simulating dynamic dispatch

[oleg@pobox.com]

> i'm hoping to be able to simulate a sort of dynamic dispatch based on
> class instances.

It seems you want to dispatch based not on a type but on the
constraint of a type. 

You code almost worked. Here's the a bit updated and working version.

class Foo a where { foo :: a -> Bool }
class Bar a where { bar :: a -> Bool }

data FB = forall a . Foo a => MkFoo a | forall a . Bar a => MkBar a

instance Foo FB where
    foo (MkFoo x) = foo x

instance Bar FB where
    bar (MkBar x) = bar x

-- some instances for the test    
instance Foo Int where
    foo x = x == 0
    
instance Bar Char where
    bar x = x == 'a'
    

test x = case x of
          (MkFoo a) -> Just $ foo a
          (MkBar a) -> Just $ bar a
--	  _         -> Nothing


-- *Main> test $ MkFoo (0::Int)
-- Just True
-- *Main> test $ MkFoo (10::Int)
-- Just False
-- *Main> test $ MkBar 'a'      
-- Just True
-- *Main> test $ MkBar 'b'
-- Just False