在无形Coproduct上进行模式匹配的清洁方式

Question

我发现了两种可以在无形副产品上进行模式匹配的方法.我也搜索了这个主题并找到了这个

import shapeless._

object ShapelessEx3 extends App {
   case class Red()
   case class Green()
   case class Blue()
   type Colors = Red :+: Green :+: Blue :+: CNil
   val blue = Coproduct[Colors](Blue())
   val green = Coproduct[Colors](Green())
   printColor1(blue)
   printColor2(green)

   def printColor1(c: Colors) : Unit = {
      (c.select[Red], c.select[Green], c.select[Blue]) match {
         case (Some(_), None, None) => println("Color is red")
         case (None, Some(_), None) => println("Color is green")
         case (None, None, Some(_)) => println("Color is blue")
         case _ => println("unknown color")
      }
   }
   def printColor2(c: Colors) : Unit = {
      c match {
         case Inl(Red()) => println("color is red")
         case Inr(Inl(Green())) => println("color is green")
         case Inr(Inr(Inl(Blue()))) => println("color is blue")
         case _ => println("unknown color")
      }
   }
}

但是,这两种功能都喜欢很嘈杂case (None, Some(_), None)或Inr(Inr(Inl(Blue()))).我怎么能这么简单的模式匹配呢

c match {
  case Red() =>
  case Green() => 
  case Blue() => 
}

Answer 1

你可以使用Poly1例如

import shapeless._
case class Red()
case class Green()
case class Blue()
type Colors = Red :+: Green :+: Blue :+: CNil
val blue = Coproduct[Colors](Blue())
val green = Coproduct[Colors](Green())

object printColor extends Poly1 {
  implicit def caseRed = at[Red] {r=> println("red") ;r    }
  implicit def caseBlue = at[Blue]{b=> println("blue");b  }
  implicit def caseGreen= at[Green]{g =>println("green");g }
}                                                             
green map printColor
blue map printColor