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cppdraft/exec/sched.md

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+[exec.sched]
+
+# 33 Execution control library [[exec]](./#exec)
+
+## 33.6 Schedulers [exec.sched]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L1099)
+
+The [scheduler](#concept:scheduler "33.6 Schedulers [exec.sched]") concept defines
+the requirements of a scheduler type ([[exec.async.ops]](exec.async.ops "33.3 Asynchronous operations"))[.](#1.sentence-1)
+
+schedule is a customization point object
+that accepts a scheduler[.](#1.sentence-2)
+
+A valid invocation of schedule is a schedule-expression[.](#1.sentence-3)
+
+namespace std::execution {template<class Sch>concept [scheduler](#concept:scheduler "33.6 Schedulers [exec.sched]") =[derived_from](concept.derived#concept:derived_from "18.4.3 Concept derived_­from [concept.derived]")<typename remove_cvref_t<Sch>::scheduler_concept, scheduler_t> &&[*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]")<Sch> &&requires(Sch&& sch) {{ schedule(std::forward<Sch>(sch)) } -> [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]"); { auto(get_completion_scheduler<set_value_t>( get_env(schedule(std::forward<Sch>(sch))))) }-> [same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<remove_cvref_t<Sch>>; } &&[equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<remove_cvref_t<Sch>> &&[copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]")<remove_cvref_t<Sch>>;}
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L1122)
+
+Let Sch be the type of a scheduler and
+let Env be the type of an execution environment
+for which [sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<schedule_result_t<Sch>, Env> is satisfied[.](#2.sentence-1)
+
+Then [*sender-in-of*](exec.snd.concepts#concept:sender-in-of "33.9.3 Sender concepts [exec.snd.concepts]")<schedule_result_t<Sch>, Env> shall be modeled[.](#2.sentence-2)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L1130)
+
+No operation required by[copyable](concepts.object#concept:copyable "18.6 Object concepts [concepts.object]")<remove_cvref_t<Sch>> and[equality_comparable](concept.equalitycomparable#concept:equality_comparable "18.5.4 Concept equality_­comparable [concept.equalitycomparable]")<remove_cvref_t<Sch>> shall exit via an exception[.](#3.sentence-1)
+
+None of these operations,
+nor a scheduler type's schedule function,
+shall introduce data races
+as a result of potentially concurrent ([[intro.races]](intro.races "6.10.2.2 Data races")) invocations
+of those operations from different threads[.](#3.sentence-2)
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L1141)
+
+For any two values sch1 and sch2 of some scheduler type Sch,sch1 == sch2 shall return true only if both sch1 and sch2 share
+the same associated execution resource[.](#4.sentence-1)
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L1148)
+
+For a given scheduler expression sch,
+the expressionget_completion_scheduler<set_value_t>(get_env(schedule(sch))) shall compare equal to sch[.](#5.sentence-1)
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L1154)
+
+For a given scheduler expression sch,
+if the expression get_domain(sch) is well-formed,
+then the expression get_domain(get_env(schedule(sch))) is also well-formed and has the same type[.](#6.sentence-1)
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L1160)
+
+A scheduler type's destructor shall not block
+pending completion of any receivers
+connected to the sender objects returned from schedule[.](#7.sentence-1)
+
+[*Note [1](#note-1)*:
+
+The ability to wait for completion of submitted function objects
+can be provided by the associated execution resource of the scheduler[.](#7.sentence-2)
+
+— *end note*]