include/ftl/future.h

/*
 * 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>
#include <variant>

#include <ftl/details/future.h>

namespace android::ftl {

// Thin wrapper around FutureImpl<T> (concretely std::future<T> or std::shared_future<T>) with
// extensions for pure values (created via ftl::yield) and continuations.
//
// See also SharedFuture<T> shorthand below.
//
template <typename T, template <typename> class FutureImpl = std::future>
class Future final : public details::BaseFuture<Future<T, FutureImpl>, T, FutureImpl> {
  using Base = details::BaseFuture<Future, T, FutureImpl>;

  friend Base;                                            // For BaseFuture<...>::self.
  friend details::BaseFuture<Future<T>, T, std::future>;  // For BaseFuture<...>::share.

 public:
  // Constructs an invalid future.
  Future() : future_(std::in_place_type<FutureImpl<T>>) {}

  // Constructs a future from its standard counterpart, implicitly.
  Future(FutureImpl<T>&& f) : future_(std::move(f)) {}

  bool valid() const {
    return std::holds_alternative<T>(future_) || std::get<FutureImpl<T>>(future_).valid();
  }

  // Forwarding functions. Base::share is only defined when FutureImpl is std::future, whereas the
  // following are defined for either FutureImpl:
  using Base::get;

  // Attaches a continuation to the future. The continuation is a function that maps T to either R
  // or ftl::Future<R>. In the former case, the chain wraps the result in a future as if by
  // ftl::yield.
  //
  //   auto future = ftl::yield(123);
  //   ftl::Future<char> futures[] = {ftl::yield('a'), ftl::yield('b')};
  //
  //   auto chain =
  //       ftl::Future(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 F, typename R = std::invoke_result_t<F, T>>
  auto then(F&& op) && -> Future<details::future_result_t<R>> {
    return defer(
        [](auto&& f, F&& op) {
          R r = op(f.get());
          if constexpr (std::is_same_v<R, details::future_result_t<R>>) {
            return r;
          } else {
            return r.get();
          }
        },
        std::move(*this), std::forward<F>(op));
  }

 private:
  template <typename V>
  friend Future<V> yield(V&&);

  template <typename V, typename... Args>
  friend Future<V> yield(Args&&...);

  template <typename... Args>
  Future(details::ValueTag, Args&&... args)
      : future_(std::in_place_type<T>, std::forward<Args>(args)...) {}

  std::variant<T, FutureImpl<T>> future_;
};

template <typename T>
using SharedFuture = Future<T, std::shared_future>;

// Deduction guide for implicit conversion.
template <typename T, template <typename> class FutureImpl>
Future(FutureImpl<T>&&) -> Future<T, FutureImpl>;

// 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 V>
inline Future<V> yield(V&& value) {
  return {details::ValueTag{}, std::move(value)};
}

template <typename V, typename... Args>
inline Future<V> yield(Args&&... args) {
  return {details::ValueTag{}, std::forward<Args>(args)...};
}

// 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 Future(std::async(std::launch::deferred, std::forward<F>(f), std::forward<Args>(args)...));
}

}  // namespace android::ftl