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/*
* 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;
using Base::wait_for;
// 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
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