In fact it turned out that the example code I posted did not exhibit the
memory leak at all. It just took a /very long time/ to complete
(compared to a Java version), but it did complete. My complete code,
which also counted the instances of a given number from the array, does
however exhibit the leak. It is here:
quick guess, and useful pattern-to-avoid: tail-recursive functions with
non-strict accumulators may be tail recursive, but they build up unevaluated
expressions representing the accumulations; when those are forced by
inspection, the evaluator descends non-tail-recursively into those possibly
deep accumulations (..(0+1)..+1), possibly resulting in stack overflows.
the worker in genSeries inspects its parameters at each call, keeping
them evaluated; the worker in countNumbers inspects only its first two
parameters, possibly (depending on optimizations) leaving acc
unevaluated. try: worker lo (i-1) $! acc
hth,
claus
module Main where
import Data.Array.IO
import System.Random
type Buffer = IOUArray Int Int
-- | Triangular Probability Density Function, equivalent to a roll of
two dice.
-- The number sums have different probabilities of surfacing.
tpdf :: (Int, Int) -> IO Int
tpdf (low, high) = do
first <- getStdRandom (randomR (low, high))
second <- getStdRandom (randomR (low, high))
return ((first + second) `div` 2)
-- | Fills an array with dither generated by the specified function.
genSeries :: Buffer -> ((Int, Int) -> IO Int) -> (Int, Int) -> IO ()
genSeries buf denfun lims =
let worker low i
| i >= low = do
r <- denfun lims
writeArray buf i r
worker low (i - 1)
| otherwise = return ()
in do
(lo, hi) <- getBounds buf
worker lo hi
countNumbers :: Buffer -> Int -> IO Int
countNumbers buf x =
let worker lo i acc
| i >= lo = do
n <- readArray buf i
if n == x
then worker lo (i - 1) (acc + 1)
else worker lo (i - 1) acc
| otherwise = return acc
in do
(lo, hi) <- getBounds buf
worker lo hi 0
main = do
buf <- newArray_ (0, 10000000) :: IO Buffer
genSeries buf tpdf (2, 12)
sevens <- countNumbers buf 7
putStrLn ("Magic number sevens: " ++ show sevens)
return 0
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