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.
- Find something that belongs in cats
- Let us know you are working on it
- Build the project
- Implement your contribution
- Write tests
- Write documentation
- Write examples
- 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 firstname.lastname@example.org:typelevel/cats.git
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
- Will take longer: To build JVM only, just use the
JS only. And if you want the default project to be
catsJVM, just copy the
.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.
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
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.
- Tests for cats-core go into the tests module, under the
- Tests for additional modules, such as ‘jvm’, go into the tests directory within that module.
- Cats tests should extend
CatsSuiteintegrates ScalaTest with Discipline for law checking, and imports all syntax and standard instances for convenience.
- The first parameter to the
checkAllmethod 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
Validateddata 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
Applicativetype class instance for the
Validateddata 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
- Note that custom serialization tests are not required for instances of type classes which come from
algebra, such as
Monoid, because the
algebralaws include a test for serialization.
Write about checking laws
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
- The menu structure for these pages is in
Generating the Site
Previewing the site
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
In a shell, navigate to the generated site directory in
Start jekyll with
Navigate to http://localhost:4000/cats/ in your browser
Make changes to your site, and run
sbt docs/makeMicrositeto regenerate the site. The changes should be reflected as soon as you run
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.
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
Submit a pull request
Before you open a pull request, you should make sure that
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
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.