1/*
2 * Copyright 2015-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#pragma once
18
19#include <type_traits>
20
21namespace folly {
22
23/// In functional programming, the degenerate case is often called "unit". In
24/// C++, "void" is often the best analogue. However, because of the syntactic
25/// special-casing required for void, it is frequently a liability for template
26/// metaprogramming. So, instead of writing specializations to handle cases like
27/// SomeContainer<void>, a library author may instead rule that out and simply
28/// have library users use SomeContainer<Unit>. Contained values may be ignored.
29/// Much easier.
30///
31/// "void" is the type that admits of no values at all. It is not possible to
32/// construct a value of this type.
33/// "unit" is the type that admits of precisely one unique value. It is
34/// possible to construct a value of this type, but it is always the same value
35/// every time, so it is uninteresting.
36struct Unit {
37 constexpr bool operator==(const Unit& /*other*/) const {
38 return true;
39 }
40 constexpr bool operator!=(const Unit& /*other*/) const {
41 return false;
42 }
43};
44
45constexpr Unit unit{};
46
47template <typename T>
48struct lift_unit {
49 using type = T;
50};
51template <>
52struct lift_unit<void> {
53 using type = Unit;
54};
55template <typename T>
56using lift_unit_t = typename lift_unit<T>::type;
57
58template <typename T>
59struct drop_unit {
60 using type = T;
61};
62template <>
63struct drop_unit<Unit> {
64 using type = void;
65};
66template <typename T>
67using drop_unit_t = typename drop_unit<T>::type;
68
69} // namespace folly
70