SKB – Scala Traversable


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 Traversable.

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 Traversable !

Going a bit further in Functional Programming concepts with Traversable

We already saw a few concepts so far such as Functor and Applicative.
Let’s dig deeper !

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 (ntaMRDzYQ3uMMgXbQZcECg).

More information about Scala Traversable

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

Traversable solves a specific problem. How to apply an operation to many inputs at once without leaving the “Box/Container/F” ?

Without Traversable, we would have to perform something like this:

val input: List[Int] = List(1,2,3)
val operation: Int => F[Int] = ???

val output: List[F[Int]] = i => operation(i))

But how do we manage to get F[List[A]] instead of List[F[A]] ? Traversable !

To implement traverse we re-use what we built in the previous episodes: Functor and Applicative. So we can chain the operation and wrap into F.

With traverse which allows you to apply an operation to many element and stay inside F, we also get for free sequence which allow you to flip List[F[A]] inside out into F[List[A]]. You will see it in Future.sequence for instance that is very convenient to wait or combine many Future together.

As a note: is the same as input.traverse(f).

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


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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