a question about *** Exception: stack overflow ..
On Thu, Apr 23, 2009 at 7:02 AM, Mozhgan Kabiri
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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On Thu, Apr 23, 2009 at 9:06 PM, Luke Palmer
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]
:-)
Yeah, but this prelude sum function suffers from the same stack overflow thing (which is a design mistake I think): c:\>ghci GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> sum [0..10^6] *** Exception: stack overflow Prelude> So the easiest way to fix this is by defining a strict sum: import Data.List (foldl') sum' = foldl' (+) 0 Or if you prefer to see how this works internally: sum' xs = aux 0 xs where aux s [] = s aux s (x:xs) = let s' = x+s in s' `seq` aux s' xs An interesting thing I learned from this mailing list is that the stack overflow does not occur because the huge addition expression (0+1+2+3+4+5+6+7+...) is build up in memory, but because the evaluation of this gigantic expression *after* it has been completely build up in memory. One can demonstrate that the addition expression is first build up in memory like this: c:\>ghci GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> sum [0..10^100] <interactive>: out of memory
participants (2)
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Luke Palmer -
Peter Verswyvelen