modules

The list of arguments to a module are

• config: option definitions (setting options)
• lib: Nixpkg's library functions
• pkgs: reference to Nixpkgs
• options: option declarations (defining options)
• modulesPath: path to Nixpkg's NixOS modules

These are documented in NixOS Manual - Writing NixOS Modules.

Using lib instead of pkgs.lib can sometime prevent infinite recursion errors.