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<section id='main' class="content"><h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>chan</a>::<wbr><a class="macro" href=''>chan_select</a></span><span class='out-of-band'><span id='render-detail'>
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</span><a class='srclink' href='../src/chan/lib.rs.html#1470-1530' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">chan_select</span> {
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">select</span>:<span class="ident">ident</span>, <span class="ident">default</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">default</span>:<span class="ident">expr</span>, $(
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<span class="macro-nonterminal">$</span><span class="macro-nonterminal">chan</span>:<span class="ident">ident</span>.<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meth</span>:<span class="ident">ident</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">send</span>:<span class="ident">expr</span>)<span class="kw-2">*</span>)
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$(<span class="op">-></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">pat</span>)<span class="op">*</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">code</span>:<span class="ident">expr</span>,
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)<span class="op">+</span>) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">select</span>:<span class="ident">ident</span>, <span class="ident">default</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">default</span>:<span class="ident">expr</span>, $(
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<span class="macro-nonterminal">$</span><span class="macro-nonterminal">chan</span>:<span class="ident">ident</span>.<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meth</span>:<span class="ident">ident</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">send</span>:<span class="ident">expr</span>)<span class="kw-2">*</span>)
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$(<span class="op">-></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">pat</span>)<span class="op">*</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">code</span>:<span class="ident">expr</span>
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),<span class="op">+</span>) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">select</span>:<span class="ident">ident</span>, $(
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<span class="macro-nonterminal">$</span><span class="macro-nonterminal">chan</span>:<span class="ident">ident</span>.<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meth</span>:<span class="ident">ident</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">send</span>:<span class="ident">expr</span>)<span class="kw-2">*</span>)
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$(<span class="op">-></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">pat</span>)<span class="op">*</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">code</span>:<span class="ident">expr</span>,
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)<span class="op">+</span>) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">select</span>:<span class="ident">ident</span>, $(
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<span class="macro-nonterminal">$</span><span class="macro-nonterminal">chan</span>:<span class="ident">ident</span>.<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meth</span>:<span class="ident">ident</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">send</span>:<span class="ident">expr</span>)<span class="kw-2">*</span>)
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$(<span class="op">-></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">pat</span>)<span class="op">*</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">code</span>:<span class="ident">expr</span>
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),<span class="op">+</span>) <span class="op">=></span> { ... };
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(<span class="ident">default</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">default</span>:<span class="ident">expr</span>) <span class="op">=></span> { ... };
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(<span class="ident">default</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">default</span>:<span class="ident">expr</span>,) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">select</span>:<span class="ident">ident</span>, <span class="ident">default</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">default</span>:<span class="ident">expr</span>) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">select</span>:<span class="ident">ident</span>, <span class="ident">default</span> <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">default</span>:<span class="ident">expr</span>,) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">select</span>:<span class="ident">ident</span>) <span class="op">=></span> { ... };
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() <span class="op">=></span> { ... };
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($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tt</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=></span> { ... };
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}</pre>
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</div><div class='docblock'><p>Synchronize on at most one channel send or receive operation.</p>
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<p>This is a <em>heterogeneous</em> select. Namely, it supports any mix of
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asynchronous, synchronous or rendezvous channels, any mix of send or
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receive operations and any mix of types on channels.</p>
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<p>Here is how select operates:</p>
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<ol>
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<li>It first examines all send and receive operations. If one or more of
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them can succeed without blocking, then it randomly selects <em>one</em>,
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executes the operation and runs the code in the corresponding arm.</li>
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<li>If all operations are blocked and there is a <code>default</code> arm, then the
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code in the <code>default</code> arm is executed.</li>
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<li>If all operations are blocked and there is no <code>default</code> arm, then
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<code>Select</code> will subscribe to all channels involved. <code>Select</code> will be
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notified when state in one of the channels has changed. This will wake
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<code>Select</code> up, and it will retry the steps in (1). If all operations remain
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blocked, then (3) is repeated.</li>
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</ol>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>Which one synchronizes first?</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">thread</span>;
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<span class="kw">let</span> (<span class="ident">asend</span>, <span class="ident">arecv</span>) <span class="op">=</span> <span class="ident">chan</span>::<span class="ident">sync</span>(<span class="number">0</span>);
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<span class="kw">let</span> (<span class="ident">bsend</span>, <span class="ident">brecv</span>) <span class="op">=</span> <span class="ident">chan</span>::<span class="ident">sync</span>(<span class="number">0</span>);
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<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">||</span> <span class="ident">asend</span>.<span class="ident">send</span>(<span class="number">5</span>));
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<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">||</span> <span class="ident">brecv</span>.<span class="ident">recv</span>());
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<span class="macro">chan_select</span><span class="macro">!</span> {
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<span class="ident">arecv</span>.<span class="ident">recv</span>() <span class="op">-></span> <span class="ident">val</span> <span class="op">=></span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"arecv received: {:?}"</span>, <span class="ident">val</span>);
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},
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<span class="ident">bsend</span>.<span class="ident">send</span>(<span class="number">10</span>) <span class="op">=></span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"bsend sent"</span>);
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},
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}</pre>
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<p>See the "failure modes" section below for more examples of the syntax.</p>
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<h1 id="example-empty-select" class="section-header"><a href="#example-empty-select">Example: empty select</a></h1>
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<p>An empty select, <code>chan_select! {}</code> will block indefinitely.</p>
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<h1 id="warning" class="section-header"><a href="#warning">Warning</a></h1>
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<p><code>chan_select!</code> is simultaneously the most wonderful and horrifying thing
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in this crate.</p>
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<p>It is wonderful because it is essential for the
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composition of channel operations in a concurrent program. Without select,
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channels becomes much less expressive.</p>
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<p>It is horrifying because the macro used to define it is <em>extremely</em>
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sensitive. My hope is that it is simply my own lack of creativity at fault
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and that others can help me fix it, but we may just be fundamentally stuck
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with something like this until a proper compiler plugin can rescue us.</p>
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<h1 id="failure-modes" class="section-header"><a href="#failure-modes">Failure modes</a></h1>
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<p>When I say that this macro is sensitive, what I mean is, "if you misstep
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on the syntax, you will be slapped upside the head with an irrelevant
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error message."</p>
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<p>Consider this:</p>
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<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><pre class="rust rust-example-rendered ignore">
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<span class="macro">chan_select</span><span class="macro">!</span> {
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<span class="ident">default</span> <span class="op">=></span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">" ."</span>);
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<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_millis</span>(<span class="number">50</span>));
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}
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<span class="ident">tick</span>.<span class="ident">recv</span>() <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"tick."</span>),
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<span class="ident">boom</span>.<span class="ident">recv</span>() <span class="op">=></span> { <span class="macro">println</span><span class="macro">!</span>(<span class="string">"BOOM!"</span>); <span class="kw">return</span>; },
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}</pre>
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<p>The compiler will tell you that the "recursion limit reached while
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expanding the macro."</p>
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<p>The actual problem is that <strong>every</strong> arm requires a trailing comma,
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regardless of whether the arm is wrapped in a <code>{ ... }</code> or not. So it
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should be written <code>default => { ... },</code>. (I'm told that various highly
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skilled individuals could remove this restriction.)</p>
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<p>Here's another. Can you spot the problem? I swear it's not commas this
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time.</p>
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<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><pre class="rust rust-example-rendered ignore">
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<span class="macro">chan_select</span><span class="macro">!</span> {
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<span class="ident">tick</span>.<span class="ident">recv</span>() <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"tick."</span>),
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<span class="ident">boom</span>.<span class="ident">recv</span>() <span class="op">=></span> { <span class="macro">println</span><span class="macro">!</span>(<span class="string">"BOOM!"</span>); <span class="kw">return</span>; },
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<span class="ident">default</span> <span class="op">=></span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">" ."</span>);
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<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_millis</span>(<span class="number">50</span>));
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},
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}</pre>
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<p>This produces the same "recursion limit" error as above.</p>
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<p>The actual problem is that the <code>default</code> arm <em>must</em> come first (or it must
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be omitted completely).</p>
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<p>Yet another:</p>
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<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><pre class="rust rust-example-rendered ignore">
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<span class="macro">chan_select</span><span class="macro">!</span> {
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<span class="ident">default</span> <span class="op">=></span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">" ."</span>);
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<span class="ident">thread</span>::<span class="ident">sleep</span>(<span class="ident">Duration</span>::<span class="ident">from_millis</span>(<span class="number">50</span>));
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},
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<span class="ident">tick</span>().<span class="ident">recv</span>() <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"tick."</span>),
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<span class="ident">boom</span>.<span class="ident">recv</span>() <span class="op">=></span> { <span class="macro">println</span><span class="macro">!</span>(<span class="string">"BOOM!"</span>); <span class="kw">return</span>; },
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}</pre>
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<p>Again, you'll get the same "recursion limit" error.</p>
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<p>The actual problem is that the channel operations must be of the form
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<code>ident.recv()</code> or <code>ident.send()</code>. You cannot use an arbitrary expression
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in place of <code>ident</code> that evaluates to a channel! To fix this, you must
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rebind <code>tick()</code> to an identifier outside of <code>chan_select!</code>.</p>
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