[Haskell-cafe] Monad transformer, liftIO

[Creighton Hogg]

On Fri, Apr 3, 2009 at 11:49 AM, Michael Roth <mroth at nessie.de> wrote:
> 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].