Draft for overlays using flake.nix outputs.flakeModule

flake.nix

@@ -58,5 +58,7 @@
       nixosModules = {
         notDetected = ./nixos/modules/installer/scan/not-detected.nix;
       };
+
+      flakeModules.default = ./pkgs/top-level/flakeModule.nix;
     };
 }

pkgs/top-level/flakeModule.nix

@@ -0,0 +1,86 @@
+{ lib, config, ... }:
+let
+  inherit (lib) mkOption types;
+
+  nixpkgsOverlayType = types.mkOptionType {
+    name = "nixpkgsOverlay";
+    description = "A nixpkgs overlay function";
+    descriptionClass = "noun";
+    check = lib.isFunction;
+    merge = _loc: defs:
+      let
+        logWarning =
+          # TODO: do not warn when lib.mkOrder/mkBefore/mkAfter are used unambiguously
+          if builtins.length defs > 1
+          then builtins.trace "WARNING[nixpkgs.flakeModule]: Multiple overlays are applied in arbitrary order." null
+          else null;
+        overlays =
+          map (x: x.value)
+            # TODO: lib.warnIf to replace the seq + if
+            (builtins.seq
+              logWarning
+              defs);
+      in
+      lib.composeManyExtensions overlays;
+  };
+
+  mkPkgsFromArgsType = types.mkOptionType {
+    name = "mkPkgsFromArgs";
+    description = "A function that returns `pkgs` from a set of arguments";
+    descriptionClass = "noun";
+    check = lib.isFunction;
+  };
+
+  pkgsType = types.mkOptionType {
+    name = "nixpkgs";
+    description = "An evaluation of Nixpkgs; the top level attribute set of packages";
+    check = builtins.isAttrs;
+  };
+
+  nixpkgsSubmodule = with types; submodule {
+    options = {
+      overlays = mkOption {
+        type = listOf nixpkgsOverlayType;
+        description = "Nixpkgs overlays";
+        default = [ ];
+      };
+
+      mkPkgsFromArgs = mkOption {
+        type = mkPkgsFromArgsType;
+        description = "Returns `pkgs` from `system";
+        internal = true;
+        default = args:
+          import ./. ({
+            localSystem = { system = builtins.currentSystem; };
+          } // args);
+      };
+
+      allPkgsPerSystem = mkOption {
+        type = types.lazyAttrsOf types.unspecified;
+        description = "Attribute set of `pkgs` named by `system`";
+        internal = true;
+        default =
+          let
+            mkPkgsFromSystem = system: config.nixpkgs.mkPkgsFromArgs { inherit system; };
+          in
+          lib.genAttrs lib.systems.flakeExposed config.nixpkgs.mkPkgsFromSystem;
+      };
+
+      # TODO: develop or eliminate
+      pkgs = mkOption {
+        internal = true;
+        type = pkgsType;
+      };
+    };
+  };
+
+in
+{
+  options = {
+    nixpkgs = lib.mkOption {
+      type = nixpkgsSubmodule;
+      description = "Configuration of nixpkgs";
+      default = { };
+    };
+  };
+}