Scala's "This" and MapLike

Say I wanted to extend Scala's MapLike trait with a concrete implementation, IntIntM开发者_StackOverflow社区ap. In order to do so, I need to implement the following methods,

def get(key: A): Option[B]
def iterator: Iterator[(A, B)]
def + [B1 >: B](kv: (A, B1)): This
def -(key: A): This

What is the This type? Should my overriding method signature be,

override def +=(kv: (Int, Int)): IntIntMap = {
  // logic
}

Or just scala.reflect.This? What about the class definition? Should it be,

class IntIntMap(...) extends MapLike[Int,Int,This] { ... }

or something else entirely?

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You should extend MapLike[Int, Int, IntIntMap], and This becomes IntIntMap.

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You should have a look first at The Architecture of Scala Collections. It shows you how to integrate you own collections.

However, you will have some trouble implementing the + method in you example, because it should allow to add values matching a supertype of Int and returning an appropriate Map. Since This should be a Map[Int,Int] you will run into trouble.

I would rather recommend to just have a type definition somewhere:

type MapIntInt = Map[Int,Int]

and eventually use implicits to bring specific methods.

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Not a reserved word, not the name of some class, but the third type parameter in MapLike. Type declaration is MapLike[K, +V, +This <: MapLike[K, V, This]. It could be called any other way. Most of the time, the This should be the actual implementor class, hence the name. in your case

class IntIntMap extends MapLike [Int, Int, IntIntMap]. 

Without that parameter, + result type would have be declared MapLike, not IntIntMap. On +, that would not be a problem, because you have to define it, and doing that you can change the result type. But methods that you don't need to redefine and which are implemented using + (such as the ++ would still return MapLike.