ValidationT.hs

import Control.Applicative(Alternative(..), liftA2)

newtype ValidationT f a b =
  ValidationT (f (Either a b))

instance Functor f => Functor (ValidationT f a) where
  fmap f (ValidationT x) =
    ValidationT (fmap (fmap f) x)

instance Applicative f => Applicative (ValidationT f a) where
  pure =
    ValidationT . pure . pure
  ValidationT f <*> ValidationT x =
    ValidationT (liftA2 (<*>) f x)

-- Monad required: second effect will not run if first succeeds
instance (Alternative f, Monad f) => Alternative (ValidationT f a) where
  ValidationT a <|> ValidationT b =
    ValidationT (a >>= either (const b) (pure . pure))
  empty =
    ValidationT (Right <$> empty)