libconfig/build-config/reflect-cpp/include/rfl/Result.hpp

#ifndef RFL_RESULT_HPP_
#define RFL_RESULT_HPP_

#ifdef REFLECTCPP_USE_STD_EXPECTED
#include <expected>
#endif

#include <array>
#include <functional>
#include <new>
#include <stdexcept>
#include <string>
#include <type_traits>

namespace rfl {

/// Defines the error class to be returned when something went wrong
class Error {
 public:
  Error(const std::string& _what) : what_(_what) {}
  Error(std::string&& _what) : what_(std::move(_what)) {}

  Error(const Error& e) = default;
  Error(Error&& e) = default;

  Error& operator=(const Error&) = default;
  Error& operator=(Error&&) = default;

  /// Returns the error message, equivalent to .what() in std::exception.
  const std::string& what() const & { return what_; }
  /// Moves the error message out of Error object and leaves what_ in a moved from state
  std::string what() && { return std::move(what_); }

 private:
  /// Documents what went wrong
  std::string what_;
};

/// To be returned when there is nothing to return, but there might be an error.
struct Nothing {};

/// This implementation is for cases where std::expected is defined
#ifdef REFLECTCPP_USE_STD_EXPECTED

template <class E>
using Unexpected = std::unexpected<E>;

template <class T>
using Result = std::expected<T, rfl::Error>;

/// This implementation is for cases where std::expected is not defined
#else  // REFLECTCPP_USE_STD_EXPECTED

template <class E>
struct Unexpected {
  Unexpected(E&& _err) : err_{std::forward<E>(_err)} {}

  Unexpected(const E& _err) : err_{_err} {}

  Unexpected(Unexpected&&) = default;

  Unexpected(const Unexpected&) = default;

  Unexpected& operator=(Unexpected&&) = default;

  Unexpected& operator=(const Unexpected&) = default;

  const E& error() const& { return err_; }

  E&& error() && { return std::move(err_); }

  E& error() & { return err_; }

 private:
  E err_;
};

/// The Result class is used for monadic error handling.
template <class T>
class Result {
  static_assert(!std::is_same<T, Error>(), "The result type cannot be Error.");

  using TOrErr = std::array<unsigned char, std::max(sizeof(T), sizeof(Error))>;

 public:
  // using Type = T;
  using value_type = T;
  using error_type = rfl::Error;

  Result(const T& _val) : success_(true) { new (&get_t()) T(_val); }

  Result(T&& _val) noexcept : success_(true) {
    new (&get_t()) T(std::move(_val));
  }

  Result(const Unexpected<Error>& _err) : success_(false) {
    new (&get_err()) Error(_err.error());
  }
  Result(Unexpected<Error>&& _err) : success_(false) {
    new (&get_err()) Error(std::move(_err.error()));
  }

  Result(Result<T>&& _other) noexcept : success_(_other.success_) {
    move_from_other(_other);
  }

  Result(const Result<T>& _other) : success_(_other.success_) {
    copy_from_other(_other);
  }

  template <class U, typename std::enable_if<std::is_convertible_v<U, T>,
                                             bool>::type = true>
  Result(Result<U>&& _other) : success_(_other && true) {
    auto temp = std::forward<Result<U> >(_other).transform(
        [](U&& _u) { return T(std::forward<U>(_u)); });
    move_from_other(temp);
  }

  template <class U, typename std::enable_if<std::is_convertible_v<U, T>,
                                             bool>::type = true>
  Result(const Result<U>& _other) : success_(_other && true) {
    auto temp = _other.transform([](const U& _u) { return T(_u); });
    move_from_other(temp);
  }

  ~Result() { destroy(); }

  /// Monadic operation - F must be a function of type T -> Result<U>.
  template <class F>
  auto and_then(const F& _f) && {
    /// Result_U is expected to be of type Result<U>.
    using Result_U = typename std::invoke_result<F, T>::type;
    if (success_) {
      return Result_U(_f(std::move(*this).get_t()));
    } else {
      return Result_U(std::move(*this).get_err());
    }
  }

  /// Monadic operation - F must be a function of type T -> Result<U>.
  template <class F>
  auto and_then(const F& _f) const& {
    /// Result_U is expected to be of type Result<U>.
    using Result_U = typename std::invoke_result<F, T>::type;
    if (success_) {
      return Result_U(_f(get_t()));
    } else {
      return Result_U(get_err());
    }
  }

  /// Returns true if the result contains a value, false otherwise.
  operator bool() const noexcept { return success_; }

  /// Allows access to the underlying value. Careful: Will result in undefined
  /// behavior, if the result contains an error.
  T&& operator*() && noexcept { return std::move(*this).get_t(); }

  /// Allows access to the underlying value. Careful: Will result in undefined
  /// behavior, if the result contains an error.
  T& operator*() & noexcept { return get_t(); }

  /// Allows read access to the underlying value. Careful: Will result in
  /// undefined behavior, if the result contains an error.
  const T& operator*() const& noexcept { return get_t(); }

  /// Assigns the underlying object.
  Result<T>& operator=(const Result<T>& _other) {
    if (this == &_other) {
      return *this;
    }
    destroy();
    success_ = _other.success_;
    copy_from_other(_other);
    return *this;
  }

  /// Assigns the underlying object.
  Result<T>& operator=(Result<T>&& _other) noexcept {
    if (this == &_other) {
      return *this;
    }
    destroy();
    success_ = _other.success_;
    move_from_other(_other);
    return *this;
  }

  Result<T>& operator=(Unexpected<Error>&& _err) noexcept {
    destroy();
    success_ = false;
    new (&get_err()) Error(_err.error());
    return *this;
  }

  Result<T>& operator=(const Unexpected<Error>& _err) noexcept {
    destroy();
    success_ = false;
    new (&get_err()) Error(_err.error());
    return *this;
  }

  /// Assigns the underlying object.
  template <class U, typename std::enable_if<std::is_convertible_v<U, T>,
                                             bool>::type = true>
  auto& operator=(const Result<U>& _other) {
    const auto to_t = [](const U& _u) -> T { return _u; };
    t_or_err_ = _other.transform(to_t).t_or_err_;
    return *this;
  }

  /// Expects a function that takes of type Error -> Result<T> and returns
  /// Result<T>.
  template <class F>
  Result<T> or_else(const F& _f) && {
    if (success_) {
      return std::move(*this).get_t();
    } else {
      return _f(std::move(*this).get_err());
    }
  }

  /// Expects a function that takes of type Error -> Result<T> and returns
  /// Result<T>.
  template <class F>
  Result<T> or_else(const F& _f) const& {
    if (success_) {
      return get_t();
    } else {
      return _f(get_err());
    }
  }

  /// Functor operation - F must be a function of type T -> U.
  template <class F>
  auto transform(const F& _f) && {
    /// Result_U is expected to be of type Result<U>.
    using U = std::invoke_result_t<F, T>;
    if (success_) {
      return rfl::Result<U>(_f(std::move(*this).get_t()));
    } else {
      return rfl::Result<U>(rfl::Unexpected(std::move(*this).get_err()));
    }
  }

  /// Functor operation - F must be a function of type T -> U.
  template <class F>
  auto transform(const F& _f) const& {
    /// Result_U is expected to be of type Result<U>.
    using U = typename std::invoke_result<F, T>::type;
    if (success_) {
      return rfl::Result<U>(_f(get_t()));
    } else {
      return rfl::Result<U>(get_err());
    }
  }

  /// Returns the value if the result does not contain an error, throws an
  /// exceptions if not. Similar to .unwrap() in Rust.
  T&& value() && {
    if (success_) {
      return std::move(*this).get_t();
    } else {
      throw std::runtime_error(get_err().what());
    }
  }

  /// Returns the value if the result does not contain an error, throws an
  /// exceptions if not. Similar to .unwrap() in Rust.
  T& value() & {
    if (success_) {
      return get_t();
    } else {
      throw std::runtime_error(get_err().what());
    }
  }

  /// Returns the value if the result does not contain an error, throws an
  /// exceptions if not. Similar to .unwrap() in Rust.
  const T& value() const& {
    if (success_) {
      return get_t();
    } else {
      throw std::runtime_error(get_err().what());
    }
  }

  /// Returns the value or a default.
  T&& value_or(T&& _default) && noexcept {
    if (success_) {
      return std::move(*this).get_t();
    } else {
      return std::forward<T>(_default);
    }
  }

  /// Returns the value or a default.
  T value_or(const T& _default) const& noexcept {
    if (success_) {
      return get_t();
    } else {
      return _default;
    }
  }

  template <class G = rfl::Error>
  rfl::Error error_or(G&& _default) && {
    if (success_) {
      return std::forward<G>(_default);
    } else {
      return std::move(*this).get_err();
    }
  }

  //  As specified by the standard :
  //  https://en.cppreference.com/w/cpp/utility/expected
  //  Observers
  template <class G = rfl::Error>
  rfl::Error error_or(G&& _default) const& {
    if (success_) {
      return std::forward<G>(_default);
    } else {
      return get_err();
    }
  }

  bool has_value() const noexcept { return success_; }

  Error& error() && {
    if (success_) throw std::runtime_error("Expected does not contain value");
    return std::move(*this).get_err();
  }

  Error& error() & {
    if (success_) throw std::runtime_error("Expected does not contain value");
    return get_err();
  }

  const Error& error() const& {
    if (success_) throw std::runtime_error("Expected does not contain value");
    return get_err();
  }

  T* operator->() noexcept { return &get_t(); }

  const T* operator->() const noexcept { return &get_t(); }

  template <class F>
  rfl::Result<T> transform_error(F&& f) && {
    static_assert(
        std::is_same<std::invoke_result_t<F, rfl::Error>, rfl::Error>(),
        "A function passed to transform_error must return an error.");
    if (!has_value()) {
      return rfl::Result<T>{std::invoke(f, std::move(*this).get_err())};
    } else {
      return rfl::Result<T>{std::move(*this).value()};
    }
  }

  template <class F>
  rfl::Result<T> transform_error(F&& f) const& {
    static_assert(
        std::is_same<std::invoke_result_t<F, rfl::Error>, rfl::Error>(),
        "A function passed to transform_error must return an error.");
    if (!has_value()) {
      return rfl::Result<T>{std::invoke(f, get_err())};
    } else {
      return rfl::Result<T>{value()};
    }
  }

 private:
  void copy_from_other(const Result<T>& _other) {
    if (success_) {
      new (&get_t()) T(_other.get_t());
    } else {
      new (&get_err()) Error(_other.get_err());
    }
  }

  void destroy() {
    if (success_) {
      if constexpr (std::is_destructible_v<std::remove_cv_t<T> >) {
        get_t().~T();
      }
    } else {
      get_err().~Error();
    }
  }

  T&& get_t() && noexcept {
    return std::move(*std::launder(reinterpret_cast<T*>(t_or_err_.data())));
  }

  T& get_t() & noexcept {
    return *std::launder(reinterpret_cast<T*>(t_or_err_.data()));
  }

  const T& get_t() const& noexcept {
    return *std::launder(reinterpret_cast<const T*>(t_or_err_.data()));
  }

  Error&& get_err() && noexcept {
    return std::move(*std::launder(reinterpret_cast<Error*>(t_or_err_.data())));
  }

  Error& get_err() & noexcept {
    return *std::launder(reinterpret_cast<Error*>(t_or_err_.data()));
  }

  const Error& get_err() const& noexcept {
    return *std::launder(reinterpret_cast<const Error*>(t_or_err_.data()));
  }

  void move_from_other(Result<T>& _other) noexcept {
    if (success_) {
      new (&get_t()) T(std::move(_other.get_t()));
    } else {
      new (&get_err()) Error(std::move(_other.get_err()));
    }
  }
  /// Signifies whether this was a success.
  bool success_;

  /// The underlying data, can either be T or Error.
  alignas(std::max(alignof(T), alignof(Error))) TOrErr t_or_err_;
};

#endif

/// Shorthand for unexpected error.
inline Unexpected<Error> error(const std::string& _what) {
  return Unexpected<Error>(Error(_what));
}

inline Unexpected<Error> error(std::string&& _what) {
  return Unexpected<Error>(Error(std::move(_what)));
}

/// Shorthand for unexpected error.
inline Unexpected<Error> error(const Error& _err) {
  return Unexpected<Error>(_err);
}

}  // namespace rfl

#endif