Thanks Jonas,<div><br></div><div>I feel much better already:</div><div><br></div><blockquote class="webkit-indent-blockquote" style="margin: 0 0 0 40px; border: none; padding: 0px;"><div><div><font class="Apple-style-span" face="'courier new', monospace">import Control.Concurrent</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace">import Control.Exception</font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace">import Control.Monad</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace">import Network</font></div></div><div><div><span class="Apple-style-span" style="font-family: 'courier new', monospace; ">import System.IO</span></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace">import System.IO.Error (isEOFError)</font></div></div></blockquote><div><div><br></div></div><blockquote class="webkit-indent-blockquote" style="margin: 0 0 0 40px; border: none; padding: 0px;">
<div><div><font class="Apple-style-span" face="'courier new', monospace">main = withSocketsDo $ do</font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace"> sListen <- listenOn (PortNumber 8000)</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace"> putStrLn "Listening on Port 8000"</font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace"> forkIO $ forever $ do</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace"> (sSession, hostname, port) <- accept sListen</font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace"> putStrLn ("Connected to " ++ hostname ++ ":" ++ show port)</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace"> forkIO $ echoLines sSession</font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace"> putStrLn "Press <CTRL-D> to quit."</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace"> exitOn</font><span class="Apple-style-span" style="font-family: 'courier new', monospace; ">Eof</span></div></div><div><div>
<font class="Apple-style-span" face="'courier new', monospace"><br></font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace">echoLines h = try (hGetLine h) >>= either</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace"> (\e -> if isEOFError e then print e else ioError e)</font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace"> (putStrLn >=> const (echoLines h))</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace"><br></font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace">exitOnEof = try getLine >>= either</font></div>
</div><div><div><font class="Apple-style-span" face="'courier new', monospace"> (\e -> unless (isEOFError e) $ ioError e)</font></div></div><div><div><font class="Apple-style-span" face="'courier new', monospace"> (const exitOnEof)</font></div>
</div></blockquote><div><div><br></div><div>I also worked out I didn't <font class="Apple-style-span" face="'courier new', monospace">void </font>by making <font class="Apple-style-span" face="'courier new', monospace">processLines </font>(now <font class="Apple-style-span" face="'courier new', monospace">echoLines h</font>) be <font class="Apple-style-span" face="'courier new', monospace">forkIO</font>'s argument rather than <font class="Apple-style-span" face="'courier new', monospace">forkIO</font>'s result.</div>
<div><br></div><div>Cheers,</div><div><br></div><div>-John</div><br><div class="gmail_quote">2011/6/28 Jonas Almström Duregård <span dir="ltr"><<a href="mailto:jonas.duregard@chalmers.se">jonas.duregard@chalmers.se</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">There is the <span style="font-family:courier new,monospace">void</span> function in Control.Monad:<br><a href="http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Monad.html#v:void" target="_blank">http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Monad.html#v:void</a><br>
<br>Instead of using <span style="font-family:courier new,monospace">return ()</span> you can just use <span style="font-family:courier new,monospace">void processLine</span>.<br><br>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:<br>
<br> <span style="font-family:courier new,monospace"> let processLine = void $ forkIO $</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> try (hGetLine sSession) >>= either</span><br style="font-family:courier new,monospace">
<span style="font-family:courier new,monospace"> (\e -> if isEOFError e <br> then print e <br> else ioError e)</span><br style="font-family:courier new,monospace"><span style="font-family:courier new,monospace"> (putStrLn >=> const processLine)</span><div>
<div></div><div class="h5"><br>
<br>On 28 June 2011 12:58, John Ky <<a href="mailto:newhoggy@gmail.com" target="_blank">newhoggy@gmail.com</a>> wrote:<br>> Hi Eric, Ivan,<br>> On 28 June 2011 18:32, Erik de Castro Lopo <<a href="mailto:mle%2Bhs@mega-nerd.com" target="_blank">mle+hs@mega-nerd.com</a>> wrote:<br>
>><br>>> The hlint program would have flagged both of those and possibly<br>>> others. See:<br>><br>> Cool!<br>> It didn't flag either for me, but it recommended replacing ++ (show port)<br>
> with ++ show port, if then else with unless, putStrLn (show x) with print x,<br>> and do stuff with stuff.<br>> All useful to know.<br>> On 28 June 2011 18:16, Ivan Lazar<br>> Miljenovic <<a href="mailto:ivan.miljenovic@gmail.com" target="_blank">ivan.miljenovic@gmail.com</a>> wrote:<br>
>><br>>> I don't think you need all those return () everywhere...<br>><br>> <br>> You're right. At some point I added it in to (try to) make the compiler<br>> happy, but it must have been or become unnecessary.<br>
> I still need two though because forkIO (and therefore my processLine<br>> function) returns IO ThreadId, but the last line for do notation must be<br>> return () (see below).<br>> On 28 June 2011 18:16, Ivan Lazar<br>
> Miljenovic <<a href="mailto:ivan.miljenovic@gmail.com" target="_blank">ivan.miljenovic@gmail.com</a>> wrote:<br>>><br>>> And at the end, why do you do "line <- getLine" when you don't use the<br>
>> result?<br>
><br>> Oh that. I was trying to figure out a way to terminate by program. I've<br>> now changed it to exit on EOF.<br>> Here is my second attempt. Is it much better?:<br>><br>> import Control.Concurrent<br>
> import Control.Exception<br>> import Control.Monad<br>> import Network<br>> import System.IO<br>> import System.IO.Error (isEOFError)<br>> main = withSocketsDo $ do<br>> sListen <- listenOn (PortNumber 8000)<br>
> putStrLn "Listening on Port 8000"<br>> forkIO $ forever $ do<br>> (sSession, hostname, port) <- accept sListen<br>> putStrLn ("Connected to " ++ hostname ++ ":" ++ show port)<br>
> let processLine = forkIO $ do<br>> lineResult <- try (hGetLine sSession)<br>> case lineResult of<br>> Right line -> do<br>> putStrLn line<br>> processLine<br>
> return ()<br>> Left e -> if isEOFError e<br>> then print e<br>> else ioError e<br>> processLine<br>> return()<br>> putStrLn "Press <CTRL-D> to quit."<br>
> let processStdIn = do<br>> lineResult <- try getLine<br>> case lineResult of<br>> Right line -> processStdIn<br>> Left e -> unless (isEOFError e) $ ioError e<br>> processStdIn<br>
><br>> Thanks for the suggestions.<br>> Cheers,<br>> -John<br>><br></div></div><div><div></div><div class="h5">> _______________________________________________<br>> Haskell-Cafe mailing list<br>> <a href="mailto:Haskell-Cafe@haskell.org" target="_blank">Haskell-Cafe@haskell.org</a><br>
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</div></div></blockquote></div><br></div>