All the libraries I have come across that provide Swift concurrency functionality either depend on an Objective-C library or are wrappers around the C based GCD. (iOS and OS X)
I'm just wondering if anyone knows of a pure Swift concurrency library? No dependencies. Or if not how might we go about making one?
The Swift language doesn't have constructs to support concurrency yet. Rumor has it that those are going to be added to a future version of the language, but in the meantime, I don't think you can do this without OS support.
GCD makes for fairly clean concurrent solutions in Swift, since GCD blocks are Swift closures. It's not platform-indpendent, however. (At least not outside the Apple ecosystem.)
I guess you could write a concurrency library that would run on all POSIX-compliant OS's by using POSIX, but Windows isn't really POSIX-compliant, so that might not be a perfect solution either.
In Swift 3 the GCD interface has been cleaned up and made much more "Swifty". Now there is a Dispatch class, that has class methods to do the things that used to use global functions.