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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 `fold` mod in crate `syn`."><meta name="keywords" content="rust, rustlang, rust-lang, fold"><title>syn::fold - 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 mod"><!--[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='../../syn/index.html'><div class='logo-container'><img src='../../rust-logo.png' alt='logo'></div></a><p class='location'>Module fold</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li></ul></div><p class='location'><a href='../index.html'>syn</a></p><script>window.sidebarCurrent = {name: 'fold', ty: 'mod', 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'>−</span>]</a></span><a class='srclink' href='../../src/syn/gen/fold.rs.html#4-3236' title='goto source code'>[src]</a></span><span class='in-band'>Module <a href='../index.html'>syn</a>::<wbr><a class="mod" href=''>fold</a></span></h1><div class='docblock'><p>Syntax tree traversal to transform the nodes of an owned syntax tree.</p>
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<p>Each method of the <a href="../../syn/fold/trait.Fold.html"><code>Fold</code></a> trait is a hook that can be overridden to
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customize the behavior when transforming the corresponding type of node.
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By default, every method recursively visits the substructure of the
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input by invoking the right visitor method of each of its fields.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Fold</span> {
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<span class="comment">/* ... */</span>
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<span class="kw">fn</span> <span class="ident">fold_expr_binary</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">node</span>: <span class="ident">ExprBinary</span>) <span class="op">-</span><span class="op">></span> <span class="ident">ExprBinary</span> {
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<span class="ident">fold_expr_binary</span>(<span class="self">self</span>, <span class="ident">node</span>)
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}
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<span class="comment">/* ... */</span>
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}
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">fold_expr_binary</span><span class="op"><</span><span class="ident">V</span><span class="op">></span>(<span class="ident">v</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">V</span>, <span class="ident">node</span>: <span class="ident">ExprBinary</span>) <span class="op">-</span><span class="op">></span> <span class="ident">ExprBinary</span>
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<span class="kw">where</span>
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<span class="ident">V</span>: <span class="ident">Fold</span> <span class="op">+</span> <span class="question-mark">?</span><span class="ident">Sized</span>,
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{
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<span class="ident">ExprBinary</span> {
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<span class="ident">attrs</span>: <span class="ident">node</span>
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.<span class="ident">attrs</span>
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.<span class="ident">into_iter</span>()
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.<span class="ident">map</span>(<span class="op">|</span><span class="ident">attr</span><span class="op">|</span> <span class="ident">v</span>.<span class="ident">fold_attribute</span>(<span class="ident">attr</span>))
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.<span class="ident">collect</span>(),
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<span class="ident">left</span>: <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">v</span>.<span class="ident">fold_expr</span>(<span class="kw-2">*</span><span class="ident">node</span>.<span class="ident">left</span>)),
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<span class="ident">op</span>: <span class="ident">v</span>.<span class="ident">fold_bin_op</span>(<span class="ident">node</span>.<span class="ident">op</span>),
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<span class="ident">right</span>: <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">v</span>.<span class="ident">fold_expr</span>(<span class="kw-2">*</span><span class="ident">node</span>.<span class="ident">right</span>)),
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}
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}
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<span class="comment">/* ... */</span></pre></div>
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<p><em>This module is available if Syn is built with the <code>"fold"</code> feature.</em></p>
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<br>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>This fold inserts parentheses to fully parenthesizes any expression.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="comment">// [dependencies]</span>
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<span class="comment">// quote = "1.0"</span>
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<span class="comment">// syn = { version = "1.0", features = ["fold", "full"] }</span>
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<span class="kw">use</span> <span class="ident">quote</span>::<span class="ident">quote</span>;
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">fold</span>::{<span class="ident">fold_expr</span>, <span class="ident">Fold</span>};
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<span class="kw">use</span> <span class="ident">syn</span>::{<span class="ident">token</span>, <span class="ident">Expr</span>, <span class="ident">ExprParen</span>};
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<span class="kw">struct</span> <span class="ident">ParenthesizeEveryExpr</span>;
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<span class="kw">impl</span> <span class="ident">Fold</span> <span class="kw">for</span> <span class="ident">ParenthesizeEveryExpr</span> {
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<span class="kw">fn</span> <span class="ident">fold_expr</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">expr</span>: <span class="ident">Expr</span>) <span class="op">-</span><span class="op">></span> <span class="ident">Expr</span> {
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<span class="ident">Expr</span>::<span class="ident">Paren</span>(<span class="ident">ExprParen</span> {
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<span class="ident">attrs</span>: <span class="ident">Vec</span>::<span class="ident">new</span>(),
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<span class="ident">expr</span>: <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">fold_expr</span>(<span class="self">self</span>, <span class="ident">expr</span>)),
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<span class="ident">paren_token</span>: <span class="ident">token</span>::<span class="ident">Paren</span>::<span class="ident">default</span>(),
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})
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}
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}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">code</span> <span class="op">=</span> <span class="macro">quote</span><span class="macro">!</span> { <span class="ident">a</span>() <span class="op">+</span> <span class="ident">b</span>(<span class="number">1</span>) <span class="op">*</span> <span class="ident">c</span>.<span class="ident">d</span> };
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<span class="kw">let</span> <span class="ident">expr</span>: <span class="ident">Expr</span> <span class="op">=</span> <span class="ident">syn</span>::<span class="ident">parse2</span>(<span class="ident">code</span>).<span class="ident">unwrap</span>();
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<span class="kw">let</span> <span class="ident">parenthesized</span> <span class="op">=</span> <span class="ident">ParenthesizeEveryExpr</span>.<span class="ident">fold_expr</span>(<span class="ident">expr</span>);
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{}"</span>, <span class="macro">quote</span><span class="macro">!</span>(#<span class="ident">parenthesized</span>));
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<span class="comment">// Output: (((a)()) + (((b)((1))) * ((c).d)))</span>
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}</pre></div>
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</div><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table><tr class='module-item'><td><a class="trait" href="trait.Fold.html" title='syn::fold::Fold trait'>Fold</a></td><td class='docblock-short'><p>Syntax tree traversal to transform the nodes of an owned syntax tree.</p>
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</td></tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table><tr class='module-item'><td><a class="fn" href="fn.fold_abi.html" title='syn::fold::fold_abi fn'>fold_abi</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_angle_bracketed_generic_arguments.html" title='syn::fold::fold_angle_bracketed_generic_arguments fn'>fold_angle_bracketed_generic_arguments</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_arm.html" title='syn::fold::fold_arm fn'>fold_arm</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_attr_style.html" title='syn::fold::fold_attr_style fn'>fold_attr_style</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_attribute.html" title='syn::fold::fold_attribute fn'>fold_attribute</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_bare_fn_arg.html" title='syn::fold::fold_bare_fn_arg fn'>fold_bare_fn_arg</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_bin_op.html" title='syn::fold::fold_bin_op fn'>fold_bin_op</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_binding.html" title='syn::fold::fold_binding fn'>fold_binding</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_block.html" title='syn::fold::fold_block fn'>fold_block</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_bound_lifetimes.html" title='syn::fold::fold_bound_lifetimes fn'>fold_bound_lifetimes</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_const_param.html" title='syn::fold::fold_const_param fn'>fold_const_param</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_constraint.html" title='syn::fold::fold_constraint fn'>fold_constraint</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_data.html" title='syn::fold::fold_data fn'>fold_data</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_data_enum.html" title='syn::fold::fold_data_enum fn'>fold_data_enum</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_data_struct.html" title='syn::fold::fold_data_struct fn'>fold_data_struct</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_data_union.html" title='syn::fold::fold_data_union fn'>fold_data_union</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_derive_input.html" title='syn::fold::fold_derive_input fn'>fold_derive_input</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_expr.html" title='syn::fold::fold_expr fn'>fold_expr</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_expr_array.html" title='syn::fold::fold_expr_array fn'>fold_expr_array</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_expr_assign.html" title='syn::fold::fold_expr_assign fn'>fold_expr_assign</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_expr_assign_op.html" title='syn::fold::fold_expr_assign_op fn'>fold_expr_assign_op</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_expr_async.html" title='syn::fold::fold_expr_async fn'>fold_expr_async</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_expr_await.html" title='syn::fold::fold_expr_await fn'>fold_expr_await</a></td><td class='docblock-short'></td></tr><tr class='module-item'><td><a class="fn" href="fn.fold_expr_binary.html" title='syn::fold::fold_expr_binary fn'>fold_expr_binary</a></td>
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