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Re: Questions on XML Schema "all" and RELAX NG "interleave"


schema all
"Alessandro Triglia" <sandro@m...> writes:

> One concrete question I might ask is, If I have an "all" group of 3
> elements in a schema, and an instance contains the 3 elements in some
> order (any order being valid), how much does it matter if a tool
> re-orders the elements on reception, and presents them to the user in a
> fixed order?   Could I justify such a behavior of the tool by saying
> that the order in an "all" group is not important?   What are the
> chances that the order *is* important and the user absolutely wants it
> to be preserved?

The chances ought to be high if the <all> group is being used
responsibly (which it often isn't, in my experience).  Let's approach
this from the other side -- if an element is declared to contain a,
then b, then c as children, then the relative order of a, b and c in
an instance carries no semantics.  From this Occam suggests that in
cases where there is no intention to convey different semantics with
different order among siblings, a sequence should always be used, to
avoid potential misunderstanding.  _If_ this argument is persuasive,
then you should _only_ use <all> when order _does_ matter, in which
case re-arrangement changes the meaning of the document and should be
avoided.

> Another possible question is, How does a user (either sending or
> receiving) benefit, in general, in RELAX NG, from the ability to
> interleave, say, elements A (multiple), B (multiple), and C (multiple)
> of an "interleave", as opposed to sending all As, then all Bs, then all
> Cs?  Or possibly all Cs, then all Bs, then all As?   Why is "interleave"
> more useful than "all", and why is "all" more useful than "sequence"?
> What do these things enable me to do, which a mere "sequence" does not?
> Why do I need the extra flexibility?

As per the above, if you need to reflect some underlying ordering,
then you need a *ed choice or an all.  Suppose for instance you have
a queue of transactions to be processed, say some deposits and some
withdrawals.  If you wish their order in the queue to be represented
by their order in the document, you can't simply put all the deposits
first followed by all the withdrawals.

Hope this helps

ht
-- 
  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@c...
		     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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