34 Execution control library [exec]

schedule obtains a schedule sender ([exec.async.ops]) from a scheduler.

The name schedule denotes a customization point object.

For a subexpression sch, the expression schedule(sch) is expression-equivalent to sch.schedule().

Mandates: The type of sch.schedule() satisfies sender.

If the expression get_completion_scheduler<set_value_t>(get_env(sch.schedule())) == sch is ill-formed or evaluates to false, the behavior of calling schedule(sch) is undefined.

just, just_error, and just_stopped are sender factories whose asynchronous operations complete synchronously in their start operation with a value completion operation, an error completion operation, or a stopped completion operation, respectively.

The names just, just_error, and just_stopped denote customization point objects.

Let just-cpo be one of just, just_error, or just_stopped.

For a pack of subexpressions ts, let Ts be the pack of types decltype((ts)).

The expression just-cpo(ts...) is ill-formed if

• (2.1)
  (movable-value<Ts> &&...) is false, or
• (2.2)
  just-cpo is just_error and sizeof...(ts) == 1 is false, or
• (2.3)
  just-cpo is just_stopped and sizeof...(ts) == 0 is false.

Otherwise, it is expression-equivalent to make-sender(just-cpo, product-typets...).

For just, just_error, and just_stopped, let set-cpo be set_value, set_error, and set_stopped, respectively.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for just-cpo as follows: namespace std::execution { template<> struct impls-for<decayed-typeof<just-cpo>> : default-impls { static constexpr auto start = [](auto& state, auto& rcvr) noexcept -> void { auto& [...ts] = state; set-cpo(std::move(rcvr), std::move(ts)...); }; }; }

read_env is a sender factory for a sender whose asynchronous operation completes synchronously in its start operation with a value completion result equal to a value read from the receiver's associated environment.

read_env is a customization point object.

For some query object q, the expression read_env(q) is expression-equivalent to make-sender(read_env, q).

The exposition-only class template impls-for ([exec.snd.general]) is specialized for read_env as follows: namespace std::execution { template<> struct impls-for<decayed-typeof<read_env>> : default-impls { static constexpr auto start = [](auto query, auto& rcvr) noexcept -> void { TRY-SET-VALUE(rcvr, query(get_env(rcvr))); }; }; }