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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: "unconstrained subtype in component declaration" vs "discriminant
 in constraint must appear alone"
Date: Mon, 22 Feb 2021 16:55:18 +0100
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On 2021-02-22 15:20, Mehdi Saada wrote:

> I just intend to initialize the value once and done. It is a default value, not a dynamic predicate.

There are three categories of "once":

1. Once per compilation, static
2. Once per object instantiation, subtype/dynamic constraint
3. Once per object update
------
and any time any place, variable.

As J-P said, you can have whatever expressions in #1, just remove 
discriminants.

In other cases you can add more discriminants and a constructing 
function. E.g. replace

    type T (L : Positive) is
       A : S (0..L - 1);
    end record;

with

    type T (<>) is private;
    function Create (L : Positive) return T;
private
    type T (L : Positive; Upper : Natural) is
       A : S (0..Upper);
    end record;

    function Create (L : Natural) return T is
    begin
       return Result : T (L, L - 1) do
          ... -- Some initialization
       end return;
    end Create;

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