At 2009-02-02 19:18 +0000, Michael Kay wrote:

>Just to reinforce this, 4 validators (Saxon-SA, Xerces, XSV, and Ms .NET)
>agree that the instance document
>
><doc/>
>
>is valid against the schema
>
><xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
>         <xsd:element name="doc" type="foo"/>
>         <xsd:complexType name="foo">
>           <xsd:sequence>
>                 <xsd:element name="e1" minOccurs="0" maxOccurs="0"/>
>           </xsd:sequence>
>         </xsd:complexType>
></xsd:schema>
>
>but invalid against
>
><xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
>         <xsd:element name="doc" type="foo"/>
>         <xsd:complexType name="foo">
>           <xsd:choice>
>                 <xsd:element name="e1" minOccurs="0" maxOccurs="0"/>
>                 <xsd:element name="e2"/>
>           </xsd:choice>
>         </xsd:complexType>
></xsd:schema>
>
>I'm having great difficulty seeing how it can be valid against a particle P
>but invalid against a choice of which P is one of the options.

But if you took the perspective that the particle doesn't even exist 
because of the minOccurs="0" and maxOccurs="0", then the above two 
schemas would be equivalent to the schemas written as:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
         <xsd:element name="doc" type="foo"/>
         <xsd:complexType name="foo">
           <xsd:sequence>
           </xsd:sequence>
         </xsd:complexType>
</xsd:schema>

.... and ....

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
         <xsd:element name="doc" type="foo"/>
         <xsd:complexType name="foo">
           <xsd:choice>
                 <xsd:element name="e2"/>
           </xsd:choice>
         </xsd:complexType>
</xsd:schema>

Which would, in fact, be valid against the first schema but not the second.

>But that seems to be the weight of opinion...

I think it is explicit.

http://www.w3.org/TR/xmlschema-1/#declare-element states:

   "the corresponding schema components are as follows (unless
    minOccurs=maxOccurs=0, in which case the item corresponds
    to no component at all):"

Which tells me that my rewriting of the schemas is equivalent to the 
original ... there is no schema component for a declaration with 
minOccurs=maxOccurs=0, which means that when <doc/> is being tested 
with the second schema, there is no schema component e1 with 
minOccurs="0" for it to match against.

Of course there would be if maxOccurs="1" or any other value and 
minOccurs="0", in which case it would have matched.

I hope this helps.

. . . . . . . . . . . Ken


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