charParse.swift

func char(str: String) -> ParseFunction {
    var letters = Set<Character>(str.characters)
    
    return { (target: String, position: Int) -> ParseResult in
      let range = stringRange(target, start: position, advance: 1)
      let char: Character = target.substringWithRange(range).characters.first!

      if letters.contains(char) {
        return (true, [String(char)], position)
      } else {
        return (false, [nil], position)
      }
    }
}

/*-----*/
let charParse = char("abcdef")
charParse(target: "apple", position: 0) // -> (true,["a"], 0)

stringRange.swift

func stringRange(target: String, start: Int, advance: Int) -> Range<String.CharacterView.Index> {

  let idx = target.startIndex

  let range = Range(start: idx.advancedBy(start),
                    end:   idx.advancedBy(start).advancedBy(advance) )

  return range
}

/*---------------------*/

let target = "apple"
let position = 0
let range = stringRange(target, start: position, advance: 1)
let sub = target.substringWithRange(range) // => "a"