[Haskell-cafe] how to implement daemon start and stop directives?

Belka lambda-belka at yandex.ru
Thu Jan 22 03:11:39 EST 2009

> You can abstract this pattern:
> -- runs its argument in an infinite loop, and returns an action that stops
> the loop
> daemon :: IO () -> IO (IO ())
> daemon action = do
>     stopvar <- atomically $ newTVar False
>     let run = do
>           stop <- atomically $ readTVar stopvar
>           if stop then return () else (action >> run)
>     forkIO run
>     return (atomically $ writeTVar stopvar True)
> TVars are overkill here, actually, an IORef would be just fine, I think.
> Luke

Thanks, Luke!

Why do you write "return (atomically $ writeTVar stopvar True)" in the end?

Actually, I'm more interested in technical details how to communicate from
shell with background process - how to send commands to it. Currently
looking into POSIX libraries hope to find answers there... 
Also, maybe a FIFO-pipe-file would solve my problem. Imagine writing a
command in it, while one of daemon's thread is locked-while-awaits for
anything to come out from the other side of the pipe...

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