On Thu, Apr 23, 2009 at 7:02 AM, Mozhgan Kabiri <mozhgan_kch@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

" Upgrade to Internet Explorer 8 Optimised for MSN. " Download Now