[Haskell-cafe] infinite list of random elements

Chad Scherrer chad.scherrer at gmail.com
Mon Jul 30 20:01:17 EDT 2007

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

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?


> 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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