Example4.scala

import zio._
import zio.clock.Clock
import zio.duration._

class Example4(emailService: EmailService) {
  // Schedule an email to each address, but send at most 10 in parallel
  def welcomeUsers(emails: List[String]): RIO[Clock, Unit] =
    ZIO.foreachParN_(10)(emails)(sendWelcomeEmail)

  // Try to send a welcome email at most 5 times,
  // sleeping for 1 second between attempts
  def sendWelcomeEmail(recipient: String): RIO[Clock, Unit] =
    emailService
      .send(recipient, "Welcome")
      .retry(
        Schedule.spaced(1.second).both(Schedule.recurs(5))
      )
}

trait EmailService {
  def send(to: String, body: String): Task[Unit]
}