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Implement a chat server

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Contents

1 Introduction

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 server

We start with a simple server. The structure of this server begins with a
main
method which will create a reusable socket, open up a TCP connection on port 4242 which will allow a maximum of two queued connections.
-- 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
(Socket, SockAddr)
— this corresponds to a new socket object which can be used to send and receive data for a given connection. This socket object is then closed at the end of our
runConn
method. The
SockAddr
, as you can see from the
runConn
method, is largely uninteresting for this use-case and will simply be the initial socket address of 4242.

3 Using System.IO for sockets

Network.Socket
incorrectly represent binary data in
send
and
recv
and, as a result, use of these functions is not advised and may lead to bugs.
Network.Socket
actually recommends using these same methods defined in the ByteString module. However, to keep things simple, we'll stick to
System.IO
for input and output. Importing our new module and turning our
Socket
into a
Handle
now looks like the following:
-- in the imports our ChatServer.hs add:
import System.IO
 
-- and we'll change our `runConn` function to look like:
runConn :: (Socket, SockAddr) -> IO ()
runConn (sock, _) = do
    hdl <- socketToHandle sock ReadWriteMode
    hSetBuffering hdl NoBuffering
    hPutStrLn hdl "Hello!"
    hClose hdl

4 Concurrency

So 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
forkIO
:
import Control.Concurrent
[...]
mainLoop sock = do
    conn <- accept sock
    forkIO (runConn conn)
    mainLoop sock

5 Adding communication between threads

This seems to be a hard problem. Luckily, the
Control.Concurrent.Chan
module provides exactly what we need: channels with a single write and multiple read ends. First we decide on a message type. Let's use a string for now:
type Msg = String
main
will have to create a channel, and pass it to
mainLoop
.
import Control.Concurrent.Chan
[...]
main = do
    [...]
    chan <- newChan
    mainLoop sock chan
mainLoop
in turn will pass it to
runConn
.
mainLoop :: Socket -> Chan Msg -> IO ()
mainLoop sock chan = do
    conn <- accept sock
    forkIO (runConn conn chan)
    mainLoop sock chan
And finally,
runConn
will duplicate the channel and read from it.
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
Note that
runConn
now actually forks another worker thread for sending messages to the connected user.

6 Cleanups and final code

(thumbnail)
Screenshot :)

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!