support using co_await with folly::Optional when it is available

Summary:
Coroutines can be used to simulate the monadic "do" notion of Haskell. Use coroutines to enable monadic composition with folly::Optional.

```
Optional<int> f() {
  return 7;
}
Optional<int> g(int) {
  return folly::none;
}

void MonadicOptional() {
  Optional<int> r = []() -> Optional<int> {
    int x = co_await f();
    assert(x == 7);
    int y = co_await g(x);
    assert(false); // not executed
    co_return y;
  }();
  assert(!r.hasValue());
}
```

Reviewed By: yfeldblum

Differential Revision: D5763371

fbshipit-source-id: 9babc682244f38da7006d0b3a8444fd4efec1747

folly/Optional.h

@@ -67,6 +67,9 @@
 
 namespace folly {
 
+template <class Value>
+class Optional;
+
 namespace detail {
 struct NoneHelper {};
 
@@ -74,7 +77,10 @@ struct NoneHelper {};
 // If exceptions are disabled, std::terminate() will be called instead of
 // throwing OptionalEmptyException when the condition fails.
 [[noreturn]] void throw_optional_empty_exception();
-}
+
+template <class Value>
+struct OptionalPromiseReturn;
+} // namespace detail
 
 typedef int detail::NoneHelper::*None;
 
@@ -133,6 +139,12 @@ class Optional {
     storage_.construct(std::forward<Args>(args)...);
   }
 
+  // Used only when an Optional is used with coroutines on MSVC
+  /* implicit */ Optional(const detail::OptionalPromiseReturn<Value>& p)
+      : Optional{} {
+    p.promise_->value_ = this;
+  }
+
   void assign(const None&) {
     clear();
   }
@@ -519,3 +531,95 @@ struct hash<folly::Optional<T>> {
   }
 };
 FOLLY_NAMESPACE_STD_END
+
+// Enable the use of folly::Optional with `co_await`
+// Inspired by https://github.com/toby-allsopp/coroutine_monad
+#if FOLLY_HAS_COROUTINES
+#include <experimental/coroutine>
+
+namespace folly {
+namespace detail {
+template <typename Value>
+struct OptionalPromise;
+
+template <typename Value>
+struct OptionalPromiseReturn {
+  Optional<Value> storage_;
+  OptionalPromise<Value>* promise_;
+  /* implicit */ OptionalPromiseReturn(OptionalPromise<Value>& promise) noexcept
+      : promise_(&promise) {
+    promise.value_ = &storage_;
+  }
+  OptionalPromiseReturn(OptionalPromiseReturn&& that) noexcept
+      : OptionalPromiseReturn{*that.promise_} {}
+  ~OptionalPromiseReturn() {}
+  /* implicit */ operator Optional<Value>() & {
+    return std::move(storage_);
+  }
+};
+
+template <typename Value>
+struct OptionalPromise {
+  Optional<Value>* value_ = nullptr;
+  OptionalPromise() = default;
+  OptionalPromise(OptionalPromise const&) = delete;
+  // This should work regardless of whether the compiler generates:
+  //    folly::Optional<Value> retobj{ p.get_return_object(); } // MSVC
+  // or:
+  //    auto retobj = p.get_return_object(); // clang
+  OptionalPromiseReturn<Value> get_return_object() noexcept {
+    return *this;
+  }
+  std::experimental::suspend_never initial_suspend() const noexcept {
+    return {};
+  }
+  std::experimental::suspend_never final_suspend() const {
+    return {};
+  }
+  template <typename U>
+  void return_value(U&& u) {
+    *value_ = static_cast<U&&>(u);
+  }
+  void unhandled_exception() {
+    // Technically, throwing from unhandled_exception is underspecified:
+    // https://github.com/GorNishanov/CoroutineWording/issues/17
+    throw;
+  }
+};
+
+template <typename Value>
+struct OptionalAwaitable {
+  Optional<Value> o_;
+  bool await_ready() const noexcept {
+    return o_.hasValue();
+  }
+  Value await_resume() {
+    return o_.value();
+  }
+  template <typename CoroHandle>
+  void await_suspend(CoroHandle h) const {
+    // make sure the coroutine returns an empty Optional:
+    h.promise().value_->clear();
+    // Abort the rest of the coroutine:
+    h.destroy();
+  }
+};
+} // namespace detail
+
+template <typename Value>
+detail::OptionalAwaitable<Value>
+/* implicit */ operator co_await(Optional<Value> o) {
+  return {std::move(o)};
+}
+} // namespace folly
+
+// This makes std::optional<Value> useable as a coroutine return type..
+FOLLY_NAMESPACE_STD_BEGIN
+namespace experimental {
+template <typename Value, typename... Args>
+struct coroutine_traits<folly::Optional<Value>, Args...> {
+  using promise_type = folly::detail::OptionalPromise<Value>;
+};
+} // experimental
+FOLLY_NAMESPACE_STD_END
+#endif // FOLLY_HAS_COROUTINES

folly/Portability.h

@@ -425,3 +425,7 @@ constexpr auto kMscVer = 0;
 #else
 #define FOLLY_CPP14_CONSTEXPR inline
 #endif
+
+#if __cpp_coroutines >= 201703L || _MSC_VER
+#define FOLLY_HAS_COROUTINES 1
+#endif

folly/test/OptionalCoroutinesTest.cpp

+/*
+ * Copyright 2017 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/Optional.h>
+#include <folly/Portability.h>
+#include <folly/portability/GTest.h>
+
+#if FOLLY_HAS_COROUTINES
+using folly::Optional;
+
+Optional<int> f1() {
+  return 7;
+}
+Optional<double> f2(int x) {
+  return 2.0 * x;
+}
+Optional<int> f3(int x, double y) {
+  return (int)(x + y);
+}
+
+TEST(Optional, CoroutineSuccess) {
+  auto r0 = []() -> Optional<int> {
+    auto x = co_await f1();
+    EXPECT_EQ(7, x);
+    auto y = co_await f2(x);
+    EXPECT_EQ(2.0 * 7, y);
+    auto z = co_await f3(x, y);
+    EXPECT_EQ((int)(2.0 * 7 + 7), z);
+    co_return z;
+  }();
+  EXPECT_TRUE(r0.hasValue());
+  EXPECT_EQ(21, *r0);
+}
+
+Optional<int> f4(int, double) {
+  return folly::none;
+}
+
+TEST(Optional, CoroutineFailure) {
+  auto r1 = []() -> Optional<int> {
+    auto x = co_await f1();
+    auto y = co_await f2(x);
+    auto z = co_await f4(x, y);
+    EXPECT_FALSE(true);
+    co_return z;
+  }();
+  EXPECT_TRUE(!r1.hasValue());
+}
+
+Optional<int> throws() {
+  throw 42;
+}
+
+TEST(Optional, CoroutineException) {
+  try {
+    auto r2 = []() -> Optional<int> {
+      auto x = co_await throws();
+      EXPECT_FALSE(true);
+      co_return x;
+    }();
+    EXPECT_FALSE(true);
+  } catch (/* nolint */ int i) {
+    EXPECT_EQ(42, i);
+  } catch (...) {
+    EXPECT_FALSE(true);
+  }
+}
+#endif