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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="Apply a `Parser`, producing `None` on `ErrMode::Backtrack`."><title>opt in winnow::combinator - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="winnow" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../winnow/index.html">winnow</a><span class="version">0.6.5</span></h2></div><div class="sidebar-elems"><h2><a href="index.html">In winnow::combinator</a></h2></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../../winnow/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Function <a href="../index.html">winnow</a>::<wbr><a href="index.html">combinator</a>::<wbr><a class="fn" href="#">opt</a><button id="copy-path" title="Copy item path to clipboard"><img src="../../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../../src/winnow/combinator/core.rs.html#97-115">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>pub fn opt<Input: <a class="trait" href="../stream/trait.Stream.html" title="trait winnow::stream::Stream">Stream</a>, Output, Error, ParseNext>(
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parser: ParseNext
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) -> impl <a class="trait" href="../trait.Parser.html" title="trait winnow::Parser">Parser</a><Input, <a class="enum" href="https://doc.rust-lang.org/1.76.0/core/option/enum.Option.html" title="enum core::option::Option">Option</a><Output>, Error><div class="where">where
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ParseNext: <a class="trait" href="../trait.Parser.html" title="trait winnow::Parser">Parser</a><Input, Output, Error>,
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Error: <a class="trait" href="../error/trait.ParserError.html" title="trait winnow::error::ParserError">ParserError</a><Input>,</div></code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Apply a <a href="../trait.Parser.html" title="trait winnow::Parser"><code>Parser</code></a>, producing <code>None</code> on <a href="../error/enum.ErrMode.html#variant.Backtrack" title="variant winnow::error::ErrMode::Backtrack"><code>ErrMode::Backtrack</code></a>.</p>
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<p>To chain an error up, see <a href="fn.cut_err.html" title="fn winnow::combinator::cut_err"><code>cut_err</code></a>.</p>
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<h2 id="example"><a href="#example">Example</a></h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>winnow::combinator::opt;
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<span class="kw">use </span>winnow::ascii::alpha1;
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<span class="kw">fn </span>parser(i: <span class="kw-2">&</span>str) -> IResult<<span class="kw-2">&</span>str, <span class="prelude-ty">Option</span><<span class="kw-2">&</span>str>> {
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opt(alpha1).parse_peek(i)
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}
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<span class="macro">assert_eq!</span>(parser(<span class="string">"abcd;"</span>), <span class="prelude-val">Ok</span>((<span class="string">";"</span>, <span class="prelude-val">Some</span>(<span class="string">"abcd"</span>))));
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<span class="macro">assert_eq!</span>(parser(<span class="string">"123;"</span>), <span class="prelude-val">Ok</span>((<span class="string">"123;"</span>, <span class="prelude-val">None</span>)));</code></pre></div>
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</div></details></section></div></main></body></html>
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