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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><a class='srclink' href='../src/synom/lib.rs.html#383-395' 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="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>Fail to parse if condition is false, otherwise parse the given parser.</p>
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<p>This is typically used inside of <code>option!</code> or <code>alt!</code>.</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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<pre class="rust rust-example-rendered">
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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="attribute">#[<span class="ident">macro_use</span>]</span> <span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">synom</span>;
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<span class="kw">use</span> <span class="ident">syn</span>::<span class="ident">parse</span>::<span class="ident">boolean</span>;
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>, <span class="ident">PartialEq</span>)]</span>
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<span class="kw">struct</span> <span class="ident">VariadicBools</span> {
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<span class="ident">data</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">bool</span><span class="op">></span>,
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<span class="ident">variadic</span>: <span class="ident">bool</span>,
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}
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<span class="comment">// Parse one or more comma-separated booleans, possibly ending in "..." to</span>
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<span class="comment">// indicate there may be more.</span>
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<span class="macro">named</span><span class="macro">!</span>(<span class="ident">variadic_bools</span> <span class="op">-></span> <span class="ident">VariadicBools</span>, <span class="macro">do_parse</span><span class="macro">!</span>(
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<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">","</span>), <span class="ident">boolean</span>) <span class="op">>></span>
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<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">","</span>)) <span class="op">>></span>
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<span class="comment">// Only allow "..." if there is a comma after the last boolean. Using</span>
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<span class="comment">// `cond_reduce!` is more convenient here than using `cond!`. The</span>
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<span class="comment">// alternatives are:</span>
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<span class="comment">//</span>
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<span class="comment">// - `cond!(c, option!(p))` or `option!(cond!(c, p))`</span>
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<span class="comment">// Gives `Some(Some("..."))` for variadic and `Some(None)` or `None`</span>
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<span class="comment">// which both mean not variadic.</span>
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<span class="comment">// - `cond_reduce!(c, option!(p))`</span>
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<span class="comment">// Incorrect; would fail to parse if there is no trailing comma.</span>
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<span class="comment">// - `option!(cond_reduce!(c, p))`</span>
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<span class="comment">// Gives `Some("...")` for variadic and `None` otherwise. Perfect!</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">trailing_comma</span>.<span class="ident">is_some</span>(), <span class="macro">punct</span><span class="macro">!</span>(<span class="string">"..."</span>))) <span class="op">>></span>
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(<span class="ident">VariadicBools</span> {
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<span class="ident">data</span>: <span class="ident">data</span>,
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<span class="ident">variadic</span>: <span class="ident">variadic</span>.<span class="ident">is_some</span>(),
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})
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));
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">"true, true"</span>;
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<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">"variadic bools"</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, <span class="ident">VariadicBools</span> {
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<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>],
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<span class="ident">variadic</span>: <span class="bool-val">false</span>,
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});
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<span class="kw">let</span> <span class="ident">input</span> <span class="op">=</span> <span class="string">"true, ..."</span>;
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<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">"variadic bools"</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">parsed</span>, <span class="ident">VariadicBools</span> {
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<span class="ident">data</span>: <span class="macro">vec</span><span class="macro">!</span>[<span class="bool-val">true</span>],
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<span class="ident">variadic</span>: <span class="bool-val">true</span>,
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});
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}</pre>
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