Hello Arrow community. I maintain <https://github.com/fraktalio/fmodel> and I am adopting (experimen...

# arrow

[Ivan Dugalic]

Hello Arrow community. I maintain https://github.com/fraktalio/fmodel and I am adopting (experimenting with) Arrow 2.0.0-SNAPSHOT (with context receivers). I will explain the case in the thread and your toughs/ideas are much appreciated:

[Ivan Dugalic]

Lets say we have this function which is handling some Command/C and storing new State/S:

[Ivan Dugalic]

context (StateComputation<C, S, E>, StateRepository<C, S>, Raise<Error>)
suspend fun <C, S, E> C.handleWithEffect(): S =
    catch({
        fetchState().computeNewState(this@handleWithEffect).save()
    }) {
        raise(CommandHandlingFailed(this, it))
    }

[Ivan Dugalic]

Now, lets handle the Flow of Commands by using ☝️function:

[Ivan Dugalic]

OPTION1:

[Ivan Dugalic]

context (StateComputation<C, S, E>, StateRepository<C, S>)
fun <C, S, E> Flow<C>.handleWithEffect(): Flow<Effect<Error, S>> =
    map { effect { it.handleWithEffect() } }.catch { emit(effect { raise(CommandPublishingFailed(it)) }) }

[Ivan Dugalic]

OPTION2:

[Ivan Dugalic]

context (StateComputation<C, S, E>, StateRepository<C, S>, Raise<Error>)
fun <C, S, E> Flow<C>.handleWithEffect2(): Flow<S> =
    map { it.handleWithEffect() }.catch { raise(CommandPublishingFailed(it) ) }

[Ivan Dugalic]

OPTION2 is more intuitive to me, but OPTION1 seems to be a better contract. What do you think?

[simon.vergauwen]

Hey @Ivan Dugalic,

Option 1 is safer since it always wraps the effect correctly, int he second option you need to be sure that collect is called inside effect { }.

[Ivan Dugalic]

Thanks @simon.vergauwen! Indeed.

[Ivan Dugalic]

My observation is that people could end-up in OPTION2, especially once you use context receivers to put Raise<Error> in the scope. It is tempting to return Flow<S> 🙂 rather then returning explicitly Flow<Effect<Error, S>>

[Ivan Dugalic]

I am also considering returning Flow<Either<Error, S>> rather then Flow<Effect<Error, S>> . What do you think? Do you see any cons of using Effect?

[simon.vergauwen]

Either is definitely a good option also.

I would say that Either is better understood, and it prevents leaking raise operator. It's a good type to use in this case.

Effect has the ability to be "traced", which also might be useful. Tracing POC

Exposing both is probably not useful, since it'll result in signature clashes all over the place.