chromium/third_party/grpc/src/src/core/lib/promise/poll.h

// Copyright 2021 gRPC authors.
//
// 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.

#ifndef GRPC_SRC_CORE_LIB_PROMISE_POLL_H
#define GRPC_SRC_CORE_LIB_PROMISE_POLL_H

#include <grpc/support/port_platform.h>

#include <string>
#include <utility>

#include <grpc/support/log.h>

#include "src/core/lib/gprpp/construct_destruct.h"

namespace grpc_core {

// A type that signals a Promise is still pending and not yet completed.
// Allows writing 'return Pending{}' and with automatic conversions gets
// upgraded to a Poll<> object.
struct Pending { … };

// A type that contains no value. Useful for simulating 'void' in promises that
// always need to return some kind of value.
struct Empty { … };

// The result of polling a Promise once.
//
// Can be either pending - the Promise has not yet completed, or ready -
// indicating that the Promise has completed AND should not be polled again.
template <typename T>
class Poll { … };

template <>
class Poll<Pending>;

template <typename T>
bool operator==(const Poll<T>& a, const Poll<T>& b) { … }

template <typename T, typename U>
Poll<T> poll_cast(Poll<U> poll) { … }

// PollTraits tells us whether a type is Poll<> or some other type, and is
// leveraged in the PromiseLike/PromiseFactory machinery to select the
// appropriate implementation of those concepts based upon the return type of a
// lambda, for example (via enable_if).
template <typename T>
struct PollTraits { … };

PollTraits<Poll<T>>;

// Convert a poll to a string
template <typename T, typename F>
std::string PollToString(
    const Poll<T>& poll,
    F t_to_string = [](const T& t) { … }

}  // namespace grpc_core

#endif  // GRPC_SRC_CORE_LIB_PROMISE_POLL_H