Tour of Scala
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 $username_"
}
val realBeyoncé = new VerifiedTweeter("Beyoncé")
realBeyoncé.tweet("Just spilled my glass of lemonade") // prints "real Beyoncé: Just spilled my glass of lemonade"
real Beyoncé: Just spilled my glass of lemonade
defined trait User defined trait Tweeter defined class VerifiedTweeter realBeyoncé: VerifiedTweeter = ammonite.$sess.cmd0$Helper$VerifiedTweeter@36bbcf25