[Haskell-cafe] a question about *** Exception: stack overflow ..
Luke Palmer
lrpalmer at gmail.com
Thu Apr 23 15:06:18 EDT 2009
On Thu, Apr 23, 2009 at 7:02 AM, Mozhgan Kabiri <mozhgan_kch at hotmail.com>wrote:
> Hi Luck ,
>
> I got you email from the Haskell Cafe list. Hope you don't mind.
> Recently I was running a simple program in Haskell and keep getting ***
> Exception: stack overflow error !
>
> I don't know how to solve it or handle it ! I tried to find the same
> problem on the web in order to get some guide but it wasn't successful.Hope
> you can give me some clue.
>
In the future, why don't you ask this kind of question on haskell-cafe or
haskell-beginners?
>
>
> Here is the code :
>
> import Control.Monad
> import Text.Printf
>
> sumit :: Int -> Int
> sumit n = sumCal 0 n 0 where
> sumCal i n sum | i < n = sumCal (i+1) n (sum+i)
> | otherwise = sum
>
This is probably your problem. If you do tail recursion, it needs to be
strict. If you don't strictly tail recurse, you end up returning a massive
thunk (i.e. a lazy expression like 0+1+2+3+4+5+6+7+8+9+10+11+12+...) which
needs to be evaluated all in one go, which will overflow your stack. It is
a little tricky, I know...
sumCal i n sum | i < n = sumCal (i+1) n $! sum+i
| otherwise = sum
However, there is a function "sum" in the prelude, so you can do this much
more simply:
sumit :: Int -> Int
sumit n = sum [1..n]
:-)
Luke
>
> loopOut :: Int -> IO()
> loopOut n | n <= 1000000
> = do
> loopIn 0 (1000000000*n) n 0
> loopOut (n*10)
> | otherwise = printf "Done!"
>
>
> loopIn :: Int -> Int -> Int -> Int -> IO()
> loopIn i ub n sum | i < ub
> = do
> loopIn (i+1) ub n (sumit n)
> printf "n=%10d sum=%15d" n sum
> | otherwise = printf "\n"
>
> main :: IO ()
> main
> =do
> printf "Started .."
> loopOut 1000
>
> Actually the next step it to parallelize the code with STM.But even at this
> stage it doesn't work ! So, if you can help me with this,I'd be grateful.
>
> Thanks,
> Mozhgan
>
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