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GHC 9.14.1 · lts/ghc-9.14.x · 7a6c33e · 2026-10-12

Modulethreads-0.5.1.8Haskell2010

Control.Concurrent.Thread.Group

This module extends Control.Concurrent.Thread with the ability to wait for a group of threads to terminate.

This module exports equivalently named functions from Control.Concurrent, (GHC.Conc), and Control.Concurrent.Thread. Avoid ambiguities by importing this module qualified. May we suggest:

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

Contents

Forking threads

Synopsis

newtype ThreadGroup
value new
value nrOfRunning
value wait
value waitN
value forkIO
value forkOS
value forkOn
value forkIOWithUnmask
value forkOnWithUnmask

Packagethreads-0.5.1.8
Exports10
LanguageHaskell2010
LicenceBSD-3-Clause
SourceGroup.hs

newtypenewtype ThreadGroup

A ThreadGroup can be understood as a counter which counts the number of threads that were added to the group minus the ones that have terminated.

More formally a ThreadGroup has the following semantics:

new initializes the counter to 0.
Forking a thread increments the counter.
When a forked thread terminates, whether normally or by raising an exception, the counter is decremented.
nrOfRunning yields a transaction that returns the counter.
wait blocks as long as the counter is greater than 0.
waitN blocks as long as the counter is greater or equal to the specified number.

Instances1Eq

Eq ThreadGroupDefined in threads-0.5.1.8 · Control.Concurrent.Thread.Group

valuenew :: IO ThreadGroup

Create an empty group of threads.

valuenrOfRunning :: ThreadGroup -> STM Int

Yield a transaction that returns the number of running threads in the group.

Note that because this function yields a STM computation, the returned number is guaranteed to be consistent inside the transaction.

valuewait :: ThreadGroup -> IO ()

Block until all threads in the group have terminated.

Note that: wait = waitN 1.

valuewaitN :: Int -> ThreadGroup -> IO ()

Block until there are fewer than N running threads in the group.

Forking threads

valueforkIO :: ThreadGroup -> IO a -> IO (ThreadId, IO (Result a))

Same as Control.Concurrent.Thread.Thread.forkIO but additionaly adds the thread to the group.

valueforkOS :: ThreadGroup -> IO a -> IO (ThreadId, IO (Result a))

Same as Control.Concurrent.Thread.Thread.forkOS but additionaly adds the thread to the group.

valueforkOn :: Int -> ThreadGroup -> IO a -> IO (ThreadId, IO (Result a))

Same as Control.Concurrent.Thread.Thread.forkOn but additionaly adds the thread to the group.

valueforkIOWithUnmask

:: ThreadGroup
-> (forall b. IO b -> IO b) -> IO a
-> IO (ThreadId, IO (Result a))

Same as Control.Concurrent.Thread.Thread.forkIOWithUnmask but additionaly adds the thread to the group.

valueforkOnWithUnmask

:: Int
-> ThreadGroup
-> (forall b. IO b -> IO b) -> IO a
-> IO (ThreadId, IO (Result a))

Like Control.Concurrent.Thread.Thread.forkOnWithUnmask but additionaly adds the thread to the group.