diff --git a/lang/builders/include/builders_utils.hpp b/lang/builders/include/builders_utils.hpp
new file mode 100644
index 0000000..ec17a44
--- /dev/null
+++ b/lang/builders/include/builders_utils.hpp
@@ -0,0 +1,46 @@
+#pragma once
+
+#include "executor.hpp"
+#include "name_tree.hpp"
+#include "nodes_storage.hpp"
+
+namespace builders {
+
+using Code = parser::ParseTree;
+using Exprs = nodes::ExpressionStorage;
+using Types = nodes::TypeStorage;
+using Names = names::NameTree;
+
+using Executor = utils::Executor<Exprs, Types, Names>;
+
+using Node = nodes::Node_<nodes::NodePart<utils::Pos>>; // TODO
+
+template <typename N>
+using Task = utils::Task<Executor, N, utils::None, parser::ParseTree::Node>;
+
+template <typename N> struct BuilderTask {
+  static_assert(false);
+};
+
+template <typename N> struct BuilderTaskBase : public Task<N> {
+  using Task<N>::Task;
+
+  template <typename OtherN>
+  OtherN Run(const parser::ParseTree::Node &node,
+             const utils::None &args = {}) {
+    BuilderTask<OtherN> task(this->executor);
+    return task(node, args);
+  }
+};
+
+// TODO
+// template <>
+// struct BuilderTask<nodes::ExpressionProxy>
+//     : public BuilderTaskBase<nodes::ExpressionProxy> {
+//   using BuilderTaskBase<nodes::ExpressionProxy>::BuilderTaskBase;
+
+//   nodes::ExpressionProxy operator()(const parser::ParseTree::Node &expr,
+//                                     const utils::None &args) override;
+// };
+
+} // namespace builders
diff --git a/lang/utils/include/executor.hpp b/lang/utils/include/executor.hpp
index ad588ac..4ea51d9 100644
--- a/lang/utils/include/executor.hpp
+++ b/lang/utils/include/executor.hpp
@@ -4,6 +4,8 @@
 
 namespace utils {
 
+struct None {};
+
 template <typename Exec, typename Ret, typename Args, typename Node> class Task;
 
 template <typename State> class ExecutorState {
@@ -39,7 +41,8 @@ private:
   Log log_;
 };
 
-template <typename Exec, typename Ret, typename Args, typename Node> class Task {
+template <typename Exec, typename Ret, typename Args, typename Node>
+class Task {
 public:
   Task(Exec &executor) : executor(executor) {}
 
@@ -47,14 +50,15 @@ public:
 
   //
 
-  // template <typename N, typename NArgs> Ret Run(const N& node, NArgs&& args) {
+  // template <typename N, typename NArgs> Ret Run(const N& node, NArgs&& args)
+  // {
   //   Task<Exec, Ret, NArgs, N> task(executor);
   //   return task(node, std::forward<Args>(args));
   // }
 
   //
 
-  virtual Ret operator()(const Node& node, const Args& args) = 0;
+  virtual Ret operator()(const Node &node, const Args &args = {}) = 0;
 
   //