Control.Concurrent.MSampleVar

MSampleVar is a safer version of the Control.Concurrent.SampleVar in base. The same problem as QSem(N) is being fixed, that of handling waiters that die before being woken normally. For Control.Concurrent.SampleVar in base this error can lead to thinking a full SampleVar is really empty and cause writeSampleVar to hang. The MSampleVar in this module is immune to this error, and has a simpler implementation.

Sample Variables

data MSampleVar a

Sample variables are slightly different from a normal MVar:

• Reading an empty MSampleVar causes the reader to block. (same as takeMVar on empty MVar)
• Reading a filled MSampleVar empties it and returns value. (same as takeMVar)
• Try reading a filled MSampleVar returns a Maybe value. (same as tryTakeMVar)
• Writing to an empty MSampleVar fills it with a value, and potentially, wakes up a blocked reader (same as for putMVar on empty MVar).
• Writing to a filled MSampleVar overwrites the current value. (different from putMVar on full MVar.)

The readers queue in FIFO order, with the lead reader joining the writers in a second FIFO queue to access the stored value. Thus writers can jump the queue of non-leading waiting readers to update the value, but the lead reader has to wait on all previous writes to finish before taking the value.

This design choice emphasises that each reader sees the most up-to-date value possible while still guaranteeing progress.

Instances

Typeable1 MSampleVar

Eq (MSampleVar a)

newEmptySV :: IO (MSampleVar a)

newEmptySV allocates a new MSampleVar in an empty state. No futher allocation is done when using the MSampleVar.

newSV :: a -> IO (MSampleVar a)

newSV allocates a new MSampleVar containing the passed value. The value is not evalated or forced, but stored lazily. No futher allocation is done when using the MSampleVar.

emptySV :: MSampleVar a -> IO ()

If the MSampleVar is full, forget the value and leave it empty. Otherwise, do nothing. This avoids any the FIFO queue of blocked readSV threads.

emptySV can block and be interrupted, in which case it does nothing. If emptySV returns then it left the MSampleVar in an empty state.

readSV :: MSampleVar a -> IO a

Wait for a value to become available, then take it and return. The queue of blocked readSV threads is a fair FIFO queue.

readSV can block and be interrupted, in which case it takes nothing. If 'readSV returns normally then it has taken a value.

writeSV :: MSampleVar a -> a -> IO ()

Write a value into the MSampleVar, overwriting any previous value that was there.

writeSV can block and be interrupted, in which case it does nothing.

isEmptySV :: MSampleVar a -> IO Bool

isEmptySV can block and be interrupted, in which case it does nothing. If isEmptySV returns then it reports the momentary status the MSampleVar. Using this value without producing unwanted race conditions is left up to the programmer.