diff --git a/sprout/tuple/tuple/tuple_decl.hpp b/sprout/tuple/tuple/tuple_decl.hpp
index 59194d49..3c0d458e 100644
--- a/sprout/tuple/tuple/tuple_decl.hpp
+++ b/sprout/tuple/tuple/tuple_decl.hpp
@@ -297,7 +297,7 @@ namespace sprout {
 			SPROUT_CXX14_CONSTEXPR void
 			assign_impl(Tuple&& t, sprout::index_tuple<Indexes...>) {
 				typedef typename std::decay<Tuple>::type type;
-				sprout::swallow({(base_type::template get<Indexes>(*this) = sprout::move(type::template get<Indexes>(SPROUT_FORWARD(Tuple, t))), 0)...});
+				sprout::swallow({((base_type::template get<Indexes>(*this) = sprout::move(type::template get<Indexes>(SPROUT_FORWARD(Tuple, t)))), 0)...});
 			}
 			template<sprout::index_t... Indexes>
 			SPROUT_CONSTEXPR std::tuple<Types...>
diff --git a/sprout/utility/swallow.hpp b/sprout/utility/swallow.hpp
index 0546b240..83ce2137 100644
--- a/sprout/utility/swallow.hpp
+++ b/sprout/utility/swallow.hpp
@@ -13,11 +13,49 @@
 
 namespace sprout {
 	//
+	// swallow_t
 	// swallow
 	//
+	// example:
+	//	swallow({(void)pack, 0}...);
+	//	((void)pack, ..., swallow);	// right fold, since C++17
+	//	(swallow, ..., (void)pack);	// left fold, since C++17
+	//
+	struct swallow_t {
+	public:
+		template<typename T>
+		inline SPROUT_CXX14_CONSTEXPR void
+		operator()(std::initializer_list<T>) const SPROUT_NOEXCEPT {}
+	};
+	namespace {
+		SPROUT_STATIC_CONSTEXPR sprout::swallow_t swallow = {};
+	}	// anonymous-namespace
 	template<typename T>
-	inline SPROUT_CXX14_CONSTEXPR void
-	swallow(std::initializer_list<T>) SPROUT_NOEXCEPT {}
+	inline SPROUT_CONSTEXPR sprout::swallow_t
+	operator,(sprout::swallow_t, T&&) SPROUT_NOEXCEPT {
+		return sprout::swallow;
+	}
+	template<typename T>
+	inline SPROUT_CONSTEXPR sprout::swallow_t
+	operator,(T&&, sprout::swallow_t) SPROUT_NOEXCEPT {
+		return sprout::swallow;
+	}
+
+	//
+	// unused_t
+	// unused
+	//
+	struct unused_t {
+	public:
+		template<typename... Args>
+		SPROUT_CONSTEXPR sprout::unused_t const&
+		operator()(Args&&...) const SPROUT_NOEXCEPT {
+			return *this;
+		}
+	};
+	namespace {
+		SPROUT_STATIC_CONSTEXPR sprout::unused_t unused = {};
+	}	// anonymous-namespace
 }	// namespace sprout
 
 #endif	// #ifndef SPROUT_UTILITY_SWALLOW_HPP