Data.AVar

AVars are a form of transactional variables. They internally use a tail recursive function to carry the state of the variable, and allow for use in concurrent systems, where actions are guaranteed to happen. They are designed to cope with exceptions thrown by any modifying functions; any exception thrown during a transaction will either be passed back to the caller or ignored, and the variable keeps on running.

They are handy for applications like keeping track of resources by incrementing and decrementing the variable. They should not be used in a way which you would read the variable, then modify it based on the result recieved, but rather using the provided functions. If this was not done, the variable's value is very likely to have changed in the mean time.

Types

data AVar a

AVars are the means through communication with the variable are conducted. They contain a Chan that is connected to the variable, and is read by the variable's handler function.

data Transaction a

A Transaction describes what should happen to a variable. They are only used internally, and are here just for reference.

Constructors

Put a

puts the a into the variable

Get (MVar a)

reads the variable

Mod (a -> a) (MVar (Maybe SomeException))

modifies the variable

JustMod (a -> a)

Just modifies the variable (unless an exception occurs)

forall b . Mod' (a -> (a, b)) (MVar (Either SomeException b))

modifies the variable, returning the b result to the caller

Atom (a -> Bool) (a -> a) (a -> a) (MVar (Either SomeException Bool))

conditionally modifies a variable

functions on AVars

putAVar :: AVar a -> a -> IO ()

putAVar replaces the currect value in the variable with the given x

newAVar :: a -> IO (AVar a)

newAVar creates a new variable. It forks off the handler that does the work for the variable itself and creates a new AVar.

putMVar :: MVar a -> a -> IO ()

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.

modAVar :: AVar a -> (a -> a) -> IO (Maybe SomeException)

modAVar takes a function from a to a, and returns Nothing if nothing went wrong, or Just e, where e is an exception thrown by the function.

modAVar' :: AVar a -> (a -> (a, b)) -> IO (Either SomeException b)

modAVar' is like modAVar, but it modifies the variable, along with returning a result of type b, within an Either e b.

justModAVar :: AVar a -> (a -> a) -> IO ()

justModAVar will attempt to run the given function on the variable. It does not report back on its sucess or failure, and if the function produces an exception, the variable is left unchanged. It should be used when you just want to modify the variable, and keep running, without waiting for the action to complete.

getAVar :: AVar a -> IO a

getAVar reads the current value inside the AVar.

condModAVar :: AVar a -> (a -> Bool) -> (a -> a) -> (a -> a) -> IO (Either SomeException Bool)

condModAVar applies the first finction to the current value in the AVar, and if true will modify the value using the second function if it results in True, or the third function if it results in Fasle.

swapAVar :: AVar a -> a -> IO (Either SomeException a)

swapAVar takes a new value, puts it into the AVar, and returns the old value.