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Re: Changes to make S&AS consistent with RDF Semantics document

From: <herman.ter.horst@philips.com>
Date: Fri, 5 Dec 2003 13:08:35 +0100
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>, "Jeremy Carroll" <jjc@hplb.hpl.hp.com>, Sandro Hawke <sandro@w3.org>
Cc: horrocks@cs.man.ac.uk, www-webont-wg@w3.org
Message-ID: <OF282409A1.8266695B-ONC1256DF3.0036D4EF-C1256DF3.0042C132@diamond.philips.com>

[...]

>> However, there still remain questions.
>> Whether you like it or not, rdf:XMLLiteral is part of OWL, in the sense
>> that each OWL DL consistency checker or OWL Full consistency checker 
that
>> is worthy of the name should recognize that the RDF graph G that I 
>> mentioned,
>>      v p l
>>      p rdfs:range rdfs:Literal
>> where l is an ill-typed XML literal, is inconsistent. 
>
>I dispute this claim.  I have an OWL DL reasoner that does not perform 
this
>check.

Where I wrote "worthy of the name", I had in mind 
"in accordance with S&AS".  There is no uncertainty about what
is specified by S&AS about consistency and entailment for OWL Full
and OWL DL.

The last messages of Peter and Jeremy make clear that the OWL design
works with an implementation-dependent choice of datatypes that
should include string and integer, but which does not necessarily
include XMLLiteral.

It seems that the semantics of this design is not yet completely
reflected in S&AS.

The implementation-dependent choice of datatypes is reflected in a
datatype map, defined in Section 3 of S&AS, which is indeed assumed
to contain string and integer and not required to include XMLLiteral.
However, even if the datatype map does not include XMLLiteral,
then the definitions of OWL DL/FULL entailment/consistency in 
Section 5 of S&AS work as if XMLLiteral is included; the reason
for this is that OWL is a semantic extension of RDFS, and RDFS
semantics always includes XMLLiteral.

In other words, S&AS *specifies*, normatively, the above graph 
G to be inconsistent, for OWL DL as well as OWL Full, for any 
datatype map.
   (Proof: G has no RDFS interpretation, and therefore no
    D-interpretation for any datatype map D, whether D contains
    XMLLiteral or not.  So G has no OWL DL or OWL Full interpretation
    for any datatype map.)

This seems to be behind the contradiction of the above cited
statements of Peter and me.

For OWL DL entailment, S&AS could be brought in line with the actual
OWL design in the way I described below:
>> One way to solve this would be to restrict the definition now in 
>> Section 5.4 of OWL DL entailment to datatype maps with XMLLiteral,
>> and to state another definition of OWL DL entailment for datatype maps 
>> without XMLLiteral, in terms of the direct semantics and the mapping T,
>> as this is a known case of conflict between the direct semantics
>> and OWL DL semantics.
>> This would realize the desire that XMLLiteral is not required in
>> OWL DL.
This would also solve the problem I mentioned below, that now S&AS
seems to define three semantics for OWL, instead of only the 
two semantics OWL Full and OWL DL sanctioned by WG decisions.

For OWL Full, it seems that the problem that S&AS does not really 
allow the possibility to exclude XMLLiteral from the semantics, 
cannot be solved in S&AS alone.
A possible solution would be that in the RDF Semantics document,
the XMLLiteral-related semantic conditions are moved from
Section 3 on RDF to Section 5 on datatypes, with the possibility
to leave XMLLiteral out of the datatype map.  This would not be
a trivial operation, since XMLLiteral is now included in the 
RDF and RDFS entailment lemmas.
Of course, when the RDF Semantics document would be changed in
such a way, then the suggestion of the preceding paragraph focusing
on OWL DL would not be needed.


===

>
>> Whether XMLLiteral is in or out of the datatype map D, OWL DL 
entailment 
>> as well as OWL Full entailment now always incorporates the meaning of 
>> rdf:XMLLiteral, inherited from the RDFS Semantics. 
>
>Sure, but so what?
>
>> In the light of this, what do you mean with your statement above:
>> >... rdf:XMLLiteral is not a required part of OWL.
>
>The OWL documents imply that rdf:XMLLiteral is not a required part of 
OWL,
>as indicated by the definition of OWL datatype maps.
>
>> By leaving out of Section 3 the assumption about XMLLiteral, in the 
>> current way, the abstract syntax-based, direct semantics is not 
>> just another side of the same coin as the OWL DL semantics.
>> It looks like the current editor's version of S&AS has three 
inequivalent
>> versions of OWL semantics: OWL Full, OWL DL, and a third version that 
>> might
>> be described as 'abstract syntax OWL without XMLLiteral'.
>> Each of these versions is labeled normative in S&AS.
>
>Yes, to some extent.
>
>> Doesn't this contradict the WG decision about semantic layering, to 
have
>> two semantics versions, now called OWL Full and OWL DL?
>
>Not really, at least in my opinion.
>
>> One way to solve this would be to restrict the definition now in 
>> Section 5.4 of OWL DL entailment to datatype maps with XMLLiteral,
>> and to state another definition of OWL DL entailment for datatype maps 
>> without XMLLiteral, in terms of the direct semantics and the mapping T,
>> as this is a known case of conflict between the direct semantics
>> and OWL DL semantics.
>> This would realize the desire that XMLLiteral is not required in
>> OWL DL.
>> 
>> Another, simpler, way to rule out the third version of the semantics 
>> would be to adopt the suggestion (*) that I give above, in Section 3 
>> of S&AS.
>
>Yes, this *would* be possible.  However, it would be a *change* in the
>definition of OWL.
>

[...]

Herman
Received on Friday, 5 December 2003 07:09:16 GMT

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