RE: response to semantics comment pfps-01

 

-----Original Message-----
From: ext pat hayes [mailto:phayes@ai.uwf.edu]
Sent: 04 February, 2003 00:51
To: w3c-rdfcore-wg@w3.org
Subject: response to semantics comment pfps-01


Some XML Schema primitive datatypes are impossible to use as RDF
datatypes.  Therefore XSD intepretations are ill-defined.

The problematic datatypes include:
duration - because equality in its value space is not well defined
QName - because there is no fixed lexical-to-value mapping
ENTITY - because there is no fixed value space
NOTATION - because there is no fixed lexical space
----


I don't consider these to be fatal problems which make it 'impossible' to use these datatypes.  

 
I'm not particularly confortable with that statement. RDF Datatyping is based on explicit characteristics
which should be embodied by each and every member of rdfs:Datatype. If some XML Schema datatype
fails to have any of those characteristics, then they are not valid instances of rdfs:Datatype and I
would expect not usable with RDF. I.e., I would consider those to be fatal problems.
 
To use a datatype with RDF, i.e. to specify it as the value of the attribute rdf:datatype is to assert that
it is a member of rdfs:Datatype. And to assert that a datatype is a member of rdfs:Datatype is to
assert that it has a value space, a lexical space, and an N:1 mapping from lexical to value space
where N >= 1.
 
Thus rdf:datatype="&xsd;ENTITY" would be an error, if so asserted, as it in fact has no fixed value
space, etc.
 

 
 What is true however is that these are examples of *underdetermined* datatypes, ie datatypes about which the available information is incomplete in some way.

 
My hope is that it is the specs which are incomplete and that the XML Schema WG could clarify the definitions
of these datatypes in a manner that would allow them to be treated as valid members of rdfs:Datatype.
 


For example, the set of durations is underdetermined; what this means is that *any* set which conforms to the XSD spec will suffice as a value space of xsd:duration, and there may be more than one such set. There may therefore be more XSD interpretations than the XML authors intended. As usual in such situations, the effect on inference is that some inferences simply cannot be made, eg it is simply unknown whether or not one xsd:duration literal can be substituted for another, so to this extent the corresponding inference rule does not apply. The text makes it clear that these rules apply only in cases where certain kinds of information are provided by the datatype information source.

In sum, I don't think there is any need to do anything about this issue, except possibly to add some explanatory prose to clarify the point.


 
 
Explanations, yes, but let's be careful about exactly what we're saying.
 
Cheers,
 
Patrick


Pat

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