haskell udp example found in the wild

UDPEchoServer.hs

--
-- UDPEchoServer.hs: Exactly what the name says, a datagram echo server.
--


module Main (main) where

import Network.Socket
import System.Posix.Directory
import System.Posix.Files
import System.Posix.IO
import System.Posix.Process
import System.Exit


echoPort = 9900
maxline = 1500

--
-- The daemon infrastructure
--

main :: IO ()
main = do
           pid <- forkProcess child
           exitImmediately ExitSuccess


child :: IO ()
child = do
           -- Set up the working directory, mask and standard i/o
           -- for a daemon process (these will be inherited by
           -- the forked process):

           changeWorkingDirectory "/"
           setFileCreationMask 0

           mapM_ closeFd [stdInput, stdOutput, stdError]
           nullFd <- openFd "/dev/null" ReadWrite Nothing  
defaultFileFlags
           mapM_ (dupTo nullFd) [stdInput, stdOutput, stdError]

           closeFd nullFd

           createSession     -- This child becomes a process and session
                             -- group leader. This prevents the child of
                             -- this process (the daemon) from
                             -- ever getting a controlling terminal.
           pid' <- forkProcess echoserver

           exitImmediately ExitSuccess

--
-- The echo server daemon
--

echoserver :: IO ()
echoserver = do
           withSocketsDo $ do
                   sock <- socket AF_INET Datagram 0
                   bindSocket sock (SockAddrInet echoPort iNADDR_ANY)
                   socketEcho sock


socketEcho :: Socket -> IO ()
socketEcho sock = do
           (mesg, recv_count, client) <- recvFrom sock maxline
           send_count <- sendTo sock mesg client
           socketEcho sock