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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Magic ordering
Date: 16 Mar 2005 15:03:51 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2005-03-16T15:03:51-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Tue, 15 Mar 2005 12:54:30 +0000, Marius Amado Alves wrote:
> 
> > (Sorry I changed the subject. I'm just that kind of guy :-)
> > 
> > On 15 Mar 2005, at 12:01, Hans Malherbe wrote:
> > 
> >> I have to ask myself though: If I am denied access to information that
> >> lets me  implement a < operator, am I supposed to be able to sort it?
> > 
> > This is one of my favourites. There's always a way. Bit by bit 
> > comparison, an arbitrary index, 'Address comparison...
> 
> All of which might appear wrong. The objects might be non-contiguous, have
> replicas etc.
> 
> Comparison should reflect the semantics of compared things. There is no
> universal way to do it.

It seems to me that there are two sorts of "<" operator here.

For one, "<" represents some arbitrary order.  It has to have
certain properties, like always returning the same answer on 
the same args, and X=Y implies that X<Y returns False, and
X<Y and Y<Z implies X<Z, and so forth.  You can use this to
create a container, where you don't care about the order,
just that there is *some* order.

For the other, "<" represents some semantic property of the type.
Like the usual order of integers, or lexicographic order for
character strings.

It seems to me that the compiler could provide the former,
which allows creating containers, and you can ask Is_Present,
and you can iterate in an arbitrary order.

The compiler cannot provide the latter, but if the programmer provides
it, you can have containers with iterators that use a *sensible* order.

- Bob