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   Re: XLink a special case in the self-describing Web?

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  • From: Eve.Maler@East.Sun.COM (Eve Maler - Sun Microsystems)
  • To: "Steven R. Newcomb" <srn@techno.com>, <xml-dev@xml.org>
  • Date: Fri, 19 May 2000 22:23:46 +0-200

>Please correct me if I'm wrong about any of this.
>(1) XML Namespaces do not provide a way for a single element to
>    conform to an element type in each of several schemas.  Therefore,
>    there is no way for a single element to be recognized as
>    conforming to both the X:Foo and the Y:Bar element types.

XML namespaces don't necessarily provide a way for "conforming"
to any schema.  But your final sentence is correct.

>(2) XLink is now just attributes; the element type can be anything.
>    This permits a single element to be recognized as an XLink and as
>    whatever else it may be.  (Whatever else it may be, it may still
>    only be one element type in one single namespace, as far as I
>    know.)  This is a kind of sleight-of-hand: XLink elements are
>    still XLink elements; we still expect certain combinations of
>    attributes to appear in certain contexts and not in others.

XLink the namespace is just a set of attributes; this is just as much
a legitimate namespace as any other.  The "sleight of hand" comes
in the form of the XLink-as-application semantics, which is really
separate from XLink-the-namespace.

>So, if my above understandings are correct, I tentatively conclude
>from this that XLink is not a namespace or a schema in the usual
>sense, because, among all of the kinds of element definitions that are
>possible, only the XLink element types are, de facto, exempt from the
>"one element, one element type name in one semantic space of element
>type names" rule.

What about the "xml" namespace itself?  It has no element types in it.
I don't think this holds up.

>Can anybody create sets of attributes, just as has been done with
>XLink, that will constitute a semantic space, and thus effectively
>have elements identify themselves as conforming to certain element
>types without requiring that the generic identifier be used to
>identify the element type?  If anybody can already do this, is this a
>methodology to which XML Schemas can provide validation services, by
>checking to see whether all of the attributes have been used in
>syntactically valid ways?  If so, how?

Hmm, I think I just gave an example!  There is some difficulty in 
using XML Schema in an extremely "tight" way to validate the use of
XML "architectural forms," but this doesn't necessarily preclude the 
creation of such sets of forms and specification of their constraints 
by other means.


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