This pattern is totally embedded into the non-monadic applicative Either sometimes called Validation data Validation e a = Collect e | Value

This pattern is totally embedded into the non-monadic applicative Either sometimes called Validation data Validation e a = Collect e | Value a deriving Functor instance Monoid e => Applicative (Validation e) where pure a = Value a Collect le <*> Collect re = Collect (le <> re) Collect e <*> _ = Collect e _ <*> Collect e = Collect e Value f <*> Value a = Value (f a) There is no monad instance for this which lets us scoot forward through a computation and and all of the errors at once.