buttonCb :: Ref Button -> IO ()
buttonCb b' = do
  l' <- getLabel b'
  if (l' == "Hello world")
    then setLabel b' "Goodbye world"
    else setLabel b' "Hello world"
buttonCb :: Ref Button -> IO ()
buttonCb b' = do
  l' <- getLabel b'
  if (l' == "Hello world")
    then setLabel b' "Goodbye world"
    else setLabel b' "Hello world"
buttonCb b' = do
  l' <- getLabel b'
  if (l' == "Hello world")
    then setLabel b' "Goodbye world"
    else setLabel b' "Hello world"