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diff --git a/doc/manual/src/contributing/experimental-features.md b/doc/manual/src/contributing/experimental-features.md new file mode 100644 index 000000000..f1db22751 --- /dev/null +++ b/doc/manual/src/contributing/experimental-features.md @@ -0,0 +1,91 @@ +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. |