Re: How to write TYPECASE in Ada 95?

[tmoran@bix.com (Tom Moran)]

Dispatching is certainly better than a case statement - if you use it.

Consider an existing package of the form:

package Grocer is
  type Fruit is abstract tagged ...
  type Apple is new Fruit ...
  ...
  type Watermelon is new Fruit ...
  ...
  function Best(Budget : in Money) return Fruit'class;
end Grocer;

And now you are writing a new package which, among other things, does

with Grocer;
 use Grocer;
package Meal is
  procedure Serve(Dish : in Apple);
  ...
  procedure Serve(Dish : in Watermelon);
end Meal;

Suppose a user wants, not unreasonably, to write
...
   Meal.Serve(Dish => Grocer.Best(Budget=>1.00));

Unless you can go back and modify Grocer, it appears you must include
in Meal
  procedure Serve_Fruit(Dish : in Fruit'class);
with a body like
  procedure Serve_Fruit(Dish : in Fruit'class) is
  begin
    if Dish in Apple'class then Serve(Apple(Dish));
    ...
    elsif Dish in Watermelon'class then Serve(Watermelon(Dish));
    else raise Heck;
    end if;
  end Serve_Fruit;

Is there a better way?