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323 lines
12 KiB
Haskell
323 lines
12 KiB
Haskell
{-# LANGUAGE ExistentialQuantification #-}
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{-# LANGUAGE RankNTypes #-}
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-- | Concurrent monads with a fixed scheduler: internal types and
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-- functions.
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module Control.Monad.Conc.Fixed.Internal where
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import Control.Monad (liftM, mapAndUnzipM)
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import Control.Monad.Cont (Cont, runCont)
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import Data.Map (Map)
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import Data.Maybe (catMaybes, fromJust, isNothing)
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import qualified Data.Map as M
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-- * Types
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-- | The underlying monad is based on continuations over Actions.
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type M n r a = Cont (Action n r) a
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-- | CVars are represented as a reference containing a maybe value,
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-- and a list of things blocked on it.
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type R r a = r (Maybe a, [Block])
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-- | Doing this with a typeclass proved to be really hard, so here's a
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-- dict of methods for different implementations to override!
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data Fixed c n r t = F
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{ newRef :: forall a. a -> n (r a)
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-- ^ Create a new reference
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, readRef :: forall a. r a -> n a
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-- ^ Read a reference.
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, writeRef :: forall a. r a -> a -> n ()
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-- ^ Overwrite the contents of a reference.
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, liftN :: forall a. n a -> c t a
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-- ^ Lift an action from the underlying monad
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, getCont :: forall a. c t a -> M n r a
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-- ^ Unpack the continuation-based computation from its wrapping
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-- type.
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}
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-- * Running @Conc@ monads
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-- | Scheduling is done in terms of a trace of 'Action's. Blocking can
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-- only occur as a result of an action, and they cover (most of) the
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-- primitives of the concurrency. 'spawn' is absent as it can be
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-- derived from 'new', 'fork' and 'put'.
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data Action n r =
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AFork (Action n r) (Action n r)
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| forall a. APut (R r a) a (Action n r)
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| forall a. ATryPut (R r a) a (Bool -> Action n r)
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| forall a. AGet (R r a) (a -> Action n r)
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| forall a. ATake (R r a) (a -> Action n r)
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| forall a. ATryTake (R r a) (Maybe a -> Action n r)
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| ANew (n (Action n r))
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| ALift (n (Action n r))
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| AStop
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-- | Every live thread has a unique identitifer. These are implemented
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-- as integers, but you shouldn't assume they are necessarily
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-- contiguous.
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type ThreadId = Int
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-- | A @Scheduler@ maintains some internal state, @s@, takes the
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-- 'ThreadId' of the last thread scheduled, and the list of runnable
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-- threads (which will never be empty). It produces a 'ThreadId' to
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-- schedule, and a new state.
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--
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-- Note: In order to prevent deadlock, the 'Conc' runtime will assume
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-- that a deadlock situation has arisen if the scheduler attempts to
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-- (a) schedule a blocked thread, or (b) schedule a nonexistant
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-- thread. In either of those cases, the computation will be halted.
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type Scheduler s = s -> ThreadId -> [ThreadId] -> (ThreadId, s)
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-- | One of the outputs of the runner is a @Trace@, which is just a
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-- log of threads and actions they have taken.
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type Trace = [(ThreadId, ThreadAction)]
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-- | All the actions that a thread can perform.
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data ThreadAction =
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Fork ThreadId
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-- ^ Start a new thread.
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| New
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-- ^ Create a new 'CVar'.
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| Put [ThreadId]
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-- ^ Put into a 'CVar', possibly waking up some threads.
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| BlockedPut
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-- ^ Get blocked on a put.
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| TryPut Bool [ThreadId]
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-- ^ Try to put into a 'CVar', possibly waking up some threads.
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| Read
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-- ^ Read from a 'CVar'.
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| BlockedRead
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-- ^ Get blocked on a read.
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| Take [ThreadId]
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-- ^ Take from a 'CVar', possibly waking up some threads.
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| BlockedTake
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-- ^ Get blocked on a take.
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| TryTake Bool [ThreadId]
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-- ^ Try to take from a 'CVar', possibly waking up some threads.
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| Lift
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-- ^ Lift an action from the underlying monad.
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| Stop
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-- ^ Cease execution and terminate.
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deriving (Eq, Show)
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-- | Run a concurrent computation with a given 'Scheduler' and initial
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-- state, returning a 'Just' if it terminates, and 'Nothing' if a
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-- deadlock is detected.
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runFixed :: (Monad (c t), Monad n) => Fixed c n r t
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-> Scheduler s -> s -> c t a -> n (Maybe a)
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runFixed fixed sched s ma = liftM (\(a,_,_) -> a) $ runFixed' fixed sched s ma
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-- | Variant of 'runConc' which returns the final state of the
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-- scheduler and an execution trace.
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runFixed' :: (Monad (c t), Monad n) => Fixed c n r t
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-> Scheduler s -> s -> c t a -> n (Maybe a, s, Trace)
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runFixed' fixed sched s ma = do
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ref <- newRef fixed Nothing
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let c = getCont fixed $ ma >>= liftN fixed . writeRef fixed ref . Just
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let threads = M.fromList [(0, (runCont c $ const AStop, False))]
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(s', trace) <- runThreads fixed [] (negate 1) sched s threads ref
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out <- readRef fixed ref
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return (out, s', reverse trace)
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-- * Running threads
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-- | A @Block@ is used to determine what sort of block a thread is
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-- experiencing.
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data Block = WaitFull ThreadId | WaitEmpty ThreadId deriving Eq
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-- | Threads are represented as a tuple of (next action, is blocked).
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type Threads n r = Map ThreadId (Action n r, Bool)
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-- | Run a collection of threads, until there are no threads left.
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--
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-- A thread is represented as a tuple of (next action, is blocked).
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--
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-- Note: this returns the trace in reverse order, because it's more
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-- efficient to prepend to a list than append. As this function isn't
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-- exposed to users of the library, this is just an internal gotcha to
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-- watch out for.
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runThreads :: (Monad (c t), Monad n) => Fixed c n r t
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-> Trace -> ThreadId -> Scheduler s -> s -> Threads n r -> r (Maybe a) -> n (s, Trace)
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runThreads fixed sofar prior sched s threads ref
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| isTerminated = return (s, sofar)
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| isDeadlocked = writeRef fixed ref Nothing >> return (s, sofar)
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| isNonexistant = writeRef fixed ref Nothing >> return (s, sofar)
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| isBlocked = writeRef fixed ref Nothing >> return (s, sofar)
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| otherwise = do
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(threads', act) <- stepThread (fst $ fromJust thread) fixed chosen threads
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let sofar' = (chosen, act) : sofar
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runThreads fixed sofar' chosen sched s' threads' ref
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where
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(chosen, s') = if prior == -1 then (0, s) else sched s prior $ M.keys runnable
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runnable = M.filter (not . snd) threads
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thread = M.lookup chosen threads
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isBlocked = snd $ fromJust thread
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isNonexistant = isNothing thread
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isTerminated = 0 `notElem` M.keys threads
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isDeadlocked = M.null runnable
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-- | Run a single thread one step, by dispatching on the type of
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-- 'Action'.
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stepThread :: (Monad (c t), Monad n)
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=> Action n r
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepThread (AFork a b) = stepFork a b
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stepThread (APut ref a c) = stepPut ref a c
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stepThread (ATryPut ref a c) = stepTryPut ref a c
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stepThread (AGet ref c) = stepGet ref c
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stepThread (ATake ref c) = stepTake ref c
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stepThread (ATryTake ref c) = stepTryTake ref c
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stepThread (ANew na) = stepNew na
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stepThread (ALift na) = stepLift na
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stepThread AStop = stepStop
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-- | Start a new thread, assigning it a unique 'ThreadId'
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stepFork :: (Monad (c t), Monad n)
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=> Action n r -> Action n r
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepFork a b _ i threads =
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let (threads', newid) = launch a threads
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in return (goto b i threads', Fork newid)
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-- | Put a value into a @CVar@, blocking the thread until it's empty.
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stepPut :: (Monad (c t), Monad n)
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=> R r a -> a -> Action n r
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepPut ref a c fixed i threads = do
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(val, blocks) <- readRef fixed ref
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case val of
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Just _ -> do
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threads' <- block fixed ref WaitEmpty i threads
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return (threads', BlockedPut)
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Nothing -> do
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writeRef fixed ref (Just a, blocks)
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(threads', woken) <- wake fixed ref WaitFull threads
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return (goto c i threads', Put woken)
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-- | Try to put a value into a @CVar@, without blocking.
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stepTryPut :: (Monad (c t), Monad n)
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=> R r a -> a -> (Bool -> Action n r)
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepTryPut ref a c fixed i threads = do
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(val, blocks) <- readRef fixed ref
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case val of
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Just _ -> return (goto (c False) i threads, TryPut False [])
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Nothing -> do
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writeRef fixed ref (Just a, blocks)
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(threads', woken) <- wake fixed ref WaitFull threads
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return (goto (c True) i threads', TryPut True woken)
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-- | Get the value from a @CVar@, without emptying, blocking the
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-- thread until it's full.
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stepGet :: (Monad (c t), Monad n)
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=> R r a -> (a -> Action n r)
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepGet ref c fixed i threads = do
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(val, _) <- readRef fixed ref
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case val of
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Just val' -> return (goto (c val') i threads, Read)
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Nothing -> do
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threads' <- block fixed ref WaitFull i threads
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return (threads', BlockedRead)
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-- | Take the value from a @CVar@, blocking the thread until it's
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-- full.
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stepTake :: (Monad (c t), Monad n)
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=> R r a -> (a -> Action n r)
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepTake ref c fixed i threads = do
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(val, blocks) <- readRef fixed ref
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case val of
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Just val' -> do
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writeRef fixed ref (Nothing, blocks)
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(threads', woken) <- wake fixed ref WaitEmpty threads
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return (goto (c val') i threads', Take woken)
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Nothing -> do
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threads' <- block fixed ref WaitFull i threads
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return (threads', BlockedTake)
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-- | Try to take the value from a @CVar@, without blocking.
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stepTryTake :: (Monad (c t), Monad n)
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=> R r a -> (Maybe a -> Action n r)
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepTryTake ref c fixed i threads = do
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(val, blocks) <- readRef fixed ref
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case val of
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Just _ -> do
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writeRef fixed ref (Nothing, blocks)
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(threads', woken) <- wake fixed ref WaitEmpty threads
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return (goto (c val) i threads', TryTake True woken)
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Nothing -> return (goto (c Nothing) i threads, TryTake False [])
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-- | Create a new @CVar@. This is exactly the same as lifting a value,
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-- except by separating the two we can (a) produce a more useful
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-- trace, and (b) make smarter pre-emption decisions.
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stepNew :: (Monad (c t), Monad n)
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=> n (Action n r)
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepNew na _ i threads = do
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a <- na
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return (goto a i threads, New)
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-- | Lift an action from the underlying monad into the @Conc@
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-- computation.
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stepLift :: (Monad (c t), Monad n)
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=> n (Action n r)
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-> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepLift na _ i threads = do
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a <- na
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return (goto a i threads, Lift)
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-- | Kill the current thread.
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stepStop :: (Monad (c t), Monad n)
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=> Fixed c n r t -> ThreadId -> Threads n r -> n (Threads n r, ThreadAction)
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stepStop _ i threads = return (kill i threads, Stop)
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-- * Manipulating threads
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-- | Replace the @Action@ of a thread.
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goto :: Action n r -> ThreadId -> Threads n r -> Threads n r
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goto a = M.alter $ \(Just (_, b)) -> Just (a, b)
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-- | Block a thread on a @CVar@.
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block :: (Monad (c t), Monad n) => Fixed c n r t
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-> R r a -> (ThreadId -> Block) -> ThreadId -> Threads n r -> n (Threads n r)
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block fixed ref typ tid threads = do
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(val, blocks) <- readRef fixed ref
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writeRef fixed ref (val, typ tid : blocks)
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return $ M.alter (\(Just (a, _)) -> Just (a, True)) tid threads
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-- | Start a thread with the next free ID.
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launch :: Action n r -> Threads n r -> (Threads n r, ThreadId)
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launch a m = (M.insert k (a, False) m, k) where
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k = succ . maximum $ M.keys m
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-- | Kill a thread.
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kill :: ThreadId -> Threads n r -> Threads n r
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kill = M.delete
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-- | Wake every thread blocked on a @CVar@ read/write.
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wake :: (Monad (c t), Monad n) => Fixed c n r t
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-> R r a -> (ThreadId -> Block) -> Threads n r -> n (Threads n r, [ThreadId])
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wake fixed ref typ m = do
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(m', woken) <- mapAndUnzipM wake' (M.toList m)
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return (M.fromList m', catMaybes woken)
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where
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wake' a@(tid, (act, True)) = do
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let blck = typ tid
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(val, blocks) <- readRef fixed ref
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if blck `elem` blocks
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then writeRef fixed ref (val, filter (/= blck) blocks) >> return ((tid, (act, False)), Just tid)
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else return (a, Nothing)
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wake' a = return (a, Nothing)
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