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我想将一些来自不同类型层次结构的对象存储到List[Any]或类似的容器中,但稍后对它们执行隐式转换以执行类型类之类的操作。这是一个例子:

abstract class Price[A] {
  def price(a: A): Int
}

trait Car
case class Prius(year: Int) extends Car
trait Food
case class FriedChicken() extends Food

object Def {
  // implicit object AnyPrices extends Price[Any] {
  //   def price(any: Any) = 0
  // }

  // implicit object PriusPrices extends Price[Prius] {
  //   def price(car: Prius) = 100
  // }

  implicit object CarPrices extends Price[Car] {
    def price(car: Car) = 100
  }

  implicit object FoodPrices extends Price[Food] {
    def price(food: Food) = 5
  }
}

def implicitPrice[A: Price](x: A) = implicitly[Price[A]].price(x)

import Def._  
val stuff: List[Any] = List(Prius(2010), FriedChicken())
stuff map { implicitPrice(_) }

上面的代码抛出如下错误:

error: could not find implicit value for evidence parameter of type Price[Any]
       stuff map { implicitPrice(_) }
                                ^

如果你取消注释AnyPrices,你会得到List(0,0),但这不是我所期望的。我是否必须将清单存储到列表中才能正常工作?

此外,List(Prius(2010)) map { implicitPrice(_) }也不起作用,因为它想要Price[Prius]并且Price[Car]不够好。有没有办法让它更灵活?

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1 回答 1

0

所以,一旦对象减少到Any. 我的使用尝试也失败了,因为即使我有对象Manifest,我似乎也无法Any投入。TManifest[T]

import reflect.Manifest._
def add [A, B >: A](stuff: A, list: List[(B, Manifest[_])])(implicit m: Manifest[A]) = (stuff, m) :: list
val stuff2 = add(Prius(2000), add(FriedChicken(), Nil))
stuff2 map { x =>
  val casted = x._2.erasure.cast(x._1)
  implicitPrice(casted)
}

给我

error: could not find implicit value for evidence parameter of type Price[Any]

所以似乎我必须先解决问题,Price然后再将它们放入List

abstract class Price[A] {
  def price(a: Any): Int
}

trait Car
case class Prius(year: Int) extends Car
trait Food
case class FriedChicken() extends Food

object Def {  
  implicit object PriusPrices extends Price[Prius] {
    def price(car: Any) = 100
  }

  implicit object FriedChickenPrices extends Price[FriedChicken] {
    def price(food: Any) = 5
  }
}

import Def._  

def add [A, B >: A](stuff: A, list: List[(B, Price[_])])(implicit p: Price[A]) = (stuff, p) :: list
val stuff = add(Prius(2000), add(FriedChicken(), Nil))
stuff map { x => x._2.price(x._1) }
于 2010-09-29T23:49:57.033 回答