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Re: Defining data in reverse chronological order

From: <noah_mendelsohn@us.ibm.com>
Date: Fri, 13 Jun 2003 16:44:10 -0400
To: "C Baines" <cbaines@westroadchurch.org.uk>
Cc: xmlschema-dev@w3.org
Message-ID: <OF988F11B0.B4983648-ON85256D44.0066FF5A@lotus.com>

Charles Baines asks:

> Which returns to the question - can <sequence> define
> the order of elements by an attribute value common to
> each element without having to physically list the
> elements in the order required?

No, <sequence> constrains the order based striclty on
element name.  There have been proposals to add various
forms of what are collectively called co-occurrence
constraints, though I haven't seen many requests for
sorted sequences of the sort you suggest.  Of course,
there is no end to what users might want...prime numbered
entries first?  Sounds bizarre, but a mathematician
might want it.  Unless you're willing to move away from
a declarative language to something Turing complete
(I.e. a full programming language for the constraints),
you'll always wind up trying to make reasonable tradeoffs
between complexity in the schema language and its 
ability to express complex constraints.  I think many
of us have a very strong preference for retaining a 
language that's basically declarative, as it is so
much easier to reason about and build tools for
such languages.

Noah Mendelsohn                              Voice: 1-617-693-4036
IBM Corporation                                Fax: 1-617-693-8676
One Rogers Street
Cambridge, MA 02142
Received on Friday, 13 June 2003 16:45:19 UTC

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