AFAICT, NixOS has no equivalent to the type https://guix.gnu.org/manual/en/html_node/operating_002dsyste... , which makes it clear what an OS contains, and allows you to query the OS.

Instead, NixOS has many options, whose only structure is that they form a tree. When trying to understand a host, it's unclear how to begin to analyze this tree: no option is at a higher level of abstraction than any other, or if it is, it's difficult to infer that.

Arguably I could just look at my host's config, and that will tell me what this host does, but it doesn't. It actually describes my how machine differs from the default NixOS machine. (How do I learn what a default NixOS machine do? Query for all services "enabled" option is set to true?)

With Guix, I follow the code: how is this host's OS "services" field populated.