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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: gnat.string_split , howto manipulate slice numbers
Date: Tue, 29 Oct 2019 20:37:15 +0100
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Date: 2019-10-29T20:37:15+01:00
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On 2019-10-29 19:33, J-P. Rosen wrote:
> Le 29/10/2019 à 17:47, Simon Wright a écrit :
>>     type Slice_Number is new Natural;
>>
>> For me, this carries declaring a type rather than a subtype further than
>> necessary. A subtype? Or why not just use Natural?
> Integer (or its subtypes) should be avoided in general. It is non
> portable, and carries no information to the reader about the purpose of
> the type. Making appropriate types that cannot be mixed is key to the
> philosophy of Ada ("strong typing", you know...).

Another reason is to distinguish index/key from position. When both are 
subtypes of integer some bugs may slip through, e.g.

    A (A'Length - 1)

This is broken code, but the compiler cannot detect it.

Or when you instantiate a generic container:

generic
    type Position_Type is range <>;
    type Index_Type is private;
    type Element_Type is private;
package Generic_Container is
    ...
    function Get (Container : Container_Type; Position : Position_Type)
       return Element_Type;
    function Get (Container : Container_Type; Key : Key_Type)
       return Element_Type;

It fails when instantiated with subtypes of same integer type.

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