example refactor snowflake module system

nix/snow/flake/modules/debug.nix

@@ -0,0 +1,78 @@
+{ config, flake-parts-lib, lib, options, getSystem, extendModules, ... }:
+let
+  inherit (lib)
+    mapAttrs
+    mkIf
+    mkOption
+    optionalAttrs
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkPerSystemOption
+    ;
+
+  mkDebugConfig = { config, options, extendModules }: config // {
+    inherit config;
+    inherit (config) _module;
+    inherit options;
+    inherit extendModules;
+  };
+in
+{
+  options = {
+    debug = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to add the attributes `debug`, `allSystems` and `currentSystem`
+        to the flake output. When `true`, this allows inspection of options via
+        `nix repl`.
+
+        ```
+        $ nix repl
+        nix-repl> :lf .
+        nix-repl> currentSystem._module.args.pkgs.hello
+        «derivation /nix/store/7vf0d0j7majv1ch1xymdylyql80cn5fp-hello-2.12.1.drv»
+        ```
+
+        Each of `debug`, `allSystems.<system>` and `currentSystem` is an
+        attribute set consisting of the `config` attributes, plus the extra
+        attributes `_module`, `config`, `options`, `extendModules`. So note that
+        these are not part of the `config` parameter, but are merged in for
+        debugging convenience.
+
+         - `debug`: The top-level options
+         - `allSystems`: The `perSystem` submodule applied to the configured `systems`.
+         - `currentSystem`: Shortcut into `allSystems`. Only available in impure mode.
+           Works for arbitrary system values.
+
+        See [Expore and debug option values](../debug.html) for more examples.
+      '';
+    };
+    perSystem = mkPerSystemOption
+      ({ options, config, extendModules, ... }: {
+        _file = ./formatter.nix;
+        options = {
+          debug = mkOption {
+            description = ''
+              Values to return in e.g. `allSystems.<system>` when
+              [`debug = true`](#opt-debug).
+            '';
+            type = types.lazyAttrsOf types.raw;
+          };
+        };
+        config = {
+          debug = mkDebugConfig { inherit config options extendModules; };
+        };
+      });
+  };
+
+  config = mkIf config.debug {
+    flake = {
+      debug = mkDebugConfig { inherit config options extendModules; };
+      allSystems = mapAttrs (_s: c: c.debug) config.allSystems;
+    } // optionalAttrs (builtins?currentSystem) {
+      currentSystem = (getSystem builtins.currentSystem).debug;
+    };
+  };
+}