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   Re: What are schemata

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  • From: Paul Prescod <papresco@technologist.com>
  • To: xml-dev@ic.ac.uk
  • Date: Mon, 01 Jun 1998 07:57:24 -0400

Sean Mc grath wrote:
> To my mind, a schema is an assertion about the structure of a unit
> of data. 

I disagree a little. The schema itself is not the assertion. The HTML DTD
does not assert anything. Some other declaration *points to* (or in the
DOCTYPE case, includes) the schema. That declaration is the assertion.

> Parsing w.r.t. to a schema both tests the veracity of the
> assertion and generates a "view" of the data as an abstract
> data strcture.

I agree 100% with the first statement. I'm not sure about the second. It
is certainly *convenient* for a schema to in some way massage the input to
generate a new "view" of the data, (e.g. DTD's can add extra defaulted
attributes, and strip away ignorable whitespace). 

But should view-generation actually be the responsibility of the schema? I
don't know. In the database world, a view is something explicit and
distinct from a schema. The view has its own schema, in fact.

Since for XSchema we are experimentally stripping the schema concept down
to its minimal core, I think that we should remove the view-creating
responsibility from schemata, at least temporarily. Practically speaking,
that would mean that XSchema processors would not be responsible for
annotating or changing the parse tree in any way. They would only be
responsible for verifying conformance or not.

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

Three things see no end: A loop with exit code done wrong
A semaphore untested, and the change that comes along

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