Difference between this and self in self-type annotations?

Question

In various Scala literature I see some self-type annotations using \"this\" and others using \"self\":

trait A { this: B => ... }
trait A { self: B =>          


        

Answer 1

There is a difference in that this always refers to the object defined by the innermost template.

The expression this can appear in the statement part of a template or compound type. It stands for the object being defined by the innermost template or compound type enclosing the reference. If this is a compound type, the type of this is that compound type. If it is a template of a class or object definition with simple name C, the type of this is the same as the type of C.this. (Scala Ref. §6.5)

So, if you call your self-type foo, you could still refer to it as this (unless, of course, you are in an inner template in which case this will refer to the object defined by it – and unless you don’t give the inner template’s self-type the same name) but obviously not the other way round.

Answer 2

All three forms are valid, and have the effect that B is assumed as the type of this in class A.

The first two variants

trait A { self: B => ... }
trait A { foo: B => ... }

introduce self (respectively, foo) as an alias for this in trait A. This is useful for accessing the this reference from an inner class. I.e. you could then use self instead of A.this when accessing the this reference of the trait A from a class nested within it. Example:

class MyFrame extends JFrame { frame =>    
  getContentPane().add( new JButton( "Hide" ) {
    addActionListener( new ActionListener {
      def actionPerformed( e: ActionEvent ) {
        // this.setVisible( false ) --> shadowed by JButton!
        frame.setVisible( false )
      }
    })
  })
}

The third variant,

trait A { this: B => ... }

does not introduce an alias for this; it just sets the self type.