Hello, a friend and I were recently experimenting with mutable arrays. We tried to implement a simple dot product on STUArrays using a 'while' loop. Unfortunately, every implementation we produced caused a stack overflow. Switching to other implementations of 'while' or to IOUArrays did not help us.
We were using ghc-6.4.1 on Linux x86, with gcc 3.3.6. It runs perfectly, and is actually quite fast, when we increase the stack space. :) > import Control.Monad.ST > import Data.STRef > import Data.Array.ST > import Control.Monad.Fix > import Control.Monad > > while :: STRef s Bool -> ST s () -> ST s () > while b c = readSTRef b >>= \v -> when v (c >> while b c) > > dot :: STUArray s Int Double -> STUArray s Int Double -> ST s Double > dot x y = do > let (l,r) = bounds x > a <- newSTRef 0.0 > e <- newSTRef l > b <- newSTRef True > while b (do > ev <- readSTRef e > av <- readSTRef a > xe <- readArray x ev > ye <- readArray y ev > writeSTRef b (ev<r) > writeSTRef e (ev+1) > writeSTRef a (av+xe*ye)) > readSTRef a > > main = do > let d = runST (do > x <- newArray (1, 1000000) 1.0 > y <- newArray (1, 1000000) 2.0 > dot x y) > putStrLn $ show d Unfortunately, I did not keep every 'while' implementation we tried, just this one: > while b c = fix (\d -> readSTRef b >>= > (\v -> if v then c >> d else return ())) We also changed the 'dot' code to use a 'for' loop instead, using IOUArrays in this case: > for :: Int -> IO () -> IO () > for 0 c = return () > for n c = c >> for (n-1) c Needless to say, it did not help. Any hints why the stack overflows would be greatly appreciated. Thanks, Gunnar. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
