What does an empty element denote?

["Costello, Roger L." <costello@mitre.org>]

Hi Folks,

 

This XML Schema declares an element with a content model that is a simple type:

 

    <xs:element name="Test1">
        <xs:simpleType>
            <xs:restriction base="xs:int">
                <xs:minInclusive value="0" />
                <xs:maxInclusive value="9" />
            </xs:restriction>
        </xs:simpleType>
    </xs:element>

 

In an XML instance document, the value of <Test1> can be 0, 1, …, 9.

 

The content model of Test1 denotes this set: {0, 1, …, 9}

 

Now consider this XML Schema, it declares an element with a content model that is a complex type with an empty sequence:

 

    <xs:element name="Test2">
        <xs:complexType>
            <xs:sequence/>
        </xs:complexType>
    </xs:element>

 

Lastly, consider this XML Schema, it declares an element with a content model that is a complex type that is an empty choice:
    
    <xs:element name="Test3">
        <xs:complexType>
            <xs:choice/>
        </xs:complexType>
    </xs:element>

 

Michael Sperberg-McQueen wrote an article [1] which says that a content model which is an empty sequence denotes a set with an empty string, { å }. So the value of <Test2> must be a string of length zero. Here’s how a string of length zero is physically expressed in an XML instance document:

    <Test2></Test2>

 

MSM says that a content model that is an empty choice denotes an empty set, {}.  That puzzled me, so I created an XML instance document containing <Test3>:

    <Test3></Test3>

 

and validated it against the schema. It validated!

 

Okay, I’m confused. In an XML instance document, does an empty element denote that the element’s value is å (the empty string, i.e., a string of length zero), or does an empty element denote that the element has no value, i.e., the set of allowable values is the empty set?

 

Can you give a concrete example that illustrates how Test2’s content is different than Test3’s content?

 

/Roger

 

[1] http://conferences.idealliance.org/extreme/html/2005/SperbergMcQueen01/EML2005SperbergMcQueen01.html#id2624489