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+[exec.let]
+
+# 33 Execution control library [[exec]](./#exec)
+
+## 33.9 Senders [[exec.snd]](exec.snd#exec.let)
+
+### 33.9.12 Sender adaptors [[exec.adapt]](exec.adapt#exec.let)
+
+#### 33.9.12.10 execution::let_value, execution::let_error, execution::let_stopped [exec.let]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3847)
+
+let_value, let_error, and let_stopped transform
+a sender's value, error, and stopped completions, respectively,
+into a new child asynchronous operation
+by passing the sender's result datums to a user-specified callable,
+which returns a new sender that is connected and started[.](#1.sentence-1)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3854)
+
+For let_value, let_error, and let_stopped,
+let *set-cpo* beset_value, set_error, and set_stopped, respectively[.](#2.sentence-1)
+
+Let the expression *let-cpo* be one oflet_value, let_error, or let_stopped[.](#2.sentence-2)
+
+For a subexpression sndr,
+let *let-env*(sndr) be expression-equivalent to
+the first well-formed expression below:
+
+- [(2.1)](#2.1)
+
+*SCHED-ENV*(get_completion_scheduler<*decayed-typeof*<*set-cpo*>>(get_env(sndr)))
+
+- [(2.2)](#2.2)
+
+*MAKE-ENV*(get_domain, get_domain(get_env(sndr)))
+
+- [(2.3)](#2.3)
+
+(void(sndr), env<>{})
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3872)
+
+The names let_value, let_error, and let_stopped denote
+pipeable sender adaptor objects[.](#3.sentence-1)
+
+For subexpressions sndr and f,
+let F be the decayed type of f[.](#3.sentence-2)
+
+If decltype((sndr)) does not satisfy [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") or
+if decltype((f)) does not satisfy [*movable-value*](exec.general#concept:movable-value "33.1 General [exec.general]"),
+the expression *let-cpo*(sndr, f) is ill-formed[.](#3.sentence-3)
+
+If F does not satisfy [invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]"),
+the expression let_stopped(sndr, f) is ill-formed[.](#3.sentence-4)
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3883)
+
+Otherwise,
+the expression *let-cpo*(sndr, f) is expression-equivalent to:transform_sender(*get-domain-early*(sndr), *make-sender*(*let-cpo*, f, sndr)) except that sndr is evaluated only once[.](#4.sentence-1)
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3891)
+
+The exposition-only class template *impls-for* ([[exec.snd.expos]](exec.snd.expos "33.9.2 Exposition-only entities"))
+is specialized for *let-cpo* as follows:
+
+[ð](#lib:impls-for%3cdecayed-typeof%3clet-cpo%3e%3e)
+
+namespace std::execution {template<class State, class Rcvr, class... Args>void *let-bind*(State& state, Rcvr& rcvr, Args&&... args); // *exposition only*template<>struct *impls-for*<*decayed-typeof*<*let-cpo*>> : *default-impls* {static constexpr auto *get-state* = *see below*; static constexpr auto *complete* = *see below*; template<class Sndr, class... Env>static consteval void *check-types*(); };}
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3911)
+
+Let *receiver2* denote the following exposition-only class template:namespace std::execution {template<class Rcvr, class Env>struct *receiver2* {using receiver_concept = receiver_t; template<class... Args>void set_value(Args&&... args) && noexcept { execution::set_value(std::move(*rcvr*), std::forward<Args>(args)...); }template<class Error>void set_error(Error&& err) && noexcept { execution::set_error(std::move(*rcvr*), std::forward<Error>(err)); }void set_stopped() && noexcept { execution::set_stopped(std::move(*rcvr*)); }decltype(auto) get_env() const noexcept {return *see below*; } Rcvr& *rcvr*; // *exposition only* Env *env*; // *exposition only*};}
+
+Invocation of the function *receiver2*::get_env returns an object e such that
+
+- [(6.1)](#6.1)
+
+decltype(e) models [*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]") and
+
+- [(6.2)](#6.2)
+
+given a query object q,
+the expression e.query(q) is expression-equivalent
+to *env*.query(q) if that expression is valid;
+otherwise,
+if the type of q satisfies [*forwarding-query*](execution.syn#concept:forwarding-query "33.4 Header <execution> synopsis [execution.syn]"),e.query(q) is expression-equivalent
+to get_env(*rcvr*).query(q);
+otherwise,e.query(q) is ill-formed[.](#6.sentence-2)
+
+[ð](#lib:check-types,impls-for%3cdecayed-typeof%3clet-cpo%3e%3e)
+
+`template<class Sndr, class... Env>
+  static consteval void check-types();
+`
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3966)
+
+*Effects*: Equivalent to:using LetFn = remove_cvref_t<*data-type*<Sndr>>;auto cs = get_completion_signatures<*child-type*<Sndr>, *FWD-ENV-T*(Env)...>();auto fn = []<class... Ts>(*decayed-typeof*<*set-cpo*>(*)(Ts...)) {if constexpr (!*is-valid-let-sender*) // *see below*throw *unspecified-exception*();};
+cs.*for-each*(*overload-set*(fn, [](auto){})); where *unspecified-exception* is
+a type derived from exception, and
+where *is-valid-let-sender* is true if and only if
+all of the following are true:
+
+- [(7.1)](#7.1)
+
+([constructible_from](concept.constructible#concept:constructible_from "18.4.11 Concept constructible_from [concept.constructible]")<decay_t<Ts>, Ts> &&...)
+
+- [(7.2)](#7.2)
+
+[invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]")<LetFn, decay_t<Ts>&...>
+
+- [(7.3)](#7.3)
+
+[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]")<invoke_result_t<LetFn, decay_t<Ts>&...>>
+
+- [(7.4)](#7.4)
+
+sizeof...(Env) == 0 || [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<invoke_result_t<LetFn, decay_t<Ts>&...>, *env-t*  
+...>
+
+where *env-t* is the packdecltype(*let-cpo*.transform_env(declval<Sndr>(), declval<Env>()))[.](#7.sentence-1)
+
+[8](#8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L3993)
+
+*impls-for*<*decayed-typeof*<*let-cpo*>>::*get-state* is initialized with a callable object equivalent to the following:[]<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) requires *see below* {auto& [_, fn, child] = sndr; using fn_t = decay_t<decltype(fn)>; using env_t = decltype(*let-env*(child)); using args_variant_t = *see below*; using ops2_variant_t = *see below*; struct *state-type* { fn_t *fn*; // *exposition only* env_t *env*; // *exposition only* args_variant_t *args*; // *exposition only* ops2_variant_t *ops2*; // *exposition only*}; return *state-type*{*allocator-aware-forward*(std::forward_like<Sndr>(fn), rcvr), *let-env*(child), {}, {}};}
+
+[9](#9)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4015)
+
+Let Sigs be a pack of the arguments
+to the completion_signatures specialization named bycompletion_signatures_of_t<*child-type*<Sndr>, *FWD-ENV-T*(env_of_t<Rcvr>)>[.](#9.sentence-1)
+
+Let LetSigs be a pack of those types in Sigs with a return type of *decayed-typeof*<*set-cpo*>[.](#9.sentence-2)
+
+Let *as-tuple* be an alias template
+such that *as-tuple*<Tag(Args...)> denotes
+the type *decayed-tuple*<Args...>[.](#9.sentence-3)
+
+Then args_variant_t denotes
+the type variant<monostate, *as-tuple*<LetSigs>...> except with duplicate types removed[.](#9.sentence-4)
+
+[10](#10)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4028)
+
+Given a type Tag and a pack Args,
+let *as-sndr2* be an alias template
+such that *as-sndr2*<Tag(Args...)> denotes
+the type *call-result-t*<F, decay_t<Args>&...>[.](#10.sentence-1)
+
+Then ops2_variant_t denotes
+the typevariant<monostate, connect_result_t<*as-sndr2*<LetSigs>, *receiver2*<Rcvr, env_t>>...> except with duplicate types removed[.](#10.sentence-2)
+
+[11](#11)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4040)
+
+The [*requires-clause*](temp.pre#nt:requires-clause "13.1 Preamble [temp.pre]") constraining the above lambda is satisfied
+if and only if
+the types args_variant_t and ops2_variant_t are well-formed[.](#11.sentence-1)
+
+[12](#12)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4045)
+
+The exposition-only function template *let-bind* has effects equivalent to:using args_t = *decayed-tuple*<Args...>;auto mkop2 = [&] {return connect( apply(std::move(state.fn),
+ state.args.template emplace<args_t>(std::forward<Args>(args)...)), *receiver2*{rcvr, std::move(state.env)});};
+start(state.ops2.template emplace<decltype(mkop2())>(*emplace-from*{mkop2}));
+
+[13](#13)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4059)
+
+*impls-for*<*decayed-typeof*<let-cpo>>::*complete* is initialized with a callable object equivalent to the following:[]<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept -> void {if constexpr ([same_as](concept.same#concept:same_as "18.4.2 Concept same_as [concept.same]")<Tag, *decayed-typeof*<*set-cpo*>>) {*TRY-EVAL*(rcvr, *let-bind*(state, rcvr, std::forward<Args>(args)...)); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); }}
+
+[14](#14)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4073)
+
+Let sndr and env be subexpressions, and
+let Sndr be decltype((sndr))[.](#14.sentence-1)
+
+If[*sender-for*](exec.snd.concepts#concept:sender-for "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, *decayed-typeof*<*let-cpo*>> is false,
+then the expression *let-cpo*.transform_env(sndr, env) is ill-formed[.](#14.sentence-2)
+
+Otherwise, it is equal to:auto& [_, _, child] = sndr;return *JOIN-ENV*(*let-env*(child), *FWD-ENV*(env));
+
+[15](#15)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L4087)
+
+Let the subexpression out_sndr denote
+the result of the invocation *let-cpo*(sndr, f) or
+an object equal to such, and
+let the subexpression rcvr denote a receiver
+such that the expression connect(out_sndr, rcvr) is well-formed[.](#15.sentence-1)
+
+The expression connect(out_sndr, rcvr) has undefined behavior
+unless it creates an asynchronous operation ([[exec.async.ops]](exec.async.ops "33.3 Asynchronous operations")) that,
+when started:
+
+- [(15.1)](#15.1)
+
+invokes f when *set-cpo* is called
+with sndr's result datums,
+
+- [(15.2)](#15.2)
+
+makes its completion dependent on
+the completion of a sender returned by f, and
+
+- [(15.3)](#15.3)
+
+propagates the other completion operations sent by sndr[.](#15.sentence-2)