Haskell Quiz/Text Munger/Solution Tel

A somewhat naive solution using State transformers to pass StdGen through the process.

{-# 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 :: StateT StdGen IO ()
loop = do l <- lift getLine
          gen <- get
          let (munge, gen2) = runState (mungeLine l) gen
          put gen2
          lift $ putStrLn munge
          loop 
          
mungeLine :: String -> State StdGen String
mungeLine = liftM unwords . mapM mungeWord . words

mungeWord :: String -> State StdGen String
mungeWord str = do gen <- get
                   let (randPairs :: [(Int, Char)], gen2) = runState (zipWithRand (,) mid) gen
                   put gen2
                   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) => (a -> b -> c) -> [b] -> State StdGen [c]
zipWithRand fn xs = do rands <- sequence $ map (const randomST) xs 
                       return $ zipWith fn rands xs

-- random promoted inside the State monad for threading the StdGen
randomST :: Random a => State StdGen a
randomST = do gen <- get
              let (x, gen2) = random gen
              put gen2
              return x