ghcide-2.9.0.0: The core of an IDE

Safe Haskell | None
Language | GHC2021

Control.Concurrent.Strict

Synopsis

- modifyVar' :: Var a -> (a -> a) -> IO a
- modifyVarIO' :: Var a -> (a -> IO a) -> IO a
- modifyVar :: Var a -> (a -> IO (a, b)) -> IO b
- modifyVar_ :: Var a -> (a -> IO a) -> IO ()
- once :: IO a -> IO (IO a)
- data ThreadId
- forkIO :: IO () -> IO ThreadId
- forkOS :: IO () -> IO ThreadId
- forkOn :: Int -> IO () -> IO ThreadId
- forkIOWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- forkOnWithUnmask :: Int -> ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- myThreadId :: IO ThreadId
- data MVar a
- newEmptyMVar :: IO (MVar a)
- newMVar :: a -> IO (MVar a)
- takeMVar :: MVar a -> IO a
- readMVar :: MVar a -> IO a
- putMVar :: MVar a -> a -> IO ()
- tryTakeMVar :: MVar a -> IO (Maybe a)
- tryPutMVar :: MVar a -> a -> IO Bool
- tryReadMVar :: MVar a -> IO (Maybe a)
- isEmptyMVar :: MVar a -> IO Bool
- addMVarFinalizer :: MVar a -> IO () -> IO ()
- getNumCapabilities :: IO Int
- setNumCapabilities :: Int -> IO ()
- killThread :: ThreadId -> IO ()
- throwTo :: Exception e => ThreadId -> e -> IO ()
- yield :: IO ()
- threadCapability :: ThreadId -> IO (Int, Bool)
- mkWeakThreadId :: ThreadId -> IO (Weak ThreadId)
- withMVar :: MVar a -> (a -> IO b) -> IO b
- modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
- swapMVar :: MVar a -> a -> IO a
- withMVarMasked :: MVar a -> (a -> IO b) -> IO b
- modifyMVar :: MVar a -> (a -> IO (a, b)) -> IO b
- modifyMVarMasked_ :: MVar a -> (a -> IO a) -> IO ()
- modifyMVarMasked :: MVar a -> (a -> IO (a, b)) -> IO b
- mkWeakMVar :: MVar a -> IO () -> IO (Weak (MVar a))
- threadWaitRead :: Fd -> IO ()
- threadWaitWrite :: Fd -> IO ()
- threadWaitReadSTM :: Fd -> IO (STM (), IO ())
- threadWaitWriteSTM :: Fd -> IO (STM (), IO ())
- threadDelay :: Int -> IO ()
- rtsSupportsBoundThreads :: Bool
- forkOSWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId
- isCurrentThreadBound :: IO Bool
- runInBoundThread :: IO a -> IO a
- runInUnboundThread :: IO a -> IO a
- data QSem
- newQSem :: Int -> IO QSem
- waitQSem :: QSem -> IO ()
- signalQSem :: QSem -> IO ()
- data QSemN
- newQSemN :: Int -> IO QSemN
- waitQSemN :: QSemN -> Int -> IO ()
- signalQSemN :: QSemN -> Int -> IO ()
- data Chan a
- newChan :: IO (Chan a)
- writeChan :: Chan a -> a -> IO ()
- readChan :: Chan a -> IO a
- dupChan :: Chan a -> IO (Chan a)
- getChanContents :: Chan a -> IO [a]
- writeList2Chan :: Chan a -> [a] -> IO ()
- forkFinally :: IO a -> (Either SomeException a -> IO ()) -> IO ThreadId
- data Barrier a
- data Var a
- data Lock
- withNumCapabilities :: Int -> IO a -> IO a
- onceFork :: IO a -> IO (IO a)
- newLock :: IO Lock
- withLock :: Lock -> IO a -> IO a
- withLockTry :: Lock -> IO a -> IO (Maybe a)
- newVar :: a -> IO (Var a)
- readVar :: Var a -> IO a
- writeVar :: Var a -> a -> IO ()
- writeVar' :: Var a -> a -> IO ()
- modifyVar_' :: Var a -> (a -> IO a) -> IO ()
- withVar :: Var a -> (a -> IO b) -> IO b
- newBarrier :: IO (Barrier a)
- signalBarrier :: Partial => Barrier a -> a -> IO ()
- waitBarrier :: Barrier a -> IO a
- waitBarrierMaybe :: Barrier a -> IO (Maybe a)

# Documentation

modifyVar' :: Var a -> (a -> a) -> IO a #

Strict modification that returns the new value

modifyVarIO' :: Var a -> (a -> IO a) -> IO a #

Strict modification that returns the new value

modifyVar :: Var a -> (a -> IO (a, b)) -> IO b #

modifyVar_ :: Var a -> (a -> IO a) -> IO () #

once :: IO a -> IO (IO a) #

Given an action, produce a wrapped action that runs at most once. If the function raises an exception, the same exception will be reraised each time.

    let x ||| y = do t1 <- onceFork x; t2 <- onceFork y; t1; t2
    \(x :: IO Int) -> void (once x) == pure ()
    \(x :: IO Int) -> join (once x) == x
    \(x :: IO Int) -> (do y <- once x; y; y) == x
    \(x :: IO Int) -> (do y <- once x; y ||| y) == x

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 would be difficult to correct while continuing to support `threadStatus`.

#### Instances

Instances details

NFData ThreadId | Since: deepseq-1.4.0.0
Instance details Defined in Control.DeepSeq Methods rnf :: ThreadId -> () #
Show ThreadId | @since base-4.2.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods showsPrec :: Int -> ThreadId -> ShowS # show :: ThreadId -> String # showList :: [ThreadId] -> ShowS #
Eq ThreadId | @since base-4.2.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods (==) :: ThreadId -> ThreadId -> Bool # (/=) :: ThreadId -> ThreadId -> Bool #
Ord ThreadId | @since base-4.2.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods compare :: ThreadId -> ThreadId -> Ordering # (<) :: ThreadId -> ThreadId -> Bool # (<=) :: ThreadId -> ThreadId -> Bool # (>) :: ThreadId -> ThreadId -> Bool # (>=) :: ThreadId -> ThreadId -> Bool # max :: ThreadId -> ThreadId -> ThreadId # min :: ThreadId -> ThreadId -> ThreadId #
Hashable ThreadId |
Instance details Defined in Data.Hashable.Class Methods hashWithSalt :: Int -> ThreadId -> Int # hash :: ThreadId -> Int #
PrimUnlifted ThreadId |
Instance details Defined in Data.Primitive.Unlifted.Class Associated Types
type Unlifted ThreadId |
Instance details Defined in Data.Primitive.Unlifted.Class type Unlifted ThreadId = ThreadId#
Methods toUnlifted# :: ThreadId -> Unlifted ThreadId # fromUnlifted# :: Unlifted ThreadId -> ThreadId #
type Unlifted ThreadId |
Instance details Defined in Data.Primitive.Unlifted.Class type Unlifted ThreadId = ThreadId#

forkIO :: IO () -> IO ThreadId #

Creates 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, unbound thread. Foreign calls made by this thread are not guaranteed to be made by any particular OS thread; if you need foreign calls to be made by a particular OS thread, then use `forkOS` instead.

The new thread inherits the masked state of the parent (see `mask`).

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.

WARNING: Exceptions in the new thread will not be rethrown in the thread that created it. This means that you might be completely unaware of the problem if/when this happens. You may want to use the async library instead.

forkOS :: IO () -> IO ThreadId #

Like `forkIO`, this sparks off a new thread to run the `IO` computation passed as the first argument, and returns the `ThreadId` of the newly created thread.

However, `forkOS` creates a bound thread, which is necessary if you need to call foreign (non-Haskell) libraries that make use of thread-local state, such as OpenGL (see Control.Concurrent).

Using `forkOS` instead of `forkIO` makes no difference at all to the scheduling behaviour of the Haskell runtime system. It is a common misconception that you need to use `forkOS` instead of `forkIO` to avoid blocking all the Haskell threads when making a foreign call; this isn't the case. To allow foreign calls to be made without blocking all the Haskell threads (with GHC), it is only necessary to use the `-threaded` option when linking your program, and to make sure the foreign import is not marked `unsafe`.

forkOn :: Int -> IO () -> IO ThreadId #

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

The `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 `getNumCapabilities`.

GHC note: the number of capabilities is specified by the `+RTS -N` 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 performance in some cases - experimentation is recommended).

@since base-4.4.0.0

forkIOWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId #

Like `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.

@since base-4.4.0.0

forkOnWithUnmask :: Int -> ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId #

Like `forkIOWithUnmask`, but the child thread is pinned to the given CPU, as with `forkOn`.

@since base-4.4.0.0

myThreadId :: IO ThreadId #

Returns the `ThreadId` of the calling thread (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 box, which may be empty or full.

#### Instances

Instances details

NFData1 MVar | Since: deepseq-1.4.3.0
Instance details Defined in Control.DeepSeq Methods liftRnf :: (a -> ()) -> MVar a -> () #
NFData (MVar a) | NOTE: Only strict in the reference and not the referenced value. Since: deepseq-1.4.2.0
Instance details Defined in Control.DeepSeq Methods rnf :: MVar a -> () #
Eq (MVar a) | Compares the underlying pointers. @since base-4.1.0.0
Instance details Defined in GHC.Internal.MVar Methods (==) :: MVar a -> MVar a -> Bool # (/=) :: MVar a -> MVar a -> Bool #
PrimUnlifted (MVar a) |
Instance details Defined in Data.Primitive.Unlifted.Class Associated Types
type Unlifted (MVar a) |
Instance details Defined in Data.Primitive.Unlifted.Class type Unlifted (MVar a) = MVar# RealWorld a
Methods toUnlifted# :: MVar a -> Unlifted (MVar a) # fromUnlifted# :: Unlifted (MVar a) -> MVar a #
type Unlifted (MVar a) |
Instance details Defined in Data.Primitive.Unlifted.Class type Unlifted (MVar a) = MVar# RealWorld a

newEmptyMVar :: IO (MVar a) #

Create an `MVar` which is initially empty.

newMVar :: a -> IO (MVar a) #

Create an `MVar` which contains the supplied value.

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.

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 `MVar`s.

readMVar :: MVar a -> IO a #

Atomically read the contents of an `MVar`. If the `MVar` is currently empty, `readMVar` will wait until it is full. `readMVar` is guaranteed to receive the next `putMVar`.

`readMVar` is multiple-wakeup, so when multiple readers are blocked on an `MVar`, all of them are woken up at the same time. The runtime guarantees that all woken threads complete their `readMVar` operation.

Compatibility note: Prior to base 4.7, `readMVar` was a combination of `takeMVar` and `putMVar`. This mean that in the presence of other threads attempting to `putMVar`, `readMVar` could block. Furthermore, `readMVar` would not receive the next `putMVar` if there was already a pending thread blocked on `takeMVar`. The old behavior can be recovered by implementing 'readMVar as follows:

    readMVar :: MVar a -> IO a
    readMVar m =
      mask_ $ do
        a <- takeMVar m
        putMVar m a
        return a

putMVar :: MVar a -> a -> IO () #

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 `MVar`s.

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.

tryReadMVar :: MVar a -> IO (Maybe a) #

A non-blocking version of `readMVar`. The `tryReadMVar` function returns immediately, with `Nothing` if the `MVar` was empty, or ``Just` a` if the `MVar` was full with contents `a`.

@since base-4.7.0.0

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 () #

getNumCapabilities :: IO Int #

Returns the number of Haskell threads that can run truly simultaneously (on separate physical processors) at any given time. To change this value, use `setNumCapabilities`.

@since base-4.4.0.0

setNumCapabilities :: Int -> IO () #

Set 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 value. The initial value is given by the `+RTS -N` runtime flag.

This is also the number of threads that will participate in parallel garbage collection. It is strongly recommended that the number of capabilities is not set larger than the number of physical processor cores, and it may often be beneficial to leave one or more cores free to avoid contention with other processes in the machine.

@since base-4.5.0.0

killThread :: ThreadId -> IO () #

`killThread` raises the `ThreadKilled` exception in the given thread (GHC only).

    killThread tid = throwTo tid ThreadKilled

throwTo :: Exception e => ThreadId -> e -> IO () #

`throwTo` raises an arbitrary exception in the target thread (GHC only).

Exception delivery synchronizes between the source and target thread: `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. Exception delivery is also atomic with respect to other exceptions. Atomicity is a useful property to have when dealing with race conditions: e.g. 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 `throwTo` will 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" (http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm). 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, `throwTo` 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 `throwTo`.

If the target of `throwTo` is the calling thread, then the behaviour is the same as `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 `unsafePerformIO` or `unsafeInterleaveIO`, that computation is not permanently replaced by the exception, but is suspended as if it had received an asynchronous exception.

Note that if `throwTo` is called with the current thread as the target, the exception will be thrown even if the thread is currently inside `mask` or `uninterruptibleMask`.

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.

threadCapability :: ThreadId -> IO (Int, Bool) #

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 `forkOn`.

@since base-4.4.0.0

mkWeakThreadId :: ThreadId -> IO (Weak ThreadId) #

Make a weak pointer to a `ThreadId`. It can be important to do this if you want to hold a reference to a `ThreadId` while still allowing the thread to receive the `BlockedIndefinitely` family of exceptions (e.g. `BlockedIndefinitelyOnMVar`). Holding a normal `ThreadId` reference will prevent the delivery of `BlockedIndefinitely` exceptions because the reference could be used as the target of `throwTo` at any time, which would unblock the thread.

Holding a `Weak ThreadId`, on the other hand, will not prevent the thread from receiving `BlockedIndefinitely` exceptions. It is still possible to throw an exception to a `Weak ThreadId`, but the caller must use `deRefWeak` first to determine whether the thread still exists.

@since base-4.6.0.0

withMVar :: MVar a -> (a -> IO b) -> IO b #

`withMVar` is an exception-safe wrapper for operating on the contents of an `MVar`. This operation is exception-safe: it will replace the original contents of the `MVar` if an exception is raised (see Control.Exception). However, it is only atomic if there are no other producers for this `MVar`. In other words, it cannot guarantee that, by the time `withMVar` gets the chance to write to the MVar, the value of the MVar has not been altered by a write operation from another thread.

modifyMVar_ :: MVar a -> (a -> IO a) -> IO () #

An exception-safe wrapper for modifying the contents of an `MVar`. Like `withMVar`, `modifyMVar` will replace the original contents of the `MVar` if an exception is raised during the operation. This function is only atomic if there are no other producers for this `MVar`. In other words, it cannot guarantee that, by the time `modifyMVar_` gets the chance to write to the MVar, the value of the MVar has not been altered by a write operation from another thread.

swapMVar :: MVar a -> a -> IO a #

Take a value from an `MVar`, put a new value into the `MVar` and return the value taken. This function is atomic only if there are no other producers for this `MVar`. In other words, it cannot guarantee that, by the time `swapMVar` gets the chance to write to the MVar, the value of the MVar has not been altered by a write operation from another thread.

withMVarMasked :: MVar a -> (a -> IO b) -> IO b #

Like `withMVar`, but the `IO` action in the second argument is executed with asynchronous exceptions masked.

@since base-4.7.0.0

modifyMVar :: MVar a -> (a -> IO (a, b)) -> IO b #

A slight variation on `modifyMVar_` that allows a value to be returned (`b`) in addition to the modified value of the `MVar`.

modifyMVarMasked_ :: MVar a -> (a -> IO a) -> IO () #

Like `modifyMVar_`, but the `IO` action in the second argument is executed with asynchronous exceptions masked.

@since base-4.6.0.0

modifyMVarMasked :: MVar a -> (a -> IO (a, b)) -> IO b #

Like `modifyMVar`, but the `IO` action in the second argument is executed with asynchronous exceptions masked.

@since base-4.6.0.0

mkWeakMVar :: MVar a -> IO () -> IO (Weak (MVar a)) #

Make a `Weak` pointer to an `MVar`, using the second argument as a finalizer to run when the `MVar` is garbage-collected

@since base-4.6.0.0

threadWaitRead :: Fd -> IO () #

Block the current thread until data is available to read on the given file descriptor (GHC only).

This will throw an `IOError` if the file descriptor was closed while this thread was blocked. To safely close a file descriptor that has been used with `threadWaitRead`, use `closeFdWith`.

threadWaitWrite :: Fd -> IO () #

Block the current thread until data can be written to the given file descriptor (GHC only).

This will throw an `IOError` if the file descriptor was closed while this thread was blocked. To safely close a file descriptor that has been used with `threadWaitWrite`, use `closeFdWith`.

threadWaitReadSTM :: Fd -> IO (STM (), IO ()) #

Returns an STM action that can be used to wait for data to read from a file descriptor. The second returned value is an IO action that can be used to deregister interest in the file descriptor.

Since: base-4.7.0.0

threadWaitWriteSTM :: Fd -> IO (STM (), IO ()) #

Returns an STM action that can be used to wait until data can be written to a file descriptor. The second returned value is an IO action that can be used to deregister interest in the file descriptor.

Since: base-4.7.0.0

threadDelay :: Int -> IO () #

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.

Be careful not to exceed `maxBound :: Int`, which on 32-bit machines is only 2147483647 μs, less than 36 minutes. Consider using `Control.Concurrent.Thread.Delay.delay` from `unbounded-delays` package.

rtsSupportsBoundThreads :: Bool #

`True` if bound threads are supported. If `rtsSupportsBoundThreads` is `False`, `isCurrentThreadBound` will always return `False` and both `forkOS` and `runInBoundThread` will fail.

forkOSWithUnmask :: ((forall a. IO a -> IO a) -> IO ()) -> IO ThreadId #

Like `forkIOWithUnmask`, but the child thread is a bound thread, as with `forkOS`.

isCurrentThreadBound :: IO Bool #

Returns `True` if the calling thread is bound, that is, if it is safe to use foreign libraries that rely on thread-local state from the calling thread.

runInBoundThread :: IO a -> IO a #

Run the `IO` computation passed as the first argument. If the calling thread is not bound, a bound thread is created temporarily. `runInBoundThread` doesn't finish until the `IO` computation finishes.

You can wrap a series of foreign function calls that rely on thread-local state with `runInBoundThread` so that you can use them without knowing whether the current thread is bound.

runInUnboundThread :: IO a -> IO a #

Run the `IO` computation passed as the first argument. If the calling thread is bound, an unbound thread is created temporarily using `forkIO`. `runInBoundThread` doesn't finish until the `IO` computation finishes.

Use this function only in the rare case that you have actually observed a performance loss due to the use of bound threads. A program that doesn't need its main thread to be bound and makes heavy use of concurrency (e.g. a web server), might want to wrap its `main` action in `runInUnboundThread`.

Note that exceptions which are thrown to the current thread are thrown in turn to the thread that is executing the given computation. This ensures there's always a way of killing the forked thread.

data QSem #

`QSem` is a quantity semaphore in which the resource is acquired and released in units of one. It provides guaranteed FIFO ordering for satisfying blocked `waitQSem` calls.

The pattern

    bracket_ waitQSem signalQSem (...)

is safe; it never loses a unit of the resource.

newQSem :: Int -> IO QSem #

Build a new `QSem` with a supplied initial quantity. The initial quantity must be at least 0.

waitQSem :: QSem -> IO () #

Wait for a unit to become available.

signalQSem :: QSem -> IO () #

Signal that a unit of the `QSem` is available.

data QSemN #

`QSemN` is a quantity semaphore in which the resource is acquired and released in arbitrary amounts. It provides guaranteed FIFO ordering for satisfying blocked `waitQSemN` calls.

The pattern

    bracket_ (waitQSemN n) (signalQSemN n) (...)

is safe; it never loses any of the resource.

newQSemN :: Int -> IO QSemN #

Build a new `QSemN` with a supplied initial quantity. The initial quantity must be at least 0.

waitQSemN :: QSemN -> Int -> IO () #

Wait for the specified quantity to become available.

signalQSemN :: QSemN -> Int -> IO () #

Signal that a given quantity is now available from the `QSemN`.

data Chan a #

`Chan` is an abstract type representing an unbounded FIFO channel.

#### Instances

Instances details

Eq (Chan a) | Since: base-4.4.0.0
Instance details Defined in Control.Concurrent.Chan Methods (==) :: Chan a -> Chan a -> Bool # (/=) :: Chan a -> Chan a -> Bool #
MonadReader (ReactorChan, IdeState) (ServerM c) # |
Instance details Defined in Development.IDE.LSP.Server Methods ask :: ServerM c (ReactorChan, IdeState) # local :: ((ReactorChan, IdeState) -> (ReactorChan, IdeState)) -> ServerM c a -> ServerM c a # reader :: ((ReactorChan, IdeState) -> a) -> ServerM c a #

newChan :: IO (Chan a) #

Build and return a new instance of `Chan`.

writeChan :: Chan a -> a -> IO () #

Write a value to a `Chan`.

readChan :: Chan a -> IO a #

Read the next value from the `Chan`. Blocks when the channel is empty. Since the read end of a channel is an `MVar`, this operation inherits fairness guarantees of `MVar`s (e.g. threads blocked in this operation are woken up in FIFO order).

Throws `BlockedIndefinitelyOnMVar` when the channel is empty and no other thread holds a reference to the channel.

dupChan :: Chan a -> IO (Chan a) #

Duplicate a `Chan`: the duplicate channel begins empty, but data written to either channel from then on will be available from both. Hence this creates a kind of broadcast channel, where data written by anyone is seen by everyone else.

(Note that a duplicated channel is not equal to its original. So: `fmap (c /=) $ dupChan c` returns `True` for all `c`.)

getChanContents :: Chan a -> IO [a] #

Return a lazy list representing the contents of the supplied `Chan`, much like `hGetContents`.

writeList2Chan :: Chan a -> [a] -> IO () #

Write an entire list of items to a `Chan`.

forkFinally :: IO a -> (Either SomeException a -> IO ()) -> IO ThreadId #

Fork a thread and call the supplied function when the thread is about to terminate, with an exception or a returned value. The function is called with asynchronous exceptions masked.

    forkFinally action and_then =
      mask $ \restore ->
        forkIO $ try (restore action) >>= and_then

This function is useful for informing the parent when a child terminates, for example.

Since: base-4.6.0.0

data Barrier a #

Starts out empty, then is filled exactly once. As an example:

    bar <- newBarrier
    forkIO $ do ...; val <- ...; signalBarrier bar val
    print =<< waitBarrier bar

Here we create a barrier which will contain some computed value. A thread is forked to fill the barrier, while the main thread waits for it to complete. A barrier has similarities to a future or promise from other languages, has been known as an IVar in other Haskell work, and in some ways is like a manually managed thunk.

data Var a #

Like an `MVar`, but must always be full. Used to operate on a mutable variable in a thread-safe way. As an example:

    hits <- newVar 0
    forkIO $ do ...; modifyVar_ hits (+1); ...
    i <- readVar hits
    print ("HITS",i)

Here we have a variable which we modify atomically, so modifications are not interleaved. This use of `MVar` never blocks on a put. No modifyVar operation should ever block, and they should always complete in a reasonable timeframe. A `Var` should not be used to protect some external resource, only the variable contained within. Information from a `readVar` should not be subsequently inserted back into the `Var`.

data Lock #

Like an `MVar`, but has no value. Used to guarantee single-threaded access, typically to some system resource. As an example:

    lock <- newLock
    let output = withLock lock . putStrLn
    forkIO $ do ...; output "hello"
    forkIO $ do ...; output "world"

Here we are creating a lock to ensure that when writing output our messages do not get interleaved. This use of `MVar` never blocks on a put. It is permissible, but rare, that a withLock contains a withLock inside it - but if so, watch out for deadlocks.

withNumCapabilities :: Int -> IO a -> IO a #

On GHC 7.6 and above with the `-threaded` flag, brackets a call to `setNumCapabilities`. On lower versions (which lack `setNumCapabilities`) this function just runs the argument action.

onceFork :: IO a -> IO (IO a) #

Like `once`, but immediately starts running the computation on a background thread.

    \(x :: IO Int) -> join (onceFork x) == x
    \(x :: IO Int) -> (do a <- onceFork x; a; a) == x

newLock :: IO Lock #

Create a new `Lock`.

withLock :: Lock -> IO a -> IO a #

Perform some operation while holding `Lock`. Will prevent all other operations from using the `Lock` while the action is ongoing.

withLockTry :: Lock -> IO a -> IO (Maybe a) #

Like `withLock` but will never block. If the operation cannot be executed immediately it will return `Nothing`.

newVar :: a -> IO (Var a) #

Create a new `Var` with a value.

readVar :: Var a -> IO a #

Read the current value of the `Var`.

writeVar :: Var a -> a -> IO () #

Write a value to become the new value of `Var`.

writeVar' :: Var a -> a -> IO () #

Strict variant of `writeVar`

modifyVar_' :: Var a -> (a -> IO a) -> IO () #

Strict variant of `modifyVar_`

withVar :: Var a -> (a -> IO b) -> IO b #

Perform some operation using the value in the `Var`, a restricted version of `modifyVar`.

newBarrier :: IO (Barrier a) #

Create a new `Barrier`.

signalBarrier :: Partial => Barrier a -> a -> IO () #

Write a value into the Barrier, releasing anyone at `waitBarrier`. Any subsequent attempts to signal the `Barrier` will throw an exception.

waitBarrier :: Barrier a -> IO a #

Wait until a barrier has been signaled with `signalBarrier`.

waitBarrierMaybe :: Barrier a -> IO (Maybe a) #

A version of `waitBarrier` that never blocks, returning `Nothing` if the barrier has not yet been signaled.