fix a extensibility and test for errors

nixite/html.nix

@@ -28,27 +28,31 @@ in rec {
         else
           keyvalue key value) attrs)));
 
-  tag = t: {
-    tag = t;
-    attrs = {};
+  tag = t:
+    ({
+      tag = t;
+      attrs = { };
+    } // baseTag);
+
+  baseTag = {
     __toString = self: toString (self "");
     __functor = self: child:
       (if !(isTag child) then
         (if isSet child then
-          self // ({attrs = self.attrs // child;})
+          incorporateAttrs self child
         else
-        throw "tag child must be tag, list, or string, got ${
-          builtins.typeOf child
-        }"
-        )
-      else {
-        tag = t;
-        attrs = self.attrs;
-        inherit child;
-        __toString = toHTML;
-      });
+          throw "tag child must be tag, list, or string, got ${
+            builtins.typeOf child
+          }")
+      else
+        self // {
+          inherit child;
+          __toString = toHTML;
+        });
   };
 
+  incorporateAttrs = self: attrs: self // ({ attrs = self.attrs // attrs; });
+
   isSet = a: builtins.isAttrs a && !a ? __toString;
 
   isTag = tag: