On 16/04/2010, at 18:06, Mathieu Boespflug wrote:
shortestPath :: [(Int, Int, Int)] -> UArray Int Int shortestPath g = runSTUArray $ do let mnew = newArray (0, SIZE * SIZE) 1 mread arr i j = unsafeRead arr (i * SIZE + j) mwrite arr i j x = unsafeWrite arr (i * SIZE + j) x unsafeIOToST $ hSetBuffering stdout LineBuffering unsafeIOToST $ putStrLn "Allocating ..." pm <- mnew unsafeIOToST $ putStrLn "Allocating ... done" let loop1 SIZE = return () loop1 k = let loop2 SIZE = return () loop2 i = let loop3 SIZE = return () loop3 j = do xij <- mread pm i j xik <- mread pm i k xkj <- mread pm k j mwrite pm i j (min xij (xik + xkj)) loop3 (j + 1) in loop3 0 >> loop2 (i + 1) in loop2 0 >> loop1 (k + 1) loop1 0 return pm
In general, GHC doesn't like nested loops. You might want to try the following structure: loop1 SIZE = return () loop1 k = loop2 k 0 loop2 k SIZE = loop1 (k+1) loop2 k i = loop3 k i 0 loop3 k i SIZE = loop2 k (i+1) loop3 k i j = do ... loop3 k i (j+1) And, as Max suggested, the llvm backend ought to improve things. Roman