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