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API Design Guidelines

Reviewer notes

This review is part of a group of three related reviews, running concurrently:

These reviews are running concurrently because they interact strongly (e.g., an API change in the standard library will correspond to a particular guideline, or an importer rule implements a particular guideline, etc.). Because of these interactions, and to keep discussion manageable, we ask that you:

  • Please get a basic understanding of all three documents before posting review commentary
  • Please post your review of each individual document in response to its review announcement. It's okay (and encouraged) to make cross-references between the documents in your review where it helps you make a point.


The design of commonly-used libraries has a large impact on the overall feel of a programming language. Great libraries feel like an extension of the language itself, and consistency across libraries elevates the overall development experience. To aid in the construction of great Swift libraries, one of the major goals for Swift 3 is to define a set of API design guidelines and to apply those design guidelines consistently.

Proposed solution

The proposed API Design Guidelines are available at

The sources for these guidelines are available at Pull requests for trivial copyediting changes are most welcome. More substantive changes should be handled as part of the review process.

Impact on existing code

The existence of API Design Guidelines has no specific impact on existing code. However, two companion proposals that apply these guidelines to the Standard Library and via the Clang importer will have a massive impact on existing code, changing a significant number of APIs.

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