scala - 使用泛型类型的类型进行过滤

Question

我有以下代码(简化):

sealed trait Funding {
  id: Int
}

final case class FlatRate(id: Int, days: Int, amount: BigDecimal) extends Funding
final case class PerItem(id: Int, amount: BigDecimal, discount: BigDecimal) extends Funding

final case class AppliedFunding[+A <: Funding](
  person: String
)

我有一些返回“通用”AppliedFunding 列表的代码，我想获得具有正确类型的特定 AppliedFunding。伪代码:

val allFunding: List[AppliedFunding[Funding]] = <...>
val flatrateFunding: List[AppliedFunding[FlatRate]] = allFunding.someMagicFunction

代替 someMagicFunction 我正在寻找一种方法来过滤出正确的 AppliedFunding 实例并以特定类型返回它们。我可以执行 allFunding.collect { ... }，但无论我尝试什么，它仍然会返回 List[AppliedFunding[Funding]]。

非常感谢任何帮助。

Answer 1

下面是两种方法。补充一点:collect 是极少数情况下 PartialFunction 是好东西

sealed trait Foo
final case class Bar(i:Int) extends Foo
final case class Baz(d:Double) extends Foo

final case class Container[F <: Foo](s:String, f:F) 


val all:List[Container[Foo]] = List(
  Container("bar", Bar(3)),
  Container("bar", Bar(4)),
  Container("bar", Bar(5)),
  Container("bar", Bar(6)),
  Container("bar", Bar(7)),
  Container("baz", Baz(1.5)),
  Container("baz", Baz(2.5)),
  Container("baz", Baz(3.5)),
  Container("baz", Baz(4.5)),
  Container("baz", Baz(5.5))
  )


val bars:List[Container[Bar]] = all.collect { case Container(s, Bar(i)) => Container(s, Bar(i))}
val baz:List[Container[Baz]] = all.foldRight(List.empty[Container[Baz]]) (
    (elem, lst) => elem match {
      case Container(s, Baz(d)) => Container(s, Baz(d)) :: lst
      case _ => lst
    }
  )

println(bars)
println(baz)