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amiller/gist:3485737

Created August 27, 2012 05:00
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gistfile1.hs
data Trunk m x = Tip | Bin (m x) (m x)
instance (Monad m) => Functor (Trunk m) where
fmap f (Bin x y) = Bin (x >>= return . f) (y >>= return . f)
fmap _ Tip = Tip
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