[Haskell-beginners] calling polymorphic function in Selenium question

[MH]

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.
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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

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