Hi! I'm learning Haskell, and now I'm trying to make framework for solving searching problems, such as Knight's Tour. For small boards it answers instantly. For 7x8 board - 23 seconds. For 8x8 board - more than 30 minutes (it hasn't finished yet). Where is the root of the evil? --program module Main where import Data.List import Data.Array.Unboxed import qualified Data.Array.IArray as IArr import Data.Ix data SResult = Good | GoodProcess | Process | Bad data SDPSearch a p r = SDPSearch (a -> p -> [a]) --expand (p -> p) --update (a -> p -> SResult) --sort ([a] -> r) --result runSDPSearch :: SDPSearch a c b -> [a] -> c -> b runSDPSearch (SDPSearch e u s r) list p = r (rec list params) where params = iterate u p rec [] _ = [] rec (l:lp) pr@(n:np) = case s l n of Good -> l : rec lp pr GoodProcess -> l : (rec (e l n) np) ++ (rec lp pr) Process -> (rec (e l n) np) ++ (rec lp pr) Bad -> rec lp pr main = do (a, b) <- (break (== ' ')) `fmap` getLine print (knightTour (read a) (read b)) knightTour :: Int -> Int -> UArray (Int, Int) Int knightTour a b = runSDPSearch (SDPSearch e u s r) [((1, 1), sArray)] 2 where size = a * b range = ((1, 1), (a, b)) sArray = listArray range (1 : (replicate (size - 1) 0)) allTurns :: Array (Int, Int) [(Int, Int)] allTurns = IArr.listArray range [turns x y | x <- [1..a], y <- [1..b]] where shifts = [(1, 2),(1, -2),(2, 1),(2, -1),(-1, 2),(-1, -2),(-2, 1),(-2, -1)] turns x y = [(x+i, y+j) | (i, j) <- shifts, inRange range (x+i, y+j)] e ((x, y), arr) p = [(t, arr // [(t, p)]) | t <- changes] where changes = [t | t <- allTurns ! (x, y), arr ! t == 0] s el p | p == size = Good | otherwise = Process u = (+ 1) r l | not (null l) = snd (head l) | otherwise = error "No solutions!"