Control.Concurrent.CVar

Combinators using CVars. These provide many of the helpful functions found in Control.Concurrent.MVar, but for CVars.

CVars

newEmptyCVar :: MonadConc m => m (CVar m a)

Create a new empty CVar.

newCVar :: MonadConc m => a -> m (CVar m a)

Create a new CVar containing a value.

takeCVar :: MonadConc m => CVar m a -> m a

Take a value from a CVar. This "empties" the CVar, allowing a new value to be put in. This will block if there is no value in the CVar already, until one has been put.

putCVar :: MonadConc m => CVar m a -> a -> m ()

Put a value into a CVar. If there is already a value there, this will block until that value has been taken, at which point the value will be stored.

readCVar :: MonadConc m => CVar m a -> m a

Block until a value is present in the CVar, and then return it. As with readMVar, this does not "remove" the value, multiple reads are possible.

swapCVar :: MonadConc m => CVar m a -> a -> m a

Swap the contents of a CVar, and return the value taken. This function is atomic only if there are no other producers fro this CVar.

tryTakeCVar :: MonadConc m => CVar m a -> m (Maybe a)

Attempt to take a value from a CVar non-blockingly, returning a Just (and emptying the CVar) if there was something there, otherwise returning Nothing.

tryPutCVar :: MonadConc m => CVar m a -> a -> m Bool

Attempt to put a value in a CVar non-blockingly, returning True (and filling the CVar) if there was nothing there, otherwise returning False.

isEmptyCVar :: MonadConc m => CVar m a -> m Bool

Check if a CVar is empty.

withCVar :: MonadConc m => CVar m a -> (a -> m b) -> m b

Operate on the contents of a CVar, replacing the contents after finishing. This operation is exception-safe: it will replace the original contents of the CVar if an exception is raised. However, it is only atomic if there are no other producers for this CVar.

withCVarMasked :: MonadConc m => CVar m a -> (a -> m b) -> m b

Like withCVar, but the IO action in the second argument is executed with asynchronous exceptions masked.

modifyCVar_ :: MonadConc m => CVar m a -> (a -> m a) -> m ()

An exception-safe wrapper for modifying the contents of a CVar. Like withCVar, modifyCVar will replace the original contents of the CVar if an exception is raised during the operation. This function is only atomic if there are no other producers for this CVar.

modifyCVar :: MonadConc m => CVar m a -> (a -> m (a, b)) -> m b

A slight variation on modifyCVar_ that allows a value to be returned (b) in addition to the modified value of the CVar.

modifyCVarMasked_ :: MonadConc m => CVar m a -> (a -> m a) -> m ()

Like modifyCVar_, but the IO action in the second argument is executed with asynchronous exceptions masked.

modifyCVarMasked :: MonadConc m => CVar m a -> (a -> m (a, b)) -> m b

Like modifyCVar, but the IO action in the second argument is executed with asynchronous exceptions masked.

Binary semaphores

A common use of CVars is in making binary semaphores to control mutual exclusion over a resource, so a couple of helper functions are provided.

lock :: MonadConc m => CVar m () -> m ()

Put a () into a CVar, claiming the lock. This is atomic.

unlock :: MonadConc m => CVar m () -> m ()

Empty a CVar, releasing the lock. This is atomic.