RecursiveUpdates.hs

{-# LANGUAGE FlexibleContexts, FlexibleInstances
  , MultiParamTypeClasses, TemplateHaskell, UndecidableInstances #-}

{-

This example is showing how to handle recursive updates.

We do this by making two exchange rate calculators. The first is
recursive, the second is not. See RecursiveObserver monad (included
with AutoForms) for explanation of recursive udpates in this context.

-}

module RecursiveUpdates where

import Graphics.UI.AF.WxFormAll

euroToDollarRate :: Float
euroToDollarRate = 1.4414

main = startWx "Recursive Updates" $ 
       do addCom $ exchangeRateCalculater "Recursive" setValue
          addCom $ exchangeRateCalculater "Non-recursive" nonRecursiveSetValue

exchangeRateCalculater :: String -> (ComH Float -> Float -> WxAct ()) -> EC Float
exchangeRateCalculater label' updateFun = label label' $ builderToCom $
       do euros   <- addCom $ label "Euros" $ mkCom 100
          dollars <- addCom $ label "Dollars" $ mkCom (100 * euroToDollarRate)
          addListener (do euros' <- getValue euros
                          updateFun dollars (euros' * euroToDollarRate)
                      ) euros
          addListener (do dollars' <- getValue dollars
                          updateFun euros (dollars' / euroToDollarRate)
                      ) dollars
          return euros