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

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Tue, 02 Dec 2003 11:56:28 -0500 (EST)
Message-Id: <20031202.115628.67696335.pfps@research.bell-labs.com>
To: herman.ter.horst@philips.com
Cc: horrocks@cs.man.ac.uk, www-webont-wg@w3.org

From: herman.ter.horst@philips.com
Subject: Re: Changes to make S&AS consistent with RDF Semantics document
Date: Tue, 2 Dec 2003 17:31:09 +0100

> Peter,
> 
> It could be very helpful, and I am very interested to 
> hear your opinion, as the main editor of S&AS, about 
> three points that remain in the process to make 
> S&AS consistent with the latest RDF Semantics document.
> These points are described below, copied from 
> recent messages to webont.
> 
> ===
> 
> Both S&AS and RDF Semantics define datatype maps
> to be partial maps from URI references to datatypes.
> RDF Semantics assumes, in addition, that each datatype
> map contains rdf:XMLLiteral.
> S&AS assumes, in addition, that each datatype map
> contains xsd:string and xsd:integer.
> 
> S&AS could be made consistent with RDF Semantics
> by correcting the third definition in Section 3.1 
> in for example the following way:
> 
> "As in RDF, a datatype map D is a partial mapping from
> URI references to datatypes that maps rdf:XMLLiteral
> to the built-in XML Literal datatype defined in the
> RDF Concepts and Abstract Syntax document [RDF Concepts].
> In addition, it is assumed that datatype maps
> map xsd:string and xsd:integer to the appropriate 
> XML Schema datatypes."

I do not think that this change is appropriate, because rdf:XMLLiteral is
not a required part of OWL.

> A corresponding addition to the first definition in 
> Section 5.2 would then be needed.
> 
> It should be noted that if XMLLiteral is not added
> by default to each datatype map, as in the RDF Semantics,
> document, then S&AS seems to be inconsistent.
> To see this, consider the following example:
> 
> RDF graph G, just two triples
>   v p l
>   p rdfs:range rdfs:Literal
> where l is an ill-typed XML literal.
> Since G has no rdfs-interpretations, it has no D-interpretations
> for any datatype map D, and also no OWL DL interpretations
> for any datatype map D, so G is OWL DL inconsistent.
> 
> Abstract syntax ontology O, containing
>   Individual(v value(p l))
>   DataProperty(p range(rdfs:Literal))
> If D is a datatype map that does not contain rdf:XMLLiteral,
> then O is consistent.

Agreed.

> It is clear that the translation of O with the mapping T of 
> S&AS contains the RDF graph G.
> This contradicts the corollary to Theorem 1 in Section 5.4
> in S&AS, for any datatype map D that does not contain
> rdf:XMLLiteral.
> (I am using here my first comment above, that Theorem 1 etc.
> should be read as holding for a certain datatype map.)

Agreed.

This is an area that needs updating in OWL, now that RDF finally has
workable datatypes.  The need for updating has been known for some time
now, and wording was put in S&AS that S&AS depended on the details for RDF
datatyping that had not yet been finalized.

My suggestion would be to weaken the corollary, adding the condition that
the corollary only holds when the datatype map includes the RDF mapping for
rdf:XMLLiteral.  Other theorems would have to be similarly changed.

> ===
> 
> (Section 3.1: second bulleted condition:)
> >-It is now assumed that LV contains each Unicode string
> >and each pair of two Unicode strings.

This should actually be weakened to pairs of Unicode strings and language
tags, or whatever the RDF model theory says.  Again this is something that
has undergone recent change in the RDF model theory.

> >For the correspondence with Section 5, it would be
> >sufficient to assume only that plain literals in
> >V (and L) are contained in LV.
> On further reflection, it seems that not only the assumption
> about plain literals but also the assumption about
> typed literals could be weakened.
> The condition could be rephrased, for example, as follows:
> "LV, the literal values of I, is a subset of R that
> contains the values of plain literals in V, and,
> for each datatype d in D and well-typed literal
> "v"^^d in V, the value L2V(d)(v)."

Part of this would work, except that it would have to refer to the RDF
model theory to pick up the ``value'' for literals.  I would oppose
weaking the requirement that all values (and not just mentioned values)
appear in LV.  I think that it is a mistake for the RDF model theory to be
worded this way.

> It seems that this assumption would suffice for
> all the normative text in S&AS.
> A motivation to make this change is that similar 
> unneccesary assumptions in the RDF Semantics led
> to many complications and the need for a similar
> change.

I do not think that this is correct.  The RDF model theory has been changed
several times in attempts to restrict the inferential closure of a finite
RDF graph.  This lead to a number of problems, which I think that OWL has
never had.

> In case you want to compare the corresponding earlier problems with
> the RDF Semantics, here is a URL:
> http://lists.w3.org/Archives/Public/www-rdf-comments/2003OctDec/0161.html
> 
> ===
> 
> >-In order to make the definition more strongly parallel
> >to the definition of ill-typed literals (and datatype
> >clash) in the RDF Semantics document, it seems to be better
> >to replace, in the last bulleted condition (in Section 3.1),
> >   R-V(d)
> >by
> >   R-LV

Agreed.

> Herman ter Horst

peter
Received on Tuesday, 2 December 2003 11:59:04 GMT

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