data Result t
	= Ok t
	| Err String
	deriving (Show, Eq)

instance Functor Result where
	fmap f (Ok  x)   = Ok (f x)
	fmap _ (Err msg) = Err msg

instance Applicative Result where
	pure = Ok
	Ok  f   <*> Ok  x   = Ok (f x)
	Ok  f   <*> Err msg = Err msg
	Err msg <*> _       = Err msg

instance Monad Result where
    (Ok v)    >>= k = k v
    (Err msg) >>= k = Err msg
    return v = Ok v
    fail msg = Err msg


data Item = Item {
	id    :: Int,
	name  :: Maybe String
	} deriving (Show, Eq)

getId :: Maybe Int -> Result Int
getId  Nothing  = Err "No ID provided"
getId (Just id) = Ok id

getItem :: Int -> Result Item
getItem id
	| id `rem` 2 == 0  =  Ok  ( Item id  Nothing)
	| id `rem` 3 == 0  =  Ok  ( Item id (Just ("item-" ++ show id)))
	| otherwise        =  Err ("Item not found, ID = " ++ show id)

getName :: Item -> Result String
getName (Item id Nothing   ) = Err ("Item has no name, ID = " ++ show id)
getName (Item _ (Just name)) = Ok name

doSomethingClever :: Int -> String -> String
doSomethingClever id name = show id ++ " => " ++ name

tryToDoSomethingClever :: Maybe Int -> Result String
tryToDoSomethingClever idParam = do
	id   <- getId idParam
	item <- getItem id
	name <- getName item
	return (doSomethingClever id name)

demoInputs = [Nothing, Just 1, Just 2, Just 3]

main = mapM_ (print . tryToDoSomethingClever) demoInputs