[exec.stopped.opt]

# 33 Execution control library [[exec]](./#exec)

## 33.9 Senders [[exec.snd]](exec.snd#exec.stopped.opt)

### 33.9.12 Sender adaptors [[exec.adapt]](exec.adapt#exec.stopped.opt)

#### 33.9.12.14 execution​::​stopped_as_optional [exec.stopped.opt]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4789)

stopped_as_optional maps a sender's stopped completion operation
into a value completion operation as a disengaged optional[.](#1.sentence-1)

The sender's value completion operation
is also converted into an optional[.](#1.sentence-2)

The result is a sender that never completes with stopped,
reporting cancellation by completing with a disengaged optional[.](#1.sentence-3)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4797)

The name stopped_as_optional denotes a pipeable sender adaptor object[.](#2.sentence-1)

For a subexpression sndr, let Sndr be decltype((sndr))[.](#2.sentence-2)

The expression stopped_as_optional(sndr) is expression-equivalent to:transform_sender(*get-domain-early*(sndr), *make-sender*(stopped_as_optional, {}, sndr)) except that sndr is only evaluated once[.](#2.sentence-3)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4806)

The exposition-only class template *impls-for* ([[exec.snd.expos]](exec.snd.expos "33.9.2 Exposition-only entities"))
is specialized for stopped_as_optional_t as follows:

[ð](#lib:impls-for%3cstopped_as_optional_t%3e)

namespace std::execution {template<>struct *impls-for*<stopped_as_optional_t> : *default-impls* {template<class Sndr, class... Env>static consteval void *check-types*() {*default-impls*::*check-types*<Sndr, Env...>(); if constexpr (!requires {requires (![same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<void, *single-sender-value-type*<*child-type*<Sndr>, *FWD-ENV-T*(Env)...>>); })throw *unspecified-exception*(); }};} where *unspecified-exception* is
a type derived from exception[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4829)

Let sndr and env be subexpressions
such that Sndr is decltype((sndr)) andEnv is decltype((env))[.](#4.sentence-1)

If [*sender-for*](exec.snd.concepts#concept:sender-for "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, stopped_as_optional_t> is false then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed;
otherwise,
if [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<*child-type*<Sndr>, *FWD-ENV-T*(Env)> is false,
the expression stopped_as_optional.transform_sender(sndr, env) is equivalent to *not-a-sender*();
otherwise, it is equivalent to:auto&& [_, _, child] = sndr;using V = *single-sender-value-type*<*child-type*<Sndr>, *FWD-ENV-T*(Env)>;return let_stopped( then(std::forward_like<Sndr>(child), []<class... Ts>(Ts&&... ts) noexcept(is_nothrow_constructible_v<V, Ts...>) {return optional<V>(in_place, std::forward<Ts>(ts)...); }), []() noexcept { return just(optional<V>()); });
