Well, the actual problem I am trying to solve involves properly reclaiming elements in a circularly linked list (next and prev pointers are TVars). I have a linked list and I need to be able to remove values from the list when all references to the node no longer exist, not counting the linked list references themselves. Using Weak pointers inside the list itself doesn't work, since if an element is collected, you also lose the information needed to stitch up the list. Originally, I had a wacky plan involving weak pointers in the nodes themselves pointing to sentinal nodes, when the sentinal was collected, I then know I can delete the node. The idea was that I can lazily delete entire chains of nodes rather than one by one. I gave up on that idea. (deRefWeak not working in STM was sort of a show stopper, and it was overly complicated) So now I have a scheme whereby I attach a finalizer to a proxy thunk.
data TimeStamp = TimeStamp TS
data TS = TS { tsWord :: TVar Word64, tsPrev :: TVar TS, tsNext :: TVar TS }
so, the finalizer attached to 'TimeStamp' values simply deletes the value it points to from the linked list. The module interface ensures that only 'TimeStamp' values can escape and each has a finalizer attached. the interface is quite simple:
newTimeStamp :: IO TimeStamp insertAfter :: TimeStamp -> IO TimeStamp
now, the problem is that I want to export insertAfter in the 'STM' monad, not the IO one. however, I am not sure how to pull it off. I cannot ensure the finalizer is only attached once to the node, if I use 'unsafeIOToSTM' then STM retrying could end up created multiple finalized nodes, possibly prematurely deleting the element from the linked list. basically, what would be really nice is if there were something like
registerCommitIO :: IO () -> STM ()
where all IO actions registered with this function (within an atomically block) are executed exactly once if and only if the atomically block commits. The IO action is not run within the STM block, notably atomically $ do foo; registerCommitIO bar; baz is equivalent to atomically (do foo; baz) >> bar I found I needed the equivalent of 'touchForeignPtr' for arbitrary objects (which I was able to do with the touch# primitive, but
touch :: a -> IO () touchSTM :: a -> STM ()
would both be at home in System.Mem.Weak. While I am wishing for things,
unsafePerformSTM :: STM a -> a
would be really handy too :) John attached is my module in question, so, my challenge is basically to make insertAfter/insertBefore have STM types and to make the 'Ord' instance not fail with "nested atomically" error when evaluated within an STM block (so I can get rid of compareTimeStamp). {-# OPTIONS_GHC -fglasgow-exts #-} module TimeStamp( TimeStamp(), newTimeStamp, compareTimeStamp, insertAfter, insertBefore ) where import Data.Word import System.IO.Unsafe import Control.Concurrent.STM import System.Mem.Weak import GHC.Prim import GHC.IO import GHC.Conc import Foreign.ForeignPtr data TimeStamp = TimeStamp TS data TS = TS { tsWord :: TVar Word64, tsPrev :: TVar TS, tsNext :: TVar TS } instance Eq TimeStamp where TimeStamp a == TimeStamp b = tsWord a == tsWord b instance Ord TimeStamp where compare x y = unsafePerformIO . atomically $ compareTimeStamp x y {-# NOINLINE theBase #-} theBase :: TS theBase = unsafePerformIO $ newTimeStampSpace newTimeStampSpace :: IO TS newTimeStampSpace = mdo bot <- newTVarIO 0 botNext <- newTVarIO beginningOfTime botPrev <- newTVarIO beginningOfTime let beginningOfTime = TS { tsWord = bot, tsNext = botNext, tsPrev = botPrev } return beginningOfTime newTimeStamp :: IO TimeStamp newTimeStamp = insertAfter (TimeStamp theBase) deleteTimeStamp :: TS -> IO () deleteTimeStamp ts = do atomically $ do tsn <- readTVar (tsNext ts) tsp <- readTVar (tsPrev ts) writeTVar (tsPrev tsn) tsp writeTVar (tsNext tsp) tsn writeTVar (tsWord ts) (error "time stamp was deleted") compareTimeStamp :: TimeStamp -> TimeStamp -> STM Ordering compareTimeStamp t1@(TimeStamp ts1) t2@(TimeStamp ts2) = do bw <- readTVar $ tsWord theBase x <- readTVar (tsWord ts1) y <- readTVar (tsWord ts2) unsafeIOToSTM $ touchTS t1 unsafeIOToSTM $ touchTS t2 return $ compare (x + bw) (y + bw) insertAfter' :: TS -> STM (TS,TimeStamp) insertAfter' ts = do nts <- newTS doInsertAfter nts ts return (nts,TimeStamp nts) insertAfter :: TimeStamp -> IO TimeStamp insertAfter t@(TimeStamp ts) = do (tts,ts) <- atomically $ insertAfter' ts touchTS t addFinalizer ts (deleteTimeStamp tts) return ts insertBefore :: TimeStamp -> IO TimeStamp insertBefore t@(TimeStamp ts) = do (tts,ts) <- atomically $ readTVar (tsPrev ts) >>= insertAfter' touchTS t addFinalizer ts (deleteTimeStamp tts) return ts touchTS :: TimeStamp -> IO () touchTS ts = touchForeignPtr (unsafeCoerce# ts) doInsertAfter :: TS -> TS -> STM () doInsertAfter ts1 ts2 = do v0 <- readTVar (tsWord ts2) let makeRoom j ts = do ts <- readTVar (tsNext ts) wj' <- if tsWord ts /= tsWord ts2 then readTVar (tsWord ts) else return (maxBound + v0) if fromIntegral (wj' - v0) <= j*j then makeRoom (j + 1) ts else relabel (wj' - v0) j relabel _ 1 = return () relabel wj j = do rl ts2 [ ((toInteger wj * k) `div` j) + fromIntegral v0 | k <- [ 1 .. j - 1] ] rl ts [] = return () rl ts (n:ns) = do ts <- readTVar (tsNext ts) writeTVar (tsWord ts) (fromIntegral n) rl ts ns makeRoom 1 ts2 tn <- readTVar (tsNext ts2) wn <- readTVar (tsWord tn) writeTVar (tsNext ts1) tn writeTVar (tsPrev ts1) ts2 writeTVar (tsNext ts2) ts1 writeTVar (tsPrev tn) ts1 bw <- readTVar $ tsWord theBase let avg = a1 `div` 2 + (a2 `div` 2) + (a1 `mod` 2 + a2 `mod` 2) `div` 2 a1 = v0 - bw a2 = if tsWord tn == tsWord theBase then maxBound else wn - bw writeTVar (tsWord ts1) avg showTimeStamp :: TimeStamp -> IO () showTimeStamp (TimeStamp ts) = atomically (f ts) >>= print where f t = do x <- readTVar (tsWord t) tsn <- readTVar (tsNext t) if tsWord tsn == tsWord ts then return [x] else fmap (x:) (f tsn) showTimeStamps :: IO () showTimeStamps = showTimeStamp (TimeStamp theBase) newTS = do w <- newTVar undefined next <- newTVar undefined prev <- newTVar undefined return $ TS w next prev