, 19 Jan 2003 23:12:00 -0800

> >From PFPS:
> >
> >>In trying to make the OWL semantics correspond to the RDF semantics I came
> >>up with the following problems in RDF datatyping:
> >>
> >>1/ A datatype is an element of IR, because the RDF MT says that datatypes are
> >>denoted by URI references.  However, rdf:XMLLiteral is said to be a datatype
> >>  but rdf:XMLLiteral is a URI reference.  Something is wrong here.
> 
> Nothing is wrong here. "rdf:XMLLiteral" is a uriref, and it denotes 
> rdf:XMLLiteral, a resource which is a datatype. Peter seems to be 
> making a use/mention confusion. But in any case, IR *can* contain 
> urirefs, eg one can quote a uriref as a string.

As far as I can see, "rdf:XMLLiteral" is a string, rdf:XMLLiteral is a
QName that is used in RDF as if it was a URI reference, and
I(rdf:XMLLiteral), for I some RDF interpretation, is a resource.
It thus appears to me that it is the RDF Semantics document that is making
a use/mention confusion when it refers to rdf:XMLLiteral as a datatype.
Of course, there is nothing against a resource being a URI reference, but I
don't think that that is the intent.

> >>2/ XSD-interpretations include in their datatypes the XML Schema datatypes
> >>that are problematic when removed from XML documents or have other
> >>problems.
> 
> They might, but that is not our problem or our business. We are 
> required by charter to integrate RDF with the XSD datatypes.

Well, perhaps, but I would think that the RDF Core WG would like to have it
be that XSD-interpretations actually make sense.  

> >>  XSD-interpretations also include, for example, the datatype
> >>named FOO, which is not defined as an XML datatype.
> 
> I fail to follow this point. It seems to be false, insofar as I can 
> understand it.

From Section 3.4,

     In any XSD-interpretation, any uriref of the form
     http://www.w3.org/2001/XMLSchema#sss will be understood to denote the
     datatype named sss in [XML-SCHEMA2].

What then does http://www.w3.org/2001/XMLSchema#FOO denote?

> >>3/ A datatype has to be more than is specified in the RDF MT.  Except for
> >>XSD-interpretations, which explicitly mention the URI-reference to datatype
> >>relationship, there is no way of tying the intended URI-reference for a
> >>datatype to that datatype.
> 
> We do not specify it, but that is not the same as saying that it 
> cannot be done. Seems to me that it can be done simply by fiat: I can 
> publish a document which simply asserts that the uriref 
> http://www.coginst.uwf.edu/~phayes/myexample#octalinteger denotes the 
> datatype consisting of the integers with octal lexical forms. Viola.

Sure you can do this, but is this sort of thing part of Datatyped
interpretations?  If so, what is the form of the documents that make the
connection?  If not, then how can I, in datatyped RDF, make the connection
between http://www.coginst.uwf.edu/~phayes/myexample#octalinteger and
the datatype that turns octal numeral strings into integers in the usual
way?  I don't see any way, in RDF, of so doing.

> I anticipate that agreed methods of formally specifying the 
> denotations of Urirefs will emerge. But that is not our business.

Well then, how can I, in datatyped RDF, arrange it so that 

	"10"^^http://www.coginst.uwf.edu/~phayes/myexample#octalinteger

denotes the integer usually written as 8?   If I can't, then what use is
RDF datatyping?

> >>  For example, if I have D containing a datatype
> >>for integers and a datatype for strings, there is no way to require that a
> >>particular URI reference, say ex:int, denotes the integer datatype.
> 
> Who is to say whether or not there is no way to require a URIref to 
> denote a datatype? The RDF MT does not specify how this is to be 
> done, but there is no reason why it should. But it is not impossible 
> for someone to specify this in any way they feel like doing. I do not 
> think that this is, or should be, an issue we need to be concerned 
> with: it is part of a much larger issue of attaching meanings to 
> urirefs.

Then what good is RDF datatyping?

> >>It probably makes more sense to say that a datatype is a four-tuple,
> >>consisting of a URI reference, a lexical space, a value space, and a
> >>lexical-to-value mapping.
> 
> We could do that, but since we say that they are triples and are 
> denoted by urirefs, the two formulations seem to me to be essentially 
> equivalent.

Well the difference would be that in a D-interpretation with a datatype
<u,l,v,l2v>, the denotion of the URI reference u would be that datatype.

> Pat

[...]

Peter F. Patel-Schneider
Bell Labs Research
Lucent Technologies
Received on Tuesday, 21 January 2003 09:49:38 EST

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