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POExcluding certain elements from a specified set in Parsing Expressive Grammar (PEG.js)?
 

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  1. POExcluding certain elements from a specified set in Parsing Expressive Grammar (PEG.js)?
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    <p>I am writing a lexer for Haskell using JavaScript and Parsing Expression Grammar, the implementation I use being <a href="http://pegjs.majda.cz/" rel="nofollow">PEG.js</a>.<br> I have a problem with making it work for reserved words, as demonstrated in a simplified form here:</p> <pre><code>program = ( word / " " )+ word = ( reserved / id ) id = ( "a" / "b" )+ reserved = ( "aa" ) </code></pre> <p>The point here is to get a series of tokens that are either arbitrary sequences of a:s and/or b:s or the sequence "aa", and they are separated by spaces.<br> What I really get is either that <em>every</em> token that is not a space is recognized as <code>id</code> or that a token that should be recognised as <code>id</code> has all initial pairs of a:s eaten up as <code>reserved</code>, e.g.<br> "aab" gets recognized as <code>reserved "aa"</code> followed by <code>id "b"</code>.</p> <p>The way the Haskell lexical specification solves this ambiguity is to specify id like this: </p> <pre><code>id = ( "a" / "b" )+[BUT NOT reserved] </code></pre> <p>I have tried replicating this using various combinations of the PEG ! and &amp; -operators to acheive the same effect, but have not found a way to get this to work properly.<br> The solution:</p> <pre><code>id = !reserved ( "a" / "b" )+ </code></pre> <p>that I've seen suggested in several places does not work.<br> Is this a limitation in the particular PEG-implementation, PEG in itself or (hopefully) my methods?</p> <p>Thanks in advance!</p>
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