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.
- data AVar a
- data Transaction a
- putAVar :: AVar a -> a -> IO ()
- newAVar :: a -> IO (AVar a)
- putMVar :: MVar 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)
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
Transaction describes what should happen to a variable.
They are only used internally, and are here just for reference.
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 replaces the currect value in the variable with the given x
newAVar creates a new variable. It forks off the
handler that does the
work for the variable itself and creates a new AVar.
There are two further important properties of
putMVaris single-wakeup. That is, if there are multiple threads blocked in
putMVar, and the
MVarbecomes empty, only one thread will be woken up. The runtime guarantees that the woken thread completes its
- 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
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.