-----------------------------------------------------------------------------
module Control.Concurrent.QSemN
- ( -- * General Quantity Semaphores
- QSemN, -- abstract
- newQSemN, -- :: Int -> IO QSemN
- waitQSemN, -- :: QSemN -> Int -> IO ()
- signalQSemN -- :: QSemN -> Int -> IO ()
+ ( -- * General Quantity Semaphores
+ QSemN, -- abstract
+ newQSemN, -- :: Int -> IO QSemN
+ waitQSemN, -- :: QSemN -> Int -> IO ()
+ signalQSemN -- :: QSemN -> Int -> IO ()
) where
import Prelude
import Control.Concurrent.MVar
+import Control.Exception ( block )
import Data.Typeable
#include "Typeable.h"
INSTANCE_TYPEABLE0(QSemN,qSemNTc,"QSemN")
-- |Build a new 'QSemN' with a supplied initial quantity.
-newQSemN :: Int -> IO QSemN
-newQSemN init = do
- sem <- newMVar (init,[])
- return (QSemN sem)
+-- The initial quantity must be at least 0.
+newQSemN :: Int -> IO QSemN
+newQSemN initial =
+ if initial < 0
+ then fail "newQSemN: Initial quantity must be non-negative"
+ else do sem <- newMVar (initial, [])
+ return (QSemN sem)
-- |Wait for the specified quantity to become available
waitQSemN :: QSemN -> Int -> IO ()
-waitQSemN (QSemN sem) sz = do
+waitQSemN (QSemN sem) sz = block $ do
(avail,blocked) <- takeMVar sem -- gain ex. access
- if (avail - sz) >= 0 then
+ let remaining = avail - sz
+ if remaining >= 0 then
-- discharging 'sz' still leaves the semaphore
-- in an 'unblocked' state.
- putMVar sem (avail-sz,blocked)
+ putMVar sem (remaining,blocked)
else do
- block <- newEmptyMVar
- putMVar sem (avail, blocked++[(sz,block)])
- takeMVar block
+ b <- newEmptyMVar
+ putMVar sem (avail, blocked++[(sz,b)])
+ takeMVar b
-- |Signal that a given quantity is now available from the 'QSemN'.
signalQSemN :: QSemN -> Int -> IO ()
-signalQSemN (QSemN sem) n = do
+signalQSemN (QSemN sem) n = block $ do
(avail,blocked) <- takeMVar sem
(avail',blocked') <- free (avail+n) blocked
- putMVar sem (avail',blocked')
+ avail' `seq` putMVar sem (avail',blocked')
where
free avail [] = return (avail,[])
- free avail ((req,block):blocked)
+ free avail ((req,b):blocked)
| avail >= req = do
- putMVar block ()
- free (avail-req) blocked
+ putMVar b ()
+ free (avail-req) blocked
| otherwise = do
- (avail',blocked') <- free avail blocked
- return (avail',(req,block):blocked')
+ (avail',blocked') <- free avail blocked
+ return (avail',(req,b):blocked')