Я бы переписал ваше перечисление так:
public enum PredictTypes {
case favtasks
case importanttasks
case alltasks
case customList(listName: String)
var rawValue: String {
switch self {
case .favtasks: return "isFav == YES"
case .importanttasks: return "isImportant == YES"
case .alltasks: return ""
case .customList(let listName): return "listName == \(listName)"
}
}
}