如何在Scala中实现Lazy Chain Pattern

Question

我正在尝试定义一组"LazyChains",用于在将来的时间处理传入的消息.我希望LazyChains的API与Scala集合API(即:Seq,Stream等)无法区分.这将允许我在消息到达之前以声明方式定义过滤/转换/操作.这可能是一个众所周知的模式,我不知道它的名字,所以这使我很难找到任何结果.

这是我想要完成的一个例子:

val chainA = LazyChain()
 .filter(_ > 1)
 .map(x => x * 2)
 .foreach(println _)

val chainB = LazyChain()
 .filter(_ > 5)
 .flatMap(x => Seq(x, x))
 .foreach(println _)

 chainA.apply(2)  // should print "4"
 chainA.apply(1)  // should print nothing
 chainB.apply(10) // should print "10" - twice

Scala集合API中是否已存在此模式？如果没有,我该如何实现这个类LazyChain？

这是我目前的尝试.我似乎无法弄清楚如何使类型工作:

case class LazyChain[I, O](val filter : Option[I => Boolean],
                      val transform : I => O,
                      val action : Option[O => Unit]) {

  def filter(otherFilter : I => Boolean): LazyChain[I, O]  = {
      val newFilter = Some({ x : I => {
        filter.map(_.apply(x)).getOrElse(true) && otherFilter.apply(x)
      }})
      copy(filter = newFilter)
  }

  def map[X](otherTransform : O => X) : LazyChain[I, X] = {
    new LazyChain[I, X](
      filter = filter,
      transform = (x: I) => {
        otherTransform.apply(transform.apply(x))
      },
      /*
        type mismatch;
        [error]  found   : Option[O => Unit]
        [error]  required: Option[X => Unit]
      */
      action = action 
    )
  }

  def flatMap[X](otherTransform : O => Seq[X]) : LazyChain[I, X] = {
    new LazyChain[I, X](
      filter = filter,
      transform = (x: I) => {
        /**
           type mismatch;
             [error]  found   : Seq[X]
             [error]  required: X
        */
        otherTransform.apply(transform.apply(x))
      }
    )
  }

  def foreach(newAction : O => Unit) = {
    copy(action = Some(newAction))
  }

  def apply(element : I) = {
    if (filter.map(_.apply(element)).getOrElse(true)) {
      val postTransform = transform.apply(element)
      action.foreach(_.apply(postTransform))
    }
  }
}
object LazyChain {
  def apply[X]() : LazyChain[X, X] = {
    new LazyChain(filter = None, transform = x => x, action = None)
  }
}

Answer 1

您想要的只是用一些奇特的方法包装一个函数I => List[O]。您可以编写代码implicit class将这些方法添加到此类型中，但 Kleisli 通过各种 cats 类型类（主要是FilterFunctor.

  import cats.implicits._
  import cats.data.Kleisli

  type LazyChain[I, O] = Kleisli[List, I, O]
  def lazyChain[A]: LazyChain[A, A] = Kleisli[List, A, A](a => List(a))

  val chainA = lazyChain[Int]
    .filter(_ > 1)
    .map(x => x * 2)
    .map(println)

  val chainB = lazyChain[Int]
    .filter(_ > 5)
    .flatMapF(x => List(x, x))
    .map(println)

  chainA(2)  // should print "4"
  chainA(1)  // should print nothing
  chainB(10) // should print "10" - twice

它可能看起来有点太神奇了，所以这是一个手工版本：

case class LazyChain[A, B](run: A => List[B]) {

  def filter(f: B => Boolean): LazyChain[A, B] = chain(_.filter(f))

  def map[C](f: B => C): LazyChain[A, C] = chain(_.map(f))

  def flatMap[C](f: B => List[C]): LazyChain[A, C] = chain(_.flatMap(f))

  def chain[C](f: List[B] => List[C]): LazyChain[A, C] = LazyChain(run andThen f)
}
object LazyChain {
  def apply[I]: LazyChain[I, I] = new LazyChain(a => List(a))
}

链接转换是一个常见的问题，正如评论所说，使用 monix.Observable、iteratees 等是解决这个问题的正确方法（而不是普通的List和流自然是懒惰的。