[Haskell-cafe] Re: Generating a random list
mhasan at cs.cornell.edu
Sat Mar 1 23:06:51 EST 2008
OK, you convinced me that sort is not the problem. After all, "last (f
1000000)" overflows too, and last is a very innocent function.
I don't know how you found the size (or structure) of the thunks (I'm
not aware of a ghci functionality that can tell me that), could you let
Anyway, the problem seems to be that the values pulled from the list are
not final float values, but instead they're thunks that grow larger and
larger as more floats are pulled from the list. That's why last and sort
don't work, but print and foldl' do work. Also, an odd number of
reverses helps since sort evaluates the elements from the end of the
So I forced those values, and it fixed the problem:
-- this function returns n random floats (no thunks)
f :: Int -> [Float]
f n = take n $ randFloats $ mkStdGen 0 where
randFloats g = x `seq` x : randFloats g' where (x, g') = random g
main = print $ foldl1' (+) $ sort $ f 1000000
This works without overflow, and so does "last (f 1000000)".
Would it make sense for System.Random to do this forcing by default, too?
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