Dendritic Pattern for Nix Flakes

The Dendritic Pattern is an aspect-oriented approach to structuring Nix configurations using flake-parts. Every Nix file is a flake-parts module of the same type. Files organize by feature (aspect), not by configuration class.

Core Rules

Every file in ./modules/ is a flake-parts module. No exceptions, no other file types. This eliminates the question "what kind of Nix file is this?"

Each file implements a single feature across all configuration classes (NixOS, home-manager, nix-darwin, nixvim) that the feature applies to. The file path names the feature.

Values share through let bindings and flake-parts options, never through specialArgs or extraSpecialArgs.

Lower-level configurations (NixOS, home-manager, nix-darwin) store as option values using flake.modules.<class>.<aspect>, typically with the deferredModule type which provides beneficial merge semantics.

Structure

flake.nix           # Minimal: inputs + mkFlake + import-tree ./modules
modules/
  flake-parts.nix   # Imports flakeModules.modules (required)
  nixos.nix         # Wires configurations.nixos to flake outputs
  ssh.nix           # SSH config across all classes
  vim.nix           # Editor config across all classes
  vpn.nix           # VPN config across all classes
  desktop.nix       # Host composition from aspects
  users/
    vic.nix         # User-specific config across all classes

flake.nix stays minimal — a manifest of dependencies. All configuration logic lives in ./modules/.

Files can be freely renamed, moved, split, or reorganized. Paths carry no semantic meaning beyond naming the feature. Prefix a path segment with _ to exclude it from auto-import.

Instructions

Step: Verify Required Scaffolding

flake.modules is not a built-in flake-parts option. It lives in flake-parts.flakeModules.modules (an extra) and must be explicitly imported. Without it, multiple aspect files defining flake.modules produces: The option 'flake.modules' is defined multiple times.

Before creating aspect modules, verify a module imports this extra:

# modules/flake-parts.nix
{ lib, inputs, ... }:
{
  imports = [ inputs.flake-parts.flakeModules.modules ];
  config.flake.modules = lib.mkDefault {};
}

Also verify a wiring module exists for each configuration class in use. A wiring module defines a configurations.<class> option and maps it to flake outputs.

For example, a NixOS wiring module:

# modules/nixos.nix
{ lib, config, ... }:
{
  options.configurations.nixos = lib.mkOption {
    type = lib.types.lazyAttrsOf (lib.types.submodule {
      options.module = lib.mkOption {
        type = lib.types.deferredModule;
      };
    });
    default = {};
  };

  config.flake = {
    nixosConfigurations = lib.mapAttrs
      (name: { module }: lib.nixosSystem { modules = [ module ]; })
      config.configurations.nixos;

    checks =
      config.flake.nixosConfigurations
      |> lib.mapAttrsToList (
        name: nixos: {
          ${nixos.config.nixpkgs.hostPlatform.system} = {
            "configurations:nixos:${name}" =
              nixos.config.system.build.toplevel;
          };
        }
      )
      |> lib.mkMerge;
  };
}

Equivalent wiring modules follow the same shape for home-manager and nix-darwin, mapping to perSystem.legacyPackages.homeConfigurations or flake.darwinConfigurations respectively.

Step: Create Aspect Modules

Each file implements a single feature across all relevant configuration classes. Place all modules under ./modules/.

# modules/ssh.nix
{ config, ... }:
let
  scpPort = 2277;
in {
  flake.modules = {
    nixos.ssh = {
      services.openssh.enable = true;
      services.openssh.ports = [ scpPort ];
      networking.firewall.allowedTCPPorts = [ scpPort ];
    };

    darwin.ssh = {
      # macOS built-in SSH server config
    };

    homeManager.ssh = {
      # ~/.ssh/config, authorized_keys, private key secrets
    };
  };

  perSystem = { pkgs, ... }: {
    # Custom packages using SSH facilities
  };
}

The let binding shares scpPort across all classes without specialArgs. Everything needed for SSH lives in one file.

The flake.modules option is typed lazyAttrsOf (lazyAttrsOf deferredModule), keyed by class then aspect name. The deferredModule type merges values when multiple files contribute to the same aspect.

Step: Compose Hosts

Hosts compose by selecting aspects through config.flake.modules:

# modules/desktop.nix
{ config, ... }:
let
  inherit (config.flake.modules) nixos;
in
{
  configurations.nixos.desktop.module = {
    imports = [
      nixos.ssh
      nixos.vpn
      nixos.shell
    ];
    nixpkgs.hostPlatform = "x86_64-linux";
  };
}

Adding a feature to a host means adding one import to the list. Aspects are referenced through config.flake.modules.<class>, not through inputs.self. The same composition pattern applies to home-manager, nix-darwin, or any other configuration class with a wiring module.

Step: Auto-Import with import-tree

Because every file is a flake-parts module, bulk-import the entire ./modules/ directory:

# flake.nix
{
  inputs = {
    flake-parts.url = "github:hercules-ci/flake-parts";
    import-tree.url = "github:vic/import-tree";
  };

  outputs = inputs:
    inputs.flake-parts.lib.mkFlake { inherit inputs; }
    {
      imports = [ (inputs.import-tree ./modules) ];
    };
}

import-tree recursively imports all .nix files, ignoring paths containing /_. No manual import lists to maintain.

Step: Share Values Across Aspects

For values needed by multiple aspect files, define flake-parts options — never use specialArgs or extraSpecialArgs:

# modules/network-config.nix
{ lib, ... }:
{
  options.network.domain = lib.mkOption {
    type = lib.types.str;
    default = "example.com";
  };
}

# modules/ssh.nix
{ config, ... }:
{
  flake.modules.nixos.ssh = {
    services.openssh.banner = "Welcome to ${config.network.domain}";
  };
}

For values shared within a single file, use let bindings.

Optional: Flake Input Management with flake-file

Modules can declare their own required flake inputs:

# modules/vim.nix
{ inputs, ... }:
{
  flake-file.inputs.nixvim.url = "github:nix-community/nixvim";

  flake.modules.homeManager.vim = {
    # use inputs.nixvim
  };
}

vic/flake-file manages flake.nix automatically, keeping input declarations close to the code that uses them. This is optional — inputs can stay in flake.nix.

Anti-Patterns

Never use specialArgs or extraSpecialArgs. Share values through let bindings or flake-parts options.

Never organize by configuration class. Do not create nixos/, home-manager/, darwin/ directories. One file per feature, spanning all relevant classes.

Never manually list imports in flake.nix. Use import-tree or equivalent auto-import.

Never scatter a feature across multiple files by class. If SSH config lives in nixos/ssh.nix and home-manager/ssh.nix, consolidate into one file: modules/ssh.nix.

The pattern is not a framework or library. It is a convention for organizing flake-parts modules. It requires no code dependencies beyond flake-parts itself.

Common Issues

The option 'flake.modules' is defined multiple times

Cause: Missing import of inputs.flake-parts.flakeModules.modules. Solution: Verify modules/flake-parts.nix imports the extra. See the scaffolding step above.

Aspect module has no effect on host configuration

Cause: The aspect is defined but not imported in the host composition module. Solution: Add the aspect to the host's imports list via config.flake.modules.<class>.<aspect>.

Values not shared between configuration classes

Cause: Using specialArgs or passing values through function arguments instead of let bindings or flake-parts options. Solution: Define shared values in let at the top of the aspect file, or create a flake-parts option if multiple files need the value.

Aspect value needs access to lower-level module arguments

Cause: The aspect body needs NixOS/home-manager config or pkgs. Solution: Write the aspect value as a function:

flake.modules.nixos.shell = nixosArgs: {
  users.users.admin.shell = nixosArgs.config.programs.fish.package;
};

Here the outer config is flake-parts top-level config, while nixosArgs.config is the NixOS evaluation config.

Examples

Example: Adding a new feature aspect

Input: "Add tmux configuration to the flake." Actions:

1. Create modules/tmux.nix with flake.modules.homeManager.tmux
2. Add homeManager.tmux to host composition imports Result: Tmux config available to hosts that import it.

Example: Splitting a growing feature

Input: "The neovim module is too large." Actions:

1. Create modules/neovim/default.nix for core config
2. Create modules/neovim/_lang/go.nix for language-specific config (prefixed with _ so import-tree skips it; default.nix imports it explicitly or the _ prefix allows toggling by renaming) Result: Feature split into sub-files without changing the aspect interface.

Further Reading

See examples for detailed patterns including user modules, incremental features, deferredModule merge semantics, and community sharing through Dendrix.

References

• Pattern definition: github.com/mightyiam/dendritic
• Auto-import library: github.com/vic/import-tree
• Flake input management: github.com/vic/flake-file
• Dendrix community configs: dendrix.oeiuwq.com