import Control.Concurrent.MVar
+import Control.Exception ( mask_ )
+
+import Data.Functor ( (<$>) )
+
-- |
-- Sample variables are slightly different from a normal 'MVar':
--
-- * Writing to a filled 'SampleVar' overwrites the current value.
-- (different from 'putMVar' on full 'MVar'.)
-type SampleVar a
- = MVar (Int, -- 1 == full
- -- 0 == empty
- -- <0 no of readers blocked
- MVar a)
+newtype SampleVar a = SampleVar ( MVar ( Int -- 1 == full
+ -- 0 == empty
+ -- <0 no of readers blocked
+ , MVar a
+ )
+ )
+ deriving (Eq)
-- |Build a new, empty, 'SampleVar'
newEmptySampleVar :: IO (SampleVar a)
newEmptySampleVar = do
v <- newEmptyMVar
- newMVar (0,v)
+ SampleVar <$> newMVar (0,v)
-- |Build a 'SampleVar' with an initial value.
newSampleVar :: a -> IO (SampleVar a)
newSampleVar a = do
v <- newMVar a
- newMVar (1,v)
+ SampleVar <$> newMVar (1,v)
-- |If the SampleVar is full, leave it empty. Otherwise, do nothing.
emptySampleVar :: SampleVar a -> IO ()
-emptySampleVar v = do
- (readers, var) <- takeMVar v
+emptySampleVar (SampleVar v) = mask_ $ do
+ s@(readers, var) <- takeMVar v
if readers > 0 then do
- takeMVar var
+ _ <- takeMVar var
putMVar v (0,var)
else
- putMVar v (readers,var)
+ putMVar v s
-- |Wait for a value to become available, then take it and return.
readSampleVar :: SampleVar a -> IO a
-readSampleVar svar = do
+readSampleVar (SampleVar svar) = mask_ $ do
--
-- filled => make empty and grab sample
-- not filled => try to grab value, empty when read val.
--
(readers,val) <- takeMVar svar
- putMVar svar (readers-1,val)
+ let readers' = readers-1
+ readers' `seq` putMVar svar (readers',val)
takeMVar val
-- |Write a value into the 'SampleVar', overwriting any previous value that
-- was there.
writeSampleVar :: SampleVar a -> a -> IO ()
-writeSampleVar svar v = do
+writeSampleVar (SampleVar svar) v = mask_ $ do
--
-- filled => overwrite
-- not filled => fill, write val
--
- (readers,val) <- takeMVar svar
+ s@(readers,val) <- takeMVar svar
case readers of
- 1 ->
- swapMVar val v >>
- putMVar svar (1,val)
- _ ->
- putMVar val v >>
- putMVar svar (min 1 (readers+1), val)
+ 1 ->
+ swapMVar val v >>
+ putMVar svar s
+ _ ->
+ putMVar val v >>
+ let readers' = min 1 (readers+1)
+ in readers' `seq` putMVar svar (readers', val)
-- | Returns 'True' if the 'SampleVar' is currently empty.
--
-- you see the result of 'isEmptySampleVar'.
--
isEmptySampleVar :: SampleVar a -> IO Bool
-isEmptySampleVar svar = do
+isEmptySampleVar (SampleVar svar) = do
(readers, _) <- readMVar svar
return (readers == 0)