RE: [DM]Some doubts/comments

Hi Mike,
  Thank you very much for a lucid explanation. I am
completely convinced by your views.  

Regards,
Mukul

--- Michael Kay <mhk@mhk.me.uk> wrote:

> 
> > 3) In Section 2.5 "Sequences" its mentioned, "If
> > sequences are combined, the result is always a
> > flattened sequence. In other words, appending (d
> e) to
> > (a b c) produces a sequence of length 5: (a b c d
> e).
> > It does not produces a sequence of length 4: 
> > (a b c (d e)), such a nested sequence never
> occurs"
> > 
> > I would have wished nested sequences! I am curious
> to
> > know, why nested sequences are not allowed in the
> > language ? 
> 
> This came up as a detailed proposal during the last
> call stage, and I think
> the proposal failed to win support at that stage for
> three main reasons: (a)
> because it was too disruptive (we all want to get
> finished), (b) because the
> facility was felt to be unnecessary feature creep,
> and (c) because nested
> sequences don't seem to correspond to anything that
> exists in XML.
> 
> The data model supports sequences of nodes because
> they arise naturally in
> XML and are useful as the result of a query. It
> supports sequences of atomic
> values because they correspond to XML Schema list
> types, and because they
> are useful as intermediate results when doing
> aggregations (e.g. sum(),
> max(), min()). It supports sequences that mix atomic
> values and nodes
> because on balance it is better to allow them than
> to impose a restriction -
> they are handy in constructs such as max((@a, 0)).
> But nested sequences
> don't arise naturally in XML, in fact they provide a
> data structure that
> competes directly with XML trees. It would give you
> two different kinds of
> tree structure in the data model, with different
> constraints and different
> operators, which I think would be very confusing to
> users. It's already
> confusing enough to many people that you can't
> access nodes in a sequence
> using the sibling axis. (It wouldn't make sense,
> because a node can be a
> member of any number of different sequences, but it
> can only belong to one
> XML tree.)
> 
> Michael Kay
> (speaking for himself) 


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Received on Tuesday, 9 November 2004 16:12:55 UTC