28 lines
6.5 KiB
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28 lines
6.5 KiB
HTML
<!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 `from_fn` fn in crate `nom`."><meta name="keywords" content="rust, rustlang, rust-lang, from_fn"><title>nom::lib::std::iter::from_fn - 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: 'from_fn', 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 1.34.0'>1.34.0</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#544-549' 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=''>from_fn</a></span></h1><pre class='rust fn'>pub fn from_fn<T, F>(f: F) -> <a class="struct" href="../../../../nom/lib/std/iter/struct.FromFn.html" title="struct nom::lib::std::iter::FromFn">FromFn</a><F> <span class="where fmt-newline">where<br> F: <a class="trait" href="../../../../nom/lib/std/prelude/v1/v1/trait.FnMut.html" title="trait nom::lib::std::prelude::v1::v1::FnMut">FnMut</a>() -> <a class="enum" href="../../../../nom/lib/std/prelude/v1/v1/enum.Option.html" title="enum nom::lib::std::prelude::v1::v1::Option">Option</a><T>, </span></pre><div class='docblock'><p>Creates a new iterator where each iteration calls the provided closure
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<code>F: FnMut() -> Option<T></code>.</p>
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<p>This allows creating a custom iterator with any behavior
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without using the more verbose syntax of creating a dedicated type
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and implementing the <code>Iterator</code> trait for it.</p>
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<p>Note that the <code>FromFn</code> iterator doesn’t make assumptions about the behavior of the closure,
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and therefore conservatively does not implement <a href="trait.FusedIterator.html"><code>FusedIterator</code></a>,
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or override <a href="trait.Iterator.html#method.size_hint"><code>Iterator::size_hint</code></a> from its default <code>(0, None)</code>.</p>
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<p>The closure can use captures and its environment to track state across iterations. Depending on
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how the iterator is used, this may require specifying the <code>move</code> keyword on the closure.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>Let’s re-implement the counter iterator from <a href="index.html">module-level documentation</a>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">count</span> <span class="op">=</span> <span class="number">0</span>;
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<span class="kw">let</span> <span class="ident">counter</span> <span class="op">=</span> <span class="ident">std</span>::<span class="ident">iter</span>::<span class="ident">from_fn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
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<span class="comment">// Increment our count. This is why we started at zero.</span>
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<span class="ident">count</span> <span class="op">+</span><span class="op">=</span> <span class="number">1</span>;
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<span class="comment">// Check to see if we've finished counting or not.</span>
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<span class="kw">if</span> <span class="ident">count</span> <span class="op"><</span> <span class="number">6</span> {
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<span class="prelude-val">Some</span>(<span class="ident">count</span>)
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} <span class="kw">else</span> {
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<span class="prelude-val">None</span>
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}
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});
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">counter</span>.<span class="ident">collect</span>::<span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="kw">_</span><span class="op">></span><span class="op">></span>(), <span class="kw-2">&</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>]);</pre></div>
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