This is undoubtedly nothing more than timing issues. Remember that the main thread exiting will kill the entire process, automatically killing all other threads as side effect. So the question is how much the thread manages to get done before that happens. If you disable output buffering, you may find that "f" or "fo" sometimes gets written before process exit. On Thu, Nov 29, 2018 at 1:43 PM Johannes Waldmann < johannes.waldmann@htwk-leipzig.de> wrote:
Dear Cafe,
I am surprised by the behaviour of the program below (the interesting property is whether it will output "foo").
Behaviours (plural) actually: it seems to depend on optimisation level, on omit-yields, and on very small changes in the source code:
It does print (mostly), when compiled with -O0. It does not, when compiled with -O2. With -O2 -fno-omit-yields, it will print. With -O0 -fno-omit-yields, and when I remove the two newTVar in the beginning, it will mostly not print.
How come?
These differences already occur with the last two lines replaced by "forever $ return ()", so the STM stuff may be inessential here. But that's the context where I came across the problem.
- J.W.
import Control.Concurrent.STM import Control.Concurrent ( forkIO ) import Control.Monad ( forever ) import System.IO
main = do
atomically $ newTVar "bar" atomically $ newTVar False
forkIO $ putStrLn "foo"
x <- atomically $ newTVar False forever $ atomically $ writeTVar x True
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-- brandon s allbery kf8nh allbery.b@gmail.com