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Re: extension and restriction

From: Jeni Tennison <jeni@jenitennison.com>
Date: Tue, 4 Dec 2001 20:22:45 +0000
Message-ID: <3321998942.20011204202245@jenitennison.com>
To: Niko Suave <niko@alum.mit.edu>
CC: xmlschema-dev@w3.org
Hi Niko,

> I know that an extended type is always the same as the base + some
> extra; so, how does that relate to attributes? Does this mean that
> an extended type may not redefine an existing attribute of the base?

Yes, that's right.

> My gut says that extension2 is clearly invalid (clearly, it could be
> implemented as an extension followed by a restriction). My
> understanding of the rules (see below) would suggest that the
> use="prohibited" clause actually removes the corresponding attribute
> use from the list of the class, and therefore the attribute use list
> for 'base' is not a subset of the attribute use list for extension2,
> and so the extension is not valid.

Yes, that's right.

> If this is correct, then I would like to know how that relates to a
> wildcard: what if you have a prohibited attribute that would
> otherwise be allowed by a wildcard? Since it seems that Attribute
> Uses do not include the option of something being explicitely
> prohibited, that attribute would presumably be okay.

I don't understand exactly what you mean here. Are you talking about
something like:

<xs:complexType name="base">
  <xs:attribute name="name" type="xs:string"/>
  <xs:attribute name="dead" type="xs:boolean"/>
  <xs:anyAttribute namespace="##other" />
</xs:complexType>

<xs:complexType name="extension">
  <xs:complexContent>
    <xs:extension base="base">
      <xs:attribute name="xlink:href" type="xs:anyURI"
                    use="prohibited" />
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

That isn't OK because you're trying to restrict what attributes are
allowed on elements of the extension type. If you're talking about:

<xs:complexType name="base">
  <xs:attribute name="name" type="xs:string"/>
  <xs:attribute name="dead" type="xs:boolean"/>
  <xs:attribute name="xlink:href" type="xs:anyURI" use="prohibited" />
</xs:complexType>

<xs:complexType name="extension">
  <xs:complexContent>
    <xs:extension base="base">
      <xs:anyAttribute namespace="##other" />
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

Then I think that *is* OK because you're extending the permitted
attributes.

> I'm not sure about extension3, but the rules would seem to suggest
> that it is okay since every attribute use in the base corresponds to
> an attribute use in extension3 with the same name, target namesapce,
> and type def but a different {Required} property. Is this correct?

I don't think so. The fact that you *can* make an attribute required
when you derive by restriction (see "Schema Component Constraint:
Derivation Valid (Restriction, Complex)") is a good indication that
you *can't* make an attribute required when you derive by extension :)
But I agree that it's not all that clear in the Rec that this isn't
allowed.

Cheers,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/
Received on Tuesday, 4 December 2001 15:34:33 GMT

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