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diff --git a/Haskell-book/15/semigroup/src/Mem.hs b/Haskell-book/15/semigroup/src/Mem.hs
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+module Mem where
+
+newtype Mem s a =
+    Mem {
+        runMem :: s -> (a,s)
+    }
+
+instance Monoid a => Monoid (Mem s a) where
+    mempty = Mem $ \x -> (mempty, x)
+    mappend (Mem f1) (Mem f2) = Mem f
+        where f x = ((mappend (fst $ f1 x) (fst $ f2 x)), snd $ f2 $ snd $ f1 x)