problem23.hs

gistfile1.hs

{-
 - Problem 23
 - Extract a given number of randomly selected elements from a list.
 - Example:
 - Prelude System.Random>rnd_select "abcdefgh" 3 >>= show
 - "eda"
 -}

pure_rnd_select :: [a] -> Int -> [a]
pure_rnd_select x y = unsafePerformIO $ rnd_select x y

rnd_select :: [a] -> Int -> IO [a]
rnd_select _  0 = return []
rnd_select [] _ = return []
rnd_select l n = do r <- randomRIO (0, (length l) - 1)
                    result <- (rnd_select ((take r l) ++ (drop (r+1) l)) (n - 1))
                    return ((l!!r) : result)

problem23 = test ["Extract given number of randomly selected elements from list"
                  ~: (length $ (pure_rnd_select "abcdefgh" 3))
                  ~=? 3 ]