Re: Discriminant computation problem

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

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