Doing some haskell

nov3.hs

-- Functors
-- fmap :: (a -> b) -> f a -> f b
class MyFunctor f where
  fmap :: (a -> b) -> f a -> f b

data Perhaps a = Surely a | Naught deriving (Show)

instance Eq a => Eq (Perhaps a) where
  (Surely x) == (Surely y) = x == y
  Naught == Naught = True
  _ == _ = False

instance MyFunctor Perhaps where
  fmap _ Naught = Naught
  fmap f (Surely a) = Surely (f a)

-- Applicative Functors
-- pure :: a -> f a
-- <*> :: f (a -> b) -> f a -> f b
class MyApplicative f where
  pure :: a -> f a
  apply :: f (a -> b) -> f a -> f b

instance MyApplicative Perhaps where
  pure x = Surely x
  apply (Surely f) (Surely x) = Surely (f x)
  apply _ Naught = Naught
  apply Naught _ = Naught

-- Monads
-- (\x -> Just 10) <*> (Just 1)

-- Main.fmap (Surely (\x -> x + 1)) (Surely 10)

main :: IO ()
main = putStr ""