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