Output random letters from [a-zA-Z] (haskell)

gistfile1.txt

-- Output random letters from [a-zA-Z].

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

main :: IO ()
main = do
  args <- getArgs
  -- Default to 15 chars if no arg is supplied.
  let length = case args of (n:_) -> read n
                            [] -> 15
  randomString <- mixedpw length
  putStrLn randomString

-- "If" as a function.  Apparently this does not exist elsewhere!
if' :: Bool -> a -> a -> a
if' True x _ = x
if' False _ y = y

-- Random password N digits long from the set [a-zA-Z].
mixedpw :: Int -> IO String
mixedpw n = do
  g <- newStdGen
  g' <- newStdGen
  g'' <- newStdGen
  let lowers = take n $ randomRs ('a', 'z') g
      uppers = take n $ randomRs ('A', 'Z') g'
      mix = take n $ randoms g''
  return $ zipWith3 if' mix lowers uppers

-- Lower case random password.
lowerpw :: Int -> IO String
lowerpw n = do
  g <- newStdGen
  return $ take n $ randomRs ('a', 'z') g