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隐式转换对任何一个都有效吗?

typeclass implicit scala

1

Eugene Yokota · 技术社区 · 15 年前

List[Any] 或类似的容器,但稍后对其执行隐式转换以执行类似类型类的操作。下面是一个例子:

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(_) }

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)

也, List(Prius(2010)) map { implicitPrice(_) } 也不起作用因为它想 Price[Prius] 和 Price[Car]

1 回复 | 直到 15 年前

1

0

Eugene Yokota 15 年前

Any . 我尝试使用 Manifest 任何 进入之内 T Manifest[T] 对象。

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)
}

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

看来我得把事情解决成 Price 在我插进去之前 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) }