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Bag Element

From: Orion Adrian <oadrian@hotmail.com>
Date: Tue, 23 Mar 2004 18:03:22 -0500
To: www-xml-schema-comments@w3.org
Message-ID: <BAY1-F7621EfXx2jvls00008ef2@hotmail.com>

I have to agree with the previous posters comments on bag elements.  While 
this construct is possible to accomplish with existing constructs, they are 
essentially unusable for such a common construct.

The following accomplishes what a bag element would accomplish for the head 
element in XHTML 1.0, though I find this very unwieldy.

  <xs:group name="head.misc">
    <xs:sequence>
      <xs:choice minOccurs="0" maxOccurs="unbounded">
        <xs:element ref="script"/>
        <xs:element ref="style"/>
        <xs:element ref="meta"/>
        <xs:element ref="link"/>
        <xs:element ref="object"/>
      </xs:choice>
    </xs:sequence>
  </xs:group>

  <xs:element name="head">
    <xs:annotation>
      <xs:documentation>
      content model is "head.misc" combined with a single
      title and an optional base element in any order
      </xs:documentation>
    </xs:annotation>
    <xs:complexType>
      <xs:sequence>
        <xs:group ref="head.misc"/>
        <xs:choice>
          <xs:sequence>
            <xs:element ref="title"/>
            <xs:group ref="head.misc"/>
            <xs:sequence minOccurs="0">
              <xs:element ref="base"/>
              <xs:group ref="head.misc"/>
            </xs:sequence>
          </xs:sequence>
          <xs:sequence>
            <xs:element ref="base"/>
            <xs:group ref="head.misc"/>
            <xs:element ref="title"/>
            <xs:group ref="head.misc"/>
          </xs:sequence>
        </xs:choice>
      </xs:sequence>
      <xs:attributeGroup ref="i18n"/>
      <xs:attribute name="id" type="xs:ID"/>
      <xs:attribute name="profile" type="URI"/>
    </xs:complexType>
  </xs:element>

Something better would be:
<xs:element name="head">
    <xs:annotation>
        <xs:documentation>
        content model is "head.misc" combined with a single
        title and an optional base element in any order
        </xs:documentation>
    </xs:annotation>
    <xs:complexType>
        <xs:bag>
            <xs:element ref="title"/>
            <xs:element ref="base" minOccurs="0"/>
            <xs:element ref="script" minOccurs="0" maxOccurs="unbouned"/>
            <xs:element ref="style" minOccurs="0" maxOccurs="unbouned"/>
            <xs:element ref="meta" minOccurs="0" maxOccurs="unbouned"/>
            <xs:element ref="link" minOccurs="0" maxOccurs="unbouned"/>
            <xs:element ref="object" minOccurs="0" maxOccurs="unbouned"/>
        </xs:bag>
        <xs:attributeGroup ref="i18n"/>
        <xs:attribute name="id" type="xs:ID"/>
        <xs:attribute name="profile" type="URI"/>
    </xs:complexType>
</xs:element>

A couple of positive points of the bag construct (though I might rename it).
1) Increased readability and ease of authoring.
2) No longer requires tricks and can be contained in a single xs:element 
element.

Increased readability and usability should be enough to introduce this 
construct, but even if it isn't then two is a nice addition.

Orion Adrian

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Received on Tuesday, 23 March 2004 18:04:49 UTC

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