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)
to:
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