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

From: <herman.ter.horst@philips.com>
Date: Tue, 9 Dec 2003 15:23:37 +0100
To: www-webont-wg@w3.org
Cc: pat hayes <phayes@ihmc.us>, pfps@research.bell-labs.com, sandro@w3.org, connolly@w3.org
Message-ID: <OFB74520DA.2CBFD48A-ONC1256DF6.00485541-C1256DF7.004F1D45@diamond.philips.com>

In this message I would like to summarize the possible options
to solve the problem that the current version of S&AS does not 
properly describe the semantics design w.r.t. XMLLiteral [1] [2].
In my view, this problem needs to be fixed, and it seems to
be difficult to do that with an erratum.  If the document
is left with an error like this, how can a reader decide
for any statement of S&AS whether it is reliable?

First I give a brief summary of the options, with brief
discussion, and at the end I add some details about the options.
The options also involve the RDF Semantics document.

*Problem*: S&AS cannot exclude the semantics of XMLLiteral, although
this possibility is required by the design of OWL.
(As I explained in [2], another aspect of the problem is that now
S&AS actually describes three semantics for OWL, instead of the
two semantics OWL DL and OWL Full sanctioned by WG decisions.)

Overview of solutions:

*Possible solution 1*: no changes to RDF Semantics, and two
"local" changes to S&AS:
-1a: add OWL DL semantics without XMLLiteral by means of a
detour via the abstract syntax (details below)
-1b: exclude the possibility of having OWL Full without
XMLLiteral semantics (after all, OWL Full is *OWL Full*)

In view of 1b, this would not solve the problem completely, but
perhaps this would be acceptable?
In addition, 1b would require a small change in OWL Reference.

*Possible solution 2*: no changes to S&AS, and two changes
to RDF Semantics:
-2a: move the XMLLiteral conditions from Section 3 (on RDF) to
Section 5 (on datatypes)
-2b: allow the possibility to leave XMLLiteral out of a
datatype map.

I prefer solution 2.  This solution would require
zero-byte change to S&AS, although some editorial work
would be needed on RDF Semantics.  This seems to be the
only complete solution to the problem.
In view of [3], it seems that some nontrivial work needs 
to be done on RDF Semantics anyway before it goes to PR, 
since the central completeness claim of the document is false.
I describe in rdf-comments [4] some details related to 
solution 2.

I do not know whether solution 2 would be in conflict with 
RDF Core's design.  If that would be the case, then there is
a conflict between the designs of WebOnt and RDF Core,
and one or both designs would need to be tweaked in order
to arrive at a solution of the problem.

*Impossible solution 3*: no changes to RDF Semantics, one
change to S&AS:
-3a: delete the (RDF-)conditions on XMLLiteral from the 
definition of OWL interpretation in S&AS.

This solution seems to be excluded, since it would violate the
central assumption on the semantic layering of OWL on top of RDF,
namely that OWL is a semantic extension of RDFS.

===

Additional details about possible solutions:

-1a: details were given in [2]:
>> 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.

-2: This solution would lead to more editorial work because 
XMLLiteral appears in the RDF and RDFS entailment lemmas.  As I
discuss in my message to rdf-comments [4], this change to the
RDF Semantics document would lead to a natural generalization of 
the RDFS entailment lemma to include a large class of datatype maps, 
i.e., providing entailment rules sound and complete with the inclusion
of datatypes.


Herman ter Horst

[1] http://lists.w3.org/Archives/Public/www-webont-wg/2003Dec/0034.html
[2] http://lists.w3.org/Archives/Public/www-webont-wg/2003Dec/0035.html
[3] http://lists.w3.org/Archives/Public/www-rdf-comments/2003OctDec/0205.html
[4] http://lists.w3.org/Archives/Public/www-rdf-comments/2003OctDec/0233.html
Received on Tuesday, 9 December 2003 09:27:48 GMT

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