recognizing eol in scala parser combinators - scala

Question

I'm trying to make a very simple parser with parser combinators (to parse something similar to BNF). I've checked several blog posts that explain the matter (the ones top-ranked at Google (for me)) and I think I understand it but the tests say otherwise.
I've checked the questions in StackOverflow and while some could maybe be applied and useful whenever I try to apply them something else breaks, so best way to to is going through an specific example:
This is my main:
def main(args: Array[String]) {
val parser: BaseParser = new BaseParser
val eol = sys.props("line.separator")
val test = s"a = b ${eol} a = c ${eol}"
System.out.println(test)
parser.parse(test)
}
This is the parser:
import com.github.trylks.tests.parser.ParserClasses._
import scala.util.parsing.combinator.syntactical._
import scala.util.parsing.combinator.ImplicitConversions
import scala.util.parsing.combinator.PackratParsers
class BaseParser extends StandardTokenParsers with ImplicitConversions with PackratParsers {
val eol = sys.props("line.separator")
lexical.delimiters += ("=", "|", "*", "[", "]", "(", ")", ";", eol)
def rules = rep1sep(rule, eol) ^^ { Rules(_) }
def rule = id ~ "=" ~ repsep(expression, "|") ^^ flatten3 { (e1: ID, _: Any, e3: List[Expression]) => Rule(e1, e3) }
def expression: Parser[Expression] = (element | parenthesized | optional) ^^ { x => x } // and sequence and repetition, but that's another problem...
def parenthesized: Parser[Expression] = "(" ~> expression <~ ")" ^^ { x => x }
def optional: Parser[Expression] = "[" ~> expression <~ "]" ^^ { Optional(_) }
def element: Parser[Element] = (id | constant) ^^ { x => x }
def constant: Parser[Constant] = stringLit ^^ { Constant(_) }
def id: Parser[ID] = ident ^^ { ID(_) }
def parse(text: String): Option[Rules] = {
val s = rules(new lexical.Scanner(text))
s match {
case Success(res, next) => {
println("Success!\n" + res.toString)
Some(res)
}
case Error(msg, next) => {
println("error: " + msg)
None
}
case Failure(msg, next) => {
println("failure: " + msg)
None
}
}
}
}
These are the classes that you are missing from the previous part of the code:
object ParserClasses {
abstract class Element extends Expression
case class ID(value: String) extends Element {
override def toString(): String = value
}
case class Constant(value: String) extends Element {
override def toString(): String = value
}
abstract class Expression
case class Optional(value: Expression) extends Expression {
override def toString() = s"[$value]"
}
case class Rule(head: ID, body: List[Expression]) {
override def toString() = s"$head = ${body.mkString(" | ")}"
}
case class Rules(rules: List[Rule]) {
override def toString() = rules.mkString("\n")
}
}
The problem is: as the code is now, it doesn't work, it parses only one rule (not both). If I replace eol with ";" (in the main and the parser) then it works (at least for this test).
Most people seem to prefer regex parsers, every blog explaining parser combinators doesn't get into details about the traits that could be extended or not, so I have no idea about those differences or why there are several (I say this because it may be important to understand why the code doesn't work). The problem is: If I try to use regex parsers then I get errors for all the strings that I have specified in the parsers "=", "*", etc.