////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2018 ArangoDB GmbH, Cologne, Germany
///
/// 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.
///
/// Copyright holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Simon Grätzer
////////////////////////////////////////////////////////////////////////////////

#ifndef ARANGOD_FUTURES_UNIT_H
#define ARANGOD_FUTURES_UNIT_H 1

namespace arangodb {
namespace futures {
/// In functional programming, the degenerate case is often called "unit". In
/// C++, "void" is often the best analogue. However, because of the syntactic
/// special-casing required for void, it is frequently a liability for template
/// metaprogramming. So, instead of writing specializations to handle cases like
/// SomeContainer<void>, a library author may instead rule that out and simply
/// have library users use SomeContainer<Unit>. Contained values may be ignored.
/// Much easier.
///
/// "void" is the type that admits of no values at all. It is not possible to
/// construct a value of this type.
/// "unit" is the type that admits of precisely one unique value. It is
/// possible to construct a value of this type, but it is always the same value
/// every time, so it is uninteresting.
struct Unit {
  constexpr bool operator==(const Unit& /*other*/) const { return true; }
  constexpr bool operator!=(const Unit& /*other*/) const { return false; }
};

constexpr Unit unit{};

template <typename T>
struct lift_unit {
  using type = T;
};
template <>
struct lift_unit<void> {
  using type = Unit;
};
template <typename T>
using lift_unit_t = typename lift_unit<T>::type;

template <typename T>
struct drop_unit {
  using type = T;
};
template <>
struct drop_unit<Unit> {
  using type = void;
};
template <typename T>
using drop_unit_t = typename drop_unit<T>::type;

}  // namespace futures
}  // namespace arangodb

#endif