Talk:99 questions/Solutions/23

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The more elegant solution using randomRs does not save the new random state and is thus dangerous to use if randomness is important.

some notes

the signature of removeAt is:

   removeAt :: Int -> [a] -> (a, [a])

so we have to swap the orders of parameters in the calls of removeAt from:

   removeAt l (k+1)


   removeAt (k+1) l

and we have to extract the remaining list from the result as well:

   snd $ removeAt (k+1) l

so the full version should be:

   rnd_select2 :: RandomGen g => [a] -> Int -> g -> ([a], g)
   rnd_select2 _ 0 gen = ([], gen)
   rnd_select2 [] _ gen = ([], gen)
   rnd_select2 l count gen
      | count == (length l) = (l, gen)
      | otherwise           =  rnd_select2 (snd $ removeAt (k+1) l) count gen'
                           where (k, gen') =
                                   randomR (0, (length l) - 1) gen
   rnd_selectIO :: [a] -> Int -> IO [a]
   rnd_selectIO l count = getStdRandom $ rnd_select2 l count