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Re: Clarification on owl:real sought

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Wed, 07 Jan 2009 14:36:02 -0500 (EST)
Message-Id: <20090107.143602.135206197.pfps@research.bell-labs.com>
To: der@hplb.hpl.hp.com
Cc: public-owl-dev@w3.org

[This is not an "official" response.  You might consider elevating this
to a formal comment, perhaps because you want better explanation to show
up in the documents.]

owl:real is slightly strange as a datatype, in that it has uncountably
many values in its value space.  This means that there is no way to have
elements of its lexical space for all of its values.  (Well, we could, I
suppose, but that might have some computational consequences for OWL,
and even for storing and parsing OWL documents. :-) )

I think that at one time, the OWL WG did discuss the idea of having
lexical elements for certain interesting owl:real values (like pi or the
square root of 2).  The current status, however, is that the lexical
space for owl:real is empty.  (Remember that empty is a perfectly
reasonable set!)

This does not mean that elements of the value space of owl:real cannot
be used in OWL ontologies.  For example, "1/3"^^owl:rational denotes an
element of the value space of owl:real (just one that also an element of
the value space of owl:rational).

This illustrates a subtle difference between the treatment of datatypes
in OWL and in XML Schema (but one that is described in the OWL

I hope that this answers your question.

Peter F. Patel-Schneider

From: Dave Reynolds <der@hplb.hpl.hp.com>
Subject: Clarification on owl:real sought
Date: Wed, 07 Jan 2009 11:39:17 +0000

> [This is not a formal comment, just seeking understanding.]
> In the OWL 2 Syntax specification [1] section 4 it states that every
> datatype in the datatype map is described by a value space, a lexical
> space and a facet space. 
> In section 4.1 it lists owl:real as such a datatype but then says that
> owl:real does not "directly provide any lexical values". Could someone
> explain in what sense owl:real is a datatype if there are no lexical
> values? 
> Since owl:real is included in the OWL 2 RL profile that seems to imply
> it is intended to be used in OWL 2 RL documents which would seem to
> imply lexical values. 
> Dave
> [1] http://www.w3.org/TR/owl2-syntax/
> [2] http://www.w3.org/TR/owl2-profiles/#Feature_Overview_3
> -- Hewlett-Packard Limited
> Registered Office: Cain Road, Bracknell, Berks RG12 1HN
> Registered No: 690597 England
> Ivan Herman wrote:
> > The W3C OWL Working Group has just published seven drafts for OWL 2,
> > including the structural specification, direct and RDF based semantics,
> > serialization in RDF or in XML, Profiles, conformance and test cases. See
> > http://www.w3.org/blog/SW/2008/10/10/seven_owl_2_drafts_published
> > for more details and pointers to the documents themselves. The Working
> > group seeks public feedback on the drafts; send your comments to
> > public-owl-comments@w3.org. Please, send your comments until 2008-10-23.
> > Cheers
> > Ivan
Received on Wednesday, 7 January 2009 19:35:34 UTC

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