xml-dev - Re: Schemata are not just constraints [was: "RDF + Topic Maps" = TheFutu

[Jonathan Borden <jborden@mediaone.net>]

Eric van der Vlist wrote:


> Jonathan,
>
> Jonathan Borden wrote:

> >
> > As DTDs and XML Schemata provide syntactic constraints, RDF Schemata
provide
> > semantic constraints (which for the purposes of this discussion is
defined
> > as placing constrainsts on arcs and nodes in a directed graph). So it is
> > true that RDF Schema provides no more than a constraint mechanism, but I
> > offer that this is precisely how ontologies are built.
>
> Even if sometimes I'd wish they are only this, I think that it's quite
> limitative to restrict schemata (either syntaxic or semantic) to a set
> of constraints.

    Please don't equate the term "provide constraints" to a stating that
schemata are *restricted* to a set of constraints.

    A schema has two broad functions:

1) contraints
2) transforms (e.g. attribute defaulting)

(if this isn't clear see Schematron!)

    Yet formally one might consider all constraints a type of transform, one
that returns either 'true' or 'false' depending on the input. In any case it
remains useful to consider schemata as providing constraints.

    Similarly one can formally define a transform given input and output
constraints. So the two are inextricably intertwined.

Jonathan Borden
The Open Healthcare Group
http://www.openhealth.org