I don't like the solution, but the implementation seems good.

@lheckemann what would be a better solution?

Replacing NIX_PATH with Something Better™ (Flakes?). I don't have any wonderful short-term ideas, unfortunately.

Replacing NIX_PATH with Something Better™ (Flakes?). I don't have any wonderful short-term ideas, unfortunately.

I don't see how flakes will be able to replace the include paths at all. It might work for custom package sources but not for things like passing extra inputs to your builds. The user we encountered on the weekend had to use it to pass some SSH configuration into the build process. Also being able to use <mypkgs> in any expression on my system can probably not be replaced by whatever.flakes { url= https://notgithub.com/foo/bar; branch = …} since it becomes very repetitive.

Do not we have some functor that would modify list merge rule?

E.g. option = mergeList [ foo ]; vs. option = mkForce xxx [ foo ]

mkAfter does not seem to do anything. This, however, does:

{lib, ...}: {
  nix = {
    nixPath = lib.mkOptionDefault [
      "nixpkgs=/home/jtojnar/Projects/nixpkgs"
    ];
  };
}

$ nixos-option -I nixos-config=/etc/nixos/configuration.nix nix.nixPath
Value:
[ "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos" "nixos-config=/etc/nixos/configuration.nix" "/nix/var/nix/profiles/per-user/root/channels" "nixpkgs=/home/jtojnar/Projects/nixpkgs" ]

Default:
[ "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos" "nixos-config=/etc/nixos/configuration.nix" "/nix/var/nix/profiles/per-user/root/channels" ]

An alternative would be moving the default value from options to config section so it has the same priority (cf. environment.systemPath). But it would be a BC break.

IMHO we should avoid ad hoc extra... options (since you could add such an option for any list-typed option, which is rather ugly). If the issue is that the default search path gets overriden silently when you set a value, the solution is as @jtojnar suggested, i.e. change the default into a definition at regular priority:

config = {
  nix.nixPath = [
    "nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos"
    "nixos-config=/etc/nixos/configuration.nix"
    "/nix/var/nix/profiles/per-user/root/channels"
  ];
};

Maybe using mkAfter to ensure that it doesn't override the user settings.

I agree with @edolstra about mkAfter. If Nix path search algorithm works "whoever is first, wins" then this is enough to allow both "append" and "override single" intents. The release notes should be added though.

Thank you for your contributions.
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Still something I want to get back to eventually.. Keeping this open serves as a todo item for me.

I marked this as stale due to inactivity. → More info

Is this still an issue to resolve? What happens when -I NIX_PATH is passed to nix? It still seems to work, so I assume it's already falling back to default values or is that an incorrect assumption?