[Haskell-cafe] infinite list of random elements

Chad Scherrer chad.scherrer at gmail.com
Tue Jul 31 16:20:36 EDT 2007

Ok, that looks good, but what if I need some random values elsewhere
in the program? This doesn't return a new generator (and it can't
because you never get to the end of the list). Without using IO or ST,
you'd have to thread the parameter by hand or use the State monad,
right? This is where I was leaking space before.

Actually, this makes me wonder... I think what killed it before was
that the state was threaded lazily through the various (= very many)
calls. I suppose a State' monad, strict in the state, could help here.
I wonder how performance for this would compare with IO or ST. Might
have to try that sometime...


On 7/31/07, Lennart Augustsson <lennart at augustsson.net> wrote:
> No leak in sight.
>   -- Lennart
> import Random
> import Array
> randomElts :: RandomGen g => g -> [a] -> [a]
> randomElts _ [] = []
> randomElts g xs = map (a!) rs
>    where a = listArray (1, n) xs
>         rs = randomRs (1, n) g
>      n = length xs
> main = do
>     g <- getStdGen
>     let xs = randomElts g [10,2,42::Int]
>     print $ sum $ take 1000000 xs

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