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Swift Evolution Process

Swift is a powerful and intuitive programming language that is designed to make writing and maintaining correct programs easier. Swift is growing and evolving, guided by a community-driven process referred to as the Swift evolution process. This document outlines the Swift evolution process and how a feature grows from a rough idea into something that can improve the Swift development experience for millions of programmers.

Scope

The Swift evolution process covers all changes to the Swift language and the public interface of the Swift standard library, including new language features and APIs (no matter how small), changes to existing language features or APIs, removal of existing features, and so on. Smaller changes, such as bug fixes, optimizations, or diagnostic improvements can be contributed via the normal contribution process; see Contributing to Swift.

Goals

The Swift evolution process aims to leverage the collective ideas, insights, and experience of the Swift community to improve the Swift development experience. Its two primary goals are:

  • Engage the wider Swift community in the ongoing evolution of Swift, and
  • Maintain the vision and conceptual coherence of Swift.

There is a natural tension between these two goals. Open evolution processes are, by nature, chaotic. Yet, maintaining a coherent vision for something as complicated as a programming language requires some level of coordination. The Swift evolution process aims to strike a balance that best serves the Swift community as a whole.

Participation

Everyone is welcome to propose, discuss, and review ideas to improve the Swift language and standard library in the Evolution section of the Swift forums. Before posting a review, please see the section "What goes into a review?" below.

The Swift core team is responsible for the strategic direction of Swift. Core team members initiate, participate in, and manage the public review of proposals and have the authority to accept or reject changes to Swift.

What goes into a review?

The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:

  • What is your evaluation of the proposal?
  • Is the problem being addressed significant enough to warrant a change to Swift?
  • Does this proposal fit well with the feel and direction of Swift?
  • If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
  • How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

Please state explicitly whether you believe that the proposal should be accepted into Swift.

How to propose a change

  • Check prior proposals: many ideas come up frequently, and may either be in active discussion on the forums, or may have been discussed already and have joined the Commonly Rejected Proposals list. Please search the forums for context before proposing something new.
  • Consider the goals of the upcoming Swift release: Each major Swift release is focused on a specific set of goals described early in the release cycle. When proposing a change to Swift, please consider how your proposal fits in with the larger goals of the upcoming Swift release. Proposals that are clearly out of scope for the upcoming Swift release will not be brought up for review. If you can't resist discussing a proposal that you know is out of scope, please include the tag [Out of scope] in the subject.
  • Socialize the idea: propose a rough sketch of the idea in the "pitches" section of the Swift forums, the problems it solves, what the solution looks like, etc., to gauge interest from the community.
  • Develop the proposal: expand the rough sketch into a complete proposal, using the proposal template, and continue to refine the proposal on the forums. Prototyping an implementation and its uses along with the proposal is required because it helps ensure both technical feasibility of the proposal as well as validating that the proposal solves the problems it is meant to solve.
  • Request a review: initiate a pull request to the swift-evolution repository to indicate to the core team that you would like the proposal to be reviewed. When the proposal is sufficiently detailed and clear, and addresses feedback from earlier discussions of the idea, the pull request will be accepted. The proposal will be assigned a proposal number as well as a core team member to manage the review.
  • Address feedback: in general, and especially during the review period, be responsive to questions and feedback about the proposal.

Preparing an implementation

When you are ready to request a review, a pull request with an implementation is required in addition to your proposal. Proposals that can ship as part of the [Standard Library Preview package] should be paired with a pull request against the swift-evolution-staging repository. All other proposals should be paired with an implementation pull request against the main Swift repository.

The preview package can accept new types, new protocols, and extensions to existing types and protocols that can be implemented without access to standard library internals or other non-public features. For more information about the kinds of changes that can be implemented in the preview package, see SE-0264.

Review process

The review process for a particular proposal begins when a member of the core team accepts a pull request of a new or updated proposal into the swift-evolution repository. That core team member becomes the review manager for the proposal. The proposal is assigned a proposal number (if it is a new proposal), and then enters the review queue. If your proposal's accompanying implementation takes the form of a package, the review manager will merge your pull request into a new branch in the swift-evolution-staging repository.

The review manager will work with the proposal authors to schedule the review. Reviews usually last a single week, but can run longer for particularly large or complex proposals.

When the scheduled review period arrives, the review manager will post the proposal to the "Proposal reviews" section of the Swift forums with the proposal title and update the list of active reviews. To avoid delays, it is important that the proposal authors be available to answer questions, address feedback, and clarify their intent during the review period.

After the review has completed, the core team will make a decision on the proposal. The review manager is responsible for determining consensus among the core team members, then reporting their decision to the proposal authors and forums. The review manager will update the proposal's state in the swift-evolution repository to reflect that decision.

Proposal states

A given proposal can be in one of several states:

  • Awaiting review: The proposal is awaiting review. Once known, the dates for the actual review will be placed in the proposal document. When the review period begins, the review manager will update the state to Active review.
  • Scheduled for review (FULL_MONTH_NAME DAY...FULL_MONTH_NAME DAY): The public review of the proposal in the Swift forums has been scheduled for the specified date range.
  • Active review (FULL_MONTH_NAME DAY...FULL_MONTH_NAME DAY): The proposal is undergoing public review in the Swift forums. The review will continue through the specified date range.
  • Returned for revision: The proposal has been returned from review for additional revision to the current draft.
  • Withdrawn: The proposal has been withdrawn by the original submitter.
  • Deferred: Consideration of the proposal has been deferred because it does not meet the goals of the upcoming major Swift release. Deferred proposals will be reconsidered when scoping the next major Swift release.
  • Accepted: The proposal has been accepted and is either awaiting implementation or is actively being implemented.
  • Accepted with revisions: The proposal has been accepted, contingent upon the inclusion of one or more revisions.
  • Rejected: The proposal has been considered and rejected.
  • Implemented (Swift VERSION): The proposal has been implemented. Append the version number in parentheses—for example: Implemented (Swift 2.2). If the proposal's implementation spans multiple version numbers, write the version number for which the implementation will be complete.

Review announcement

When a proposal enters review, a new topic will be posted to the "Proposal Reviews" section of the Swift forums using the following template:


Hello Swift community,

The review of "<<PROPOSAL NAME>>" begins now and runs through <<REVIEW END DATE>>. The proposal is available here:

https://linkToProposal

Reviews are an important part of the Swift evolution process. All review feedback should be either on this forum thread or, if you would like to keep your feedback private, directly to the review manager. When emailing the review manager directly, please keep the proposal link at the top of the message.

What goes into a review?

The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:

  • What is your evaluation of the proposal?
  • Is the problem being addressed significant enough to warrant a change to Swift?
  • Does this proposal fit well with the feel and direction of Swift?
  • If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
  • How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

More information about the Swift evolution process is available at

https://github.com/apple/swift-evolution/blob/master/process.md

Thank you,

-<<REVIEW MANAGER NAME>>

Review Manager


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