编写一个类型类，选择依赖于上下文绑定

Question

我的初始代码：

sealed trait Adder[L <: HList, U] extends DepFn2[L, Vector[U]]

object Adder {
  def apply[L <: HList, U: Ordering](implicit adder: Adder[L, U]): Aux[L, U, adder.Out] = adder

  type Aux[L <: HList, U, Out0] = Adder[L, U] { type Out = Out0 }

  implicit def found[T <: HList, U: Ordering]: Aux[Vector[U] :: T, U, Vector[U] :: T] =
    new Adder[Vector[U] :: T, U] {
      type Out = Vector[U] :: T
      def apply(l: Vector[U] :: T, collection: Vector[U]): Out = {
        (l.head ++ collection).sorted :: l.tail
      }
    }

  implicit def notFound[H, T <: HList, U: Ordering, OutT <: HList](implicit ut: Aux[T, U, OutT]): Aux[H :: T, U, H :: OutT] =
    new Adder[H :: T, U] {
      type Out = H :: OutT
      def apply(l: H :: T, collection: Vector[U]): Out = {
        val outT = ut(l.tail, collection)
        l.head :: outT
      }
    }

  implicit def empty[U: Ordering]: Aux[HNil, U, Vector[U] :: HNil] =
    new Adder[HNil, U] {
      type Out = Vector[U] :: HNil
      def apply(l: HNil, collection: Vector[U]): Out = collection :: HNil
    }
}

我发现了一个没有上下文绑定的错误 Ordering，类型通过 notFound 而不是 found 传递，这在事后看来并不令人惊讶。我试图通过添加来修复错误另一个隐式应该在没有 Ordering:

时触发

  implicit def foundNoOrdering[T <: HList, U]: Aux[Vector[U] :: T, U, Vector[U] :: T] =
    new Adder[Vector[U] :: T, U] {
      type Out = Vector[U] :: T
      def apply(l: Vector[U] :: T, collection: Vector[U]): Out = {
        l.head ++ collection :: l.tail
      }
    }

但是，这会导致 foundNoOrdering 和 found。我怎样才能有不同的代码路径取决于是否有 Ordering?

Answer 1

标准技巧是通过将隐式放入祖先特征中来降低优先级

object Adder extends LowPriorityAdderImplicits {
   implicit def found...
}

trait LowPriorityAdderImplicits {
  implicit def foundNoOrdering....
}

您会在标准库中找到其中的一些。 LowPriorityImplicits这个名字好像习惯了。

在规范中：

SLS §7.2 隐式参数

If there are several eligible arguments which match the implicit parameter’s type, a most specific one will be chosen using the rules of static overloading resolution (§6.26.3)

SLS §6.26.3：相关位太长无法完整引用，但您有一些关于

A class or object C is derived from a class or object D if one of the following holds:

• C is a subclass of D, or

• C is a companion object of a class derived from D, or

• D is a companion object of a class from which C is derived.

并且派生使其更具体并在解决方案中获得优先权。我相信其中一个只是为了隐含而制作的。

编写一个类型类，选择依赖于上下文绑定

Writing a typeclass with selection dependent on context bound

scala

shapeless