import Prelude hiding(Maybe(..), lookup)


data Maybe a = Just a | Nothing deriving (Show)
instance Monad Maybe where
  return x = Just x
  (Just x) >>= f = f x
  Nothing >>= _  = Nothing

findSomething :: Maybe Int
findSomething = return 3

findSomethingElse :: Int -> Maybe Int
findSomethingElse x = return (x+5)

lookup :: Int -> Int -> Maybe Int
lookup x y = return (x*y)

someFailure :: Maybe a
someFailure = Nothing

findThemAll :: Maybe Int
findThemAll = do
  x <- findSomething
  y <- findSomethingElse x
  lookup x y

findThemAllFail :: Maybe Int
findThemAllFail = do
  x <- findSomething
  someFailure
  y <- findSomethingElse x
  lookup x y
  
main = print findThemAll >> print findThemAllFail