1 Differences between R0 and R1

• Improve examples to be clearer, and fully expanded into function call form.
• Add reference to range.adapter.
• Remove is_noexcept_sender.
• Remove just_error.
• Clarified use of parameter packs of values and errors.
• Removed confusing use of on in addition to via in the final example.

2 Introduction

In [P0443R11] we have included the fundamental principles described in [P1660R0], and the fundamental requirement to customize algorithms. In recent discussions we have converged to an understanding of the submit operation on a sender acting as a fundamental interoperation primitive, and algorithm customization giving us full flexibility to optimize, to offload and to avoid synchronization in chains of mutually compatible algorithm customizations.

As a starting point, in [P0443R11] we only include a bulk_execute algorithm, that satisfies the core requirement we planned with P0443 to provide scalar and bulk execution. To make the C++23 solution completely practical, we should extend the set of algorithms, however. This paper suggests an expanded initial set that enables early useful work chains. This set is intended to act as a discussion focus for us to discuss one by one, and to analyze the finer constraints of the wording to make sure we do not over-constrain the design.

In the long run we expect to have a much wider set of algorithms, potentially covering the full set in the current C++20 parallel algorithms. The precise customization of these algorithms is open to discussion: they may be individually customized and individually defaulted, or they may be optionally individually customized but defaulted in a tree such that customizing one is known to accelerate dependencies. It is open to discussion how we achieve this and that is an independent topic, beyond the scope of this paper.

2.1 Summary

Starting with [P0443R11] as a baseline we have the following customization points:

• execute(executor, invocable) -> void
• submit(sender, receiver) -> void
• schedule(scheduler) -> sender
• set_done
• set_error
• set_value

and the following Concepts:

• executor
• scheduler
• callback_signal
• callback
• sender

We propose immediately discussing the addition of the following algorithms:

• just(v)
  • returns a sender of the value v
• via(s, sch)
  • returns a sender that propagates the value or error from s on sch’s execution context
• sync_wait(s)
  • blocks and returns the value type of the sender, throwing on error
• transform(s, f)
  • returns a sender that applies f to the value passed by s, or propagates errors or cancellation
• bulk_transform(s, f)
  • returns a sender that applies f to each element in a range sent by s, or propagates errors or cancellation
• handle_error(s, f)
  • returns a sender that applies f to an error passed by s, ignoring the values or cancellation

2.2 Examples

2.2.0.1 Simple example

A very simple example of applying a function to a propagated value and waiting for it.

auto  just_sender =       // sender_to<int>
  just(3);

auto transform_sender =  // sender_to<float>
  transform(
    std::move(via_sender),
    [](int a){return a+0.5f;});

int result =              // value: 5
  sync_wait(std::move(error_sender));

In this very simple example we:

• start a chain with the value three
• apply a function to the incoming value, adding 0.5 and returning a sender of a float.
• block for the resulting value and assign the float value 3.5 to result.

Using operator| as in ranges to remove the need to pass arguments around, we can represent this as:

auto s = ;  
float f = sync_wait(
  just(3) | transform([](int a){return a+0.5f;}));              

2.2.0.2 With exception

A simple example showing how an exception that leaks out of a transform may propagate and be thrown from sync_wait.

int result = 0;
try {
  auto just_sender = just(3);
  auto via_sender = via(std::move(just_sender), scheduler1);
  auto transform_sender = transform(
    std::move(via_sender),
    [](int a){throw 2;});
  auto skipped_transform_sender = transform(
    std::move(transform_sender).
    [](){return 3;});

  result = sync_wait(std::move(skipped_transform_sender));
} catch(int a) {
 result = a;                                     // Assign 2 to result
}

In this example we:

• start a chain with an int value 3
• switch the context to one owned by scheduler1
• apply a transformation to the value 3, but this transform throws an exception rather than returning a transformed value
• skip the final transform because there is an error propagating
• block for the resulting value, seeing an exception thrown instead of a value returned
• handle the exception

As before, using operator| as in ranges to remove the need to pass arguments around, we can represent this more cleanly:

int result = 0;
try {
 result = sync_wait(
    just(3) |                           
    via(scheduler1) |                    
    transform([](int a){throw 2;}) |
    transform([](){return 3;}));
} catch(int a) {
 result = a;                                     // Assign 2 to result
}

2.2.0.3 Handle an exception

Very similar to the above, we can handle an error mid-stream

auto just_sender = just(3);
auto via_sender = via(std::move(just_sender), scheduler1);
auto transform_sender = transform(
  std::move(via_sender),
  [](int a){throw 2;});
auto skipped_transform_sender = transform(
  std::move(transform_sender).
  [](){return 3;});
auto error_handling_sender = handle_error(
  std::move(skipped_transform_sender),
  [](exception_ptr e){return just(5);});

auto result = sync_wait(std::move(error_handling_sender));

In this example we:

• start a chain with an int value 3
• switch the context to one owned by scheduler1
• apply a transformation to the value 3, but this transform throws an exception rather than returning a transformed value
• skip the final transform because there is an error propagating
• handle the error channel, applying an operation to an exception_ptr pointing to the value 2
• in handling the error we return a sender that propagates the value 5, thus recovering from the error
• block for the resulting value, assigning 5 to result

As before, using operator| as in ranges to remove the need to pass arguments around, we can represent this more cleanly:

auto s = ;
int result = sync_wait(
  just(3) |                            
  via(scheduler1) |                    
  transform([](float a){throw 2;}) |   
  transform([](){return 3;}) |         
  handle_error([](auto e){         
   return just(5);}));       

3 Impact on the standard library

3.1 Sender adapter objects

Taking inspiration from range adaptors define sender adapters.

Wording to be based on [range.adaptors] with the basic requirement that:

• operator| be overloaded for the purpose of creating pipelines over senders
• That the following are equivalent expressions:
  • algorithm(sender, args...)
  • algorithm(args...)(sender)
  • sender | algorithm(args...)
• that algorithms(args...) is a sender adaptor closure object
• TBD where sender adapters are declared

Details below are in loosely approximated wording and should be made consistent with [P0443R11] and the standard itself when finalized. We choose this set of algorithms as a basic set to allow a range of realistic, though still limited, compositions to be written against executors.

3.2 execution::just

3.2.1 Overview

just creates a sender that propagates a value inline to a submitted receiver.

Signature:

S<T...> just(T...);

where S<T...> is an implementation-defined typed_sender that that sends a set of values of type T... in its value channel.

[ Example:

int r = sync_wait(just(3));
// r==3

- end example]

3.2.2 Wording

The expression execution::just(t) returns a sender, s wrapping the value t.

• If t... are nothrow movable then execution::is_noexcept_sender(s) shall be constexpr and return true.
• When execution::submit(s, r) is called for some r, and r-value s will call execution::set_value(r, std::move(t)...), inline with the caller.
• When execution::submit(s, r) is called for some r, and l-value s will call execution::set_value(r, t...), inline with the caller.
• If moving of t throws, then will catch the exception and call execution::set_error(r, e) with the caught exception_ptr.

3.3 execution::sync_wait

3.3.1 Overview

Blocks the calling thread to wait for the passed sender to complete. Returns the value (or void if the sender carries no value), throws if an exception is propagated and throws a TBD exception type on cancellation.¹ On propagation of the set_done() signal, returns an empty optional.

T... sync_wait(S<T...>)

where S<T...> is a sender that sends zero or one values of type T... in its value channel. The existence of, and if existing the type T must be known statically and cannot be part of an overload set.

[ Example:

int r = sync_wait(just(3));
// r==3

- end example]

3.3.2 Wording

The name execution::sync_wait denotes a customization point object. The expression execution::sync_wait(S) for some subexpression S is expression-equivalent to:

• S.sync_wait() if that expression is valid.
• Otherwise, sync_wait(S), if that expression is valid with overload resolution performed in a context that includes the declaration

        template<class S>
          void sync_wait(S) = delete;

and that does not include a declaration of execution::sync_wait.

• Otherwise constructs a receiver, r over an implementation-defined synchronization primitive and passes that receiver to execution::submit(S, r). Waits on the synchronization primitive to block on completion of S.

  • If set_value is called on r, returns the passed value (or simply returns for void sender).
  • If set_error is called on r, throws the error value as an exception.
  • If set_done is called on r, throws some TBD cancellation exception type.

3.4 execution::via

3.4.1 Overview

via is a sender adapter that takes a sender and a scheduler and returns a sender that propagates the same value as the original, but does so on the sender’s execution context.

Signature:

S<T...> via(S<T...>, Scheduler);

where S<T> is an implementation-defined type that is a sender that sends a value of type T in its value channel.

[ Example:

static_thread_pool t{1};
int r = sync_wait(just(3) | via(t.scheduler()));
// r==3

3.4.2 Wording

The name execution::via denotes a customization point object. The expression execution::via(S, Sch) for some subexpressions S, Sch is expression-equivalent to:

• S.via(Sch) if that expression is valid.
• Otherwise, via(S, Sch) if that expression is valid with overload resolution performed in a context that includes the declaration

        template<class S, class Sch>
          void via(S, Sch) = delete;

• Otherwise constructs a receiver r such that when set_value, set_error or set_done is called on r the value(s) or error(s) are packaged, and a receiver r2 constructed such that when execution::set_value(r2) is called, the stored value or error is transmitted and r2 is submitted to Sch. If set_error or set_done is called on r2 the error or cancellation is propagated and the packaged values ignored.
• The returned sender’s value types match those of sender1.
• The returned sender’s execution context is that of scheduler1.

3.5 execution::transform

3.5.1 Overview

transform is a sender adapter that takes a sender and an invocable and returns a sender that propagates the value resulting from calling the invocable on the value passed by the preceding sender.

Signature:

S<T2> transform(S<T...>, invocable<T2(T...));

where S<T...> and S<T2> are implementation-defined types that is represent senders that send a value of type list T... or T2 respectively in their value channels. Note that in the general case there may be many types T... for a given sender, in which case the invocable may have to represent an overload set.

[ Example:

int r = sync_wait(just(3) | transform([](int v){return v+1;}));
// r==4

3.5.2 Wording

The name execution::transform denotes a customization point object. The expression execution::transform(S, F) for some subexpressions S and F is expression-equivalent to:

• S.transform(F) if that expression is valid.
• Otherwise, transform(S, F), if that expression is valid with overload resolution performed in a context that includes the declaration

        template<class S, class F>
          void transform(S, F) = delete;

and that does not include a declaration of execution::transform.

• Otherwise constructs a receiver, r over an implementation-defined synchronization primitive and passes that receiver to execution::submit(S, r). When some output_receiver has been passed to submit on the returned sender.

  • If set_value(r, Ts... ts) is called, calls std::invoke(F, ts...) and passes the result v to execution::set_value(output_receiver, v).
  • If F throws, catches the exception and passes it to execution::set_error(output_receiver, e).
  • If set_error(c, e) is called, passes e to execution::set_error(output_receiver, e).
  • If set_done(c) is called, calls execution::set_done(output_receiver).

3.6 execution::bulk_transform

3.6.1 Overview

bulk_transform is a sender adapter that takes a sender of a range of values and an invocable and returns a sender that executes the invocable for each element of the input range, and propagates the range of returned values.

Signature:

S<range<T2>> bulk_transform(S<range<T>>, invocable<T2(T));

where S<range<T>> and S<T2> are implementation-defined types that is represent senders that send a value of type list T or T2 respectively in their value channels. Note that in the general case there may be many types T for a given sender, in which case the invocable may have to represent an overload set.

[ Example:

std::vector<int> r = sync_wait(just(std::vector<int>{3, 4, 5}) | bulk_transform([](int v){return v+1;}));
// r=={4, 5, 6}

Note: it is TBD how precisely we should represent the intermediate data types here. Intermediate vectors would require allocator support. Purely lazy ranges may be inadequate.

3.6.2 Wording

The name execution::bulk_transform denotes a customization point object. The expression execution::bulk_transform(S, F) for some subexpressions S and F is expression-equivalent to:

• S.bulk_transform(F), if that expression is valid.
• Otherwise, bulk_transform(S, F), if that expression is valid with overload resolution performed in a context that includes the declaration

        template<class S, class F>
          void bulk_transform(S, F) = delete;

and that does not include a declaration of execution::bulk_transform.

• Otherwise constructs a receiver, r over an implementation-defined synchronization primitive and passes that receiver to execution::submit(S, r).

  • If S::value_type does not model the concept Range<T> for some T the expression ill-formed.
    
  • If set_value is called on r with some parameter input applies the equivalent of out = std::ranges::transform_view(input, F) and passes the result output to execution::set_value(output_receiver, v).
  • If set_error(r, e) is called, passes e to execution::set_error(output_receiver, e).
  • If set_done(r) is called, calls execution::set_done(output_receiver).

3.7 execution::handle_error

3.7.1 Overview

handle_error is a sender adapter that takes a sender and an invocable and returns a sender that propagates the value, error or done signal from the sender returned by the invocable.

Signature:

S<T2..., E2...> handle_error(S<T..., E...>, invocable<sender<T2..., E2...>(E...));

where S<T..., E...> and S<T2..., E2...> are implementation-defined types that is represent senders that send a value of type list T... or T2... respectively in their value channels and error type lists E... and E2... in their error channels. The invocable takes the error types E... and returns a sender over some potentially new set of types. By returning a sender the algorithm has control of error recovery as well as use cases such as logging and propagation. Note that in the general case there may be many types E... for a given sender, in which case the invocable may have to represent an overload set.

[ Example:

float r = sync_wait(
  just(3) |
  transform([](int v){throw 2.0f;}) |
  handle_error([](float e){return just(e+1);}));
// r==3.0f

3.7.2 Wording

The name execution::handle_error denotes a customization point object. The expression execution::handle_error(S, F) for some subexpressions S and F is expression-equivalent to:

• S.handle_error(F), if that expression is valid.
• Otherwise, handle_error(S, F), if that expression is valid with overload resolution performed in a context that includes the declaration

        template<class S, class F>
          void handle_error(S, F) = delete;

and that does not include a declaration of execution::handle_error.

• Otherwise constructs a receiver, r over an implementation-defined synchronization primitive and passes that receiver to execution::submit(S, r).

  • If set_value(r, v...) is called, passes v... to execution::set_value(output_receiver, v...).
  • If set_error(r, e...) is called, passes e... to f, resulting in a sender s2 and passes output_receiver to submit(s2, output_receiver).
  • If set_done(r) is called, calls execution::set_done(output_receiver).

4 Customization and example

Each of these algorithms, apart from just, is customizable on one or more sender implementations. This allows full optimization. For example, in the following simple work chain:

auto s = just(3) |                                        // s1
         via(scheduler1) |                                // s2
         transform([](int a){return a+1;}) |              // s3
         transform([](int a){return a*2;}) |              // s4
         via(scheduler2) |                                // s5
         handle_error([](auto e){return just(3);});       // s6
int r = sync_wait(s);

The result of s1 might be a just_sender<int> implemented by the standard library vendor.

via(just_sender<int>, scheduler1) has no customization defined, and this expression returns an scheduler1_via_sender<int> that is a custom type from the author of scheduler1, it will call submit on the result of s1.

s3 calls transform(scheduler1_via_sender<int>, [](int a){return a+1;}) for which the author of scheduler1 may have written a customization. The scheduler1_via_sender has stashed the value somewhere and build some work queue in the background. We do not see submit called at this point, it uses a behind-the-scenes implementation to schedule the work on the work queue. An scheduler1_transform_sender<int> is returned.

s4 implements a very similar customization, and again does not call submit. There need be no synchronization in this chain.

At s5, however, the implementor of scheduler2 does not know about the implementation of scheduler1. At this point it will call submit on the incoming scheduler1_transform_sender, forcing scheduler1’s sender to implement the necessary synchronization to map back from the behind-the-scenes optimal queue to something interoperable with another vendor’s implementation.

handle_error at s6 will be generic in terms of submit and not do anything special, this uses the default implementation in terms of submit. sync_wait similarly constructs a condition_variable and a temporary int, submits a receiver to s and waits on the condition_variable, blocking the calling thread.

r is of course the value 8 at this point assuming that neither scheduler triggered an error. If there were to be a scheduling error, then that error would propagate to handle_error and r would subsequently have the value 3.

5 References