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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Selects from a set of channel operations."><meta name="keywords" content="rust, rustlang, rust-lang, select"><title>select in crossbeam_channel - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../dark.css" disabled><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script src="../crates.js"></script><script defer src="../main.js"></script>
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</a><div class="sidebar-elems"><h2 class="location">Other items in<br><a href="index.html">crossbeam_channel</a></h2><div id="sidebar-vars" data-name="select" data-ty="macro" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../crossbeam_channel/index.html"><img class="rust-logo" src="../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../brush.svg"></button><div id="theme-choices" role="menu"></div></div><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../settings.html" title="settings"><img width="18" height="18" alt="Change settings" src="../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><h1 class="fqn"><span class="in-band">Macro <a href="index.html">crossbeam_channel</a>::<wbr><a class="macro" href="#">select</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></span><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../src/crossbeam_channel/select_macro.rs.html#1110-1116" title="goto source code">[src]</a></span></h1><div class="docblock item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules!</span> <span class="ident">select</span> {
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($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens</span> : <span class="ident">tt</span>) <span class="kw-2">*</span>) => { ... };
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}</code></pre></div>
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</div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Selects from a set of channel operations.</p>
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<p>This macro allows you to define a set of channel operations, wait until any one of them becomes
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ready, and finally execute it. If multiple operations are ready at the same time, a random one
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among them is selected.</p>
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<p>It is also possible to define a <code>default</code> case that gets executed if none of the operations are
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ready, either right away or for a certain duration of time.</p>
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<p>An operation is considered to be ready if it doesn’t have to block. Note that it is ready even
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when it will simply return an error because the channel is disconnected.</p>
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<p>The <code>select</code> macro is a convenience wrapper around <a href="struct.Select.html"><code>Select</code></a>. However, it cannot select over a
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dynamically created list of channel operations.</p>
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<h2 id="examples" class="section-header"><a href="#examples">Examples</a></h2>
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<p>Block until a send or a receive operation is selected:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">crossbeam_channel</span>::{<span class="ident">select</span>, <span class="ident">unbounded</span>};
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<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
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<span class="comment">// Since both operations are initially ready, a random one will be executed.</span>
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<span class="macro">select!</span> {
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<span class="ident">recv</span>(<span class="ident">r1</span>) -> <span class="ident">msg</span> => <span class="macro">assert_eq!</span>(<span class="ident">msg</span>, <span class="prelude-val">Ok</span>(<span class="number">10</span>)),
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<span class="ident">send</span>(<span class="ident">s2</span>, <span class="number">20</span>) -> <span class="ident">res</span> => {
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<span class="macro">assert_eq!</span>(<span class="ident">res</span>, <span class="prelude-val">Ok</span>(()));
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<span class="macro">assert_eq!</span>(<span class="ident">r2</span>.<span class="ident">recv</span>(), <span class="prelude-val">Ok</span>(<span class="number">20</span>));
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}
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}</code></pre></div>
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<p>Select from a set of operations without blocking:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">std::thread</span>;
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<span class="kw">use</span> <span class="ident">std::time::Duration</span>;
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<span class="kw">use</span> <span class="ident">crossbeam_channel</span>::{<span class="ident">select</span>, <span class="ident">unbounded</span>};
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<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="ident">thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="ident">thread::sleep</span>(<span class="ident">Duration::from_secs</span>(<span class="number">1</span>));
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<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
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});
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<span class="ident">thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="ident">thread::sleep</span>(<span class="ident">Duration::from_millis</span>(<span class="number">500</span>));
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<span class="ident">s2</span>.<span class="ident">send</span>(<span class="number">20</span>).<span class="ident">unwrap</span>();
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});
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<span class="comment">// None of the operations are initially ready.</span>
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<span class="macro">select!</span> {
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<span class="ident">recv</span>(<span class="ident">r1</span>) -> <span class="ident">msg</span> => <span class="macro">panic!</span>(),
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<span class="ident">recv</span>(<span class="ident">r2</span>) -> <span class="ident">msg</span> => <span class="macro">panic!</span>(),
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<span class="ident">default</span> => <span class="macro">println!</span>(<span class="string">"not ready"</span>),
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}</code></pre></div>
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<p>Select over a set of operations with a timeout:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">std::thread</span>;
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<span class="kw">use</span> <span class="ident">std::time::Duration</span>;
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<span class="kw">use</span> <span class="ident">crossbeam_channel</span>::{<span class="ident">select</span>, <span class="ident">unbounded</span>};
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<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="ident">thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="ident">thread::sleep</span>(<span class="ident">Duration::from_secs</span>(<span class="number">1</span>));
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<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
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});
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<span class="ident">thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="ident">thread::sleep</span>(<span class="ident">Duration::from_millis</span>(<span class="number">500</span>));
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<span class="ident">s2</span>.<span class="ident">send</span>(<span class="number">20</span>).<span class="ident">unwrap</span>();
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});
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<span class="comment">// None of the two operations will become ready within 100 milliseconds.</span>
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<span class="macro">select!</span> {
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<span class="ident">recv</span>(<span class="ident">r1</span>) -> <span class="ident">msg</span> => <span class="macro">panic!</span>(),
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<span class="ident">recv</span>(<span class="ident">r2</span>) -> <span class="ident">msg</span> => <span class="macro">panic!</span>(),
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<span class="ident">default</span>(<span class="ident">Duration::from_millis</span>(<span class="number">100</span>)) => <span class="macro">println!</span>(<span class="string">"timed out"</span>),
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}</code></pre></div>
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<p>Optionally add a receive operation to <code>select!</code> using <a href="fn.never.html"><code>never</code></a>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">std::thread</span>;
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<span class="kw">use</span> <span class="ident">std::time::Duration</span>;
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<span class="kw">use</span> <span class="ident">crossbeam_channel</span>::{<span class="ident">select</span>, <span class="ident">never</span>, <span class="ident">unbounded</span>};
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<span class="kw">let</span> (<span class="ident">s1</span>, <span class="ident">r1</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="kw">let</span> (<span class="ident">s2</span>, <span class="ident">r2</span>) <span class="op">=</span> <span class="ident">unbounded</span>();
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<span class="ident">thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="ident">thread::sleep</span>(<span class="ident">Duration::from_secs</span>(<span class="number">1</span>));
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<span class="ident">s1</span>.<span class="ident">send</span>(<span class="number">10</span>).<span class="ident">unwrap</span>();
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});
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<span class="ident">thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="ident">thread::sleep</span>(<span class="ident">Duration::from_millis</span>(<span class="number">500</span>));
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<span class="ident">s2</span>.<span class="ident">send</span>(<span class="number">20</span>).<span class="ident">unwrap</span>();
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});
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<span class="comment">// This receiver can be a `Some` or a `None`.</span>
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<span class="kw">let</span> <span class="ident">r2</span> <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="kw-2">&</span><span class="ident">r2</span>);
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<span class="comment">// None of the two operations will become ready within 100 milliseconds.</span>
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<span class="macro">select!</span> {
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<span class="ident">recv</span>(<span class="ident">r1</span>) -> <span class="ident">msg</span> => <span class="macro">panic!</span>(),
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<span class="ident">recv</span>(<span class="ident">r2</span>.<span class="ident">unwrap_or</span>(<span class="kw-2">&</span><span class="ident">never</span>())) -> <span class="ident">msg</span> => <span class="macro">assert_eq!</span>(<span class="ident">msg</span>, <span class="prelude-val">Ok</span>(<span class="number">20</span>)),
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}</code></pre></div>
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<p>To optionally add a timeout to <code>select!</code>, see the <a href="fn.never.html#examples">example</a> for <a href="fn.never.html"><code>never</code></a>.</p>
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