Zipwith产品实现与Shapeless？

Question

给定：

// Given an HList of size N, provide evidence of the sum of HList
// multiplied by _3 (length) :: _2 (length) :: _1 (length) :: HNil
// Example: input: _1 :: _2 :: _2 -> output: _3 + _4 + _2 :: HNil
trait HListProductZipped[L <: HList] {
  type Out <: HList
}
object HListProductZipped {

  type Aux[L <: HList, Out1 <: HList] = HListProductZipped[L] { type Out = Out1 }

  def apply[L <: HList](implicit ev: HListProductZipped[L]): Aux[L, ev.Out] = ev

  implicit def induct[H <: Nat, T <: HList, L <: Nat](
    implicit ev: Length.Aux[H :: T, L],
             prod: Prod[H, L],
             rest: HListProductZipped[T]
  ): HListProductZipped.Aux[H :: T, prod.Out :: rest.Out] = new HListProductZipped[H :: T] {
    type Out = prod.Out :: rest.Out
  }

  implicit val hnilHListProductZipped: HListProductZipped[HNil] = new 
    HListProductZipped[HNil] {
      type Out = HNil
    }

}

然而，它并没有像我预期的那样工作：

import shapeless._
import nat._
import net.HListProductZipped

scala> val a = HListProductZipped[_1 :: _2 :: HNil]
a: net.HListProductZipped[shapeless.::[shapeless.Succ[shapeless._0],shapeless.::[shapeless.Succ[shapeless.Succ[shapeless._0]],shapeless.HNil]]]{type Out = shapeless.::[shapeless.Succ[shapeless.Succ[shapeless._0]],shapeless.::[shapeless.Succ[shapeless.Succ[shapeless._0]],net.HListProductZipped.hnilHListProductZipped.Out]]} = net.HListProductZipped$$anon$3@130efbff

scala> val e: a.Out = _2 :: _2 :: HNil
<console>:19: error: type mismatch;
 found   : shapeless.::[shapeless.nat._2,shapeless.::[shapeless.nat._2,shapeless.HNil]]
    (which expands to)  shapeless.::[shapeless.Succ[shapeless.Succ[shapeless._0]],shapeless.::[shapeless.Succ[shapeless.Succ[shapeless._0]],shapeless.HNil]]
 required: a.Out
    (which expands to)  shapeless.::[shapeless.Succ[shapeless.Succ[shapeless._0]],shapeless.::[shapeless.Succ[shapeless.Succ[shapeless._0]],net.HListProductZipped.hnilHListProductZipped.Out]]
       val e: a.Out = _2 :: _2 :: HNil
                         ^

请告诉我我做错了什么。

Answer 1

在hnilHListProductZipped的返回类型中缺少优化，这意味着编译器不能知道它的Out是HNil。将其更改为Aux[HNil, HNil]将使此工作正常。