nixosConfiguration: preSwitchCkeck: nvd

Allow to specify a list of classes in the host/<hostname>.nix (by
default the class <hostname> is used) that will automatically load
specified outputs of inputs.

export/nixosConfigurations.nix

@@ -10,11 +10,29 @@
 { ... }@inputs:
 { host, ... }@imports:
 let
+  pre-switch-check =
+    { pkgs, lib, config, ... }:
+    {
+      config.system.preSwitchChecks.nvd = ''
+        nvd_check=y
+        if [[ -d /run/current-system ]]
+        then
+          ${lib.getExe pkgs.nvd} --nix-bin-dir ${config.nix.package}/bin diff /run/current-system "''${1}"
+          if [[ -t 0 ]]; then
+            printf 'really %s to %s? [y]es/[N]o ' "''${2}" "''${1}"
+            read -r nvd_check
+          fi
+        fi
+        [[ "''${nvd_check}" == y ]]
+      '';
+    };
+
   system =
     hostname:
     {
       namespace ? "fbs42",
       nixpkgs ? "nixpkgs",
+      classes ? [ hostname ],
     }:
     inputs.${nixpkgs}.lib.nixosSystem {
       specialArgs = { inherit inputs flake namespace; };
@@ -24,6 +42,10 @@ let
             base: builtins.attrValues (flake.lib.scan { base = "${base}/host/${hostname}"; })
           ) flake.lib.schema.base
         )
+        ++ map (class: {
+          imports = map (input: input.${namespace}.${class} or { }) (builtins.attrValues inputs);
+        }) classes
+        ++ [ pre-switch-check ]
         ++ [ { config.networking.hostName = hostname; } ]
         ++ [ nixosModules.default ]
         ++ inputs.${nixpkgs}.lib.optional (inputs ? home-manager) {