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Subject: Re: ID/IDREF problem

Eric van der Vlist wrote:
> James Clark wrote:
> >
> > In addition to ID/IDREF, XML Schemas Part 1 has a new facility for
> > identity-constraints (using the xsd:unique, xsd:key, xsd:keyref,
> > xsd:selector, xsd:field elements).  See
> > http://www.w3.org/TR/xmlschema-1/#Identity-constraint_Definition_details).
> > This is quite a good facility.  The only thing I think it lacks is
> > proper support for hierarchical references.  On the other hand the
> > elements supporting identity-constraints have the feel of a separate
> > language, which could easily be layered on top of the rest of Schema
> > Part 1.
> There is another limitation: IMO, it would be much more powerful if the
> paths in these mechanism were defined as being XPath expressions than
> being defined as paths pointing to nodes from the document.

I don't understand your point. In the XML Schema CR, the paths are
general XPath expressions.  (This is going to change to a very, very
restricted subset of XPath.)

> > We could support these XML Schema Part 1 elements for identity
> > constraints in TREX by allowing <element> elements to have arbitrary
> > elements from other namespaces with a global attribute
> > trex:role="constraint". Just like you can plug in a datatyping language
> > to TREX, so you would be able to plug in a identity-constraint language.
> More generally, wouldn't it be possible to plug a rule checking language
> ?
> Identity constraints can be seen as rules.



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