How should we read values from an Effect in the future.

[notcome]

So I decided to live on the edge and try to migrate my codebase to prerelease/1.0. One thing I noticed that the conformance to Publisher is now deprecated. However at the moment, we don't have any way to read values from an Effect, other than treating it as a Combine Publisher. What should we do instead? Will Effect become an AsyncSequence?

[tgrapperon]

Hey @notcome! Conflating Effect with Publisher<_, Never> is a byproduct of Effect being based on Publisher. Ideally, your dependencies should produce some Publisher<Output, Failure>. In other words, the dependency doesn't produce an Effect, it produces a Publisher of events. Your reducer can then use this publisher to transform this sequence of events into a sequence of Actions (that is, an Effect). You can't really read values from an Effect because an Effect isn't strictly speaking something that produces values¹. This is something that is expressed by the Store.

Once you keep this dichotomy in mind, the conversion is simple: The publisher of events becomes an async sequence of events, and your reducer can iterate over this sequence of events to produce actions for the store. There is an example of such conversion in EffectPublisher's documentation.

To sum up, pre-1.0, Effect is a Publisher and a Publisher of Actions that can't fail is an Effect, but in 1.0, Effect is an opaque type and a Publisher of Actions that can't fail is not an Effect (but it can used to create one Effect). Pre-1.0, A Publisher<Output, Never> is technically an Effect<Output>, but this doesn't make a lot of sense if Output is not the Action for some Store, as Effect's only purpose is to be consumed by a Store.

If you encounter issues while modernizing your codebase, feel free to post a few problematic bits, so we can provide more specific orientations!

Footnotes

1. It is right now because Effects are Publisher, but this will not be the case anymore in 1.0. ↩

[notcome]

Thanks! I think that made lots of sense. Lately my code was abusing Effect as a replacement for AnyPublisher, and that probably isn't a very good idea. I used it to read update from Apple and non-Apple APIs. I should probably move it to AsyncSequence, using things like AsyncChannel instead.

[tgrapperon]

I'm not certain that you need AsyncChannel, but yes, modelling your dependencies as AsyncSequences is probably a good move. You can easily convert any publisher to an async sequence using the .values property starting from iOS 15, as shown in EffectPublisher's documentation.
Please note that the de-facto erasure for AsyncSequence in TCA is to wrap it in an AsyncStream (Dependencies provides an eraseToStream() method that you can call on any AsyncSequence for this purpose).

[notcome]

I'm not certain that you need AsyncChannel

I was abusing Effect.run to provide implementations for my dependencies. So converting them to AsyncChannel feels natural, based on my limited experience with swift-async-algorithms, 😂. But certainly in many cases I can do it the functional style.