SKB – Scala List Flatten


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 List flatten.

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 List flatten !

Do you remember the map, from few SKBs ago ?

As a reminder, map allows you to transform a List[A] into a List[B].

What will happen if B is a List as well ?

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 (fGhtslb9TiiTQ5brcXpgLg).

More information about Scala List flatten

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

flatten allows you to turn a List[List[A]] into a List[A].

Try to experiment with other containers such as Option we saw in the past.

What happen if you do

val a: Option[Int] = Some(12)

val aa: Option[Option[Int]] = => Some(x))

val flattenA: Option[Int] = aa.flatten

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


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! 🙂

2 thoughts on “SKB – Scala List Flatten”

  1. flatten is indeed an interesting topic – one cool thing is that flatMap is just map followed by flatten, i.e. (ma flatMap f) == (ma map f flatten)


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