From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,3abdaa39e1c27f49
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news2.google.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Discriminant computation problem
Date: Mon, 15 Nov 2004 17:02:11 +0100
Message-ID: <1csd5r45jm41a$.119v7kqdyy147.dlg@40tude.net>
References: <NmTld.122$rc.97840@news20.bellglobal.com>
 <yjVld.906538$Gx4.504752@bgtnsc04-news.ops.worldnet.att.net>
 <7e2ad2d.0411150602.79ee1251@posting.google.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
X-Trace: news.uni-berlin.de jVZ5mZ8p02JKhBYESAxvFgAB++CyQ3y+tmoSrv3KdS6/0fqn0=
User-Agent: 40tude_Dialog/2.0.12.1
Xref: g2news1.google.com comp.lang.ada:6208
Date: 2004-11-15T17:02:11+01:00
List-Id: <comp.lang.ada>

On 15 Nov 2004 06:02:39 -0800, Sandro Magi wrote:

> I was trying to avoid the extra step of instantiating a generic
> package. Furthermore, the types from one package would not be
> interoperable with the types from another package would they? ie.
> using my Bit_Vector example and assuming the package defines an Append
> operation:
> 
> procedure Test is
>   package Bit_Vector_1024 is new Bit_Vector(1024);
>   package Bit_Vector_2048 is new Bit_Vector(2048);
> 
>   b1024 : Bit_Vector_1024.Bit_Vector;
>   b2048 : Bit_Vector_2048.Bit_Vector;
> begin
>   ...
>   (initialize some data in each vector)
>   ...
>   --this would fail to type check wouldn't it?
>   Append(Data=>b1024, To=>b2048);
> end Test;
> 
> Sorry for asking something so obvious, but I can't check this myself
> at the moment.

Bit_Vector_1024.Bit_Vector and Bit_Vector_2048.Bit_Vector are two different
types, when they are declared in Bit_Vector as types. To avoid this you
should have a common ancestor for both. There could be three variants:

1. Constrained subtypes:

package Bit_Vectors is
   type Vector is array (Integer range <>) of Boolean;
-- or
   type Vector (Size : Natural) is private;
   procedure Append (Data : Vector; To : in out Vector);
      -- Note, Append is defined here. The common "ancestor"
      -- is unconstrained Vector
   ...
end Bit_Vectors_Base;

generic
   Size : Positive;
package Bit_Vectors.Fixed_Size is
   subtype Bit_Vector is Vector (1..Size);
   ...
end Bit_Vectors.Fixed_Size;

procedure Test is
   package Bit_Vector_1024 is new Bit_Vectors.Fixed_Size (1024);
   package Bit_Vector_2048 is new Bit_Vectors.Fixed_Size (2048);
 
   b1024 : Bit_Vector_1024.Bit_Vector;
   b2048 : Bit_Vector_2048.Bit_Vector;
begin
   Append (Data => b1024, To => b2048); -- This is OK

2. Tagged types derived from the same base:

package Bit_Vectors is
   type Vector is abstract tagged ...
   procedure Append (Data : Vector'Class; To : in out Vector) is
      abstract;
      -- Note, at least one of the parameters should be
      -- class-wide. There is no full multiple dispatch in Ada!
      -- The common ancestor there is abstract Vector.
   ...
end Bit_Vectors_Base;

generic
   Size : Positive;
package Bit_Vectors.Fixed_Size is
   type Bit_Vector is new Vector with record
      -- Here the body uses Size
   end;
   procedure Append (Data : Vector'Class; To : in out Bit_Vector);
      -- You have to implement it here for the case when To
      -- is Bit_Vector, where Data is anything possible.
end Bit_Vectors.Fixed_Size;

procedure Test is
   package Bit_Vector_1024 is new Bit_Vectors.Fixed_Size (1024);
   package Bit_Vector_2048 is new Bit_Vectors.Fixed_Size (2048);
 
   b1024 : Bit_Vector_1024.Bit_Vector;
   b2048 : Bit_Vector_2048.Bit_Vector;
begin
   Append (Data => b1024, To => b2048); -- This is OK

3. Do not use generics! Ada's unconstrained types are powerful and
efficient.

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