CODEWITHSUNDEEP
haskellfunctional-programming
ShocKwav3_
ShocKwav3_

Reputation: 1760

Messages printed at the wrong moment with putStr

import Control.Monad
import Data.Char

main = forever $ do
    putStr "Give me some input: "
    l <- getLine
    putStrLn $ map toUpper l

I have been learning Haskell from learn you haskell. When i try to run this code it does not behave like it should.

l
Give me some input: L
abc
Give me some input: ABC

When i run this it doesn't print the string first and i can input. After providing some input, in this case l it returns me like this : Give me some input: L.

What it should do is, ask for input by printing Give me some input: and after putting the input it shall return the input in uppercase.

How can i fix this?

Upvotes: 1

Views: 166

Answers (2)

duplode
duplode

Reputation: 34378

As Michail points out, it is a buffering issue, which can be solved with hSetBuffering for stdout:

import Control.Monad
import Data.Char
import System.IO

main = do
    hSetBuffering stdout NoBuffering
    forever $ do
    putStr "Give me some input: "
    l <- getLine
    putStrLn $ map toUpper l

If for whatever reason you don't want to change the buffer mode for the whole program, you can use hFlush to do an extra flushing of the buffer exactly where you need it:

import Control.Monad
import Data.Char
import System.IO

main = do
    forever $ do
    putStr "Give me some input: "
    hFlush stdout
    l <- getLine
    putStrLn $ map toUpper l

Upvotes: 4

Reactormonk
Reactormonk

Reputation: 21690

Try this:

import Control.Monad
import Data.Char

main = forever $ do
    putStrLn "Give me some input: "
    l <- getLine
    putStrLn $ map toUpper l

The issue here is that putStr doesn't write a newline. And the output buffer only gets flushed on a newline.

Upvotes: 1

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