a.hs

import           Control.Monad

main = do topic <- getLine
          payload <- getLine
          let msg = Message (Topic topic) payload
          mapM_ (runAction topic payload) rules
      where
          runAction topic payload (Action (Topic t) cb)
            = when (t == topic) $ cb payload

rules :: [Action]
rules = [
         controlButton1
        ]

data Topic = Topic String
type Payload = String
data Message = Message Topic Payload
data Button = Button Topic

data OnOff = On | Off
data Action = Action Topic (Payload -> IO ())

-- FIXME: We actually need to attach this to mqtt
publish (Topic topic) msg = putStrLn (topic ++ ":" ++ msg)

stateChanged (Button bt) cb = Action bt listener
  where listener payload = case  payload of
                    "on"  -> cb On
                    "off" -> cb Off
button1 = Button (Topic "button1")

-- Definition of light class
data Light = Light Topic

setLight :: Light -> OnOff -> IO ()

setLight (Light topic) On  =  publish topic "ON"
setLight (Light topic) Off = publish topic "OFF"

light1 = Light (Topic "light1")
light2 = Light (Topic "light2")
controlButton1 = button1 `stateChanged` toggleLight
  where toggleLight onOff = mapM_ (`setLight` onOff) [light1, light2]