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cppdraft/exec/starts/on.md

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+[exec.starts.on]
+
+# 33 Execution control library [[exec]](./#exec)
+
+## 33.9 Senders [[exec.snd]](exec.snd#exec.starts.on)
+
+### 33.9.12 Sender adaptors [[exec.adapt]](exec.adapt#exec.starts.on)
+
+#### 33.9.12.5 execution​::​starts_on [exec.starts.on]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3224)
+
+starts_on adapts an input sender into a sender
+that will start on an execution agent belonging to
+a particular scheduler's associated execution resource[.](#1.sentence-1)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3229)
+
+The name starts_on denotes a customization point object[.](#2.sentence-1)
+
+For subexpressions sch and sndr,
+if decltype((  
+sch)) does not satisfy [scheduler](exec.sched#concept:scheduler "33.6 Schedulers [exec.sched]"), ordecltype((sndr)) does not satisfy [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"),starts_on(sch, sndr) is ill-formed[.](#2.sentence-2)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3236)
+
+Otherwise,
+the expression starts_on(sch, sndr) is expression-equivalent to:transform_sender(*query-with-default*(get_domain, sch, default_domain()), *make-sender*(starts_on, sch, sndr)) except that sch is evaluated only once[.](#3.sentence-1)
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3246)
+
+Let out_sndr and env be subexpressions
+such that OutSndr is decltype((out_sndr))[.](#4.sentence-1)
+
+If [*sender-for*](exec.snd.concepts#concept:sender-for "33.9.3 Sender concepts [exec.snd.concepts]")<OutSndr, starts_on_t> is false,
+then the expressions starts_on.transform_env(out_sndr, env) andstarts_on.transform_sender(out_sndr, env) are ill-formed; otherwise
+
+- [(4.1)](#4.1)
+
+starts_on.transform_env(out_sndr, env) is equivalent to:auto&& [_, sch, _] = out_sndr;return *JOIN-ENV*(*SCHED-ENV*(sch), *FWD-ENV*(env));
+
+- [(4.2)](#4.2)
+
+starts_on.transform_sender(out_sndr, env) is equivalent to:auto&& [_, sch, sndr] = out_sndr;return let_value( schedule(sch), [sndr = std::forward_like<OutSndr>(sndr)]() mutablenoexcept(is_nothrow_move_constructible_v<decay_t<OutSndr>>) {return std::move(sndr); });
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3272)
+
+Let out_sndr be a subexpression denoting
+a sender returned from starts_on(sch, sndr) or one equal to such, and
+let OutSndr be the type decltype((out_sndr))[.](#5.sentence-1)
+
+Let out_rcvr be a subexpression denoting a receiver
+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[.](#5.sentence-2)
+
+Let op be an lvalue referring to the operation state
+that results from connecting out_sndr with out_rcvr[.](#5.sentence-3)
+
+Calling start(op) shall start sndr on an execution agent of the associated execution resource of sch[.](#5.sentence-4)
+
+If scheduling onto sch fails,
+an error completion on out_rcvr shall be executed
+on an unspecified execution agent[.](#5.sentence-5)