In Java there is the idea of [100% Pure Java}](http://www.javacoffeebreak.com/faq/faq0006.html). Put simply it is the idea that your code contains no OS native calls. This idea came from the desire to have completely portable programs. As Swift migrates to different platforms I wonder if we will see this desire come to it. I assume this would be useful for server apps built with Swift. Also, it depends on the language itself. Does Swift and its standard library lend themselves to writing platform agnostic code. If the facilities to write server apps aren’t built into Swift and the standard library (like I/O or concurrency or whatever) then people will have to write and link in platform specific code which ends that dream.
I’m guessing that the easiest work is done in that there are Mac OS and Linux implementations of Swift which I assume means that 100% Pure Swift is possible on *nix based systems. I’m not sure how well the language plays with Windows since Apple hasn’t provided a Windows port.