personName.swift

func personName(firstName: String, lastName: String?) -> String {
    guard let lastName = lastName else {
        return "\(firstName)"
    }

    return "\(firstName) \(lastName)"
}

personName("Siong", lastName: "Ong") // prints "Siong Ong"
personName("Siong", lastName: nil) // prints "Siong"
personName("Siong") // This dosen't work but it should, nil should be auto-assigned to optional parameter :(