-- A simple program which pops up a window saying 'Hello World' with a
-- button saying 'Bye' which you click and it changes the message to
-- 'Goodbye'. if you click the button again the program exits.

-- This is the gtk2hs solution.

import Mogul
import Control.Concurrent.MVar

main :: IO ()
main = do
  initGUI
  -- create a top-level window
  win <- newWindow

  -- create a VBox to hold different sized widgets and but it into the window
  vbox <- newVBox False 5
  win `containerAdd` vbox
  
  -- create the two UI elements
  but <- newButtonWithLabel "Bye"
  lab <- newLabel $ Just "Hello World"

  -- add them to the VBox such that the button stays the same size and
  -- that the label spreads out over the available area when resized
  boxPackStart vbox lab PackRepel 0
  boxPackEnd vbox but PackNatural 0

  -- add a handler for the button clicked signal
  stop <- newMVar False
  but `onClicked` do
    state <- readMVar stop
    if state then mainQuit else do
      lab `labelSetText` "Goodbye"
      but `buttonSetLabel` "Exit"
      swapMVar stop True
      return ()

  -- for completeness: quit the application when the window ceases
  win `onDestroy` mainQuit

  -- show and run the demo
  widgetShowAll win
  mainGUI