How to use GHC.Event module, main thread not exit directly

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How to use GHC.Event module, main thread not exit directly

Hello everyone:
I write a server use GHC.Event module, 

{-# LANGUAGE OverloadedStrings #-}
-- A simple example of an epoll based http server in Haskell.
-- Uses two libraries:
--   * network-bytestring, bytestring-based socket IO.
--      - cabal install network-bytestring: 
--   * haskell-event, epoll-based scalable IO events
--      - git clone git://
--      - autoreconf ; then cabal install
import Network hiding (accept)
import Network.Socket (fdSocket, accept)
import Network.Socket.ByteString
import Data.ByteString.Char8
import System.Event
import System.Posix
import System.Posix.IO
main = withSocketsDo $ do
    sock <- listenOn $ PortNumber 5002
    let fd = fromIntegral (fdSocket sock)
    mgr <- new
    registerFd mgr (client sock) fd evtRead
    loop mgr
client sock _ _ = do
    (c,_) <- accept sock
    sendAll c msg
    sClose c
msg = "HTTP/1.0 200 OK\r\nContent-Length: 5\r\n\r\nPong!\r\n"

but I not find the definition of 'loop' function: loop mgr

the after is my program:
btnz@vmubuntuserver:~/work/code/echo-server-event-model$ cat Main.hs
module Main where

import Network.Socket
import Network.Socket.ByteString (recv, sendAll)
import GHC.Event as Event
import qualified Control.Exception as E (bracket)
import Data.ByteString.Internal (packChars)

hints = defaultHints {addrFlags = [AI_PASSIVE], addrSocketType = Stream}
msg = "HTTP/1.0 200 OK\r\nContent-Length: 5\r\n\r\nPong!\r\n"

main :: IO ()
main = do
    putStrLn "Hello, Haskell!"
    addr <- head <$> getAddrInfo (Just hints) Nothing (Just "3000")
    E.bracket (sock addr) close eventServer
    return ()
        sock addr = do
            s <- socket (addrFamily addr) (addrSocketType addr) (addrProtocol addr)
            setSocketOption s ReuseAddr 1
            withFdSocket s setCloseOnExecIfNeeded
            bind s $ addrAddress addr
            listen s 1024
            return s
        eventServer s = do
            fd <- withFdSocket s (\ci -> return (fromIntegral ci))
            mgr <-
            Event.registerFd mgr (service s) fd Event.evtRead Event.OneShot
--            loop mgr

service :: Socket -> FdKey -> Event -> IO ()
service s _ _ = do
    (c, _) <- accept s
    sendAll c $ packChars msg
    close c

btnz@vmubuntuserver:~/work/code/echo-server-event-model$ cabal run
Up to date
Hello, Haskell!

when I run cabal run the program exit directly.

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