I copy the Quick Hull snippet from http://darcs.haskell.org/packages/dph/examples/quickhull/QuickHull.hs The "packed" expression in "hsplitList" is much confusing to me. Why does it have to zip points with cross? Is [ p | (c, p) <- cross, c > 0.0 ] not enough? Thanks. data Point = Point !Double !Double data Line = Line Point Point instance Show Point where show (Point x y) = show (x, y) distance :: Point -> Line -> Double distance (Point xo yo) (Line (Point x1 y1) (Point x2 y2)) = (x1 - xo) * (y2 - yo) - (y1 - yo) * (x2 - xo) upper :: (a -> a -> Bool) -> [(a, b)] -> b upper above = snd . foldl1 pick where pick left@(kl, _) right@(kr, _) | kl `above` kr = left | otherwise = right hsplitList :: [Point] -> Line -> [Point] hsplitList points line@(Line p1 p2) | length packed < 2 = p1:packed | otherwise = hsplitList packed (Line p1 pm) ++ hsplitList packed (Line pm p2) where cross = [ (distance p line, p) | p <- points ] packed = [ p | (p, (c, _)) <- zip points cross, c > 0.0 ] pm = upper (>) cross quickHullList :: [Point] -> [Point] quickHullList [] = [] quickHullList points = hsplitList points (Line minx maxx) ++ hsplitList points (Line maxx minx) where xs = [ (x, p) | p@(Point x y) <- points ] minx = upper (<) xs maxx = upper (>) xs Best regards, Zhi-Qiang Lei zhiqiang.lei@gmail.com