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cppdraft/exec/schedule/from.md
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cppdraft/exec/schedule/from.md
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[exec.schedule.from]
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# 33 Execution control library [[exec]](./#exec)
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## 33.9 Senders [[exec.snd]](exec.snd#exec.schedule.from)
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### 33.9.12 Sender adaptors [[exec.adapt]](exec.adapt#exec.schedule.from)
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#### 33.9.12.7 execution::schedule_from [exec.schedule.from]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3359)
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schedule_from schedules work dependent on the completion of a sender
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onto a scheduler's associated execution resource[.](#1.sentence-1)
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[*Note [1](#note-1)*:
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schedule_from is not meant to be used in user code;
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it is used in the implementation of continues_on[.](#1.sentence-2)
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â *end note*]
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3367)
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The name schedule_from denotes a customization point object[.](#2.sentence-1)
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For some subexpressions sch and sndr,
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let Sch be decltype((sch)) andSndr be decltype((sndr))[.](#2.sentence-2)
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If Sch does not satisfy [scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]"), orSndr does not satisfy [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"),schedule_from(sch, sndr) is ill-formed[.](#2.sentence-3)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3376)
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Otherwise,
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the expression schedule_from(sch, sndr) is expression-equivalent to:transform_sender(*query-with-default*(get_domain, sch, default_domain()), *make-sender*(schedule_from, sch, sndr)) except that sch is evaluated only once[.](#3.sentence-1)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3386)
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The exposition-only class template *impls-for* ([[exec.snd.expos]](exec.snd.expos "33.9.2 Exposition-only entities"))
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is specialized for schedule_from_t as follows:
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[ð](#lib:impls-for%3cschedule_from_t%3e)
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namespace std::execution {template<>struct *impls-for*<schedule_from_t> : *default-impls* {static constexpr auto *get-attrs* = *see below*; static constexpr auto *get-state* = *see below*; static constexpr auto *complete* = *see below*; template<class Sndr, class... Env>static consteval void *check-types*(); };}
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3404)
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The member *impls-for*<schedule_from_t>::*get-attrs* is initialized with a callable object equivalent to the following lambda:[](const auto& data, const auto& child) noexcept -> decltype(auto) {return *JOIN-ENV*(*SCHED-ATTRS*(data), *FWD-ENV*(get_env(child)));}
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3413)
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The member *impls-for*<schedule_from_t>::*get-state* is initialized with a callable object equivalent to the following lambda:[]<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(*see below*)requires [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<*child-type*<Sndr>, *FWD-ENV-T*(env_of_t<Rcvr>)> {auto& [_, sch, child] = sndr; using sched_t = decltype(auto(sch)); using variant_t = *see below*; using receiver_t = *see below*; using operation_t = connect_result_t<schedule_result_t<sched_t>, receiver_t>; constexpr bool nothrow = noexcept(connect(schedule(sch), receiver_t{nullptr})); struct *state-type* { Rcvr& *rcvr*; // *exposition only* variant_t *async-result*; // *exposition only* operation_t *op-state*; // *exposition only*explicit *state-type*(sched_t sch, Rcvr& rcvr) noexcept(nothrow): *rcvr*(rcvr), *op-state*(connect(schedule(sch), receiver_t{this})) {}}; return *state-type*{sch, rcvr};}
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[ð](#lib:check-types,impls-for%3cschedule_from_t%3e)
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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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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3448)
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*Effects*: Equivalent to:get_completion_signatures<schedule_result_t<*data-type*<Sndr>>, *FWD-ENV-T*(Env)...>();auto cs = get_completion_signatures<*child-type*<Sndr>, *FWD-ENV-T*(Env)...>();*decay-copyable-result-datums*(cs); // see [[exec.snd.expos]](exec.snd.expos "33.9.2 Exposition-only entities")
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3458)
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Objects of the local class *state-type* can be used
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to initialize a structured binding[.](#8.sentence-1)
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[9](#9)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3462)
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Let Sigs be
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a pack of the arguments to the completion_signatures specialization
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named by completion_signatures_of_t<*child-type*<Sndr>, *FWD-ENV-T*(env_of_t<Rcvr>)>[.](#9.sentence-1)
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Let *as-tuple* be an alias template such that*as-tuple*<Tag(Args...)> denotes
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the type *decayed-tuple*<Tag, Args...>, and
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let *is-nothrow-decay-copy-sig* be a variable template such thatauto(*is-nothrow-decay-copy-sig*<Tag(Args...
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)>) is
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a constant expression of type bool and
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equal to (is_nothrow_constructible_v<decay_t<Args>, Args> && ...)[.](#9.sentence-2)
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Let *error-completion* be a pack consisting of
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the type set_error_t(exception_ptr) if (*is-nothrow-decay-copy-sig*<Sigs> &&...) is false,
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and an empty pack otherwise[.](#9.sentence-3)
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Then variant_t denotes
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the type variant<monostate, *as-tuple*<Sigs>..., *error-completion*...>,
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except with duplicate types removed[.](#9.sentence-4)
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[10](#10)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3481)
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receiver_t is an alias for the following exposition-only class:namespace std::execution {struct *receiver-type* {using receiver_concept = receiver_t; *state-type** *state*; // *exposition only*void set_value() && noexcept { visit([this]<class Tuple>(Tuple& result) noexcept -> void {if constexpr (<monostate, Tuple>) {auto& [tag, ...args] = result;
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tag(std::move(*state*->*rcvr*), std::move(args)...); }}, *state*->*async-result*); }template<class Error>void set_error(Error&& err) && noexcept { execution::set_error(std::move(*state*->*rcvr*), std::forward<Error>(err)); }void set_stopped() && noexcept { execution::set_stopped(std::move(*state*->*rcvr*)); }decltype(auto) get_env() const noexcept {return *FWD-ENV*(execution::get_env(*state*->*rcvr*)); }};}
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[11](#11)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3516)
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The expression in the noexcept clause of the lambda is true if the construction of the returned *state-type* object
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is not potentially throwing;
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otherwise, false[.](#11.sentence-1)
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[12](#12)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3522)
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The member *impls-for*<schedule_from_t>::*complete* is initialized with a callable object equivalent to the following lambda:[]<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept-> void {using result_t = *decayed-tuple*<Tag, Args...>; constexpr bool nothrow = (is_nothrow_constructible_v<decay_t<Args>, Args> && ...); try { state.*async-result*.template emplace<result_t>(Tag(), std::forward<Args>(args)...); } catch (...) {if constexpr (!nothrow) state.*async-result*.template emplace<tuple<set_error_t,
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exception_ptr>>(set_error, current_exception()); } start(state.*op-state*);};
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[13](#13)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3542)
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Let out_sndr be a subexpression denoting
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a sender returned from schedule_from(sch, sndr) or one equal to such,
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and let OutSndr be the type decltype((out_sndr))[.](#13.sentence-1)
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Let out_rcvr be a subexpression denoting a receiver
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that has an environment of type Env such that [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<OutSndr, Env> is true[.](#13.sentence-2)
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Let op be an lvalue referring to the operation state
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that results from connecting out_sndr with out_rcvr[.](#13.sentence-3)
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Calling start(op) shall
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start sndr on the current execution agent and
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execute completion operations on out_rcvr on an execution agent of the execution resource associated with sch[.](#13.sentence-4)
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If scheduling onto sch fails,
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an error completion on out_rcvr shall be executed
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on an unspecified execution agent[.](#13.sentence-5)
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