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Re: Really Understood W3C Schema Complex Types?

• From: Kohsuke KAWAGUCHI <kohsukekawaguchi@y...>
• To: Jeff Rafter <jeffrafter@d...>, xml-dev@l...
• Date: Tue, 04 Sep 2001 12:06:29 -0700

> > First question. How many of the schema authors know this side effect when
> > s/he innocently writes this type hierarchy?  I personally saw these kind
> > of mistakes made by relatively experienced schema authors.
> 
> Good question, probably not many-- it is outlined in the best practices doc
> and the faq I believe (along with it being demonstrated in the primer)-- but
> it is still not widely acknowledged.

When reading the primer, one understands that the complex type is
substitutable but I guess it's easy to forget it once s/he starts writing
a schema.


> That is why it is probably good practice to utilize the final attribute on
> your complexTypes.

Yes, or may utilize the block attribute (or finalDefault/blockDefault
attributes)



> How is this an example of fragility? If anything it is an example that you
> cannot do *everything* with XML Schemas... rightly so. Your first example

I don't want to say that "hey, you can't do this in XML Schema". If I
sound like so, it's my bad. I'm just trying to defend my claim that
learning W3C XML Schema is not easy.

In this particular "type fragility" topic, I wanted to demonstrate that
sometimes you can't write restriction even if it's a true restriction,
and once it happens, the result is bizarre (because the type hierarchy
will break.)


But apparently there was a problem in my example. "repeating.derived"
should be derived from "repeating" by restriction (not from "annotation".)

<xs:complexType name="repeating">
  <xs:complexContent>
    <xs:extension base="annotated">
      <xs:group ref="list.class" minOccurs="0" maxOccurs="unbounded"/>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

<xs:complexType name="repeating.derived">
  <xs:complexContent>
    <xs:restriction base="repeating">
      ....
      <xs:element ref="A"/>
      <xs:group ref="list.class" minOccurs="0" maxOccurs="unbounded"/>
    </xs:restriction>
  </xs:complexContent>
</xs:complexType>


> As far as I can tell it doesn't seem that "repeating.derived" is in any
> sense of the word a 'restriction' of "repeating" or "annotated". It should

I think it is fair to claim that "repeating.derived" is conceptually a
restriction of "repeating" because if something is valid wrt
"repeating.derived", it is always valid wrt the "repeating" type.

Or am I missing something?



> > (By the way, one of the horrible fact is that the author used a schema
> > editor to produce this schema and still he couldn't detect this error.)
> 
> That is a problem-- but new versions are being created all of the time...
> but this shouldn't be counted against the concept of complexTypes, right?

I thought it's an incident that reveals the difficulty of correctly
enforcing the restriction-ok constraint.


regards,
----------------------
K.Kawaguchi
E-Mail: kohsukekawaguchi@y...

• References:
  • Re: Really Understood W3C Schema Complex Types?
    • From: Jeff Rafter <jeffrafter@d...>

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