[dsdl-discuss] Re: Overlaying schema expressions

G. Ken Holman wrote:

> But, in my understanding, a lot of people think that this is insane.
>
> Then it is time for us to proselytize, and I have a real-world
> situation where I will try to bring this up.

OTOH, the idea that there may be more than one schema per underlies XML
Schemas type derivation
by restriction, wildcards and substitution groups. I don't believe
that is contentious. And I don't think
it is really so contentious that it is necessary sometimes to have
different orthogonal schemas, e.g.
anything + Schematron, to make up for shortcomings.

What is contentious is the idea of having parallel schemas in the same
schema language which are not
derived from one another: e.g. James' second schema for HTML to validate
exclusion exceptions for
<html:a>. This worries people because it seems to expose a short coming
of the expressiveness of
the schema language.

>> Many people think that schemas provide semantics to documents.
>
>
> I believe schemas are merely syntax checkers at the level of the
> presence of labeled branches in a hierarchical tree.
>
> I believe semantics are implemented solely by applications and how
> they wish to interpret the labelled hierarchy of information. The
> syntax of the semantics is typically prose. People have tried to
> convince me that the use of taxonomies can somehow express semantics
> formally, but I haven't grasped this concept yet.

I have a slightly different view: schemas do validate semantics, but
only where there is concretization of the
semantics into syntax. Where some value is used as a symbol, for example
an ID, its atomic semantics are entirely vested in
its syntax. (You can see RDF as a an attempt to make semantics as
'syntactic' as possible.) The trouble with saying
that schemas don't validate syntax is that we end up with a circular
definition, where we define "semantics" as
"anything that a schema cannot validate" which probably is not helpful,
because then "semantics" means only
"behaviours".

>> I strongly doubt the future of PSVI, XPath 2, XSLT 2, XQuery, etc.
>> In five years (when lots of hype around XML disappear), we will know
>> if which approach is correct.
>
> I am really curious now to read more about the basis of this faith in
> these technologies that some people have regarding semantics expressed
> in syntax ... I'm sure it will be good fodder for my justification of
> using multiple expressions of syntactic constraints for a single set
> of semantics.

Well, everything on a computer has to be expressed in syntax (or
something expressible in syntax). If you have this
absolute distinction between syntax and semantics, then "semantics"
reduces to "everything that cannot be expressed
on a computer" it seems to me. But things only have syntax because
they have semantics. So I don't really
understand the whole argument.

Cheers
Rick

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