View source for Value-equals test required

= Value-equals test required (a co-constraint use case) =

Allow one attribute to be required if the value of another attribute
is a particular named value; otherwise forbidden.

Cf. [[Attribute mutex]] and [[Simple attribute implication]] (which make
the required/optional/forbidden status of one attribute depend 
on the presence or absence, rather than the value, of the other
attribute).

Source: [http://lists.w3.org/Archives/Public/xmlschema-dev/2001Sep/0128.html]

<!-- ! TOC here -->

Other use cases: [[Co-constraint Use Cases]]

== Description ==

    Does XML handle conditional structure?
    In otherwords, can the use of a type be conditional?
    Please see the following example.


<pre><nowiki>
     <xsd:complexType name="BindType">
       <xsd:attribute name="type" use="required">
         <xsd:simpleType>
           <xsd:restriction base="xsd:NMTOKEN">
             <xsd:enumeration value="BridgedEthernet"/>
             <xsd:enumeration value="PPPOE"/>
           </xsd:restriction>
         </xsd:simpleType>
       </xsd:attribute>
    
       <!-- the following item is required if value of above attribute "type"
       is "BridgeEthernet" : can this be done ???? -->
       <xsd:attribute name="BrEthernetIP" use="optional">
         <xsd:simpleType>
           <xsd:restriction base="xsd:string">
             <xsd:minLength value="7"/>
             <xsd:maxLength value="16"/>
           </xsd:restriction>
         </xsd:simpleType>
       </xsd:attribute>
     </xsd:complexType>
</nowiki></pre>


== Analysis ==
=== FV ===
This is a value-based condition on an Attribute generating an upgrade of another attribute. Axis is attribute.

{| border="1" cellpadding="2" cellspacing="0"
|  [[CoConstraintDimensionSource]]: 
|-
|  [[CoConstraintDimensionDestination]]: 
|-
|  [[CoConstraintDimensionType]]: 
|-
|  [[CoConstraintDimensionEffect]]: 
|-
|  [[CoConstraintDimensionAxis]]: 
|}

== Possible solutions ==

=== Relax NG ===
=== Schematron ===
=== Check clause ===
One solution (proposal 1) is to add this annotation to the complex type.

<pre><nowiki>
  <xsd:annotation>
   <xsd:appinfo>
    <sch:pattern>
     <sch:rule>
      <sch:report 
       test="@type='BridgeEthernet' &amp; @BrEthernetIP = ''">
       If BindType/@type is 'BridgeEthernet', then a value is required
       for the BrEthernetIP attribute.
      </sch:report>
     </sch:rule>
    </sch:pattern>
   </xsd:appinfo>
  </xsd:annotation>
</nowiki></pre>

Another (proposal 2) would be to include the ''sch:report'' element
directly in the complex type definition.

Note that to the extent that the user conceives of this as a 
constraint on the ''BrEthernetIP'' attribute, any expression as
a constraint on the containing complex type will feel misplaced.

=== [[SchemaPath]] ===
One of the possible solutions is: 


<pre><nowiki>
     <xsd:element name="Bind">
       <xsd:alt cond="./@type='BridgedEthernet'" type="TBridged"/>
       <xsd:alt                                  type="TPPPOE"/>
     </xsd:element>

     <xsd:complexType name="TBridged">
       <xsd:attribute name="type"         use="required" type="xsd:NMTOKEN"    fixed="BridgedEthernet">
       <xsd:attribute name="BrEthernetIP" use="required" type="SevenSixteen"/>
     </xsd:complexType>

     <xsd:complexType name="TPPPOE">
       <xsd:attribute name="type"         use="required" type="xsd:NMTOKEN"    fixed="PPPOE">
     </xsd:complexType>

     <xsd:simpleType name="SevenSixteen">
       <xsd:restriction base="xsd:string">
          <xsd:minLength value="7"/>
          <xsd:maxLength value="16"/>
       </xsd:restriction>
     </xsd:simpleType>

</nowiki></pre>


and another is: 


<pre><nowiki>
     <xsd:element name="Bind">
       <xsd:alt cond="./@type='BridgedEthernet' and not ./@BrEthernetIP" type="xsd:error"/>
       <xsd:alt                                                          type="BindType">
     </xsd:element>

     <xsd:complexType name="BindType">
       <xsd:attribute name="type" use="required">
         <xsd:simpleType>
           <xsd:restriction base="xsd:NMTOKEN">
             <xsd:enumeration value="BridgedEthernet"/>
             <xsd:enumeration value="PPPOE"/>
           </xsd:restriction>
         </xsd:simpleType>
       </xsd:attribute>
       <xsd:attribute name="BrEthernetIP" use="optional">
         <xsd:simpleType>
           <xsd:restriction base="xsd:string">
             <xsd:minLength value="7"/>
             <xsd:maxLength value="16"/>
           </xsd:restriction>
         </xsd:simpleType>
       </xsd:attribute>
     </xsd:complexType>

</nowiki></pre>


=== Conditional Type ===

Inventing a syntax: 


<pre><nowiki>
     <xsd:complexType name="BindType">
       <xsd:alt cond="./@type='BridgedEthernet'">
         <xsd:attribute name="type"         use="required" type="xsd:NMTOKEN" fixed="BridgedEthernet"/>
         <xsd:attribute name="BrEthernetIP" use="optional" type="SevenSixteen"/>
       </xsd:alt>
       <xsd:alt>
         <xsd:attribute name="type"         use="required" type="xsd:NMTOKEN" fixed="PPPOE"/>
       </xsd:alt>
     </xsd:complexType>

     <xsd:simpleType name="SevenSixteen">
       <xsd:restriction base="xsd:string">
          <xsd:minLength value="7"/>
          <xsd:maxLength value="16"/>
       </xsd:restriction>
     </xsd:simpleType>

</nowiki></pre>

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[[Category:CoConstraintUseCase]]