2_2_3_traversable_result.hs

{-
GHCi> traverse (\x->[x+2,x-2]) (Ok 5)
[Ok 7,Ok 3]
GHCi> traverse (\x->[x+2,x-2]) (Error "!!!")
[Error "!!!"]
-}

data Result a = Ok a | Error String deriving (Eq,Show)

instance Functor Result where
  fmap f (Ok x) = Ok $ f x
  fmap _ (Error s) = Error s

instance Foldable Result where
  foldr f ini (Ok x) = f x ini
  foldr _ ini _ = ini

instance Traversable Result where
  traverse f (Ok x) = Ok <$> f x
  traverse _ (Error s) = pure $ Error s

ex1 = traverse (\x->[x+2,x-2]) (Ok 5)
ex2 = traverse (\x->[x+2,x-2]) (Error "!!!")