Control.Concurrent.Strict

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 ()

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

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

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

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.

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.

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 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.

Instances

Show ThreadId

Since: base-4.2.0.0

Defined in GHC.Conc.Sync

Methods

showsPrec :: Int -> ThreadId -> ShowS

show :: ThreadId -> String

showList :: [ThreadId] -> ShowS

NFData ThreadId

Since: deepseq-1.4.0.0

Defined in Control.DeepSeq

Methods

rnf :: ThreadId -> ()

Eq ThreadId

Since: base-4.2.0.0

Defined in GHC.Conc.Sync

Methods

(==) :: ThreadId -> ThreadId -> Bool

(/=) :: ThreadId -> ThreadId -> Bool

Ord ThreadId

Since: base-4.2.0.0

Defined in GHC.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

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> ThreadId -> Int

hash :: ThreadId -> Int

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

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

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.

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.

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.

rtsSupportsBoundThreads :: Bool

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

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.

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

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

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 Chan a

Chan is an abstract type representing an unbounded FIFO channel.

Instances

Eq (Chan a)

Since: base-4.4.0.0

Defined in Control.Concurrent.Chan

Methods

(==) :: Chan a -> Chan a -> Bool

(/=) :: Chan a -> Chan a -> Bool

MonadReader (ReactorChan, IdeState) (ServerM c)

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

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

Write an entire list of items to a 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 MVars (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.

newChan :: IO (Chan a)

Build and returns a new instance of Chan.

getChanContents :: Chan a -> IO [a]

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

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.)

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.

waitQSem :: QSem -> IO ()

Wait for a unit to become available

signalQSem :: QSem -> IO ()

Signal that a unit of the QSem is available

newQSem :: Int -> IO QSem

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

data QSemN

QSemN is a quantity semaphore in which the resource is acquired and released in units of one. 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.

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.

newQSemN :: Int -> IO QSemN

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

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

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.

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.

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.

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

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.

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 MVar is garbage-collected

Since: base-4.6.0.0

addMVarFinalizer :: MVar a -> IO () -> IO ()

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.

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.

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

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

myThreadId :: IO ThreadId

Returns the ThreadId of the calling thread (GHC only).

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

killThread :: ThreadId -> IO ()

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

killThread tid = throwTo tid ThreadKilled

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

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

NFData1 MVar

Since: deepseq-1.4.3.0

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

Defined in Control.DeepSeq

Methods

rnf :: MVar a -> ()

Eq (MVar a)

Since: base-4.1.0.0

Defined in GHC.MVar

Methods

(==) :: MVar a -> MVar a -> Bool

(/=) :: MVar a -> MVar a -> Bool

PrimUnlifted (MVar a)

Defined in Data.Primitive.Unlifted.Class

Associated Types

type Unlifted (MVar a) :: TYPE UnliftedRep

Methods

toUnlifted# :: MVar a -> Unlifted (MVar a)

fromUnlifted# :: Unlifted (MVar a) -> MVar a

writeUnliftedArray# :: MutableArrayArray# s -> Int# -> MVar a -> State# s -> State# s

readUnliftedArray# :: MutableArrayArray# s -> Int# -> State# s -> (# State# s, MVar a #)

indexUnliftedArray# :: ArrayArray# -> Int# -> MVar a

type Unlifted (MVar a)

Defined in Data.Primitive.Unlifted.Class

type Unlifted (MVar a) = MVar# RealWorld a

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.

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

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.

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 MVars.

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.

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 MVars.

newMVar :: a -> IO (MVar a)

Create an MVar which contains the supplied value.

newEmptyMVar :: IO (MVar a)

Create an MVar which is initially empty.

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.

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

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

waitBarrier :: Barrier a -> IO a

Wait until a barrier has been signaled with signalBarrier.

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.

newBarrier :: IO (Barrier a)

Create a new Barrier.

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

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

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

Strict variant of modifyVar_

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

Strict variant of writeVar

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

Write a value to become the new value of Var.

readVar :: Var a -> IO a

Read the current value of the Var.

newVar :: a -> IO (Var a)

Create a new Var with a value.

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

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

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.

newLock :: IO Lock

Create a new Lock.

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

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

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.

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.

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 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.