[Haskell-cafe] State monad exit

[p75213 at gmail.com]

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"?