this: SomeObject => object construct

Question

I was reading up on best practices for implementing Slick, and was examining this example. In it, there is this construct:

trait BankRepository extends BankTable { this: DBComponent =>
   ... //A bunch of code
}

I don't understand the this: DBComponent => part. In this case, DBComponent is a simple trait defined elsewhere (you can find it in the above link). What I don't understand is:

• What does the this: DBComponent => construct do. My IDE doesn't complain, but it also doesn't link to the function being executed by the =>. My intuition is that it's saying the rest of the code is a value that is returned, but I'm not clear on what is invoking it, or what the value returned exactly.
• What do I even call this construct? As with many symbol-heavy constructs it's hard to look up/find documentation of, because it's clearly dependent on context. But even describing the context is difficult. What is this construct called?

Answer 1

It's called a self type. It's basically a contract that says any class extending this trait (mixing it in) must include DBComponent. And, as such, the compiler should assume DBCompenent elements are in scope for the following code.

Here's a link to a description of it from Programming in Scala, Odersky et al, 1st Edition (a little dated but still accurate on most topics).