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Subject: Re: ID/IDREF problem


James Clark wrote:
> 
> Eric van der Vlist wrote:
> >
> > 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.)

There are some situations where it would be useful to use a XPath
function instead of a XPath expression pointing to a node in the tree.

One example are references to fragments in the same document:

<author id="jclark">
...

<product name="xt">
  <author xlink:type="simple" xlink:href="#jclark"/>
...

The match could be done if the keyref path allowed
"substring-after(@xlink:href, '#')", the result of a XPath expression
that doesn't point to anything in the document.

This would be useful to handle other cases of compounded keys where a
reference is not stricly equal to a key of composed with the key.

> 
> > > 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.

Allowing arbitrary expressions can be seen as a first step to write
context dependent rules...

Eric


> James

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