Applicative OptionalT

optT.hs

instance Monad f => Applicative (OptionalT f) where
  pure ::
    a
    -> OptionalT f a
  pure =
    OptionalT . return . pure
  (<*>) ::
    OptionalT f (a -> b)
     -> OptionalT f a
     -> OptionalT f b
  (OptionalT mf) <*> (OptionalT ma) =
    OptionalT $ do
      optf <- mf
      case optf of
        Empty -> return Empty
        Full f -> do
          opta <- ma
          case opta of
            Empty -> return Empty
            Full a -> return $ Full (f a)