Rick Jelliffe writes:

> Discussing the sequencing of particular technologies outside their
> diagnostic context puts the cart before the horse.

Yes, exactly.  Furthermore, there are likely to be tradeoffs involved in 
any decision you make.  Certain constraints are typically easier to 
express in grammar-based constraint languages.  For example, saying that 
element E must contain a sequence of A,B,C.   Certain constraints are 
typically easier to express in a language like Schematron.

Fine, so you should use each for what it does well?  Often, but not 
always.  Let's say you decide to put some constraints in W3C XML Schema 
and some in Schematron.  That can be a great approach, and many people are 
happy with it, but there are compromises involved.  For example, if a 
downstream tool (e.g. a databinding engine) wants to reason about the 
constraints on element E, it may have to look at both the Schematron rules 
and the XSD grammars together.  Not necessarily a bad thing, but 
potentially a complication.   (Indeed, that's one of the reasons for the 
architecture of the new assertion rules proposed for W3C XML Schema 1.1; 
they are influenced heavily by Schematron, but they are not a complete 
replacement.  They are, however, more directly integrated with the XSD 
type system (in Schema 1.1, XPath-based constraints are on complexTypes).  
So, you'll still want to use Schematron  or Schematron+XSD to solve some 
problems, but for many of the simpler uses of Schematron+XSD, Schema 1.1 
provides a more tightly integrated approach.)

The main point here is not to advocate Schema 1.1 assertions, Schematron, 
XSD or any other combination of technologies.  Rather it's to second what 
I take to be Rick's position:  which is you need to think hard about use 
cases and success criteria in order to judge the tradeoffs involved in 
adopting any combination of these technologies.

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Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
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