Hello, I'm trying to get my mind around Arrow (0.1...

# arrow

[Ahmed Ibrahim]

Hello, I'm trying to get my mind around Arrow (0.11.0-SNAPSHOT), and trying to implement a simple app using its FP power.
I have this class that retrieves Ids from a service, and then use theses Ids to get the details in parallel, then combine the result into a list to be used in the ui layer.

The problem now once an error happens, it just crashes even though I'm handling things with Either fold. What I'm doing wrong in my case? and how can I prevent this code from crashing?

class GetStoriesListUseCase @Inject constructor(
    private val hackerNewsService: HackerNewsService
) {

    suspend fun topStories(
        pageNumber: Int,
        pageSize: Int = 100
    ): Either<Throwable, List<StoryUIModel>> =
        readTopStoriesIds(pageSize)
            .flatMap { getTopStoriesDetails(it) }
            .suspended()

    private fun getTopStoriesDetails(topStoriesIds: List<Id<Long>>): IO<Throwable, List<StoryUIModel>> =
        topStoriesIds.map { it.value() }.parTraverse { anId ->
            IO.effect(<http://Dispatchers.IO|Dispatchers.IO>) {
                Timber.d("${Thread.currentThread().name}")
                hackerNewsService.topStoryDetail(anId).toStoryUIModel()
            }
        }

    private fun readTopStoriesIds(pageSize: Int): IO<Throwable, List<Id<Long>>> =
        IO.effect {
            hackerNewsService.topStoriesIds().map { Id.just(it) }.take(pageSize)
        }

    private fun StoryDetails.toStoryUIModel() =
        StoryUIModel(id = id, title = title, author = by, url = url)
}

[aballano]

Hi there @Ahmed Ibrahim! I assume it crashes on the topStories function?

[Ahmed Ibrahim]

Yep, that's correct, in case of network error, it crashes

[aballano]

it could be because you’re transforming an IO into a suspend, so if the IO you get has an exception (throwable, not E), it’s then mapped to how coroutines work, and they throw

[aballano]

an alternative in case you still want to use suspend is to do

someIO.attempt().suspended()

[aballano]

attempt does as follows

IO<E, A>.attempt(): IO<E, Either<Throwable, A>>

so any fatal failure will be put into the either, but it still allows you to fold over your expected error E and the value

[aballano]

otherwise I’d recommend not converting IO to suspended in order to have more functionality

[Ahmed Ibrahim]

But I thought IO<E, A> was primarily introduced to replace doing IO work gracefully instead of working with Either?

[aballano]

yes, but the problem here AFAIK is that you’re narrowing the type from IO<E, A> to a suspend function, which allows A or throws

[aballano]

so the recommended way from the coroutines world seems to be to try-catch your code, because it throws

[aballano]

that’s why you either put the error information in the result (if you wanna use suspended) with attempt or you use IO directly

[Ahmed Ibrahim]

ah, I see, that's clear now, thanks for the explanation!

[aballano]

hope it helps! Don’t hesitate to ask any more questions/feedback you might have 🙂

[Ahmed Ibrahim]

So do you think that I should return instead an IO instead of Either, and do the attempt().suspended() in the client code? and catch the exception there through fold?

[aballano]

my very personal opinion is to execute the IO directly and don’t do suspended in general, but it really depends on the project and what you want to achieve

[aballano]

using IO’s runtime has some advantages over the coroutines one that I’ll try to explain in the docs for the new version, but the most obvious one is that it doesn’t crash on errors

[Ahmed Ibrahim]

But one feedback that I have, is that it would be nice to have an IO.catch { } -> IO<Throwable, A> construct so that it might handle any error that could happen inside that block.

[aballano]

AFAIK that’s already embedded in IO, meaning that any exception that happens in its so to say “effectful constructors” (IO.effect { }, IO { }, etc) is gonna be kept, then you should be able to handle it or not with handleError , and finally execute it and obtain a value, a domain error or an exception

[Ahmed Ibrahim]

OK, now it is clear, thank you so much!
One more question regarding the parallel execution of stuff, do you think what I'm doing through parTraverse is the best way to go, or there is a better alternative that I should look into?

[aballano]

I think that’s good 👍