Question

给定一个任意case class Cc( a: String, b: Int )和一个类型类trait Tc[T]，有没有办法实现一个简单的HList（或最好是一个标记的记录），它包含每个参数的类型类实例是Cc？

def resolveTypeClasses[C] = ???
val resolved: Tc[String] :: Tc[Int] :: HNil = resolveTypeClasses[Cc]

Answer 1

由于需要解析中间Generic，很难制作单一类型参数功能。

因此，从我的noob观点来看，它应该是两个参数def，或两个def的序列，每个都采用单一类型参数。第二种变体可以实现如下：

import shapeless._
trait Tc[T]

case class Cc(a: String, b: Int)

trait ResolveTC[L] {
  type Out <: HList
  val out: Out
}

implicit object ResolveHNilTC extends ResolveTC[HNil] {
  type Out = HNil
  val out: HNil = HNil
}

implicit def resolveHListTC[X, XS <: HList](implicit tc: Tc[X], rest: ResolveTC[XS]) =
  new ResolveTC[X :: XS] {
    type Out = Tc[X] :: rest.Out
    val out = tc :: rest.out
  }

class TCResolver[C] {
  def get[L <: HList](implicit gen: Generic.Aux[C, L], resolve: ResolveTC[L]): resolve.Out = resolve.out
}

def resolveTypeClasses[C] = new TCResolver[C]

implicit case object StringInst extends Tc[String]
implicit case object IntInst extends Tc[Int]

使用该实现

val resolved = resolveTypeClasses[Cc].get

将产生

StringInst :: IntInst :: HNil

召唤类型类的HList

1 个答案: