<!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 `once` fn in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, once"><title>std::iter::once - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../../light1.35.0.css" id="themeStyle"><script src="../../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow1.35.0.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='../../std/index.html'><img src='../../rust-logo1.35.0.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='../index.html'>std</a>::<wbr><a href='index.html'>iter</a></p><script>window.sidebarCurrent = {name: 'once', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><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><a id="settings-menu" href="../../settings.html"><img src="../../wheel1.35.0.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='out-of-band'><span class='since' title='Stable since Rust version 1.2.0'>1.2.0</span><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/core/iter/sources.rs.html#374-376' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../index.html'>std</a>::<wbr><a href='index.html'>iter</a>::<wbr><a class="fn" href=''>once</a></span></h1><div class="important-traits"><div class='tooltip'>ⓘ<span class='tooltiptext'>Important traits for <a class="struct" href="../../std/iter/struct.Once.html" title="struct std::iter::Once">Once</a><T></span></div><div class="content hidden"><h3 class="important">Important traits for <a class="struct" href="../../std/iter/struct.Once.html" title="struct std::iter::Once">Once</a><T></h3><code class="content"><span class="where fmt-newline">impl<T> <a class="trait" href="../../std/iter/trait.Iterator.html" title="trait std::iter::Iterator">Iterator</a> for <a class="struct" href="../../std/iter/struct.Once.html" title="struct std::iter::Once">Once</a><T></span><span class="where fmt-newline"> type <a href='../../std/iter/trait.Iterator.html#associatedtype.Item' class="type">Item</a> = T;</span></code></div></div><pre class='rust fn'>pub fn once<T>(value: T) -> <a class="struct" href="../../std/iter/struct.Once.html" title="struct std::iter::Once">Once</a><T></pre><div class='docblock'><p>Creates an iterator that yields an element exactly once.</p>
<p>This is commonly used to adapt a single value into a <a href="trait.Iterator.html#method.chain"><code>chain</code></a> of other
kinds of iteration. Maybe you have an iterator that covers almost
everything, but you need an extra special case. Maybe you have a function
which works on iterators, but you only need to process one value.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Basic usage:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">iter</span>;
<span class="comment">// one is the loneliest number</span>
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">one</span> <span class="op">=</span> <span class="ident">iter</span>::<span class="ident">once</span>(<span class="number">1</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">Some</span>(<span class="number">1</span>), <span class="ident">one</span>.<span class="ident">next</span>());
<span class="comment">// just one, that's all we get</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="prelude-val">None</span>, <span class="ident">one</span>.<span class="ident">next</span>());<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Aiter%3B%0A%0A%2F%2F%20one%20is%20the%20loneliest%20number%0Alet%20mut%20one%20%3D%20iter%3A%3Aonce(1)%3B%0A%0Aassert_eq!(Some(1)%2C%20one.next())%3B%0A%0A%2F%2F%20just%20one%2C%20that's%20all%20we%20get%0Aassert_eq!(None%2C%20one.next())%3B%0A%7D">Run</a></pre></div>
<p>Chaining together with another iterator. Let's say that we want to iterate
over each file of the <code>.foo</code> directory, but also a configuration file,
<code>.foorc</code>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">iter</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">path</span>::<span class="ident">PathBuf</span>;
<span class="kw">let</span> <span class="ident">dirs</span> <span class="op">=</span> <span class="ident">fs</span>::<span class="ident">read_dir</span>(<span class="string">".foo"</span>).<span class="ident">unwrap</span>();
<span class="comment">// we need to convert from an iterator of DirEntry-s to an iterator of</span>
<span class="comment">// PathBufs, so we use map</span>
<span class="kw">let</span> <span class="ident">dirs</span> <span class="op">=</span> <span class="ident">dirs</span>.<span class="ident">map</span>(<span class="op">|</span><span class="ident">file</span><span class="op">|</span> <span class="ident">file</span>.<span class="ident">unwrap</span>().<span class="ident">path</span>());
<span class="comment">// now, our iterator just for our config file</span>
<span class="kw">let</span> <span class="ident">config</span> <span class="op">=</span> <span class="ident">iter</span>::<span class="ident">once</span>(<span class="ident">PathBuf</span>::<span class="ident">from</span>(<span class="string">".foorc"</span>));
<span class="comment">// chain the two iterators together into one big iterator</span>
<span class="kw">let</span> <span class="ident">files</span> <span class="op">=</span> <span class="ident">dirs</span>.<span class="ident">chain</span>(<span class="ident">config</span>);
<span class="comment">// this will give us all of the files in .foo as well as .foorc</span>
<span class="kw">for</span> <span class="ident">f</span> <span class="kw">in</span> <span class="ident">files</span> {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="ident">f</span>);
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Aiter%3B%0Ause%20std%3A%3Afs%3B%0Ause%20std%3A%3Apath%3A%3APathBuf%3B%0A%0Alet%20dirs%20%3D%20fs%3A%3Aread_dir(%22.foo%22).unwrap()%3B%0A%0A%2F%2F%20we%20need%20to%20convert%20from%20an%20iterator%20of%20DirEntry-s%20to%20an%20iterator%20of%0A%2F%2F%20PathBufs%2C%20so%20we%20use%20map%0Alet%20dirs%20%3D%20dirs.map(%7Cfile%7C%20file.unwrap().path())%3B%0A%0A%2F%2F%20now%2C%20our%20iterator%20just%20for%20our%20config%20file%0Alet%20config%20%3D%20iter%3A%3Aonce(PathBuf%3A%3Afrom(%22.foorc%22))%3B%0A%0A%2F%2F%20chain%20the%20two%20iterators%20together%20into%20one%20big%20iterator%0Alet%20files%20%3D%20dirs.chain(config)%3B%0A%0A%2F%2F%20this%20will%20give%20us%20all%20of%20the%20files%20in%20.foo%20as%20well%20as%20.foorc%0Afor%20f%20in%20files%20%7B%0A%20%20%20%20println!(%22%7B%3A%3F%7D%22%2C%20f)%3B%0A%7D%0A%7D">Run</a></pre></div>
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