Streams in FPDart about fpdart

[dfdgsdfg]

I think dart_either has a practical implementation.

fp_dart already has TaskEither.tryCatch which is generate ADT from Effect, so TaskEither.tryCatchStream would be have.

https://github.com/hoc081098/dart_either/blob/master/lib/src/dart_either.dart#L349-L384

  /// Transforms data events to [Right]s and error events to [Left]s.
  ///
  /// When the source stream emits a data event, the result stream will emit
  /// a [Right] wrapping that data event.
  ///
  /// When the source stream emits a error event, calling [errorMapper] with that error
  /// and the result stream will emits a [Left] wrapping the result.
  ///
  /// The done events will be forwarded.
  ///
  /// ### Example
  /// ```dart
  /// final Stream<int> s = Stream.fromIterable([1, 2, 3, 4]);
  /// final Stream<Either<Object, int>> eitherStream = Either.catchStreamError((e, s) => e, s);
  ///
  /// eitherStream.listen(print); // prints Either.Right(1), Either.Right(2),
  ///                             // Either.Right(3), Either.Right(4),
  /// ```
  ///
  /// ```dart
  /// final Stream<int> s = Stream.error(Exception());
  /// final Stream<Either<Object, int>> eitherStream = Either.catchStreamError((e, s) => e, s);
  ///
  /// eitherStream.listen(print); // prints Either.Left(Exception)
  /// ```
  static Stream<Either<L, R>> catchStreamError<L, R>(
    ErrorMapper<L> errorMapper,
    Stream<R> stream,
  ) =>
      stream.transform(
        StreamTransformer<R, Either<L, R>>.fromHandlers(
          handleData: (data, sink) => sink.add(Either.right(data)),
          handleError: (e, s, sink) =>
              sink.add(Either.left(errorMapper(e.throwIfFatal(), s))),
        ),
      );

/// Map [error] and [stackTrace] to a [T] value.
typedef ErrorMapper<T> = T Function(Object error, StackTrace stackTrace);

[tim-smart]

I have been working on a alternative to Stream that is more functional in nature:

https://pub.dev/packages/offset_iterator

I have recently added OffsetIterable.fromStreamEither which might be of interest! You can see it in action in this test case:
https://github.com/tim-smart/offset_iterator/blob/main/packages/offset_iterator/test/offset_iterator_test.dart#L111

[SandroMaglione]

Hi @lloydrichards

Streams are a very interesting (and complex) usecase 😅

There are some patterns used in functional programming to deal with Streams. Adding them to fpdart I think would be interesting but also hard work!

Curious to hear any opinion about this 👀

[lloydrichards]

I would like such a feature 😁 my app uses streams nearly constantly for database reading so being able to abstract my left and right makes it much easier for my application layer to return the correct state.

Currently, I rely on RxDart to add more functionality to the stream such as the .onErrorReturnWith() which gives me the correct effect, but can maybe be packaged a little better

[TheHypnoo]

@SandroMaglione
It would be interesting to prioritize this part, since Future and Streams are commonly used. As in the case of my application