Dart - Problem replicating sealed classes like kotlin - flutter

Question

I'm getting this error on a starting example that I'm writing
type 'Error<dynamic>' is not a subtype of type 'FutureOr<Success<List<TaskEntity>>>'
I've created a generic Result class like this:
enum DataStatus { local, remote, error }
enum ErrorStatus { unknown, backend_error, timeout }
#sealed
abstract class Result<T> {}
class Success<T> extends Result<T> {
final T data;
final DataStatus dataStatus;
final String message;
Success({this.data, this.dataStatus, this.message});
}
class Error extends Result {
final Exception exception;
final ErrorStatus errorStatus;
final String message;
Error({this.exception, this.errorStatus, this.message});
}
On my repository I have this method:
Future<Result<List<TaskEntity>>> getTasks() {
return remoteDataSource
.getTasks()
.then((value) => Success(
data: value.map((e) => e.toTaskEntity()).toList(),
dataStatus: DataStatus.remote))
.catchError((Object obj) {
switch (obj.runtimeType) {
case DioError:
final dioErrorResponse = (obj as DioError).response;
return Error(
exception: Exception(dioErrorResponse.statusMessage),
errorStatus: ErrorStatus.backend_error,
message: dioErrorResponse.statusMessage);
break;
default:
}
});
My approach is to use this class to return the responses and If there's an error return the error.
Right now I'm blocked because I'm getting this error
type 'Error<dynamic>' is not a subtype of type 'FutureOr<Success<List<TaskEntity>>>'
I also tried to use the freezed library but no luck.
part 'result.freezed.dart';
#freezed
abstract class Result<T> with _$Result<T> {
const factory Result.success({T data, DataStatus dataStatus, String message}) = Success<T>;
const factory Result.error({Exception exception, ErrorStatus errorStatus, String message}) = Error;
}

Answer 1

Your problem is that you never specified a T for your error class, neither directly, nor implicitely. So it is an Error<dynamic>. But an Error<dynamic> does not extend Result<List<TaskEntity>>. An Error<List<TaskEntity>> would. So you need to make sure that your Error class gets a type for T too.
To be honest the whole .then .catch thing is way to complicated for me. I would have used async and await with normal try/catch blocks. But if you want to keep it as it is, at least make sure you do this:
return Error<List<TaskEntity>>(
so your Error<> class is of the right type syntactically.
Just as an example of what I mean by making it easier by using async and await:
Future<Result<List<TaskEntity>>> getTasks() async {
try {
final results = await remoteDataSource.getTasks();
final tasks = results.map((e) => e.toTaskEntity()).toList()
return Success<List<TaskEntity>>(data: tasks, dataStatus: DataStatus.remote);
} catch (e) {
if (e is DioError) {
return Error<List<TaskEntity>>(
exception: Exception(e.response.statusMessage),
errorStatus: ErrorStatus.backend_error,
message: e.response.statusMessage);
} else {
// you are missing a case here? what do you want to return here?
}
}
}
For me this is a lot easier to read, to the point where it's obvious you are missing a case. There needs to be a return statement there.