Document NixOS module inputs

[domenkozar]

There's currently no reference documentation (that I could find) listing all module inputs.

For example modulesPath is quite uncommon.

[infinisil]

The NixOS module internally specifies these arguments:

• lib
• config
• options

The module system allows adding arbitrary other arguments however, using either _module.args, args (which is implemented with _module.args) or specialArgs (whose values can be used in imports). NixOS uses these to add some NixOS specific arguments (see https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/eval-config.nix):

• pkgs via _module.args
• baseModules via args
• extraModules via args
• modules via args
• modulesPath via specialArgs

Finally, the user can add arbitrary attributes as well using mainly _module.args

[roberth]

The NixOS module internally specifies these arguments:

The module system internally specifies [...]

It's not NixOS-specific; only pioneered with it. (I know you know; typo?)

pkgs

This one can be set through nixpkgs.pkgs as well, where at can be extended with overlays.

the user can add arbitrary attributes

Should we encourage this though? It's a flat namespace and it's not as powerful as options, iirc. So why not put your stuff in the config namespace?

[infinisil]

It's not NixOS-specific; only pioneered with it. (I know you know; typo?)

Oh yeah, should be "module system" as you suggest instead :)

Should we encourage this though? It's a flat namespace and it's not as powerful as options, iirc. So why not put your stuff in the config namespace?

I agree that in general it should be discouraged. Though defining options currently has the problem that the type that is being used can influence evaluation of the value. However, it just so happens that I worked on improving this today, see #132448

[infinisil]

I'm documenting _module.args in #165540