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   Re: What are schemata

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  • From: Paul Prescod <papresco@technologist.com>
  • To: "'xml-dev@ic.ac.uk'" <xml-dev@ic.ac.uk>
  • Date: Mon, 01 Jun 1998 07:57:13 -0400

Jarle Stabell wrote:
> So by 'schema', you mean basically what is also known as 'type'? 

Here are a few of my concerns with the word type:

 * I think that it implies a set of legal operations, (e.g. the boolean
type has logical operations, the integer type has integral operations,
etc.) But in generic markup, we want the set of operations to be
open-ended. In type systems, operations are "open-ended" in the sense that
you can build up complex ones (e.g. power) from basic ones (addition), but
in generic markup, you work directly on the data.

 * Each XSchema document will probably contain many types -- one per
element, perhaps some data types and attribute types also.

 * As with DTDs, you will probably be able to use any element type in an
XSchema as the "top", so any particular XSchema will not itself define a
particular document type, but one per possible "root element" -- again
several conceptual types.

 * Your sentence above conflates levels. Even if we use the word type, the
schema is not a type. The schema *defines* a type.

But at some level, yes, schemas define types. They define subtypes of the
type "XML document". Some subset of all XML documents conform to a
particular schema. I just don't think that the word is intuitively applied
here. The types *in* the schema (element type, attribute types, document
types) will get confused with the type the schema defines (a subtype of
XML document).

 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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