Open source Swift

Apple released Swift as an open source project in December 2015, hosted at open-source version of Swift is similar from a runtime perspective on both Linux and OS X; however, the set of libraries available differ between the two platforms.

For example, the Objective-C runtime was not present in the initial release of Swift for Linux; as a result, several methods that are delegated to Objective-C implementations are not available. "hello".hasPrefix("he") compiles and runs successfully on OS X and iOS but is a compile error in the first Swift release for Linux. In addition to missing functions, there is also a different set of modules (frameworks) between the two platforms. The base functionality on OS X and iOS is provided by the Darwin module, but on Linux, the base functionality is provided by the Glibc module. The Foundation module, which provides many of the data types that are outside of the base-collections library, is implemented in Objective-C on OS X and iOS, but on Linux, it is a clean-room reimplementation in Swift. As Swift on Linux evolves, more of this functionality will be filled in, but it is worth testing on both OS X and Linux specifically if cross platform functionality is required.

Finally, although the Swift language and core libraries have been open sourced, this does not apply to the iOS libraries or other functionality in Xcode. As a result, it is not possible to compile iOS or OS X applications from Linux, and building iOS applications and editing user interfaces is something that must be done in Xcode on OS X.