Demonstration of a traffic light with state in haskell

trafficLight.hs

#!/usr/bin/env stack
-- stack --resolver lts-10.7 --install-ghc runghc

import Control.Concurrent (threadDelay)
import Control.Monad.State

data TrafficLightColor = Red | Yellow | Green deriving Show

type TrafficLightState = (TrafficLightColor, Int) 

nextColorRule :: TrafficLightColor -> TrafficLightState
nextColorRule Red    = (Yellow,  3)
nextColorRule Yellow = (Green,  10)
nextColorRule Green  = (Red,    20)

nextColor :: TrafficLightState -> TrafficLightState
nextColor (c, 0) = nextColorRule c
nextColor (c, t) = (c, t - 1)

type TrafficLightStateMonad a = StateT TrafficLightState IO a

runTrafficLight :: TrafficLightStateMonad ()
runTrafficLight = modify nextColor

printTrafficLight :: TrafficLightStateMonad ()
printTrafficLight = liftIO . print =<< get


main = runStateT foreverTraffic initialState
    where 
        waitASecond    = liftIO $ threadDelay $ (10^6) 
        foreverTraffic = forever $ runTrafficLight >> printTrafficLight >> waitASecond
        initialState   = nextColorRule Green