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   XML Schema in Z ?

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  • From: Robert Worden <rworden@dial.pipex.com>
  • To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
  • Date: Thu, 27 Jul 2000 09:40:07 +0100

In response to Jonathan Robie's question: what should the Schema WG do 

A suggestion: in parallel with anything else you do, re-express the Schema 
spec in a  mathematical specification language such as VDM or Z. Publish a 
mathematically annotated  version of the spec.

What would Z or VDM do for Schema? They use elementary maths (mainly set 
theory) to express  more precisely what you mean. This is the best way I 
know to expose any inconsistencies,  ambiguities and gaps in a spec. Better 
even than implementing. A spec in Z is never 'just  maths'; it is maths 
with explanatory English. Reading it, trying to relate the maths and the 
 English, clears the cobwebs faster than anything.

People have noted pieces of 'tortured prose' in the schema spec. Try 
turning the tortured  prose into maths. If the maths is simple, we have 
learned something, and can probably  re-express the ideas in simpler 
English. If the maths is tortured, we have got a problem!  Then leave that 
part for release 2 and sort it out first.

Writing a Z spec is a piece of intense work, and reading it is not for 
everybody. But   getting the spec right is _much_ cheaper than implementing 
and finding problems downstream.  Michael Kay has commented that namespaces 
doubled the complexity of implementing SAXON; we  need to be sure there are 
no such complexity time-bombs in XML schema. A lot of people will 
 implement on top of it for a long time.

W3C needs to be sure it is releasing a quality specification, as Schema 
will be so central.  This would be a cost-effective way to do so.

Robert Worden


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