Add documentation for drop_value sender adaptor

docs/api.rst

@@ -75,6 +75,12 @@ The ``pika/init.hpp`` header provides functionality to manage the pika runtime.
 
 The ``pika/execution.hpp`` header provides functionality related to ``std::execution``.
 
+.. doxygenvariable:: pika::execution::experimental::drop_value
+
+.. literalinclude:: ../examples/documentation/drop_value_documentation.cpp
+   :language: c++
+   :start-at: #include
+
 .. doxygenvariable:: pika::execution::experimental::split_tuple
 
 .. literalinclude:: ../examples/documentation/split_tuple_documentation.cpp

examples/documentation/CMakeLists.txt

@@ -4,8 +4,8 @@
 # Distributed under the Boost Software License, Version 1.0. (See accompanying
 # file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
-set(example_programs hello_world_documentation init_hpp_documentation split_tuple_documentation
-                     when_all_vector_documentation
+set(example_programs drop_value_documentation hello_world_documentation init_hpp_documentation
+                     split_tuple_documentation when_all_vector_documentation
 )
 
 foreach(example_program ${example_programs})

examples/documentation/drop_value_documentation.cpp

@@ -0,0 +1,36 @@
+//  Copyright (c) 2024 ETH Zurich
+//
+//  SPDX-License-Identifier: BSL-1.0
+//  Distributed under the Boost Software License, Version 1.0. (See accompanying
+//  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <pika/execution.hpp>
+#include <pika/init.hpp>
+
+#include <fmt/printf.h>
+
+#include <tuple>
+#include <utility>
+
+struct custom_type
+{
+};
+
+int main(int argc, char* argv[])
+{
+    namespace ex = pika::execution::experimental;
+    namespace tt = pika::this_thread::experimental;
+
+    pika::start(argc, argv);
+    ex::thread_pool_scheduler sched{};
+
+    auto s = ex::just(42, custom_type{}, std::tuple("hello")) | ex::drop_value() |
+        // No matter what is sent to drop_value, it won't be sent from drop_value
+        ex::then([] { fmt::print("I got nothing...\n"); });
+    tt::sync_wait(std::move(s));
+
+    pika::finalize();
+    pika::stop();
+
+    return 0;
+}

libs/pika/execution/include/pika/execution/algorithms/drop_value.hpp

@@ -129,19 +129,22 @@ namespace pika::drop_value_detail {
 }    // namespace pika::drop_value_detail
 
 namespace pika::execution::experimental {
-    inline constexpr struct drop_value_t final
-      : pika::functional::detail::tag_fallback<drop_value_t>
+    struct drop_value_t final : pika::functional::detail::tag_fallback<drop_value_t>
     {
         template <typename Sender, PIKA_CONCEPT_REQUIRES_(is_sender_v<Sender>)>
         friend constexpr PIKA_FORCEINLINE auto tag_fallback_invoke(drop_value_t, Sender&& sender)
         {
             return drop_value_detail::drop_value_sender<Sender>{PIKA_FORWARD(Sender, sender)};
         }
 
-        // TODO: Use static operator() here, will go to customization afterwards anyway.
-        friend constexpr PIKA_FORCEINLINE auto tag_fallback_invoke(drop_value_t)
-        {
-            return detail::partial_algorithm<drop_value_t>{};
-        }
-    } drop_value{};
+        using pika::functional::detail::tag_fallback<drop_value_t>::operator();
+        auto operator()() const { return detail::partial_algorithm<drop_value_t>{}; }
+    };
+
+    /// \brief Ignores all values sent by the predecessor sender, sending none itself.
+    ///
+    /// Sender adaptor that takes any sender and returns a new sender that sends no values.
+    ///
+    /// Added in 0.6.0.
+    inline constexpr drop_value_t drop_value{};
 }    // namespace pika::execution::experimental