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cppdraft/exec/on.md
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cppdraft/exec/on.md
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[exec.on]
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# 33 Execution control library [[exec]](./#exec)
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## 33.9 Senders [[exec.snd]](exec.snd#exec.on)
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### 33.9.12 Sender adaptors [[exec.adapt]](exec.adapt#exec.on)
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#### 33.9.12.8 execution::on [exec.on]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3561)
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The on sender adaptor has two forms:
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- [(1.1)](#1.1)
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on(sch, sndr),
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which starts a sender sndr on an execution agent
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belonging to a scheduler sch's associated execution resource and
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that, upon sndr's completion,
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transfers execution back to the execution resource
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on which the on sender was started[.](#1.1.sentence-1)
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- [(1.2)](#1.2)
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on(sndr, sch, closure),
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which upon completion of a sender sndr,
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transfers execution to an execution agent
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belonging to a scheduler sch's associated execution resource,
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then executes a sender adaptor closure closure with the async results of the sender, and
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that then transfers execution back to the execution resource
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on which sndr completed[.](#1.2.sentence-1)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3582)
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The name on denotes a pipeable sender adaptor object[.](#2.sentence-1)
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For subexpressions sch and sndr,on(sch, sndr) is ill-formed if any of the following is true:
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- [(2.1)](#2.1)
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decltype((sch)) does not satisfy [scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]"), or
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- [(2.2)](#2.2)
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decltype((sndr)) does not satisfy [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") andsndr is not
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a pipeable sender adaptor closure object ([[exec.adapt.obj]](exec.adapt.obj "33.9.12.2 Closure objects")), or
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- [(2.3)](#2.3)
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decltype((sndr)) satisfies [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") andsndr is also a pipeable sender adaptor closure object[.](#2.sentence-2)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3598)
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Otherwise, if decltype((sndr)) satisfies [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"),
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the expression on(sch, sndr) is expression-equivalent to:transform_sender(*query-with-default*(get_domain, sch, default_domain()), *make-sender*(on, 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#L3608)
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For subexpressions sndr, sch, and closure, if
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- [(4.1)](#4.1)
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decltype((sch)) does not satisfy [scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]"), or
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- [(4.2)](#4.2)
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decltype((sndr)) does not satisfy [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"), or
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- [(4.3)](#4.3)
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closure is not a pipeable sender adaptor closure object ([[exec.adapt.obj]](exec.adapt.obj "33.9.12.2 Closure objects")),
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the expression on(sndr, sch, closure) is ill-formed;
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otherwise, it is expression-equivalent to:transform_sender(*get-domain-early*(sndr), *make-sender*(on, *product-type*{sch, closure}, sndr)) except that sndr is evaluated only once[.](#4.sentence-1)
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3627)
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Let out_sndr and env be subexpressions,
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let OutSndr be decltype((out_sndr)), and
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let Env be decltype((env))[.](#5.sentence-1)
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If [*sender-for*](exec.snd.concepts#concept:sender-for "33.9.3 Sender concepts [exec.snd.concepts]")<OutSndr, on_t> is false,
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then the expressions on.transform_env(out_sndr, env) andon.transform_sender(out_sndr, env) are ill-formed[.](#5.sentence-2)
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3635)
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Otherwise:
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Let *not-a-scheduler* be an unspecified empty class type[.](#6.sentence-1)
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3639)
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The expression on.transform_env(out_sndr, env) has effects equivalent to:auto&& [_, data, _] = out_sndr;if constexpr ([scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]")<decltype(data)>) {return *JOIN-ENV*(*SCHED-ENV*(std::forward_like<OutSndr>(data)), *FWD-ENV*(std::forward<Env>(env)));} else {return std::forward<Env>(env);}
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3651)
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The expression on.transform_sender(out_sndr, env) has effects equivalent to:auto&& [_, data, child] = out_sndr;if constexpr ([scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]")<decltype(data)>) {auto orig_sch =*query-with-default*(get_scheduler, env, *not-a-scheduler*()); if constexpr ([same_as](concept.same#concept:same_as "18.4.2 Concept same_as [concept.same]")<decltype(orig_sch), *not-a-scheduler*>) {return *not-a-sender*{}; } else {return continues_on( starts_on(std::forward_like<OutSndr>(data), std::forward_like<OutSndr>(child)),
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std::move(orig_sch)); }} else {auto& [sch, closure] = data; auto orig_sch = *query-with-default*( get_completion_scheduler<set_value_t>,
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get_env(child), *query-with-default*(get_scheduler, env, *not-a-scheduler*())); if constexpr ([same_as](concept.same#concept:same_as "18.4.2 Concept same_as [concept.same]")<decltype(orig_sch), *not-a-scheduler*>) {return *not-a-sender*{}; } else {return write_env( continues_on( std::forward_like<OutSndr>(closure)( continues_on( write_env(std::forward_like<OutSndr>(child), *SCHED-ENV*(orig_sch)),
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sch)),
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orig_sch), *SCHED-ENV*(sch)); }}
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[9](#9)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3689)
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Let out_sndr be a subexpression denoting
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a sender returned from on(sch, sndr) or one equal to such, and
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let OutSndr be the type decltype((out_sndr))[.](#9.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[.](#9.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[.](#9.sentence-3)
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Calling start(op) shall
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- [(9.1)](#9.1)
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remember the current scheduler, get_scheduler(get_env(rcvr));
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- [(9.2)](#9.2)
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start sndr on an execution agent belonging tosch's associated execution resource;
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- [(9.3)](#9.3)
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upon sndr's completion,
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transfer execution back to the execution resource
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associated with the scheduler remembered in step 1; and
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- [(9.4)](#9.4)
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forward sndr's async result to out_rcvr[.](#9.sentence-4)
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If any scheduling operation fails,
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an error completion on out_rcvr shall be executed
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on an unspecified execution agent[.](#9.sentence-5)
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[10](#10)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3716)
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Let out_sndr be a subexpression denoting
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a sender returned from on(sndr, sch, closure) or one equal to such, and
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let OutSndr be the type decltype((out_sndr))[.](#10.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[.](#10.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[.](#10.sentence-3)
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Calling start(op) shall
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- [(10.1)](#10.1)
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remember the current scheduler,
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which is the first of the following expressions that is well-formed:
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* [(10.1.1)](#10.1.1)
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get_completion_scheduler<set_value_t>(get_env(sndr))
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* [(10.1.2)](#10.1.2)
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get_scheduler(get_env(rcvr));
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- [(10.2)](#10.2)
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start sndr on the current execution agent;
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- [(10.3)](#10.3)
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upon sndr's completion,
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transfer execution to an agent
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owned by sch's associated execution resource;
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- [(10.4)](#10.4)
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forward sndr's async result as if by
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connecting and starting a sender closure(S),
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where S is a sender
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that completes synchronously with sndr's async result; and
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- [(10.5)](#10.5)
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upon completion of the operation started in the previous step,
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transfer execution back to the execution resource
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associated with the scheduler remembered in step 1 and
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forward the operation's async result to out_rcvr[.](#10.sentence-4)
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If any scheduling operation fails,
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an error completion on out_rcvr shall be executed on
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an unspecified execution agent[.](#10.sentence-5)
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