SKB – Scala self-referred type


This article is part of the Scala knowledge bits Series.

Periodically, I will publish new exercises so you can slowly build up knowledge about Scala.

It is designed to be done in a very short amount of time and learn a little bit each day, just to create a routine.

This episode will teach you about Scala self-referred type.

Hope you are going to enjoy it! It is designed for anyone to learn Scala from scratch and slowly learn, one Bit at a time.

After this Bit, I would love to hear your feedback in the comments down below.

Feel free to join the Discord server as well if you would like some help and support from the rest of our community.

What are we learning today?

Today we are going to learn about Scala self-referred type !

Let’s dive more into generic type and self-referred type.

The use case is how to use the children Type in the Parent trait safely.

Time to try on the exercise on your own and scroll down for more information when you are done or if you are stuck.


Here is an exercise to complete today.

If I did my job well, you should be able to guess by yourself the solution based on what you previously learned and based on the clues.

But if you get stuck, scroll down to get more information.

The goal of the exercise is to replace the ??? by a piece of code so that the exercise compiles and that’s how you win! Good luck!

You can fill the exercise right in here:

Or, if it does not load, go on to Scastie (9fDW3IfPTParnRzONDXkkA).

More information about Scala self-referred type

In this exercise you will learn (or have learned, if you have already solved the puzzle) about Scala self-referred type.

Sorry for the long exercise but I really wanted to show step by step how we end up with this solution based on the problem encountered.

In the first example, without doing anything, we are able to change the output type to the child type when we override the method but not the input type.

In the second example we now have information about the child type from inside the parent trait but there are not guarantee that A is the right children, it could be any children.

With the final solution, A must be in used in the child type which guarantee that it is the child.

Let’s summaries the syntax:

trait Name[A <: Name[A]] { this: A => 

Hopefully that will help you make the compiler your friend and catch more mistake at compile time rather than at runtime !

Feel free to go back to the exercise, modify the code to try out new things and get a better intuition for Scala self-referred type.


I hope you have learned something new or had fun during this Scala Knowledge Bit.

Please ask questions or post feedback in the comments below.

Feel free to try on the next Scala Knowledege Bit.

If you are curious about the previous Scala knowledge Bits, go check it out! 🙂

Leave a Reply

%d bloggers like this: