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)

MVars

data MVar a Source

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.

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 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.

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 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.

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 Just a if the MVar 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.

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

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 Just a if the MVar was full with contents a.

Since: 4.7.0.0

mkWeakMVar :: MVar a -> IO () -> IO (Weak (MVar a)) Source

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 () Source