Ahead of the planned 2019 release of Swift 5, Apple has released the intermediate Swift 4.1 version and is working on Swift 4.2.
Next version: Key features in Apple’s Swift 4.2
Now available in beta as part of the beta Xcode 10 IDE, the planned Version 4.2 release of Apple’s Swift language is focused on stabilizing the language’s ABI (application binary interface). Swift 4.2 will serve as an intermediary step to achieving ABI stability in the planned Swift 5.0 release due in 2019. ABI stability allows for binary compatibility between applications and libraries compiled with different Swift releases.
ABI changes in Swift 4.2, many of which tie into performance, are to be rolled out incrementally so there can be time for user feedback before changes are locked into place in the ABI.
Although Apple’s goal is stability, Swift 4.2 is not binary-compatible with previous releases. Source compatibility, however, is planned. Most source code built with the Swift 4.0 compiler should compile with the Swift 4.2 compiler and the Swift 4.1 compiler.