Re: [Haskell-beginners] Question on Lazy IO

Thank you, Michael, for kind explanation. My first approach was calling the following sentinel function before proceeding: isValidReq req = (type req) /= ERROR && (length $ hdrs req) < 10000 The hdrs of (web) request are constructed lazily, so I should do length call to wait until full request are read. With HTTP processing, I cannot figure out how to do with strict I/O. Maybe complex parsing (parsec?) is required, I think. So I changed the sentinel function using deepseq. isVaidReq req = (hdrs req) `deepseq` (type req) /= ERROR Regards, Chul-Woong 2014-07-24 16:54 GMT+09:00 Michael Snoyman :

My biggest recommendation would be not to use lazy I/O. But excluding that...

You just need to make sure that as much of the input is actually received before sending a response. You do that by forcing evaluation of the thunk. With lazy I/O, forcing evaluation ends up causing I/O to be performed, which is what you want in this case. As an example, to make sure that at least one character is received before continuing, you could use this function:

getSomeContents = do x <- getContents case x of [] -> return x _:_ -> return x

Another possibility is to use *some* strict I/O. For example, you can get the first character strictly and the rest lazily:

getSomeContents = do c <- getChar x <- getContents return $ c : x

Something else you should think about is what data representation you want to use. Everything we've done here using String, which is (1) inefficient, since it's an unpacked representation, and (2) incorrect, since HTTP data is really a series of bytes, not a series of unicode code points. But I'm going on the assumption that this exercise is more to understand I/O evaluation.

On Thu, Jul 24, 2014 at 9:07 AM, 양철웅 wrote:

...

Hi, all.

Haskell's lazy IO causes below program to write back "ECHO result" to the user even if he doesn't give any input to the program.

test1 :: IO () test1 = do l <- fmap lines getContents putStrLn "ECHO result" putStr $ unlines l

If the client logic, that is the part for feeding data to this program and wait for the response, expect the server logic to respond only after all the client's request data are feed to the server logic, the behaviour could be problematic.

I'm now writing simple web server, as you might guess, and my web server responds "HTTP/1.1" before any client's request are given. I'm using hGetContents and web server's logic is basically same with above code skeleton.

How can I tell the program to respond data only after all the requests are feed? Any directions are welcomed. Thanks.

Chul-Woong

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