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

[Philippe Poulard <Philippe.Poulard@sophia.inria.fr>]

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

Except that you will need 3000 tags to express all that people expect 
and will expect.

The set of all declarative constraints is a subset of all constraints 
expressible (not necessary declarative).

However, there is always a mean to turn a non-declarative constraint to 
a declarative one : just add the new tag that expresses what you want.
<acme:the-number-of-cells-must-be-the-same-in-all-columns-of-the-table/>
This is a curious declaration, but a declaration anyway. I can express 
it in an Active Schema without implementing this silly tag.

I only argue that instead of making a "tag race", there is a better way 
to do things : allow to mix imperative instructions with declarative 
sentences, which is applyable not only for schemata, but for operating 
tag libs in general. If there is a component behind a tag, the hierarchy 
of the components expressed with an XML document is static, but can 
becomes dynamic if alternative statements are allowed. Of course, a new 
level of indirection is added, but a great number of tags may be saved.

So, it's more simple.

  - 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/
> 

-- 
Cordialement,

               ///
              (. .)
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|      Philippe Poulard       |
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