I lifted the code below from here: http://en.wikibooks.org/wiki/Haskell/Monad_transformers Since the wiki page doesn't say what needs to be imported, I'm guessing. Not sure what is happening. Maybe someone can tell me. Michael ============== import Control.Monad import Control.Monad.Trans.Class import Data.Char newtype (Monad m) => MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) } instance Monad m => Monad (MaybeT m) where return = MaybeT . return . Just x >>= f = MaybeT $ do maybe_value <- runMaybeT x case maybe_value of Nothing -> return Nothing Just value -> runMaybeT $ f value instance Monad m => MonadPlus (MaybeT m) where mzero = MaybeT $ return Nothing mplus x y = MaybeT $ do maybe_value <- runMaybeT x case maybe_value of Nothing -> runMaybeT y Just value -> runMaybeT x instance MonadTrans MaybeT where lift = MaybeT . (liftM Just) instance Show (MaybeT m a) getValidPassword :: MaybeT IO String getValidPassword = do s <- lift getLine guard (isValid s) return s isValid :: String -> Bool isValid s = (length s > 8) && ((filter isAlphaNum s) == s) && any isDigit s && any isAlpha s askPassword :: MaybeT IO () askPassword = do lift $ putStrLn "Insert your new password:" value <- getValidPassword lift $ putStrLn "Storing in database..." ============= [michael@localhost ~]$ ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :l test5 [1 of 1] Compiling Main ( test5.hs, interpreted ) Ok, modules loaded: Main. *Main> askPassword Loading package transformers-0.2.2.0 ... linking ... done. *** Exception: stack overflow *Main>