Get an implicit instance by class name

Question

What I'm trying to do is: get an implicit instance from the class name. The main problem that I can't get an implicit instance for a class type that created at runtime.

What I have:

trait Base
case class A() extends Base
case class B() extends Base

trait Worker[T <: Base] {
  def foo(t: T): Unit
}

implicit val workerA = new Worker[A] {
  def foo(a: A): Unit =  ??? // do some A specific work
}

implicit val workerB = new Worker[B] {
  def foo(b: B): Unit = ??? // do some B specific work
}

What I want to do: somehow get an implicit instance from the class name.

  trait TypeHolder {
    type Typed <: Base
  }


  def getClassType(className: String): TypeHolder = className match {
    case "A" => new TypeHolder {
      type Typed = A
    }
    case "B" => new TypeHolder {
      type Typed = B
    }
  }

  def getWorker(typeHolder: TypeHolder)(implicit worker: Worker[typeHolder.Typed]): Worker[typeHolder.Typed] = worker

  val className: String = ConfigFactory.load().getString("class-name")
  val worker = getWorker(getClassType(className))

Error: could not find implicit value for parameter worker: Worker[typeHolder.Typed] val worker = getWorker(getClassType(className))

OlivierBlanvillain OlivierBlanvillain · Accepted Answer · 2017-09-15T11:43:37

That's impossible.

Implicit resolution is resolved at compile time: it can't be influence by a runtime value.

Get an implicit instance by class name

1 Answers