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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="Waits on multiple concurrent branches, returning when all branches complete with `Ok(_)` or on the first `Err(_)`."><meta name="keywords" content="rust, rustlang, rust-lang, try_join"><title>try_join in tokio - 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">tokio</a></h2><div id="sidebar-vars" data-name="try_join" 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="../tokio/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">tokio</a>::<wbr><a class="macro" href="#">try_join</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/tokio/macros/try_join.rs.html#103-171" 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">try_join</span> {
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(@ { ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">count</span> : <span class="ident">tt</span>) <span class="kw-2">*</span>) $(($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">skip</span> : <span class="ident">tt</span>) <span class="kw-2">*</span>) <span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span> : <span class="ident">expr</span>,) <span class="op">*</span> }) => { ... };
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(@ { ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">s</span> : <span class="ident">tt</span>) <span class="kw-2">*</span>) $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span> : <span class="ident">tt</span>) <span class="op">*</span> } <span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span> : <span class="ident">expr</span>, $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">r</span> : <span class="ident">tt</span>) <span class="kw-2">*</span>) => { ... };
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($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span> : <span class="ident">expr</span>), <span class="op">*</span> $(,) <span class="question-mark">?</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>Waits on multiple concurrent branches, returning when <strong>all</strong> branches
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complete with <code>Ok(_)</code> or on the first <code>Err(_)</code>.</p>
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<p>The <code>try_join!</code> macro must be used inside of async functions, closures, and
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blocks.</p>
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<p>Similar to <a href="macro.join.html"><code>join!</code></a>, the <code>try_join!</code> macro takes a list of async
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expressions and evaluates them concurrently on the same task. Each async
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expression evaluates to a future and the futures from each expression are
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multiplexed on the current task. The <code>try_join!</code> macro returns when <strong>all</strong>
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branches return with <code>Ok</code> or when the <strong>first</strong> branch returns with <code>Err</code>.</p>
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<h2 id="notes" class="section-header"><a href="#notes">Notes</a></h2>
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<p>The supplied futures are stored inline and does not require allocating a
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<code>Vec</code>.</p>
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<h4 id="runtime-characteristics" class="section-header"><a href="#runtime-characteristics">Runtime characteristics</a></h4>
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<p>By running all async expressions on the current task, the expressions are
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able to run <strong>concurrently</strong> but not in <strong>parallel</strong>. This means all
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expressions are run on the same thread and if one branch blocks the thread,
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all other expressions will be unable to continue. If parallelism is
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required, spawn each async expression using <a href="task/fn.spawn.html"><code>tokio::spawn</code></a> and pass the
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join handle to <code>try_join!</code>.</p>
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<h2 id="examples" class="section-header"><a href="#examples">Examples</a></h2>
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<p>Basic try_join with two branches.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">async</span> <span class="kw">fn</span> <span class="ident">do_stuff_async</span>() -> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span><span class="op">></span> {
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<span class="comment">// async work</span>
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}
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">more_async_work</span>() -> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span><span class="op">></span> {
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<span class="comment">// more here</span>
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}
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<span class="attribute">#[<span class="ident">tokio::main</span>]</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">res</span> <span class="op">=</span> <span class="macro">tokio::try_join!</span>(
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<span class="ident">do_stuff_async</span>(),
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<span class="ident">more_async_work</span>());
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<span class="kw">match</span> <span class="ident">res</span> {
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<span class="prelude-val">Ok</span>((<span class="ident">first</span>, <span class="ident">second</span>)) => {
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<span class="comment">// do something with the values</span>
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}
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<span class="prelude-val">Err</span>(<span class="ident">err</span>) => {
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<span class="macro">println!</span>(<span class="string">"processing failed; error = {}"</span>, <span class="ident">err</span>);
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}
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}
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}</code></pre></div>
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<p>Using <code>try_join!</code> with spawned tasks.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use</span> <span class="ident">tokio::task::JoinHandle</span>;
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">do_stuff_async</span>() -> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span><span class="op">></span> {
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<span class="comment">// async work</span>
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}
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">more_async_work</span>() -> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span><span class="op">></span> {
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<span class="comment">// more here</span>
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}
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">flatten</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">handle</span>: <span class="ident">JoinHandle</span><span class="op"><</span><span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">T</span>, <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span><span class="op">></span><span class="op">></span>) -> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">T</span>, <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span><span class="op">></span> {
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<span class="kw">match</span> <span class="ident">handle</span>.<span class="kw">await</span> {
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<span class="prelude-val">Ok</span>(<span class="prelude-val">Ok</span>(<span class="ident">result</span>)) => <span class="prelude-val">Ok</span>(<span class="ident">result</span>),
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<span class="prelude-val">Ok</span>(<span class="prelude-val">Err</span>(<span class="ident">err</span>)) => <span class="prelude-val">Err</span>(<span class="ident">err</span>),
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<span class="prelude-val">Err</span>(<span class="ident">err</span>) => <span class="prelude-val">Err</span>(<span class="string">"handling failed"</span>),
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}
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}
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<span class="attribute">#[<span class="ident">tokio::main</span>]</span>
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<span class="kw">async</span> <span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">handle1</span> <span class="op">=</span> <span class="ident">tokio::spawn</span>(<span class="ident">do_stuff_async</span>());
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<span class="kw">let</span> <span class="ident">handle2</span> <span class="op">=</span> <span class="ident">tokio::spawn</span>(<span class="ident">more_async_work</span>());
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<span class="kw">match</span> <span class="macro">tokio::try_join!</span>(<span class="ident">flatten</span>(<span class="ident">handle1</span>), <span class="ident">flatten</span>(<span class="ident">handle2</span>)) {
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<span class="prelude-val">Ok</span>(<span class="ident">val</span>) => {
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<span class="comment">// do something with the values</span>
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}
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<span class="prelude-val">Err</span>(<span class="ident">err</span>) => {
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<span class="macro">println!</span>(<span class="string">"Failed with {}."</span>, <span class="ident">err</span>);
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}
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}
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}</code></pre></div>
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