A Haskell noob trying to create sorta-kinda random passwords

passgen.hs

import Control.Applicative
import System.Environment
import System.Exit
import System.Random

main = getArgs >>= parseArgs

-- Exit with success.
exit :: IO a
exit = exitWith ExitSuccess

-- Generate an x long password. The initial s String is appended to the
-- password.
generatePassword :: Int -> IO String -> IO String
generatePassword x s
  | x < 1 = s
  | otherwise = generatePassword (x -1) (liftA2 (:) randChar s)

-- Print the help text.
help :: String
help = "Usage: passgen OPTION\n" ++
       "  -l x outputs a random password of x length\n" ++
       "  -h this help text\n" ++
       "  -v version of the program"

-- Parse the commandline arguments and return one of:
--   help
--   version
--   actual program output
parseArgs :: [String] -> IO ()
parseArgs ("-l":n:[])   = generatePassword (read n :: Int) (return "") >>= putStrLn >> exit
parseArgs []            = putStrLn help >> exit
parseArgs ["-h"]        = putStrLn help >> exit
parseArgs ["--help"]    = putStrLn help >> exit
parseArgs ["-v"]        = putStrLn version >> exit
parseArgs ["--version"] = putStrLn version >> exit
parseArgs (a:as)        = putStrLn help >> exit

-- Yields a random char from the validChars list.
randChar :: IO Char
randChar = randNum >>= (\i -> return (validChars !! i))

-- Yields a random integer in the 0 .. length validChars range.
randNum :: IO Int
randNum = getStdRandom (randomR (0, (length validChars) - 1))

-- A list of valid characters
validChars :: String
validChars = ['a'..'z'] ++ ['0'..'9'] ++ ['A'..'Z']

-- Print the version
version :: String
version = "0.0.1"