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POCan explain what is necesariy the extra rule on this PEG
 

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  1. POCan explain what is necesariy the extra rule on this PEG
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    <p>I'm playing with <a href="http://pegjs.majda.cz/" rel="nofollow">PEG.js</a> and reading and in the <a href="http://nathansuniversity.com/pegs6.html" rel="nofollow">Nathans University</a> i found a good "explain" how to build my own languaje but i'm stuck with this step</p> <p>i dont understand the <code>primary</code> can explain me please</p> <pre><code>start = comma comma = left: additive "," right:comma { return {tag: ",", left:left, right:right}; } / additive additive = left:multiplicative "+" right:additive { return {tag: "+", left:left, right:right}; } / multiplicative multiplicative = left:primary "*" right:multiplicative { return {tag: "*", left:left, right:right}; } / primary primary = integer / "(" comma:comma ")" # Can explain me why is necesary "(" and ")" { return comma; } integer = digits:[0-9]+ { return parseInt(digits.join(""), 10); } </code></pre> <h2>test</h2> <pre><code>var parse = wrapExceptions(PEG.buildParser(answer).parse); assert_eq(parse("1+2"), {tag:"+", left:1, right:2}, "parse 1+2"); assert_eq(parse("1+2*3"), {tag:"+", left:1, right:{tag:"*", left:2, right:3}}, "parse 1+2*3"); assert_eq(parse("1,2"), {tag:",", left:1, right:2}, "parse 1,2"); assert_eq(parse("1,2+3"), {tag:",", left:1, right:{tag:"+", left:2, right:3}}, "parse 1,2+3"); assert_eq(parse("1*2,3"), {tag:",", left:{tag:"*", left:1, right:2}, right:3}, "parse 1*2,3"); </code></pre> <p>my question is why is necesary the <code>/ "(" comma:comma ")"</code> if the entry does not have parenthesis, if remove that line the last test fails</p>
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    1. COThis may be obvious, but `primary` is either an `integer` or a `comma` surrounded by `()`s. What is exactly your doubt?
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      1. POCan explain what is necesariy the extra rule on this PEG
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