Programming language: Scala
License: Apache License 2.0
Tags: Misc    

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Scala Exercises

How it works

"Scala Exercises" brings exercises for the Stdlib, Cats, Shapeless, and many other great libraries for Scala to your browser. This includes hundreds of solvable exercises organized into several categories covering the basics of the Scala language and its most important libraries.

  • LEARN: Each category includes an explanation of the basics. Learn the concepts through simple code samples.

  • SOLVE: Each exercise is a unit test that must pass successfully—complete the exercise by filling in the blanks. Receive instant feedback as your answers are validated in real-time.

  • SHARE: The system will consider the category complete when all its exercises are successfully done. Don't forget to share your progress on social networks before moving on to the next category!

  • EDIT: After completing a category, you'll be able to go back and edit it. Add new exercises or improve existing ones by sending a pull request.

Getting Started


Scala Exercises is available at scala-exercises.org.

Local development


Installing the app locally

Get the repository

First of all, either clone the repository via git

$ git clone https://github.com/scala-exercises/scala-exercises

or download it

$ wget https://github.com/scala-exercises/scala-exercises/archive/master.zip
Configure the database

You'll need a working PostgreSQL 9.4 database and user for running the app. Once the database is running,

  • Create a user called scalaexercises_dev_user
$ sudo -u postgres psql -c "CREATE USER scalaexercises_dev_user WITH PASSWORD 'a_password';"
  • Create a db called scalaexercises_dev and grant all privileges on it to scalaexercises_dev_user
$ sudo -u postgres createdb scalaexercises_dev
$ sudo -u postgres psql -c "GRANT ALL PRIVILEGES ON DATABASE scalaexercises_dev TO scalaexercises_dev_user;"

Alternatively, you can also use Docker to run the database. The following command creates a database container and exposes it:

$ docker run --name scala-exercises-db -e POSTGRES_DB=scalaexercises_dev -e POSTGRES_PASSWORD=scalaexercises_pass -e POSTGRES_USER=scalaexercises_dev_user -p 5432:5432 -d postgres:9.4
Configure the application

Edit the server/conf/application.dev.conf configuration file with your database information.

Running the app

Go into the project's root directory, run sbt server/run with -mem option to increase the memory.

$ sbt -mem 1500 server/run

After compilation, the application will be running, listening in the 9000 port. Point your browser to localhost:9000 and start having fun!

Running the tests

To run the tests (for the server project), you need to add a test database and a test user.

  • Create a user called scalaexercises_user
$ sudo -u postgres psql -c "CREATE USER scalaexercises_user WITH PASSWORD 'scalaexercises_pass';"
  • Create a db called scalaexercises_test and grant all privileges on it to scalaexercises_user
$ sudo -u postgres createdb scalaexercises_test
$ sudo -u postgres psql -c "GRANT ALL PRIVILEGES ON DATABASE scalaexercises_test TO scalaexercises_user;"

Adding more exercises

Currently, scala-exercises includes exercises for the Scala Standard Library, Cats, and Shapeless. However, more exercises are available, like for Doobie, Functional Programming in Scala, and ScalaCheck. See the scala-exercises on github, or you can include exercises from other parties or create your own (see Contributing section).

To add additional exercises to your locally running server:

  • clone the exercises repository to a local folder
  • 'cd' into the local repository folder.
  • run sbt compile publishLocal to create a jar in your local ivy repository.
    !Note: The compile task is mandatory here otherwise the exercises will not show up in the application.
  • add a dependency to the exersises jar in the server project in the build.sbt file (~L118).

Now run sbt server/run and the application index will also display the added exercises.


Additional exercises do not show up in the application

See the Adding more exercises section. Note that, currently, the compile step is required before publishLocal for the application to be able to pickup the exercises.


If you use ensime and you have configured the sbt-ensime plugin in your sbt user global settings, it's likely you might have this issue running the application locally:

java.lang.NoClassDefFoundError: scalariform/formatter/preferences/SpacesAroundMultiImports$

In that case, you could solve this issue setting up your /.sbt/0.13/plugins/plugins.sbt file as follows:

addSbtPlugin("org.ensime" % "ensime-sbt" % "0.5.1")

dependencyOverrides in ThisBuild += "org.scalariform" %% "scalariform" % "0.1.8"

In order to avoid the error related to Github API rate limit exceeded during compilation of exercises, we recommend setting a local environment variable called GITHUB_TOKEN with a personal token that you can create here.

While creating the PostgreSQL database, you may run into problems following the previous instructions if developing on a MacOS X environment. In that case, we recommend using the following alternatives:

  • Create a user called scalaexercises_dev_user. Note that, if you installed PostgreSQL using Homebrew, your superuser may be different than postgres:
$ psql -U your_postgres_user -c "CREATE USER scalaexercises_dev_user WITH PASSWORD 'a_password';"
  • Create a db called scalaexercises_dev and grant all privileges on it to scalaexercises_dev_user:
$ createdb scalaexercises_dev
$ psql -U your_postgres_user -c "GRANT ALL PRIVILEGES ON DATABASE scalaexercises_dev TO scalaexercises_dev_user;"

Project structure

The project is split between a few directories, namely:

  • server, which contains the server code written using Play,
  • client, which contains ScalaJS code for a frontend part of the application,
  • shared, where code shared between the server and the client exists,
  • definitions, containing definitions used by other parts of the application and libraries containing exercises,
  • sbt-exercise is a sbt plugin that locates exercise libraries and processes their source code,
  • compiler for compiling exercises,
  • runtime for runtime evaluation of exercises.

The compiler and runtime directories allow exercises to be defined using regular Scala, which is compiled into an exercise library.

The site, client, and shared directories contain the website. These items depend on components in compiler and runtime.

At the moment, those subprojects are coupled tightly. Once this project is a bit more stable, the exercise compiler plugin will be published, and it will be easy to create new exercises for existing Scala libraries.