XSLT, 2nd Edition

Creating New Datatypes by Extension

Another way to create new datatypes is through extension. The most common example of this is adding an attribute to a simple type. The XML Schema spec says that simple types (xs:string, xs:integer, etc.) can’t have attributes. We can create a complex type that allows an element to have content of type xs:integer and have an attribute. Here’s how the schema looks:

<?xml version="1.0" encoding="UTF-8"?>
<!-- extension.xsd -->
<xs:schema 
  xmlns="http://www.oreilly.com/xslt"
  targetNamespace="http://www.oreilly.com/xslt"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:simpleType name="currency-type">
    <xs:restriction base="xs:string">
      <xs:enumeration value="USD"/>
      <xs:enumeration value="GBP"/>
      <xs:enumeration value="CNY"/>
      <xs:enumeration value="EUR"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:complexType name="price-type">
    <xs:simpleContent>
      <xs:extension base="xs:decimal">
        <xs:attribute name="currency" type="currency-type"/>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>
  
  <xs:element name="price" type="price-type"/>

</xs:schema>

Now we have an element whose content must be an xs:decimal, while it must also have an attribute named currency. Here’s a valid document for this schema:

<?xml version="1.0" encoding="utf-8"?>
<!-- extension.xml -->
<price
  xmlns="http://www.oreilly.com/xslt"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.oreilly.com/xslt extension.xsd"
  currency="USD">
438.92
</price>

If we change ...