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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="Parsing interface for parsing a token stream into a syntax tree node."><title>syn::parse - 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="syn" 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 mod"><!--[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="../../syn/index.html">syn</a><span class="version">2.0.48</span></h2></div><h2 class="location"><a href="#">Module parse</a></h2><div class="sidebar-elems"><section><ul class="block"><li><a href="#modules">Modules</a></li><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</a></li><li><a href="#types">Type Aliases</a></li></ul></section><h2><a href="../index.html">In crate syn</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="../../syn/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>Module <a href="../index.html">syn</a>::<wbr><a class="mod" href="#">parse</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/syn/parse.rs.html#1-1386">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Parsing interface for parsing a token stream into a syntax tree node.</p>
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<p>Parsing in Syn is built on parser functions that take in a <a href="type.ParseStream.html" title="type syn::parse::ParseStream"><code>ParseStream</code></a>
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and produce a <a href="type.Result.html" title="type syn::parse::Result"><code>Result<T></code></a> where <code>T</code> is some syntax tree node. Underlying
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these parser functions is a lower level mechanism built around the
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<a href="../buffer/struct.Cursor.html" title="struct syn::buffer::Cursor"><code>Cursor</code></a> type. <code>Cursor</code> is a cheaply copyable cursor over a range of
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tokens in a token stream.</p>
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<h2 id="example"><a href="#example">Example</a></h2>
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<p>Here is a snippet of parsing code to get a feel for the style of the
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library. We define data structures for a subset of Rust syntax including
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enums (not shown) and structs, then provide implementations of the <a href="trait.Parse.html" title="trait syn::parse::Parse"><code>Parse</code></a>
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trait to parse these syntax tree data structures from a token stream.</p>
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<p>Once <code>Parse</code> impls have been defined, they can be called conveniently from a
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procedural macro through <a href="../macro.parse_macro_input.html" title="macro syn::parse_macro_input"><code>parse_macro_input!</code></a> as shown at the bottom of
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the snippet. If the caller provides syntactically invalid input to the
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procedural macro, they will receive a helpful compiler error message
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pointing out the exact token that triggered the failure to parse.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>proc_macro::TokenStream;
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<span class="kw">use </span>syn::{braced, parse_macro_input, token, Field, Ident, <span class="prelude-ty">Result</span>, Token};
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<span class="kw">use </span>syn::parse::{Parse, ParseStream};
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<span class="kw">use </span>syn::punctuated::Punctuated;
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<span class="kw">enum </span>Item {
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Struct(ItemStruct),
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Enum(ItemEnum),
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}
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<span class="kw">struct </span>ItemStruct {
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struct_token: <span class="macro">Token!</span>[<span class="kw">struct</span>],
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ident: Ident,
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brace_token: token::Brace,
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fields: Punctuated<Field, <span class="macro">Token!</span>[,]>,
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}
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<span class="kw">impl </span>Parse <span class="kw">for </span>Item {
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<span class="kw">fn </span>parse(input: ParseStream) -> <span class="prelude-ty">Result</span><<span class="self">Self</span>> {
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<span class="kw">let </span>lookahead = input.lookahead1();
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<span class="kw">if </span>lookahead.peek(<span class="macro">Token!</span>[<span class="kw">struct</span>]) {
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input.parse().map(Item::Struct)
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} <span class="kw">else if </span>lookahead.peek(<span class="macro">Token!</span>[<span class="kw">enum</span>]) {
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input.parse().map(Item::Enum)
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} <span class="kw">else </span>{
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<span class="prelude-val">Err</span>(lookahead.error())
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}
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}
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}
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<span class="kw">impl </span>Parse <span class="kw">for </span>ItemStruct {
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<span class="kw">fn </span>parse(input: ParseStream) -> <span class="prelude-ty">Result</span><<span class="self">Self</span>> {
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<span class="kw">let </span>content;
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<span class="prelude-val">Ok</span>(ItemStruct {
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struct_token: input.parse()<span class="question-mark">?</span>,
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ident: input.parse()<span class="question-mark">?</span>,
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brace_token: <span class="macro">braced!</span>(content <span class="kw">in </span>input),
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fields: content.parse_terminated(Field::parse_named, <span class="macro">Token!</span>[,])<span class="question-mark">?</span>,
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})
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}
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}
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<span class="attr">#[proc_macro]
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</span><span class="kw">pub fn </span>my_macro(tokens: TokenStream) -> TokenStream {
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<span class="kw">let </span>input = <span class="macro">parse_macro_input!</span>(tokens <span class="kw">as </span>Item);
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<span class="comment">/* ... */
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</span>}</code></pre></div>
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<h2 id="the-synparse-functions"><a href="#the-synparse-functions">The <code>syn::parse*</code> functions</a></h2>
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<p>The <a href="../fn.parse.html" title="fn syn::parse"><code>syn::parse</code></a>, <a href="../fn.parse2.html" title="fn syn::parse2"><code>syn::parse2</code></a>, and <a href="../fn.parse_str.html" title="fn syn::parse_str"><code>syn::parse_str</code></a> functions serve
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as an entry point for parsing syntax tree nodes that can be parsed in an
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obvious default way. These functions can return any syntax tree node that
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implements the <a href="trait.Parse.html" title="trait syn::parse::Parse"><code>Parse</code></a> trait, which includes most types in Syn.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>syn::Type;
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<span class="kw">let </span>t: Type = syn::parse_str(<span class="string">"std::collections::HashMap<String, Value>"</span>)<span class="question-mark">?</span>;</code></pre></div>
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<p>The <a href="../macro.parse_quote.html" title="macro syn::parse_quote"><code>parse_quote!</code></a> macro also uses this approach.</p>
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<h2 id="the-parser-trait"><a href="#the-parser-trait">The <code>Parser</code> trait</a></h2>
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<p>Some types can be parsed in several ways depending on context. For example
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an <a href="../struct.Attribute.html" title="struct syn::Attribute"><code>Attribute</code></a> can be either “outer” like <code>#[...]</code> or “inner” like
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<code>#![...]</code> and parsing the wrong one would be a bug. Similarly <a href="../punctuated/index.html" title="mod syn::punctuated"><code>Punctuated</code></a>
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may or may not allow trailing punctuation, and parsing it the wrong way
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would either reject valid input or accept invalid input.</p>
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<p>The <code>Parse</code> trait is not implemented in these cases because there is no good
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behavior to consider the default.</p>
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<div class="example-wrap compile_fail"><a href="#" class="tooltip" title="This example deliberately fails to compile">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="comment">// Can't parse `Punctuated` without knowing whether trailing punctuation
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// should be allowed in this context.
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</span><span class="kw">let </span>path: Punctuated<PathSegment, <span class="macro">Token!</span>[::]> = syn::parse(tokens)<span class="question-mark">?</span>;</code></pre></div>
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<p>In these cases the types provide a choice of parser functions rather than a
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single <code>Parse</code> implementation, and those parser functions can be invoked
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through the <a href="trait.Parser.html" title="trait syn::parse::Parser"><code>Parser</code></a> trait.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>proc_macro::TokenStream;
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<span class="kw">use </span>syn::parse::Parser;
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<span class="kw">use </span>syn::punctuated::Punctuated;
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<span class="kw">use </span>syn::{Attribute, Expr, PathSegment, <span class="prelude-ty">Result</span>, Token};
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<span class="kw">fn </span>call_some_parser_methods(input: TokenStream) -> <span class="prelude-ty">Result</span><()> {
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<span class="comment">// Parse a nonempty sequence of path segments separated by `::` punctuation
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// with no trailing punctuation.
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</span><span class="kw">let </span>tokens = input.clone();
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<span class="kw">let </span>parser = Punctuated::<PathSegment, <span class="macro">Token!</span>[::]>::parse_separated_nonempty;
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<span class="kw">let </span>_path = parser.parse(tokens)<span class="question-mark">?</span>;
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<span class="comment">// Parse a possibly empty sequence of expressions terminated by commas with
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// an optional trailing punctuation.
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</span><span class="kw">let </span>tokens = input.clone();
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<span class="kw">let </span>parser = Punctuated::<Expr, <span class="macro">Token!</span>[,]>::parse_terminated;
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<span class="kw">let </span>_args = parser.parse(tokens)<span class="question-mark">?</span>;
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<span class="comment">// Parse zero or more outer attributes but not inner attributes.
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</span><span class="kw">let </span>tokens = input.clone();
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<span class="kw">let </span>parser = Attribute::parse_outer;
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<span class="kw">let </span>_attrs = parser.parse(tokens)<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>(())
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}</code></pre></div>
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</div></details><h2 id="modules" class="section-header"><a href="#modules">Modules</a></h2><ul class="item-table"><li><div class="item-name"><a class="mod" href="discouraged/index.html" title="mod syn::parse::discouraged">discouraged</a></div><div class="desc docblock-short">Extensions to the parsing API with niche applicability.</div></li></ul><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.Error.html" title="struct syn::parse::Error">Error</a></div><div class="desc docblock-short">Error returned when a Syn parser cannot parse the input tokens.</div></li><li><div class="item-name"><a class="struct" href="struct.Lookahead1.html" title="struct syn::parse::Lookahead1">Lookahead1</a></div><div class="desc docblock-short">Support for checking the next token in a stream to decide how to parse.</div></li><li><div class="item-name"><a class="struct" href="struct.Nothing.html" title="struct syn::parse::Nothing">Nothing</a></div><div class="desc docblock-short">An empty syntax tree node that consumes no tokens when parsed.</div></li><li><div class="item-name"><a class="struct" href="struct.ParseBuffer.html" title="struct syn::parse::ParseBuffer">ParseBuffer</a></div><div class="desc docblock-short">Cursor position within a buffered token stream.</div></li><li><div class="item-name"><a class="struct" href="struct.StepCursor.html" title="struct syn::parse::StepCursor">StepCursor</a></div><div class="desc docblock-short">Cursor state associated with speculative parsing.</div></li></ul><h2 id="traits" class="section-header"><a href="#traits">Traits</a></h2><ul class="item-table"><li><div class="item-name"><a class="trait" href="trait.Parse.html" title="trait syn::parse::Parse">Parse</a></div><div class="desc docblock-short">Parsing interface implemented by all types that can be parsed in a default
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way from a token stream.</div></li><li><div class="item-name"><a class="trait" href="trait.Parser.html" title="trait syn::parse::Parser">Parser</a></div><div class="desc docblock-short">Parser that can parse Rust tokens into a particular syntax tree node.</div></li><li><div class="item-name"><a class="trait" href="trait.Peek.html" title="trait syn::parse::Peek">Peek</a></div><div class="desc docblock-short">Types that can be parsed by looking at just one token.</div></li></ul><h2 id="types" class="section-header"><a href="#types">Type Aliases</a></h2><ul class="item-table"><li><div class="item-name"><a class="type" href="type.ParseStream.html" title="type syn::parse::ParseStream">ParseStream</a></div><div class="desc docblock-short">Input to a Syn parser function.</div></li><li><div class="item-name"><a class="type" href="type.Result.html" title="type syn::parse::Result">Result</a></div><div class="desc docblock-short">The result of a Syn parser.</div></li></ul></section></div></main></body></html>
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