algebra.scala

object Program {
  def main(args: Array[String]): Unit = {
    val pgm1 = FrontEnd.clickMe
    val pgm2 = FrontEnd.clickMe2

    val init = AppState(-4)

    val s1 = App.eval(pgm1, init)
    println(s1)

    val s2 = App.eval(pgm2, init)
    println(s2)
  }
}

sealed trait AppAction
case object Noop extends AppAction
case class AndThen(first: AppAction, second: AppAction) extends AppAction
case object Increment extends AppAction
case class Add(n: Int) extends AppAction

case class AppState(count: Int)

object App {
  def eval(action: AppAction, env: AppState): AppState = action match {
    case Noop            => env
    case AndThen(a1, a2) =>
      val stateAfterA1 = eval(a1, env)
      val stateAfterA2 = eval(a2, stateAfterA1)
      stateAfterA2
    case Increment      => env.copy(count = env.count + 1)
    case Add(n)         => env.copy(count = env.count + n)
  }

  def buildProgram(xs: AppAction*): AppAction = xs.foldLeft[AppAction](Noop) { (b,a) => AndThen(b, a) }
}

object FrontEnd {
  def clickMe: AppAction = App.buildProgram(
    Increment,
    Increment,
    Increment
  )

  def clickMe2: AppAction = App.buildProgram(
    Add(3)
  )
}