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. This allows accessing unstable functionality without unwittingly relying on it.

Experimental feature flags were first introduced in Nix 2.4. 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 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.

Currently available experimental features