{-# LANGUAGE ApplicativeDo #-}

f x y = undefined

data A a = A { runA :: a }
instance Functor A where
  fmap f (A a) = A (f a)
instance Applicative A where
  pure = A
  A f <*> A x = A (f x)

main = do
  x <- undefined
  y <- (runA $ do { z <- undefined; w <- undefined; return (f z w) })
  return y