SLIP

Scala Library Improvement Process

Scala SLIPs

(jump forward to: Table of Contents, Active SLIP List)

When do I need a SLIP?

Many changes, including bug fixes and documentation improvements can be implemented and reviewed via the normal GitHub pull request workflow. See the Scala Bugfix Contribution Guide for more details.

Anything that will affect either binary or source code compatibility will need more than just an issue and a pull request, and that's where SLIPs come in. These are for alterations on, additions to or removals from the Scala core libraries. For these, a lightweight but formal proposal and review process will be carried out according to the guidelines in this document.

Please note that this process draws heavily from the Rust language RFCs process. Any similarity in this process or document is likely intentional, and should be considered flattery to the excellent work the Rust community has done in putting their guidelines together.

Active SLIP List

• 0022-async.md

Table of Contents

• Opening
• Active SLIP List
• Table of Contents
• When you need to follow this process
• Before creating a SLIP
• What is the process
• The role of the shepherd
• The SLIP life-cycle
• Reviewing SLIPs
• Implementing a SLIP
• SLIP Postponement
• Help this is all too informal!

When you need to follow this process

You need to follow this process if you intend to make "substantial" changes to the Scala core libraries, that is, the libraries distributed as part of the standard Scala download. What constitutes a "substantial" change is evolving based on community norms, but may include the following.

• Any addition or alteration of library features that are not a bugfix.
• Removing features.
• Anything that will affect binary or source level compatibility.

Some changes do not require a SLIP:

• Rephrasing, reorganizing, refactoring, or otherwise "changing shape does not change meaning".
• Additions that strictly improve objective, numerical quality criteria (warning removal, speedup, better platform coverage, more parallelism, trap more errors, etc.)
• Additions only likely to be noticed by other developers of Scala, invisible to users of Scala.

If you submit a pull request to implement a new feature without submitting a SLIP, it may be closed with a polite request to submit an SLIP first.

Before creating a SLIP

A hastily-proposed SLIP can hurt its chances of acceptance. Low quality proposals, proposals for previously-rejected features, or those that don't fit into the near-term roadmap, may be quickly rejected, which can be demotivating for the unprepared contributor. Laying some groundwork ahead of the SLIP can make the process smoother.

Although there is no single way to prepare for submitting a SLIP, it is generally a good idea to pursue feedback from other project developers beforehand, to ascertain that the SLIP may be desirable: having a consistent impact on the project requires concerted effort toward consensus-building.

The most common preparations for writing and submitting a SLIP include talking the idea over on scala-internals or on the scala/slip gitter channel, filing and discusssing ideas on the Scala issue tracker, and occasionally posting 'pre-SLIPs' on scala-internals for early review.

As a rule of thumb, receiving encouraging feedback from long-standing project developers, and particularly members of the core team is a good indication that the SLIP is worth pursuing.

What is the process

In short, to get a major feature added to the Scala core libraries, one must first get the SLIP merged into the SLIP repo as a markdown file. At that point the SLIP is 'active' and may be implemented with the goal of eventual inclusion into the core libs.

• Fork the SLIP repo http://github.com/scala/slip
• Copy slip-template.md to text/0000-my-feature.md (where 'my-feature' is descriptive. don't assign a SLIP number yet, use 0000).
• Fill in the SLIP. Put care into the details: SLIPs that do not present convincing motivation, demonstrate understanding of the impact of the design, or are disingenuous about the drawbacks or alternatives tend to be poorly-received.
• Submit a pull request. As a pull request the SLIP will receive design feedback from the larger community, and the author should be prepared to revise it in response.
• During triage, the pull request will either be closed (for SLIPs that clearly will not be accepted) or assigned a shepherd. The shepherd is a trusted developer who is familiar with the process, who will help to move the SLIP forward, and ensure that the right people see and review it.
• Build consensus and integrate feedback. SLIPs that have broad support are much more likely to make progress than those that don't receive any comments. The shepherd assigned to your SLIP should help you get feedback from Scala developers as well.
• The shepherd may schedule meetings with the author and/or relevant stakeholders to discuss the issues in greater detail, and in some cases the topic may be discussed at the larger monthly meeting. In either case a summary from the meeting will be posted back to the SLIP pull request.
• Once both proponents and opponents have clarified and defended positions and the conversation has settled, the shepherd will take it to the core team for a final decision.
• Eventually, someone from the core team will either accept the SLIP by merging the pull request, assigning the SLIP a number (corresponding to the pull request number), at which point the SLIP is 'active', or reject it by closing the pull request.

The role of the shepherd

During triage, every SLIP will either be closed or assigned a shepherd. The role of the shepherd is to move the SLIP through the process. This starts with simply reading the SLIP in detail and providing initial feedback. The shepherd should also solicit feedback from people who are likely to have strong opinions about the SLIP. Finally, when this feedback has been incorporated and the SLIP seems to be in a steady state, the shepherd will bring it to the meeting. In general, the idea here is to "front-load" as much of the feedback as possible before the point where we actually reach a decision.

The SLIP life-cycle

Once an SLIP becomes active then authors may implement it and submit the feature as a pull request to the SLIP repo. Being 'active' is not a rubber stamp, and in particular still does not mean the feature will ultimately be merged; it does mean that in principle all the major stakeholders have agreed to the feature and are amenable to merging it.

Furthermore, the fact that a given SLIP has been accepted and is 'active' implies nothing about what priority is assigned to its implementation, nor does it imply anything about whether a Scala developer has been assigned the task of implementing the feature. While it is not necessary that the author of the SLIP also write the implementation, it is by far the most effective way to see an SLIP through to completion: authors should not expect that other project developers will take on responsibility for implementing their accepted feature.

Modifications to active SLIP's can be done in followup PR's. We strive to write each SLIP in a manner that it will reflect the final design of the feature; but the nature of the process means that we cannot expect every merged SLIP to actually reflect what the end result will be at the time of the next major release; therefore we try to keep each SLIP document somewhat in sync with the language feature as planned, tracking such changes via followup pull requests to the document.

An SLIP that makes it through the entire process to implementation is considered 'complete' and is moved to the 'complete' folder; an SLIP that fails after becoming active is 'inactive' and moves to the 'inactive' folder.

Reviewing SLIPs

While the SLIP PR is up, the shepherd may schedule meetings with the author and/or relevant stakeholders to discuss the issues in greater detail, and in some cases the topic may be discussed at the larger [weekly meeting]. In either case a summary from the meeting will be posted back to the SLIP pull request.

The core team makes final decisions about SLIPs after the benefits and drawbacks are well understood. These decisions can be made at any time, but the core team will regularly issue decisions on at least a monthly basis. When a decision is made, the SLIP PR will either be merged or closed, in either case with a comment describing the rationale for the decision. The comment should largely be a summary of discussion already on the comment thread.

Implementing a SLIP

Some accepted SLIP's represent vital features that need to be implemented right away. Other accepted SLIP's can represent features that can wait until some arbitrary developer feels like doing the work. Every accepted SLIP has an associated issue tracking its implementation in the SLIP repository; thus that associated issue can be assigned a priority via the [triage process] that the team uses for all issues in the SLIP repository.

The author of an SLIP is not obligated to implement it. Of course, the SLIP author (like any other developer) is welcome to post an implementation for review after the SLIP has been accepted.

If you are interested in working on the implementation for an 'active' SLIP, but cannot determine if someone else is already working on it, feel free to ask (e.g. by leaving a comment on the associated issue).

SLIP Postponement

Some SLIP pull requests are tagged with the 'postponed' label when they are closed (as part of the rejection process). An SLIP closed with “postponed” is marked as such because we want neither to think about evaluating the proposal nor about implementing the described feature until after the next major release, and we believe that we can afford to wait until then to do so.

Usually an SLIP pull request marked as “postponed” has already passed an informal first round of evaluation, namely the round of “do we think we would ever possibly consider making this change, as outlined in the SLIP pull request, or some semi-obvious variation of it.” (When the answer to the latter question is “no”, then the appropriate response is to close the SLIP, not postpone it.)

Help this is all too informal!

The process is intended to be as lightweight as reasonable for the present circumstances. As usual, we are trying to let the process be driven by consensus and community norms, not impose more structure than necessary.