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// 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_CORE_LIB_PROMISE_PROMISE_H
#define GRPC_CORE_LIB_PROMISE_PROMISE_H
#include <grpc/support/port_platform.h>
#include <functional>
#include <type_traits>
#include "absl/status/status.h"
#include "absl/types/optional.h"
#include "absl/types/variant.h"
#include "src/core/lib/promise/detail/promise_like.h"
#include "src/core/lib/promise/poll.h"
namespace grpc_core {
// A Promise is any functor that takes no arguments and returns Poll<T>.
// Most of the time we just pass around the functor, but occasionally
// it pays to have a type erased variant, which we define here.
template <typename T>
using Promise = std::function<Poll<T>()>;
// Helper to execute a promise immediately and return either the result or
// nothing.
template <typename Promise>
auto NowOrNever(Promise promise)
-> absl::optional<typename promise_detail::PromiseLike<Promise>::Result> {
auto r = promise_detail::PromiseLike<Promise>(std::move(promise))();
if (auto* p = absl::get_if<kPollReadyIdx>(&r)) {
return std::move(*p);
}
return {};
}
// A promise that never completes.
template <typename T>
struct Never {
Poll<T> operator()() { return Pending(); }
};
namespace promise_detail {
// A promise that immediately completes.
template <typename T>
class Immediate {
public:
explicit Immediate(T value) : value_(std::move(value)) {}
Poll<T> operator()() { return std::move(value_); }
private:
T value_;
};
} // namespace promise_detail
// Return \a value immediately
template <typename T>
promise_detail::Immediate<T> Immediate(T value) {
return promise_detail::Immediate<T>(std::move(value));
}
// Return status Ok immediately
struct ImmediateOkStatus {
Poll<absl::Status> operator()() { return absl::OkStatus(); }
};
// Typecheck that a promise returns the expected return type.
// usage: auto promise = WithResult<int>([]() { return 3; });
// NOTE: there are tests in promise_test.cc that are commented out because they
// should fail to compile. When modifying this code these should be uncommented
// and their miscompilation verified.
template <typename T, typename F>
auto WithResult(F f) ->
typename std::enable_if<std::is_same<decltype(f()), Poll<T>>::value,
F>::type {
return f;
}
} // namespace grpc_core
#endif // GRPC_CORE_LIB_PROMISE_PROMISE_H
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