learning about parser combinators (this is likely wrong code)

pcomb.swift

struct Parser<Input: CollectionType, Output> {
    typealias Function = (Input, Input.Index) -> (Output, Input.Index)?
}

func parseString(string: String) -> Parser<String, String>.Function {
    return { str, index in
        
        let end = advance(index, count(string))
        
        if str.substringWithRange(index..<end) == string {
            return (string, end)
        } else {
            return nil
        }
    }
}

func concat<C: CollectionType, T, U>(ƒ: Parser<C, T>.Function, g: Parser<C, U>.Function) -> Parser<C, (T, U)>.Function {
    return { c, index in
        if let first = ƒ(c, index), second = g(c, first.1) {
            return ((first.0, second.0), second.1)
        } else {
            return nil
        }
    }
}

func parse<Input: CollectionType, Output>(ƒ: Parser<Input, Output>.Function, input: Input) -> Output? {
    return ƒ(input, input.startIndex).flatMap { $1 == input.endIndex ? $0 : nil }
}

let parser = concat(parseString("asdf"), parseString("asdf"))
println(parse(parser, "asdfasdf"))