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. In order to understand this example, keep the reference nearby to check out the functions we use.

> {-# LANGUAGE OverloadedStrings #-}
> module Main where
> 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 "127.0.0.1" 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)