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[Resend due to finger trouble, with additions]
There are indeed many useful kinds of schema languages that don't have
widespread currency yet.
In some of my company's products we use our own little schema language
that says
* what elements are allowed or required
* what attributes are allowed or required
* what elements are only every found in first or last position
We also have "usage schemas" which sample documents and generate all the
possible grandparent/parent/child paths in the document, and checks
other documents against these.
Checking lists of tokens is indeed a very problematic area for
Schematron using the default XSLT 1 implementations.
ISO DSDL was created to give a home and official status to these kind of
little languages. If anyone can come up with a technically excellent and
implemented little schema language that helps validate some significant
kinds of markup idioms that XSD or the other ISO DSDL schema languages
do not cover well (as is *entirely* possible), I am certain the ISO SC34
WG1 group would be interested in considering it for standardization, in
typically unpanicked fashion.
To be honest, I suspect that Schematron with a particular extension
could pretty much do what Peter requires. In particular, ISO Schematron
has a macro facility called abstract patterns that allow you to be much
more declarative in labelling the participants in a schema relationship:
you could have one like
<sch:pattern name="required-child" abstract="true">
<sch:rule context="$parent">
<sch:assert test="$child">The parent should have a child</sch:assert>
</sch:rule>
</sch:pattern>
where the $ tokens are macro arguments that are replaced by their
invocation to give conventional Schematron schemas
<sch:pattern name="eg" is-a="required-child">
<sch:param name="parent" value="Angela"/>
<sch:param name="child" value="Suhai"/>
<sch:param name="position" value="1" />
</sch:pattern>
What this gives is enough markup that a custom processor can take the
schema and
generate code based on it. For example, to append a Suhai element to
the Angela
element in the first position. In fact, you might even decide not to
ever validate using the Schematron schema per se, (use it as
documentation) but to drive your superduper custom processor with the
information specified using abstract patterns!
Abstract patterns represent, I hope, a significant advance in home-made
schema languages, because not only do you get the background boring
power of XPath validation, but you also get the extra labelling required
to enable identification of the parts of constraints and assertion
tests. And that identification opens the door for re-targetting the
schema for purposes such as code generation or any kind of useful
purpose. XPaths are great because they are terse; abstract patterns
overcome the concomitant lack of declative expressiveness.
Cheers
Rick
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