tmp/tmpvpabj8gv/{from.md → to.md}
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| 1 |
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### Sender factories <a id="exec.factories">[[exec.factories]]</a>
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#### `execution::schedule` <a id="exec.schedule">[[exec.schedule]]</a>
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`schedule` obtains a schedule sender [[exec.async.ops]] from a
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scheduler.
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The name `schedule` denotes a customization point object. For a
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subexpression `sch`, the expression `schedule(sch)` is
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expression-equivalent to `sch.schedule()`.
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*Mandates:* The type of `sch.schedule()` satisfies `sender`.
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If the expression
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``` cpp
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get_completion_scheduler<set_value_t>(get_env(sch.schedule())) == sch
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```
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is ill-formed or evaluates to `false`, the behavior of calling
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`schedule(sch)` is undefined.
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#### `execution::just`, `execution::just_error`, `execution::just_stopped` <a id="exec.just">[[exec.just]]</a>
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`just`, `just_error`, and `just_stopped` are sender factories whose
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asynchronous operations complete synchronously in their start operation
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with a value completion operation, an error completion operation, or a
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stopped completion operation, respectively.
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The names `just`, `just_error`, and `just_stopped` denote customization
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point objects. Let *`just-cpo`* be one of `just`, `just_error`, or
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`just_stopped`. For a pack of subexpressions `ts`, let `Ts` be the pack
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of types `decltype((ts))`. The expression `just-cpo(ts...)` is
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ill-formed if
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- `(movable-value<Ts> &&...)` is `false`, or
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- *`just-cpo`* is `just_error` and `sizeof...(ts) == 1` is `false`, or
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- *`just-cpo`* is `just_stopped` and `sizeof...(ts) == 0` is `false`.
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Otherwise, it is expression-equivalent to
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`make-sender(just-cpo, product-type{ts...})`.
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For `just`, `just_error`, and `just_stopped`, let *`set-cpo`* be
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`set_value`, `set_error`, and `set_stopped`, respectively. The
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exposition-only class template *`impls-for`* [[exec.snd.expos]] is
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specialized for *`just-cpo`* as follows:
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``` cpp
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namespace std::execution {
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template<>
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struct impls-for<decayed-typeof<just-cpo>> : default-impls {
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static constexpr auto start =
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[](auto& state, auto& rcvr) noexcept -> void {
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auto& [...ts] = state;
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set-cpo(std::move(rcvr), std::move(ts)...);
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};
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};
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}
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```
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#### `execution::read_env` <a id="exec.read.env">[[exec.read.env]]</a>
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`read_env` is a sender factory for a sender whose asynchronous operation
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completes synchronously in its start operation with a value completion
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result equal to a value read from the receiver’s associated environment.
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`read_env` is a customization point object. For some query object `q`,
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the expression `read_env(q)` is expression-equivalent to
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`make-sender(read_env, q)`.
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The exposition-only class template *`impls-for`* [[exec.snd.expos]] is
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specialized for `read_env` as follows:
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``` cpp
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namespace std::execution {
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template<>
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struct impls-for<decayed-typeof<read_env>> : default-impls {
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static constexpr auto start =
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[](auto query, auto& rcvr) noexcept -> void {
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TRY-SET-VALUE(rcvr, query(get_env(rcvr)));
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};
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};
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template<class Sndr, class Env>
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static consteval void check-types();
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}
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```
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``` cpp
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template<class Sndr, class Env>
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static consteval void check-types();
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```
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Let `Q` be `decay_t<`*`data-type`*`<Sndr>>`.
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*Throws:* An exception of an unspecified type derived from `exception`
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if the expression `Q()(env)` is ill-formed or has type `void`, where
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`env` is an lvalue subexpression whose type is `Env`.
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