SKB – Scala constraint inheritance


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 constraint inheritance.

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 constraint inheritance !

More inheritance today, more specifically how to get help from the compiler in complex inheritance patterns

The example here is simple to illustrate the feature but imagine what it would look like in a much more complex project.

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 (1rovwDJFTRaHBS7e2c2ofg).

More information about Scala constraint inheritance

In this exercise you will learn (or have learned, if you have already solved the puzzle) about Scala constraint inheritance.

We saw multiple inheritance last time, but it can quickly become messy if a lot of your trait are depending on each other.

It would be great if we could ask the compiler to remind us that two or more trait needs to always be together for the code to work well.

Well, guess what? It is !

It is possible in Scala to create a trait that cannot be used if not mixed in with a selection of other trait. Let’s look at the syntax:

trait MyTrait { name: DependencyTrait => 

The name can be any variable name if you need to disambiguates two or more trait that might have same function names. It can also be this if you do not need an extra handler for it.

And DependencyTrait can be one trait that is required to create MyTrait but it can more than one if you replace it by trait1 with trait2 with trait3 ....

In case of failure to comply those constraint, the compiler will give you the following error:

illegal inheritance;
 self-type [Class you are working on] does not conform to [MyTrait]'s selftype [MyTrait] with [DependencyTrait]

You should have seen this error message in the exercise and you now know how to fix it.

You also now know how to befriend the compiler and making sure you don’t forget a critical trait in the mix for your classes.

Feel free to go back to the exercise, modify the code to try out new things and get a better intuition for Scala constraint inheritance.


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! 🙂

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