On Fri, Apr 3, 2009 at 11:49 AM, Michael Roth
Hello list,
maybe I'm just stupid, I'm trying to do something like this:
import Control.Monad import Control.Monad.Trans import Control.Monad.List
foobar = do a <- [1,2,3] b <- [4,5,6] liftIO $ putStrLn $ (show a) ++ " " ++ (show b) return (a+b)
main = do sums <- foobar print sums
But this apparently doesn't work... I'm total clueless how to achieve the correct solution. Maybe my mental image on the monad transformer thing is totally wrong?
Okay, so I think what you want is import Control.Monad import Control.Monad.Trans import Control.Monad.List foobar :: ListT IO Int foobar = do a <- msum . map return $ [1,2,3] b <- msum . map return $ [4,5,6] liftIO $ putStrLn $ (show a) ++ " " ++ (show b) return (a+b) main = do sums <- runListT foobar print sums There were a couple of things going on here: first, that you tried to use literal list syntax in do notation which I believe only works in the actual [] monad. Second, you didn't have the runListT acting on the foobar, which is how you go from a ListT IO Int to a IO [Int].