Scala Language Parser Combinators


ParseResult Cases

A ParseResult comes in three flavors:

  • Success, with a marker as to the start of the match and the next character to be matched.
  • Failure, with a marker as to the start of where the match was attempted. In this case the parser backtracks to that position, where it will be when parsing continues.
  • Error, which stops the parsing. No backtracking or further parsing occurs.

Basic Example

import scala.util.parsing.combinator._

class SimpleParser extends RegexParsers {
  // Define a grammar rule, turn it into a regex, and apply it the input.
  def word: Parser[String] = """[A-Z][a-z]+""".r ^^ { _.toString }

object SimpleParser extends SimpleParser {
  val parseAlice = parse(word, "Alice went to Alamo Square.")
  val parseBarb = parse(word, "barb went Upside Down.")

//Successfully finds a match
//Fails to find a match

The output will be as follows:

[1.6] parsed: Alice
res0: Unit = ()

[1.1] failure: string matching regex `[A-Z][a-z]+' expected but `b' found

barb went Upside Down.

[1.6] in the Alice example indicates that the start of the match is at position 1, and the fist character remaining to match starts at position 6.