fix #4876

[ghc-base.git] / Control / Concurrent / SampleVar.hs

diff --git a/Control/Concurrent/SampleVar.hs b/Control/Concurrent/SampleVar.hs
index 8cd1915..68e4b30 100644 (file)
--- a/Control/Concurrent/SampleVar.hs
+++ b/Control/Concurrent/SampleVar.hs
@@ -2,13 +2,11 @@
 -- |
 -- Module      :  Control.Concurrent.SampleVar
 -- Copyright   :  (c) The University of Glasgow 2001
--- License     :  BSD-style (see the file libraries/core/LICENSE)
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
 -- 
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  experimental
--- Portability :  non-portable
---
--- $Id: SampleVar.hs,v 1.2 2002/04/24 16:31:37 simonmar Exp $
+-- Portability :  non-portable (concurrency)
 --
 -- Sample variables
 --
@@ -16,13 +14,15 @@
 
 module Control.Concurrent.SampleVar
        (
+         -- * Sample Variables
          SampleVar,         -- :: type _ =
  
-        newEmptySampleVar, -- :: IO (SampleVar a)
+         newEmptySampleVar, -- :: IO (SampleVar a)
          newSampleVar,      -- :: a -> IO (SampleVar a)
-        emptySampleVar,    -- :: SampleVar a -> IO ()
-        readSampleVar,     -- :: SampleVar a -> IO a
-        writeSampleVar     -- :: SampleVar a -> a -> IO ()
+         emptySampleVar,    -- :: SampleVar a -> IO ()
+         readSampleVar,     -- :: SampleVar a -> IO a
+         writeSampleVar,    -- :: SampleVar a -> a -> IO ()
+         isEmptySampleVar,  -- :: SampleVar a -> IO Bool
 
        ) where
 
@@ -30,69 +30,95 @@ import Prelude
 
 import Control.Concurrent.MVar
 
--- Sample variables are slightly different from a normal MVar:
+import Control.Exception ( mask_ )
+
+import Data.Functor ( (<$>) )
+
+-- |
+-- 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 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)
+--  * 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 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.)
+--  * 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)
-
--- Initally, a SampleVar is empty/unfilled.
+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 <- newEmptyMVar
-   putMVar v a
-   newMVar (1,v)
+   v <- newMVar a
+   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
-   if readers >= 0 then
+emptySampleVar (SampleVar v) = mask_ $ do
+   s@(readers, var) <- takeMVar v
+   if readers > 0 then do
+     _ <- 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 (SampleVar svar) = mask_ $ do
 --
 -- filled => make empty and grab sample
 -- not filled => try to grab value, empty when read val.
 --
-readSampleVar :: SampleVar a -> IO a
-readSampleVar svar = do
    (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 (SampleVar svar) v = mask_ $ do
 --
 -- filled => overwrite
 -- not filled => fill, write val
 --
-writeSampleVar :: SampleVar a -> a -> IO ()
-writeSampleVar svar v = do
-   (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.
+--
+-- 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 (SampleVar svar) = do
+   (readers, _) <- readMVar svar
+   return (readers <= 0)
+