Why self instead of this in self reference?

Question

I understand what the result of a self type is as in

trait SpellChecker {
  self: RandomAccessSeq[char] =>
  ...
}

from http://www.markthomas.info/blog/92

but I did not understand why it is better to use self instead of this here...!? Also, if I write asfd instead of self I also don't get a compiler error... so I am not quite sure "self" is all about. I don't see that it is possible to use self like an object in one of the methods of the trait later.

Answer 1

self is an alias for your SpellChecker instance. This is useful if you have nested structures (like classes). Consider this:

trait Foo {
  self =>

  override def toString : String = "Foo here"

  class Bar {
    def print() : Unit = {
      println(s"this = $this")
      println(s"self = $self")
    }
    override def toString : String = "Bar here"
  }
}

val foo = new Foo {}
val bar = new foo.Bar()
bar.print()

Outputs:

this = Bar here 
self = Foo here

So if you want to reference the outer element you can use your alias. Using this means no alias, so this : Example => means "I dont need an alias for this, I just want to make sure this is mixed with Example". And you can name your alias however you like asfd is as fine as iLikeFries. self is just somewhat convention.