Tuesday, September 30, 2008

Some first steps with Data.Reactive

I've had a pretty busy last week and I began to go down the path of neglecting my poor, tiny blog. Don't die on me, tiny blog! I will grant you the content you desire to make you happy!

Alright, so a friend of mine asked me about Data.Reactive a few days ago and I wanted to share a couple of small examples and even questions about the proper use of this library.

For those who haven't seen it, Conal Elliot has made a slightly brain-twisty new library for doing functional reactive programming, i.e. a way to make interactive programs with clean semantics. What follows is my attempt to make a simple chat server that uses Data.Reactive to handle logins and echoing text. I'm partly cribbing from a good example on haskell cafe and unabashedly borrow some of Mr. Stephen's function definitions. I'm not claiming this is a good example of using Data.Reactive, only that it merely works and this was my way of learning about the library. If anyone with more experience can comment as to my use/misuse of the Conal's work, it would be most appreciated.

> module Main where
> import Control.Concurrent
> import Control.Concurrent.STM
> import Control.Applicative
> import Control.Monad
> import Data.Reactive
> import Network
> import System.IO

The function socketServer is going to be our introduction to using Events in Data.Reactive. It returns an event source that we can use to grab handles as they are created by the acceptConnection function. An important point that I found embarrassingly confusing is that you don't need to do any plumbing between the event and sink. The sink is just a function of type a -> IO () that populates the event whenever it is called. acceptConnection is just a simple function that accepts the the connection then passes that into the sink that we created with mkEvent.

> socketServer :: IO (Event Handle)
> socketServer = withSocketsDo $ do
> (event,sink) <- mkEvent
> socket <- listenOn (PortNumber 5000)
> forkIO $ forever $ acceptConnection socket sink
> return event
> acceptConnection :: Socket -> (Handle -> IO ()) -> IO ThreadId
> acceptConnection s sink = do
> (h,_,_) <- accept s
> hSetBuffering h NoBuffering
> forkIO $ sink h

Now seeing these functions, you might be wondering what you need to do to get at the data in the event that's returned by this function. Well, we only have to take a look at the documentation for Data.Reactive on hackage to see that Event is an instance of Functor and that this functor instance allows us to lift a function of type a -> IO b to a function Event a -> Event (IO b). Looking at the documentation again, we find a wonderful little function called runE that has type Event (IO b) -> IO a so lets put these together into one tiny program.

> main' = do
> e <- socketServer
> runE $ fmap (\h -> print h >> hClose h) e

This is a pretty braindead example, but it will print a line to stdout every time that a new connection is made to the server. Now lets go ahead and make it so that as new events occur we launch a connection handler that allows us to turn this into a real chat room. We'll also, just to be gratuitous, also use events to manage the incoming messages and their distribution.

> main = do
> e <- socketServer
> (msgEvent,sink) <- mkEvent
> runE $ fmap (forkIO . (handleCloser $ handler msgEvent sink)) e
> handler :: Event String -> (String -> IO ())-> Handle -> IO ()
> handler e sink h = do
> subscribe e (hPutStrLn h)
> forever $ hGetLine h >>= sink
> handleCloser :: (Handle -> IO ()) -> Handle -> IO ()
> handleCloser action h = catch (action h) (const $ hClose h)

Okay, so not really that much changed in this version. The big change is that we now have two sets of events, one that for the handles and one for the messages. We also use the subscribe function to launch the threads that display messages to everyone logged in, subscribe being a slightly misleadingly typed function that takes in an event and a consumer and spawns a new thread feeding the event data into the consumer. Now, all you boys and girls following along at home will probably notice a major oversight: at the moment, there's no real way to close down the chat server cleanly. Also, I cheat and don't properly handle logouts except for closing the handle when it hits an error.

I'm also working on another use of Data.Reactive with some of the SDL toys I've written. Maybe I should port my pong clone over to it, if I have copious free time.


Antoine said...

I think I get it.

I always get caught up in the plumbing when I read the FRP papers, so it's nice to see a simple example.

It looks like a very 'functional' approach to callback-style programming.

beardetected said...

Thx for this post! It would be really pleasant if you share SDL++FRP examples.

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