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Re: Question: disjointness of classes and datatypes

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 23 Aug 2001 08:28:05 -0400
To: dallsopp@signal.qinetiq.com
Cc: www-rdf-logic@w3.org
Message-Id: <20010823082805V.pfps@research.bell-labs.com>
From: David Allsopp <dallsopp@signal.qinetiq.com>
Subject: Question: disjointness of classes and datatypes
Date: Thu, 23 Aug 2001 11:38:08 +0100

> All,
> 
> Hopefully a simple question:
> 
> The March 2001 reference description of DAML+OIL 
> (http://www.daml.org/2001/03/reference.html) says
> 
> "Objects and Datatype Values - DAML+OIL divides the universe into two
> disjoint parts. One part consists of the values that belong to XML
> Schema datatypes. This part is called the datatype domain. The other
> part consists of (individual) objects that are considered to be members
> of classes described within DAML+OIL (or RDF). This part is called the
> object domain."
> 
> However, this disjointness is not expressed in the language
> specification of DAML+OIL (http://www.daml.org/2001/03/daml+oil.daml) as
> far as I can see.

From the model-theoretic semantics
(http://www.daml.org/2001/03/model-theoretic-semantics.html), which is, in
my view, the most authoritative source:

  The semantics uses a non-empty domain of discourse, <tt>AD</tt>,
  which is a collection of DAML+OIL objects.  
  Added to this object domain is the
  datatype domain, <tt>DD</tt>, which is the intended model for XML Schema
  data types, disjointly unioned with an infinite collection of other values.
  The disjoint union of <tt>AD</tt> and <tt>DD</tt> is designated by
  <tt>UD</tt>. 

So there is no way for anything to be both an object and a datatype value.

By the way, http://www.daml.org/2001/03/daml+oil.daml is more of a syntax
specification with some semantic comments.  There is lots more to the
meaning of DAML+OIL than is contained therein.

> Is there a reason for this? How should a resource that claims to be an
> instance or subclass of both daml:Class and daml:Datatype be handled -
> the reference description says this is illegal but the language spec
> doesn't. 
> 
> Also, is it illegal to explicitly declare a resource to be of type
> daml:Datatype?

Not illegal, per se, but doing so, if it is even syntactically possible,
would result in an inconsistency.

> Regards,
> 
> David Allsopp
> QinetiQ
> UK

Peter F. Patel-Schneider
Bell Labs Research

PS:  If you are interested in the meaning of DAML+OIL, then you should
probably first read the excellent semantics for RDF and RDFS from Pat
Hayes.
Received on Thursday, 23 August 2001 08:29:49 GMT

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