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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>
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
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