Wrong IO actions order using putStr and getLine

0 votes
asked Mar 23, 2010 by qwrp

I have the following code:

main = do
    putStr "Test input : "
    content <- getLine
    putStrLn content

When I run it (with runhaskell) or compile it (ghc 6.10.4) the result is like this:

asd
Test input : asd

Why is Test input : asd being printed after asd?

In the code sample on http://learnyouahaskell.com/, which uses putStr, the getLine's presented output is different than mine. When I use putStrLn the program works as expected (print, then prompt, and print).

Is it a bug in ghc, or it is the way that it should work?

1 Answer

0 votes
answered Mar 23, 2010 by josh-lee

This is because ghci disables buffering, while a program compiled with ghc has line buffering by default. You can see this by running this:

import IO
main = print =<< hGetBuffering stdout

In ghci you see NoBuffering while with runghc you get LineBuffering. Since the newline character doesn't print until after the user input, the prompt doesn't either.

Fix it by adding hFlush stdout after your prompt (or disable buffering with hSetBuffering stdout NoBuffering, but that’s probably bad).

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