naiveParser.swift

func parse(source: String) -> Person? {
    
    func startingIndex(`for` string: String, after offset: Int) -> String.Index {
        return string.index(string.startIndex, offsetBy: offset)
    }
    let lines = source.components(separatedBy: "\n")
    guard lines.count >= 4 else { return nil }
    var age: Int? = nil
    if lines[1].hasPrefix("age: ") {
        let start = startingIndex(for: lines[1], after: 5)
        age = Int(lines[1][start...start])
    }
    var name: String? = nil
    if lines[2].hasPrefix("name: ") {
        name = lines[3].substring(from: startingIndex(for: lines[3], after: 6))
    }
    var city: String? = nil
    if lines[3].hasPrefix("city: ") {
        city = lines[3].substring(from: startingIndex(for: lines[3], after: 6))
    }
    
    if let a = age, let n = name, let c = city {
        return Person(name: n, age: a, city: c)
    } else {
        return nil
    }
}

let parsedPerson = parse(source: source)
print(parsedPerson ?? "not parsed")