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fumieval/pos-monoid.hs

Created August 14, 2015 07:41
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pos-monoid.hs
data Pos a = Rel !a | Abs !a deriving (Show, Read, Eq, Ord, Functor)
instance Num a => Monoid (Pos a) where
mempty = Rel 0
_ `mappend` Abs a = Abs a
Abs a `mappend` Rel b = Abs (a + b)
Rel a `mappend` Rel b = Rel (a + b)
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