有没有办法从 Shapeless HList 中检索特定元素的 Nat 位置？

Question

给定任何 HList，例如1 :: "str" :: true :: HNil，有没有一种（简单的）方法可以找到对应于任何特定列表元素位置的 Nat，例如f(hlist, true) ==> Nat._2？是否可以对元素 type 而不是元素本身做同样的事情，例如f[String](hlist) ==> Nat._1？假设每个 element/type 只出现一次并且它在列表中的存在是有保证的。

Answer 1

尝试引入一种类型class

import shapeless.{::, DepFn0, HList, HNil, Nat, Succ}
import shapeless.nat._

trait Find[L <: HList, A] extends DepFn0 { type Out <: Nat }
object Find {
  type Aux[L <: HList, A, Out0 <: Nat] = Find[L, A] { type Out = Out0 }
  def instance[L <: HList, A, Out0 <: Nat](x: Out0): Aux[L, A, Out0] = new Find[L, A] {
    type Out = Out0
    override def apply(): Out = x
  }

  implicit def head[A, T <: HList]: Aux[A :: T, A, _0] = instance(_0)
  implicit def tail[H, T <: HList, A, N <: Nat](implicit
    find: Aux[T, A, N]
  ): Aux[H :: T, A, Succ[N]] = instance(Succ[N]())
}

def f[L <: HList, A](l: L, a: A)(implicit find: Find[L, A]): find.Out = find()

def f[A] = new PartiallyApplied[A]
class PartiallyApplied[A] {
  def apply[L <: HList](l: L)(implicit find: Find[L, A]): find.Out = find()
}

implicitly[Find.Aux[Int :: String :: Boolean :: HNil, String, _1]]
implicitly[Find.Aux[Int :: String :: Boolean :: HNil, Boolean, _2]]

val hlist = 1 :: "str" :: true :: HNil

val n = f(hlist, true)
implicitly[n.N =:= _2]

val m = f[String](hlist)
implicitly[m.N =:= _1]

或标准类型 classes

import shapeless.ops.hlist.{Collect, IsHCons, ZipWithIndex}
import shapeless.{HList, HNil, Nat, Poly1, poly}

trait Second[A] extends Poly1
object Second {
  implicit def cse[A, N <: Nat]: poly.Case1.Aux[Second[A], (A, N), N] = poly.Case1(_._2)
}

def f[L <: HList, A, L1 <: HList, L2 <: HList, N <: Nat](l: L, a: A)(implicit
  zipWithIndex: ZipWithIndex.Aux[L, L1],
  collect: Collect.Aux[L1, Second[A], L2],
  isHCons: IsHCons.Aux[L2, N, _]
): N = collect(zipWithIndex(l)).head

有没有办法从 Shapeless HList 中检索特定元素的 Nat 位置？

Is there a way to retrieve Nat position of a specific element from Shapeless HList?

scala

shapeless