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Merge pull request #161 from cplusplus/port-transfer-to-basic-sender
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port the `transfer` algorithm to `basic-sender`
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@ericniebler
ericniebler authored Dec 13, 2023
2 parents d42af37 + 07da295 commit 37b940f
Showing 1 changed file with 47 additions and 41 deletions.
88 changes: 47 additions & 41 deletions execution.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -4883,7 +4883,19 @@ enum class forward_progress_guarantee {

- otherwise, `tag_invoke(q, e3, as...)` is ill-formed.

4. For two subexpressions `r` and `e`, let <code><i>SET-VALUE</i>(r,
4. For a scheduler `sch`, let <code><i>SCHED-ATTRS</i>(sch)</code> be a
queryable object `o1` such that
<code>tag_invoke(get_completion_scheduler&lt;<i>Tag</i>>, o1)</code> is a
prvalue with the same type and value as `sch` where <i>`Tag`</i> is one
of `set_value_t` or `set_stopped_t`; and let
<code>tag_invoke(get_domain, o1)</code> be expression-equivalent to
<code>tag_invoke(get_domain, sch)</code>. Let
<code><i>SCHED-ENV</i>(sch)</code> be a queryable object `o2` such that
<code>tag_invoke(get_scheduler, o2)</code> is a prvalue with the same
type and value as `sch`, and let <code>tag_invoke(get_domain, o2)</code>
be expression-equivalent to <code>tag_invoke(get_domain, sch)</code>.

5. For two subexpressions `r` and `e`, let <code><i>SET-VALUE</i>(r,
e)</code> be `(e, set_value(r))` if the type of `e` is `void`;
otherwise, it is `set_value(r, e)`. Let <code><i>TRY-SET-VALUE</i>(r,
e)</code> be:
Expand All @@ -4899,7 +4911,7 @@ enum class forward_progress_guarantee {
if `e` is potentially-throwing, except that `r` is evaluated only once;
or <code><i>SET-VALUE</i>(r, e)</code> otherwise.

5. <pre highlight="c++">
6. <pre highlight="c++">
template&lt;class Default = default_domain, class Sender>
constexpr auto <i>completion-domain</i>(const Sender& sndr) noexcept;
</pre>
Expand All @@ -4917,7 +4929,7 @@ enum class forward_progress_guarantee {
prvalue of type `Default`.
Otherwise, <code><i>completion-domain</i>&lt;Default>(sndr)</code> is ill-formed.

6. <pre highlight="c++">
7. <pre highlight="c++">
template&lt;class Tag, class Env, class Default>
constexpr decltype(auto) <i>query-with-default</i>(Tag, const Env& env, Default&& value) noexcept(<i>see below</i>);
</pre>
Expand All @@ -4935,7 +4947,7 @@ enum class forward_progress_guarantee {
: is_nothrow_constructible_v&lt;Default, Default>
</pre>

7. <pre highlight="c++">
8. <pre highlight="c++">
template&lt;class Sender>
constexpr auto <i>get-domain-early</i>(const Sender& sndr) noexcept;
</pre>
Expand All @@ -4948,7 +4960,7 @@ enum class forward_progress_guarantee {

- `return default_domain();`

8. <pre highlight="c++">
9. <pre highlight="c++">
template&lt;class Sender, class Env>
constexpr auto <i>get-domain-late</i>(const Sender& sndr, const Env& env) noexcept;
</pre>
Expand All @@ -4975,7 +4987,7 @@ enum class forward_progress_guarantee {
its predecessor, using only its destination scheduler to select a customization. *--end note*]


9. <pre highlight="c++">
10. <pre highlight="c++">
template&lt;class... T>
struct <i>product-type</i> {
T<sub><i>0</i></sub> <i>t<sub>0</sub></i>; // exposition only
Expand All @@ -4988,7 +5000,7 @@ enum class forward_progress_guarantee {
- <span class="wg21note">An expression of type <code><i>product-type</i></code> is usable as the initializer
of a structured binding declaration [dcl.struct.bind].</span>

10. <pre highlight="c++">
11. <pre highlight="c++">
template &lt;semiregular Tag, <i>movable-value</i> Data, sender... Child>
constexpr auto <i>make-sender</i>(Tag, Data&& data, Child&&... child);
</pre>
Expand Down Expand Up @@ -5908,15 +5920,7 @@ template&lt;class Domain, class Tag, sender Sender, class... Args>
1. `on` adapts an input sender into a sender that will start on an execution
agent belonging to a particular scheduler's associated execution resource.

2. Let <code><i>replace-scheduler</i>(e, sch)</code> be an expression denoting
an object `e2` such that `get_scheduler(e2)` returns a copy of `sch`,
`get_domain(e2)` is expression-equivalent to `get_domain(sch)`, and
`tag_invoke(tag, e2, args...)` is expression-equivalent to `tag(e, args...)`
for all arguments `args...` and for all `tag` whose type satisfies
<code><i>forwarding-query</i></code> and is not `get_scheduler_t` or
`get_domain_t`.

3. The name `on` denotes a customization point object. For some subexpressions
2. The name `on` denotes a customization point object. For some subexpressions
`sch` and `s`, let `Sch` be `decltype((sch))` and `S` be `decltype((s))`. If
`Sch` does not satisfy `scheduler`, or `S` does not satisfy `sender`,
`on(sch, s)` is ill-formed. Otherwise, the expression `on(sch, s)` is
Expand All @@ -5928,7 +5932,7 @@ template&lt;class Domain, class Tag, sender Sender, class... Args>
<i>make-sender</i>(on, sch, s));
</pre>

4. Let `out_s` and `e` be subexpressions such that `OutS` is `decltype((out_s))`. If
3. Let `out_s` and `e` be subexpressions such that `OutS` is `decltype((out_s))`. If
<code><i>sender-for</i>&lt;OutS, on_t></code> is `false`, then the expressions
`on_t().transform_env(out_s, e)` and `on_t().transform_sender(out_s, e)` are ill-formed;
otherwise:
Expand All @@ -5937,7 +5941,7 @@ template&lt;class Domain, class Tag, sender Sender, class... Args>

<pre highlight="c++">
auto&& [ign1, sch, ign2] = out_s;
return <i>replace-scheduler</i>(e, sch);
return <i>JOIN-ENV</i>(<i>SCHED-ENV</i>(sch), <i>FWD-ENV</i>(e));
</pre>

- `on_t().transform_sender(out_s, e)` is equivalent to:
Expand All @@ -5951,13 +5955,14 @@ template&lt;class Domain, class Tag, sender Sender, class... Args>
});
</pre>

5. Let `out_s` be a subexpression denoting a sender returned from `on(sch, s)`
4. Let `out_s` be a subexpression denoting a sender returned from `on(sch, s)`
or one equal to such, and let `OutS` be the type `decltype((out_s))`. Let
`out_r` be a subexpression denoting a receiver that has an environment of
type `E` such that `sender_in<OutS, E>` is `true`. Let `op` be an lvalue
referring to the operation state that results from connecting `out_s` with
`out_r`. Calling `start(op)` shall start `s` on an execution agent of
the associated execution resource of `sch`.
`out_r`. Calling `start(op)` shall start `s` on an execution agent of the
associated execution resource of `sch`, or failing that, shall execute an
error completion on `out_r`.

#### `execution::transfer` <b>[exec.transfer]</b> #### {#spec-execution.senders.adapt.transfer}

Expand All @@ -5977,34 +5982,25 @@ template&lt;class Domain, class Tag, sender Sender, class... Args>
<i>make-sender</i>(transfer, sch, s));
</pre>

If a sender `S` returned from `transfer(s, sch)` is connected with a
receiver `R` with environment `E` such that
<code>transform_sender(<i>get-domain-late</i>(S, E), S, E)</code> does not
return a sender that is a result of a call to
<code>transform_sender(<i>get-domain-late</i>(S, E), schedule_from(sch, s2), E)</code>,
where `s2` is a sender that sends values equal to those sent by `s`,
the behavior of calling `connect(S, R)` is undefined.
3. The exposition-only class template <code><i>impls-for</i></code> is specialized
for `transform_t` as follows:

3. For a sender `t` returned from `transfer(s, sch)`, `get_env(t)` shall return
a queryable object `q` such that `get_domain(q)` is expression-equivalent to
`get_domain(sch)` and `get_completion_scheduler<CPO>(q)` returns a copy of
`sch`, where `CPO` is either `set_value_t` or `set_stopped_t`. <span
class="wg21note">The `get_completion_scheduler<set_error_t>` query is not
implemented, as the scheduler cannot be guaranteed in case an error is
thrown while trying to schedule work on the given scheduler object.</span>
For all other query objects <code><i>Q</i></code> whose type satisfies
<code><i>forwarding-query</i></code>, the expression <code><i>Q</i>(q,
args...)</code> shall be equivalent to <code><i>Q</i>(get_env(s),
args...)</code>.
<pre highlight="c++">
template&lt;>
struct <i>impls-for</i>&lt;transform_t> : <i>default-impls</i> {
static constexpr auto get_attrs =
[](const auto& data, const auto& child) noexcept -> decltype(auto) {
return <i>JOIN-ENV</i>(<i>SCHED-ATTRS</i>(data), <i>FWD-ENV</i>(get_env(child)));
};
};
</pre>

4. Let `s` and `e` be subexpressions such that `S` is `decltype((s))`. If
<code><i>sender-for</i>&lt;S, transfer_t></code> is `false`, then the expression
`transfer_t().transform_sender(s, e)` is ill-formed; otherwise, it
is equal to:

<pre highlight="c++">
const auto& env = e;
auto domain = <i>get-domain-late</i>(s, env);
auto [tag, data, child] = s;
return schedule_from(std::move(data), std::move(child));
</pre>
Expand All @@ -6013,6 +6009,16 @@ template&lt;class Domain, class Tag, sender Sender, class... Args>
`schedule_from(sch, s)` when it is connected with a receiver with an
execution domain that does not customize `transfer`.</span>

4. Let `out_s` be a subexpression denoting a sender returned from `transfer(s, sch)`
or one equal to such, and let `OutS` be the type `decltype((out_s))`. Let
`out_r` be a subexpression denoting a receiver that has an environment of
type `E` such that `sender_in<OutS, E>` is `true`. Let `op` be an lvalue
referring to the operation state that results from connecting `out_s` with
`out_r`. Calling `start(op)` shall start `s` on the current execution agent
and execute completion operations on `out_r` on an execution agent of the
associated execution resource of `sch`; or failing that, execute an error
completion on `out_r`.

#### `execution::schedule_from` <b>[exec.schedule.from]</b> #### {#spec-execution.senders.adaptors.schedule_from}

1. `schedule_from` schedules work dependent on the completion of a sender onto a
Expand Down

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