Referring to the type of an inner class in Scala

Question

The following code tries to mimic Polymorphic Embedding of DSLs: rather than giving the behavior in Inner, it is encoded in the useInner method of

Answer 1

The problem is as you describe, that useInner is expecting an Inner of a specific Outer instance. Since enclosing returns a generic Outer, there is really no way to tie both together that I know of.

You can force it, however:

def toBoolean(x: Outer#Inner): Boolean = {
  val outer = x.enclosing
  outer.useInner(x.asInstanceOf[outer.Inner])
}

Answer 2

You can also define your member like this:

def useInner(x:Outer#Inner) : Boolean

Or you can write like this:

abstract class Outer {
    class InnerImpl {
        def enclosing = Outer.this
    }
    final type Inner = Outer#InnerImpl
    def useInner(x:Inner) : Boolean
}

Answer 3

I suppose the type Inner is like the type this.Inner. Outer#Inner is independent of the outer instance (not a path-dependent type).

abstract class Outer {
  sealed class Inner {
    def enclosing = Outer.this
  }
  def useInner(x:Outer#Inner) : Boolean
}

def toBoolean(x:Outer#Inner) : Boolean = x.enclosing.useInner(x)