Re: Extending a schema by adding choices

Daniel Flippance <DFlippance@mpsbc.com> writes:

> We are trying to define 2 schemas, one which allows 2 options in an
> xsd:choice, and one which extends this and allows 3 options.
> If one schema defines 2 choices of elements, can this list of choices be
> extended in a deriving schema?

No.  The invariant for extension is that any instance of the derived
type has an instance of the base type as a prefix.  Extending always
operates to allow the _addition_ of material.

To get the effect you want, use a substitution group, e.g.

<xsd:complexType name="Contact">
  <xsd:complexContent>
    <xsd:sequence>
      <xsd:element ref="contact"/>
    </xsd:sequence>
  </xsd:complexContent>
</xsd:complexType>

<xsd:element name="contact" abstract="true" type="xsd:string"/>
<xsd:element name="Telephone" substitionGroup="contact" />
<xsd:element name="Fax" substitionGroup="contact" />

Then a derived schema can add

<xsd:element name="EMail" substitionGroup="contact" />

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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