I am running the following code that is using Selenium. If you look at the function "start", you will see that the return type of the function is polymorphic. Now in main function, I call start function to get selenium IO monad and sequentially call selenium commands (open, doCommand etc...). The problem that I have here is, while I can call all Selenium commands with signature (String -> Selenium String), I can't call commands with signature (String -> Selenium Bool). As I understand it, even though "start" function shall return IO (Selenium a -> IO (Either String a)), it actually return IO (Selenium String -> IO (Either String String)). How shall go about fixing this problem? I need to be able to call both types of Selenium commands 1. doCommand :: SCommand -> [String] -> Selenium String OR selectFrame :: String -> Selenium String AND 2. isTextPresent :: String -> Selenium Bool Thanks. ========================================================= module SeleniumTest where import Control.Monad.Error import Data.Maybe import Network.BSD import Network.URI import Test.Selenium.Server import Test.Selenium.Syntax infixr 0 $$ -- | Starts up a session and returns a wrapper function that will run -- commands. Gives common defaults for browser and host. start :: String -> IO (Selenium a -> IO (Either String a)) start url = do -- host <- getHostName start' Firefox "localhost" url -- | Starts up a session and returns a wrapper function that will run -- commands. start' :: Browser -> HostName -> String -> IO (Selenium a -> IO (Either String a)) start' browser host url = do let uri = fromJust (parseURI url) sel = mkSeleniumRCSession host browser uri result <- runSelenium sel startSelenium return $ runSelenium (either (\msg -> error msg) id result) ($$) :: Show t => (Selenium () -> r) -> Selenium t -> r ($$) s c = s $ do r <- c; liftIO (putStrLn $ "Result: " ++ show r); return () -- | Stops a session (in the wrapper returned by start) stop :: Selenium () stop = stopSelenium --main::IO() main = do selenium <- start "http://www.google.com" selenium $ open "/" selenium $ doCommand SWindowMaximize [] selenium $ typeText (Name "q") "stuff" selenium $ clickAndWait (Name "btnG") return selenium ===============================================================