Question

I am experimenting with parser combinators and I often run into what seems like infinite recursions. Here is the first one I ran into:

import util.parsing.combinator.Parsers
import util.parsing.input.CharSequenceReader

class CombinatorParserTest extends Parsers {

  type Elem = Char

  def notComma = elem("not comma", _ != ',')

  def notEndLine = elem("not end line", x => x != '\r' && x != '\n')

  def text = rep(notComma | notEndLine)

}

object CombinatorParserTest {

  def main(args:Array[String]): Unit = {
    val p = new CombinatorParserTest()
    val r = p.text(new CharSequenceReader(","))
    // does not get here
    println(r)
  }

}

How can I print what is going on? And why does this not finish?

Answer 1

Logging the attempts to parse notComma and notEndLine show that it is the end-of-file (shown as a CTRL-Z in the log(…)(“mesg”) output) that is being repeatedly parsed. Here’s how I modified your parser for this purpose:

def text = rep(log(notComma)("notComma") | log(notEndLine)("notEndLine"))

I’m not entirely sure what’s going on (I tried many variations on your grammar), but I think it’s something like this: The EOF is not really a character artificially introduced into the input stream, but rather a sort of perpetual condition at the end of the input. Thus this never-consumed EOF pseudo-character is repeatedly parsed as “either not a comma or not an end-of-line.”