178 lines
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<section id='main' class="content">
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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>syn</a>::<wbr><a class="macro" href=''>alt</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>−</span>]
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</a>
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</span><a class='srclink' href='../src/syn/parsers.rs.html#861-905' title='goto source code'>[src]</a></span></h1>
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<pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">alt</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">e</span>:<span class="ident">ident</span> <span class="op">|</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>:<span class="ident">tt</span>)<span class="kw-2">*</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">subrule</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="kw-2">*</span>) <span class="op">|</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>:<span class="ident">tt</span>)<span class="kw-2">*</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">subrule</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="macro-nonterminal">$</span><span class="macro-nonterminal">gen</span>:<span class="ident">expr</span> } <span class="op">|</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>:<span class="ident">tt</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">e</span>:<span class="ident">ident</span> <span class="op">=></span> { <span class="macro-nonterminal">$</span><span class="macro-nonterminal">gen</span>:<span class="ident">expr</span> } <span class="op">|</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>:<span class="ident">tt</span>)<span class="kw-2">*</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">e</span>:<span class="ident">ident</span> <span class="op">=></span> { <span class="macro-nonterminal">$</span><span class="macro-nonterminal">gen</span>:<span class="ident">expr</span> }) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">subrule</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="macro-nonterminal">$</span><span class="macro-nonterminal">gen</span>:<span class="ident">expr</span> }) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">e</span>:<span class="ident">ident</span>) <span class="op">=></span> { ... };
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">subrule</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="kw-2">*</span>)) <span class="op">=></span> { ... };
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}</pre>
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<div class='docblock'><p>Run a series of parsers, returning the result of the first one which
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succeeds.</p>
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<p>Optionally allows for the result to be transformed.</p>
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<ul>
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<li><strong>Syntax:</strong> <code>alt!(THING1 | THING2 => { FUNC } | ...)</code></li>
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<li><strong>Output:</strong> <code>T</code>, the return type of <code>THING1</code> and <code>FUNC(THING2)</code> and ...</li>
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</ul>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">macro_use</span>]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">syn</span>;
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">proc_macro2</span>;
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<span class="kw">use</span> <span class="ident">proc_macro2</span>::{<span class="ident">Ident</span>, <span class="ident">Span</span>};
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<span class="comment">// Parse any identifier token, or the `!` token in which case the</span>
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<span class="comment">// identifier is treated as `"BANG"`.</span>
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<span class="macro">named</span><span class="macro">!</span>(<span class="ident">ident_or_bang</span> <span class="op">-></span> <span class="ident">Ident</span>, <span class="macro">alt</span><span class="macro">!</span>(
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<span class="macro">syn</span><span class="macro">!</span>(<span class="ident">Ident</span>)
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<span class="op">|</span>
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<span class="macro">punct</span><span class="macro">!</span>(<span class="op">!</span>) <span class="op">=></span> { <span class="op">|</span>_<span class="op">|</span> <span class="ident">Ident</span>::<span class="ident">new</span>(<span class="string">"BANG"</span>, <span class="ident">Span</span>::<span class="ident">call_site</span>()) }
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));</pre>
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<p>The <code>alt!</code> macro is most commonly seen when parsing a syntax tree enum such
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as the <a href="enum.Item.html"><code>Item</code></a> enum.</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">impl</span> <span class="ident">Synom</span> <span class="kw">for</span> <span class="ident">Item</span> {
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<span class="macro">named</span><span class="macro">!</span>(<span class="ident">parse</span> <span class="op">-></span> <span class="self">Self</span>, <span class="macro">alt</span><span class="macro">!</span>(
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<span class="macro">syn</span><span class="macro">!</span>(<span class="ident">ItemExternCrate</span>) <span class="op">=></span> { <span class="ident">Item</span>::<span class="ident">ExternCrate</span> }
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<span class="op">|</span>
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<span class="macro">syn</span><span class="macro">!</span>(<span class="ident">ItemUse</span>) <span class="op">=></span> { <span class="ident">Item</span>::<span class="ident">Use</span> }
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<span class="op">|</span>
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<span class="macro">syn</span><span class="macro">!</span>(<span class="ident">ItemStatic</span>) <span class="op">=></span> { <span class="ident">Item</span>::<span class="ident">Static</span> }
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<span class="op">|</span>
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<span class="macro">syn</span><span class="macro">!</span>(<span class="ident">ItemConst</span>) <span class="op">=></span> { <span class="ident">Item</span>::<span class="ident">Const</span> }
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<span class="op">|</span>
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<span class="comment">/* ... */</span>
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));
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}</pre>
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<p><em>This macro is available if Syn is built with the <code>"parsing"</code> feature.</em></p>
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Accepted types are: <code>fn</code>, <code>mod</code>,
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Search functions by type signature (e.g.
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