As an exception, an explicit conditional expression with null can be used when fixing a important bug without triggering a mass rebuild.
  If this is done a follow up pull request _should_ be created to change the code to `lib.optional(s)`.

# Practical contributing advice

To contribute effectively and efficiently, you need to be aware of how the contributing process generally works.
This section aims to document the process as we live it in Nixpkgs to set expectations right and give practical tips on how to work with it.

## I opened a PR, how do I get it merged?

In order for your PR to be merged, someone with merge permissions on the repository ("committer") needs to review and merge it.
Because the group of people with merge permissions is mostly a collection of independent unpaid volunteers who do this in their own free time, this can take some time to happen.
It is entirely normal for your PR to sit around without any feedback for days, weeks or sometimes even months.
We strive to avoid the latter cases of course but the reality of it is that this does happen quite frequently.
Even when you get feedback, follow-up feedback may take similarly long.
Don't be intimidated by this and kindly ask for feedback again every so often.
If your change is good it will eventually be merged at some point.

There are some things you can do to help speed up the process of your PR being merged though.
In order to speed the process up, you need to know what needs to happen before a committer will actually hit the merge button.
This section intends to give a little overview and insight of what happens after you create your PR.

### The committer's perspective

PRs have varying quality and even the best people make mistakes.
It is the role of the committer team to assess whether any PR's changes are good changes or not.
In order for any PR to be merged, at least one committer needs to be convinced of its quality enough to merge it.

Committers typically assess three aspects of your PR:

1. Whether the change's intention is necessary and desirable
2. Whether the code quality of your changes is good
3. Whether the artefacts produced by the code are good

If you want your PR to get merged quickly and smoothly, it is in your best interest to help convince committers in these three aspects.

### How to help committers assess your PR

For the committer to judge your intention, it's best to explain why you've made your change.
This does not apply to trivial changes like version updates because the intention is obvious (though linking the changelog is appreciated).
For any more nuanced changes or even major version upgrades, it helps if you explain the background behind your change a bit.
E.g. if you're adding a package, explain what it is and why it should be in Nixpkgs.
This goes hand in hand with [Writing good commit messages](#writing-good-commit-messages).

For the code quality assessment, you cannot do anything yourself as only the committer can do this and they already have your code to look at.
In order to minimise the need for back and forth though, do take a look over your code changes yourself and try to put yourself into the shoes of someone who didn't just write that code.
Would you immediately know what the code does or why it is needed by glancing at it?
If not, reviewers will notice this and will ask you to clarify the code by refactoring it and/or adding a few explanations in code comments.
Doing this preemptively can save you and the committer a lot of time.
To better convey the "story" of your change, consider dividing your change into multiple atomic commits.
There is a balance to strike however: over-fragmentation causes friction.

The code artefacts are the hardest for committers to assess because PRs touch all sorts of components: applications, libraries, NixOS modules, editor plugins and many many other things.
Any individual committer can only really assess components that they themselves know how to use however and yet they must still be convinced somehow.