olivasj:

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: joe 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: --- joe 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: import System.IO main :: IO () main = do hSetBuffering stdout NoBuffering putStr "Please enter your name: " name <- getLine putStrLn ("Hello, " ++ name ++ ", how are you?") Which runs as: $ ./a.out Please enter your name: Don Hello, Don, how are you? Cheers, Don