我正在尝试定义一组“ LazyChains”,以用于将来处理传入的消息。我希望LazyChains的API与Scala集合API(即:Seq,Stream等)没有区别。这将使我能够在消息到达之前提前声明性地定义过滤/转换/动作。这可能是一种众所周知的模式,我不知道其名称,因此这使我很难找到任何结果。
这是我要完成的工作的一个示例:
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中是否已经存在此模式?如果没有,如何实现此类LazyChain
?
这是我目前的尝试。我似乎无法弄清楚如何使这些类型起作用:
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)
}
}
答案 0 :(得分:2)
您所要做的就是用一些精美的方法包装一个函数I => List[O]
。您可以编写implicit class
来将这些方法添加到这种类型,但是Kleisli通过各种cats类型类(主要是FilterFunctor
)免费完成了其中的大部分工作。
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
它可能看起来太神奇了,所以这是手工制作的版本:
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))
}
链式转换是一个常见的问题,正如评论所说,使用monix.Observable,iterates等是解决此问题的正确方法(而不是普通的List
,流自然是懒惰的。