A universal type class that includes all shapeless.HList operations

HListWitness.scala

import shapeless._
import shapeless.ops.hlist.ZipWithIndex.Helper
import shapeless.poly._
import shapeless.ops.hlist.{
  Length,
  LiftAll,
  Mapper,
  ToCoproduct,
  ToList,
  ZipWithIndex
}

/**
  * @author 杨博 (Yang Bo) <pop.atry@gmail.com>
  */
trait HListWitness[L <: HList] {

  type N <: Nat
  def length: Length.Aux[L, N]

  def toCoproduct: ToCoproduct[L]

  def zipWithIndexHelper[Start <: Nat]: ZipWithIndex.Helper[L, Start]

  def zipWithIndex: ZipWithIndex[L]

//  def liftAll[F[_]]: LiftAll[F, L]

  type Cases[HF <: Poly] <: HList

  def map[HF <: Poly](cases: Cases[HF]): Mapper[HF, L]

  def map[HF <: Poly](
      implicit liftAll: LiftAll.Aux[Case1[HF, ?], L, Cases[HF]])
    : Mapper[HF, L] = {
    this.map[HF](liftAll.instances)
  }

}

object HListWitness {

  implicit object HNilWitness extends HListWitness[HNil] {

    override def length = Length.hnilLength

    override type N = _0

    override def toCoproduct: ToCoproduct[HNil] = implicitly

    override def zipWithIndexHelper[Start <: Nat] = Helper.hnil[Start]

    override def zipWithIndex = ZipWithIndex.default(zipWithIndexHelper)

    type Cases[HF <: Poly] = HNil

    type MapResult[HF <: Poly, C <: Cases[HF]] = HNil
    override def map[HF <: Poly](cases: Cases[HF]) = Mapper.hnilMapper1[HF]
  }

  trait HConsWitness[H, T <: HList] extends HListWitness[H :: T] {

    val tailWitness: HListWitness[T]

    override type N = Succ[tailWitness.N]

    // OK
    override def length = {
      Length.hlistLength[H, T, tailWitness.N](tailWitness.length, implicitly)
    }

    // Can be type safe
    override def toCoproduct: ToCoproduct[H :: T] = {
      ToCoproduct.hlistToCoproduct(tailWitness.toCoproduct)
    }

    override def zipWithIndexHelper[Start <: Nat] = {
      val tailHelper = tailWitness.zipWithIndexHelper[Succ[Start]]
      Helper.hcons[H, T, tailHelper.Out, Start](tailHelper)
    }

    override def zipWithIndex = ZipWithIndex.default(zipWithIndexHelper)

    override type Cases[HF <: Poly] = Case1[HF, H] :: tailWitness.Cases[HF]

    // Require recursive typeof expression to be type safe, which is not support in Scala
    override def map[HF <: Poly](cases: Cases[HF]) = {
      Mapper.hlistMapper1[HF, H, T](cases.head, tailWitness.map(cases.tail))
    }
  }

  implicit def hconWitness[H, T <: HList](
      tailWitness0: HListWitness[T]): HConsWitness[H, T] {
    val tailWitness: tailWitness0.type
  } = {
    new {
      override val tailWitness: tailWitness0.type = tailWitness0
    } with HConsWitness[H, T]
  }
}