SKB – Scala List of Option 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 of Option 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 of Option flatten !

Recently ( as on December 2020 ), we have talked about List of Option on the Discord server so I thought we would dive in deeper here.

This specific flatten operation is not well documented but extremely useful in a lost of use cases. Today we are simply going to see how it works and later on we are going to see in what cases it can be useful.

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 (WU5TCyfhTLKRPcvMccymZg).

More information about Scala List of Option flatten

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

We saw it before but flatMap(f) is the same as .map(f).flatten. The flatten operation will combine two layers of container ( monad ) into one. So it will combine List[List[A]] into List[A] or Option[Option[A]] into Option[A] for instance.

In this episode, we are looking at what happen in the specific case of List[Option[A]] when it is flatten.

List[Option[A]] will be turned into List[A] by the flatten operation.

It might be interesting to note that Option[List[A]].flatten triggers an error.

But this answer raise a new question, what happen to the Option elements in the list !?

Remember that Option has two possible case. The first one is Some when there is an element, those will simply be opened up and they will be transformed from Some[A] to A. For the second case, None, they will be removed from the list. So you will end up with potentially less element that you started with in the list after performing the flatten operation.

It would be equivalent of doing this:

val l: List[Option[A]] = ???

val r: List[A] = l 
    case Some(_) => true
    case None => false
  .map {
     case Some(n) => n
     case None => throw new Exception("Cannot happen")

Feel free to go back to the exercise, modify the code to try out new things and get a better intuition for Scala List of Option 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! 🙂

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