Scala this aliasing and self type

Question

Is there any relationship between this aliasing and self type? Is this aliasing a special case of self type? In programm

Answer 1

You can have a self-type and this aliasing at the same time:

abstract class Parser[+T] { p: SomeAssumedType => … }

If you don’t include a type ascription, Scala will assume that the type of the variable is the type of the surrounding class, thus giving you a simple alias for this.

If you keep the name this with the ascription, then Scala expects you to initialise this class in a way that the ascription can be fulfilled.

As for the this aliasing. Here’s the situation in which this is needed:

object OuterObject { outer =>
  val member = "outer"
  object InnerObject {
    val member = "inner"
    val ref1 = member
    val ref2 = this.member
    val ref3 = outer.member

    def method1 = {
      val member = "method"
      member
    }
    def method2 = {
      val member = "method"
      this.member
    }
    def method3 = {
      val member = "method"
      outer.member
    }
  }
}

scala> OuterObject.InnerObject.ref1
res1: java.lang.String = inner

scala> OuterObject.InnerObject.ref2
res2: java.lang.String = inner

scala> OuterObject.InnerObject.ref3
res3: java.lang.String = outer

scala> OuterObject.InnerObject.method1
res4: java.lang.String = method

scala> OuterObject.InnerObject.method2
res5: java.lang.String = inner

scala> OuterObject.InnerObject.method3
res6: java.lang.String = outer