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

28 Jun 2011

      There is the void function in Control.Monad:
http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Mon...

Instead of using return () you can just use void processLine.

Also some people like to use the either function instead of matching on
Left/Right. In this case you can also avoid introducing a few names:

   let processLine = void $ forkIO $
         try (hGetLine sSession) >>= either
           (\e -> if isEOFError e
             then print e
             else ioError e)
           (putStrLn >=> const processLine)

On 28 June 2011 12:58, John Ky  wrote:
...
Hi Eric, Ivan,
On 28 June 2011 18:32, Erik de Castro Lopo  wrote:
...
The hlint program would have flagged both of those and possibly
others. See:
Cool!
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  wrote:
...
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  wrote:
...
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
            processLine
            return ()
          Left e -> if isEOFError e
            then print e
            else ioError e
    processLine
    return()
  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
  processStdIn
Thanks for the suggestions.
Cheers,
-John
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Re: [Haskell-cafe] Cleaner way to write code and handle errors?

Jonas Almström Duregård