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 $username_"
}
val realBeyoncé = new VerifiedTweeter("Beyoncé")
realBeyoncé.tweet("Just spilled my glass of lemonade") // prints "real Beyoncé: Just spilled my glass of lemonade"
Because we said this: User => in trait Tweeter, now the variable username is in scope for the tweet method. This also means that since VerifiedTweeter extends Tweeter, it must also mix-in User (using with User).
trait User:
def username: String
trait Tweeter:
this: User => // reassign this
def tweet(tweetText: String) = println(s"$username: $tweetText")
class VerifiedTweeter(val username_ : String) extends Tweeter, User: // We mixin User because Tweeter required it
def username = s"real $username_"
val realBeyoncé = 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 , User).