Haskell console program outputs in wrong order

Question

So I just made a program that asks for a number n and displays n th term of the Fibonacci sequence:

import Control.Monad (forever)

main = forever $ do
    putStrLn ""
    putStr "Which Fibonacci sequence number do you need? "
    number <- readLn :: IO Int
    putStrLn $ "The Fibonacci number you wanted is " ++ (show $ fib number) ++ "."
    putStrLn ""
    where  fib :: Int -> Integer
           fib number = fibs !! number
           fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

When I run the program in GHCi or through runhaskell, it executes fine; that is, asks me for a number, allows me to enter it on the same line and returns a number on another line:

Gallifreian@Gallifrey:~/Haskell$ ghci
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> :l Fib.hs
[1 of 1] Compiling Main             ( Fib.hs, interpreted )
Ok, modules loaded: Main.
*Main> main

Which Fibonacci sequence number do you need? 4
The Fibonacci number you wanted is 3.

However, when I run the compiled program, it returns this:

Gallifreian@Gallifrey:~/Haskell$ ./Fib

4
Which Fibonacci sequence number do you need? The Fibonacci number you wanted is 3.

I. e. waits for me to enter a number and then returns all prompts on one line. What have I done wrong? Is there a way around this?

Found answer, see http://stackoverflow.com/questions/2500459/wrong-io-actions-order-using-putstr-and-getline (I don't have any flags left to mark this as duplicate) — Markus Laire, Aug 13 '16 at 11:16
Related: https://stackoverflow.com/questions/13190314/haskell-do-monad-io-happens-out-of-order — Gallifreyan, Feb 19 '17 at 18:15

score 1 · Accepted Answer · answered Aug 13 '16 at 11:18

It looks like stdout has line buffering on, which means that the calls to putStr are only storing the output in a buffer, which is not being output until putStrLn is called. You can fix your output by using putStrLn for your prompt, or disabling buffering on stdout.

Haskell console program outputs in wrong order

1 Answers1