Scala: Do implicit conversions work on Any?

Question

I would like to store some objects from different type hierarchy into List[Any] or similar container, but perform implicit conversions on them later on to do something like type class. Here is an example:

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(_) }

The above code throws an error as follows:

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

If you uncomment AnyPrices, you'd get List(0,0), but that's not what I am expecting. Do I have to store the manifest into the list for this to work?

Also, List(Prius(2010)) map { implicitPrice(_) } doesn't work either because it wants Price[Prius] and Price[Car] isn't good enough. Is there a way to make it more flexible?

Answer 1

So, looks like I can't get a type class once the objects are reduced to Any. My attempt of using Manifest also failed, since there seems to be no way for me to cast an Any into T even if I have the Manifest[T] object.

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)
}

gives me

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

so it seems like I have to resolve things into Price before I stick them into 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) }