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<section id='main' class="content"><h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>syn</a>::<wbr><a class="macro" href=''>cond</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../src/syn/parsers.rs.html#307-323' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">cond</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">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> { ... };
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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">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> { ... };
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}</pre>
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</div><div class='docblock'><p>Execute a parser only if a condition is met, otherwise return None.</p>
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<p>If you are familiar with nom, this is nom's <code>cond_with_error</code> parser.</p>
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<ul>
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<li><strong>Syntax:</strong> <code>cond!(CONDITION, THING)</code></li>
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<li><strong>Output:</strong> <code>Some(THING)</code> if the condition is true, else <code>None</code></li>
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</ul>
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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">use</span> <span class="ident">syn</span>::{<span class="ident">Ident</span>, <span class="ident">MacroDelimiter</span>};
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">token</span>::{<span class="ident">Paren</span>, <span class="ident">Bracket</span>, <span class="ident">Brace</span>};
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">synom</span>::<span class="ident">Synom</span>;
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<span class="doccomment">/// Parses a macro call with empty input. If the macro is written with</span>
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<span class="doccomment">/// parentheses or brackets, a trailing semicolon is required.</span>
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<span class="doccomment">///</span>
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<span class="doccomment">/// Example: `my_macro!{}` or `my_macro!();` or `my_macro![];`</span>
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<span class="kw">struct</span> <span class="ident">EmptyMacroCall</span> {
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<span class="ident">name</span>: <span class="ident">Ident</span>,
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<span class="ident">bang_token</span>: <span class="macro">Token</span><span class="macro">!</span>[<span class="op">!</span>],
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<span class="ident">empty_body</span>: <span class="ident">MacroDelimiter</span>,
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<span class="ident">semi_token</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="macro">Token</span><span class="macro">!</span>[;]<span class="op">></span>,
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}
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<span class="kw">fn</span> <span class="ident">requires_semi</span>(<span class="ident">delimiter</span>: <span class="kw-2">&</span><span class="ident">MacroDelimiter</span>) <span class="op">-></span> <span class="ident">bool</span> {
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<span class="kw">match</span> <span class="kw-2">*</span><span class="ident">delimiter</span> {
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<span class="ident">MacroDelimiter</span>::<span class="ident">Paren</span>(<span class="kw">_</span>) <span class="op">|</span> <span class="ident">MacroDelimiter</span>::<span class="ident">Bracket</span>(<span class="kw">_</span>) <span class="op">=></span> <span class="bool-val">true</span>,
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<span class="ident">MacroDelimiter</span>::<span class="ident">Brace</span>(<span class="kw">_</span>) <span class="op">=></span> <span class="bool-val">false</span>,
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}
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}
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<span class="kw">impl</span> <span class="ident">Synom</span> <span class="kw">for</span> <span class="ident">EmptyMacroCall</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">do_parse</span><span class="macro">!</span>(
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<span class="ident">name</span>: <span class="macro">syn</span><span class="macro">!</span>(<span class="ident">Ident</span>) <span class="op">>></span>
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<span class="ident">bang_token</span>: <span class="macro">punct</span><span class="macro">!</span>(<span class="op">!</span>) <span class="op">>></span>
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<span class="ident">empty_body</span>: <span class="macro">alt</span><span class="macro">!</span>(
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<span class="macro">parens</span><span class="macro">!</span>(<span class="macro">epsilon</span><span class="macro">!</span>()) <span class="op">=></span> { <span class="op">|</span><span class="ident">d</span><span class="op">|</span> <span class="ident">MacroDelimiter</span>::<span class="ident">Paren</span>(<span class="ident">d</span>.<span class="number">0</span>) }
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<span class="op">|</span>
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<span class="macro">brackets</span><span class="macro">!</span>(<span class="macro">epsilon</span><span class="macro">!</span>()) <span class="op">=></span> { <span class="op">|</span><span class="ident">d</span><span class="op">|</span> <span class="ident">MacroDelimiter</span>::<span class="ident">Bracket</span>(<span class="ident">d</span>.<span class="number">0</span>) }
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<span class="op">|</span>
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<span class="macro">braces</span><span class="macro">!</span>(<span class="macro">epsilon</span><span class="macro">!</span>()) <span class="op">=></span> { <span class="op">|</span><span class="ident">d</span><span class="op">|</span> <span class="ident">MacroDelimiter</span>::<span class="ident">Brace</span>(<span class="ident">d</span>.<span class="number">0</span>) }
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) <span class="op">>></span>
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<span class="ident">semi_token</span>: <span class="macro">cond</span><span class="macro">!</span>(<span class="ident">requires_semi</span>(<span class="kw-2">&</span><span class="ident">empty_body</span>), <span class="macro">punct</span><span class="macro">!</span>(;)) <span class="op">>></span>
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(<span class="ident">EmptyMacroCall</span> {
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<span class="ident">name</span>,
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<span class="ident">bang_token</span>,
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<span class="ident">empty_body</span>,
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<span class="ident">semi_token</span>,
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})
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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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