Mixins in Scala - scala

Question

I'm trying to create a trait in Scala:
trait First {
override def greet() {
super.greet
println("First")
}
}
but compiler says:
scala/scala_learning/traits_hierarchy.scala:3: error: value greet is not a member of java.lang.Object with ScalaObject
super.greet
^
But I would like to extend with this trait some class which superclass has method greet... Is it possible in scala?

Answer 1

While there are structural types in scala, I don't think you can do that for any type that has a greet method, but for a greet method in a specific trait or class.
Then if you want to have your routine change a routine not yet defined, calling it with super, it must be tagged abstract override, not simply override.
That would be
trait Greeter { def greet }
trait First extends Greeter {
abstract override def greet = {
super.greet
println("Hi, I'm first")
}
}
Then you can have greet definition
class StandardGreeter(greeting: String) extends Greeter {
def greet = println(greeting)
}
Note that First was defined without knowledge of StandardGreeter. You mix in new classes with
class FirstGreeter(greeting: String) extends StandardGreeter(greeting) with First
or create instances directly with
new StandardGreeter("whatever") with First
You may want to read this article.

Answer 2

I believe you can do this with structural self types:
trait Foobar {
this: { def greet() } => // self type
def go() = greet()
}
class Barbar extends Foobar { def greet() = { println("hello") }}
You are defining that Foobar can only extend a class which defines a method greet(). Using the REPL:
scala> trait Foobar { this: { def greet() } =>
| def go() = greet()
| }
defined class Foobar
scala> new Foobar()
<console>:9: error: class Foobar cannot be instantiated because it does not conform to its self-type Foobar with AnyRef{def greet(): Unit}
new Foobar()
^
scala> class Barbar extends Foobar { def greet() = { println("hello") }}
defined class Barbar
scala> new Barbar().go
hello
You can then override the greet method in Foobar (but what use this is I don't know):
trait Foobar {
this: { def greet() } => // self type
override def greet() = { println("mygreet") }
def go() = greet()
}
class Barbar extends Foobar