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+-- Exercise 3-i
+-- Which of these types are `Functor`s? Give instances for the ones that are.
+
+-- `T1` is a `Functor`:
+newtype T1 a
+  = T1 (Int -> a)
+
+instance Functor T1 where
+  fmap f (T1 g) = T1 $ f <$> g
+
+newtype T2 a
+  = T2 (a -> Int)
+
+newtype T3 a
+  = T3 (a -> a)
+
+newtype T4 a
+  = T4 ((Int -> a) -> Int)
+
+-- `T5` is a `Functor`:
+newtype T5 a
+  = T5 ((a -> Int) -> Int)
+
+instance Functor T5 where
+  fmap f (T5 g) = T5 $ \f' -> g $ f' . f