[Haskell-cafe] infinite list of random elements
sebastian.sylvan at gmail.com
Mon Jul 30 18:15:11 EDT 2007
On 30/07/07, Lennart Augustsson <lennart at augustsson.net> wrote:
> 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)
(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
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