Sam Stites

Self Types in Traits

April 25, 2015

25 April 2015 - Mountain View

Last time we learned that a scala trait might extend a class. Well — that might offer up a little confusion when the superclass for any of those mixins references when looking at self references. For this reason, scala has self types.

When a trait starts with this:_Type_ => then it can only be mixed into a subclass of the given _Type_. This is similar to a trait with a supertype, when invocations to super were post-fixed with the supertype in brackets (super[_SuperType_].method).

The self type can also be used to specify a structural type — a type that specifies methods a class has without specifying the class itself. Both will compile:

trait Bar { def bar(x: String) }

trait Foo {
  this: Bar =>
  def log(y: String) {}
}

trait Foo {
  this: { def bar(x: String) } =>
  def log(y: String) {}
}