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   Re: [xml-dev] xsi:type and broken contracts

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Paul Prescod <paul@prescod.net> writes:

> "Henry S. Thompson" wrote:
> > 
> >...
> > Um, no, the content model for dl in XHTML is (dt|dd)*, which will
> > cover what your rules produce, and what they produce will look OK
> > too.  
> Let's not get hung up on the details of XHTML. There are many content
> models of the form (x,y)+ out there. And anyhow, I can come up with
> these examples all day.

Well, yes and no.  The reason the dt/dd example is interesting is
precisely that it's an open question whether the behaviour of the
original stylesheet with the new document is correct or incorrect.

If you want to be conservative, the W3C XML Schema REC lets you be
conservative, by 'block'ing the use of xsi:type and enforcing the use
of _your_ schema, not anyone elses (see the other thread running at
the moment).  If you want to be liberal, and allow people to use your
stuff with documents which include extensions, your stuff will always
do what it should with the original material, and may or may not do
anything interesting with the additional bits.

I guess that's a better way of saying what I was trying to say before
-- as long as you only rely on required bits, and don't work back from
the end in your expressions, your stylesheets and other applications
will always do the same thing with the unextended parts of instances
which exploit xsi:type to extend or restrict their inputs as they
would have without the extensions.  What happens to the extensions
themselves is anybody's guess.

Just as no-one would allow a mission-critical system involving
validation to do so against a client-supplied DTD (despite the fact
that, as you point out in your companion message, XML 1.0 _requires_
it to do so to be a conformant validating parser), but would instead
use their own, just so anyone writing a mission-critical application
involving schema-validity assessment will do so against their own
schema and either write it to 'block' the use xsi:type wrt extension,
or ignore any other schema hints, so any attempt to use foreign types
will fail (both of these strategies _are_ allowed by W3C XML Schema).

Horses for courses, take your pick.

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2002, 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/
 [mail really from me _always_ has this .sig -- mail without it is forged spam]


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