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<h1 class='fqn'><span class='in-band'>Module <a href='../index.html'>nom</a>::<wbr><a class='mod' href=''>macros</a></span><span class='out-of-band'><span id='render-detail'>
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</span><a id='src-3491' class='srclink' href='../../src/nom/macros.rs.html#1-2265' title='goto source code'>[src]</a></span></h1>
<div class='docblock'><p>Macro combinators</p>
<p>Macros are used to make combination easier,
since they often do not depend on the type
of the data they manipulate or return.</p>
<p>There is a trick to make them easier to assemble,
combinators are defined like this:</p>
<pre class='rust rust-example-rendered'>
<span class='macro'>macro_rules</span><span class='macro'>!</span> <span class='ident'>tag</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'>inp</span>: <span class='ident'>expr</span>) <span class='op'>=></span> (
{
...
}
);
);
</pre>
<p>But when used in other combinators, are Used
like this:</p>
<pre class='rust rust-example-rendered'>
<span class='macro'>named</span><span class='macro'>!</span>(<span class='ident'>my_function</span>, <span class='macro'>tag</span><span class='macro'>!</span>(<span class='string'>"abcd"</span>));
</pre>
<p>Internally, other combinators will rewrite
that call to pass the input as first argument:</p>
<pre class='rust rust-example-rendered'>
<span class='macro'>macro_rules</span><span class='macro'>!</span> <span class='ident'>named</span> (
(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>name</span>:<span class='ident'>ident</span>, <span class='macro-nonterminal'>$</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> (
<span class='kw'>fn</span> <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>name</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span>( <span class='ident'>i</span>: <span class='kw-2'>&</span><span class='lifetime'>'a</span> [<span class='ident'>u8</span>] ) <span class='op'>-></span> <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>crate</span>::<span class='ident'>IResult</span><span class='op'><</span><span class='lifetime'>'a</span>,<span class='kw-2'>&</span>[<span class='ident'>u8</span>], <span class='kw-2'>&</span>[<span class='ident'>u8</span>]<span class='op'>></span> {
<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>submac</span><span class='op'>!</span>(<span class='ident'>i</span>, $(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>args</span>)<span class='op'>*</span>)
}
);
);
</pre>
<p>If you want to call a combinator directly, you can
do it like this:</p>
<pre class='rust rust-example-rendered'>
<span class='kw'>let</span> <span class='ident'>res</span> <span class='op'>=</span> { <span class='macro'>tag</span><span class='macro'>!</span>(<span class='ident'>input</span>, <span class='string'>"abcd"</span>); }
</pre>
<p>Combinators must have a specific variant for
non-macro arguments. Example: passing a function
to filter! instead of another combinator.</p>
<pre class='rust rust-example-rendered'>
<span class='macro'>macro_rules</span><span class='macro'>!</span> <span class='ident'>filter</span>(
(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>input</span>:<span class='ident'>expr</span>, <span class='macro-nonterminal'>$</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> (
{
...
}
);
<span class='comment'>// wrap the function in a macro to pass it to the main implementation</span>
(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>input</span>:<span class='ident'>expr</span>, <span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>f</span>:<span class='ident'>expr</span>) <span class='op'>=></span> (
<span class='macro'>filter</span><span class='macro'>!</span>(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>input</span>, <span class='macro'>call</span><span class='macro'>!</span>(<span class='macro-nonterminal'>$</span><span class='macro-nonterminal'>f</span>));
);
);
</pre>
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