Control/Concurrent/QSemN.hs

{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE CPP #-}
#ifdef __GLASGOW_HASKELL__
{-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
#endif

-----------------------------------------------------------------------------
-- |
-- Module : Control.Concurrent.QSemN
-- 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)
--
-- Quantity semaphores in which each thread may wait for an arbitrary
-- \"amount\".
--
-----------------------------------------------------------------------------

module Control.Concurrent.QSemN
 {-# DEPRECATED "Control.Concurrent.QSemN will be removed in GHC 7.8. Please use an alternative, e.g. the SafeSemaphore package, instead." #-}
 ( -- * 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 ( mask_ )
import Data.Typeable

#include "Typeable.h"

-- |A 'QSemN' is a quantity semaphore, in which the available
-- \"quantity\" may be signalled or waited for in arbitrary amounts.
newtype QSemN = QSemN (MVar (Int,[(Int,MVar ())])) deriving Eq

INSTANCE_TYPEABLE0(QSemN,qSemNTc,"QSemN")

-- |Build a new 'QSemN' with a supplied initial quantity.
-- 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 = mask_ $ do
 (avail,blocked) <- takeMVar sem -- gain ex. access
 let remaining = avail - sz
 if remaining >= 0 then
 -- discharging 'sz' still leaves the semaphore
 -- in an 'unblocked' state.
 putMVar sem (remaining,blocked)
 else do
 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 = mask_ $ do
 (avail,blocked) <- takeMVar sem
 (avail',blocked') <- free (avail+n) blocked
 avail' `seq` putMVar sem (avail',blocked')
 where
 free avail [] = return (avail,[])
 free avail ((req,b):blocked)
 | avail >= req = do
 putMVar b ()
 free (avail-req) blocked
 | otherwise = do
 (avail',blocked') <- free avail blocked
 return (avail',(req,b):blocked')