GNU Guix is a functional package manager for the GNU system. Besides the usual package management features, it also supports transactional upgrades and roll-backs, per-user profiles, and much more. It is similar in design to the Nix package manager.
Ranked in these QuestionsQuestion Ranking
Pro Can setup a shell which has exactly the defined libraries available
A method which works across languages and provides a reproducible programming environment.
Pro Can always roll back to a safe state
Guix creates new profile generations for each operation. If anything goes wrong, a simple --roll-back gets you immediately back to the previous, working, generation. Because it is a purely functional package management system, generations don't affect each other, so you're back to the exact same state as before : still working.
Pro Can create independent packages
Guix pack creates packages which do not need Guix to be run.
Pro No side effects when building packages
Guix is a purely functional package management system. This means that the act of building a package does not have side effects, such as destructively updating or deleting files that may be used by other packages.
Pro Can build containers right-away, from docker to tarballs
See guix pack --help and https://www.gnu.org/software/guix/blog/2018/tarballs-the-ultimate-container-image-format/.
Pro Doesn't require root privileges
Normal users can install packages on a Guix-enabled system, or even run their own Guix instance if the system isn't Guix-enabled.
Pro Easy to add your own packages
The clean and declarative syntax makes it easy to define new packages by using an existing one as an example.
Con Updates take a long time
It's gotten better over time but both updating Guix itself and updating the installed packages can take a long time.
Con Cannot handle filetypes that have different semantics across different versions
While the functional approach that Guix takes is great for sandboxing binary artifacts of packages, it seriously lacks any power in handling configuration files or user data. It's difficult to upgrade and downgrade files where semantics and syntax can change between versions.