On 30-Jan-2001, Ashley Yakeley 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 | "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.

On 30-Jan-2001, Marcin 'Qrczak' Kowalczyk wrote:

Tue, 30 Jan 2001 00:13:41 -0800, Ashley Yakeley pisze:

...

How do I define downcast?

You can use a non-standard module Dynamic present in ghc, hbc and Hugs (I don't know if it's compatible with O'Haskell).

That lets you downcast to specific ground types, but it doesn't let you downcast to a type class constrained type variable. -- Fergus Henderson | "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.