Re: Allowable schema representations

[Eddie Robertsson]

W3C XML Schema language permits different content models for elements 
with the same name, as long as the validator can unambiguously 
determine which content model applies. Your examples are legal.

Not really. See comment below.
If I define an XML schema as follows:
<?xml version="1.0" encoding="UTF-8"?>
<!-- strange.xsd -->
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema";
                        targetNamespace="http://localhost/strange";
                        xmlns="http://localhost/strange";
elementFormDefault="qualified">
<xsd:complexType name="C1">
<xsd:sequence>
      <xsd:element name="A" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="C2">
      <xsd:sequence>
      <xsd:element name="A" type="C1"/>
<xsd:element name="A" type="xsd:string"/>
This is not legal in W3C XML Schema. In order to have two elements with 
the same name in the same content model they also _must_ have the same 
type. This constraint is violated in the above type.
The spec reads [1]:

*"Schema Component Constraint: Element Declarations Consistent*
If the {particles} <http://www.w3.org/TR/xmlschema-1/#particles> 
contains, either directly, indirectly (that is, within the {particles} 
<http://www.w3.org/TR/xmlschema-1/#particles> of a contained model 
group, recursively) or �implicitly� 
<http://www.w3.org/TR/xmlschema-1/#key-impl-cont> two or more element 
declaration particles with the same {name} 
<http://www.w3.org/TR/xmlschema-1/#e-name> and {target namespace} 
<http://www.w3.org/TR/xmlschema-1/#e-target_namespace>, then all their 
type definitions must be the same top-level definition, that is, *all* 
of the following must be true:
1 all their {type definition} 
<http://www.w3.org/TR/xmlschema-1/#type_definition>s must have a 
non-�absent� <http://www.w3.org/TR/xmlschema-1/#key-null> name.
2 all their {type definition} 
<http://www.w3.org/TR/xmlschema-1/#type_definition>s must have the same 
name.
3 all their {type definition} 
<http://www.w3.org/TR/xmlschema-1/#type_definition>s must have the same 
target namespace."

Cheers,
/Eddie
[1] http://www.w3.org/TR/xmlschema-1/#coss-modelGroup
</xsd:sequence>
</xsd:complexType>
<xsd:element name="A" type="C2"/>
</xsd:schema>
then an instance document looks like
<?xml version="1.0" encoding="UTF-8"?>
<!-- strange.xml -->
<A xmlns="http://localhost/strange";
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:schemaLocation="http://localhost/strange strange.xsd">
<A>
<A>foo</A>
</A>
<A>bar</A>
</A>
Is there anything inherently wrong with having an "A" element used in
many different ways? Xerces will certainly validate the document OK, and
I couldn't find anything in the xml-schema spec that invalidates this.
It looks strange to me, as I don't think you could have defined this
structure using a DTD. Does anyone have a view on this?
cheers
Loz
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