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
On 7/31/07, Chad Scherrer
Thanks for your responses.
Stefan, I appreciate your taking a step back for me (hard to judge what level of understanding someone is coming from), but the example you gave doesn't contradict my intuition either. I don't consider the output [IO a] a "list of tainted a's", but, as you suggest, a "list of IO actions, each returning an a". I couldn't return an IO [a], since that would force evaluation of an infinite list of random values, so I was using [IO a] as an intermediary, assuming I'd be putting it through something like (sequence . take n) rather than sequence alone. Unfortunately, I can't use your idea of just selecting one, because I don't have any way of knowing in advance how many values I'll need (in my case, that depends on the results of several layers of Map.lookup). Also, I'm using GHC 6.6, so maybe there have been recent fixes that would now allow my idea to work.
Cale, that's interesting. I wouldn't have thought this kind of laziness would work in this context.
Lennart, I prefer the purely functional approach as well, but I've been bitten several times by laziness causing space leaks in this context. I'm on a bit of a time crunch for this, so I avoided the risk.
Sebastian, this seems like a nice abstraction to me, but I don't think it's the same thing statistically. If I'm reading it right, this gives a concatenation of an infinite number of random shuffles of a sequence, rather than sampling with replacement for each value. So shuffles [1,2] g would never return [1,1,...], right?
Chad
I was thinking the best way to do this might be to first write this function:
randomElts :: [a] -> [IO a] randomElts [] = [] randomElts [x] = repeat (return x) randomElts xs = repeat r where bds = (1, length xs) xArr = listArray bds xs r = do i <- randomRIO bds return (xArr ! i)
Then I should be able to do this in ghci:
sequence . take 5 $ randomElts [1,2,3] [*** Exception: stack overflow
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