Implemented partial function application prototype

* function `Apply` enables partial supply of the template parameters of a given _function_
** unsupplied parameters may be marked by _placeholders_, i.e. approach is similar to `std::bind`
** returns a template type taking the remaining, unsupplied parameters of the partially applied _function_
* this is a prototype in the sense, that neither full application using `Apply` or partial application of higher order functions is currently supported
** e.g. a `single_type` alias type declaration is required as the variadic template `template<typename...>` doesn't directly map to `template<typename>`
* appropriate test cases document current feature set
* the goal is to enable a concise defintion of _lambda-like_ expressions for easy specialization of e.g. functors provided to higher order functions

2 files changed, 87 insertions, 0 deletions

diff --git a/src/function/apply.h b/src/function/apply.h
new file mode 100644
index 0000000..d353169
--- /dev/null
+++ b/src/function/apply.h
@@ -0,0 +1,64 @@
+#ifndef TYPEASVALUE_SRC_FUNCTION_APPLY_H_
+#define TYPEASVALUE_SRC_FUNCTION_APPLY_H_
+
+#include <type_traits>
+
+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;
+typedef detail::placeholder<3> _3;
+
+template <
+	template<typename...> class Function,
+	typename...                 Arguments
+>
+struct Apply {
+	typedef typename tav::List<Arguments...>::type argument_list;
+
+	template <typename... Partials>
+	using type = Function<
+		typename detail::resolve_placeholder<
+			typename tav::List<Partials...>::type,
+			Arguments
+		>::type...
+	>;
+
+	template <typename Partials>
+	using single_type = type<Partials>;
+};
+
+}
+
+#endif  // TYPEASVALUE_SRC_FUNCTION_APPLY_H_
diff --git a/test.cc b/test.cc
index 62c6f85..6f95ca9 100644
--- a/test.cc
+++ b/test.cc
@@ -14,6 +14,8 @@
 #include "list/generator/make_list.h"
 #include "list/generator/higher/list_tabulate.h"
 
+#include "function/apply.h"
+
 int main(int, char **) { }
 
 // equality
@@ -631,3 +633,24 @@ static_assert(
 	>::value,
 	"(count even? (list 1 3 5)) != 0"
 );
+
+// function apply
+
+static_assert(
+	std::is_same<
+		tav::Int<42>,
+		tav::Apply<tav::Multiply, tav::Int<21>, tav::_0>::type<tav::Int<2>>::type
+	>::value,
+	"((lambda (x) (* 21 x)) 2) != 42"
+);
+
+static_assert(
+	std::is_same<
+		tav::List<tav::Int<10>, tav::Int<12>, tav::Int<14>>::type,
+		tav::Map<
+			tav::Apply<tav::Add, tav::_0, tav::Int<10>>::single_type,
+			tav::List<tav::Int<0>, tav::Int<2>, tav::Int<4>>::type
+		>::type
+	>::value,
+	"(map (lambda (x) (+ x 10)) (list 0 2 4)) != (list 10 12 14)"
+);