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

1 files changed, 25 insertions, 0 deletions

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(