From a436935d8999ea799876fb9afd01a997e1bb1224 Mon Sep 17 00:00:00 2001
From: Christos Choutouridis <hoo2@hoo2.net>
Date: Thu, 8 Oct 2020 21:51:23 +0300
Subject: [PATCH] WIP: concepts rework

---
 include/utl/meta/typelist.h  | 23 ++++++++++++++++++-----
 test/tests/TmetaTypelist.cpp |  2 ++
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/include/utl/meta/typelist.h b/include/utl/meta/typelist.h
index 6e62651..6a1c4d6 100644
--- a/include/utl/meta/typelist.h
+++ b/include/utl/meta/typelist.h
@@ -244,7 +244,7 @@ namespace meta {
    //! @{
    namespace front_impl {
       template <typename L>
-      struct front_ { };
+      struct front_ { using type = nil_; };
 
       template <typename Head, typename... Tail>
       struct front_<typelist<Head, Tail...>> {
@@ -636,7 +636,7 @@ namespace meta {
 
    /*!
     * Search for the first \c Item on the \c List for which the predicate \c Pred
-    * returns true_ when `eval<invoke<Pred, Item>>`
+    * yields to \c true_ when `eval<invoke<Pred, Item>>`
     *
     * Complexity \f$ O(N) \f$
     *
@@ -684,7 +684,7 @@ namespace meta {
 
    /*!
     * Search for the first \c Item on the \c List for which the predicate \c Pred
-    * returns true_ when `eval<invoke<Pred, Item>>` and return the rest of the \c List
+    * yields to \c true_ when `eval<invoke<Pred, Item>>` and return the rest of the \c List
     * starting from that position as new typelist
     *
     * Complexity \f$ O(N) \f$
@@ -705,8 +705,21 @@ namespace meta {
    using seek = seek_if <List, same_as<T>>;
    //! @}
 
+   //! \name first_if
+   //! @{
+   /*!
+    * Search and returns the first \c Item on the \c List for which the predicate \c Pred
+    * yields to true_ when \code eval<invoke<Pred, Item>> \endcode
+    *
+    * Complexity \f$ O(N) \f$
+    *
+    * \tparam  List  A typelist
+    * \tparam  Pred  A Unary invocable predicate
+    * \return  On success the item, otherwise utl::meta::nil_
+    */
    template <typename List, typename Pred>
    using first_if = front<seek_if<List, Pred>>;
+   //! @}
 
    //! \name count_if
    //! @{
@@ -783,8 +796,8 @@ namespace meta {
    }
 
    /*!
-    * Return a new typelist with elements, the elements of \c List that satisfy the
-    * invocable \c Pred such that `eval<invoke<Pred, Item>>` is \c true_
+    * Returns a new typelist with elements, the elements of \c List that satisfy the
+    * invocable \c Pred such that `eval<invoke<Pred, Item>>` yields to \c true_
     *
     * Complexity \f$ O(N) \f$
     *
diff --git a/test/tests/TmetaTypelist.cpp b/test/tests/TmetaTypelist.cpp
index 28b5053..0f55a20 100644
--- a/test/tests/TmetaTypelist.cpp
+++ b/test/tests/TmetaTypelist.cpp
@@ -174,7 +174,9 @@ namespace TmetaTypelist {
       EXPECT_EQ (true, (std::is_same<long, at<l, int_<3>>>()));
 
       EXPECT_EQ (true, (std::is_same<char*, front<l>>()));
+      EXPECT_EQ (true, (std::is_same<nil_, front<typelist<>>>()));
       EXPECT_EQ (true, (std::is_same<short, back<l>>()));
+      EXPECT_EQ (true, (std::is_same<nil_, back<typelist<>>>()));
    }
 
    TEST(TmetaTypelist, Concat) {