NixOS - how is environment.systemPackages implemented

Question

I've started using NixOS. As far as I understand Nix is a purpose built functional language to avoid side-effects. All packages are part of a giant attribute set but as everything is lazily evaluated there is no problem. By accessing one member you basically "make something happen", but even then a "derivation" is just a value and you also need to explicitly "build" it too.

I wonder how environment.systemPackages is exactly implemented then and where I can find the source of it? (I've tried searching through nixpkgs but there were too many false positive hits.).

I just want to understand the whole process and possibly even hack up a config file so that a local user could have a declaratively managed list of packages installed (similar to systemPackages for all) versus using an imperative way of (automatically) running nix-env -i. I've seen ~/.config/nixpkgs/config.nix examples and am able to implement the packageOverrides function. But this way I would be just able to create some sort of new meta-package "including" others ones (still nothing would get automatically installed) or am I mistaken? I've heared about a thing called overlays - how does this concept fit into the picture?

Answer 1

users.users.<name?>.packages is what I was looking for.

I will try to rephrase the second part of my question (which made it quite confusing, sorry about it)

Suppose I have something like:

blah = [ pkgs.hello ];

is there any function f so that I could do:

f blah

and that would make package hello present on the system.

By now I realized that this is sort of a contradiction, because if there was, Nix wasn't free of side-effects. So I suppose Chris Stryczynski is right, there is some external tool for which environment.systemPackages is a thing but blah isn't. However I can probably implement f as

environment.systemPackages ++ blah

or something (treat the code samples as pseudo code and not as syntactically correct Nix).

Answer 2

In terms of how environment.systemPackages is implemented, I would assume this value would be retrieved from the various scripts like nixos-install, nixos-rebuild etc which can be found here: https://github.com/NixOS/nixpkgs/tree/master/nixos/modules/installer/tools

But it appears to just be a list (in the Nix language).

Having had a look at them myself, it would probably be helpful to first understand how Nix works and then the Nixos related functionality.