by writer monad

RegisterAFP.sc

import cats.data.Writer
import cats.instances.vector._
import cats.syntax.writer._

case class CashRegister(total: Int) {
  def addCash(toAdd: Int) = CashRegister(total + toAdd)
}
type Purchase = CashRegister => CashRegister
def makePurchase(amount: Int): Purchase = (r: CashRegister) => {
  println(s"Purchase in amount: $amount")
  r addCash amount
}
type PurchaseHistory = Vector[Purchase]
def execute(purchase: Purchase)
    : CashRegister => Writer[PurchaseHistory, CashRegister] =
  register => purchase(register).writer(Vector(purchase))

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

def program(r0: CashRegister) = for {
  r1 <- execute(p1)(r0)
  r2 <- execute(p2)(r1)
} yield r2

val (history, total1) = program(CashRegister(0)).run

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