From 27aaee43499c268903332c7e9e1e6ec2d193dc3a Mon Sep 17 00:00:00 2001
From: Adrian Kummerlaender
Date: Fri, 6 Feb 2015 19:26:26 +0100
Subject: Revamped to use `Cons` as a function and `Pair` as its result * this
 is analogous to _Scheme_ where a pair (dot-expression) is returned from a
 call to `cons` * `Head` and `Tail` are kept as direct references to the `CAR`
 and `CDR` values of a pair to match e.g. the math operators

---
 example/prime/prime.cc                 |  4 ++--
 src/list/cons.h                        | 27 +++++++++++++++++++++------
 src/list/generator/iota.h              |  6 +++---
 src/list/generator/make_list.h         |  6 +++---
 src/list/list.h                        | 10 +++++-----
 src/list/operation/append.h            |  4 ++--
 src/list/operation/basic.h             |  4 ++--
 src/list/operation/higher/filter.h     |  2 +-
 src/list/operation/higher/map.h        | 10 ++++------
 src/list/operation/higher/partition.h  | 10 ++++------
 src/list/operation/higher/take_while.h |  4 ++--
 src/list/operation/nth.h               | 10 +++++-----
 src/list/operation/reverse.h           |  2 +-
 src/list/operation/take.h              |  4 ++--
 src/pair.h                             | 26 ++++++++++++++++++++++++++
 src/runtime/list/for_each.h            |  2 +-
 test.cc                                | 28 ++++++++++++++--------------
 17 files changed, 98 insertions(+), 61 deletions(-)
 create mode 100644 src/pair.h

diff --git a/example/prime/prime.cc b/example/prime/prime.cc
index 93b7db5..17d437c 100644
--- a/example/prime/prime.cc
+++ b/example/prime/prime.cc
@@ -31,7 +31,7 @@ using removeMultiplesOf = tav::Remove<
 
 template <typename Candidates>
 struct Sieve {
-	typedef tav::Cons<
+	typedef typename tav::Cons<
 		tav::Head<Candidates>,
 		typename Sieve<
 			typename removeMultiplesOf<
@@ -39,7 +39,7 @@ struct Sieve {
 				tav::Head<Candidates>
 			>::type
 		>::type
-	> type;
+	>::type type;
 };
 
 template <>
diff --git a/src/list/cons.h b/src/list/cons.h
index b858c4c..7f54eb5 100644
--- a/src/list/cons.h
+++ b/src/list/cons.h
@@ -1,6 +1,8 @@
 #ifndef TYPEASVALUE_SRC_LIST_CONS_H_
 #define TYPEASVALUE_SRC_LIST_CONS_H_
 
+#include "pair.h"
+
 namespace tav {
 
 template <
@@ -8,15 +10,28 @@ template <
 	typename CDR
 >
 struct Cons {
-	typedef CAR car;
-	typedef CDR cdr;
+	typedef Pair<CAR, CDR> type;
 };
 
-template <typename Cons>
-using Car = typename Cons::car;
+template <typename Pair>
+struct Car {
+	static_assert(
+		IsPair<Pair>::type::value,
+		"Pair type required"
+	);
+
+	typedef typename Pair::car type;
+};
 
-template <typename Cons>
-using Cdr = typename Cons::cdr;
+template <typename Pair>
+struct Cdr {
+	static_assert(
+		IsPair<Pair>::type::value,
+		"Pair type required"
+	);
+
+	typedef typename Pair::cdr type;
+};
 
 }
 
diff --git a/src/list/generator/iota.h b/src/list/generator/iota.h
index 54f0f40..2d5b506 100644
--- a/src/list/generator/iota.h
+++ b/src/list/generator/iota.h
@@ -11,14 +11,14 @@ template <
 	typename Step
 >
 struct Iota {
-	typedef Cons<
+	typedef typename Cons<
 		Initial,
 		typename Iota<
 			Substract<Count, Size<1>>,
 			Add<Initial, Step>,
 			Step
 		>::type
-	> type;
+	>::type type;
 };
 
 template <
@@ -26,7 +26,7 @@ template <
 	typename Step
 >
 struct Iota<Size<1>, Initial, Step> {
-	typedef Cons<Initial, void> type;
+	typedef typename Cons<Initial, void>::type type;
 };
 
 }
diff --git a/src/list/generator/make_list.h b/src/list/generator/make_list.h
index 3bf65a2..33419cb 100644
--- a/src/list/generator/make_list.h
+++ b/src/list/generator/make_list.h
@@ -11,18 +11,18 @@ template <
 	typename Element
 >
 struct MakeList {
-	typedef Cons<
+	typedef typename Cons<
 		Element,
 		typename MakeList<
 			Substract<Count, Size<1>>,
 			Element
 		>::type
-	> type;
+	>::type type;
 };
 
 template <typename Element>
 struct MakeList<Size<1>, Element> {
-	typedef Cons<Element, void> type;
+	typedef typename Cons<Element, void>::type type;
 };
 
 }
diff --git a/src/list/list.h b/src/list/list.h
index 66394fc..2e335e2 100644
--- a/src/list/list.h
+++ b/src/list/list.h
@@ -10,15 +10,15 @@ template <
 	typename... Tail
 >
 struct List {
-	typedef Cons<
+	typedef typename Cons<
 		Head,
 		typename List<Tail...>::type
-	> type;
+	>::type type;
 };
 
 template <typename Head>
 struct List<Head> {
-	typedef Cons<Head, void> type;
+	typedef typename Cons<Head, void>::type type;
 };
 
 template <
@@ -30,10 +30,10 @@ using ListOfType = List<
 >;
 
 template <typename Cons>
-using Head = Car<Cons>;
+using Head = typename Car<Cons>::type;
 
 template <typename Cons>
-using Tail = Cdr<Cons>;
+using Tail = typename Cdr<Cons>::type;
 
 }
 
diff --git a/src/list/operation/append.h b/src/list/operation/append.h
index 9e1be79..6cf15f0 100644
--- a/src/list/operation/append.h
+++ b/src/list/operation/append.h
@@ -8,13 +8,13 @@ template <
 	typename Secondary
 >
 struct Append {
-	typedef Cons<
+	typedef typename Cons<
 		Head<Primary>,
 		typename Append<
 			Tail<Primary>,
 			Secondary
 		>::type
-	> type;
+	>::type type;
 };
 
 template <typename Secondary>
diff --git a/src/list/operation/basic.h b/src/list/operation/basic.h
index 1f4b043..cbbe22c 100644
--- a/src/list/operation/basic.h
+++ b/src/list/operation/basic.h
@@ -7,11 +7,11 @@
 
 namespace tav {
 
-template <typename Cons>
+template <typename Pair>
 using Length = Fold<
 	Apply<Add, Size<1>, _1>::pair_type,
 	Size<0>,
-	Cons
+	Pair
 >;
 
 }
diff --git a/src/list/operation/higher/filter.h b/src/list/operation/higher/filter.h
index 3fc08ee..acc1422 100644
--- a/src/list/operation/higher/filter.h
+++ b/src/list/operation/higher/filter.h
@@ -18,7 +18,7 @@ class Filter {
 		>
 		using predicate_wrapper = If<
 			Predicate<Current>::type::value,
-			Cons<Current, Previous>,
+			typename Cons<Current, Previous>::type,
 			Previous
 		>;
 
diff --git a/src/list/operation/higher/map.h b/src/list/operation/higher/map.h
index a451b2a..9cb6524 100644
--- a/src/list/operation/higher/map.h
+++ b/src/list/operation/higher/map.h
@@ -15,12 +15,10 @@ class Map {
 			typename Current,
 			typename Previous
 		>
-		struct function_wrapper {
-			typedef Cons<
-				typename Function<Current>::type,
-				Previous
-			> type;
-		};
+		using function_wrapper = Cons<
+			typename Function<Current>::type,
+			Previous
+		>;
 
 	public:
 		typedef typename Fold<function_wrapper, void, List>::type type;
diff --git a/src/list/operation/higher/partition.h b/src/list/operation/higher/partition.h
index e42b971..a0ea379 100644
--- a/src/list/operation/higher/partition.h
+++ b/src/list/operation/higher/partition.h
@@ -9,12 +9,10 @@ template <
 	template<typename> class Predicate,
 	typename                 List
 >
-struct Partition {
-	typedef Cons<
-		typename Filter<Predicate, List>::type,
-		typename Remove<Predicate, List>::type
-	> type;
-};
+using Partition = Cons<
+	typename Filter<Predicate, List>::type,
+	typename Remove<Predicate, List>::type
+>;
 
 }
 
diff --git a/src/list/operation/higher/take_while.h b/src/list/operation/higher/take_while.h
index 27f58ed..d04dc2d 100644
--- a/src/list/operation/higher/take_while.h
+++ b/src/list/operation/higher/take_while.h
@@ -12,10 +12,10 @@ template <
 struct TakeWhile {
 	typedef typename If<
 		Predicate<Head<Current>>::type::value,
-		Cons<
+		typename Cons<
 			Head<Current>,
 			typename TakeWhile<Predicate, Tail<Current>>::type
-		>,
+		>::type,
 		void
 	>::type type;
 };
diff --git a/src/list/operation/nth.h b/src/list/operation/nth.h
index 8eb88ba..6020dcb 100644
--- a/src/list/operation/nth.h
+++ b/src/list/operation/nth.h
@@ -7,18 +7,18 @@ namespace tav {
 
 template <
 	typename Index,
-	typename Cons
+	typename Pair
 >
 struct Nth {
 	typedef typename Nth<
 		Substract<Index, Size<1>>,
-		Tail<Cons>
+		Tail<Pair>
 	>::type type;
 };
 
-template <typename Cons>
-struct Nth<Size<0>, Cons> {
-	typedef Head<Cons> type;
+template <typename Pair>
+struct Nth<Size<0>, Pair> {
+	typedef Head<Pair> type;
 };
 
 template <typename Index>
diff --git a/src/list/operation/reverse.h b/src/list/operation/reverse.h
index 632fa5b..83691f8 100644
--- a/src/list/operation/reverse.h
+++ b/src/list/operation/reverse.h
@@ -16,7 +16,7 @@ class Reverse {
 		struct reversed_append {
 			typedef typename Append<
 				Previous,
-				Cons<Current, void>
+				typename Cons<Current, void>::type
 			>::type type;
 		};
 
diff --git a/src/list/operation/take.h b/src/list/operation/take.h
index db25640..c396c28 100644
--- a/src/list/operation/take.h
+++ b/src/list/operation/take.h
@@ -10,13 +10,13 @@ template <
 	typename Current
 >
 struct Take {
-	typedef Cons<
+	typedef typename Cons<
 		Head<Current>,
 		typename Take<
 			Substract<Count, Size<1>>,
 			Tail<Current>
 		>::type
-	> type;
+	>::type type;
 };
 
 template <typename Current>
diff --git a/src/pair.h b/src/pair.h
new file mode 100644
index 0000000..20bef4e
--- /dev/null
+++ b/src/pair.h
@@ -0,0 +1,26 @@
+#ifndef TYPEASVALUE_SRC_PAIR_H_
+#define TYPEASVALUE_SRC_PAIR_H_
+
+#include <type_traits>
+
+namespace tav {
+
+namespace detail { struct pair_tag { }; }
+
+template <
+	typename CAR,
+	typename CDR
+>
+struct Pair : detail::pair_tag {
+	typedef CAR car;
+	typedef CDR cdr;
+
+	typedef Pair<CAR, CDR> type;
+};
+
+template <typename Type>
+using IsPair = std::is_base_of<detail::pair_tag, Type>;
+
+}
+
+#endif  // TYPEASVALUE_SRC_PAIR_H_
diff --git a/src/runtime/list/for_each.h b/src/runtime/list/for_each.h
index 8986135..1ca2034 100644
--- a/src/runtime/list/for_each.h
+++ b/src/runtime/list/for_each.h
@@ -21,7 +21,7 @@ template <
 	typename std::enable_if<!std::is_void<Current>::value, std::size_t>::type = 0
 >
 void for_each(const Function& function) {
-	function(Head<Current>::value);
+	function(Head<Current>::type::value);
 
 	for_each<Tail<Current>, Function>(function);
 }
diff --git a/test.cc b/test.cc
index da3193f..9037e25 100644
--- a/test.cc
+++ b/test.cc
@@ -189,8 +189,8 @@ static_assert(
 	std::is_same<
 		tav::Int<1>,
 		tav::Car<
-			tav::Cons<tav::Int<1>, void>
-		>
+			tav::Cons<tav::Int<1>, void>::type
+		>::type
 	>::value,
 	"(car (cons 1 void)) != 1"
 );
@@ -199,8 +199,8 @@ static_assert(
 	std::is_same<
 		tav::Int<1>,
 		tav::Car<
-			tav::Cons<tav::Int<1>, tav::Int<2>>
-		>
+			tav::Cons<tav::Int<1>, tav::Int<2>>::type
+		>::type
 	>::value,
 	"(car (cons 1 2)) != 1"
 );
@@ -209,8 +209,8 @@ static_assert(
 	std::is_same<
 		tav::Int<2>,
 		tav::Cdr<
-			tav::Cons<tav::Int<1>, tav::Int<2>>
-		>
+			tav::Cons<tav::Int<1>, tav::Int<2>>::type
+		>::type
 	>::value,
 	"(cdr (cons 1 2)) != 2"
 );
@@ -220,9 +220,9 @@ static_assert(
 		tav::Int<2>,
 		tav::Car<
 			tav::Cdr<
-				tav::Cons<tav::Int<1>, tav::Cons<tav::Int<2>, tav::Int<3>>>
-			>
-		>
+				tav::Cons<tav::Int<1>, tav::Cons<tav::Int<2>, tav::Int<3>>::type>::type
+			>::type
+		>::type
 	>::value,
 	"(car (cdr (cons 1 (cons 2 3)))) != 2"
 );
@@ -231,7 +231,7 @@ static_assert(
 
 static_assert(
 	std::is_same<
-		tav::Cons<tav::Int<1>, void>,
+		tav::Pair<tav::Int<1>, void>,
 		tav::List<tav::Int<1>>::type
 	>::value,
 	"(list 1) != (cons 1 void)"
@@ -239,7 +239,7 @@ static_assert(
 
 static_assert(
 	std::is_same<
-		tav::Cons<tav::Int<1>, tav::Cons<tav::Int<2>, void>>,
+		tav::Pair<tav::Int<1>, tav::Pair<tav::Int<2>, void>>,
 		tav::List<tav::Int<1>, tav::Int<2>>::type
 	>::value,
 	"(list 1 2) != (cons 1 (cons 2 void))"
@@ -249,7 +249,7 @@ static_assert(
 
 static_assert(
 	std::is_same<
-		tav::Cons<tav::Int<1>, void>,
+		tav::Pair<tav::Int<1>, void>,
 		tav::ListOfType<int, 1>::type
 	>::value,
 	"(list 1) != (cons 1 void)"
@@ -257,7 +257,7 @@ static_assert(
 
 static_assert(
 	std::is_same<
-		tav::Cons<tav::Int<1>, tav::Cons<tav::Int<2>, void>>,
+		tav::Pair<tav::Int<1>, tav::Pair<tav::Int<2>, void>>,
 		tav::ListOfType<int, 1, 2>::type
 	>::value,
 	"(list 1 2) != (cons 1 (cons 2 void))"
@@ -463,7 +463,7 @@ static_assert(
 
 static_assert(
 	std::is_same<
-		tav::Cons<
+		tav::Pair<
 			tav::List<tav::Int<1>, tav::Int<3>>::type,
 			tav::List<tav::Int<2>>::type
 		>,
-- 
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