SKB – Scala foldLeft


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 foldLeft.

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 foldLeft !

Let me introduce accumulators and aggregations.

foldLeft is the generic concept that is under most of the function programming transformations. You can replace map, flatMap, filter and more by a foldLeft.

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 (1YO5x72fSE686gtMiAxKzQ).

More information about Scala foldLeft

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

In this exercise, you can see two use cases of foldLeft. But first let’s explain the syntax:

foldLeft(initialValue) { case (accumulator, currentElement) => 
    // return the new value of the accumulator
Note that when currentElement is the first element of the List, then accumulator is equal to initialValue. Also, if the the list is empty, then the returned value will be the initialValue.

The returned value can be anything, for instance:

foldLeft(List.empty) { case (accumulator, currentElement) => 
    accumulator :+ currentElement 
would return a new List with the same content as the input list.

An other example:

foldLeft(0) { case (accumulator, currentElement) => 
    accumulator + currentElement 
would return the total sum of the item of the List. This is similar to the first example in today’s exercise. And scala provide a shortcut for it: .sum, this would be a special case of the exercise when the initial value is 0.

In the second use case, there is a bit more going on. It uses pattern matching to implement different behavior based on the current element and create a new list element by element.

As an extra exercise, try to compute the average of the list by changing the initialized value from startFold to (0, 0) and modify the function to aggregate the values.

You can also try to implement map, flatMap and, filter using foldLeft. Share your solution with our community on Discord !

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


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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