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Date: Thu, 9 Apr 2020 13:45:50 -0700 (PDT)
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Subject: Re: Using Generic Pasckages
From: Jere <jhb.chat@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:58339
Date: 2020-04-09T13:45:50-07:00
List-Id: <comp.lang.ada>

On Thursday, April 9, 2020 at 4:40:11 AM UTC-4, ldries46 wrote:
> I have created some generic packages. tese are packages that only do the=
=20
> same thing but with other types. Till now I did only need these packages=
=20
> without interaction with between packages using the same type. I just=20
> can declare them:
> Package AA is new BB(type); then calling them as
> A :=3D AA.Get_Value;
> No other declaration seemed to be neccesary .
>=20
> Now I have the following situation:
> Package AA is new BB(integer);
> Package CC is new BB(integer);
> and a case where I have several statements using Package AA in one=20
> condition and CC the other case.
> I want=C2=A0 to do that by using:
> if D then EE :=3D AA; else EE :=3D CC; end if;
> But I cannot find in the documentation=C2=A0 how the declaration of EE sh=
ould=20
> be made.
>=20
> Of course there is the possibility to create the same progrram without=20
> using EE but that is far less readable and thus creating more possible=20
> errors.
>=20
> If I have to do this without the generic packages I already have the=20
> program alse will be less readable.

You've already had suggestions for polymorphism or using if/else=20
scaffolding.  For run time decisions like this, if both packages
are using the same types, I would recommend polymorphism.  However
if you prefer to stick with generics, you make the scaffolding a bit
less painful by using another generic.  Assuming your generic BB has
a specification like this:

    generic
        type Some_Type is private;
    package BB is
        procedure Set_Value(V : Some_Type);
        function Get_Value return Some_Type;
    end BB;

You can abstract out all your logic using it with another generic
similar to:

    generic
        with package EE is new BB(Some_Type =3D> Integer);
    procedure Do_Stuff;
   =20
    procedure Do_Stuff is
   =20
        V : Integer :=3D 23;
   =20
    begin
        Put_Line(Integer'Image(EE.Get_Value));
        EE.Set_Value(V);
        Put_Line(Integer'Image(EE.Get_Value));
       =20
        -- All your other BB logic you want to do

    end Do_Stuff;

then you pair it with the if/else or case structure=20
and locally declared blocks:

    Put_Line("Hello, world!");
    AA.Set_Value(10);
    CC.Set_Value(20);
 =20
    if D then=20
        declare
            procedure P is new Do_Stuff(AA);
        begin
            P;
        end;
    else
        declare
            procedure P is new Do_Stuff(CC);
        begin
            P;
        end;
    end if;

It still has some scaffolding, but it is more=20
readable then tons of if/else blocks scattered
throughout the code.  If your packages don't=20
have to rely on Integer, you can change the=20
specification to:

    generic
        with package EE is new BB(<>);
    procedure Do_Stuff;

But if your logic actually does rely on knowing
it is an integer type, you will get compiler errors.

Full compilable example below:

with Ada.Text_IO; use Ada.Text_IO;
procedure Hello is
   =20
    generic
        type Some_Type is private;
    package BB is
        procedure Set_Value(V : Some_Type);
        function Get_Value return Some_Type;
    end BB;
   =20
    package body BB is
        Value : Some_Type;
       =20
        procedure Set_Value(V : Some_Type) is
        begin
            Value :=3D V;
        end Set_Value;
       =20
        function Get_Value return Some_Type is (Value);
    end BB;
   =20
    package AA is new BB(Integer);
    package CC is new BB(Integer);
   =20
    generic
        with package EE is new BB(<>);
    procedure Do_Stuff_1;
   =20
    procedure Do_Stuff_1 is
   =20
        V : EE.Some_Type :=3D EE.Get_Value;
   =20
    begin
        EE.Set_Value(V);
    end Do_Stuff_1;
   =20
    generic
        with package EE is new BB(Some_Type =3D> Integer);
    procedure Do_Stuff_2;
   =20
    procedure Do_Stuff_2 is
   =20
        V : Integer :=3D 23;
   =20
    begin
        Put_Line(Integer'Image(EE.Get_Value));
        EE.Set_Value(V);
        Put_Line(Integer'Image(EE.Get_Value));
    end Do_Stuff_2;
   =20
    D : Boolean :=3D True;

begin
    Put_Line("Hello, world!");
    AA.Set_Value(10);
    CC.Set_Value(20);
 =20
    if D then=20
        declare
            procedure P is new Do_Stuff_2(AA);
        begin
            P;
        end;
    else
        declare
            procedure P is new Do_Stuff_2(CC);
        begin
            P;
        end;
    end if;
 =20
end Hello;