struct Parser<A> {
  let run: (inout Substring) -> A?

  func map<B>(_ f: @escaping (A) -> B) -> Parser<B> {
    return Parser<B> { input in
      self.run(&input).map(f)
    }
  }
}

let char = Parser<Character> { str in
  guard let match = str.first else { return nil }
  str.removeFirst()
  return match
}

let northSouth = char
  .map {
    $0 == "N" ? 1.0
      : $0 == "S" ? -1
      : nil
}