W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > November 2001

Re: heading toward datatyping telecon

From: <Patrick.Stickler@nokia.com>
Date: Mon, 5 Nov 2001 12:32:44 +0200
Message-ID: <2BF0AD29BC31FE46B78877321144043114C05E@trebe003.NOE.Nokia.com>
To: w3c-rdfcore-wg@w3.org

> FWIW, when I reviewed XML schema data types some time ago, I was deeply 
> uncomfortable that the primitive value spaces seemed to be defined (and 
> constrained) in terms of their lexical representations.  This seemed maybe

> justified because XML schema was, by design, all about values that were 
> represented in a text-based format (i.e. XML).  But I'm not convinced that

> the considerations apply equivalently to RDF.

But exactly how is an RDF literal not a lexical representation?!

Insofar as associating a data type with a literal, both the value
space and lexical space are relevant.

Insofar as making logical inferences about the values themselves
and their relation to other values, compliance with range constraints,
etc. etc. then the lexical space is not relevant.

However, if a given standardized data type such as xsd:integer
*does* define a particular lexical space, then IMO clasifications
of literals which do not conform to that lexical space as xsd:integer
is wrong -- as that will confuse, if not choke, applications which
are expecting a valid xsd:integer lexical form (as they should have
the right to do).

The lexical validity of sub-types with regards to their super-types
is important in the context of ontological transparency whereby
a given value may be defined in terms of a very specific data
type yet a given query (and the resultant knowledge) is defined
in terms of a more general data type, and the response must be
encoded in a *lexical* form that is valid.

Thus, it is not acceptable to e.g. create a sub-type hexInteger
of type integer which has a lexical form that is invalid for
integer because a system may recieve a query for the value of
a property that has a range of 'integer' yet the knowledge
is defined via a sub-property having a range of hexInteger, and
the resultant response would encode a hexInteger literal as
the value of a integer property, which is invalid.

Thus, per the XML Schema specification, a nonNegativeInteger
lexical form is also a valid integer lexical form is also a valid
decimal lexical form, etc. These data types are very well defined,
and the hierarchical equivalence issues were obviously well
understood by the folks who wrote it -- and of course, the whole
concept of manditory validity of instances of sub-classes in
super-classes is at the very heart of the XML Schema model. Sub-types
are defined only be restriction, not by deviation which does not
conform to all superclasses. This characteristic should *not*
be discarded by RDF in the interpretation of literals by defined
data type by assuming that rdf:type only applies to value space
and not also to lexical space.

Cheers,

Patrick

--
               
Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com
Received on Monday, 5 November 2001 05:33:21 EST

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