fp in scala EXERCISE 6.11 CandyMachineの実装

CandyMachine.scala

case class State[S, +A](run: S => (A, S)) {
  def map[B](f: A => B): State[S, B] = State { s0 =>
    val (a, s1) = run(s0)
    (f(a), s1)
  }

  def map2[B, C](that: State[S, B])(f: (A, B) => C): State[S, C] = State { s0 =>
    val (a, s1) = this.run(s0)
    val (b, s2) = that.run(s1)
    (f(a, b), s2)
  }

  def flatMap[B](f: A => State[S, B]): State[S, B] = State { s0 =>
    val (a, s1) = run(s0)
    f(a).run(s1)
  }
}

object State {
  def unit[S, A](a: A) = State[S, A](s => (a, s))
  
  def sequence[S, A](aList: List[State[S, A]]): State[S, List[A]] = aList match {
    case x :: xs => x.map2(sequence(xs))(_ :: _)
    case _ => unit[S, List[A]](Nil)
  }
}

sealed trait Input
case object Coin extends Input
case object Turn extends Input
case class Machine(locked: Boolean, candies: Int, coins: Int)

object CandyMachine {
  def coin: State[Machine, Unit] = State {
    case m@Machine(_, 0, _) => ((), m)
    case m@Machine(false, _, _) => ((), m)
    case Machine(true, n, c) => ((), Machine(false, n, c+1))
  }

  def turn: State[Machine, Unit] = State {
    case m@Machine(_, 0, _) => ((), m)
    case m@Machine(true, _, _) => ((), m)
    case Machine(false, n, c) => ((), Machine(true, n-1, c))
  }

  def input(i: Input) = if(i == Coin) coin else turn
  def simulateMachine(inputs: List[Input]): State[Machine, (Int, Int)] = State{ m0 => 
    val (_, m) = State.sequence(inputs.map(input)).run(m0)
    ((m.coins, m.candies), m)
  }
}

val tInitMachine = Machine(true, 5, 10)
val tInputs = List.fill(4)(List(Coin, Turn)).flatten
CandyMachine.simulateMachine(tInputs).run(tInitMachine)._1.ensuring(_ == (14, 1))