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 <cwyang@aranetworks.com> 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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