I would file a bug on the GHC bug tracker: http://hackage.haskell.org/trac/ghc/newticket?type=bug You will need to use the guest login to trac, though (in grey text at the bottom of the page). I've also CCed the GHC-users list, as there are folks over there that might have knowledge of the concurrency libraries. Take care, Antoine On Sat, Jan 1, 2011 at 7:03 PM, Eric Stansifer wrote:

Hi,

I didn't get a response to my question on haskell-cafe, perhaps libraries is a more appropriate place to ask.

doc:  http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.0.0/Contr... source:  http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.0.0/src/C...

The isEmptySampleVar function,

isEmptySampleVar :: SampleVar a -> IO Bool isEmptySampleVar (SampleVar svar) = do   (readers, _) <- readMVar svar   return (readers == 0)

returns False whenever readers < 0.  However, readers < 0 occurs when there are threads waiting on an empty SampleVar.

The documentation on SampleVar is a bit sparse on explaining the intended behavior;  I wouldn't have expected this behavior if I hadn't read the source.  Can someone clarify the semantics of SampleVar?

For a contrived example, consider

do_something = threadDelay 100000     -- 100 ms

produce, consume :: SampleVar Int -> IO () produce svar = do    do_something    b <- isEmptySampleVar svar    if b then    randomIO >>= writeSampleVar svar    else   return ()    produce svar

consume svar = do    x <- readSampleVar svar    print x    consume svar

main = do    svar <- newEmptySampleVar    forkIO $ produce svar    forkIO $ consume svar    threadDelay 1000000       -- one second

This code deadlocks instead of printing random numbers.

Thanks, Eric

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Wouldn't the following implementation of SampleVars be simpler (and slightly more efficient) that the current one? The difference is that instead of keeping an Int that represents the number of readers, I keep a Bool that represents whether the SampleVar is empty. While your example runs without deadlocking, I haven't yet fully checked the correctness though... ----------------------------------------------------------------------------- -- | -- 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 import Control.Exception ( mask_ ) import Data.Functor ( (<$>) ) -- | -- Sample variables are slightly different from a normal 'MVar': -- -- * Reading an empty 'SampleVar' causes the reader to block. -- (same as 'takeMVar' on empty 'MVar') -- -- * Reading a filled 'SampleVar' empties it and returns value. -- (same as 'takeMVar') -- -- * Writing to an empty 'SampleVar' fills it with a value, and -- potentially, wakes up a blocked reader (same as for 'putMVar' on -- empty 'MVar'). -- -- * Writing to a filled 'SampleVar' overwrites the current value. -- (different from 'putMVar' on full 'MVar'.) newtype SampleVar a = SampleVar ( MVar ( Bool -- is empty? , MVar a ) ) deriving (Eq) -- |Build a new, empty, 'SampleVar' newEmptySampleVar :: IO (SampleVar a) newEmptySampleVar = do v <- newEmptyMVar SampleVar <$> newMVar (True, v) -- |Build a 'SampleVar' with an initial value. newSampleVar :: a -> IO (SampleVar a) newSampleVar a = do v <- newMVar a SampleVar <$> newMVar (False, v) -- |If the SampleVar is full, leave it empty. Otherwise, do nothing. emptySampleVar :: SampleVar a -> IO () emptySampleVar (SampleVar svar) = mask_ $ do s@(empty, val) <- takeMVar svar if empty then putMVar svar s else takeMVar val >> putMVar svar (True, val) -- |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. -- s@(empty, val) <- takeMVar svar if empty then putMVar svar s else putMVar svar (True, 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 -- s@(empty, val) <- takeMVar svar if empty then do putMVar val v putMVar svar (False, val) else do swapMVar val v putMVar svar s -- | 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) = fst <$> readMVar svar ----------------------------------------------------------------------------- Regards, Bas