module Twist where

data Twist a b = Twil | Twons a (Twist b a)

tmap :: (a->c) -> (b->d) -> Twist a b -> Twist c d
tmap f g Twil = Twil
tmap f g (Twons a bas) = Twons (f a) (tmap g f bas)

list Twil = []
list (Twons x xs) = x : list xs

instance (Show a, Show b) => Show (Twist a b) where
  show Twil = "[]"
  show abs = "[" ++ sep (list (tmap show show abs)) ++ "]"
             where sep = foldl1 (\x y -> x ++ ", " ++ y)

twip :: [a] -> [b] -> Twist a b
twip (x:xs) ys = Twons x (twip ys xs)
twip [] ys = Twil

twap :: (a->c) -> (b->c) -> Twist a b -> [c]
twap f g Twil = []
twap f g (Twons a bs) = f a : twap g f bs

len :: Twist a b -> Int
len Twil = 0
len (Twons _ abs) = 1 + len abs


{- Sample runs:

	> twip "abcd" [1..5]
	['a', 1, 'b', 2, 'c', 3, 'd', 4]
	> twap ord id $$
	[97,1,98,2,99,3,100,4]
	> sum $$
	404
	> twip "abcd" [1..5]
	['a', 1, 'b', 2, 'c', 3, 'd', 4]
	> tmap (:[]) even $$
	["a", False, "b", True, "c", False, "d", True]
	> len $$
	8

-}