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.
If you're after a more unsafe interface to AVars, see Data.AVar.Unsafe, which will throw the errors returned fromt he variable.
- data AVar a
- putAVar :: AVar a -> a -> IO ()
- modAVar :: AVar a -> (a -> a) -> IO (Maybe SomeException)
- modAVar' :: AVar a -> (a -> (a, b)) -> IO (Either SomeException b)
- justModAVar :: AVar a -> (a -> a) -> IO ()
- getAVar :: AVar a -> IO a
- condModAVar :: AVar a -> (a -> Bool) -> (a -> a) -> (a -> a) -> IO (Either SomeException Bool)
- swapAVar :: AVar a -> a -> IO (Either SomeException a)
- newAVar :: a -> IO (AVar a)
AVars are the means through which communication with the variable are
conducted. They contain a Chan that is
connected to the variable, and
is read by the variable's
putAVar replaces the currect value in the variable with the given x
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' is like modAVar, but it modifies the variable, along with
returning a result of type b, within an Either e b.
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.
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 takes a new value, puts it into the AVar, and returns the old value.