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Newsgroups: comp.lang.ada
Subject: Re: Generic Package
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From: Markus E Leypold
 <development-2006-8ecbb5cc8aREMOVETHIS@ANDTHATm-e-leypold.de>
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Date: Fri, 27 Apr 2007 13:43:58 +0200
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Date: 2007-04-27T13:43:58+02:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Thu, 26 Apr 2007 22:52:58 +0100, Simon Wright wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>> 
>>> You cannot iterate a relational table, because there is no order
>>> defined on it.
>> 
>> Why would that stop me iterating over all the rows?
>
> Because iterating presumes following an order. If there is no *any* order,
> which one would you follow? 

None. Iteration gives the values in a certain sequence, any time you
iterate. But it doesn't follow an order in the case of unordered
collections. Next time you iterate the elements might come in another
sequence.

> It is clear that we could enumerate anything on
> a real computer, but that would be same as Unchecked_Conversion, it would
> break the abstraction. 

So I'm having an unchecked operation if I enumerate elements of a set
(sets are unordered). How embarrasing. Theat would mean, that there is
no efficient way to get at the elements of a given subset of the
natural numbers. I'd have to

  for I in 0 ... infinity: if (I in M) then ... else ...


> I guess, one should first create a "result set", then enumerate it
> according to some order relation. That thing could be iterated
> through.

What nonsense.

Regards -- Markus