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POParsing data with Parsec and omitting comments
 

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    <p>I am trying to write a Haksell Parsec Parser that parses input data from a file into the LogLine datatype as follows:</p> <pre><code>--Final parser that holds the indvidual parsers. final :: Parser [LogLine] final = do{ logLines &lt;- sepBy1 logLine eol ; return logLines } --The logline token declaration logLine :: Parser LogLine logLine = do name &lt;- plainValue -- parse the name (identifier) many1 space -- parse and throw away a space args1 &lt;- bracketedValue -- parse the first arguments many1 space -- throw away the second sapce args2 &lt;- bracketedValue -- parse the second list of arguments many1 space -- constant &lt;- plainValue -- parse the constant identifier space weighting &lt;- plainValue --parse the weighting double space return $ LogLine name args1 args2 constant weighting </code></pre> <p>It parses everything just fine, but now I need to add comments to the file, and I have to modify the parser so that it ignores them. It should support single-line comments only beginning with "--" and ending with a '\n' I've tried defining the comment token as follows:</p> <pre><code>comments :: Parser String comments = do string "--" comment &lt;- (manyTill anyChar newline) return "" </code></pre> <p>And then plugging it into the <code>final</code> parser like so:</p> <pre><code>final :: Parser [LogLine] final = do optional comments logLines &lt;- sepBy1 logLine (comments&lt;|&gt;newline) optional comments return logLines </code></pre> <p>It compiles fine, but it does not parse. I've tried several minor modifications but the best result was parsing everything up to the first comment, so I'm beginning to think that this is not the way to do it. PS: I've seen this <a href="https://stackoverflow.com/questions/12940490/how-would-i-strip-out-comments-from-a-file-with-parsec">Similar Question</a>, but it is slightly different from what I'm trying to achieve.</p>
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    1. USAtanas Bozhkov
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    1. COI suspect that at least part of your problem is that `logLine` requires exactly one space before and after `weighting`. Depending on your format, making them consume any amount of spaces (`' '` only or all whitespace) may be the way to go. The separator could then be `optional comments` or `comments <|> fmap return newline` (Note: `comments <|> newline` shouldn't compile since `comments` returns `[Char]` and `newline` a `Char`.)
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      1. USDaniel Fischer
    2. COOkay the format of the input file is such that on each row you have either a logLine or a comment, you can't have more than one of them on the same line. Something like this: `name arg1 arg2 c1 weight --comment goes here. name2 arg1 arg2 c2 weight2` I know that requiring the extra space after the weight is alogical, but if I remove it, it doesn't parse for whatever reason. Something that I want to ask you, when you're saying the _separator_ do you mean that I should define a new separator or just do something like: `logLines <- sepBy1 (logLine) (comments <|> fmap return newline)`
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      1. USAtanas Bozhkov
    3. COSorry the newlines didn't come up. The example should have been as so: name arg1 arg2 c1 weight \n --comment goes here.\n name2 arg1 arg2 c2 weight2
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