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diff --git a/Haskell-book/23/Moi.hs b/Haskell-book/23/Moi.hs
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+module Moi where
+
+newtype Moi s a = Moi { runMoi :: s -> (a, s) }
+
+instance Functor (Moi s) where
+    -- fmap :: (a -> b) -> Moi s a -> Moi s b
+    fmap f (Moi g) = Moi (\x -> h $ g x)
+        where h (a, b) = (f a, b)
+
+instance Applicative (Moi s) where
+    -- pure :: a -> Moi s a
+    pure a = Moi (\s -> (a, s))
+    -- (<*>) :: Moi s (a -> b) -> Moi s a -> Moi s b
+    (Moi f) <*> (Moi g) = Moi (\s -> ((fst (f s)) (fst (g s)), s))
+
+instance Monad (Moi s) where
+    return = pure
+    -- (>>=) :: Moi s a -> (a -> Moi s b) -> Moi s b
+    (Moi f) >>= g = Moi (\s -> (runMoi (g (fst (f s)))) (snd (f s)))
+
+get :: Moi s s
+get = Moi $ \a -> (a, a)
+
+put :: s -> Moi s ()
+put s = Moi $ \_ -> ((), s)
+
+exec :: Moi s a -> s -> s
+exec (Moi sa) s = snd (sa s)
+
+eval :: Moi s a -> s -> a
+eval (Moi sa) s = fst (sa s)
+
+modify :: (s -> s) -> Moi s ()
+modify f = Moi $ \a -> ((), f a)