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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 `repeat_with` fn in crate `futures_util`."><meta name="keywords" content="rust, rustlang, rust-lang, repeat_with"><title>futures_util::stream::repeat_with - 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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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../futures_util/index.html'><div class='logo-container rust-logo'><img src='../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class="location"><a href="../index.html">futures_util</a>::<wbr><a href="index.html">stream</a></p><div id="sidebar-vars" data-name="repeat_with" data-ty="fn" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><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" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" class="help-button">?</button>
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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="in-band">Function <a href="../index.html">futures_util</a>::<wbr><a href="index.html">stream</a>::<wbr><a class="fn" href="">repeat_with</a></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/futures_util/stream/repeat_with.rs.html#92-94" title="goto source code">[src]</a></span></h1><pre class="rust fn">pub fn repeat_with<A, F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>() -> A>(repeater: F) -> <a class="struct" href="../../futures_util/stream/struct.RepeatWith.html" title="struct futures_util::stream::RepeatWith">RepeatWith</a><F></pre><div class="docblock"><p>Creates a new stream that repeats elements of type <code>A</code> endlessly by
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applying the provided closure, the repeater, <code>F: FnMut() -> A</code>.</p>
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<p>The <code>repeat_with()</code> function calls the repeater over and over again.</p>
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<p>Infinite stream like <code>repeat_with()</code> are often used with adapters like
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[<code>stream.take()</code>], in order to make them finite.</p>
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<p>If the element type of the stream you need implements <a href="https://doc.rust-lang.org/nightly/core/clone/trait.Clone.html" title="Clone"><code>Clone</code></a>, and
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it is OK to keep the source element in memory, you should instead use
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the [<code>stream.repeat()</code>] function.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>Basic usage:</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">stream</span>::{<span class="self">self</span>, <span class="ident">StreamExt</span>};
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<span class="comment">// let's assume we have some value of a type that is not `Clone`</span>
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<span class="comment">// or which don't want to have in memory just yet because it is expensive:</span>
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">PartialEq</span>, <span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">Expensive</span>;
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<span class="comment">// a particular value forever:</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">things</span> <span class="op">=</span> <span class="ident">stream</span>::<span class="ident">repeat_with</span>(<span class="op">|</span><span class="op">|</span> <span class="ident">Expensive</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="ident">Expensive</span>), <span class="ident">things</span>.<span class="ident">next</span>().<span class="kw">await</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="ident">Expensive</span>), <span class="ident">things</span>.<span class="ident">next</span>().<span class="kw">await</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="ident">Expensive</span>), <span class="ident">things</span>.<span class="ident">next</span>().<span class="kw">await</span>);</pre></div>
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<p>Using mutation and going finite:</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">stream</span>::{<span class="self">self</span>, <span class="ident">StreamExt</span>};
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<span class="comment">// From the zeroth to the third power of two:</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">curr</span> <span class="op">=</span> <span class="number">1</span>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">pow2</span> <span class="op">=</span> <span class="ident">stream</span>::<span class="ident">repeat_with</span>(<span class="op">|</span><span class="op">|</span> { <span class="kw">let</span> <span class="ident">tmp</span> <span class="op">=</span> <span class="ident">curr</span>; <span class="ident">curr</span> <span class="kw-2">*</span><span class="op">=</span> <span class="number">2</span>; <span class="ident">tmp</span> })
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.<span class="ident">take</span>(<span class="number">4</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="number">1</span>), <span class="ident">pow2</span>.<span class="ident">next</span>().<span class="kw">await</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="number">2</span>), <span class="ident">pow2</span>.<span class="ident">next</span>().<span class="kw">await</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="number">4</span>), <span class="ident">pow2</span>.<span class="ident">next</span>().<span class="kw">await</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="number">8</span>), <span class="ident">pow2</span>.<span class="ident">next</span>().<span class="kw">await</span>);
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<span class="comment">// ... and now we're done</span>
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">None</span>, <span class="ident">pow2</span>.<span class="ident">next</span>().<span class="kw">await</span>);</pre></div>
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