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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. Offering hundreds of solvable exercises organized into several categories covering the basics of the Scala language and it's 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

Local development


Installing the app locally

Get the repository

First of all, either clone the repository via git

$ git clone

or download it

$ wget
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/ 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, 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 follow:

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 locally an environment variable called GITHUB_TOKEN with a personal token which 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 alternative ones:

  • 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 which 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.


Contributions welcome! Please join our Gitter channel to get involved.

Feel free to open an issue if you notice a bug, have an idea for a feature, or have a question about the code. Pull requests are also gladly accepted.

In the same fashion, if you're interested in providing your own content for your library (or a third-party's), you can find more information on how to do it in the Developer Guide.

People are expected to follow the Typelevel Code of Conduct when discussing Scala Exercises on the Github page, Gitter channel, or other venues.

We hope that our community will be respectful, helpful, and kind. If you find yourself embroiled in a situation that becomes heated, or that fails to live up to our expectations, you should disengage and contact one of the project maintainers in private.


Copyright (C) 2015-2016 47 Degrees, LLC. Reactive, scalable software solutions.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.