or_else accepts actions returning void

cmake/Findgimo-catch2.cmake

@@ -1,4 +1,4 @@
-#          Copyright Dominic (DNKpp) Koepke 2025 - 2026.
+#          Copyright Dominic (DNKpp) Koepke 2025-2026.
 # Distributed under the Boost Software License, Version 1.0.
 #    (See accompanying file LICENSE_1_0.txt or copy at
 #          https://www.boost.org/LICENSE_1_0.txt)
@@ -8,7 +8,7 @@ include(Gimo-get_cpm)
 # fixes reporting in clion
 set(CATCH_CONFIG_CONSOLE_WIDTH 800 CACHE STRING "")
 
-CPMAddPackage("gh:catchorg/Catch2@3.13.0")
+CPMAddPackage("gh:catchorg/Catch2@3.15.0")
 
 if (Catch2_ADDED)
 	include("${Catch2_SOURCE_DIR}/extras/Catch.cmake")

include/gimo/algorithm/OrElse.hpp

@@ -1,4 +1,4 @@
-//          Copyright Dominic (DNKpp) Koepke 2025.
+//          Copyright Dominic (DNKpp) Koepke 2025-2026.
 // Distributed under the Boost Software License, Version 1.0.
 //    (See accompanying file LICENSE_1_0.txt or copy at
 //          https://www.boost.org/LICENSE_1_0.txt)
@@ -21,31 +21,31 @@
 namespace gimo::detail::or_else
 {
     template <typename Nullable, typename Action>
-    consteval Nullable* print_diagnostics()
+    consteval Nullable& print_diagnostics()
     {
         if constexpr (!std::is_invocable_v<Action>)
         {
             static_assert(always_false_v<Nullable>, "The or_else algorithm requires an action invocable without any arguments.");
         }
-        else if constexpr (!std::same_as<Nullable, std::invoke_result_t<Action>>)
+        else if constexpr (!std::same_as<Nullable, std::invoke_result_t<Action>> && !std::is_void_v<std::invoke_result_t<Action>>)
         {
-            static_assert(always_false_v<Nullable>, "The or_else algorithm requires an action returning the same nullable type.");
+            static_assert(always_false_v<Nullable>, "The or_else algorithm requires an action returning the same nullable type or void.");
         }
 
-        return nullptr;
+        return std::declval<Nullable&>();
     }
 
     template <typename Action, nullable Nullable>
     [[nodiscard]]
-    constexpr std::remove_cvref_t<Nullable> on_value([[maybe_unused]] Action&& action, Nullable&& opt)
+    constexpr std::remove_cvref_t<Nullable> on_value(Action&& /*action*/, Nullable&& opt)
     {
         return std::forward<Nullable>(opt);
     }
 
     template <typename Action, nullable Nullable, typename Next, typename... Steps>
     [[nodiscard]]
     constexpr auto on_value(
-        [[maybe_unused]] Action&& action,
+        Action&& /*action*/,
         Nullable&& opt,
         Next&& next,
         Steps&&... steps)
@@ -57,9 +57,17 @@ namespace gimo::detail::or_else
 
     template <typename Action, nullable Nullable>
     [[nodiscard]]
-    constexpr std::remove_cvref_t<Nullable> on_null(Action&& action, [[maybe_unused]] Nullable&& opt)
+    constexpr std::remove_cvref_t<Nullable> on_null(Action&& action, Nullable&& /*opt*/)
     {
-        return std::invoke(std::forward<Action>(action));
+        if constexpr (std::is_void_v<std::invoke_result_t<Action>>)
+        {
+            std::invoke(std::forward<Action>(action));
+            return null_v<Nullable>;
+        }
+        else
+        {
+            return std::invoke(std::forward<Action>(action));
+        }
     }
 
     template <nullable Nullable, typename Action, typename Next, typename... Steps>
@@ -72,13 +80,24 @@ namespace gimo::detail::or_else
             std::forward<Steps>(steps)...);
     }
 
+    template <nullable Nullable, typename Action, typename Next, typename... Steps>
+        requires std::is_void_v<std::invoke_result_t<Action>>
+    [[nodiscard]]
+    constexpr auto on_null(Action&& action, Nullable&& opt, Next&& next, Steps&&... steps)
+    {
+        return std::forward<Next>(next).on_null(
+            or_else::on_null(std::forward<Action>(action), std::forward<Nullable>(opt)),
+            std::forward<Steps>(steps)...);
+    }
+
     struct traits
     {
         template <nullable Nullable, typename Action>
         static constexpr bool is_applicable_on = requires {
             requires std::same_as<
-                std::remove_cvref_t<Nullable>,
-                std::remove_cvref_t<std::invoke_result_t<Action>>>;
+                         std::remove_cvref_t<Nullable>,
+                         std::remove_cvref_t<std::invoke_result_t<Action>>>
+                         || std::is_void_v<std::invoke_result_t<Action>>;
         };
 
         template <typename Action, nullable Nullable, typename... Steps>
@@ -94,7 +113,7 @@ namespace gimo::detail::or_else
             }
             else
             {
-                return *or_else::print_diagnostics<Nullable, Action>();
+                return or_else::print_diagnostics<Nullable, Action>();
             }
         }
 
@@ -111,7 +130,7 @@ namespace gimo::detail::or_else
             }
             else
             {
-                return *or_else::print_diagnostics<Nullable, Action>();
+                return or_else::print_diagnostics<Nullable, Action>();
             }
         }
     };
@@ -133,7 +152,7 @@ namespace gimo
      * \return A Pipeline step containing the `or_else` algorithm.
      * \details
      * - **On Value**: Propagates the value state immediately (i.e., `action` is not executed).
-     * - **On Null**: Invokes the `action`. The `action` **must** return a `nullable` type.
+     * - **On Null**: Invokes the `action`. The `action` must either return the **exact same** `nullable` type or `void`.
      * \see https://en.wikipedia.org/wiki/Monad_(functional_programming)
      */
     template <typename Action>

test/compile-errors/or_else/action-return-mismatch.cpp

@@ -1,4 +1,4 @@
-//          Copyright Dominic (DNKpp) Koepke 2025.
+//          Copyright Dominic (DNKpp) Koepke 2025-2026.
 // Distributed under the Boost Software License, Version 1.0.
 //    (See accompanying file LICENSE_1_0.txt or copy at
 //          https://www.boost.org/LICENSE_1_0.txt)
@@ -8,7 +8,7 @@
 
 /*
 <begin-expected-compile-error>
-The or_else algorithm requires an action returning the same nullable type\.
+The or_else algorithm requires an action returning the same nullable type or void\.
 <end-expected-compile-error>
 */
 

test/unit-tests/algorithm/OrElse.cpp

@@ -1,4 +1,4 @@
-//          Copyright Dominic (DNKpp) Koepke 2025.
+//          Copyright Dominic (DNKpp) Koepke 2025-2026.
 // Distributed under the Boost Software License, Version 1.0.
 //    (See accompanying file LICENSE_1_0.txt or copy at
 //          https://www.boost.org/LICENSE_1_0.txt)
@@ -180,3 +180,22 @@ TEMPLATE_LIST_TEST_CASE(
         and finally::returns(1337);
     CHECK(1337 == pipeline.apply(std::optional<int>{}));
 }
+
+TEST_CASE(
+    "gimo::or_else accepts actions returning void.",
+    "[algorithm]")
+{
+    mimicpp::Mock<void()> action{};
+    auto const pipeline = or_else(std::ref(action));
+
+    SECTION("When a value is contained, the action is not invoked.")
+    {
+        CHECK(1337 == pipeline.apply(std::optional{1337}));
+    }
+
+    SECTION("When no value is contained, the action is invoked and null is returned.")
+    {
+        SCOPED_EXP action.expect_call();
+        CHECK(!pipeline.apply(std::optional<int>{}));
+    }
+}