Page475

Page475_trimmed.hs

module Page475 where

-- Ex 5
either' :: (a -> c) -> (b -> c) -> Either a b -> c
either' f g eab =
  case eab of
    Left a  -> f a
    Right b -> g b

-- Ex 6
rightVal :: Either a b -> b
rightVal (Right x) = x

eitherMaybe'' :: (b -> c) -> Either a b -> Maybe c
eitherMaybe'' f eab =
  case eab of
    Left _ -> Nothing
    _      -> Just (either' (f . g (rightVal eab)) f eab)
    where
      g :: b -> a -> b
      g b a = b