[Haskell-beginners] Re: Simple Haskell daemon

[Chaddaï Fouché]

On Tue, Nov 16, 2010 at 10:11 PM, Amy de Buitléir <amy at nualeargais.ie> wrote:
> 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.

Even recursively it doesn't return anything, or rather it always ends
up returning Nothing since that's what the base case returns and the
recursive case don't modify it, also finalise already returns (), so
you can write :

> loop :: (a -> IO a) -> (a -> IO ()) -> a -> IO ()
> loop work finalise d = do
>  timeToStop <- readMVar termReceived
>  if timeToStop
>    then finalise d
>    else work d >>= loop work finalise

To avoid the need to repeat the same parameters at each call of the
recursive function, you can use a "worker-wrapper" (? not sure of the
term) transformation :

> loop :: (a -> IO a) -> (a -> IO ()) -> a -> IO ()
> loop work finalise d = loop' d
>   where
>     loop' d = do
>       timeToStop <- readMVar termReceived
>       if timeToStop
>         then finalise d
>         else work d >>= loop'

Which finally can be directly included in your start function :

> 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
>   where
>     loop d = do
>       timeToStop <- readMVar termReceived
>       if timeToStop
>         then finalise d
>         else work d >>= loop

Defining functions at the top level instead may be clearer or more
useful in certain cases, but I'm not sure this is one of those cases
(you don't want to export loop, start and loop bodies are very short,
and you can simplify and optimize loop body by making it a local
function).
-- 
Jedaï