5.scala

import util.control.ControlThrowable

case object RestartException extends ControlThrowable

def restart: Nothing = throw RestartException

def attempt[A](f: => A): Attempt[A] = new Attempt(f)

class Attempt[A](f: => A) {
  def fallback(catcher: PartialFunction[Throwable, A]): A = {
    var stop = true
    var result = null.asInstanceOf[A]
    do {
      try {
        result = f
        stop = true
      } catch {
        case t: Throwable if catcher isDefinedAt t => {
          try {
            result = catcher(t)
            stop = true
          } catch {
            case RestartException => stop = false
          }
        }
        case t: Throwable => throw t
      }
    } while(!stop)
    result
  }
}