Now that you mention Quake 3, this article [1] by John Carmack himself on the subject is interesting, and then someone reviewed the source code from Doom 3 [2]

Now that you mention Quake 3, this article [1] by John Carmack himself on the subject is interesting, and then someone reviewed the source code from Doom 3 [2] where apparently they put some of that in practice. [1] https://web.archive.org/web/20130819160454/http://www.altdevblogaday.com/2012/04/26/functional-programming-in-c/ https://web.archive.org/web/20130819160454/http://www.altdev... [2] http://kotaku.com/5975610/the-exceptional-beauty-of-doom-3s-source-code?post=56177550 http://kotaku.com/5975610/the-exceptional-beauty-of-doom-3s-...