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   Re: Names and schemas

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  • From: Paul Prescod <papresco@technologist.com>
  • To: xml-dev@ic.ac.uk
  • Date: Mon, 01 Jun 1998 12:15:54 -0400

Steven R. Newcomb wrote:
> If that's true, then that's GREAT!
> However, I keep stumbling over a problem in my own mind.  (Maybe it's
> just too obvious for me to see it.)  

No. It's an excellent question.

> Using your defined terms, please
> explain the usefulness and/or purpose -- in terms of how it furthers
> the cause of reliable, vendor-neutral information interchange -- of
> declaring that a real or conceptual object exists, in the absence of
> any interchangeable definition of what that object is, or what
> constraints it must conform to in order to be processable.

You are right: knowing that an object exists is not enough. To do
something useful with it we need more. In some cases we will need
verification and thus will want a schema or architecture. In some cases we
will need behaviour, such as that provided by a stylesheet or Java class.
In some cases we will need a semantic definition: an RDF or some other
knowledge-rep description.

In some (perhaps most!) cases we will want more than one of the above e.g.
two stylesheets, a semantic description, a couple of schemas and a Java
class. (note that I personally think that a schema is just another
behavioural specification, like a stylesheet or Java class...I separate
them out in this discussion for those who don't agree)

So namespaces are not enough, but what you pair them with will depend on
your needs. I suspect that in the business you and I are in, the schema
will be most important. But XML is predicated on the idea that sometimes
you want to do processing without a schema and I think that it is
demonstrably true that sometimes you want more than one to apply to the
same element, based on its type.

I also believe that the association between schemas, stylesheets and java
classes can occur in many places. It could be in the document, in its
"dictionary", in one of the schemas, in the java class and even in the
stylesheet. Those all seem like appropriate places to do the association
in certain situations. For example, a stylesheet can "protect itself" from
being used on inappropriate data by embedding a reference to a schema. A
schema could reference a stylesheet fragment in its per-element meta-data.

So my point is that the whole thing is complicated and highly variable. I
don't think that we should make a rule that namespaces must be defined by
schemas, and especially not in any specific schema language (even
archforms). In some circumstances, a Java .JAR file could be as valid a
namespace definition structure as a schema.

 Paul Prescod  - http://itrc.uwaterloo.ca/~papresco

Three things it is far better that only you should know:
How much you're paid, the schedule pad, and what is just for show

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