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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. Yes. James
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