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>