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  <title>Rename `join` and `nest` in `async_scope` proposal</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Rename
<code class="sourceCode cpp">join</code> and
<code class="sourceCode cpp">nest</code> in
<code class="sourceCode cpp">async_scope</code> proposal</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>
      P3706R0
      [<a href="https://wg21.link/P3706">Latest</a>]
      [<a href="https://wg21.link/P3706/status">Status</a>]
    </td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2025-05-19</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Ruslan Arutyunyan<br>&lt;<a href="mailto:ruslan.arutyunyan@intel.com" class="email">ruslan.arutyunyan@intel.com</a>&gt;<br>
      Ville Voutilainen<br>&lt;<a href="mailto:ville.voutilainen@gmail.com" class="email">ville.voutilainen@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#motivation" id="toc-motivation"><span class="toc-section-number">1</span> Motivation<span></span></a></li>
<li><a href="#rename_join" id="toc-rename_join"><span class="toc-section-number">2</span> Rename
<code class="sourceCode cpp">join</code> member
functions<span></span></a></li>
<li><a href="#rename_nest" id="toc-rename_nest"><span class="toc-section-number">3</span> Rename
<code class="sourceCode cpp">nest</code><span></span></a></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">4</span> Proposal<span></span></a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">5</span> Wording<span></span></a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">6</span> References<span></span></a></li>
</ul>
</div>
<h1 class="unnumbered unlisted" id="abstract">Abstract<a href="#abstract" class="self-link"></a></h1>
<p>This paper proposes renaming <code class="sourceCode cpp">join</code>
and <code class="sourceCode cpp">nest</code> methods in
<code class="sourceCode cpp">async_scope</code> proposal <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span></p>
<h1 data-number="1" id="motivation"><span class="header-section-number">1</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p><span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> provides a one of the
important pieces for <span><a href="https://wg21.link/exec">[exec]</a></span> (<span class="citation" data-cites="P2300R10"><a href="https://wg21.link/p2300r10" role="doc-biblioref">[P2300R10]</a></span> proposal) to complete a
picture. The facilities allow to group the asynchronous work by
associating it with scopes and then wait at some point for all
associated work to be completed. It a bridge from the unstructured
concurrency to the structured one with saying that individual pieces of
work (possibly on different execution contexts) become the one, combined
work. The facilities also support</p>
<ul>
<li>associating the work and run it (eager execution)</li>
<li>associating the work but getting back a sender to run it later (lazy
execution)</li>
</ul>
<p>Despite all the advantages, some names might be arguably misleading
because they are either not following the existing practices or might
have a different meaning in the same or very close domains: concurrency
and parallelism.</p>
<p><span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> has a section that ruminates
about naming. But this discussion never happened in LEWG and/or in
reflector.</p>
<p><span class="citation" data-cites="P3685R0"><a href="https://isocpp.org/files/papers/P3685R0.pdf" role="doc-biblioref">[P3685R0]</a></span> started making improvements by
proposing <code class="sourceCode cpp">async_scope_token</code> -&gt;
<code class="sourceCode cpp">scope_token</code> renaming. We want to
make it even better with this proposal by renaming
<code class="sourceCode cpp">join</code> and
<code class="sourceCode cpp">nest</code>. There are other candidates to
be renamed as well, however the existing names are also fine and authors
cannot come up with better alternative, so other than
<code class="sourceCode cpp">join</code> and
<code class="sourceCode cpp">nest</code> the rest is out of scope for
this paper.</p>
<h1 data-number="2" id="rename_join"><span class="header-section-number">2</span> Rename
<code class="sourceCode cpp">join</code> member functions<a href="#rename_join" class="self-link"></a></h1>
<p>The name <code class="sourceCode cpp">join</code> is problematic
because we already have <code class="sourceCode cpp">join</code> in the
standard for threads but in a different way. <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> tells about similarity between
<code class="sourceCode cpp">std<span class="op">::</span>thread<span class="op">::</span>join</code>
and <code class="sourceCode cpp">execution<span class="op">::</span>counting_scope<span class="op">::</span>join</code>,
however there is a fundamental difference:
<code class="sourceCode cpp">join</code> for
<code class="sourceCode cpp">std<span class="op">::</span>thread</code>
is a blocking call. It blocks the current thread and waits till its
completion. <code class="sourceCode cpp">counting_scope<span class="op">::</span>join</code>,
on the other hand, is not blocking. It returns a sender that signals
when all the work associated with a scope is completed. Consider the
following comparison table:</p>
<table>
<caption><blockquote>
<p><code class="sourceCode cpp">thread<span class="op">::</span>join</code>
vs <code class="sourceCode cpp">counting_scope<span class="op">::</span>join</code></p>
</blockquote></caption>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">thread<span class="op">::</span>join</code></strong>
</div></th>
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">counting_scope<span class="op">::</span>join</code></strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div>

<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a><span class="co">// create the thread doing some work</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a>std<span class="op">::</span>thread t1<span class="op">([]{</span> <span class="co">/* some work */</span> <span class="op">})</span>;</span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a><span class="co">// block a current thread and waits for t1 completion</span></span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a>t1<span class="op">.</span>join<span class="op">()</span>;</span>
<span id="cb1-15"><a href="#cb1-15" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-16"><a href="#cb1-16" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-17"><a href="#cb1-17" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-18"><a href="#cb1-18" aria-hidden="true" tabindex="-1"></a></span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">namespace</span> ex <span class="op">=</span> std<span class="op">::</span>execution;</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a>my_thread_pool pool;</span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a>ex<span class="op">::</span>counting_scope scope;</span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a><span class="co">// create sender to run within async scope</span></span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a><span class="kw">auto</span> snd <span class="op">=</span> ex<span class="op">::</span>transfer_just<span class="op">(</span>pool<span class="op">.</span>get_scheduler<span class="op">())</span> <span class="op">|</span></span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a>               ex<span class="op">::</span>then<span class="op">([]</span> <span class="op">{</span> <span class="co">/* some work */</span> <span class="op">})</span>;</span>
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-10"><a href="#cb2-10" aria-hidden="true" tabindex="-1"></a><span class="co">// associate snd and launch the work</span></span>
<span id="cb2-11"><a href="#cb2-11" aria-hidden="true" tabindex="-1"></a>ex<span class="op">::</span>spawn<span class="op">(</span>snd, scope<span class="op">.</span>get_token<span class="op">())</span>;</span>
<span id="cb2-12"><a href="#cb2-12" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-13"><a href="#cb2-13" aria-hidden="true" tabindex="-1"></a><span class="co">// !!! Has no effect. Returns sender that is discarded</span></span>
<span id="cb2-14"><a href="#cb2-14" aria-hidden="true" tabindex="-1"></a>scope<span class="op">.</span>join<span class="op">()</span>;</span>
<span id="cb2-15"><a href="#cb2-15" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-16"><a href="#cb2-16" aria-hidden="true" tabindex="-1"></a><span class="co">// Users have to call sync_wait explicitly on the</span></span>
<span id="cb2-17"><a href="#cb2-17" aria-hidden="true" tabindex="-1"></a><span class="co">// returned sender by join to get the expected behavior</span></span>
<span id="cb2-18"><a href="#cb2-18" aria-hidden="true" tabindex="-1"></a>this_thread<span class="op">::</span>sync_wait<span class="op">(</span>scope<span class="op">.</span>join<span class="op">())</span>;</span></code></pre></div>

</div></td>
</tr>
</tbody>
</table>
<p>As you can see in the example above, <code class="sourceCode cpp">counting_scope<span class="op">::</span>join</code>
does not block as some might expect. It returns a sender that can signal
that the all associated work with the scope is completed. Somebody might
argue that this is a quality of implementation: implementers can put
<code class="sourceCode cpp"><span class="op">[[</span><span class="at">nodiscard</span><span class="op">]]</span></code>
attribute on the function and emit a warning. While this is true it does
not eliminate the fact that the name itself it not helpful to recognize
such behavior.</p>
<p>In <span class="citation" data-cites="libunifex"><a href="https://github.com/facebookexperimental/libunifex" role="doc-biblioref">[libunifex]</a></span> there is a v1 of
<code class="sourceCode cpp">async_scope</code> (the latest one is v2 as
far as we can tell) that names this function as
<code class="sourceCode cpp">complete</code>. While this name is better
in our opinion, it still does not give any clue that the function itself
is non-blocking.</p>
<p>Together with authors of <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> and some other people we
considered a bunch of names. For example,
<code class="sourceCode cpp">complete</code>,
<code class="sourceCode cpp">cleanup</code>,
<code class="sourceCode cpp">drain</code>,
<code class="sourceCode cpp">completes</code>,
<code class="sourceCode cpp">completes_when</code>,
<code class="sourceCode cpp">done</code>, etc.</p>
<p>It seems like people converge to have
<code class="sourceCode cpp">when_</code> prefix, similar to
<code class="sourceCode cpp">when_all</code>. Given that we mentioned a
term <em>completion</em> several times in this paper, we suggest to
rename <code class="sourceCode cpp">join</code> function to
<code class="sourceCode cpp">when_completed</code> because in our
opinion it’s much clearer and gives users better understanding of what’s
going on.</p>
<p>The last line of the code in the column with
<code class="sourceCode cpp">counting_scope</code> looks like this with
out proposal:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>this_thread<span class="op">::</span>sync_wait<span class="op">(</span>scope<span class="op">.</span>when_completed<span class="op">())</span>;</span></code></pre></div>
<h1 data-number="3" id="rename_nest"><span class="header-section-number">3</span> Rename
<code class="sourceCode cpp">nest</code><a href="#rename_nest" class="self-link"></a></h1>
<p>There is a term that is called <em>nested parallelism</em>. It means
that some piece of work spawns more work from a asynchronous/parallel
context.</p>
<p>One of the APIs that creates a nested parallelism (or nested work) in
<span class="citation" data-cites="P2300R10"><a href="https://wg21.link/p2300r10" role="doc-biblioref">[P2300R10]</a></span> is
<code class="sourceCode cpp">let_value</code>. It, basically, runs the
returned sender from the passed function and waits for its completion to
send the computed values further. This
returned-by-the-passed-function-sender is a nested work. <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> itself contains the example
with nested parallelism in the example with handling a tree using
<code class="sourceCode cpp">spawn_future</code> and
<code class="sourceCode cpp">let_value</code>.</p>
<p>However, <code class="sourceCode cpp">nest</code> name itself doesn’t
imply nested parallelism at all. That’s why it’s confusing. When users
call <code class="sourceCode cpp">nest</code> they basically want to
<code class="sourceCode cpp">associate</code> the piece of work with the
scope, or <code class="sourceCode cpp">attach</code> the piece of work
to the scope. The execution of the work is deferred.</p>
<p>The API <code class="sourceCode cpp">nest</code> might be also called
like that by historical reasons: previously <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> proposal had
<code class="sourceCode cpp">nest</code> as one of the required
operations on scopes by <code class="sourceCode cpp">async_scope</code>
concept. The current state is that the
<code class="sourceCode cpp">async_scope</code> concept was replaced by
the lower level one named
<code class="sourceCode cpp">async_scope_token</code> (hopefully, just
<code class="sourceCode cpp">scope_token</code> after applying <span class="citation" data-cites="P3685R0"><a href="https://isocpp.org/files/papers/P3685R0.pdf" role="doc-biblioref">[P3685R0]</a></span>) with requiring
<code class="sourceCode cpp">try_associate</code> and
<code class="sourceCode cpp">deassociate</code> methods (plus
<code class="sourceCode cpp">wrap</code>). Also, v1
<code class="sourceCode cpp">async_scope</code> in <span class="citation" data-cites="libunifex"><a href="https://github.com/facebookexperimental/libunifex" role="doc-biblioref">[libunifex]</a></span> had the
<code class="sourceCode cpp">attach</code> name instead of
<code class="sourceCode cpp">nest</code>.</p>
<p>In our desire to avoid confusion and also taking into account the
previous experience we can recommend two names instead of
<code class="sourceCode cpp">nest</code>. It’s either
<code class="sourceCode cpp">associate</code> or
<code class="sourceCode cpp">attach</code>. Given that <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> talks about association a lot,
this is the preferred option, unless authors of <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> think that the similarity to
<code class="sourceCode cpp">try_associate</code> is more confusing than
helpful. If this is the case then
<code class="sourceCode cpp">attach</code> is also a good choice.</p>
<h1 data-number="4" id="proposal"><span class="header-section-number">4</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<ul>
<li>Rename <code class="sourceCode cpp">join</code> to
<code class="sourceCode cpp">when_completed</code></li>
<li>Rename <code class="sourceCode cpp">nest</code> to
<code class="sourceCode cpp">associate</code></li>
</ul>
<h1 data-number="5" id="wording"><span class="header-section-number">5</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P3149R9"><a href="https://wg21.link/p3149r9" role="doc-biblioref">[P3149R9]</a></span> is still in active LWG review
phase the wording for renaming does not make a lot of sense. We will
need to work with <code class="sourceCode cpp">async_scope</code>
authors to make sure that all the changes are incorporated from this
proposal, if it passes.</p>
<h1 data-number="6" id="bibliography"><span class="header-section-number">6</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" data-entry-spacing="1" role="doc-bibliography">
<div id="ref-libunifex" class="csl-entry" role="doc-biblioentry">
[libunifex] libunifex. <a href="https://github.com/facebookexperimental/libunifex"><div class="csl-block">https://github.com/facebookexperimental/libunifex</div></a>
</div>
<div id="ref-P2300R10" class="csl-entry" role="doc-biblioentry">
[P2300R10] Eric Niebler, Michał Dominiak, Georgy Evtushenko, Lewis
Baker, Lucian Radu Teodorescu, Lee Howes, Kirk Shoop, Michael Garland,
Bryce Adelstein Lelbach. 2024-06-28. `std::execution`. <a href="https://wg21.link/p2300r10"><div class="csl-block">https://wg21.link/p2300r10</div></a>
</div>
<div id="ref-P3149R9" class="csl-entry" role="doc-biblioentry">
[P3149R9] Ian Petersen, Jessica Wong; Dietmar Kühl; Ján Ondrušek; Kirk
Shoop; Lee Howes; Lucian Radu Teodorescu; Ruslan Arutyunyan; 2025-02-18.
async_scope — Creating scopes for non-sequential concurrency. <a href="https://wg21.link/p3149r9"><div class="csl-block">https://wg21.link/p3149r9</div></a>
</div>
<div id="ref-P3685R0" class="csl-entry" role="doc-biblioentry">
[P3685R0] Robert Leahy. Rename
<code class="sourceCode cpp">async_scope_token</code>. <a href="https://isocpp.org/files/papers/P3685R0.pdf"><div class="csl-block">https://isocpp.org/files/papers/P3685R0.pdf</div></a>
</div>
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