diff --git a/sprout/detail/digits_to_int.hpp b/sprout/detail/digits_to_int.hpp
new file mode 100644
index 00000000..d7ff55ed
--- /dev/null
+++ b/sprout/detail/digits_to_int.hpp
@@ -0,0 +1,22 @@
+#ifndef SPROUT_DETAIL_DIGITS_TO_INT_HPP
+#define SPROUT_DETAIL_DIGITS_TO_INT_HPP
+
+#include <type_traits>
+#include <sprout/config.hpp>
+
+namespace sprout {
+	namespace detail {
+		template<typename IntType, char... Chars>
+		struct digits_to_int;
+		template<typename IntType, char C>
+		struct digits_to_int<IntType, C>
+			: public std::integral_constant<IntType, IntType(C - 48)>
+		{};
+		template<typename IntType, char Head, char... Tail>
+		struct digits_to_int<IntType, Head, Tail...>
+			: public std::integral_constant<IntType, 10 * IntType(Head - 48) + sprout::detail::digits_to_int<IntType, Tail...>::value>
+		{};
+	}	// namespace detail
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_DETAIL_DIGITS_TO_INT_HPP
diff --git a/sprout/functional/bind.hpp b/sprout/functional/bind.hpp
index 022187b2..90dee84f 100644
--- a/sprout/functional/bind.hpp
+++ b/sprout/functional/bind.hpp
@@ -3,5 +3,7 @@
 
 #include <sprout/config.hpp>
 #include <sprout/functional/bind/bind.hpp>
+#include <sprout/functional/bind/placeholder.hpp>
+#include <sprout/functional/bind/udl.hpp>
 
 #endif	// #ifndef SPROUT_FUNCTIONAL_BIND_HPP
diff --git a/sprout/functional/bind/bind.hpp b/sprout/functional/bind/bind.hpp
index cf620dd5..f1152a47 100644
--- a/sprout/functional/bind/bind.hpp
+++ b/sprout/functional/bind/bind.hpp
@@ -15,10 +15,14 @@
 #include <sprout/functional/ref.hpp>
 #include <sprout/functional/mem_fn.hpp>
 #include <sprout/functional/type_traits/weak_result_type.hpp>
+#include <sprout/functional/bind/placeholder.hpp>
 
 namespace sprout {
 	// 20.8.9 bind
 
+	//
+	// is_bind_expression
+	//
 	template<typename T>
 	struct is_bind_expression
 		: public std::false_type
@@ -36,99 +40,6 @@ namespace sprout {
 		: public sprout::is_bind_expression<T>
 	{};
 
-	template<typename T>
-	struct is_placeholder
-		: public std::integral_constant<int, 0>
-	{};
-	template<typename T>
-	struct is_placeholder<T const>
-		: public sprout::is_placeholder<T>
-	{};
-	template<typename T>
-	struct is_placeholder<T volatile>
-		: public sprout::is_placeholder<T>
-	{};
-	template<typename T>
-	struct is_placeholder<T const volatile>
-		: public sprout::is_placeholder<T>
-	{};
-
-	//
-	// placeholder
-	//
-	template<int N>
-	struct placeholder {};
-
-	namespace placeholders {
-		namespace {
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<1> _1 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<2> _2 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<3> _3 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<4> _4 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<5> _5 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<6> _6 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<7> _7 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<8> _8 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<9> _9 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<10> _10 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<11> _11 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<12> _12 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<13> _13 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<14> _14 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<15> _15 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<16> _16 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<17> _17 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<18> _18 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<19> _19 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<20> _20 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<21> _21 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<22> _22 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<23> _23 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<24> _24 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<25> _25 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<26> _26 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<27> _27 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<28> _28 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<29> _29 = {};
-			SPROUT_STATIC_CONSTEXPR sprout::placeholder<30> _30 = {};
-		}	// anonymous-namespace
-	}	// namespace placeholders
-	using sprout::placeholders::_1;
-	using sprout::placeholders::_2;
-	using sprout::placeholders::_3;
-	using sprout::placeholders::_4;
-	using sprout::placeholders::_5;
-	using sprout::placeholders::_6;
-	using sprout::placeholders::_7;
-	using sprout::placeholders::_8;
-	using sprout::placeholders::_9;
-	using sprout::placeholders::_10;
-	using sprout::placeholders::_11;
-	using sprout::placeholders::_12;
-	using sprout::placeholders::_13;
-	using sprout::placeholders::_14;
-	using sprout::placeholders::_15;
-	using sprout::placeholders::_16;
-	using sprout::placeholders::_17;
-	using sprout::placeholders::_18;
-	using sprout::placeholders::_19;
-	using sprout::placeholders::_20;
-	using sprout::placeholders::_21;
-	using sprout::placeholders::_2;
-	using sprout::placeholders::_23;
-	using sprout::placeholders::_24;
-	using sprout::placeholders::_25;
-	using sprout::placeholders::_26;
-	using sprout::placeholders::_27;
-	using sprout::placeholders::_28;
-	using sprout::placeholders::_29;
-	using sprout::placeholders::_30;
-
-	template<int N>
-	struct is_placeholder<sprout::placeholder<N> >
-		: public std::integral_constant<int, N>
-	{};
-
 	namespace detail {
 		struct no_tuple_element;
 
@@ -509,6 +420,9 @@ namespace sprout {
 		}
 	};
 
+	//
+	// is_bind_expression
+	//
 	template<typename Signature>
 	struct is_bind_expression<sprout::binder<Signature> >
 		: public std::true_type
diff --git a/sprout/functional/bind/placeholder.hpp b/sprout/functional/bind/placeholder.hpp
new file mode 100644
index 00000000..541ac947
--- /dev/null
+++ b/sprout/functional/bind/placeholder.hpp
@@ -0,0 +1,104 @@
+#ifndef SPROUT_FUNCTIONAL_BIND_PLACEHOLDER_HPP
+#define SPROUT_FUNCTIONAL_BIND_PLACEHOLDER_HPP
+
+#include <type_traits>
+#include <sprout/config.hpp>
+
+namespace sprout {
+	//
+	// placeholder
+	//
+	template<int N>
+	struct placeholder {};
+
+	namespace placeholders {
+		namespace {
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<1> _1 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<2> _2 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<3> _3 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<4> _4 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<5> _5 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<6> _6 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<7> _7 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<8> _8 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<9> _9 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<10> _10 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<11> _11 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<12> _12 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<13> _13 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<14> _14 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<15> _15 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<16> _16 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<17> _17 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<18> _18 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<19> _19 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<20> _20 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<21> _21 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<22> _22 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<23> _23 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<24> _24 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<25> _25 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<26> _26 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<27> _27 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<28> _28 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<29> _29 = {};
+			SPROUT_STATIC_CONSTEXPR sprout::placeholder<30> _30 = {};
+		}	// anonymous-namespace
+	}	// namespace placeholders
+	using sprout::placeholders::_1;
+	using sprout::placeholders::_2;
+	using sprout::placeholders::_3;
+	using sprout::placeholders::_4;
+	using sprout::placeholders::_5;
+	using sprout::placeholders::_6;
+	using sprout::placeholders::_7;
+	using sprout::placeholders::_8;
+	using sprout::placeholders::_9;
+	using sprout::placeholders::_10;
+	using sprout::placeholders::_11;
+	using sprout::placeholders::_12;
+	using sprout::placeholders::_13;
+	using sprout::placeholders::_14;
+	using sprout::placeholders::_15;
+	using sprout::placeholders::_16;
+	using sprout::placeholders::_17;
+	using sprout::placeholders::_18;
+	using sprout::placeholders::_19;
+	using sprout::placeholders::_20;
+	using sprout::placeholders::_21;
+	using sprout::placeholders::_2;
+	using sprout::placeholders::_23;
+	using sprout::placeholders::_24;
+	using sprout::placeholders::_25;
+	using sprout::placeholders::_26;
+	using sprout::placeholders::_27;
+	using sprout::placeholders::_28;
+	using sprout::placeholders::_29;
+	using sprout::placeholders::_30;
+
+	//
+	// is_placeholder
+	//
+	template<typename T>
+	struct is_placeholder
+		: public std::integral_constant<int, 0>
+	{};
+	template<typename T>
+	struct is_placeholder<T const>
+		: public sprout::is_placeholder<T>
+	{};
+	template<typename T>
+	struct is_placeholder<T volatile>
+		: public sprout::is_placeholder<T>
+	{};
+	template<typename T>
+	struct is_placeholder<T const volatile>
+		: public sprout::is_placeholder<T>
+	{};
+	template<int N>
+	struct is_placeholder<sprout::placeholder<N> >
+		: public std::integral_constant<int, N>
+	{};
+}	// namespace sprout
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_BIND_PLACEHOLDER_HPP
diff --git a/sprout/functional/bind/udl.hpp b/sprout/functional/bind/udl.hpp
new file mode 100644
index 00000000..558461bf
--- /dev/null
+++ b/sprout/functional/bind/udl.hpp
@@ -0,0 +1,48 @@
+#ifndef SPROUT_FUNCTIONAL_BIND_UDL_HPP
+#define SPROUT_FUNCTIONAL_BIND_UDL_HPP
+
+#include <sprout/config.hpp>
+#include <sprout/functional/bind/placeholder.hpp>
+
+#if SPROUT_USE_USER_DEFINED_LITERALS
+
+#include <sprout/type_traits/identity.hpp>
+#include <sprout/detail/digits_to_int.hpp>
+
+namespace sprout {
+	//
+	// placeholders_result
+	//
+	template<char... Chars>
+	struct placeholders_result
+		: public sprout::identity<
+			sprout::placeholder<sprout::detail::digits_to_int<int, Chars...>::value>
+		>
+	{};
+
+	namespace placeholders {
+		namespace udl {
+			//
+			// _
+			//
+			template<char... Chars>
+			SPROUT_CONSTEXPR typename sprout::placeholders_result<Chars...>::type
+			operator"" _() {
+				typedef typename sprout::placeholders_result<Chars...>::type type;
+				return type();
+			}
+		}	// namespace udl
+	}	// namespace placeholders
+
+	namespace udl {
+		namespace placeholders {
+			using sprout::placeholders::udl::operator"" _;
+		}	// namespace placeholders
+
+		using sprout::placeholders::udl::operator"" _;
+	}	// namespace udl
+}	// namespace sprout
+
+#endif	// #if SPROUT_USE_USER_DEFINED_LITERALS
+
+#endif	// #ifndef SPROUT_FUNCTIONAL_BIND_UDL_HPP
diff --git a/sprout/functional/ref.hpp b/sprout/functional/ref.hpp
index bd231c67..e779d0ab 100644
--- a/sprout/functional/ref.hpp
+++ b/sprout/functional/ref.hpp
@@ -161,7 +161,7 @@ namespace sprout {
 		// invocation
 		template<typename... Args>
 		SPROUT_CONSTEXPR typename std::result_of<T& (Args&&...)>::type
-		operator() (Args&&... args) const {
+		operator()(Args&&... args) const {
 			return (*t_)(sprout::forward<Args>(args)...);
 		}
 	};
@@ -285,7 +285,7 @@ namespace sprout {
 	// unwrap_ref
 	//
 	template<typename T>
-	inline typename sprout::unwrap_reference<T>::type&
+	inline SPROUT_CONSTEXPR typename sprout::unwrap_reference<T>::type&
 	unwrap_ref(T& t) {
 		return t;
 	}
diff --git a/sprout/index_tuple/udl.hpp b/sprout/index_tuple/udl.hpp
index aafdc9db..0855236e 100644
--- a/sprout/index_tuple/udl.hpp
+++ b/sprout/index_tuple/udl.hpp
@@ -6,33 +6,18 @@
 
 #if SPROUT_USE_USER_DEFINED_LITERALS
 
-#include <type_traits>
 #include <sprout/index_tuple/make_index_tuple.hpp>
+#include <sprout/detail/digits_to_int.hpp>
 
 namespace sprout {
-	namespace detail {
-		template<typename IntType, char... Chars>
-		struct digits_to_int;
-		template<typename IntType, char C>
-		struct digits_to_int<IntType, C>
-			: public std::integral_constant<IntType, IntType(C - 48)>
-		{};
-		template<typename IntType, char Head, char... Tail>
-		struct digits_to_int<IntType, Head, Tail...>
-			: public std::integral_constant<IntType, 10 * IntType(Head - 48) + sprout::detail::digits_to_int<IntType, Tail...>::value>
-		{};
-	}	// namespace detail
-
 	//
 	// indexes_result
+	// uindexes_result
 	//
 	template<char... Chars>
 	struct indexes_result
 		: public sprout::make_index_tuple<sprout::detail::digits_to_int<sprout::index_t, Chars...>::value>
 	{};
-	//
-	// uindexes_result
-	//
 	template<char... Chars>
 	struct uindexes_result
 		: public sprout::make_uindex_tuple<sprout::detail::digits_to_int<sprout::uindex_t, Chars...>::value>
@@ -42,16 +27,13 @@ namespace sprout {
 		namespace indexes {
 			//
 			// _indexes
+			// _uindexes
 			//
 			template<char... Chars>
 			SPROUT_CONSTEXPR typename sprout::indexes_result<Chars...>::type
 			operator"" _indexes() {
 				return sprout::indexes_result<Chars...>::make();
 			}
-
-			//
-			// _uindexes
-			//
 			template<char... Chars>
 			SPROUT_CONSTEXPR typename sprout::uindexes_result<Chars...>::type
 			operator"" _uindexes() {