Re: How to write TYPECASE in Ada 95?

[Matthew Heaney <matthew_heaney@acm.org>]

tmoran@bix.com (Tom Moran) writes:

> 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?


It seems like the real problem in your example is that Fruit doesn't
include any primitive operations for Serve'ing.

If you want to extend a type with arbitrary behavior, you could take a
look at using the Visitor pattern.  Ada implementations of this pattern
are available at the ACM patterns archive.

<http://www.acm.org/archives/patterns.html>