W3C home > Mailing lists > Public > www-xml-schema-comments@w3.org > October to December 2000

Re: Closure for XML Schemas

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 08 Dec 2000 11:29:35 +0000
To: Murali Mani <mani@CS.UCLA.EDU>
Cc: www-xml-schema-comments@w3.org
Message-ID: <f5b8zprw3q8.fsf@cogsci.ed.ac.uk>
Murali Mani <mani@CS.UCLA.EDU> writes:

> My previous question to the schema WG was about CFGs appearing in the
> content model. You had replied that it was handled in a way similar to
> what RELAX does -- by not allowing recursion in model groups.
> I had another question now -- will there be a definition of closure
> properties under boolean set operations, such as union, intersection and
> difference for XML Schema? I believe, XML Schema will not be closed under
> union and difference. It should not be surprising that most query
> operations also will not be closed under XML Schema. I believe one of the
> usage scenarios of XML Schema is for query?

It's clear that this isn't possible without a major change to the XML
Schema design, which depends on the Unique Attribution constraint in a
number of ways.  This means that the union of two types may be
undefined, e.g.

(a?,b*) U (b*,c)

> And databases do not work without closure. Also, I do not think we can
> obtain closure by restrictions on XML Schema. XML Schema is not closed
> because it is much restricted and less expressive than, say RELAX.

That restriction buys both implementation simplicity and expressive
power in other areas (annotation, keys, etc.) which were judged very

> Is this something which XML Schema will consider?, in such a case, I find
> it better that for some purposes, we use a closed schema language. I
> believe RELAX will be closed under all boolean set operations as well as
> query operations (from the properties of regular tree languages). 
> Therefore will it be possible for the Schema WG to combine forces with
> RELAX, and define something when we require such closure properties. From
> where I stand, it appears that RELAX is not being given any consideration,
> which I believe it deserves.

We considered this issue long and hard, but came to a different
conclusion than you evidently have.  

> I would appreciate the views of the Schema WG on this.

What you've got is my personal views:  I don't speak for the WG except 
when empowered by explicit WG decisions.

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/
Received on Friday, 8 December 2000 06:29:40 UTC

This archive was generated by hypermail 2.3.1 : Wednesday, 7 January 2015 14:49:54 UTC