|Portability||non-portable (GHC extensions)|
|Basic concurrency stuff.
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
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 ().
|Forking and suchlike
Sparks off a new thread to run the IO computation passed as the
first argument, and returns the ThreadId of the newly created
The new thread will be a lightweight thread; if you want to use a foreign
library that uses thread-local storage, use Control.Concurrent.forkOS instead.
GHC note: the new thread inherits the blocked state of the parent
The newly created thread has an exception handler that discards the
exceptions BlockedIndefinitelyOnMVar, BlockedIndefinitelyOnSTM, and
ThreadKilled, and passes all other exceptions to the uncaught
exception handler (see setUncaughtExceptionHandler).
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
|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
|Returns the ThreadId of the calling thread (GHC only).
killThread raises the ThreadKilled exception in the given
thread (GHC only).
killThread tid = throwTo tid ThreadKilled
throwTo raises an arbitrary exception in the target thread (GHC only).
throwTo does not return until the exception has been raised in the
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.
Whatever work the target thread was doing when the exception was
raised is not lost: the computation is suspended until required by
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
There is no guarantee that the exception will be delivered promptly,
although the runtime will endeavour to ensure that arbitrary
delays don't occur. In GHC, an exception can only be raised when a
thread reaches a safe point, where a safe point is where memory
allocation occurs. Some loops do not perform any memory allocation
inside the loop and therefore cannot be interrupted by a throwTo.
Blocked throwTo is fair: if multiple threads are trying to throw an
exception to the same target thread, they will succeed in FIFO order.
|Internal function used by the RTS to run sparks.
|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
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
(http://www.informatik.uni-kiel.de/~fhu/chd/) may choose to overload
labelThread for their purposes as well.
|The current status of a thread
|ThreadRunning||the thread is currently runnable or running
|ThreadFinished||the thread has finished
|ThreadBlocked BlockReason||the thread is blocked on some resource
|ThreadDied||the thread received an uncaught exception
|BlockedOnMVar||blocked on on MVar
|BlockedOnBlackHole||blocked on a computation in progress by another thread
|BlockedOnException||blocked in throwTo
|BlockedOnSTM||blocked in retry in an STM transaction
|BlockedOnForeignCall||currently in a foreign call
|BlockedOnOther||blocked on some other resource. Without -threaded,
I/O and threadDelay show up as BlockedOnOther, with -threaded
they show up as BlockedOnMVar.
Suspends the current thread for a given number of microseconds
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.
|Set the value of returned TVar to True after a given number of
microseconds. The caveats associated with threadDelay also apply.
|Block the current thread until data is available to read on the
given file descriptor (GHC only).
|Block the current thread until data can be written to the
given file descriptor (GHC only).
|A monad supporting atomic memory transactions.
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 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)
|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
|Exception handling within STM actions.
|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 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.
|Shared memory locations that support atomic memory transactions.
|Create a new TVar holding a value supplied
|IO version of newTVar. This is useful for creating top-level
TVars using System.IO.Unsafe.unsafePerformIO, because using
atomically inside System.IO.Unsafe.unsafePerformIO isn't
|Return the current value stored in a TVar
Return the current value stored in a TVar.
This is equivalent to
readTVarIO = atomically . readTVar
but works much faster, because it doesn't perform a complete
transaction, it just reads the current value of the TVar.
|Write the supplied value into a TVar
Unsafely performs IO in the STM monad. Beware: this is a highly
dangerous thing to do.
- The STM implementation will often run transactions multiple
times, so you need to be prepared for this if your IO has any
- The STM implementation will abort transactions that are known to
be invalid and need to be restarted. This may happen in the middle
of unsafeIOToSTM, so make sure you don't acquire any resources
that need releasing (exception handlers are ignored when aborting
the transaction). That includes doing any IO using Handles, for
example. Getting this wrong will probably lead to random deadlocks.
- The transaction may have seen an inconsistent view of memory when
the IO runs. Invariants that you expect to be true throughout
your program may not be true inside a transaction, due to the
way transactions are implemented. Normally this wouldn't be visible
to the programmer, but using unsafeIOToSTM can expose it.
|Produced by Haddock version 2.6.1|