On Sun, Jan 9, 2011 at 10:11 PM, Lennart Augustsson
That looks like it looses the efficiency of the underlying representation.
Yes, I don't think one can retain that cleanly without using restricted monads to exclude things like liftM ($42) (mplus (return pred) (return succ)) or just liftM ($42) (return pred) Maybe one can hack something to achieve mplus :: Ord a => Set a -> Set a -> Set a mplus a b = Set (\k -> S.union (a >>- ret) (b >>- ret) `bind` k) where ret = S.singleton bind = flip Data.Foldable.foldMap mplus :: not (Ord a) => Set a -> Set a -> Set a mplus a b = Set (\k -> S.union (a >>- k) (b >>- k)) using overloading with undecidable instances (?) (and defining a Monoid instance for the Set monad in terms of the MonadPlus instance) Reminds me of instance chains.. [1] Sebastian [1]: http://portal.acm.org/citation.cfm?id=1863596