Cool! Indeed, Rez does something similar on the package management side. For Nix, this is almost a side-effect, however. Nix's primary task is to describe how

Cool! Indeed, Rez does something similar on the package management side. For Nix, this is almost a side-effect, however. Nix's primary task is to describe how to build packages, and the entire OS is just another package that groups all dependencies. You can make a single change to a file everything depends on and it will result in a new OS. Nix can evaluate the entire system configuration in seconds, and build or download missing binaries in parallel. As a result you can e.g. have a complete OS based on a different glibc (maybe with some patch you like) installed and running alongside the normal OS without the glibc patch. Packages that do not use glibc are simply shared.