1 files changed, 109 insertions, 0 deletions
diff --git a/include/ftl/future.h b/include/ftl/future.h
new file mode 100644
index 0000000000..dd6358fa7d
--- /dev/null
+++ b/include/ftl/future.h
@@ -0,0 +1,109 @@
+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <future>
+#include <type_traits>
+#include <utility>
+
+namespace android::ftl {
+
+// Creates a future that defers a function call until its result is queried.
+//
+//   auto future = ftl::defer([](int x) { return x + 1; }, 99);
+//   assert(future.get() == 100);
+//
+template <typename F, typename... Args>
+inline auto defer(F&& f, Args&&... args) {
+  return std::async(std::launch::deferred, std::forward<F>(f), std::forward<Args>(args)...);
+}
+
+// Creates a future that wraps a value.
+//
+//   auto future = ftl::yield(42);
+//   assert(future.get() == 42);
+//
+//   auto ptr = std::make_unique<char>('!');
+//   auto future = ftl::yield(std::move(ptr));
+//   assert(*future.get() == '!');
+//
+template <typename T>
+inline std::future<T> yield(T&& v) {
+  return defer([](T&& v) { return std::forward<T>(v); }, std::forward<T>(v));
+}
+
+namespace details {
+
+template <typename T>
+struct future_result {
+  using type = T;
+};
+
+template <typename T>
+struct future_result<std::future<T>> {
+  using type = T;
+};
+
+template <typename T>
+using future_result_t = typename future_result<T>::type;
+
+// Attaches a continuation to a future. The continuation is a function that maps T to either R or
+// std::future<R>. In the former case, the chain wraps the result in a future as if by ftl::yield.
+//
+//   auto future = ftl::yield(123);
+//   std::future<char> futures[] = {ftl::yield('a'), ftl::yield('b')};
+//
+//   std::future<char> chain =
+//       ftl::chain(std::move(future))
+//           .then([](int x) { return static_cast<std::size_t>(x % 2); })
+//           .then([&futures](std::size_t i) { return std::move(futures[i]); });
+//
+//   assert(chain.get() == 'b');
+//
+template <typename T>
+struct Chain {
+  // Implicit conversion.
+  Chain(std::future<T>&& f) : future(std::move(f)) {}
+  operator std::future<T>&&() && { return std::move(future); }
+
+  T get() && { return future.get(); }
+
+  template <typename F, typename R = std::invoke_result_t<F, T>>
+  auto then(F&& op) && -> Chain<future_result_t<R>> {
+    return defer(
+        [](auto&& f, F&& op) {
+          R r = op(f.get());
+          if constexpr (std::is_same_v<R, future_result_t<R>>) {
+            return r;
+          } else {
+            return r.get();
+          }
+        },
+        std::move(future), std::forward<F>(op));
+  }
+
+  std::future<T> future;
+};
+
+}  // namespace details
+
+template <typename T>
+inline auto chain(std::future<T>&& f) -> details::Chain<T> {
+  return std::move(f);
+}
+
+}  // namespace android::ftl