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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
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