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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>synom</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'>&#x2212;</span>]
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               </span><a class='srclink' href='../src/synom/lib.rs.html#383-395' title='goto source code'>[src]</a></span></h1>
<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">cond_reduce</span> {
    (<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">=&gt;</span> { ... };
    (<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">=&gt;</span> { ... };
}</pre>
<div class='docblock'><p>Fail to parse if condition is false, otherwise parse the given parser.</p>
<p>This is typically used inside of <code>option!</code> or <code>alt!</code>.</p>
<ul>
<li><strong>Syntax:</strong> <code>cond_reduce!(CONDITION, THING)</code></li>
<li><strong>Output:</strong> <code>THING</code></li>
</ul>

<pre class="rust rust-example-rendered">
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">syn</span>;
<span class="attribute">#[<span class="ident">macro_use</span>]</span> <span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">synom</span>;

<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">parse</span>::<span class="ident">boolean</span>;

<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>, <span class="ident">PartialEq</span>)]</span>
<span class="kw">struct</span> <span class="ident">VariadicBools</span> {
    <span class="ident">data</span>: <span class="ident">Vec</span><span class="op">&lt;</span><span class="ident">bool</span><span class="op">&gt;</span>,
    <span class="ident">variadic</span>: <span class="ident">bool</span>,
}

<span class="comment">// Parse one or more comma-separated booleans, possibly ending in &quot;...&quot; to</span>
<span class="comment">// indicate there may be more.</span>
<span class="macro">named</span><span class="macro">!</span>(<span class="ident">variadic_bools</span> <span class="op">-&gt;</span> <span class="ident">VariadicBools</span>, <span class="macro">do_parse</span><span class="macro">!</span>(
    <span class="ident">data</span>: <span class="macro">separated_nonempty_list</span><span class="macro">!</span>(<span class="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;,&quot;</span>), <span class="ident">boolean</span>) <span class="op">&gt;&gt;</span>
    <span class="ident">trailing_comma</span>: <span class="macro">option</span><span class="macro">!</span>(<span class="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;,&quot;</span>)) <span class="op">&gt;&gt;</span>
    <span class="comment">// Only allow &quot;...&quot; if there is a comma after the last boolean. Using</span>
    <span class="comment">// `cond_reduce!` is more convenient here than using `cond!`. The</span>
    <span class="comment">// alternatives are:</span>
    <span class="comment">//</span>
    <span class="comment">//   - `cond!(c, option!(p))` or `option!(cond!(c, p))`</span>
    <span class="comment">//     Gives `Some(Some(&quot;...&quot;))` for variadic and `Some(None)` or `None`</span>
    <span class="comment">//     which both mean not variadic.</span>
    <span class="comment">//   - `cond_reduce!(c, option!(p))`</span>
    <span class="comment">//     Incorrect; would fail to parse if there is no trailing comma.</span>
    <span class="comment">//   - `option!(cond_reduce!(c, p))`</span>
    <span class="comment">//     Gives `Some(&quot;...&quot;)` for variadic and `None` otherwise. Perfect!</span>
    <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">trailing_comma</span>.<span class="ident">is_some</span>(), <span class="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;...&quot;</span>))) <span class="op">&gt;&gt;</span>
    (<span class="ident">VariadicBools</span> {
        <span class="ident">data</span>: <span class="ident">data</span>,
        <span class="ident">variadic</span>: <span class="ident">variadic</span>.<span class="ident">is_some</span>(),
    })
));

<span class="kw">fn</span> <span class="ident">main</span>() {
    <span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">&quot;true, true&quot;</span>;
    <span class="kw">let</span> <span class="ident">parsed</span> <span class="op">=</span> <span class="ident">variadic_bools</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;variadic bools&quot;</span>);
    <span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, <span class="ident">VariadicBools</span> {
        <span class="ident">data</span>: <span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</span>, <span class="bool-val">true</span>],
        <span class="ident">variadic</span>: <span class="bool-val">false</span>,
    });

    <span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">&quot;true, ...&quot;</span>;
    <span class="kw">let</span> <span class="ident">parsed</span> <span class="op">=</span> <span class="ident">variadic_bools</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;variadic bools&quot;</span>);
    <span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, <span class="ident">VariadicBools</span> {
        <span class="ident">data</span>: <span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</span>],
        <span class="ident">variadic</span>: <span class="bool-val">true</span>,
    });
}</pre>
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