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.
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.
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.
Everyone is welcome to propose, discuss, and review ideas to improve the Swift language and standard library on the swift-evolution mailing list. 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.
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.
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Check prior proposals: many ideas come up frequently, and may either be in active discussion on the mailing list, or may have been discussed already and have joined the Commonly Rejected Proposals list. Please check the mailing list archives and this list for context before proposing something new.
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Socialize the idea: propose a rough sketch of the idea on the swift-evolution mailing list, the problems it solves, what the solution looks like, etc., to gauge interest from the community.
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Develop the proposal: expand the rough sketch into a complete proposal, using the proposal template, and continue to refine the proposal on the evolution mailing list. Prototyping an implementation and its uses along with the proposal is encouraged, 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.
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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.
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Address feedback: in general, and especially during the review period, be responsive to questions and feedback about the proposal.
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), then enters the review queue.
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 swift-evolution mailing list with the subject "[Review]" followed by 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 mailing list. The review manager will update the proposal's state in the swift-evolution repository to reflect that decision.
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 and updated in the review schedule. When the review period begins, the review manager will update the state to under review.
- Under review: the proposal is undergoing public review on the swift-evolution mailing list.
- Under revision: the proposal is undergoing revision by the author(s) based on feedback from the review.
- 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. Once a proposal enters the "accepted" state, it gets placed into its target Swift release.
- Rejected: the proposal has been considered and rejected.
When a proposal enters review, an email using the following template will be sent to the swift-evolution-announce and swift-evolution mailing lists:
Hello Swift community,
The review of "<<PROPOSAL NAME>>" begins now and runs through <<REVIEW END DATE>>. The proposal is available here:
https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-proposal.md
Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at
or, if you would like to keep your feedback private, directly to the review manager. When replying, please try to keep the proposal link at the top of the message:
Proposal link:
Reply text
Other replies
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