Implicit resolution from trait

Question

I have a use case similar to the situation following:

trait A {
  implicit val x = "hello"
}

class B {

  // somehow bring x into scope here???

  def run(x: Int)(implicit y: String) = y + x
}

println((new B).run(3))

I understand that I need to bring x defined in the trait in the implicit scope of B. I've already tried the following:

# attempt 1 #
class B extends A { .... } /// doesn't work

# attempt 2 #
class B extends A {

  val x1 = implicitly[String]  /// doesn't work either

  def run(x: Int)(implicit y: String) = y + x
}

Please explain what am I missing here (or, point me to relevant theory topic which I can study, fairly new to scala).

thoredge · Accepted Answer · 2015-06-26 12:49:12Z

2

The value of 'implicit y' will be resolved in your println-line where it is not available. You are making the variable implicitly available within the body of the class, but resolution of implicit String is not needed there.

Implicit isn't magic; if you can't reach the implicit variable explicitly then so can't the compiler.

What problem are you really trying to solve?

edited Jun 26, 2015 at 12:49

answered Jun 26, 2015 at 12:40

thoredge

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Comments

curious · Accepted Answer · 2015-06-26 12:20:26Z

2

Wrap your whole code in a object and extend trait A in class B :

  object P {
    trait A {
      implicit val x = "hello"
    }
    class B extends A {
      def run(y: Int) = y + x
    }
   def f = println(new B().run(3))
  }

Output :

scala> P.f
3hello

answered Jun 26, 2015 at 12:20

curious

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2 Comments

gashu Over a year ago

this works. Although if it's not too much, can you explain why does this work?

curious Over a year ago

Why is answered by @thoredge in his ans.

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