expected.h

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/******************************************************************************
The MIT License(MIT)
 
Embedded Template Library.
https://github.com/ETLCPP/etl
https://www.etlcpp.com
 
Copyright(c) 2022 John Wellbelove
 
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#ifndef ETL_EXPECTED_INCLUDED
#define ETL_EXPECTED_INCLUDED

#include "platform.h"
#include "error_handler.h"
#include "exception.h"
#include "initializer_list.h"
#include "invoke.h"
#include "memory.h"
#include "type_traits.h"
#include "utility.h"
#include "variant.h"
 
namespace etl
{
  // Forward declaration for is_expected
  template <typename TValue, typename TError>
  class expected;
 
  template <typename T>
  struct is_expected : etl::false_type
  {
  };
 
  template <typename TValue, typename TError>
  struct is_expected<expected<TValue, TError> > : etl::true_type
  {
  };
 
  //***************************************************************************
  //***************************************************************************
  class expected_exception : public etl::exception
  {
  public:
 
    expected_exception(string_type reason_, string_type file_name_, numeric_type line_number_)
      : exception(reason_, file_name_, line_number_)
    {
    }
  };
 
  //***************************************************************************
  //***************************************************************************
  class expected_invalid : public etl::expected_exception
  {
  public:
 
    expected_invalid(string_type file_name_, numeric_type line_number_)
      : expected_exception(ETL_ERROR_TEXT("expected:invalid", ETL_EXPECTED_FILE_ID"A"), file_name_, line_number_)
    {
    }
  };
 
  //***************************************************************************
  //***************************************************************************
  template <typename TError>
  class unexpected
  {
  public:
 
    typedef TError error_type;
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR unexpected(const unexpected& other)
      : error_value(other.error_value)
    {
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR unexpected(unexpected&& other)
      : error_value(etl::move(other.error_value))
    {
    }
#endif
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR explicit unexpected(const TError& e)
      : error_value(e)
    {
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR explicit unexpected(TError&& e)
      : error_value(etl::forward<TError>(e))
    {
    }
 
    //*******************************************
    //*******************************************
    template <typename... Args >
    ETL_CONSTEXPR explicit unexpected(etl::in_place_t, Args&&... args)
      : error_value(etl::forward<Args>(args)...)
    {
    }
#endif
 
#if ETL_HAS_INITIALIZER_LIST
    //*******************************************
    //*******************************************
    template <typename U, typename... Args>
    ETL_CONSTEXPR explicit unexpected(etl::in_place_t, std::initializer_list<U> init, Args&&... args)
      : error_value(init, etl::forward<Args>(args)...)
    {
    }
#endif
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 etl::unexpected<TError>& operator=(const etl::unexpected<TError>& rhs)
    {
#if ETL_USING_CPP11
      ETL_STATIC_ASSERT(etl::is_copy_constructible<TError>::value, "Error not copy assignable");
#endif
 
      error_value = rhs.error_value;
      return *this;
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 etl::unexpected<TError>& operator=(etl::unexpected<TError>&& rhs)
    {
      ETL_STATIC_ASSERT(etl::is_move_constructible<TError>::value, "Error not move assignable");
 
      error_value = etl::move(rhs.error_value);
      return *this;
    }
#endif
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 TError& error() & ETL_NOEXCEPT
    {
      return error_value;
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 const TError& error() const& ETL_NOEXCEPT
    {
      return error_value;
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 TError&& error() && ETL_NOEXCEPT
    {
      return etl::move(error_value);
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 TError&& error() const&& ETL_NOEXCEPT
    {
      return etl::move(error_value);
    }
#else
    //*******************************************
    //*******************************************
    const TError& error() const
    {
      return error_value;
    }
#endif
 
    //*******************************************
    //*******************************************
    void swap(etl::unexpected<TError>& other)
    {
      using ETL_OR_STD::swap;
 
      swap(error_value, other.error_value);
    }
 
  private:
 
    TError error_value;
  };
 
  //*****************************************************************************
  //*****************************************************************************
  struct unexpect_t
  {
    ETL_CONSTEXPR14 explicit unexpect_t() {}
  };
 
#if ETL_USING_CPP17
  inline ETL_CONSTEXPR unexpect_t unexpect{};
#else
  static const unexpect_t unexpect;
#endif
 
  //*****************************************************************************
  //*****************************************************************************
  template <typename TValue, typename TError>
  class expected
  {
  public:
 
    typedef etl::expected<TValue, TError> this_type;
    typedef TValue                        value_type;
    typedef TError                        error_type;
    typedef etl::unexpected<TError>       unexpected_type;
 
#if ETL_USING_CPP11
    template <typename U>
    using rebind = expected<U, TError>;
#endif
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected() ETL_NOEXCEPT
      : storage(etl::in_place_index_t<Value_Type>(), value_type())
    {
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(const value_type& value_) ETL_NOEXCEPT
      : storage(etl::in_place_index_t<Value_Type>(), value_)
    {
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(value_type&& value_) ETL_NOEXCEPT
      : storage(etl::in_place_index_t<Value_Type>(), etl::move(value_))
    {
    }
#endif
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(const expected& other) ETL_NOEXCEPT
      : storage(other.storage)
    {
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(expected&& other) ETL_NOEXCEPT
      : storage(etl::move(other.storage))
    {
    }
#endif
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    template <typename G, typename etl::enable_if< !etl::is_convertible<const G&, TError>::value, bool>::type = false>
    ETL_CONSTEXPR14 explicit expected(const etl::unexpected<G>& ue)
      : storage(etl::in_place_index_t<Error_Type>(), ue.error())
    {
    }
 
    template <typename G, typename etl::enable_if< etl::is_convertible<const G&, TError>::value, bool>::type = false>
    ETL_CONSTEXPR14 expected(const etl::unexpected<G>& ue)
      : storage(etl::in_place_index_t<Error_Type>(), ue.error())
    {
    }
#else
    template <typename G>
    explicit expected(const etl::unexpected<G>& ue)
      : storage(etl::in_place_index_t<Error_Type>(), ue.error())
    {
    }
#endif
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    template <typename G, typename etl::enable_if< !etl::is_convertible<const G&, TError>::value, bool>::type = false>
    ETL_CONSTEXPR14 explicit expected(etl::unexpected<G>&& ue)
      : storage(etl::in_place_index_t<Error_Type>(), etl::move(ue.error()))
    {
    }
 
    template <typename G, typename etl::enable_if< etl::is_convertible<const G&, TError>::value, bool>::type = false>
    ETL_CONSTEXPR14 expected(etl::unexpected<G>&& ue)
      : storage(etl::in_place_index_t<Error_Type>(), etl::move(ue.error()))
    {
    }
#endif
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 explicit expected(etl::in_place_t) ETL_NOEXCEPT
      : storage(value_type())
    {
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    template <typename... Args>
    ETL_CONSTEXPR14 explicit expected(etl::in_place_t, Args&&... args)
      : storage(etl::in_place_index_t<Value_Type>(), etl::forward<Args>(args)...)
    {
    }
 
  #if ETL_HAS_INITIALIZER_LIST
    //*******************************************
    //*******************************************
    template <typename U, typename... Args>
    ETL_CONSTEXPR14 explicit expected(etl::in_place_t, std::initializer_list<U> il, Args&&... args)
      : storage(etl::in_place_index_t<Value_Type>(), il, etl::forward<Args>(args)...)
    {
    }
  #endif
 
    //*******************************************
    //*******************************************
    template <typename... Args>
    ETL_CONSTEXPR14 explicit expected(etl::unexpect_t, Args&&... args)
      : storage(error_type(etl::forward<Args>(args)...))
    {
    }
 
  #if ETL_HAS_INITIALIZER_LIST
    //*******************************************
    //*******************************************
    template <typename U, typename... Args>
    ETL_CONSTEXPR14 explicit expected(etl::unexpect_t, std::initializer_list<U> il, Args&&... args)
      : storage(error_type(il, etl::forward<Args>(args)...))
    {
    }
  #endif
#endif
 
    //*******************************************
    //*******************************************
    this_type& operator=(const this_type& other)
    {
      ETL_STATIC_ASSERT(etl::is_copy_constructible<TValue>::value && etl::is_copy_constructible<TError>::value, "Not copy assignable");
 
      storage = other.storage;
 
      return *this;
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    this_type& operator=(this_type&& other)
    {
      ETL_STATIC_ASSERT(etl::is_move_constructible<TValue>::value && etl::is_move_constructible<TError>::value, "Not move assignable");
 
      storage = etl::move(other.storage);
 
      return *this;
    }
#endif
 
    //*******************************************
    //*******************************************
    expected& operator=(const value_type& value)
    {
      ETL_STATIC_ASSERT(etl::is_copy_constructible<TValue>::value, "Value not copy assignable");
 
      storage.template emplace<Value_Type>(value);
 
      return *this;
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    expected& operator=(value_type&& value)
    {
      ETL_STATIC_ASSERT(etl::is_move_constructible<TValue>::value, "Value not move assignable");
 
      storage.template emplace<Value_Type>(etl::move(value));
 
      return *this;
    }
#endif
 
    //*******************************************
    //*******************************************
    expected& operator=(const unexpected_type& ue)
    {
#if ETL_USING_CPP11
      ETL_STATIC_ASSERT(etl::is_copy_constructible<TError>::value, "Error not copy assignable");
#endif
 
      storage.template emplace<Error_Type>(ue.error());
 
      return *this;
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    expected& operator=(unexpected_type&& ue)
    {
      ETL_STATIC_ASSERT(etl::is_move_constructible<TError>::value, "Error not move assignable");
 
      storage.template emplace<Error_Type>(etl::move(ue.error()));
 
      return *this;
    }
#endif
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 value_type& value() &
    {
      return etl::get<Value_Type>(storage);
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 const value_type& value() const&
    {
      return etl::get<Value_Type>(storage);
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 value_type&& value() &&
    {
      return etl::move(etl::get<Value_Type>(storage));
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 const value_type&& value() const&&
    {
      return etl::move(etl::get<Value_Type>(storage));
    }
#else
    //*******************************************
    //*******************************************
    const value_type& value() const
    {
      return etl::get<Value_Type>(storage);
    }
#endif
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 bool has_value() const ETL_NOEXCEPT
    {
      return (storage.index() == Value_Type);
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 ETL_EXPLICIT operator bool() const ETL_NOEXCEPT
    {
      return has_value();
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    template <typename U>
    ETL_NODISCARD ETL_CONSTEXPR14 etl::enable_if_t<etl::is_convertible<U, value_type>::value, value_type> value_or(U&& default_value) const&
    {
      if (has_value())
      {
        return value();
      }
      else
      {
        return static_cast<value_type>(etl::forward<U>(default_value));
      }
    }
 
    //*******************************************
    //*******************************************
    template <typename U>
    ETL_NODISCARD ETL_CONSTEXPR14 etl::enable_if_t<etl::is_convertible<U, value_type>::value, value_type> value_or(U&& default_value) &&
    {
      if (has_value())
      {
        return etl::move(value());
      }
      else
      {
        return static_cast<value_type>(etl::forward<U>(default_value));
      }
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 error_type& error() & ETL_NOEXCEPT
    {
      return etl::get<Error_Type>(storage);
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 const error_type& error() const& ETL_NOEXCEPT
    {
      return etl::get<Error_Type>(storage);
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 error_type&& error() && ETL_NOEXCEPT
    {
      return etl::move(etl::get<Error_Type>(storage));
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 const error_type&& error() const&& ETL_NOEXCEPT
    {
      return etl::move(etl::get<Error_Type>(storage));
    }
 
    //*******************************************
    //*******************************************
    void swap(this_type& other)
    {
      using ETL_OR_STD::swap;
 
      swap(storage, other.storage);
    }
 
    //*******************************************
    //*******************************************
    template <typename... Args>
    ETL_CONSTEXPR14 value_type& emplace(Args&&... args) ETL_NOEXCEPT
    {
      storage.template emplace<value_type>(etl::forward<Args>(args)...);
 
      return value();
    }
 
      //*******************************************
      //*******************************************
  #if ETL_HAS_INITIALIZER_LIST
    template <typename U, typename... Args>
    ETL_CONSTEXPR14 value_type& emplace(std::initializer_list<U> il, Args&&... args) ETL_NOEXCEPT
    {
      storage.template emplace<value_type>(il, etl::forward<Args>(args)...);
 
      return value();
    }
  #endif
#else
    //*******************************************
    //*******************************************
    template <typename U>
    value_type value_or(const U& default_value) const
    {
      if (has_value())
      {
        return value();
      }
      else
      {
        return default_value;
      }
    }
 
    //*******************************************
    //*******************************************
    const error_type& error() const
    {
      return etl::get<Error_Type>(storage);
    }
#endif
 
    //*******************************************
    //*******************************************
    value_type* operator->()
    {
      ETL_ASSERT_OR_RETURN_VALUE(has_value(), ETL_ERROR(expected_invalid), ETL_NULLPTR);
 
      return etl::addressof(etl::get<value_type>(storage));
    }
 
    //*******************************************
    //*******************************************
    const value_type* operator->() const
    {
      ETL_ASSERT_OR_RETURN_VALUE(has_value(), ETL_ERROR(expected_invalid), ETL_NULLPTR);
 
      return etl::addressof(etl::get<value_type>(storage));
    }
 
    //*******************************************
    //*******************************************
    value_type& operator*() ETL_LVALUE_REF_QUALIFIER
    {
      ETL_ASSERT(has_value(), ETL_ERROR(expected_invalid));
 
      return etl::get<value_type>(storage);
    }
 
    //*******************************************
    //*******************************************
    const value_type& operator*() const ETL_LVALUE_REF_QUALIFIER
    {
      ETL_ASSERT_OR_RETURN_VALUE(has_value(), ETL_ERROR(expected_invalid), ETL_NULLPTR);
 
      return etl::get<value_type>(storage);
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    value_type&& operator*() &&
    {
      ETL_ASSERT_OR_RETURN_VALUE(has_value(), ETL_ERROR(expected_invalid), ETL_NULLPTR);
 
      return etl::move(etl::get<value_type>(storage));
    }
 
    //*******************************************
    //*******************************************
    const value_type&& operator*() const&&
    {
      ETL_ASSERT(has_value(), ETL_ERROR(expected_invalid));
 
      return etl::move(etl::get<value_type>(storage));
    }
#endif
 
#if ETL_USING_CPP11
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TValue&>::type>::type>
    auto transform(F&& f) & -> expected<U, TError>
    {
      return transform_impl<F, this_type&, U, TValue&>(etl::forward<F>(f), *this);
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TValue&>::type>::type>
    auto transform(F&& f) const& -> expected<U, TError>
    {
      return transform_impl<F, const this_type&, U, const TValue&>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TValue&&>::type>::type>
    auto transform(F&& f) && -> expected<U, TError>
    {
      return transform_impl<F, this_type&&, U, TValue&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TValue&&>::type>::type>
    auto transform(F&& f) const&& -> expected<U, TError>
    {
      return transform_impl<F, const this_type&&, U, const TValue&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TValue&>::type>::type>
    auto and_then(F&& f) & -> U
    {
      return and_then_impl<F, this_type&, U, TValue&>(etl::forward<F>(f), *this);
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TValue&>::type>::type>
    auto and_then(F&& f) const& -> U
    {
      return and_then_impl<F, const this_type&, U, const TValue&>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TValue&&>::type>::type>
    auto and_then(F&& f) && -> U
    {
      return and_then_impl<F, this_type&&, U, TValue&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TValue&&>::type>::type>
    auto and_then(F&& f) const&& -> U
    {
      return and_then_impl<F, const this_type&&, U, const TValue&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&>::type>::type>
    auto or_else(F&& f) & -> U
    {
      return or_else_impl<F, this_type&, U, TError&>(etl::forward<F>(f), *this);
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&>::type>::type>
    auto or_else(F&& f) const& -> U
    {
      return or_else_impl<F, const this_type&, U, const TError&>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&&>::type>::type>
    auto or_else(F&& f) && -> U
    {
      return or_else_impl<F, this_type&&, U, TError&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&&>::type>::type>
    auto or_else(F&& f) const&& -> U
    {
      return or_else_impl<F, const this_type&&, U, const TError&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&>::type>::type>
    auto transform_error(F&& f) & -> expected<TValue, U>
    {
      return transform_error_impl<F, this_type&, U, TError&>(etl::forward<F>(f), *this);
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&>::type>::type>
    auto transform_error(F&& f) const& -> expected<TValue, U>
    {
      return transform_error_impl<F, const this_type&, U, const TError&>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&&>::type>::type>
    auto transform_error(F&& f) && -> expected<TValue, U>
    {
      return transform_error_impl<F, this_type&&, U, TError&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&&>::type>::type>
    auto transform_error(F&& f) const&& -> expected<TValue, U>
    {
      return transform_error_impl<F, const this_type&&, U, const TError&&>(etl::forward<F>(f), etl::move(*this));
    }
#endif
 
  private:
 
    enum
    {
      Uninitialised,
      Value_Type,
      Error_Type
    };
 
    typedef etl::variant<etl::monostate, value_type, error_type> storage_type;
    storage_type                                                 storage;
 
#if ETL_USING_CPP11
    template < typename F, typename TExp, typename TRet, typename TValueRef, typename = typename etl::enable_if<!etl::is_void<TRet>::value>::type>
    auto transform_impl(F&& f, TExp&& exp) const -> expected<TRet, TError>
    {
      if (exp.has_value())
      {
        return expected<TRet, TError>(etl::invoke(etl::forward<F>(f), etl::forward<TValueRef>(etl::get<Value_Type>(exp.storage))));
      }
      else
      {
        return expected<TRet, TError>(unexpected<TError>(etl::forward<TExp>(exp).error()));
      }
    }
 
    template < typename F, typename TExp, typename TRet, typename TValueRef, typename = typename etl::enable_if<etl::is_void<TRet>::value>::type>
    auto transform_impl(F&& f, TExp&& exp) const -> expected<void, TError>
    {
      if (exp.has_value())
      {
        etl::invoke(etl::forward<F>(f), etl::forward<TValueRef>(etl::get<Value_Type>(exp.storage)));
        return expected<void, TError>();
      }
      else
      {
        return expected<void, TError>(unexpected<TError>(etl::forward<TExp>(exp).error()));
      }
    }
 
    template < typename F, typename TExp, typename TRet, typename TValueRef,
               typename = typename etl::enable_if< !etl::is_void<TRet>::value && etl::is_expected<TRet>::value
                                                   && etl::is_same<typename TRet::error_type, TError>::value>::type>
    auto and_then_impl(F&& f, TExp&& exp) const -> TRet
    {
      if (exp.has_value())
      {
        return etl::invoke(etl::forward<F>(f), etl::forward<TValueRef>(etl::get<Value_Type>(exp.storage)));
      }
      else
      {
        return TRet(unexpected<TError>(etl::forward<TExp>(exp).error()));
      }
    }
 
    template < typename F, typename TExp, typename TRet, typename TErrorRef,
               typename = typename etl::enable_if< !etl::is_void<TRet>::value && etl::is_expected<TRet>::value
                                                   && etl::is_same<typename TRet::value_type, TValue>::value>::type>
    auto or_else_impl(F&& f, TExp&& exp) const -> TRet
    {
      if (exp.has_value())
      {
        return TRet(etl::forward<TExp>(exp).value());
      }
      else
      {
        return etl::invoke(etl::forward<F>(f), etl::forward<TErrorRef>(etl::get<Error_Type>(exp.storage)));
      }
    }
 
    template < typename F, typename TExp, typename TRet, typename TErrorRef, typename = typename etl::enable_if<!etl::is_void<TRet>::value>::type>
    auto transform_error_impl(F&& f, TExp&& exp) const -> expected<TValue, TRet>
    {
      if (exp.has_value())
      {
        return expected<TValue, TRet>(etl::forward<TExp>(exp).value());
      }
      else
      {
        return expected<TValue, TRet>(unexpected<TRet>(etl::invoke(etl::forward<F>(f), etl::forward<TErrorRef>(etl::get<Error_Type>(exp.storage)))));
      }
    }
#endif
  };
 
  //*****************************************************************************
  //*****************************************************************************
  template <typename TError>
  class expected<void, TError>
  {
  public:
 
    typedef etl::expected<void, TError> this_type;
    typedef void                        value_type;
    typedef TError                      error_type;
    typedef etl::unexpected<TError>     unexpected_type;
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected()
    {
    }
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(const unexpected_type& ue_)
      : storage(ue_.error())
    {
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(unexpected_type&& ue_)
      : storage(etl::move(ue_.error()))
    {
    }
#endif
 
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(const this_type& other)
      : storage(other.storage)
    {
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_CONSTEXPR14 expected(this_type&& other)
      : storage(etl::move(other.storage))
    {
    }
#endif
 
    //*******************************************
    //*******************************************
    this_type& operator=(const this_type& other)
    {
      ETL_STATIC_ASSERT(etl::is_copy_constructible<TError>::value, "Not copy assignable");
 
      storage = other.storage;
      return *this;
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    this_type& operator=(this_type&& other)
    {
      ETL_STATIC_ASSERT(etl::is_move_constructible<TError>::value, "Not move assignable");
 
      storage = etl::move(other.storage);
      return *this;
    }
#endif
 
    //*******************************************
    //*******************************************
    expected& operator=(const unexpected_type& ue)
    {
#if ETL_USING_CPP11
      ETL_STATIC_ASSERT(etl::is_copy_constructible<TError>::value, "Error not copy assignable");
#endif
 
      storage.template emplace<Error_Type>(ue.error());
      return *this;
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    expected& operator=(unexpected_type&& ue)
    {
      ETL_STATIC_ASSERT(etl::is_move_constructible<TError>::value, "Error not move assignable");
 
      storage.template emplace<Error_Type>(etl::move(ue.error()));
      return *this;
    }
#endif
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 bool has_value() const ETL_NOEXCEPT
    {
      return (storage.index() != Error_Type);
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 ETL_EXPLICIT operator bool() const ETL_NOEXCEPT
    {
      return has_value();
    }
 
#if ETL_USING_CPP11
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 error_type& error() & ETL_NOEXCEPT
    {
      return etl::get<Error_Type>(storage);
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 const error_type& error() const& ETL_NOEXCEPT
    {
      return etl::get<Error_Type>(storage);
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 error_type&& error() && ETL_NOEXCEPT
    {
      return etl::move(etl::get<Error_Type>(storage));
    }
 
    //*******************************************
    //*******************************************
    ETL_NODISCARD ETL_CONSTEXPR14 const error_type&& error() const&& ETL_NOEXCEPT
    {
      return etl::move(etl::get<Error_Type>(storage));
    }
#else
    //*******************************************
    //*******************************************
    const error_type& error() const
    {
      return etl::get<Error_Type>(storage);
    }
#endif
 
    //*******************************************
    //*******************************************
    void swap(this_type& other)
    {
      using ETL_OR_STD::swap;
 
      swap(storage, other.storage);
    }
 
#if ETL_USING_CPP11
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto transform(F&& f) & -> expected<U, TError>
    {
      return transform_impl<F, this_type&, U>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto transform(F&& f) const& -> expected<U, TError>
    {
      return transform_impl<F, const this_type&, U>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto transform(F&& f) && -> expected<U, TError>
    {
      return transform_impl<F, this_type&&, U>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto transform(F&& f) const&& -> expected<U, TError>
    {
      return transform_impl<F, const this_type&&, U>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto and_then(F&& f) & -> U
    {
      return and_then_impl<F, this_type&, U>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto and_then(F&& f) const& -> U
    {
      return and_then_impl<F, const this_type&, U>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto and_then(F&& f) && -> U
    {
      return and_then_impl<F, this_type&&, U>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void>::type>::type>
    auto and_then(F&& f) const&& -> U
    {
      return and_then_impl<F, const this_type&&, U>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&>::type>::type>
    auto or_else(F&& f) & -> U
    {
      return or_else_impl<F, this_type&, U, TError&>(etl::forward<F>(f), *this);
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&>::type>::type>
    auto or_else(F&& f) const& -> U
    {
      return or_else_impl<F, const this_type&, U, const TError&>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&&>::type>::type>
    auto or_else(F&& f) && -> U
    {
      return or_else_impl<F, this_type&&, U, TError&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&&>::type>::type>
    auto or_else(F&& f) const&& -> U
    {
      return or_else_impl<F, const this_type&&, U, const TError&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&>::type>::type>
    auto transform_error(F&& f) & -> expected<void, U>
    {
      return transform_error_impl<F, this_type&, U, TError&>(etl::forward<F>(f), *this);
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&>::type>::type>
    auto transform_error(F&& f) const& -> expected<void, U>
    {
      return transform_error_impl<F, const this_type&, U, const TError&>(etl::forward<F>(f), *this);
    }
 
    template <typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, TError&&>::type>::type>
    auto transform_error(F&& f) && -> expected<void, U>
    {
      return transform_error_impl<F, this_type&&, U, TError&&>(etl::forward<F>(f), etl::move(*this));
    }
 
    template < typename F, typename U = typename etl::remove_cvref< typename etl::invoke_result<F, void, const TError&&>::type>::type>
    auto transform_error(F&& f) const&& -> expected<void, U>
    {
      return transform_error_impl<F, const this_type&&, U, const TError&&>(etl::forward<F>(f), *this);
    }
#endif
 
  private:
 
    enum
    {
      Uninitialised,
      Error_Type
    };
 
    etl::variant<etl::monostate, error_type> storage;
 
#if ETL_USING_CPP11
    template < typename F, typename TExp, typename TRet, typename = typename etl::enable_if<!etl::is_void<TRet>::value>::type>
    auto transform_impl(F&& f, TExp&& exp) const -> expected<TRet, TError>
    {
      if (exp.has_value())
      {
        return expected<TRet, TError>(etl::invoke(etl::forward<F>(f)));
      }
      else
      {
        return expected<TRet, TError>(unexpected<TError>(etl::forward<TExp>(exp).error()));
      }
    }
 
    template < typename F, typename TExp, typename TRet, typename = typename etl::enable_if<etl::is_void<TRet>::value>::type>
    auto transform_impl(F&& f, TExp&& exp) const -> expected<void, TError>
    {
      if (exp.has_value())
      {
        etl::invoke(etl::forward<F>(f));
        return expected<void, TError>();
      }
      else
      {
        return expected<void, TError>(unexpected<TError>(etl::forward<TExp>(exp).error()));
      }
    }
 
    template < typename F, typename TExp, typename TRet,
               typename = typename etl::enable_if< !etl::is_void<TRet>::value && etl::is_expected<TRet>::value
                                                   && etl::is_same<typename TRet::error_type, TError>::value>::type>
    auto and_then_impl(F&& f, TExp&& exp) const -> TRet
    {
      if (exp.has_value())
      {
        return etl::invoke(etl::forward<F>(f));
      }
      else
      {
        return TRet(unexpected<TError>(etl::forward<TExp>(exp).error()));
      }
    }
 
    template <typename F, typename TExp, typename TRet, typename TErrorRef,
              typename = typename etl::enable_if< !etl::is_void<TRet>::value && etl::is_expected<TRet>::value
                                                  && etl::is_same<typename TRet::value_type, void>::value>::type>
    auto or_else_impl(F&& f, TExp&& exp) const -> TRet
    {
      if (exp.has_value())
      {
        return TRet();
      }
      else
      {
        return etl::invoke(etl::forward<F>(f), etl::forward<TErrorRef>(etl::get<Error_Type>(exp.storage)));
      }
    }
 
    template < typename F, typename TExp, typename TRet, typename TErrorRef, typename = typename etl::enable_if<!etl::is_void<TRet>::value>::type>
    auto transform_error_impl(F&& f, TExp&& exp) const -> expected<void, TRet>
    {
      if (exp.has_value())
      {
        return expected<void, TRet>();
      }
      else
      {
        return expected<void, TRet>(unexpected<TRet>(etl::invoke(etl::forward<F>(f), etl::forward<TErrorRef>(etl::get<Error_Type>(exp.storage)))));
      }
    }
#endif
  };
 
  //*******************************************
  //*******************************************
  template <typename TValue, typename TError, typename TValue2, typename TError2>
  ETL_CONSTEXPR14 bool operator==(const etl::expected<TValue, TError>& lhs, const etl::expected<TValue2, TError2>& rhs)
  {
    if (lhs.has_value() != rhs.has_value())
    {
      return false;
    }
    if (lhs.has_value())
    {
      return lhs.value() == rhs.value();
    }
    return lhs.error() == rhs.error();
  }
 
  //*******************************************
  template <typename TValue, typename TError, typename TValue2>
  ETL_CONSTEXPR14 bool operator==(const etl::expected<TValue, TError>& lhs, const TValue2& rhs)
  {
    if (!lhs.has_value())
    {
      return false;
    }
    return lhs.value() == rhs;
  }
 
  //*******************************************
  template <typename TValue, typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator==(const etl::expected<TValue, TError>& lhs, const etl::unexpected<TError2>& rhs)
  {
    if (lhs.has_value())
    {
      return false;
    }
    return lhs.error() == rhs.error();
  }
 
  //*******************************************
  template <typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator==(const etl::expected<void, TError>& lhs, const etl::expected<void, TError2>& rhs)
  {
    if (lhs.has_value() != rhs.has_value())
    {
      return false;
    }
    if (lhs.has_value())
    {
      return true;
    }
    return lhs.error() == rhs.error();
  }
 
  //*******************************************
  template <typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator==(const etl::expected<void, TError>& lhs, const etl::unexpected<TError2>& rhs)
  {
    if (lhs.has_value())
    {
      return false;
    }
    return lhs.error() == rhs.error();
  }
 
  //*******************************************
  template <typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator==(const etl::unexpected<TError>& lhs, const etl::unexpected<TError2>& rhs)
  {
    return lhs.error() == rhs.error();
  }
 
  //*******************************************
  template <typename TValue, typename TError, typename TValue2, typename TError2>
  ETL_CONSTEXPR14 bool operator!=(const etl::expected<TValue, TError>& lhs, const etl::expected<TValue2, TError2>& rhs)
  {
    return !(lhs == rhs);
  }
 
  //*******************************************
  template <typename TValue, typename TError, typename TValue2>
  ETL_CONSTEXPR14 bool operator!=(const etl::expected<TValue, TError>& lhs, const TValue2& rhs)
  {
    return !(lhs == rhs);
  }
 
  //*******************************************
  template <typename TValue, typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator!=(const etl::expected<TValue, TError>& lhs, const etl::unexpected<TError2>& rhs)
  {
    return !(lhs == rhs);
  }
 
  //*******************************************
  template <typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator!=(const etl::expected<void, TError>& lhs, const etl::expected<void, TError2>& rhs)
  {
    return !(lhs == rhs);
  }
 
  //*******************************************
  template <typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator!=(const etl::expected<void, TError>& lhs, const etl::unexpected<TError2>& rhs)
  {
    return !(lhs == rhs);
  }
 
  //*******************************************
  template <typename TError, typename TError2>
  ETL_CONSTEXPR14 bool operator!=(const etl::unexpected<TError>& lhs, const etl::unexpected<TError2>& rhs)
  {
    return !(lhs == rhs);
  }
} // namespace etl
 
//*******************************************
//*******************************************
template <typename TValue, typename TError>
ETL_CONSTEXPR14 void swap(etl::expected<TValue, TError>& lhs, etl::expected<TValue, TError>& rhs)
{
  lhs.swap(rhs);
}
 
//*******************************************
//*******************************************
template <typename TError>
ETL_CONSTEXPR14 void swap(etl::unexpected<TError>& lhs, etl::unexpected<TError>& rhs)
{
  lhs.swap(rhs);
}
 
#endif