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

1 files changed, 86 insertions, 0 deletions

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_