| <!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 for either one of two differently-typed futures to complete."><meta name="keywords" content="rust, rustlang, rust-lang, select"><title>select in futures::prelude::future - 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 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class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../../../futures/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../../futures/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In futures::prelude::future</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Function <a href="../../index.html">futures</a>::<wbr><a href="../index.html">prelude</a>::<wbr><a href="index.html">future</a>::<wbr><a class="fn" 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></h1><span class="out-of-band"><a class="srclink" href="../../../src/futures_util/future/select.rs.html#84-87">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust fn"><code>pub fn select<A, B>(future1: A, future2: B) -> <a class="struct" href="../../future/struct.Select.html" title="struct futures::future::Select">Select</a><A, B><span class="notable-traits"><span class="notable-traits-tooltip">ⓘ<span class="notable-traits-tooltiptext"><span class="docblock"><span class="notable">Notable traits for <a class="struct" href="../../future/struct.Select.html" title="struct futures::future::Select">Select</a><A, B></span><code class="content"><span class="where fmt-newline">impl<A, B> <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a> for <a class="struct" href="../../future/struct.Select.html" title="struct futures::future::Select">Select</a><A, B><span class="where fmt-newline">where<br> A: <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>,<br> B: <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>,</span></span><span class="where fmt-newline"> type <a href="../../future/trait.Future.html#associatedtype.Output" class="associatedtype">Output</a> = <a class="enum" href="../../future/enum.Either.html" title="enum futures::future::Either">Either</a><(<A as <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a>>::<a class="associatedtype" href="../../future/trait.Future.html#associatedtype.Output" title="type futures::future::Future::Output">Output</a>, B), (<B as <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a>>::<a class="associatedtype" href="../../future/trait.Future.html#associatedtype.Output" title="type futures::future::Future::Output">Output</a>, A)>;</span></code></span></span></span></span><span class="where fmt-newline">where<br> A: <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>,<br> B: <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>,</span></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Waits for either one of two differently-typed futures to complete.</p> |
| <p>This function will return a new future which awaits for either one of both |
| futures to complete. The returned future will finish with both the value |
| resolved and a future representing the completion of the other work.</p> |
| <p>Note that this function consumes the receiving futures and returns a |
| wrapped version of them.</p> |
| <p>Also note that if both this and the second future have the same |
| output type you can use the <code>Either::factor_first</code> method to |
| conveniently extract out the value at the end.</p> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <p>A simple example</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::{ |
| pin_mut, |
| future::Either, |
| future::<span class="self">self</span>, |
| }; |
| |
| <span class="comment">// These two futures have different types even though their outputs have the same type. |
| </span><span class="kw">let </span>future1 = <span class="kw">async </span>{ |
| future::pending::<()>().<span class="kw">await</span>; <span class="comment">// will never finish |
| </span><span class="number">1 |
| </span>}; |
| <span class="kw">let </span>future2 = <span class="kw">async </span>{ |
| future::ready(<span class="number">2</span>).<span class="kw">await |
| </span>}; |
| |
| <span class="comment">// 'select' requires Future + Unpin bounds |
| </span><span class="macro">pin_mut!</span>(future1); |
| <span class="macro">pin_mut!</span>(future2); |
| |
| <span class="kw">let </span>value = <span class="kw">match </span>future::select(future1, future2).<span class="kw">await </span>{ |
| Either::Left((value1, <span class="kw">_</span>)) => value1, <span class="comment">// `value1` is resolved from `future1` |
| // `_` represents `future2` |
| </span>Either::Right((value2, <span class="kw">_</span>)) => value2, <span class="comment">// `value2` is resolved from `future2` |
| // `_` represents `future1` |
| </span>}; |
| |
| <span class="macro">assert!</span>(value == <span class="number">2</span>);</code></pre></div> |
| <p>A more complex example</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::future::{<span class="self">self</span>, Either, Future, FutureExt}; |
| |
| <span class="comment">// A poor-man's join implemented on top of select |
| |
| </span><span class="kw">fn </span>join<A, B>(a: A, b: B) -> <span class="kw">impl </span>Future<Output=(A::Output, B::Output)> |
| <span class="kw">where </span>A: Future + Unpin, |
| B: Future + Unpin, |
| { |
| future::select(a, b).then(|either| { |
| <span class="kw">match </span>either { |
| Either::Left((x, b)) => b.map(<span class="kw">move </span>|y| (x, y)).left_future(), |
| Either::Right((y, a)) => a.map(<span class="kw">move </span>|x| (x, y)).right_future(), |
| } |
| }) |
| }</code></pre></div> |
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