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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Generic Package
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Date: Sat, 28 Apr 2007 19:35:18 +0200
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Date: 2007-04-28T19:35:02+02:00
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On Fri, 27 Apr 2007 13:43:58 +0200, Markus E Leypold wrote:

> "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,

"Sequence" is defined as an ordered [and countable] set.

>> 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.

Yes, because that set is a representation [model] of some other [domain
space] set which could be fundamentally unordered and/or uncountable. As an
example consider merits of the loop:

   for X in 0.0..1.0 loop

Fortunately the above is illegal in Ada. Not because it were impossible:

declare
   X : Float := 0.0;
begin
   while X <= 1.0 loop
      ...
      X := Float'Succ (X);
   end loop;

but because it is a mess.

After interating reals we could proceed to complex numbers...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de