extract home-manager assertions into own module

nix/hmModules/assertions/default.nix

@@ -0,0 +1,6 @@
+{
+  imports = [
+    # files
+    ./gpg-agent.nix
+  ];
+}

nix/hmModules/assertions/gpg-agent.nix

@@ -0,0 +1,31 @@
+{
+  config,
+  lib,
+  osConfig ? null,
+  ...
+}:
+let
+  cfg = config.services.gpg-agent;
+  hwSmartcards = osConfig.hardware.gpgSmartcards.enable;
+  scDaemon = cfg.enable && cfg.enableScDaemon;
+in
+{
+  config = lib.mkIf (!builtins.isNull osConfig) {
+
+    assertions = [
+      {
+        assertion = scDaemon -> hwSmartcards;
+        message = ''
+          gpg-agent’s scDaemon is enabled but NixOS hardware.gpgSmartcards is disabled
+        '';
+      }
+    ];
+
+    warnings = [
+      (lib.mkIf (hwSmartcards && !scDaemon) ''
+        NixOS hardware.gpgSmartcards is enabled but gpg-agent’s scDaemon is disabled
+      '')
+    ];
+
+  };
+}

nix/hmModules/default.nix

@@ -1,9 +1,13 @@
-{ ... }@flakeArg:
+{ lib, self, ... }@flakeArg:
 {
 
+  assertions.imports = lib.singleton ./assertions;
+
   # combination of all my custom modules
   # these should not change anything until you enable their custom options
   default.imports = [
+    # flake
+    self.assertions
     # directories
     ./extends
   ];

nix/nixos-modules/frontend/home.nix

@@ -18,23 +18,6 @@ let
 in
 {
 
-  # TODO exclude in own home-manager module
-  assertions =
-    let
-      hwSmartcards = osConfig.hardware.gpgSmartcards.enable;
-      scDaemon = with config.services.gpg-agent; enable && enableScDaemon;
-    in
-    [
-      {
-        assertion = hwSmartcards -> scDaemon;
-        message = "hardware.gpgSmartcards is enabled on system side but gpg-agent’s scDaemon is disabled";
-      }
-      {
-        assertion = scDaemon -> hwSmartcards;
-        message = "gpg-agent’s scDaemon is enabled but hardware.gpgSmartcards is disabled on system side";
-      }
-    ];
-
   home.stateVersion = osConfig.system.stateVersion;
 
   home.file = {