scala implicit

implicit 可用于变量 伴生对象 参数 方法

object Math {
  trait NumberLike[T] {
    def divide(x: T, y: Int): T
  }
  object NumberLikes {
    implicit object NumberLikeDouble extends NumberLike[Double] {
      def divide(x: Double, y: Int): Double = {
        println("o-------------")
        x / y
      }
    }
  }
  class Number extends NumberLike[Double] {
    def divide(x: Double, y: Int): Double = {
      println("t-------------")
      123.0
    }
  }
  implicit val test = new Number
}


object Statistics {
  import Math.NumberLike
  def mean[T](xs: Vector[T])(implicit ev: NumberLike[T]): T = {
    println(ev.isInstanceOf[NumberLike[T]])
    ev.divide(xs.reduce(ev.plus(_, _)), xs.size)
  }
}

测试对于mean的隐含参数，伴生对象的优先级高于变量，还有貌似泛型的类型可以根据参数推断，所以可以省略