Implicit parameter resolution - setting the precedence

问题

I am trying to create a typeclass Default that supplies the default value for a given type. Here is what I have come up with so far:

trait Default[A] {
  def value: A
}

object Default {
  def withValue[A](a: A) = new Default[A] {
    def value = a
  }

  def default[A : Default]: A = implicitly[Default[A]].value

  implicit val forBoolean = withValue(false)

  implicit def forNumeric[A : Numeric] = 
    withValue(implicitly[Numeric[A]].zero)

  implicit val forChar = withValue(' ')

  implicit val forString = withValue("")

  implicit def forOption[A] = withValue(None : Option[A])

  implicit def forAnyRef[A >: Null] = withValue(null : A)
}

case class Person(name: String, age: Int)

case class Point(x: Double, y: Double)

object Point {
  implicit val pointDefault = Default withValue Point(0.0, 0.0)
}

object Main {
  def main(args: Array[String]): Unit = {
    import Default.default
    println(default[Int])
    println(default[BigDecimal])
    println(default[Option[String]])
    println(default[String])
    println(default[Person])
    println(default[Point])
  }
}

The above implementation behaves as expected, except for the cases of BigInt and BigDecimal (and other user defined types that are instances of Numeric) where it gives null instead of zero. What should I do so that forNumeric takes precedence over forAnyRef and I get the behavior I expect?

回答1:

The forAnyRef implicit is chosen because it is more specific than forNumeric according to §6.26.3 “Overloading Resolution” of the Scala reference. There is a way to reduce its priority by moving it to a trait that Default extends, like this:

trait LowerPriorityImplicits extends LowestPriorityImplicits {
  this: Default.type =>

  implicit def forAnyRef[A >: Null] = withValue(null: A)

}

object Default extends LowerPriorityImplicits {
  // as before, without forAnyRef
}

But that's only part of the trick, because now both forAnyRef and forNumeric are as specific as each other, and you'll get an ambiguous-implicit error. Why is that? Well, forAnyRef gets an extra specificity point because it has a non-trivial constraint on A: A >: Null. What you can do then, to add a nontrivial constraint to forNumeric, is to double it in Default:

implicit def forNumericVal[A <: AnyVal: Numeric] = withValue(implicitly[Numeric[A]].zero)

implicit def forNumericRef[A <: AnyRef: Numeric] = withValue(implicitly[Numeric[A]].zero)

Now, this additional constraint makes forNumericVal and forNumericRef more specific that forAnyRef for types where a Numeric is available.

回答2:

Here is another way to solve the problem, doesn't require any code duplication:

trait Default[A] {
  def value: A
}

object Default extends LowPriorityImplicits {
  def withValue[A](a: A) = new Default[A] {
    def value = a
  }

  def default[A : Default]: A = implicitly[Default[A]].value

  implicit val forBoolean = withValue(false)

  implicit def forNumeric[A : Numeric] = 
    withValue(implicitly[Numeric[A]].zero)

  implicit val forChar = withValue(' ')

  implicit val forString = withValue("")

  implicit def forOption[A] = withValue(None : Option[A])
}

trait LowPriorityImplicits { this: Default.type =>
  implicit def forAnyRef[A](implicit ev: Null <:< A) = withValue(null : A)
}

case class Person(name: String, age: Int)

case class Point(x: Double, y: Double)

object Point {
  implicit val pointDefault = Default withValue Point(0.0, 0.0)
}

object Main {
  import Default.default

  def main(args: Array[String]): Unit = {
    println(default[Int])
    println(default[BigDecimal])
    println(default[Option[String]])
    println(default[String])
    println(default[Person])
    println(default[Point])
  }
}

来源：https://stackoverflow.com/questions/6966825/implicit-parameter-resolution-setting-the-precedence

标签

scala

implicit