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"

Constructive comments from #haskell IRC channel:

ThomasLocke: you can shorten "randNum >>= (\i -> return (validChars !! i))" to "fmap (validChars !!) randNum"
ThomasLocke: generatePassword could be a pure function which takes a random number generator instead.
ThomasLocke: "take 10 . filter notValidChar . randoms $ mkStdGen 1"
ThomasLocke: you could put "parseArgs _ = putStrLn help >> exit" as the last clause, and remove the other ones that print the help message
ThomasLocke: That's not entirely secure since filtering invalid characters might introduce some weird bias in the generated passwords
ThomasLocke: assuming >>= and fmap are implemented correctly (which they are for IO), "x >>= \y -> return (f y)" can always be reduced to "fmap f x"
:t state random
(MonadState s m, RandomGen s, Random a) => m a
aweinstock: Why use "state random" instead of randoms?
:t randoms
(RandomGen g, Random a) => g -> [a]
randoms just produces an infinite list
:t \size -> replicateM size (state random)
(MonadState s m, RandomGen s, Random a) => Int -> m [a]
:t randoms
(RandomGen g, Random a) => g -> [a]
:t randoms (mkStdGen 1) :: String
> randoms (mkStdGen 1) :: String
"\39335\14070\DC1\959535\514122\923211\263434\59659\367628\671400\744213\298...
> filter isAlpha $ randoms (mkStdGen 1)
"\39335\14070\70334\138983\37111\27202\139598\25181\165723\29657\11327\31112...
dot_Laptop: Don't use read :\