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46 lines
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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 `once_with` fn in crate `nom`."><meta name="keywords" content="rust, rustlang, rust-lang, once_with"><title>nom::lib::std::iter::once_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="../../../../dark.css"><link rel="stylesheet" type="text/css" href="../../../../light.css" id="themeStyle"><script src="../../../../storage.js"></script><noscript><link rel="stylesheet" href="../../../../noscript.css"></noscript><link rel="shortcut icon" href="../../../../favicon.ico"><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='../../../../nom/index.html'><div class='logo-container'><img src='../../../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a>::<wbr><a href='index.html'>iter</a></p><script>window.sidebarCurrent = {name: 'once_with', 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="../../../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></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><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 class='since' title='Stable since Rust version 1.43.0'>1.43.0</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='https://doc.rust-lang.org/nightly/src/core/iter/sources.rs.html#500-502' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a>::<wbr><a href='index.html'>iter</a>::<wbr><a class="fn" href=''>once_with</a></span></h1><pre class='rust fn'>pub fn once_with<A, F>(gen: F) -> <a class="struct" href="../../../../nom/lib/std/iter/struct.OnceWith.html" title="struct nom::lib::std::iter::OnceWith">OnceWith</a><F> <span class="where fmt-newline">where<br> F: <a class="trait" href="../../../../nom/lib/std/prelude/v1/v1/trait.FnOnce.html" title="trait nom::lib::std::prelude::v1::v1::FnOnce">FnOnce</a>() -> A, </span></pre><div class='docblock'><p>Creates an iterator that lazily generates a value exactly once by invoking
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the provided closure.</p>
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<p>This is commonly used to adapt a single value generator into a <a href="trait.Iterator.html#method.chain"><code>chain</code></a> of
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other kinds of iteration. Maybe you have an iterator that covers almost
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everything, but you need an extra special case. Maybe you have a function
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which works on iterators, but you only need to process one value.</p>
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<p>Unlike <a href="fn.once.html"><code>once</code></a>, this function will lazily generate the value on request.</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">std</span>::<span class="ident">iter</span>;
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<span class="comment">// one is the loneliest number</span>
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<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_with</span>(<span class="op">|</span><span class="op">|</span> <span class="number">1</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">one</span>.<span class="ident">next</span>());
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<span class="comment">// just one, that's all we get</span>
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<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>());</pre></div>
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<p>Chaining together with another iterator. Let's say that we want to iterate
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over each file of the <code>.foo</code> directory, but also a configuration file,
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<code>.foorc</code>:</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">std</span>::<span class="ident">iter</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">path</span>::<span class="ident">PathBuf</span>;
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<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>();
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<span class="comment">// we need to convert from an iterator of DirEntry-s to an iterator of</span>
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<span class="comment">// PathBufs, so we use map</span>
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<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>());
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<span class="comment">// now, our iterator just for our config file</span>
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<span class="kw">let</span> <span class="ident">config</span> <span class="op">=</span> <span class="ident">iter</span>::<span class="ident">once_with</span>(<span class="op">|</span><span class="op">|</span> <span class="ident">PathBuf</span>::<span class="ident">from</span>(<span class="string">".foorc"</span>));
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<span class="comment">// chain the two iterators together into one big iterator</span>
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<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>);
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<span class="comment">// this will give us all of the files in .foo as well as .foorc</span>
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<span class="kw">for</span> <span class="ident">f</span> <span class="kw">in</span> <span class="ident">files</span> {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="ident">f</span>);
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}</pre></div>
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