After officially delivering Swift 4.2, the Swift team is now focusing on Swift 5 by kicking off the final phase of its release process. Planned to be released early 2019, Swift 5 aims to bring ABI stability to the language while preserving source compatibility.
ABI stability can be roughly understood as “binary compatibility” across Swift versions. ABI stability would enable linking a framework into a program irrespective of which compiler versions were used to compile them, provided both compilers produce code conforming to the upcoming Swift ABI. This is a crucial feature for third party framework developers as well as to broaden Swift adoption inside of Apple. ABI stability was originally planned for Swift 3, but then got delayed due to the large amount of new language features that were in the workings.
As noted, ABI stability does not include what Apple is calling module stability, which can be described as interface to libraries being forward-compatible with future compiler versions. In other words, while ABI stability affects the linkability of a framework into a program at runtime, module stability concerns the possibility of using a library built with a previous compiler version (e.g., Swift 5) when building a program using a newer compiler (e.g., Swift 6). Module stability is surely desirable to have, since it can make developers life easier on a number of accounts, but is not critical. It is not yet clear whether Module stability will make it into Swift 5 or not.
Swift 5 will also include a number of new language features, some of which have already been implemented, so there are no doubts about them making into the release. Those include, among others:
- Identity key path, which provides a way to reference the entire value a key path is applied to using the
selfpseudo-property. count(where:), which adds the possibility to count the number of elements in aSequencethat satisfy a boolean expression.- Raw string delimiters, which allow developers to include any characters that would otherwise require escaping in a string delimited through
"""as opposed to using simple quotes (").
Other features that are still under development include flattening nested optionals from try?, simd vectors, user-defined dynamically "callable" types, and more.
All new language features will not break source compatibility, meaning that the majority of sources compatible with the Swift 4.2 compiler should compile with the Swift 5.0 compiler. Swift versions previous to 4.2 will not be source compatible, though, and should be upgraded at least to Swift 4.2. The development team will regularly post downloadable snapshots of intermediate releases towards 5.0.