Re: [xml-dev] A single, all-encompassing data validation language - good

["Chris Scott" <scott.chris@gmail.com>]

> Up till this date, grammar-based and rule-based languages have been
> kept separate:
>
>     Grammar-based Languages: XML Schema, Relax NG, DTD
>
>     Rule-based Languages: Schematron, RuleML
>
> What do you think about XML Schema working group incorporating
> rule-based capabilities into the language?

Since we already have ISO Schematron, why not define a standard way to
include Schematron in XML Schema?  Unless you're trying to get around
some sort of shortcoming of Schematron, why reinvent the wheel?
Creating documents in multiple namespaces is already fairly common,
xsl+xsl:fo is one good example.

I haven't read all of XML Schema 1.1 yet, so forgive me if this is
something already planned.

> 1. XML Schemas is already quite large and complex.  This will make it
> larger and more complex.

If the assertion section is orthogonal to the structure definition,
the complexity barrier doesn't seem as high.  If it's easy to use one,
or the other or both, then I think this concern is mitigated.

> 3. Possible performance degradation since, for example, validation
> can't be halted when grammar requirements fail.

Yes, I can see possible performance degradation, especially since the
naive method would be to do a two pass validation step.  That's
exactly what we'd have to do in a pipeline though.

> 4. Replacing one grammar language with another becomes prohibitive
> (example: you may want to replace XML Schemas with Relax NG)

XML Schema + Schematron ~ RNG + Schematron

If there is support in both communities for rules based validation,
and I believe there is, then consolidation around existing standards
should ease grammar translations.

> QUESTIONS
>
> 1. Can you add to the above list?  What other advantages and
> disadvantages are there?

I can see implementation difficulties arising from having rules for
the same element in two unrelated pieces of a validating schema.
Along those lines, I can also see a moderate decrease in readability
because of this separation.

> 2. Is grammar validation of a fundamentally different nature than rule
> validation?

I think so.  Which is why I think it makes sense to define a
Schematron inclusion as a top level element instead of modifying
existing XML Schema structures.

> 3. If so, is it reasonable to merge two fundamentally different things?

Well validation is validation, right? :)

> 4. Is it in the best interest of the marketplace to have a single,
> all-encompassing data validation language, or is it better to have
> multiple data validation languages that work together?

Obviously I believe the latter.  Why not use ISO Schematron if it's
already there?  Am I missing any show stoppers?

Thanks,

~Chris