uncons without pattern matching

Tail.hs

{-# LANGUAGE ImpredicativeTypes #-}
{-# LANGUAGE RankNTypes #-}
module Tail where

type CL a = forall c. (a -> c -> c) -> c -> c

t :: CL a  -> (forall c. (a -> (Bool -> c) -> (Bool -> c))
               -> (Bool -> c)
               -> (Bool -> c))
t fold cons nil = fold myCons myNil
  where myNil = nil
        myCons _ as True = as False
        myCons a as False = cons a (\_ -> as False) False

ctail :: CL a -> CL a
ctail l cons nil = t l (\x xs b -> cons x (xs b)) (\_ -> nil) True

chead :: CL a -> a
chead l = l const undefined

uncons :: CL a -> (a, CL a)
uncons l = (chead l, ctail l)