# SKB – Scala extractor pattern

## 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 extractor pattern.

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 extractor pattern !

Today we are going to combine several things we know to learn about extractor pattern.

It might be a bit complicated but I am sure you are going to be fine !

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 (m2wkxempSUqHNni7KEz1Zg).

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

The goal here is to be able to only keep the element of the `List` that are of a given type.

We start from a list of type A : `List[A]`. And we want to keep the element of type B, with B being the child class of A. We saw few exercise ago the syntax : `B <: A`.

We also saw the `implicit` proof using the syntax: `A : PROOF`. In this case we want `B` to be a `ClassTag` as well as being a child of `A`. Which is why we now have this syntax: `B <: A: ClassTag`.

Have you tried removing the `ClassTag` ? You are seeing a `Warning` talking about `abstract type B is unchecked since it is eliminated by erasure`. We are going to dive deeper into this in another episode. But for now, you only need to know that to mitigate the "type erasure" problem, we need to introduce a proof to be `ClassTag`.

Now that we have our extractor, we can filter down the `List[Animal]` to only keep the `Cat` or the `Dog` !

I am sure you are going to need this pattern in one of your project one day ! Just let us know on the Discord server how that has been useful to you !

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

## Conclusion

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