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websockets example
==================
This is the Haskell implementation of the example for the WebSockets library. We
implement a simple multi-user chat program. A live demo of the example is
available [here](http://jaspervdj.be/websockets-example). In order to understand
this example, keep the [reference](http://jaspervdj.be/websockets/reference)
nearby to check out the functions we use.
> {-# LANGUAGE OverloadedStrings #-}
> import Data.Char (isPunctuation, isSpace)
> import Data.Monoid (mappend)
> import Data.Text (Text)
> import Control.Exception (finally)
> import Control.Monad (forM_, forever)
> import Control.Concurrent (MVar, newMVar, modifyMVar_, modifyMVar, readMVar)
> import qualified Data.Text as T
> import qualified Data.Text.IO as T
> import qualified Network.WebSockets as WS
We represent a client by their username and a `WS.Connection`. We will see how we
obtain this `WS.Connection` later on.
> type Client = (Text, WS.Connection)
The state kept on the server is simply a list of connected clients. We've added
an alias and some utility functions, so it will be easier to extend this state
later on.
> type ServerState = [Client]
Create a new, initial state:
> newServerState :: ServerState
> newServerState = []
Get the number of active clients:
> numClients :: ServerState -> Int
> numClients = length
Check if a user already exists (based on username):
> clientExists :: Client -> ServerState -> Bool
> clientExists client = any ((== fst client) . fst)
Add a client (this does not check if the client already exists, you should do
this yourself using `clientExists`):
> addClient :: Client -> ServerState -> ServerState
> addClient client clients = client : clients
Remove a client:
> removeClient :: Client -> ServerState -> ServerState
> removeClient client = filter ((/= fst client) . fst)
Send a message to all clients, and log it on stdout:
> broadcast :: Text -> ServerState -> IO ()
> broadcast message clients = do
> T.putStrLn message
> forM_ clients $ \(_, conn) -> WS.sendTextData conn message
The main function first creates a new state for the server, then spawns the
actual server. For this purpose, we use the simple server provided by
`WS.runServer`.
> main :: IO ()
> main = do
> state <- newMVar newServerState
> WS.runServer "0.0.0.0" 9160 $ application state
Our main application has the type:
> application :: MVar ServerState -> WS.ServerApp
Note that `WS.ServerApp` is nothing but a type synonym for
`WS.PendingConnection -> IO ()`.
Our application starts by accepting the connection. In a more realistic
application, you probably want to check the path and headers provided by the
pending request.
We also fork a pinging thread in the background. This will ensure the connection
stays alive on some browsers.
> application state pending = do
> conn <- WS.acceptRequest pending
> WS.forkPingThread conn 30
When a client is succesfully connected, we read the first message. This should
be in the format of "Hi! I am Jasper", where Jasper is the requested username.
> msg <- WS.receiveData conn
> clients <- readMVar state
> case msg of
Check that the first message has the right format:
> _ | not (prefix `T.isPrefixOf` msg) ->
> WS.sendTextData conn ("Wrong announcement" :: Text)
Check the validity of the username:
> | any ($ fst client)
> [T.null, T.any isPunctuation, T.any isSpace] ->
> WS.sendTextData conn ("Name cannot " `mappend`
> "contain punctuation or whitespace, and " `mappend`
> "cannot be empty" :: Text)
Check that the given username is not already taken:
> | clientExists client clients ->
> WS.sendTextData conn ("User already exists" :: Text)
All is right! We're going to allow the client, but for safety reasons we *first*
setup a `disconnect` function that will be run when the connection is closed.
> | otherwise -> flip finally disconnect $ do
We send a "Welcome!", according to our own little protocol. We add the client to
the list and broadcast the fact that he has joined. Then, we give control to the
'talk' function.
> modifyMVar_ state $ \s -> do
> let s' = addClient client s
> WS.sendTextData conn $
> "Welcome! Users: " `mappend`
> T.intercalate ", " (map fst s)
> broadcast (fst client `mappend` " joined") s'
> return s'
> talk conn state client
> where
> prefix = "Hi! I am "
> client = (T.drop (T.length prefix) msg, conn)
> disconnect = do
> -- Remove client and return new state
> s <- modifyMVar state $ \s ->
> let s' = removeClient client s in return (s', s')
> broadcast (fst client `mappend` " disconnected") s
The talk function continues to read messages from a single client until he
disconnects. All messages are broadcasted to the other clients.
> talk :: WS.Connection -> MVar ServerState -> Client -> IO ()
> talk conn state (user, _) = forever $ do
> msg <- WS.receiveData conn
> readMVar state >>= broadcast
> (user `mappend` ": " `mappend` msg)
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