- From: <Patrick.Stickler@nokia.com>
- Date: Mon, 5 Nov 2001 12:32:44 +0200
- 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 UTC