Haskell Quiz/Text Munger/Solution Dolio

This solution makes use of the random monad elsewhere on the wiki.

module Main where
import Control.Monad
import Data.Char
import Data.List
import System.Random
import MonadRandom

permute [] = return []
permute l  = do r <- getRandomR (0, n)
                let (a, e:b) = splitAt r l
                l' <- permute (a ++ b)
                return $ e : l'
 where n = length l - 1

munge l
    | isAlpha h && length l > 2 = ((h:) . (++[f])) `liftM` permute m
    | otherwise                 = return l
 where h = head l
       f = last l
       m = tail (init l)

main = do g <- getStdGen
          interact $ flip evalRand g . liftM join . mapM munge . groupBy f
 where f a b = isAlpha a == isAlpha b