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types and instances in DTDs and XML Schemas
- From: Alexander Nakhimovsky <firstname.lastname@example.org>
- To: Xml-Dev <email@example.com>
- Date: Sat, 25 Aug 2001 12:33:42 -0400
In XML 1.0, the element's content model is its type. Instances of the type
are found in instance documents. If one type contains another type, the
containment relationship is visible in the instance document.
XML Schema "types" are used within the schema to define element content
models. They are twice removed from instance documents. They are, in
effect, tree-structured parameter entities, with relations such as
"extention-of" or "restriction-of" defined on them. These relations are
completely invisible in instance documents. Any computer program that
relies on those relations either must have access to the XML schema
document or work with (as yet unspecified) PSVI, which has to be produced
by the validating parser. There may well be useful programs that rely on
Schema type relations, but it seems unreasonable to burden every validating
parser with the task of understanding those relations and translating them
into a "heavily-decorated DOM" data structure.
So, it would be a good thing, IMO, if XML Schema were *re-factored* into
the validating part (such as RELAX NG) and a "complex-type-relations" part,
for use in specialized applications.