Monad Transformers 1

trans1.hs

import Control.Monad
import Control.Monad.Trans
import Control.Monad.Trans.Maybe

isValid :: String -> Bool
isValid [] = False
isValid (x:_) = x == 'a'

getPsw :: MaybeT IO String
getPsw = do s <- lift $ getLine
            guard (isValid s)
            return s

test = runMaybeT getPsw