Can someone explain to me the advantage of using Nix over containers? What do they offer that are not provided with using docker or other container platform.

Strictly compared to containers, the big advantages are reproducibility and lower overhead.

Overhead: Windows and macOS can't run Linux-based containers natively. Instead, there's always a full Linux virtual machine running in the background acting as an intermediary and host for your containers. Nix can conjure arbitrary native development environments on a per-command or per-terminal basis, giving you all the performance of directly running tools without the risk of clashing with systemwide software.

Reproducibility: Nix provides much stronger guarantees about the exact versions of software you're running. It effectively gives you a lockfile for your entire dependency chain, all the way down to libc. Containers tend to be more stateful: everyone on your team may be using the same Dockerfile, but if you build an image from it two weeks apart, you're probably going to get very different outputs due to things like your apt-get update step returning new versions of packages. This doesn't happen with Nix.

The beauty is that this isn't either/or; you can actually use Nix to generate OCI container images which are thus fully specified and repeatable.

Adding to this, my understanding is that Nix does not guarantee any Nix derivation/package will seamlessly run on Linux, Mac, and Windows. So in this one aspect it is less capable than Docker, i.m.h.o.

i.e. if a package depends on the systemd package https://search.nixos.org/packages?channel=unstable&show=syst... , Nix will not automatically find a replacement to run the package on Mac. But it may be possible to manually work around this with https://github.com/LnL7/nix-darwin

More on building Docker images with Nix: https://nix.dev/tutorials/building-and-running-docker-images

Nix runs on Windows exactly like Docker runs on Windows— only inside a Linux VM. If you ship a Linux VM on Mac or Windows like people usually do for Docker, you're free to run the Linux version of a Nix package on those platforms.

> i.e. if a package depends on the systemd package https://search.nixos.org/packages?channel=unstable&show=syst... , Nix will not automatically find a replacement to run the package on Mac. But it may be possible to manually work around this with https://github.com/LnL7/nix-darwin

Nix-Darwin doesn't do anything for packaging issues, it just offers an alternative module system for declaratively managing services and configurations in a NixOS-like way.

Docker containers also don't normally do any kind of service management, they're single-process images. They're not really comparable to orchestrated services managed by NixOS or Nix-Darwin modules. But if your concern is just shipping the same thing and you don't care about what's managing the services, a whole NixOS VM isn't any less efficient than some other VM in which you run `docker-compose` or Kubernetes.

That said, there are Nix-y ways to ship one or more supervised processes in a way that's portable across the platforms that Nix supports. One way is with a manually tailored supervisord config with a nix-shell or something like devenv.sh. Another strategy would be to leverage something an abstraction layer like this one: https://github.com/svanderburg/nix-processmgmt