flake-module: add flake-parts module (#5229)

Add a flake-parts module, output as flakeModules.home-manager and flakeModules.default.

The module defines options for flake.homeModules and flake.homeConfigurations, based on the respective nixos equivalents; flake.nixosModules and flake.nixosConfigurations.

docs/manual/nix-flakes.md

@@ -30,6 +30,7 @@ nix-flakes/prerequisites.md
 nix-flakes/standalone.md
 nix-flakes/nixos.md
 nix-flakes/nix-darwin.md
+nix-flakes/flake-parts.md
 ```
 
 

docs/manual/nix-flakes/flake-parts.md

@@ -0,0 +1,40 @@
+# flake-parts module {#sec-flakes-flake-parts-module}
+
+When using [flake-parts](https://flake.parts)
+you may wish to import home-manager's flake module,
+`flakeModules.home-manager`.
+
+``` nix
+{
+  description = "flake-parts configuration";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    home-manager.url = "github:nix-community/home-manager";
+    home-manager.inputs.nixpkgs.follows = "nixpkgs";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+  };
+
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      imports = [
+        # Import home-manager's flake module
+        inputs.home-manager.flakeModules.home-manager
+      ];
+      flake = {
+        # Define `homeModules`, `homeConfigurations`,
+        # `nixosConfigurations`, etc here
+      };
+      # See flake.parts for more features, such as `perSystem`
+    };
+}
+```
+
+The flake module defines the `flake.homeModules` and `flake.homeConfigurations`
+options, allowing them to be properly merged if they are defined in multiple
+modules.
+
+If you are only defining `homeModules` and/or `homeConfigurations` once in a
+single module, flake-parts should work fine without importing
+`flakeModules.home-manager`.
+

flake-module.nix

@@ -0,0 +1,32 @@
+{ lib, flake-parts-lib, moduleLocation, ... }:
+let inherit (lib) toString mapAttrs mkOption types;
+in {
+  options = {
+    flake = flake-parts-lib.mkSubmoduleOptions {
+      homeConfigurations = mkOption {
+        type = types.lazyAttrsOf types.raw;
+        default = { };
+        description = ''
+          Instantiated Home-Manager configurations.
+
+          `homeConfigurations` is for specific installations. If you want to expose
+          reusable configurations, add them to `homeModules` in the form of modules, so
+          that you can reference them in this or another flake's `homeConfigurations`.
+        '';
+      };
+      homeModules = mkOption {
+        type = types.lazyAttrsOf types.unspecified;
+        default = { };
+        apply = mapAttrs (k: v: {
+          _file = "${toString moduleLocation}#homeModules.${k}";
+          imports = [ v ];
+        });
+        description = ''
+          Home-Manager modules.
+
+          You may use this for reusable pieces of configuration, service modules, etc.
+        '';
+      };
+    };
+  };
+}

flake.nix

@@ -19,6 +19,11 @@
       # unofficial; deprecated in Nix 2.8
       darwinModule = self.darwinModules.default;
 
+      flakeModules = rec {
+        home-manager = import ./flake-module.nix;
+        default = home-manager;
+      };
+
       templates = {
         standalone = {
           path = ./templates/standalone;