Control.Concurrent.MVar

base-4.8.2.0: Basic libraries

Copyright | (c) The University of Glasgow 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | experimental
Portability | non-portable (concurrency)
Safe Haskell | Trustworthy
Language | Haskell2010

Control.Concurrent.MVar

Contents

- `MVar`s

Description

An ``MVar` t` is mutable location that is either empty or contains a value of type `t`. It has two fundamental operations: `putMVar` which fills an `MVar` if it is empty and blocks otherwise, and `takeMVar` which empties an `MVar` if it is full and blocks otherwise. They can be used in multiple different ways:

1. As synchronized mutable variables,
2. As channels, with `takeMVar` and `putMVar` as receive and send, and
3. As a binary semaphore ``MVar` ()`, with `takeMVar` and `putMVar` as wait and signal.

They were introduced in the paper "Concurrent Haskell" by Simon Peyton Jones, Andrew Gordon and Sigbjorn Finne, though some details of their implementation have since then changed (in particular, a put on a full `MVar` used to error, but now merely blocks.)

### Applicability

`MVar`s offer more flexibility than `IORef`s, but less flexibility than `STM`. They are appropriate for building synchronization primitives and performing simple interthread communication; however they are very simple and susceptible to race conditions, deadlocks or uncaught exceptions. Do not use them if you need perform larger atomic operations such as reading from multiple variables: use `STM` instead.

In particular, the "bigger" functions in this module (`readMVar`, `swapMVar`, `withMVar`, `modifyMVar_` and `modifyMVar`) are simply the composition of a `takeMVar` followed by a `putMVar` with exception safety. These only have atomicity guarantees if all other threads perform a `takeMVar` before a `putMVar` as well; otherwise, they may block.

### Fairness

No thread can be blocked indefinitely on an `MVar` unless another thread holds that `MVar` indefinitely. One usual implementation of this fairness guarantee is that threads blocked on an `MVar` are served in a first-in-first-out fashion, but this is not guaranteed in the semantics.

### Gotchas

Like many other Haskell data structures, `MVar`s are lazy. This means that if you place an expensive unevaluated thunk inside an `MVar`, it will be evaluated by the thread that consumes it, not the thread that produced it. Be sure to `evaluate` values to be placed in an `MVar` to the appropriate normal form, or utilize a strict MVar provided by the strict-concurrency package.

### Ordering

`MVar` operations are always observed to take place in the order they are written in the program, regardless of the memory model of the underlying machine. This is in contrast to `IORef` operations which may appear out-of-order to another thread in some cases.

### Example

Consider the following concurrent data structure, a skip channel. This is a channel for an intermittent source of high bandwidth information (for example, mouse movement events.) Writing to the channel never blocks, and reading from the channel only returns the most recent value, or blocks if there are no new values. Multiple readers are supported with a `dupSkipChan` operation.

A skip channel is a pair of `MVar`s. The first `MVar` contains the current value, and a list of semaphores that need to be notified when it changes. The second `MVar` is a semaphore for this particular reader: it is full if there is a value in the channel that this reader has not read yet, and empty otherwise.

        data SkipChan a = SkipChan (MVar (a, [MVar ()])) (MVar ())

        newSkipChan :: IO (SkipChan a)
        newSkipChan = do
            sem <- newEmptyMVar
            main <- newMVar (undefined, [sem])
            return (SkipChan main sem)

        putSkipChan :: SkipChan a -> a -> IO ()
        putSkipChan (SkipChan main _) v = do
            (_, sems) <- takeMVar main
            putMVar main (v, [])
            mapM_ (sem -> putMVar sem ()) sems

        getSkipChan :: SkipChan a -> IO a
        getSkipChan (SkipChan main sem) = do
            takeMVar sem
            (v, sems) <- takeMVar main
            putMVar main (v, sem:sems)
            return v

        dupSkipChan :: SkipChan a -> IO (SkipChan a)
        dupSkipChan (SkipChan main _) = do
            sem <- newEmptyMVar
            (v, sems) <- takeMVar main
            putMVar main (v, sem:sems)
            return (SkipChan main sem)

This example was adapted from the original Concurrent Haskell paper. For more examples of `MVar`s being used to build higher-level synchronization primitives, see `Chan` and `QSem`.

Synopsis

- data MVar a
- newEmptyMVar :: IO (MVar a)
- newMVar :: a -> IO (MVar a)
- takeMVar :: MVar a -> IO a
- putMVar :: MVar a -> a -> IO ()
- readMVar :: MVar a -> IO a
- swapMVar :: MVar a -> a -> IO a
- tryTakeMVar :: MVar a -> IO (Maybe a)
- tryPutMVar :: MVar a -> a -> IO Bool
- isEmptyMVar :: MVar a -> IO Bool
- withMVar :: MVar a -> (a -> IO b) -> IO b
- withMVarMasked :: MVar a -> (a -> IO b) -> IO b
- modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
- 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
- tryReadMVar :: MVar a -> IO (Maybe a)
- mkWeakMVar :: MVar a -> IO () -> IO (Weak (MVar a))
- addMVarFinalizer :: MVar a -> IO () -> IO ()

# `MVar`s

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) |

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.

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.

readMVar :: MVar a -> IO a

Atomically read the contents of an `MVar`. If the `MVar` is currently empty, `readMVar` will wait until its 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

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`.

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.

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`.

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: 4.7.0.0

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`.

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: 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: 4.6.0.0

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: 4.7.0.0

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 `MVar` is garbage-collected

Since: 4.6.0.0

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

Deprecated: use `mkWeakMVar` instead