On 30/07/07, Lennart Augustsson
Why this obsession with IO? There should be no IO involved in this, except for getting the initial generator. Using IO just confuses what is going on.
Indeed. Here's my version: -- first define a shuffle function, completely pure! pick 1 (x:xs) = (x, xs) pick n (x:xs) = (y, x:ys) where (y, ys) = pick (n-1) xs shuffle gen xs = shuffle' gen xs (length xs) shuffle' _ [] _ = [] shuffle' gen xs n = p : shuffle' gen' xs' (n-1) where (r, gen') = randomR (1,n) gen (p, xs') = pick r xs -- a function for giving us an infinite list of generators from -- an initial generator gens g = unfoldr (Just . split) g -- shuffles the given list an infinite number of times -- with a different generator each time shuffles xs g = zipWith shuffle (gens g) (repeat xs) You can then pass in whatever generator you wish to shuffles. E.g. create a pure one with mkStdGen or create one in the IO monad with newStdGen. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862