Hi,
This is quite a neat generalisation of forkIO, and something I've wanted
in the past.
My comment would be about the MonadIO m requirement for ForkableMonad.
I understand that conceptually it's a nice thing to have. But
practically, I don't think it's necessary, and could be a little
restrictive -- I could imagine potentially having a newtype-wrapped
monad that doesn't have a MonadIO instance, but does have a
ForkableMonad instance. I tried removing the MonadIO constraint, and it
works as long as you add a Monad constraint either to the class or to
the ReaderT and StateT instances. That depends if you can imagine an
instance of ForkableMonad that wasn't a Monad (an applicative
perhaps)... probably not, especially given the name.
In short: I recommend changing MonadIO m to Monad m on the class.
I came up with this instance for ContT:
instance (ForkableMonad m) => ForkableMonad (ContT r m) where
forkIO act = lift $ forkIO (runContT act (const $ return undefined))
I don't know if that's useful and/or correct, though.
Thanks,
Neil.
David Anderson wrote:
Dear Haskellers,
I'm happy, and only slightly intimidated, to announce the initial
release of forkable-monad.
The short version is that forkable-monad exports a replacement forkIO
that lets you do this:
type MyMonad = ReaderT Config (StateT Ctx IO)
startThread :: MyMonad ThreadId
startThread = forkIO threadMain
threadMain :: MyMonad ()
threadMain = forever $ liftIO $ putStrLn "Painless monad stack forking!"
Note the lack of monad stack deconstruction and reconstruction to
transport it over to the new thread. You'll find the details in the
Haddock documentation for the module.
forkable-monad is available:
* On hackage: http://hackage.haskell.org/package/forkable-monad
* Via cabal: cabal install forkable-monad
* Source and issue tracker: http://code.google.com/p/forkable-monad/
Feedback is of course welcome. As this is my first published Haskell
code and Hackage upload, I expect there will be quite a bit!
- Dave
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