P2505R4 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++

Contact: jeff@crystalclearsoftware.com

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, this draft also proposes the addition of two additional functions for expected to facilitate additional cases:

transform_error apply a function to change the value, otherwise call a function with error type
error_or a value to return when there is not an error

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` to `expected<void, E>` return

Consider the following code.

struct E{};
auto x = expected<int, E>(4).transform([](int){});

The original specifications and implementations would fail to compile this code. A reasonable alternative is to return expected<void, E>. The specification has been changed to allow this behavior.

`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.

Use of deducing-this in the specification

This question has been asked several times including in the LEWG 2022-05-03 meeting. While less concise, the author prefers to keep the specification consistent with the rest of the c++23 library and especially std::optional. Limited access to compilers that implement deducing-this make it difficult to confirm correct behavior of a deducing-this based specification. In addition, the current specification does not preclude changing to a deducing-this based consistently with other library facilities in a future release.

Adding `error_or` functions

It has been suggested that error_or functions be added to the proposal. While the author is sympathetic to the suggestion, since consistency with optional for c++23 is the intent these are not proposed. These can be added later or if LEWG would prefer the author can investigate adding them in this cycle.

At 2022-06-07 LEWG telecom poll was taken to add this function. The wording is added in R4 of the paper.

POLL: Add error_or to expected for C++23.

Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
4	7	2	2	0

Attendance: 21

Author Position: N

Outcome: Consensus in favor.

As an example usage consider the following code:

Before

//before
file create_file(char const*, std::error_code&);
bool test_and_report(const error_code&);

std::error_code ec;
file f = create_file("file", ec);
if (test_and_report(ec)) { return; }

After

expected<file, error_code> create_file(char const*);
bool test_and_report(const error_code&);

auto f = create_file("file");

if (test_and_report(f.error_or({}))) { return; }

Free versus member functions

One design question that has been raised is the use of member functions instead of free functions. The author is sympathetic to this view, but believes this can only be done by changing both optional and expected. The expressed benefits of free functions would be:

better compatibility with future library facilities
more generic code – singular specification

While in theory, free functions provide consistency with some future version of the library where functions such as then and upon_error from P2300 std::execution would be part of the standard library. However, then is specified using concepts such as std::execution::sender which are not appropriate for a generalized then functions for optional and expected. In fact, attempting to specify this as free functions now might trample on names for future library free functions.

Since the author is unaware of any existing practice with providing these functions and the purpose of the paper is consistency with monadic optional functions already voted into the working draft, the author will not pursue this option. A fully thought out paper with existing practice would be required to make this change.

Note that the author's experience with std::expected implementing json parsing is that expected would be extremely cumbersome for the task without these functions. So shipping C++23 without these functions for an unclear future benefit would be a major mistake for users.

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.

Rishabh Dwivedi has also implemented the proposal based on the draft wording https://github.com/RishabhRD/expected.

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&&) &&;
template<class G = E> constexpr E error_or(G&&) const&;
template<class G = E> constexpr E error_or(G&&) &&;

// [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 following at the end [expected.object.observe] Observers:

1 template <class G = E> constexpr E error_or(G&& e) const&;

Mandates: is_copy_constructible_v<E> is true and is_convertible<G, E> is true.

Returns: std::forward<G>(e) if has_value() is true, error() otherwise.

2 template <class G = E> constexpr E error_or(G&& e) i&&;

Mandates: is_move_constructible_v<E> is true and is_convertible<G, E> is true.

Returns: std::forward<G>(e) if has_value() is true, std::move(error()) otherwise.

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

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_copy_constructible_v<E> is true

Mandates: U is a specialization of expected and is_same_v<U::error_type, E> is true,

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_move_constructible_v<E> is true

Mandates: U is a specialization of expected and is_same_v<U::error_type, E> is true,

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_copy_constructible_v<T> is true.

Mandates: G is a specialization of expected and is_same_v<G::value_type, T> is true,

Effects: Equivalent to:

if (has_value()) {
    return G(in_place, 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_move_constructible_v<T> is true

Mandates: G is a specialization of expected and is_same_v<G::value_type, T> is true.

Effects: Equivalent to:

if (has_value()) { 
   return G(in_place, std::move(value()));
}
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(value())>>.

Mandates: U is void or a non-array object type other than in_place_t, unexpect_t, or a specialization of unexpected. If is_void_v is false, the declaration U u(invoke(std::forward<F>(f), value())); is well-formed.

Constraints:

is_copy_constructible_v<E> is true, and

Effects:

If has_value() is false, returns expected<U,E>(unexpect, error()), otherwise
if is_void_v is false, returns an expected<U, E> object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward<F>(f), value()), otherwise
evaluates invoke(std::forward<F>(f), value()) and then returns expected<U,E>().

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(value()))>>.

Constraints: is_move_constructible_v<E> is true, and

Effects:

If has_value() is false, returns expected<U,E>(unexpect, std::move(error())), otherwise
if is_void_v is false, returns an expected<U, E> object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward<F>(f), std::move(value())), otherwise
evaluates invoke(std::forward<F>(f), std::move(value())) and then returns expected<U,E>().

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())>>.

Mandates: G is a non-array object type other than in_place_t, unexpect_t, or a specialization of unexpected. The declaration G g(invoke(std::forward<F>(f), error())); is well-formed.

Constraints: is_copy_constructible_v<T> is true

Returns:

If has_value() is true, expected<T,G>(in_place, value()), otherwise
an expected<T, G> object whose has_val member is false and unex member is direct-non-list-initialized with invoke(std::forward<F>(f), error()), otherwise

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()))>>.

Mandates: G is a non-array object type other than in_place_t, unexpect_t, or a specialization of unexpected. The declaration G g(invoke(std::forward<F>(f), std::move(error()))); is well-formed.

Constraints: is_move_constructible_v<T> is true, and

Returns:

If has_value() is true, expected<T,G>(in_place, std::move(value())), otherwise
an expected<T, G> object whose has_val member is false and unex member is direct-non-list-initialized with invoke(std::forward<F>(f), std::move(error()))

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

After error functions add the new functions:

constexpr E&& error() &&;
template<class G = E> constexpr E error_or(G&&) const&;
template<class G = E> constexpr E error_or(G&&) &&;

// �.�.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&&;

Add following at the end [expected.void.observe] Observers:

1 template <class G = E> constexpr E error_or(G&& e) const&;

Mandates: is_copy_constructible_v<E> is true and is_convertible<G, E> is true.

Returns: std::forward<G>(e) if has_value() is true, error() otherwise.

2 template <class G = E> constexpr E error_or(G&& e) i&&;

Mandates: is_move_constructible_v<E> is true and is_convertible<G, E> is true.

Returns: std::forward<G>(e) if has_value() is true, std::move(error()) otherwise.

Add a new section [expected.void.monadic] Monadic operations [expected.void.observe] and [expected.void.eq]

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>>.

Constraints: is_copy_constructible_v<E> is true.

Mandates: U is a specialization of expected and is_same_v<U::error_type, E> is true.

Effects: Equivalent to:

if (has_value()) { 
   return invoke(std::forward<F>(f));
}
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>>.

Constraints: is_move_constructible_v<E> is true

Mandates: U is a specialization of expected and is_same_v<U::error_type, E> is true.

Effects: Equivalent to:

if (has_value()) { 
   return invoke(std::forward<F>(f));
}
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()))>>.

Mandates: G is a specialization of expected and is_same_v<G::value_type, T> is true.

Effects: Equivalent to:

if (has_value()) {
    return G();
} 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()))>>.

Mandates: G is a specialization of expected and is_same_v<G::value_type, T> is true.

Effects: Equivalent to:

if (has_value()) { 
   return G();
}
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>>.

Constraints: is_copy_constructible_v<E> is true

Effects:

If has_value() is false, returns expected<U,E>(unexpect, error()), otherwise
if is_void_v is false, returns an expected<U, E> object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward<F>(f)), otherwise
evaluates invoke(std::forward<F>(f)) and then returns expected<U,E>().

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>>.

Constraints: is_move_constructible_v<E> is true, and

Effects:

If has_value() is false, returns expected<U,E>(unexpect, std::move(error())), otherwise
if is_void_v is false, returns an expected<U, E> object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward<F>(f)), otherwise
evaluates invoke(std::forward<F>(f)) and then returns expected<U,E>().

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())>>.

Mandates: G is a non-array object type other than in_place_t, unexpect_t, or a specialization of unexpected. The declaration G g(invoke(std::forward<F>(f), error())); is well-formed.

Returns:

If has_value() is true, expected<T,G>(), otherwise
an expected<T, G> object whose has_val member is false and unex member is direct-non-list-initialized with invoke(std::forward<F>(f), error()), otherwise

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()))>>.

Returns:

If has_value() is true, expected<T,G>(), otherwise
an expected<T, G> object whose has_val member is false and unex member is direct-non-list-initialized with invoke(std::forward<F>(f), std::move(error()))

Acknowledgements

Thanks to Jonathan Wakely for implementation and wording feedback.
Thanks to Tomasz Kaminski for extensive wording input and suggestions on error_or.
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 as well as shepherding in LEWG.

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
3	2022-06-05	Add a design discussion of why deducing-this is not utilized
		Add a discussion of free function versus member functions
		mandate `transform` return type not be `unexpected`, `inplace_t`, etc
		design discussion and wording for `transform` to `expected<void, E>`
		design discussion of `error_or` functions
4	2022-06-15	Add an `error_or` function that mirrors `value_or` for basic case
		Update design section and intro with `error_or` information
		Incorporate wording suggestions from Tomasz K.
		Add link to Rishabh D. implementation

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

P2505R4 Monadic Functions for std::expected

Table of Contents

Introduction

Motivation and Scope

Design Considerations

expected<void, E>

transform to expected<void, E> return

transform_error function

Changing of error return types

Use of deducing-this in the specification

Adding error_or functions

Free versus member functions

Other Languages

Implementations

Wording

Feature Test Macro

Class template expected [expected.expected]

Add following at the end [expected.object.observe] Observers:

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

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

Add following at the end [expected.void.observe] Observers:

Add a new section [expected.void.monadic] Monadic operations [expected.void.observe] and [expected.void.eq]

Acknowledgements

Revision History

References

P2505R4 Monadic Functions for `std::expected`

`expected<void, E>`

`transform` to `expected<void, E>` return

`transform_error` function

Adding `error_or` functions