Hi, I'm having trouble making use of MonadError within another Monad, in this case IO. I've blundered around for a while, trying various combinations of things, but I don't think I've fully cottoned-on to nesting of monads. Following is some code which does not compile, but hopefully shows you what my intentions are. I'd appreciate it if someone could show me how to use the nested monads in this situation. Thanks. module Test where import Control.Monad.Error import Data.Char import System.IO instance Error Int where noMsg = 1 strMsg = length f :: IO (Either String String) f = do n <- readLn if n == 2 then return $ throwError "I don't like strings with 2 characters." else do s <- mapErrs2 (g n) putStrLn s g :: Int -> IO (Either String String) g 0 = throwError "I won't do zero length strings." g 1 = do c <- getChar c' <- mapErrs (h c) return [c'] g n = do c <- getChar cs <- g (n-1) c' <- mapErrs (h c) return (c':cs) h :: Char -> Either Int Char h c | isUpper c = throwError 200 | otherwise = return $ toUpper c mapErrs :: Either Int Char -> Either String Char mapErrs (Right c) = Right c mapErrs (Left 200) = Left "The String contained uppercase characters." mapErrs (Left i) = Left ("Unrecognised failure in h, code = " ++ (show i)) mapErrs2 :: Either String String -> Either String String mapErrs2 (Right s) = Right s mapErrs2 (Left e) = Left ("g Error: " ++ e)