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Avoiding Lock-In to Legacy Protocol Designs

Introduction

We propose to deprecate or move protocols that shouldn't be a part of the standard library's public API going forward.

Swift-evolution threads: Late Pitch, Review

Motivation

We've always known that when Swift reached ABI stability (now slated for Swift 4), we would be committed to supporting many of the standard library's design decisions for years to come.

We only realized very recently that, although Swift 3.0 is not shipping with a stable ABI, the promise that Swift 3.0 code will work with Swift 4.0 means that many of the standard library's protocols will be locked down now. Especially where these protocols show up in refinement hierarchies, we won't be able to keep Swift 3 code working in the future without carrying them forward into future standard library binaries.

Proposed solution

The proposed changes are as follows:

  • Deprecate the Indexable protocols with a message indicating that they will be gone in Swift 4. These protocols are implementation details of the standard library designed to work around language limitations that we expect to be gone in Swift 4. There's no reason for anyone to ever touch these; users should always use a corresponding Collection protocol (e.g. instead of MutableIndexable, use MutableCollection).

    See this pull request for the detailed design. (This pull request was merged prematurely but will be reverted if the change isn't approved).

  • Deprecate the ExpressibleByStringInterpolation protocol with a message indicating that its design is expected to change. We know this protocol to be mis-designed and limited, but there's no time to fix it for Swift 3. If we knew how the new design should look, we might be able to calculate that the current API is supportable in a forward-compatible way (that's the case for Comparable, for example). Unfortunately, we do not know that yet.

    See this pull request for the detailed design.

  • Rename Streamable to TextOutputStreamable and add a deprecated Streamable typealias for it. Now that OutputStream been renamed to TextOutputStream, we should also move Streamable out of the way, at least to reduce confusion if and when the name is reused for other purposes.

    See the following pull requests for the detailed design. (These pull requests were merged prematurely but will be reverted if the change isn't approved). Multiple pull requests were used to keep the contiguous integration system healthy.

Detailed design

See the pull requests referenced above.

Impact on existing code

No code will stop compiling or behave differently due to this change, though the deprecations will produce warnings. Automatic migration will handle the renaming of Streamable, but other adjustments to suppress the warnings will have to be made manually.

Alternatives considered

We considered deprecating much more than just protocols, but on reconsideration, we think our other legacy APIs do not result in the same kind of lock-in for the standard library.