Question

My meta-question would be “How do I ask this question intelligibly?” but perhaps an example would make it clearer:

trait Toolbox {
  trait Bolt
  trait Wrench {
    def tighten(b : Bolt) 
  }

  def getWrench : Wrench
  def getBolt : Bolt
}


object MetricToolbox extends Toolbox {
...
}

object EnglishToolbox extends Toolbox {
...
}

Of course, a metric wrench can only tighten a metric bolt; ditto for tools from the English toolbox. My question is, how do I express that in a type-safe way so the following won’t compile:

MetricToolbox.getWrench tighten EnglishToolbox.getBolt

but the following will:

def doTighten(box : Toolbox) = box.getWrench tighten box.getBolt

I know for a fact that this is possible because I heard Martin Odersky say it with his own mouth at the Scala meetup last night but at that moment, my wife called my cell and I had to scurry out of the room before he explained how.

EDIT didierd pointed out that my code worked as written. He’s right — I’m a little unclear on the details but it does. The natural reverse questions is, how do I not do that? The answer was pretty simple: put those classes at the same level as the main trait.

trait Nail
trait Hammer {
  def pound(n : Nail)
}

trait Toolbox {
  trait Bolt
  trait Wrench {
    def tighten(b : Bolt) 
  }

  def getWrench : Wrench
  def getBolt : Bolt
  def getNail : Nail
  def getHammer : Hammer
}

 MetricToolbox.getHammer pound EnglishToolbox.getNail // this DOES compile

Answer 1

As written, it already works. As Bolt and Wrench are nested inside Toolbox, the Bolt that types the argument of tighten means a Bolt of this exact toolbox, not a Bolt of any toolbox. They are written respectively someToolbox.Bolt, which means a Bolt of the exact instance someToolBox and Toolbox#Bolt, which means the Bolt defined in the type Toolbox, whatever the insatance toolbox. So Toolbox#Bolt is a common supertype of all x.Bolt.

Here Bolt without qualification means this.Bolt, this being the enclosing Toolbox. So you cannot mix.