Oliver,

I think the substitionGroup concept is what you require to treat the types
polymorphically. Here substitutable types must be derived from the same base
type (which may or may not be abstract). If you are validating via schema
the enclosing type can reference the substitution group rather than the
explicit types. Also, whilst I have never personally looked at it I
understand that XSLT template <<match>> and XPath <<instance of>> or <<treat
as>> operators respect substitution groups if you use a schema aware
processor.

I suppose the difference with OO languages such as Java is that you have to
declare substitution explicitly - in Java simply subclassing another class
(or implementing an interface) is enough.

Michael


On Tue, Oct 6, 2009 at 1:37 AM, Jim Tivy <jimt@b...> wrote:

> Hi Oliver
>
> Not sure I fully follow - could you provide the XML Schema types and
> syntax?
> I am not sure if a and aa are types or elements - seems like you may mean
> they are both?
>
> Jim
>
>
> -----Original Message-----
> From: Olivier Rossel [mailto:olivier.rossel@g...]
> Sent: Monday, October 05, 2009 2:04 AM
> To: xml-dev@l...
> Subject: [xml-dev] Schemas and the open world assumption.
>
> Hello everyone.
>
> Sorry to be 5 years late, but it is just today that I question myself
> about derivation by extension and closed world assumption.
>
> Please do not hesitate to comment the following points:
>
> If I define "a" as being a sequence of b,c:
> <a>
>  <b/>
>  <c/>
> </a>
>
> and i extend "a" into "aa" that extends that sequence with d:
> <aa>
>  <b/>
>  <c/>
>  <d/>
> </aa>
>
> then any "aa" will not validate against the definition of "a".
> right?
> this sounds like a MAJOR difference with OO paradigm (where any "aa" is
> also a "a").
>
> that is what i call the closed world assumption in xml validation.
>
> considering i need a more open world approach, i plan
> to relax my schema by defining "a" in this way:
> <a>
>  <b/>
>  <c/>
>  <xsd:any>
> </a>
>
> then i feel like i could extend my "a" definition  without breaking
> the "subclass" philosophy.
>
> can anyone comment that point of view?
> i am especially interested in possible pitfalls i could have missed in
> using the "any" statement.
> i am also interested in best practices when defining modular expandable
> models.
>
> any help is very welcome.
>
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