Control.Concurrent.STM.TMVar

stm-2.2.0.1: Software Transactional Memory

Portability | non-portable (requires STM)
Stability | experimental
Maintainer | libraries@haskell.org

Control.Concurrent.STM.TMVar

Contents

- TMVars

Description

TMVar: Transactional MVars, for use in the STM monad (GHC only)

Synopsis

- data TMVar a
- newTMVar :: a -> STM (TMVar a)
- newEmptyTMVar :: STM (TMVar a)
- newTMVarIO :: a -> IO (TMVar a)
- newEmptyTMVarIO :: IO (TMVar a)
- takeTMVar :: TMVar a -> STM a
- putTMVar :: TMVar a -> a -> STM ()
- readTMVar :: TMVar a -> STM a
- swapTMVar :: TMVar a -> a -> STM a
- tryTakeTMVar :: TMVar a -> STM (Maybe a)
- tryPutTMVar :: TMVar a -> a -> STM Bool
- isEmptyTMVar :: TMVar a -> STM Bool

# TMVars

data TMVar a

A `TMVar` 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

Typeable1 TMVar |
Eq (TMVar a) |

newTMVar :: a -> STM (TMVar a)

Create a `TMVar` which contains the supplied value.

newEmptyTMVar :: STM (TMVar a)

Create a `TMVar` which is initially empty.

newTMVarIO :: a -> IO (TMVar a)

`IO` version of `newTMVar`. This is useful for creating top-level `TMVar`s using `System.IO.Unsafe.unsafePerformIO`, because using `atomically` inside `System.IO.Unsafe.unsafePerformIO` isn't possible.

newEmptyTMVarIO :: IO (TMVar a)

`IO` version of `newEmptyTMVar`. This is useful for creating top-level `TMVar`s using `System.IO.Unsafe.unsafePerformIO`, because using `atomically` inside `System.IO.Unsafe.unsafePerformIO` isn't possible.

takeTMVar :: TMVar a -> STM a

Return the contents of the `TMVar`. If the `TMVar` is currently empty, the transaction will `retry`. After a `takeTMVar`, the `TMVar` is left empty.

putTMVar :: TMVar a -> a -> STM ()

Put a value into a `TMVar`. If the `TMVar` is currently full, `putTMVar` will `retry`.

readTMVar :: TMVar a -> STM a

This is a combination of `takeTMVar` and `putTMVar`; ie. it takes the value from the `TMVar`, puts it back, and also returns it.

swapTMVar :: TMVar a -> a -> STM a

Swap the contents of a `TMVar` for a new value.

tryTakeTMVar :: TMVar a -> STM (Maybe a)

A version of `takeTMVar` that does not `retry`. The `tryTakeTMVar` function returns `Nothing` if the `TMVar` was empty, or ``Just` a` if the `TMVar` was full with contents `a`. After `tryTakeTMVar`, the `TMVar` is left empty.

tryPutTMVar :: TMVar a -> a -> STM Bool

A version of `putTMVar` that does not `retry`. The `tryPutTMVar` function attempts to put the value `a` into the `TMVar`, returning `True` if it was successful, or `False` otherwise.

isEmptyTMVar :: TMVar a -> STM Bool

Check whether a given `TMVar` is empty.

Notice that the boolean value returned is just a snapshot of the state of the `TMVar`. By the time you get to react on its result, the `TMVar` may have been filled (or emptied) - so be extremely careful when using this operation. Use `tryTakeTMVar` instead if possible.