[dsdl-discuss] Re: Overlaying schema expressions

G. Ken Holman wrote:

> If an organization publishes a normative W3C Schema expression for a
> document model, and I want to create a restricted subset whose
> instances validate against the original document model, I feel I can
> keep the namespace of the original vocabulary and just write a
> secondary set of constraints that would be applied to a given document
> before or after, but always in conjunction with, the normative schema
> expression.
>
> Does this sit well with people, or is the act of creating a new
> document model (the subset) obligating me to come up with a new
> namespace?

That namespace don't force a single schema comes from
 a) history, that schemas evolve and change, such as the HTML DTDs
 b) the intent of namespaces as being for modularity, where namespaces
(on which modern schema languages are
based) were developed to allow mix and match
c) the explicit statements in other W3C specs, notably in XML schemas
where the schema used to validate
is located by hints by applications, and is not retrieved from the
namespace URI as such. So it is perfectly
fine to make a schema for, say, XHTML, that allows ANY everywhere: that
is a schema, just not
"the" official schema.
d) XML Schemas <appinfo> element was provided to allow the expression of
additional constraints,
such as embedded Schematron constraints. Even XML Spy supports embedded
Schematron constraints
(their technical director wrote an articly in XML Journal about it).
Those constraints restrict in some
way the XML Schema constraints.

As for subsetting, W3C XML Schemas provides derivation by restriction,
where the same namespace
is used by a particular kind of subset is used (where every document
that is valid against the
restricted schema is also valid against the full schema).

I think the particular cases that are problematic are
 1) you have a restricted schema that is not valid against the standard
public base schema,
and you make this publically available as a standard
 2) you use someone else's namespace for your own names

I don't think either of these are acceptable, but for reasons of fair
play. Presumably an architectural
forms system could be used to say "my ZZZZ element is their ZZZZ element".

But I don't see anything that would, for example, prevent you from
putting out your own schema
for XHTML using any schema language you like. If the schema has been
patented (grrr) such
as MS is doing, that might be different though.

Cheers
Rick

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