Hello, hello! How do we switch CoroutineContexts w...

# arrow

[Danilo Herrera]

Hello, hello! How do we switch CoroutineContexts within IO.fx { ... }? I tried starting the block with continueOn(<http://Dispatchers.IO|Dispatchers.IO>) but for some reason it hangs.

[Bob Glamm]

https://arrow-kt.io/docs/effects/async/#continueon

[Danilo Herrera]

Thanks for replying, Bob! Does this mean that continueOn is meant to be used with async and not fx?

[Bob Glamm]

ugh. Sorry, I totally misread your question. Just based on reading the continueOn example it may be that you will need to use that combinator outside of the fx block

[Bob Glamm]

because chain in the example is Async.just(...).continueOn(...).flatMap(...).continueOn(...).flatMap(...)

[Bob Glamm]

I'm not sure if that translates to `IO.fx { ... }`'s syntax

[Danilo Herrera]

Ahh ok, I think I'm misusing the APIs with this example:

IO.fx {
            continueOn(<http://Dispatchers.IO|Dispatchers.IO>)
            val inventory = !requestOperations.getInventory()
            continueOn(Dispatchers.Main)
            inventoryLiveData.value = inventory
        }
            .attempt()
            .unsafeRunSync()
            .fold(
                ifLeft = {
                    // handle error
                },
                ifRight = {
                    // completed
                }
            )

[Bob Glamm]

So I'm wondering if you can have something like this:

IO.unit.continueOn(Dispatchers.Main).flatMap { IO.fx { ... } }

[Bob Glamm]

but I'll admit I don't know if that works or even yields the correct behavior

[Danilo Herrera]

well, that definitely worked!

[Bob Glamm]

I'm happy to hear that! I think continueOn has to happen at IO "program" boundaries

[Bob Glamm]

basically IO.continueOn(...).flatMap(IO).continueOn(...).flatMap(IO)...

[Bob Glamm]

but @simon.vergauwen or @pakoito or @raulraja will have to chime in on whether that assumption is correct or not

[pakoito]

when you do unsafeRunSync you’re blocking the current thread, which I’ll assume is the main thread. Then inside the IO you try to return back but it’s blocked

[pakoito]

use unsafeRunAsync { /* NOTHING */ } and it you want to display something on the UI do it on the fx block

[pakoito]

it’s a bad habit we got with Rx

[pakoito]

myOperation.observeOn(Main).subscribe { ui() }

where it should be

myOperation.observeOn(Main).flatMap { ui() }.subscribe { /* unrecoverable exception + crash */ }

[pakoito]

note that subscribe is unsafeRunAsync, whereas unsafeRynSync is blockingFirst

[Danilo Herrera]

so would you handle errors within unsafeRunAsync?

[Danilo Herrera]

unsafeRunSync was the precise reason it was hanging! Got it to work with the async version:

IO.fx {
            continueOn(<http://Dispatchers.IO|Dispatchers.IO>)
            val inventory = !requestOperations.getInventory()
            continueOn(Dispatchers.Main)
            inventoryLiveData.value = inventory
        }
            .unsafeRunAsync {
                it.fold(
                    ifLeft = {
                        // handle error
                    },
                    ifRight = {
                        // completed
                    }
                )
            }

[raulraja]

You can use continue on and arbitrary parMapN inside Fx with your own dispatchers

[raulraja]

It's an effect

[raulraja]

Anything can go inside IO

[Danilo Herrera]

I'm slowly but surely getting the picture!

[pakoito]

so would you handle errors within unsafeRunAsync

.handleErrorWith before you run it

[pakoito]

and you can do that at any layer

[pakoito]

per operation

!requestOperations.getInventory()
  .handleError { showErrorUI() }

for the whole thing

IO. fx { ... }
  .handleErrorWith { 
    effect(UI) { showErrorUI()  } 
  }