StackableTraitPattern.scala

// cf. http://www.artima.com/scalazine/articles/stackable_trait_pattern.html

trait Request {
  type Response
}

trait Context {
  def handle[R](req: Request { type Response = R }): R
}

object Foo extends Request {
  case class Response(i: Int)
}

object Bar extends Request {
  case class Response(s: String)
}

trait HandlesFoo extends Context {
  abstract override def handle[R](req: Request { type Response = R }): R = 
    req match {
      case Foo => Foo.Response(123)
      case _   => super.handle(req)
    }
}

trait HandlesBar extends Context {
  abstract override def handle[R](req: Request { type Response = R }): R = 
    req match {
      case Bar => Bar.Response("456")
      case _   => super.handle(req)
    }
}

// not possible:
// object Attempt1 extends HandlesFoo

trait FinallyFails extends Context {
  def handle[R](req: Request { type Response = R }): R = 
    sys.error(s"Unsupported request $req")
}

object Attempt2 extends FinallyFails with HandlesFoo with HandlesBar

object Baz extends Request {
  type Response = Unit
}

import scala.util.Try

Try(Attempt2.handle(Baz))

Attempt2.handle(Foo)
Attempt2.handle(Bar)