[Haskell-beginners] Question on Lazy IO

[Santtu Keskinen]

Hi Chul-Woong,

Deepseq package can be used to make sure a list is actually fully
constructed before it's used.

import System.IO
import Control.DeepSeq
main :: IO ()
main = do
    s <- getContents
    s `deepseq` hClose stdin
    putStrLn "ECHO result"
    putStr s

Cheers


2014-07-24 9:07 GMT+03:00 양철웅 <cwyang at aranetworks.com>:

> 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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