Implement continuables as return types for coroutines

* Closes #4

doc/tutorial-awaiting-continuables.dox

@@ -96,16 +96,34 @@ result.get_value();
 result.get_exception();
 \endcode
 
+\section tutorial-awaiting-continuables-return Using continuables as return type from coroutines
+
+It is possible to use a \ref continuable_base as return type from coroutines.
 
-\note It isn't possible as of now to use a \ref continuable_base
-      as return type from coroutines:
 \code{.cpp}
-cti::continuable<int> do_sth() {
+cti::continuable<> resolve_async_void() {
+  co_await http_request("github.com");
+  // ...
+  co_return;
+}
+
+cti::continuable<int> resolve_async() {
   co_await http_request("github.com");
   // ...
   co_return 0;
 }
 \endcode
 
+Additionally it's possible to return multiple return values from coroutines
+by wrapping those in a tuple like type:
+
+\code{.cpp}
+cti::continuable<int, int, int> resolve_async_multiple() {
+  co_await http_request("github.com");
+  // ...
+  co_return std::make_tuple(0, 1, 2);
+}
+\endcode
+
 */
 }

include/continuable/continuable-base.hpp

@@ -48,6 +48,7 @@
 
 #ifdef CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
 #include <continuable/detail/awaiting.hpp>
+#include <experimental/coroutine>
 #endif // CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
 
 namespace cti {
@@ -670,16 +671,32 @@ public:
   /// result.get_exception();
   /// ```
   ///
-  /// \attention Note that it isn't possible as of now to use a continuable
-  ///            as return type from coroutines as depicted below:
+  /// \note  Using continuable_base as return type for coroutines
+  ///        is supported. The coroutine is initially stopped and
+  ///        resumed when the continuation is requested in order to
+  ///        keep the lazy evaluation semantics of the continuable_base.
   /// ```cpp
-  /// cti::continuable<int> do_sth() {
+  /// cti::continuable<> resolve_async_void() {
+  ///   co_await http_request("github.com");
+  ///   // ...
+  ///   co_return;
+  /// }
+  ///
+  /// cti::continuable<int> resolve_async() {
   ///   co_await http_request("github.com");
   ///   // ...
   ///   co_return 0;
   /// }
   /// ```
-  ///            Propably this will be added in a future version of the library.
+  /// It's possible to return multiple return values from coroutines
+  /// by wrapping those in a tuple like type:
+  /// ```cpp
+  /// cti::continuable<int, int, int> resolve_async_multiple() {
+  ///   co_await http_request("github.com");
+  ///   // ...
+  ///   co_return std::make_tuple(0, 1, 2);
+  /// }
+  /// ```
   ///
   /// \since     2.0.0
   auto operator co_await() && {

include/continuable/continuable-coroutine.hpp

@@ -0,0 +1,66 @@
+/*
+
+                        /~` _  _ _|_. _     _ |_ | _
+                        \_,(_)| | | || ||_|(_||_)|(/_
+
+                    https://github.com/Naios/continuable
+                                   v3.0.0
+
+  Copyright(c) 2015 - 2018 Denis Blank <denis.blank at outlook dot com>
+
+  Permission is hereby granted, free of charge, to any person obtaining a copy
+  of this software and associated documentation files(the "Software"), to deal
+  in the Software without restriction, including without limitation the rights
+  to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
+  copies of the Software, and to permit persons to whom the Software is
+  furnished to do so, subject to the following conditions :
+
+  The above copyright notice and this permission notice shall be included in
+  all copies or substantial portions of the Software.
+
+  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
+  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+  SOFTWARE.
+**/
+
+#ifndef CONTINUABLE_COROUTINE_HPP_INCLUDED
+#define CONTINUABLE_COROUTINE_HPP_INCLUDED
+
+#include <continuable/continuable-base.hpp>
+#include <continuable/continuable-types.hpp>
+#include <continuable/detail/features.hpp>
+#include <continuable/detail/types.hpp>
+
+#if defined(CONTINUABLE_HAS_EXCEPTIONS)
+#include <exception>
+#endif // CONTINUABLE_HAS_EXCEPTIONS
+
+#ifdef CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
+#include <continuable/detail/awaiting.hpp>
+#endif // CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
+
+/// \cond false
+#ifdef CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
+// As far as I know there is no other way to implement this specialization...
+// NOLINTNEXTLINE(cert-dcl58-cpp)
+namespace std {
+namespace experimental {
+template <typename Data, typename... Args, typename... FunctionArgs>
+struct coroutine_traits<
+    cti::continuable_base<Data,
+                          cti::detail::hints::signature_hint_tag<Args...>>,
+    FunctionArgs...> {
+
+  using promise_type =
+      cti::detail::awaiting::promise_type<cti::promise<Args...>, Args...>;
+};
+} // namespace experimental
+} // namespace std
+#endif // CONTINUABLE_HAS_EXPERIMENTAL_COROUTINE
+/// \endcond
+
+#endif // CONTINUABLE_COROUTINE_HPP_INCLUDED

include/continuable/continuable-promise-base.hpp

@@ -76,6 +76,11 @@ public:
   /// Constructor accepting the data object
   explicit promise_base(Data data) : data_(std::move(data)) {
   }
+  /// \cond false
+  /// Constructor for constructing an empty promise
+  explicit promise_base(detail::types::promise_no_init_tag) {
+  }
+  /// \endcond
 
   /// Constructor accepting any object convertible to the data object
   template <typename OData, std::enable_if_t<std::is_convertible<

include/continuable/continuable.hpp

@@ -47,6 +47,7 @@ namespace cti {}
 
 #include <continuable/continuable-base.hpp>
 #include <continuable/continuable-connections.hpp>
+#include <continuable/continuable-coroutine.hpp>
 #include <continuable/continuable-promise-base.hpp>
 #include <continuable/continuable-promisify.hpp>
 #include <continuable/continuable-trait.hpp>

include/continuable/detail/awaiting.hpp

@@ -34,10 +34,12 @@
 
 #include <cassert>
 #include <experimental/coroutine>
+#include <tuple>
 
 #include <continuable/detail/expected.hpp>
 #include <continuable/detail/features.hpp>
 #include <continuable/detail/hints.hpp>
+#include <continuable/detail/traits.hpp>
 #include <continuable/detail/types.hpp>
 #include <continuable/detail/util.hpp>
 
@@ -120,25 +122,91 @@ template <typename T>
 constexpr auto create_awaiter(T&& continuable) {
   return awaitable<std::decay_t<T>>(std::forward<T>(continuable));
 }
+
+/// This makes it possible to take the coroutine_handle over on suspension
+struct handle_takeover {
+  coroutine_handle<>& handle_;
+
+  bool await_ready() noexcept {
+    return false;
+  }
+
+  void await_suspend(coroutine_handle<> handle) noexcept {
+    handle_ = handle;
+  }
+
+  void await_resume() noexcept {
+  }
+};
+
+/// The type which is passed to the compiler that describes the properties
+/// of a continuable_base used as coroutine promise type.
+template <typename Promise, typename... Args>
+struct promise_type;
+
+/// Implements the resolving method return_void and return_value accordingly
+template <typename Base>
+struct promise_resolver_base;
+
+template <typename Promise>
+struct promise_resolver_base<promise_type<Promise>> {
+  void return_void() {
+    auto me = static_cast<promise_type<Promise>*>(this);
+    me->promise_.set_value();
+  }
+};
+template <typename Promise, typename T>
+struct promise_resolver_base<promise_type<Promise, T>> {
+  void return_value(T value) {
+    auto me = static_cast<promise_type<Promise, T>*>(this);
+    me->promise_.set_value(std::move(value));
+  }
+};
+template <typename Promise, typename... Args>
+struct promise_resolver_base<promise_type<Promise, Args...>> {
+  template <typename T>
+  void return_value(T&& tuple_like) {
+    auto me = static_cast<promise_type<Promise, Args...>*>(this);
+    traits::unpack(std::move(me->promise_), std::forward<T>(tuple_like));
+  }
+};
+
+template <typename Promise, typename... Args>
+struct promise_type
+    : promise_resolver_base<promise_type<Promise, Args...>> {
+
+  coroutine_handle<> handle_;
+  Promise promise_;
+
+  explicit promise_type() : promise_(types::promise_no_init_tag{}) {
+  }
+
+  auto get_return_object() {
+    return [this](auto&& promise) {
+      promise_ = std::forward<decltype(promise)>(promise);
+      handle_.resume();
+    };
+  }
+
+  handle_takeover initial_suspend() {
+    return {handle_};
+  }
+
+  std::experimental::suspend_never final_suspend() {
+    return {};
+  }
+
+  void unhandled_exception() noexcept {
+#if defined(CONTINUABLE_HAS_EXCEPTIONS)
+    promise_.set_exception(std::current_exception());
+#else  // CONTINUABLE_HAS_EXCEPTIONS
+       // Returning error types from coroutines isn't supported
+    cti::detail::util::trap();
+#endif // CONTINUABLE_HAS_EXCEPTIONS
+  }
+};
 } // namespace awaiting
 } // namespace detail
 } // namespace cti
 
-// As far as I know there is no other was to implement this specialization...
-// NOLINTNEXTLINE(cert-dcl58-cpp)
-namespace std {
-namespace experimental {
-template <typename Data, typename... Args, typename... FunctionArgs>
-struct coroutine_traits<
-    cti::continuable_base<Data,
-                          cti::detail::hints::signature_hint_tag<Args...>>,
-    FunctionArgs...> {
-
-  static_assert(cti::detail::traits::fail<Data>::value,
-                "Using a continuable as return type from co_return "
-                "expressions isn't supported yet!");
-};
-} // namespace experimental
-} // namespace std
-
 #endif // CONTINUABLE_DETAIL_UTIL_HPP_INCLUDED

include/continuable/detail/types.hpp

@@ -77,6 +77,8 @@ public:
   using T::operator();
 };
 
+/// Tag for constructing an empty promise_base .
+struct promise_no_init_tag {};
 } // namespace types
 } // namespace detail
 } // namespace cti

test/unit-test/multi/test-continuable-await.cpp

@@ -32,42 +32,9 @@
 
 #include <tuple>
 
-namespace std {
-namespace experimental {
-template <class... T>
-struct coroutine_traits<void, T...> {
-  struct promise_type {
-    void get_return_object() {
-    }
-
-    void set_exception(exception_ptr const&) noexcept {
-    }
-
-    // FIXME This throws errors in MSVC but is required in clang
-#ifndef _MSC_VER
-    void unhandled_exception() {
-      GTEST_FATAL_FAILURE_("Unhandled async exception!");
-    }
-#endif // _MSC_VER
-
-    suspend_never initial_suspend() noexcept {
-      return {};
-    }
-
-    suspend_never final_suspend() noexcept {
-      return {};
-    }
-
-    void return_void() noexcept {
-    }
-  };
-};
-} // namespace experimental
-} // namespace std
-
 /// Resolves the given promise asynchonously
-template <typename S, typename T>
-void resolve_async(S&& supplier, T promise) {
+template <typename S>
+cti::continuable<> resolve_async(S&& supplier) {
   // 0 args
   co_await supplier();
 
@@ -79,21 +46,34 @@ void resolve_async(S&& supplier, T promise) {
   std::tuple<int, int> a2 = co_await supplier(1, 2);
   EXPECT_EQ(a2, std::make_tuple(1, 2));
 
-  promise.set_value();
   co_return;
 }
 
+template <typename S>
+cti::continuable<int> resolve_async_one(S&& supplier) {
+  // Pseudo wait
+  co_await supplier();
+
+  co_return 4644;
+}
+
+template <typename S>
+cti::continuable<int, int, int, int> resolve_async_multiple(S&& supplier) {
+  // Pseudo wait
+  co_await supplier();
+
+  co_return std::make_tuple(0, 1, 2, 3);
+}
+
 TYPED_TEST(single_dimension_tests, are_awaitable) {
   auto const& supply = [&](auto&&... args) {
     // Supplies the current tested continuable
     return this->supply(std::forward<decltype(args)>(args)...);
   };
 
-  EXPECT_ASYNC_RESULT(
-      this->supply().then(cti::make_continuable<void>([&](auto promise) {
-        // Resolve the cotinuable through a coroutine
-        resolve_async(supply, std::move(promise));
-      })));
+  EXPECT_ASYNC_RESULT(resolve_async(supply));
+  EXPECT_ASYNC_RESULT(resolve_async_one(supply), 4644);
+  EXPECT_ASYNC_RESULT(resolve_async_multiple(supply), 0, 1, 2, 3);
 }
 
 #ifndef CONTINUABLE_WITH_NO_EXCEPTIONS
@@ -109,8 +89,8 @@ struct await_exception : std::exception {
 };
 
 /// Resolves the given promise asynchonously through an exception
-template <typename S, typename T>
-void resolve_async_exceptional(S&& supplier, T promise) {
+template <typename S>
+cti::continuable<> resolve_async_exceptional(S&& supplier) {
   // 0 args
   co_await supplier();
 
@@ -128,7 +108,6 @@ void resolve_async_exceptional(S&& supplier, T promise) {
   EXPECT_THROW(co_await supplier().then([] { throw await_exception{}; }),
                await_exception);
 
-  promise.set_value();
   co_return;
 }
 
@@ -138,23 +117,29 @@ TYPED_TEST(single_dimension_tests, are_awaitable_with_exceptions) {
     return this->supply(std::forward<decltype(args)>(args)...);
   };
 
-  ASSERT_ASYNC_COMPLETION(
-      this->supply().then(cti::make_continuable<void>([&](auto promise) {
-        // Resolve the cotinuable through a coroutine
-        resolve_async_exceptional(supply, std::move(promise));
-      })));
+  ASSERT_ASYNC_COMPLETION(resolve_async_exceptional(supply));
 }
 
-// TODO Implement this later
-//
-//  static cti::continuable<int> async_await() {
-//   co_await cti::make_continuable<void>([](auto&& promise) {
-//     // ...
-//     promise.set_value();
-//   });
-//
-//   co_return 1;
-// }
+/// Resolves the given promise asynchonously through an exception
+template <typename S>
+cti::continuable<> resolve_coro_exceptional(S&& supplier) {
+  // Pseudo wait
+  co_await supplier();
+
+  throw await_exception{};
+
+  co_return;
+}
+
+TYPED_TEST(single_dimension_tests, are_awaitable_with_exceptions_from_coro) {
+  auto const& supply = [&](auto&&... args) {
+    // Supplies the current tested continuable
+    return this->supply(std::forward<decltype(args)>(args)...);
+  };
+
+  ASSERT_ASYNC_EXCEPTION_RESULT(resolve_coro_exceptional(supply),
+                                await_exception{})
+}
 
 #endif // CONTINUABLE_WITH_NO_EXCEPTIONS