Perhaps a more sensible example: $ cat t.hs import Control.Concurrent import Control.Monad spawnThread :: IO ThreadId spawnThread = forkIO $ forM_ [0..] $ \i -> threadDelay 1000000 $ ghci-7.10.1 t.hs -v0 *Main> t0 <- spawnThread *Main> :break 8 Breakpoint 0 activated at t.hs:8:9-27 *Main> *** Ignoring breakpoint *** Ignoring breakpoint *** Ignoring breakpoint *** Ignoring breakpoint :q $ Thanks, Facundo On Wed, May 20, 2015 at 12:51 PM, Facundo Domínguez wrote:

Hello, I have a multi-threaded and interactive application that sometimes stops responding, and it would be helpful being able to inspect the state when it doesn't.

I thought the GHCi debugger could be useful here, however I see no way to signal a thread and have GHCi show me its state.

Here is an example GHCi session:

$ cat t.hs import Control.Concurrent import Control.Exception import Control.Monad import System.IO

spawnThread :: String -> IO ThreadId spawnThread label = forkIO $ flip finally (putStrLn $ "bye " ++ label) $ forM_ [0..] $ \i -> threadDelay 1000000 $ ghci-7.10.1 t.hs -fbreak-on-exception -v0 *Main> hSetBuffering stdout LineBuffering *Main> t0 <- spawnThread "t0" *Main> throwTo t0 (ErrorCall "end") *Main>

It looks like I'm not getting the "bye t0" message when stopping the thread while using -fbreak-on-exception.

Is the debugger supposed to be used like this? Otherwise, is there any way in which I could inspect the state of a running thread?

Thanks, Facundo