xml-dev - Re: XML Schemas and extension

[ht@cogsci.ed.ac.uk (Henry S. Thompson)]

Jeff Rafter <jeffrafter@earthlink.net> writes:

> I know that we seemed to finish the "ref'ing vs. inlining" discussion and
> all seemed pretty satisfied, but I was wondering how all of this fits into
> type derivation when using extension.
> 
> I know that,
> 
>  "A complex type which extends another does so by having additional content
> model particles at the end of the other definition's content model, or by
> having additional attribute declarations, or both.
> 
> NOTE: This specification allows only appending, and not other kinds of
> extensions. This decision simplifies application processing required to cast
> instances from derived to base type. Future versions may allow more kinds of
> extension, requiring more complex transformations to effect casting. "
> 
> I have read through the spec. and can't seem to come up with an answer to
> the following, unless it fits into the above cited "future revision" caveat
> : Can an element in be redeclared in a derived (by extension) complexType?
> 
> <complexType name="Foo">
>   <element name="bar" type="int">
> </complexType>
> 
> <complexType name="Foo2" base="Foo" derivedBy="extension">
>   <element name="bar" type="int">
>   <element name="bar2" type="int">
> </complexType>
> 
> I apologize if there is an obvious answer in the specification that I am
> missing or not comprehending.

Derivation by extension can be understood as syntactic sugar.  It's
the effective type definition that has to satisfy the constraints on
types such as no-duplicate-elt-name-with-different-types.  So consider 
the effective type definition:

<complexType name="Foo2">
  <sequence>
   <element name="bar" type="int">
   <element name="bar" type="int">
   <element name="bar2" type="int">
  </sequence>
</complexType>

Nothing wrong with that, so the derivation is OK.

ht
-- 
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
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	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
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