[Haskell-cafe] Performance problem with random numbers
ntupel at googlemail.com
Sat Oct 13 14:59:15 EDT 2007
On Sat, 2007-10-13 at 13:35 -0400, Brandon S. Allbery KF8NH wrote:
> For starters, look into "seq". Try applying it to any expression
> using a generated random number. This should force evaluation to
> occur somewhere other than when random is trying to figure out what
> StdGen value it's been told to use as its initial state.
Ok, but I still wonder where that might be. random and randomR are used
in a function named "next" as show here:
next :: (Array Int a, Array Int a, UArray Int Double) -> StdGen -> (a,
next (xs, as, rs) g =
let n = length $ indices xs
(x1, g1) = randomR (0, n - 1) g
(x2, g2) = random g1
r = rs!x1
if x2 <= r
then (xs!x1, g2)
else (as!x1, g2)
x1 and x2 are used in the same function so I assume this already
requires their evaluation. The only function that calls "next" is
randomList :: (Array Int a, Array Int a, UArray Int Double) -> StdGen ->
randomList t g =
let (n, g') = next t g
n:randomList t g'
Cf. my original e-mail for the complete program.
> Alternately you can put all the uses in IO and use
> Control.Exception.evaluate (or even print). This is probably not
> what you want to do in your actual production code, however.
Right. This is not what I want.
Many thanks again,
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