Scala Self Traits

[Joe San]

Why would the Wrong trait result in a compile time failure in the following code?

scala> trait User { def name: String }
defined trait User

scala> trait Tweeter {
     | user: User =>
     | def tweet(msg: String) = println(s"$name: $msg")
     | }
defined trait Tweeter

scala> trait Correct extends Tweeter with User { }
defined trait Correct

scala> trait Wrong extends Tweeter
<console>:9: error: illegal inheritance;
 self-type Wrong does not conform to Tweeter's selftype Tweeter with User
       trait Wrong extends Tweeter
                           ^

scala> 

Does it mean that I always have to extend Tweeter with a User?

[Joe San]

I'm slowly trying to get a hang of it. In essence this is Scala's native way of doing DI! But what I don't get it:

Tweeter has-a User, then why would one need to mix User in when extending Tweeter? Scala's language features just seem like an abyss!

[Joe San]

The rule is anybody that extends Tweeter (trait, abstract class, object, class) should either declare User as a self type as declared in Tweeter or bloody mix it in!

[Sidharth Khattri]

Self type annotation is used for dependency injection for traits which basically what Cake Pattern is.
Consider it as Tweeter needs an implementation of User instead of considering it as Tweeter has-a User. So, whenever you would like to use Tweeter, you'd have to provide an implementation of User.

[Jesper de Jong]

The self-type annotation in your trait Tweeter does not mean that Tweeter has-a User, it says that Tweeter is-a user, or rather that anything that extends Tweeter must also be a User.

It puts a restriction on traits and classes that extend Tweeter; the restriction is that those must also extend User.

You can use this for mix-ins. So, Tweeter is a trait that adds extra functionality for things that already extend User.