Hi all! For Arrow.Fx does it make sense to have all impure functions have IO return types (in additi...

# arrow

[tavish pegram]

Hi all! For Arrow.Fx does it make sense to have all impure functions have IO return types (in addition to the suspend modifier) or is it best practice to just use IO to run the effects at the boundary of the system and stick to suspend to indicate impurity?

For example
suspend fun printAString(s: String): Unit
suspend fun printAString(s: String): IO<Unit>

At the moment I am attempting to stick to a ports and adapters architecture with suspend for impure functions generally, IO return types at the service level, and actually unwrapping/running the IO at the controllers. The data is also the boundary but it didn't make a lot of sense to me to unwrap anything there so I've just been using suspend in the repos.

As a follow-up, does anyone know if it is possible to custom alias the modifier to something like impure to distinguish from coroutine suspend methods? Or am I thinking about that incorrectly? Like
impure fun printAString(s: String): Unit
Is that something we might want to use Arrow.Meta for?

Thanks!

[Marcin Gryszko]

Should you second example be fun printAString(s: String): IO<Unit>?

In my view, returning IO documents sufficiently that the function has side effects.

[tim]

As these operations occur at the boundary, why wouldn't you resolve the IO inside this function and return an Either/Option rather than IO itself?

Unless I'm missing something, the IO has to be resolved at some point and it seem to me at the boundary seems like a good place to do this?

[stojan]

you would either use suspend OR IO , both suspend and IO doesn't really make sense 🙂

[stojan]

you can also convert from suspend to IO using IO.effect or from IO to suspend using .suspended()

[tavish pegram]

thanks for the responses! I will talk with my team about what is preferred: IO return types or suspend.

@tim sorry I should clarify that they are not occurring at the boundary, just running the effects on the boundary. So like a service method returns side effects so I'm running them at the controller.

[tim]

I definitely don't understand this well enough 🙂

[stojan]

in certain cases you have to use IO , e.g. switching threads, racing, parMap , parTraverse etc....

[stojan]

https://kotlinlang.slack.com/archives/C5UPMM0A0/p1588325627015600