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Subject: Re: ID/IDREF problem


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.

> 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.

Eric
-- 
Rendez-vous  Paris pour net2001.
                         http://www.mynet2001.net/pgmonline2001/it2.html
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Eric van der Vlist       Dyomedea                    http://dyomedea.com
http://xmlfr.org         http://4xt.org              http://ducotede.com
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