P2505R2 Monadic Functions for `std::expected`

Introduction
Motivation and Scope
Design Considerations
Other Languages
Implementations
Wording
Acknowledgements
Revision History
References

Authors: Jeff Garland

Audience: LEWG, LWG

Project: ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++

Introduction

With the final plenary vote of P0798 Monadic Functions for std::optional complete, we now have an design inconsistency with std::expected. P0323 std::expected has now also been voted into the working draft for C++23. This proposal corrects the inconsistency by adding 4 functions to std::expected and is targeted at C++23. The author believes this should be treated as a consistency/bug fix still in scope for C++23.

The intent from P0798 clearly states that std::expected should gain the same functions:

There is a proposal for std::expected which would benefit from many of these same ideas. If the idea to add monadic interfaces to standard library classes on a case-by-case basis is chosen rather than a unified non-member function interface, then compatibility between this proposal and the std::expected one should be maintained

Motivation and Scope

The following 3 functions are added to std::optional, but are currently not part of std::expected.

and_then compose a chain of functions returning an expected
or_else returns if it has a value, otherwise it calls a function with the error type
transform apply a function to change the value (and possibly the type)

After feedback on the R0, this draft also proposes the addition of a fourth function for expected to facilitate additional cases:

transform_error apply a function to change the value, otherwise call a function with error type

For example, given the following:

using time_expected = expected<boost::posix_time::ptime, std::string>;

time_expected from_iso_str( std::string time )
{
  try {
    ptime t  = boost::posix_time::from_iso_string( time );
    return t;
  }
  catch( std::exception& e )
  {
     return unexpected( e.what() + " str was: "s + time);
  }
}

// for use with transform
ptime next_day( boost::posix_time::ptime t )
{
   return t + boost::gregorian::days(1);
}

// for use with or_else
void print_error( std::string error )
{
   cout << error << endl;
}


//valid iso string
const std::string ts ( "20210726T000000" );

Before the change we'd write this:

Before

time_expected d = from_iso_str( ts );
if (d)
{
   ptime t = next_day( *d );
} 
else
{
   print_error( d.error() );
}

And after, this:

After

// chain a series of functions until there's an error
auto d = from_iso_str ( ts )
               .or_else( print_error )
               .transform( next_day ) ;

Jonathan Wakely has a live demo illustrating the api here https://godbolt.org/z/3a1j6d63a. For more examples see: https://godbolt.org/z/K7rbhx9oo.

Design Considerations

`expected<void, E>`

Compared with optional, expected provides the capability for void as a function return type. This complicates the design compared to optional. As currently specified the void specialization of expected, expected<void, T> removes the value_or function since void cannot be initialized to anything else.

While the primary implementation for this paper excludes transform for expected<void, E> and the original version removed this overload, feedback from reviewers suggested the overload is valuable. Thus the new version allows for this overload.

`transform_error` function

The R0 version of this paper did not include this function to match optional, but several reviewers noted the important utility of this case. Note that in the reference implementation of this paper, the function is called map_error. The R1 version of the paper names the function transform_error after discussions with various reviewers.

changing of error return types

One complication of the monadic functions for expected as compared to optional is the ability to change the return error return type on subsequent function calls. Optional, of course, can only return std::nullopt_t.

Thus we would like to support use cases like the following:


std::expected<SomeUserType, InternalError> initialize();

std::expected<SomeUserType, UserError>
makeUserError(InternalError err) 
{ 
   return std::unexpected( UserError(err) ); 
};

std::expected<SomeUserType, UserError> result = initialize().or_else( makeUserError );

Note that the changing of the error type applies to the or_else and transform_error overloads only.

Other Languages

Rust provides a result type with similar functions monadic functions.

https://doc.rust-lang.org/std/result/enum.Result.html

Implementations

Jonathan Wakely has implemented the full proposal here https://github.com/jwakely/gcc/blob/expected/libstdc%2B%2B-v3/include/std/expected.

The core of the proposal was originally implemented by Sy Brand https://github.com/TartanLlama/expected with documentation here.

Wording

Feature Test Macro

Update the __cpp_lib_expected feature test macro from P0323. Although this could be considered optional since expected is not yet a c++ feature in any implementations.

Class template expected [expected.expected]

After value_or functions add the new functions:

template<class U> constexpr T value_or(U&&) &&;

// [expected.monadic], monadic operations
template <class F> constexpr auto and_then(F&& f) &;
template <class F> constexpr auto and_then(F&& f) &&;
template <class F> constexpr auto and_then(F&& f) const&;
template <class F> constexpr auto and_then(F&& f) const&&;
template <class F> constexpr auto or_else(F&& f) &;
template <class F> constexpr auto or_else(F&& f) &&;
template <class F> constexpr auto or_else(F&& f) const&;
template <class F> constexpr auto or_else(F&& f) const&&;
template <class F> constexpr auto transform(F&& f) &;
template <class F> constexpr auto transform(F&& f) &&;
template <class F> constexpr auto transform(F&& f) const&;
template <class F> constexpr auto transform(F&& f) const&&;
template <class F> constexpr auto transform_error(F&& f) &;
template <class F> constexpr auto transform_error(F&& f) &&;
template <class F> constexpr auto transform_error(F&& f) const&;
template <class F> constexpr auto transform_error(F&& f) const&&;
// [expected.object.swap], swap

Add a new sub-clause Monadic operations for [expected.monadic] between [expected.observe] and [expected.equality]

1 template <class F> constexpr auto and_then(F&& f) &;

2 template <class F> constexpr auto and_then(F&& f) const&;

Let U be remove_cvref_t<invoke_result_t<F, decltype(value())>>.

Constraints:

is_same_v<U::error_type, E> is true,
is_copy_constructible_v<E> is true, and
is_copy_constructible_v<U> is true.

Mandates: U is a specialization of expected.

Effects: Equivalent to:

if (has_value()) { 
   return invoke(std::forward<F>(f), value());
}
else { 
   return U(unexpect, error());
}

1 template <class F> constexpr auto and_then(F&& f) &&;

2 template <class F> constexpr auto and_then(F&& f) const&&;

Let U be remove_cvref_t<invoke_result_t<F, decltype(std::move(value()))>>.

Constraints:

is_same_v<U::error_type, E> is true,
is_move_constructible_v<E> is true, and
is_move_constructible_v<U> is true.

Mandates: U is a specialization of expected.

Effects: Equivalent to:

if (has_value()) { 
   return invoke(std::forward<F>(f), std::move(value()));
}
else {
   return U(unexpect, std::move(error()));
}

1 template <class F> constexpr auto or_else(F&& f) &;

2 template <class F> constexpr auto or_else(F&& f) const&;

Let G be remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>.

Constraints:

is_same_v<G::value_type, T> is true,
is_copy_constructible_v<T> is true, and
is_copy_constructible_v<G> is true.

Mandates: G is a specialization of expected.

Effects: Equivalent to:

if (has_value()) {
    return G(value());
} else {
    return invoke(std::forward<F>(), error());
}

1 template <class F> constexpr auto or_else(F&& f) &&;

2 template <class F> constexpr auto or_else(F&& f) const&&;

Let G be remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>.

Constraints:

is_same_v<G::value_type, T> is true,
is_move_constructible_v<T> is true, and
is_move_constructible_v<G> is true.

Mandates: G is a specialization of expected.

Effects: Equivalent to:

if (has_value()) { 
   return std::move(*this);
}
else { 
   return invoke(std::forward<F>(f)(std::move(error())));
}

1 template <class F> constexpr auto transform(F&& f) &;

2 template <class F> constexpr auto transform(F&& f) const&;

Let U be remove_cvref_t<invoke_result_t<F, decltype(error())>>.

Constraints:

is_same_v<U::error_type, E> is true,
is_copy_constructible_v<E> is true, and
is_copy_constructible_v<U> is true.

Effects: Equivalent to:

if (has_value()) {
   return expected<U,E>(invoke(std::forward<F>(f), value()));
}
else {
   return U(unexpect, error());
}

1 template <class F> constexpr auto transform(F&& f) &&;

2 template <class F> constexpr auto transform(F&& f) const&&;

Let U be remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>.

Constraints:

is_same_v<U::error_type, E> is true,
is_move_constructible_v<E> is true, and
is_move_constructible_v<U> is true.

Effects: Equivalent to:

if (has_value()) { 
   return expected<U,E>(invoke(std::forward<F>(f), std::move(value())));
}
else { 
   return U(unexpect, std::move(error()));
}

1 template <class F> constexpr auto transform_error(F&& f) &;

2 template <class F> constexpr auto transform_error(F&& f) const&;

Let G be remove_cvref_t<invoke_result_t<F, decltype(error())>>.

Constraints:

is_copy_constructible_v<T> is true, and
is_copy_constructible_v<G> is true.

Effects: Equivalent to:

if (has_value()) { 
    return expected<T, G>(value());
} 
else { 
   return invoke(std::forward<F>(f)(error()));
}

1 template <class F> constexpr auto transform_error(F&& f) &&;

2 template <class F> constexpr auto transform_error(F&& f) const&&;

Let G be remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>.

Constraints:

is_move_constructible_v<T> is true, and
is_move_constructible_v<G> is true.

Effects: Equivalent to:

if (has_value()) { 
   return expected<T, G>(std::move(value()))
}
else { 
   return invoke(std::forward<F>(f)(std::move(error())));
}

Partial specialization of expected for void types [expected.void]

After �.�.8.6, observers – add

// �.�.8.7, monadic
template <class F> constexpr auto and_then(F&& f) &;
template <class F> constexpr auto and_then(F&& f) &&;
template <class F> constexpr auto and_then(F&& f) const&;
template <class F> constexpr auto and_then(F&& f) const&&;
template <class F> constexpr auto or_else(F&& f) &;
template <class F> constexpr auto or_else(F&& f) &&;
template <class F> constexpr auto or_else(F&& f) const&;
template <class F> constexpr auto or_else(F&& f) const&&;
template <class F> constexpr auto transform(F&& f) &;
template <class F> constexpr auto transform(F&& f) &&;
template <class F> constexpr auto transform(F&& f) const&;
template <class F> constexpr auto transform(F&& f) const&&;
template <class F> constexpr auto transform_error(F&& f) &;
template <class F> constexpr auto transform_error(F&& f) &&;
template <class F> constexpr auto transform_error(F&& f) const&;
template <class F> constexpr auto transform_error(F&& f) const&&;

Partial specialization of expected for void [expected.void.observe]

After section �.�.8.5 Observers [expected.void.observe]

Add section �.�.8.6 Monadic [expected.void.monadic]

1 template <class F> constexpr auto and_then(F&& f) &;

2 template <class F> constexpr auto and_then(F&& f) const&;

3 template <class F> constexpr auto and_then(F&& f) &&;

4 template <class F> constexpr auto and_then(F&& f) const&&;

Let U be remove_cvref_t<invoke_result_t<F>>.

Constraints: is_same_v<U::error_type, E> is true. E models copy_constructible for the first two overloads and move_constructible for the second two overloads.

Mandates: U is a specialization of expected.

Effects: Equivalent to:

if (has_value()) { 
   return std::move(std::forward<F>(f)());
}
else { 
   return U(unexpect, std::move(error()));
}

1 template <class F> constexpr expected or_else(F&& f) &;

2 template <class F> constexpr expected or_else(F&& f) const&;

3 template <class F> constexpr expected or_else(F&& f) &&;

4 template <class F> constexpr expected or_else(F&& f) const &&;

Let G be remove_cvref_t<invoke_result_t<F, decltype(error())>>.

Constraints: E models copy_constructible for the first two overloads and move_constructible for the second two overloads.

Mandates: G is a specialization of expected.

Effects: Equivalent to:

if (has_value()) { 
    return std::move(*this);
} 
else { 
   return std::forward<F>(f)();
}

1 template <class F> constexpr auto transform(F&& f) &;

2 template <class F> constexpr auto transform(F&& f) const&;

3 template <class F> constexpr auto transform(F&& f) &&;

4 template <class F> constexpr auto transform(F&& f) const&&;

Let U be remove_cvref_t<invoke_result_t<F>>.

Constraints: is_same_v<U::error_type, E> is true. E models copy_constructible for the first two overloads and move_constructible for the second two overloads.

Effects: Equivalent to:

if (has_value()) {
   return expected<U,E>(std::forward<F>(f)());
}
else {
  return U(unexpect, std::move(error()));
}

1 template <class F> constexpr auto transform_error(F&& f) &;

2 template <class F> constexpr auto transform_error(F&& f) const&;

3 template <class F> constexpr auto transform_error(F&& f) &&;

4 template <class F> constexpr auto transform_error(F&& f) &&;

Let G be remove_cvref_t<invoke_result_t<F, decltype(error())>>.

Constraints: E models copy_constructible for the first two overloads and move_constructible for the second two overloads.

Mandates: G is a specialization of expected.

Effects: Equivalent to:

if (has_value()) { 
    return std::move(*this);
} 
else { 
   return std::move(std::forward<F>(f)(error()));
}

Acknowledgements

Thanks to Jonathan Wakely for implemenation and wording feedback.
Thanks to Sy Brand for the original work on optional and expected.
Thanks to Broneck Kozicki for early feedback.
Thanks to Arthur O'Dwyer for design and wording feedback.
Thanks to Barry Revzin for design and wording feedback.

Revision History

Version	Date	Changes
0	2021-12-12	Initial Draft
1	2022-02-10	added `transform_or`
		allow `transform` on `expected<void, E>`
		updated status of P0323 to plenary approved
		improved examples
		added missing overloads for `or_else`
		`or_else` signature and wording fixes
		added design discussion of changing return types
		variety of wording fixes
2	2022-04-15	updates to reference Wakely implmentation and examples
		remove unneeded invoke calls from void overloads
		`expected.object.swap` instead of modifiers in synopsis
		change constraint wording to use `is_copy_constructible_v` form for consistency
		change Let U for move overloads to `decltype(std::move(...))`
		fix `transform_error` and `or_else` constraints to be T instead of E
		rename `transform_or` to `transform_error`
		remove the `is_same_v` constraint on `transform_error`
		change `tranform_error` true case to return `expected<T,U>`
		remove note: U will implicitly model either copy or move constructible…
		change all the `if (*this)` to `if (has_value())` to match latest expected wording
		remove the `in_place` tag in `transform` overloads
		remove void from `invoke_results` for `expected<void,E> and_then` and `transform`.
		change error returns for `and_then~/~transform` to `return U(unexpect, error())`
		change Let U to Let G for `transform_error` and `or_else` for less confusion

References

[P0798] Sy Brand "Monadic Functions for std::optional" https://wg21.link/P0798
C++ draft [optional.nomadic] http://eel.is/c++draft/optional.monadic
[P0323] Vicente Botet, JF Bastien, Jonathan Wakely std::expected https://wg21.link/P0323
Sy Brand expected https://github.com/TartanLlama/expected
Sy Brand expected docs https://tl.tartanllama.xyz/en/latest/api/expected.html#tl-expected
Jonathan Wakely full implemenation https://github.com/jwakely/gcc/blob/expected/libstdc%2B%2B-v3/include/std/expected
Jonathan Wakely live demo https://godbolt.org/z/3a1j6d63a
Rust Result https://doc.rust-lang.org/std/result/enum.Result.html
More complete examples https://godbolt.org/z/va5fzx11f

P2505R2 Monadic Functions for std::expected

Table of Contents

Introduction

Motivation and Scope

Design Considerations

expected<void, E>

transform_error function

changing of error return types

Other Languages

Implementations

Wording

Feature Test Macro

Class template expected [expected.expected]

Add a new sub-clause Monadic operations for [expected.monadic] between [expected.observe] and [expected.equality]

Partial specialization of expected for void types [expected.void]

Partial specialization of expected for void [expected.void.observe]

Acknowledgements

Revision History

References

P2505R2 Monadic Functions for `std::expected`

`expected<void, E>`

`transform_error` function