MyMaybeMonad.hs

data MyMaybe a =
  MyNothing
  | MyJust a
  deriving (Show)

instance Functor MyMaybe where
  fmap _ MyNothing = MyNothing
  fmap f (MyJust a) = MyJust (f a)

instance Applicative MyMaybe


instance Monad MyMaybe where
  return = MyJust
  (>>=) MyNothing _ = MyNothing
  (>>=) (MyJust a) f = f a

maybeAddFive :: MyMaybe Int -> MyMaybe Int
maybeAddFive =
  fmap (+ 5)

heavyCalculation1 :: MyMaybe Int -> MyMaybe Int
heavyCalculation1 number = do
  addResult <- (+ 5) <$> number
  mulResult <- (* addResult) <$> (MyJust 2)
  return mulResult -- this is actually redundant, see below

heavyCalculation2 :: MyMaybe Int -> MyMaybe Int
heavyCalculation2 number = do
  addResult <- (+ 5) <$> number
  (* addResult) <$> MyJust 2 

-- this time with explicit bind instead of do notation
heavyCalculation3 :: MyMaybe Int -> MyMaybe Int
heavyCalculation3 number =
  (+ 5) <$> number >>=
  (\addResult -> (* addResult) <$> MyJust 2)



main :: IO ()
main = do
  print $ maybeAddFive $ MyJust 2
  print $ maybeAddFive MyNothing