retroarch: add module

add a module for configuring retroarch through home-manager

modules/programs/retroarch.nix

@@ -0,0 +1,91 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  cfg = config.programs.retroarch;
+
+  enabledCores = lib.filterAttrs (_: core: core.enable) cfg.cores;
+in
+{
+  meta.maintainers = [
+    lib.hm.maintainers.jtrrll
+  ];
+
+  options.programs.retroarch = {
+    enable = lib.mkEnableOption "RetroArch";
+
+    package = lib.mkPackageOption pkgs "retroarch" {
+      default = "retroarch-bare";
+    };
+
+    finalPackage = lib.mkOption {
+      type = lib.types.package;
+      readOnly = true;
+      description = ''
+        Resulting RetroArch package.
+      '';
+    };
+
+    cores = lib.mkOption {
+      type = lib.types.attrsOf (
+        lib.types.submodule (
+          { name, ... }:
+          {
+            options = {
+              enable = lib.mkEnableOption "RetroArch core";
+              package = lib.mkPackageOption pkgs [ "libretro" name ] { };
+            };
+          }
+        )
+      );
+      default = { };
+      example = lib.literalExpression ''
+        {
+          mgba.enable = true;  # Uses pkgs.libretro.mgba
+          snes9x = {
+            enable = true;
+            package = pkgs.libretro.snes9x2010;
+          };
+          custom-core = {
+            enable = true;
+            package = pkgs.callPackage ./custom-core.nix { };
+          };
+        }
+      '';
+      description = ''
+        RetroArch cores to enable. You can provide custom core packages.
+      '';
+    };
+
+    settings = lib.mkOption {
+      type = lib.types.attrsOf lib.types.str;
+      default = { };
+      example = {
+        input_max_users = "4";
+        menu_scale_factor = "0.950000";
+        netplay_nickname = "username";
+        video_driver = "vulkan";
+        video_fullscreen = "true";
+      };
+      description = ''
+        RetroArch configuration settings.
+
+        See <https://github.com/libretro/RetroArch/blob/master/retroarch.cfg>
+        for available configuration options.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    programs.retroarch.finalPackage = (
+      cfg.package.wrapper {
+        inherit (cfg) settings;
+        cores = lib.mapAttrsToList (_: core: core.package) enabledCores;
+      }
+    );
+    home.packages = [ cfg.finalPackage ];
+  };
+}