Hi, 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 documentation for Control.Concurrent.SampleVar implies that a SampleVar is either "filled" or "empty".  The source code suggests that there are three possible states:  "full" (when readers == 1), "empty" (when readers == 0), and "empty with blocked readers" (when readers < 0).  In particular, the isEmptySampleVar function, isEmptySampleVar :: SampleVar a -> IO Bool isEmptySampleVar (SampleVar svar) = do   (readers, _) <- readMVar svar   return (readers == 0) returns false in the third state, when readers < 0.  Can someone clarify the semantics of SampleVar? Thanks, Eric