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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Surprise in array concatenation
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Date: Thu, 1 Sep 2005 15:59:28 +0200
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On Thu, 01 Sep 2005 13:42:37 +0200, Georg Bauhaus wrote:

> Dmitry A. Kazakov wrote:
> 
>> A more interesting question is why Empty'First does not raise any
>> exception.
> 
> Because 'First does not denote an element?

Because it does not exist. Consider a discrete type with one or even no
elements.

>> After all, there is no any lower bound of an empty index range.
> 
> ? What's the bound of a range? (as opposed to a bounded array)

The bound of the index range. Array itself has no any bounds, its index may
have them.

>> Provided, that empty arrays are all same, of course. If not, then another
>> interesting question would appear: how to make an empty array with the
>> lower bound Integer'First?
> 
> Carrying the arguement further, how to reference an element
> outside the machine's storage?

This way:

   raise Constraint_Error; -- (:-))

--------------
The problem with the current (Ada 83) design is that it tries to abstract
away trivial mathematical facts:

1. Not all sets have bounds
2. Even if a set is bounded it does not necessarily contain the bound.

Further trivial fact:

3. [a, a-1] is not an empty set. It might be an interval of negative
length, whatever meaning that may have.

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