stm-2.1.1.2: Software Transactional MemoryContentsIndex
Control.Concurrent.STM.TMVar
Portabilitynon-portable (requires STM)
Stabilityexperimental
Maintainerlibraries@haskell.org
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.
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 TMVars 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 TMVars 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.

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