data Result a = Success a | Error String

instance (Show a) => Show (Result a) where
    show (Success a) = show a
    show (Error string) = string

instance (Eq a) => Eq (Result a) where
    (==) (Success a) (Success b) = (a==b)
    (==) (Error a) (Error b) = (a==b)
    (==) _ _ = False

instance Monad Result where
    return a = Success a
    (>>=) (Success a) f = f a
    (>>=) (Error a) _ = Error a

instance Functor Result where
    fmap = map'

instance Applicative Result where
    pure a = Success a
    (<*>) (Success f) (Success a) = Success (f a)
    (<*>) (Error a) (Error b) = Error (a++b)
    (<*>) (Error f) _ = Error f
    (<*>) _ (Error a) = Error a

map':: (a->b)->Result a->Result b
map' f (Success a) = Success (f a)