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Document the concept of “experimental feature” (#5930)

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Add a page explaining what “experimental features” are, when and how they should be used

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>
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Théophane Hufschmitt 2023-04-06 13:25:22 +02:00 committed by GitHub
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doc/manual/src/SUMMARY.md.in
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@ -95,6 +95,7 @@
- [Glossary](glossary.md)
- [Contributing](contributing/contributing.md)
- [Hacking](contributing/hacking.md)
- [Experimental Features](contributing/experimental-features.md)
- [CLI guideline](contributing/cli-guideline.md)
- [Release Notes](release-notes/release-notes.md)
- [Release X.Y (202?-??-??)](release-notes/rl-next.md)

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This section describes the notion of *experimental features*, and how it fits into the big picture of the development of Nix.
# What are experimental features?
Experimental features are considered unstable, which means that they can be changed or removed at any time.
Users must explicitly enable them by toggling the associated [experimental feature flags](@docroot@/command-ref/conf-file.md#conf-experimental-features).
This allows accessing unstable functionality without unwittingly relying on it.
Experimental feature flags were first introduced in [Nix 2.4](@docroot@/release-notes/rl-2.4.md).
Before that, Nix did have experimental features, but they were not guarded by flags and were merely documented as unstable.
This was a source of confusion and controversy.
# When should a new feature be marked experimental?
A change in the Nix codebase should be guarded by an experimental feature flag if it is considered likely to be reverted or adapted in a backwards-incompatible manner after gathering more experience with it in practice.
Examples:
- Changes to the Nix language, such as new built-ins, syntactic or semantic changes, etc.
- Changes to the command-line interface
# Lifecycle of an experimental feature
Experimental features have to be treated on a case-by-case basis.
However, the standard workflow for an experimental feature is as follows:
- A new feature is implemented in a *pull request*
- It is guarded by an experimental feature flag that is disabled by default
- The pull request is merged, the *experimental* feature ends up in a release
- Using the feature requires explicitly enabling it, signifying awareness of the potential risks
- Being experimental, the feature can still be changed arbitrarily
- The feature can be *removed*
- The associated experimental feature flag is also removed
- The feature can be declared *stable*
- The associated experimental feature flag is removed
- There should be enough evidence of users having tried the feature, such as feedback, fixed bugs, demonstrations of how it is put to use
- Maintainers must feel confident that:
- The feature is designed and implemented sensibly, that it is fit for purpose
- Potential interactions are well-understood
- Stabilising the feature will not incur an outsized maintenance burden in the future
The following diagram illustrates the process:
```
.------.
| idea |
'------'
|
discussion, design, implementation
|
| .-------.
| | |
v v |
.--------------. review
| pull request | |
'--------------' |
| ^ | |
| | '-------'
.---' '----.
| |
merge user feedback,
| (breaking) changes
| |
'---. .----'
| |
v |
+--------------+
.---| experimental |----.
| +--------------+ |
| |
decision to stabilise decision against
| keeping the feature
| |
v v
+--------+ +---------+
| stable | | removed |
+--------+ +---------+
```
# Relation to the RFC process
Experimental features and [RFCs](https://github.com/NixOS/rfcs/) both allow approaching substantial changes while minimizing the risk.
However they serve different purposes:
- An experimental feature enables developers to iterate on and deliver a new idea without committing to it or requiring a costly long-running fork.
It is primarily an issue of *implementation*, targeting Nix developers and early testers.
- The goal of an RFC is to make explicit all the implications of a change:
Explain why it is wanted, which new use-cases it enables, which interface changes it requires, etc.
It is primarily an issue of *design* and *communication*, targeting the broader community.
This means that experimental features and RFCs are orthogonal mechanisms, and can be used independently or together as needed.