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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Generic Package
Date: Tue, 1 May 2007 14:14:46 -0500
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Date: 2007-05-01T14:14:46-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:1ozvzzh59ebq8$.yeh9do8s3hig$.dlg@40tude.net...
> On Tue, 01 May 2007 15:42:21 +0200, Georg Bauhaus wrote:
...
> > Yes, in the sense that the specifics of memory layout and such are
> > unknown and irrelevant to the client; in particular, no order needs
> > to be specified.
>
> Apart from obvious uselessness of contracts referencing to the memory
> layout, the above is self-contradictory. Memory layout defines an order.
It
> also is a part of the contract. Hence the interface is ordered.

And now all is clear.

It is impossible to implement a set with having some form of order. So, for
us practitioners, an "unordered set" is simply a set with no defined order.
I can see that you are arguing about a idealized set unordered set that
cannot actually exist - an abstraction that is irrelevant in practice. No
wonder people are confused.

There is a reason that Ada.Containers has ordered and hashed sets, but not a
mathematically unordered one. That because the latter is impossible to
implement without a suboptimal mapping into one of the other forms. The net
effect is that even if you could use such a form, you would take a massive
performance hit to do so (all operations being O(N)). As such, there is no
reason to consider such forms or worry about their properties.

                                        Randy.