[Haskell-cafe] Very Basic IO question
Donald Bruce Stewart
dons at cse.unsw.edu.au
Sat Mar 3 02:09:12 EST 2007
> I am VERY new to Haskell, and just getting my feet wet with functional
> programming in general. I've been going over a few examples online, but I
> can't figure out the behavior I'm seeing on a very basic example:
> module Main where
> import System.IO
> main :: IO ()
> main = do
> putStrLn "Please enter your name: "
> name <- getLine
> putStrLn ("Hello, " ++ name ++ ", how are you?")
> This example works as I would expect. The prompt displays with a new
> line, the input is entered and echoed:
> Please enter your name:
> Hello, joe, how are you?
> However, changing 'putStrLn' to 'putStr' does not do what I would expect.
> The prompt doesn't get displayed until after there is input:
> Please enter your name: Hello, joe, how are you?
> Why does it exhibit this behavior? I'm using a ghc 6.6 built from source
> from 20070227. The regular 6.6 release had the same behavior. Any help
> would be greatly appreciated.
In short: buffering.
You're not flushing the output buffer till \n appears.
To ask for no buffering, you would write:
main :: IO ()
main = do
hSetBuffering stdout NoBuffering
putStr "Please enter your name: "
name <- getLine
putStrLn ("Hello, " ++ name ++ ", how are you?")
Which runs as:
Please enter your name: Don
Hello, Don, how are you?
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