draconisplusplus/include/rfl/Result.hpp

#ifndef RFL_RESULT_HPP_
#define RFL_RESULT_HPP_

#include <array>
#include <iostream>
#include <optional>
#include <ranges>
#include <span>
#include <stdexcept>
#include <string>
#include <tuple>
#include <type_traits>
#include <variant>
#include <vector>

#include "internal/is_array.hpp"
#include "internal/to_std_array.hpp"

namespace rfl {

  /// To be returned
  class Error {
   public:
    Error(const std::string& _what) : what_(_what) {}

    ~Error() = default;

    /// Returns the error message, equivalent to .what() in std::exception.
    const std::string& what() const { return what_; }

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

  /// Can be used when we are simply interested in whether an operation was
  /// successful.
  struct Nothing {};

  /// 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;

    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 Error& _err) : success_(false) {
      new (&get_err()) Error(_err);
    }

    Result(Error&& _err) noexcept : success_(false) {
      new (&get_err()) Error(std::move(_err));
    }

    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(); }

    /// Returns Result<U>, if successful and error otherwise.
    /// Inspired by .and(...) in the Rust std::result type.
    template <class U>
    Result<U> and_other(const Result<U>& _r) const noexcept {
      const auto f = [&](const auto& _) { return _r; };
      return and_then(f);
    }

    /// 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::forward<T>(get_t())));
      } else {
        return Result_U(std::forward<Error>(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());
      }
    }

    /// Results types can be iterated over, which even make it possible to use
    /// them within a std::range.
    T* begin() noexcept {
      if (success_) {
        return &get_t();
      } else {
        return nullptr;
      }
    }

    /// Results types can be iterated over, which even make it possible to use
    /// them within a std::range.
    const T* begin() const noexcept {
      if (success_) {
        return &get_t();
      } else {
        return nullptr;
      }
    }

    /// Results types can be iterated over, which even make it possible to use
    /// them within a std::range.
    T* end() noexcept {
      if (success_) {
        return &get_t() + 1;
      } else {
        return nullptr;
      }
    }

    /// Results types can be iterated over, which even make it possible to use
    /// them within a std::range.
    const T* end() const noexcept {
      if (success_) {
        return &get_t() + 1;
      } else {
        return nullptr;
      }
    }

    /// Returns an std::optional<error> if this does in fact contain an error
    /// or std::nullopt otherwise.
    std::optional<Error> error() const noexcept {
      if (success_) {
        return std::nullopt;
      } else {
        return 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 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;
    }

    /// 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::forward<T>(get_t());
      } else {
        return _f(std::forward<Error>(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());
      }
    }

    /// Returns the value contained if successful or the provided result r if
    /// not.
    Result<T> or_other(const Result<T>& _r) const noexcept {
      const auto f = [&](const auto& _) { return _r; };
      return or_else(f);
    }

    /// 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 = typename std::invoke_result<F, T>::type;
      if (success_) {
        return rfl::Result<U>(_f(std::forward<T>(get_t())));
      } else {
        return rfl::Result<U>(std::forward<Error>(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 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::forward<T>(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;
      }
    }

   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<
                          T> /*&& !internal::is_array_v<T>*/) {
          get_t().~T();
        }
      } else {
        get_err().~Error();
      }
    }

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

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

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

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

    void move_from_other(Result<T>& _other) {
      if (success_) {
        new (&get_t()) T(std::move(_other.get_t()));
      } else {
        new (&get_err()) Error(std::move(_other.get_err()));
      }
    }

   private:
    /// 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_;
  };

} // namespace rfl

#endif