134 lines
4.3 KiB
C++
134 lines
4.3 KiB
C++
/*
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* Copyright 2020 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <future>
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#include <type_traits>
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#include <utility>
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#include <variant>
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#include <ftl/details/future.h>
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namespace android::ftl {
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// Thin wrapper around FutureImpl<T> (concretely std::future<T> or std::shared_future<T>) with
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// extensions for pure values (created via ftl::yield) and continuations.
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//
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// See also SharedFuture<T> shorthand below.
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//
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template <typename T, template <typename> class FutureImpl = std::future>
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class Future final : public details::BaseFuture<Future<T, FutureImpl>, T, FutureImpl> {
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using Base = details::BaseFuture<Future, T, FutureImpl>;
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friend Base; // For BaseFuture<...>::self.
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friend details::BaseFuture<Future<T>, T, std::future>; // For BaseFuture<...>::share.
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public:
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// Constructs an invalid future.
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Future() : future_(std::in_place_type<FutureImpl<T>>) {}
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// Constructs a future from its standard counterpart, implicitly.
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Future(FutureImpl<T>&& f) : future_(std::move(f)) {}
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bool valid() const {
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return std::holds_alternative<T>(future_) || std::get<FutureImpl<T>>(future_).valid();
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}
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// Forwarding functions. Base::share is only defined when FutureImpl is std::future, whereas the
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// following are defined for either FutureImpl:
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using Base::get;
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// Attaches a continuation to the future. The continuation is a function that maps T to either R
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// or ftl::Future<R>. In the former case, the chain wraps the result in a future as if by
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// ftl::yield.
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//
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// auto future = ftl::yield(123);
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// ftl::Future<char> futures[] = {ftl::yield('a'), ftl::yield('b')};
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//
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// auto chain =
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// ftl::Future(std::move(future))
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// .then([](int x) { return static_cast<std::size_t>(x % 2); })
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// .then([&futures](std::size_t i) { return std::move(futures[i]); });
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//
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// assert(chain.get() == 'b');
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//
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template <typename F, typename R = std::invoke_result_t<F, T>>
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auto then(F&& op) && -> Future<details::future_result_t<R>> {
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return defer(
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[](auto&& f, F&& op) {
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R r = op(f.get());
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if constexpr (std::is_same_v<R, details::future_result_t<R>>) {
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return r;
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} else {
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return r.get();
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}
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},
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std::move(*this), std::forward<F>(op));
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}
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private:
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template <typename V>
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friend Future<V> yield(V&&);
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template <typename V, typename... Args>
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friend Future<V> yield(Args&&...);
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template <typename... Args>
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Future(details::ValueTag, Args&&... args)
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: future_(std::in_place_type<T>, std::forward<Args>(args)...) {}
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std::variant<T, FutureImpl<T>> future_;
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};
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template <typename T>
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using SharedFuture = Future<T, std::shared_future>;
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// Deduction guide for implicit conversion.
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template <typename T, template <typename> class FutureImpl>
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Future(FutureImpl<T>&&) -> Future<T, FutureImpl>;
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// Creates a future that wraps a value.
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//
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// auto future = ftl::yield(42);
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// assert(future.get() == 42);
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//
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// auto ptr = std::make_unique<char>('!');
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// auto future = ftl::yield(std::move(ptr));
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// assert(*future.get() == '!');
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//
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template <typename V>
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inline Future<V> yield(V&& value) {
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return {details::ValueTag{}, std::move(value)};
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}
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template <typename V, typename... Args>
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inline Future<V> yield(Args&&... args) {
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return {details::ValueTag{}, std::forward<Args>(args)...};
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}
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// Creates a future that defers a function call until its result is queried.
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//
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// auto future = ftl::defer([](int x) { return x + 1; }, 99);
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// assert(future.get() == 100);
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//
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template <typename F, typename... Args>
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inline auto defer(F&& f, Args&&... args) {
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return Future(std::async(std::launch::deferred, std::forward<F>(f), std::forward<Args>(args)...));
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}
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} // namespace android::ftl
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