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Re: Overriding of a fixed value

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 14 Feb 2002 11:35:11 +0000
To: Jeni Tennison <jeni@jenitennison.com>
Cc: Maikel Jansen <maikel.jansen@asml.com>, xmlschema-dev@w3.org
Message-ID: <f5bd6z8mh3k.fsf@cogsci.ed.ac.uk>
Jeni Tennison <jeni@jenitennison.com> writes:

> > Consider the following complex type definition:
> >
> > <xs:complexType name="type0">
> >   <xs:simpleContent>
> >     <xs:extension base="int"/>
> >   </xs:simpleContent>
> >   <xs:attribute name="description" fixed="a description"/>
> > </xs:complexType>
> 
> That isn't a legal complex type definition. The xs:attribute should be
> within the xs:extension element, giving:
> 
> <xs:complexType name="type0">
>   <xs:simpleContent>
>     <xs:extension base="int">
>       <xs:attribute name="description" fixed="a description" />
>     </xs:extension>
>   </xs:simpleContent>
> </xs:complexType>
> 
> > Now, I want to express a type called type1 in an XML schema, such
> > that type1 is an inheritance type of type0 and type1 overrides the
> > (fixed) value of the attribute 'desc' of type0.
> 
> You cannot derive this type by restriction, as you cannot change the
> fixed value constraint of the attribute if you derive by restriction.
> I *think* that you can derive it by extension (I don't see anything
> banning it in the Rec), as follows:
> 
> <xs:complexType name="type1">
>   <xs:simpleContent>
>     <xs:extension base="type0">
>       <xs:attribute name="description" fixed="another description" />
>     </xs:extension>
>   </xs:simpleContent>
> </xs:complexType>

That's broken -- it produces a type with two attributes called "description".

> But I think that you would get a cleaner hierarchy if you had a
> general type with the (unfixed) description attribute:
> 
> <xs:complexType name="type">
>   <xs:simpleContent>
>     <xs:extension base="int">
>       <xs:attribute name="description" />
>     </xs:extension>
>   </xs:simpleContent>
> </xs:complexType>
> 
> And then derive the type0 and type1 from that type by restriction:
> 
> <xs:complexType name="type0">
>   <xs:simpleContent>
>     <xs:restriction base="type">
>       <xs:attribute name="description" fixed="a description" />
>     </xs:restriction>
>   </xs:simpleContent>
> </xs:complexType>
> 
> <xs:complexType name="type1">
>   <xs:simpleContent>
>     <xs:restriction base="type">
>       <xs:attribute name="description" fixed="another description" />
>     </xs:restriction>
>   </xs:simpleContent>
> </xs:complexType>

That looks good to me.

ht
-- 
  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 Thursday, 14 February 2002 06:37:19 GMT

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