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GHC.Conc

Portability | non-portable (GHC extensions)
Stability | internal
Maintainer | cvs-ghc@haskell.org

Description
Basic concurrency stuff.
Synopsis

data ThreadId = ThreadId ThreadId#
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 ()
threadWaitRead :: Int -> IO ()
threadWaitWrite :: Int -> 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 ()

Documentation
data ThreadId

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 `ThreadId`s. 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 ().

Constructors

ThreadId ThreadId# |

Instances

Eq ThreadId
Ord ThreadId
Show ThreadId

myThreadId :: IO ThreadId
Returns the `ThreadId` of the calling thread (GHC only).
killThread :: ThreadId -> IO ()

`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 ()

`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.

par :: a -> b -> b
pseq :: a -> b -> b
yield :: IO ()
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 ()

`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.

threadDelay :: Int -> IO ()
The `threadDelay` operation will cause the current thread to suspend for a given number of microseconds (GHC only). Note that the resolution used by the Haskell runtime system's internal timer together with the fact that the thread may take some time to be rescheduled after the time has expired, means that the accuracy is more like 1/50 second.
threadWaitRead :: Int -> IO ()
Block the current thread until data is available to read on the given file descriptor (GHC only).
threadWaitWrite :: Int -> IO ()
Block the current thread until data can be written to the given file descriptor (GHC only).
data MVar a

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.
Instances

Eq (MVar a)

newMVar :: a -> IO (MVar a)
Create an `MVar` which contains the supplied value.
newEmptyMVar :: IO (MVar a)
Create an `MVar` which is initially empty.
takeMVar :: MVar a -> IO a
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. If several threads are competing to take the same `MVar`, one is chosen to continue at random when the `MVar` becomes full.
putMVar :: MVar a -> a -> IO ()
Put a value into an `MVar`. If the `MVar` is currently full, `putMVar` will wait until it becomes empty. If several threads are competing to fill the same `MVar`, one is chosen to continue at random with the `MVar` becomes empty.
tryTakeMVar :: MVar a -> IO (Maybe a)
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 Bool
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 Bool
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 ()
Add a finalizer to an `MVar` (GHC only). See Foreign.ForeignPtr and System.Mem.Weak for more about finalizers.
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