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RE: [xml-dev] XML spec and XSD

> > 2) Isn't it the case that some of the complexities of XSD 
> are that way 
> > to allow for that validation speed?
> >   
> Do you have an example?  (I imagine it causes some 
> simplifications as well as some complexities.)

Some of the restrictions are certainly there for that reason, like the
subset of XPath permitted in identity constraints, the ban on backtracking
(UPA etc), and the rules in conditional type assignment that restrict you to
looking at attributes of the current element.

I'm always suspicious of such rules: on the whole I think it's better to let
users decide where to make the trade-off between performance and
functionality; and I think writers of specifications are sometimes
unimaginative when they consider implementation options. There's a classic
in XSD, the rules for restriction of xs:all content models, where the spec
explicitly says that a rule is there only for implementation convenience,
and it actually causes my implementation a great deal of inconvenience to
enforce it.

As for streaming, I think we'll find in the fullness of time that a
reasonably large subset of the assertions that people write (either in XSD
1.1 or in Schematron) can actually be evaluated in streaming mode.


Michael Kay

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