Scala之旅 10 隐参数

一个带有隐参数的方法能够像正常方法一样调用，增加的implicit关键字没有任何作用。如果一个方法没有指明隐参数，那么一个隐含参数将自动提供。

下面示例定义了一个方法，通过add和unit来计算一个列表的元素之和。需要注意的是隐含值不能是top-level的，应该是模板的成员。

abstract class SemiGroup[A] {
  def add(x: A, y: A): A
}
abstract class Monoid[A] extends SemiGroup[A] {
  def unit: A
}
object ImplicitTest extends Application {
  implicit object StringMonoid extends Monoid[String] {
    def add(x: String, y: String): String = x concat y
    def unit: String = ""
  }
  implicit object IntMonoid extends Monoid[Int] {
    def add(x: Int, y: Int): Int = x + y
    def unit: Int = 0
  }
  def sum[A](xs: List[A])(implicit m: Monoid[A]): A =
    if (xs.isEmpty) m.unit
    else m.add(xs.head, sum(xs.tail))

  println(sum(List(1, 2, 3)))
  println(sum(List("a", "b", "c")))
}

运行结果为：

6

abc

注意sum方法，  

def sum[A](xs: List[A])(implicit m: Monoid[A]): A =
    if (xs.isEmpty) m.unit
    else m.add(xs.head, sum(xs.tail))

一种迭代的方法,sum(xs.tail)计算除链表头以外的链表之和。

  /** Selects all elements except the first.
   *  $orderDependent
   *  @return   a $coll consisting of all elements of this $coll
   *           except the first one.
   *  @throws `UnsupportedOperationException` if the $coll is empty.
   */
  def tail: Repr = {
    if (isEmpty) throw new UnsupportedOperationException("empty.tail")
    drop(1)
  }

  /** Selects all elements except first ''n'' ones.
   *  $orderDependent
   *  @param  n    the number of elements to drop from this $coll.
   *  @return  a $coll consisting of all elements of this $coll except the first `n` ones, or else the
   *          empty $coll, if this $coll has less than `n` elements.
   */
  def drop(n: Int): Repr = {
    val b = newBuilder
    if (n >= 0) b.sizeHint(this, -n)
    var i = 0
    for (x <- this) {
      if (i >= n) b += x
      i += 1
    }
    b.result
  }