Hi Keith,

If you validate that in oXygen it will also quote you in the error 
report the following link from the spec:
http://www.w3.org/TR/xmlschema-1/#cos-element-consistent

One thing that may get you closer to what you want is to use xsi:type on 
the first parameter to specify a restricted type, but that needs to be 
placed in the instance, like below

<params xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
     <param xsi:type="p1">a</param>
     <param>x</param>
</params>

with a schema like

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
     <xs:element name="params">
         <xs:complexType>
             <xs:sequence>
                 <xs:element minOccurs="2" maxOccurs="2" name="param" 
type="xs:string"/>
             </xs:sequence>
         </xs:complexType>
     </xs:element>
     <xs:simpleType name="p1">
         <xs:restriction base="xs:string">
             <xs:enumeration value="a"/>
             <xs:enumeration value="b"/>
             <xs:enumeration value="c"/>
         </xs:restriction>
     </xs:simpleType>
</xs:schema>

One possibility is to embed Schematron in your schema, below you can 
find a working sample:

<xs:element name="params">
     <xs:annotation>
         <xs:appinfo>
             <sch:pattern name="Test the first parameter"
                 xmlns:sch="http://purl.oclc.org/dsdl/schematron">
                 <sch:rule context="params">
                     <sch:assert test="param[1]='a' or param[1]='b' or 
param[1]='c'">
                         The first param must be a, b or c
                     </sch:assert>
                 </sch:rule>
             </sch:pattern>
         </xs:appinfo>
     </xs:annotation>
     <xs:complexType>
         <xs:sequence>
             <xs:element minOccurs="2" maxOccurs="2" name="param" 
type="xs:string"/>
         </xs:sequence>
     </xs:complexType>
</xs:element>

Best Regards,
George
--
George Cristian Bina
<oXygen/> XML Editor, Schema Editor and XSLT Editor/Debugger
http://www.oxygenxml.com


Keith Hassen wrote:
> Hi,
> 
> I have what I suspect is a simple question about element consistency in 
> a schema.  I have recently run across a few scenarios that require me to 
> define something like the following:
> 
>   <xs:element name="params">
>     <xs:complexType>
>       <xs:sequence>
>         <xs:element minOccurs="1" maxOccurs="1" name="param">
>           <xs:simpleType>
>             <xs:restriction base="xs:string">
>               <xs:enumeration value="a"/>
>               <xs:enumeration value="b"/>
>               <xs:enumeration value="c"/>
>             </xs:restriction>
>           </xs:simpleType>
>         </xs:element>
>         <xs:element minOccurs="1" maxOccurs="1" name="param" 
> type="xs:string"/>
>       </xs:sequence>
>     </xs:complexType>
>   </xs:element>
> 
> The definition above isn't ambiguous (which would break the unique 
> particle attribution rule), but it is not *consistent* vis-a-vis the 
> types.  The Xerces parser reports a "cos-element-consistent" error, 
> which I understand, but which I am hoping to circumvent while 
> maintaining strict parsing rules.
> 
> Is there a way to define two unambiguous elements with the same name but 
> are of different types?  I believe the answer here is "it can't be 
> done", but thought I'd throw this at the wolves to see if I'm right. :)
> 
> (Incidentally, the above example is a snippet from a schema that is 
> attempting a representation of an XML-RPC message set, where the first 
> parameter is always a constrained list of possible values, while the 
> second parameter is any string).
> 
> -K
> 
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