Self-type

Self-types are a way to declare that a trait must be mixed into another trait, even though it doesn’t directly extend it. That makes the members of the dependency available without imports.

A self-type is a way to narrow the type of this or another identifier that aliases this. The syntax looks like normal function syntax but means something entirely different.

To use a self-type in a trait, write an identifier, the type of another trait to mix in, and a => (e.g. someIdentifier: SomeOtherTrait =>).

trait User {
  def username: String
}

trait Tweeter {
	this: User =>  // reassign this
	def tweet(tweetText: String) = println(s"$username: $tweetText")
}

class VerifiedTweeter(val username_ : String) extends Tweeter with User {  // We mixin User because Tweeter required it
	def username = s"real $username_"
}

val realBeyoncé = new VerifiedTweeter("Beyoncé")
realBeyoncé.tweet("Just spilled my glass of lemonade")  // prints "real Beyoncé: Just spilled my glass of lemonade"

Because we said this: User => in trait Tweeter, now the variable username is in scope for the tweet method. This also means that since VerifiedTweeter extends Tweeter, it must also mix-in User (using with User).