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Re: redefine

From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
Date: 13 Apr 2001 15:50:35 +0100
To: Ian Stokes-Rees <ijs@decisionsoft.com>
Cc: "xmlschema-dev@w3.org" <xmlschema-dev@w3.org>
Message-ID: <f5bofu07t1g.fsf@cogsci.ed.ac.uk>
Ian Stokes-Rees <ijs@decisionsoft.com> writes:

> What is the motivation for redefine?  It seems to me that there are some
> serious problems with redefining complex types defined in other schema
> files but in the same namespace.

<redefine> provides a modest facility within the language to
reconstruct what people would do outside the language if it weren't
provided, namely hand-edit copies of existing schema documents to
change them.  This may be for specialisation, for versioning, or whatever.

> The effect would be that an instance document which was set in a
> particular namespace would have different understandings of a certain
> document depending on which schema it happened to use (or perhaps even
> the order in which it read in the schemas or applied them to the
> validation of the document).

There's no way we can stop this, with or without <redefine>.  It will
always be open to someone to provide a schema document for a namespace 
which differs from what the document author had in mind, whether
legitimately or not.

> Since xsi:schemalocation is only meant to be a hint to a schema
> validator, it doesn't seem to make sense to have two different
> definitions of the same type within a given namespace depending on which
> schema file you happen to read.

Once we recognised that in at least some situations, it's the consumer 
of a document who needs to control what schema is used, not the
producer, it became inevitable that the connection between instance
and schema could not be made rigid.  Even the connection between
instance and DTD is nowhere near as rigid as one might at first
believe.

In the WG's view, managing instance-schema connections is not the
business of the XML Schema language design itself.  After all, only
serious work with digital signatures and packaging could _really_
provide guarantees about rigid connections, and then only with
additional superstructure beyond the XML Schema level.

ht
-- 
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/
Received on Friday, 13 April 2001 10:50:38 GMT

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