Safe Haskell | Safe-Inferred |
---|---|
Language | Haskell2010 |
Documentation
module Control.Concurrent.MVar
class Access io => MVarAccess io where Source
newEmptyMVar' :: io (MVar a) Source
Create an MVar
which is initially empty.
newMVar' :: a -> io (MVar a) Source
Create an MVar
which contains the supplied value.
takeMVar' :: MVar a -> io a Source
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 intakeMVar
, and theMVar
becomes full, only one thread will be woken up. The runtime guarantees that the woken thread completes itstakeMVar
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 usingMVar
s.
putMVar' :: MVar a -> a -> io () Source
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 inputMVar
, and theMVar
becomes empty, only one thread will be woken up. The runtime guarantees that the woken thread completes itsputMVar
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 usingMVar
s.
readMVar' :: MVar a -> io a Source
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 Source
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) Source
A non-blocking version of takeMVar
. The tryTakeMVar
function
returns immediately, with Nothing
if the MVar
was empty, or
if the Just
aMVar
was full with contents a
. After tryTakeMVar
,
the MVar
is left empty.
tryPutMVar' :: MVar a -> a -> io Bool Source
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) Source
A non-blocking version of readMVar
. The tryReadMVar
function
returns immediately, with Nothing
if the MVar
was empty, or
if the Just
aMVar
was full with contents a
.
Since: 4.7.0.0
isEmptyMVar' :: MVar a -> io Bool Source
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 Source
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 Source
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 () Source
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 Source
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 () Source
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 Source
Like modifyMVar
, but the IO
action in the second argument is executed with
asynchronous exceptions masked.
Since: 4.6.0.0
class (WeakMemAccess io, MVarAccess io) => WeakMVarAccess io where Source