Scala查看边界链接问题

Question

I know view bounds may be deprecated soon. Please ignore that.

The following code compiles if only one of the last 3 implicit conversions are uncommented. Is this a compiler bug?

object Viewable extends App {
  /** Speed of light in m/s */
  val C: Double = 299293458d

  /** @param weight in kilograms */
  case class Matter(name: String, weight: Double) {
    /** @return matter-energy equivalence in megajoules */
    def energy: Double = weight * C * C / 1000000d

    def megaJouleMsg: String = f"$name's mass-energy equivalence is $energy%.0f megajoules."
  }

  case class Animal(name: String, height: Double, weight: Double)
  case class Vegetable(name: String, height: Double, weight: Double)
  case class Mineral(name: String, weight: Double)

  case class Bug(name: String, height: Double, weight: Double, canFly: Boolean)
  case class Whale(name: String, height: Double, weight: Double, hasTeeth: Boolean)

  case class AppleTree(name: String, height: Double, weight: Double, age: Int)
  case class Grass(name: String, height: Double, weight: Double, edible: Boolean)

  case class Sand(name: String, color: String, weight: Double)
  case class Rock(name: String, color: String, weight: Double)

  implicit def sandToMineral(sand: Sand) = Mineral(sand.name, sand.weight)
  implicit def rockToMineral(rock: Rock) = Mineral(rock.name, rock.weight)

  implicit def appleTreeToVegetable(tree: AppleTree) = Vegetable(tree.name,  tree.height,  tree.weight)
  implicit def grassToVegetable(grass: Grass)        = Vegetable(grass.name, grass.height, grass.weight)

  implicit def bugToAnimal(bug: Bug)       = Animal(bug.name, bug.height, bug.weight)
  implicit def whaleToAnimal(whale: Whale) = Animal(whale.name, whale.height, whale.weight)

  implicit def animalToMatter[X <% Animal](animal: X)          = Matter(animal.name,    animal.weight)
  implicit def vegetableToMatter[X <% Vegetable](vegetable: X) = Matter(vegetable.name, vegetable.weight)
  implicit def mineralToMatter[X <% Mineral](mineral: X)       = Matter(mineral.name,   mineral.weight)

  println(Animal("Poodle", 1.0, 8.0).megaJouleMsg)
  println(AppleTree("Spartan", 2.3, 26.2, 12).megaJouleMsg)
  println(Rock("Quartz crystal", "white", 2.3).megaJouleMsg)
}

The error messages are:

type mismatch;
 found   : solutions.Viewable.Animal
 required: ?{def megaJouleMsg: ?}
Note that implicit conversions are not applicable because they are ambiguous:
 both method animalToMatter in object Viewable of type [X](animal: X)(implicit evidence$1: X => solutions.Viewable.Animal)solutions.Viewable.Matter
 and method vegetableToMatter in object Viewable of type [X](vegetable: X)(implicit evidence$2: X => solutions.Viewable.Vegetable)solutions.Viewable.Matter
 are possible conversion functions from solutions.Viewable.Animal to ?{def megaJouleMsg: ?}
  println(Animal("Poodle", 1.0, 8.0).megaJouleMsg)
                ^

type mismatch;
 found   : solutions.Viewable.AppleTree
 required: ?{def megaJouleMsg: ?}
Note that implicit conversions are not applicable because they are ambiguous:
 both method animalToMatter in object Viewable of type [X](animal: X)(implicit evidence$1: X => solutions.Viewable.Animal)solutions.Viewable.Matter
 and method vegetableToMatter in object Viewable of type [X](vegetable: X)(implicit evidence$2: X => solutions.Viewable.Vegetable)solutions.Viewable.Matter
 are possible conversion functions from solutions.Viewable.AppleTree to ?{def megaJouleMsg: ?}
  println(AppleTree("Spartan", 2.3, 26.2, 12).megaJouleMsg)
                   ^

Answer 1

Well the compiler gives the error precisely. This is because there is more than one function which can Animal => Matter . ie your below 3 functions:

  implicit def animalToMatter[X](animal: X) (implicit ev: X => Animal) =  Matter(animal.name,    animal.weight)
  implicit def vegetableToMatter[X ](vegetable: X) (implicit ev: X => Vegetable) = Matter(vegetable.name, vegetable.weight)
  implicit def mineralToMatter[X ](mineral: X) (implicit ev: X => Mineral)  = Matter(mineral.name,   mineral.weight)

are equally eligible to be called when you do Animal("Poodle", 1.0, 8.0).megaJouleMsg .

When you call megaJouleMsg the compiler looks for any implicit function available that can take Animal and return an object containing method megaJouleMsg ( Matter in your case). Now all the 3 functions can take Animal (there is no constraint coded any where) and can return Matter . So the compiler gets confused on which function to calls

Solution : From what it appears, the view bound is not required. This will do:

  implicit def animalToMatter(animal: Animal)  = Matter(animal.name,    animal.weight)
  implicit def vegetableToMatter(vegetable: Vegetable) = Matter(vegetable.name, vegetable.weight)
  implicit def mineralToMatter(mineral: Mineral)       = Matter(mineral.name,   mineral.weight)

  scala> Animal("Poodle", 1.0, 8.0).megaJouleMsg
  res1: String = Poodle's mass-energy equivalence is 716612592013 megajoules.

Edit: Looks like the confusion is because of X <% Animal . In animalToMatter function, it expects an implicit argument which can take X and return Animal . Now if you see in your scope there is no such function X => Animal . But because you are passing Animal("Poodle", 1.0, 8.0) it doesn't need any implicit function because it has already obtained Animal .

In short to repeat the compiler process on seeing Animal("Poodle", 1.0, 8.0).megaJouleMsg :

1. Sees all functions in the current scope which can take Animal and return Matter .
2. All the 3 functions can take Animal and return Matter . Note that vegetableToMatter and mineralToMatter will fail if accepted. ie though they are eligible but they will not succeed because they dont have any implicit function available that can X => Animal
3. Throws Error

As an example consider this:

scala> implicit def f[T <% Int](n:T) = Matter("",1)  
warning: there were 1 feature warning(s); re-run with -feature for details
f: [T](n: T)(implicit evidence$1: T => Int)Matter

scala> 1.megaJouleMsg
res2: String = 's mass-energy equivalence is 89576574002 megajoules.

scala> "a".megaJouleMsg          
<console>:12: error: No implicit view available from String => Int.
              "a".megaJouleMsg          
              ^
<console>:12: error: value megaJouleMsg is not a member of String
              "a".megaJouleMsg      

Notice the error? It gives:

No implicit view available from String => Int.

and not just value megaJouleMsg is not a member of String

Which simply means "a" was eligible for the implicit function f but f could not find a function that could convert it to Int and hence it gives error No implicit view available... . If it was not eligible for function f , then it would just throw error megaJouleMsg is not a member...