Re: Ranges and (non)static constraints

["Stuart" <stuart@0.0>]

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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:g2go52hf14qn.16tb2xiy7ilrf.dlg@40tude.net...

> On Thu, 16 Nov 2006 18:18:47 +0100, Jean-Pierre Rosen wrote:
>> Dmitry A. Kazakov a �crit :
>>> On Thu, 16 Nov 2006 12:02:15 +0100, Maciej Sobczak wrote:
>>>
>>>> type T is range 1 .. N;
>>>> type U is new Integer range 1 .. M;
>>>>
>>>> N must be static, but M does not have to.
>>>> Why and what is the real difference between T and U?
...
>> Actually, the second form should never be used: you are relying on
>> Integer, a non-portable type that plays absolutely (or almost) no
>> special role in Ada. Why derive from Integer, rather than from
>> Long_Integer, or anything else?
...
>> If you want dynamic bounds, remember that anything "dynamic" has to have
>> an upper limit at some point. It is thus better to write:
>>
>> type Biggest_T is range 1 .. Absolute_Max_Expectable_Value;
>> subtype T is Biggest_T range 1 .. N;

> Well, this is nice in theory, which I strongly support. But this theory
> applies only to application software, where Absolute_Max_Expectable_Value
> is determinable from the problem space.
>
> When developing portable libraries, and Ada is one of the best choices
> there, isn't it? Then the upper bound often becomes indeterminable. So
> people are forced to use [new] Integer. ARM does this as well by defining
> the type String based on Integer. Only in generics we have a choice to 
> say:

Surely there is System.Max_Int!
   [ARM 13.7 (23)] The largest (most positive) value allowed for the 
expression
  of a signed_integer_type_definition.

So you would have:
   type Biggest_T is range 1..System.Max_Int;
  subtype T is Biggest_T range 1..N;

-- 
Stuart