SKB – Scala for-comprehension


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 for-comprehension.

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 for-comprehension !

Using a lot of map and flatMap can make the code very hard to read as it goes into deep functions of functions.

Scala has a way of handling those cases. It is called a for-comprehension.

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 (Ghu5hkwTQsyazXKdcNujTw).

More information about Scala for-comprehension

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

A for-comprehension allow you to chain map and flatMap together in an easy to read form.

For instance, those two snippets of code are equivalent:

for { n <- list } yield { n + 1 }
and n => n + 1 )

You can also filter the input using for-comprehension.

for {
    n <- list
   if n == 2
} yield {
This snippet is equivalent to:
input.withFilter(n => n == 2)

In general, everything you can do with pattern matching, you can do within a for-comprehension. The left side of the <- behave similar to a pattern matching.

Sometimes, it can be hard to convert in your head back and forth between for-comprehension and map and flatMap modes. Some IDEs, such as IntelliJ, starting with version 2020, allows you to de-sugar the code and convert the for-comprehension intomap and flatMap.

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


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 for-comprehension”

  1. .withFilter(i => id + n % modFilter == 0)
    here since i is not used inside, we can replace it with _ to be more expressive.


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