scala - scalaz.Validation loopSuccess和loopFailure如何工作

有人可以用真实的例子来解释scalaz.Validation方法下的工作原理吗？
我的意思是loopSuccess和loopFailure。

源代码段(scalaz7):

scalaz.Validation:

/** Spin in tail-position on the success value of this validation. */
def loopSuccess[EE >: E, AA >: A, X](success: AA => X \/ Validation[EE, AA], failure: EE => X): X =
Validation.loopSuccess(this, success, failure)

/** Spin in tail-position on the failure value of this validation. */
def loopFailure[EE >: E, AA >: A, X](success: AA => X, failure: EE => X \/ Validation[EE, AA]): X =
Validation.loopFailure(this, success, failure)

随播对象:

object Validation extends ValidationFunctions with ValidationInstances {

  /** Spin in tail-position on the success value of the given validation. */
  @annotation.tailrec
  final def loopSuccess[E, A, X](d: Validation[E, A], success: A => X \/ Validation[E, A], failure: E => X): X =
    d match {
      case Failure(e) => failure(e)
      case Success(a) => success(a) match {
        case -\/(x) => x
        case \/-(q) => loopSuccess(q, success, failure)
      }
    }

  /** Spin in tail-position on the failure value of the given validation. */
  @annotation.tailrec
  final def loopFailure[E, A, X](d: Validation[E, A], success: A => X, failure: E => X \/ Validation[E, A]): X =
    d match {
      case Failure(e) => failure(e) match {
        case -\/(x) => x
        case \/-(q) => loopFailure(q, success, failure)
      }
      case Success(a) => success(a)
    }

}

最佳答案

这类似于trampoline。对于loopSuccess，您需要提供一个初始值和一个将您带入下一个状态的函数。可能存在以下3种状态:

X.left           // stop processing with X as the result
Success(a).right // no result, run the next iteration with this value
Failure(e).right // stop processing, run the failure function on this result and return it

对于loopFailure来说，将Failure和Success反向执行相同的操作，因此，您将继续运行直到返回左键或Success为止。

这是使用loopSuccess的示例:

import scalaz._
import Scalaz._

object TestLoopSuccess extends App {
  // check if a number divides another, returning a Failure for division by zero
  val divides : Int => Int => Validation[String,Boolean] = { div => num =>
    if(div == 0) "division by zero".failure
    else (num % div == 0).success
  }

  val allDivide : Int => List[Int] => String \/ Validation[Int,List[Int]] = { div => nums =>
    nums match {
      // empty list means we are done, so we return a left
      case Nil => "All numbers divide".left

      // process the head of the list and return a right
      case x::xs => divides(div)(x).flatMap { divides =>
        if(divides)
          // head divides, so process more of the list
          xs.success
        else
          // head does not divide, so we are done
          "%d is not a multiple of %d".format(x,div).failure
      }.right
    }
  }

  println(Validation.loopSuccess(List(2,4,6,8).success[String], allDivide(0), identity[String])) // "division by zero"
  println(Validation.loopSuccess(List(2,4,6,8).success[String], allDivide(2), identity[String])) // "All numbers divide"
  println(Validation.loopSuccess(List(2,4,7,8).success[String], allDivide(2), identity[String])) // "7 is not a multiple of 2"
}