by state monad

RegisterAFP2.sc

import cats.data.State

case class CashRegister(total: Int) {
  def addCash(toAdd: Int) = CashRegister(total + toAdd)
}
type Purchase = CashRegister => CashRegister
def makePurchase(amount: Int): Purchase = (r: CashRegister) => {
  println("Purchase in amount: " + amount)
  r addCash amount
}
type PurchaseHistory = Vector[Purchase]
def execute(p: Purchase): State[(PurchaseHistory, CashRegister), Unit] =
  State { case (h, r) => ((h :+ p, p(r)), ()) }

// test -------------------------------------------------------------------
val p1 = makePurchase(100)
val p2 = makePurchase(50)

def program = for {
  _ <- execute(p1)
  _ <- execute(p2)
} yield ()

val (history, total1) = program.runS((Vector.empty, CashRegister(0))).value

// 再実行
history.foldLeft(CashRegister(10))((acc, p) => p(acc))