Parser Combinators

Remarks#

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
println(SimpleParser.parseAlice)
//Fails to find a match
println(SimpleParser.parseBarb)

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.