Hey :slightly_smiling_face: I have a question abou...

# arrow

[Mervyn McCreight]

Hey 🙂 I have a question about the saga-pattern implementation of arrow (https://arrow-kt.io/learn/resilience/saga/).
Does it only work for exceptions, or can a saga action also use the raise-dsl or other forms or typed error handling?

Looking at the implementation (https://github.com/arrow-kt/arrow/blob/main/arrow-libs/resilience/arrow-resilience/src/commonMain/kotlin/arrow/resilience/Saga.kt) I’d assume it only works for actions throwing exceptions in case of a failure, but I may also be wrong 🙂

[simon.vergauwen]

It works with Raise! All DSL from Arrow do! Including cancellation on Arrow Fx Coroutines, Saga, Resource, AutoClose, etc.

Small example:

either {
  saga {
    saga({ println("Action A") }) { println("Rollback A") }
    raise("failed")
    1
  }.transact()
}

There is however a big difference between either { saga { } } or saga { either { } }. The first is a computation that can fail with E, which inside is using a saga. Since Saga sees raise as a failure, it will rollback action A.

In the latter case, we have a Saga that computes an Either. So any Left.bind, or raise, that is encountered will not fail the Saga but would return Either.Left if transact is called on the Saga.

Hope that helps! I think we should probably add this example to the documentation ☺️ I find it really neat that all DSLs can be nested, and that depending on the order you can get different behavior.

[Mervyn McCreight]

Thanks for the detailed explanation!

IMO it would be helpful to explain that in the documentation as well, but it might also be just me 🙂

> I find it really neat that all DSLs can be nested, and that depending on the order you can get different behavior.
👍 - similar to how it is with parMap, ...

[simon.vergauwen]

Yes, I've been meaning to add it website for a while.. 😕

[Mervyn McCreight]

FYI: It works like a charm and I love it 😄

[simon.vergauwen]

Awesome, happy to hear! arrow intensifies