| There’s a few reasons, but mostly is due to economics and the way Apple builds its own software. Swift came out of the iOS side of the company, and was sponsored to make writing iOS apps easier. It was therefore constrained to fit into a particular shape; in particular, Objective-C compatibility was a must have. Many of the frameworks and libraries on iOS/macOS are implemented in Objective-C including “std” libraries like Foundation. When building Swift for Linux/Windows, there isn’t an Objective-C ecosystem, and rather than introduce one, instead the language just doesn’t have support for that. As a result, the “std” libraries are a complete rewrite of the libraries available on macOS. Even being able to include Posix basics used a different library name; Darwin vs GlibC. [There was a long battle to try and get a meta import of LibC which would import the right one on different platforms.] So there are really two dialects of the language: Apple Swift and Linux/Windows Swift. They share the same overall feel but is like the difference between Diesel and Petrol/Gas. Same overall outcome, different completely under the hood. Two other reasons exist; firstly, the internal builds are a fork of the open source project, so when a new feature lands on an iOS device, code and libraries are written to support that (eg SwiftUI) and nothing related to that fork lands until after Apple have announced it. What that means is you get giant blobs of diffs on an annual basis; in some cases, reverting bugs that have been fixed already in the open source world (because the internal version was forked six months previously). The other reason is that Swift depends on a forked version of llvm/clang, to the extent where you have to install those to be able to compile Swift programs. Many upstream distributions already ship their own clang, which is essentially incompatible with Swift, but they want to follow their philosophy of only one version of a library (and don’t want to replace their own version with a Swift specific one). So, Swift on iOS is a primary platform, and if you are writing iOS apps is the standard way. Every other platform is a second class citizen and software project management at Apple is not well suited to an open source workflow; since the iOS team are calling the shots on swift (and paying for most of it, to be fair) then the language evolves for their benefit primarily and then others as a side effect. |