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Trac3r-rust/doc/syn/macro.cond_reduce.html

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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 `cond_reduce` macro in crate `syn`."><meta name="keywords" content="rust, rustlang, rust-lang, cond_reduce"><title>syn::cond_reduce - 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">&#9776;</div><a href='../syn/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'>syn</a></p><script>window.sidebarCurrent = {name: 'cond_reduce', 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 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="../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'>&#x2212;</span>]</a></span><a class='srclink' href='../src/syn/parsers.rs.html#386-402' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>syn</a>::<wbr><a class="macro" href=''>cond_reduce</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">cond_reduce</span> {
(<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">cond</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">&gt;</span> { ... };
(<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">cond</span>:<span class="ident">expr</span>) <span class="op">=</span><span class="op">&gt;</span> { ... };
(<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">cond</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">&gt;</span> { ... };
}</pre></div>
</div><div class='docblock'><p>Execute a parser only if a condition is met, otherwise fail to parse.</p>
<p>This is typically used inside of <a href="macro.option.html"><code>option!</code></a> or <a href="macro.alt.html"><code>alt!</code></a>.</p>
<ul>
<li><strong>Syntax:</strong> <code>cond_reduce!(CONDITION, THING)</code></li>
<li><strong>Output:</strong> <code>THING</code></li>
</ul>
<p>The subparser may be omitted in which case it defaults to <a href="macro.epsilon.html"><code>epsilon!</code></a>.</p>
<ul>
<li><strong>Syntax:</strong> <code>cond_reduce!(CONDITION)</code></li>
<li><strong>Output:</strong> <code>()</code></li>
</ul>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="attribute">#[<span class="ident">macro_use</span>]</span>
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">syn</span>;
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">Type</span>;
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">token</span>::<span class="ident">Paren</span>;
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">punctuated</span>::<span class="ident">Punctuated</span>;
<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">synom</span>::<span class="ident">Synom</span>;
<span class="doccomment">/// Parses a possibly variadic function signature.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Example: `fn(A) or `fn(A, B, C, ...)` or `fn(...)`</span>
<span class="doccomment">/// Rejected: `fn(A, B...)`</span>
<span class="kw">struct</span> <span class="ident">VariadicFn</span> {
<span class="ident">fn_token</span>: <span class="macro">Token</span><span class="macro">!</span>[<span class="kw">fn</span>],
<span class="ident">paren_token</span>: <span class="ident">Paren</span>,
<span class="ident">types</span>: <span class="ident">Punctuated</span><span class="op">&lt;</span><span class="ident">Type</span>, <span class="macro">Token</span><span class="macro">!</span>[,]<span class="op">&gt;</span>,
<span class="ident">variadic</span>: <span class="prelude-ty">Option</span><span class="op">&lt;</span><span class="macro">Token</span><span class="macro">!</span>[...]<span class="op">&gt;</span>,
}
<span class="comment">// Example of using `cond_reduce!` inside of `option!`.</span>
<span class="kw">impl</span> <span class="ident">Synom</span> <span class="kw">for</span> <span class="ident">VariadicFn</span> {
<span class="macro">named</span><span class="macro">!</span>(<span class="ident">parse</span> <span class="op">-</span><span class="op">&gt;</span> <span class="self">Self</span>, <span class="macro">do_parse</span><span class="macro">!</span>(
<span class="ident">fn_token</span>: <span class="macro">keyword</span><span class="macro">!</span>(<span class="kw">fn</span>) <span class="op">&gt;</span><span class="op">&gt;</span>
<span class="ident">params</span>: <span class="macro">parens</span><span class="macro">!</span>(<span class="macro">do_parse</span><span class="macro">!</span>(
<span class="ident">types</span>: <span class="macro">call</span><span class="macro">!</span>(<span class="ident">Punctuated</span>::<span class="ident">parse_terminated</span>) <span class="op">&gt;</span><span class="op">&gt;</span>
<span class="comment">// Allow, but do not require, an ending `...` but only if the</span>
<span class="comment">// preceding list of types is empty or ends with a trailing comma.</span>
<span class="ident">variadic</span>: <span class="macro">option</span><span class="macro">!</span>(<span class="macro">cond_reduce</span><span class="macro">!</span>(<span class="ident">types</span>.<span class="ident">empty_or_trailing</span>(), <span class="macro">punct</span><span class="macro">!</span>(...))) <span class="op">&gt;</span><span class="op">&gt;</span>
(<span class="ident">types</span>, <span class="ident">variadic</span>)
)) <span class="op">&gt;</span><span class="op">&gt;</span>
({
<span class="kw">let</span> (<span class="ident">paren_token</span>, (<span class="ident">types</span>, <span class="ident">variadic</span>)) <span class="op">=</span> <span class="ident">params</span>;
<span class="ident">VariadicFn</span> {
<span class="ident">fn_token</span>,
<span class="ident">paren_token</span>,
<span class="ident">types</span>,
<span class="ident">variadic</span>,
}
})
));
}</pre></div>
<p><em>This macro is available if Syn is built with the <code>&quot;parsing&quot;</code> feature.</em></p>
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