public struct Contact: Decodable {
    let contact_ID: Int
}
  
enum ContactDataMapper {
    
    static func map(_ data: Data, from response: URLResponse) throws -> [Contact] {
        guard let contacts = try? JSONDecoder().decode([Contact].self, from: data) else {
            throw APIError.invalidData
        }
        return contacts
    }
}

public enum APIError: Swift.Error {
    case invalidData
    case serverDefined(String)
    case connectivity
}