Hi, while still working on optimizing (naively programmed) primefactors i watched a very strange behavior of ghc. The last version below takes 2.34 minutes on my system for computing 2^61+1 = 3*768614,336404,564651. Importing Data.Char without anywhere using it reduces this time to 1.34 minute - a remarkable speed up. System is WindowsXP on 2.2GHZ Intel, 512MB Ram. I give the complete code here - hopefully all tabs are (4) blanks. Can this be reproduced ? I compile just with --make -O2. module Main where import IO import System.Exit import Data.Char main = do hSetBuffering stdin LineBuffering putStrLn "Number to decompose ?" s <- getLine if s == [] then exitWith ExitSuccess else do putStrLn (show$primefactors$read s) main data DivIter = DivIter {dividend :: Integer, divisor :: Integer, bound :: Integer, result :: [Integer]} intsqrt m = floor (sqrt $ fromInteger m) primefactors :: Integer -> [Integer] primefactors n | n<2 = [] | even n = o2 ++ (primefactors o1) | otherwise = if z/=1 then result res ++[z] else result res where res = divisions (DivIter {dividend = o1, divisor = 3, bound = intsqrt(o1), result = o2}) z = dividend res -- is 1 sometimes (o1,o2) = twosect (n,[]) twosect :: (Integer,[Integer]) -> (Integer,[Integer]) twosect m |odd (fst m) = m |even (fst m) = twosect (div (fst m) 2, snd m ++ [2]) found :: DivIter -> DivIter found x = x {dividend = xidiv, bound = intsqrt(xidiv), result = result x ++ [divisor x]} where xidiv = (dividend x) `div` (divisor x) d2 :: DivIter -> DivIter d2 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 2} |otherwise = found x d4 :: DivIter -> DivIter d4 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 4} |otherwise = found x d6 :: DivIter -> DivIter d6 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 6} |otherwise = found x divisions :: DivIter -> DivIter divisions y |or[divisor y == 3, divisor y == 5] = divisions (d2 y) |divisor y <= bound y = divisions (d6$d2$d6$d4$d2$d4$d2$d4 y) |otherwise = y