xml-dev - Re: RDF Schema Question: range of values of an rdfs:Class?

[Dan Brickley <Daniel.Brickley@bristol.ac.uk>]

On Tue, 4 May 1999, Roger L. Costello wrote:

> Question: if I create an RDF Schema Class, then what is its rdf:range of
> values?
[...] 
> In the example it says that the possible values for the maritalStatus
> property are: {Married, Divorced, Single, Widowed}.  Here is where I
> have a question.  How did the range of possible values for maritalStatus
> suddenly get restricted to these four values?  In the example it creates
> four instances of the MaritalStatus Class:
> 
> <MaritalStatus rdf:ID="Married"/>
> <MaritalStatus rdf:ID="Divorced"/>
> <MaritalStatus rdf:ID="Single"/>
> <MaritalStatus rdf:ID="Widowed"/>
> 
> Presumably, this is where the four values were gotten from.  Something
> seems fishy here.  

I see your concern. The main thing to say on this is that it is a trust
issue:

	"Whether resources declared to be of type MaritalStatus in
	another graph are trusted is an application level decision."
	(from http://www.w3.org/TR/PR-rdf-schema/ 7.1)

In other words, the core schema machinery only lets you say that
'maritalStatus' makes sense for values that are of rdf:type
MaritalStatus. What this minimalistic spec doesn't itself give you is a
comprehensive framework for figuring out whose data to trust. How one
sets about this is likely to depend on details of your application; in
some contexts, for example, it might be rather useful to learn about new
instances of such classes. In others, this could be a security
concern (as could trusting the contents of random XML or RDF data in general).

Earlier working drafts listed this topic as an open issue (which was
resolved ultimately in favour of minimalism). Here's the text from 
http://www.w3.org/TR/1998/WD-rdf-schema-19980814/

	C.12. Class Sealing

	We would like to define a mechanism for 'sealing' an RDF Class, so that
	it becomes illegal to make certain RDF
	statements involving it. This is loosely analogous to the notion of
	'final' classes in Java / OO programming. We might,
	for example, want to stop people creating subclasses of the class, or
	creating property types which have that class as
	their domain. The degree of sophistication required of the class sealing
	mechanism is as yet unclear: we might (for example) wish to
	consider the feasibility of using digital signatures in this context.
 


		I am having difficulty relating this to my Java
> background.  In Java, if I create a class MaritalStatus:
[...]

The analogy with Java can be overstretched, but this was a good
comparison. In both cases, any object/resource that was known to be of
the appropriate type would be allowed. The difference is that in RDF/Web
context, we won't necessarily believe every assertion that some resource
is of a certain type. 

hope this helps,

dan




--
Daniel.Brickley@bristol.ac.uk                  
Institute for Learning and Research Technology http://www.ilrt.bris.ac.uk/
University of Bristol,  Bristol BS8 1TN, UK.   phone:+44(0)117-9287096


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