<!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 `core`."><meta name="keywords" content="rust, rustlang, rust-lang, from_fn"><title>core::iter::from_fn - 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='../../core/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'>core</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="../../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 1.34.0'>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='../../src/core/iter/sources.rs.html#531-535' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../index.html'>core</a>::<wbr><a href='index.html'>iter</a>::<wbr><a class="fn" href=''>from_fn</a></span></h1><div class="important-traits"><div class='tooltip'>ⓘ<span class='tooltiptext'>Important traits for <a class="struct" href="../../core/iter/struct.FromFn.html" title="struct core::iter::FromFn">FromFn</a><F></span></div><div class="content hidden"><h3 class="important">Important traits for <a class="struct" href="../../core/iter/struct.FromFn.html" title="struct core::iter::FromFn">FromFn</a><F></h3><code class="content"><span class="where fmt-newline">impl<T, F> <a class="trait" href="../../core/iter/trait.Iterator.html" title="trait core::iter::Iterator">Iterator</a> for <a class="struct" href="../../core/iter/struct.FromFn.html" title="struct core::iter::FromFn">FromFn</a><F> <span class="where fmt-newline">where<br> F: <a class="trait" href="../../core/ops/trait.FnMut.html" title="trait core::ops::FnMut">FnMut</a>() -> <a class="enum" href="../../core/option/enum.Option.html" title="enum core::option::Option">Option</a><T>, </span></span><span class="where fmt-newline"> type <a href='../../core/iter/trait.Iterator.html#associatedtype.Item' class="type">Item</a> = T;</span></code></div></div><pre class='rust fn'>pub fn from_fn<T, F>(f: F) -> <a class="struct" href="../../core/iter/struct.FromFn.html" title="struct core::iter::FromFn">FromFn</a><F> <span class="where fmt-newline">where<br> F: <a class="trait" href="../../core/ops/trait.FnMut.html" title="trait core::ops::FnMut">FnMut</a>() -> <a class="enum" href="../../core/option/enum.Option.html" title="enum core::option::Option">Option</a><T>, </span></pre><div class='docblock'><p>Creates a new iterator where each iteration calls the provided closure
<code>F: FnMut() -> Option<T></code>.</p>
<p>This allows creating a custom iterator with any behavior
without using the more verbose syntax of creating a dedicated type
and implementing the <code>Iterator</code> trait for it.</p>
<p>Note that the <code>FromFn</code> iterator doesn’t make assumptions about the behavior of the closure,
and therefore conservatively does not implement <a href="trait.FusedIterator.html"><code>FusedIterator</code></a>,
or override <a href="trait.Iterator.html#method.size_hint"><code>Iterator::size_hint</code></a> from its default <code>(0, None)</code>.</p>
<p>The closure can use captures and its environment to track state across iterations. Depending on
how the iterator is used, this may require specifying the <code>move</code> keyword on the closure.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Let’s re-implement the counter iterator from <a href="index.html">module-level documentation</a>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<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>;
<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="comment">// Increment our count. This is why we started at zero.</span>
<span class="ident">count</span> <span class="op">+=</span> <span class="number">1</span>;
<span class="comment">// Check to see if we've finished counting or not.</span>
<span class="kw">if</span> <span class="ident">count</span> <span class="op"><</span> <span class="number">6</span> {
<span class="prelude-val">Some</span>(<span class="ident">count</span>)
} <span class="kw">else</span> {
<span class="prelude-val">None</span>
}
});
<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="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>]);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20mut%20count%20%3D%200%3B%0Alet%20counter%20%3D%20std%3A%3Aiter%3A%3Afrom_fn(move%20%7C%7C%20%7B%0A%20%20%20%20%2F%2F%20Increment%20our%20count.%20This%20is%20why%20we%20started%20at%20zero.%0A%20%20%20%20count%20%2B%3D%201%3B%0A%0A%20%20%20%20%2F%2F%20Check%20to%20see%20if%20we've%20finished%20counting%20or%20not.%0A%20%20%20%20if%20count%20%3C%206%20%7B%0A%20%20%20%20%20%20%20%20Some(count)%0A%20%20%20%20%7D%20else%20%7B%0A%20%20%20%20%20%20%20%20None%0A%20%20%20%20%7D%0A%7D)%3B%0Aassert_eq!(counter.collect%3A%3A%3CVec%3C_%3E%3E()%2C%20%26%5B1%2C%202%2C%203%2C%204%2C%205%5D)%3B%0A%7D">Run</a></pre></div>
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