When I want to put some text before reading an input in Haskell, I tried writing it like this:
putStr "enter value: "
var <- getLine
However the output requires the user input before it displays the text:
[input]
enter value:
When I use putStrLn instead of putStr, it displays as it should:
enter value:
[input]
Why do these two statements function differently? Is it the addition of the newline?
putStr "enter value: " actually writes to an output buffer, which is flushed to the actual standard output only later on, when the buffer becomes full or when a newline is found.
This is roughly the same mechanism found in the C programming language.
So, even if putStr "enter value: " is run before getLine, we don't see the output message, yet, which feels wrong.
The solution is to flush the standard output handle explicitly.
import System.IO
-- ...
putStr "enter value: "
hFlush stdout
var <- getLine
