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Re: Circular types in XML Schemas

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 01 May 2002 20:26:43 +0100
To: Jeni Tennison <jeni@jenitennison.com>
Cc: "Morris Matsa" <mmatsa@us.ibm.com>, Yuri de Wit <yuri.dewit@metaserver.com>, "'xmlschema-dev@w3.org'" <xmlschema-dev@w3.org>
Message-ID: <f5bsn5boej0.fsf@cogsci.ed.ac.uk>
Jeni Tennison <jeni@jenitennison.com> writes:

> Hi Morris,
> > I believe that necessarily infinitely recursive unsatisfiable types
> > are forbidden in Schemas. This question was answered about a year
> > ago on this list [1], and the part of the schema spec quoted then
> > was [2]. Please tell me if I'm wrong.
> Hmm... I don't think that the definition of an emptiable group
> (http://www.w3.org/TR/xmlschema-1/#cos-group-emptiable) relates to the
> question of infinitely recursive unsatisfiable types. As far as I can
> tell, the only places in which this Schema Component Constraint is
> used is when assessing whether it's OK to derive one type from another
> type, and whether an element's default/fixed value is OK.
> There might be something else specifying a constraint like this
> somewhere, of course, but I don't think that's it. If there were such
> a constraint, I guess it would be a constraint on model groups that
> stated that a model group whose minimum effective total range is more
> than 0 cannot contain an element particle at any level whose minimum
> effective total range is more than 0 and whose type's content involves
> that model group at any level. But I can't see such a constraint
> anywhere.

There isn't one.  It would be very expensive to enforce, and not in at 
least my judgement worth it.

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2002, 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/
 [mail really from me _always_ has this .sig -- mail without it is forged spam]
Received on Wednesday, 1 May 2002 15:26:57 UTC

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