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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: syntaxic exploration
Date: Sun, 24 Dec 2017 15:39:56 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:49623
Date: 2017-12-24T15:39:56+02:00
List-Id: <comp.lang.ada>

On 17-12-24 05:37 , Robert Eachus wrote:
> On Saturday, December 23, 2017 at 11:23:32 AM UTC-5, Jeffrey R.
> Carter wrote:
>> On 12/22/2017 11:11 PM, Niklas Holsti wrote:
>>>
>>> I think the aggregate form (null array) was suggested some time
>>> ago, in analogy with the existing (null record). It looks good to
>>> me, but as I remember, there was some objection. One problem is
>>> that if the index type has exactly one value, then a null array
>>> with that index type cannot exist (because then A'First = A'Last
>>> for any such array A).

   [snip]

> A more complex, and potentially ugly case is for multidimensional
> arrays.  Defining (null array) as being empty in all dimensions with
> Foo'Range(n) = Bar'First..Bar'Pred(Bar'First) where Bar is the nth
> index SUBtype for Foo works.

In other words, the same as the current rule for the null string literal "".

> In general, you should be surprised if Foo'Last(N) for a null
> array doesn't raise Constraint_Error.

That is not true in current Ada, as I understand it.

That would be very bad -- it would mean that any general operation that 
loops over an array that is potentially null would have to first check 
the 'Length. Fortunately this is not the case.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .