compose.hs

newtype Identity a =
    Identity { runIdentity :: a }
    deriving (Eq, Show)

newtype Compose f g a =
    Compose { getCompose :: f (g a) }
    deriving (Eq, Show)

instance (Functor f, Functor g) =>
        Functor (Compose f g) where
    fmap f (Compose fga) =
        Compose $ (fmap . fmap) f fga

instance (Applicative f, Applicative g) =>
        Applicative (Compose f g) where
    pure :: a -> Compose f g a
    pure a = undefined

    (<*>) :: Compose f g (a -> b)
          -> Compose f g a
          -> Compose f g b
    (Compose f) <*> (Compose a) = Compose (f a)