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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include <memory>
#include <concepts>
#include <variant>
#include <exception>
#include <stdexcept>
#include <future>
namespace vespalib::coro {
struct UnavailableResultException : std::runtime_error {
using std::runtime_error::runtime_error;
};
// concept indicating that R may be used to receive T
template <typename R, typename T>
concept receiver_of = requires(R r, T t, std::exception_ptr e) {
r.set_value(std::move(t));
r.set_error(e);
r.set_done();
};
// concept indicating that R is a completion callback accepting T
template <typename R, typename T>
concept completion_callback_for = requires(R r, T t) {
r(std::move(t));
};
/**
* Simple value wrapper that stores the result observed by a receiver
* (value/error/done). A receiver is the continuation of an
* asynchronous operation in the world of executors.
**/
template <std::movable T>
class Received {
private:
std::variant<std::exception_ptr,T> _value;
std::exception_ptr normalize_error() const {
if (auto ex = std::get<0>(_value)) {
return ex;
} else {
return std::make_exception_ptr(UnavailableResultException("tried to access the result of a canceled operation"));
}
}
public:
Received() : _value() {}
template <typename RET>
void set_value(RET &&value) { _value.template emplace<1>(std::forward<RET>(value)); }
void set_error(std::exception_ptr exception) { _value.template emplace<0>(exception); }
void set_done() { _value.template emplace<0>(nullptr); }
bool has_value() const { return (_value.index() == 1); }
bool has_error() const { return (_value.index() == 0) && bool(std::get<0>(_value)); }
bool was_canceled() const { return !has_value() && !has_error(); }
std::exception_ptr get_error() const {
return has_value() ? std::exception_ptr() : std::get<0>(_value);
}
T &get_value() & {
if (_value.index() == 1) {
return std::get<1>(_value);
} else {
std::rethrow_exception(normalize_error());
}
}
T &&get_value() && { return std::move(get_value()); }
template <typename R>
requires completion_callback_for<R,Received>
void forward(R &&r) {
r(std::move(*this));
}
template <typename R>
requires receiver_of<R,T>
void forward(R &r) {
if (_value.index() == 1) {
r.set_value(std::get<1>(std::move(_value)));
} else {
if (auto ex = std::get<0>(_value)) {
r.set_error(ex);
} else {
r.set_done();
}
}
}
void forward(std::promise<T> &r) {
if (_value.index() == 1) {
r.set_value(std::get<1>(std::move(_value)));
} else {
r.set_exception(normalize_error());
}
}
};
static_assert(receiver_of<Received<int>, int>);
static_assert(receiver_of<Received<std::unique_ptr<int>>, std::unique_ptr<int>>);
}
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