module Foo where

import Monad
import System.IO.Unsafe (unsafePerformIO)
import Data.Array.MArray
import Data.Array.IO

import MyST

hist f xs = do
    arr <- f $ newArray ('a','z') 0
    flip mapM_ xs $ \x -> do
        n <- readArray arr x
        writeArray arr x (n+1)
    filter ((/=0) . snd) `liftM` getAssocs arr

idSTArray :: ST s (STArray s i e) -> ST s (STArray s i e)
idSTArray = id

idIOArray :: IO (IOArray i e) -> IO (IOArray i e)
idIOArray = id

histST, histIO :: [Char] -> [(Char, Int)]
histST xs = runST           (hist idSTArray xs)
histIO xs = unsafePerformIO (hist idIOArray xs)