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Re: [xml-dev] Rick Jelliffe quotable quote on the purpose of schemas


I want to throw my hat in the ring here from my own experience at 
constructing these things (yes i know - in the database, not document 

However, the point is that the discussion will continue until we 
recognise the fundamental difference between "structure" and "constraint".

For my money I'd be happy if it took 2 documents to provide a valid 
description of what an xml document could contain. The first simply and 
only outlines structures. In effect it could easily be a prototype xml 
document with one of all possible structures, possibly with some sort of 
ellipsis to indicate recursive inclusion - or we could stick with dtd's.

The second would be an XPath type document that lists restraints similar 
to XSD.

 From my perspective (and the commercial view) this means I could have 
eg some sort of generic description of say a purchase order and then a 
constraint based instance that was my requirements - what must be 
included, optional, values limits, etc.

This was the basic approach in the ANSI EDI standards and worked very 
well to build what must remain the most successful B2B system.

This is a long way of saying that I think trying to get one tool to do 2 
fundamentally different things is always going to less than optimal.


Michael Kay wrote:
>> "The flaw with grammars is that they only allow to constraint 
>> content models in a declarative manner
> There's nothing wrong with constraints being declarative - in fact, they
> definitely should be - the flaw is with the expressive power of a grammar as
> a contraint language.
> This is all well known. And in fact, XML Schema itself has some
> extra-grammatical constraints, namely uniqueness and referential
> constraints. But there's still a camp that believes (wrongly, in my view)
> that the expressive power of a schema language should be restricted for
> performance reasons.
> Michael Kay
> http://www.saxonica.com/
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