applicative bug

bug.hs

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ApplicativeDo #-}

module ApplicativeDont (foo) where

data M a
data N a
instance Functor M
instance Applicative M
instance Functor N
instance Applicative N
instance Monad N

newtype Alias = Alias (forall a. a -> M a)

foo :: N (M ())
foo = do
  pure ()
  Alias mt <- undefined
  pure ()
  pure $ (\c b -> ()) <$> mt 'a' <*> mt False