Funny type.

[Koen Claessen]

Jay Cox complained that the following is not possible:

 | data S m a = Nil | Cons a (m (S m a))
 |
 | instance (Show a, Show (m (S m a))) => Show (S m a)  where
 |   show Nil = "Nil"
 |   show (Cons x y) = "Cons " ++ show x ++ " " ++ show y

Ken Shan answered:

 | Here's how I've been handling such situations:
 |
 |     data S m a = Nil | Cons a (m (S m a))
 |
 |     -- "ShowF f" means that the functor f "preserves Show"
 |     class ShowF f where
 |         showsPrecF :: (Int -> a -> ShowS) -> (Int -> f a -> ShowS)

Actually, this class definition can be simplified to:

  class ShowF f where
    showsPrecF :: Show a => Int -> f a -> ShowS

And the rest of Ken's code accordingly.

/Koen.

--
Koen Claessen         http://www.cs.chalmers.se/~koen
phone:+46-31-772 5424      mailto:koen@cs.chalmers.se
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Chalmers University of Technology, Gothenburg, Sweden