Haskell Quiz/Text Munger/Solution Tel

{-# OPTIONS -fglasgow-exts #-}
 
module Main where
 
import System.Random
import Data.Ord (comparing)
import Data.List (sortBy)
import Control.Monad.State.Lazy
 
main :: IO ()
main = getStdGen >>= evalStateT loop
 
loop :: (MonadState StdGen m, MonadIO m) => m ()
loop = do l <- liftIO getLine
          munge <- mungeLine l
          liftIO $ putStrLn munge
          loop
 
mungeLine :: MonadState StdGen m => String -> m String
mungeLine = liftM unwords . mapM mungeWord . words
 
mungeWord :: MonadState StdGen m => String -> m String
mungeWord str = do (randPairs :: [(Int, Char)]) <- zipWithRand (,) mid
                   return (first ++ map snd (sortBy (comparing fst) randPairs) ++ last)
    where (first, mid, last) = splitEnds str
 
splitEnds :: Eq a => [a] -> ([a], [a], [a])
splitEnds (x:[]) = ([x], [], [])
splitEnds xs = ([head xs], (init . tail) xs, [last xs]) 
 
-- zip across a list, passing a (Random t => t) in
zipWithRand :: (Random a, MonadState StdGen m) => (a -> b -> c) -> [b] -> m [c]
zipWithRand fn xs = do rands <- mapM (const randomST) xs 
                       return $ zipWith fn rands xs
 
-- random promoted inside some MonadState for threading the StdGen
randomST :: (MonadState StdGen m, Random a) => m a
randomST = do gen <- get
              let (x, gen2) = random gen
              put gen2
              return x