Re: combine

[James Clark <jjc@thaiopensource.com>]

There's an important aspect of derivation by extension that this does not
capture.

Suppose I have a schema for invoice that has shippingAddress and
invoiceAddress elements, and suppose that the content of these types is
declared to be the complex type Address.  If I have understood XML Schema
correctly, once you derive UKAddress from Address you are allowed to do

<shippingAddress xsi:type="UKAddress">
  ...
  <postCode>W8 6UT</postCode>
</shippingAddress>

In other words, deriving from Address changes what is allowed by a reference
to Address.

Personally, I abhor xsi:type, but I think the functionality it is providing
is useful.   Given a schema like this:

<define name="Address">
...
</define>

<start>
  <element name="invoice">
     <element name="shippingAddress">
        <ref name="Address"/>
     </element>
    <element name="invoiceAddress">
        <ref name="Address"/>
    </element>
    ...
  </element>
</start>

the requirement is to be able to create a schema in which addresses also
allow a postCode. Derivation by extension in XML Schema supports this.  In
RELAX NG, combine="group" provides similar functionality.

<grammar>

<include href="invoice.rng"/>

<define name="Address" combine="group">
  <element name="postCode">
    <text/>
  </element>
</define>

</grammar>

> We do not need "combine" for mimicking "derivation by extension".
> Here is a rewrite of an example of "derivation by extension" in
> XML Schema Part 0.
>
> <grammar>
>
> <start>
> <element name="dmy">
> <element name="uk">
>   <ref name="UKAddress"/>
> </element>
> <element name="us">
>   <ref name="USAddress"/>
> </element>
> </element>
> </start>
>
> <define name="Address">
>    <element name="name"><anyString/></element>
>    <element name="street"><anyString/></element>
>    <element name="city"><anyString/></element>
> </define>
>
>  <define name="USAddress">
>   <ref name="Address"/>
>   <element name="state"><anyString/></element>
>   <element name="zip"><anyString/></element>
>  </define>
>
>  <define name="UKAddress">
>   <ref name="Address"/>
>   <element name="postcode"><anyString/></element>
>   <attribute name="exportCode"><anyString/></attribute>
>  </define>
>
> </grammar>
>
> Cheers,
>
> Makoto
>
>
>