scala - Shapeless中TypeClass特征的emptyCoproduct和coproduct方法的目的是什么

我尚不完全清楚Shapeless中emptyCoProduct特性的coproduct和TypeClass方法的目的是什么。

什么时候可以使用TypeClass特性而不是ProductTypeClass？

这两种方法的实现方式有哪些示例？

最佳答案

假设我有一个简单的类型类:

trait Weight[A] { def apply(a: A): Int }

object Weight {
  def apply[A](f: A => Int) = new Weight[A] { def apply(a: A) = f(a) }
}

还有一些实例:

implicit val stringWeight: Weight[String] = Weight(_.size)
implicit def intWeight: Weight[Int] = Weight(identity)

和一个案例类:

case class Foo(i: Int, s: String)

还有一个ADT:

sealed trait Root
case class Bar(i: Int) extends Root
case class Baz(s: String) extends Root

我可以为我的类型类定义一个ProductTypeClass实例:

import shapeless._

implicit object WeightTypeClass extends ProductTypeClass[Weight] {
  def emptyProduct: Weight[HNil] = Weight(_ => 0)
  def product[H, T <: HList](hw: Weight[H], tw: Weight[T]): Weight[H :: T] =
    Weight { case (h :: t) => hw(h) + tw(t) }
  def project[F, G](w: => Weight[G], to: F => G, from: G => F): Weight[F] =
    Weight(f => w(to(f)))
}

并像这样使用它:

scala> object WeightHelper extends ProductTypeClassCompanion[Weight]
defined object WeightHelper

scala> import WeightHelper.auto._
import WeightHelper.auto._

scala> implicitly[Weight[Foo]]
res0: Weight[Foo] = Weight$$anon$1@4daf1b4d

scala> implicitly[Weight[Bar]]
res1: Weight[Bar] = Weight$$anon$1@1cb152bb

scala> implicitly[Weight[Baz]]
res2: Weight[Baz] = Weight$$anon$1@74930887

但!

scala> implicitly[Weight[Root]]
<console>:21: error: could not find implicit value for parameter e: Weight[Root]
              implicitly[Weight[Root]]
                        ^

这是一个问题，它使我们的自动类型类实例派生对于ADT几乎毫无用处。幸运的是，我们可以改用TypeClass:

implicit object WeightTypeClass extends TypeClass[Weight] {
  def emptyProduct: Weight[HNil] = Weight(_ => 0)
  def product[H, T <: HList](hw: Weight[H], tw: Weight[T]): Weight[H :: T] =
    Weight { case (h :: t) => hw(h) + tw(t) }
  def project[F, G](w: => Weight[G], to: F => G, from: G => F): Weight[F] =
    Weight(f => w(to(f)))
  def emptyCoproduct: Weight[CNil] = Weight(_ => 0)
  def coproduct[L, R <: Coproduct]
    (lw: => Weight[L], rw: => Weight[R]): Weight[L :+: R] = Weight {
      case Inl(h) => lw(h)
      case Inr(t) => rw(t)
    }
}

然后:

scala> object WeightHelper extends TypeClassCompanion[Weight]
defined object WeightHelper

scala> import WeightHelper.auto._
import WeightHelper.auto._

scala> implicitly[Weight[Root]]
res0: Weight[Root] = Weight$$anon$1@7bc44e19

以上所有其他内容仍然可以正常使用。

总结一下:Shapeless的Coproduct是ADT的一种抽象，通常，应为类型类提供TypeClass的实例，而不是尽可能地仅提供ProductTypeClass。

关于scala - Shapeless中TypeClass特征的emptyCoproduct和coproduct方法的目的是什么，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/25517069/