CONTRIBUTING.md

How to contribute changes

👍🎉 First off, thank you for taking time to contribute! 🎉👍

The following is a set of guidelines for proposing changes to the OCaml distribution. These are just guidelines, not rules, use your best judgment and feel free to propose changes to this document itself in a pull request.

This document assumes that you have a patch against the sources of the compiler distribution, that you wish to submit to the OCaml maintainers upstream. See INSTALL.adoc for details on how to build the compiler distribution from sources. See HACKING.adoc for details on how to modify the sources.

Contribution

Modifying the sources is far from the only way to contribute to the OCaml distribution. Bug reports (in particular when they come with a reproducible example), simple typos or clarifications in the documentation also help, and help evaluating and integrating existing change proposals also help. Providing good answers on the discussion forums, or asking the good questions that highlight deficiencies in existing documentations, also help.

There are also many valuable ways to contribute to the wider OCaml ecosystem that do not involve changes to the OCaml distribution.

The rest of the document is concerned with the form of change proposals against the OCaml distribution. (Code changes, but also improvement to documentation or implementation comments, which are valuable changes on their own.)

Workflow

All changes to the OCaml distribution need to be processed through the GitHub Pull Request (PR) system. In order to propose a change, a contributor thus needs to have a GitHub account, fork the ocaml/ocaml repository, create a branch for the proposal on their fork and submit it as a Pull Request on the upstream repository. (If you are not yet familiar with GitHub, don't worry, all these steps are actually quite easy!)

The current rule is that a PR needs to get an explicit approval from one of the core maintainer in order to be merged. Reviews by external contributors are very much appreciated.

Since core maintainers cannot push directly without going through an approved PR, they need to be able to apply small changes to the contributed branches themselves. Such changes include fixing conflicts, adjusting a Changelog entry, or applying some code changes required by the reviewers. Contributors are thus strongly advised to check the Allow edits from maintainer flag on their PRs in the GitHub interface. Failing to do so might significantly delay the inclusion of an otherwise perfectly ok contribution.

Maintainers

The current list of maintainers is as follows:

• @alainfrisch Alain Frisch
• @Armael Armaël Guéneau
• @avsm Anil Madhavapeddy
• @chambart Pierre Chambart
• @damiendoligez Damien Doligez
• @dra27 David Allsopp
• @Engil Enguerrand
• @garrigue Jacques Garrigue
• @gasche Gabriel Scherer
• @jhjourdan Jacques-Henri Jourdan
• @kayceesrk KC Sivaramakrishnan
• @let-def Frédéric Bour
• @lpw25 Leo White
• @lthls Vincent Laviron
• @maranget Luc Maranget
• @mshinwell Mark Shinwell
• @nojb Nicolás Ojeda Bär
• @Octachron Florian Angeletti
• @sadiqj Sadiq Jaffer
• @shindere Sébastien Hinderer
• @stedolan Stephen Dolan
• @trefis Thomas Refis
• @xavierleroy Xavier Leroy
• @yallop Jeremy Yallop

Coding guidelines

You should not leave trailing whitespace; not have line longer than 80 columns, not use tab characters (spaces only), and not use non-ASCII characters. These typographical rules can be checked with the script tools/check-typo, see HACKING.adoc: check-typo.

Otherwise, there are no strongly enforced guidelines specific to the compiler -- and, as a result, the style may differ in the different parts of the compiler. The general OCaml Programming Guidelines are good to keep in mind, and otherwise we strive for good taste and local consistency (following the code located around your change).

If you strongly feel that a style-related change would improve quality of the existing code (for example, giving more descriptive names to some variables throughout a module, factoring repeated code patterns as auxiliary functions, or adding comments to document a part of the code that you had trouble understanding), you can have code cleanup commits at the beginning of your patch series, or submit code cleanups as your change proposal. Those cleanups should remain separate commits from the functional changes in the rest of the patch series; it is easier to review commits that are specifically marked as exactly preserving the code semantics.

Test you must.

Whenever applicable, merge requests must come with tests exercising the affected features: regression tests for bug fixes, and correctness tests for new features (including corner cases and failure cases). Warnings and errors should also be tested.

See testsuite/HACKING.adoc for details on how to write tests and run the testsuite.

Adding tests is also a way to make sure reviewers see working (and failing) examples of the feature you fix, extend or introduce, rather than just an abstract description of it.

Description of the proposed change

In the merge request interface

The description of the merge request must contain a precise explanation of the proposed change.

Before going into the implementation details, you should include a summary of the change, a justification of why it is beneficial, and a high-level description of the design of the proposed change with example use cases.

Changes have a cost, they require review work and may accidentally introduce new bugs. Communicating as clearly as you can the benefits of your PR will reassure and motivate potential reviewers.

In the patches

If some of the explanations you provide for the merge request would make sense as comments in the code, or documentation in the manual, you should include them there as well.

In-code comments help make the codebase more accessible to newcomers (many places in the compiler could benefit from a few extra explanations), and they are also useful to code reviewers. In particular, any subtlety in code that cannot be made self-explanatory should come with an explanation in comment. If you add some non-obvious code specifically to fix a bug, include the issue number in comments.

Do not assume that code reviewers are all experts in the existing codebase. If you use subtle code, add a comment, even if the same kind of code is used somewhere else in the same module. (If this is a common and useful domain-specific idiom that is already explained somewhere, pointing to this explanation in your commit message is better than adding redundant explanations.)

User documentation

Changes affecting the compiler libraries should be reflected in the documentation comments of the relevant .mli files. After running make html_doc, you can find the HTML Standard Library documentation at ./api_docgen/html/libref/index.html.

It is recommended to include changes to the OCaml Reference Manual (in particular for any change in the surface language), which is now part of the main repository (under manual/). To build the full manual, see the instructions in manual/README.md.

Finally, changes in command-line options should be integrated in the manual, but also in the man pages present in the man/ sub-directory of the OCaml distribution.

Changelog

Any user-visible change should have a Changes entry:

• in the right section (named sections if major feature, generic "Bug fixes" and "Feature requests" otherwise)

• using the label "*" if it breaks existing programs, "-" otherwise

• with all relevant issue and PR numbers #{N}, in ascending numerical order (separated by commas if necessary)

• maintaining the order: the entries in each section should be sorted by issue/PR number (the first of each entry, if more than one is available)

• with a concise readable description of the change (possibly taken from a commit message, but it should make sense to end-users reading release notes)

• crediting the people that worked on the feature. The people that wrote the code should be credited of course, but also substantial code reviews or design advice, and the reporter of the bug (if applicable) or designer of the feature request (if novel).

• following the format

    {label} {issue number(s)}: {readable description}
            ({credits})
  
  note that the `{credits}` should be on their own line, aligned with the
  issue number for readability
  (`{readable description}` can be multiline to not overflow 80
  columns, and should be aligned with the issue number as well.)
  

This changelog can be included in the main commit, if the merge request is just one patch, or as a separate commit, if it's a patch series and no particular commit feels best suited to receive the Changelog entry.

(Do not under-estimate the importance of a good changelog. Users do read the release notes, and things forgotten from the changelog will cause pain or regrets down the line.)

Clean patch series

Clean patch series are useful, both during the review process and for code maintenance after it has been merged. Before submitting your request, you should rebase your patch series:

• on top of the OCaml branch in which you want to merge (usually trunk), solving any conflicts.

• into a few well-separated, self-contained patches (github PRs can generate gazillions of micro-changes)

• erasing history that does not make sense after the issue is merged (back-and-forth between different designs, etc. The PR number allows interested people to go back to the original discussion if needed.)

• bisectable: the distribution should be in a good state after the application of each patch (in particular, later commits that fix bugs in previous commits should always be squashed into the commit they fix)

• with readable commit messages (this is for future developers needing to understand a change that happened in the past). Commit messages should not overflow 80 columns, with the following format:

    {one-liner header description (with issue number if applicable)}
    {blank line}
    {one or several paragraphs of explanation if needed}
  

During review, you may make many other changes to the patch series. You can rebase it on the fly (if you git push -f on the branch of the pull request in your personal clone, Github will update the pull request automatically; remember to always create a new branch for any) or wait until the discussion has converged, once we agree the request is ready for merging. Doing a good rebase is grunt work that takes some time and care (use git log -u to make sure the rebase patches make sense), but:

• It is easier and faster to do for the author of the patch than for others (if rebasing against the current trunk creates a conflict with another change you don't understand well, feel free to ask).

• Maintainers are usually short on time, and asking them to do a rebase means they have less time to review and merge other contributions.

• The long-term benefits of keeping a clean, bisectable history cannot be overstated. Imagine that in three years, under the pressure of a coming release, a contributor ends up somewhere in the middle of your patch series, wondering if or why it is the cause of a specific issue. Wasting his or her time then (with a "yolo" commit message, a big ugly commit of unrelated changes, or an un-testable intermediary state) is a sure way to generate ill will.

Contributing to the standard library

Contributions to the standard library are very welcome. See the dedicated stdlib/CONTRIBUTING.md for more information.

Contributing optimizations

Contributions to improve the compiler's optimization capabilities are welcome. However, due to the potential risks involved with such changes, we ask the following of contributors when submitting pull requests:

• Explain the benefits of the optimization (faster code, smaller code, improved cache behaviour, lower power consumption, increased compilation speed).

• Explain when the optimization does and does not apply.

• Explain when, if ever, the optimization may be detrimental.

• Provide benchmark measurements to justify the expected benefits. Measurements should ideally include experiments with full-scale applications as well as with microbenchmarks. Which kinds of measurements are appropriate will vary depending on the optimization; some optimizations may have to be measured indirectly (for example, by measuring cache misses for a code size optimization). Measurements showing clear benefits when combined with some other optimization/change are acceptable.

• At least some of the measurements provided should be from experiments on open source code.

• If assistance is sought with benchmarking then this should be made clear on the initial pull request submission.

• Justify the correctness of the optimization, and discuss a testing strategy to ensure that it does not introduce bugs. The use of formal methods to increase confidence is encouraged.

A major criterion in assessing whether to include an optimisation in the compiler is the balance between the increased complexity of the compiler code and the expected benefits of the benchmark. Contributors are asked to bear this in mind when making submissions.

Collective maintenance

Proposing changes to the OCaml compiler contribution generates "maintenance work" for other people. Maintenance work includes, for example:

• reviewing Pull Requests or language change proposals,

• considering change suggestions and giving feedback to turn them into actionable issues,

• implementing bug fixes or feature requests of general interest,

• improving the documentation of the tools or other usability aspects,

• or documenting or clarifying the codebase to preserve and improve our ability to change it in the future.

Doing this collective maintenance work is a selfless task, and we typically have much fewer people willing to to do it than people willing to submit new language features or generally evolve the codebase for their own specific needs. Without a collective effort to participate, we end up with a handful of people doing the vast majority of this collective maintenance work. This is exhausting, does not scale, and slows down the pace of improvement of the compiler distribution.

To keep a healthy open source project, we need the total maintenance work performed by all contributors to scale proportionally with the total demand for maintenance work they generate. This can only work if as many contributors as possible perform some (possibly small) amount of maintenance work: collective maintenance. One could use the metaphor of a shared house: things work well when most people, not just a few people, participate to the house chores.

If your contributions generate maintenance work for others -- in particular, if you spend a substantial effort working on a change to the language or compiler codebase meant to be eventually proposed upstream -- we expect that you will spend a fraction of your contribution time on maintenance tasks, typically on the parts of the compiler codebase that you are already working on. This approach is good for the project, and also for you: helping maintain the codebase will improve the quality of your own contributions, and the social ties created by infrequent collaboration with other contributors will be useful when submitting your own work.

Note: we have been asked whether groups of contributors could balance maintenance work at the level of the whole group, rather than individual contributors -- for example a company where some frequent OCaml contributors would do less maintenance and others would do more to compensate. Yes, that sounds reasonable, but also harder to balance than encouraging everyone to play nice individually.

Contributor License Agreement

We distinguish two kind of contributions:

• Small changes that do not bear a specific mark of their authors (another developer recreating the change without access to the original patch would write an indistinguishable patch), and are thus not protected by copyright, do not require any particular paperwork. This is convenient for everyone, and of course does not mean that those contributions are of lesser importance. (For example a bugfix can be obvious once a bug is understood, reported and reproduced, and yet invaluable for users.)

• Larger changes that are covered by copyright. For them, we require contributors to sign a Contributor License Agreement (CLA), which gives INRIA (Institut National de Recherche en Informatique et en Automatique) the rights to integrate the contribution, maintain it, evolve it, and redistribute it under the license of its choice. This is not a copyright assignment (as requested by the Free Software Foundation for example), contributors retain the copyright on their contribution, and can use it as they see fit. The OCaml CLA is lightly adapted from the CLA of the Apache Foundation, and is available in two versions: for individual contributors and for corporations.

You must understand that, by proposing a contribution for integration in the OCaml distribution, you accept that it be considered under one of those regimes. In particular, in all cases you give INRIA the permission to freely re-license the OCaml distribution including the contribution.

This ability to re-license allows INRIA to provide members of the Caml Consortium with a license on the Caml code base that is more permissive than the public license.

How to sign the CLA

If your contribution is large enough, you should sign the CLA. If you are contributing on your own behalf, you should sign the individual CLA. For corporate contributions, if your employer has not already done so, they should sign the corporate CLA. Review the CLA, sign it, and send it -- scanned PDF by email, or postail mail -- to Xavier Leroy (contact info).