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+-- Exercise 6.4-iv
+-- There is also a monad transformer version of `Cont`. Implement it.
+newtype ContT t a = ContT
+  { unContT :: forall r. (a -> t r) -> t r
+  }
+
+instance Functor (ContT t) where
+  fmap f (ContT a) = ContT $ \c -> a $ c . f
+
+instance Applicative (ContT t) where
+  pure a = ContT $ \c -> c a
+  ContT f <*> ContT a = ContT $ \c -> f $ \c' -> a $ c . c'
+
+instance Monad (ContT t) where
+  ContT a >>= f = ContT $ \c -> a $ \c' -> unContT (f c') c