by io monad

RegisterAFP2_IO.sc

// program 中のregisterの持ち回りを避ける

import cats.data.StateT
import cats.effect.IO

case class CashRegister(total: Int) {
  def addCash(toAdd: Int) = CashRegister(total + toAdd)
}
type Purchase = CashRegister => IO[CashRegister]
def makePurchase(amount: Int): Purchase = (r: CashRegister) => for {
  _ <- IO { println ("Purchase in amount: " + amount) }
} yield r addCash amount

type PurchaseHistory = Vector[Purchase]
def execute(p: Purchase): StateT[IO, (PurchaseHistory, CashRegister), Unit] =
  StateT { case (h, r) => p(r) map { r2 => ((h :+ p, r2), ()) }}

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

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

val io1 = program.runS((Vector.empty, CashRegister(0)))
val (history, last) = io1.unsafeRunSync()

val io2 = history.foldLeft(IO.pure(CashRegister(10)))(_ flatMap _)
io2.unsafeRunSync()