{-# LANGUAGE CPP #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE GeneralizedNewtypeDeriving#-} {-# LANGUAGE InstanceSigs #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE UndecidableInstances #-} -- XXX -- | -- Module : Streamly.Internal.Data.Stream.Ahead -- Copyright : (c) 2017 Harendra Kumar -- -- License : BSD3 -- Maintainer : streamly@composewell.com -- Stability : experimental -- Portability : GHC -- -- module Streamly.Internal.Data.Stream.Ahead ( AheadT , Ahead , aheadly , ahead ) where import Control.Concurrent.MVar (putMVar, takeMVar) import Control.Exception (assert) import Control.Monad (void, when) import Control.Monad.Base (MonadBase(..), liftBaseDefault) import Control.Monad.Catch (MonadThrow, throwM) -- import Control.Monad.Error.Class (MonadError(..)) import Control.Monad.IO.Class (MonadIO(..)) import Control.Monad.Reader.Class (MonadReader(..)) import Control.Monad.State.Class (MonadState(..)) import Control.Monad.Trans.Class (MonadTrans(lift)) import Control.Monad.Trans.Control (MonadBaseControl (..)) import Data.Heap (Heap, Entry(..)) import Data.IORef (IORef, readIORef, atomicModifyIORef, writeIORef) import Data.Maybe (fromJust) #if __GLASGOW_HASKELL__ < 808 import Data.Semigroup (Semigroup(..)) #endif import GHC.Exts (inline) import qualified Data.Heap as H import Streamly.Internal.Data.Stream.SVar (fromSVar) import Streamly.Internal.Data.SVar import Streamly.Internal.Data.Stream.StreamK (IsStream(..), Stream, mkStream, foldStream, foldStreamShared) import qualified Streamly.Internal.Data.Stream.StreamK as K import qualified Streamly.Internal.Data.Stream.StreamD as D import Prelude hiding (map) #include "Instances.hs" ------------------------------------------------------------------------------- -- Ahead ------------------------------------------------------------------------------- -- Lookahead streams can execute multiple tasks concurrently, ahead of time, -- but always serve them in the same order as they appear in the stream. To -- implement lookahead streams efficiently we assign a sequence number to each -- task when the task is picked up for execution. When the task finishes, the -- output is tagged with the same sequence number and we rearrange the outputs -- in sequence based on that number. -- -- To explain the mechanism imagine that the current task at the head of the -- stream has a "token" to yield to the outputQueue. The ownership of the token -- is determined by the current sequence number is maintained in outputHeap. -- Sequence number is assigned when a task is queued. When a thread dequeues a -- task it picks up the sequence number as well and when the output is ready it -- uses the sequence number to queue the output to the outputQueue. -- -- The thread with current sequence number sends the output directly to the -- outputQueue. Other threads push the output to the outputHeap. When the task -- being queued on the heap is a stream of many elements we evaluate only the -- first element and keep the rest of the unevaluated computation in the heap. -- When such a task gets the "token" for outputQueue it evaluates and directly -- yields all the elements to the outputQueue without checking for the -- "token". -- -- Note that no two outputs in the heap can have the same sequence numbers and -- therefore we do not need a stable heap. We have also separated the buffer -- for the current task (outputQueue) and the pending tasks (outputHeap) so -- that the pending tasks cannot interfere with the current task. Note that for -- a single task just the outputQueue is enough and for the case of many -- threads just a heap is good enough. However we balance between these two -- cases, so that both are efficient. -- -- For bigger streams it may make sense to have separate buffers for each -- stream. However, for singleton streams this may become inefficient. However, -- if we do not have separate buffers, then the streams that come later in -- sequence may hog the buffer, hindering the streams that are ahead. For this -- reason we have a single element buffer limitation for the streams being -- executed in advance. -- -- This scheme works pretty efficiently with less than 40% extra overhead -- compared to the Async streams where we do not have any kind of sequencing of -- the outputs. It is especially devised so that we are most efficient when we -- have short tasks and need just a single thread. Also when a thread yields -- many items it can hold lockfree access to the outputQueue and do it -- efficiently. -- -- XXX Maybe we can start the ahead threads at a lower cpu and IO priority so -- that they do not hog the resources and hinder the progress of the threads in -- front of them. -- Left associated ahead expressions are expensive. We start a new SVar for -- each left associative expression. The queue is used only for right -- associated expression, we queue the right expression and execute the left. -- Thererefore the queue never has more than on item in it. -- -- XXX Also note that limiting concurrency for cases like "take 10" would not -- work well with left associative expressions, because we have no visibility -- about how much the left side of the expression would yield. -- -- XXX It may be a good idea to increment sequence numbers for each yield, -- currently a stream on the left side of the expression may yield many -- elements with the same sequene number. We can then use the seq number to -- enforce yieldMax and yieldLImit as well. -- Invariants: -- -- * A worker should always ensure that it pushes all the consecutive items in -- the heap to the outputQueue especially the items on behalf of the workers -- that have already left when we were holding the token. This avoids deadlock -- conditions when the later workers completion depends on the consumption of -- earlier results. For more details see comments in the consumer pull side -- code. {-# INLINE underMaxHeap #-} underMaxHeap :: SVar Stream m a -> Heap (Entry Int (AheadHeapEntry Stream m a)) -> IO Bool underMaxHeap sv hp = do (_, len) <- readIORef (outputQueue sv) -- XXX simplify this let maxHeap = case maxBufferLimit sv of Limited lim -> Limited $ max 0 (lim - fromIntegral len) Unlimited -> Unlimited case maxHeap of Limited lim -> do active <- readIORef (workerCount sv) return $ H.size hp + active <= fromIntegral lim Unlimited -> return True -- Return value: -- True => stop -- False => continue preStopCheck :: SVar Stream m a -> IORef (Heap (Entry Int (AheadHeapEntry Stream m a)) , Maybe Int) -> IO Bool preStopCheck sv heap = -- check the stop condition under a lock before actually -- stopping so that the whole herd does not stop at once. withIORef heap $ \(hp, _) -> do heapOk <- underMaxHeap sv hp takeMVar (workerStopMVar sv) let stop = do putMVar (workerStopMVar sv) () return True continue = do putMVar (workerStopMVar sv) () return False if heapOk then case yieldRateInfo sv of Nothing -> continue Just yinfo -> do rateOk <- isBeyondMaxRate sv yinfo if rateOk then continue else stop else stop abortExecution :: IORef ([Stream m a], Int) -> SVar Stream m a -> Maybe WorkerInfo -> Stream m a -> IO () abortExecution q sv winfo m = do reEnqueueAhead sv q m incrementYieldLimit sv sendStop sv winfo -- XXX In absence of a "noyield" primitive (i.e. do not pre-empt inside a -- critical section) from GHC RTS, we have a difficult problem. Assume we have -- a 100,000 threads producing output and queuing it to the heap for -- sequencing. The heap can be drained only by one thread at a time, any thread -- that finds that heap can be drained now, takes a lock and starts draining -- it, however the thread may get prempted in the middle of it holding the -- lock. Since that thread is holding the lock, the other threads cannot pick -- up the draining task, therefore they proceed to picking up the next task to -- execute. If the draining thread could yield voluntarily at a point where it -- has released the lock, then the next threads could pick up the draining -- instead of executing more tasks. When there are 100,000 threads the drainer -- gets a cpu share to run only 1:100000 of the time. This makes the heap -- accumulate a lot of output when we the buffer size is large. -- -- The solutions to this problem are: -- 1) make the other threads wait in a queue until the draining finishes -- 2) make the other threads queue and go away if draining is in progress -- -- In both cases we give the drainer a chance to run more often. -- processHeap :: (MonadIO m, MonadBaseControl IO m) => IORef ([Stream m a], Int) -> IORef (Heap (Entry Int (AheadHeapEntry Stream m a)), Maybe Int) -> State Stream m a -> SVar Stream m a -> Maybe WorkerInfo -> AheadHeapEntry Stream m a -> Int -> Bool -- we are draining the heap before we stop -> m () processHeap q heap st sv winfo entry sno stopping = loopHeap sno entry where stopIfNeeded ent seqNo r = do stopIt <- liftIO $ preStopCheck sv heap if stopIt then liftIO $ do -- put the entry back in the heap and stop requeueOnHeapTop heap (Entry seqNo ent) seqNo sendStop sv winfo else runStreamWithYieldLimit True seqNo r loopHeap seqNo ent = case ent of AheadEntryNull -> nextHeap seqNo AheadEntryPure a -> do -- Use 'send' directly so that we do not account this in worker -- latency as this will not be the real latency. -- Don't stop the worker in this case as we are just -- transferring available results from heap to outputQueue. void $ liftIO $ send sv (ChildYield a) nextHeap seqNo AheadEntryStream r -> if stopping then stopIfNeeded ent seqNo r else runStreamWithYieldLimit True seqNo r nextHeap prevSeqNo = do res <- liftIO $ dequeueFromHeapSeq heap (prevSeqNo + 1) case res of Ready (Entry seqNo hent) -> loopHeap seqNo hent Clearing -> liftIO $ sendStop sv winfo Waiting _ -> if stopping then do r <- liftIO $ preStopCheck sv heap if r then liftIO $ sendStop sv winfo else processWorkQueue prevSeqNo else inline processWorkQueue prevSeqNo processWorkQueue prevSeqNo = do work <- dequeueAhead q case work of Nothing -> liftIO $ sendStop sv winfo Just (m, seqNo) -> do yieldLimitOk <- liftIO $ decrementYieldLimit sv if yieldLimitOk then if seqNo == prevSeqNo + 1 then processWithToken q heap st sv winfo m seqNo else processWithoutToken q heap st sv winfo m seqNo else liftIO $ abortExecution q sv winfo m -- We do not stop the worker on buffer full here as we want to proceed to -- nextHeap anyway so that we can clear any subsequent entries. We stop -- only in yield continuation where we may have a remaining stream to be -- pushed on the heap. singleStreamFromHeap seqNo a = do void $ liftIO $ sendYield sv winfo (ChildYield a) nextHeap seqNo -- XXX when we have an unfinished stream on the heap we cannot account all -- the yields of that stream until it finishes, so if we have picked up -- and executed more actions beyond that in the parent stream and put them -- on the heap then they would eat up some yield limit which is not -- correct, we will think that our yield limit is over even though we have -- to yield items from unfinished stream before them. For this reason, if -- there are pending items in the heap we drain them unconditionally -- without considering the yield limit. runStreamWithYieldLimit continue seqNo r = do _ <- liftIO $ decrementYieldLimit sv if continue -- see comment above -- && yieldLimitOk then do let stop = do liftIO (incrementYieldLimit sv) nextHeap seqNo foldStreamShared st (yieldStreamFromHeap seqNo) (singleStreamFromHeap seqNo) stop r else liftIO $ do let ent = Entry seqNo (AheadEntryStream r) liftIO $ requeueOnHeapTop heap ent seqNo incrementYieldLimit sv sendStop sv winfo yieldStreamFromHeap seqNo a r = do continue <- liftIO $ sendYield sv winfo (ChildYield a) runStreamWithYieldLimit continue seqNo r {-# NOINLINE drainHeap #-} drainHeap :: (MonadIO m, MonadBaseControl IO m) => IORef ([Stream m a], Int) -> IORef (Heap (Entry Int (AheadHeapEntry Stream m a)), Maybe Int) -> State Stream m a -> SVar Stream m a -> Maybe WorkerInfo -> m () drainHeap q heap st sv winfo = do r <- liftIO $ dequeueFromHeap heap case r of Ready (Entry seqNo hent) -> processHeap q heap st sv winfo hent seqNo True _ -> liftIO $ sendStop sv winfo data HeapStatus = HContinue | HStop data WorkerStatus = Continue | Suspend processWithoutToken :: (MonadIO m, MonadBaseControl IO m) => IORef ([Stream m a], Int) -> IORef (Heap (Entry Int (AheadHeapEntry Stream m a)), Maybe Int) -> State Stream m a -> SVar Stream m a -> Maybe WorkerInfo -> Stream m a -> Int -> m () processWithoutToken q heap st sv winfo m seqNo = do -- we have already decremented the yield limit for m let stop = do liftIO (incrementYieldLimit sv) -- If the stream stops without yielding anything, and we do not put -- anything on heap, but if heap was waiting for this seq number -- then it will keep waiting forever, because we are never going to -- put it on heap. So we have to put a null entry on heap even when -- we stop. toHeap AheadEntryNull mrun = runInIO $ svarMrun sv r <- liftIO $ mrun $ foldStreamShared st (\a r -> toHeap $ AheadEntryStream $ K.cons a r) (toHeap . AheadEntryPure) stop m res <- restoreM r case res of Continue -> workLoopAhead q heap st sv winfo Suspend -> drainHeap q heap st sv winfo where -- XXX to reduce contention each CPU can have its own heap toHeap ent = do -- Heap insertion is an expensive affair so we use a non CAS based -- modification, otherwise contention and retries can make a thread -- context switch and throw it behind other threads which come later in -- sequence. newHp <- liftIO $ atomicModifyIORef heap $ \(hp, snum) -> let hp' = H.insert (Entry seqNo ent) hp in assert (heapIsSane snum seqNo) ((hp', snum), hp') when (svarInspectMode sv) $ liftIO $ do maxHp <- readIORef (maxHeapSize $ svarStats sv) when (H.size newHp > maxHp) $ writeIORef (maxHeapSize $ svarStats sv) (H.size newHp) heapOk <- liftIO $ underMaxHeap sv newHp status <- case yieldRateInfo sv of Nothing -> return HContinue Just yinfo -> case winfo of Just info -> do rateOk <- liftIO $ workerRateControl sv yinfo info if rateOk then return HContinue else return HStop Nothing -> return HContinue if heapOk then case status of HContinue -> return Continue HStop -> return Suspend else return Suspend data TokenWorkerStatus = TokenContinue Int | TokenSuspend processWithToken :: (MonadIO m, MonadBaseControl IO m) => IORef ([Stream m a], Int) -> IORef (Heap (Entry Int (AheadHeapEntry Stream m a)), Maybe Int) -> State Stream m a -> SVar Stream m a -> Maybe WorkerInfo -> Stream m a -> Int -> m () processWithToken q heap st sv winfo action sno = do -- Note, we enter this function with yield limit already decremented -- XXX deduplicate stop in all invocations let stop = do liftIO (incrementYieldLimit sv) return $ TokenContinue (sno + 1) mrun = runInIO $ svarMrun sv r <- liftIO $ mrun $ foldStreamShared st (yieldOutput sno) (singleOutput sno) stop action res <- restoreM r case res of TokenContinue seqNo -> loopWithToken seqNo TokenSuspend -> drainHeap q heap st sv winfo where singleOutput seqNo a = do continue <- liftIO $ sendYield sv winfo (ChildYield a) if continue then return $ TokenContinue (seqNo + 1) else do liftIO $ updateHeapSeq heap (seqNo + 1) return TokenSuspend -- XXX use a wrapper function around stop so that we never miss -- incrementing the yield in a stop continuation. Essentiatlly all -- "unstream" calls in this function must increment yield limit on stop. yieldOutput seqNo a r = do continue <- liftIO $ sendYield sv winfo (ChildYield a) yieldLimitOk <- liftIO $ decrementYieldLimit sv if continue && yieldLimitOk then do let stop = do liftIO (incrementYieldLimit sv) return $ TokenContinue (seqNo + 1) foldStreamShared st (yieldOutput seqNo) (singleOutput seqNo) stop r else do let ent = Entry seqNo (AheadEntryStream r) liftIO $ requeueOnHeapTop heap ent seqNo liftIO $ incrementYieldLimit sv return TokenSuspend loopWithToken nextSeqNo = do work <- dequeueAhead q case work of Nothing -> do liftIO $ updateHeapSeq heap nextSeqNo workLoopAhead q heap st sv winfo Just (m, seqNo) -> do yieldLimitOk <- liftIO $ decrementYieldLimit sv let undo = liftIO $ do updateHeapSeq heap nextSeqNo reEnqueueAhead sv q m incrementYieldLimit sv if yieldLimitOk then if seqNo == nextSeqNo then do let stop = do liftIO (incrementYieldLimit sv) return $ TokenContinue (seqNo + 1) mrun = runInIO $ svarMrun sv r <- liftIO $ mrun $ foldStreamShared st (yieldOutput seqNo) (singleOutput seqNo) stop m res <- restoreM r case res of TokenContinue seqNo1 -> loopWithToken seqNo1 TokenSuspend -> drainHeap q heap st sv winfo else -- To avoid a race when another thread puts something -- on the heap and goes away, the consumer will not get -- a doorBell and we will not clear the heap before -- executing the next action. If the consumer depends -- on the output that is stuck in the heap then this -- will result in a deadlock. So we always clear the -- heap before executing the next action. undo >> workLoopAhead q heap st sv winfo else undo >> drainHeap q heap st sv winfo -- XXX the yield limit changes increased the performance overhead by 30-40%. -- Just like AsyncT we can use an implementation without yeidlimit and even -- without pacing code to keep the performance higher in the unlimited and -- unpaced case. -- -- XXX The yieldLimit stuff is pretty invasive. We can instead do it by using -- three hooks, a pre-execute hook, a yield hook and a stop hook. In fact these -- hooks can be used for a more general implementation to even check predicates -- and not just yield limit. -- XXX we can remove the sv parameter as it can be derived from st workLoopAhead :: (MonadIO m, MonadBaseControl IO m) => IORef ([Stream m a], Int) -> IORef (Heap (Entry Int (AheadHeapEntry Stream m a)), Maybe Int) -> State Stream m a -> SVar Stream m a -> Maybe WorkerInfo -> m () workLoopAhead q heap st sv winfo = do r <- liftIO $ dequeueFromHeap heap case r of Ready (Entry seqNo hent) -> processHeap q heap st sv winfo hent seqNo False Clearing -> liftIO $ sendStop sv winfo Waiting _ -> do -- Before we execute the next item from the work queue we check -- if we are beyond the yield limit. It is better to check the -- yield limit before we pick up the next item. Otherwise we -- may have already started more tasks even though we may have -- reached the yield limit. We can avoid this by taking active -- workers into account, but that is not as reliable, because -- workers may go away without picking up work and yielding a -- value. -- -- Rate control can be done either based on actual yields in -- the output queue or based on any yield either to the heap or -- to the output queue. In both cases we may have one issue or -- the other. We chose to do this based on actual yields to the -- output queue because it makes the code common to both async -- and ahead streams. -- work <- dequeueAhead q case work of Nothing -> liftIO $ sendStop sv winfo Just (m, seqNo) -> do yieldLimitOk <- liftIO $ decrementYieldLimit sv if yieldLimitOk then if seqNo == 0 then processWithToken q heap st sv winfo m seqNo else processWithoutToken q heap st sv winfo m seqNo -- If some worker decremented the yield limit but then -- did not yield anything and therefore incremented it -- later, then if we did not requeue m here we may find -- the work queue empty and therefore miss executing -- the remaining action. else liftIO $ abortExecution q sv winfo m ------------------------------------------------------------------------------- -- WAhead ------------------------------------------------------------------------------- -- XXX To be implemented. Use a linked queue like WAsync and put back the -- remaining computation at the back of the queue instead of the heap, and -- increment the sequence number. -- The only difference between forkSVarAsync and this is that we run the left -- computation without a shared SVar. forkSVarAhead :: (IsStream t, MonadAsync m) => t m a -> t m a -> t m a forkSVarAhead m1 m2 = mkStream $ \st yld sng stp -> do sv <- newAheadVar st (concurrently (toStream m1) (toStream m2)) workLoopAhead foldStream st yld sng stp (fromSVar sv) where concurrently ma mb = mkStream $ \st yld sng stp -> do liftIO $ enqueue (fromJust $ streamVar st) mb foldStream st yld sng stp ma -- | Polymorphic version of the 'Semigroup' operation '<>' of 'AheadT'. -- Merges two streams sequentially but with concurrent lookahead. -- -- @since 0.3.0 {-# INLINE ahead #-} ahead :: (IsStream t, MonadAsync m) => t m a -> t m a -> t m a ahead m1 m2 = mkStream $ \st yld sng stp -> case streamVar st of Just sv | svarStyle sv == AheadVar -> do liftIO $ enqueue sv (toStream m2) -- Always run the left side on a new SVar to avoid complexity in -- sequencing results. This means the left side cannot further -- split into more ahead computations on the same SVar. foldStream st yld sng stp m1 _ -> foldStreamShared st yld sng stp (forkSVarAhead m1 m2) -- | XXX we can implement it more efficienty by directly implementing instead -- of combining streams using ahead. {-# INLINE consMAhead #-} {-# SPECIALIZE consMAhead :: IO a -> AheadT IO a -> AheadT IO a #-} consMAhead :: MonadAsync m => m a -> AheadT m a -> AheadT m a consMAhead m r = fromStream $ K.yieldM m `ahead` (toStream r) ------------------------------------------------------------------------------ -- AheadT ------------------------------------------------------------------------------ -- | The 'Semigroup' operation for 'AheadT' appends two streams. The combined -- stream behaves like a single stream with the actions from the second stream -- appended to the first stream. The combined stream is evaluated in the -- speculative style. This operation can be used to fold an infinite lazy -- container of streams. -- -- @ -- import "Streamly" -- import qualified "Streamly.Prelude" as S -- import Control.Concurrent -- -- main = do -- xs \<- S.'toList' . 'aheadly' $ (p 1 |: p 2 |: nil) <> (p 3 |: p 4 |: nil) -- print xs -- where p n = threadDelay 1000000 >> return n -- @ -- @ -- [1,2,3,4] -- @ -- -- Any exceptions generated by a constituent stream are propagated to the -- output stream. -- -- The monad instance of 'AheadT' may run each monadic continuation (bind) -- concurrently in a speculative manner, performing side effects in a partially -- ordered manner but producing the outputs in an ordered manner like -- 'SerialT'. -- -- @ -- main = S.drain . 'aheadly' $ do -- n <- return 3 \<\> return 2 \<\> return 1 -- S.yieldM $ do -- threadDelay (n * 1000000) -- myThreadId >>= \\tid -> putStrLn (show tid ++ ": Delay " ++ show n) -- @ -- @ -- ThreadId 40: Delay 1 -- ThreadId 39: Delay 2 -- ThreadId 38: Delay 3 -- @ -- -- @since 0.3.0 newtype AheadT m a = AheadT {getAheadT :: Stream m a} deriving (MonadTrans) -- | A serial IO stream of elements of type @a@ with concurrent lookahead. See -- 'AheadT' documentation for more details. -- -- @since 0.3.0 type Ahead = AheadT IO -- | Fix the type of a polymorphic stream as 'AheadT'. -- -- @since 0.3.0 aheadly :: IsStream t => AheadT m a -> t m a aheadly = K.adapt instance IsStream AheadT where toStream = getAheadT fromStream = AheadT consM = consMAhead (|:) = consMAhead ------------------------------------------------------------------------------ -- Semigroup ------------------------------------------------------------------------------ {-# INLINE mappendAhead #-} {-# SPECIALIZE mappendAhead :: AheadT IO a -> AheadT IO a -> AheadT IO a #-} mappendAhead :: MonadAsync m => AheadT m a -> AheadT m a -> AheadT m a mappendAhead m1 m2 = fromStream $ ahead (toStream m1) (toStream m2) instance MonadAsync m => Semigroup (AheadT m a) where (<>) = mappendAhead ------------------------------------------------------------------------------ -- Monoid ------------------------------------------------------------------------------ instance MonadAsync m => Monoid (AheadT m a) where mempty = K.nil mappend = (<>) ------------------------------------------------------------------------------ -- Monad ------------------------------------------------------------------------------ {-# INLINE concatMapAhead #-} {-# SPECIALIZE concatMapAhead :: (a -> AheadT IO b) -> AheadT IO a -> AheadT IO b #-} concatMapAhead :: MonadAsync m => (a -> AheadT m b) -> AheadT m a -> AheadT m b concatMapAhead f m = fromStream $ K.concatMapBy ahead (\a -> K.adapt $ f a) (K.adapt m) {-# INLINE apAhead #-} apAhead :: MonadAsync m => AheadT m (a -> b) -> AheadT m a -> AheadT m b apAhead (AheadT m1) (AheadT m2) = let f x1 = K.concatMapBy ahead (pure . x1) m2 in AheadT $ K.concatMapBy ahead f m1 instance (Monad m, MonadAsync m) => Applicative (AheadT m) where {-# INLINE pure #-} pure = AheadT . K.yield {-# INLINE (<*>) #-} (<*>) = apAhead instance MonadAsync m => Monad (AheadT m) where return = pure {-# INLINE (>>=) #-} (>>=) = flip concatMapAhead ------------------------------------------------------------------------------ -- Other instances ------------------------------------------------------------------------------ MONAD_COMMON_INSTANCES(AheadT, MONADPARALLEL)