TaskQL

protocol Query {
    func serialized() -> String
}

protocol Field: Query {

}

protocol Argument {
    // ...
}

protocol Object: Query {
    associatedtype FieldType: Field
    
    var fields: [FieldType] { get }
    var arguments: [Argument]? { get }
}

// ---

enum TaskField: Field {
    case id, title
    
    func serialized() -> String {
        switch self {
        case .id:
            return "id"
        case .title:
            return "title"
        }
    }
}

struct TaskQuery: Object {
    var fields: [TaskField]
    var arguments: [Argument]?
    
    func serialized() -> String {
        return "tasks {\(fields.map({ $0.serialized() }).joined(separator: " "))}"
    }
}

enum UserField: Field {
    case id, firstName, lastName, tasks(TaskQuery)
    
    func serialized() -> String {
        switch self {
        case .id:
            return "id"
        case .firstName:
            return "firstName"
        case .lastName:
            return "lastName"
        case .tasks(let taskQuery):
            return taskQuery.serialized()
        }
    }
}

struct UserQuery: Object {
    var fields: [UserField]
    var arguments: [Argument]?
    
    func serialized() -> String {
        return "user {\(fields.map({ $0.serialized() }).joined(separator: " "))}"
    }
}

let taskQuery = TaskQuery(fields: [.id, .title], arguments: nil)
let userQuery = UserQuery(fields: [.id, .tasks(taskQuery)], arguments: nil)

userQuery.serialized()