From 77771f1d6d0f53afee937b8e017b46f0427de914 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Mon, 22 Nov 2021 22:44:28 +0100
Subject: [PATCH] Add overlay

---
 all-modules.nix     |  1 +
 modules/overlay.nix | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 modules/overlay.nix

diff --git a/all-modules.nix b/all-modules.nix
index c9049f6..f6e508c 100644
--- a/all-modules.nix
+++ b/all-modules.nix
@@ -7,6 +7,7 @@
     ./modules/flake.nix
     ./modules/legacyPackages.nix
     ./modules/nixosModules.nix
+    ./modules/overlay.nix
     ./modules/packages.nix
     ./modules/perSystem.nix
   ];
diff --git a/modules/overlay.nix b/modules/overlay.nix
new file mode 100644
index 0000000..88405ae
--- /dev/null
+++ b/modules/overlay.nix
@@ -0,0 +1,32 @@
+{ config, lib, flake-modules-core-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-modules-core-lib)
+    mkSubmoduleOptions
+    ;
+in
+{
+  options = {
+    flake = mkSubmoduleOptions {
+      overlay = mkOption {
+        # uniq should be ordered: https://github.com/NixOS/nixpkgs/issues/147052
+        # also update description when done
+        type = types.uniq (types.functionTo (types.functionTo (types.lazyAttrsOf types.unspecified)));
+        # This eta expansion exists for the sole purpose of making nix flake check happy.
+        apply = f: final: prev: f final prev;
+        default = _: _: { };
+        defaultText = lib.literalExpression or lib.literalExample ''final: prev: {}'';
+        description = ''
+          An overlay.
+
+          Note that this option's type is not mergeable. While overlays can be
+          composed, the order of composition is significant, but the module
+          system does not guarantee deterministic definition ordering.
+        '';
+      };
+    };
+  };
+}