add lib for helper functions

flake.nix

@@ -1,7 +1,8 @@
 {
   description = "Declaratively update your data.";
   inputs.nixpkgs.url = "nixpkgs/nixos-unstable";
-  outputs = { ... }: {
+  outputs = { nixpkgs, ... }: {
+    lib = builtins.import ./lib.nix { pkgs = nixpkgs; };
     nixosModules = rec {
       fooud = builtins.import ./module.nix;
       default = fooud;

lib.nix

@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+let
+  post-recv = dest: pkgs.writeScriptBin "post-recv" ''
+    git clone . ${dest}
+  '';
+in
+let
+  lib.git = {
+    inherit post-recv;
+  };
+in
+  lib