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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Generic Package
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Date: Mon, 30 Apr 2007 14:16:29 +0200
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Date: 2007-04-30T14:16:07+02:00
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On Mon, 30 Apr 2007 12:30:29 +0200, Georg Bauhaus wrote:

> On Sun, 2007-04-29 at 10:19 +0200, Dmitry A. Kazakov wrote:
> 
> My subprogram can hardly be arranged to be called with an element
> more than once by For_Each, or not at all (instantiation not
> being recursive in Ada):
> "your subprogram is called with each element once" is part of the
> contract of For_Each.

The point is that you cannot state this contract formally. It is equivalent
to the well-ordering principle, I guess.

> So it is not a question of trusting the iterator but rather
> a mere consequence of trust in the iterator being defined
> as required by the specification of iterator.

But you could not present any formal specification of.

Consider the program that counts elements of a set:

   C : Protected_Integer;
   procedure Count is
   begin
      C.Inc;
   end Count;
begin
   C.Set (0);
   foreach S do Count;

How could you prove that C.Get = S'Length?

>>> Is there iteration in the following SETL expression?
>>> 
>>>   Result := { x : x in {1, 3, 24, 17, 11} | P(x) };
>> 
>> Iteration in what? In the language construct? In Result? Consider a
>> possibility that Result were computed at compile time. Would it still be
>> iteration?
> 
> That's the point, there is iteration in this set former, but it
> happens behind the scene: on an AS-IF basis, it does not even matter
> when (or even if) iteration is happening;

Right, that is _the_ point. What is the reason to call this "iteration,"
rather than an operation constructing a new set?

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