Debug Configurations

builtins.trace

Print values during evaluation:

den.aspects.foo = { user, ... }@context:
  (builtins.trace context {
    nixos = { };
  });

builtins.break

Drop into a REPL at any evaluation point:

den.aspects.foo = { user, ... }@context:
  (builtins.break context {
    nixos = { };
  });

Trace Context Keys

See which contexts are being applied:

den.default.includes = [
  (context: builtins.trace (builtins.attrNames context) { })
];

REPL Inspection

Load your flake and explore interactively:

$ nix repl
nix-repl> :lf .
nix-repl> nixosConfigurations.igloo.config.networking.hostName
"igloo"

Expose den for Inspection

Temporarily expose the den attrset as a flake output:

{ den, ... }: {
  flake.den = den;  # remove when done
}

Then in REPL:

nix-repl> :lf .
nix-repl> den.aspects.igloo
nix-repl> den.hosts.x86_64-linux.igloo

Manually Resolve an Aspect

Test how an aspect resolves for a specific class:

nix-repl> module = den.aspects.foo.resolve { class = "nixos"; aspect-chain = []; }
nix-repl> config = (lib.evalModules { modules = [ module ]; }).config

For parametric aspects, apply context first:

nix-repl> aspect = den.aspects.foo { host = den.hosts.x86_64-linux.igloo; }
nix-repl> module = aspect.resolve { class = "nixos"; aspect-chain = []; }

Inspect a Host’s Main Module

nix-repl> module = den.hosts.x86_64-linux.igloo.mainModule
nix-repl> config = (lib.nixosSystem { modules = [ module ]; }).config

Common Issues

Duplicate values in lists: Your function matches too many contexts. Use den.lib.take.exactly to restrict matching:

den.lib.take.exactly ({ host }: { nixos.x = 1; })

Missing attribute: The context doesn’t have the expected parameter. Trace context keys to see what’s available.

Infinite recursion: Aspects including each other in a cycle. Check your includes chains for circular dependencies.