by eff monad

RegisterAFP3.sc

import cats.data.{State, Writer}
import org.atnos.eff._
import org.atnos.eff.state._
import org.atnos.eff.writer._
import org.atnos.eff.syntax.all._

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 Stack = Fx.fx2[State[CashRegister, ?], Writer[Purchase, ?]]
type _writerVector[R]  = Writer[Purchase, ?]    |= R
type _stateRegister[R] = State[CashRegister, ?] |= R
def execute[R :_stateRegister :_writerVector](p: Purchase): Eff[R, Unit] = for {
  _ <- tell(p)
  _ <- modify(p(_))
} yield ()

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

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

val ((_, last), history) = program.runState(CashRegister(0)).runWriter.run

// 再実行
val total2 = history.foldLeft(CashRegister(10))((acc, p) => p(acc))