# SKB – Scala List of Option flatten

## Introduction

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.

## Exercise

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.

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
.filter{
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`.

## Conclusion

I hope you have learned something new or had fun during this Scala Knowledge Bit.