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POScala parser combinators and newline-delimited text
 

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  1. POScala parser combinators and newline-delimited text
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    <p>I am writing a Scala parser combinator grammar that reads newline-delimited word lists, where lists are separated by one or more blank lines. Given the following string:</p> <pre><code>cat mouse horse apple orange pear </code></pre> <p>I would like to have it return <code>List(List(cat, mouse, horse), List(apple, orange, pear))</code>.</p> <p>I wrote this basic grammar which treats word lists as newline-delimited words. Note that I had to override the default definition of <code>whitespace</code>.</p> <pre><code>import util.parsing.combinator.RegexParsers object WordList extends RegexParsers { private val eol = sys.props("line.separator") override val whiteSpace = """[ \t]+""".r val list: Parser[List[String]] = repsep( """\w+""".r, eol) val lists: Parser[List[List[String]]] = repsep(list, eol) def main(args: Array[String]) { val s = """cat |mouse |horse | |apple |orange |pear""".stripMargin println(parseAll(lists, s)) } } </code></pre> <p>This incorrectly treats blank lines as empty word lists, i.e. it returns</p> <pre><code>[8.1] parsed: List(List(cat, mouse, horse), List(), List(apple, orange, pear)) </code></pre> <p>(Note the empty list in the middle.)</p> <p>I can put an optional end of line at the end of each list.</p> <pre><code>val list: Parser[List[String]] = repsep( """\w+""".r, eol) &lt;~ opt(eol) </code></pre> <p>This handles the case where there is a single blank line between lists, but has the same problem with multiple blank lines.</p> <p>I tried changing the <code>lists</code> definition to allow multiple end-of-line delimiters:</p> <pre><code>val lists:Parser[List[List[String]]] = repsep(list, rep(eol)) </code></pre> <p>but this hangs on the above input.</p> <p>What is the correct grammar that will handle multiple blank lines as delimiters?</p>
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