Changed logic to typeswitch and added comments.

And also added my name to contrib list as promised before.

CONTRIBUTORS

@@ -6,3 +6,4 @@ Dusty Wilson
 Marek Majkowski
 Peter van Dijk
 Omri Bahumi
+Alex Sergeyev

msg.go

@@ -577,13 +577,16 @@ func packStructValue(val reflect.Value, msg []byte, off int, compression map[str
 		default:
 			return lenmsg, &Error{err: "bad kind packing"}
 		case reflect.Interface:
-			if data, ok := fv.Interface().(PrivateRdata); ok {
+			// PrivateRR is the only RR implementation that has interface field.
+			// therefore it's expected that this interface would be PrivateRdata
+			switch data := fv.Interface().(type) {
+			case PrivateRdata:
 				n, err := data.WriteByteSlice(msg[off:])
 				if err != nil {
 					return lenmsg, err
 				}
 				off += n
-			} else {
+			default:
 				return lenmsg, &Error{err: "bad kind interface packing"}
 			}
 		case reflect.Slice:
@@ -880,13 +883,16 @@ func unpackStructValue(val reflect.Value, msg []byte, off int) (off1 int, err er
 		default:
 			return lenmsg, &Error{err: "bad kind unpacking"}
 		case reflect.Interface:
-			if data, ok := fv.Interface().(PrivateRdata); ok {
+			// PrivateRR is the only RR implementation that has interface field.
+			// therefore it's expected that this interface would be PrivateRdata
+			switch data := fv.Interface().(type) {
+			case PrivateRdata:
 				n, err := data.ParseByteSlice(msg[off:rdend])
 				if err != nil {
 					return lenmsg, err
 				}
 				off += n
-			} else {
+			default:
 				return lenmsg, &Error{err: "bad kind interface unpacking"}
 			}
 		case reflect.Slice: