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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Help with designing data structure for novice Ada programmer
Date: Sun, 9 Jan 2022 11:10:55 +0100
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On 2022-01-09 10:49, Aleksy Grabowski wrote:

> I also have some concrete question, how to properly implement optional
> element? Right now I have something like this:
> 
>      generic
>          type T is private;
>      package TypeUtils is
>          type Optional(exists : Boolean) is
>          record
>              case exists is
>                  when True => value : T;
>                  when False => null;
>              end case;
>          end record;
>      end TypeUtils;
> 
> But it looks like it doesn't work, because it depends on the 
> discriminant `exists' but it is set once in .ads file and I can't modify
> it in runtime.

Provide a default value for the discriminant:

    type Optional (Defined : Boolean := False) is record
        case Present is
           when True =>
              Value : T;
           when False =>
              null;
        end case;
    end record;

Now it is a definite type and you can place it into another record type:

    type Container is record
       ...
       Optional_Field : Optional;
       ...
    end record;

and you can always update it:

    X : Container;

    X.Optional_Field := (True, Value_1);
    ...
    X.Optional_Field := (Defined => False);
    ...
    X.Optional_Field := (True, Value_2);
    ...

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