Control.Concurrent.SampleVar

Sample variables

Sample Variables

data SampleVar a

Sample variables are slightly different from a normal MVar:

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

Instances

Eq (SampleVar a)

newEmptySampleVar :: IO (SampleVar a)

Build a new, empty, SampleVar

newSampleVar :: a -> IO (SampleVar a)

Build a SampleVar with an initial value.

emptySampleVar :: SampleVar a -> IO ()

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

readSampleVar :: SampleVar a -> IO a

Wait for a value to become available, then take it and return.

writeSampleVar :: SampleVar a -> a -> IO ()

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

isEmptySampleVar :: SampleVar a -> IO Bool

Returns True if the SampleVar is currently empty.

Note that this function is only useful if you know that no other threads can be modifying the state of the SampleVar, because otherwise the state of the SampleVar may have changed by the time you see the result of isEmptySampleVar.