On Thu, 18 Nov 2010, Yannick Duch�ne (Hibou57) wrote:

> Formally speaking, you do not have multiple
> implementations of a single specification. The closest thing actually is
> generic package. But that is not easy to do with generics: you will have to
> give the generic instance all of its custom implementation via generic
> parameters (methods and others).

Why is that a big deal? The instance of a polymorphic data structure must 
somehow be told which operations to use, anyway.

Here is, how it should look like (not checked by a compiler).


-----------
The specification:

generic
  procedure Insert(Container: in out Store;
                   Key: Key_Type; Item: Payload;
                   Did_Overwrite: out Boolean)
  procedure Lookup(Container: Store;
                   Key: Key_Type; Item: out Payload;
                   Found: out Boolean);
  procedure Delete(Container: in out Store;
                   Key: Key_Type;
                   Found: out Boolean);
package Do_Something is
  ...
end Do_Something;

-----------
Two possible instantiations:

package Do_Something_Simple is new
  Do_Something(Linked_List.Insert,
               Linked_List.Lookup,
               Linked_List.Delete);
  -- use a linked list as the internal data struture
  -- constant time insert, linear time lookup and delete

package Do_Something_Fast is new
  Do_Something(AVL.Insert,
               AVL.Lookup,
               AVL.Delete);
  -- use an AVL tree, as the internal data structure
  -- logarithmic time for all of insert, lookup and delete

To me, it seems that generics are quite good for eactly that purpose ...

Stefan

-- 
------ Stefan Lucks   --  Bauhaus-University Weimar  --   Germany  ------
               Stefan dot Lucks at uni minus weimar dot de
------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------