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/*
* Copyright 2022 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 <type_traits>
#include <utility>
namespace android::ftl {
// The unit type, and its only value.
constexpr struct Unit {
} unit;
constexpr bool operator==(Unit, Unit) {
return true;
}
constexpr bool operator!=(Unit, Unit) {
return false;
}
// Adapts a function object F to return Unit. The return value of F is ignored.
//
// As a practical use, the function passed to ftl::Optional<T>::transform is not allowed to return
// void (cf. https://wg21.link/P0798R8#mapping-functions-returning-void), but may return Unit if
// only its side effects are meaningful:
//
// ftl::Optional opt = "food"s;
// opt.transform(ftl::unit_fn([](std::string& str) { str.pop_back(); }));
// assert(opt == "foo"s);
//
template <typename F>
struct UnitFn {
F f;
template <typename... Args>
Unit operator()(Args&&... args) {
return f(std::forward<Args>(args)...), unit;
}
};
template <typename F>
constexpr auto unit_fn(F&& f) -> UnitFn<std::decay_t<F>> {
return {std::forward<F>(f)};
}
namespace details {
// Identity function for all T except Unit, which maps to void.
template <typename T>
struct UnitToVoid {
template <typename U>
static auto from(U&& value) {
return value;
}
};
template <>
struct UnitToVoid<Unit> {
template <typename U>
static void from(U&&) {}
};
} // namespace details
} // namespace android::ftl
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