|Portability||non-portable (GHC extensions)|
Basic concurrency stuff.
- data ThreadId = ThreadId ThreadId#
- forkIO :: IO () -> IO ThreadId
- forkIOUnmasked :: IO () -> IO ThreadId
- forkIOWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- forkOn :: Int -> IO () -> IO ThreadId
- forkOnIO :: Int -> IO () -> IO ThreadId
- forkOnIOUnmasked :: Int -> IO () -> IO ThreadId
- forkOnWithUnmask :: Int -> ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- numCapabilities :: Int
- getNumCapabilities :: IO Int
- numSparks :: IO Int
- childHandler :: SomeException -> IO ()
- myThreadId :: IO ThreadId
- killThread :: ThreadId -> IO ()
- throwTo :: Exception e => ThreadId -> e -> IO ()
- par :: a -> b -> b
- pseq :: a -> b -> b
- runSparks :: IO ()
- yield :: IO ()
- labelThread :: ThreadId -> String -> IO ()
- data ThreadStatus
- data BlockReason
- threadStatus :: ThreadId -> IO ThreadStatus
- threadCapability :: ThreadId -> IO (Int, Bool)
- newtype STM a = STM (State# RealWorld -> (#State# RealWorld, a#))
- atomically :: STM a -> IO a
- retry :: STM a
- orElse :: STM a -> STM a -> STM a
- throwSTM :: Exception e => e -> STM a
- catchSTM :: Exception e => STM a -> (e -> STM a) -> STM a
- alwaysSucceeds :: STM a -> STM ()
- always :: STM Bool -> STM ()
- data TVar a = TVar (TVar# RealWorld a)
- newTVar :: a -> STM (TVar a)
- newTVarIO :: a -> IO (TVar a)
- readTVar :: TVar a -> STM a
- readTVarIO :: TVar a -> IO a
- writeTVar :: TVar a -> a -> STM ()
- unsafeIOToSTM :: IO a -> STM a
- withMVar :: MVar a -> (a -> IO b) -> IO b
- modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
- setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
- getUncaughtExceptionHandler :: IO (SomeException -> IO ())
- reportError :: SomeException -> IO ()
- reportStackOverflow :: IO ()
- sharedCAF :: a -> (Ptr a -> IO (Ptr a)) -> IO a
ThreadId is an abstract type representing a handle to a thread.
ThreadId is an instance of
Ord instance implements an arbitrary total ordering over
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
This misfeature will hopefully be corrected at a later date.
Note: Hugs does not provide any operations on other threads;
ThreadId as a synonym for ().
Forking and suchlike
The new thread will be a lightweight thread; if you want to use a foreign
library that uses thread-local storage, use
GHC note: the new thread inherits the masked state of the parent
The newly created thread has an exception handler that discards the
ThreadKilled, and passes all other exceptions to the uncaught
exception handler (see
This function is deprecated; use
forkIO, but the child thread is passed a function that can
be used to unmask asynchronous exceptions. This function is
typically used in the following way
... mask_ $ forkIOWithUnmask $ \unmask -> catch (unmask ...) handler
so that the exception handler in the child thread is established with asynchronous exceptions masked, meanwhile the main body of the child thread is executed in the unmasked state.
Note that the unmask function passed to the child thread should only be used in that thread; the behaviour is undefined if it is invoked in a different thread.
forkIO, but lets you specify on which processor the thread
should run. Unlike a
forkIO thread, a thread created by
will stay on the same processor for its entire lifetime (
threads can migrate between processors according to the scheduling
forkOn is useful for overriding the scheduling policy when
you know in advance how best to distribute the threads.
Int argument specifies a capability number (see
getNumCapabilities). Typically capabilities correspond to physical
processors, but the exact behaviour is implementation-dependent. The
value passed to
forkOn is interpreted modulo the total number of
capabilities as returned by
GHC note: the number of capabilities is specified by the
option when the program is started. Capabilities can be fixed to
actual processor cores with
+RTS -qa if the underlying operating
system supports that, although in practice this is usually unnecessary
(and may actually degrade perforamnce in some cases - experimentation
This function is deprecated; use
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 processor cores on
Strictly speaking it is better to use
the number of capabilities might vary at runtime.
Returns the number of Haskell threads that can run truly
simultaneously (on separate physical processors) at any given time.
The number passed to
forkOn is interpreted modulo this
An implementation in which Haskell threads are mapped directly to
OS threads might return the number of physical processor cores in
the machine, and
forkOn would be implemented using the OS's
affinity facilities. An implementation that schedules Haskell
threads onto a smaller number of OS threads (like GHC) would return
the number of such OS threads that can be running simultaneously.
GHC notes: this returns the number passed as the argument to the
+RTS -N flag. In current implementations, the value is fixed
when the program starts and never changes, but it is possible that
in the future the number of capabilities might vary at runtime.
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 another thread.
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
mask or not. However, in GHC a foreign call
can be annotated as
interruptible, in which case a
cause the RTS to attempt to cause the call to return; see the GHC
documentation for more details.
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). Unlike other interruptible operations, however,
is always interruptible, even if it does not actually block.
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
If the target of
throwTo is the calling thread, then the behaviour
is the same as
Control.Exception.throwIO, except that the exception
is thrown as an asynchronous exception. This means that if there is
an enclosing pure computation, which would be the case if the current
IO operation is inside
computation is not permanently replaced by the exception, but is
suspended as if it had received an asynchronous exception.
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).
The current status of a thread
the thread is currently runnable or running
the thread has finished
the thread is blocked on some resource
the thread received an uncaught exception
blocked on on
blocked on a computation in progress by another thread
currently in a foreign call
returns the number of the capability on which the thread is currently
running, and a boolean indicating whether the thread is locked to
that capability or not. A thread is locked to a capability if it
was created with
A monad supporting atomic memory transactions.
Perform a series of STM actions atomically.
You cannot use
atomically inside an
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.)
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 whole retries.
Throwing an exception in
STM aborts the transaction and propagates the
throw e `seq` x ===> throw e throwSTM e `seq` x ===> x
The first example will cause the exception
e to be raised,
whereas the second one won't. In fact,
throwSTM will only cause
an exception to be raised when it is used within the
throwSTM variant should be used in preference to
raise an exception within the
STM monad because it guarantees
ordering with respect to other
STM operations, whereas
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.
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
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 side effects.
- The STM implementation will abort transactions that are known to
be invalid and need to be restarted. This may happen in the middle
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
unsafeIOToSTMcan expose it.