Another list monad

List.hs

module List where

data List a = Nil | Cons a (List a) deriving (Eq, Show)

instance Functor List where
  fmap f Nil = Nil
  fmap f (Cons a as) = Cons (f a) (fmap f as)

instance Applicative List where
  pure a = Cons a Nil

  (<*>) = undefined

append :: List a -> List a -> List a
append (Cons a as) bs = Cons a (append as bs)
append Nil bs = bs

join :: List (List a) -> List a
join Nil = Nil
join (Cons l ls) = append (join ls) l

instance Monad List where
  a >>= f = join $ fmap f a

toList :: [a] -> List a
toList [] = Nil
toList (a:as) = Cons a (toList as)

fromList :: List a -> [a]
fromList Nil = []
fromList (Cons a as) = a : fromList as