折叠提供的 HList

Question

我知道我的问题可能是 "XY problem"，所以这里简要回顾一下我想要实现的目标。假设我有以下类型：

trait Provider[T] { def provide: T }

我希望能够将从该类型提供的多个值组合成一个（每个提供的值可以有不同的类型）。组合发生在程序的不同部分，让我们定义类型 Combiner（尝试将 shapeless 应用于我的问题）：

trait Combiner[Out, Providers <: HList] { 
  def combine(providers: Providers): Out 
}

我得出以下结论：

class ShapelessCombiner[Out, Providers <: HList, P](implicit
  zero: Out,
  folder: LeftFolder.Aux[Providers, Out, P, Out]
) extends Combiner[Out, Providers] {
  def combine(providers: Providers): Out = folder(folder, zero)
}

为了指定用于折叠的函数，我创建了辅助方法：

def shapelessCombiner[Out, Providers <: HList](
  fun: Poly
)(implicit
  zero: Out,
  folder: LeftFolder.Aux[Providers, Out, fun.type, Out]
) = new ShapelessCombiner[Out, Providers]

一切都很好，只是它不起作用。我正在尝试使用以下代码对其进行测试：

implicit val zero = Json.obj()

//incorrect implementation, but compilation is important here
object poly extends Poly2 {
implicit def caseInt =
    at[Json, Provider[Int]]((j, p) => j deepMerge Json.fromInt(p.provide))
  implicit def caseString =
    at[Json, Provider[String]]((j, p) => j deepMerge Json.fromString(p.provide))
}
def hlistInstance: Provider[Int] :: Provider[String] :: HNil = ???
val directResult = hlistInstance.foldLeft(zero)(poly) //compiles
val combiner = shapelessCombiner[Json, Provider[Int] :: Provider[String] :: HNil](poly)//do not compile, LeftFolder.AUX not found

我的问题是：

a) 我该如何解决这个编译问题，或者

b) 有没有 different/better 方法来解决我最初的问题

编辑

事实证明我的错误是由错误的导入引起的，我以某种方式导入了 tuple.LeftFolder 而不是 hlist.LeftFolder，愚蠢的错误

Answer 1

以下代码编译通过

  import io.circe.Json
  import shapeless.{::, HList, HNil, Poly, Poly2}
  import shapeless.ops.hlist.LeftFolder

  trait Provider[T] {
    def provide: T
  }

  trait Combiner[Out, Providers <: HList] {
    def combine(providers: Providers): Out
  }

  class ShapelessCombiner[Out, Providers <: HList, P](implicit
                                                      zero: Out,
                                                      folder: LeftFolder.Aux[Providers, Out, P, Out]
                                                     ) extends Combiner[Out, Providers] {
    def combine(providers: Providers): Out = folder(providers, zero)
  }

  def shapelessCombiner[Out, Providers <: HList](
                                                  fun: Poly
                                                )(implicit
                                                  zero: Out,
                                                  folder: LeftFolder.Aux[Providers, Out, fun.type, Out]
                                                ) = new ShapelessCombiner[Out, Providers, fun.type]

  implicit val zero = Json.obj()

  object poly extends Poly2 {
    implicit def caseInt =
      at[Json, Provider[Int]]((j, p) => j deepMerge Json.fromInt(p.provide))
    implicit def caseString =
      at[Json, Provider[String]]((j, p) => j deepMerge Json.fromString(p.provide))
  }
  def hlistInstance: Provider[Int] :: Provider[String] :: HNil = ???
  val directResult = hlistInstance.foldLeft(zero)(poly)
  val combiner = shapelessCombiner[Json, Provider[Int] :: Provider[String] :: HNil](poly)

折叠提供的 HList

Folding over provided HList

scala

implicits

shapeless

scala-2.12

circe