Scala variable scoping question

Question

I have a scala syntax question - say I have a simple dependency pattern construct like the following

trait Master {
  val foobar

  object SubObject extends SubObject {
      foobar = foobar
  }
}

trait SubObject {
  val foobar
}

Obviously, this will not compile, since the reference foobar = foobar is ambiguous.

How do I specify that the RHS of the expression should reference Master's foobar variable? Is there some sort of special usage of 'this' or 'self' that I should know about?

Andrzej Doyle · Accepted Answer · 2011-05-23 16:13:49Z

6

You can use the Master.this qualifier to specifically reference the outer scope, something like the following:

trait Master {
  val foobar = "Hello world"

  object SubObject extends SubObject {
      val foobar = Master.this.foobar
  }
}

trait SubObject {
  val foobar:String
}

answered May 23, 2011 at 16:13

Andrzej Doyle

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Dave Griffith · Accepted Answer · 2011-05-23 16:16:18Z

5

I believe the easiest way is to use a self-type definition. In addition to a bunch of cool type-theoretic effects, you can use a self-type to create an alias for "this". (Haven't tested this)

trait Master {
  master =>
  val foobar

  object SubObject extends SubObject {
      foobar = master.foobar
  }
}

trait SubObject {
  val foobar
}

answered May 23, 2011 at 16:16

Dave Griffith

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3 Comments

Dave Griffith Over a year ago

Hmm, perhaps not the easiest way, and you should probably give the check-mark to Andrzej's solution, but you should know about the self-type as alias trick as well. Comes in handy in some otherwise tricky edge-cases

dpratt Over a year ago

I initially started using self types and a simple cake pattern, and I'll likely end up back at that eventually, but I wanted to have a simple Environment object that creates everything for me and doesn't necessarily have to be mixed in to everything that needs it.

Dave Griffith Over a year ago

Also, I don't necessarily like to use "object" for this sort of dependency injection form (or, indeed, for anything other than companion objects). Slightly less verbose to say something like "lazy val subObject = new SubObject{ foobar = master.foobar}"

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