懒惰地生成组合的惯用 Scala 方式

Question

我想生成一些值的组合，如下面的代码所示：

object ContinueGenerate {

  val foods = List("A", "B", "C")
  val places = List("P1", "P2", "P3")
  val communities = List("C1", "C2", "C3", "C4")

  case class Combination(food: String, place: String, community: String)

  def allCombinations() = {
    for {
      food <- foods; place <- places; community <- communities
    } yield Combination(food, place, community)
  }

  def main(args: Array[String]) {
    allCombinations foreach println
  }

}

然而，这种方法的问题在于，所有数据都是一次性生成的。当 foods、places 和 communities 的大小变得非常大时，这是一个大问题。除了这三个参数之外，还可以有其他参数。

所以我希望能够以连续的方式生成组合，以便仅在请求时生成组合。

Scala 的惯用方式是什么？

Answer 1

您使用流：

object ContinueGenerate {

  val foods = Stream("A", "B", "C")
  val places = Stream("P1", "P2", "P3")
  val communities = Stream("C1", "C2", "C3", "C4")

  case class Combination(food: String, place: String, community: String)

  def allCombinations() = {
    for {
      food <- foods; place <- places; community <- communities
    } yield Combination(food, place, community)
  }

  def main(args: Array[String]) {
    allCombinations foreach println
  }

}

一个Stream缓存了所有的数据。如果您只想迭代一次，请改用 Iterator，它应该垃圾收集已经遍历的元素。

Answer 2

您可以通过在每个列表上使用 View 来完成此操作。在下面的代码中，我添加了一个副作用，因此当为每个元素调用 yield 时它是可见的。

val foods = List("A", "B", "C")
val places = List("P1", "P2", "P3")
val communities = List("C1", "C2", "C3", "C4")

case class Combination(food: String, place: String, community: String)

def allCombinations() =
  for {
    food <- foods; place <- places; community <- communities
  } yield {
    val comb = Combination(food, place, community)
    println(comb)
    comb
  }

//Prints all items
val combinations = allCombinations()

def allCombinationsView() =
  for {
    //Use a view of each list
    food <- foods.view; place <- places.view; community <- communities.view
  } yield {
    val comb = Combination(food, place, community)
    println(comb)
    comb
  }
//Prints nothing
val combinationsView = allCombinationsView()

//Prints 5 items
val five = combinationsView.take(5).toList

懒惰地生成组合的惯用 Scala 方式

Idiomatic Scala way of generating combinations lazily

combinations

scala