[Haskell-cafe] Re: Waiting for thread to finish

[Ben Franksen]

Belatedly I realized that this answer should have been going to the list:
---------------
ChrisK wrote:
On Mittwoch, 28. November 2007, you wrote:
> A safer gimmick...
>
> Ben Franksen wrote:
> > tickWhileDoing :: String -> IO a -> IO a
> > tickWhileDoing msg act = do
> >   hPutStr stderr msg >> hPutChar stderr ' ' >> hFlush stderr
> >   start_time <- getCPUTime
> >   tickerId <- forkIO ticker
>
> ... an async exception here will leave the ticker runnning....
>
> >   res <- act `finally` killThread tickerId

Thanks for spotting this loophole. I keep forgetting people tend to hit 
Ctrl-C whenever they feel like it... ;-) Thinking some more about this, I 
realise that the async exception could also come from somewhere inside the 
Haskell program (e.g. from a killThread like I did myself in the next 
line.) So the fix below makes this whole things more robust indeed.

> The best way to make this safe that I know of is:
> >   res <- block $ do
> >     tickerId <- forkIO ticker
> >     unblock act `finally` killThread tickerId

Yes.

Cheers
Ben