On 2001-05-27T22:46:37-0500, Jay Cox wrote:
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
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) -- "instance ShowF []" is based on showList instance ShowF [] where showsPrecF sh p [] = showString "[]" showsPrecF sh p (x:xs) = showChar '[' . sh 0 x . showl xs where showl [] = showChar ']' showl (x:xs) = showChar ',' . sh 0 x . showl xs -- S preserves ShowF instance (ShowF m) => ShowF (S m) where showsPrecF sh p Nil = showString "Nil" showsPrecF sh p (Cons x y) = showString "Cons " . sh 0 x . showChar ' ' . showsPrecF (showsPrecF sh) 0 y -- Now we can define "instance Show (S m a)" as desired instance (Show a, ShowF m) => Show (S m a) where showsPrec = showsPrecF showsPrec You could call it the poor man's generic programming... -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig
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