Hi, I am playing around with the State monad and queues. At the moment I have the following code: {-# LANGUAGE ViewPatterns, FlexibleContexts #-} module Main where import Criterion.Main import Control.Monad.State.Lazy import Data.Maybe (fromJust) import Data.Sequence ((<|), ViewR ((:>))) import qualified Data.Sequence as S -------------------------------------------------------- data Queue a = Queue { enqueue :: [a], dequeue :: [a] } deriving (Eq, Show) -- adds an item push :: a -> Queue a -> Queue a push a q = Queue (a:enqueue q) (dequeue q) pop :: Queue a -> Maybe (a, Queue a) pop q = if null (dequeue q) then go $ Queue [] (reverse (enqueue q)) else go q where go (Queue _ []) = Nothing go (Queue en (x:de)) = Just (x, Queue en de) queueTst :: Int -> Queue Int -> Queue Int queueTst 0 q = q queueTst n q | even n = queueTst (n - 1) (push (100 + n) q) | otherwise = queueTst (n - 1) (if popped == Nothing then q else snd (fromJust popped)) where popped = pop q ------------------------------------------------------------- pushS :: a -> S.Seq a -> S.Seq a pushS a s = a <| s pushS' :: a -> State (S.Seq a) (Maybe a) pushS' a = do s <- get put (a <| s) return Nothing pushS'' :: a -> State (S.Seq a) (Maybe a) pushS'' a = get >>= (\g -> put (a <| g)) >> return Nothing popS :: S.Seq a -> Maybe (a, S.Seq a) popS (S.viewr -> S.EmptyR) = Nothing popS (S.viewr -> s:>r) = Just (r,s) popS' :: State (S.Seq a) (Maybe a) popS' = do se <- get let sl = popS'' se put $ snd sl return $ fst sl where popS'' (S.viewr -> S.EmptyR) = (Nothing, S.empty) popS'' (S.viewr -> beg:>r) = (Just r, beg) queueTstS :: Int -> S.Seq Int -> S.Seq Int queueTstS 0 s = s queueTstS n s | even n = queueTstS (n - 1) (pushS (100 + n) s) | otherwise = queueTstS (n - 1) (if popped == Nothing then s else snd (fromJust popped)) where popped = popS s queueTstST :: Int -> State (S.Seq Int) (Maybe Int) queueTstST n = if (n > 0) then if even n then pushS' (100 + n) >> queueTstST (n - 1) else popS' >> queueTstST (n - 1) else return Nothing main1 :: IO () main1 = defaultMain [ bench "Twin Queue" $ whnf (queueTst 550) (Queue [500,499..1] []) , bench "Sequence Queue" $ whnf (queueTstS 550) (S.fromList [500,499..1]) , bench "State Queue" $ whnf (runState (queueTstST 550)) (S.fromList [500,499..1]) ] -------------------------------------------------------------------------------------------------------------------------------------------------- In the function "queueTstST" is there a way to exit while retaining the last "Maybe value" rather than with "Nothing"?