Set a option with its default value in nixos configuration

I want to add a new path into nix.nixPath. I have some options to do this, such as edting NIX_PATH in shell’s profiles.

But I want to set it in configuration.nix. I can get default value from website or nixpkgs repo.
I wonder how I can get it using nix expression in configuration.nix. I know it is lazy in config, so using argument named config cannot work.

For example(what i want to do):

{ config, ... }:
{ a = config.a ++ [ sth ]; }

List options are merged, not replaced, so { a = [ sth ]; } should be enough.

my system version is 19.03, and it is replaced, not merged.
The only merged one is systemPackgaes, the other, such as nix.nixPath is not for me

Give

nix.nixPath = mkOptionDefault [
  "my-path=/the/path/you/want/to/add"
];

a try if you want to merge with the option’s default value.

This can work. Thanks.

As an alternative, more literal approach, this could work:

{ config, options, ... }:
{ nix.nixPath = options.nix.nixPath.default ++ [sth]; }