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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>std</a>::<wbr><a class="macro" href=''>select</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version '></span><span id='render-detail'>
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</span><a class='srclink' href='../src/std/macros.rs.html#250-264' title='goto source code'>[src]</a></span></h1>
<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">select</span> {
(
$(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">name</span>:<span class="ident">pat</span> <span class="op">=</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">rx</span>:<span class="ident">ident</span>.<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meth</span>:<span class="ident">ident</span>() <span class="op">=></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">code</span>:<span class="ident">expr</span>),<span class="op">+</span>
) <span class="op">=></span> { ... };
}</pre>
<div class='stability'><div class='stab unstable'><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>mpsc_select </code><a href="https://github.com/rust-lang/rust/issues/27800">#27800</a>)</div></div><div class='docblock'><p>A macro to select an event from a number of receivers.</p>
<p>This macro is used to wait for the first event to occur on a number of
receivers. It places no restrictions on the types of receivers given to
this macro, this can be viewed as a heterogeneous select.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="attribute">#![<span class="ident">feature</span>(<span class="ident">mpsc_select</span>)]</span>
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">thread</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">mpsc</span>;
<span class="comment">// two placeholder functions for now</span>
<span class="kw">fn</span> <span class="ident">long_running_thread</span>() {}
<span class="kw">fn</span> <span class="ident">calculate_the_answer</span>() <span class="op">-></span> <span class="ident">u32</span> { <span class="number">42</span> }
<span class="kw">let</span> (<span class="ident">tx1</span>, <span class="ident">rx1</span>) <span class="op">=</span> <span class="ident">mpsc</span>::<span class="ident">channel</span>();
<span class="kw">let</span> (<span class="ident">tx2</span>, <span class="ident">rx2</span>) <span class="op">=</span> <span class="ident">mpsc</span>::<span class="ident">channel</span>();
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span><span class="op">||</span> { <span class="ident">long_running_thread</span>(); <span class="ident">tx1</span>.<span class="ident">send</span>(()).<span class="ident">unwrap</span>(); });
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span><span class="op">||</span> { <span class="ident">tx2</span>.<span class="ident">send</span>(<span class="ident">calculate_the_answer</span>()).<span class="ident">unwrap</span>(); });
<span class="macro">select</span><span class="macro">!</span> {
_ <span class="op">=</span> <span class="ident">rx1</span>.<span class="ident">recv</span>() <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"the long running thread finished first"</span>),
<span class="ident">answer</span> <span class="op">=</span> <span class="ident">rx2</span>.<span class="ident">recv</span>() <span class="op">=></span> {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"the answer was: {}"</span>, <span class="ident">answer</span>.<span class="ident">unwrap</span>());
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Bfeature(mpsc_select)%5D%0A%0Afn%20main()%20%7B%0Ause%20std%3A%3Athread%3B%0Ause%20std%3A%3Async%3A%3Ampsc%3B%0A%0A%2F%2F%20two%20placeholder%20functions%20for%20now%0Afn%20long_running_thread()%20%7B%7D%0Afn%20calculate_the_answer()%20-%3E%20u32%20%7B%2042%20%7D%0A%0Alet%20(tx1%2C%20rx1)%20%3D%20mpsc%3A%3Achannel()%3B%0Alet%20(tx2%2C%20rx2)%20%3D%20mpsc%3A%3Achannel()%3B%0A%0Athread%3A%3Aspawn(move%7C%7C%20%7B%20long_running_thread()%3B%20tx1.send(()).unwrap()%3B%20%7D)%3B%0Athread%3A%3Aspawn(move%7C%7C%20%7B%20tx2.send(calculate_the_answer()).unwrap()%3B%20%7D)%3B%0A%0Aselect!%20%7B%0A%20%20%20%20_%20%3D%20rx1.recv()%20%3D%3E%20println!(%22the%20long%20running%20thread%20finished%20first%22)%2C%0A%20%20%20%20answer%20%3D%20rx2.recv()%20%3D%3E%20%7B%0A%20%20%20%20%20%20%20%20println!(%22the%20answer%20was%3A%20%7B%7D%22%2C%20answer.unwrap())%3B%0A%20%20%20%20%7D%0A%7D%0Adrop(rx1.recv())%3B%0Adrop(rx2.recv())%3B%0A%7D&version=nightly">Run</a></pre>
<p>For more information about select, see the <code>std::sync::mpsc::Select</code> structure.</p>
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