23.hs

import Control.Monad.State.Lazy
import System.Random
import RemoveAt

choose = do
  (gen, xs) <- get
  let (idx, gen') = randomR (1, length xs) gen
  let (val, xs') = removeAt idx xs
  put (gen', xs')
  return val

chooseN gen 0 _ = (gen, [])
chooseN gen n xs =
  let (result, (gen', xs')) = runState choose (gen, xs)
      (gen'', rest) = chooseN gen' (n - 1) xs'
  in (gen'', result:rest)

rnd_select :: [a] -> Int -> IO [a]
rnd_select xs n = do
  gen <- getStdGen
  let (gen', choice) = (chooseN gen n xs)
  setStdGen gen'
  return choice