Automatic override priority based on provider type

[Benna96]

I have a config that goes something like this:

{
  den.hosts.x86_64-linux.igloo.users.tux = { };
  den.homes.x86_64-linux."tux@igloo" = {};
  
  den.aspects.tux.homeManager = {config, ...}: {
    my.wallpaper.light = ./wallpapers/tux-light.png;
    my.wallpaper.dark = ./wallpapers/tux-dark.png;
  };
  
  den.aspects."tux@igloo".homeManager = {config, ...}: {
    my.wallpaper.light = ./wallpapers/optimized-for-igloo/tux-light.png;
    my.wallpaper.dark = ./wallpapers/optimized-for-igloo/tux-dark.png;
  };

It causes an error, since there's conflicting definitions. Fixable by using lib.mkOverride to make one override the other, e.g. by using lib.mkForce in the home aspect:

{lib, ...}
  ...
  
  den.aspects.tux.homeManager = {config, ...}: {
    ...
  };
  
  den.aspects."tux@igloo".homeManager = {config, ...}: lib.mkForce {
    ...
  };
}

Is there a way to define priority overriding in just one place, depending on aspect type? e.g. have all homeManager definitions from home aspects automatically get lib.mkForce applied to them upon merging?

(my.wallpaper is a custom nixos module option, I'm in the process of migrating to den.)

[theutz]

I'm not sure exactly what you're asking for here. Can you write out what your ideal API would be, maybe? Might help us figure out if 1) something like that exists already, or 2) if there's a better way to achieve what you want. :D

[Benna96]

Probably would want to define them in schema, maybe something like

# Priorities are defined in pkgs.lib.modules:
# https://github.com/NixOS/nixpkgs/blob/6e90d09d59dde7442b03cde4a2682b415f78848c/lib/modules.nix#L1564

# Lower number trumps higher number
# If not defined, either don't apply a priority, or apply 100 which is the default

# I assume `default` is for aspects that aren't hosts, users, or homes?
den.schema.default.overridePriorities.homeManager = 1000; # mkDefault
# den.schema.host.overridePriorities.homeManager = 100;
# den.schema.user.overridePriorities.homeManager = 100;
den.schema.home.overridePriorities.homeManager = 50; # mkForce

Which would turn

den.aspects.tux.homeManager = {
  my.wallpaper = ./wallpaper-tux-uses-by-default.png;
}

den.aspects."tux@igloo".homeManager = {
  my.wallpaper = ./wallpaper-tux-uses-on-igloo.png;
}

into

den.aspects.tux.homeManager = /*lib.mkOverride 100*/ {
  my.wallpaper = ./wallpaper-tux-uses-by-default.png;
}

den.aspects."tux@igloo".homeManager = lib.mkOverride 50 {
  my.wallpaper = ./wallpaper-tux-uses-on-igloo.png;
}

It'd be nice to have automatic override priorities depending on the aspect's type. Would simplify writing configs so I don't need to add mkDefault and mkForce to places.

On the other hand, it hides the priority from the place where you write the config, which could e.g. result in your config silently getting overridden instead of throwing a "conflicting definitions" error.

It's fine if it doesn't exist. It's just something I found myself hoping exists, at least for now when I'm migrating my existing config to den.

[sini]

I've been working on a custom module system and honestly I am looking to drop support for weighted overrides apart from mkForce/mkDefault. Arbitrarily weighted merge defaults from who knows where in the import tree just feel gross/like a bad idea. I'm instead implementing graph position based accumulation. There's also not a way to express with nixpkgs.lib eval module the necessary complexity to apply a recursive decent weighted merge strategy -- though possibly with gen there could be; I'm not not sure if we should.

[sini]

A rough sketch of what would be involved with gen: https://gist.github.com/sini/690d4902ff869395758d48a3505faf5a