parse.scala

  type ParseState = Int
  type ParseResult[A] = (A, ParseState)
  type Parser[A] = ParseState => ParseResult[A]

  def map[A, B](p: Parser[A])(fn: A => B): Parser[B] =
    p(_) match {
      case (a, n) => (fn(a), n)
    }

  def flatMap[A, B](p: Parser[A])(fn: A => Parser[B]): Parser[B] =
    p(_) match {
      case (a, n) => fn(a)(n)
    }

  def expect[A, B](p: Parser[A], expected: A, p2: Parser[B]): Parser[B] =
    flatMap(p){a =>
      require(a == expected, a.toString)
      p2
    }