{-
-- Interaction elements, i.e., widgets:
data Widget = Entry         [ConfItem] -- simple text entry
            | Button        EventHandler [ConfItem]
            | Label         [ConfItem]
            | TextEdit      [ConfItem]
            | Scale Int Int [ConfItem] -- scaling input
            ...
            | Row    [ConfCollection] [Widget]
            | Col    [ConfCollection] [Widget]
            | Matrix [ConfCollection] [[Widget]]

-- Widget references: abstract type
data WidgetRef = ...

-- Configuration of widgets:
data ConfItem = Text String
              | Background String
              | WRef WidgetRef
              | Cmd EventHandler
              | ...

data ConfCollection = CenterAlign | LeftAlign | ...

-- The type of event handlers:
type EventHandler = GuiPort -> IO () -- GuiPort: reference on window

-- Basic operations for event handlers:

-- Gets the value (a String) of a widget in a GUI:
getValue :: WidgetRef -> GuiPort -> IO String

-- Puts a string value into a widget in a GUI:
setValue :: WidgetRef -> String -> GuiPort -> IO ()

-- Updates a string value in a widget in a GUI:
updateValue :: (String -> String) -> WidgetRef -> GuiPort -> IO ()
updateValue upd wref gp = do
  val <- getValue wref gp
  setValue wref (upd val) gp

-- Terminates the GUI:
exitGUI :: GuiPort -> IO ()

-- To start a widget as a GUI:
runGUI :: String -> Widget -> IO ()

-}

import Graphics.UI -- in package gui: install by `cypm add gui`

counterWidget :: Widget
counterWidget =
  Col [] [Label [Text "A simple counter"],
          Entry [Text "0", Background "yellow", WRef entryref],
          Row [] [Button (updateValue incrText entryref) [Text "Increment"],
                  Button (setValue entryref "0")         [Text "Reset"],
                  Button exitGUI                         [Text "Stop"]]]
 where
  entryref free

  incrText s = show ((read s :: Int) + 1)

main :: IO ()
main = runGUI "A simple counter" counterWidget


main4 :: IO ()
main4 = runGUI "Counter Matrix"
               (Matrix [] [[counterWidget, counterWidget],
                           [counterWidget, counterWidget]])