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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>syn</a>::<wbr><a class="macro" href=''>cond_reduce</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#390-406' 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">cond_reduce</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="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 class='docblock'><p>Execute a parser only if a condition is met, otherwise fail to parse.</p>
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<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>
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<ul>
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<li><strong>Syntax:</strong> <code>cond_reduce!(CONDITION, THING)</code></li>
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<li><strong>Output:</strong> <code>THING</code></li>
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</ul>
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<p>The subparser may be omitted in which case it defaults to <a href="macro.epsilon.html"><code>epsilon!</code></a>.</p>
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<ul>
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<li><strong>Syntax:</strong> <code>cond_reduce!(CONDITION)</code></li>
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<li><strong>Output:</strong> <code>()</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">Type</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>;
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">punctuated</span>::<span class="ident">Punctuated</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 possibly variadic function signature.</span>
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<span class="doccomment">///</span>
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<span class="doccomment">/// Example: `fn(A) or `fn(A, B, C, ...)` or `fn(...)`</span>
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<span class="doccomment">/// Rejected: `fn(A, B...)`</span>
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<span class="kw">struct</span> <span class="ident">VariadicFn</span> {
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<span class="ident">fn_token</span>: <span class="macro">Token</span><span class="macro">!</span>[<span class="kw">fn</span>],
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<span class="ident">paren_token</span>: <span class="ident">Paren</span>,
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<span class="ident">types</span>: <span class="ident">Punctuated</span><span class="op"><</span><span class="ident">Type</span>, <span class="macro">Token</span><span class="macro">!</span>[,]<span class="op">></span>,
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<span class="ident">variadic</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="comment">// Example of using `cond_reduce!` inside of `option!`.</span>
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<span class="kw">impl</span> <span class="ident">Synom</span> <span class="kw">for</span> <span class="ident">VariadicFn</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">fn_token</span>: <span class="macro">keyword</span><span class="macro">!</span>(<span class="kw">fn</span>) <span class="op">>></span>
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<span class="ident">params</span>: <span class="macro">parens</span><span class="macro">!</span>(<span class="macro">do_parse</span><span class="macro">!</span>(
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<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">>></span>
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<span class="comment">// Allow, but do not require, an ending `...` but only if the</span>
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<span class="comment">// preceding list of types is empty or ends with a trailing comma.</span>
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<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">>></span>
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(<span class="ident">types</span>, <span class="ident">variadic</span>)
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)) <span class="op">>></span>
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({
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<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>;
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<span class="ident">VariadicFn</span> {
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<span class="ident">fn_token</span>,
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<span class="ident">paren_token</span>,
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<span class="ident">types</span>,
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<span class="ident">variadic</span>,
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}
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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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