Re: [xml-dev] RNG vs. XSD : is the use of abstract types andpolymorphis

[John Cowan <cowan@mercury.ccil.org>]

Costello, Roger L. scripsit:

> If the reason is (1) then I would like to know why abstract types and
> polymorphism are not appropriate for a schema language for XML? Are
> there dangers in designing schemas for XML that use abstract types
> and polymorphism? Does James Clark and MURATA Makoto recommend, when
> using XML Schemas, avoiding its abstract types and polymorphism?

See http://www.thaiopensource.com/relaxng/design.html#section:15 for
James's thinking on inheritance in schema languages.

I would add that XML Schema expresses three kinds of constraints:
schema-instance constraints (the schema limits what the instance can
contain), schema-schema constraints (parts of the schema limits what
other parts of the schema can contain), and instance-instance constraints
(parts of the instance limit what other parts of the instance can contain,
as specified by the schema).  Type extension and restriction belong to
the second class of constraints, but RELAX NG expresses only the first
class (except for DTD-compatible ID and IDREF(S), which are a trivial
example of the third class).

-- 
John Cowan  http://ccil.org/~cowan    cowan@ccil.org
There are books that are at once excellent and boring.  Those that at
once leap to the mind are Thoreau's Walden, Emerson's Essays, George
Eliot's Adam Bede, and Landor's Dialogues.  --Somerset Maugham