Re: Wanted: Ada STL. Reward: Ada's Future

[Matthew Heaney <matthew_heaney@acm.org>]

Brian Rogoff <bpr@shell5.ba.best.com> writes:

> Another strength of C++ is in automatic instantiation of generic 
> subprograms. This reduces the number of instantiations, and makes code a 
> lot more readable, IMO, in the same way that overloading properly used 
> makes code more readable.

You can get some of this in Ada, by using default params for generic
formal subprograms:

generic

   type Item_Type is private;

   type Stack_Type is limited private;
   
   with procedure Push
     (Item : in     Item_Type;
      On   : in out Stack_Type) is <>;

package GP is ...;


If Push is directly visible at the point of instantiation, then you
don't need to specify explicitly:

  package Item_Stacks is new Stacks (Item_Type);
  use Item_Stacks;

  package P is new GP (Item_Type, Stack_Type);


Another, lesser-used technique is to provide default parameters by name:

generic
  ...
package GP is

...
   procedure Do_Nothing;

   generic
      with procedure Do_At_End_Of_Line is Do_Nothing;
   package GQ is 

      ...
   end GQ;

end GP;

A client that doesn't need to do an end-of-line processing, doesn't need
to say anything else in his instantiation:

  package Q is new P.GQ;

Of course, he may want to do something different from the default:

  procedure Write_EOL;

  package Q is new P.GQ (Write_EOL);