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 Waffle.io page. Choose a card from the “Ready” column. Before you start working on it, 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.

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 git@github.com:typelevel/cats.git

To build Cats you should have sbt and Node.js installed. 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(JS). 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.

Write code

TODO

Should this just link to a separate doc? This might get large.

Write about implicit params as discussed in https://github.com/typelevel/cats/issues/27

Write about type class methods on data structures as described in https://github.com/typelevel/cats/issues/25

Write about https://github.com/typelevel/cats/pull/36#issuecomment-72892359

Attributions

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 ScalaTest 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.

TODO

Write about checking laws

Contributing documentation

source for the documentation

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

  • The source for the static pages is in docs/src/site
  • The source for the tut compiled pages is in docs/src/main/tut

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 jekyll

    gem install jekyll

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

  3. Start jekyll with jekyll serve

  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 tut 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.

Publishing the site to github.

The git.remoteRepo variable in docs/build.sbt controls which repository you will push to. Ensure that this variable points to a repo you wish to push to, and that it is one for which you have push access, then run sbt ghpagesPushSite

Write examples

TODO

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. You can get an overview of who is working on what via Waffle.io.

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 Typelevel 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.