[Haskell-beginners] Re: Simple Haskell daemon

[Amy de Buitléir]

Patrick LeBoutillier <patrick.leboutillier <at> gmail.com> writes:

> 
> 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 ().

Thank you Patrick, that make the interface easier for the user to understand.

The "loop" function does return something when it is called recursively, but the 
outer call never returns anything, so my first try was to have two separate 
functions.

----- code snippet -----
loop :: (a -> IO a) -> (a -> IO ()) -> a -> IO ()
loop work finalise d = loop' work finalise d >> 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
----- end of code snippet -----

But then I thought that looked more complicated, so I went with this. 
(CountingExample.hs is unchanged.)

----- 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 ())
  -- | Returns nothing.
  -> IO ()
start initialise work finalise = 
    installHandler sigTERM (Catch handleTERM) Nothing 
        >> initialise 
        >>= loop work finalise 
        >> return ()