text: /*
   * Copyright (c) Facebook, Inc. and its affiliates.
   *
   * 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 <folly/Utility.h>
  #include <folly/functional/Invoke.h>
  
  #include <stdexcept>
  #include <tuple>
  #include <utility>
  
  namespace folly {
  
  template <class T>
  Try<T>::Try(Try<T>&& t) noexcept(std::is_nothrow_move_constructible<T>::value)
      : contains_(t.contains_) {
    if (contains_ == Contains::VALUE) {
      new (&value_) T(std::move(t.value_));
    } else if (contains_ == Contains::EXCEPTION) {
      new (&e_) exception_wrapper(std::move(t.e_));
    }
  }
  
  template <class T>
  template <class T2>
  Try<T>::Try(typename std::enable_if<
              std::is_same<Unit, T2>::value,
              Try<void> const&>::type t) noexcept
      : contains_(Contains::NOTHING) {
    if (t.hasValue()) {
      contains_ = Contains::VALUE;
      new (&value_) T();
    } else if (t.hasException()) {
      contains_ = Contains::EXCEPTION;
      new (&e_) exception_wrapper(t.exception());
    }
  }
  
  Try<void>::Try(const Try<Unit>& t) noexcept : hasValue_(!t.hasException()) {
    if (t.hasException()) {
      new (&e_) exception_wrapper(t.exception());
    }
  }
  
  template <class T>
  Try<T>& Try<T>::operator=(Try<T>&& t) noexcept(
      std::is_nothrow_move_constructible<T>::value) {
    if (this == &t) {
      return *this;
    }
  
    destroy();
  
    if (t.contains_ == Contains::VALUE) {
      new (&value_) T(std::move(t.value_));
    } else if (t.contains_ == Contains::EXCEPTION) {
      new (&e_) exception_wrapper(std::move(t.e_));
    }
  
    contains_ = t.contains_;
  
    return *this;
  }
  
  template <class T>
  Try<T>::Try(const Try<T>& t) noexcept(
      std::is_nothrow_copy_constructible<T>::value) {
    static_assert(
        std::is_copy_constructible<T>::value,
        "T must be copyable for Try<T> to be copyable");
    contains_ = t.contains_;
    if (contains_ == Contains::VALUE) {
      new (&value_) T(t.value_);
    } else if (contains_ == Contains::EXCEPTION) {
      new (&e_) exception_wrapper(t.e_);
    }
  }
  
  template <class T>
  Try<T>& Try<T>::operator=(const Try<T>& t) noexcept(
      std::is_nothrow_copy_constructible<T>::value) {
    static_assert(
        std::is_copy_constructible<T>::value,
        "T must be copyable for Try<T> to be copyable");
  
    if (this == &t) {
      return *this;
    }
  
    destroy();
  
    if (t.contains_ == Contains::VALUE) {
      new (&value_) T(t.value_);
    } else if (t.contains_ == Contains::EXCEPTION) {
      new (&e_) exception_wrapper(t.e_);
    }
  
    contains_ = t.contains_;
  
    return *this;
  }
  
  template <class T>
  Try<T>::~Try() {
    if (LIKELY(contains_ == Contains::VALUE)) {
      value_.~T();
    } else if (UNLIKELY(contains_ == Contains::EXCEPTION)) {
      e_.~exception_wrapper();
    }
  }
  
  template <typename T>
  template <typename... Args>
  T& Try<T>::emplace(Args&&... args) noexcept(
      std::is_nothrow_constructible<T, Args&&...>::value) {
    this->destroy();
    new (&value_) T(static_cast<Args&&>(args)...);
    contains_ = Contains::VALUE;
    return value_;
  }
  
  template <typename T>
  template <typename... Args>
  exception_wrapper& Try<T>::emplaceException(Args&&... args) noexcept(
      std::is_nothrow_constructible<exception_wrapper, Args&&...>::value) {
    this->destroy();
    new (&e_) exception_wrapper(static_cast<Args&&>(args)...);
    contains_ = Contains::EXCEPTION;
    return e_;
  }
  
  template <class T>
  T& Try<T>::value() & {
    throwIfFailed();
    return value_;
  }
  
  template <class T>
  T&& Try<T>::value() && {
    throwIfFailed();
    return std::move(value_);
  }
  
  template <class T>
  const T& Try<T>::value() const& {
    throwIfFailed();
    return value_;
  }
  
  template <class T>
  const T&& Try<T>::value() const&& {
    throwIfFailed();
    return std::move(value_);
  }
  
  template <class T>
  void Try<T>::throwIfFailed() const {
    switch (contains_) {
      case Contains::VALUE:
        return;
      case Contains::EXCEPTION:
        e_.throw_exception();
      case Contains::NOTHING:
      default:
        throw_exception<UsingUninitializedTry>();
    }
  }
  
  template <class T>
  void Try<T>::destroy() noexcept {
    auto oldContains = std::exchange(contains_, Contains::NOTHING);
    if (LIKELY(oldContains == Contains::VALUE)) {
      value_.~T();
    } else if (UNLIKELY(oldContains == Contains::EXCEPTION)) {
      e_.~exception_wrapper();
    }
  }
  
  Try<void>& Try<void>::operator=(const Try<void>& t) noexcept {
    if (t.hasException()) {
      if (hasException()) {
        e_ = t.e_;
      } else {
        new (&e_) exception_wrapper(t.e_);
        hasValue_ = false;
      }
    } else {
      if (hasException()) {
        e_.~exception_wrapper();
        hasValue_ = true;
      }
    }
    return *this;
  }
  
  template <typename... Args>
  exception_wrapper& Try<void>::emplaceException(Args&&... args) noexcept(
      std::is_nothrow_constructible<exception_wrapper, Args&&...>::value) {
    if (hasException()) {
      e_.~exception_wrapper();
    }
    new (&e_) exception_wrapper(static_cast<Args&&>(args)...);
    hasValue_ = false;
    return e_;
  }
  
  void Try<void>::throwIfFailed() const {
    if (hasException()) {
      e_.throw_exception();
    }
  }
  
  template <typename F>
  typename std::enable_if<
      !std::is_same<invoke_result_t<F>, void>::value,
      Try<invoke_result_t<F>>>::type
  makeTryWithNoUnwrap(F&& f) {
    using ResultType = invoke_result_t<F>;
    try {
      return Try<ResultType>(f());
    } catch (std::exception& e) {
      return Try<ResultType>(exception_wrapper(std::current_exception(), e));
    } catch (...) {
      return Try<ResultType>(exception_wrapper(std::current_exception()));
    }
  }
  
  template <typename F>
  typename std::
      enable_if<std::is_same<invoke_result_t<F>, void>::value, Try<void>>::type
      makeTryWithNoUnwrap(F&& f) {
    try {
      f();
      return Try<void>();
    } catch (std::exception& e) {
      return Try<void>(exception_wrapper(std::current_exception(), e));
    } catch (...) {
      return Try<void>(exception_wrapper(std::current_exception()));
    }
  }
  
  template <typename F>
  typename std::
      enable_if<!isTry<invoke_result_t<F>>::value, Try<invoke_result_t<F>>>::type
      makeTryWith(F&& f) {
    return makeTryWithNoUnwrap(std::forward<F>(f));
  }
  
  template <typename F>
  typename std::enable_if<isTry<invoke_result_t<F>>::value, invoke_result_t<F>>::
      type
      makeTryWith(F&& f) {
    using ResultType = invoke_result_t<F>;
    try {
      return f();
    } catch (std::exception& e) {
      return ResultType(exception_wrapper(std::current_exception(), e));
    } catch (...) {
      return ResultType(exception_wrapper(std::current_exception()));
    }
  }
  
  template <typename T, typename... Args>
  T* tryEmplace(Try<T>& t, Args&&... args) noexcept {
    try {
      return std::addressof(t.emplace(static_cast<Args&&>(args)...));
    } catch (const std::exception& ex) {
      t.emplaceException(std::current_exception(), ex);
      return nullptr;
    } catch (...) {
      t.emplaceException(std::current_exception());
      return nullptr;
    }
  }
  
  void tryEmplace(Try<void>& t) noexcept {
    t.emplace();
  }
  
  template <typename T, typename Func>
  T* tryEmplaceWith(Try<T>& t, Func&& func) noexcept {
    static_assert(
        std::is_constructible<T, folly::invoke_result_t<Func>>::value,
        "Unable to initialise a value of type T with the result of 'func'");
    try {
      return std::addressof(t.emplace(static_cast<Func&&>(func)()));
    } catch (const std::exception& ex) {
      t.emplaceException(std::current_exception(), ex);
      return nullptr;
    } catch (...) {
      t.emplaceException(std::current_exception());
      return nullptr;
    }
  }
  
  template <typename Func>
  bool tryEmplaceWith(Try<void>& t, Func&& func) noexcept {
    static_assert(
        std::is_void<folly::invoke_result_t<Func>>::value,
        "Func returns non-void. Cannot be used to emplace Try<void>");
    try {
      static_cast<Func&&>(func)();
      t.emplace();
      return true;
    } catch (const std::exception& ex) {
      t.emplaceException(std::current_exception(), ex);
      return false;
    } catch (...) {
      t.emplaceException(std::current_exception());
      return false;
    }
  }
  
  namespace try_detail {
  
  /**
   * Trait that removes the layer of Try abstractions from the passed in type
   */
  template <typename Type>
  struct RemoveTry;
  template <template <typename...> class TupleType, typename... Types>
  struct RemoveTry<TupleType<folly::Try<Types>...>> {
    using type = TupleType<Types...>;
  };
  
  template <std::size_t... Indices, typename Tuple>
  auto unwrapTryTupleImpl(std::index_sequence<Indices...>, Tuple&& instance) {
    using std::get;
    using ReturnType = typename RemoveTry<typename std::decay<Tuple>::type>::type;
    return ReturnType{(get<Indices>(std::forward<Tuple>(instance)).value())...};
  }
  } // namespace try_detail
  
  template <typename Tuple>
  auto unwrapTryTuple(Tuple&& instance) {
    using TupleDecayed = typename std::decay<Tuple>::type;
    using Seq = std::make_index_sequence<std::tuple_size<TupleDecayed>::value>;
    return try_detail::unwrapTryTupleImpl(Seq{}, std::forward<Tuple>(instance));
  }
  
  template <typename T>
  void tryAssign(Try<T>& t, Try<T>&& other) noexcept {
    try {
      t = std::move(other);
    } catch (const std::exception& ex) {
      t.emplaceException(std::current_exception(), ex);
    } catch (...) {
      t.emplaceException(std::current_exception());
    }
  }
  
  // limited to the instances unconditionally forced by the futures library
  extern template class Try<Unit>;
  
  } // namespace folly
  