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...
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>>My SUV is bigger than your bike. Stay out of the damn road!
>Kiss my reflector, SUV-boy
I'm too busy sucking on my tailpipe, bike dude.
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