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   RE: [xml-dev] Subtyping in XML

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On Fri, 6 Sep 2002, Jeff Lowery wrote:

> > The specific argument is that XML Schema should not allow
> > derivation by
> > extension by default. And people should be very careful if
> > they choose to
> > do it explicitly.
> I can agree with that.  It  should at least be a configurable option in any
> XML Schema editor.


> That's a bit of a strawman, IMHO.  It may have been the intent of the WG to
> produce OO in XML, but regardless it seems natural to associate a collection
> of properties with a concept.

XML has neither the concept of properties nor the concept of concept. ;) I
do know a language that has these concepts though.

> C++.  So there may be a tight corrollary between methodless public structs
> in C++ (or methodless classes with public data members in Java) and XML.
> Both associate fields with types. What's a type? Well, that's another
> lengthy thread so I won't go there.

XML associates fields with types? Where do you see anything like that in
the XML specification. The closest thing to "field" in XML is attribute.
But we've been talking about element content.

> > Does
> > your relational
> > schema map from relations into objects?
> No, but a normalized relational schema defines domains, represented by
> tables.  Domains are the spittin' cousins of types.

Relational schemas are optimized for speed of lookup. Objects are
optimized for ease of programming. XML is optimized for ease of reading
and writing (as a serialization format!). Mapping between the worlds is
possible but it will usually involve a process of defining the mapping!

> But it seems that such an approach would immediately be abstracted away with
> some syntactic sugar, right?  You want to hand type an AF mapping instead of
> typing "base="? Evidently your keyboard doensn't have a hole worn in the
> middle of the backspace key like mine does.

An AF mapping does not involve that much more typing than base=. But it is
more flexible because you a third party can define the mapping from
subtypes to supertypes.

> > There are too many details to go into but the gist is that
> > archforms are
> > ackward but they have the right ideas, IMO.
> Ah, so your backspace key is worn through, then...

I didn't claim that AFs were perfect. They just point in the right
direction. It is the responsibility of an AF processor to present a view
of the data that is recognizable to the process (i.e. adheres strictly to
the schema the process has agreed to). XML Schema did not define anything
like that.

 Paul Prescod


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