A Tour of Scala: Implicit Parameters

A method with implicit parameters can be applied to arguments just like a normal method. In this case theimplicit label has no effect. However, if such a method misses arguments for its implicit parameters, such arguments will be automatically provided.

The actual arguments that are eligible to be passed to an implicit parameter fall into two categories:

• First, eligible are all identifiers x that can be accessed at the point of the method call without a prefix and that denote an implicit definition or an implicit parameter.
• Second, eligible are also all members of companion modules of the implicit parameter's type that are labeled implicit.

In the following example we define a method sum which computes the sum of a list of elements using the monoid's add and unit operations. Please note that implicit values can not be top-level, they have to be members of a template.

    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")))
    }
  

Here is the output of the Scala program:

6
abc