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Re: Questions about optional and abset

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 03 May 2000 09:34:30 +0100
To: Peter Canning <canning@vitria.com>
Cc: www-xml-schema-comments@w3.org
Message-ID: <f5bg0s06nhl.fsf@cogsci.ed.ac.uk>
Peter Canning <canning@vitria.com> writes:

> The structure specification identifies some schema component
> properties (e.g. the "min occurs" property in the "Attribute
> Declaration" component) as optional and states (section 3 paragraph 1)
> that optional properties that are missing have "absent" as their
> value.  It also describes some properties (e.g. the "target namespace"
> property in the "Attribute Declaration" component) as mandatory, but
> includes "absent" in the list of legal values.
> What is the difference between an optional property, and a mandatory
> property whose value can be "absent"?

Good question.  I think (leaving aside the question of what happens
when a required property is absent without leave because of a failed
reference) that {target namespace} is the only such property, and we
should revisit the nomenclature here.  There was a change in
terminology in this area very late in the publication process, and
some more work needs to be done.

The intended distinction is that {target namespace} is _always_
relevant, it value always significant, even when it is the 'absent'
value == no namespace.  For e.g. {min occurs}, there are
circumstances, e.g. for top-level attribute declarations, where the
property is irrelevant, the value never looked at and hence not

  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 Wednesday, 3 May 2000 04:34:33 UTC

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