Aspects & Functors

What is an aspect?

An aspect is a composable unit of configuration that bundles modules for one or more Nix classes (nixos, darwin, homeManager, or custom) in a single attrset. Aspects declare what behavior applies — one concern, all platforms. They compose via includes (DAG edges) and provides (named sub-aspects).

den.aspects.gaming = { host, user }: {
  nixos = { pkgs, ... }: { programs.steam.enable = true; };
  homeManager = { pkgs, ... }: { programs.mangohud.enable = true; };
  includes = [ den.aspects.performance ];
};

The `__functor` Pattern

Any Nix attrset with __functor can be called as a function:

let
  counter = {
    value = 42;
    __functor = self: n: self.value + n;
  };
in counter 8  # => 50

The __functor receives self (the attrset itself) and returns a function.

Aspects as Functors

Every aspect in Den has a default __functor that inspects context parameters to decide what to produce. Bare attrsets and functions work directly — den.lib.parametric is no longer needed:

# Bare functions work directly — no wrapper required
{
  nixos.foo = 1;
  includes = [
    ({ host }: { nixos.networking.hostName = host.hostName; })
    ({ user }: { homeManager.home.username = user.userName; })
  ];
}

Aspect Structure

Aspects have three kinds of attributes:

Owned Configurations

Direct settings per Nix class:

den.aspects.igloo = {
  nixos.networking.hostName = "igloo";
  darwin.nix-homebrew.enable = true;
  homeManager.programs.vim.enable = true;
};

Includes

A list forming a dependency graph:

den.aspects.igloo.includes = [
  # Static: applied unconditionally
  { nixos.programs.vim.enable = true; }

  # Parametric: applied when context matches argument shape
  ({ host, ... }: { nixos.time.timeZone = "UTC"; })

  # Reference to another aspect
  den.aspects.tools
];

Provides

Named sub-aspects scoped to this aspect, accessible via den.aspects.igloo._.name or den.aspects.igloo.provides.name:

den.aspects.igloo.provides.gpu = { host, ... }:
  lib.optionalAttrs (host ? gpu) {
    nixos.hardware.nvidia.enable = true;
  };

Resolution

Aspects are resolved for a specific class using the den.lib.aspects.resolve API.

module = den.lib.aspects.resolve "nixos" den.aspects.igloo;

Resolution collects all nixos attrs from owned configs, walks includes recursively, applies parametric dispatch, deduplicates, and produces a single deferredModule containing all NixOS configuration.