Question

Is there any relationship between this aliasing and self type?
Is this aliasing a special case of self type?
In programming in scala 2nd P776, the author said:

abstract class Parser[+T] extends … { p =>

You saw syntax like this in Section 29.4, where it was used to give a
self type to a trait.

but, the syntax for self type doesn’t look like this, it’s like:

this:SomeAssumedType =>

And another question is why this aliasing is useful? I can’t see there’s any sense to give this reference an alias, because it’s already a conventional alias for the current object reference, however in the Play framework source code, I saw lots of codes (especially, the anorm part) like:

trait RowParser[+A] extends (Row => SqlResult[A]) {

parent =>

Why does this make sense?

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