For high level applications, Foundation is almost always imported and can pretty much be assumed. It's kind of a de facto standard library extension. The proble

For high level applications, Foundation is almost always imported and can pretty much be assumed. It's kind of a de facto standard library extension. The problem as you put it is that what's available on macOS/iOS is not fully available on other platforms. So you can't just take some software that uses Foundation and compile it for a non-apple system without potentially running into a bunch of things you need to implement or polyfill or whatever you want to call it. Foundation has been growing its support for other platforms over the years, so the situation has been getting better. But it's been hard to take the "open source cross platform" goal of Swift seriously when they there is clearly a priority given to apple platforms.