One problem with such monad implementations is efficiency. Let's define
step :: (MonadPlus m) => Int -> m Int step i = choose [i, i + 1]
-- repeated application of step on 0: stepN :: (Monad m) => Int -> m (S.Set Int) stepN = runSet . f where f 0 = return 0 f n = f (n-1) >>= step
Then `stepN`'s time complexity is exponential in its argument. This is because `ContT` reorders the chain of computations to right-associative, which is correct, but changes the time complexity in this unfortunate way. If we used Set directly, constructing a left-associative chain, it produces the result immediately:
The example is excellent. And yet, the efficient genuine Set monad is possible. BTW, a simpler example to see the problem with the original CPS monad is to repeat choose [1,1] >> choose [1,1] >>choose [1,1] >> return 1 and observe exponential behavior. But your example is much more subtle. Enclosed is the efficient genuine Set monad. I wrote it in direct style (it seems to be faster, anyway). The key is to use the optimized choose function when we can. {-# LANGUAGE GADTs, TypeSynonymInstances, FlexibleInstances #-} module SetMonadOpt where import qualified Data.Set as S import Control.Monad data SetMonad a where SMOrd :: Ord a => S.Set a -> SetMonad a SMAny :: [a] -> SetMonad a instance Monad SetMonad where return x = SMAny [x] m >>= f = collect . map f $ toList m toList :: SetMonad a -> [a] toList (SMOrd x) = S.toList x toList (SMAny x) = x collect :: [SetMonad a] -> SetMonad a collect [] = SMAny [] collect [x] = x collect ((SMOrd x):t) = case collect t of SMOrd y -> SMOrd (S.union x y) SMAny y -> SMOrd (S.union x (S.fromList y)) collect ((SMAny x):t) = case collect t of SMOrd y -> SMOrd (S.union y (S.fromList x)) SMAny y -> SMAny (x ++ y) runSet :: Ord a => SetMonad a -> S.Set a runSet (SMOrd x) = x runSet (SMAny x) = S.fromList x instance MonadPlus SetMonad where mzero = SMAny [] mplus (SMAny x) (SMAny y) = SMAny (x ++ y) mplus (SMAny x) (SMOrd y) = SMOrd (S.union y (S.fromList x)) mplus (SMOrd x) (SMAny y) = SMOrd (S.union x (S.fromList y)) mplus (SMOrd x) (SMOrd y) = SMOrd (S.union x y) choose :: MonadPlus m => [a] -> m a choose = msum . map return test1 = runSet (do n1 <- choose [1..5] n2 <- choose [1..5] let n = n1 + n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] -- Values to choose from might be higher-order or actions test1' = runSet (do n1 <- choose . map return $ [1..5] n2 <- choose . map return $ [1..5] n <- liftM2 (+) n1 n2 guard $ n < 7 return n) -- fromList [2,3,4,5,6] test2 = runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return (i,j,k)) -- fromList [(3,4,5),(4,3,5),(6,8,10),(8,6,10)] test3 = runSet (do i <- choose [1..10] j <- choose [1..10] k <- choose [1..10] guard $ i*i + j*j == k * k return k) -- fromList [5,10] -- Test by Petr Pudlak -- First, general, unoptimal case step :: (MonadPlus m) => Int -> m Int step i = choose [i, i + 1] -- repeated application of step on 0: stepN :: Int -> S.Set Int stepN = runSet . f where f 0 = return 0 f n = f (n-1) >>= step -- it works, but clearly exponential {- *SetMonad> stepN 14 fromList [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14] (0.09 secs, 31465384 bytes) *SetMonad> stepN 15 fromList [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15] (0.18 secs, 62421208 bytes) *SetMonad> stepN 16 fromList [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] (0.35 secs, 124876704 bytes) -} -- And now the optimization chooseOrd :: Ord a => [a] -> SetMonad a chooseOrd x = SMOrd (S.fromList x) stepOpt :: Int -> SetMonad Int stepOpt i = chooseOrd [i, i + 1] -- repeated application of step on 0: stepNOpt :: Int -> S.Set Int stepNOpt = runSet . f where f 0 = return 0 f n = f (n-1) >>= stepOpt {- stepNOpt 14 fromList [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14] (0.00 secs, 515792 bytes) stepNOpt 15 fromList [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15] (0.00 secs, 515680 bytes) stepNOpt 16 fromList [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] (0.00 secs, 515656 bytes) stepNOpt 30 fromList [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30] (0.00 secs, 1068856 bytes) -}