Contributor guide

About this document

This guide is for people who would like to be involved in building Cats.

This guide assumes that you have some experience doing Scala development. If you get stuck on any of these steps, please feel free to ask for help.

How can I help?

Cats follows a standard fork and pull model for contributions via GitHub pull requests.

Below is a list of the steps that might be involved in an ideal contribution. If you don’t have the time to go through every step, contribute what you can, and someone else will probably be happy to follow up with any polishing that may need to be done.

If you want to touch up some documentation or fix typos, feel free to skip these steps and jump straight to submitting a pull request.

  1. Find something that belongs in cats
  2. Let us know you are working on it
  3. Build the project
  4. Implement your contribution
  5. Write tests
  6. Write documentation
  7. Write examples
  8. Submit pull request

Find something that belongs in Cats

Looking for a way that you can help out? Check out our open issues and look for ones tagged as help wanted or low-hanging fruit. These issues are the easiest way to start contributing, but if you find other items that catch your eye, you’re most than welcome to tackle them!

Make sure that it’s not already assigned to someone and that nobody has left a comment saying that they are working on it!

(Of course, you can also comment on an issue someone is already working on and offer to collaborate.)

Have an idea for something new? That’s great! We recommend that you make sure it belongs in Cats before you put effort into creating a pull request. The preferred ways to do that are to either:

Things that belong in Cats generally have the following characteristics:

  • Their behavior is governed by well-defined laws.
  • They provide general abstractions.

Laws help keep types consistent, and remove ambiguity or sensitivity about how particular instances can behave. We’ve found that types with laws are often more useful than lawless types

(In some cases, lawless type classes and instances are useful. We intend to support some of these in a future module.)

By staying general, Cats’ abstractions are widely-applicable, and not tied to particular libraries or strategies. Rather than being a library to work with databases, HTTP requests, etc, Cats provides abstractions used to build those libraries.

Cats (and especially cats-core) is intended to be lean and modular. Some great ideas are not a great fit, either due to their size or their complexity. In these cases, creating your own library that depends on Cats is probably the best plan.

Cats subprojects

Cats has other companion projects, described next:

  • cats-effect: a project aimed to provide a standard IO type for the Cats ecosystem, as well as a set of typeclasses (and associated laws) which characterize general effect types.
  • cats-mtl: provides transformer typeclasses for Cats’ Monads, Applicatives and Functors.
  • mouse: a small companion to the Cats functional programming library for Scala. It includes convenience extension methods for Scala standard library classes, including some found in scalaz that are not in Cats.

Let us know you are working on it

If there is already a GitHub issue for the task you are working on, leave a comment to let people know that you are working on it. If there isn’t already an issue and it is a non-trivial task, it’s a good idea to create one (and note that you’re working on it). This prevents contributors from duplicating effort.

Build the project

First you’ll need to checkout a local copy of the code base:

git clone

To build Cats you should have sbt and Node.js installed. If you’d like, you can use the Nix Cats development environment.

Run sbt, and then use any of the following commands:

  • compile: compile the code
  • console: launch a REPL
  • test: run the tests
  • unidoc: generate the documentation
  • scalastyle: run the style-checker on the code
  • validate: run tests, style-checker, and doc generation

Scala and Scala.js

Cats cross-compiles to both JVM and JavaScript runtimes. If you are not used to working with cross-compiling builds, the first things that you will notice is that builds:

  • Will take longer: To build JVM only, just use the catsJVM, or catsJS for JS only. And if you want the default project to be catsJVM, just copy the file scripts/sbtrc-JVM to .sbtrc in the root directory.

  • May run out of memory: We suggest you use Paul Philips’s sbt script that will use the settings from Cats.

Editor Setup Tips


  • Be warned, IntelliJ is currently not 100% accurate at reporting compilation errors, there will be cases that it reports errors incorrectly. If you simply don’t want to see the errors, a quick an easy work around is to disable Type-Aware Highlighting by clicking the [T] icon in the bottom toolbar.

  • There is an open issue with the IntelliJ scala plugin, which prevents it from configuring similacrum correctly when importing the cats project. The work around for this issue is to set val CompileTime = Provided in build.sbt. Note: Be careful not to commit this change.

  • IntelliJ does have support for kind-projector. However, it is not always seamless. If you are unable to get IntelliJ to recognise the special symbols that kind-project provides, such as ? Lambda[X => G[F[A]]] or λ[X => G[F[A]]] try upgrading to the early access preview (EAP) version of the scala plugin. This can be done under Settings > Languages & Frameworks > Scala > Updates

Write code

See guidelines.


If your contribution has been derived from or inspired by other work, please state this in its ScalaDoc comment and provide proper attribution. When possible, include the original authors’ names and a link to the original work.

Write tests

  • Tests for cats-core go into the tests module, under the cats.tests package.
  • Tests for additional modules, such as ‘jvm’, go into the tests directory within that module.
  • Cats tests should extend CatsSuite. CatsSuite integrates with Discipline for law checking, and imports all syntax and standard instances for convenience.
  • The first parameter to the checkAll method provided by Discipline, is the name of the test and will be output to the console as part of the test execution. By convention:
  • When checking laws, this parameter generally takes a form that describes the data type being tested. For example the name “Validated[String, Int]” might be used when testing a type class instance that the Validated data type supports.
  • An exception to this is serializability tests, where the type class name is also included in the name. For example, in the case of Validated, the serializability test would take the form, “Applicative[Validated[String, Int]]”, to indicate that this test is verifying that the Applicative type class instance for the Validated data type is serializable.
  • This convention helps to ensure clear and easy to understand output, with minimal duplication in the output.
  • It is also a goal that, for every combination of data type and supported type class instance:
  • Appropriate law checks for that combination are included to ensure that the instance meets the laws for that type class.
  • A serializability test for that combination is also included, such that we know that frameworks which rely heavily on serialization, such as Spark, will have strong compatibility with Cats.
  • Note that custom serialization tests are not required for instances of type classes which come from algebra, such as Monoid, because the algebra laws include a test for serialization.
  • For testing your laws, it is advised to check this guide.

Binary compatibility

It is important to verify that the feature you are implementing is compatible with Scala 2.12.x and Scala 2.13.x (Scala <2.11.x is not supported). When you submit a PR, Travis makes this check, but it is time-expensive, so you can assure this step beforehand by issuing the command ++2.12.13, which sets the cats’ Scala version to 2.12.13 and then run mimaReportBinaryIssues.

A summary of these steps is as follows:

$ sbt
> ++2.12.13
> mimaReportBinaryIssues

This procedure will report if there are any binary compatibility issues that should be fixed.

As a side note, the latter command uses sbt-mima (shorthand for “Migration Manager”) plugin and you can find more information about it here.

Contributing documentation

source for the documentation

The documentation for this website is stored alongside the source, in the docs subproject.

  • The source for the tut compiled pages is in docs/src/main/mdoc
  • The menu structure for these pages is in docs/src/main/resources/microsite/data/menu.yml

Generating the Site

run sbt docs/makeMicrosite

Previewing the site

  1. Install jekyll locally. Depending on your platform, you might do this with:

    yum install jekyll

    apt-get install ruby-full; gem install jekyll

    gem install jekyll

    Or just dropping into a nix-shell if you are using the Nix Cats development environment.

  2. In a shell, navigate to the generated site directory in cats-docs/target/site

  3. Start jekyll with jekyll serve -b /cats

  4. Navigate to http://localhost:4000/cats/ in your browser

  5. Make changes to your site, and run sbt docs/makeMicrosite to regenerate the site. The changes should be reflected as soon as you run makeMicrosite.

Compiler verified documentation

We use mdoc to compile source code which appears in the documentation, this ensures us that our examples should always compile, and our documentation has a better chance of staying up-to-date.

Write examples

One of the best ways to provide examples is doctest, here is an example. Doctest is a sbt plugin which generates tests based on the syntax mentioned above and runs when sbt’s test task is invoked. You can find more information in the plugin documentation.

Submit a pull request

Before you open a pull request, you should make sure that sbt validate runs successfully. Travis will run this as well, but it may save you some time to be alerted to style problems earlier.

If your pull request addresses an existing issue, please tag that issue number in the body of your pull request or commit message. For example, if your pull request addresses issue number 52, please include “fixes #52”.

If you make changes after you have opened your pull request, please add them as separate commits and avoid squashing or rebasing. Squashing and rebasing can lead to a tidier git history, but they can also be a hassle if somebody else has done work based on your branch.

How did we do?

Getting involved in an open source project can be tough. As a newcomer, you may not be familiar with coding style conventions, project layout, release cycles, etc. This document seeks to demystify the contribution process for the cats project.

It may take a while to familiarize yourself with this document, but if we are doing our job right, you shouldn’t have to spend months poring over the project source code or lurking the Gitter room before you feel comfortable contributing. In fact, if you encounter any confusion or frustration during the contribution process, please create a GitHub issue and we’ll do our best to improve the process.

Getting in touch

Discussion around Cats is currently happening in the Gitter channel as well as on Github issue and PR pages.

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.

People are expected to follow the Scala Code of Conduct when discussing Cats 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. We hope to avoid letting minor aggressions and misunderstandings escalate into larger problems.

If you are being harassed, please contact one of us immediately so that we can support you.

Nix Cats Development Environment

Since Cats development can include the Scala runtime, the Scala.js runtime, the Cats website, and more; a number of dependencies (sbt, Node.js, Jekyll, etc) can be needed to work on Cats. Managing these dependencies globally can be a hassle and can lead to version conflicts. To make this easier to manage in an isolated development environment, Cats provides a shell.nix for anyone using the Nix package manager.

To use the Nix-based Cats development environment:

  1. Install the Nix package manager.
  2. At the root level of the Cats repository, run nix-shell --pure. This will drop you into a minimal bash shell that has just the required dependencies on the PATH. Note that the first time that you run this it will take some extra time to download the necessary dependencies into your local Nix store.
  3. Run sbt, jekyll, etc as required from the nix-shell.
  4. When you are finished you can exit the nix-shell.