Unified `Iota` and `MakeList` using nested structure generator

* `detail::generate_nested_structure` offers a higher order nested structure constructor for procedural list generation
* renamed `Fold` implementation details

7 files changed, 74 insertions, 79 deletions

diff --git a/README.md b/README.md
index 50d6dff..ba07515 100644
--- a/README.md
+++ b/README.md
@@ -6,17 +6,15 @@ This library is a expanded reimplementation of my previous attempt at this probl
 
 ## Example
 
-```cpp
-// λ (length (filter odd? (list 1 2 3)))
-// 2
-
-const std::size_t count = tav::Length<
-	tav::Filter<
-		tav::Odd,
-		tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>::type
-	>::type
->::type::value;
-```
+	// λ (length (filter odd? (list 1 2 3)))
+	// 2
+	
+	const std::size_t count = tav::Length<
+		tav::Filter<
+			tav::Odd,
+			tav::List<tav::Int<1>, tav::Int<2>, tav::Int<3>>
+		>
+	>::value;
 
 ## Current features
 
diff --git a/src/function/detail/apply.h b/src/function/detail/apply.h
index 3df80ef..aca0af4 100644
--- a/src/function/detail/apply.h
+++ b/src/function/detail/apply.h
@@ -13,17 +13,12 @@ namespace detail {
 struct placeholder_tag { };
 
 template <typename Type>
-using is_placeholder = Boolean<
-	std::is_base_of<placeholder_tag, Type>::value
->;
+using is_placeholder = Eval<std::is_base_of<placeholder_tag, Type>>;
 
 template <int Index>
 struct placeholder : placeholder_tag { };
 
-template <
-	typename Partials,
-	typename Argument
->
+template <typename, typename Argument>
 struct resolve_placeholder {
 	typedef Argument type;
 };
@@ -33,7 +28,7 @@ template <
 	int      Index
 >
 struct resolve_placeholder<Partials, placeholder<Index>> {
-	typedef tav::Nth<Size<Index>, Partials> type;
+	typedef Nth<Size<Index>, Partials> type;
 };
 
 template <typename... Arguments>
diff --git a/src/list/generator/detail/generate_nested_structure.h b/src/list/generator/detail/generate_nested_structure.h
new file mode 100644
index 0000000..84db78d
--- /dev/null
+++ b/src/list/generator/detail/generate_nested_structure.h
@@ -0,0 +1,39 @@
+#ifndef TYPEASVALUE_SRC_LIST_GENERATOR_DETAIL_GENERATE_NESTED_STRUCTURE_H_
+#define TYPEASVALUE_SRC_LIST_GENERATOR_DETAIL_GENERATE_NESTED_STRUCTURE_H_
+
+namespace tav {
+
+namespace detail {
+
+template <
+	template <typename, typename> class Structure,
+	template <typename>           class Generator,
+	typename                            Initial,
+	typename                            Count
+>
+struct generate_nested_structure {
+	typedef Structure<
+		Initial,
+		Eval<generate_nested_structure<
+			Structure,
+			Generator,
+			Generator<Initial>,
+			Substract<Count, Size<1>>
+		>>
+	> type;
+};
+
+template <
+	template <typename, typename> class Structure,
+	template <typename>           class Generator,
+	typename                            Initial
+>
+struct generate_nested_structure<Structure, Generator, Initial, Size<0>> {
+	typedef void type;
+};
+
+}
+
+}
+
+#endif  // TYPEASVALUE_SRC_LIST_GENERATOR_DETAIL_GENERATE_NESTED_STRUCTURE_H_
diff --git a/src/list/generator/iota.h b/src/list/generator/iota.h
index 73b7cc8..1def67e 100644
--- a/src/list/generator/iota.h
+++ b/src/list/generator/iota.h
@@ -1,44 +1,24 @@
 #ifndef TYPEASVALUE_SRC_LIST_GENERATOR_IOTA_H_
 #define TYPEASVALUE_SRC_LIST_GENERATOR_IOTA_H_
 
+#include "pair.h"
 #include "operation/math.h"
+#include "function/apply.h"
+#include "detail/generate_nested_structure.h"
 
 namespace tav {
 
-namespace detail {
-
-template <
-	typename Count,
-	typename Initial,
-	typename Step
->
-struct Iota {
-	typedef Cons<
-		Initial,
-		Eval<Iota<
-			Substract<Count, Size<1>>,
-			Add<Initial, Step>,
-			Step
-		>>
-	> type;
-};
-
-template <
-	typename Initial,
-	typename Step
->
-struct Iota<Size<1>, Initial, Step> {
-	typedef Cons<Initial, void> type;
-};
-
-}
-
 template <
 	typename Count,
 	typename Initial,
 	typename Step
 >
-using Iota = Eval<detail::Iota<Count, Initial, Step>>;
+using Iota = Eval<detail::generate_nested_structure<
+	Pair,
+	Apply<Add, Step, _0>::template function,
+	Initial,
+	Count
+>>;
 
 }
 
diff --git a/src/list/generator/make_list.h b/src/list/generator/make_list.h
index 863dce2..ec49651 100644
--- a/src/list/generator/make_list.h
+++ b/src/list/generator/make_list.h
@@ -1,39 +1,21 @@
 #ifndef TYPEASVALUE_SRC_LIST_GENERATOR_MAKE_LIST_H_
 #define TYPEASVALUE_SRC_LIST_GENERATOR_MAKE_LIST_H_
 
-#include "list/cons.h"
-#include "operation/math.h"
+#include "pair.h"
+#include "detail/generate_nested_structure.h"
 
 namespace tav {
 
-namespace detail {
-
-template <
-	typename Count,
-	typename Element
->
-struct MakeList {
-	typedef Cons<
-		Element,
-		Eval<MakeList<
-			Substract<Count, Size<1>>,
-			Element
-		>>
-	> type;
-};
-
-template <typename Element>
-struct MakeList<Size<1>, Element> {
-	typedef Cons<Element, void> type;
-};
-
-}
-
 template <
 	typename Count,
 	typename Element
 >
-using MakeList = Eval<detail::MakeList<Count, Element>>;
+using MakeList = Eval<detail::generate_nested_structure<
+	Pair,
+	Eval,
+	Element,
+	Count
+>>;
 
 }
 
diff --git a/src/list/operation/higher/fold.h b/src/list/operation/higher/fold.h
index 7364fe5..3f39b40 100644
--- a/src/list/operation/higher/fold.h
+++ b/src/list/operation/higher/fold.h
@@ -10,10 +10,10 @@ template <
 	typename                           Initial,
 	typename                           Current
 >
-struct Fold {
+struct fold_pair {
 	typedef Function<
 		Head<Current>,
-		Eval<Fold<Function, Initial, Tail<Current>>>
+		Eval<fold_pair<Function, Initial, Tail<Current>>>
 	> type;
 };
 
@@ -21,7 +21,7 @@ template <
 	template<typename, typename> class Function,
 	typename                           Initial
 >
-struct Fold<Function, Initial, void> {
+struct fold_pair<Function, Initial, void> {
 	typedef Initial type;
 };
 
@@ -32,7 +32,7 @@ template <
 	typename                           Initial,
 	typename                           Current
 >
-using Fold = Eval<detail::Fold<Function, Initial, Current>>;
+using Fold = Eval<detail::fold_pair<Function, Initial, Current>>;
 
 }
 
diff --git a/src/list/operation/nth.h b/src/list/operation/nth.h
index 0a56ba3..1ffb1f0 100644
--- a/src/list/operation/nth.h
+++ b/src/list/operation/nth.h
@@ -1,8 +1,9 @@
 #ifndef TYPEASVALUE_SRC_LIST_OPERATION_NTH_H_
 #define TYPEASVALUE_SRC_LIST_OPERATION_NTH_H_
 
-#include "operation/math.h"
 #include "drop.h"
+#include "conditional/if.h"
+#include "operation/math.h"
 
 namespace tav {