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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="API documentation for the Rust `select` fn in crate `futures_util`."><meta name="keywords" content="rust, rustlang, rust-lang, select"><title>futures_util::future::select - Rust</title><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="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><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/futures_util/future/select.rs.html#84-88" title="goto source code">[src]</a></span><span class="in-band">Function <a href="../index.html">futures_util</a>::<wbr><a href="index.html">future</a>::<wbr><a class="fn" href="">select</a></span></h1><pre class="rust fn">pub fn select<A, B>(future1: A, future2: B) -> <a class="struct" href="../../futures_util/future/struct.Select.html" title="struct futures_util::future::Select">Select</a><A, B><span class="notable-traits"><span class="notable-traits-tooltip">ⓘ<div class="notable-traits-tooltiptext"><span class="docblock"><h3 class="notable">Notable traits for <a class="struct" href="../../futures_util/future/struct.Select.html" title="struct futures_util::future::Select">Select</a><A, B></h3><code class="content"><span class="where fmt-newline">impl<A, B> <a class="trait" href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html" title="trait core::future::future::Future">Future</a> for <a class="struct" href="../../futures_util/future/struct.Select.html" title="struct futures_util::future::Select">Select</a><A, B> <span class="where fmt-newline">where<br> A: <a class="trait" href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html" title="trait core::future::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="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html" title="trait core::future::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="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html#associatedtype.Output" class="type">Output</a> = <a class="enum" href="../../futures_util/future/enum.Either.html" title="enum futures_util::future::Either">Either</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">(</a>A::<a class="type" href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html#associatedtype.Output" title="type core::future::future::Future::Output">Output</a>, B<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">)</a>, <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">(</a>B::<a class="type" href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html#associatedtype.Output" title="type core::future::future::Future::Output">Output</a>, A<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">)</a>>;</span></code></span></div></span></span> <span class="where fmt-newline">where<br> A: <a class="trait" href="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html" title="trait core::future::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="https://doc.rust-lang.org/nightly/core/future/future/trait.Future.html" title="trait core::future::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></pre><div class="docblock"><p>Waits for either one of two differently-typed futures to complete.</p>
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<p>This function will return a new future which awaits for either one of both
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futures to complete. The returned future will finish with both the value
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resolved and a future representing the completion of the other work.</p>
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<p>Note that this function consumes the receiving futures and returns a
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wrapped version of them.</p>
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<p>Also note that if both this and the second future have the same
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output type you can use the <code>Either::factor_first</code> method to
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conveniently extract out the value at the end.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>A simple example</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">futures</span>::{
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<span class="ident">pin_mut</span>,
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<span class="ident">future</span>::<span class="ident">Either</span>,
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<span class="ident">future</span>::<span class="self">self</span>,
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};
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<span class="comment">// These two futures have different types even though their outputs have the same type.</span>
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<span class="kw">let</span> <span class="ident">future1</span> <span class="op">=</span> <span class="kw">async</span> {
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<span class="ident">future</span>::<span class="ident">pending</span>::<span class="op"><</span>()<span class="op">></span>().<span class="kw">await</span>; <span class="comment">// will never finish</span>
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<span class="number">1</span>
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};
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<span class="kw">let</span> <span class="ident">future2</span> <span class="op">=</span> <span class="kw">async</span> {
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<span class="ident">future</span>::<span class="ident">ready</span>(<span class="number">2</span>).<span class="kw">await</span>
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};
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<span class="comment">// 'select' requires Future + Unpin bounds</span>
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<span class="macro">pin_mut</span><span class="macro">!</span>(<span class="ident">future1</span>);
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<span class="macro">pin_mut</span><span class="macro">!</span>(<span class="ident">future2</span>);
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<span class="kw">let</span> <span class="ident">value</span> <span class="op">=</span> <span class="kw">match</span> <span class="ident">future</span>::<span class="ident">select</span>(<span class="ident">future1</span>, <span class="ident">future2</span>).<span class="kw">await</span> {
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<span class="ident">Either</span>::<span class="ident">Left</span>((<span class="ident">value1</span>, <span class="kw">_</span>)) <span class="op">=</span><span class="op">></span> <span class="ident">value1</span>, <span class="comment">// `value1` is resolved from `future1`</span>
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<span class="comment">// `_` represents `future2`</span>
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<span class="ident">Either</span>::<span class="ident">Right</span>((<span class="ident">value2</span>, <span class="kw">_</span>)) <span class="op">=</span><span class="op">></span> <span class="ident">value2</span>, <span class="comment">// `value2` is resolved from `future2`</span>
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<span class="comment">// `_` represents `future1`</span>
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};
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<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">value</span> <span class="op">=</span><span class="op">=</span> <span class="number">2</span>);</pre></div>
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<p>A more complex example</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">futures</span>::<span class="ident">future</span>::{<span class="self">self</span>, <span class="ident">Either</span>, <span class="ident">Future</span>, <span class="ident">FutureExt</span>};
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<span class="comment">// A poor-man's join implemented on top of select</span>
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<span class="kw">fn</span> <span class="ident">join</span><span class="op"><</span><span class="ident">A</span>, <span class="ident">B</span><span class="op">></span>(<span class="ident">a</span>: <span class="ident">A</span>, <span class="ident">b</span>: <span class="ident">B</span>) <span class="op">-</span><span class="op">></span> <span class="kw">impl</span> <span class="ident">Future</span><span class="op"><</span><span class="ident">Output</span><span class="op">=</span>(<span class="ident">A</span>::<span class="ident">Output</span>, <span class="ident">B</span>::<span class="ident">Output</span>)<span class="op">></span>
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<span class="kw">where</span> <span class="ident">A</span>: <span class="ident">Future</span> <span class="op">+</span> <span class="ident">Unpin</span>,
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<span class="ident">B</span>: <span class="ident">Future</span> <span class="op">+</span> <span class="ident">Unpin</span>,
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{
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<span class="ident">future</span>::<span class="ident">select</span>(<span class="ident">a</span>, <span class="ident">b</span>).<span class="ident">then</span>(<span class="op">|</span><span class="ident">either</span><span class="op">|</span> {
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<span class="kw">match</span> <span class="ident">either</span> {
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<span class="ident">Either</span>::<span class="ident">Left</span>((<span class="ident">x</span>, <span class="ident">b</span>)) <span class="op">=</span><span class="op">></span> <span class="ident">b</span>.<span class="ident">map</span>(<span class="kw">move</span> <span class="op">|</span><span class="ident">y</span><span class="op">|</span> (<span class="ident">x</span>, <span class="ident">y</span>)).<span class="ident">left_future</span>(),
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<span class="ident">Either</span>::<span class="ident">Right</span>((<span class="ident">y</span>, <span class="ident">a</span>)) <span class="op">=</span><span class="op">></span> <span class="ident">a</span>.<span class="ident">map</span>(<span class="kw">move</span> <span class="op">|</span><span class="ident">x</span><span class="op">|</span> (<span class="ident">x</span>, <span class="ident">y</span>)).<span class="ident">right_future</span>(),
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}
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})
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}</pre></div>
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