From 43972b416ebff01d27490b6e5e0fa23b433ab5fc Mon Sep 17 00:00:00 2001
From: John Wellbelove <john.wellbelove@etlcpp.com>
Date: Mon, 9 Feb 2026 10:56:23 +0000
Subject: [PATCH] Added etl::type_list_remove, etl::type_list_remove_if,
 etl::type_list_unique, etl::type_list_pop_front, etl::type_list_pop_back

---
 include/etl/type_list.h | 173 ++++++++++++++++++++++++++++++++++++++-
 test/test_type_list.cpp | 176 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 343 insertions(+), 6 deletions(-)

diff --git a/include/etl/type_list.h b/include/etl/type_list.h
index afc158f4..6f357b84 100644
--- a/include/etl/type_list.h
+++ b/include/etl/type_list.h
@@ -79,6 +79,7 @@ namespace etl
 
   private:
 
+    // A type_list cannot be instantiated, so delete the constructor and assignment operators.
     type_list() ETL_DELETE;
     type_list(const type_list&) ETL_DELETE;
     type_list& operator =(const type_list&) ETL_DELETE;
@@ -109,6 +110,7 @@ namespace etl
 
   private:
 
+    // A type_list cannot be instantiated, so delete the constructor and assignment operators.
     type_list() ETL_DELETE;
     type_list(const type_list&) ETL_DELETE;
     type_list& operator =(const type_list&) ETL_DELETE;
@@ -418,10 +420,10 @@ namespace etl
   private:
 
     ETL_STATIC_ASSERT((etl::is_type_list<TTypeList>::value), "TTypeList must be an etl::type_list");
-    ETL_STATIC_ASSERT(Index <= etl::TTypeList::size,         "Index out of range");
+    ETL_STATIC_ASSERT(Index <= TTypeList::size,         "Index out of range");
 
     using index_sequence_for_prefix = etl::make_index_sequence<Index>;
-    using index_sequence_for_suffix = etl::make_index_sequence_with_offset<Index, etl::TTypeList::size - Index>;
+    using index_sequence_for_suffix = etl::make_index_sequence_with_offset<Index, TTypeList::size - Index>;
 
     using prefix = etl::type_list_select_from_index_sequence_t<TTypeList, index_sequence_for_prefix>;
     using suffix = etl::type_list_select_from_index_sequence_t<TTypeList, index_sequence_for_suffix>;
@@ -437,6 +439,173 @@ namespace etl
   using type_list_insert_t = typename etl::type_list_insert<TTypeList, T, Index>::type;
 #endif
 
+  //***************************************************************************
+  /// Remove a type at an index in a type_list.
+  //***************************************************************************
+  template <typename TTypeList, size_t Index>
+  struct type_list_remove
+  {
+  private:
+
+    ETL_STATIC_ASSERT((etl::is_type_list<TTypeList>::value), "TTypeList must be an etl::type_list");
+    ETL_STATIC_ASSERT(Index < TTypeList::size,               "Index out of range");
+
+    using index_sequence_for_prefix = etl::make_index_sequence<Index>;
+    using index_sequence_for_suffix = etl::make_index_sequence_with_offset<Index + 1, TTypeList::size - Index - 1>;
+
+    using prefix = etl::type_list_select_from_index_sequence_t<TTypeList, index_sequence_for_prefix>;
+    using suffix = etl::type_list_select_from_index_sequence_t<TTypeList, index_sequence_for_suffix>;
+
+  public:
+
+    // Concatenate the prefix and suffix to create the new type list with the Index element removed.
+    using type = etl::type_list_cat_t<prefix, suffix>;
+  };
+
+#if ETL_USING_CPP11
+  template <typename TTypeList, size_t Index>
+  using type_list_remove_t = typename etl::type_list_remove<TTypeList, Index>::type;
+#endif
+
+  //***************************************************************************
+  // Remove types that satisfy a predicate from a type_list.
+  //***************************************************************************
+  namespace private_type_list
+  {
+    template <typename TTypeList, template <typename> class TPredicate>
+    struct type_list_remove_if_impl;
+
+    template <template <typename> class TPredicate>
+    struct type_list_remove_if_impl<etl::type_list<>, TPredicate>
+    {
+      using type = etl::type_list<>;
+    };
+
+    template <typename Head, typename... Tail, template <typename> class TPredicate>
+    struct type_list_remove_if_impl<etl::type_list<Head, Tail...>, TPredicate>
+    {
+    private:
+
+      using rest = typename type_list_remove_if_impl<etl::type_list<Tail...>, TPredicate>::type;
+
+    public:
+
+      using type = typename etl::conditional<TPredicate<Head>::value,
+                                             rest,
+                                             etl::type_list_prepend_t<rest, Head>>::type;
+    };
+  }
+
+  //***************************************************************************
+  /// Remove types that satisfy a predicate from a type_list.
+  /// Predicate must be: template <typename T> struct Pred : etl::bool_constant<...> {};
+  //***************************************************************************
+  template <typename TTypeList, template <typename> class TPredicate>
+  struct type_list_remove_if
+  {
+  private:
+
+    ETL_STATIC_ASSERT((etl::is_type_list<TTypeList>::value), "TTypeList must be an etl::type_list");
+
+  public:
+
+    using type = typename private_type_list::type_list_remove_if_impl<TTypeList, TPredicate>::type;
+  };
+
+#if ETL_USING_CPP11
+  template <typename TTypeList, template <typename> class TPredicate>
+  using type_list_remove_if_t = typename etl::type_list_remove_if<TTypeList, TPredicate>::type;
+#endif
+
+  //***************************************************************************
+  /// Removes the first type from a type_list.
+  //***************************************************************************
+  template <typename TTypeList>
+  struct type_list_pop_front
+  {
+  private:
+
+    ETL_STATIC_ASSERT((etl::is_type_list<TTypeList>::value), "TTypeList must be an etl::type_list");
+    ETL_STATIC_ASSERT(TTypeList::size > 0U,                  "Cannot pop_front from an empty type_list");
+
+  public:
+
+    using type = typename TTypeList::tail;
+  };
+
+#if ETL_USING_CPP11
+  template <typename TTypeList>
+  using type_list_pop_front_t = typename etl::type_list_pop_front<TTypeList>::type;
+#endif
+
+  //***************************************************************************
+  /// Removes the last type from a type_list.
+  //***************************************************************************
+  template <typename TTypeList>
+  struct type_list_pop_back
+  {
+  private:
+
+    ETL_STATIC_ASSERT((etl::is_type_list<TTypeList>::value), "TTypeList must be an etl::type_list");
+    ETL_STATIC_ASSERT(TTypeList::size > 0U,                  "Cannot pop_back from an empty type_list");
+
+  public:
+
+    using type = typename etl::type_list_remove<TTypeList, TTypeList::size - 1U>::type;
+  };
+
+#if ETL_USING_CPP11
+  template <typename TTypeList>
+  using type_list_pop_back_t = typename etl::type_list_pop_back<TTypeList>::type;
+#endif
+
+  //***************************************************************************
+  // Remove duplicate types from a type_list, preserving the first occurrence.
+  //***************************************************************************
+  namespace private_type_list
+  {
+    template <typename TTypeList, typename TResult>
+    struct type_list_unique_impl;
+
+    // Base case: empty list, return the result.
+    template <typename TResult>
+    struct type_list_unique_impl<etl::type_list<>, TResult>
+    {
+      using type = TResult;
+    };
+
+    // Recursive case: Check if the head is already in the result, if not add it, then recurse on the tail.
+    template <typename Head, typename... Tail, typename TResult>
+    struct type_list_unique_impl<etl::type_list<Head, Tail...>, TResult>
+    {
+    private:
+
+      using next_result = etl::conditional_t<etl::type_list_contains<TResult, Head>::value,
+                                             TResult,
+                                             etl::type_list_append_t<TResult, Head>>;
+
+    public:
+
+      using type = typename type_list_unique_impl<etl::type_list<Tail...>, next_result>::type;
+    };
+  }
+
+  //***************************************************************************
+  /// Defines a new type_list by removing duplicate types from a given type_list, preserving the first occurrence.
+  //***************************************************************************
+  template <typename TTypeList>
+  struct type_list_unique
+  {
+    ETL_STATIC_ASSERT((etl::is_type_list<TTypeList>::value), "TTypeList must be an etl::type_list");
+
+    using type = typename private_type_list::type_list_unique_impl<TTypeList, etl::type_list<>>::type;
+  };
+
+#if ETL_USING_CPP11
+  template <typename TTypeList>
+  using type_list_unique_t = typename etl::type_list_unique<TTypeList>::type;
+#endif
+
   //***************************************************************************
   /// Checks that two type lists are convertible.
   /// Static asserts if the type lists are not the same length.
diff --git a/test/test_type_list.cpp b/test/test_type_list.cpp
index 7a343808..82fe9496 100644
--- a/test/test_type_list.cpp
+++ b/test/test_type_list.cpp
@@ -40,6 +40,11 @@ namespace
   struct B { static constexpr int id = 1; };
   struct C { static constexpr int id = 2; };
 
+  template <typename T>
+  struct is_type_b : etl::bool_constant<std::is_same<T, B>::value>
+  {
+  };
+
   // Convenience comparator for types that expose a constexpr integral ID (ascending)
   template <typename T1, typename T2>
   struct by_asencding_id : etl::bool_constant<(T1::id < T2::id)>
@@ -376,7 +381,7 @@ namespace
       using expected = etl::type_list<A>;
 
       CHECK((etl::is_same<result, expected>::value));
-      //CHECK_EQUAL(1U, etl::type_list_size<result>::value);
+      CHECK_EQUAL(1U, etl::type_list_size<result>::value);
     }
 
     //*************************************************************************
@@ -394,9 +399,172 @@ namespace
       CHECK((etl::is_same<result2, expected2>::value));
       CHECK((etl::is_same<result3, expected3>::value));
       
-      //CHECK_EQUAL(3U, etl::type_list_size<result1>::value);
-      //CHECK_EQUAL(3U, etl::type_list_size<result2>::value);
-      //CHECK_EQUAL(3U, etl::type_list_size<result3>::value);
+      CHECK_EQUAL(3U, etl::type_list_size<result1>::value);
+      CHECK_EQUAL(3U, etl::type_list_size<result2>::value);
+      CHECK_EQUAL(3U, etl::type_list_size<result3>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_remove_from_empty_list)
+    {
+      // Uncomment to generate static_assert error.
+
+      //using list     = etl::type_list<>;
+      //using result1  = etl::type_list_remove_t<list, 0>;
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_remove_from_single_list)
+    {
+      using list     = etl::type_list<A>;
+      using result1  = etl::type_list_remove_t<list, 0>;
+      using expected1 = etl::type_list<>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+
+      CHECK_EQUAL(0U, etl::type_list_size<result1>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_remove_from_multiple_list)
+    {
+      using list     = etl::type_list<A, B, C>;
+      using result1  = etl::type_list_remove_t<list, 0>;
+      using result2  = etl::type_list_remove_t<list, 1>;
+      using result3  = etl::type_list_remove_t<list, 2>;
+      using expected1 = etl::type_list<B, C>;
+      using expected2 = etl::type_list<A, C>;
+      using expected3 = etl::type_list<A, B>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+      CHECK((etl::is_same<result2, expected2>::value));
+      CHECK((etl::is_same<result3, expected3>::value));
+
+      CHECK_EQUAL(2U, etl::type_list_size<result1>::value);
+      CHECK_EQUAL(2U, etl::type_list_size<result2>::value);
+      CHECK_EQUAL(2U, etl::type_list_size<result3>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_remove_if_from_empty_list)
+    {
+      using list      = etl::type_list<>;
+      using result1   = etl::type_list_remove_if_t<list, is_type_b>;
+      using expected1 = etl::type_list<>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+
+      CHECK_EQUAL(0U, etl::type_list_size<result1>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_remove_if_from_single_list)
+    {
+      using list1     = etl::type_list<A>;
+      using list2     = etl::type_list<B>;
+      using result1   = etl::type_list_remove_if_t<list1, is_type_b>;
+      using result2   = etl::type_list_remove_if_t<list2, is_type_b>;
+      using expected1 = etl::type_list<A>;
+      using expected2 = etl::type_list<>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+      CHECK((etl::is_same<result2, expected2>::value));
+
+      CHECK_EQUAL(1U, etl::type_list_size<result1>::value);
+      CHECK_EQUAL(0U, etl::type_list_size<result2>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_remove_if_from_multiple_list)
+    {
+      using list     = etl::type_list<A, B, C>;
+      using result1  = etl::type_list_remove_if_t<list, is_type_b>;
+      using expected1 = etl::type_list<A, C>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+
+      CHECK_EQUAL(2U, etl::type_list_size<result1>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_pop_front_empty_list)
+    {
+      // Uncomment to generate static_assert error.
+      
+      //using list     = etl::type_list<>;
+      //using result1  = etl::type_list_pop_front_t<list>;
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_pop_front_from_non_empty_list)
+    {
+      using list     = etl::type_list<A, B, C>;
+      using result1  = etl::type_list_pop_front_t<list>;
+      using result2  = etl::type_list_pop_front_t<result1>;
+      using result3  = etl::type_list_pop_front_t<result2>;
+      using expected1 = etl::type_list<B, C>;
+      using expected2 = etl::type_list<C>;
+      using expected3 = etl::type_list<>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+      CHECK((etl::is_same<result2, expected2>::value));
+      CHECK((etl::is_same<result3, expected3>::value));
+
+      CHECK_EQUAL(2U, etl::type_list_size<result1>::value);
+      CHECK_EQUAL(1U, etl::type_list_size<result2>::value);
+      CHECK_EQUAL(0U, etl::type_list_size<result3>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_pop_back_empty_list)
+    {
+      // Uncomment to generate static_assert error.
+
+      //using list     = etl::type_list<>;
+      //using result1  = etl::type_list_pop_front_t<list>;
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_pop_back_from_non_empty_list)
+    {
+      using list     = etl::type_list<A, B, C>;
+      using result1  = etl::type_list_pop_back_t<list>;
+      using result2  = etl::type_list_pop_back_t<result1>;
+      using result3  = etl::type_list_pop_back_t<result2>;
+      using expected1 = etl::type_list<A, B>;
+      using expected2 = etl::type_list<A>;
+      using expected3 = etl::type_list<>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+      CHECK((etl::is_same<result2, expected2>::value));
+      CHECK((etl::is_same<result3, expected3>::value));
+
+      CHECK_EQUAL(2U, etl::type_list_size<result1>::value);
+      CHECK_EQUAL(1U, etl::type_list_size<result2>::value);
+      CHECK_EQUAL(0U, etl::type_list_size<result3>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_unique_for_empty_list)
+    {
+      using list     = etl::type_list<>;
+      using result   = etl::type_list_unique_t<list>;
+      using expected = etl::type_list<>;
+
+      CHECK((etl::is_same<result, expected>::value));
+      CHECK_EQUAL(0U, etl::type_list_size<result>::value);
+    }
+
+    //*************************************************************************
+    TEST(test_type_list_unique_for_non_empty_list)
+    {
+      using list      = etl::type_list<A, B, A, C, C, B, C>;
+      using result1   = etl::type_list_unique_t<list>;
+      using expected1 = etl::type_list<A, B, C>;
+
+      CHECK((etl::is_same<result1, expected1>::value));
+
+      CHECK_EQUAL(3U, etl::type_list_size<result1>::value);
     }
   }
 #endif