[Haskell-cafe] Cleaner way to write code and handle errors?

John Ky newhoggy at gmail.com
Tue Jun 28 12:58:59 CEST 2011

Hi Eric, Ivan,

On 28 June 2011 18:32, Erik de Castro Lopo <mle+hs at mega-nerd.com> wrote:

> The hlint program would have flagged both of those and possibly
> others. See:


It didn't flag either for me, but it recommended replacing ++ (show
port)with ++
show port, if then else with unless, putStrLn (show x) with print x, and do
stuff with stuff.

All useful to know.

On 28 June 2011 18:16, Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>

> I don't think you need all those return () everywhere...

You're right.  At some point I added it in to (try to) make the compiler
happy, but it must have been or become unnecessary.

I still need two though because forkIO (and therefore my processLine function)
returns IO ThreadId, but the last line for do notation must be return
()(see below).

On 28 June 2011 18:16, Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>

> And at the end, why do you do "line <- getLine" when you don't use the
> result?

Oh that.  I was trying to figure out a way to terminate by program.  I've
now changed it to exit on EOF.

Here is my second attempt.  Is it much better?:

import Control.Concurrent
import Control.Exception
import Control.Monad
import Network
import System.IO
import System.IO.Error (isEOFError)

main = withSocketsDo $ do
  sListen <- listenOn (PortNumber 8000)
  putStrLn "Listening on Port 8000"
  forkIO $ forever $ do
    (sSession, hostname, port) <- accept sListen
    putStrLn ("Connected to " ++ hostname ++ ":" ++ show port)
    let processLine = forkIO $ do
        lineResult <- try (hGetLine sSession)
        case lineResult of
          Right line -> do
            putStrLn line
            return ()
          Left e -> if isEOFError e
            then print e
            else ioError e
  putStrLn "Press <CTRL-D> to quit."
  let processStdIn = do
      lineResult <- try getLine
      case lineResult of
        Right line -> processStdIn
        Left e -> unless (isEOFError e) $ ioError e

Thanks for the suggestions.


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