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POInelegant structural repetition in parser combinators
 

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    <p>While parsing some complex text, where I need to split out regular expression definitions for reuse and readability reasons, I seem to be often ending up with Scala code of this general structure (<code>pn</code> - a regex pattern, <code>vn</code> a variable):</p> <pre><code>val cp1 = p1 ~ p2 ~ p3 ~ p4 ~ p5 ~ p6 ^^ case { dummy1 ~ v2 ~ dummy3 ~ v4 ~ dummy5 ~ v6 =&gt; ACaseClass(v2, v4, v6) } </code></pre> <p>The obvious issue is the readability and maintainability of the code as new patterns need to be inserted because of the separation of the useful matches (<code>vn</code>) from the placeholder ones (<code>dummyn</code>).</p> <p>So, is there a neater way to express the intent? Could I use <code>_</code> instead for every <code>dummyn</code>?</p> <p>In the SNOBOL language, one could write <code>(pat . var)</code> or <code>(pat $ var)</code> which would assign the result of the match to the variable; similarly, in the latest regex syntax we have named capture groups <code>(?P&lt;name&gt;pat)</code>. The intent is clearly to keep the match capture variable close to the pattern.</p> <p>So, what I would like to write is something along the general lines of:</p> <pre><code>val cp1 = p1 ~ ( p2 $$ v2 ) ~ p3 ~ ( p4 $$ v4 ) ~ p5 ~ ( p6 $$ v6 ) $=&gt; ACaseClass(v2, v4, v6) </code></pre> <p>Obviously I am assuming some sort of new operators <code>$$</code> and <code>$=&gt;</code> which enable this simpler syntax.</p> <p>Conceivably macros could help, but they are rather beyond my abilities at present. Any input welcome!</p>
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