add support for assertions and warnings

modules/system/default.nix

@@ -8,6 +8,12 @@ let
 
   cfg = config.system;
 
+  failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);
+
+
+  throwAssertions = res: if (failedAssertions != []) then throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}" else res;
+  showWarnings = res: fold (w: x: builtins.trace "warning: ${w}" x) res config.warnings;
+
 in
 
 {
@@ -42,11 +48,34 @@ in
       default = pkgs.lib.nixpkgsVersion;
     };
 
+    assertions = mkOption {
+      type = types.listOf types.unspecified;
+      internal = true;
+      default = [];
+      example = [ { assertion = false; message = "you can't enable this for that reason"; } ];
+      description = ''
+        This option allows modules to express conditions that must
+        hold for the evaluation of the system configuration to
+        succeed, along with associated error messages for the user.
+      '';
+    };
+
+    warnings = mkOption {
+      internal = true;
+      default = [];
+      type = types.listOf types.str;
+      example = [ "The `foo' service is deprecated and will go away soon!" ];
+      description = ''
+        This option allows modules to show warnings to users during
+        the evaluation of the system configuration.
+      '';
+    };
+
   };
 
   config = {
 
-    system.build.toplevel = stdenvNoCC.mkDerivation {
+    system.build.toplevel = throwAssertions (showWarnings (stdenvNoCC.mkDerivation {
       name = "darwin-system-${cfg.darwinLabel}";
       preferLocalBuild = true;
 
@@ -80,7 +109,7 @@ in
         echo -n "$darwinLabel" > $out/darwin-version
         echo -n "$system" > $out/system
       '';
-    };
+    }));
 
   };
 }