From 20cd0191fcac3526afceba5712ef8f7f593f6014 Mon Sep 17 00:00:00 2001
From: Denis Blank <denis.blank@outlook.com>
Date: Mon, 14 Jan 2019 21:10:17 +0100
Subject: [PATCH] For loops provide loop_result loop_break and loop_continue
 for better readability

---
 include/continuable/detail/core/types.hpp     |  5 ++
 include/continuable/operations/loop.hpp       | 51 ++++++++++++++++++-
 .../test-continuable-operations-loop.cpp      | 20 +++++++-
 3 files changed, 73 insertions(+), 3 deletions(-)

diff --git a/include/continuable/detail/core/types.hpp b/include/continuable/detail/core/types.hpp
index 0ec8737..395188d 100644
--- a/include/continuable/detail/core/types.hpp
+++ b/include/continuable/detail/core/types.hpp
@@ -31,6 +31,7 @@
 #ifndef CONTINUABLE_DETAIL_TYPES_HPP_INCLUDED
 #define CONTINUABLE_DETAIL_TYPES_HPP_INCLUDED
 
+#include <type_traits>
 #include <utility>
 #include <continuable/detail/features.hpp>
 #include <continuable/detail/utility/identity.hpp>
@@ -84,6 +85,10 @@ class plain_tag {
   T value_;
 
 public:
+  template <typename O, std::enable_if_t<std::is_constructible<
+                            T, std::decay_t<O>>::value>* = nullptr>
+  /* implicit */ plain_tag(O&& value) : value_(std::forward<O>(value)) {
+  }
   explicit plain_tag(T value) : value_(std::move(value)) {
   }
 
diff --git a/include/continuable/operations/loop.hpp b/include/continuable/operations/loop.hpp
index dd4f569..a2f9142 100644
--- a/include/continuable/operations/loop.hpp
+++ b/include/continuable/operations/loop.hpp
@@ -32,6 +32,7 @@
 #define CONTINUABLE_OPERATIONS_LOOP_HPP_INCLUDED
 
 #include <utility>
+#include <continuable/continuable-primitives.hpp>
 #include <continuable/continuable-result.hpp>
 #include <continuable/detail/operations/loop.hpp>
 
@@ -42,11 +43,30 @@ namespace cti {
 /// Can be used to create an asynchronous loop.
 ///
 /// The callable will be called repeatedly until it returns a
-/// cti::continuable_base which then resolves to present cti::result.
+/// cti::continuable_base which then resolves to a present cti::result.
+///
+/// For better readability cti::loop_result, cti::loop_break and
+/// cti::loop_continue are provided which can be used as following:
+/// ```cpp
+/// auto while_answer_not_yes() {
+///   return loop([] {
+///     return ask_something().then([](std::string answer) -> loop_result<> {
+///       if (answer == "yes") {
+///         return loop_break();
+///       } else {
+///         return loop_continue();
+///       }
+///     });
+///   });
+/// }
+/// ```
 ///
 /// \param callable The callable type which must return a cti::continuable_base
 ///        which then resolves to a cti::result of arbitrary values.
 ///
+/// \param args The arguments that are passed to the callable upon
+///             each invocation.
+///
 /// \since 4.0.0
 ///
 template <typename Callable, typename... Args>
@@ -55,6 +75,34 @@ auto loop(Callable&& callable, Args&&... args) {
                                   std::forward<Args>(args)...);
 }
 
+/// Can be used to indicate a specific result inside an asynchronous loop.
+///
+/// See cti::loop for details.
+///
+/// \since 4.0.0
+template <typename... T>
+using loop_result = plain_t<result<T...>>;
+
+/// Can be used to create a loop_result which causes the loop to be
+/// cancelled and resolved with the given arguments.
+///
+/// See cti::loop for details.
+///
+/// \since 4.0.0
+template <typename... T>
+auto loop_break(T&&... args) {
+  return make_plain(make_result(std::forward<T>(args)...));
+}
+
+/// Can be used to create a loop_result which causes the loop to be repeated.
+///
+/// See cti::loop for details.
+///
+/// \since 4.0.0
+inline auto loop_continue() noexcept {
+  return empty_result{};
+}
+
 ///
 /// \since 4.0.0
 ///
@@ -64,6 +112,7 @@ auto range_loop(Callable&& callable, Iterator begin, Iterator end) {
       detail::operations::make_range_looper(std::forward<Callable>(callable),
                                             begin, end));
 }
+
 /// \}
 } // namespace cti
 
diff --git a/test/unit-test/multi/test-continuable-operations-loop.cpp b/test/unit-test/multi/test-continuable-operations-loop.cpp
index 40d7a90..ace6d18 100644
--- a/test/unit-test/multi/test-continuable-operations-loop.cpp
+++ b/test/unit-test/multi/test-continuable-operations-loop.cpp
@@ -52,6 +52,22 @@ TYPED_TEST(single_dimension_tests, operations_loop_completion) {
                       CANARY, 2, CANARY);
 }
 
+TYPED_TEST(single_dimension_tests, operations_loop_until) {
+  int i = 0;
+
+  ASSERT_ASYNC_COMPLETION(loop([&i] {
+    return make_ready_continuable(++i).then([](int i) -> loop_result<> {
+      if (i == 3) {
+        return loop_break();
+      } else {
+        return loop_continue();
+      }
+    });
+  }));
+
+  ASSERT_EQ(i, 3);
+}
+
 TYPED_TEST(single_dimension_tests, operations_loop_looping) {
   int i = 0;
 
@@ -61,7 +77,7 @@ TYPED_TEST(single_dimension_tests, operations_loop_looping) {
         ++i;
         return make_ready_continuable();
       },
-      0, 10));
+      0, 3));
 
-  ASSERT_EQ(i, 10);
+  ASSERT_EQ(i, 3);
 }