24 lines
6.3 KiB
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24 lines
6.3 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 `opt` macro in crate `nom`."><meta name="keywords" content="rust, rustlang, rust-lang, opt"><title>nom::opt - 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 macro"><!--[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></p><script>window.sidebarCurrent = {name: 'opt', ty: 'macro', 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 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/nom/macros.rs.html#989-1007' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>nom</a>::<wbr><a class="macro" href=''>opt</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">opt</span> {
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">submac</span>:<span class="macro">ident</span><span class="macro">!</span>( $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">args</span>:<span class="ident">tt</span>)<span class="op">*</span> )) <span class="op">=</span><span class="op">></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">f</span>:<span class="ident">expr</span>) <span class="op">=</span><span class="op">></span> { ... };
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}</pre></div>
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</div><div class='docblock'><p><code>opt!(I -> IResult<I,O>) => I -> IResult<I, Option<O>></code>
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make the underlying parser optional</p>
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<p>returns an Option of the returned type. This parser returns <code>Some(result)</code> if the child parser
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succeeds,<code>None</code> if it fails, and <code>Incomplete</code> if it did not have enough data to decide</p>
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<p><em>Warning</em>: if you are using <code>opt</code> for some kind of optional ending token (like an end of line),
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you should combine it with <code>complete</code> to make sure it works.</p>
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<p>As an example, <code>opt!(tag!("\r\n"))</code> will return <code>Incomplete</code> if it receives an empty input,
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because <code>tag</code> does not have enough input to decide.
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On the contrary, <code>opt!(complete!(tag!("\r\n")))</code> would return <code>None</code> as produced value,
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since <code>complete!</code> transforms an <code>Incomplete</code> in an <code>Error</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="macro">named</span><span class="macro">!</span>( <span class="ident">o</span><span class="op"><</span><span class="kw-2">&</span>[<span class="ident">u8</span>], <span class="prelude-ty">Option</span><span class="op"><</span><span class="kw-2">&</span>[<span class="ident">u8</span>]<span class="op">></span> <span class="op">></span>, <span class="macro">opt</span><span class="macro">!</span>( <span class="macro">tag</span><span class="macro">!</span>( <span class="string">"abcd"</span> ) ) );
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<span class="kw">let</span> <span class="ident">a</span> <span class="op">=</span> <span class="string">b"abcdef"</span>;
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<span class="kw">let</span> <span class="ident">b</span> <span class="op">=</span> <span class="string">b"bcdefg"</span>;
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">o</span>(<span class="kw-2">&</span><span class="ident">a</span>[..]), <span class="prelude-val">Ok</span>((<span class="kw-2">&</span><span class="string">b"ef"</span>[..], <span class="prelude-val">Some</span>(<span class="kw-2">&</span><span class="string">b"abcd"</span>[..]))));
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">o</span>(<span class="kw-2">&</span><span class="ident">b</span>[..]), <span class="prelude-val">Ok</span>((<span class="kw-2">&</span><span class="string">b"bcdefg"</span>[..], <span class="prelude-val">None</span>)));</pre></div>
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