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

1 files changed, 21 insertions, 6 deletions

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;
+};
 
 }