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RE: RE: Postional predicates de-mystified

Subject: RE: RE: Postional predicates de-mystified
From: "Michael Kay" <michael.h.kay@xxxxxxxxxxxx>
Date: Fri, 4 Jan 2002 15:52:17 -0000
RE:  RE: Postional predicates de-mystified
> Curious about the shift from node-set to sequence then.
> Was it because of all the discussion (confusion?) on the list
> re sets?

I think it started with the fact that XML Schema supports lists as a data
type, for example IDREFS is a list of strings. XQuery wanted to support
lists of nodes (without them, what is the result of a sort operation?), and
so came the idea of a data model based on sequences. There was then
considerable debate about whether the model should support sets and
sequences as separate types, with an eventual consensus that operations that
naturally produced sets (like path and union expressions) would be defined
as producing a sequence in canonical order, as this kept the model simpler.

The confusion about XPath 1.0 node-sets may have had an influence. It wasn't
confined to this list either - I've heard people within W3C, people with a
mathematical training who should know better - claim that XPath 1.0
node-sets were really sequences all the time.

Mike Kay


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