[Haskell-cafe] Memory leak or wrong use of Array ?

L.Guo leaveye.guo at gmail.com
Fri Sep 14 21:35:14 EDT 2007

Hi Stuart.

Thanks for your advice about thunk, though I do not understand *thunk*
very well. Is there any other discriptions about thunk ?

I have tried the *seq* operation. When input is 10,000,000, the memory
still "leak", and there is still a "stack overflow".

I changed some mapM_ to sequence . map f, and tried to save some division.

The key functions now looks like this:

  proportions n = unsafePerformIO $ do
      arr <- newArray (2,n) (False,1/1) :: Fractional t => IO (IOArray Int (Bool,t))
      sequence_ $ map (sieve arr n) [2..n]
      factors <- getElems arr
      return . map (\(n,(b,f)) -> (f,n)) $ zip [2..n] factors
      sieve arr ubound p = do
          (b,o) <- readArray arr p
          if b then return () else
            sequence_ . map (update arr (toRational p)) . takeWhile (<=ubound) $ iterate (+p) p
      update arr p i = do
          (_,o) <- readArray arr i
          --writeArray arr i (True,o*(p-1)/p)
          let val = o * p / (p-1)
          val `seq` return () -- force the thunk
          writeArray arr i (True, val)
  solutionOf = snd . minimum
             . filter (\(f,n) -> isPerm (floor $ toRational n/f) n) . proportions


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