Why isn't my IO executed in order?

Question

Hi, I'm having some fun learning about the haskell IO. However in my recent exploration of it I have encountered some problems with IO not executing in order, even inside a do construct. In the following code I am just keeping track of what cards are left, where the card is a tuple of chars (one for suit and one for value) then the user is continously asked for wich cards have been played. I want the putStr to be executed between each input, and not at the very end like it is now.

module Main where
main = doLoop cards
doLoop xs = do  putStr $ show xs
                s <- getChar
                n <- getChar
                doLoop $ remove (s,n) xs
suits = "SCDH"
vals = "A23456789JQK"
cards = [(s,n) | s <- suits, n <- vals]
type Card = (Char,Char)
remove :: Card -> [Card] -> [Card]
remove card xs = filter (/= card) xs

Answer 1

If the problem is what I think it is, your problem is that Haskell's IO is buffered: this question explains what's happening. When you run a compiled Haskell program, GHC stores output in the buffer and only periodically flushes it to the screen; it does so if (a) the buffer is too full, (b) if a newline is printed, or (c) if you call hFlush stdout.

The other problem you may be seeing is that getChar may not fire until a newline is read, but then the newline is in your input stream; you could perhaps solve this with an extra getChar to swallow the newline, but there should probably be a better way.

Answer 2

absz's answer is correct, Haskell's buffered IO is what's causing you trouble. Here's one way to rewrite your doLoop to have the effect you're looking for:

doLoop xs = do  putStrLn $ show xs
                input <- getLine
                let s:n:_ = input
                doLoop $ remove (s,n) xs

The two changes: use putStrLn to append a newline and flush the output (which is probably what you want), and use getLine to grab the input a line at a time (again, probably what you want).

Answer 3

Buffering, in the form of putStr, is your problem, as others have pointed out.

Also, a style point: putStrLn $ show xs is the same as print xs