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RE: Circular types in XML Schemas

From: Yuri de Wit <yuri.dewit@metaserver.com>
Date: Wed, 1 May 2002 11:32:59 -0400 (EDT)
Message-ID: <D4DC61068C33D51186A700306E061976A42DFD@bianca.metaserver.com>
To: "'Jeni Tennison'" <jeni@jenitennison.com>
Cc: "'xmlschema-dev@w3.org'" <xmlschema-dev@w3.org>
Jeni, 

What does the XML Schema specification says about infinite recursive types,
i.e. the ones that do not have a minOccurs="0" as a loophole?

Like the following error schema:
 <complexType name="error">
        <sequence>
            <element name="name" type="xsd:string"/>
            <element name="code" type="xsd:string" />
            <element name="message" type="xsd:string" />
            <element name="description" type="xsd:string" minOccurs="0"/>
            <element name="source" type="xsd:string" minOccurs="0"/>
            <element name="content" type="xsd:base64Binary" minOccurs="0"/>
            <element name="error" type="ms:error"/>
        </sequence>
 </complexType>

Thanks,

Yuri

> -----Original Message-----
> From: Jeni Tennison [mailto:jeni@jenitennison.com]
> Sent: Wednesday, May 01, 2002 05:12
> To: Yuri de Wit
> Cc: 'xmlschema-dev@w3.org'
> Subject: Re: Circular types in XML Schemas
> 
> 
> Hi Yuri,
> 
> > The XML Schema specification constrains how groups may be used in
> > circular dependencies. My understanding is that the circularity
> > constraint applies only to model groups and nested items and not to
> > top level types or elements.
> 
> Circularity constraints apply to types in that a type cannot be
> derived from itself (clause 3 of
> http://www.w3.org/TR/xmlschema-1/#ct-props-correct), but you're right
> that it's fine for a complex type to contain an element declaration
> whose type is that complex type. That isn't circular.
> 
> Similarly, it isn't circular to have:
> 
> <xs:group name="error">
>   <xs:sequence>
>     ...
>     <xs:element name="error">
>       <xs:complexType>
>         <xs:group ref="ms:error" />
>       </xs:complexType>
>     </xs:element>
>   </xs:sequence>
> </xs:group>
> 
> The circularity constraint on model groups states that the particles
> of the group, at any depth, cannot include the group itself. In the
> case above, the particles of the ms:error group are a sequence,
> various other things, and an error element. The particles that are
> part of the content type of the error element aren't particles of the
> group -- as far as a validator is concerned, the element declaration
> is a black box.
> 
> > For instance, consider the following XML Schema. Is it a valid XML
> > Schema with a recursive type reference? I can see problems if
> > minOccurs=0 is not specified, but is it a valid schema based on the
> > XML Schema spec?
> >
> > Is it valid?
> >
> > <complexType name="error">
> >        <sequence>
> >            <element name="name" type="xsd:string"/>
> >            <element name="code" type="xsd:string" />
> >            <element name="message" type="xsd:string" />
> >            <element name="description" type="xsd:string" 
> minOccurs="0"/>
> >            <element name="source" type="xsd:string" minOccurs="0"/>
> >            <element name="content" type="xsd:base64Binary" 
> minOccurs="0"/>
> >            <element name="error" type="ms:error" minOccurs="0"/>
> >        </sequence>
> > </complexType>
> 
> Yes, that's fine.
> 
> Cheers,
> 
> Jeni
> 
> ---
> Jeni Tennison
> http://www.jenitennison.com/
> 
Received on Friday, 3 May 2002 05:18:23 UTC

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