Hey There, I am trying to write a hash-algorithm that in fact is working, but as you might have guessed the problem is the performance :) At the moment I am 40 times worse than the same implementation in C. My problem is, I need mutable arrays which are the heart of that hash. The algorithm is round-based and within each round I need the value of certain positions in that array. At the moment I work with an unboxed ST-Array at the crucial part --- snip --- test list = runSTUArray $ do a_arr <- newListArray (0, 1752) list let round i = do ain <- readArray a_arr (i-n) at0 <- readArray a_arr (i-t0) at1 <- readArray a_arr (i-t1) at2 <- readArray a_arr (i-t2) at3 <- readArray a_arr (i-t3) at4 <- readArray a_arr (i-t4) let li = ls $ (i - n) mod 16 ri = rs $ (i - n) mod 16 writeArray a_arr i $ (step4 li) . (step3 ri) . (step2 at1 at2 at3 at4) $ (step1 ain at0 i) mapM_ round [n..(n+t-1)] return a_arr --- snap --- I also played around with peekElemOff and pokeElemOff, but this isn't much faster. It took ~30sec with my Haskell-Code while the C-implementation need only ~1sec. Is there someone who may have some hints for me, what I can do better, or just lead me in the right direction? :) Thanks a lot in advance -- Matthias