dependent_type_question.scala

sealed trait Key {
    type Inner
    def inner: Inner
}

trait IntKey extends Key {
    type Inner = Int
}
trait StringKey extends Key {
    type Inner = String
}

object Key {
  def work(m: Key): m.Inner = m match {
   case i: IntKey => i.inner
   case s: StringKey => s.inner
  }
}

Pretty neat compiler error message:

<console>:12: error: type mismatch;
 found   : i.scala.Inner
    (which expands to)  Int
 required: m.Inner
          case i: IntKey => i.inner
                              ^
<console>:13: error: type mismatch;
 found   : s.java.lang.Inner
    (which expands to)  String
 required: m.Inner
          case s: StringKey => s.inner
                                 ^