# Pastebin 1sjEEDOV
data Validation e a = Error [e] | OK a deriving Functor

instance Applicative (Validation e) where
  pure = OK
  Error e1 <*> Error e2 = Error (e1 <> e2)
  Error e1 <*> OK _ = Error e1
  OK _ <*> Error e2 = Error e2
  OK f <*> OK x = OK (f x)