SKB – Scala Either


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 Either.

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 Either !

Either, you will see, is kind of a in-between Try and Option.

Try to experiment to see the similarity and differences.

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 (bzjEzMzhQmSOiJm2bejFkw).

More information about Scala Either

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

Similar to Option, you can create Either in two ways:

  • Right which would be similar to Some
  • Left which, over time, became similar to None, except you can store information. I am saying “over time” because up until Scala v2.12+, Either was not Right-bias. Left and Right were just two Types. But now, Left is accepting to carry error messages and Right to be the channel for successes.

You can test which Type is Either using isLeft and isRight. You can also use map, flatMap, etc… once again. You must be starting to know those functions pretty well by now. This is where the Right-bias come into play. map will take a function that modify the type contained in Right and will not do anything if the Either contains a Left.

If you would like to specifically act on Left or Right, you can use the .left and .right methods to project the Either on one side of the other. For instance, to modify the content of the Left when it is there, you can do

One major difference with Try is that Either will not catch the Exception. If an exception is thrown inside an Either it will propagate. Inside a Try it will be captured inside the Failure channel.

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


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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