Reimplemented `TakeWhile` and `DropWhile` in terms of `ListIndex`

* new higher order list operation `ListIndex` returns the index of the first list element satisfying a given predicate function
** this enables finding the split index required for both `TakeWhile` and `DropWhile`
** actual list manipulation is then performed using lower order `Take` or `Drop`
** analogously to `list-index` in SRFI-1
** added appropriate test case
* added higher order predicate utility function `utility::predicate_assurance`
** as its name implies this function enables assuring that a value satisfies a given predicate
*** if this is not the case a surrogate value is returned instead
** this is used to return a appropriate size value if the `ListIndex` call fails because no element satisfies its predicate
** `detail::predicate_negator` was moved from `Remove`'s implementation details into the newly introduced `utility` namespace
*** as it is required by both `TakeWhile` and `DropWhile` in addition to `Remove`
* continuation of 8e49cc6

6 files changed, 138 insertions, 66 deletions

diff --git a/src/list/operation/higher/drop_while.h b/src/list/operation/higher/drop_while.h
index 021698f..1207020 100644
--- a/src/list/operation/higher/drop_while.h
+++ b/src/list/operation/higher/drop_while.h
@@ -1,38 +1,28 @@
 #ifndef TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_DROP_WHILE_H_
 #define TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_DROP_WHILE_H_
 
-#include "conditional/if.h"
+#include "list_index.h"
+#include "list/operation/drop.h"
+#include "utility/predicate.h"
 
 namespace tav {
 
-namespace detail {
-
-template <
-	template<typename> class Predicate,
-	typename                 Current
->
-struct DropWhile {
-	typedef If<
-		Eval<Predicate<Head<Current>>>,
-		Eval<DropWhile<Predicate, Tail<Current>>>,
-		Current
-	> type;
-};
-
-template <
-	template<typename> class Predicate
->
-struct DropWhile<Predicate, void> {
-	typedef void type;
-};
-
-}
-
 template <
 	template<typename> class Predicate,
-	typename                 Current
+	typename                 List
 >
-using DropWhile = Eval<detail::DropWhile<Predicate, Current>>;
+using DropWhile = Drop<
+	typename utility::predicate_assurance<
+		utility::predicate_negator<std::is_void>::template function,
+		Length<List>
+	>::template assure<
+		ListIndex<
+			utility::predicate_negator<Predicate>::template function,
+			List
+		>
+	>,
+	List
+>;
 
 }
 
diff --git a/src/list/operation/higher/list_index.h b/src/list/operation/higher/list_index.h
new file mode 100644
index 0000000..bbf43ee
--- /dev/null
+++ b/src/list/operation/higher/list_index.h
@@ -0,0 +1,45 @@
+#ifndef TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_LIST_INDEX_H_
+#define TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_LIST_INDEX_H_
+
+#include "operation/math.h"
+
+namespace tav {
+
+namespace detail {
+
+template <
+	template<typename> class Predicate,
+	typename                 List,
+	typename                 Index = Size<0>
+>
+struct index_of_first {
+	typedef If<
+		Eval<Predicate<Head<List>>>,
+		Index,
+		Eval<index_of_first<
+			Predicate,
+			Tail<List>,
+			Add<Index, Size<1>>
+		>>
+	> type;
+};
+
+template <
+	template<typename> class Predicate,
+	typename                 Index
+>
+struct index_of_first<Predicate, void, Index> {
+	typedef void type;
+};
+
+}
+
+template <
+	template<typename> class Predicate,
+	typename                 List
+>
+using ListIndex = Eval<detail::index_of_first<Predicate, List>>;
+
+}
+
+#endif  // TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_LIST_INDEX_H_
diff --git a/src/list/operation/higher/remove.h b/src/list/operation/higher/remove.h
index 598f5a6..6770359 100644
--- a/src/list/operation/higher/remove.h
+++ b/src/list/operation/higher/remove.h
@@ -2,25 +2,16 @@
 #define TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_REMOVE_H_
 
 #include "filter.h"
+#include "utility/predicate.h"
 
 namespace tav {
 
-namespace detail {
-
-template <template<typename> class Predicate>
-struct predicate_negator {
-	template <typename Element>
-	using function = Not<Predicate<Element>>;
-};
-
-}
-
 template <
 	template<typename> class Predicate,
 	typename                 List
 >
 using Remove = Filter<
-	detail::predicate_negator<Predicate>::template function,
+	utility::predicate_negator<Predicate>::template function,
 	List
 >;
 
diff --git a/src/list/operation/higher/take_while.h b/src/list/operation/higher/take_while.h
index dec5551..f9fc3aa 100644
--- a/src/list/operation/higher/take_while.h
+++ b/src/list/operation/higher/take_while.h
@@ -1,41 +1,28 @@
 #ifndef TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_TAKE_WHILE_H_
 #define TYPEASVALUE_SRC_LIST_OPERATION_HIGHER_TAKE_WHILE_H_
 
-#include "conditional/if.h"
+#include "list_index.h"
+#include "list/operation/take.h"
+#include "utility/predicate.h"
 
 namespace tav {
 
-namespace detail {
-
-template <
-	template<typename> class Predicate,
-	typename                 Current
->
-struct TakeWhile {
-	typedef If<
-		Eval<Predicate<Head<Current>>>,
-		Cons<
-			Head<Current>,
-			Eval<TakeWhile<Predicate, Tail<Current>>>
-		>,
-		void
-	> type;
-};
-
-template <
-	template<typename> class Predicate
->
-struct TakeWhile<Predicate, void> {
-	typedef void type;
-};
-
-}
-
 template <
 	template<typename> class Predicate,
-	typename                 Current
+	typename                 List
 >
-using TakeWhile = Eval<detail::TakeWhile<Predicate, Current>>;
+using TakeWhile = Take<
+	typename utility::predicate_assurance<
+		utility::predicate_negator<std::is_void>::template function,
+		Length<List>
+	>::template assure<
+		ListIndex<
+			utility::predicate_negator<Predicate>::template function,
+			List
+		>
+	>,
+	List
+>;
 
 }
 
diff --git a/src/utility/predicate.h b/src/utility/predicate.h
new file mode 100644
index 0000000..f06fcee
--- /dev/null
+++ b/src/utility/predicate.h
@@ -0,0 +1,34 @@
+#ifndef TYPEASVALUE_SRC_UTILITY_PREDICATE_H_
+#define TYPEASVALUE_SRC_UTILITY_PREDICATE_H_
+
+#include "conditional/if.h"
+#include "operation/logic.h"
+
+namespace tav {
+
+namespace utility {
+
+template <
+	template<typename> class Predicate,
+	typename                 Surrogate
+>
+struct predicate_assurance {
+	template <typename Value>
+	using assure = If<
+		Eval<Predicate<Value>>,
+		Value,
+		Surrogate
+	>;
+};
+
+template <template<typename> class Predicate>
+struct predicate_negator {
+	template <typename Element>
+	using function = Not<Predicate<Element>>;
+};
+
+}
+
+}
+
+#endif  // TYPEASVALUE_SRC_UTILITY_PREDICATE_H_
diff --git a/test.cc b/test.cc
index 2f95561..6a957b1 100644
--- a/test.cc
+++ b/test.cc
@@ -12,6 +12,7 @@
 #include "list/operation/higher/remove.h"
 #include "list/operation/higher/partition.h"
 #include "list/operation/higher/query.h"
+#include "list/operation/higher/list_index.h"
 #include "list/operation/higher/find.h"
 #include "list/operation/higher/take_while.h"
 #include "list/operation/higher/drop_while.h"
@@ -906,6 +907,30 @@ static_assert(
 	"(count even? (list 1 3 5)) != 0"
 );
 
+// list index
+
+static_assert(
+	std::is_same<
+		tav::Size<2>,
+		tav::ListIndex<
+			tav::Even,
+			tav::List<tav::Int<1>, tav::Int<3>, tav::Int<4>, tav::Int<6>>
+		>
+	>::value,
+	"(list-index even? (list 1 3 4 6)) != 2"
+);
+
+static_assert(
+	std::is_same<
+		void,
+		tav::ListIndex<
+			tav::Even,
+			tav::List<tav::Int<1>, tav::Int<3>, tav::Int<5>>
+		>
+	>::value,
+	"(list-index even? (list 1 3 5)) != void"
+);
+
 // list find
 
 static_assert(