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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.

Contribution

Adding or modifying code 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. We currently have more contributors willing to propose changes than contributors willing to review other people's changes, so more eyes on the existing change requests is a good way to increase the integration bandwidth of external contributions.

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.)

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.

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). For regression tests, testing other aspects of the feature (in particular, related edge cases) that are not currently covered is a good way to catch other instances of bugs -- this did happen several times in the past. Warnings and errors should also be tested.

Tests go in the sub-directories of testsuite/tests. Running make all in testsuite/ runs all tests (this takes a few minutes), and you can use make one DIR=tests/foo to run the tests of a specific sub-directory. There are many kind of tests already, so the easiest way to start is to extend or copy an existing test.

In general, running a test produces one (or several) .result file, that are compared to one (or several) .reference file present in the repository; the test succeeds if they are identical. If your patch breaks a test, diffing the .result and .reference file is a way to see what went wrong. Some reasonable compiler changes affect the compiler output in way that make those outputs differ (for example slight modifications of warning or error messages may break all tests checking warnings). If you are positive that the new .result file is correct (and that the change in behavior does not endanger backward compatibility), you can replace the old .reference file with it. Finally, when adding new tests, do not forget to include your .reference files (but not .result) in the versioned repository.

Testing 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.

Run tests before sending a PR

You should run all the tests before creating the merge request or pushing new commits (even if Travis will also do it for you): make tests (this takes a few minutes).

Unfortunately some of the lib-threads test are non-deterministic and fail once in a while (it's hard to test these well). If they consistently break after your change, you should investigate, but if you only see a transient failure once and your change has no reason to affect threading, it's probably not your fault.

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 in the implementation details, you should include a summary of the change, and a high-level description of the design of the proposed change, with example use-cases.

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.

It is recommended to included 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/).

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 Changelog 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 the issue number PR#{N} if from mantis, GPR#{N} if from github (several numbers separated by commas can be used)
maintaining the order: each section lists Mantis PRs first in ascending numerical order, followed by Github PRs
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. There is some widespread belief in the community than the stdlib is somehow "frozen" and that its evolutions are mostly driven by the need of the OCaml compiler itself. Let's be clear: this is just plain wrong. The compiler is happy with its own local utility functions, and many recent additions to the stdlib are not used by the compiler.

Another common and wrong idea is that core OCaml maintainers don't really care about the standard library. This is not true, and won't be unless one of the "alternative standard" libraries really gains enough "market share" in the community.

So: please contribute!

Obviously, the proposals to evolve the standard library will be evaluated with very high standards, similar to those applied to the evolution of the surface langage, and much higher than those for internal compiler changes (optimizations, etc).

A key property of the standard library is its stability. Backward compatibility is not an absolute technical requirement (any addition to/of a module can break existing code, formally), but breakage should be limited as much as possible (and assessed, when relevant). A corollary is that any addition creates a long-term support commitment. For instance, once a concrete type or function is made public, changing the exposed definition cannot be done easily.

There is no plan to extend dramatically the functional domain covered by the standard library. For instance, proposals to include support for XML, JSON, or network protocols are very likely to be rejected. Such domains are better treated by external libraries. Small additions to existing modules are much simpler to get in, even more so (but not necessarily) when:

they cannot easily be implemented externally, or when
they facilitate communication between independent external libraries, or when
they fill obvious gaps.

Of course, standard guidelines apply as well: proper documentation, proper tests, portability (yes, also Windows!), good justification for why the change is desirable and why it should go into stdlib.

So: be prepared for some serious review process! But yes, yes, contributions are welcome and appreciated. Promised.

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).

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