I'm dipping a toe in the shallow end of the monad pool. I would appreciate any feedback on the first function here, "mutateList". It seems to be working fine, but I have this feeling that I'm doing some unnecessary mucking about with state and I don't know how to fix it. Since both randomListSelection and mutateGene update the state, it seems unnecessary for mutateList to get and put the state as well. What do ye think? I have a list of 16-bit unsigned words. I want to randomly select an element in the list. In the selected element, I want to flip a bit at random. ---------- The code ---------- {-# LANGUAGE PackageImports #-} import "mtl" Control.Monad.State import System.Random import Data.Word import Data.Bits -- | ***** LOOK HERE ***** -- | Flip a random bit in a random element in the list. mutateList :: [Word16] -> State StdGen [Word16] mutateList xs = do g <- get let ((i, x), g') = runState (randomListSelection xs) g let (x', g'') = runState (mutateGene x) g' put g'' return (replaceElement xs i x') -- | ***** LOOK HERE ***** -- | Choose an element at random from a list and return the element and its index randomListSelection :: [a] -> State StdGen (Int, a) randomListSelection xs = do g <- get let s = length xs let (i, g') = randomR (0,s-1) g put g' return (i, xs !! i) -- | Randomly flip a bit in this gene. mutateGene :: Word16 -> State StdGen Word16 mutateGene x = do g <- get let (i, g') = randomR (0,16) g put g' return (x `complementBit` i) -- | Replace an element in a list with a new element. replaceElement -- | The list :: [a] -- | Index of the element to replace. -> Int -- | The new element. -> a -- | The updated list. -> [a] replaceElement xs i x = fore ++ (x : aft) where fore = take i xs aft = drop (i+1) xs