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

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  • From: John Cowan <jcowan@reutershealth.com>
  • To: "Steven R. Newcomb" <srn@techno.com>, "xml-dev@xml.org" <xml-dev@xml.org>
  • Date: Fri, 19 May 2000 14:07:42 -0400

"Steven R. Newcomb" wrote:

> (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.

That is correct as stated, but it reflects a misconception.  Namespaces
provide nothing but a superficial mapping of GIs and attribute names into
so-called "universal names", which are simply an ordered pair of the
form {string, Name} where "string" has the syntax of a URI.  The SGML/XML
semantic model is unchanged.

Suppose that prefix X: is mapped to the string "http://www.example/ns/X"
Then X:Foo just means the name {"http://www.example/ns/X", "Foo"},
This underlying name cannot be surfaced directly as an XML GI or attribute
name for purely syntactic reasons: Namespaces provides an escape hatch.

There is nothing semantic about XML Namespaces; they are just a convention
like beginning all *your* names with "SRN." would be, but with wider

Note that by definition X:Foo and Y:Bar cannot represent the same universal
name, but X:Foo and Y:Foo can and do if the prefixes X: and Y: are bound
to the same string.

> 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.

The namespace for XLink at present is "http://www.w3.org/1999/xlink".
The current XLink draft gives meaning to the universal attribute names
{"http://www.w3.org/1999/xlink", "type"}, {"http://www.w3.org/1999/xlink",
"href"}, and others, but does not give meaning to any universal GIs.
You are free to use a name such as {"http://www.w3.org/1999/xlink", "simple-link"}
as a GI if you want, but XLink does not say what it means.

> 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?

Yes, indeed.  If one is sufficiently canny, one can do so in conformance
to the rules of SGML architectures as well, as I believe XLink does.

> 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?

I think so, but I don't know exactly how.

Schlingt dreifach einen Kreis um dies! || John Cowan <jcowan@reutershealth.com>
Schliesst euer Aug vor heiliger Schau,  || http://www.reutershealth.com
Denn er genoss vom Honig-Tau,           || http://www.ccil.org/~cowan
Und trank die Milch vom Paradies.            -- Coleridge (tr. Politzer)

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