[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)
