Implement a chat server
This page describes how to implement a simple chat server. The server should support multiple connected users. Messages sent to the server are broadcast to all currently connected users. For this tutorial we'll use Network.Socket, which provides low-level bindings to the C-socket API.
2 Simple socket serverWe start with a simple server. The structure of this server begins with a
-- in ChatServer.hs import Network.Socket main :: IO () main = do sock <- socket AF_INET Stream 0 -- create socket setSocketOption sock ReuseAddr 1 -- make socket immediately reusable - eases debugging. bind sock (SockAddrInet 4242 iNADDR_ANY) -- listen on TCP port 4242. listen sock 2 -- set a max of 2 queued connections mainLoop sock -- unimplemented
In our main loop we'll build out the socket-server equivalent of a "Hello World!" example. For a given socket we'll: accept a connection, relay a simple "Hello World!", close the connection, and recurse on the original socket.
-- in ChatServer.hs mainLoop :: Socket -> IO () mainLoop sock = do conn <- accept sock -- accept a connection and handle it runConn conn -- run our server's logic mainLoop sock -- repeat runConn :: (Socket, SockAddr) -> IO () runConn (sock, _) = do send sock "Hello!\n" close sock
Notice that accepting a socket has a return type of
3 Using System.IO for sockets
import System.IO [...] runConn (sock, _) = do hdl <- socketToHandle sock ReadWriteMode hSetBuffering hdl NoBuffering hPutStrLn hdl "Hi!" hClose hdl
4 ConcurrencySo far the server can only handle one connection at a time. This is ok for just writing a message but won't work for a chat server. We can fix this quite easily though, using
import Control.Concurrent [...] mainLoop sock = do conn <- accept sock forkIO (runConn conn) mainLoop sock
5 Adding communication between threadsThis seems to be a hard problem. Luckily, the
type Msg = String
import Control.Concurrent.Chan [...] main = do [...] chan <- newChan mainLoop sock chan
mainLoop :: Socket -> Chan Msg -> IO () mainLoop sock chan = do conn <- accept sock forkIO (runConn conn chan) mainLoop sock chan
import Control.Monad import Control.Monad.Fix (fix) [...] runConn :: (Socket, SockAddr) -> Chan Msg -> IO () runConn (sock, _) chan = do let broadcast msg = writeChan chan msg hdl <- socketToHandle sock ReadWriteMode hSetBuffering hdl NoBuffering chan' <- dupChan chan -- fork off thread for reading from the duplicated channel forkIO $ fix $ \loop -> do line <- readChan chan' hPutStrLn hdl line loop -- read lines from socket and echo them back to the user fix $ \loop -> do line <- liftM init (hGetLine hdl) broadcast line loop
6 Cleanups and final code
There are two major problems left in the code. First, the code has a memory leak, because the original channel is never read by anyone. This can be fixed by adding another thread just for that purpose.
Secondly, closing connections is not handled gracefully at all. This requires exception handling.
The code below fixes the first issue and mostly fixes the second one, and adds a few cosmetic improvements:
- messages are not echoed back to the user they came from.
- every connection is associated with a name.
-- with apologies for the lack of comments :) import Network.Socket import System.IO import Control.Exception import Control.Concurrent import Control.Concurrent.Chan import Control.Monad import Control.Monad.Fix (fix) type Msg = (Int, String) main :: IO () main = do chan <- newChan sock <- socket AF_INET Stream 0 setSocketOption sock ReuseAddr 1 bind sock (SockAddrInet 4242 iNADDR_ANY) listen sock 2 forkIO $ fix $ \loop -> do (_, msg) <- readChan chan loop mainLoop sock chan 0 mainLoop :: Socket -> Chan Msg -> Int -> IO () mainLoop sock chan nr = do conn <- accept sock forkIO (runConn conn chan nr) mainLoop sock chan $! nr+1 runConn :: (Socket, SockAddr) -> Chan Msg -> Int -> IO () runConn (sock, _) chan nr = do let broadcast msg = writeChan chan (nr, msg) hdl <- socketToHandle sock ReadWriteMode hSetBuffering hdl NoBuffering hPutStrLn hdl "Hi, what's your name?" name <- liftM init (hGetLine hdl) broadcast ("--> " ++ name ++ " entered.") hPutStrLn hdl ("Welcome, " ++ name ++ "!") chan' <- dupChan chan reader <- forkIO $ fix $ \loop -> do (nr', line) <- readChan chan' when (nr /= nr') $ hPutStrLn hdl line loop handle (\(SomeException _) -> return ()) $ fix $ \loop -> do line <- liftM init (hGetLine hdl) case line of "quit" -> hPutStrLn hdl "Bye!" _ -> do broadcast (name ++ ": " ++ line) loop killThread reader broadcast ("<-- " ++ name ++ " left.") hClose hdl
Have fun chatting!