tmp/tmp3km8cle8/{from.md → to.md}
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| 1 |
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#### `execution::then`, `execution::upon_error`, `execution::upon_stopped` <a id="exec.then">[[exec.then]]</a>
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`then` attaches an invocable as a continuation for an input sender’s
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value completion operation. `upon_error` and `upon_stopped` do the same
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for the error and stopped completion operations, respectively, sending
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the result of the invocable as a value completion.
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The names `then`, `upon_error`, and `upon_stopped` denote pipeable
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sender adaptor objects. Let the expression *`then-cpo`* be one of
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`then`, `upon_error`, or `upon_stopped`. For subexpressions `sndr` and
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`f`, if `decltype((sndr))` does not satisfy `sender`, or `decltype((f))`
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does not satisfy `movable-value`, `then-cpo(sndr, f) `is ill-formed.
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Otherwise, the expression `then-cpo(sndr, f)` is expression-equivalent
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to:
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``` cpp
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transform_sender(get-domain-early(sndr), make-sender(then-cpo, f, sndr))
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```
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except that `sndr` is evaluated only once.
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For `then`, `upon_error`, and `upon_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 *`then-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<then-cpo>> : default-impls {
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static constexpr auto complete =
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[]<class Tag, class... Args>
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(auto, auto& fn, auto& rcvr, Tag, Args&&... args) noexcept -> void {
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if constexpr (same_as<Tag, decayed-typeof<set-cpo>>) {
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TRY-SET-VALUE(rcvr,
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invoke(std::move(fn), std::forward<Args>(args)...));
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} else {
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Tag()(std::move(rcvr), std::forward<Args>(args)...);
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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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```
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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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*Effects:* Equivalent to:
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``` cpp
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auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>();
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auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {
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if constexpr (!invocable<remove_cvref_t<data-type<Sndr>>, Ts...>)
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throw unspecified-exception();
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};
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cs.for-each(overload-set{fn, [](auto){}});
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```
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where *`unspecified-exception`* is a type derived from `exception`.
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The expression `then-cpo(sndr, f)` has undefined behavior unless it
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returns a sender `out_sndr` that
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- invokes `f` or a copy of such with the value, error, or stopped result
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datums of `sndr` for `then`, `upon_error`, and `upon_stopped`,
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respectively, using the result value of `f` as `out_sndr`’s value
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completion, and
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- forwards all other completion operations unchanged.
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