Re: Circular types in XML Schemas

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.

ht
-- 
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2002, part-time member of W3C Team
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Received on Wednesday, 1 May 2002 15:26:57 UTC