Debugging Haskell

Jerry, JiJie
Sat, 9 Mar 2002 23:01:21 +0800

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Good day everyone, I was fiddling around with this tiny echo
client/server haskell program from 'The Great Language Shootout'
site ( and got stuck. 

The code (attached) has been reformatted with minimal API tweak
(mkPortNumber, writeSocket, readSocket) to please my ghc-5.02.2, and
all what I get is something stuck forever after the first

$ ./echo 3
Client wrote: Hello there sailor
Server recv: Hello there sailor
Server read: Hello there sailor
Server wrote: Hello there sailor

After adding all these print statement, I still don't have a clue
what's jammed there. Hope someone here can shred some light.

BTW, I'd also like to take this chance to ask how to debug a haskell
program in general?


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-- $Id: echo.ghc,v 1.2 2001/05/01 20:19:52 doug Exp $
-- Haskell echo/client server
-- written by Brian Gregor
-- compile with:
-- ghc -O -o echo -package net -package concurrent -package lang echo.hs
module Main where

import SocketPrim
import Concurrent
import System (getArgs,exitFailure)
import Exception(finally)
import MVar

server_sock :: IO (Socket)
server_sock = do
    s <- socket AF_INET Stream 6
    setSocketOption s ReuseAddr 1
    -- bindSocket s (SockAddrInet (mkPortNumber portnum) iNADDR_ANY)
    bindSocket s (SockAddrInet (PortNum portnum) iNADDR_ANY)
    listen s 2
    return s

echo_server s = do
    (s', clientAddr) <- accept s
    proc <- read_data s' 0
    putStrLn ("server processed "++(show proc)++" bytes")
    sClose s'
        read_data sock totalbytes = do
            -- (str,i) <- readSocket sock 19
            str <- recv sock 19
            -- if (i >= 19) 
            putStr ("Server recv: " ++ str)
            if ((length str) >= 19) 
                then do
                    putStr ("Server read: " ++ str)
                    -- writ <- writeSocket sock str
                    writ <- send sock str
                    putStr ("Server wrote: " ++ str)
                    read_data sock $! (totalbytes+(length $! str))
                    -- read_data sock (totalbytes+(length str))
                else do
                    putStr ("server read: " ++ str)
                    return totalbytes

local       = ""        
message     = "Hello there sailor\n"
portnum     = 7001

client_sock = do
    s <- socket AF_INET Stream 6
    ia <- inet_addr local
    -- connect s (SockAddrInet (mkPortNumber portnum) ia)
    connect s (SockAddrInet (PortNum portnum) ia)
    return s

echo_client n = do
    s <- client_sock
    drop <- server_echo s n
    sClose s
        server_echo sock n = if n > 0
            then do 
                -- writeSocket sock message
                send sock message
                putStr ("Client wrote: " ++ message)
                -- (str,i) <- readSocket sock 19
                str <- recv sock 19
                if (str /= message)
                    then do
                        putStr ("Client read error: " ++ str ++ "\n")
                    else do
                        putStr ("Client read success")
                        server_echo sock (n-1)
            else do 
                putStr "Client read nil\n"
                return []

main = do 
    ~[n] <- getArgs
    -- server & client semaphores
    -- get the server socket
    ssock <- server_sock 
    -- fork off the server
    s <- myForkIO (echo_server ssock)
    -- fork off the client
    c <- myForkIO (echo_client (read n::Int))
    -- let 'em run until they've signaled they're done
    join s
    putStr("join s")
    join c
    putStr("join c")

-- these are used to make the main thread wait until
-- the child threads have exited
myForkIO :: IO () -> IO (MVar ())
myForkIO io = do
    mvar <- newEmptyMVar
    forkIO (io `finally` putMVar mvar ())
    return mvar

join :: MVar () -> IO ()
join mvar = readMVar mvar