9

有人可以用现实世界的例子来解释下面的方法是如何工作的scalaz.Validation吗？我的意思是loopSuccess和loopFailure。

源代码（scalaz7）的片段：

scalaz.验证：

/** 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)

伴侣对象：

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)
    }

}
4

1 回答 1

5

这类似于蹦床。对于 loopSuccess，您提供一个初始值和一个带您进入下一个状态的函数。有 3 种可能的下一个状态：

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

For loopFailure 对失败和成功做同样的事情，所以你继续运行，直到你返回一个左或一个成功。

下面是一个使用 loopSuccess 的例子：

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"
}
于 2012-12-28T16:50:52.370 回答