/usr/lib/hugs/packages/base/Control/Concurrent/SampleVar.hs is in libhugs-base-bundled 98.200609.21-5.4.
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-- |
-- Module : Control.Concurrent.SampleVar
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable (concurrency)
--
-- Sample variables
--
-----------------------------------------------------------------------------
module Control.Concurrent.SampleVar
(
-- * Sample Variables
SampleVar, -- :: type _ =
newEmptySampleVar, -- :: IO (SampleVar a)
newSampleVar, -- :: a -> IO (SampleVar a)
emptySampleVar, -- :: SampleVar a -> IO ()
readSampleVar, -- :: SampleVar a -> IO a
writeSampleVar, -- :: SampleVar a -> a -> IO ()
isEmptySampleVar, -- :: SampleVar a -> IO Bool
) where
import Prelude
import Control.Concurrent.MVar
-- |
-- 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'.)
type SampleVar a
= MVar (Int, -- 1 == full
-- 0 == empty
-- <0 no of readers blocked
MVar a)
-- |Build a new, empty, 'SampleVar'
newEmptySampleVar :: IO (SampleVar a)
newEmptySampleVar = do
v <- newEmptyMVar
newMVar (0,v)
-- |Build a 'SampleVar' with an initial value.
newSampleVar :: a -> IO (SampleVar a)
newSampleVar a = do
v <- newEmptyMVar
putMVar v a
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
if readers > 0 then do
takeMVar var
putMVar v (0,var)
else
putMVar v (readers,var)
-- |Wait for a value to become available, then take it and return.
readSampleVar :: SampleVar a -> IO a
readSampleVar svar = 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)
takeMVar val
-- |Write a value into the 'SampleVar', overwriting any previous value that
-- was there.
writeSampleVar :: SampleVar a -> a -> IO ()
writeSampleVar svar v = do
--
-- filled => overwrite
-- not filled => fill, write val
--
(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)
-- | 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'.
--
isEmptySampleVar :: SampleVar a -> IO Bool
isEmptySampleVar svar = do
(readers,val) <- readMVar svar
return (readers == 0)
|