parboiled2 Illegal rule composition

Question

I am writing an cron parser, but compiler complains illegal rule composition,

What's wrong with my parser?

import org.parboiled2._

sealed trait Part
case class Fixed(points: Seq[Int]) extends Part
case class Range(start: Int, end: Int) extends Part
case class Every(start: Int, interval: Int) extends Part
case object Full extends Part
case object Ignore extends Part

class CronParser(val input: ParserInput) extends Parser {

  def number = rule { capture(digits) ~> (_.toInt) }

  def digits = rule { oneOrMore(CharPredicate.Digit) }

  def fixed = rule { oneOrMore(number).separatedBy(",") ~> Fixed }

  def range = rule { digits ~ '-' ~ digits ~> Range }

  def every= rule { digits ~ '/' ~ digits ~> Every }

  def full= rule { '*' ~ push(Full) }

  def ignore = rule { '?' ~ push(Ignore) }

  def part = rule { fixed | range | every | full | ignore }

  def expr = rule { part ~ part ~ part ~ part ~ part}
}

Answer 1

You're using digits where I think you want to be using number. The following should work just fine:

class CronParser(val input: ParserInput) extends Parser {
  def number = rule { capture(digits) ~> (_.toInt) }
  def digits = rule { oneOrMore(CharPredicate.Digit) }
  def fixed = rule { oneOrMore(number).separatedBy(",") ~> Fixed }
  def range = rule { number ~ '-' ~ number ~> Range }
  def every = rule { number ~ '/' ~ number ~> Every }
  def full = rule { '*' ~ push(Full) }
  def ignore = rule { '?' ~ push(Ignore) }
  def part = rule { fixed | range | every | full | ignore }
  def expr = rule { part ~ part ~ part ~ part ~ part }
}

The problem was that digits doesn't push a value, which meant that range, etc. were rules that wanted to pop values off the stack, and these weren't able to be composed with ~.