-- A simple desk calculator showing the combination of GUIs with
-- local state where the state is stored in an object.
-- Therefore, the GUI runs as a constraint.

import Tk
import Ports
import Char

data CalcMsg = Button Char | Display String

-- state contains value of current operand and an accumulator function:
calcMgr :: (Int,Int->Int) -> [CalcMsg] -> Success
calcMgr (d,f) (Display s : ms) = s=:=(show d) &> calcMgr (d,f) ms
calcMgr (d,f) (Button b  : ms)
 | isDigit b = calcMgr (10*d + ord b - ord '0', f) ms
 | b=='+' = calcMgr (0,((f d) + )) ms
 | b=='-' = calcMgr (0,((f d) - )) ms
 | b=='*' = calcMgr (0,((f d) * )) ms
 | b=='/' = calcMgr (0,((f d) `div` )) ms
 | b=='=' = calcMgr (f d, id) ms
 | b=='C' = calcMgr (0, id) ms


-- the GUI needs a reference to the calculator object: parameter "calculator"
calc_GUI calculator =
  TkCol [] [
    TkEntry [TkRef display, TkText "0", TkBackground "yellow"],
    TkRow [] (map cbutton ['1','2','3','+']),
    TkRow [] (map cbutton ['4','5','6','-']),
    TkRow [] (map cbutton ['7','8','9','*']),
    TkRow [] (map cbutton ['C','0','=','/'])]
  where display free

        cbutton c = TkButton (button_pressed c) [TkText [c]]

        button_pressed c gp = let d free in
                               send (Button c) calculator &>
                               send (Display d) calculator &>
                               tkCSetValue display d gp

runCalcOnPort gp
 | let cm free in
   newObject calcMgr (0,id) cm & runWidgetOnPort (calc_GUI cm) gp
 = done

main = openWish "Calculator" >>= runCalcOnPort