main = do
 void initGUI -- allocates some resources and prepares GTK+ for work, also parses command line arguments
 window <- windowNew -- :: Window
 set window [ windowTitle := "Calculator", windowResizable := False, windowDefaultWidth := 230, windowDefaultHeight := 250 ]

 display <- entryNew -- Entry - a single line text entry widget
 set display [ entryEditable := False, entryXalign := 1 , entryText := "0" ]

 grid <- gridNew
 gridSetRowHomogeneous grid True
 let attach x y w h item = gridAttach grid item x y w h
 attach 0 0 5 1 display
  mkBtn "MC" display >>= attach 0 1 1 1
  mkBtn "MR" display >>= attach 1 1 1 1
  mkBtn "MS" display >>= attach 2 1 1 1
  mkBtn "M+" display >>= attach 3 1 1 1
  mkBtn "M-" display >>= attach 4 1 1 1
  mkBtn "âĘŘ" display >>= attach 0 2 1 1
  mkBtn "CE" display >>= attach 1 2 1 1
  mkBtn "C" display >>= attach 2 2 1 1
  mkBtn "Âś" display >>= attach 3 2 1 1
  mkBtn "âĹŽ" display >>= attach 4 2 1 1
  mkBtn "7" display >>= attach 0 3 1 1
  mkBtn "8" display >>= attach 1 3 1 1
  mkBtn "9" display >>= attach 2 3 1 1
  mkBtn "Ãů" display >>= attach 3 3 1 1
  mkBtn "%" display >>= attach 4 3 1 1
  mkBtn "4" display >>= attach 0 4 1 1
  mkBtn "5" display >>= attach 1 4 1 1
  mkBtn "6" display >>= attach 2 4 1 1
  mkBtn "*" display >>= attach 3 4 1 1
  mkBtn "1/x" display >>= attach 4 4 1 1
  mkBtn "1" display >>= attach 0 5 1 1
  mkBtn "2" display >>= attach 1 5 1 1
  mkBtn "3" display >>= attach 2 5 1 1
  mkBtn "âĂŞ" display >>= attach 3 5 1 1
  mkBtn "=" display >>= attach 4 5 1 2
  mkBtn "0" display >>= attach 0 6 2 1
  mkBtn "." display >>= attach 2 6 1 1
  mkBtn "+" display >>= attach 3 6 1 1
  containerAdd window grid

 widgetShowAll window
 mainGUI -- main loop

 btnOnActivated :: Entry -> String -> IO ()
  btnOnActivated display label = do
  text <- get display entryText
  case label of
  "âĹŽ" -> set display [ entryText := show (sqrt (read text)) ]
  otherwise -> set display [ entryText := text++label ]

 mkBtn :: String -> Entry -> IO Button
  mkBtn label display= do
  btn <- buttonNew
  set btn [ buttonLabel := label ]
  on btn buttonActivated (btnOnActivated display label)
  return btn