And Guix has put that in an beautiful form. There are two things which make Guix special:

1. The package definitions are just a normal, battle-proven, very well defined general-purpose, functional-style supporting programming language (Scheme).

2. There is no conceptual difference between a package definition in the public Guix system, and a self-written package definition which a developers makes to build and test his own package, or to build and run a specific piece of software. The difference is equally small as between using an Emacs package, and configuring that package in ones .emacs configuration file.