Apple's intention when open-sourcing Swift was to provide a cross-platform, general-purpose programming language that is ready to use. The Swift standard library provides core language features and common collection types. However, this does not provide everything needed to get up and running.
Therefore, Apple provides a framework called Foundation to help you perform common programming tasks that aren't covered by the core Swift language and the standard library.
The Foundation framework that you will use when developing for Apple platforms is closed-sourced, which means the underlying code is not accessible and only the API is visible. However, when Apple open-sourced Swift and made it available for Linux, it became necessary to provide the Foundation framework as well. To this end, Apple has released an open source, Swift-based version...
