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.)

newEmptySV :: IO (MSampleVar a)

newSV :: a -> IO (MSampleVar a)

emptySV :: MSampleVar a -> IO ()

If the MSampleVar is full, leave it empty. Otherwise, do nothing.

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.

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