[Haskell-cafe] Problem with System.Random.randoms

[Luke Palmer]

2008/12/7 S. Günther <h8spawn at googlemail.com>

> Hi,
>
> I have a small problem with System.Random.randoms. I need a rather
> large number of random numbers but the following program consumes a
> huge amount of memory. I terminated it when it used up more than 2 Gb:
>
> module Main where
>
> import System.Random
>
> n :: Int
> n = maxBound
>
> main = do
>  g <- getStdGen
>  print $ length $ take n $ ((randoms g)::[Int])


I think the problem is that the list spine is being forced, but not the
elements, so the generator is becoming a rather massive thunk.  This is
peculiar to your benchmark, and would probably not occur in practice when
you are actually using the random numbers.

Try:

strictTake 0 _ = []
strictTake n [] = []
strictTake n (x:xs) = x `seq` (x : strictTake (n-1) xs)

And use that instead of take.

Again, this strictTake is probably not necessary in your actual application,
it's just to fix your benchmark.

Luke


>
>
> On the other hand using
>  take n $ [1..]
> it runs in constant space.
> Am I doing something wrong? Or should I just abandon randoms and use
> the more primitive functions in System.Random?
>
> Thanks in advance
> S. Günther
>
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