Keep track of the program variables in Haskell like imperative programs

Question

I'm having a hard time finding out how to make something change every time the user interacts with my program. It's hard to explain so here's an example (Haskell + wxhaskell):

simulate :: Int -> Frame () -> IO ()
simulate qNr window = do
 fdata <- readFile "qarchive"
 case (split (listTake (split fdata '\n') qNr 0) '#') of
  (qst:a:b:c:coralt:answer:x) -> do
   -- GUI Controls Initialization (buttons,text,etc...) 
   nextButton <- button window [text := "Next Question ->", on command := set infoField [text := "Next Question"], position := pt 185 155]
   return ()

main :: IO ()
main = start gui

gui :: IO ()
gui = do
 window <- frame [text := "Question program", clientSize := sz 640 480]
 headerText <- staticText window [text := "Title Text", fontSize := 20, position := pt 5 5]
 simulate 0 window
 return ()

I want some widgets to change when the "Next Question" button is pressed. I want to change these widgets to some values I read from a file. How can I keep track of what the current question number is? I cannot actually increment questionNumber as a variable, since Haskell doesn't permit such things. I guess there's another way to do it.

Example:

Initialize GUI
Read data from file
If button is pressed, increment question number by 1 and change widgets.

How do you tackle this kind of problem in a functional manner?

Answer 1

The arguments to functions are your variables. As the user enters new values, pass those values to functions.

For example, a simple program that updates a value based on user input:

main = loop 0

loop n = do
    print n
    v <- read `fmap` getLine
    loop (n + v)

Note the recursive calls to 'loop' have a different value passed each time, based on what the user provided.

This is a fundamental way of thinking in functional programming -- what would be a local, mutable variable in a loop in an imperative program becomes a parameter to a recursive function in a functional program.