Add functor option for flake's __functor attribute

This allows for conveniently making flakes callable.

Setting this is expected to be uncommon in general, but having the
option is useful as flakelight module flakes can use this to reduce the
boilerplate in using them.

api_guide.md

@@ -684,6 +684,32 @@ These can be paths, which is preferred as it results in better debug output:
 }
 ```
 
+### functor
+
+The `functor` option allows you to make your flake callable.
+
+If it is set to a function, that function will be set as the `__functor`
+attribute of your flake outputs.
+
+Flakelight uses it so that calling your `flakelight` input calls
+`flakelight.lib.mkFlake`.
+
+As an example:
+
+```nix
+{
+  inputs.flakelight.url = "github:accelbread/flakelight";
+  outputs = { flakelight, ... }:
+    flakelight ./. {
+      outputs.testvalue = 5;
+      functor = self: x: x + self.testvalue;
+    }
+}
+```
+
+With the above flake, another flake that has imports it with the name `addFive`
+would be able to call `addFive 4` to get 9.
+
 ### meta
 
 The following options are available for configuring the meta attributes of the
@@ -746,6 +772,7 @@ The following options can be autoloaded (no module args):
 - nixosModule
 - homeModule
 - flakelightModule
+- functor
 
 ### flakelight
 

builtinModules/functor.nix

@@ -0,0 +1,19 @@
+# flakelight -- Framework for simplifying flake setup
+# Copyright (C) 2023 Archit Gupta <archit@accelbread.com>
+# SPDX-License-Identifier: MIT
+
+{ config, lib, ... }:
+let
+  inherit (lib) mkOption mkIf;
+  inherit (lib.types) functionTo nullOr raw uniq;
+in
+{
+  options.functor = mkOption {
+    type = nullOr (uniq (functionTo (functionTo raw)));
+    default = null;
+  };
+
+  config.outputs = mkIf (config.functor != null) {
+    __functor = config.functor;
+  };
+}

builtinModules/nixDir.nix

@@ -43,6 +43,7 @@ in
       homeConfigurations = autoImportArgs' [ "homeConfigurations" "home" ];
       flakelightModule = autoImport' "flakelightModule";
       flakelightModules = autoImportArgs' "flakelightModules";
+      functor = autoImport' "functor";
     in
     mkMerge [
       { _module.args = { inherit autoloadArgs; }; }
@@ -78,5 +79,6 @@ in
       (mkIf (homeConfigurations != null) { inherit homeConfigurations; })
       (mkIf (flakelightModule != null) { inherit flakelightModule; })
       (mkIf (flakelightModules != null) { inherit flakelightModules; })
+      (mkIf (functor != null) { inherit functor; })
     ];
 }

default.nix

@@ -2,14 +2,14 @@
 # Copyright (C) 2023 Archit Gupta <archit@accelbread.com>
 # SPDX-License-Identifier: MIT
 
-localInputs:
+nixpkgs:
 let
   inherit (builtins) isAttrs isPath readDir;
-  inherit (localInputs.nixpkgs.lib) attrNames composeManyExtensions
+  inherit (nixpkgs.lib) attrNames composeManyExtensions
     filter findFirst genAttrs getValues hasSuffix isFunction isList
     mapAttrsToList pathExists pipe removePrefix removeSuffix evalModules
     mkDefault mkOptionType singleton;
-  inherit (localInputs.nixpkgs.lib.types) coercedTo functionTo listOf;
+  inherit (nixpkgs.lib.types) coercedTo functionTo listOf;
 
   builtinModules = mapAttrsToList (k: _: ./builtinModules + ("/" + k))
     (readDir ./builtinModules);
@@ -17,7 +17,7 @@ let
   mkFlake = src: root: (evalModules {
     specialArgs.modulesPath = ./builtinModules;
     modules = builtinModules ++ [
-      { inputs.nixpkgs = mkDefault localInputs.nixpkgs; }
+      { inputs.nixpkgs = mkDefault nixpkgs; }
       { _module.args = { inherit src flakelight; }; }
       root
     ];
@@ -94,7 +94,4 @@ let
     let v = autoImport dir name; in
     if isFunction v then v args else v;
 in
-{
+flakelight
-  lib = flakelight;
-  __functor = _: mkFlake;
-}

flake.nix

@@ -6,10 +6,11 @@
   description =
     "A modular Nix flake framework for simplifying flake definitions";
   inputs.nixpkgs.url = "nixpkgs/nixos-unstable";
-  outputs = inputs:
+  outputs = { nixpkgs, ... }:
-    let flakelight = import ./. inputs; in
+    let lib = import ./. nixpkgs; in
-    flakelight ./. {
+    lib.mkFlake ./. {
-      outputs = flakelight;
+      outputs = { inherit lib; };
+      functor = _: lib.mkFlake;
       templates = import ./templates;
       checks.statix = pkgs: "${pkgs.statix}/bin/statix check";
     };