[Haskell-beginners] Re: Simple Haskell daemon

[Patrick LeBoutillier]

Amy,

Here's a small suggestion:

If the start and loop functions always returns Nothing, I think it may
be cleaner to
simply make them return ().


Patrick


On Tue, Nov 16, 2010 at 3:28 PM, Amy de Buitléir <amy at nualeargais.ie> wrote:
> Chaddaï Fouché <chaddai.fouche <at> gmail.com> writes:
>
>>
>> To use a typeclass there is pretty strange and doesn't have much
>> advantage, it would be much easier and nicer to just write start to
>> take some functions as parameters...
>>
>
> Doh! Yes, I kind of overlooked the obvious there. Thank you so much! Here's my
> new version, in case anyone Googles this.
>
> ----- Daemon.hs -----
> module Daemon where
>
> import System.Exit
> import System.IO.Unsafe
> import System.Posix.Signals
> import Control.Concurrent
>
> termReceived = unsafePerformIO (newMVar False)
>
> handleTERM :: IO ()
> handleTERM = swapMVar termReceived True >> return ()
>
> loop :: (a -> IO a) -> (a -> IO ()) -> a -> IO (Maybe a)
> loop work finalise d = do
>  timeToStop <- readMVar termReceived
>  if timeToStop
>    then finalise d >> return Nothing
>    else work d >>= loop work finalise
>
> start
>  -- | This function will be invoked when the daemon starts.
>  :: IO a
>  -- | This function will be invoked in the main loop.
>  -> (a -> IO a)
>  -- | This function will be invoked when the daemon shuts down.
>  -> (a -> IO ())
>  -- | The result will always be Nothing.
>  -> IO (Maybe a)
> start initialise work finalise =
>    installHandler sigTERM (Catch handleTERM) Nothing >> initialise >>= loop
> work finalise
>
> ----- CountingExample.hs -----
>
> module CountingExample where
>
> import Daemon
>
> initialise = do
>  putStrLn "Starting up"
>  return 0
>
> work i = do
>  putStrLn (show i)
>  return (i+1)
>
> finalise i = do
>  putStrLn "Shutting down"
>  return ()
>
> main = start initialise work finalise
>
>
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-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada