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

Eugene Kirpichov ekirpichov at gmail.com
Thu Jan 22 04:14:37 EST 2009

Although I am not Luke, I will answer :)
The code creates a TVar 'stopvar', whose truth means to the 'run' loop
that it's time to return ().
So, if we're going to return an action that stops the loop, we're
going to return an action that sets stopvar to True.
That's precisely what return (atomically $ writeTVar stopvar True)
does. In case you're wondering about the atomically, it's because
writeTVar .... has type STM () and whereas we need to 'return' an IO
(). For precisely the same reason is atomically present in the run
loop, since the loop is also in the IO monad.

2009/1/22 Belka <lambda-belka at yandex.ru>:
>> 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...
> Belka
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Евгений Кирпичев
Разработчик Яндекс.Маркета

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