如何派生作用于特定子类型的 HList 的无形操作

Question

我试图创建一个无形操作，它作用于给定类型的对象的 HList。但是，我无法弄清楚如何使它在作为该对象子类型的对象的 HList 上工作。这是一个例子：

    trait Transform[I, O] { def f: I => O }

    trait Transformer[I, TH <: HList] {
      type Out <: HList
      def transform(input: I, transforms: TH): Out
    }
    object Transformer {
      type Aux[I, Transforms <: HList, Out0] = Transformer[I, Transforms] { type Out = Out0 }

      def apply[I, Transforms <: HList](implicit transformer: Transformer[I, Transforms]): Aux[I, Transforms, transformer.Out] = transformer

      implicit def hnilTransformer[I]: Aux[I, HNil, HNil] =
        new Transformer[I, HNil] {
          type Out = HNil
          override def transform(input: I, transforms: HNil): HNil = HNil
        }

      implicit def hconsTransformer[I, O, TIn <: HList, TOut <: HList]
        (implicit t: Transformer.Aux[I, TIn, TOut]): Aux[I, Transform[I, O] :: TIn, O :: TOut] =
          new Transformer[I, Transform[I, O] :: TIn] {
            type Out = O :: TOut
            override def transform(input: I, transforms: Transform[I, O] :: TIn): Out = {
              transforms.head.f(input) :: t.transform(input, transforms.tail)
            }
          }
    }

    def applyTransforms[I, TH <: HList](transforms: TH)(input: I)
                                       (implicit transformer: Transformer[I, TH]): transformer.Out = {
      transformer.transform(input, transforms)
    }

    val double = new Transform[Int, Int] { def f = _ * 2 }
    val int2Str = new Transform[Int, String] { def f = _.toString }

    applyTransforms(double :: int2Str :: HNil)(4) // shouldBe 8 :: "4" :: HNil

    trait SubTransform[I, O] extends Transform[I, O]
    val doubleSub = new SubTransform[Int, Int] { def f = _ * 2 }
    val int2StrSub = new SubTransform[Int, String] { def f = _.toString }

    applyTransforms(doubleSub :: int2StrSub :: HNil)(4) // fails to compile

第一个 applyTransforms 编译成功但第二个失败并出现错误：

ShapelessSpec.scala:124: could not find implicit value for parameter transformer: Transformer[Int,shapeless.::[SubTransform[Int,Int],shapeless.::[SubTransform[Int,String],shapeless.HNil]]]
    applyTransforms(doubleSub :: int2StrSub :: HNil)(4)

我怀疑问题出在 hconsTransformer 上，但我不明白为什么这不应该在 SubTransform 的范围内。

Answer 1

如果您希望它与 Transform

的子类型一起使用，请修改您的递归步骤

implicit def hconsTransformer[I, O, TIn <: HList, TOut <: HList, X]
(implicit t: Transformer.Aux[I, TIn, TOut], ev: X <:< Transform[I, O]): Aux[I, X :: TIn, O :: TOut] =
  new Transformer[I, X :: TIn] {
    type Out = O :: TOut
    override def transform(input: I, transforms: X :: TIn): Out = {
      transforms.head.f(input) :: t.transform(input, transforms.tail)
    }
  }

或使Transformer相对于TH

逆变

trait Transformer[I, -TH <: HList] { ...

如何派生作用于特定子类型的 HList 的无形操作

How to derive a shapeless op which acts on an HList of a specific subtype

scala

shapeless