Implicit parameter resolution for higher kinded types

Question

Consider the following code:

object foo {

    trait Bar[Q[_]]

    implicit object OptionBar extends Bar[Option]

    def test[T, C[_]](c: C[T])(implicit ba         


        

Answer 1

That's because Some(42) is a more specific type than Option[Int]. It is a Some[Int]. See alternative coding below:

object foo {

    trait Bar[Q[_]]

    implicit object OptionBar extends Bar[Option]

    def test[T, C[_]](c: C[T])(implicit bar: Bar[C]) = ()

    def main(args: Array[String]) {
      test(Option(42))
    }
}

Answer 2

It's not going to work in all cases, but as stated, you can try this:

object foo {
  trait Bar[Q[_]]

  implicit object OptionBar extends Bar[Option]

  def test[T, C[_], D](c: D)(implicit bar: Bar[C], ev: D <:< C[T]) = ()

  def main(args: Array[String]) {
    test(Some(42)) //???
  }
}

Interestingly, this doesn't infer, although it expresses the same thing:

def test[T, C[_], D <: C[T]](c: D)(implicit bar: Bar[C]) = ()

To learn more about <:<, see:

• What do <:<, <%<, and =:= mean in Scala 2.8, and where are they documented?
• <:< operator in scala