import Control.Monad
import Data.Char

getPassphrase :: IO (Maybe String)
getPassphrase = do s <- getLine
                 if isValid s then return $ Just s
                              else return Nothing

-- The validation test could be anything we want it to be.
isValid :: String -> Bool
isValid s = length s >= 8
            && any isAlpha s
            && any isNumber s
            && any isPunctuation s