scott-lists.hs

{-# LANGUAGE RankNTypes #-}

data Z
data S n

type L a = forall b. (a -> b -> b) -> b -> b

consL a fold = \cons nil -> cons a (fold cons nil)

(@:) = consL

infixr @:

nilL = \cons nil -> nil

fromList :: [a] -> L a
fromList = foldr consL nilL

showL :: Show a => L a -> String
showL fold = fold (\x rest -> show x ++ " @: " ++ rest) "nilL"

tailL :: L a -> L a
tailL fold = fst $ fold (\x (_, xs) -> (xs, x@:xs)) (undefined, nilL)

main :: IO ()
main = putStrLn $ showL $ tailL $ fromList [1, 2, 3, 4, 5]