Revamped partial function application

* moved internals into separate header i.e. the `detail` namespace relating to `Apply`
* implemented automatic alias selection by implementing aliae of the basic variadic `type` template alias in different base classes
** variadic partial application is implemented in `detail::apply_variadic`
*** `detail::apply_single` and `detail::apply_pair` define aliae to `detail::apply_variadic`'s `type` template alias
*** both restricted aliae derive from `detail::apply_variadic`
** `Apply` derives from any of the aliae defining base classes depending on the count of placeholders as determined by `detail::count_placeholders`
*** `Apply` is guaranteed to always be derived from `detail::apply_variadic` one way or the other
* changed functions, test cases and examples depending on `Apply` accordingly
** `Length` had to be reimplemented without `Apply` as it doesn't allow usage of aliae expecting a different count of arguments anymore
*** this is a advantage in the sense that core functionality of _TypeAsValue_ now doesn't depend on this complex partial application implementation anymore
*** such functionality may be reimplemented separately from `Apply`
* removed unnecessary `tav` namespace prefixes

2 files changed, 97 insertions, 51 deletions

diff --git a/src/function/apply.h b/src/function/apply.h
index 8e780e0..21364a8 100644
--- a/src/function/apply.h
+++ b/src/function/apply.h
@@ -1,43 +1,12 @@
 #ifndef TYPEASVALUE_SRC_FUNCTION_APPLY_H_
 #define TYPEASVALUE_SRC_FUNCTION_APPLY_H_
 
-#include <type_traits>
+#include "conditional/if.h"
 
-#include "list/list.h"
-#include "list/operation/nth.h"
+#include "detail/apply.h"
 
 namespace tav {
 
-namespace detail {
-
-struct placeholder_tag { };
-
-template <typename Type>
-using is_placeholder = tav::Boolean<
-	std::is_base_of<placeholder_tag, Type>::value
->;
-
-template <int Index>
-struct placeholder : placeholder_tag { };
-
-template <
-	typename Partials,
-	typename Argument
->
-struct resolve_placeholder {
-	typedef Argument type;
-};
-
-template <
-	typename Partials,
-	int      Index
->
-struct resolve_placeholder<Partials, placeholder<Index>> {
-	typedef typename Nth<Size<Index>, Partials>::type type;
-};
-
-}
-
 typedef detail::placeholder<0> _0;
 typedef detail::placeholder<1> _1;
 typedef detail::placeholder<2> _2;
@@ -47,24 +16,15 @@ template <
 	template<typename...> class Function,
 	typename...                 Arguments
 >
-struct Apply {
-	template <typename... Partials>
-	using variadic_type = Function<
-		typename detail::resolve_placeholder<
-			typename tav::List<Partials...>::type,
-			Arguments
-		>::type...
-	>;
-
-	template <typename Partial>
-	using single_type = variadic_type<Partial>;
-
-	template <typename Partial0, typename Partial1>
-	using pair_type = variadic_type<Partial0, Partial1>;
-
-	template <typename Partial0, typename Partial1, typename Partial2>
-	using triple_type = variadic_type<Partial0, Partial1, Partial2>;
-};
+struct Apply : If<
+	(detail::count_placeholders<Arguments...>::type::value > 2),
+	detail::apply_variadic<Function, Arguments...>,
+	typename If<
+		detail::count_placeholders<Arguments...>::type::value == 2,
+		detail::apply_pair<Function, Arguments...>,
+		detail::apply_single<Function, Arguments...>
+	>::type
+>::type { };
 
 }
 
diff --git a/src/function/detail/apply.h b/src/function/detail/apply.h
new file mode 100644
index 0000000..bf60be1
--- /dev/null
+++ b/src/function/detail/apply.h
@@ -0,0 +1,86 @@
+#ifndef TYPEASVALUE_SRC_FUNCTION_DETAIL_APPLY_H_
+#define TYPEASVALUE_SRC_FUNCTION_DETAIL_APPLY_H_
+
+#include <type_traits>
+
+#include "list/list.h"
+#include "list/operation/nth.h"
+#include "list/operation/higher/query.h"
+
+namespace tav {
+namespace detail {
+
+struct placeholder_tag { };
+
+template <typename Type>
+using is_placeholder = tav::Boolean<
+	std::is_base_of<placeholder_tag, Type>::value
+>;
+
+template <int Index>
+struct placeholder : placeholder_tag { };
+
+template <
+	typename Partials,
+	typename Argument
+>
+struct resolve_placeholder {
+	typedef Argument type;
+};
+
+template <
+	typename Partials,
+	int      Index
+>
+struct resolve_placeholder<Partials, placeholder<Index>> {
+	typedef typename Nth<Size<Index>, Partials>::type type;
+};
+
+template <typename... Arguments>
+using count_placeholders = Count<
+	is_placeholder,
+	typename List<Arguments...>::type
+>;
+
+template <
+	template<typename...> class Function,
+	typename...                 Arguments
+>
+struct apply_variadic {
+	template <typename... Partials>
+	using type = Function<
+		typename resolve_placeholder<
+			typename tav::List<Partials...>::type,
+			Arguments
+		>::type...
+	>;
+};
+
+template <
+	template<typename...> class Function,
+	typename...                 Arguments
+>
+struct apply_single : apply_variadic<Function, Arguments...> {
+	template <typename Partial0>
+	using type = typename apply_variadic<
+		Function,
+		Arguments...
+	>::template type<Partial0>;
+};
+
+template <
+	template<typename...> class Function,
+	typename...                 Arguments
+>
+struct apply_pair : apply_variadic<Function, Arguments...> {
+	template <typename Partial0, typename Partial1>
+	using type = typename apply_variadic<
+		Function,
+		Arguments...
+	>::template type<Partial0, Partial1>;
+};
+
+}
+}
+
+#endif  // TYPEASVALUE_SRC_FUNCTION_DETAIL_APPLY_H_