View source for Needs choice inside all

= Needs choice inside all (a co-constraint use case) =
source: Kevin Burges
[http://lists.w3.org/Archives/Public/xmlschema-dev/2001Sep/0148.html]

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

== Description ==

    What I would like is:


<pre><nowiki>
                      __ name (minOccurs=0)
                     /
                    /                         __ field (minOccurs=1)
    column --[ALL]-----[CHOICE minOccurs=1]--|__ formula (minOccurs=1)
                    \
                     \__ width (minOccurs=0)
    
</nowiki></pre>


    But it appears I cannot have a `choice` section inside the `all`.

== Analysis ==
=== FV ===
This is is an UNCONDITIONED RELAXATION on an <all> structure in an ELEMENT

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

=== MSM ===
While it's clear that the original poster's idea could be expressed in
an SGML all group, it's not clear to me that the sample solutions below for
[[SchemaPath]] and conditional types are correct in saying that this could
be handled by the recently agreed extensions to ''all'':  we agreed
to allow ''maxOccurs'' to exceed 1, and to specify interleave semantics.
Did we agree to allow choices inside ''all''-groups?

Of course, an unextended ''all''-group could handle the substantive
requirement today, either using a wrapper element:
<pre><nowiki> <xsd:complexType name="column">
  <xsd:all>
   <xsd:element ref="name"    minOccurs="0"/>
   <xsd:element ref="payload" minOccurs="1"/>
   <xsd:element ref="width"   minOccurs="0"/>
  </xsd:all>
 </xsd:complexType>

 <xsd:element name="payload">
  <xsd:complexType>
   <xsd:choice>
    <xsd:element ref="field"/>
    <xsd:element ref="formula"/>
   </xsd:choice>
  </xsd:complexType>
 </xsd:element>
</nowiki></pre>


or using a substitution group:

<pre><nowiki> <xsd:complexType name="column">
  <xsd:all>
   <xsd:element ref="name"    minOccurs="0"/>
   <xsd:element ref="payload" minOccurs="1"/>
   <xsd:element ref="width"   minOccurs="0"/>
  </xsd:all>
 </xsd:complexType>
 <xsd:element name="payload" abstract="true"/>
 <xsd:element name="field" substitutionGroup="payload"> ... </xsd:element>
 <xsd:element name="formula" substitutionGroup="payload"> ... </xsd:element>

</nowiki></pre>


== Possible solutions ==

=== Relax NG ===
=== Schematron ===
=== Check clause ===
A check-clause solution might be:

<pre><nowiki>
 <xsd:complexType name="choice_inside_all">
  <xsd:annotation><xsd:appinfo><sch:pattern><sch:rule>
      <sch:assert test="count(my:name) &lt; 2
       and count(my:width) &lt; 2
       and count(my:field or my:formula) = 1">
       One of field or formula must occur, not more, not fewer.
      </sch:report>
     </sch:rule></sch:pattern></xsd:appinfo></xsd:annotation>
  <xsd:choice minOccurs="0" maxOccurs="3">
   <xsd:element name="my:name"/>
   <xsd:element name="my:field"/>
   <xsd:element name="my:formula"/>
   <xsd:element name="my:width"/>
  </xsd:choice>
 </xsd:complexType>
</nowiki></pre>

This illustrates one danger of a powerful predicate language in check clauses:
that constraints which can be expressed in a relatively simple way in the
content model will instead be expressed in a less direct way in a predicate.
(Of course, directness may be in the eye of the beholder.  It's quite possible
that the schema author might feel that the predicate approach allowed a simpler,
clearer, and more direct expression of what was really desired.)

=== [[SchemaPath]] ===
This appears to be solvable with current extension of <all>. But, if you want to have it using co-constraints, one of the possible solutions is: 


<pre><nowiki>

<xsd:element name="column">
  <xsd:alt cond="./field and ./formula"           type="xsd:error"/>
  <xsd:alt cond="not(./field) and not(./formula)" type="xsd:error"/>
  <xsd:alt>
     <xsd:complexType>
        <xsd:all>
           <xsd:element ref="name"    minOccurs="0"/>
           <xsd:element ref="field"   minOccurs="0"/>
           <xsd:element ref="formula" minOccurs="0"/>
           <xsd:element ref="width"   minOccurs="0"/>
        </xsd:all>
     </xsd:complexType>
  </xsd:alt>
</xsd:element>

</nowiki></pre>


=== Conditional Type ===
This appears to be solvable with current extension of <all>. But, if you want to have it using co-constraints, one of the possible solutions is: 


<pre><nowiki>

<xsd:element name="column" type="tColumn"/>

<xsd:complexType name="tColumn">
  <xsd:alt cond="./field or ./formula and not(./field and ./formula)">
     <xsd:all>
        <xsd:element ref="name"    minOccurs="0"/>
        <xsd:element ref="field"   minOccurs="0"/>
        <xsd:element ref="formula" minOccurs="0"/>
        <xsd:element ref="width"   minOccurs="0"/>
     </xsd:all>
  </xsd:alt>
</xsd:complexType>

</nowiki></pre>

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[[Category:CoConstraintUseCase]]