random order

What is an efficient way to randomize the order of something, for example characters in a string or places in an array, without repeating anything before you ran trough all characters/arrayplaces?

>>4
import Monad
import Random
import Data.Array

randomize array =
   foldM step array $ reverse [lo..hi]
   where step a i = fmap (swap a i) $ randomRIO (lo, i)
     swap a i j = a // [(j, a!i), (i, a!j)]
     (lo, hi) = bounds array