8.2.1. Redefining of Simple and Complex Types

Simple and complex types are redefined by deriving them (by restriction for simple types and by restriction or extension for complex types) inside the xs:redefine element. We can apply this to our last example. The definition of bookTmp is currently used to describe the book element though derivation:

<xs:element name="book">
  <xs:complexType>
    <xs:complexContent>
      <xs:restriction base="bookTmp">
        <xs:sequence>
          <xs:element ref="isbn"/>
          <xs:element ref="title"/> 
          <xs:element ref="author" minOccurs="0"
            maxOccurs="unbounded"/> 
          <xs:element ref="character" minOccurs="0"
            maxOccurs="unbounded"/>
        </xs:sequence>
        <xs:attribute name="id" type="bookID"/>
        <xs:attribute ref="available"/>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>
</xs:element>

Instead of doing this, we can also redefine the definition of the book complex type. The new schema to define the complex types is then:

<?xml version="1.0"?> 
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:complexType name="elementWithID">
    <xs:attribute ref="id"/>
  </xs:complexType>
  <xs:complexType name="book">
    <xs:complexContent>
      <xs:extension base="elementWithID">
        <xs:sequence>
          <xs:element ref="isbn"/>
          <xs:element ref="title"/> 
          <xs:element ref="author" minOccurs="0"
            maxOccurs="unbounded"/> 
          <xs:element ref="character" minOccurs="0"
            maxOccurs="unbounded"/>
        </xs:sequence>
        <xs:attribute ref="available"/>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
  <xs:complexType name="personType">
    <xs:complexContent>
      <xs:extension base="elementWithID">
        <xs:sequence>
          <xs:element ref="name"/>
          <xs:element ref="born"/>
          <xs:element ref="dead" minOccurs="0"/>
          <xs:element ref="qualification" minOccurs="0"/>
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
</xs:schema>

The redefinition--note how a book complex type is redefined using a base type with the same name, which would be forbidden anywhere else--and usage of the book element looks like:

<xs:redefine schemaLocation="complex-types2.xsd">
  <xs:complexType name="book">
    <xs:complexContent>
      <xs:restriction base="book">
        <xs:sequence>
          <xs:element ref="isbn"/>
          <xs:element ref="title"/> 
          <xs:element ref="author" minOccurs="0"
            maxOccurs="unbounded"/> 
          <xs:element ref="character" minOccurs="0"
            maxOccurs="unbounded"/>
        </xs:sequence>
        <xs:attribute name="id" type="bookID"/>
        <xs:attribute ref="available"/>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>
</xs:redefine>
          
<xs:element name="book" type="book"/>

8.2.2. Redefinition of Element and Attribute Groups

The redefinition of complex and simple types seems quite natural and should not be much of a surprise, since it builds on things we've discussed in detail in previous chapters. The new part of xs:redefine is that element and attribute groups--which cannot be derived--can also be redefined. Redefinition of element and attribute groups is done without any special schema element: a group redefinition that contains a reference to itself is considered an extension; otherwise, it's considered a restriction. These two methods have their own rules and semantics, which are similar but not identical to the rules and semantics of the derivation of complex types. These deserve a specific description. As we will see, the general principles are the same, and the asymmetry between extension and restriction is preserved for group redefinitions.