How to make certain IO operations in Haskell strict?

Question

I want to write a Haskell program that outputs a prompt and accepts user input on the same line that the prompt was outputted to. In order to do this I believe I need to force evaluation of the function putStr.

echoWithPrompt :: IO ()
echoWithPrompt = do
                  putStr "input: "
                  echo <- getLine :: IO String
                  putStrLn echo


main :: IO ()
main = do
        echoWithPrompt

Running this as is will not print "input: " until after the user has inputted a line. I have tried things like

write <- putStr "input: "
echo <- write `deepseq` getLine :: IO String

or

putStr $! "input: "

to try to force the message to print but to no avail.

That is only due to flushing. It has nothing to do with laziness. — Willem Van Onsem, Oct 01 '22 at 20:11

score 2 · Accepted Answer · answered Oct 01 '22 at 20:14

That is only due to flushing. It has nothing to do with laziness. You can flush the stdout with:

import System.IO(hFlush, stdout)

echoWithPrompt :: IO ()
echoWithPrompt = do
    putStr "input: "
    hFlush stdout
    echo <- getLine
    putStrLn echo

or shorter:

import System.IO(hFlush, stdout)

echoWithPrompt :: IO ()
echoWithPrompt = putStr "input: " >> hFlush stdout >> getLine >>= putStrLn

How to make certain IO operations in Haskell strict?

1 Answers1