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)