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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: Tue, 02 Jun 1998 16:11:25 -0400

Robin Cover wrote:
> I'm made to feel more in doubt than ever about the level of abstraction
> that's envisioned for "schema" (and XSchema) in this discussion.  It's
> not clear to me that the principal goal of a new schema language would
> be to define classes of markup, with (syntactic?) constraints, though
> certainly that could be one role.

I don't think that the role of schemas would be merely to define syntactic
constraints. Usually you would use a grammar or regular expression to do

My definition of schema is: "A document that defines a class of data
objects in some data model." So a "database schema" defines a class of
"databases", usually in the "relational" data model. XSchema should define
a class of "elements" (and "documents") in the (still implicit) "XML data

Angle brackets are not part of the (implicit) XML data model -- they are
part of its syntax. So a schema language would not override or change
those characters. The characters between tags (content) are part of the
data model. It might make sense for a schema language to express
contraints on those characters.
> From what realm(s) of discourse are these definitions being drawn?  

I don't know where that definition comes from. It is my application of the
intuitive notion of a schema and my experience (but not expertise) with
database systems.

> That said (and perhaps too much), I welcome clarifications from
> Paul and others on what is meant by "schema" and what authorities
> (domains) are being used for the definition. and (especially)
> the extent to which it's agreed that a schema should (not) be
> constrained by the notions of particular markup models.

I'm not sure what *exactly* you mean by markup models. The schema works on
a data model. If you can translate some random document conforming to a
random markup language into something that fits that model, then the
schema applies. Otherwise it does not. Simple "low-level" schemas will
usually express bias towards one markup language or the other. One could
imagine a markup language with no concept of attribute, or with one that
could have deep structures. XSchema would probably not apply to the
latter, and might not work "as well" with the former. On the other hand, a
higher-level schema might not care about concepts like elements and
attributes at all. For instance RDF's data model is radically different
from XML's (and thus XSchemas).

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

Three things never anger: First, the one who runs your deck
The one who does the backup, and the one who signs your check 

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