SKB – Scala Foldable


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 Foldable.

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 Foldable !

After learning about Functor, the next piece with no dependency is Foldable.

Similarly to Functor, this concept is taken from Category Theory and used in Functional Programing.

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 (q7avzZbtSreqhyrrdrp1kA).

More information about Scala Foldable

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

The operation fold allows is an aggregation. It takes a starting element and combine it with the Foldable type following the recipe provided by the method f.

Fold can the used to implement reduce. You should try it ! The difference with reduce is that the start element is either the identity of the operation provided in f, meaning an element that does not change the value, for instance empty string "" for the string concatenation operation or the 0 for the + operation in Int type. It is possible to implement a version of reduce where the start element is simply the first element to be combined in the fold if you have access to a function head for instance. You will have to handle cases when the Foldable provided is empty too.

As you can see fold implementation is relying on recursion. We talked about recursion in a previous episode.

Now you have seen Functor and Foldable !

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


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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