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RE: An analysis of whether we should include rdf:XMLLiteral into OWL 2 (ACTION-244)

From: Michael Schneider <schneid@fzi.de>
Date: Sat, 15 Nov 2008 13:07:33 +0100
Message-ID: <0EF30CAA69519C4CB91D01481AEA06A0E06749@judith.fzi.de>
To: "Ivan Herman" <ivan@w3.org>
Cc: "W3C OWL Working Group" <public-owl-wg@w3.org>, "Boris Motik" <boris.motik@comlab.ox.ac.uk>
Ivan Herman wrote:

>On the other hand, by not including that datatype, we would create a
>backward incompatibility for OWL 1 which explicitly list rdf:XMLLiteral
>as built in[1,2]. 

I am confused by citation [1]. The text there starts with a list of datatypes that are obviously NOT mandatory, but CAN (aka MAY) be supported: 

    The following XML Schema datatypes can 
    be used in OWL as built-in datatypes

Later, the text continues by:

    The built-in RDF datatype, rdf:XMLLiteral, 
    is also an OWL built-in datatype.

Now this latter formulation really sounds like a MUST to me ("is also"). But why the "also"? The preceding text does not say for any datatype that it *IS* a built-in datatype, only that it *CAN* be used as such. And isn't it true that only xsd:string and xsd:integer have to be supported by an OWL DL compliant implementation? I would therefore think that the text also means for rdf:XMLLiteral that it only MAY be used as datatype for OWL DL, and that the actual text is only a typo or confusingly written. In this case, this would mean that not having rdf:XMLLiteral in the OWL 2 DL datatype map would NOT be a backwards compatibility issue.

For OWL Full, things are much clearer. According to Ivan's second citation [2], OWL Full interpretations are defined w.r.t. a datatype map for which it is *required* to include rdf:XMLLiteral. Actually, this requirement is not really an addition to OWL Full, but is inherited from the definition of datatype maps in the RDF Semantics spec. In [3], it is stated that

    Every datatype map is understood 
    to contain <rdf:XMLLiteral, x>

The RDF Semantics spec additionally contains other strong dependencies on rdf:XMLLiteral: Several of the RDF semantic conditions are explicitly about rdf:XMLLiteral, see [4], and this datatype is also treated by the RDFS axiomatic triples [5].

So rdf:XMLLiteral is supported in OWL 2 Full by inheritance from RDF (the current RDF-Based Semantics only lists the URI and cites the RDF Semantics spec). However, I don't see why rdf:XMLLiteral should also be in OWL 2 DL. As I said, I don't necessarily read the cited statement as a MUST. 

Michael

[1] <http://www.w3.org/TR/2004/REC-owl-semantics-20040210/syntax.html#owl_built_in_datatypes>
[2] <http://www.w3.org/TR/2004/REC-owl-semantics-20040210/rdfs.html#5.2>
[3] <http://www.w3.org/TR/rdf-mt/#defDatatypeMap> 
[4] <http://www.w3.org/TR/rdf-mt/#defXMLdatatype>
[5] <http://www.w3.org/TR/rdf-mt/#RDFS_axiomatic_triples>  

--
Dipl.-Inform. Michael Schneider
FZI Forschungszentrum Informatik Karlsruhe
Abtl. Information Process Engineering (IPE)
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Email: Michael.Schneider@fzi.de
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