How can I define a return type of a lambda function in Dart?

Question

I try to create a repository with type safe parameters. For this, I created the function parameter as a typedef and use the Dartz package, to be able to return either a failure or the desired result.

The issue is, that I have to create a specific function, if I want to pass an argument of the typedef type.

typedef FailOrModel<T> = Future<Either<Failure, T>> Function();

class Repository {

  Future<Either<Failure, T>> _runAfterSync<T>(FailOrModel func) async {
    await synchronize().then(
      (value) => value.fold(
        (failure) {
          log.e('Synchronization error: $failure');
        },
        (_) {},
      ),
    );
    return await func();
  }
  
  Future<Either<Failure, Storage>> getStorage(int id) async {
    // Todo: Find out why lambda does not work
    Future<Either<Failure, Storage>> func() async {
      try {
        final model = await localDataSource.getStorage(id);
        return Right(model);
      } on CacheException {
        log.e('Could not load storage with id $id');
        return Left(CacheFailure());
      }
    }

    return await _runAfterSync<Storage>(func);
  }
}

When I pass the identical function body as a lambda function directly, as I would like to, I get a runtime error:

  Future<Either<Failure, Storage>> getStorage(int id) async {
    return await _runAfterSync<Storage>(() async {
      try {
        final model = await localDataSource.getStorage(id);
        return Right(model);
      } on CacheException {
        log.e('Could not load storage with id $id');
        return Left(CacheFailure());
      }
    });
  }

type 'Right<Failure, dynamic>' is not a subtype of type 'FutureOr<Either<Failure, Storage>>'

It is not a big deal, but maybe someone could enlighten me, how I am able to use a lambda function here, or why I get this error, if I use a lambda function.

Answer 1

The expected return type of getStorage(int) from a Future is Either<Failure, Storage>. The mismatch occurs because you're trying to return Right<Failure, dynamic>.

As what have been mentioned in the comments, you can pass FailOrModel<T> in _runAfterSync to solve the issue.