I recently upgraded to ghc 7.0.2 from 6.12.3, along with the Haskell
platform, and noticed that the following code no longer works as expected:
waitFor tvar = atomically $ do
count <- readTVar tvar
check (count == 0)
worker tchan tvar = loop
where loop = do
putStrLn "checking"
finished <- atomically $ isEmptyTChan tchan
threadDelay 50000
if finished
then atomically $ do val <- readTVar tvar
writeTVar tvar $ (subtract 1) val
else (atomically $ readTChan tchan) >> loop
test = do
tchan <- newTChanIO
pure $ forM_ [1..5] $ writeTChan tchan -- THIS LINE
tvar <- newTVarIO 1
forkIO $ worker tchan tvar
waitFor tvar
putStrLn "DONE"
This will work if I use atomically, but otherwise it does nothing, tchan
remains empty afterwards.
I'm not clear how this should behave.
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