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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>synom</a>::<wbr><a class="macro" href=''>cond</a></span><span class='out-of-band'><span id='render-detail'>
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<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">cond</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>Conditionally execute the given parser.</p>
<p>If you are familiar with nom, this is nom&#39;s <code>cond_with_error</code> parser.</p>
<ul>
<li><strong>Syntax:</strong> <code>cond!(CONDITION, THING)</code></li>
<li><strong>Output:</strong> <code>Some(THING)</code> if the condition is true, else <code>None</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="comment">// Parses a tuple of booleans like `(true, false, false)`, possibly with a</span>
<span class="comment">// dotdot indicating omitted values like `(true, true, .., true)`. Returns</span>
<span class="comment">// separate vectors for the booleans before and after the dotdot. The second</span>
<span class="comment">// vector is None if there was no dotdot.</span>
<span class="macro">named</span><span class="macro">!</span>(<span class="ident">bools_with_dotdot</span> <span class="op">-&gt;</span> (<span class="ident">Vec</span><span class="op">&lt;</span><span class="ident">bool</span><span class="op">&gt;</span>, <span class="prelude-ty">Option</span><span class="op">&lt;</span><span class="ident">Vec</span><span class="op">&lt;</span><span class="ident">bool</span><span class="op">&gt;&gt;</span>), <span class="macro">do_parse</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="ident">before</span>: <span class="macro">separated_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">after</span>: <span class="macro">option</span><span class="macro">!</span>(<span class="macro">do_parse</span><span class="macro">!</span>(
<span class="comment">// Only allow comma if there are elements before dotdot, i.e. cannot</span>
<span class="comment">// be `(, .., true)`.</span>
<span class="macro">cond</span><span class="macro">!</span>(<span class="op">!</span><span class="ident">before</span>.<span class="ident">is_empty</span>(), <span class="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;,&quot;</span>)) <span class="op">&gt;&gt;</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">after</span>: <span class="macro">many0</span><span class="macro">!</span>(<span class="macro">preceded</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="comment">// Only allow trailing comma if there are elements after dotdot,</span>
<span class="comment">// i.e. cannot be `(true, .., )`.</span>
<span class="macro">cond</span><span class="macro">!</span>(<span class="op">!</span><span class="ident">after</span>.<span class="ident">is_empty</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="ident">after</span>)
)) <span class="op">&gt;&gt;</span>
<span class="comment">// Allow trailing comma if there is no dotdot but there are elements.</span>
<span class="macro">cond</span><span class="macro">!</span>(<span class="op">!</span><span class="ident">before</span>.<span class="ident">is_empty</span>() <span class="op">&amp;&amp;</span> <span class="ident">after</span>.<span class="ident">is_none</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="macro">punct</span><span class="macro">!</span>(<span class="string">&quot;)&quot;</span>) <span class="op">&gt;&gt;</span>
(<span class="ident">before</span>, <span class="ident">after</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, false, false)&quot;</span>;
<span class="kw">let</span> <span class="ident">parsed</span> <span class="op">=</span> <span class="ident">bools_with_dotdot</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;bools with dotdot&quot;</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, (<span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</span>, <span class="bool-val">false</span>, <span class="bool-val">false</span>], <span class="prelude-val">None</span>));
<span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">&quot;(true, true, .., true)&quot;</span>;
<span class="kw">let</span> <span class="ident">parsed</span> <span class="op">=</span> <span class="ident">bools_with_dotdot</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;bools with dotdot&quot;</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, (<span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</span>, <span class="bool-val">true</span>], <span class="prelude-val">Some</span>(<span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</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">bools_with_dotdot</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;bools with dotdot&quot;</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, (<span class="macro">vec</span><span class="macro">!</span>[], <span class="prelude-val">Some</span>(<span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</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">bools_with_dotdot</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;bools with dotdot&quot;</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, (<span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</span>, <span class="bool-val">true</span>], <span class="prelude-val">Some</span>(<span class="macro">vec</span><span class="macro">!</span>[])));
<span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">&quot;(..)&quot;</span>;
<span class="kw">let</span> <span class="ident">parsed</span> <span class="op">=</span> <span class="ident">bools_with_dotdot</span>(<span class="ident">input</span>).<span class="ident">expect</span>(<span class="string">&quot;bools with dotdot&quot;</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, (<span class="macro">vec</span><span class="macro">!</span>[], <span class="prelude-val">Some</span>(<span class="macro">vec</span><span class="macro">!</span>[])));
}</pre>
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