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diff --git a/Control/Concurrent/STM/StatusCache.hs b/Control/Concurrent/STM/StatusCache.hs
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+---------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Concurrent.STM.StatusCache
+--
+--
+-- Maintainer  :  joe@jerkface.net
+-- Stability   :  experimental
+--
+-- Motivation: Suppose you must communicate state changes over a stream.  If
+-- the stream is consumed slowly, then it may occur that backlogged state
+-- changes are obsoleted by newer changes in state.  In this case, it is
+-- desirable to discard the obsolete messages from the stream.
+--
+-- A streamed status message might be very large, and it would be wasteful in
+-- both time and space, to treat it as a monolithic blob that must be built
+-- completely before it can be sent.  Therefore, we require that each message
+-- consist of a stream of smaller chunks of type @x@ and we require predicates
+-- that indicate when a chunk starts a new message and when it ends a message.
+--
+-- In the folowing example, our chunk type is Char and complete messages are
+-- delimited by the characters '(' and ')'.  We process the input stream
+-- \"(aaa(a))(bb)(cc(c)cc)\" first with a delayed processor and then again with
+-- an efficient dedicated thread. The result follows:
+--
+-- > Backlogged consumer: (cc(c)cc)
+-- > Fast consumer: (aaa(a))(bb)(cc(c)cc)
+--
+-- The complete source code:
+--
+-- > import Control.Monad      (when, forever, (>=>))
+-- > import Control.Monad.STM  (atomically)
+-- > import Control.Concurrent (forkIO, threadDelay)
+-- > import System.IO          (hFlush, stdout)
+-- > import qualified Control.Concurrent.STM.StatusCache as Cache
+-- >
+-- > main = do q <- atomically $ Cache.new (== '(') (==')')
+-- >           backlog q "(aaa(a))(bb)(cc(c)cc)"
+-- >           fast    q "(aaa(a))(bb)(cc(c)cc)"
+-- >
+-- > while pred body = pred >>= flip when (body >> while pred body)
+-- >
+-- > backlog q xs = do putStr $ "Backlogged consumer: "
+-- >                   mapM_ (atomically . Cache.push q) xs
+-- >                   while (atomically $ fmap not $ Cache.isEmpty q) $ do
+-- >                       c <- atomically $ Cache.pull q
+-- >                       putChar c
+-- >                   putStrLn ""
+-- >                   hFlush stdout
+-- >
+-- > fast q xs = do putStr "Fast consumer: "
+-- >                forkIO $ forever $ do
+-- >                        c <- atomically $ Cache.pull q
+-- >                        putChar c
+-- >                        hFlush stdout
+-- >                mapM_ (atomically . Cache.push q >=> const (threadDelay 10000))
+-- >                      xs
+-- >                putStrLn ""
+--
+-- As shown above, it is intended that this module be imported qualified.
+--
+{-# LANGUAGE DoAndIfThenElse #-}
+module Control.Concurrent.STM.StatusCache
+    ( StatusCache
+    , new
+    , push
+    , pull
+    , pullDepth
+    , isStopper
+    , isStarter
+    , isEmpty
+    ) where
+import Control.Monad
+import Control.Monad.STM
+import Control.Concurrent.STM.TVar
+import Control.Concurrent.STM.TChan
+
+data StatusCache x =
+    StatusCache { feed :: TVar (TChan x)
+                , cache :: TVar (TChan x)
+                , feedFlag :: TVar Bool
+                , pushDepth :: TVar Int
+                , pullDepth :: TVar Int -- ^ current nesting depth of next-to-pull data
+                , isStarter :: x -> Bool -- ^ True if the given chunk begins a message.
+                , isStopper :: x -> Bool -- ^ True if the given chunk ends a message.
+                }
+
+-- | @new isStart isStop@
+--
+-- The @isStart@ and @isStop@ predicates indicate when an element
+-- begins or ends a message.
+new :: (x -> Bool) -> (x -> Bool) -> STM (StatusCache x)
+new isStart isStop = do
+    feed <- newTChan >>= newTVar
+    cache <- newTChan >>= newTVar
+    flag <- newTVar True
+    pushd <- newTVar 0
+    pulld <- newTVar 0
+    return StatusCache { feed = feed
+                       , cache = cache
+                       , feedFlag = flag
+                       , pushDepth = pushd
+                       , pullDepth = pulld
+                       , isStarter = isStart
+                       , isStopper = isStop
+                       }
+
+
+-- | Pull off a chunk from the 'StatusCache' for processing.
+--
+-- If chunks are not pulled off quickly, they may be obsoleted
+-- and discarded when new messages are 'push'ed.
+pull :: StatusCache x -> STM x
+pull q = do
+    hasCache <- readTVar (feedFlag q)
+    exhausted <- readTVar (feed q) >>= isEmptyTChan
+    when (hasCache && exhausted) $ do
+        next <- newTChan >>= swapTVar (cache q)
+        writeTVar (feedFlag q) False
+        writeTVar (feed q) next
+    chan <- readTVar $ feed q
+    exhausted <- isEmptyTChan chan
+    if exhausted then retry
+    else do
+        v <- readTChan chan
+        when (isStarter q v) $ do
+            depth <- readTVar (pullDepth q)
+            modifyTVar' (pullDepth q) (+1)
+            when (depth==0)
+                $ writeTVar (feedFlag q) False
+        when (isStopper q v)
+            $ modifyTVar' (pullDepth q) (subtract 1)
+        return v
+
+-- | Enqueue a chunk into the 'StatusCache'.
+push :: StatusCache a -> a -> STM ()
+push q v = do
+    shouldCache <- readTVar (feedFlag q)
+    depth <- readTVar (pushDepth q)
+    when (isStopper q v) $ do
+        modifyTVar' (pushDepth q) (subtract 1)
+        when (depth==0)
+            $ writeTVar (feedFlag q) True
+    when (isStarter q v)
+        $ modifyTVar' (pushDepth q) (+1)
+    chan <-
+        if shouldCache then do
+            when (depth==0 && isStarter q v)
+                $ newTChan
+                  >>= writeTVar (cache q)
+            readTVar $ cache q
+        else do
+            readTVar $ feed q
+    writeTChan chan v
+
+-- | True when the 'StatusCache' is completely exhuasted.
+isEmpty :: StatusCache x -> STM Bool
+isEmpty q = do
+    empty_feed  <- readTVar (feed q) >>= isEmptyTChan
+    empty_cache <- readTVar (cache q) >>= isEmptyTChan
+    return $ empty_feed && empty_cache
diff --git a/Control/Concurrent/STM/UpdateStream.hs b/Control/Concurrent/STM/UpdateStream.hs
new file mode 100644
index 00000000..b01fc0bc
--- /dev/null
+++ b/Control/Concurrent/STM/UpdateStream.hs
@@ -0,0 +1,169 @@
+---------------------------------------------------------------------------
+-- |
+-- Module      :  Control.Concurrent.STM.UpdateStream
+--
+--
+-- Maintainer  :  joe@jerkface.net
+-- Stability   :  experimental
+--
+-- An UpdateSream consists of pass-through messages that are queued up and 
+-- slotted messages which might be obsoleted and discarded if they are not
+-- consumed before newer slotted messages with the same slot value occur.
+-- 
+-- Slots are implemented with "Control.Concurrent.STM.StatusCache" and it is
+-- recommended that you read that documentation first.
+--
+-- For example, the output
+--
+-- > Backlogged consumer: (set x = 5)(set y = 100)(Hello)(Joe)(was)(here)
+-- > Fast consumer: (set x = 2)(Hello)(set y = 23)(Joe)(set y = 100)(was)(set x = 5)(here)
+--
+-- is produced by the following code:
+--
+-- > import Control.Monad      (when, forever, (>=>))
+-- > import Control.Monad.STM  (atomically)
+-- > import Control.Concurrent (forkIO, threadDelay)
+-- > import System.IO          (hFlush, stdout)
+-- > import qualified Control.Concurrent.STM.UpdateStream as Cache
+-- > 
+-- > messages :: [(Maybe String, Char)]
+-- > messages = concat
+-- >     [ slot "x" "(set x = 2)"
+-- >     , message  "(Hello)"
+-- >     , slot "y" "(set y = 23)"
+-- >     , message  "(Joe)"
+-- >     , slot "y" "(set y = 100)"
+-- >     , message  "(was)"
+-- >     , slot "x" "(set x = 5)"
+-- >     , message  "(here)"
+-- >     ]
+-- >  where
+-- >     slot v cs = map ((,) (Just v)) cs
+-- >     message cs = map ((,) Nothing) cs
+-- > 
+-- > main = do
+-- >     q <- atomically $ Cache.new (== '(') (==')')
+-- >     let go = mapM_ (atomically . (uncurry $ Cache.push q)
+-- >                      >=> const (threadDelay 10000))
+-- >                    messages
+-- >         slowly = do
+-- >             while (atomically $ fmap not $ Cache.isEmpty q) $ do
+-- >                 c <- atomically $ Cache.pull q
+-- >                 putChar c
+-- >             putStrLn ""
+-- >             hFlush stdout
+-- >                           where while pred body =
+-- >                                  pred >>= flip when (body >> while pred body)
+-- >         quickly = forkIO . forever $ do
+-- >                     c <- atomically $ Cache.pull q
+-- >                     putChar c
+-- >                     hFlush stdout
+-- >     putStr $ "Backlogged consumer: "
+-- >     go >> slowly
+-- >     putStr "Fast consumer: "
+-- >     quickly >> go
+-- >     putStrLn ""
+--
+module Control.Concurrent.STM.UpdateStream
+    ( UpdateStream
+    , new
+    , push
+    , pull
+    , isStopper
+    , isStarter
+    , isEmpty
+    ) where
+
+import Control.Monad
+import Control.Monad.STM
+import Control.Concurrent.STM.TVar
+import Control.Concurrent.STM.TChan
+import Control.Concurrent.STM.StatusCache (StatusCache)
+import qualified Control.Concurrent.STM.StatusCache as Status
+import Data.Map (Map)
+import qualified Data.Map as Map
+import Data.Maybe
+import Data.Foldable (foldlM)
+
+data UpdateStream slot x =
+    UpdateStream { cache :: TVar (Map slot (StatusCache x))
+                 , events :: TChan x
+                 , inMessage :: TVar (Maybe (StatusCache x))
+                 , isStarter :: x -> Bool -- ^ True if the given chunk begins a message.
+                 , isStopper :: x -> Bool -- ^ True if the given chunk ends a message.
+                 }
+
+-- | @new isStart isStop@
+--
+-- The @isStart@ and @isStop@ predicates indicate when an element
+-- begins or ends a message.
+new :: Ord slot => (x->Bool) -> (x->Bool) -> STM (UpdateStream slot x)
+new isStart isStop = do
+    cache <- newTVar Map.empty
+    events <- newTChan
+    inMessage <- newTVar Nothing
+    return UpdateStream { cache = cache
+                        , events = events
+                        , inMessage = inMessage
+                        , isStarter = isStart
+                        , isStopper = isStop
+                        }
+
+-- | Enqueue a chunk into the 'UpdateStream'
+--
+-- If a slot is provided, then the message may be obsoleted when a new message
+-- starts with the same slot value.  Otherwise, the chunk is preserved.  Note
+-- that the same slot value must be passed again for every chunk of the message
+-- as it is not remembered.
+--
+-- Note that although a slot value is provided on each push, it is assumed that
+-- messages will be pushed in contiguous streams.  To change this, we would
+-- need to add a mutable integer to track the nesting level of the primary
+-- (non-slotted) stream.
+push :: Ord slot => UpdateStream slot x -> Maybe slot -> x -> STM ()
+push u Nothing x = writeTChan (events u) x
+push u (Just n) x = do
+    smap <- readTVar (cache u)
+    scache <-
+        case Map.lookup n smap of
+            Just scache -> return scache
+            Nothing -> do
+                scache <- Status.new (isStarter u) (isStopper u)
+                modifyTVar (cache u) (Map.insert n scache)
+                return scache
+
+    Status.push scache x
+
+-- | Pull off a chunk from the 'UpdateStream' for processing.
+-- 
+-- If chunks are not pulled off quickly, they may be obsoleted
+-- and discarded when new messages are 'push'ed.
+pull :: Ord slot => UpdateStream slot x -> STM x
+pull u = do
+    mbscache <- readTVar $ inMessage u
+    let action =
+         case mbscache of
+            Just scache -> do
+                x <- Status.pull scache
+                nesting <- readTVar (Status.pullDepth scache)
+                when (nesting==0) $ writeTVar (inMessage u) Nothing
+                return x
+            Nothing -> do
+                cs <- fmap (Map.toList) $ readTVar (cache u)
+                cs <- filterM (return . not <=< Status.isEmpty . snd)
+                              cs
+                maybe (readTChan $ events u)
+                      (\(n,scache) -> do
+                            writeTVar (inMessage u) (Just scache) -- (Just n)
+                            Status.pull scache)
+                      (listToMaybe cs)
+    x <- action
+    return x
+
+-- | True when the 'UpdateStream' is completely exhuasted.
+isEmpty :: UpdateStream slot x -> STM Bool
+isEmpty q = do
+    e <- isEmptyTChan (events q)
+    qs <- readTVar (cache q)
+    d <- foldlM (\b s -> fmap (b &&) $ Status.isEmpty s) True qs
+    return $ e && d