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RDF/XML syntax for datatyping, XML Schema validation, and all that jazz...

From: <Patrick.Stickler@nokia.com>
Date: Fri, 23 Aug 2002 12:11:06 +0300
Message-ID: <A03E60B17132A84F9B4BB5EEDE57957B5FBA9F@trebe006.europe.nokia.com>
To: <w3c-rdfcore-wg@w3.org>

	
I've been playing around with being able to use an XML Schema
validator to validate typed literals in RDF/XML instances in
a way that lets RDF and XML Schema interoperate in this regard.

Unfortunately, I didn't succeed. The main problem is providing
for both local and global idioms in XML Schema. I've not been
able to find a way to define a property element which can either
have an xsd:type for its simple data content, or alternatively,
a locally specified type.

If we have

   <ex:age>10</ex:age>
   <ex:age><xsd:integer>10</xsd:integer></ex:age>

then we can say 

   <xsd:element name="xsd:integer" type="xsd:integer"/>

and either

   <xsd:element name="ex:age" type="xsd:integer"/>

or

   <xsd:element name="ex:age">
      <xsd:element name="xsd:integer"/>
   </xsd:element>

but we can't do both for ex:age. There's no way in XML Schema
to constrain the data content of a mixed content model, so even
though the <xsd:integer> inline element is constrained by a 
separate rule, if we define ex:age to have mixed content, the
simple implicit inlined idiom can't be validated.

Alternatively, if we adopt a datatyping property such as

   <ex:age>10</ex:age>
   <ex:age xsd:integer="10"/>

we can define in XML Schema

   <xsd:attribute name="xsd:integer" type="xsd:integer"/>

   <xsd:element name="ex:age" type="xsd:integer"/>

but in the case of

   <ex:age xsd:integer="10"/>

the latter rule complains since the null string is not
in fact a valid lexical form for xsd:integer.

So it appears that it is just too hairy to try to get XML Schema
to grok datatyping in RDF/XML in a way that provides for full
interchangability of local and global idioms.

Cest la vie.

--

Therefore...

I think that going with the rdf:type attribute is the most optimal
representation, and leave validation to be done based on the
knowledge in the graph -- even if one extracts an XML representation
that can be validated by an XML Schema validator.

So, we'd have either

   <ex:age>10</ex:age>
or
   <ex:age rdf:type="&xsd;integer">10</ex:age>

which would give us the triples

   ?s ex:age "10" .
and
   ?s ex:age <&xsd;integer>"10" .

respectively.

This is analogous to the presently legal representation of
locally typed URIref denoted resources, such as

   <ex:friend rdf:type="&foo;Person" rdf:resource="#Bill"/>

which gives us the triples

   ?s ex:friend Bill .
   Bill rdf:type foo:Person .

The similarity between

   <ex:age rdf:type="&xsd;integer">10</ex:age>
and
   <ex:friend rdf:type="&foo;Person" rdf:resource="#Bill"/>

is significant, in that both identify typed resources, and
the semantics of

   ... <&xsd;integer>"10" .
and
   Bill rdf:type foo:Person .

are essentially the same. Both the typed literal node and
the URIref node denote a resource that has the specified
rdf:type -- only we use a more compact representation in
the case of the typed literal node, since literals can't
be subjects. If they could, we'd just say

   "10" rdf:type xsd:integer .

where the similarity between the typed literal and URIref 
cases becomes quite clear.

Now, at present, having both literal data content and
attributes for a property element is not legal. So,
parsers simply need to be tweaked to allow an rdf:type
attribute on property elements which have literal content
and in such cases generate a typed literal node. So this
is a simple extension to the RDF/XML syntax, not a change
to how parsing works at present. The triples you get now
will continue to be what you get, but extending the syntax
to allow the local literal typing will allow you to get
triples with typed literal nodes as well.

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 Friday, 23 August 2002 05:11:15 EDT

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