concurrent-extra-0.3: Extra concurrency primitives

MaintainerBas van Dijk <> , Roel van Dijk <>



Standard threads extended with the ability to wait for their termination.

Inspired by:

This module re-implements several functions from Control.Concurrent. Avoid ambiguities by importing one or both qualified. We suggest importing this module like:

 import qualified Control.Concurrent.Thread as Thread ( ... )



data ThreadId α Source

A ThreadId α is an abstract type representing a handle to a thread that is executing or has executed a computation of type IO α.

ThreadId α is an instance of Eq, Ord and Show, where the Ord instance implements an arbitrary total ordering over ThreadIds. The Show instance lets you convert an arbitrary-valued ThreadId to string form; showing a ThreadId value is occasionally useful when debugging or diagnosing the behaviour of a concurrent program.


threadId :: ThreadId α -> ThreadIdSource

Extract the underlying ThreadId (Conctrol.Concurrent.ThreadId).

forkIO :: IO α -> IO (ThreadId α)Source

Sparks off a new thread to run the given IO computation and returns the ThreadId of the newly created thread.

The new thread will be a lightweight thread; if you want to use a foreign library that uses thread-local storage, use forkOS instead.

GHC note: the new thread inherits the blocked state of the parent (see block).

The newly created thread has an exception handler that discards the exceptions BlockedOnDeadMVar, BlockedIndefinitely, and ThreadKilled, and passes all other exceptions to the uncaught exception handler (see setUncaughtExceptionHandler).

forkOS :: IO α -> IO (ThreadId α)Source

Like forkIO, this sparks off a new thread to run the given IO computation and returns the ThreadId of the newly created thread.

Unlike forkIO, forkOS creates a bound thread, which is necessary if you need to call foreign (non-Haskell) libraries that make use of thread-local state, such as OpenGL (see Control.Concurrent).

Using forkOS instead of forkIO makes no difference at all to the scheduling behaviour of the Haskell runtime system. It is a common misconception that you need to use forkOS instead of forkIO to avoid blocking all the Haskell threads when making a foreign call; this isn't the case. To allow foreign calls to be made without blocking all the Haskell threads (with GHC), it is only necessary to use the -threaded option when linking your program, and to make sure the foreign import is not marked unsafe.

wait :: ThreadId α -> IO (Either SomeException α)Source

Block until the given thread is terminated.

  • Returns Right x if the thread terminated normally and returned x.
  • Returns Left e if some exception e was thrown in the thread and wasn't caught.

waitTimeout :: ThreadId α -> Integer -> IO (Maybe (Either SomeException α))Source

Block until the given thread is terminated or until a timer expires.

  • Returns Nothing if a timeout occurred.
  • Returns Just the result wait would return when the thread finished within the specified time.

The timeout is specified in microseconds.

isRunning :: ThreadId α -> IO BoolSource

Returns True if the given thread is currently running.

Notice that this observation is only a snapshot of a thread's state. By the time a program reacts on its result it may already be out of date.

killThread :: ThreadId α -> IO ()Source

killThread terminates the given thread (GHC only). Any work already done by the thread isn't lost: the computation is suspended until required by another thread. The memory used by the thread will be garbage collected if it isn't referenced from anywhere. The killThread function is defined in terms of throwTo.

This function blocks until the target thread is terminated. It is a no-op if the target thread has already completed.

killThreadTimeout :: ThreadId α -> Integer -> IO BoolSource

Like killThread but with a timeout. Returns True if the target thread was terminated within the given amount of time, False otherwise.

The timeout is specified in microseconds.

Note that even when a timeout occurs, the target thread can still terminate at a later time as a direct result of calling this function.

throwTo :: Exception e => ThreadId α -> e -> IO ()Source

throwTo raises an arbitrary exception in the target thread (GHC only).

throwTo does not return until the exception has been raised in the target thread. The calling thread can thus be certain that the target thread has received the exception. This is a useful property to know when dealing with race conditions: eg. if there are two threads that can kill each other, it is guaranteed that only one of the threads will get to kill the other.

If the target thread is currently making a foreign call, then the exception will not be raised (and hence throwTo will not return) until the call has completed. This is the case regardless of whether the call is inside a block or not.

Important note: the behaviour of throwTo differs from that described in the paper "Asynchronous exceptions in Haskell" ( In the paper, throwTo is non-blocking; but the library implementation adopts a more synchronous design in which throwTo does not return until the exception is received by the target thread. The trade-off is discussed in Section 9 of the paper. Like any blocking operation, throwTo is therefore interruptible (see Section 5.3 of the paper).

There is currently no guarantee that the exception delivered by throwTo will be delivered at the first possible opportunity. In particular, a thread may unblock and then re-block exceptions without receiving a pending throwTo. This is arguably undesirable behaviour.