base- Basic librariesSource codeContentsIndex
Portabilitynon-portable (GHC extensions)
Forking and suchlike
Basic concurrency stuff.
data ThreadId = ThreadId ThreadId#
forkIO :: IO () -> IO ThreadId
forkOnIO :: Int -> IO () -> IO ThreadId
numCapabilities :: Int
childHandler :: Exception -> IO ()
myThreadId :: IO ThreadId
killThread :: ThreadId -> IO ()
throwTo :: ThreadId -> Exception -> IO ()
par :: a -> b -> b
pseq :: a -> b -> b
yield :: IO ()
labelThread :: ThreadId -> String -> IO ()
threadDelay :: Int -> IO ()
registerDelay :: Int -> IO (TVar Bool)
threadWaitRead :: Fd -> IO ()
threadWaitWrite :: Fd -> IO ()
data MVar a
newMVar :: a -> IO (MVar a)
newEmptyMVar :: IO (MVar a)
takeMVar :: MVar a -> IO a
putMVar :: MVar a -> a -> IO ()
tryTakeMVar :: MVar a -> IO (Maybe a)
tryPutMVar :: MVar a -> a -> IO Bool
isEmptyMVar :: MVar a -> IO Bool
addMVarFinalizer :: MVar a -> IO () -> IO ()
data STM a
atomically :: STM a -> IO a
retry :: STM a
orElse :: STM a -> STM a -> STM a
catchSTM :: STM a -> (Exception -> STM a) -> STM a
alwaysSucceeds :: STM a -> STM ()
always :: STM Bool -> STM ()
data TVar a
newTVar :: a -> STM (TVar a)
newTVarIO :: a -> IO (TVar a)
readTVar :: TVar a -> STM a
writeTVar :: TVar a -> a -> STM ()
unsafeIOToSTM :: IO a -> STM a
signalHandlerLock :: MVar ()
ensureIOManagerIsRunning :: IO ()
data ThreadId Source

A ThreadId is an abstract type representing a handle to a thread. ThreadId 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.

Note: in GHC, if you have a ThreadId, you essentially have a pointer to the thread itself. This means the thread itself can't be garbage collected until you drop the ThreadId. This misfeature will hopefully be corrected at a later date.

Note: Hugs does not provide any operations on other threads; it defines ThreadId as a synonym for ().

ThreadId ThreadId#
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Forking and suchlike
forkIO :: IO () -> IO ThreadIdSource

Sparks off a new thread to run the IO computation passed as the first argument, 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).

forkOnIO :: Int -> IO () -> IO ThreadIdSource

Like forkIO, but lets you specify on which CPU the thread is created. Unlike a forkIO thread, a thread created by forkOnIO will stay on the same CPU for its entire lifetime (forkIO threads can migrate between CPUs according to the scheduling policy). forkOnIO is useful for overriding the scheduling policy when you know in advance how best to distribute the threads.

The Int argument specifies the CPU number; it is interpreted modulo numCapabilities (note that it actually specifies a capability number rather than a CPU number, but to a first approximation the two are equivalent).

numCapabilities :: IntSource
the value passed to the +RTS -N flag. This is the number of Haskell threads that can run truly simultaneously at any given time, and is typically set to the number of physical CPU cores on the machine.
childHandler :: Exception -> IO ()Source
myThreadId :: IO ThreadIdSource
Returns the ThreadId of the calling thread (GHC only).
killThread :: ThreadId -> 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:

 killThread tid = throwTo tid (AsyncException ThreadKilled)
throwTo :: ThreadId -> Exception -> 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 8 of the paper. Like any blocking operation, throwTo is therefore interruptible (see Section 4.3 of the paper).

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

par :: a -> b -> bSource
pseq :: a -> b -> bSource
yield :: IO ()Source
The yield action allows (forces, in a co-operative multitasking implementation) a context-switch to any other currently runnable threads (if any), and is occasionally useful when implementing concurrency abstractions.
labelThread :: ThreadId -> String -> IO ()Source

labelThread stores a string as identifier for this thread if you built a RTS with debugging support. This identifier will be used in the debugging output to make distinction of different threads easier (otherwise you only have the thread state object's address in the heap).

Other applications like the graphical Concurrent Haskell Debugger ( may choose to overload labelThread for their purposes as well.

threadDelay :: Int -> IO ()Source

Suspends the current thread for a given number of microseconds (GHC only).

There is no guarantee that the thread will be rescheduled promptly when the delay has expired, but the thread will never continue to run earlier than specified.

registerDelay :: Int -> IO (TVar Bool)Source
Set the value of returned TVar to True after a given number of microseconds. The caveats associated with threadDelay also apply.
threadWaitRead :: Fd -> IO ()Source
Block the current thread until data is available to read on the given file descriptor (GHC only).
threadWaitWrite :: Fd -> IO ()Source
Block the current thread until data can be written to the given file descriptor (GHC only).
data MVar aSource
An MVar (pronounced "em-var") is a synchronising variable, used for communication between concurrent threads. It can be thought of as a a box, which may be empty or full.
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newMVar :: a -> IO (MVar a)Source
Create an MVar which contains the supplied value.
newEmptyMVar :: IO (MVar a)Source
Create an MVar which is initially empty.
takeMVar :: MVar a -> IO aSource

Return the contents of the MVar. If the MVar is currently empty, takeMVar will wait until it is full. After a takeMVar, the MVar is left empty.

There are two further important properties of takeMVar:

  • takeMVar is single-wakeup. That is, if there are multiple threads blocked in takeMVar, and the MVar becomes full, only one thread will be woken up. The runtime guarantees that the woken thread completes its takeMVar operation.
  • When multiple threads are blocked on an MVar, they are woken up in FIFO order. This is useful for providing fairness properties of abstractions built using MVars.
putMVar :: MVar a -> a -> IO ()Source

Put a value into an MVar. If the MVar is currently full, putMVar will wait until it becomes empty.

There are two further important properties of putMVar:

  • putMVar is single-wakeup. That is, if there are multiple threads blocked in putMVar, and the MVar becomes empty, only one thread will be woken up. The runtime guarantees that the woken thread completes its putMVar operation.
  • When multiple threads are blocked on an MVar, they are woken up in FIFO order. This is useful for providing fairness properties of abstractions built using MVars.
tryTakeMVar :: MVar a -> IO (Maybe a)Source
A non-blocking version of takeMVar. The tryTakeMVar function returns immediately, with Nothing if the MVar was empty, or Just a if the MVar was full with contents a. After tryTakeMVar, the MVar is left empty.
tryPutMVar :: MVar a -> a -> IO BoolSource
A non-blocking version of putMVar. The tryPutMVar function attempts to put the value a into the MVar, returning True if it was successful, or False otherwise.
isEmptyMVar :: MVar a -> IO BoolSource

Check whether a given MVar is empty.

Notice that the boolean value returned is just a snapshot of the state of the MVar. By the time you get to react on its result, the MVar may have been filled (or emptied) - so be extremely careful when using this operation. Use tryTakeMVar instead if possible.

addMVarFinalizer :: MVar a -> IO () -> IO ()Source
Add a finalizer to an MVar (GHC only). See Foreign.ForeignPtr and System.Mem.Weak for more about finalizers.
data STM aSource
A monad supporting atomic memory transactions.
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atomically :: STM a -> IO aSource

Perform a series of STM actions atomically.

You cannot use atomically inside an unsafePerformIO or unsafeInterleaveIO. Any attempt to do so will result in a runtime error. (Reason: allowing this would effectively allow a transaction inside a transaction, depending on exactly when the thunk is evaluated.)

However, see newTVarIO, which can be called inside unsafePerformIO, and which allows top-level TVars to be allocated.

retry :: STM aSource
Retry execution of the current memory transaction because it has seen values in TVars which mean that it should not continue (e.g. the TVars represent a shared buffer that is now empty). The implementation may block the thread until one of the TVars that it has read from has been udpated. (GHC only)
orElse :: STM a -> STM a -> STM aSource
Compose two alternative STM actions (GHC only). If the first action completes without retrying then it forms the result of the orElse. Otherwise, if the first action retries, then the second action is tried in its place. If both actions retry then the orElse as a whole retries.
catchSTM :: STM a -> (Exception -> STM a) -> STM aSource
Exception handling within STM actions.
alwaysSucceeds :: STM a -> STM ()Source
alwaysSucceeds adds a new invariant that must be true when passed to alwaysSucceeds, at the end of the current transaction, and at the end of every subsequent transaction. If it fails at any of those points then the transaction violating it is aborted and the exception raised by the invariant is propagated.
always :: STM Bool -> STM ()Source
always is a variant of alwaysSucceeds in which the invariant is expressed as an STM Bool action that must return True. Returning False or raising an exception are both treated as invariant failures.
data TVar aSource
Shared memory locations that support atomic memory transactions.
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newTVar :: a -> STM (TVar a)Source
Create a new TVar holding a value supplied
newTVarIO :: a -> IO (TVar a)Source
IO version of newTVar. This is useful for creating top-level TVars using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.
readTVar :: TVar a -> STM aSource
Return the current value stored in a TVar
writeTVar :: TVar a -> a -> STM ()Source
Write the supplied value into a TVar
unsafeIOToSTM :: IO a -> STM aSource
Unsafely performs IO in the STM monad.
signalHandlerLock :: MVar ()Source
ensureIOManagerIsRunning :: IO ()Source
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