O'Haskell OOP Polymorphic Functions

[Fergus Henderson]

On 30-Jan-2001, Ashley Yakeley <ashley@semantic.org> wrote:
> At 2001-01-17 17:03, Lennart Augustsson wrote:
> 
> >You seem to want dynamic type tests.
...
> >You might want to look at existential types; it is a similar feature.
> 
> I seem to run into a similar problem:
> 
> --
> class BaseClass s
> data Base = forall a. BaseClass a => Base a
> 
> class (BaseClass s) => DerivedClass s
> data Derived = forall a. DerivedClass a => Derived a
> 
> upcast :: Derived -> Base
> upcast (Derived d) = Base d
> 
> downcast :: Base -> Maybe Derived
> --
> 
> How do I define downcast?

class BaseClass s where
	downcast_to_derived :: s -> Maybe Derived

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.