scala - 无形:从副产品映射到不同的副产品

Question

在下面，我试图制作一个多态函数来转换 RawFeatureValue成RefinedFeatureValue .

import shapeless._

object test {
  type RawFeatureValue = Int :+: Double :+: String :+: CNil
  type RefinedFeatureValue = Int :+: Double :+: CNil

  private object convert extends Poly1 {
    implicit def caseInt = at[Int](i => i)
    implicit def caseDouble = at[Double](d => d)
    implicit def caseString = at[String](s => s.hashCode)
  }

  val a = Coproduct[RawFeatureValue](12)
  val b: RefinedFeatureValue = a map convert
}

但是，结果类型是 Int :+: Double :+: Int :+: CNil与 RefinedFeatureValue 不兼容.

[error]  found   : shapeless.:+:[Int,shapeless.:+:[Double,shapeless.:+:[Int,shapeless.CNil]]]
[error]  required: test.RefinedFeatureValue
[error]     (which expands to)  shapeless.:+:[Int,shapeless.:+:[Double,shapeless.CNil]]
[error]   val b: RefinedFeatureValue = a map convert
[error]                                  ^

我如何告诉无形的两个Int s 应该被视为一个？

Answer 1

我能想到的最直接的方法是将每个元素映射到您的目标副产品，然后统一结果:

import shapeless._

type RawFeatureValue = Int :+: Double :+: String :+: CNil
type RefinedFeatureValue = Int :+: Double :+: CNil

object convert extends Poly1 {
  implicit val caseInt = at[Int](Coproduct[RefinedFeatureValue](_))
  implicit val caseDouble = at[Double](Coproduct[RefinedFeatureValue](_))
  implicit val caseString = at[String](s =>
    Coproduct[RefinedFeatureValue](s.hashCode))
}

这按预期工作:

scala> val a = Coproduct[RawFeatureValue](12)
a: RawFeatureValue = 12

scala> val b: RefinedFeatureValue = a.map(convert).unify
b: RefinedFeatureValue = 12

scala> val c = Coproduct[RawFeatureValue]("foo")
c: RawFeatureValue = foo

scala> val d: RefinedFeatureValue = c.map(convert).unify
d: RefinedFeatureValue = 101574

这个解决方案还不错，但它似乎对于单个操作来说可能足够有用。