What is the difference between self-types and trait subclasses?

Question

A self-type for a trait A:

trait B
trait A { this: B => }

says that \"A cannot be mixed into a concrete cl

Answer 1

Self types allow you to define cyclical dependencies. For example, you can achieve this:

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

Inheritance using extends does not allow that. Try:

trait A extends B
trait B extends A
error:  illegal cyclic reference involving trait A

In the Odersky book, look at section 33.5 (Creating spreadsheet UI chapter) where it mentions:

In the spreadsheet example, class Model inherits from Evaluator and thus gains access to its evaluation method. To go the other way, class Evaluator defines its self type to be Model, like this:

package org.stairwaybook.scells
trait Evaluator { this: Model => ...

Hope this helps.