Control.Concurrent.MVar

module Control.Concurrent.MVar
	( 
	  -- * @MVar@s
	  MVar		-- abstract
	, newEmptyMVar  -- :: IO (MVar a)
	, newMVar 	-- :: a -> IO (MVar a)
	, takeMVar 	-- :: MVar a -> IO a
	, putMVar  	-- :: MVar a -> a -> IO ()
	, readMVar 	-- :: MVar a -> IO a
	, swapMVar 	-- :: MVar a -> a -> IO a
	, tryTakeMVar   -- :: MVar a -> IO (Maybe a)
	, tryPutMVar    -- :: MVar a -> a -> IO Bool
	, isEmptyMVar	-- :: MVar a -> IO Bool
	, withMVar	-- :: MVar a -> (a -> IO b) -> IO b
	, modifyMVar_ 	-- :: MVar a -> (a -> IO a) -> IO ()
	, modifyMVar 	-- :: MVar a -> (a -> IO (a,b)) -> IO b

    ) where

import Hugs.ConcBase ( MVar, newEmptyMVar, newMVar, takeMVar, putMVar,
		  tryTakeMVar, tryPutMVar, isEmptyMVar,
		)

import Prelude
import Control.Exception as Exception

{-|
  This is a combination of 'takeMVar' and 'putMVar'; ie. it takes the value
  from the 'MVar', puts it back, and also returns it.
-}
readMVar :: MVar a -> IO a
readMVar m =
  block $ do
    a <- takeMVar m
    putMVar m a
    return a

-- |Swap the contents of an 'MVar' for a new value.
swapMVar :: MVar a -> a -> IO a
swapMVar mvar new =
  block $ do
    old <- takeMVar mvar
    putMVar mvar new
    return old

{-|
  'withMVar' is a safe wrapper for operating on the contents of an
  'MVar'.  This operation is exception-safe: it will replace the
  original contents of the 'MVar' if an exception is raised (see
  "Control.Exception").
-}
withMVar :: MVar a -> (a -> IO b) -> IO b
withMVar m io = 
  block $ do
    a <- takeMVar m
    b <- Exception.catch (unblock (io a))
      	    (\e -> do putMVar m a; throw e)
    putMVar m a
    return b

{-|
  A safe wrapper for modifying the contents of an 'MVar'.  Like 'withMVar', 
  'modifyMVar' will replace the original contents of the 'MVar' if an
  exception is raised during the operation.
-}
modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
modifyMVar_ m io = 
  block $ do
    a  <- takeMVar m
    a' <- Exception.catch (unblock (io a))
      	    (\e -> do putMVar m a; throw e)
    putMVar m a'

{-|
  A slight variation on 'modifyMVar_' that allows a value to be
  returned (@b@) in addition to the modified value of the 'MVar'.
-}
modifyMVar :: MVar a -> (a -> IO (a,b)) -> IO b
modifyMVar m io = 
  block $ do
    a      <- takeMVar m
    (a',b) <- Exception.catch (unblock (io a))
      	        (\e -> do putMVar m a; throw e)
    putMVar m a'
    return b